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Sample records for active phonon modes

  1. Infra red active modes due to coupling of cyclotron excitation and LO phonons in polar semiconductor

    NASA Astrophysics Data System (ADS)

    Agrawal, Ratna; Dubey, Swati; Ghosh, S.

    2013-06-01

    Effects of free carrier concentration, external magnetic field and Callen effective charge on infra red active modes in a polar semiconductor have been analytically investigated using simple harmonic oscillator model. Callen effective charge considerably enhances reflectivity and shifts minima towards lower values of energy. Presence of magnetic field leads towards the coupling of collective cyclotron excitations with LO phonon giving rise to maximum reflectivity whereas cyclotron resonance absorption results into minimum reflectivity.

  2. Phonons in active microfluidic crystals

    NASA Astrophysics Data System (ADS)

    Tsang, Alan Cheng Hou; Kanso, Eva

    2016-11-01

    One-dimensional crystals of driven particles confined in quasi two-dimensional microfluidic channels have been shown to exhibit propagating sound waves in the form of 'phonons', including both transverse and longitudinal normal modes. Here, we focus on one-dimensional crystals of motile particles in uniform external flows. We study the propagation of phonons in the context of an idealized model that accounts for hydrodynamic interactions among the motile particles. We obtain a closed-form analytical expression for the dispersion relation of the phonons. In the moving frame of reference of the crystals, the traveling directions of the phonons depend on the intensity of the external flow, and are exactly opposite for the transverse and longitudinal modes. We further investigate the stability of the phonons and show that the longitudinal mode is linearly stable, whereas the transverse mode is subject to an instability arising from the activity and orientation dynamics of the motile particles. These findings are important for understanding the propagation of disturbances and instabilities in confined motile particles, and could generate practical insights into the transport of motile cells in microfluidic devices.

  3. Phonon-plasmon coupled modes in GaN

    NASA Astrophysics Data System (ADS)

    Dyson, A.

    2009-04-01

    The phonon lifetime in GaN is known to exhibit a dependence on electron density. Recent noise measurements have also shown the lifetime to be temperature dependent. The source of these dependences is the coupling of the phonon and plasmon populations through the dielectric function. The effect of this anharmonicity is illustrated by comparing the frequency and wavevector dependent coupled-mode momentum relaxation rate with the phonon momentum relaxation rate obtained by Callen. A simple model that includes the anharmonic interaction and phonon migration yields phonon lifetimes depending on both electron density and temperature.

  4. Temperature dependence of the Raman-active phonon frequencies in indium sulfide

    NASA Astrophysics Data System (ADS)

    Gasanly, N. M.; Özkan, H.; Aydinli, A.; Yilmaz, İ.

    1999-03-01

    The temperature dependence of the Raman-active mode frequencies in indium sulfide was measured in the range from 10 to 300 K. The analysis of the temperature dependence of the A g intralayer optical modes show that Raman frequency shift results from the change of harmonic frequency with volume expansion and anharmonic coupling to phonons of other branches. The pure-temperature contribution (phonon-phonon coupling) is due to three- and four-phonon processes.

  5. Topological Phonon Modes in a Two-Dimensional Wigner Crystal

    NASA Astrophysics Data System (ADS)

    Ji, Wen-Cheng; Shi, Jun-Ren

    2017-03-01

    We investigate the spin-orbit coupling effect in a two-dimensional Wigner crystal. We show that sufficiently strong spin-orbit coupling and an appropriate sign of g-factor could transform the Wigner crystal to a topological phonon system. We demonstrate the existence of chiral phonon edge modes in finite size samples, as well as the robustness of the modes in the topological phase. We explore the possibility of realizing the topological phonon system in two-dimensional Wigner crystals confined in semiconductor quantum wells/heterostructure. We find that the spin-orbit coupling is too weak for driving a topological phase transition in these systems. We argue that one may look for the topological phonon system in correlated Wigner crystals with emergent effective spin-orbit coupling.

  6. Lattice parameters and Raman-active phonon modes of β-(Al{sub x}Ga{sub 1−x}){sub 2}O{sub 3}

    SciTech Connect

    Kranert, Christian Jenderka, Marcus; Lenzner, Jörg; Lorenz, Michael; Wenckstern, Holger von; Schmidt-Grund, Rüdiger; Grundmann, Marius

    2015-03-28

    We present X-ray diffraction and Raman spectroscopy investigations of a (100)-oriented (Al{sub x}Ga{sub 1–x}){sub 2}O{sub 3} thin film on MgO (100) and bulk-like ceramics in dependence on their composition. The thin film grown by pulsed laser deposition has a continuous lateral composition spread allowing to determine precisely the dependence of the phonon mode properties and lattice parameters on the chemical composition. For x < 0.4, we observe the single-phase β-modification. Its lattice parameters and phonon energies depend linearly on the composition. We determined the slopes of these dependencies for the individual lattice parameters and for nine Raman lines, respectively. While the lattice parameters of the ceramics follow Vegard's rule, deviations are observed for the thin film. This deviation has only a small effect on the phonon energies, which show a reasonably good agreement between thin film and ceramics.

  7. CO2 Infrared Phonon Modes in Interstellar Ice Mixtures

    NASA Astrophysics Data System (ADS)

    Cooke, Ilsa R.; Fayolle, Edith C.; Öberg, Karin I.

    2016-11-01

    CO2 ice is an important reservoir of carbon and oxygen in star- and planet-forming regions. Together with water and CO, CO2 sets the physical and chemical characteristics of interstellar icy grain mantles, including desorption and diffusion energies for other ice constituents. A detailed understanding of CO2 ice spectroscopy is a prerequisite to characterize CO2 interactions with other volatiles both in interstellar ices and in laboratory experiments of interstellar ice analogs. We report laboratory spectra of the CO2 longitudinal optical (LO) phonon mode in pure CO2 ice and in CO2 ice mixtures with H2O, CO, and O2 components. We show that the LO phonon mode position is sensitive to the mixing ratio of various ice components of astronomical interest. In the era of the James Webb Space Telescope, this characteristic could be used to constrain interstellar ice compositions and morphologies. More immediately, LO phonon mode spectroscopy provides a sensitive probe of ice mixing in the laboratory and should thus enable diffusion measurements with higher precision than has been previously possible.

  8. Optical phonon modes in rhombohedral boron monosulfide under high pressure

    SciTech Connect

    Cherednichenko, Kirill A.; Sokolov, Petr S.; Solozhenko, Vladimir L.; Kalinko, Aleksandr; Le Godec, Yann; Polian, Alain; Itié, Jean-Paul

    2015-05-14

    Raman spectra of rhombohedral boron monosulfide (r-BS) were measured under pressures up to 34 GPa at room temperature. No pressure-induced structural phase transition was observed, while strong pressure shift of Raman bands towards higher wavenumbers has been revealed. IR spectroscopy as a complementary technique has been used in order to completely describe the phonon modes of r-BS. All experimentally observed bands have been compared with theoretically calculated ones and modes assignment has been performed. r-BS enriched by {sup 10}B isotope was synthesized, and the effect of boron isotopic substitution on Raman spectra was observed and analyzed.

  9. Infrared dielectric anisotropy and phonon modes of rutile TiO2

    NASA Astrophysics Data System (ADS)

    Schöche, S.; Hofmann, T.; Korlacki, R.; Tiwald, T. E.; Schubert, M.

    2013-04-01

    Spectroscopic ellipsometry in the mid-infrared and far-infrared spectral range and generalized ellipsometry in the mid-infrared spectral range are used to investigate the anisotropic dielectric response of rutile TiO2. The ordinary and extraordinary dielectric function tensor components and all infrared active phonon mode parameters of single crystalline rutile TiO2 are determined with high accuracy for wavelengths from 3 μm to 83 μm. The data were acquired from samples of (001), (100), and (111) surfaces cut from bulk single crystals. A factorized model dielectric function is employed in order to determine the frequencies and damping parameters of the transverse and longitudinal phonon modes with A2u and Eu symmetries. The bands of total reflection of s- and p-polarized light in dependence of the angle of incidence for highly symmetric sample cuts and orientations are derived. Excellent agreement with phonon modes reported in literature is obtained. Introduction of two additional modes for ordinary as well as extraordinary component of the dielectric function tensor was necessary to most accurately match the experimental data. The spectral position of the additional modes is compared to the calculated phonon density of states. The low-frequency dielectric constants are calculated from the determined phonon mode parameters and the high-frequency dielectric constants by applying the Lyddanne-Sachs-Teller relation. The presented data revise existing infrared optical function data and will be suitable for interpretation of any kind of infrared spectra for bulk TiO2 single crystal substrates, thin films, and TiO2 nanostructures.

  10. Linear mode-mixing of phonons with trapped ions

    NASA Astrophysics Data System (ADS)

    Marshall, Kevin; James, Daniel F. V.

    2017-01-01

    We propose a method to manipulate the normal modes in a chain of trapped ions using only two lasers. Linear chains of trapped ions have proven experimentally to be highly controllable quantum systems with a variety of refined techniques for preparation, evolution, and readout; however, typically for quantum information processing applications people have been interested in using the internal levels of the ions as the computational basis. We analyze the case where the motional degrees of freedom of the ions are the quantum system of interest, and where the internal levels are leveraged to facilitate interactions. In particular, we focus on an analysis of mode-mixing of phonons in different normal modes to mimic the quantum optical equivalent of a beam splitter.

  11. Photo-excited charge carriers suppress sub-terahertz phonon mode in silicon at room temperature

    NASA Astrophysics Data System (ADS)

    Liao, Bolin; Maznev, A. A.; Nelson, Keith A.; Chen, Gang

    2016-10-01

    There is a growing interest in the mode-by-mode understanding of electron and phonon transport for improving energy conversion technologies, such as thermoelectrics and photovoltaics. Whereas remarkable progress has been made in probing phonon-phonon interactions, it has been a challenge to directly measure electron-phonon interactions at the single-mode level, especially their effect on phonon transport above cryogenic temperatures. Here we use three-pulse photoacoustic spectroscopy to investigate the damping of a single sub-terahertz coherent phonon mode by free charge carriers in silicon at room temperature. Building on conventional pump-probe photoacoustic spectroscopy, we introduce an additional laser pulse to optically generate charge carriers, and carefully design temporal sequence of the three pulses to unambiguously quantify the scattering rate of a single-phonon mode due to the electron-phonon interaction. Our results confirm predictions from first-principles simulations and indicate the importance of the often-neglected effect of electron-phonon interaction on phonon transport in doped semiconductors.

  12. Photo-excited charge carriers suppress sub-terahertz phonon mode in silicon at room temperature

    PubMed Central

    Liao, Bolin; Maznev, A. A.; Nelson, Keith A.; Chen, Gang

    2016-01-01

    There is a growing interest in the mode-by-mode understanding of electron and phonon transport for improving energy conversion technologies, such as thermoelectrics and photovoltaics. Whereas remarkable progress has been made in probing phonon–phonon interactions, it has been a challenge to directly measure electron–phonon interactions at the single-mode level, especially their effect on phonon transport above cryogenic temperatures. Here we use three-pulse photoacoustic spectroscopy to investigate the damping of a single sub-terahertz coherent phonon mode by free charge carriers in silicon at room temperature. Building on conventional pump–probe photoacoustic spectroscopy, we introduce an additional laser pulse to optically generate charge carriers, and carefully design temporal sequence of the three pulses to unambiguously quantify the scattering rate of a single-phonon mode due to the electron–phonon interaction. Our results confirm predictions from first-principles simulations and indicate the importance of the often-neglected effect of electron–phonon interaction on phonon transport in doped semiconductors. PMID:27731406

  13. Phonons in Wurtzite (GaN) mu(AIN) nu Superlattices: Non-Monotoneous Dependence of the Number of Raman-Active Modes on Superlattice Period

    DTIC Science & Technology

    1999-06-18

    The symmetry of wurtzite (GaN)(sub m) (AlN)(sub n), is established to depend on the numbers of monolayers of constituent materials in the primitive ... cell being C?4 to 6V! (odd m+n) or C?1 to 3V! (even m+n). As a result it was shown that the number of Raman active modes depends non-monotonically on m+n.

  14. Anisotropy, phonon modes, and free charge carrier parameters in monoclinic β -gallium oxide single crystals

    NASA Astrophysics Data System (ADS)

    Schubert, M.; Korlacki, R.; Knight, S.; Hofmann, T.; Schöche, S.; Darakchieva, V.; Janzén, E.; Monemar, B.; Gogova, D.; Thieu, Q.-T.; Togashi, R.; Murakami, H.; Kumagai, Y.; Goto, K.; Kuramata, A.; Yamakoshi, S.; Higashiwaki, M.

    2016-03-01

    We derive a dielectric function tensor model approach to render the optical response of monoclinic and triclinic symmetry materials with multiple uncoupled infrared and far-infrared active modes. We apply our model approach to monoclinic β -Ga2O3 single-crystal samples. Surfaces cut under different angles from a bulk crystal, (010) and (2 ¯01 ), are investigated by generalized spectroscopic ellipsometry within infrared and far-infrared spectral regions. We determine the frequency dependence of 4 independent β -Ga2O3 Cartesian dielectric function tensor elements by matching large sets of experimental data using a point-by-point data inversion approach. From matching our monoclinic model to the obtained 4 dielectric function tensor components, we determine all infrared and far-infrared active transverse optic phonon modes with Au and Bu symmetry, and their eigenvectors within the monoclinic lattice. We find excellent agreement between our model results and results of density functional theory calculations. We derive and discuss the frequencies of longitudinal optical phonons in β -Ga2O3 . We derive and report density and anisotropic mobility parameters of the free charge carriers within the tin-doped crystals. We discuss the occurrence of longitudinal phonon plasmon coupled modes in β -Ga2O3 and provide their frequencies and eigenvectors. We also discuss and present monoclinic dielectric constants for static electric fields and frequencies above the reststrahlen range, and we provide a generalization of the Lyddane-Sachs-Teller relation for monoclinic lattices with infrared and far-infrared active modes. We find that the generalized Lyddane-Sachs-Teller relation is fulfilled excellently for β -Ga2O3 .

  15. Phonon Heat Conduction In Nanostructures: Ballistic, Coherent, Localized, Hydrodynamic, and Divergent Modes

    NASA Astrophysics Data System (ADS)

    Chen, Gang

    In this talk, we will discuss different modes of heat conduction in nanostructures. Ballistic transport happens when phonon mean free path is longer than the characteristic size of the structure. We will discuss how we compute phonon mean free path distributions based on first-principles and measure the distributions with optical pump-probe techniques by exploring ballistic phonon transport processes. In superlattice structures, ballistic phonon transport across the whole thickness of the superlattices implies phase coherence. We observed this coherent transport in GaAs/AlAs superlattices with fixed periodic thickness and varying number of periods. Simulations show that although high frequency phonons are scattering by roughness, remaining long wavelength phonons maintain their phase and traverse the superlattices ballistically. Accessing the coherent heat conduction regime opens a new venue for phonon engineering. We show further that phonon heat conduction localization happens in GaAs/AlAs superlattice by placing ErAs nanodots at interfaces. This heat-conduction localization phenomenon is confirmed by nonequilibrium atomic Green's function simulation. These ballistic and localization effects can be exploited to improve thermoelectric energy conversion materials via reducing their thermal conductivity. In another opposite, we will discuss phonon hydrodynamic transport mode in graphene via first-principle simulations. In this mode, phonons drift with an average velocity under a temperature gradient, similar to fluid flow in a pipe. Conditions for observing such phonon hydrodynamic modes will be discussed. Finally, we will talk about the one-dimensional nature of heat conduction in polymer chains. Such 1D nature can lead to divergent thermal conductivity. Inspired by simulation, we have experimentally demonstrated high thermal conductivity in ultra-drawn polyethylene nanofibers and sheets. Work supported by DOE Office of Basic Energy Sciences under Award Number: DE

  16. Temperature dependence of Raman-active phonons and anharmonic interactions in layered hexagonal BN

    NASA Astrophysics Data System (ADS)

    Cuscó, Ramon; Gil, Bernard; Cassabois, Guillaume; Artús, Luis

    2016-10-01

    We present a Raman scattering study of optical phonons in hexagonal BN for temperatures ranging from 80 to 600 K. The experiments were performed on high-quality, single-crystalline hexagonal BN platelets. The observed temperature dependence of the frequencies and linewidths of both Raman active E2 g optical phonons is analyzed in the framework of anharmonic decay theory, and possible decay channels are discussed in the light of density-functional theory calculations. With increasing temperature, the E2g high mode displays strong anharmonic interactions, with a linewidth increase that indicates an important contribution of four-phonon processes and a marked frequency downshift that can be attributed to a substantial effect of the four-phonon scattering processes (quartic anharmonicity). In contrast, the E2g low mode displays a very narrow linewidth and weak anharmonic interactions, with a frequency downshift that is primarily accounted for by the thermal expansion of the interlayer spacing.

  17. An informatics based analysis of the impact of isotope substitution on phonon modes in graphene

    SciTech Connect

    Broderick, Scott; Srinivasan, Srikant; Rajan, Krishna; Ray, Upamanyu; Balasubramanian, Ganesh

    2014-06-16

    It is shown by informatics that the high frequency short ranged modes exert a significant influence in impeding thermal transport through isotope substituted graphene nanoribbons. Using eigenvalue decomposition methods, we have extracted features in the phonon density of states spectra that reveal correlations between isotope substitution and phonon modes. This study also provides a data driven computational framework for the linking of materials chemistry and transport properties in 2D systems.

  18. Frequency stabilization of the zero-phonon line of a quantum dot via phonon-assisted active feedback

    SciTech Connect

    Hansom, Jack; Schulte, Carsten H. H.; Matthiesen, Clemens; Stanley, Megan J.; Atatüre, Mete

    2014-10-27

    We report on the feedback stabilization of the zero-phonon emission frequency of a single InAs quantum dot. The spectral separation of the phonon-assisted component of the resonance fluorescence provides a probe of the detuning between the zero-phonon transition and the resonant driving laser. Using this probe in combination with active feedback, we stabilize the zero-phonon transition frequency against environmental fluctuations. This protocol reduces the zero-phonon fluorescence intensity noise by a factor of 22 by correcting for environmental noise with a bandwidth of 191 Hz, limited by the experimental collection efficiency. The associated sub-Hz fluctuations in the zero-phonon central frequency are reduced by a factor of 7. This technique provides a means of stabilizing the quantum dot emission frequency without requiring access to the zero-phonon emission.

  19. Coherent phonon optics in a chip with an electrically controlled active device.

    PubMed

    Poyser, Caroline L; Akimov, Andrey V; Campion, Richard P; Kent, Anthony J

    2015-02-05

    Phonon optics concerns operations with high-frequency acoustic waves in solid media in a similar way to how traditional optics operates with the light beams (i.e. photons). Phonon optics experiments with coherent terahertz and sub-terahertz phonons promise a revolution in various technical applications related to high-frequency acoustics, imaging, and heat transport. Previously, phonon optics used passive methods for manipulations with propagating phonon beams that did not enable their external control. Here we fabricate a phononic chip, which includes a generator of coherent monochromatic phonons with frequency 378 GHz, a sensitive coherent phonon detector, and an active layer: a doped semiconductor superlattice, with electrical contacts, inserted into the phonon propagation path. In the experiments, we demonstrate the modulation of the coherent phonon flux by an external electrical bias applied to the active layer. Phonon optics using external control broadens the spectrum of prospective applications of phononics on the nanometer scale.

  20. Coherent phonon optics in a chip with an electrically controlled active device

    PubMed Central

    Poyser, Caroline L.; Akimov, Andrey V.; Campion, Richard P.; Kent, Anthony J.

    2015-01-01

    Phonon optics concerns operations with high-frequency acoustic waves in solid media in a similar way to how traditional optics operates with the light beams (i.e. photons). Phonon optics experiments with coherent terahertz and sub-terahertz phonons promise a revolution in various technical applications related to high-frequency acoustics, imaging, and heat transport. Previously, phonon optics used passive methods for manipulations with propagating phonon beams that did not enable their external control. Here we fabricate a phononic chip, which includes a generator of coherent monochromatic phonons with frequency 378 GHz, a sensitive coherent phonon detector, and an active layer: a doped semiconductor superlattice, with electrical contacts, inserted into the phonon propagation path. In the experiments, we demonstrate the modulation of the coherent phonon flux by an external electrical bias applied to the active layer. Phonon optics using external control broadens the spectrum of prospective applications of phononics on the nanometer scale. PMID:25652241

  1. Detection of phonon and phason modes in intrinsic colloidal quasicrystals by reconstructing their structure in hyperspace

    NASA Astrophysics Data System (ADS)

    Hielscher, J.; Martinsons, M.; Schmiedeberg, M.; Kapfer, S. C.

    2017-03-01

    Phasons are additional degrees of freedom which occur in quasicrystals alongside the phonons known from conventional periodic crystals. The rearrangements of particles that are associated with a phason mode are hard to interpret in physical space. We reconstruct the quasicrystal structure by an embedding into extended higher-dimensional space, where phasons correspond to displacements perpendicular to the physical space. In dislocation-free decagonal colloidal quasicrystals annealed with Brownian dynamics simulations, we identify thermal phonon and phason modes. Finite phononic strain is pinned by phasonic excitations even after cooling down to zero temperature. For the phasonic displacements underlying the flip pattern, the reconstruction method gives an approximation within the limits of a multi-mode harmonic ansatz, and points to fundamental limitations of a harmonic picture for phasonic excitations in intrinsic colloidal quasicrystals.

  2. Detection of phonon and phason modes in intrinsic colloidal quasicrystals by reconstructing their structure in hyperspace.

    PubMed

    Hielscher, J; Martinsons, M; Schmiedeberg, M; Kapfer, S C

    2017-03-08

    Phasons are additional degrees of freedom which occur in quasicrystals alongside the phonons known from conventional periodic crystals. The rearrangements of particles that are associated with a phason mode are hard to interpret in physical space. We reconstruct the quasicrystal structure by an embedding into extended higher-dimensional space, where phasons correspond to displacements perpendicular to the physical space. In dislocation-free decagonal colloidal quasicrystals annealed with Brownian dynamics simulations, we identify thermal phonon and phason modes. Finite phononic strain is pinned by phasonic excitations even after cooling down to zero temperature. For the phasonic displacements underlying the flip pattern, the reconstruction method gives an approximation within the limits of a multi-mode harmonic ansatz, and points to fundamental limitations of a harmonic picture for phasonic excitations in intrinsic colloidal quasicrystals.

  3. Dynamic Jahn-Teller viewpoint for generation mechanism of asymmetric modes of coherent phonons

    NASA Astrophysics Data System (ADS)

    Kayanuma, Yosuke; Nakamura, Kazutaka G.

    2017-03-01

    We propose a dynamic Jahn-Teller approach to elucidate the generation mechanism of asymmetric modes of coherent phonons induced in crystals by irradiation with a short optical pulse in the opaque energy region. This is a natural extension of the impulsive excitation model of symmetric modes to multi dimensions in the configuration coordinate space. We show that the two generation mechanisms of coherent phonons coexist in this case, namely the impulsive absorption (IA) mechanism and impulsive stimulated Raman scattering (ISRS) mechanism. The dependence of the phonon amplitude on the polarization of the pump pulse is exactly the same in IA and ISRS processes and is in agreement with the prediction of the argument based on Raman tensors. The dependence of the excitation efficiency of the coherent phonons on the frequency of the pump pulse is calculated using a simplified model of the optical response function of the crystal. Generally, the IA mechanism predominates in the opaque region, although ISRS makes a comparable contribution to phonon generation in the near-edge opaque region. The initial phase of the coherent phonon is always cosine-like in IA but depends on the excitation frequency in ISRS.

  4. Mode-selective phonon excitation in gallium nitride using mid-infrared free-electron laser

    NASA Astrophysics Data System (ADS)

    Kagaya, Muneyuki; Yoshida, Kyohei; Zen, Heishun; Hachiya, Kan; Sagawa, Takashi; Ohgaki, Hideaki

    2017-02-01

    The single-phonon mode was selectively excited in a solid-state sample. A mid-infrared free-electron laser, which was tuned to the target phonon mode, was irradiated onto a crystal cooled to a cryogenic temperature, where modes other than the intended excitation were suppressed. An A 1(LO) vibrational mode excitation on GaN(0001) face was demonstrated. Anti-Stokes Raman scattering was used to observe the excited vibrational mode, and the appearance and disappearance of the scattering band at the target wavenumber were confirmed to correspond to on and off switching of the pump free-electron laser and were fixed to the sample vibrational mode. The sum-frequency generation signals of the pump and probe lasers overlapped the Raman signals and followed the wavenumber shift of the pump laser.

  5. Optical absorption in semiconductor quantum dots coupling to dispersive phonons of infinite modes

    NASA Astrophysics Data System (ADS)

    Ding, Zhiwen; Wang, Qin; Zheng, Hang

    2012-10-01

    Optical absorption spectrum of semiconductor quantum dot is investigated by means of an analytical approach based on the Green's function for different forms of coupling strength in an unified method by using the standard model with valence and conduction band levels coupled to dispersive quantum phonons of infinite modes. The analytical expression of the optical absorption coefficient in semiconductor quantum dots is obtained and by this expression the line shape and the peak position of the absorption spectrum are procured. The relation between the properties of absorption spectrum and the forms of coupling strength is clarified, which can be referenced for choosing the proper form of the coupling strength or spectral density to control the features of absorption spectrum of quantum dot. The coupling and confinement induced energy shift and intensity decrease in the absorption spectrum are determined precisely for a wide range of parameters. The results show that the activation energy of the optical absorption is reduced by the effect of exciton-phonon coupling and photons with lower frequencies could also be absorbed in absorption process. With increase of the coupling constant, the line shape of optical absorption spectrum broadens and the peak position moves to lower photon energy with a rapid decrease in intensity at the same time. Both the coupling induced red shift and the confinement induced blue shift conduce to decrease in the intensity of absorption spectrum. Furthermore, this method may have application potential to other confined quantum systems.

  6. Overtones of interlayer shear modes in the phonon-assisted emission spectrum of hexagonal boron nitride

    NASA Astrophysics Data System (ADS)

    Vuong, T. Q. P.; Cassabois, G.; Valvin, P.; Jacques, V.; Cuscó, R.; Artús, L.; Gil, B.

    2017-01-01

    We address the intrinsic optical properties of hexagonal boron nitride in deep ultraviolet. We show that the fine structure of the phonon replicas arises from overtones involving up to six low-energy interlayer shear modes. These lattice vibrations are specific to layered compounds since they correspond to the shear rigid motion between adjacent layers, with a characteristic energy of about 6-7 meV. We obtain a quantitative interpretation of the multiplet observed in each phonon replica under the assumption of a cumulative Gaussian broadening as a function of the overtone index, and with a phenomenological line broadening taken identical for all phonon types. We show from our quantitative interpretation of the full emission spectrum above 5.7 eV that the energy of the involved phonon mode is 6.8 ±0.5 meV, in excellent agreement with temperature-dependent Raman measurements of the low-energy interlayer shear mode in hexagonal boron nitride. We highlight the unusual properties of this material where the optical response is tailored by the phonon group velocities in the middle of the Brillouin zone.

  7. Spectroscopy of infrared-active phonons in high-temperature superconductors

    NASA Technical Reports Server (NTRS)

    Litvinchuk, A. P.; Thomsen, C.; Cardona, M.; Borjesson, L.

    1995-01-01

    For a large variety of superconducting materials both experimental and theoretical lattice dynamical studies have been performed to date. The assignment of the observed infrared- and Raman-active phonon modes to the particular lattice eigenmodes is generally accepted. We will concentrate here upon the analysis of the changes of the infrared-phonon parameters (frequency and linewidth) upon entering the superconducting state which, as will be shown, may provide information on the magnitude of the superconductivity-related gap and its dependence on the superconducting transition temperature Tc.

  8. Correlation effects and phonon modes softening with doping in Ba₁-xKxBiO₃.

    PubMed

    Korotin, Dm M; Novoselov, D; Anisimov, V I

    2014-05-14

    The monoclinic crystal structure of the undoped BaBiO₃ can be described as a cubic perovskite which is distorted by the frozen breathing and tilting phonon modes of the BiO₆ octahedra. The phonon mode softening is experimentally observed (Braden et al 1996 Europhys. Lett. 34 531) in Ba₁-xKxBiO₃ through potassium doping followed by a transition into an ideal cubic perovskite structure at x = 0.37, close to the emergence of superconductivity. In our previous paper (Korotin et al 2012 J. Phys.: Condens. Matter 24 415603) we demonstrated that it is necessary to take into account correlation effects using the DFT+U method in Wannier functions as a basis to obtain a good agreement between the calculated and experimental values of crystal structure distortion and the energy gap in BaBiO₃. In the present work, using the same method, we calculated the breathing mode phonon frequencies as a function of the potassium doping level in Ba₁-xKxBiO₃. The obtained frequencies are in good agreement with experimental values and the breathing mode softening with doping is reproduced, contrary to calculations made without consideration of correlation effects. We show that the cubic crystal structure becomes stable at x = 0.30 in agreement with the experimental transition to cubic perovskite at x = 0.37. The possible connections between the correlation effects, phonon mode softening and superconductivity in Ba₁-xKxBiO₃ are discussed.

  9. Hot carrier relaxation in CdTe via phonon-plasmon modes.

    PubMed

    Zhong, Y; Ostach, D; Scholz, M; Epp, S W; Techert, S; Schlichting, I; Ullrich, J; Krasniqi, F S

    2017-03-08

    Carrier and lattice dynamics of laser excited CdTe was studied by time-resolved reflectivity for excitation fluences spanning about three orders of magnitude, from 0.064 to 6.14 mJ cm(-2). At fluences below 1 mJ cm(-2) the transient reflectivity is dominated by the dynamics of hybrid phonon-plasmon modes. At fluences above 1 mJ cm(-2) the time-dependent reflectivity curves show a complex interplay between band-gap renormalization, band filling, carrier dynamics and recombination. A framework that accounts for such complex dynamics is presented and used to model the time-dependent reflectivity data. This model suggests that the excess energy of the laser-excited hot carriers is reduced much more efficiently by emitting hybrid phonon-plasmon modes rather than bare longitudinal optical phonons.

  10. Dynamically coupled plasmon-phonon modes in GaP: An indirect-gap polar semiconductor

    NASA Astrophysics Data System (ADS)

    Ishioka, Kunie; Brixius, Kristina; Höfer, Ulrich; Rustagi, Avinash; Thatcher, Evan M.; Stanton, Christopher J.; Petek, Hrvoje

    2015-11-01

    The ultrafast coupling dynamics of coherent optical phonons and the photoexcited electron-hole plasma in the indirect gap semiconductor GaP are investigated by experiment and theory. For below-gap excitation and probing by 800-nm light, only the bare longitudinal optical (LO) phonons are observed. For above-gap excitation with 400-nm light, the photoexcitation creates a high density, nonequilibrium e -h plasma, which introduces an additional, faster decaying oscillation due to an LO phonon-plasmon coupled (LOPC) mode. The LOPC mode frequency exhibits very similar behavior for both n - and p -doped GaP, downshifting from the LO to the transverse optical (TO) phonon frequency limits with increasing photoexcited carrier density. We assign the LOPC mode to the LO phonons coupled with the photoexcited multicomponent plasma. For the 400-nm excitation, the majority of the photoexcited electrons are scattered from the Γ valley into the satellite X valley, while the light and spin-split holes are scattered into the heavy hole band, within 30 fs. The resulting mixed plasma is strongly damped, leading to the LOPC frequency appearing in the reststrahlen gap. Due to the large effective masses of the X electrons and heavy holes, the coupled mode appears most distinctly at carrier densities ≳5 ×1018cm-3 . We perform theoretical calculations of the nuclear motions and the electronic polarizations following an excitation with an ultrashort optical pulse to obtain the transient reflectivity responses of the coupled modes. We find that, while the longitudinal diffusion of photoexcited carriers is insignificant, the lateral inhomogeneity of the photoexcited carriers due to the laser intensity profile should be taken into account to reproduce the major features of the observed coupled mode dynamics.

  11. Selective coherent phonon-mode generation in single-wall carbon nanotubes.

    PubMed

    Nugraha, Ahmad R T; Hasdeo, Eddwi H; Saito, Riichiro

    2017-02-08

    The pulse-train technique within ultrafast pump-probe spectroscopy is theoretically investigated to excite a specific coherent phonon mode while suppressing the other phonon modes generated in single-wall carbon nanotubes (SWNTs). In particular, we focus on the selectivity of the radial breathing mode (RBM) and the G-band for a given SWNT. We find that if the repetition period of the pulse train matches with the integer multiple of the RBM phonon period, the RBM amplitude can be maintained while the amplitudes of the other modes are suppressed. As for the G-band, when we apply a repetition period of a half-integer multiple of the RBM period, the RBM can be suppressed because of destructive interference, while the G-band still survives. It is also possible to keep the G-band and suppress the RBM by applying a repetition period that matches with the integer multiple of the G-band phonon period. However, in this case we have to use a large number of laser pulses having a property of "magic ratio" of the G-band and RBM periods.

  12. Selective coherent phonon-mode generation in single-wall carbon nanotubes

    NASA Astrophysics Data System (ADS)

    Nugraha, Ahmad R. T.; Hasdeo, Eddwi H.; Saito, Riichiro

    2017-02-01

    The pulse-train technique within ultrafast pump-probe spectroscopy is theoretically investigated to excite a specific coherent phonon mode while suppressing the other phonon modes generated in single-wall carbon nanotubes (SWNTs). In particular, we focus on the selectivity of the radial breathing mode (RBM) and the G-band for a given SWNT. We find that if the repetition period of the pulse train matches with the integer multiple of the RBM phonon period, the RBM amplitude can be maintained while the amplitudes of the other modes are suppressed. As for the G-band, when we apply a repetition period of a half-integer multiple of the RBM period, the RBM can be suppressed because of destructive interference, while the G-band still survives. It is also possible to keep the G-band and suppress the RBM by applying a repetition period that matches with the integer multiple of the G-band phonon period. However, in this case we have to use a large number of laser pulses having a property of “magic ratio” of the G-band and RBM periods.

  13. Observation of coherent delocalized phonon-like modes in DNA under physiological conditions

    PubMed Central

    González-Jiménez, Mario; Ramakrishnan, Gopakumar; Harwood, Thomas; Lapthorn, Adrian J.; Kelly, Sharon M.; Ellis, Elizabeth M.; Wynne, Klaas

    2016-01-01

    Underdamped terahertz-frequency delocalized phonon-like modes have long been suggested to play a role in the biological function of DNA. Such phonon modes involve the collective motion of many atoms and are prerequisite to understanding the molecular nature of macroscopic conformational changes and related biochemical phenomena. Initial predictions were based on simple theoretical models of DNA. However, such models do not take into account strong interactions with the surrounding water, which is likely to cause phonon modes to be heavily damped and localized. Here we apply state-of-the-art femtosecond optical Kerr effect spectroscopy, which is currently the only technique capable of taking low-frequency (GHz to THz) vibrational spectra in solution. We are able to demonstrate that phonon modes involving the hydrogen bond network between the strands exist in DNA at physiologically relevant conditions. In addition, the dynamics of the solvating water molecules is slowed down by about a factor of 20 compared with the bulk. PMID:27248361

  14. Thermal transport in bismuth telluride quintuple layer: mode-resolved phonon properties and substrate effects

    PubMed Central

    Shao, Cheng; Bao, Hua

    2016-01-01

    The successful exfoliation of atomically-thin bismuth telluride (Bi2Te3) quintuple layer (QL) attracts tremendous research interest in this strongly anharmonic quasi-two-dimensional material. The thermal transport properties of this material are not well understood, especially the mode-wise properties and when it is coupled with a substrate. In this work, we have performed molecular dynamics simulations and normal mode analysis to study the mode-resolved thermal transport in freestanding and supported Bi2Te3 QL. The detailed mode-wise phonon properties are calculated and the accumulated thermal conductivities with respect to phonon mean free path (MFP) are constructed. It is shown that 60% of the thermal transport is contributed by phonons with MFP longer than 20 nm. Coupling with a-SiO2 substrate leads to about 60% reduction of thermal conductivity. Through varying the interfacial coupling strength and the atomic mass of substrate, we also find that phonon in Bi2Te3 QL is more strongly scattered by interfacial potential and its transport process is less affected by the dynamics of substrate. Our study provides an in-depth understanding of heat transport in Bi2Te3 QL and is helpful in further tailoring its thermal property through nanostructuring. PMID:27263656

  15. On-chip optical mode conversion based on dynamic grating in photonic-phononic hybrid waveguide

    PubMed Central

    Chen, Guodong; Zhang, Ruiwen; Sun, Junqiang

    2015-01-01

    We present a scheme for reversible and tunable on-chip optical mode conversion based on dynamic grating in a hybrid photonic-phononic waveguide. The dynamic grating is built up through the acousto-optic effect and the theoretical model of the optical mode conversion is developed by considering the geometrical deformation and refractive index change. Three kinds of mode conversions are able to be realized using the same hybrid waveguide structure in a large bandwidth by only changing the launched acoustic frequency. The complete mode conversion can be achieved by choosing a proper acoustic power under a given waveguide length. PMID:25996236

  16. Phonon dispersion in hypersonic two-dimensional phononic crystal membranes

    NASA Astrophysics Data System (ADS)

    Graczykowski, B.; Sledzinska, M.; Alzina, F.; Gomis-Bresco, J.; Reparaz, J. S.; Wagner, M. R.; Sotomayor Torres, C. M.

    2015-02-01

    We investigate experimentally and theoretically the acoustic phonon propagation in two-dimensional phononic crystal membranes. Solid-air and solid-solid phononic crystals were made of square lattices of holes and Au pillars in and on 250 nm thick single crystalline Si membrane, respectively. The hypersonic phonon dispersion was investigated using Brillouin light scattering. Volume reduction (holes) or mass loading (pillars) accompanied with second-order periodicity and local resonances are shown to significantly modify the propagation of thermally activated GHz phonons. We use numerical modeling based on the finite element method to analyze the experimental results and determine polarization, symmetry, or three-dimensional localization of observed modes.

  17. Manifestations of nonlinear optical effects in a novel SRS-active crystal—natural topaz, Al2(F1-x(OH)x)2SiO4: many-phonon χ(3)-lasing, more than sesqui-octave Stokes and anti-Stokes multi-wavelength comb lasing, cascaded and cross-cascaded χ(3)↔χ(3) Raman-induced interactions under single- and dual-wavelength picosecond collinear coherent pumping, THG and combined SRS-promoting phonon modes

    NASA Astrophysics Data System (ADS)

    Kaminskii, A. A.; Lux, O.; Rhee, H.; Eichler, H. J.; Yoneda, H.; Shirakawa, A.; Ueda, K.; Rückamp, R.; Bohatý, L.; Becker, P.

    2013-07-01

    Natural crystals of topaz, Al2(F1-x(OH)x)2SiO4 were found to be an attractive Raman gain material and a subject for the investigation of different χ(3)-nonlinear optical effects. We present several manifestations of photon-phonon interactions related to SRS and RFWM processes initiated by picosecond excitations at room and cryogenic (≈9 K) temperature. Among them are octave-spanning Stokes and anti-Stokes generation in the visible and near-IR spectral range, combined SRS-active phonon modes, cross-cascaded up-conversion, χ(3)↔χ(3) lasing, as well as THG via self-sum frequency parametric generation. All recorded Raman-induced lasing lines are identified and attributed to the promoting χ(3)-vibration transitions. Based on the experimental data, theoretical simulations employing Fourier analysis are performed to demonstrate the potential of wide SRS frequency combs in terms of ultra-short pulse generation. On the 50th anniversary of the discovery of stimulated Raman scattering.

  18. Orbital angular momentum mode division filtering for photon-phonon coupling

    NASA Astrophysics Data System (ADS)

    Zhu, Zhi-Han; Sheng, Li-Wen; Lv, Zhi-Wei; He, Wei-Ming; Gao, Wei

    2017-01-01

    Stimulated Brillouin scattering (SBS), a fundamental nonlinear interaction between light and acoustic waves occurring in any transparency material, has been broadly studied for several decades and gained rapid progress in integrated photonics recently. However, the SBS noise arising from the unwanted coupling between photons and spontaneous non-coherent phonons in media is inevitable. Here, we propose and experimentally demonstrate this obstacle can be overcome via a method called orbital angular momentum mode division filtering. Owing to the introduction of a new distinguishable degree-of-freedom, even extremely weak signals can be discriminated and separated from a strong noise produced in SBS processes. The mechanism demonstrated in this proof-of-principle work provides a practical way for quasi-noise-free photonic-phononic operation, which is still valid in waveguides supporting multi-orthogonal spatial modes, permits more flexibility and robustness for future SBS devices.

  19. Orbital angular momentum mode division filtering for photon-phonon coupling.

    PubMed

    Zhu, Zhi-Han; Sheng, Li-Wen; Lv, Zhi-Wei; He, Wei-Ming; Gao, Wei

    2017-01-10

    Stimulated Brillouin scattering (SBS), a fundamental nonlinear interaction between light and acoustic waves occurring in any transparency material, has been broadly studied for several decades and gained rapid progress in integrated photonics recently. However, the SBS noise arising from the unwanted coupling between photons and spontaneous non-coherent phonons in media is inevitable. Here, we propose and experimentally demonstrate this obstacle can be overcome via a method called orbital angular momentum mode division filtering. Owing to the introduction of a new distinguishable degree-of-freedom, even extremely weak signals can be discriminated and separated from a strong noise produced in SBS processes. The mechanism demonstrated in this proof-of-principle work provides a practical way for quasi-noise-free photonic-phononic operation, which is still valid in waveguides supporting multi-orthogonal spatial modes, permits more flexibility and robustness for future SBS devices.

  20. Pressure Effect on the Einstein-Like Phonon Mode in Superconducting YB6

    NASA Astrophysics Data System (ADS)

    Orendáč, Mat.; Gabáni, S.; Gažo, E.; Pristáš, G.; Flachbart, K.; Mori, T.; Wang, X.; Kamenev, K.

    2016-12-01

    YB6 is known as a conventional type-II BCS superconductor in which the strong coupling superconductivity with 2Δ /kBTc≈ 4.1 is mediated by the Einstein-like phonon mode of Y atoms located at ω E≈ 8 meV. We have investigated the pressure effect on ω E by Raman scattering up to 14 GPa. The received linear pressure increase in this mode dω E/dp≈ 0.21 meV/GPa yields an isothermal Grüneisen coefficient γ T= -partial ln ω E/partial ln V = 3.85 . Moreover, the pressure effect on the electron-phonon interaction λ calculated from the McMillan-Allen-Dynes expression for the superconducting transition temperature was determined to be partial ln λ/partial ln V \\cong 7.2.

  1. Coupling of phonon-polariton modes at dielectric-dielectric interfaces by the ATR technique

    NASA Astrophysics Data System (ADS)

    Cocoletzi, G. H.; Olvera Hernández, J.; Martínez Montes, G.

    1989-08-01

    We report the calculated ATR dispersion relation of the interface phonon-polariton modes in the prism-dielectric-dielectric configuration. Comparison of electromagnetic dispersion relations (EMDR) with the ATR dispersion relations are presented for three different interfaces: I) GaAs/GaP, II) CdF2/CaF2 and III) CaF2/GaP in two propagation windows, using the Otto and Kretschmann geometries for p-polarized light. We have studied the three cases using angle and frequency scans for each window and geometry. The results indicate that it is possible to excite and detect phonon-polariton modes at the dielectric-dielectric interface.

  2. Orbital angular momentum mode division filtering for photon-phonon coupling

    PubMed Central

    Zhu, Zhi-Han; Sheng, Li-Wen; Lv, Zhi-Wei; He, Wei-Ming; Gao, Wei

    2017-01-01

    Stimulated Brillouin scattering (SBS), a fundamental nonlinear interaction between light and acoustic waves occurring in any transparency material, has been broadly studied for several decades and gained rapid progress in integrated photonics recently. However, the SBS noise arising from the unwanted coupling between photons and spontaneous non-coherent phonons in media is inevitable. Here, we propose and experimentally demonstrate this obstacle can be overcome via a method called orbital angular momentum mode division filtering. Owing to the introduction of a new distinguishable degree-of-freedom, even extremely weak signals can be discriminated and separated from a strong noise produced in SBS processes. The mechanism demonstrated in this proof-of-principle work provides a practical way for quasi-noise-free photonic-phononic operation, which is still valid in waveguides supporting multi-orthogonal spatial modes, permits more flexibility and robustness for future SBS devices. PMID:28071736

  3. Axial interface optical phonon modes in a double-nanoshell system.

    PubMed

    Kanyinda-Malu, C; Clares, F J; de la Cruz, R M

    2008-07-16

    Within the framework of the dielectric continuum (DC) model, we analyze the axial interface optical phonon modes in a double system of nanoshells. This system is constituted by two identical equidistant nanoshells which are embedded in an insulating medium. To illustrate our results, typical II-VI semiconductors are used as constitutive polar materials of the nanoshells. Resolution of Laplace's equation in bispherical coordinates for the potentials derived from the interface vibration modes is made. By imposing the usual electrostatic boundary conditions at the surfaces of the two-nanoshell system, recursion relations for the coefficients appearing in the potentials are obtained, which entails infinite matrices. The problem of deriving the interface frequencies is reduced to the eigenvalue problem on infinite matrices. A truncating method for these matrices is used to obtain the interface phonon branches. Dependences of the interface frequencies on the ratio of inter-nanoshell separation to core size are obtained for different systems with several values of nanoshell interdistance. Effects due to the change of shell and embedding materials are also investigated in interface phonon modes.

  4. 'Soft' phonon modes, structured diffuse scattering and the crystal chemistry of Fe-bearing sphalerites

    SciTech Connect

    Withers, Ray L. . E-mail: withers@rsc.anu.edu.au; Welberry, T.R.; Pring, Allan; Tenailleau, Cristophe; Liu Yun

    2005-03-15

    Electron diffraction has been used to carefully investigate the reciprocal lattices of a range of iron-bearing sphalerites looking for evidence of Fe clustering and/or Fe/Zn ordering in the form of either additional satellite reflections or a structured diffuse intensity distribution accompanying the strong Bragg reflections of the underlying sphalerite-type average structure. While a highly structured diffuse intensity distribution in the form of transverse polarized {l_brace}110{r_brace}* sheets of diffuse intensity has been detected and found to be characteristic of all compositions, it does not appear to arise from Fe clustering and/or Fe/Zn ordering. Rather inherently low frequency, and therefore strongly thermally excited, phonon modes propagating along reciprocal space directions perpendicular to each of the six <110> real space directions of the average structure are suggested to be responsible for these {l_brace}110{r_brace}* sheets of diffuse intensity. Monte Carlo simulation (for a range of Zn-S, Zn-Zn and S-S interaction strengths) and subsequent Fourier transformation is used to confirm the existence of these low-frequency phonon modes of distortion as well as to show that they are an intrinsic, predictable property of the corner-connected tetrahedral structure of sphalerite. The low-frequency phonon modes involve coupled (Zn, Fe) and S motion in one-dimensional strings along <110> real space directions.

  5. Tunable evolutions of wave modes and bandgaps in quasi-1D cylindrical phononic crystals

    NASA Astrophysics Data System (ADS)

    Meidani, Mehrashk; Kim, Eunho; Li, Feng; Yang, Jinkyu; Ngo, Duc

    2015-01-01

    We investigate the tunable characteristics of mechanical waves propagating in quasi-1D phononic crystals composed of horizontally stacked short cylinders at various contact angles and offsets. According to the Hertzian contact theory, elastic compression of laterally-touching cylindrical bodies exhibits a various range of contact stiffness depending on their alignment angles. In this study, we first assemble cylindrical particles in various combinations of inclination angles and systematically examine their forming mechanisms of frequency bandgaps. We also investigate the effect of the rattling motions of cylindrical particles by introducing asymmetric center-of-mass offsets with respect to their contact points. We find that the frequency responses of these quasi-1D phononic crystals evolve into multiple band structures as we employ higher deviations of contact angles and offsets. We calculate the dispersive behavior of propagating waves using a discrete particle model for simple zero-offset cases, while we use a finite element method for simulating the rattling motions of particles under non-zero offsets. We report branching behavior of frequency band structures and the evolution of their vibration modes as we manipulate the contact angles and offsets of the phononic crystals. This study implies that we can leverage the versatile wave filtering characteristics of quasi-1D phononic crystals to construct tunable wave filtering devices for engineering applications.

  6. Nanocrystalline diamond: Effect of confinement, pressure, and heating on phonon modes

    NASA Astrophysics Data System (ADS)

    Lipp, Magnus J.; Baonza, Valentín García; Evans, William J.; Lorenzana, Hector E.

    1997-09-01

    Micro- and nanocrystalline systems exhibit properties that differ markedly from bulk systems. Diamond, a prototypical system, demonstrates a broadening, shift, and emergence of Raman phonon modes that are believed to originate from finite-size effects. Such information should be useful in constraining confinement models developed to describe the state of these mesoscopic systems. For example, previous investigations have analyzed crystallite size and stresses in scientifically and technologically relevant environments, including chemical-vapor-deposition diamond films and diamond nanocomposites. We have experimentally measured the effect on the diamond Raman phonon modes due to confinement, pressure, and heating effects. At ambient pressure, we present Raman measurements for diamond crystallites ranging from 6 nm to 10 μm, which were synthesized by both static and dynamic techniques. The Raman spectra obtained from the statically synthesized samples exhibit a characteristic strong and narrow diamond band, while those dynamically synthesized exhibit both diamond and graphiticlike features. A redshift of the diamond Raman band is observed for decreasing particle size. However, the pressure dependence of the phonon is about the same as that for the bulk system up to 30 kbar for crystallite sizes between 6 and 10 nm. Our measurements also indicate that heating effects from the incident laser dramatically affect the measured Raman spectra. This result leads us to an explanation for discrepancies among previously published results. We show that crystallite size and stress information cannot be determined without compensating for heating effects. Lastly, the phonon confinement model is able to explain the shifts of the Raman modes with size.

  7. Coherent phonon spectroscopy of non-fully symmetric modes using resonant terahertz excitation

    SciTech Connect

    Huber, T. Huber, L.; Johnson, S. L.; Ranke, M.; Ferrer, A.

    2015-08-31

    We use intense terahertz (THz) frequency electromagnetic pulses generated via optical rectification in an organic crystal to drive vibrational lattice modes in single crystal Tellurium. The coherent modes are detected by measuring the polarization changes of femtosecond laser pulses reflecting from the sample surface, resulting in a phase-resolved detection of the coherent lattice motion. We compare the data to a model of Lorentz oscillators driven by the near-single-cycle broadband THz pulse. The demonstrated technique of optically probed coherent phonon spectroscopy with THz frequency excitation could prove to be a viable alternative to other time-resolved spectroscopic methods like standard THz time domain spectroscopy.

  8. Planar modes free piezoelectric resonators using a phononic crystal with holes.

    PubMed

    Aragón, J L; Quintero-Torres, R; Domínguez-Juárez, J L; Iglesias, E; Ronda, S; Montero de Espinosa, F

    2016-09-01

    By using the principles behind phononic crystals, a periodic array of circular holes made along the polarization thickness direction of piezoceramic resonators are used to stop the planar resonances around the thickness mode band. In this way, a piezoceramic resonator adequate for operation in the thickness mode with an in phase vibration surface is obtained, independently of its lateral shape. Laser vibrometry, electric impedance tests and finite element models are used to corroborate the performances of different resonators made with this procedure. This method can be useful in power ultrasonic devices, physiotherapy and other external medical power ultrasound applications where piston-like vibration in a narrow band is required.

  9. Flexocoupling impact on the generalized susceptibility and soft phonon modes in the ordered phase of ferroics

    SciTech Connect

    Morozovska, Anna N.; Vysochanskii, Yulian M.; Varenyk, Oleksandr V.; Silibin, Maxim V.; Kalinin, Sergei V.; Eliseev, Eugene A.

    2015-09-29

    The impact of the flexoelectric effect on the generalized susceptibility and soft phonon dispersion is not well known in the long-range-ordered phases of ferroics. Within the Landau-Ginzburg-Devonshire approach we obtained analytical expressions for the generalized susceptibility and phonon dispersion relations in the ferroelectric phase. The joint action of the static and dynamic flexoelectric effects induces nondiagonal components of the generalized susceptibility, whose amplitude is proportional to the convolution of the spontaneous polarization with the flexocoupling constants. The flexocoupling essentially broadens the k spectrum of the generalized susceptibility and leads to an additional “pushing away” of the optical and acoustic soft mode phonon branches. The degeneracy of the transverse optical and acoustic modes disappears in the ferroelectric phase in comparison with the paraelectric phase due to the joint action of flexoelectric coupling and ferroelectric nonlinearity. Lastly, the results obtained might be mainly important for theoretical analyses of a broad spectrum of experimental data, including neutron and Brillouin scattering.

  10. Flexocoupling impact on the generalized susceptibility and soft phonon modes in the ordered phase of ferroics

    DOE PAGES

    Morozovska, Anna N.; Vysochanskii, Yulian M.; Varenyk, Oleksandr V.; ...

    2015-09-29

    The impact of the flexoelectric effect on the generalized susceptibility and soft phonon dispersion is not well known in the long-range-ordered phases of ferroics. Within the Landau-Ginzburg-Devonshire approach we obtained analytical expressions for the generalized susceptibility and phonon dispersion relations in the ferroelectric phase. The joint action of the static and dynamic flexoelectric effects induces nondiagonal components of the generalized susceptibility, whose amplitude is proportional to the convolution of the spontaneous polarization with the flexocoupling constants. The flexocoupling essentially broadens the k spectrum of the generalized susceptibility and leads to an additional “pushing away” of the optical and acoustic softmore » mode phonon branches. The degeneracy of the transverse optical and acoustic modes disappears in the ferroelectric phase in comparison with the paraelectric phase due to the joint action of flexoelectric coupling and ferroelectric nonlinearity. Lastly, the results obtained might be mainly important for theoretical analyses of a broad spectrum of experimental data, including neutron and Brillouin scattering.« less

  11. Localization and fractal spectra of optical phonon modes in quasiperiodic structures

    NASA Astrophysics Data System (ADS)

    Anselmo, D. H. A. L.; Dantas, A. L.; Medeiros, S. K.; Albuquerque, E. L.; Freire, V. N.

    2005-04-01

    The dispersion relation and localization profile of confined optical phonon modes in quasiperiodic structures, made up of nitride semiconductor materials, are analyzed through a transfer-matrix approach. The quasiperiodic structures are characterized by the nature of their Fourier spectrum, which can be dense pure point (Fibonacci sequences) or singular continuous (Thue-Morse and Double-period sequences). These substitutional sequences are described in terms of a series of generations that obey peculiar recursion relations and/or inflation rules. We present a quantitative analysis of the localization and magnitude of the allowed band widths in the optical phonons spectra of these quasiperiodic structures, as well as how they scale as a function of the number of generations of the sequences.

  12. Size and environment dependence of surface phonon modes of gallium arsenide nanowires as measured by Raman spectroscopy.

    PubMed

    Spirkoska, D; Abstreiter, G; Fontcuberta I Morral, A

    2008-10-29

    Gallium arsenide nanowires were synthesized by gallium-assisted molecular beam epitaxy. By varying the growth time, nanowires with diameters ranging from 30 to 160 nm were obtained. Raman spectra of the nanowire ensembles were measured. The small linewidth of the optical phonon modes agree with an excellent crystalline quality. A surface phonon mode was also revealed, as a shoulder at lower frequencies of the longitudinal optical mode. In agreement with the theory, the surface mode shifts to lower wavenumbers when the diameter of the nanowires is decreased or the environment dielectric constant increased.

  13. Unambiguous phonon mode assignment in multiferroic BiFeO3 single crystals

    NASA Astrophysics Data System (ADS)

    Beekman, Christianne; Cheong, Sang-Wook; Burch, Kenneth

    2012-02-01

    In Bismuth ferrite (BiFeO3) antiferromagnetic and ferroelectric order parameters coexist at room temperature, making this material an excellent candidate for new functionalities, such as electrical control of magnetism. Despite extensive reports on Raman scattering experiments on single crystals and thin films, controversy still remains in the observation and assignment of the phonon mode symmetries. We present polarized micro-Raman spectroscopy of single crystals ((1 0 0)cubic surface) with uniform ferroelectric polarization. Careful examination of the Raman spectra upon crystal rotation enables us to unambiguously assign the (A1, Ex and Ey) modes. We will show that ambiguity is easily introduced by slight misalignment of the crystal and that the crystal rotation is necessary to reach unambiguous mode assignment. Our method not only results in proper Raman mode assignment, which is necessary to describe the phonons critical for the multiferroic behavior, it also allows study of symmetry breaking and may provide a way to non-invasively check the ferroelectric polarization direction.

  14. Phonon-roton modes of liquid 4He beyond the roton in MCM-41

    SciTech Connect

    Azuah, Richard T; Omar Diallo, Souleymane; Adams, Mark A.; Kirichek, Oleg; Glyde, Henry R

    2013-01-01

    We present neutron scattering measurements of the phonon-roton (P-R) mode of superfluid 4He confined in 47 A MCM-41 at T = 0.5 K at wave vectors, Q, beyond the roton wave vector (QR = 1.92 A-1). Measurements beyond the roton require access to high wave vectors (up to Q = 4 A-1) with excellent energy resolution and high statistical precision. The present results show for the first time that at T = 0.5 K the P-R mode in MCM-41 extends out to wave-vector Q 3.6 A-1 with the same energy and zero width (within precision) as observed in bulk superfluid 4He. Layer modes in the roton region are also observed. Specifically, the P-R mode energy, !Q, increases with Q for Q > QR and reaches a plateau at a maximum energy !Q = 2 where is the roton energy, = 0.74 0.01 meV in MCM-41. This upper limit means the P-R mode decays to two rotons when its energy exceeds 2 . It also means that the P-R mode does not decay to two layers modes. If the P-R could decay to two layer modes, !Q would plateau at a lower energy, !Q = 2 L where L = 0.60 meV is the energy of the roton like minimum of the layer mode. The observation of the P-R mode with energy up to 2 shows that the P-R mode and the layer modes are independent modes with apparently little interaction between them.

  15. Phononic crystal surface mode coupling and its use in acoustic Doppler velocimetry.

    PubMed

    Cicek, Ahmet; Salman, Aysevil; Kaya, Olgun Adem; Ulug, Bulent

    2016-02-01

    It is numerically shown that surface modes of two-dimensional phononic crystals, which are Bloch modes bound to the interface between the phononic crystal and the surrounding host, can couple back and forth between the surfaces in a length scale determined by the separation of two surfaces and frequency. Supercell band structure computations through the finite-element method reveal that the surface band of an isolated surface splits into two bands which support either symmetric or antisymmetric hybrid modes. When the surface separation is 3.5 times the lattice constant, a coupling length varying between 30 and 48 periods can be obtained which first increases linearly with frequency and, then, decreases rapidly. In the linear regime, variation of coupling length can be used as a means of measuring speeds of objects on the order of 0.1m/s by incorporating the Doppler shift. Speed sensitivity can be improved by increasing surface separation at the cost of larger device sizes.

  16. The hydrogen-bond network of water supports propagating optical phonon-like modes

    SciTech Connect

    Elton, Daniel C.; Fernández-Serra, Marivi

    2016-01-04

    The local structure of liquid water as a function of temperature is a source of intense research. This structure is intimately linked to the dynamics of water molecules, which can be measured using Raman and infrared spectroscopies. The assignment of spectral peaks depends on whether they are collective modes or single-molecule motions. Vibrational modes in liquids are usually considered to be associated to the motions of single molecules or small clusters. Using molecular dynamics simulations, here we find dispersive optical phonon-like modes in the librational and OH-stretching bands. We argue that on subpicosecond time scales these modes propagate through water’s hydrogen-bond network over distances of up to 2 nm. In the long wavelength limit these optical modes exhibit longitudinal–transverse splitting, indicating the presence of coherent long-range dipole–dipole interactions, as in ice. Lastly, our results indicate the dynamics of liquid water have more similarities to ice than previously thought.

  17. The hydrogen-bond network of water supports propagating optical phonon-like modes

    DOE PAGES

    Elton, Daniel C.; Fernández-Serra, Marivi

    2016-01-04

    The local structure of liquid water as a function of temperature is a source of intense research. This structure is intimately linked to the dynamics of water molecules, which can be measured using Raman and infrared spectroscopies. The assignment of spectral peaks depends on whether they are collective modes or single-molecule motions. Vibrational modes in liquids are usually considered to be associated to the motions of single molecules or small clusters. Using molecular dynamics simulations, here we find dispersive optical phonon-like modes in the librational and OH-stretching bands. We argue that on subpicosecond time scales these modes propagate through water’smore » hydrogen-bond network over distances of up to 2 nm. In the long wavelength limit these optical modes exhibit longitudinal–transverse splitting, indicating the presence of coherent long-range dipole–dipole interactions, as in ice. Lastly, our results indicate the dynamics of liquid water have more similarities to ice than previously thought.« less

  18. The hydrogen-bond network of water supports propagating optical phonon-like modes

    PubMed Central

    Elton, Daniel C.; Fernández-Serra, Marivi

    2016-01-01

    The local structure of liquid water as a function of temperature is a source of intense research. This structure is intimately linked to the dynamics of water molecules, which can be measured using Raman and infrared spectroscopies. The assignment of spectral peaks depends on whether they are collective modes or single-molecule motions. Vibrational modes in liquids are usually considered to be associated to the motions of single molecules or small clusters. Using molecular dynamics simulations, here we find dispersive optical phonon-like modes in the librational and OH-stretching bands. We argue that on subpicosecond time scales these modes propagate through water's hydrogen-bond network over distances of up to 2 nm. In the long wavelength limit these optical modes exhibit longitudinal–transverse splitting, indicating the presence of coherent long-range dipole–dipole interactions, as in ice. Our results indicate the dynamics of liquid water have more similarities to ice than previously thought. PMID:26725363

  19. Microscopic theory of cooperative spin crossover: Interaction of molecular modes with phonons

    SciTech Connect

    Palii, Andrew E-mail: klokishner@yahoo.com; Ostrovsky, Serghei; Reu, Oleg; Klokishner, Sophia E-mail: klokishner@yahoo.com; Tsukerblat, Boris; Decurtins, Silvio; Liu, Shi-Xia

    2015-08-28

    In this article, we present a new microscopic theoretical approach to the description of spin crossover in molecular crystals. The spin crossover crystals under consideration are composed of molecular fragments formed by the spin-crossover metal ion and its nearest ligand surrounding and exhibiting well defined localized (molecular) vibrations. As distinguished from the previous models of this phenomenon, the developed approach takes into account the interaction of spin-crossover ions not only with the phonons but also a strong coupling of the electronic shells with molecular modes. This leads to an effective coupling of the local modes with phonons which is shown to be responsible for the cooperative spin transition accompanied by the structural reorganization. The transition is characterized by the two order parameters representing the mean values of the products of electronic diagonal matrices and the coordinates of the local modes for the high- and low-spin states of the spin crossover complex. Finally, we demonstrate that the approach provides a reasonable explanation of the observed spin transition in the [Fe(ptz){sub 6}](BF{sub 4}){sub 2} crystal. The theory well reproduces the observed abrupt low-spin → high-spin transition and the temperature dependence of the high-spin fraction in a wide temperature range as well as the pronounced hysteresis loop. At the same time within the limiting approximations adopted in the developed model, the evaluated high-spin fraction vs. T shows that the cooperative spin-lattice transition proves to be incomplete in the sense that the high-spin fraction does not reach its maximum value at high temperature.

  20. Scattering of carriers by coupled plasmon-phonon modes in bulk polar semiconductors and polar semiconductor heterostructures

    NASA Astrophysics Data System (ADS)

    Hauber, Anna; Fahy, Stephen

    2017-01-01

    We present a general treatment of carrier scattering by coupled phonon-plasmon collective modes in polar semiconductors, taking anharmonic phonon decay into account and self-consistently calculating carrier momentum relaxation rates and carrier mobility in a parabolic band model. We iteratively solve the weak-field Boltzmann equations for carriers and collective modes and obtain their nonequilibrium distribution functions. Both the scattering rates and the anharmonic decay of the coupled modes are expressed through the total dielectric function of the semiconductor, consisting of a damped lattice dielectric function, and a temperature dependent random phase approximation dielectric function for the carrier plasma. We show that the decay of the coupled modes has a significant effect on the contribution to the mobility limited by carrier-coupled mode scattering. We also propose a scalar quantity, the phonon dissipation weight factor, with which this effect can be estimated from an analytic expression. We apply this treatment to dynamically screened electron-longitudinal optical phonon scattering in bulk polar semiconductors, and to dynamically screened remote phonon scattering in polar heterostructures where monolayers of MoS2 are sandwiched between various polar dielectrics. We find that a dynamic treatment of the remote phonon scattering yields mobilities up to 75% higher than a static screening approximation does for structures which consist of a monolayer of MoS2 between hafnia and silica. Moreover, we show that accounting for the nonzero thickness of the MoS2 interface layer has an important effect on the calculated mobility in the same structure.

  1. (Ga,In)P: A standard alloy in the classification of phonon mode behavior

    NASA Astrophysics Data System (ADS)

    Pagès, O.; Chafi, A.; Fristot, D.; Postnikov, A. V.

    2006-04-01

    Contrary to a broadly accepted assumption we show that random (Ga,In)P is not an exception with respect to the crude classification of the phonon mode behavior of random mixed crystals in terms of 1-bond→1-mode systems or 2-bond→1-mode systems, as established from the simple criterion derived by Elliott [R. J. Elliott , Rev. Mod. Phys. 46, 465 (1974)]. Consistent understanding of the puzzling Raman/infrared behavior of (Ga,In)P, that has been a subject of controversy, is achieved via a basic version of our 1-bond→2-mode model originally developed for (Zn,Be)-chalcogenides, that exhibit a large contrast in the bond properties, and recently extended under a simplified form to the usual (Ga,In)As alloy. The Raman/infrared features from (Ga,In)P are accordingly re-assigned, with considerable change with respect to the previous approaches. In particular the In impurity mode, previously assigned within (˜390cm-1) the optical band of the host GaP compound (368-403cm-1) , is re-assigned below it (˜350cm-1) . Accordingly the Ga-P and In-P transverse optical branches do not overlap, which reconciles (Ga,In)P with the Elliott’s criterion. Besides, we show that the idea of two bond lengths per species in alloys, supported by our 1-bond→2-phonon picture, opens an attractive area for the discussion of spontaneous ordering in GaInP2 , and mixed crystals in general. Essentially this is because it allows to play with the related competition effects regarding the minimization of the local strain energy due to the bond length mismatch between the parent compounds. In particular the unsuspected issue of intrinsic limit to spontaneous ordering comes out ( η˜0.5 in GaInP2 ). The whole discussion is supported by detailed re-examination of the (Ga,In)P Raman/infrared data in the literature, full contour modeling of the transverse and longitudinal optical Raman lineshapes via our phenomenological 1-bond→2-mode model, and first-principles bond length calculations concerned

  2. Evolution of anatase surface active sites probed by in situ sum-frequency phonon spectroscopy.

    PubMed

    Cao, Yue; Chen, Shiyou; Li, Yadong; Gao, Yi; Yang, Deheng; Shen, Yuen Ron; Liu, Wei-Tao

    2016-09-01

    Surface active sites of crystals often govern their relevant surface chemistry, yet to monitor them in situ in real atmosphere remains a challenge. Using surface-specific sum-frequency spectroscopy, we identified the surface phonon mode associated with the active sites of undercoordinated titanium ions and conjoint oxygen vacancies, and used it to monitor them on anatase (TiO2) (101) under ambient conditions. In conjunction with theory, we determined related surface structure around the active sites and tracked the evolution of oxygen vacancies under ultraviolet irradiation. We further found that unlike in vacuum, the surface oxygen vacancies, which dominate the surface reactivity, are strongly regulated by ambient gas molecules, including methanol and water, as well as weakly associated species, such as nitrogen and hydrogen. The result revealed a rich interplay between prevailing ambient species and surface reactivity, which can be omnipresent in environmental and catalytic applications of titanium dioxides.

  3. Evolution of anatase surface active sites probed by in situ sum-frequency phonon spectroscopy

    PubMed Central

    Cao, Yue; Chen, Shiyou; Li, Yadong; Gao, Yi; Yang, Deheng; Shen, Yuen Ron; Liu, Wei-Tao

    2016-01-01

    Surface active sites of crystals often govern their relevant surface chemistry, yet to monitor them in situ in real atmosphere remains a challenge. Using surface-specific sum-frequency spectroscopy, we identified the surface phonon mode associated with the active sites of undercoordinated titanium ions and conjoint oxygen vacancies, and used it to monitor them on anatase (TiO2) (101) under ambient conditions. In conjunction with theory, we determined related surface structure around the active sites and tracked the evolution of oxygen vacancies under ultraviolet irradiation. We further found that unlike in vacuum, the surface oxygen vacancies, which dominate the surface reactivity, are strongly regulated by ambient gas molecules, including methanol and water, as well as weakly associated species, such as nitrogen and hydrogen. The result revealed a rich interplay between prevailing ambient species and surface reactivity, which can be omnipresent in environmental and catalytic applications of titanium dioxides. PMID:27704049

  4. Tunable waveguide and cavity in a phononic crystal plate by controlling whispering-gallery modes in hollow pillars

    NASA Astrophysics Data System (ADS)

    Jin, Yabin; Fernez, Nicolas; Pennec, Yan; Bonello, Bernard; Moiseyenko, Rayisa P.; Hémon, Stéphanie; Pan, Yongdong; Djafari-Rouhani, Bahram

    2016-02-01

    We investigate the properties of a phononic crystal plate with hollow pillars and introduce the existence of whispering-gallery modes (WGMs). We show that by tuning the inner radius of the hollow pillar, these modes can merge inside both Bragg and low frequency band gaps, deserving phononic crystal and acoustic metamaterial applications. These modes can be used as narrow pass bands for which the quality factor can be greatly enhanced by the introduction of an additional cylinder between the hollow cylinder and the plate. We discuss some functionalities of these confined WGM in both Bragg and low frequency gaps for wavelength division in multiplexer devices using heteroradii pillars introduced into waveguide and cavity structures.

  5. Experimental demonstration of mode-selective phonon excitation of 6H-SiC by a mid-infrared laser with anti-Stokes Raman scattering spectroscopy

    SciTech Connect

    Yoshida, Kyohei; Hachiya, Kan; Okumura, Kensuke; Mishima, Kenta; Inukai, Motoharu; Torgasin, Konstantin; Omer, Mohamed; Sonobe, Taro; Zen, Heishun; Negm, Hani; Kii, Toshiteru; Masuda, Kai; Ohgaki, Hideaki

    2013-10-28

    Mode-selective phonon excitation by a mid-infrared laser (MIR-FEL) is demonstrated via anti-Stokes Raman scattering measurements of 6H-silicon carbide (SiC). Irradiation of SiC with MIR-FEL and a Nd-YAG laser at 14 K produced a peak where the Raman shift corresponds to a photon energy of 119 meV (10.4 μm). This phenomenon is induced by mode-selective phonon excitation through the irradiation of MIR-FEL, whose photon energy corresponds to the photon-absorption of a particular phonon mode.

  6. Enhancement of surface phonon modes in the Raman spectrum of ZnSe nanoparticles on adsorption of 4-mercaptopyridine

    SciTech Connect

    Islam, Syed K.; Lombardi, John R.

    2014-02-21

    By chemically etching a thin film of crystalline ZnSe with acid, we observe a strong Raman enhancement of the surface phonon modes of ZnSe on adsorption of a molecule (4-mercaptopyridine). The surface is composed of oblate hemi-ellipsoids, which has a large surface-to-bulk ratio. The assignment of the observed modes (at 248 and 492 cm{sup −1}) to a fundamental and first overtone of the surface optical mode is consistent with observations from high-resolution electron energy loss spectroscopy as well as calculations.

  7. Acoustic phonon modes in asymmetric AlxGa1-xN/GaN/AlyGa1-yN quantum wells

    NASA Astrophysics Data System (ADS)

    Zan, Y. H.; Ban, S. L.; Chai, Y. J.; Qu, Y.

    2017-02-01

    Using an elastic continuum model, the dispersion relations and phonon modes of propagating, confined, half space and interface acoustic phonons in asymmetric AlxGa1-xN/GaN/Al1-yGayN quantum wells (QWs) have been solved theoretically with the varieties of Al components x and y. Contrary to the previous conclusions, some regulations for the existence of the above different acoustic phonons are revealed as well as the transition conditions among these modes are also discussed. With increase of wave vectors, the dispersion relations split into several groups. Because the classification of these groups is related to the eigen frequencies of bulk materials forming QWs, phonon modes in these groups will be confined or propagating in different layers of QWs. Furthermore, the gradients of the dispersion relations' asymptotes are the velocities of longitudinal and the transverse acoustic phonons propagating in bulk materials in turns. The properties of the dispersion relations and their phonon modes are also analyzed in depth based on the cut-off conditions. By the changing of Al components x and y, the bottom of these groups will be modified to adjust eigen frequencies of AlxGa1-xN or Al1-yGayN layers. But the propagation properties of acoustic phonon modes will remain unchanged in each section.

  8. Correlation between surface phonon mode and luminescence in nanocrystalline CdS thin films: An effect of ion beam irradiation

    SciTech Connect

    Kumar, Pragati Agarwal, Avinash; Saxena, Nupur; Singh, Fouran; Gupta, Vinay

    2014-07-28

    The influence of swift heavy ion irradiation (SHII) on surface phonon mode (SPM) and green emission in nanocrystalline CdS thin films grown by chemical bath deposition is studied. The SHII of nanocrystalline CdS thin films is carried out using 70 MeV Ni ions. The micro Raman analysis shows that asymmetry and broadening in fundamental longitudinal optical (LO) phonon mode increases systematically with increasing ion fluence. To analyze the role of phonon confinement, spatial correlation model (SCM) is fitted to the experimental data. The observed deviation of SCM to the experimental data is further investigated by fitting the micro Raman spectra using two Lorentzian line shapes. It is found that two Lorentzian functions (LFs) provide better fitting than SCM fitting and facilitate to identify the contribution of SPM in the observed distortion of LO mode. The behavior of SPM as a function of ion fluence is studied to correlate the observed asymmetry (Γ{sub a}/Γ{sub b}) and full width at half maximum of LO phonon mode and to understand the SHII induced enhancement of SPM. The ion beam induced interstitial and surface state defects in thin films, as observed by photoluminescence (PL) spectroscopy studies, may be the underlying reason for enhancement in SPM. PL studies also show enhancement in green luminescence with increase in ion fluence. PL analysis reveals that the variation in population density of surface state defects after SHII is similar to that of SPM. The correlation between SPM and luminescence and their dependence on ion irradiation fluence is explained with the help of thermal spike model.

  9. Soft phonon modes driven reduced thermal conductivity in self-compensated Sn1.03Te with Mn doping

    NASA Astrophysics Data System (ADS)

    Acharya, Somnath; Pandey, Juhi; Soni, Ajay

    2016-09-01

    The key challenge for superior thermoelectric performance of SnTe is optimization of very high hole concentration (˜1021 cm-3) arising from inherent Sn vacancies. Partial control of charge carriers can be achieved by self-compensation via careful filling of the vacancies using excess Sn, although high thermal conductivity remained a concern. In this context, with deliberate doping, an anharmonicity in phonon dispersion can be generated to obtain a poor thermal conductivity. We report on point defects and soft phonon mode driven poor thermal conductivity in self-compensated Sn1.03Te with Mn doping. The obvious modification in the electronic band structure has been demonstrated by four times enhancement in thermopower for Sn0.93Mn0.1Te from Sn1.03Te, and metallic behavior of temperature dependent resistivity. The observed soft phonon mode and impurity localized mode in Raman spectra have been explained based on the created anharmonicity in Sn1.03Te crystal with Mn doping.

  10. Dynamic disorder, phonon lifetimes, and the assignment of modes to the vibrational spectra of methylammonium lead halide perovskites.

    PubMed

    Leguy, Aurélien M A; Goñi, Alejandro R; Frost, Jarvist M; Skelton, Jonathan; Brivio, Federico; Rodríguez-Martínez, Xabier; Weber, Oliver J; Pallipurath, Anuradha; Alonso, M Isabel; Campoy-Quiles, Mariano; Weller, Mark T; Nelson, Jenny; Walsh, Aron; Barnes, Piers R F

    2016-10-21

    We present Raman and terahertz absorbance spectra of methylammonium lead halide single crystals (MAPbX3, X = I, Br, Cl) at temperatures between 80 and 370 K. These results show good agreement with density-functional-theory phonon calculations. Comparison of experimental spectra and calculated vibrational modes enables confident assignment of most of the vibrational features between 50 and 3500 cm(-1). Reorientation of the methylammonium cations, unlocked in their cavities at the orthorhombic-to-tetragonal phase transition, plays a key role in shaping the vibrational spectra of the different compounds. Calculations show that these dynamic effects split Raman peaks and create more structure than predicted from the independent harmonic modes. This explains the presence of extra peaks in the experimental spectra that have been a source of confusion in earlier studies. We discuss singular features, in particular the torsional vibration of the C-N axis, which is the only molecular mode that is strongly influenced by the size of the lattice. From analysis of the spectral linewidths, we find that MAPbI3 shows exceptionally short phonon lifetimes, which can be linked to low lattice thermal conductivity. We show that optical rather than acoustic phonon scattering is likely to prevail at room temperature in these materials.

  11. Optical phonon modes in InGaN/GaN dot-in-a-wire heterostructures grown by molecular beam epitaxy

    NASA Astrophysics Data System (ADS)

    Titus, J.; Nguyen, H. P. T.; Mi, Z.; Perera, A. G. U.

    2013-03-01

    We report on the studies of optical phonon modes in nearly defect-free GaN nanowires embedded with intrinsic InGaN quantum dots by using oblique angle transmission infrared spectroscopy. These phonon modes are dependent on the nanowire fill-factor, doping densities of the nanowires, and the presence of InGaN dots. These factors can be applied for potential phonon based photodetectors whose spectral responses can be tailored by varying a combination of these three parameters. The optical anisotropy along the growth (c-) axis of the GaN nanowire contributes to the polarization agility of such potential photodetectors.

  12. Coupling of Low-energy Electrons in the Optimally Doped Bi2Sr2CaCu2O8+δ Superconductor to an Optical Phonon Mode

    SciTech Connect

    Rameau, J.D.; Yang, H.-B.; Gu, G. D.; Johnson, P. D.

    2009-11-24

    Laser-based photoemission with photons of energy 6 eV is used to examine the fine details of the very low-energy electron dispersion and associated dynamics in the nodal region of optimally doped Bi2212. A 'kink' in the dispersion in the immediate vicinity of the Fermi energy is associated with scattering from an optical phonon previously identified in Raman studies. The identification of this phonon as the appropriate mode is confirmed by comparing the scattering rates observed experimentally with the results of calculated scattering rates based on the properties of the phonon mode.

  13. Phonon modes of A(Co 1/2Mn 1/2) O 3 ( A=La, Nd, Dy, Ho, Yb)

    NASA Astrophysics Data System (ADS)

    Gao, F.; Lewis, R. A.; Wang, X. L.; Dou, S. X.

    2001-09-01

    Phonon energies in cobaltite/manganites A(Co1/2Mn1/2)O3, where A is a lanthanide, have been determined by far-infrared spectroscopy. The phonon energies systematically shift and split and new modes appear as the mass of the lanthanide is increased through the series A=La, Nd, Dy, Ho, Yb. The behavior of the phonon modes correlates with the magnetic properties of this series of compounds, in particular with the appearance of metamagnetism for the compounds with smaller ions on the A site.

  14. Sound and Noisy Light: Tuning Phonon Modes in Photo-switchable Nanostructures

    NASA Astrophysics Data System (ADS)

    Sklan, Sophia; Grossman, Jeffrey

    2014-03-01

    The coupling of light to structural vibrations is well known and results in phenomena like phonon polaritons, acousto-optics (where phonons modulate optical properties), and optomechanics (where light creates or absorbs phonons). Here we consider the question of whether light could also be used to modulate the properties of phonons. We examine photo-isomers (which change their shape under exposure to light), embedded in a nanostructure designed to amplify the effects of photo-switching. To isolate the effects of photo-isomerization (jump photo-switching and shot noise), we apply a combination of analytic and computational techniques to analyze the stochastic dynamics of a toy model of this system. Particular attention is paid to applying this model to explore the potential applications of the photo-switchable nanostructure.

  15. Growth and decay of discrete nonlinear Schrodinger breathers interacting with internal modes or standing-wave phonons

    PubMed

    Johansson; Aubry

    2000-05-01

    We investigate the long-time evolution of weakly perturbed single-site breathers (localized stationary states) in the discrete nonlinear Schrodinger equation. The perturbations we consider correspond to time-periodic solutions of the linearized equations around the breather, and can be either (i) spatially localized or (ii) spatially extended. For case (i), which corresponds to the excitation of an internal mode of the breather, we find that the nonlinear interaction between the breather and its internal mode always leads to a slow growth of the breather amplitude and frequency. In case (ii), corresponding to interaction between the breather and a standing-wave phonon, the breather will grow provided that the wave vector of the phonon is such that the generation of radiating higher harmonics at the breather is possible. In other cases, breather decay is observed. This condition yields a limit value for the breather frequency above which no further growth is possible. We also discuss another mechanism for breather growth and destruction which becomes important when the amplitude of the perturbation is non-negligible, and which originates from the oscillatory instabilities of the nonlinear standing-wave phonons.

  16. Phonons in Ge nanowires

    NASA Astrophysics Data System (ADS)

    Peelaers, H.; Partoens, B.; Peeters, F. M.

    2009-09-01

    The phonon spectra of thin freestanding, hydrogen passivated, Ge nanowires are calculated by ab initio techniques. The effect of confinement on the phonon modes as caused by the small diameters of the wires is investigated. Confinement causes a hardening of the optical modes and a softening of the longitudinal acoustic modes. The stability of the nanowires, undoped or doped with B or P atoms, is investigated using the obtained phonon spectra. All considered wires were stable, except for highly doped, very thin nanowires.

  17. Modification of the G-phonon mode of graphene by nitrogen doping

    SciTech Connect

    Lukashev, Pavel V. Hurley, Noah; Zhao, Liuyan; Pasupathy, Abhay; Paudel, Tula R.; Tsymbal, Evgeny Y.; Schiros, Theanne; Pinczuk, Aron; He, Rui

    2016-01-25

    The effect of nitrogen doping on the phonon spectra of graphene is analyzed. In particular, we employ first-principles calculations and scanning Raman analysis to investigate the dependence of phonon frequencies in graphene on the concentration of nitrogen dopants. We demonstrate that the G phonon frequency shows oscillatory behavior as a function of nitrogen concentration. We analyze different mechanisms which could potentially be responsible for this behavior, such as Friedel charge oscillations around the localized nitrogen impurity atom, the bond length change between nitrogen impurity and its nearest neighbor carbon atoms, and the long-range interactions of the nitrogen point defects. We show that the bond length change and the long range interaction of point defects are possible mechanisms responsible for the oscillatory behavior of the G frequency as a function of nitrogen concentration. At the same time, Friedel charge oscillations are unlikely to contribute to this behavior.

  18. The effects of substrate phonon mode scattering on transport in carbon nanotubes.

    PubMed

    Perebeinos, Vasili; Rotkin, Slava V; Petrov, Alexey G; Avouris, Phaedon

    2009-01-01

    Carbon nanotubes (CNTs) have large intrinsic carrier mobility due to weak acoustic phonon scattering. However, unlike two-dimensional metal-oxide-semiconductor field effect transistors (MOSFETs), substrate surface polar phonon (SPP) scattering has a dramatic effect on the CNTFET mobility, due to the reduced vertical dimensions of the latter. We find that for the van der Waals distance between CNT and an SiO2 substrate, the low-field mobility at room temperature is reduced by almost an order of magnitude depending on the tube diameter. We predict additional experimental signatures of the SPP mechanism in dependence of the mobility on density, temperature, tube diameter, and CNT-substrate separation.

  19. Structural And Phonon Modes Of Multiferroic Bi0.9Ca0.1Fe0.9Co0.1O3 Nanoparticles

    NASA Astrophysics Data System (ADS)

    Varshney, Dinesh; Das, Geeta; Kumar, Ashwini

    2011-07-01

    Multiferroic Bi0.9Ca0.1Fe0.9Co0.1O3 [BCFCO] nanoparticles of 23 nm were successfully prepared by chemical co-precipitation method. The x-ray diffraction patterns confirmed the formation of single-phase perovskite structure. Rietveld-refinement of crystal structure parameters revealed the existence of rhombohedral R3c symmetry. Raman spectrum identifies the five active optical phonon modes (A1-2, E-2, A1-4, E-8, E-9). The BCFCO nanomaterials sample shows the lower frequency shift in Raman modes as compared to pure BiFeO3 and is attributed to the structural distortion at Fe site.

  20. Semiconductor superlattice diodes for detection of terahertz photons: The role of hybridization of the plasma and polar-optical phonon modes

    SciTech Connect

    Ignatov, Anatoly A.

    2014-08-28

    The current (voltage) responsivity of a superlattice-based diode detector has been studied theoretically in the terahertz frequency band that includes the region of the polar-optical phonon frequencies. Within the framework of an equivalent circuit approach, the electro-dynamical model which allows one to analyze the responsivity taking into account the hybridization of the plasma and polar-optical phonon modes both in the substrate and in the cladding layers of the diode has been suggested. It has been shown that the presence of the plasma and polar-optical phonon modes gives rise to strong features in the frequency dependence of the responsivity, i.e., to the resonance dips and peaks at frequencies of hybridized plasmons and polar-optical phonons. It has been suggested that by judicious engineering of the superlattice-based diodes, it would be possible to enhance substantially their responsivity in the terahertz frequency band.

  1. Quantifying electron-phonon coupling in CdTe1-xSex nanocrystals via coherent phonon manipulation

    NASA Astrophysics Data System (ADS)

    Spann, B. T.; Xu, X.

    2014-08-01

    We employ ultrafast transient absorption spectroscopy with temporal pulse shaping to manipulate coherent phonon excitation and quantify the strength of electron-phonon coupling in CdTe1-xSex nanocrystals (NCs). Raman active CdSe and CdTe longitudinal optical phonon (LO) modes are excited and probed in the time domain. By temporally controlling pump pulse pairs to coherently excite and cancel coherent phonons in the CdTe1-xSex NCs, we estimate the relative amount of optical energy that is coupled to the coherent CdSe LO mode.

  2. Longitudinal phonon modes in a ZnSe/ZnS(x)Se(1-x) lattice-mismatched superlattice

    NASA Astrophysics Data System (ADS)

    Xia, Hua; Jiang, S. S.; Zhang, Wei; Zhang, X. K.; Guan, Z. P.; Fan, X. W.

    1994-11-01

    Longitudinal acoustic and optical phonon modes of a ZnSe/ZnS(x)Se(1-x) (x approximately equals 0.20) lattice-mismatched superlattice, prepared with atmospheric metal organic chemical vapor deposition method, have been investigated by light scattering measurements. Despite a lattice mismatch as large as 1% between the alternating layers, the measured longitudinal elastic constants are in agreement with the calculated values of an unstrained effective medium model. Furthermore, a correlative study was made by fitting the spectra to a spatial correlation model, which reproduces line shapes of the observed confined longitudinal-optical modes without incorporating the strain effects. The results demonstrate that a combination of Brillouin and Raman spectroscopy provides a good method to determine accurately the elastic constants and strain information of the lattice-mismatched superlattices and heterostructures.

  3. Phonon modes at the 2H-NbSe2 surface observed by grazing incidence inelastic x-ray scattering.

    PubMed

    Murphy, B M; Requardt, H; Stettner, J; Serrano, J; Krisch, M; Müller, M; Press, W

    2005-12-16

    We have determined the dispersion of acoustic and optical surface phonon modes 2H-NbSe2 at the by inelastic x-ray scattering under grazing incidence conditions. Already, at room temperature, an anomaly is observed close to the charge density wave -vector position located at about one-third along the Gamma-M direction of the Brillouin zone. Our results indicate that the anomaly for the surface mode occurs at a lower energy than that measured in bulk sensitive geometry in the same experiment, showing evidence of a modified behavior in the uppermost layers. We demonstrate that inelastic x-ray scattering in grazing incidence conditions provides a unique tool to selectively study either surface or bulk lattice dynamics in a single experiment.

  4. Close correlation between magnetic properties and the soft phonon mode of the structural transition in BaFe2As2 and SrFe2As2

    DOE PAGES

    Parshall, D.; Pintschovius, L.; Niedziela, Jennifer L.; ...

    2015-04-27

    Pmore » arent compounds of Fe-based superconductors undergo a structural phase transition from a tetragonal to an orthorhombic structure. We investigated the temperature dependence of the frequencies of TA phonons that extrapolate to the shear vibrational mode at the zone center, which corresponds to the orthorhombic deformation of the crystal structure at low temperatures in BaFe2As2 and SrFe2As2. We found that acoustic phonons at small wave vectors soften gradually towards the transition from high temperatures, tracking the increase of the size of slowly fluctuating magnetic domains. On cooling below the transition to base temperature the phonons harden, following the square of the magnetic moment (which we find is proportional to the anisotropy gap). Finally, our results provide evidence for close correlation between magnetic and phonon properties in Fe-based superconductors.« less

  5. Effects of alloy disorder and confinement on phonon modes and Raman scattering in Si{sub x}Ge{sub 1−x} nanocrystals: A microscopic modeling

    SciTech Connect

    Vasin, A. S.; Vikhrova, O. V.; Vasilevskiy, M. I.

    2014-04-14

    Confinement and alloy disorder effects on the lattice dynamics and Raman scattering in Si{sub 1−x}Ge{sub x} nanocrystals (NCs) are investigated numerically employing two different empirical inter-atomic potentials. Relaxed NCs of different compositions (x) were built using the Molecular Dynamics method and applying rigid boundary conditions mimicking the effect of surrounding matrix. The resulting variation of bond lengths with x was checked against Vegard's law and the NC phonon modes were calculated using the same inter-atomic potential. The localization of the principal Raman-active (Si-Si, Si-Ge, and Ge-Ge) modes is investigated by analysing representative eigenvectors and their inverse participation ratio. The dependence of the position and intensity of these modes upon x and NC size is presented and compared to previous calculated results and available experimental data. In particular, it is argued that the composition dependence of the intensity of the Si-Ge and Ge-Ge modes does not follow the fraction of the corresponding nearest-neighbour bonds as it was suggested by some authors. Possible effects of alloy segregation are considered by comparing the calculated properties of random and clustered Si{sub x}Ge{sub 1−x} NCs. It is found that the Si-Si mode and Ge-Ge mode are enhanced and blue-shifted (by several cm{sup −1}for the Si-Si mode), while the intensity of the Si-Ge Raman mode is strongly suppressed by clustering.

  6. Observation of Raman active phonon with Fano lineshape in quasi-one-dimensional superconductor K2Cr3As3

    NASA Astrophysics Data System (ADS)

    Zhang, W.-L.; Li, H.; Dai, X.; L, H. W.; Shi, Y.-G.; Luo, J. L.; Hu, Jiangping; Richard, P.; Ding, H.; Extreme Condition Team; Condensed Matter Theory Team

    We study the polarization-resolved phononic Raman scattering in the recent discovered quasi-one-dimensional superconductor K2Cr3As3. With support from first-principles calculations, we characterize several phonons, among which one mode has a Fano lineshape, indicative of an electron-phonon coupling. While the common expectation of an electron-phonon coupling is the conventional superconducting mechanism, we show that this mode is related to the in-plane Cr vibration, which modulates the exchange coupling between the first nearest Cr neighbors. Our result support the presence of magnetic fluctuations coupled to the electrons via the lattice. We acknowledge MOST (2010CB923000, 2011CBA001000, 2011CBA00102, 2012CB821403 and 2013CB921703), NSFC (11004232, 11034011/A0402, 11234014, 11274362 and 11474330) of China and by the Strategic Priority Research Program (B) of the Chinese Academy of Sciences, Grant No. XDB07020100.

  7. First-principles mode-by-mode analysis for electron-phonon scattering channels and mean free path spectra in GaAs

    NASA Astrophysics Data System (ADS)

    Liu, Te-Huan; Zhou, Jiawei; Liao, Bolin; Singh, David J.; Chen, Gang

    2017-02-01

    We present a first-principles framework to investigate the electron scattering channels and transport properties for polar materials by combining the exact solution of the linearized electron-phonon (e-ph) Boltzmann transport equation in its integral-differential form associated with the e-ph coupling matrices obtained from the polar Wannier interpolation scheme. No ad hoc parameter is required throughout this calculation, and GaAs, a well-studied polar material, is used as an example to demonstrate this method. In this work, the long-range and short-range contributions as well as the intravalley and intervalley transitions in the e-ph interactions (EPIs) have been quantitatively addressed. Promoted by such mode-by-mode analysis, we find that in GaAs, the piezoelectric scattering is comparable to deformation-potential scattering for electron scatterings by acoustic phonons in EPI even at room temperature, and it makes a significant contribution to mobility. Furthermore, we achieved good agreement with experimental data for the mobility, and we identified that electrons with mean free paths between 130 and 210 nm provide the dominant contribution to the electron transport at 300 K. Such information provides a deeper understanding of the electron transport in GaAs, and the presented framework can be readily applied to other polar materials.

  8. Bond-stretching phonon mode in stripe ordered orthorhombic Nd1.67Sr0.33NiO4.

    NASA Astrophysics Data System (ADS)

    Hücker, M.; Fukuda, T.; Gu, G. D.; Tranquada, J. M.; Baron, A. Q. R.; Hill, J. P.

    2007-03-01

    Inelastic x-ray scattering (IXS) experiments on Nd1.67Sr0.33NiO4 have been performed to study electron-phonon interactions in this charge stripe ordered nickelate. Resurgent interest in such phenomena has been triggered by recent results on the high temperature superconductors, where a kink in the electron dispersion as well as striking anomalies in high- energy optical phonon modes have been observed. A significant softening of the bond-stretching-phonon mode for Q||[100] was also observed in the tetragonal nickelate La1.69Sr0.31NiO4 with inelastic neutron scattering. Moreover, this compound shows an apparent splitting of the bond-stretching mode along the [110] direction. Here we present first IXS results for Q||[110] on the orthorhombic compound Nd1.67Sr0.33NiO4, which is characterized by domains with unidirectional stripe order. By probing different spots on the sample with different domain distribution, a weak contrast between the phonon spectra has been observed. We discuss these differences in terms of phonons propagating parallel and perpendicular to the stripe direction, as well as the anisotropic, i.e. orthorhombic, lattice structure.

  9. Phonons and hybrid modes in the high and low temperature far infrared dynamics of hexagonal TmMnO3.

    PubMed

    Massa, Néstor E; del Campo, Leire; De Sousa Meneses, Domingos; Echegut, Patrick; Martínez-Lope, María Jesús; Alonso, José Antonio

    2014-07-09

    We report on temperature dependent TmMnO3 far infrared emissivity and reflectivity spectra from 1910 K to 4 K. At the highest temperature the number of infrared bands is lower than that predicted for centrosymmetric P63/mmc (D(4)(6h)) (Z = 2) space group due to high temperature anharmonicity and possible defect induced bitetrahedra misalignments. On cooling, at ~1600 ± 40 K, TmMnO3 goes from non-polar to an antiferroelectric-ferroelectric polar phase reaching the ferroelectric onset at ~700 K. Room temperature reflectivity is fitted using 19 oscillators and this number of phonons is maintained down to 4 K. A weak phonon anomaly in the band profile at 217 cm(-1) (4 K) suggests subtle Rare Earth magneto-electric couplings at ~TN and below. A low energy collective excitation is identified as a THz instability associated with room temperature eg electrons in a d-orbital fluctuating environment. It condenses into two modes that emerge pinned to the E-type antiferromagnetic order hardening simultaneously down to 4 K. They obey power laws with TN as the critical temperature and match known zone center magnons. The one peaking at 26 cm(-1), with critical exponent β=0.42 as for antiferromagnetic order in a hexagonal lattice, is dependent on the Rare Earth ion. The higher frequency companion at ~50 cm(-1), with β=0.25, splits at ~TN into two peaks. The weaker band of the two is assimilated to the upper branch of the gap opening in the transverse acoustical (TA) phonon branch crossing the magnetic dispersion found in YMnO3. (Petit et al 2007 Phys. Rev. Lett. 99 266604). The stronger second band at ~36 cm(-1) corresponds to the lower branch of the TA gap. We assign both excitations as zone center magneto-electric hybrid quasiparticles, concluding that in NdMnO3 perovskite the equivalent picture corresponds to an instability which may be driven by an external field to transform NdMnO3 into a multiferroic compound by perturbation enhancing the TA

  10. Mode resolved modeling of phonon-structure interactions in semiconductor nanocomposites

    NASA Astrophysics Data System (ADS)

    Feser, Joseph

    Introducing nanoscale inhomogeneities into semiconductor alloys is a known route to enhance the scattering of long wavelength phonons and to subsequently reduce thermal conductivity. For key applications such as thermoelectric energy conversion materials, this must be done efficiently to avoid harming electronic functionality. Thus, key questions arise such as what type (i.e. contrast mechanisms), shape, size, and number density of particles should be used. This talk presents two new theoretical developments in this area from our group: (1) The use of continuum mechanics to analytically calculate exact phonon scattering cross sections of cylindrical and spherical shaped elastic discontinuities across the Mie regime, and their subsequent use in Boltzmann transport models of thermal transport and (2) the development of a new frequency-domain atomistic approach to simulate the scattering cross section of nanoparticles of arbitrary complexity for wavevectors spanning the entire Brillouin zone, and which can accommodate very large atomistic systems. Its advantages compared to Atomistic Green's functions and molecular dynamics will be discussed.

  11. Planar ring-shaped phononic crystal anchoring boundaries for enhancing the quality factor of Lamb mode resonators

    NASA Astrophysics Data System (ADS)

    Binci, L.; Tu, C.; Zhu, H.; Lee, J. E.-Y.

    2016-11-01

    We report the use of planar ring-shaped phononic crystals (PnCs) as anchor boundaries of very-high-frequency band piezoelectric-on-silicon Lamb mode resonators for the purpose of enhancing their quality factor (Q). Here, we exploit the acoustic bandgap associated with the PnC anchoring boundaries to reduce acoustic energy leakage out of the micromechanical resonator. The proposed approach provides greater mechanical robustness (by merit of interlocking the cells in a matrix) and the possibility of electrical routing through the PnC cells. We experimentally show enhancements in Q by a factor of three using the proposed approach of hybridizing planar ring-shaped PnCs with micromechanical resonators. The effect of these PnCs on resonator Q is further corroborated by their effects in suppressing transmission when incorporated into a delay line.

  12. Phonon Transport at Crystalline Si/Ge Interfaces: The Role of Interfacial Modes of Vibration

    PubMed Central

    Gordiz, Kiarash; Henry, Asegun

    2016-01-01

    We studied the modal contributions to heat conduction at crystalline Si and crystalline Ge interfaces and found that more than 15% of the interface conductance arises from less than 0.1% of the modes in the structure. Using the recently developed interface conductance modal analysis (ICMA) method along with a new complimentary methodology, we mapped the correlations between modes, which revealed that a small group of interfacial modes, which exist between 12–13 THz, exhibit extremely strong correlation with other modes in the system. It is found that these interfacial modes (e.g., modes with large eigen vectors for interfacial atoms) are enabled by the degree of anharmonicity near the interface, which is higher than in the bulk, and therefore allows this small group of modes to couple to all others. The analysis sheds light on the nature of localized vibrations at interfaces and can be enlightening for other investigations of localization. PMID:26979787

  13. Temperature dependence of phonon-defect interactions: phonon scattering vs. phonon trapping

    PubMed Central

    Bebek, M. B.; Stanley, C. M.; Gibbons, T. M.; Estreicher, S. K.

    2016-01-01

    The interactions between thermal phonons and defects are conventionally described as scattering processes, an idea proposed almost a century ago. In this contribution, ab-initio molecular-dynamics simulations provide atomic-level insight into the nature of these interactions. The defect is the Si|X interface in a nanowire containing a δ-layer (X is C or Ge). The phonon-defect interactions are temperature dependent and involve the trapping of phonons for meaningful lengths of time in defect-related, localized, vibrational modes. No phonon scattering occurs and the momentum of the phonons released by the defect is unrelated to the momentum of the phonons that generated the excitation. The results are extended to the interactions involving only bulk phonons and to phonon-defect interactions at high temperatures. These do resemble scattering since phonon trapping occurs for a length of time short enough for the momentum of the incoming phonon to be conserved. PMID:27535463

  14. Predictions of thermal expansion coefficients of rare-earth zirconate pyrochlores: A quasi-harmonic approximation based on stable phonon modes

    NASA Astrophysics Data System (ADS)

    Lan, Guoqiang; Ouyang, Bin; Xu, Yushuai; Song, Jun; Jiang, Yong

    2016-06-01

    Rare-earth (RE) pyrochlores are considered as promising candidate materials for the thermal barrier coating. In this study, we performed first-principles calculations, augmented by quasi-harmonic phonon calculations, to investigate the thermal expansion behaviors of several RE2Zr2O7 (RE = La, Nd, Sm, Gd) pyrochlores. Our findings show that RE2Zr2O7 pyrochlores exhibit low-lying optical phonon frequencies that correspond to RE-cation rattling vibrational modes. These frequencies become imaginary upon volume expansion, preventing correct determination of the free energy versus volume relation and thereby quantification of thermal expansion using QH phonon calculations. To address this challenge, we proposed a QH approximation approach based on stable phonon modes where the RE-cation rattling modes were systematically eliminated. This approach is shown to provide accurate predictions of the coefficients of thermal expansion (CTEs) of RE2Zr2O7 pyrochlores, in good agreement with experimental measurements and data from first-principles molecular dynamics simulations. In addition, we showed that the QH Debye model considerably overestimates the magnitudes and wrongly predicts the trend for the CTEs of RE2Zr2O7 pyrochlores.

  15. Effects of two-mode transverse optical phonons in bulk wurtzite AlGaN on electronic mobility in AlGaN/GaN quantum wells

    NASA Astrophysics Data System (ADS)

    Gu, Z.; Ban, S. L.; Jiang, D. D.; Qu, Y.

    2017-01-01

    The two-mode property of bulk transverse optical (TO) phonons in ternary mixed crystals of wurtzite AlxGa1-xN has been investigated by introducing impurity modes in a modified random-element isodisplacement model. Based on the dielectric continuous model, the uniaxial model, and the Lei-Ting balance equation, the effects of the two-mode property on electrostatic potentials of interface optical and confined optical phonons in AlGaN/GaN quantum wells, as well as their influences on the electronic mobility (EM), are discussed by a component-dependent weight model. Our results indicate that the total EM decreases to a minimum at first and then increases slowly with x under the influences of the competitions from the eight branches of phonons. The further calculation shows that the total EM decreases with the increment of temperature in the range of 200 K < T < 400 K and reduction of well width d. As a comparison, the EM is calculated for an Al0.58Ga0.42N/GaN quantum well at room temperature, and our result is 1263.0 cm2/Vs, which is 1.44 times of the experiment value. Our result is expected since the difference between our theory and the experiment is mainly due to the neglect of interface-roughness and other secondary scattering mechanisms. Consequently, the two-mode property of bulk TO phonons in ternary mixed crystals does affect obviously on the electron transport in the quantum wells. And our component-dependent weight model could be extended to study the electric properties influenced by optical phonons in other related heterostructures.

  16. Softening of Roton and Phonon Modes in a Bose-Einstein Condensate with Spin-Orbit Coupling

    NASA Astrophysics Data System (ADS)

    Ji, Si-Cong; Zhang, Long; Xu, Xiao-Tian; Wu, Zhan; Deng, Youjin; Chen, Shuai; Pan, Jian-Wei

    2015-03-01

    Roton-type excitations usually emerge from strong correlations or long-range interactions, as in superfluid helium or dipolar ultracold atoms. However, in a weakly short-range interacting quantum gas, the recently synthesized spin-orbit (SO) coupling can lead to various unconventional phases of superfluidity and give rise to an excitation spectrum of roton-maxon character. Using Bragg spectroscopy, we study a SO-coupled Bose-Einstein condensate of 87Rb atoms and show that the excitation spectrum in a "magnetized" phase clearly possesses a two-branch and roton-maxon structure. As Raman coupling strength Ω is decreased, a roton-mode softening is observed, as a precursor of the phase transition to a stripe phase that spontaneously breaks spatially translational symmetry. The measured roton gaps agree well with theoretical calculations. Furthermore, we determine sound velocities both in the magnetized and in the nonmagnetized phases, and a phonon-mode softening is observed around the phase transition in between. The validity of the f -sum rule is examined.

  17. Excitation of surface phonon polariton modes in gold gratings with silicon carbide substrate and their potential sensing applications

    NASA Astrophysics Data System (ADS)

    Zheng, Gaige; Xu, Linhua; Zou, Xiujuan; Liu, Yuzhu

    2017-02-01

    We demonstrate the excitation of surface phonon polaritons (SPhPs) in the mid-infrared (mid-IR) Reststrahlen band (10.288 μm-12.563 μm) on patterned surfaces with silicon carbide (SiC) substrate and gold (Au) gratings. The very large negative permittivity of Au limits its applications in the mid-IR range, and to couple incident light to SPhPs modes, their momentum mismatch can be compensated by patterning Au grating onto the surface of SiC substrate. Samples were fabricated and characterized experimentally by Fourier transform infrared reflection (FTIR) spectroscopy. The optical properties were also simulated by the rigorous coupled wave analysis (RCWA) method. Reflection dips are observed for light polarized vertical to the grating lines (TM-polarized), which are attributed to the coupling of electromagnetic (EM) waves into the SPhP modes. In addition, we present small-volume index sensing with analyte specificity based on mid-IR SPhPs in the fabricated configuration.

  18. Enhanced plane wave expansion analysis for the band structure of bulk modes in two-dimensional high-contrast solid-solid phononic crystals

    NASA Astrophysics Data System (ADS)

    Baboly, Mohammadhosein Ghasemi; Soliman, Yasser; Su, Mehmet F.; Reinke, Charles M.; Leseman, Zayd C.; El-Kady, Ihab

    2014-11-01

    Plane wave expansion analyses that use the inverse rule to obtain the Fourier coefficients of the elastic tensor instead of the more conventional Laurent's rule, exhibit faster convergence rates for solid-solid phononic crystals. In this work, the band structure convergence of calculations using the inverse rule is investigated and applied to the case of high acoustic impedance contrast solid-solid phononic crystals, previously known for convergence difficulties. Results are contrasted to those obtained with the conventional plane wave expansion method. The inverse rule is found to converge at a much rate for all ranges of impedance contrast, and the ratio between the computational times needed to obtain a convergent band structure for a high-contrast solid-solid phononic crystal with the conventional plane wave expansion method using 1369 reciprocal lattice vectors is as large as 6800:1. This ratio decreases for material sets with lower impedance contrast; however, the inverse rule is still faster for a given error threshold for even the lowest impedance contrast phononic crystals reported in the literature. This convergence enhancement is a major factor in reconsidering the plane wave expansion method as an important tool in obtaining propagating elastic modes in phononic crystals.

  19. Phonon Mode Transformation Across the Orthohombic-Tetragonal Phase Transition in a Lead Iodide Perovskite CH3NH3PbI3: A Terahertz Time-Domain Spectroscopy Approach.

    PubMed

    La-o-Vorakiat, Chan; Xia, Huanxin; Kadro, Jeannette; Salim, Teddy; Zhao, Daming; Ahmed, Towfiq; Lam, Yeng Ming; Zhu, Jian-Xin; Marcus, Rudolph A; Michel-Beyerle, Maria-Elisabeth; Chia, Elbert E M

    2016-01-07

    We study the temperature-dependent phonon modes of the organometallic lead iodide perovskite CH3NH3PbI3 thin film across the terahertz (0.5-3 THz) and temperature (20-300 K) ranges. These modes are related to the vibration of the Pb-I bonds. We found that two phonon modes in the tetragonal phase at room temperature split into four modes in the low-temperature orthorhombic phase. By use of the Lorentz model fitting, we analyze the critical behavior of this phase transition. The carrier mobility values calculated from the low-temperature phonon mode frequencies, via two theoretical approaches, are found to agree reasonably with the experimental value (∼2000 cm(2) V(-1) s(-1)) from a previous time-resolved THz spectroscopy work. Thus, we have established a possible link between terahertz phonon modes and the transport properties of perovskite-based solar cells.

  20. Ultrafast optical generation of coherent phonons in CdTe1-xSex quantum dots

    NASA Astrophysics Data System (ADS)

    Bragas, A. V.; Aku-Leh, C.; Costantino, S.; Ingale, Alka; Zhao, J.; Merlin, R.

    2004-05-01

    We report on the impulsive generation of coherent optical phonons in CdTe0.68Se0.32 nanocrystallites embedded in a glass matrix. Pump-probe experiments using femtosecond laser pulses were performed by tuning the laser central energy to resonate with the absorption edge of the nanocrystals. We identify two longitudinal optical phonons, one longitudinal acoustic phonon and a fourth mode of a mixed longitudinal-transverse nature. The amplitude of the optical phonons as a function of the laser central energy exhibits a resonance that is well described by a model based on impulsive stimulated Raman scattering. The phases of the coherent phonons reveal coupling between different modes. At low power density excitations, the frequency of the optical coherent phonons deviates from values obtained from spontaneous Raman scattering. This behavior is ascribed to the presence of electronic impurity states which modify the nanocrystal dielectric function and, thereby, the frequency of the infrared-active phonons.

  1. Optical properties of amorphous Ge1- x Se x and Ge1- x- y Se x As y thin films — optical gap bowing and phonon modes

    NASA Astrophysics Data System (ADS)

    Lee, Hosuk; So, Hyeon Seob; Lee, Hosun; Shin, Hae-Young; Yoon, Seokhyun; Ahn, Hyung-Woo; Kim, Su-Dong; Lee, Suyoun; Jeong, Doo-Seok; Cheong, Byung-ki

    2014-06-01

    We investigated the optical properties of Ge1- x Se x and Ge1- x- y Se x As y amorphous films by using spectroscopic ellipsometry and Raman spectroscopy. The dielectric functions and absorption coefficients ( α) of the amorphous films were determined from the measured ellipsometric angles (Ψ,Δ). We obtained the optical gap energies and the Urbach energies from the absorption coefficients and found a strong bowing effect in the optical gap energy of Ge1- x- y Se x As y , where the endpoint binaries were Ge0.50Se0.50 and Ge0.31As0.69. Based on the correlation between the optical gap energies and the Urbach energies, we attributed the large bowing parameter to electronic disorder. Using Raman spectroscopy, we measured the phonon modes and discussed the composition dependence of the phonon peak frequencies and lineshapes in terms of structural units. Based on the composition dependence of the phonons in Ge1- x- y Se x As y , we identified the phonon modes of Ge0.31As0.69. A resonant Raman phenomenon was observed in Ge0.40Se0.60 at a laser excitation of 514 nm (2.41 eV). We verified that this laser energy corresponded to the transition energy of Ge0.40Se0.60 by using the second derivative of the dielectric function of Ge0.40Se0.60.

  2. The anharmonic phonon decay rate in group-III nitrides

    NASA Astrophysics Data System (ADS)

    Srivastava, G. P.

    2009-04-01

    Measured lifetimes of hot phonons in group-III nitrides have been explained theoretically by considering three-phonon anharmonic interaction processes. The basic ingredients of the theory include full phonon dispersion relations obtained from the application of an adiabatic bond charge model and crystal anharmonic potential within the isotropic elastic continuum model. The role of various decay routes, such as Klemens, Ridley, Vallée-Bogani and Barman-Srivastava channels, in determining the lifetimes of the Raman active zone-centre longitudinal optical (LO) modes in BN (zincblende structure) and A1(LO) modes in AlN, GaN and InN (wurtzite structure) has been quantified.

  3. Density response of the t-J model and renormalization of breathing and half-breathing phonon modes: A slave-fermion calculation

    NASA Astrophysics Data System (ADS)

    Horsch, P.; Khaliullin, G.; Oudovenko, V.

    The density fluctuation spectrum N( k, ω) is calculated for the t- J model in the low-doping regime using a slave-fermion method for the constrained fermions. The obtained results for N( k, ω) are in good agreement with diagonalization results. The density response is characterized by incoherent, momentum dependent spectral functions reaching up to energies ∼ 8 t and a low-energy structure at energy ∼ J due to transitions in the quasiparticle band. N( k, ω) is shown to lead to a strong renormalization of planar bond-streching and breathing phonon modes with a large phonon linewidth at intermediate momenta caused by the low-energy response. Our results are consistent with recent neutron scattering data, showing the peculiar behavior of these modes.

  4. Bandgap measurements and the peculiar splitting of E2H phonon modes of InxAl1-xN nanowires grown by plasma assisted molecular beam epitaxy

    NASA Astrophysics Data System (ADS)

    Tangi, Malleswararao; Mishra, Pawan; Janjua, Bilal; Ng, Tien Khee; Anjum, Dalaver H.; Prabaswara, Aditya; Yang, Yang; Albadri, Abdulrahman M.; Alyamani, Ahmed Y.; El-Desouki, Munir M.; Ooi, Boon S.

    2016-07-01

    The dislocation free InxAl1-xN nanowires (NWs) are grown on Si(111) by nitrogen plasma assisted molecular beam epitaxy in the temperature regime of 490 °C-610 °C yielding In composition ranges over 0.50 ≤ x ≤ 0.17. We study the optical properties of these NWs by spectroscopic ellipsometry (SE), photoluminescence, and Raman spectroscopies since they possesses minimal strain with reduced defects comparative to the planar films. The optical bandgap measurements of InxAl1-xN NWs are demonstrated by SE where the absorption edges of the NW samples are evaluated irrespective of substrate transparency. A systematic Stoke shift of 0.04-0.27 eV with increasing x was observed when comparing the micro-photoluminescence spectra with the Tauc plot derived from SE. The micro-Raman spectra in the NWs with x = 0.5 showed two-mode behavior for A1(LO) phonons and single mode behavior for E2H phonons. As for x = 0.17, i.e., high Al content, we observed a peculiar E2H phonon mode splitting. Further, we observe composition dependent frequency shifts. The 77 to 600 K micro-Raman spectroscopy measurements show that both AlN- and InN-like modes of A1(LO) and E2H phonons in InxAl1-xN NWs are redshifted with increasing temperature, similar to that of the binary III group nitride semiconductors. These studies of the optical properties of the technologically important InxAl1-xN nanowires will path the way towards lasers and light-emitting diodes in the wavelength of the ultra-violet and visible range.

  5. Compositional behavior of Raman-active phonons in Pb (Z r1 -xT ix)O3 ceramics

    NASA Astrophysics Data System (ADS)

    Buixaderas, E.; Gregora, I.; Savinov, M.; Hlinka, J.; Jin, Li; Damjanovic, D.; Malic, B.

    2015-01-01

    A systematic study of the Raman spectra of Pb (Z r1 -xT ix)O3 (PZT) ceramics has been performed in a broad temperature interval (10-600 K) and a broad Ti/Zr concentration range around the morphotropic phase boundary (x =0.25 -0.70 ) . The number of the spectral components was estimated by a standard fitting procedure with damped harmonic oscillators as well as by counting the number of peaks and shoulders with the help of a purposely designed mathematical analysis based on frequency derivatives of the Raman spectra. This last method proves to be very useful to study Raman spectra of disordered materials. For the case of PZT, the comparison with the Raman modes of PbTi O3 allows us to assign the phonon bands on both sides of the morphotropic phase boundary, and the crossover from the tetragonal to rhombohedral phase spectra is clearly visible. However, there are no indications of a systematic splitting of the E -symmetry modes into monoclinic A'-A'' doublets in the morphotropic samples. Detailed adjusting of the response function to the spectrum requires to assume additional Raman-active modes, but this holds for a much broader concentration range than that of the anticipated monoclinic phase. In particular, the lowest frequency transverse optic mode of E -symmetry (soft mode of the ferroelectric phase transition) is split into two components, a THz frequency anharmonic (central modelike) component and a resonant component (at ω ˜80 c m-1 ), both related to the same normal coordinate. The additional Raman band appearing in this frequency range (ω ˜65 c m-1) at low temperatures is rather associated with the antiphase tilt vibrations of the oxygen octahedra.

  6. Composition dependence of the in-plane Cu-O bond-stretching LO phonon mode in YBa2Cu3O6+x

    SciTech Connect

    Stercil, F.; Egami, T.; Mook Jr, Herbert A; Yethiraj, Mohana; Chung, J.-H.; Arai, M.; Frost, C.; Dogan, F.

    2008-01-01

    An inelastic pulsed neutron scattering study was performed on the dependence of the dispersion and spectral intensity of the in-plane Cu-O bond-stretching LO phonon mode on doped charge density. The measurements were made in the time-of-flight mode with the multiangle position sensitive spectrometer of the ISIS facility on single crystals of YBa{sub 2}Cu{sub 3}O{sub 6+x} (x=0.15, 0.35, 0.6, 0.7, and 0.95). The focus of the study is the in-plane Cu-O bond-stretching LO phonon mode, which is known for strong electron-phonon coupling and unusual dependence on composition and temperature. It is shown that the dispersions for the samples with x=0.35, 0.6, and 0.7 are similar to the superposition of those for x=0.15 and 0.95 samples, and cannot be explained in terms of the structural anisotropy. It is suggested that the results are consistent with the model of nanoscale electronic phase separation, with the fraction of the phases being dependent on the doped charge density.

  7. Phonons and defects in semiconductors and nanostructures: Phonon trapping, phonon scattering, and heat flow at heterojunctions

    SciTech Connect

    Estreicher, S. K. Gibbons, T. M.; Kang, By.; Bebek, M. B.

    2014-01-07

    Defects in semiconductors introduce vibrational modes that are distinct from bulk modes because they are spatially localized in the vicinity of the defect. Light impurities produce high-frequency modes often visible by Fourier-transform infrared absorption or Raman spectroscopy. Their vibrational lifetimes vary by orders of magnitude and sometimes exhibit unexpectedly large isotope effects. Heavy impurities introduce low-frequency modes sometimes visible as phonon replicas in photoluminescence bands. But other defects such as surfaces or interfaces exhibit spatially localized modes (SLMs) as well. All of them can trap phonons, which ultimately decay into lower-frequency bulk phonons. When heat flows through a material containing defects, phonon trapping at localized modes followed by their decay into bulk phonons is usually described in terms of phonon scattering: defects are assumed to be static scattering centers and the properties of the defect-related SLMs modes are ignored. These dynamic properties of defects are important. In this paper, we quantify the concepts of vibrational localization and phonon trapping, distinguish between normal and anomalous decay of localized excitations, discuss the meaning of phonon scattering in real space at the atomic level, and illustrate the importance of phonon trapping in the case of heat flow at Si/Ge and Si/C interfaces.

  8. Einstein Modes in the Phonon Density of States of the Single-filled Skutterudite Yb0.2Co4Sb12

    SciTech Connect

    Dimitrov, I.; Li, Q.; Manley, M.E.; Shapiro, S.M.; Yang, J.; Zhang, W.; Chen, L.D.; Jie, Q.; Ehlers, G.; Podlesnyak, A.; Camacho, J.

    2010-11-10

    Measurements of the phonon density of states by inelastic neutron time-of-flight scattering and specific-heat measurements along with first-principles calculations, provide compelling evidence for the existence of an Einstein oscillator (rattler) at {omega}{sub E1} {approx} 5.0 meV in the filled skutterudite Yb{sub 0.2}Co{sub 4}Sb{sub 12}. Multiple dispersionless modes in the measured density of states of Yb{sub 0.2}Co{sub 4}Sb{sub 12} at intermediate transfer energies (14 {le} {omega} {le} 20 meV) are exhibited in both the experimental and theoretical density of states of the Yb-filled specimen. A peak at 12.4 meV is shown to coincide with a second Einstein mode at {omega}{sub E2} {approx} 12.8 meV obtained from heat-capacity data. The local modes at intermediate transfer energies are attributed to altered properties of the host CoSb{sub 3} cage as a result of Yb filling. It is suggested that these modes are owed to a complementary mechanism for the scattering of heat-carrying phonons in addition to the mode observed at {omega}{sub E1} {approx} 5.0 meV. Our observations offer a plausible explanation for the significantly higher dimensionless figures of merit of filled skutterudites, compared to their parent compounds.

  9. Observation of acoustic-phonon-like mode driven by magnetic imbalance between neighboring Fe atoms in Fe1+yTe (y < 0 . 12)

    NASA Astrophysics Data System (ADS)

    Fobes, David; Zaliznyak, Igor; Xu, Zhijun; Gu, Genda; Tranquada, John M.; He, Xu-Gang; Ku, Wei; Garlea, Ovidiu

    2014-03-01

    We have studied the evolution with temperature of the low-energy inelastic spectra of Fe1+yTe (y < 0 . 12), a parent compound of the iron-chalcogenide superconductor family, revealing an acoustic mode at an unexpected position. Recently, we found evidence for the formation of a bond-order wave leading to ferro-orbital order in the monoclinic phase, in part due to the observation of an elastic structural peak at (100) in the low-temperature monoclinic phase [D. Fobes, et al., arXiv:1307.7162]. In the inelastic spectra we observe a sharp acoustic-phonon-like mode dispersing out of the (100) position in the monoclinic phase. Surprisingly, the mode survives in the tetragonal phase, despite the absence of a Bragg peak at (100); such a peak is forbidden by symmetry. LDA calculations suggest this mode could involve significant magnetic scattering. By assuming in-phase virtual displacement of the Fe atoms from their equilibrium position in a frozen phonon calculation, we have found a small but significant imbalance in the magnetic moments between the two Fe atoms within the unit cell, suggesting magnetic contribution to the mode. Work at BNL supported by Office of Basic Energy Sciences, US DOE, under Contract No. DE-AC02-98CH10886. Research conducted at ORNL Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, US DOE.

  10. Pressure-induced low-lying phonon modes softening and enhanced thermal resistance in β -M g2A l4S i5O18

    NASA Astrophysics Data System (ADS)

    Li, Yiran; Tian, Zhilin; Luo, Yixiu; Wang, Jiemin; Sun, Luchao; Zheng, Liya; Wang, Jingyang

    2017-02-01

    Lattice thermal conductivities of β -M g2A l4S i5O18 were predicted at various hydrostatic pressures based on some theoretical models. An abnormal decrement on lattice thermal conductivity is observed for compressed crystal structure. A rigorous analysis of structural stability, bonding characteristics, vibration modes, group velocities, and mode Grüneisen parameters helps us to recognize the origin of this anomalous behavior. We attribute the negative dependent trend to the softening of low frequency phonons and strengthening of anharmonicity at elevated pressure, both of which arise from the specific corner-linked tetrahedral framework in the crystal structure. To validate theoretical calculations, we synthesized pure and dense β -M g2A l4S i5O18 ceramic by using a two-step processing method and determined its intrinsic lattice thermal conductivity by successfully eliminating the phonon scattering from defects and high-temperature thermal radiation. The experimental intrinsic values agreed quite well with the theoretical predictions. This paper reports an anomalous pressure-induced reduction of lattice thermal conductivity and also provides a key insight into the interesting phonon modification mechanism through tailoring the crystal structure of complex compounds.

  11. Additional and canonical phonon modes in Hg1-xCdxTe(0.06≤x≤0.7)

    NASA Astrophysics Data System (ADS)

    Polit, J.; Sheregii, E. M.; Cebulski, J.; Kisiel, A.; Robouch, B. V.; Marcelli, A.; Mycielski, A.

    2010-07-01

    In this experimental work a conception of the phonon spectra of the Hg1-xCdxTe(x=0.06-0.7) solid solution is presented which explains the presence of additional lines in the region 100-115cm-1 . Data of the optical reflectivity measurements obtained in far and middle infrared regions for eleven compositions of these alloys in the temperature range from 20 to 293 K using the synchrotron radiation ( DAΦNE -LIGHT in LNF, Italy) as source are analyzed. Analyses were performed on samples of different types ( n and p type) of conductivity as well as the temperature dependences of the line intensity under consideration in the region from 70 to 118cm-1 . The model of two valley potential of the mercury atom in the Hg1-xCdxTe lattice is used for interpretation of the additional phonon modes.

  12. Observation of Odd Symmetry Surface Phonon Modes on NICKEL(100) and SILVER(100) Surfaces Using the New Multichannel High Resolution Electron Energy Loss Spectrometer

    NASA Astrophysics Data System (ADS)

    Jeong, Eue-Jin

    A multichannel detection high resolution electron analyzer has been constructed and tested. The capabilities of achieving out-of-plane scattering geometry, high resolution and high sensitivity has made it possible to detect for the first time the odd-symmetry surface phonon modes on Ni(100) and Ag(100) surfaces. Initial tests were performed to verify the performance of the spectrometer. The best instrumental resolution obtained was 3.5 meV and analyzer count rate could be maintained at 1000 Hz in large angle scattering geometry. This represents an improvement in performance of a factor of at least 50 compared to the existing conventional single channel spectrometers. Odd symmetry surface phonon modes found on Ag(100) surfaces have been measured to be 3.2 meV which agrees closely with available calculations. An additional interesting feature found to be interesting is that the background width of odd symmetry modes appear to be narrower than that of the even symmetry mode scattering data. This effect has not been predicted or explained by theory. As it stands now, the large angle high-resolution electron energy loss cross sections are not completely characterized by theory.

  13. Temperature dependence of phonon modes, dielectric functions, and interband electronic transitions in Cu2ZnSnS4 semiconductor films.

    PubMed

    Li, Wenwu; Jiang, Kai; Zhang, Jinzhong; Chen, Xiangui; Hu, Zhigao; Chen, Shiyou; Sun, Lin; Chu, Junhao

    2012-07-28

    The quaternary semiconductor Cu(2)ZnSnS(4) (CZTS) has attracted a lot of attention as a possible absorber material for solar cells due to its direct bandgap and high absorption coefficient. In this study, photovoltaic CZTS nanocrystalline film with a grain size of about 10 nm has been grown on a c-plane sapphire substrate by radio-frequency magnetron sputtering. With increasing the temperature from 86 to 323 K, the A(1) phonon mode shows a red shift of about 9 cm(-1) due to the combined effects of thermal expansion and the anharmonic coupling to the other phonons. Optical and electronic properties of the CZTS film have been investigated by transmittance spectra in the temperature range of 8-300 K. Near-infrared-ultraviolet dielectric functions have been extracted with the Tauc-Lorentz dispersion model. The fundamental band gap E(0), and higher-energy critical points E(1) and E(2) are located at 1.5, 3.6, and 4.2 eV, respectively. Owing to the influences of electron-phonon interaction and the lattice expansion, the three interband transitions present a red shift trend with increasing temperature. It was found that the absorption coefficient in the visible region increases due to the modifications of electronic band structures. The detailed study of the optical properties of CZTS film can provide an experimental basis for CZTS-based solar cell applications.

  14. Air-coupled method to investigate the lowest-order antisymmetric Lamb mode in stubbed and air-drilled phononic plates

    NASA Astrophysics Data System (ADS)

    Zhang, Dongbo; Zhao, Jinfeng; Bonello, Bernard; Li, Libing; Wei, Jianxin; Pan, Yongdong; Zhong, Zheng

    2016-08-01

    In this work, we applied a robust and fully air-coupled method to investigate the propagation of the lowest-order antisymmetric Lamb (A0) mode in both a stubbed and an air-drilled phononic-crystal (PC) plate. By measuring simply the radiative acoustic waves of A0 mode close to the plate surface, we observed the band gaps for the stubbed PC plate caused by either the local resonance or the Bragg scattering, in frequency ranges in good agreement with theoretical predictions. We measured then the complete band gap of A0 mode for the air-drilled PC plate, in good agreement with the band structures. Finally, we compared the measurements made using the air-coupled method with those obtained by the laser ultrasonic technique.

  15. Multi-enhanced-phonon scattering modes in Ln-Me-A sites co-substituted LnMeA11O19 ceramics.

    PubMed

    Lu, Haoran; Wang, Chang-An; Huang, Yong; Xie, Huimin

    2014-10-29

    Authors reported an effective path to decrease the thermal conductivity while to increase the coefficient of thermal expansion, thus enhancing the thermo-physical properties of the LnMeA11O19-type magnetoplumbite LaMgAl11O19 by simultaneously substituting La(3+), Mg(2+) and Al(3+) ions with large ionic radius Ba(2+), Zn(2+) and Ti(4+), respectively. The mechanism behind the lowered thermal conductivity was mainly due to the multi-enhanced-phonon scattering modes in Ln-Me-A sites co-substituted LnMeA11O19 ceramics. These modes involve the following four aspects, namely, point defect mechanism, the intrinsic scattering in the complex crystal cell and materials with stepped surface to localize phonon vibrational modes, as well as nano-platelet-like structure to incorporate additional grain boundary scattering. This study provides novel thoughts for promising candidate materials of even lower thermal conductivity for the next generation thermal barrier coatings.

  16. Multi-Enhanced-Phonon Scattering Modes in Ln-Me-A Sites co-substituted LnMeA11O19 Ceramics

    PubMed Central

    Lu, Haoran; Wang, Chang-An; Huang, Yong; Xie, Huimin

    2014-01-01

    Authors reported an effective path to decrease the thermal conductivity while to increase the coefficient of thermal expansion, thus enhancing the thermo-physical properties of the LnMeA11O19-type magnetoplumbite LaMgAl11O19 by simultaneously substituting La3+, Mg2+ and Al3+ ions with large ionic radius Ba2+, Zn2+ and Ti4+, respectively. The mechanism behind the lowered thermal conductivity was mainly due to the multi-enhanced-phonon scattering modes in Ln-Me-A sites co-substituted LnMeA11O19 ceramics. These modes involve the following four aspects, namely, point defect mechanism, the intrinsic scattering in the complex crystal cell and materials with stepped surface to localize phonon vibrational modes, as well as nano-platelet-like structure to incorporate additional grain boundary scattering. This study provides novel thoughts for promising candidate materials of even lower thermal conductivity for the next generation thermal barrier coatings. PMID:25351166

  17. Multi-Enhanced-Phonon Scattering Modes in Ln-Me-A Sites co-substituted LnMeA11O19 Ceramics

    NASA Astrophysics Data System (ADS)

    Lu, Haoran; Wang, Chang-An; Huang, Yong; Xie, Huimin

    2014-10-01

    Authors reported an effective path to decrease the thermal conductivity while to increase the coefficient of thermal expansion, thus enhancing the thermo-physical properties of the LnMeA11O19-type magnetoplumbite LaMgAl11O19 by simultaneously substituting La3+, Mg2+ and Al3+ ions with large ionic radius Ba2+, Zn2+ and Ti4+, respectively. The mechanism behind the lowered thermal conductivity was mainly due to the multi-enhanced-phonon scattering modes in Ln-Me-A sites co-substituted LnMeA11O19 ceramics. These modes involve the following four aspects, namely, point defect mechanism, the intrinsic scattering in the complex crystal cell and materials with stepped surface to localize phonon vibrational modes, as well as nano-platelet-like structure to incorporate additional grain boundary scattering. This study provides novel thoughts for promising candidate materials of even lower thermal conductivity for the next generation thermal barrier coatings.

  18. Phonon engineering for nanostructures.

    SciTech Connect

    Aubry, Sylvie; Friedmann, Thomas Aquinas; Sullivan, John Patrick; Peebles, Diane Elaine; Hurley, David H.; Shinde, Subhash L.; Piekos, Edward Stanley; Emerson, John Allen

    2010-01-01

    Understanding the physics of phonon transport at small length scales is increasingly important for basic research in nanoelectronics, optoelectronics, nanomechanics, and thermoelectrics. We conducted several studies to develop an understanding of phonon behavior in very small structures. This report describes the modeling, experimental, and fabrication activities used to explore phonon transport across and along material interfaces and through nanopatterned structures. Toward the understanding of phonon transport across interfaces, we computed the Kapitza conductance for {Sigma}29(001) and {Sigma}3(111) interfaces in silicon, fabricated the interfaces in single-crystal silicon substrates, and used picosecond laser pulses to image the thermal waves crossing the interfaces. Toward the understanding of phonon transport along interfaces, we designed and fabricated a unique differential test structure that can measure the proportion of specular to diffuse thermal phonon scattering from silicon surfaces. Phonon-scale simulation of the test ligaments, as well as continuum scale modeling of the complete experiment, confirmed its sensitivity to surface scattering. To further our understanding of phonon transport through nanostructures, we fabricated microscale-patterned structures in diamond thin films.

  19. Comparison of junctionless and inversion-mode p-type metal-oxide-semiconductor field-effect transistors in presence of hole-phonon interactions

    SciTech Connect

    Dib, E.; Carrillo-Nuñez, H.; Cavassilas, N.; Bescond, M.

    2016-01-28

    Junctionless transistors are being considered as one of the alternatives to conventional metal-oxide field-effect transistors. In this work, it is then presented a simulation study of silicon double-gated p-type junctionless transistors compared with its inversion-mode counterpart. The quantum transport problem is solved within the non-equilibrium Green's function formalism, whereas hole-phonon interactions are tackled by means of the self-consistent Born approximation. Our findings show that junctionless transistors should perform as good as a conventional transistor only for ultra-thin channels, with the disadvantage of requiring higher supply voltages in thicker channel configurations.

  20. Phonon-assisted relaxation in a frustrated antiferromagnet

    SciTech Connect

    Ehlers, Georg

    2006-01-01

    A thermally activated magnetic relaxation is observed using neutron spin-echo in the pyrochlore slab (kagom{acute e} bilayer) compound SrCr{sub 9x}Ga{sub 12-9x}O{sub 19} (x=0.95) in a restricted temperature range, 4K < T < 4K, above a cross-over to a low temperature relaxation regime with a weaker temperature dependence. The activation energy of the thermally activated relaxation, of the order of 7 meV, coincides with the energy of a phonon mode observed with neutron and Raman spectroscopy, indicating a phonon-assisted regime. The experimental observation of phonon-assisted process gives additional insight to the importance of spin-phonon coupling in frustrated magnets with regard to the models mostly based on purely magnetic interactions.

  1. A novel system-bath Hamiltonian for vibration-phonon coupling: Formulation, and application to the relaxation of Si-H and Si-D bending modes of H/D:Si(100)-(2 × 1)

    NASA Astrophysics Data System (ADS)

    Lorenz, U.; Saalfrank, P.

    2017-01-01

    We present a rigorous method to set up a system-bath Hamiltonian for the coupling of adsorbate vibrations (the system) to surface phonons (the bath). The Hamiltonian is straightforward to derive and exact up to second order in the environment coordinates, thus capable of treating one- and two-phonon contributions to vibration-phonon coupling. The construction of the Hamiltonian uses orthogonal coordinates for system and bath modes, is based on an embedded cluster approach, and generalizes previous Hamiltonians of a similar type, but avoids several (additional) approximations. While the parametrization of the full Hamiltonian is in principle feasible by a first principles quantum mechanical treatment, here we adopt in the spirit of a QM/MM model a combination of density functional theory ("QM", for the system) and a semiempirical forcefield ("MM", for the bath). We apply the Hamiltonian to a fully H-covered Si(100)-(2 × 1) surface, using Fermi's Golden Rule to obtain vibrational relaxation rates of various H-Si bending modes of this system. As in earlier work it is found that the relaxation is dominated by two-phonon contributions because of an energy gap between the Si-H bending modes and the Si phonon bands. We obtain vibrational lifetimes (of the first excited state) on the order of 2 ps at T = 0 K. The lifetimes depend only little on the type of bending mode (symmetric vs. antisymmetric, parallel vs. perpendicular to the Si2H2 dimers). They decrease by a factor of about two when heating the surface to 300 K. We also study isotope effects by replacing adsorbed H atoms by deuterium, D. The Si-D bending modes are shifted into the Si phonon band of the solid, opening up one-phonon decay channels and reducing the lifetimes to few hundred fs.

  2. Coherent phonon modes of crystalline and amorphous Ge{sub 2}Sb{sub 2}Te{sub 5} thin films: A fingerprint of structure and bonding

    SciTech Connect

    Shalini, A.; Liu, Y.; Srivastava, G. P.; Hicken, R. J.; Katmis, F.; Braun, W.

    2015-01-14

    Femtosecond optical pump-probe measurements have been made upon epitaxial, polycrystalline, and amorphous thin films of Ge{sub 2}Sb{sub 2}Te{sub 5} (GST). A dominant coherent optical phonon mode of 3.4 THz frequency is observed in time-resolved anisotropic reflectance (AR) measurements of epitaxial films, and is inferred to have 3-dimensional T{sub 2}-like character based upon the dependence of its amplitude and phase on pump and probe polarization. In contrast, the polycrystalline and amorphous phases exhibit a comparatively weak mode of about 4.5 THz frequency in both reflectivity (R) and AR measurements. Raman microscope measurements confirm the presence of the modes observed in pump-probe measurements, and reveal additional modes. While the Raman spectra are qualitatively similar for all three phases of GST, the mode frequencies are found to be different within experimental error, ranging from 3.2 to 3.6 THz and 4.3 to 4.7 THz, indicating that the detailed crystallographic structure has a significant effect upon the phonon frequency. While the lower frequency (3.6 THz) mode of amorphous GST is most likely associated with GeTe{sub 4} tetrahedra, modes in epitaxial (3.4 THz) and polycrystalline (3.2 THz) GST could be associated with either GeTe{sub 6} octahedra or Sb-Te bonds within defective octahedra. The more polarizable Sb-Te bonds are the most likely origin of the higher frequency (4.3–4.7 THz) mode, although the influence of Te-Te bonds cannot be excluded. The effect of high pump fluence, which leads to irreversible structural changes, has been explored. New modes with frequency of 3.5/3.6 THz in polycrystalline/amorphous GST may be associated with Sb{sub 2}Te{sub 3} or GeTe{sub 4} tetrahedra, while a 4.2 THz mode observed in epitaxial GST may be related to segregation of Sb.

  3. Coherent Phonon Rabi Oscillations with a High-Frequency Carbon Nanotube Phonon Cavity.

    PubMed

    Zhu, Dong; Wang, Xin-He; Kong, Wei-Cheng; Deng, Guang-Wei; Wang, Jiang-Tao; Li, Hai-Ou; Cao, Gang; Xiao, Ming; Jiang, Kai-Li; Dai, Xing-Can; Guo, Guang-Can; Nori, Franco; Guo, Guo-Ping

    2017-02-08

    Phonon-cavity electromechanics allows the manipulation of mechanical oscillations similar to photon-cavity systems. Many advances on this subject have been achieved in various materials. In addition, the coherent phonon transfer (phonon Rabi oscillations) between the phonon cavity mode and another oscillation mode has attracted many interest in nanoscience. Here, we demonstrate coherent phonon transfer in a carbon nanotube phonon-cavity system with two mechanical modes exhibiting strong dynamical coupling. The gate-tunable phonon oscillation modes are manipulated and detected by extending the red-detuned pump idea of photonic cavity electromechanics. The first- and second-order coherent phonon transfers are observed with Rabi frequencies 591 and 125 kHz, respectively. The frequency quality factor product fQm ∼ 2 × 10(12) Hz achieved here is larger than kBTbase/h, which may enable the future realization of Rabi oscillations in the quantum regime.

  4. GHz spurious mode free AlN lamb wave resonator with high figure of merit using one dimensional phononic crystal tethers

    NASA Astrophysics Data System (ADS)

    Wu, Guoqiang; Zhu, Yao; Merugu, Srinivas; Wang, Nan; Sun, Chengliang; Gu, Yuandong

    2016-07-01

    This letter reports a spurious mode free GHz aluminum nitride (AlN) lamb wave resonator (LWR) towards high figure of merit (FOM). One dimensional gourd-shape phononic crystal (PnC) tether with large phononic bandgaps is employed to reduce the acoustic energy dissipation into the substrate. The periodic PnC tethers are based on a 1 μm-thick AlN layer with 0.26 μm-thick Mo layer on top. A clean spectrum over a wide frequency range is obtained from the measurement, which indicates a wide-band suppression of spurious modes. Experimental results demonstrate that the fabricated AlN LWR has an insertion loss of 5.2 dB and a loaded quality factor (Q) of 1893 at 1.02 GHz measured in air. An impressive ratio of the resistance at parallel resonance (Rp) to the resistance at series resonance (Rs) of 49.8 dB is obtained, which is an indication of high FOM for LWR. The high Rp to Rs ratio is one of the most important parameters to design a radio frequency filter with steep roll-off.

  5. Hypersonic phonon propagation in one-dimensional surface phononic crystal

    NASA Astrophysics Data System (ADS)

    Graczykowski, B.; Sledzinska, M.; Kehagias, N.; Alzina, F.; Reparaz, J. S.; Sotomayor Torres, C. M.

    2014-03-01

    Hypersonic, thermally activated surface acoustic waves propagating in the surface of crystalline silicon patterned with periodic stripes were studied by Brillouin light scattering. Two characteristic directions (normal and parallel to the stripes) of surface acoustic waves propagation were examined exhibiting a distinctive propagation behavior. The measured phononic band structure exhibits diverse features, such as zone folding, band gap opening, and hybridization to local resonance for waves propagating normal to the stripes, and a variety of dispersive modes propagating along the stripes. Experimental results were supported by theoretical calculations performed using finite element method.

  6. Active control of multiple resistive wall modes

    NASA Astrophysics Data System (ADS)

    Brunsell, P. R.; Yadikin, D.; Gregoratto, D.; Paccagnella, R.; Liu, Y. Q.; Bolzonella, T.; Cecconello, M.; Drake, J. R.; Kuldkepp, M.; Manduchi, G.; Marchiori, G.; Marrelli, L.; Martin, P.; Menmuir, S.; Ortolani, S.; Rachlew, E.; Spizzo, G.; Zanca, P.

    2005-12-01

    A two-dimensional array of saddle coils at Mc poloidal and Nc toroidal positions is used on the EXTRAP T2R reversed-field pinch (Brunsell P R et al 2001 Plasma Phys. Control. Fusion 43 1457) to study active control of resistive wall modes (RWMs). Spontaneous growth of several RWMs with poloidal mode number m = 1 and different toroidal mode number n is observed experimentally, in agreement with linear MHD modelling. The measured plasma response to a controlled coil field and the plasma response computed using the linear circular cylinder MHD model are in quantitive agreement. Feedback control introduces a linear coupling of modes with toroidal mode numbers n, n' that fulfil the condition |n - n'| = Nc. Pairs of coupled unstable RWMs are present in feedback experiments with an array of Mc × Nc = 4 × 16 coils. Using intelligent shell feedback, the coupled modes are generally not controlled even though the field is suppressed at the active coils. A better suppression of coupled modes may be achieved in the case of rotating modes by using the mode control feedback scheme with individually set complex gains. In feedback with a larger array of Mc × Nc = 4 × 32 coils, the coupling effect largely disappears, and with this array, the main internal RWMs n = -11, -10, +5, +6 are all simultaneously suppressed throughout the discharge (7 8 wall times). With feedback there is a two-fold extension of the pulse length, compared to discharges without feedback.

  7. Phonon anomalies in some iron telluride materials

    DOE PAGES

    C. C. Homes; Dai, Y. M.; Schneeloch, J.; ...

    2016-03-21

    In this paper, the detailed temperature dependence of the infrared-active mode in Fe1.03Te (TN ≃ 68 K) and Fe1.13Te (TN ≃ 56 K) has been examined, and the position, width, strength, and asymmetry parameter have been determined using an asymmetric Fano profile superimposed on an electronic background. In both materials the frequency of the mode increases as the temperature is reduced; however, there is also a slight asymmetry in the line shape, indicating that the mode is coupled to either spin or charge excitations. Below TN there is an anomalous decrease in frequency, and the mode shows little temperature dependence,more » at the same time becoming more symmetric, suggesting a reduction in spin- or electron-phonon coupling. The frequency of the infrared-active mode and the magnitude of the shift below TN are predicted reasonably well by first-principles calculations; however, the predicted splitting of the mode is not observed. In superconducting FeTe0.55Se0.45 (Tc ≃ 14 K) the infrared-active Eu mode displays asymmetric line shape at all temperatures, which is most pronounced between 100 – 200 K, indicating the presence of either spin- or electron-phonon coupling, which may be a necessary prerequisite for superconductivity in this class of materials.« less

  8. Phonons in twisted bilayer graphene

    NASA Astrophysics Data System (ADS)

    Cocemasov, Alexandr I.; Nika, Denis L.; Balandin, Alexander A.

    2013-07-01

    We theoretically investigate phonon dispersion in AA-stacked, AB-stacked, and twisted bilayer graphene with various rotation angles. The calculations are performed using the Born-von Karman model for the intralayer atomic interactions and the Lennard-Jones potential for the interlayer interactions. It is found that the stacking order affects the out-of-plane acoustic phonon modes the most. The difference in the phonon densities of states in the twisted bilayer graphene and in AA- or AB-stacked bilayer graphene appears in the phonon frequency range 90-110 cm-1. Twisting bilayer graphene leads to the emergence of different phonon branches—termed hybrid folded phonons—which originate from the mixing of phonon modes from different high-symmetry directions in the Brillouin zone. The frequencies of the hybrid folded phonons depend strongly on the rotation angle and can be used for noncontact identification of the twist angles in graphene samples. The obtained results and the tabulated frequencies of phonons in twisted bilayer graphene are important for the interpretation of experimental Raman data and in determining the thermal conductivity of these material systems.

  9. Activation of coherent lattice phonon following ultrafast molecular spin-state photo-switching: A molecule-to-lattice energy transfer

    PubMed Central

    Marino, A.; Cammarata, M.; Matar, S. F.; Létard, J.-F.; Chastanet, G.; Chollet, M.; Glownia, J. M.; Lemke, H. T.; Collet, E.

    2015-01-01

    We combine ultrafast optical spectroscopy with femtosecond X-ray absorption to study the photo-switching dynamics of the [Fe(PM-AzA)2(NCS)2] spin-crossover molecular solid. The light-induced excited spin-state trapping process switches the molecules from low spin to high spin (HS) states on the sub-picosecond timescale. The change of the electronic state (<50 fs) induces a structural reorganization of the molecule within 160 fs. This transformation is accompanied by coherent molecular vibrations in the HS potential and especially a rapidly damped Fe-ligand breathing mode. The time-resolved studies evidence a delayed activation of coherent optical phonons of the lattice surrounding the photoexcited molecules. PMID:26798836

  10. Phononic crystal diffraction gratings

    NASA Astrophysics Data System (ADS)

    Moiseyenko, Rayisa P.; Herbison, Sarah; Declercq, Nico F.; Laude, Vincent

    2012-02-01

    When a phononic crystal is interrogated by an external source of acoustic waves, there is necessarily a phenomenon of diffraction occurring on the external enclosing surfaces. Indeed, these external surfaces are periodic and the resulting acoustic diffraction grating has a periodicity that depends on the orientation of the phononic crystal. This work presents a combined experimental and theoretical study on the diffraction of bulk ultrasonic waves on the external surfaces of a 2D phononic crystal that consists of a triangular lattice of steel rods in a water matrix. The results of transmission experiments are compared with theoretical band structures obtained with the finite-element method. Angular spectrograms (showing frequency as a function of angle) determined from diffraction experiments are then compared with finite-element simulations of diffraction occurring on the surfaces of the crystal. The experimental results show that the diffraction that occurs on its external surfaces is highly frequency-dependent and has a definite relation with the Bloch modes of the phononic crystal. In particular, a strong influence of the presence of bandgaps and deaf bands on the diffraction efficiency is found. This observation opens perspectives for the design of efficient phononic crystal diffraction gratings.

  11. Optimum design of phononic crystal perforated plate structures for widest bandgap of fundamental guided wave modes and maximized in-plane stiffness

    NASA Astrophysics Data System (ADS)

    Hedayatrasa, Saeid; Abhary, Kazem; Uddin, Mohammad; Ng, Ching-Tai

    2016-04-01

    This paper presents a topology optimization of single material phononic crystal plate (PhP) to be produced by perforation of a uniform background plate. The primary objective of this optimization study is to explore widest exclusive bandgaps of fundamental (first order) symmetric or asymmetric guided wave modes as well as widest complete bandgap of mixed wave modes (symmetric and asymmetric). However, in the case of single material porous phononic crystals the bandgap width essentially depends on the resultant structural integration introduced by achieved unitcell topology. Thinner connections of scattering segments (i.e. lower effective stiffness) generally lead to (i) wider bandgap due to enhanced interfacial reflections, and (ii) lower bandgap frequency range due to lower wave speed. In other words higher relative bandgap width (RBW) is produced by topology with lower effective stiffness. Hence in order to study the bandgap efficiency of PhP unitcell with respect to its structural worthiness, the in-plane stiffness is incorporated in optimization algorithm as an opposing objective to be maximized. Thick and relatively thin Polysilicon PhP unitcells with square symmetry are studied. Non-dominated sorting genetic algorithm NSGA-II is employed for this multi-objective optimization problem and modal band analysis of individual topologies is performed through finite element method. Specialized topology initiation, evaluation and filtering are applied to achieve refined feasible topologies without penalizing the randomness of genetic algorithm (GA) and diversity of search space. Selected Pareto topologies are presented and gradient of RBW and elastic properties in between the two Pareto front extremes are investigated. Chosen intermediate Pareto topology, even not extreme topology with widest bandgap, show superior bandgap efficiency compared with the results reported in other works on widest bandgap topology of asymmetric guided waves, available in the literature

  12. Local phonon mode in thermoelectric Bi{sub 2}Te{sub 2}Se from charge neutral antisites

    SciTech Connect

    Tian, Yao; Osterhoudt, Gavin B.; Burch, Kenneth S.; Jia, Shuang; Cava, R. J.

    2016-01-25

    Local modes caused by defects play a significant role in the thermal transport properties of thermoelectrics. Of particular interest are charge-neutral defects that suppress thermal conductivity, without significantly reducing electrical transport. Here, we report a temperature dependent Raman study that identifies such a mode in a standard thermoelectric material, Bi{sub 2}Te{sub 2}Se. One of the modes observed, whose origin has been debated for decades, was shown most likely to be an antisite defect induced local mode. The anomalous temperature independent broadening of the local mode is ascribed to the random arrangement of Se atoms. The temperature renormalization of all modes is well explained by an anharmonic model–Klemens's model.

  13. Raman scattering study of background electron density in InN: a hydrodynamical approach to the LO-phonon-plasmon coupled modes.

    PubMed

    Cuscó, R; Alarcón-Lladó, E; Ibáñez, J; Yamaguchi, T; Nanishi, Y; Artús, L

    2009-10-14

    We use a hydrodynamical approach to analyse the long-wavelength LO-phonon-plasmon coupled modes observed in a set of high-quality MBE-grown InN epilayers with electron densities varying over one order of magnitude, from ∼2 × 10(18) to ∼2 × 10(19)  cm(-3). The samples were characterized by scanning electron microscopy, x-ray diffraction and Hall measurements. The correlation observed between the E(2)(high) mode frequency, and hence residual strain, and the electron density measured in the layers indicates that the differences in background electron density may be associated with threading dislocations. Owing to the low Raman signal, only the L(-) branch of the coupled modes can be unambiguously observed. The frequency of the L(-) Raman peak is, however, sensitive enough to the free electron density to allow its determination from lineshape fits to the spectra. These were carried out using an extended hydrodynamical model. Given the small bandgap energy and large conduction band nonparabolicity of InN, suitable expressions for the optical effective mass and mean square velocity that enter the hydrodynamical model were derived. Electron density values extracted from L(-) lineshape fits agree reasonably well with Hall determinations.

  14. Phonon Mode Transformation across the Orthohombic-Tetragonal Phase Transition in a Lead-Iodide Perovskite CH3NH3PbI3: a Terahertz Time-Domain Spectroscopy Approach

    NASA Astrophysics Data System (ADS)

    Chia, Elbert E. M.; La-O-Vorakiat, Chan; Kadro, Jeannette; Salim, Teddy; Zhao, Daming; Ahmed, Towfiq; Lam, Yeng Ming; Zhu, Jian-Xin; Marcus, Rudolph; Michel-Beyerle, Maria-Elisabeth

    Using terahertz time-domain spectroscopy (THz-TDS), we study the temperature-dependent phonon modes of the organometallic lead iodide perovskite CH3NH3PbI3 thin film across the terahertz (0.5-3 THz) and temperature (20-300 K) ranges. These modes are related to the vibration of the Pb-I bonds. We found that two phonon modes in the tetragonal phase at room temperature split into four modes in the low-temperature orthorhombic phase. By use of the Lorentz model fitting, we analyze the critical behavior of this phase transition. King Mongkut's University of Technology Thonburi (Grant No. SCI58-003), Singapore MOE Tier 1 (RG13/12, RG123/14), ONR, ARO, NTU Biophysics Center, LANL LDRD, LANL CINT.

  15. Nonlinear phononics using atomically thin membranes

    NASA Astrophysics Data System (ADS)

    Midtvedt, Daniel; Isacsson, Andreas; Croy, Alexander

    2014-09-01

    Phononic crystals and acoustic metamaterials are used to tailor phonon and sound propagation properties by utilizing artificial, periodic structures. Analogous to photonic crystals, phononic band gaps can be created, which influence wave propagation and, more generally, allow engineering of the acoustic properties of a system. Beyond that, nonlinear phenomena in periodic structures have been extensively studied in photonic crystals and atomic Bose-Einstein condensates in optical lattices. However, creating nonlinear phononic crystals or nonlinear acoustic metamaterials remains challenging and only few examples have been demonstrated. Here, we show that atomically thin and periodically pinned membranes support coupled localized modes with nonlinear dynamics. The proposed system provides a platform for investigating nonlinear phononics.

  16. Normal Modes Expose Active Sites in Enzymes

    PubMed Central

    Glantz-Gashai, Yitav; Samson, Abraham O.

    2016-01-01

    Accurate prediction of active sites is an important tool in bioinformatics. Here we present an improved structure based technique to expose active sites that is based on large changes of solvent accessibility accompanying normal mode dynamics. The technique which detects EXPOsure of active SITes through normal modEs is named EXPOSITE. The technique is trained using a small 133 enzyme dataset and tested using a large 845 enzyme dataset, both with known active site residues. EXPOSITE is also tested in a benchmark protein ligand dataset (PLD) comprising 48 proteins with and without bound ligands. EXPOSITE is shown to successfully locate the active site in most instances, and is found to be more accurate than other structure-based techniques. Interestingly, in several instances, the active site does not correspond to the largest pocket. EXPOSITE is advantageous due to its high precision and paves the way for structure based prediction of active site in enzymes. PMID:28002427

  17. Measuring phonons in protein crystals

    NASA Astrophysics Data System (ADS)

    Niessen, Katherine A.; Snell, Edward; Markelz, A. G.

    2013-03-01

    Using Terahertz near field microscopy we find orientation dependent narrow band absorption features for lysozyme crystals. Here we discuss identification of protein collective modes associated with the observed features. Using normal mode calculations we find good agreement with several of the measured features, suggesting that the modes arise from internal molecular motions and not crystal phonons. Such internal modes have been associated with protein function.

  18. Electrons and Phonons in Semiconductor Multilayers

    NASA Astrophysics Data System (ADS)

    Ridley, B. K.

    2014-08-01

    Introduction; 1. Simple models of the electron-phonon interaction; 2. Quantum confinement of carriers; 3. Quasicontinuum theory of lattice vibrations; 4. Bulk vibratory modes in an isotropic continuum; 5. Optical modes in a quantum well; 6. Superlattice modes; 7. Optical modes in various structures; 8. Electron-phonon interaction in a quantum well; 9. Other scattering mechanisms; 10. Quantum screening; 11. The electron distribution function; 12. Spin relaxation; 13. Electrons and phonons in the Wurtzite lattice; 14. Nitride heterostructures; 15. Terahertz sources; References; Index.

  19. Wide-Stopband Aperiodic Phononic Filters

    NASA Technical Reports Server (NTRS)

    Rostem, Karwan; Chuss, David; Denis, K. L.; Wollack, E. J.

    2016-01-01

    We demonstrate that a phonon stopband can be synthesized from an aperiodic structure comprising a discrete set of phononic filter stages. Each element of the set has a dispersion relation that defines a complete bandgap when calculated under a Bloch boundary condition. Hence, the effective stopband width in an aperiodic phononic filter (PnF) may readily exceed that of a phononic crystal with a single lattice constant or a coherence scale. With simulations of multi-moded phononic waveguides, we discuss the effects of finite geometry and mode-converting junctions on the phonon transmission in PnFs. The principles described may be utilized to form a wide stopband in acoustic and surface wave media. Relative to the quantum of thermal conductance for a uniform mesoscopic beam, a PnF with a stopband covering 1.6-10.4 GHz is estimated to reduce the thermal conductance by an order of magnitude at 75 mK.

  20. Observation of phonons in multiferroic BiFeO3 single crystals by Raman scattering

    NASA Astrophysics Data System (ADS)

    Fukumura, H.; Matsui, S.; Harima, H.; Takahashi, T.; Itoh, T.; Kisoda, K.; Tamada, M.; Noguchi, Y.; Miyayama, M.

    2007-09-01

    We have grown BiFeO3 bulk single crystals by a flux method and characterized the phonon spectra in detail by Raman scattering in the temperature range 4-1100 K. All the 13 Raman-active phonon modes predicted by group theory, 4A1+9E, were observed at low temperature and successfully assigned by a polarized Raman measurement. Moreover, drastic spectral changes in the Raman spectra were observed at temperatures 600-700 K and 1000-1100 K. These features are discussed from the viewpoint of phonon coupling with the magnetic ordering and the structural phase transition, respectively.

  1. Temperature Dependence of Phonons in Pyrolitic Graphite

    DOE R&D Accomplishments Database

    Brockhouse, B. N.; Shirane, G.

    1977-01-01

    Dispersion curves for longitudinal and transverse phonons propagating along and near the c-axis in pyrolitic graphite at temperatures between 4°K and 1500°C have been measured by neutron spectroscopy. The observed frequencies decrease markedly with increasing temperature (except for the transverse optical ''rippling'' modes in the hexagonal planes). The neutron groups show interesting asymmetrical broadening ascribed to interference between one phonon and many phonon processes.

  2. Temperature dependence of coherent phonons in TbVO4 crystal probed by ultrafast optical spectroscopy

    NASA Astrophysics Data System (ADS)

    Jin, Z.; Ma, H.; Li, D.; Wang, L.; Ma, G.; Guo, F.; Chen, J.

    2011-07-01

    Coherent optical phonons in terbium vanadate (TbVO4) are investigated by using femtosecond time-resolved pump-probe spectroscopy at temperatures from 20 to 300 K. Combined with the Raman spectrum, the coherent phonon mode is attributed to an optical phonon mode of B1g symmetry. The main generation mechanism of the coherent optical phonons is revealed to be the impulsive stimulated Raman scattering. The temperature dependence of the dephasing time reveals that the main mechanism of the coherent phonon population decay is anharmonic phonon-phonon coupling, which causes a redshift of the coherent phonon frequency with increasing temperature.

  3. Effects of substitution, pressure, and temperature on the phonon mode in layered-rocksalt-type Li{sub (1−x)/2}Ga{sub (1−x)/2}Zn{sub x}O (x = 0.036–0.515) alloys

    SciTech Connect

    Tan, Lijie; Hu, Qiwei; Lei, Li Jiang, Xiaodong; Gao, Shangpan; He, Duanwei

    2015-11-14

    ZnO-based semiconductor alloys, Li{sub (1−x)/2}Ga{sub (1−x)/2}Zn{sub x}O (x = 0.036–0.515) with a layered-rocksalt-type structure, have been prepared under high pressure. The composition, pressure, and temperature dependence of phonons have been studied by Raman spectroscopy. We observe two disorder-activated Raman (DAR) modes when the Zn composition x increases: a broad Raman peak at ca. 400 cm{sup −1} and a left-shoulder peak at ca. 530 cm{sup −1} on the low-frequency side of A{sub 1g} mode at ca. 580 cm{sup −1}, which can be explained by reference to the phonon density of states for rocksalt-type ZnO. With the increase of the pressure and temperature, the left-shoulder DAR mode induced by substitution does not change at the same pace with the A{sub 1g} mode at Brillouin-zone center. We find that ion substitution can be seen as a kind of chemical pressure, and the chemical pressure caused by internal substitution and the physical pressure caused by external compression have equivalent effects on the shortening of correlation length, the distortion of crystal lattice, and the change of atomic occupation.

  4. Phonon manipulation with phononic crystals.

    SciTech Connect

    Kim Bongsang; Hopkins, Patrick Edward; Leseman, Zayd C.; Goettler, Drew F.; Su, Mehmet F.; El-Kady, Ihab Fathy; Reinke, Charles M.; Olsson, Roy H., III

    2012-01-01

    In this work, we demonstrated engineered modification of propagation of thermal phonons, i.e. at THz frequencies, using phononic crystals. This work combined theoretical work at Sandia National Laboratories, the University of New Mexico, the University of Colorado Boulder, and Carnegie Mellon University; the MESA fabrication facilities at Sandia; and the microfabrication facilities at UNM to produce world-leading control of phonon propagation in silicon at frequencies up to 3 THz. These efforts culminated in a dramatic reduction in the thermal conductivity of silicon using phononic crystals by a factor of almost 30 as compared with the bulk value, and about 6 as compared with an unpatterned slab of the same thickness. This work represents a revolutionary advance in the engineering of thermoelectric materials for optimal, high-ZT performance. We have demonstrated the significant reduction of the thermal conductivity of silicon using phononic crystal structuring using MEMS-compatible fabrication techniques and in a planar platform that is amenable to integration with typical microelectronic systems. The measured reduction in thermal conductivity as compared to bulk silicon was about a factor of 20 in the cross-plane direction [26], and a factor of 6 in the in-plane direction. Since the electrical conductivity was only reduced by a corresponding factor of about 3 due to the removal of conductive material (i.e., porosity), and the Seebeck coefficient should remain constant as an intrinsic material property, this corresponds to an effective enhancement in ZT by a factor of 2. Given the number of papers in literature devoted to only a small, incremental change in ZT, the ability to boost the ZT of a material by a factor of 2 simply by reducing thermal conductivity is groundbreaking. The results in this work were obtained using silicon, a material that has benefitted from enormous interest in the microelectronics industry and that has a fairly large thermoelectric power

  5. Phonon Cooling by an Optomechanical Heat Pump.

    PubMed

    Dong, Ying; Bariani, F; Meystre, P

    2015-11-27

    We propose and analyze theoretically a cavity optomechanical analog of a heat pump that uses a polariton fluid to cool mechanical modes coupled to a single precooled phonon mode via external modulation of the substrate of the mechanical resonator. This approach permits us to cool phonon modes of arbitrary frequencies not limited by the cavity-optical field detuning deep into the quantum regime from room temperature.

  6. Effect of crystal structure and cationic order on phonon modes across ferroelectric phase transformation in Pb(Fe{sub 0.5-x}Sc{sub x}Nb{sub 0.5})O{sub 3} bulk ceramics

    SciTech Connect

    Mallesham, B.; Ranjith, R.; Viswanath, B.

    2016-01-15

    Pb(Fe{sub 0.5-x}Sc{sub x}Nb{sub 0.5})O{sub 3} [(PFSN) (0 ≤ x ≤ 0.5)] multiferroic relaxors were synthesized and the temperature dependence of phonon modes across ferroelectric to paraelectric transition was studied. With varying Sc content from x = 0 to 0.25 the structure remains monoclinic and with further addition (x = 0.3 - 0.5) the structure transforms into rhombohedral symmetry. Structural refinement studies showed that the change in crystal structure from monoclinic to rhombohedral symmetry involves a volume increment of 34-36%. Associated changes in the tolerance factor (1.024 ≤ t ≤ 0.976) and bond angles were observed. Structure assisted B′-B″ cation ordering was confirmed through the superlattice reflections in selected area electron diffraction (SAED) pattern of Pb(Sc{sub 0.5}Nb{sub 0.5})O{sub 3} (x = 0.5). Cation ordering is also evident from the evolution of Pb-O phonon mode in Raman spectra of compositions with rhombohedral symmetry (x ≥ 0.3). The high temperature Raman scattering studies show that the B-localized mode [F{sub 1u}, ∼250 cm{sup −1}] and BO{sub 6} octahedral rotational mode [F{sub 1g}, ∼200 cm{sup −1}], both originating from polar nano regions (PNRs) behave like coupled phonon modes in rhombohedral symmetry. However, in monoclinic symmetry they behave independently across the transition. Softening of B localized mode across the transition followed by the hardening for all compositions confirms the diffusive nature of the ferroelectric transformation. The presence of correlation between the B localized and BO{sub 6} rotational modes introduces a weak relaxor feature for systems with rhombohedral symmetry in PFSN ceramics, which was confirmed from the macroscopic dielectric studies.

  7. Actively mode-locked Raman fiber laser.

    PubMed

    Yang, Xuezong; Zhang, Lei; Jiang, Huawei; Fan, Tingwei; Feng, Yan

    2015-07-27

    Active mode-locking of Raman fiber laser is experimentally investigated for the first time. An all fiber connected and polarization maintaining loop cavity of ~500 m long is pumped by a linearly polarized 1120 nm Yb fiber laser and modulated by an acousto-optic modulator. Stable 2 ns width pulse train at 1178 nm is obtained with modulator opening time of > 50 ns. At higher power, pulses become longer, and second order Raman Stokes could take place, which however can be suppressed by adjusting the open time and modulation frequency. Transient pulse evolution measurement confirms the absence of relaxation oscillation in Raman fiber laser. Tuning of repetition rate from 392 kHz to 31.37 MHz is obtained with harmonic mode locking.

  8. Polar optical phonons in semiconducting CdS nanocrystals

    SciTech Connect

    Belogorokhov, A. I. Belogorokhov, I. A.; Miranda, R. P.; Vasilevskii, M. I.; Gavrilov, S. A.

    2007-02-15

    We have experimentally and theoretically studied IR-active optical phonons, which are spatially confined in the volume of semiconducting CdS nanocrystals of various shapes synthesized in a dielectric matrix (porous aluminum oxide). Within an approach admitting the mixing of all expected types of vibrations, the complete sets of phonon modes are determined for a spherical quantum dot (QD) and a cylindrical quantum wire (QW) in this matrix. Based on these results, the polarizability spectra of QDs and QWs, as well as the effective dielectric function of a composite material containing such nanoparticles, are calculated for the far-IR wavelength range. It is established that the spectrum of the dielectric function exhibits specific features in the region between the transverse and longitudinal optical phonon frequencies of the massive semiconductor material. These features explain the rather unusual structure of the IR spectra of the composite samples studied.

  9. Phonon anomalies in some iron telluride materials

    SciTech Connect

    C. C. Homes; Dai, Y. M.; Schneeloch, J.; Zhong, R. D.; Gu, G. D.

    2016-03-21

    In this paper, the detailed temperature dependence of the infrared-active mode in Fe1.03Te (TN ≃ 68 K) and Fe1.13Te (TN ≃ 56 K) has been examined, and the position, width, strength, and asymmetry parameter have been determined using an asymmetric Fano profile superimposed on an electronic background. In both materials the frequency of the mode increases as the temperature is reduced; however, there is also a slight asymmetry in the line shape, indicating that the mode is coupled to either spin or charge excitations. Below TN there is an anomalous decrease in frequency, and the mode shows little temperature dependence, at the same time becoming more symmetric, suggesting a reduction in spin- or electron-phonon coupling. The frequency of the infrared-active mode and the magnitude of the shift below TN are predicted reasonably well by first-principles calculations; however, the predicted splitting of the mode is not observed. In superconducting FeTe0.55Se0.45 (Tc ≃ 14 K) the infrared-active Eu mode displays asymmetric line shape at all temperatures, which is most pronounced between 100 – 200 K, indicating the presence of either spin- or electron-phonon coupling, which may be a necessary prerequisite for superconductivity in this class of materials.

  10. Enhanced electron-phonon coupling for a semiconductor charge qubit in a surface phonon cavity

    PubMed Central

    Chen, J. C. H.; Sato, Y.; Kosaka, R.; Hashisaka, M.; Muraki, K.; Fujisawa, T.

    2015-01-01

    Electron-phonon coupling is a major decoherence mechanism, which often causes scattering and energy dissipation in semiconductor electronic systems. However, this electron-phonon coupling may be used in a positive way for reaching the strong or ultra-strong coupling regime in an acoustic version of the cavity quantum electrodynamic system. Here we propose and demonstrate a phonon cavity for surface acoustic waves, which is made of periodic metal fingers that constitute Bragg reflectors on a GaAs/AlGaAs heterostructure. Phonon band gap and cavity phonon modes are identified by frequency, time and spatially resolved measurements of the piezoelectric potential. Tunneling spectroscopy on a double quantum dot indicates the enhancement of phonon assisted transitions in a charge qubit. This encourages studying of acoustic cavity quantum electrodynamics with surface phonons. PMID:26469629

  11. Engineering dissipation with phononic spectral hole burning.

    PubMed

    Behunin, R O; Kharel, P; Renninger, W H; Rakich, P T

    2017-03-01

    Optomechanics, nano-electromechanics, and integrated photonics have brought about a renaissance in phononic device physics and technology. Central to this advance are devices and materials supporting ultra-long-lived photonic and phononic excitations that enable novel regimes of classical and quantum dynamics based on tailorable photon-phonon coupling. Silica-based devices have been at the forefront of such innovations for their ability to support optical excitations persisting for nearly 1 billion cycles, and for their low optical nonlinearity. While acoustic phonon modes can persist for a similar number of cycles in crystalline solids at cryogenic temperatures, it has not been possible to achieve such performance in silica, as silica becomes acoustically opaque at low temperatures. We demonstrate that these intrinsic forms of phonon dissipation are greatly reduced (by >90%) by nonlinear saturation using continuous drive fields of disparate frequencies. The result is a form of steady-state phononic spectral hole burning that produces a wideband transparency window with optically generated phonon fields of modest (nW) powers. We developed a simple model that explains both dissipative and dispersive changes produced by phononic saturation. Our studies, conducted in a microscale device, represent an important step towards engineerable phonon dynamics on demand and the use of glasses as low-loss phononic media.

  12. Engineering dissipation with phononic spectral hole burning

    NASA Astrophysics Data System (ADS)

    Behunin, R. O.; Kharel, P.; Renninger, W. H.; Rakich, P. T.

    2016-12-01

    Optomechanics, nano-electromechanics, and integrated photonics have brought about a renaissance in phononic device physics and technology. Central to this advance are devices and materials supporting ultra-long-lived photonic and phononic excitations that enable novel regimes of classical and quantum dynamics based on tailorable photon-phonon coupling. Silica-based devices have been at the forefront of such innovations for their ability to support optical excitations persisting for nearly 1 billion cycles, and for their low optical nonlinearity. While acoustic phonon modes can persist for a similar number of cycles in crystalline solids at cryogenic temperatures, it has not been possible to achieve such performance in silica, as silica becomes acoustically opaque at low temperatures. We demonstrate that these intrinsic forms of phonon dissipation are greatly reduced (by >90%) by nonlinear saturation using continuous drive fields of disparate frequencies. The result is a form of steady-state phononic spectral hole burning that produces a wideband transparency window with optically generated phonon fields of modest (nW) powers. We developed a simple model that explains both dissipative and dispersive changes produced by phononic saturation. Our studies, conducted in a microscale device, represent an important step towards engineerable phonon dynamics on demand and the use of glasses as low-loss phononic media.

  13. Scanning Tunneling Microscopy Observation of Phonon Condensate.

    PubMed

    Altfeder, Igor; Voevodin, Andrey A; Check, Michael H; Eichfeld, Sarah M; Robinson, Joshua A; Balatsky, Alexander V

    2017-02-22

    Using quantum tunneling of electrons into vibrating surface atoms, phonon oscillations can be observed on the atomic scale. Phonon interference patterns with unusually large signal amplitudes have been revealed by scanning tunneling microscopy in intercalated van der Waals heterostructures. Our results show that the effective radius of these phonon quasi-bound states, the real-space distribution of phonon standing wave amplitudes, the scattering phase shifts, and the nonlinear intermode coupling strongly depend on the presence of defect-induced scattering resonance. The observed coherence of these quasi-bound states most likely arises from phase- and frequency-synchronized dynamics of all phonon modes, and indicates the formation of many-body condensate of optical phonons around resonant defects. We found that increasing the strength of the scattering resonance causes the increase of the condensate droplet radius without affecting the condensate fraction inside it. The condensate can be observed at room temperature.

  14. Scanning Tunneling Microscopy Observation of Phonon Condensate

    PubMed Central

    Altfeder, Igor; Voevodin, Andrey A.; Check, Michael H.; Eichfeld, Sarah M.; Robinson, Joshua A.; Balatsky, Alexander V.

    2017-01-01

    Using quantum tunneling of electrons into vibrating surface atoms, phonon oscillations can be observed on the atomic scale. Phonon interference patterns with unusually large signal amplitudes have been revealed by scanning tunneling microscopy in intercalated van der Waals heterostructures. Our results show that the effective radius of these phonon quasi-bound states, the real-space distribution of phonon standing wave amplitudes, the scattering phase shifts, and the nonlinear intermode coupling strongly depend on the presence of defect-induced scattering resonance. The observed coherence of these quasi-bound states most likely arises from phase- and frequency-synchronized dynamics of all phonon modes, and indicates the formation of many-body condensate of optical phonons around resonant defects. We found that increasing the strength of the scattering resonance causes the increase of the condensate droplet radius without affecting the condensate fraction inside it. The condensate can be observed at room temperature. PMID:28225066

  15. Modelling of the phonon strain shift coefficients in Si1-xGex alloys

    NASA Astrophysics Data System (ADS)

    Pezzoli, F.; Sanguinetti, S.; Bonera, E.; Grilli, E.; Guzzi, M.

    2007-12-01

    In the Raman spectrum of epitaxial Si1-xGex alloys, the composition effect, that determines the three mode peak position, is superimposed to the phonon shift caused by strain. Therefore, the experimental spectra interpretation remains not straightforward, unless vibrational properties calculations of such systems are available. In this paper, reticular dynamic calculations within a modified Keating model, the Anharmonic Keating, are discussed. A new set of model parameters is introduced, providing simple and predictive supercell calculations to investigate accurately the three Raman-active phonon modes in Si1-xGex alloys. The recalculated AK model is successfully employed to model the zone centre Raman-active phonon modes of relaxed and hydrostatically stressed Si1-xGex alloys. The results of the calculations are in quantitative agreement with experimental data reported in literature.

  16. Squeezed Phonons: Modulating Quantum Fluctuations of Atomic Displacements.

    NASA Astrophysics Data System (ADS)

    Hu, Xuedong; Nori, Franco

    1997-03-01

    We have studied phonon squeezed states and also put forward several proposals for their generation(On phonon parametric process, X. Hu and F. Nori, Phys. Rev. Lett. 76), 2294 (1996); on polariton mechanism, X. Hu and F. Nori, Phys. Rev. B 53, 2419 (1996); on second-order Raman scattering, X. Hu and F. Nori, preprint.. Here, we compare the relative merits and limitations of these approaches, including several factors that will limit the amount of phonon squeezing. In particular, we investigate the effect of the initial thermal states on the phonon modes. Using a model for the phonon density matrix, we also study the mixing of the phonon squeezed states with thermal states, which describes the decay of the phonon coherence. Finally, we calculate the maximum possible squeezing from a phonon parametric process limited by phonon decay.

  17. Anharmonicity of Zone-Center Optical Phonons: Raman Scattering Spectra of GaSe0.5S0.5 Layered Crystal

    NASA Astrophysics Data System (ADS)

    Gasanly, N. M.; Aydinli, A.; Aydinli, A.; Kocabaş, C.; Özkan, H.

    The temperature dependencies (10-300 K) of the eight Raman-active mode frequencies and linewidths in GaSe0.5S0.5 layered crystal have been measured in the frequency range from 10 to 320 cm-1. We observed softening and broadening of the optical phonon lines with increasing temperature. Comparison of the experimental data with the theories of the shift and broadening of the interlayer and intralayer phonon lines showed that the temperature dependencies can be explained by the contributions from thermal expansion, lattice anharmonicity and crystal disorder. The purely anharmonic contribution (phonon-phonon coupling) is found to be due to three-phonon processes. It was established that the effect of crystal disorder on the broadening of phonon lines is greater for GaSe0.5S0.5 than for binary compounds GaSe and GaS.

  18. Phononic crystal devices

    DOEpatents

    El-Kady, Ihab F [Albuquerque, NM; Olsson, Roy H [Albuquerque, NM

    2012-01-10

    Phononic crystals that have the ability to modify and control the thermal black body phonon distribution and the phonon component of heat transport in a solid. In particular, the thermal conductivity and heat capacity can be modified by altering the phonon density of states in a phononic crystal. The present invention is directed to phononic crystal devices and materials such as radio frequency (RF) tags powered from ambient heat, dielectrics with extremely low thermal conductivity, thermoelectric materials with a higher ratio of electrical-to-thermal conductivity, materials with phononically engineered heat capacity, phononic crystal waveguides that enable accelerated cooling, and a variety of low temperature application devices.

  19. PROMINENCE ACTIVATION BY CORONAL FAST MODE SHOCK

    SciTech Connect

    Takahashi, Takuya; Asai, Ayumi; Shibata, Kazunari

    2015-03-01

    An X5.4 class flare occurred in active region NOAA11429 on 2012 March 7. The flare was associated with a very fast coronal mass ejection (CME) with a velocity of over 2500 km s{sup −1}. In the images taken with the Solar Terrestrial Relations Observatory-B/COR1, a dome-like disturbance was seen to detach from an expanding CME bubble and propagated further. A Type-II radio burst was also observed at the same time. On the other hand, in extreme ultraviolet images obtained by the Solar Dynamic Observatory/Atmospheric Imaging Assembly (AIA), the expanding dome-like structure and its footprint propagating to the north were observed. The footprint propagated with an average speed of about 670 km s{sup −1} and hit a prominence located at the north pole and activated it. During the activation, the prominence was strongly brightened. On the basis of some observational evidence, we concluded that the footprint in AIA images and the ones in COR1 images are the same, that is, the MHD fast mode shock front. With the help of a linear theory, the fast mode Mach number of the coronal shock is estimated to be between 1.11 and 1.29 using the initial velocity of the activated prominence. Also, the plasma compression ratio of the shock is enhanced to be between 1.18 and 2.11 in the prominence material, which we consider to be the reason for the strong brightening of the activated prominence. The applicability of linear theory to the shock problem is tested with a nonlinear MHD simulation.

  20. Controlling thermal emission of phonon by magnetic metasurfaces.

    PubMed

    Zhang, X; Liu, H; Zhang, Z G; Wang, Q; Zhu, S N

    2017-02-03

    Our experiment shows that the thermal emission of phonon can be controlled by magnetic resonance (MR) mode in a metasurface (MTS). Through changing the structural parameter of metasurface, the MR wavelength can be tuned to the phonon resonance wavelength. This introduces a strong coupling between phonon and MR, which results in an anticrossing phonon-plasmons mode. In the process, we can manipulate the polarization and angular radiation of thermal emission of phonon. Such metasurface provides a new kind of thermal emission structures for various thermal management applications.

  1. Controlling thermal emission of phonon by magnetic metasurfaces

    PubMed Central

    Zhang, X.; Liu, H.; Zhang, Z. G.; Wang, Q.; Zhu, S. N.

    2017-01-01

    Our experiment shows that the thermal emission of phonon can be controlled by magnetic resonance (MR) mode in a metasurface (MTS). Through changing the structural parameter of metasurface, the MR wavelength can be tuned to the phonon resonance wavelength. This introduces a strong coupling between phonon and MR, which results in an anticrossing phonon-plasmons mode. In the process, we can manipulate the polarization and angular radiation of thermal emission of phonon. Such metasurface provides a new kind of thermal emission structures for various thermal management applications. PMID:28157206

  2. Controlling thermal emission of phonon by magnetic metasurfaces

    NASA Astrophysics Data System (ADS)

    Zhang, X.; Liu, H.; Zhang, Z. G.; Wang, Q.; Zhu, S. N.

    2017-02-01

    Our experiment shows that the thermal emission of phonon can be controlled by magnetic resonance (MR) mode in a metasurface (MTS). Through changing the structural parameter of metasurface, the MR wavelength can be tuned to the phonon resonance wavelength. This introduces a strong coupling between phonon and MR, which results in an anticrossing phonon-plasmons mode. In the process, we can manipulate the polarization and angular radiation of thermal emission of phonon. Such metasurface provides a new kind of thermal emission structures for various thermal management applications.

  3. New First Order Raman-active Modes in Few Layered Transition Metal Dichalcogenides

    PubMed Central

    Terrones, H.; Corro, E. Del; Feng, S.; Poumirol, J. M.; Rhodes, D.; Smirnov, D.; Pradhan, N. R.; Lin, Z.; Nguyen, M. A. T.; Elías, A. L.; Mallouk, T. E.; Balicas, L.; Pimenta, M. A.; Terrones, M.

    2014-01-01

    Although the main Raman features of semiconducting transition metal dichalcogenides are well known for the monolayer and bulk, there are important differences exhibited by few layered systems which have not been fully addressed. WSe2 samples were synthesized and ab-initio calculations carried out. We calculated phonon dispersions and Raman-active modes in layered systems: WSe2, MoSe2, WS2 and MoS2 ranging from monolayers to five-layers and the bulk. First, we confirmed that as the number of layers increase, the E′, E″ and E2g modes shift to lower frequencies, and the A′1 and A1g modes shift to higher frequencies. Second, new high frequency first order A′1 and A1g modes appear, explaining recently reported experimental data for WSe2, MoSe2 and MoS2. Third, splitting of modes around A′1 and A1g is found which explains those observed in MoSe2. Finally, exterior and interior layers possess different vibrational frequencies. Therefore, it is now possible to precisely identify few-layered STMD. PMID:24572993

  4. Tunable phonon-induced transparency in bilayer graphene nanoribbons.

    PubMed

    Yan, Hugen; Low, Tony; Guinea, Francisco; Xia, Fengnian; Avouris, Phaedon

    2014-08-13

    In the phenomenon of plasmon-induced transparency, which is a classical analogue of electromagnetically induced transparency (EIT) in atomic gases, the coherent interference between two plasmon modes results in an optical transparency window in a broad absorption spectrum. With the requirement of contrasting lifetimes, typically one of the plasmon modes involved is a dark mode that has limited coupling to the electromagnetic radiation and possesses relatively longer lifetime. Plasmon-induced transparency not only leads to light transmission at otherwise opaque frequency regions but also results in the slowing of light group velocity and enhanced optical nonlinearity. In this article, we report an analogous behavior, denoted as phonon-induced transparency (PIT), in AB-stacked bilayer graphene nanoribbons. Here, light absorption due to the plasmon excitation is suppressed in a narrow window due to the coupling with the infrared active Γ-point optical phonon, whose function here is similar to that of the dark plasmon mode in the plasmon-induced transparency. We further show that PIT in bilayer graphene is actively tunable by electrostatic gating and estimate a maximum slow light factor of around 500 at the phonon frequency of 1580 cm(-1), based on the measured spectra. Our demonstration opens an avenue for the exploration of few-photon nonlinear optics and slow light in this novel two-dimensional material.

  5. Focusing and waveguiding of Lamb waves in micro-fabricated piezoelectric phononic plates.

    PubMed

    Chiou, Meng-Jhen; Lin, Yu-Ching; Ono, Takahito; Esashi, Masayoshi; Yeh, Sih-Ling; Wu, Tsung-Tsong

    2014-09-01

    This paper presents results on the numerical and experimental studies of focusing and waveguiding of the lowest anti-symmetric Lamb wave in micro-fabricated piezoelectric phononic plates. The phononic structure was based on an AT-cut quartz plate and consisted of a gradient-index phononic crystal (GRIN PC) lens and a linear phononic plate waveguide. The band structures of the square-latticed AT-cut quartz phononic crystal plates with different filling ratios were analyzed using the finite element method. The design of a GRIN PC plate lens which is attached with a linear phononic plate waveguide is proposed. In designing the waveguide, propagation modes in square-latticed PC plates with different waveguide widths were studied and the results were served for the experimental design. In the micro-fabrication, deep reactive ion etching (Deep-RIE) process with a laboratory-made etcher was utilized to fabricate both the GRIN PC plate lens and the linear phononic waveguide on an 80 μm thick AT-cut quartz plate. Interdigital transducers were fabricated directly on the quartz plate to generate the lowest anti-symmetric Lamb waves. A vibro-meter was used to detect the wave fields and the measured results on the focusing and waveguiding of the piezoelectric GRIN PC lens and waveguide are in good accordance with the numerical predictions. The results of this study may serve as a basis for developing an active micro plate lens and related devices.

  6. The manifestation of spin-phonon coupling in CaMnO{sub 3}

    SciTech Connect

    Goian, V. Kamba, S.; Borodavka, F.; Nuzhnyy, D.; Savinov, M.; Belik, A. A.

    2015-04-28

    Recently predicted presence of spin-phonon coupling in the CaMnO{sub 3} is experimentally confirmed in infrared (IR), Raman and time-domain THz spectra. Most of phonon frequencies seen below 350 cm{sup −1} exhibit significant shifts on cooling below antiferromagnetic phase transition at T{sub N} ≅ 120 K. Moreover, several new modes activate in the IR and Raman spectra on cooling below T{sub N}. Sum of phonon contributions to static permittivity exhibits small but reliable anomaly at T{sub N}. On the other hand, the spin-phonon coupling is not manifested in temperature dependence of radio-frequency permittivity, because intrinsic permittivity is screened by extrinsic contribution from conductivity, which enhances the permittivity to giant values.

  7. Jahn-Teller Effect in the Phonon Properties of Defective SrTiO3 from First Principles

    SciTech Connect

    Evarestov, Robert; Blokhin, Evgeny; Gryaznov, D.; Kotomin, Eugene Alexej; Merkle, R.; Maier, Joachim

    2012-05-17

    Using a hybrid density functional theory method combined with linear combination of atomic orbitals basis set and periodic supercell approach, the atomic, electronic structure, and phonon properties were calculated for SrTiO3 containing either Fe4+ substituting host Ti4+ ions or neutral oxygen vacancies Vo. For both defects, the Jahn–Teller effect occurs, thus reducing the cubic symmetry of a perfect crystal and leading to the appearance of both Raman- and infrared-active vibrational modes. The calculated phonon densities of states and group-theoretical analysis of defect-induced phonon frequencies were used for the interpretation of the relevant experimental data, once defect-induced local modes are identified. The temperature dependence of the Vo formation energy based on the calculated Gibbs free energy was also compared with experiments, and the phonon contribution therein estimated.

  8. Optical, phonon and efficient visible and infrared photocatalytic activity of Cu doped ZnS micro crystals

    NASA Astrophysics Data System (ADS)

    Prasad, Neena; Balasubramanian, Karthikeyan

    2017-02-01

    We report, the enhanced photocatalytic behaviour of Cu doped ZnS micro crystals. ZnS and different concentrations of Cu doped ZnS microcrystals were prepared. X-ray diffraction confirms the crystalline and phase of the particles. Morphology and sizes were studied using Scanning Electron Microscopy (SEM). Recorded optical absorption spectra show a band for around 365 nm for pure ZnS, but there is a broad band in the near infrared regime for the Cu-doped ZnS microcrystals which are attributed to the d-d transitions of Cu2 + ions. Phonon properties of as-prepared samples were investigated using Raman spectroscopy. Present work we investigate the potential of ZnS and Cu doped ZnS as a photocatalyst. For this from the degradation of methylene blue dye in aqueous media the photocatalytic activity of pure and highest doped ZnS samples with the irradiation of white light and infrared, enhanced photocatalytic activity were observed. Mechanism of white light an IR light based photocatalytic activity is explained based on the electron-hole pair production.

  9. Lattice thermal conductivity of TixZryHf1 -x -yNiSn half-Heusler alloys calculated from first principles: Key role of nature of phonon modes

    NASA Astrophysics Data System (ADS)

    Eliassen, Simen N. H.; Katre, Ankita; Madsen, Georg K. H.; Persson, Clas; Løvvik, Ole Martin; Berland, Kristian

    2017-01-01

    In spite of their relatively high lattice thermal conductivity κℓ, the X NiSn (X =Ti , Zr, or Hf) half-Heusler compounds are good thermoelectric materials. Previous studies have shown that κℓ can be reduced by sublattice alloying on the X site. To cast light on how the alloy composition affects κℓ, we study this system using the phonon Boltzmann-transport equation within the relaxation time approximation in conjunction with density functional theory. The effect of alloying through mass-disorder scattering is explored using the virtual crystal approximation to screen the entire ternary TixZryHf1 -x -yNiSn phase diagram. The lowest lattice thermal conductivity is found for the TixHf1 -xNiSn compositions; in particular, there is a shallow minimum centered at Ti0.5Hf0.5NiSn with κℓ taking values between 3.2 and 4.1 W/mK when the Ti content varies between 20% and 80%. Interestingly, the overall behavior of mass-disorder scattering in this system can only be understood from a combination of the nature of the phonon modes and the magnitude of the mass variance. Mass-disorder scattering is not effective at scattering acoustic phonons of low energy. By using a simple model of grain boundary scattering, we find that nanostructuring these compounds can scatter such phonons effectively and thus further reduce the lattice thermal conductivity; for instance, Ti0.5Hf0.5NiSn with a grain size of L =100 nm experiences a 42% reduction of κℓ compared to that of the single crystal.

  10. Broadband radiation modes: estimation and active control.

    PubMed

    Berkhoff, Arthur P

    2002-03-01

    In this paper we give a formulation of the most efficiently radiating vibration patterns of a vibrating body, the radiation modes, in the time domain. The radiation modes can be used to arrive at efficient weighting schemes for an array of sensors in order to reduce the controller dimensionality. Because these particular radiation modes are optimum in a broadband sense, they are termed broadband radiation modes. Methods are given to obtain these modes from measured data. The broadband radiation modes are used for the design of an actuator array in a feedback control system to reduce the sound power radiated from a plate. Three methods for the design of the actuator are compared, taking into account the reduction of radiated sound power in the controlled frequency range, but also the possible increase of radiated sound power in the uncontrolled frequency range.

  11. Plasmon-phonon coupling in graphene-hyperbolic bilayer heterostructures

    NASA Astrophysics Data System (ADS)

    Yin, Ge; Yuan, Jun; Jiang, Wei; Zhu, Jianfei; Ma, Yungui

    2016-11-01

    Polar dielectrics are important optical materials enabling the subwavelength manipulation of light in infrared due to their capability to excite phonon polaritons. In practice, it is highly desired to actively modify these hyperbolic phonon polaritons (HPPs) to optimize or tune the response of the device. In this work, we investigate the plasmonic material, a monolayer graphene, and study its hybrid structure with three kinds of hyperbolic thin films grown on SiO2 substrate. The inter-mode hybridization and their tunability have been thoroughly clarified from both the band dispersions and the mode patterns numerically calculated through a transfer matrix method. Our results show that these hybrid multilayer structures are of strong potentials for applications in plasmonic waveguides, modulators and detectors in infrared. Project supported by the National Natural Science Foundation of China (Grant No. 61271085) and the Natural Science Foundation of Zhejiang Province, China (Grant No. LR15F050001).

  12. Phonon spectral functions of photo-generated hot carrier plasmas: effects of carrier screening and plasmon-phonon coupling.

    PubMed

    Yi, Kyung-Soo; Kim, Hye-Jung

    2017-02-15

    We investigate spectral behavior of phonon spectral functions in an interacting multi-component hot carrier plasma. Spectral analysis of various phonon spectral functions is performed considering carrier-phonon channels of polar and nonpolar optical phonons, acoustic deformation-potential, and piezoelectric Coulomb couplings. Effects of phonon self-energy corrections are examined at finite temperature within a random phase approximation extended to include the effects of dynamic screening, plasmon-phonon coupling, and local-field corrections of the plasma species. We provide numerical data for the case of a photo-generated electron-hole plasma formed in a wurtzite GaN. Our result shows the clear significance of the multiplicity of the plasma species in the phonon spectral functions of a multi-component plasma giving rise to a variety of spectral behaviors of carrier-phonon coupled collective modes. A useful sum rule on the plasma-species-resolved dielectric functions is also found.

  13. Phonon spectral functions of photo-generated hot carrier plasmas: effects of carrier screening and plasmon-phonon coupling

    NASA Astrophysics Data System (ADS)

    Yi, Kyung-Soo; Kim, Hye-Jung

    2017-02-01

    We investigate spectral behavior of phonon spectral functions in an interacting multi-component hot carrier plasma. Spectral analysis of various phonon spectral functions is performed considering carrier-phonon channels of polar and nonpolar optical phonons, acoustic deformation-potential, and piezoelectric Coulomb couplings. Effects of phonon self-energy corrections are examined at finite temperature within a random phase approximation extended to include the effects of dynamic screening, plasmon-phonon coupling, and local-field corrections of the plasma species. We provide numerical data for the case of a photo-generated electron-hole plasma formed in a wurtzite GaN. Our result shows the clear significance of the multiplicity of the plasma species in the phonon spectral functions of a multi-component plasma giving rise to a variety of spectral behaviors of carrier-phonon coupled collective modes. A useful sum rule on the plasma-species-resolved dielectric functions is also found.

  14. Manipulation of Phonons with Phononic Crystals

    SciTech Connect

    Leseman, Zayd Chad

    2015-07-09

    There were three research goals associated with this project. First, was to experimentally demonstrate phonon spectrum control at THz frequencies using Phononic Crystals (PnCs), i.e. demonstrate coherent phonon scattering with PnCs. Second, was to experimentally demonstrate analog PnC circuitry components at GHz frequencies. The final research goal was to gain a fundamental understanding of phonon interaction using computational methods. As a result of this work, 7 journal papers have been published, 1 patent awarded, 14 conference presentations given, 4 conference publications, and 2 poster presentations given.

  15. Phonon Scattering Dynamics of Thermophoretic Motion in Carbon Nanotube Oscillators.

    PubMed

    Prasad, Matukumilli V D; Bhattacharya, Baidurya

    2016-04-13

    Using phonon wave packet molecular dynamics simulations, we find that anomalous longitudinal acoustic (LA) mode phonon scattering in low to moderate energy ranges is responsible for initiating thermophoretic motion in carbon nanotube oscillators. The repeated scattering of a single mode LA phonon wave packet near the ends of the inner nanotube provides a net unbalanced force that, if large enough, initiates thermophoresis. By applying a coherent phonon pulse on the outer tube, which generalizes the single mode phonon wave packet, we are able to achieve thermophoresis in a carbon nanotube oscillator. We also find the nature of the unbalanced force on end-atoms to be qualitatively similar to that under an imposed thermal gradient. The thermodiffusion coefficient obtained for a range of thermal gradients and core lengths suggest that LA phonon scattering is the dominant mechanism for thermophoresis in longer cores, whereas for shorter cores, it is the highly diffusive mechanism that provides the effective force.

  16. Relaxation of a hot-electron-two-mode-phonon system in highly excited CdS1-xSex crystals

    NASA Astrophysics Data System (ADS)

    Žukauskas, A.; Juršėnas, S.

    1995-02-01

    An investigation of the electron-hole-plasma effective-temperature relaxation in highly excited CdS1-xSex mixed crystals is presented. The slow (~100-ps) relaxation stage, attributed to the depopulation of the fragments (decay products) of the initially produced nonequilibrium LO phonons, is examined with variation of the alloy composition. The relevant relaxation time dependence on x exhibiting a remarkable drop at small CdSe mole fractions is analyzed in terms of a two-route energy relaxation model considering hot-carrier plasma and two generations of nonequilibrium phonons each originating from both pure constituents of the alloy. The disorder-enhanced cross relaxation between two sublattices of the alloy is inferred to account for the experimental results.

  17. Aspect-ratio driven evolution of high-order resonant modes and near-field distributions in localized surface phonon polariton nanostructures

    PubMed Central

    Ellis, Chase T.; Tischler, Joseph G.; Glembocki, Orest J.; Bezares, Francisco J.; Giles, Alexander J.; Kasica, Richard; Shirey, Loretta; Owrutsky, Jeffrey C.; Chigrin, Dmitry N.; Caldwell, Joshua D.

    2016-01-01

    Polar dielectrics have garnered much attention as an alternative to plasmonic metals in the mid- to long-wave infrared spectral regime due to their low optical losses. As such, nanoscale resonators composed of these materials demonstrate figures of merit beyond those achievable in plasmonic equivalents. However, until now, only low-order, phonon-mediated, localized polariton resonances, known as surface phonon polaritons (SPhPs), have been observed in polar dielectric optical resonators. In the present work, we investigate the excitation of 16 distinct high-order, multipolar, localized surface phonon polariton resonances that are optically excited in rectangular pillars etched into a semi-insulating silicon carbide substrate. By elongating a single pillar axis we are able to significantly modify the far- and near-field properties of localized SPhP resonances, opening the door to realizing narrow-band infrared sources with tailored radiation patterns. Such control of the near-field behavior of resonances can also impact surface enhanced infrared optical sensing, which is mediated by polarization selection rules, as well as the morphology and strength of resonator hot spots. Furthermore, through the careful choice of polar dielectric material, these results can also serve as the guiding principles for the generalized design of optical devices that operate from the mid- to far-infrared. PMID:27622525

  18. Phonon thermal transport through tilt grain boundaries in strontium titanate

    SciTech Connect

    Zheng, Zexi; Chen, Xiang; Yang, Shengfeng; Xiong, Liming; Chen, Youping; Deng, Bowen; Chernatynskiy, Aleksandr

    2014-08-21

    In this work, we perform nonequilibrium molecular dynamics simulations to study phonon scattering at two tilt grain boundaries (GBs) in SrTiO{sub 3}. Mode-wise energy transmission coefficients are obtained based on phonon wave-packet dynamics simulations. The Kapitza conductance is then quantified using a lattice dynamics approach. The obtained results of the Kapitza conductance of both GBs compare well with those obtained by the direct method, except for the temperature dependence. Contrary to common belief, the results of this work show that the optical modes in SrTiO{sub 3} contribute significantly to phonon thermal transport, accounting for over 50% of the Kapitza conductance. To understand the effect of the GB structural disorder on phonon transport, we compare the local phonon density of states of the atoms in the GB region with that in the single crystalline grain region. Our results show that the excess vibrational modes introduced by the structural disorder do not have a significant effect on phonon scattering at the GBs, but the absence of certain modes in the GB region appears to be responsible for phonon reflections at GBs. This work has also demonstrated phonon mode conversion and simultaneous generation of new modes. Some of the new modes have the same frequency as the initial wave packet, while some have the same wave vector but lower frequencies.

  19. Edge phonons in black phosphorus

    PubMed Central

    Ribeiro, H. B.; Villegas, C. E. P.; Bahamon, D. A.; Muraca, D.; Castro Neto, A. H.; de Souza, E. A. T.; Rocha, A. R.; Pimenta, M. A.; de Matos, C. J. S.

    2016-01-01

    Black phosphorus has recently emerged as a new layered crystal that, due to its peculiar and anisotropic crystalline and electronic band structures, may have important applications in electronics, optoelectronics and photonics. Despite the fact that the edges of layered crystals host a range of singular properties whose characterization and exploitation are of utmost importance for device development, the edges of black phosphorus remain poorly characterized. In this work, the atomic structure and behaviour of phonons near different black phosphorus edges are experimentally and theoretically studied using Raman spectroscopy and density functional theory calculations. Polarized Raman results show the appearance of new modes at the edges of the sample, and their spectra depend on the atomic structure of the edges (zigzag or armchair). Theoretical simulations confirm that the new modes are due to edge phonon states that are forbidden in the bulk, and originated from the lattice termination rearrangements. PMID:27412813

  20. Edge phonons in black phosphorus

    NASA Astrophysics Data System (ADS)

    Ribeiro, H. B.; Villegas, C. E. P.; Bahamon, D. A.; Muraca, D.; Castro Neto, A. H.; de Souza, E. A. T.; Rocha, A. R.; Pimenta, M. A.; de Matos, C. J. S.

    2016-07-01

    Black phosphorus has recently emerged as a new layered crystal that, due to its peculiar and anisotropic crystalline and electronic band structures, may have important applications in electronics, optoelectronics and photonics. Despite the fact that the edges of layered crystals host a range of singular properties whose characterization and exploitation are of utmost importance for device development, the edges of black phosphorus remain poorly characterized. In this work, the atomic structure and behaviour of phonons near different black phosphorus edges are experimentally and theoretically studied using Raman spectroscopy and density functional theory calculations. Polarized Raman results show the appearance of new modes at the edges of the sample, and their spectra depend on the atomic structure of the edges (zigzag or armchair). Theoretical simulations confirm that the new modes are due to edge phonon states that are forbidden in the bulk, and originated from the lattice termination rearrangements.

  1. Phonon-assisted nonlinear optical processes in ultrashort-pulse pumped optical parametric amplifiers

    NASA Astrophysics Data System (ADS)

    Isaienko, Oleksandr; Robel, István

    2016-03-01

    Optically active phonon modes in ferroelectrics such as potassium titanyl phosphate (KTP) and potassium titanyl arsenate (KTA) in the ~7–20 THz range play an important role in applications of these materials in Raman lasing and terahertz wave generation. Previous studies with picosecond pulse excitation demonstrated that the interaction of pump pulses with phonons can lead to efficient stimulated Raman scattering (SRS) accompanying optical parametric oscillation or amplification processes (OPO/OPA), and to efficient polariton-phonon scattering. In this work, we investigate the behavior of infrared OPAs employing KTP or KTA crystals when pumped with ~800-nm ultrashort pulses of duration comparable to the oscillation period of the optical phonons. We demonstrate that under conditions of coherent impulsive Raman excitation of the phonons, when the effective χ(2) nonlinearity cannot be considered instantaneous, the parametrically amplified waves (most notably, signal) undergo significant spectral modulations leading to an overall redshift of the OPA output. The pump intensity dependence of the redshifted OPA output, the temporal evolution of the parametric gain, as well as the pump spectral modulations suggest the presence of coupling between the nonlinear optical polarizations PNL of the impulsively excited phonons and those of parametrically amplified waves.

  2. Phonon-assisted nonlinear optical processes in ultrashort-pulse pumped optical parametric amplifiers

    DOE PAGES

    Isaienko, Oleksandr; Robel, Istvan

    2016-03-15

    Optically active phonon modes in ferroelectrics such as potassium titanyl phosphate (KTP) and potassium titanyl arsenate (KTA) in the ~7–20 THz range play an important role in applications of these materials in Raman lasing and terahertz wave generation. Previous studies with picosecond pulse excitation demonstrated that the interaction of pump pulses with phonons can lead to efficient stimulated Raman scattering (SRS) accompanying optical parametric oscillation or amplification processes (OPO/OPA), and to efficient polariton-phonon scattering. In this work, we investigate the behavior of infrared OPAs employing KTP or KTA crystals when pumped with ~800-nm ultrashort pulses of duration comparable to themore » oscillation period of the optical phonons. We demonstrate that under conditions of coherent impulsive Raman excitation of the phonons, when the effective χ(2) nonlinearity cannot be considered instantaneous, the parametrically amplified waves (most notably, signal) undergo significant spectral modulations leading to an overall redshift of the OPA output. Furthermore, the pump intensity dependence of the redshifted OPA output, the temporal evolution of the parametric gain, as well as the pump spectral modulations suggest the presence of coupling between the nonlinear optical polarizations PNL of the impulsively excited phonons and those of parametrically amplified waves.« less

  3. Phonon-assisted nonlinear optical processes in ultrashort-pulse pumped optical parametric amplifiers

    SciTech Connect

    Isaienko, Oleksandr; Robel, Istvan

    2016-03-15

    Optically active phonon modes in ferroelectrics such as potassium titanyl phosphate (KTP) and potassium titanyl arsenate (KTA) in the ~7–20 THz range play an important role in applications of these materials in Raman lasing and terahertz wave generation. Previous studies with picosecond pulse excitation demonstrated that the interaction of pump pulses with phonons can lead to efficient stimulated Raman scattering (SRS) accompanying optical parametric oscillation or amplification processes (OPO/OPA), and to efficient polariton-phonon scattering. In this work, we investigate the behavior of infrared OPAs employing KTP or KTA crystals when pumped with ~800-nm ultrashort pulses of duration comparable to the oscillation period of the optical phonons. We demonstrate that under conditions of coherent impulsive Raman excitation of the phonons, when the effective χ(2) nonlinearity cannot be considered instantaneous, the parametrically amplified waves (most notably, signal) undergo significant spectral modulations leading to an overall redshift of the OPA output. Furthermore, the pump intensity dependence of the redshifted OPA output, the temporal evolution of the parametric gain, as well as the pump spectral modulations suggest the presence of coupling between the nonlinear optical polarizations PNL of the impulsively excited phonons and those of parametrically amplified waves.

  4. Phonon-assisted nonlinear optical processes in ultrashort-pulse pumped optical parametric amplifiers

    PubMed Central

    Isaienko, Oleksandr; Robel, István

    2016-01-01

    Optically active phonon modes in ferroelectrics such as potassium titanyl phosphate (KTP) and potassium titanyl arsenate (KTA) in the ~7–20 THz range play an important role in applications of these materials in Raman lasing and terahertz wave generation. Previous studies with picosecond pulse excitation demonstrated that the interaction of pump pulses with phonons can lead to efficient stimulated Raman scattering (SRS) accompanying optical parametric oscillation or amplification processes (OPO/OPA), and to efficient polariton-phonon scattering. In this work, we investigate the behavior of infrared OPAs employing KTP or KTA crystals when pumped with ~800-nm ultrashort pulses of duration comparable to the oscillation period of the optical phonons. We demonstrate that under conditions of coherent impulsive Raman excitation of the phonons, when the effective χ(2) nonlinearity cannot be considered instantaneous, the parametrically amplified waves (most notably, signal) undergo significant spectral modulations leading to an overall redshift of the OPA output. The pump intensity dependence of the redshifted OPA output, the temporal evolution of the parametric gain, as well as the pump spectral modulations suggest the presence of coupling between the nonlinear optical polarizations PNL of the impulsively excited phonons and those of parametrically amplified waves. PMID:26975881

  5. Electron-interface phonon interaction in multiple quantum well structures

    NASA Astrophysics Data System (ADS)

    Sun, J. P.; Teng, H. B.; Haddad, G. I.; Stroscio, M. A.

    1998-08-01

    Intersubband relaxation rates due to electron interactions with the interface phonons are evaluated for multiple quantum well structures designed for step quantum well lasers operating at mid-infrared to submillimetre wavelengths. The interface phonon modes and electron-phonon interaction Hamiltonians for the structures are derived using the transfer matrix method, based on the macroscopic dielectric continuum model, whereas the electron wavefunctions are obtained by solving the Schrödinger equation. Fermi's golden rule is employed to calculate the electron relaxation rates between the subbands in these structures. The relaxation rates for two different structures are examined and compared with those calculated using the bulk phonon modes and the Fröhlich interaction Hamiltonian. The sum rule for the relationship between the form factors of the various localized phonon modes and the bulk phonon modes is verified. The results obtained in this work illustrate that the transfer matrix method provides a convenient way for deriving the properties of the interface phonon modes in different structures of current interest and that, for preferential electron relaxation in intersubband laser structures, the effects of the interface phonon modes are significant and should be considered for optimal design of these laser structures.

  6. Ultrafast Structure Switching through Nonlinear Phononics

    NASA Astrophysics Data System (ADS)

    Juraschek, D. M.; Fechner, M.; Spaldin, N. A.

    2017-02-01

    We describe a mechanism by which nonlinear phononics allows ultrafast coherent and directional control of transient structural distortions. With ErFeO3 as a model system, we use density functional theory to calculate the structural properties as input into an anharmonic phonon model that describes the response of the system to a pulsed optical excitation. We find that the trilinear coupling of two orthogonal infrared-active phonons to a Raman-active phonon causes a transient distortion of the lattice. In contrast to the quadratic-linear coupling that has been previously explored, the direction of the distortion is determined by the polarization of the exciting light, introducing a novel mechanism for nonlinear phononic switching. Since the occurrence of the coupling is determined by the symmetry of the system we propose that it is a universal feature of orthorhombic and tetragonal perovskites.

  7. Theory of coherent phonon spectroscopy in carbon nanotubes

    NASA Astrophysics Data System (ADS)

    Sanders, G. D.; Stanton, C. J.; Lim, Y. S.; Yee, K. J.; Kim, J. H.; Haroz, E. H.; Booshehri, L. G.; Kono, J.

    2008-03-01

    We develop a theory for the generation and detection of coherent phonons in single wall carbon nanotubes. Coherent phonons are generated in the nanotube by ultrafast laser pulses via the deformation potential electron-phonon interaction with the photogenerated carriers. The electronic states are treated in a tight binding formalism which gives a description of the states over the nanotube Brillouin zone while the nanotube phonon modes are treated in a valence force field model that includes bond-stretching, in-plane and out-of-plane bond-bending, and bond-twisting interactions. Equations of motion for the coherent phonon amplitudes are obtained in a density matrix formalism and we find that the coherent phonon amplitudes satisfy driven oscillator equations. In coherent phonon spectroscopy the coherent phonons are detected by ultrafast pump probe differential transmission measurements. We find that for uniform illumination with a 5 fs pump pulse only the q = 0 radial breathing mode and a high frequency G mode are strongly excited. We will discuss excitation strengths for different coherent phonon modes and compare to recent experiments.

  8. Far-Infrared and Raman Spectroscopy Investigation of Phonon Modes in Amorphous and Crystalline Epitaxial GeTe-Sb2Te3 Alloys

    NASA Astrophysics Data System (ADS)

    Bragaglia, V.; Holldack, K.; Boschker, J. E.; Arciprete, F.; Zallo, E.; Flissikowski, T.; Calarco, R.

    2016-06-01

    A combination of far-infrared and Raman spectroscopy is employed to investigate vibrational modes and the carrier behavior in amorphous and crystalline ordered GeTe-Sb2Te3 alloys (GST) epitaxially grown on Si(111). The infrared active GST mode is not observed in the Raman spectra and vice versa, indication of the fact that inversion symmetry is preserved in the metastable cubic phase in accordance with the Fm3 space group. For the trigonal phase, instead, a partial symmetry break due to Ge/Sb mixed anion layers is observed. By studying the crystallization process upon annealing with both the techniques, we identify temperature regions corresponding to the occurrence of different phases as well as the transition from one phase to the next. Activation energies of 0.43 eV and 0.08 eV for the electron conduction are obtained for both cubic and trigonal phases, respectively. In addition a metal-insulator transition is clearly identified to occur at the onset of the transition between the disordered and the ordered cubic phase.

  9. Far-Infrared and Raman Spectroscopy Investigation of Phonon Modes in Amorphous and Crystalline Epitaxial GeTe-Sb2Te3 Alloys

    PubMed Central

    Bragaglia, V.; Holldack, K.; Boschker, J. E.; Arciprete, F.; Zallo, E.; Flissikowski, T.; Calarco, R.

    2016-01-01

    A combination of far-infrared and Raman spectroscopy is employed to investigate vibrational modes and the carrier behavior in amorphous and crystalline ordered GeTe-Sb2Te3 alloys (GST) epitaxially grown on Si(111). The infrared active GST mode is not observed in the Raman spectra and vice versa, indication of the fact that inversion symmetry is preserved in the metastable cubic phase in accordance with the Fm3 space group. For the trigonal phase, instead, a partial symmetry break due to Ge/Sb mixed anion layers is observed. By studying the crystallization process upon annealing with both the techniques, we identify temperature regions corresponding to the occurrence of different phases as well as the transition from one phase to the next. Activation energies of 0.43 eV and 0.08 eV for the electron conduction are obtained for both cubic and trigonal phases, respectively. In addition a metal-insulator transition is clearly identified to occur at the onset of the transition between the disordered and the ordered cubic phase. PMID:27340085

  10. Phonon assisted resonant tunneling and its phonons control

    NASA Astrophysics Data System (ADS)

    Kusmartsev, F. V.; Krevchik, V. D.; Semenov, M. B.; Filatov, D. O.; Shorokhov, A. V.; Bukharaev, A. A.; Dakhnovsky, Y.; Nikolaev, A. V.; Pyataev, N. A.; Zaytsev, R. V.; Krevchik, P. V.; Egorov, I. A.; Yamamoto, K.; Aringazin, A. K.

    2016-09-01

    We observe a series of sharp resonant features in the tunneling differential conductance of InAs quantum dots. We found that dissipative quantum tunneling has a strong influence on the operation of nanodevices. Because of such tunneling the current-voltage characteristics of tunnel contact created between atomic force microscope tip and a surface of InAs/GaAs quantum dots display many interesting peaks. We found that the number, position, and heights of these peaks are associated with the phonon modes involved. To describe the found effect we use a quasi-classical approximation. There the tunneling current is related to a creation of a dilute instanton-anti-instanton gas. Our experimental data are well described with exactly solvable model where one charged particle is weakly interacting with two promoting phonon modes associated with external medium. We conclude that the characteristics of the tunnel nanoelectronic devices can thus be controlled by a proper choice of phonons existing in materials, which are involved.

  11. Symmetry-adapted phonon analysis of nanotubes

    NASA Astrophysics Data System (ADS)

    Aghaei, Amin; Dayal, Kaushik; Elliott, Ryan S.

    2013-02-01

    The characteristics of phonons, i.e. linearized normal modes of vibration, provide important insights into many aspects of crystals, e.g. stability and thermodynamics. In this paper, we use the Objective Structures framework to make concrete analogies between crystalline phonons and normal modes of vibration in non-crystalline but highly symmetric nanostructures. Our strategy is to use an intermediate linear transformation from real-space to an intermediate space in which the Hessian matrix of second derivatives is block-circulant. The block-circulant nature of the Hessian enables us to then follow the procedure to obtain phonons in crystals: namely, we use the Discrete Fourier Transform from this intermediate space to obtain a block-diagonal matrix that is readily diagonalizable. We formulate this for general Objective Structures and then apply it to study carbon nanotubes of various chiralities that are subjected to axial elongation and torsional deformation. We compare the phonon spectra computed in the Objective Framework with spectra computed for armchair and zigzag nanotubes. We also demonstrate the approach by computing the Density of States. In addition to the computational efficiency afforded by Objective Structures in providing the transformations to almost-diagonalize the Hessian, the framework provides an important conceptual simplification to interpret the phonon curves. Our findings include that, first, not all non-optic long-wavelength modes are zero energy and conversely not all zero energy modes are long-wavelength; second, the phonon curves accurately predict both the onset as well as the soft modes for instabilities such as torsional buckling; and third, unlike crystals where phonon stability does not provide information on stability with respect to non-rank-one deformation modes, phonon stability in nanotubes is sufficient to guarantee stability with respect to all perturbations that do not involve structural modes. Our finding of characteristic

  12. Quantitative analysis of the temperature dependency in Raman active vibrational modes of molybdenum disulfide atomic layers.

    PubMed

    Najmaei, Sina; Ajayan, Pulickel M; Lou, J

    2013-10-21

    Raman spectroscopy is utilized to quantify the temperature dependency of the vibrational modes in molybdenum disulfide (MoS2) atomic layers. These analyses are essential for understanding the structural properties and phononic behaviors of this two-dimensional (2D) material. We quantitatively analyze the temperature dependent shifts of the Raman peak positions in the temperature range from 300 to 550 K, and find that both planar and out-of-plane characteristic modes are highly sensitive to temperature variations. This temperature dependency is linear and can be fully explained by the first-order temperature coefficient. Using a semi-quantitative model, we evaluate the contributions of the material's thermal expansion and intrinsic temperature effects to this dependency. We reveal that the dominating source of shift in the peak position of planar mode E2g(1) for samples of all thicknesses investigated is the four-phonon process. In addition to the four-phonon process, thermal expansion plays a significant role in the temperature dependency of the out-of-plane mode, A1g. The thickness dependency of the temperature coefficient for MoS2 and a drastic change in behaviors of samples from bi- to single-layered are also demonstrated. We further explore the role of defects in the thermal properties of MoS2 by examining the temperature dependency of Raman modes in CVD-grown samples.

  13. Phononic Origins of Friction in Carbon Nanotube Oscillators.

    PubMed

    Prasad, Matukumilli V D; Bhattacharya, Baidurya

    2017-03-01

    Phononic coupling can have a significant role in friction between nanoscale surfaces. We find frictional dissipation per atom in carbon nanotube (CNT) oscillators to depend significantly on interface features such as contact area, commensurability, and by end-capping of the inner core. We perform large-scale phonon wavepacket MD simulations to study phonon coupling between a 250 nm long (10,10) outer tube and inner cores of four different geometries. Five different phonon polarizations known to have dominant roles in thermal transport are selected, and transmission coefficient plots for a range of phonon energies along with phonon scattering dynamics at specific energies are obtained. We find that the length of interface affects friction only through LA phonon scattering and has a significant nonlinear effect on total frictional force. Incommensurate contact does not always give rise to superlubricity: the net effect of two competing interaction mechanisms shown by longitudinal and transverse phonons decides the role of commensurability. Capping of the core has no effect on acoustic phonons but destroys the coherence of transverse optical phonons and creates diffusive scattering. In contrast, the twisting and radial breathing phonon modes have perfect transmission at all energies and can be deemed as the enablers of ultralow friction in CNT oscillators. Our work suggests that tuning of interface geometries can give rise to desirable friction properties in nanoscale devices.

  14. Probing Phonons in Plutonium

    NASA Astrophysics Data System (ADS)

    Wong, Joe

    2004-03-01

    The phonon spectra of plutonium and its alloys have been sought after in the past few decades following the discovery of this actinide element in 1941, but with no success. This was due to a combination of the high neutron absorption cross section of 239Pu, the common isotope, and non-availability of large single crystals of any Pu-bearing materials. We have recent designed a high resolution inelastic x-ray scattering experiment using a bright synchrotron x-ray beam at the European Sychrotron Radiation Facility (ESRF), Grenoble and mapped the full phonon dispersion curves of an fcc delta-phase polycrystalline Pu-Ga alloy (1). Several unusual features including, a large elastic anisotropy, a small shear elastic modulus C', a Kohn-like anomaly in the T1[011] branch, and a pronounced softening of the [111] transverse modes are found. These features can be related to the phase transitions of plutonium and to strong coupling between the lattice structure and the 5f valence instabilities. Our results also provide a critical test for theoretical treatments of highly correlated 5f electron systems as exemplified by recent dynamical mean field theory (DMFT) calculations for d-plutonium.(2) This work was performed in collaboration with Dr. M. Krisch (ESRF)) and Prof. T.-C. Chiang (UIU), and under the auspices of the U. S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48. 1. Joe Wong et al. Science, vol.301, 1078 (2003) 2. X. Dai et al. Science, vol.300, 953 (2003)

  15. Spin-phonon coupling in multiferroic YbMnO3 studied by Raman scattering

    NASA Astrophysics Data System (ADS)

    Fukumura, H.; Hasuike, N.; Harima, H.; Kisoda, K.; Fukae, K.; Yoshimura, T.; Fujimura, N.

    2009-02-01

    Hexagonal YbMnO3 bulk polycrystals were prepared and studied by Raman scattering in the temperature range of 15-300 K. A total of 15 phonon modes of A1, E1 and E2 type were identified. Some E2 phonon modes showed anomalous temperature variations in frequency at TN~80 K, suggesting a coupling between the spin and phonon systems below TN. As another evidence of spin-phonon coupling, softening of an A1-phonon mode for the O-Mn vibration was observed at ~TN. Substitution of Mn by Al suggests this view.

  16. Hot carriers relaxation in highly excited polar semiconductors: Hot phonons versus phonon-plasmon coupling

    NASA Astrophysics Data System (ADS)

    Tea, Eric; Hamzeh, Hani; Aniel, Frédéric

    2011-12-01

    We present a study of the photo-excited charge carriers relaxation dynamics in polar semiconductors comparing calculations to pump probe experiments. Hot carrier densities in the 1018cm-3 range can easily be photo-generated using moderately intense optical excitations. This can lead to known phenomena, namely, hot phonon populations and the coupling of polar optical phonons with plasmon modes. However, these two phenomena can affect the hot carriers relaxation and have never been examined together. This is a problem for the theoretical study of future Hot Carrier Solar Cells, where the conditions allow both of these phenomena to occur. The charge carriers dynamics and the coupling of polar optical phonons with plasmon modes are treated by a Full Band Ensemble Monte Carlo simulation code featuring a self-consistent dielectric function. To take into consideration hot phonon populations and the subsequent phonon bottleneck for the carriers relaxation, the charge carriers simulation code is coupled to a phonon dedicated Ensemble Monte Carlo code. This enables for the first time an accurate study of both the charge carriers and phonon systems dynamics, the latter being most of the time overly simplified in previous studies. The present work explores to which extent the two aforementioned phenomena affect the photo-generated charge carriers relaxation in GaAs and can be easily adapted to other polar semiconductors.

  17. Phononic Phase Conjugation in an Optomechanical System

    NASA Astrophysics Data System (ADS)

    Buchmann, Lukas; Wright, Ewan; Meystre, Pierre

    2013-05-01

    We study theoretically the phase conjugation of a phononic field in an optomechanical system with two mechanical modes coupled to a common optical field. Phase conjugation becomes the dominant process for an appropriate choice of driving field parameters, and he effective coupling coefficients between phonon modes can result in amplification and entanglement, phase-conjugation or a mixture thereof. We discuss surprising consequences of mechanical phase-conjugation that could lead to the preparation of mechanical states with negative temperature, the improvement of quantum memories and the study of the quantum-classical transition. Supported by DARPA ORCHID program.

  18. Phonon anharmonicity of monoclinic zirconia and yttrium-stabilized zirconia

    DOE PAGES

    Li, Chen W.; Smith, Hillary L.; Lan, Tian; ...

    2015-04-13

    Inelastic neutron scattering measurements on monoclinic zirconia (ZrO2) and 8 mol% yttrium-stabilized zirconia were performed at temperatures from 300 to 1373 ωK. We reported temperature-dependent phonon densities of states (DOS) and Raman spectra obtained at elevated temperatures. First-principles lattice dynamics calculations with density functional theory gave total and partial phonon DOS curves and mode Grüneisen parameters. These mode Grüneisen parameters were used to predict the experimental temperature dependence of the phonon DOS with partial success. However, substantial anharmonicity was found at elevated temperatures, especially for phonon modes dominated by the motions of oxygen atoms. Yttrium-stabilized zirconia (YSZ) was somewhat moremore » anharmonic and had a broader phonon spectrum at low temperatures, owing in part to defects in its structure. YSZ also has a larger vibrational entropy than monoclinic zirconia.« less

  19. Phonon anharmonicity of monoclinic zirconia and yttrium-stabilized zirconia

    SciTech Connect

    Li, Chen W.; Smith, Hillary L.; Lan, Tian; Niedziela, Jennifer L.; Munoz, Jorge A.; Keith, J. Brian; Mauger, L.; Abernathy, Douglas L; Fultz, B.

    2015-04-13

    Inelastic neutron scattering measurements on monoclinic zirconia (ZrO2) and 8 mol% yttrium-stabilized zirconia were performed at temperatures from 300 to 1373 ωK. We reported temperature-dependent phonon densities of states (DOS) and Raman spectra obtained at elevated temperatures. First-principles lattice dynamics calculations with density functional theory gave total and partial phonon DOS curves and mode Grüneisen parameters. These mode Grüneisen parameters were used to predict the experimental temperature dependence of the phonon DOS with partial success. However, substantial anharmonicity was found at elevated temperatures, especially for phonon modes dominated by the motions of oxygen atoms. Yttrium-stabilized zirconia (YSZ) was somewhat more anharmonic and had a broader phonon spectrum at low temperatures, owing in part to defects in its structure. YSZ also has a larger vibrational entropy than monoclinic zirconia.

  20. Multiple Quantum Wells for P T -Symmetric Phononic Crystals

    NASA Astrophysics Data System (ADS)

    Poshakinskiy, A. V.; Poddubny, A. N.; Fainstein, A.

    2016-11-01

    We demonstrate that the parity-time symmetry for sound is realized in laser-pumped multiple-quantum-well structures. Breaking of the parity-time symmetry for the phonons with wave vectors corresponding to the Bragg condition makes the structure a highly selective acoustic wave amplifier. Single-mode distributed feedback phonon lasing is predicted for structures with realistic parameters.

  1. Topological Phonons and Weyl Lines in Three Dimensions

    NASA Astrophysics Data System (ADS)

    Stenull, Olaf; Kane, C. L.; Lubensky, T. C.

    2016-08-01

    Topological mechanics and phononics have recently emerged as an exciting field of study. Here we introduce and study generalizations of the three-dimensional pyrochlore lattice that have topologically protected edge states and Weyl lines in their bulk phonon spectra, which lead to zero surface modes that flip from one edge to the opposite as a function of surface wave number.

  2. Bloch oscillations in the presence of plasmons and phonons

    PubMed

    Ghosh; Jonsson; Wilkins

    2000-07-31

    The coupling between Bloch oscillating electrons and longitudinal optical phonons in a superlattice leads to resonant phonon excitation but no gap in the Bloch-phonon spectrum. In addition, we predict a sharp transition from plasma to Bloch oscillations at nu(B) = 2nu(P). From a microscopic description with phenomenological dampings, we numerically map out the behavior of coupled Bloch-plasmon-phonon modes for a wide range of parameters, and mimic experimental conditions. Our results are in good agreement with recent experiments by Dekorsy et al. [Phys. Rev. Lett. 85, 1080 (2000)].

  3. Thermal transport and anharmonic phonons in strained monolayer hexagonal boron nitride

    PubMed Central

    Li, Shasha; Chen, Yue

    2017-01-01

    Thermal transport and phonon-phonon coupling in monolayer hexagonal boron nitride (h-BN) under equibiaxial strains are investigated from first principles. Phonon spectra at elevated temperatures have been calculated from perturbation theory using the third-order anharmonic force constants. The stiffening of the out-of-plane transverse acoustic mode (ZA) near the Brillouin zone center and the increase of acoustic phonon lifetimes are found to contribute to the dramatic increase of thermal transport in strained h-BN. The transverse optical mode (TO) at the K point, which was predicted to lead to mechanical failure of h-BN, is found to shift to lower frequencies at elevated temperatures under equibiaxial strains. The longitudinal and transverse acoustic modes exhibit broad phonon spectra under large strains in sharp contrast to the ZA mode, indicating strong in-plane phonon-phonon coupling. PMID:28262786

  4. Thermal transport and anharmonic phonons in strained monolayer hexagonal boron nitride.

    PubMed

    Li, Shasha; Chen, Yue

    2017-03-06

    Thermal transport and phonon-phonon coupling in monolayer hexagonal boron nitride (h-BN) under equibiaxial strains are investigated from first principles. Phonon spectra at elevated temperatures have been calculated from perturbation theory using the third-order anharmonic force constants. The stiffening of the out-of-plane transverse acoustic mode (ZA) near the Brillouin zone center and the increase of acoustic phonon lifetimes are found to contribute to the dramatic increase of thermal transport in strained h-BN. The transverse optical mode (TO) at the K point, which was predicted to lead to mechanical failure of h-BN, is found to shift to lower frequencies at elevated temperatures under equibiaxial strains. The longitudinal and transverse acoustic modes exhibit broad phonon spectra under large strains in sharp contrast to the ZA mode, indicating strong in-plane phonon-phonon coupling.

  5. Thermal transport and anharmonic phonons in strained monolayer hexagonal boron nitride

    NASA Astrophysics Data System (ADS)

    Li, Shasha; Chen, Yue

    2017-03-01

    Thermal transport and phonon-phonon coupling in monolayer hexagonal boron nitride (h-BN) under equibiaxial strains are investigated from first principles. Phonon spectra at elevated temperatures have been calculated from perturbation theory using the third-order anharmonic force constants. The stiffening of the out-of-plane transverse acoustic mode (ZA) near the Brillouin zone center and the increase of acoustic phonon lifetimes are found to contribute to the dramatic increase of thermal transport in strained h-BN. The transverse optical mode (TO) at the K point, which was predicted to lead to mechanical failure of h-BN, is found to shift to lower frequencies at elevated temperatures under equibiaxial strains. The longitudinal and transverse acoustic modes exhibit broad phonon spectra under large strains in sharp contrast to the ZA mode, indicating strong in-plane phonon-phonon coupling.

  6. Interaction Induced Electron Self-Interference in a Semiconductor: The Phonon Staircase Effect

    NASA Astrophysics Data System (ADS)

    Kenrow, J. A.; El Sayed, K.; Stanton, C. J.

    1997-06-01

    The exact quantum mechanics of a model semiconductor system of an electron with a discrete and equidistant energy spectrum interacting with a single phonon mode is presented. An electron initially excited into a coherent superposition of states interacts with phonons thereby creating a self-interference in time which reduces the emission of phonons to isolated bursts. This self-interference effect gives rise to steplike behavior in the relaxation kinetics of the electron and phonons. We show that this ``phonon staircase'' effect is a consequence of a correlated initial electron distribution and the violation of energy conservation in the electron-phonon interaction on short time scales.

  7. Probing phonons in plutonium

    SciTech Connect

    Wong, Joe; Krisch, M.; Farber, D.; Occelli, F.; Schwartz, A.; Chiang, T.C.; Wall, M.; Boro, C.; Xu, Ruqing

    2010-11-16

    high resolution inelastic x-ray scattering (HRIXS) capability on ID28. The complete PDCs for an fcc Pu-0.6 wt% Ga alloy are plotted in Figure 2, and represent the first full set of phonon dispersions ever determined for any Pu-bearing materials. The solid curves (red) are calculated using a standard Born-von Karman (B-vK) force constant model. An adequate fit to the experimental data is obtained if interactions up to the fourth-nearest neighbours are included. The dashed curves (blue) are recent dynamical mean field theory (DMFT) results by Dai et al. The elastic moduli calculated from the slopes of the experimental phonon dispersion curves near the {Lambda} point are: C{sub 11} = 35.3 {+-} 1.4 GPa, C{sub 12} = 25.5 {+-} 1.5 GPa and C{sub 44} = 30.53 {+-} 1.1 GPa. These values are in excellent agreement with those of the only other measurement on a similar alloy (1 wt % Ga) using ultrasonic techniques as well as with those recently calculated from a combined DMFT and linear response theory for pure {delta}-Pu. Several unusual features, including a large elastic anisotropy, a small shear elastic modulus C{prime}, a Kohn-like anomaly in the T{sub 1}[011] branch, and a pronounced softening of the [111] transverse modes are found. These features can be related to the phase transitions of plutonium and to strong coupling between the lattice structure and the 5f valence instabilities. The HRIXS results also provide a critical test for theoretical treatments of highly correlated 5f electron systems as exemplified by recent dynamical mean field theory (DMFT) calculations for {delta}-plutonium. The experimental-theoretical agreements shown in Figure 2 in terms of a low shear elastic modulus C{prime}, a Kohn-like anomaly in the T{sub 1}[011] branch, and a large softening of the T[111] modes give credence to the DMFT approach for the theoretical treatment of 5f electron systems of which {delta}-Pu is a classic example. However, quantitative differences remain. These are the

  8. CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES Phonon States and Dispersive Spectra of Polar Optical Phonons in Quasi-One-Dimensional Nanowires of Wurtzite ZnO and Zinc-Blend MgO Semiconductors

    NASA Astrophysics Data System (ADS)

    Zhang, Li

    2011-01-01

    Within the framework of the macroscopic dielectric continuum model and Loudon's uniaxial crystal model, the phonon modes of a wurtzite/zinc-blende one-dimensional (1D) cylindrical nanowire (NW) are derived and studied. The analytical phonon states of phonon modes are given. It is found that there exist two types of polar phonon modes, i.e. interface optical (IO) phonon modes and the quasi-confined (QC) phonon modes existing in 1D wurtzite/zinc-blende NWs. Via the standard procedure of field quantization, the Fröhlich electron-phonon interaction Hamiltonians are obtained. Numerical calculations of dispersive behavior of these phonon modes on a wurtzite/zinc-blende ZnO/MgO NW are performed. The frequency ranges of the IO and QC phonon modes of the ZnO/MgO NWs are analyzed and discussed. It is found that the IO modes only exist in one frequency range, while QC modes may appear in three frequency ranges. The dispersive properties of the IO and QC modes on the free wave-number kz and the azimuthal quantum number m are discussed. The analytical Hamiltonians of electron-phonon interaction obtained here are quite useful for further investigating phonon influence on optoelectronics properties of wurtzite/zinc-blende 1D NW structures.

  9. Active Control of Linear Periodic System with Two Unstable Modes.

    DTIC Science & Technology

    1982-12-01

    tV;;;.~II.~9 - B ~ZV ~- p1 . ,,~ >. ~ ACTIVE CONTROL OF LINEAR PERIODIC SYSTEM WITH TWO UNSTABLE MODES THESIS by Gregory E. Myers, B.S.E. 2nd Lt...PERIODIC SYSTEM WITH TWO UNSTABLE MODES THESIS Presented to the Faculty of the School of Engineering of the Air Force Institute of Technology Air University...December 1982 Approved for public release; distribution unlimited -ow PREFACE This thesis is a continuation of the work done by Yeakel in the control of

  10. Phonon dynamics of graphene on metals

    NASA Astrophysics Data System (ADS)

    Taleb, Amjad Al; Farías, Daniel

    2016-03-01

    The study of surface phonon dispersion curves is motivated by the quest for a detailed understanding of the forces between the atoms at the surface and in the bulk. In the case of graphene, additional motivation comes from the fact that thermal conductivity is dominated by contributions from acoustic phonons, while optical phonon properties are essential to understand Raman spectra. In this article, we review recent progress made in the experimental determination of phonon dispersion curves of graphene grown on several single-crystal metal surfaces. The two main experimental techniques usually employed are high-resolution electron energy loss spectroscopy (HREELS) and inelastic helium atom scattering (HAS). The different dispersion branches provide a detailed insight into the graphene-substrate interaction. Softening of optical modes and signatures of the substrate‧s Rayleigh wave are observed for strong graphene-substrate interactions, while acoustic phonon modes resemble those of free-standing graphene for weakly interacting systems. The latter allows determining the bending rigidity and the graphene-substrate coupling strength. A comparison between theory and experiment is discussed for several illustrative examples. Perspectives for future experiments are discussed.

  11. Phononic Frequency Comb via Intrinsic Three-Wave Mixing.

    PubMed

    Ganesan, Adarsh; Do, Cuong; Seshia, Ashwin

    2017-01-20

    Optical frequency combs have resulted in significant advances in optical frequency metrology and found wide applications in precise physical measurements and molecular fingerprinting. A direct analogue of frequency combs in the phononic or acoustic domain has not been reported to date. In this Letter, we report the first clear experimental evidence for a phononic frequency comb. We show that the phononic frequency comb is generated through the intrinsic coupling of a driven phonon mode with an autoparametrically excited subharmonic mode. The experiments depict the comb generation process evidenced by a spectral response consisting of equally spaced discrete and phase coherent comb lines. Through systematic experiments at different drive frequencies and amplitudes, we portray the well-connected process of phononic frequency comb formation and define the attributes to control the features associated with comb formation in such a system. In addition to the demonstration of frequency comb, the interplay between the nonlinear resonances and the well-known Duffing phenomenon is also observed.

  12. Phononic Frequency Comb via Intrinsic Three-Wave Mixing

    NASA Astrophysics Data System (ADS)

    Ganesan, Adarsh; Do, Cuong; Seshia, Ashwin

    2017-01-01

    Optical frequency combs have resulted in significant advances in optical frequency metrology and found wide applications in precise physical measurements and molecular fingerprinting. A direct analogue of frequency combs in the phononic or acoustic domain has not been reported to date. In this Letter, we report the first clear experimental evidence for a phononic frequency comb. We show that the phononic frequency comb is generated through the intrinsic coupling of a driven phonon mode with an autoparametrically excited subharmonic mode. The experiments depict the comb generation process evidenced by a spectral response consisting of equally spaced discrete and phase coherent comb lines. Through systematic experiments at different drive frequencies and amplitudes, we portray the well-connected process of phononic frequency comb formation and define the attributes to control the features associated with comb formation in such a system. In addition to the demonstration of frequency comb, the interplay between the nonlinear resonances and the well-known Duffing phenomenon is also observed.

  13. Phonon localization drives polar nanoregions in a relaxor ferroelectric.

    PubMed

    Manley, M E; Lynn, J W; Abernathy, D L; Specht, E D; Delaire, O; Bishop, A R; Sahul, R; Budai, J D

    2014-04-10

    Relaxor ferroelectrics exemplify a class of functional materials where interplay between disorder and phase instability results in inhomogeneous nanoregions. Although known for about 30 years, there is no definitive explanation for polar nanoregions (PNRs). Here we show that ferroelectric phonon localization drives PNRs in relaxor ferroelectric PMN-30%PT using neutron scattering. At the frequency of a preexisting resonance mode, nanoregions of standing ferroelectric phonons develop with a coherence length equal to one wavelength and the PNR size. Anderson localization of ferroelectric phonons by resonance modes explains our observations and, with nonlinear slowing, the PNRs and relaxor properties. Phonon localization at additional resonances near the zone edges explains competing antiferroelectric distortions known to occur at the zone edges. Our results indicate the size and shape of PNRs that are not dictated by complex structural details, as commonly assumed, but by phonon resonance wave vectors. This discovery could guide the design of next generation relaxor ferroelectrics.

  14. Effect of phonon focusing on Knudsen flow of phonon gas in single-crystal nanowires made of spintronics materials

    NASA Astrophysics Data System (ADS)

    Kuleev, I. I.; Bakharev, S. M.; Kuleev, I. G.; Ustinov, V. V.

    2017-01-01

    Effect of anisotropy of elastic energy on the phonon propagation in single-crystal nanowires made of Fe, Cu, MgO, InSb, and GaAs materials that are used to fabricate spintronics devices in the regime of the Knudsen flow of phonon gas has been studied. A new method of analyzing the focusing of quasi-transverse modes has been suggested, which made it possible to determine the average values of the densities of phonon states in the regions of focusing and defocusing slow and fast quasi-transverse modes. The effect of phonon focusing on the anisotropy of heat conductivity and lengths of the phonon free paths has been analyzed for all acoustic modes that exist in spintronics nanostructures. It has been shown that for all the nanowires investigated the angular dependences of the free paths of fast and slow transverse modes in the {100} and {110} planes correlate with the angular dependences of the densities of phonon states for these modes. Directions of the heat flux that ensure the maximum and minimum phonon heat conductivity in the nanowires have been determined.

  15. Phonon dispersion in red mercuric iodide

    SciTech Connect

    Sim, H.; Chang, Y. ); James, R.B. )

    1994-02-15

    We present theoretical studies of phonon modes of undoped HgI[sub 2] in its red tetragonal form. A rigid-ion model including the Coulomb interaction is used which gives the best fit to the neutron scattering, infrared reflectivity, and Raman scattering data. The calculated sound velocities are also in accord with experiment.

  16. ``Forbidden'' phonon in the iron chalcogenide series

    NASA Astrophysics Data System (ADS)

    Fobes, David M.; Zaliznyak, Igor A.; Xu, Zhijun; Gu, Genda; Tranquada, John M.

    2015-03-01

    Recently, we uncovered evidence for the formation of a bond-order wave (BOW) leading to ferro-orbital order at low temperature, acting to stabilize the bicollinear AFM order, in the iron-rich parent compound, Fe1+yTe. Investigating the inelastic spectra centered near (100) in Fe1+yTe, a signature peak for the BOW formation in the monoclinic phase, we observed an acoustic phonon dispersion in both tetragonal and monoclinic phases. While a structural Bragg peak accompanies the mode in the monoclinic phase, in the tetragonal phase Bragg scattering at this Q is forbidden by symmetry, and we observed no elastic peak. This phonon mode was also observed in superconducting FeTe0.6Se0.4, where structural and magnetic transitions are suppressed. LDA frozen phonon calculations suggested that this mode could result from a spin imbalance between neighboring Fe atoms, but polarized neutron measurements revealed no additional magnetic scattering. We propose that this ``forbidden'' phonon mode may originate from dynamically broken symmetry, perhaps related to the strong dynamic spin correlations in these materials. Work at BNL was supported by BES, US DOE, under Contract No. DE-AC02-98CH10886. Research at ORNL's HFIR and SNS sponsored by Scientific User Facilities Division, BES, US DOE. We acknowledge the support of NIST, in providing neutron research facilities.

  17. Modes of active deformation in Eastern Hispaniola

    NASA Astrophysics Data System (ADS)

    García-Senz, J.; Pérez-Estaún, A.

    2012-04-01

    Eastern Hispaniola and the Puerto Rico Island are the emerged part of a doubly vergent thrust wedge formed by oblique arc-continent collision with subduction and underthrusting of the North America Plate in the Puerto Rico trench and underthrusting of the Caribbean crust in The Muertos trough (Dolan et al. 1998, Mann et al., 2002, ten Brink et al. 2010). In the relatively small area of Eastern Hispaniola several types of active crustal deformation have been recognized: 1) At the prowedge of the orogene, the rear of the accretionary prism is cut by the strike-slip Septentrional Fault, bounding a sliver plate (Mann et al, 2002). Recent detailed mapping and aeromagnetic surveys in the onshore part of the prism (Samaná Peninsula and Septentrional Cordillera, Sysmin Team) revealed that the internal structure of the sliver is made of parallel bands of sigmoidal, left-lateral, NW-SE thrust splays, bounded by steep strike-slip faults. We interpreted these structures as transpressional strike-slip duplex. It is worth to note the similarity between the strike and dip of the thrust splays and the 303, 62, 74 focal mechanism calculated by Russo and Villaseñor (1995) for the thrust event of the August 4, 1946 Hispaniola earthquake. 2) The uplifted core of the orogen extends between the accretionary prism and the beginning of the Muertos retrowedge. Half of this area is occupied by the Oriental Cordillera, a recent uplift of cretaceous island-arc rocks arching the Late Neogene reef. The rest of the territory is the Caribbean Coastal Plain modelled on the Late Neogene reef. The Oriental Cordillera is made of two en echelon left-stepping uplifts: the domal-shaped Haitises and the rhombohedral-shaped Seibo (García-Senz et al, 2007); the latter share structural similarities and scaling relations with the 90° neutral stepover model of McClay and Bonora (2001). Therefore we interpret it as a restraining stepover developed over a blind splay of the Septentrional Fault, and the main

  18. Optical phonon lasing and its detection in transport through semiconduc- tor double quantum dots

    NASA Astrophysics Data System (ADS)

    Okuyama, Rin; Eto, Mikio; Brandes, Tobias

    2014-03-01

    We theoretically propose optical phonon lasing for a double quantum dot (DQD) fabricated in a semiconductor substrate. No additional cavity or resonator is required. We show that the DQD couples to only two phonon modes that act as a natural cavity. The pumping to the upper level is realized by an electric current through the DQD under a finite bias. Using the rate equation in the Born-Markov-Secular approximation, we analyze the enhanced phonon emission when the level spacing in the DQD is tuned to the phonon energy. We find the phonon lasing when the pumping rate is much larger than the phonon decay rate, whereas anti-bunching of phonon emission is observed when the pumping rate is smaller.[1] Our theory can be also applicable to DQDs embedded in nanomechanical resonators to control the vibrating modes. We discuss detection of amplified modes using the electric current and its noise through the DQD, and another DQD fabricated nearby.

  19. Renormalisation of Nonequilibrium Phonons Under Strong Perturbative Influences.

    NASA Astrophysics Data System (ADS)

    Mehta, Sushrut Madhukar

    Effects of strong perturbative influences, namely the presence of a narrow distribution of acoustic phonons, and the presence of an electron plasma, on the dynamics of nonequilibrium, near zone center, longitudinal optical phonons in GaP have been investigated in two separate experiments. The study of the effects of the interaction between the LO phonons and a heavily populated, narrow distribution of acoustic phonons lead to the observation of a new optically driven nonequilibrium phonon state. Time Resolved Coherent Antistokes Raman Scattering (TR-CARS), with picosecond resolution, was used to investigate the new mode. In order to achieve high occupation numbers in the acoustic branch, the picosecond laser pulses used were amplified up to 1.0 GW/cm^2 peak power per laser beam. An important characteristic property of the new state which differentiates it from the well known LO phonon state is the fact that rather than having the single decay rate observed under thermal equilibrium, the new state has two decay rates. Moreover, these two decay rates depend strongly on the distribution of the acoustic phonon occupation number. The coupling of the LO phonons with an electron plasma, on the other hand, was investigated by measurements of the shape of the Raman scattered line associated with the phonon-plasmon coupled mode. The plasma was generated by thermal excitation of carriers in doped samples. It was possible to study a large variety of plasma excitations by controlling the concentration of the dopant and the ambient temperature. A complete, self consistant model based on standard dielectric response theory is presented, and applied to the measurements of the phonon-plasmon coupled mode. It is possible to recover, via this model, the effective coupled mode damping rate, the plasma damping rate, and the plasma frequency as functions of ambient temperature, or the carrier concentration.

  20. Evidence of superconductivity-induced phonon spectra renormalization in alkali-doped iron selenides

    DOE PAGES

    Opačić, M.; Lazarević, N.; Šćepanović, M.; ...

    2015-11-16

    Polarized Raman scattering spectra of superconducting KxFe2-ySe2 and nonsuperconducting K0.8Fe1.8Co0.2Se2 single crystals were measured in a temperature range from 10 K up to 300 K. Two Raman active modes from the I4/mmm phase and seven from the I4/m phase are observed in frequency range from 150 to 325 cm -1 in both compounds, suggesting that K0.8Fe1.8Co0.2Se2 single crystal also has two-phase nature. Temperature dependence of Raman mode energy is analyzed in terms of lattice thermal expansion and phonon-phonon interaction. Temperature dependence of Raman mode linewidth is considered as temperature-induced anharmonic effects. It is shown that change of Raman mode energymore » with temperature is dominantly driven by thermal expansion of the crystal lattice. Abrupt change of the A1g mode energy near TC was observed in KxFe2-ySe2 , whereas it is absent in K0.8Fe1.8Co0.2Se2. Phonon energy hardening at low temperatures in the superconducting sample is a consequence of superconductivity-induced redistribution of the electronic states below critical temperature.« less

  1. Intrinsic phonon properties of double-walled carbon nanotubes

    NASA Astrophysics Data System (ADS)

    Tran, H. N.; Levshov, D. I.; Nguyen, V. C.; Paillet, M.; Arenal, R.; Than, X. T.; Zahab, A. A.; Yuzyuk, Y. I.; Phan, N. M.; Sauvajol, J.-L.; Michel, T.

    2017-03-01

    Double-walled carbon nanotubes (DWNT) are made of two concentric and weakly van der Waals coupled single-walled carbon nanotubes (SWNT). DWNTs are the simplest systems for studying the mechanical and electronic interactions between concentric carbon layers. In this paper we review recent results concerning the intrinsic features of phonons of DWNTs obtained from Raman experiments performed on index-identified DWNTs. The effect of the interlayer distance on the strength of the mechanical and electronic coupling between the layers, and thus on the frequencies of the Raman-active modes, namely the radial breathing-like modes (RBLMs) and G-modes, are evidenced and discussed. Invited talk at 8th International Workshop on Advanced Materials Science and Nanotechnology (IWAMSN2016), 8–12 November 2016, Ha Long City, Vietnam.

  2. Active mode locking of lasers by piezoelectrically induced diffraction modulation

    NASA Astrophysics Data System (ADS)

    Krausz, F.; Turi, L.; Kuti, Cs.; Schmidt, A. J.

    1990-04-01

    A new amplitude-modulation mode-locking technique is presented. Acoustic waves are generated directly on the faces of a resonant photoelastic medium. The created standing waves cause a highly efficient diffraction modulation of light. The modulation depth of standing-wave mode lockers is related to material and drive parameters and a figure of merit is introduced. With a lithium niobate crystal modulation depths over 10 are achieved at 1.054 μm and 1 W of radio frequency power. Using this device for the active mode locking of a continuous-wave Nd:glass laser pulses as short as 3.8 ps are produced at a repetition rate of 66 MHz. Limitations of amplitude-modulation mode locking by standing acoustic waves are discussed.

  3. Active mode-locked lasers and other photonic devices using electro-optic whispering gallery mode resonators

    NASA Technical Reports Server (NTRS)

    Matsko, Andrey B. (Inventor); Ilchenko, Vladimir (Inventor); Savchenkov, Anatoliy (Inventor); Maleki, Lutfollah (Inventor)

    2006-01-01

    Techniques and devices using whispering gallery mode (WGM) optical resonators, where the optical materials of the WGM resonators exhibit an electro-optical effect to perform optical modulation. Examples of actively mode-locked lasers and other devices are described.

  4. Phonon-Josephson resonances in atomtronic circuits

    NASA Astrophysics Data System (ADS)

    Bidasyuk, Y. M.; Prikhodko, O. O.; Weyrauch, M.

    2016-09-01

    We study the resonant excitation of sound modes from Josephson oscillations in Bose-Einstein condensates. From the simulations for various setups using the Gross-Pitaevskii mean-field equations and Josephson equations we observe additional tunneling currents induced by resonant phonons. The proposed experiment may be used for spectroscopy of phonons as well as other low-energy collective excitations in Bose-Einstein condensates. We also argue that the observed effect may mask the observation of Shapiro resonances if not carefully controlled.

  5. Optical phonon spectra of GaP nanoparticles prepared by nanochemistry

    NASA Astrophysics Data System (ADS)

    Manciu, F. S.; Furis, M.; McCombe, B. D.; Sahoo, Y.; Macrae, D. J.; Prasad, P. N.

    2003-03-01

    We have used IR transmission spectroscopy to study optically active phonon modes of GaP nanoparticles and obtained information about the morphology, crystallinity, and surface interactions. GaP nanoparticles were fabricated by colloidal nanochemistry and synthesized in several batches with different surfactants: Trioctylphosphineoxide and Dodecylamine. Samples for the IR studies were prepared in the form of pellets, by embedding them in a polycrystalline CsI matrix. The IR transmission measurements were carried out with a Fourier Transform Spectrometer covering the range from 20 - 600 cm-1. The transmittance spectra for all samples exhibit common absorption bands. A TO(Γ) band at 360 cm-1 demonstrates the crystallinity of the GaP nanoparticles. Evidence for two predicted surface optical phonon modes was found in all samples, as well as other weak features that we attribute to GaP two phonon and combination phonon bands. The crystallinity and the stoichiometry of the samples were also examined and characterized by Transmission Electron Microscopy, Electron Diffraction, and Energy Dispersive Spectroscopy. Supported by the DURINT program through the AFOSR under grant # F496200110358.

  6. Polaron action for multimode dispersive phonon systems

    NASA Astrophysics Data System (ADS)

    Kornilovitch, P. E.

    2006-03-01

    The path-integral approach to the tight-binding polaron is extended to multiple optical phonon modes of arbitrary dispersion and polarization. The nonlinear lattice effects are neglected. Only one electron band is considered. The electron-phonon interaction is of the density-displacement type, but can be of arbitrary spatial range and shape. Feynman’s analytical integration of ion trajectories is performed by transforming the electron-ion forces to the basis in which the phonon dynamical matrix is diagonal. The resulting polaron action is derived for the periodic and shifted boundary conditions in imaginary time. The former can be used for calculating polaron thermodynamics while the latter for the polaron mass and spectrum. The developed formalism is the analytical basis for numerical analysis of such models by path-integral Monte Carlo methods.

  7. Mapping gigahertz vibrations in a plasmonic-phononic crystal

    NASA Astrophysics Data System (ADS)

    Kelf, Timothy A.; Hoshii, Wataru; Otsuka, Paul H.; Sakuma, Hirotaka; Veres, Istvan A.; Cole, Robin M.; Mahajan, Sumeet; Baumberg, Jeremy J.; Tomoda, Motonobu; Matsuda, Osamu; Wright, Oliver B.

    2013-02-01

    We image the gigahertz vibrational modes of a plasmonic-phononic crystal at sub-micron resolution by means of an ultrafast optical technique, using a triangular array of spherical gold nanovoids as a sample. Light is strongly coupled to the plasmonic modes, which interact with the gigahertz phonons by a process akin to surface-enhanced stimulated Brillouin scattering. A marked enhancement in the observed optical reflectivity change at the centre of a void on phononic resonance is likely to be caused by this mechanism. By comparison with numerical simulations of the vibrational field, we identify resonant breathing deformations of the voids and elucidate the corresponding mode shapes. We thus establish scanned optomechanical probing of periodic plasmonic-phononic structures as a new means of investigating their coupled excitations on the nanoscale.

  8. Raman selection rule for surface optical phonons in ZnS nanobelts

    NASA Astrophysics Data System (ADS)

    Ho, Chih-Hsiang; Varadhan, Purushothaman; Wang, Hsin-Hua; Chen, Cheng-Ying; Fang, Xiaosheng; He-Hau, Jr.

    2016-03-01

    We report Raman scattering results for high-quality wurtzite ZnS nanobelts (NBs) grown by chemical vapor deposition. In the Raman spectrum, the ensembles of ZnS NBs exhibit first order phonon modes at 274 cm-1 and 350 cm-1, corresponding to A1/E1 transverse optical and A1/E1 longitudinal optical phonons, in addition to a strong surface optical (SO) phonon mode at 329 cm-1. The existence of the SO band is confirmed by its shift with different surrounding dielectric media. Polarization dependent Raman spectra were recorded on a single ZnS NB and for the first time a SO phonon band has been detected on a single nanobelt. Different selection rules for the SO phonon mode are shown from their corresponding E1/A1 phonon modes, and were attributed to the breaking of anisotropic translational symmetry on the NB surface.

  9. Near-Field Infrared Pump-Probe Imaging of Surface Phonon Coupling in Boron Nitride Nanotubes.

    PubMed

    Gilburd, Leonid; Xu, Xiaoji G; Bando, Yoshio; Golberg, Dmitri; Walker, Gilbert C

    2016-01-21

    Surface phonon modes are lattice vibrational modes of a solid surface. Two common surface modes, called longitudinal and transverse optical modes, exhibit lattice vibration along or perpendicular to the direction of the wave. We report a two-color, infrared pump-infrared probe technique based on scattering type near-field optical microscopy (s-SNOM) to spatially resolve coupling between surface phonon modes. Spatially varying couplings between the longitudinal optical and surface phonon polariton modes of boron nitride nanotubes are observed, and a simple model is proposed.

  10. Photon-Phonon-Enhanced Infrared Rectification in a Two-Dimensional Nanoantenna-Coupled Tunnel Diode

    NASA Astrophysics Data System (ADS)

    Kadlec, Emil A.; Jarecki, Robert L.; Starbuck, Andrew; Peters, David W.; Davids, Paul S.

    2016-12-01

    The interplay of strong infrared photon-phonon coupling with electromagnetic confinement in nanoscale devices is demonstrated to have a large impact on ultrafast photon-assisted tunneling in metal-oxide-semiconductor (MOS) structures. Infrared active optical phonon modes in polar oxides lead to strong dispersion and enhanced electric fields at material interfaces. We find that the infrared dispersion of SiO2 near a longitudinal optical phonon mode can effectively impedance match a photonic surface mode into a nanoscale tunnel gap that results in large transverse-field confinement. An integrated 2D nanoantenna structure on a distributed large-area MOS tunnel-diode rectifier is designed and built to resonantly excite infrared surface modes and is shown to efficiently channel infrared radiation into nanometer-scale gaps in these MOS devices. This enhanced-gap transverse-electric field is converted to a rectified tunneling displacement current resulting in a dc photocurrent. We examine the angular and polarization-dependent spectral photocurrent response of these 2D nanoantenna-coupled tunnel diodes in the photon-enhanced tunneling spectral region. Our 2D nanoantenna-coupled infrared tunnel-diode rectifier promises to impact large-area thermal energy harvesting and infrared direct detectors.

  11. From Modal Mixing to Tunable Functional Switches in Nonlinear Phononic Crystals

    NASA Astrophysics Data System (ADS)

    Ganesh, R.; Gonella, S.

    2015-02-01

    We introduce a paradigm for spatial and modal wave manipulation based on nonlinear phononic crystals and explore its potential for engineering wave control systems with tunable, adaptive, and multifunctional characteristics. Our approach exploits nonlinear mechanisms to stretch the frequency signature of the wave response and distribute it over multiple modes, thereby activating a mixture of modal characteristics and enabling functionalities associated with high-frequency optical modes, even while operating in the low-frequency regime. To elucidate the versatility of this approach, we consider different granular crystal configurations that span the available landscape of crystal topologies and wave control functionalities. The ability to switch between complementary functionalities allows rethinking nonlinear phononic crystals as programmable acoustic ports that form the building blocks of a new structural logic framework enabled by nonlinearity.

  12. Actively mode-locked diode laser with a mode spacing stability of ∼6 × 10{sup -14}

    SciTech Connect

    Zakharyash, V F; Kashirsky, A V; Klementyev, V M

    2015-10-31

    We have studied mode spacing stability in an actively mode-locked external-cavity semiconductor laser. It has been shown that, in the case of mode spacing pulling to the frequency of a highly stable external microwave signal produced by a hydrogen standard (stability of 4 × 10{sup -14} over an averaging period τ = 10 s), this configuration ensures a mode spacing stability of 5.92 × 10{sup -14} (τ = 10 s). (control of radiation parameters)

  13. Electron-phonon interaction model and prediction of thermal energy transport in SOI transistor.

    PubMed

    Jin, Jae Sik; Lee, Joon Sik

    2007-11-01

    An electron-phonon interaction model is proposed and applied to thermal transport in semiconductors at micro/nanoscales. The high electron energy induced by the electric field in a transistor is transferred to the phonon system through electron-phonon interaction in the high field region of the transistor. Due to this fact, a hot spot occurs, which is much smaller than the phonon mean free path in the Si-layer. The full phonon dispersion model based on the Boltzmann transport equation (BTE) with the relaxation time approximation is applied for the interactions among different phonon branches and different phonon frequencies. The Joule heating by the electron-phonon scattering is modeled through the intervalley and intravalley processes for silicon by introducing average electron energy. The simulation results are compared with those obtained by the full phonon dispersion model which treats the electron-phonon scattering as a volumetric heat source. The comparison shows that the peak temperature in the hot spot region is considerably higher and more localized than the previous results. The thermal characteristics of each phonon mode are useful to explain the above phenomena. The optical mode phonons of negligible group velocity obtain the highest energy density from electrons, and resides in the hot spot region without any contribution to heat transport, which results in a higher temperature in that region. Since the acoustic phonons with low group velocity show the higher energy density after electron-phonon scattering, they induce more localized heating near the hot spot region. The ballistic features are strongly observed when phonon-phonon scattering rates are lower than 4 x 10(10) S(-1).

  14. Shear viscosity due to phonons in superfluid neutron stars

    NASA Astrophysics Data System (ADS)

    Manuel, Cristina; Tolos, Laura

    2011-12-01

    We compute the contribution of phonons to the shear viscosity η in superfluid neutron stars, assuming neutron pairing in a S01 channel. We use a Boltzmann equation amended by a collision term that takes into account the binary collisions of phonons. We use effective field theory techniques to extract the phonon scattering rates, written as a function of the equation of state of the system. Our formulation is rather general, and can be used to extract the shear viscosity due to binary collisions of phonons for other superfluids, such as the cold Fermi gas in the unitarity limit. We find that η∝1/T5, the proportionality factor depending on the equation of state of the system. Our results indicate that the phonon contribution to η cannot be ignored and might have relevant effects in the dynamics of the different oscillation modes of the star.

  15. Suppression of phonon transport in molecular Christmas trees.

    PubMed

    Lambert, Colin John; Famili, Marjan; Grace, Iain; Sadeghi, Hatef

    2017-02-27

    Minimising the phonon thermal conductance of self-assembled molecular films, whilst preserving their electrical properties, is highly desirable, both for thermal management at the nanoscale and for the design of high-efficiency thermoelectric materials. Here we highlight a new strategy for minimising the phonon thermal conductance of Christmas-tree-like molecules composed of a long trunk, along which phonons can propagate, attached to pendant molecular branches. We demonstrate that phonon transport along the trunk is suppressed by Fano resonances associated with internal vibrational modes of the branches and that thermal conductance is suppressed most-effectively in molecules with pendant branches of different lengths. As examples, we use density functional theory to demonstrate the reduction in phonon transport in tree-like molecules formed from alkane or acene trunks with various pendant branches.

  16. Phononic Crystal Tunable via Ferroelectric Phase Transition

    NASA Astrophysics Data System (ADS)

    Xu, Chaowei; Cai, Feiyan; Xie, Shuhong; Li, Fei; Sun, Rong; Fu, Xianzhu; Xiong, Rengen; Zhang, Yi; Zheng, Hairong; Li, Jiangyu

    2015-09-01

    Phononic crystals (PCs) consisting of periodic materials with different acoustic properties have potential applications in functional devices. To realize more smart functions, it is desirable to actively control the properties of PCs on demand, ideally within the same fabricated system. Here, we report a tunable PC made of Ba0.7Sr0.3Ti O3 (BST) ceramics, wherein a 20-K temperature change near room temperature results in a 20% frequency shift in the transmission spectra induced by a ferroelectric phase transition. The tunability phenomenon is attributed to the structure-induced resonant excitation of A0 and A1 Lamb modes that exist intrinsically in the uniform BST plate, while these Lamb modes are sensitive to the elastic properties of the plate and can be modulated by temperature in a BST plate around the Curie temperature. The study finds opportunities for creating tunable PCs and enables smart temperature-tuned devices such as the Lamb wave filter or sensor.

  17. Meditation leads to reduced default mode network activity beyond an active task.

    PubMed

    Garrison, Kathleen A; Zeffiro, Thomas A; Scheinost, Dustin; Constable, R Todd; Brewer, Judson A

    2015-09-01

    Meditation has been associated with relatively reduced activity in the default mode network, a brain network implicated in self-related thinking and mind wandering. However, previous imaging studies have typically compared meditation to rest, despite other studies having reported differences in brain activation patterns between meditators and controls at rest. Moreover, rest is associated with a range of brain activation patterns across individuals that has only recently begun to be better characterized. Therefore, in this study we compared meditation to another active cognitive task, both to replicate the findings that meditation is associated with relatively reduced default mode network activity and to extend these findings by testing whether default mode activity was reduced during meditation, beyond the typical reductions observed during effortful tasks. In addition, prior studies had used small groups, whereas in the present study we tested these hypotheses in a larger group. The results indicated that meditation is associated with reduced activations in the default mode network, relative to an active task, for meditators as compared to controls. Regions of the default mode network showing a Group × Task interaction included the posterior cingulate/precuneus and anterior cingulate cortex. These findings replicate and extend prior work indicating that the suppression of default mode processing may represent a central neural process in long-term meditation, and they suggest that meditation leads to relatively reduced default mode processing beyond that observed during another active cognitive task.

  18. Electron-phonon interaction and scattering in Si and Ge: Implications for phonon engineering

    SciTech Connect

    Tandon, Nandan; Albrecht, J. D.; Ram-Mohan, L. R.

    2015-07-28

    We report ab-initio results for electron-phonon (e-ph) coupling and display the existence of a large variation in the coupling parameter as a function of electron and phonon dispersion. This variation is observed for all phonon modes in Si and Ge, and we show this for representative cases where the initial electron states are at the band gap edges. Using these e-ph matrix elements, which include all possible phonon modes and electron bands within a relevant energy range, we evaluate the imaginary part of the electron self-energy in order to obtain the associated scattering rates. The temperature dependence is seen through calculations of the scattering rates at 0 K and 300 K. The results provide a basis for understanding the impacts of phonon scattering vs. orientation and geometry in the design of devices, and in analysis of transport phenomena. This provides an additional tool for engineering the transfer of energy from carriers to the lattice.

  19. Slow light and slow acoustic phonons in optophononic resonators

    NASA Astrophysics Data System (ADS)

    Villafañe, V.; Soubelet, P.; Bruchhausen, A. E.; Lanzillotti-Kimura, N. D.; Jusserand, B.; Lemaître, A.; Fainstein, A.

    2016-11-01

    Slow and confined light have been exploited in optoelectronics to enhance light-matter interactions. Here we describe the GaAs/AlAs semiconductor microcavity as a device that, depending on the excitation conditions, either confines or slows down both light and optically generated acoustic phonons. The localization of photons and phonons in the same place of space amplifies optomechanical processes. Picosecond laser pulses are used to study through time-resolved reflectivity experiments the coupling between photons and both confined and slow acoustic phonons when the laser is tuned either with the cavity (confined) optical mode or with the stop-band edge (slow) optical modes. A model that fully takes into account the modified propagation of the acoustic phonons and light in these resonant structures is used to describe the laser detuning dependence of the coherently generated phonon spectra and amplitude under these different modes of laser excitation. We observe that confined light couples only to confined mechanical vibrations, while slow light can generate both confined and slow coherent vibrations. A strong enhancement of the optomechanical coupling using confined photons and vibrations, and also with properly designed slow photon and phonon modes, is demonstrated. The prospects for the use of these optoelectronic devices in confined and slow optomechanics are addressed.

  20. Close correlation between magnetic properties and the soft phonon mode of the structural transition in BaFe2As2 and SrFe2As2

    SciTech Connect

    Parshall, D.; Pintschovius, L.; Niedziela, Jennifer L.; Castellan, J. -P.; Lamago, D.; Mittal, R.; Wolf, Th.; Reznik, Dmitry

    2015-04-27

    Parent compounds of Fe-based superconductors undergo a structural phase transition from a tetragonal to an orthorhombic structure. We investigated the temperature dependence of the frequencies of TA phonons that extrapolate to the shear vibrational mode at the zone center, which corresponds to the orthorhombic deformation of the crystal structure at low temperatures in BaFe2As2 and SrFe2As2. We found that acoustic phonons at small wave vectors soften gradually towards the transition from high temperatures, tracking the increase of the size of slowly fluctuating magnetic domains. On cooling below the transition to base temperature the phonons harden, following the square of the magnetic moment (which we find is proportional to the anisotropy gap). Finally, our results provide evidence for close correlation between magnetic and phonon properties in Fe-based superconductors.

  1. Phonon-lifetimes in demixing systems

    NASA Astrophysics Data System (ADS)

    Davaasambuu, J.; Güthoff, F.; Petri, M.; Hradil, K.; Schober, H.; Ollivier, J.; Eckold, G.

    2012-06-01

    The dynamics of silver-alkali halide mixed single crystals (AgxNa1-xBr, x = 0.23, 0.35, 0.40 and 0.70) were studied by inelastic neutron scattering during the process of spinodal decomposition. Using the thermal three-axes spectrometer PUMA as well as the time-of-flight spectrometer IN5, the time evolution of phonons was observed in time-resolved, stroboscopic measurements. Complementary to the study of long wavelength acoustic phonons, as studied previously, we extended these investigations to Brillouin-zone boundary modes that are particularly sensitive to variations of the local structure. Starting from the homogeneous mixed phase the behaviour of these modes during demixing is observed in real-time. A simple dynamical model based on local structure variants helps to interpret the results. It is shown that the phonon lifetimes vary strongly during the phase separation and increase drastically during the coarsening process. Up to a critical size of precipitates of about 10 nm, zone-boundary modes are found to be strongly damped, while beyond the line widths are reduced to the experimental resolution. This finding leads to the conclusion that the typical mean free path of these modes is of the order of 10 nm, which corresponds to 20 unit cells.

  2. Phonon Analysis in Multiphonon Transitions

    NASA Astrophysics Data System (ADS)

    Huang, Kun; Gu, Zongquan

    In the investigation of multiphonon transitions, single-mode or single-frequency models are widely used. In view of the fact that such oversimplified models can be seriously inadequate, the present work bridges the gap between the complexity of the general formal theory and the simplicity required for concrete applications by introducing the concept of multi-frequency models. That is, the theory is so formulated that a general system can be approximated by multi-frequency models of any degree of elaboration. A statistical thermodynamic formalism is developed for treating such multi-frequency models, which, on the one hand, greatly reduces the labour of calculation with such models and, on the other hand, leads directly to a simple statistical distribution law for numbers of phonons of each frequency participating in a multiphonon transition. Applications of the theory to concrete models lead to certain general conclusions on frequency dispersion effects in multiphonon transitions. The use of the theory is further demonstrated by fully accounting for the paradoxical experimental results reported by Jia and Yen that the isotopic substitution of H by D in CsMn Cl3· 2H2O reduces the multiphonon nonradiative transition probability of excited Mn2+ ion by more than ten-fold, and yet leaves the corresponding luminescence phonon sideband little changed. In the last section of the paper, the relation between the statistical thermodynamic formalism and existing multiphonon transition theory is elucidated, thereby the theoretical basis of the statistical formalism becomes clearly defined.

  3. Robust Phonon-Plasmon Coupling in Quasifreestanding Graphene on Silicon Carbide.

    PubMed

    Koch, R J; Fryska, S; Ostler, M; Endlich, M; Speck, F; Hänsel, T; Schaefer, J A; Seyller, Th

    2016-03-11

    Using inelastic electron scattering in combination with dielectric theory simulations on differently prepared graphene layers on silicon carbide, we demonstrate that the coupling between the 2D plasmon of graphene and the surface optical phonon of the substrate cannot be quenched by modification of the interface via intercalation. The intercalation rather provides additional modes like, e.g., the silicon-hydrogen stretch mode in the case of hydrogen intercalation or the silicon-oxygen vibrations for water intercalation that couple to the 2D plasmons of graphene. Furthermore, in the case of bilayer graphene with broken inversion symmetry due to charge imbalance between the layers, we observe a similar coupling of the 2D plasmon to an internal infrared-active mode, the LO phonon mode. The coupling of graphene plasmons to vibrational modes of the substrate surface and internal infrared active modes is envisioned to provide an excellent tool for tailoring the plasmon band structure of monolayer and bilayer graphene for plasmonic devices such as plasmon filters or plasmonic waveguides. The rigidity of the effect furthermore suggests that it may be of importance for other 2D materials as well.

  4. Molecular Detection and Phonon Filtering in Heat-Transfer Spectroscopy

    NASA Astrophysics Data System (ADS)

    Walczak, Kamil; Yerkes, Kirk

    2014-03-01

    We examine heat transport carried by acoustic phonons in the systems composed of nanoscale chains of masses coupled to two thermal baths of different temperatures. Thermal conductance is obtained by using linearized Landauer formula for heat flux with phonon transmission probability calculated within atomistic Green's functions (AGF) method. AGF formalism is extended onto dissipative chains of masses with harmonic coupling beyond nearest-neighbor approximation, while atomistic description of heat reservoirs is also included into computational scheme. The resonant structure of phonon transmission spectrum is analyzed with respect to reservoir-dimensionality effects, molecular damping, and mass-to-mass harmonic coupling. Analysis of transmission zeros (antiresonances) and their accompanied Fano-shape resonances are discussed as a result of interference effects between different vibrational modes. Specifically, we show that the heat-transfer-based characterization method may be used to identify individual molecules or filter out specific phonon modes from the whole frequency spectrum. This work is supported by AFOSR grant.

  5. Preface: Phonons 2007

    NASA Astrophysics Data System (ADS)

    Perrin, Bernard

    2007-06-01

    Conference logo The conference PHONONS 2007 was held 15-20 July 2007 in the Conservatoire National des Arts et Métiers (CNAM) Paris, France. CNAM is a college of higher technology for training students in the application of science to industry, founded by Henri Grégoire in 1794. This was the 12th International Conference on Phonon Scattering in Condensed Matter. This international conference series, held every 3 years, started in France at Sainte-Maxime in 1972. It was then followed by meetings at Nottingham (1975), Providence (1979), Stuttgart (1983), Urbana-Champaign (1986), Heidelberg (1989), Ithaca (1992), Sapporo (1995), Lancaster (1998), Dartmouth (2001) and St Petersburg (2004). PHONONS 2007 was attended by 346 delegates from 37 different countries as follows: France 120, Japan 45, Germany 25, USA 25, Russia 21, Italy 13, Poland 9, UK 9, Canada 7, The Netherlands 7, Finland 6, Spain 6, Taiwan 6, Greece 4, India 4, Israel 4, Ukraine 4, Serbia 3, South Africa 3, Argentina 2, Belgium 2, China 2, Iran 2, Korea 2, Romania 2, Switzerland 2, and one each from Belarus, Bosnia-Herzegovina, Brazil, Bulgaria, Egypt, Estonia, Mexico, Moldova, Morocco, Saudi Arabia, Turkey. There were 5 plenary lectures, 14 invited talks and 84 oral contributions; 225 posters were presented during three poster sessions. The first plenary lecture was given by H J Maris who presented fascinating movies featuring the motion of a single electron in liquid helium. Robert Blick gave us a review on the new possibilities afforded by nanotechnology to design nano-electomechanical systems (NEMS) and the way to use them to study elementary and fundamental processes. The growing interest for phonon transport studies in nanostructured materials was demonstrated by Arun Majumdar. Andrey Akimov described how ultrafast acoustic solitons can monitor the optical properties of quantum wells. Finally, Maurice Chapellier told us how

  6. Phonon Dispersion in Chiral Single-Wall Carbon Nanotubes

    NASA Astrophysics Data System (ADS)

    Mu, Weihua; Vamivakas, Anthony Nickolas; Fang, Yan; Wang, Bolin

    The method to obtain phonon dispersion of achiral single-wall carbon nanotubes (SWNTs) from 6×6 matrix proposed by Mahan and Jeon7 has been extended to chiral SWNTs. The number of calculated phonon modes of a chiral SWNT (10, 1) is much larger than that of a zigzag one (10, 0) because the number of atoms in the translational unit cell of chiral SWNT is larger than that of an achiral one even though they have relative similar radius. The possible application of our approach to other models with more phonon potential terms beyond Mahan and Jeon's model is discussed.

  7. Phonon anharmonicity and components of the entropy in palladium and platinum

    NASA Astrophysics Data System (ADS)

    Shen, Yang; Li, Chen W.; Tang, Xiaoli; Smith, Hillary L.; Fultz, B.

    2016-06-01

    Inelastic neutron scattering was used to measure the phonon density of states in fcc palladium and platinum metal at temperatures from 7 K to 1576 K. Both phonon-phonon interactions and electron-phonon interactions were calculated by methods based on density functional theory (DFT) and were consistent with the measured shifts and broadenings of phonons with temperature. Unlike the longitudinal modes, the characteristic transverse modes had a nonlinear dependence on temperature owing to the requirement for a population of thermal phonons for upscattering. Kohn anomalies were observed in the measurements at low temperature and were reproduced by calculations based on DFT. Contributions to the entropy from phonons and electrons were assessed and summed to obtain excellent agreement with prior calorimetric data. The entropy from thermal expansion is positive for both phonons and electrons but larger for phonons. The anharmonic phonon entropy is negative in Pt, but in Pd it changes from positive to negative with increasing temperature. Owing to the position of the Fermi level on the electronic DOS, the electronic entropy was sensitive to the adiabatic electron-phonon interaction in both Pd and Pt. The adiabatic EPI depended strongly on thermal atom displacements.

  8. Phonon waveguides for electromechanical circuits.

    PubMed

    Hatanaka, D; Mahboob, I; Onomitsu, K; Yamaguchi, H

    2014-07-01

    Nanoelectromechanical systems (NEMS), utilizing localized mechanical vibrations, have found application in sensors, signal processors and in the study of macroscopic quantum mechanics. The integration of multiple mechanical elements via electrical or optical means remains a challenge in the realization of NEMS circuits. Here, we develop a phonon waveguide using a one-dimensional array of suspended membranes that offers purely mechanical means to integrate isolated NEMS resonators. We demonstrate that the phonon waveguide can support and guide mechanical vibrations and that the periodic membrane arrangement also creates a phonon bandgap that enables control of the phonon propagation velocity. Furthermore, embedding a phonon cavity into the phonon waveguide allows mobile mechanical vibrations to be dynamically switched or transferred from the waveguide to the cavity, thereby illustrating the viability of waveguide-resonator coupling. These highly functional traits of the phonon waveguide architecture exhibit all the components necessary to permit the realization of all-phononic NEMS circuits.

  9. Surface Phonons and Polaritons.

    DTIC Science & Technology

    1976-01-01

    for an impurity in the surface of a crystal could be observed in the one phonon cross section for the resonant absorption or e.ission of ,—rays by...localized at the surface. The w5 — dependence has a simple physical origin. It is well known that the cross section for scattering of bulk phonons by a...propagate. In Section II of the present Chapter we present the theory underlying the surface induced vibrational properties of crystals which we have

  10. Active/passive mode-locked laser oscillator

    DOEpatents

    Fountain, William D.; Johnson, Bertram C.

    1977-01-01

    A Q-switched/mode-locked Nd:YAG laser oscillator employing simultaneous active (electro-optic) and passive (saturable absorber) loss modulation within the optical cavity is described. This "dual modulation" oscillator can produce transform-limited pulses of duration ranging from about 30 psec to about 5 nsec with greatly improved stability compared to other mode-locked systems. The pulses produced by this system lack intrapulse frequency or amplitude modulation, and hence are idealy suited for amplification to high energies and for other applications where well-defined pulses are required. Also, the pulses of this system have excellent interpulse characteristics, wherein the optical noise between the individual pulses of the pulse train has a power level well below the power of the peak pulse of the train.

  11. Active flat optics using a guided mode resonance.

    PubMed

    Kim, Soo Jin; Brongersma, Mark L

    2017-01-01

    Dynamically-controlled flat optics relies on achieving active and effective control over light-matter interaction in ultrathin layers. A variety of metasurface designs have achieved efficient amplitude and phase modulation. Particularly, noteworthy progress has been made with the incorporation of newly emerging electro-optical materials into such metasurfaces, including graphene, phase change materials, and transparent conductive oxides. In this Letter, we demonstrate dynamic light-matter interaction in a silicon-based subwavelength grating that supports a guided mode resonance. By overcoating the grating with indium tin oxide as an electrically tunable material, its reflectance can be tuned from 4% to 86%. Guided mode resonances naturally afford higher optical quality factors than the optical antennas used in the construction of metasurfaces. As such, they facilitate more effective control over the flow of light within the same layer thickness.

  12. Classification of topological phonons in linear mechanical metamaterials

    PubMed Central

    Süsstrunk, Roman

    2016-01-01

    Topological phononic crystals, alike their electronic counterparts, are characterized by a bulk–edge correspondence where the interior of a material dictates the existence of stable surface or boundary modes. In the mechanical setup, such surface modes can be used for various applications such as wave guiding, vibration isolation, or the design of static properties such as stable floppy modes where parts of a system move freely. Here, we provide a classification scheme of topological phonons based on local symmetries. We import and adapt the classification of noninteracting electron systems and embed it into the mechanical setup. Moreover, we provide an extensive set of examples that illustrate our scheme and can be used to generate models in unexplored symmetry classes. Our work unifies the vast recent literature on topological phonons and paves the way to future applications of topological surface modes in mechanical metamaterials. PMID:27482105

  13. Photons, phonons, and plasmons with orbital angular momentum in plasmas

    DOE PAGES

    Chen, Qiang; Qin, Hong; Liu, Jian

    2017-02-06

    Exact eigen modes with orbital angular momentum (OAM) in the complex media of unmagnetized homogeneous plasmas are studied. Three exact eigen modes with OAM are derived, i.e., photons, phonons, and plasmons. The OAM of different plasma components are closely related to the charge polarities. For photons, the OAM of electrons and ions are of the same magnitude but opposite direction, and the total OAM is carried by the field. For the phonons and plasmons, their OAM are carried by the electrons and ions. Lastly, the OAM modes in plasmas and their characteristics can be explored for potential applications in plasmamore » physics and accelerator physics.« less

  14. Photons, phonons, and plasmons with orbital angular momentum in plasmas

    PubMed Central

    Chen, Qiang; Qin, Hong; Liu, Jian

    2017-01-01

    Exact eigen modes with orbital angular momentum (OAM) in the complex media of unmagnetized homogeneous plasmas are studied. Three exact eigen modes with OAM are derived, i.e., photons, phonons, and plasmons. The OAM of different plasma components are closely related to the charge polarities. For photons, the OAM of electrons and ions are of the same magnitude but opposite direction, and the total OAM is carried by the field. For the phonons and plasmons, their OAM are carried by the electrons and ions. The OAM modes in plasmas and their characteristics can be explored for potential applications in plasma physics and accelerator physics. PMID:28164998

  15. Photons, phonons, and plasmons with orbital angular momentum in plasmas

    NASA Astrophysics Data System (ADS)

    Chen, Qiang; Qin, Hong; Liu, Jian

    2017-02-01

    Exact eigen modes with orbital angular momentum (OAM) in the complex media of unmagnetized homogeneous plasmas are studied. Three exact eigen modes with OAM are derived, i.e., photons, phonons, and plasmons. The OAM of different plasma components are closely related to the charge polarities. For photons, the OAM of electrons and ions are of the same magnitude but opposite direction, and the total OAM is carried by the field. For the phonons and plasmons, their OAM are carried by the electrons and ions. The OAM modes in plasmas and their characteristics can be explored for potential applications in plasma physics and accelerator physics.

  16. Enhanced electron-phonon coupling in graphene with periodically distorted lattice

    NASA Astrophysics Data System (ADS)

    Pomarico, E.; Mitrano, M.; Bromberger, H.; Sentef, M. A.; Al-Temimy, A.; Coletti, C.; Stöhr, A.; Link, S.; Starke, U.; Cacho, C.; Chapman, R.; Springate, E.; Cavalleri, A.; Gierz, I.

    2017-01-01

    Electron-phonon coupling directly determines the stability of cooperative order in solids, including superconductivity, charge, and spin density waves. Therefore, the ability to enhance or reduce electron-phonon coupling by optical driving may open up new possibilities to steer materials' functionalities, potentially at high speeds. Here, we explore the response of bilayer graphene to dynamical modulation of the lattice, achieved by driving optically active in-plane bond stretching vibrations with femtosecond midinfrared pulses. The driven state is studied by two different ultrafast spectroscopic techniques. First, terahertz time-domain spectroscopy reveals that the Drude scattering rate decreases upon driving. Second, the relaxation rate of hot quasiparticles, as measured by time- and angle-resolved photoemission spectroscopy, increases. These two independent observations are quantitatively consistent with one another and can be explained by a transient threefold enhancement of the electron-phonon coupling constant. The findings reported here provide useful perspective for related experiments, which reported the enhancement of superconductivity in alkali-doped fullerites when a similar phonon mode was driven.

  17. Interaction of optical and interface phonons and their anisotropy in GaAs/AlAs superlattices: Experiment and calculations

    SciTech Connect

    Volodin, V. A.; Sachkov, V. A.; Sinyukov, M. P.

    2015-05-15

    The angular anisotropy of interface phonons and their interaction with optical phonons in (001) GaAs/AlAs superlattices are calculated and experimentally studied. Experiments were performed by Raman light scattering in different scattering geometries for phonons with the wave vector directed normally to the superlattice and along its layers. Phonon frequencies were calculated by the extended Born method taking the Coulomb interaction into account in the rigid-ion approximation. Raman scattering spectra were calculated in the Volkenshtein bond-polarizability approximation. Calculations confirmed that the angular anisotropy of phonons observed in experiments appears due to interaction (mixing) of optical phonons, in which atoms are mainly displaced normally to superlattices, with interface phonons (TO-IF modes). In the scattering geometry, when the wave vector lies in the plane of superlattice layers, the mixed TO-IF modes are observed under nonresonance conditions. The Raman spectra for TO-IF modes depend on the mixing of atoms at heteroboundaries.

  18. ThermoPhonon

    SciTech Connect

    Zarkevich, Nikolai

    2014-11-24

    ThermoPhonon is a stand-alone code, which can be integrated into other software packages. Typically, it is used together with a density functional theory (DFT) code (such as VASP, Wien2k, AbInit, SIESTA) and a phonon code (such as Phonopy or Phon). The workflow is the following. Molecular dynamics (MD) in a supercell at a given temperature T is performed using another code. After sufficient equilibration, the output in the form of atomic positions and forces for a large number of selected MD steps is recorded into a file. If needed, one can modify this file by applying additional constraints, such as enforced crystal symmetry or subtracted motion of the center of mass. ThermoPhonon reads the file with atomic positions and forces and writes a new file with the force constants. Force constants can be used by another code (such as Phonopy or Phon) to produce phonon spectrum for plotting, in the assumption of known equilibrium atomic positions provided in a separate file.

  19. Phonons, Atoms, and Waves

    ERIC Educational Resources Information Center

    Reid, John S.

    1977-01-01

    Discussed are how the thermal vibrations of a solid are described in terms of lattice waves, how these waves interact with other waves, or with themselves, and how one is led from such a description in terms of waves to the concept of a phonon. (Author/MA)

  20. Temperature Dependence of Brillouin Light Scattering Spectra of Acoustic Phonons in Silicon

    NASA Astrophysics Data System (ADS)

    Somerville, Kevin; Klimovich, Nikita; An, Kyongmo; Sullivan, Sean; Weathers, Annie; Shi, Li; Li, Xiaoqin

    2015-03-01

    Thermal management represents an outstanding challenge in many areas of technology. Electrons, optical phonons, and acoustic phonons are often driven out of local equilibrium in electronic devices or during laser-material interaction processes. Interest in non-equilibrium transport processes has motivated the development of Raman spectroscopy as a local temperature sensor of optical phonons and intermediate frequency acoustic phonons, whereas Brillouin light scattering (BLS) has recently been explored as a temperature sensor of low-frequency acoustic phonons. Here, we report temperature dependent BLS spectra of silicon, with Raman spectra taken simultaneously for comparison. The origins of the observed temperature dependence of the BLS peak position, linewidth, and intensity are examined in order to evaluate their potential use as temperature sensors for acoustic phonons. We determine that the integrated BLS intensity can be used measure the temperature of specific acoustic phonon modes. This work is supported by National Science Foundation (NSF) Thermal Transport Processes Program under Grant CBET-1336968.

  1. Magnetic relaxations arising from spin-phonon interactions in the nonthermally activated temperature range for a double-decker terbium phthalocyanine single molecule magnet.

    PubMed

    Fukuda, Takamitsu; Shigeyoshi, Natsuko; Yamamura, Tomoo; Ishikawa, Naoto

    2014-09-02

    Magnetic relaxations arising from spin-phonon interactions for a magnetically diluted double-decker terbium phthalocyanine single molecule magnet, dil1, in the nonthermally activated temperature range have been investigated. While the relaxation time, τ, is independent of the external static magnetic field, H(dc), in the high temperature range, where linear relationships between -ln τ and T(-1) are observed in the Arrhenius plot, magnetic field dependences for τ are observed in the lower temperature range. The τ(-1) vs H(dc) plot at 12 K fits the quadric curve when H(dc) < 12 kOe, while linear relationships are observed in the τ(-1) vs T plots in the temperature range of 12-20 K. These results indicate that the direct process is the dominant magnetic relaxation pathway in the nonthermally activated temperature range, while the contribution from the Raman process, if any, is not observable. We emphasize in this paper that the contribution from the thermal relaxation processes and the quantum tunneling of magnetizations (QTMs) to the experimentally observed magnetic relaxations must be evaluated carefully in order to avoid confusion between the thermal and quantum-mechanical relaxation pathways.

  2. Phonon properties of americium phosphide

    NASA Astrophysics Data System (ADS)

    Arya, B. S.; Aynyas, Mahendra; Sanyal, S. P.

    2016-05-01

    Phonon properties of AmP have been studied by using breathing shell models (BSM) which includes breathing motion of electrons of the Am atoms due to f-d hybridization. The phonon dispersion curves, specific heat calculated from present model. The calculated phonon dispersion curves of AmP are presented follow the same trend as observed in uranium phosphide. We discuss the significance of this approach in predicting the phonon dispersion curves of these compounds and examine the role of electron-phonon interaction.

  3. Environmental stability of actively mode locked fibre lasers

    NASA Astrophysics Data System (ADS)

    Hill, Calum H.; Lee, Stephen T.; Reid, Derryck T.; Baili, Ghaya; Davies, John

    2016-10-01

    Lasers developed for defence related applications typically encounter issues with reliability and meeting desired specification when taken from the lab to the product line. In particular the harsh environmental conditions a laser has to endure can lead to difficulties. This paper examines a specific class of laser, namely actively mode-locked fibre lasers (AMLFLs), and discusses the impact of environmental perturbations. Theoretical and experimental results have assisted in developing techniques to improve the stability of a mode-locked pulse train for continuous operation. Many of the lessons learned in this research are applicable to a much broader category of lasers. The AMLFL consists of a fibre ring cavity containing a semiconductor optical amplifier (SOA), an isolator, an output coupler, a circulator, a bandpass filter and a modulator. The laser produces a train of 6-ps pulses at 800 nm with a repetition rate in the GHz regime and a low-noise profile. This performance is realisable in a laboratory environment. However, even small changes in temperature on the order of 0.1 °C can cause a collapse of mode-locked dynamics such that the required stability cannot be achieved without suitable feedback. Investigations into the root causes of this failure were performed by changing the temperature of components that constitute the laser resonator and observing their properties. Several different feedback mechanisms have been investigated to improve laser stability in an environment with dynamic temperature changes. Active cavity length control will be discussed along with DC bias control of the Mach-Zehnder modulator (MZM).

  4. Variable-Range Hopping through Marginally Localized Phonons

    NASA Astrophysics Data System (ADS)

    Banerjee, Sumilan; Altman, Ehud

    2016-03-01

    We investigate the effect of coupling Anderson localized particles in one dimension to a system of marginally localized phonons having a symmetry protected delocalized mode at zero frequency. This situation is naturally realized for electrons coupled to phonons in a disordered nanowire as well as for ultracold fermions coupled to phonons of a superfluid in a one-dimensional disordered trap. To determine if the coupled system can be many-body localized we analyze the phonon-mediated hopping transport for both the weak and strong coupling regimes. We show that the usual variable-range hopping mechanism involving a low-order phonon process is ineffective at low temperature due to discreteness of the bath at the required energy. Instead, the system thermalizes through a many-body process involving exchange of a diverging number n ∝-log T of phonons in the low temperature limit. This effect leads to a highly singular prefactor to Mott's well-known formula and strongly suppresses the variable range hopping rate. Finally, we comment on possible implications of this physics in higher dimensional electron-phonon coupled systems.

  5. Variable-Range Hopping through Marginally Localized Phonons.

    PubMed

    Banerjee, Sumilan; Altman, Ehud

    2016-03-18

    We investigate the effect of coupling Anderson localized particles in one dimension to a system of marginally localized phonons having a symmetry protected delocalized mode at zero frequency. This situation is naturally realized for electrons coupled to phonons in a disordered nanowire as well as for ultracold fermions coupled to phonons of a superfluid in a one-dimensional disordered trap. To determine if the coupled system can be many-body localized we analyze the phonon-mediated hopping transport for both the weak and strong coupling regimes. We show that the usual variable-range hopping mechanism involving a low-order phonon process is ineffective at low temperature due to discreteness of the bath at the required energy. Instead, the system thermalizes through a many-body process involving exchange of a diverging number n∝-logT of phonons in the low temperature limit. This effect leads to a highly singular prefactor to Mott's well-known formula and strongly suppresses the variable range hopping rate. Finally, we comment on possible implications of this physics in higher dimensional electron-phonon coupled systems.

  6. Evidence for strong electron-phonon interaction from inelastic tunneling of Cooper pairs in c-direction in Bi 2Sr 2CaCu 2O 8 break junctions

    NASA Astrophysics Data System (ADS)

    Ponomarev, Ya. G.; Tsokur, E. B.; Sudakova, M. V.; Tchesnokov, S. N.; Shabalin, M. E.; Lorenz, M. A.; Hein, M. A.; Müller, G.; Piel, H.; Aminov, B. A.

    1999-07-01

    A reproducible fine structure at subgap voltages in the I( U)-characteristics of Bi 2Sr 2CaCu 2O 8 break junctions has been observed and investigated. The structure is detectable only in the presence of an a.c. Josephson current. The position of the dips, composing the structure in the d I/d U-characteristics, is independent of the gap parameter Δ, the temperature T and the geometry of the contacts. The overall form of the fine structure is in good agreement with the Raman scattering spectra of the phonon modes in this material. We attribute this structure to an inelastic (phonon assisted) tunneling of Cooper pairs, which is accompanied by the emission of coherent Raman-active optical phonons at resonance voltages Ures=ℏ ωphon/2 e. These results hint for strong electron-phonon interaction in this material.

  7. High temperature phonon dispersion in graphene using classical molecular dynamics

    SciTech Connect

    Anees, P. Panigrahi, B. K.; Valsakumar, M. C.

    2014-04-24

    Phonon dispersion and phonon density of states of graphene are calculated using classical molecular dynamics simulations. In this method, the dynamical matrix is constructed based on linear response theory by computing the displacement of atoms during the simulations. The computed phonon dispersions show excellent agreement with experiments. The simulations are done in both NVT and NPT ensembles at 300 K and found that the LO/TO modes are getting hardened at the Γ point. The NPT ensemble simulations capture the anharmonicity of the crystal accurately and the hardening of LO/TO modes is more pronounced. We also found that at 300 K the C-C bond length reduces below the equilibrium value and the ZA bending mode frequency becomes imaginary close to Γ along K-Γ direction, which indicates instability of the flat 2D graphene sheets.

  8. Phonovoltaic. I. Harvesting hot optical phonons in a nanoscale p -n junction

    NASA Astrophysics Data System (ADS)

    Melnick, Corey; Kaviany, Massoud

    2016-03-01

    The phonovoltaic (pV) cell is similar to the photovoltaic. It harvests nonequilibrium (hot) optical phonons (Ep ,O) more energetic than the band gap (Δ Ee ,g) to generate power in a p-n junction. We examine the theoretical electron-phonon and phonon-phonon scattering rates, the Boltzmann transport of electrons, and the diode equation and hydrodynamic simulations to describe the operation of a pV cell and develop an analytic model predicting its efficiency. Our findings indicate that a pV material with Ep ,O≃Δ Ee ,g≫kBT , where kBT is the thermal energy, and a strong interband electron-phonon coupling surpasses the thermoelectric limit, provided the optical phonon population is excited in a nanoscale cell, enabling the ensuing local nonequilibrium. Finding and tuning a material with these properties is challenging. In Paper II [C. Melnick and M. Kaviany, Phys. Rev. B 93, 125203 (2016), 10.1103/PhysRevB.93.125203], we tune the band gap of graphite within density functional theory through hydrogenation and the application of isotropic strains. The band gap is tuned to resonate with its energetic optical phonon modes and calculate the ab initio electron-phonon and phonon-phonon scattering rates. While hydrogenation degrades the strong electron-phonon coupling in graphene such that the figure of merit vanishes, we outline the methodology for a continued material search.

  9. Acoustic-optical phonon up-conversion and hot-phonon bottleneck in lead-halide perovskites

    PubMed Central

    Yang, Jianfeng; Wen, Xiaoming; Xia, Hongze; Sheng, Rui; Ma, Qingshan; Kim, Jincheol; Tapping, Patrick; Harada, Takaaki; Kee, Tak W.; Huang, Fuzhi; Cheng, Yi-Bing; Green, Martin; Ho-Baillie, Anita; Huang, Shujuan; Shrestha, Santosh; Patterson, Robert; Conibeer, Gavin

    2017-01-01

    The hot-phonon bottleneck effect in lead-halide perovskites (APbX3) prolongs the cooling period of hot charge carriers, an effect that could be used in the next-generation photovoltaics devices. Using ultrafast optical characterization and first-principle calculations, four kinds of lead-halide perovskites (A=FA+/MA+/Cs+, X=I−/Br−) are compared in this study to reveal the carrier-phonon dynamics within. Here we show a stronger phonon bottleneck effect in hybrid perovskites than in their inorganic counterparts. Compared with the caesium-based system, a 10 times slower carrier-phonon relaxation rate is observed in FAPbI3. The up-conversion of low-energy phonons is proposed to be responsible for the bottleneck effect. The presence of organic cations introduces overlapping phonon branches and facilitates the up-transition of low-energy modes. The blocking of phonon propagation associated with an ultralow thermal conductivity of the material also increases the overall up-conversion efficiency. This result also suggests a new and general method for achieving long-lived hot carriers in materials. PMID:28106061

  10. Acoustic-optical phonon up-conversion and hot-phonon bottleneck in lead-halide perovskites.

    PubMed

    Yang, Jianfeng; Wen, Xiaoming; Xia, Hongze; Sheng, Rui; Ma, Qingshan; Kim, Jincheol; Tapping, Patrick; Harada, Takaaki; Kee, Tak W; Huang, Fuzhi; Cheng, Yi-Bing; Green, Martin; Ho-Baillie, Anita; Huang, Shujuan; Shrestha, Santosh; Patterson, Robert; Conibeer, Gavin

    2017-01-20

    The hot-phonon bottleneck effect in lead-halide perovskites (APbX3) prolongs the cooling period of hot charge carriers, an effect that could be used in the next-generation photovoltaics devices. Using ultrafast optical characterization and first-principle calculations, four kinds of lead-halide perovskites (A=FA(+)/MA(+)/Cs(+), X=I(-)/Br(-)) are compared in this study to reveal the carrier-phonon dynamics within. Here we show a stronger phonon bottleneck effect in hybrid perovskites than in their inorganic counterparts. Compared with the caesium-based system, a 10 times slower carrier-phonon relaxation rate is observed in FAPbI3. The up-conversion of low-energy phonons is proposed to be responsible for the bottleneck effect. The presence of organic cations introduces overlapping phonon branches and facilitates the up-transition of low-energy modes. The blocking of phonon propagation associated with an ultralow thermal conductivity of the material also increases the overall up-conversion efficiency. This result also suggests a new and general method for achieving long-lived hot carriers in materials.

  11. Acoustic-optical phonon up-conversion and hot-phonon bottleneck in lead-halide perovskites

    NASA Astrophysics Data System (ADS)

    Yang, Jianfeng; Wen, Xiaoming; Xia, Hongze; Sheng, Rui; Ma, Qingshan; Kim, Jincheol; Tapping, Patrick; Harada, Takaaki; Kee, Tak W.; Huang, Fuzhi; Cheng, Yi-Bing; Green, Martin; Ho-Baillie, Anita; Huang, Shujuan; Shrestha, Santosh; Patterson, Robert; Conibeer, Gavin

    2017-01-01

    The hot-phonon bottleneck effect in lead-halide perovskites (APbX3) prolongs the cooling period of hot charge carriers, an effect that could be used in the next-generation photovoltaics devices. Using ultrafast optical characterization and first-principle calculations, four kinds of lead-halide perovskites (A=FA+/MA+/Cs+, X=I-/Br-) are compared in this study to reveal the carrier-phonon dynamics within. Here we show a stronger phonon bottleneck effect in hybrid perovskites than in their inorganic counterparts. Compared with the caesium-based system, a 10 times slower carrier-phonon relaxation rate is observed in FAPbI3. The up-conversion of low-energy phonons is proposed to be responsible for the bottleneck effect. The presence of organic cations introduces overlapping phonon branches and facilitates the up-transition of low-energy modes. The blocking of phonon propagation associated with an ultralow thermal conductivity of the material also increases the overall up-conversion efficiency. This result also suggests a new and general method for achieving long-lived hot carriers in materials.

  12. Significant reduction of lattice thermal conductivity due to phonon confinement in graphene nanoribbons

    NASA Astrophysics Data System (ADS)

    Nissimagoudar, A. S.; Sankeshwar, N. S.

    2014-06-01

    Lattice thermal conductivity, κp, of suspended and supported graphene nanoribbons (GNRs) is studied over a wide temperature range, taking into account the dispersive nature of confined acoustic phonon modes. Employing a modified Callaway model, an expression for κp is developed, considering the explicit contributions from in-plane longitudinal, transverse, and torsional acoustic, and out-of-plane flexural acoustic phonon modes. Numerical calculations of κp(T) are presented assuming the confined acoustic phonons to be scattered by sample boundaries, impurities, and other phonons via both normal and umklapp processes. The effect of phonon confinement is to modify the phonon group velocities and the temperature dependence of κp. In a suspended 5-nm-wide GNR at room temperature, a decrease in κp by ˜70% is predicted. Our study brings out the relative importance of the contributing phonon modes and reveals the influence of flexural phonons on κp as a marked shoulder at low temperatures. The role of the various sample-dependent scattering mechanisms is examined. The substrate, in supported GNRs, is shown to curtail the phonon mean free path and suppress the low-temperature κp. Our results are in good agreement with recent experimental data of Bae et al. [M. H. Bae, Z. Li, Z. Aksamija, P. N. Martin, F. Xiong, Z. Y. Ong, I. Knezevic, and E. Pop, Nat. Commun. 4, 1734 (2013), 10.1038/ncomms2755] for supported GNRs.

  13. Platelet Serotonin Transporter Function Predicts Default-Mode Network Activity

    PubMed Central

    Kasess, Christian H.; Meyer, Bernhard M.; Hofmaier, Tina; Diers, Kersten; Bartova, Lucie; Pail, Gerald; Huf, Wolfgang; Uzelac, Zeljko; Hartinger, Beate; Kalcher, Klaudius; Perkmann, Thomas; Haslacher, Helmuth; Meyer-Lindenberg, Andreas; Kasper, Siegfried; Freissmuth, Michael; Windischberger, Christian; Willeit, Matthäus; Lanzenberger, Rupert; Esterbauer, Harald; Brocke, Burkhard; Moser, Ewald; Sitte, Harald H.; Pezawas, Lukas

    2014-01-01

    Background The serotonin transporter (5-HTT) is abundantly expressed in humans by the serotonin transporter gene SLC6A4 and removes serotonin (5-HT) from extracellular space. A blood-brain relationship between platelet and synaptosomal 5-HT reuptake has been suggested, but it is unknown today, if platelet 5-HT uptake can predict neural activation of human brain networks that are known to be under serotonergic influence. Methods A functional magnetic resonance study was performed in 48 healthy subjects and maximal 5-HT uptake velocity (Vmax) was assessed in blood platelets. We used a mixed-effects multilevel analysis technique (MEMA) to test for linear relationships between whole-brain, blood-oxygen-level dependent (BOLD) activity and platelet Vmax. Results The present study demonstrates that increases in platelet Vmax significantly predict default-mode network (DMN) suppression in healthy subjects independent of genetic variation within SLC6A4. Furthermore, functional connectivity analyses indicate that platelet Vmax is related to global DMN activation and not intrinsic DMN connectivity. Conclusion This study provides evidence that platelet Vmax predicts global DMN activation changes in healthy subjects. Given previous reports on platelet-synaptosomal Vmax coupling, results further suggest an important role of neuronal 5-HT reuptake in DMN regulation. PMID:24667541

  14. Surface phononic graphene

    NASA Astrophysics Data System (ADS)

    Yu, Si-Yuan; Sun, Xiao-Chen; Ni, Xu; Wang, Qing; Yan, Xue-Jun; He, Cheng; Liu, Xiao-Ping; Feng, Liang; Lu, Ming-Hui; Chen, Yan-Feng

    2016-12-01

    Strategic manipulation of wave and particle transport in various media is the key driving force for modern information processing and communication. In a strongly scattering medium, waves and particles exhibit versatile transport characteristics such as localization, tunnelling with exponential decay, ballistic, and diffusion behaviours due to dynamical multiple scattering from strong scatters or impurities. Recent investigations of graphene have offered a unique approach, from a quantum point of view, to design the dispersion of electrons on demand, enabling relativistic massless Dirac quasiparticles, and thus inducing low-loss transport either ballistically or diffusively. Here, we report an experimental demonstration of an artificial phononic graphene tailored for surface phonons on a LiNbO3 integrated platform. The system exhibits Dirac quasiparticle-like transport, that is, pseudo-diffusion at the Dirac point, which gives rise to a thickness-independent temporal beating for transmitted pulses, an analogue of Zitterbewegung effects. The demonstrated fully integrated artificial phononic graphene platform here constitutes a step towards on-chip quantum simulators of graphene and unique monolithic electro-acoustic integrated circuits.

  15. Surface phononic graphene.

    PubMed

    Yu, Si-Yuan; Sun, Xiao-Chen; Ni, Xu; Wang, Qing; Yan, Xue-Jun; He, Cheng; Liu, Xiao-Ping; Feng, Liang; Lu, Ming-Hui; Chen, Yan-Feng

    2016-12-01

    Strategic manipulation of wave and particle transport in various media is the key driving force for modern information processing and communication. In a strongly scattering medium, waves and particles exhibit versatile transport characteristics such as localization, tunnelling with exponential decay, ballistic, and diffusion behaviours due to dynamical multiple scattering from strong scatters or impurities. Recent investigations of graphene have offered a unique approach, from a quantum point of view, to design the dispersion of electrons on demand, enabling relativistic massless Dirac quasiparticles, and thus inducing low-loss transport either ballistically or diffusively. Here, we report an experimental demonstration of an artificial phononic graphene tailored for surface phonons on a LiNbO3 integrated platform. The system exhibits Dirac quasiparticle-like transport, that is, pseudo-diffusion at the Dirac point, which gives rise to a thickness-independent temporal beating for transmitted pulses, an analogue of Zitterbewegung effects. The demonstrated fully integrated artificial phononic graphene platform here constitutes a step towards on-chip quantum simulators of graphene and unique monolithic electro-acoustic integrated circuits.

  16. Diagnostic for two-mode variable valve activation device

    SciTech Connect

    Fedewa, Andrew M

    2014-01-07

    A method is provided for diagnosing a multi-mode valve train device which selectively provides high lift and low lift to a combustion valve of an internal combustion engine having a camshaft phaser actuated by an electric motor. The method includes applying a variable electric current to the electric motor to achieve a desired camshaft phaser operational mode and commanding the multi-mode valve train device to a desired valve train device operational mode selected from a high lift mode and a low lift mode. The method also includes monitoring the variable electric current and calculating a first characteristic of the parameter. The method also includes comparing the calculated first characteristic against a predetermined value of the first characteristic measured when the multi-mode valve train device is known to be in the desired valve train device operational mode.

  17. Resonant phase jump with enhanced electric field caused by surface phonon polariton in terahertz region.

    PubMed

    Okada, Takanori; Nagai, Masaya; Tanaka, Koichiro

    2008-04-14

    We investigated surface phonon polariton in cesium iodide with terahertz time-domain attenuated total reflection method in Otto configuration, which gives us both information on amplitude and phase of surface electromagnetic mode directly. Systematic experiments with precise control of the distance between a prism and an active material show that the abrupt change of pi-phase jump appears sensitively under polariton picture satisfied when the local electric field at the interface becomes a maximum. This demonstration will open the novel phase-detection terahertz sensor using the active medium causing the strong enhancement of terahertz electric field.

  18. Temperature induced phonon behaviour in germanium selenide thin films probed by Raman spectroscopy

    NASA Astrophysics Data System (ADS)

    Taube, A.; Łapińska, A.; Judek, J.; Wochtman, N.; Zdrojek, M.

    2016-08-01

    Here we report a detailed study of temperature-dependent phonon properties of exfoliated germanium selenide thin films (several tens of nanometers thick) probed by Raman spectroscopy in the 70-350 K temperature range. The temperature-dependent behavior of the positions and widths of the Raman modes was nonlinear. We concluded that the observed effects arise from anharmonic phonon-phonon interactions and are explained by the phenomenon of optical phonon decay into acoustic phonons. At temperatures above 200 K, the position of the Raman modes tended to be linearly dependent, and the first order temperature coefficients χ were  -0.0277, -0.0197 and  -0.031 cm-1 K-1 for B 3g , A g(1) and A g(2) modes, respectively.

  19. Structural and phonon transmission study of Ge-Au-Ge eutectically bonded interfaces

    SciTech Connect

    Knowlton, W.B. |

    1995-07-01

    This thesis presents a structural analysis and phonon transparency investigation of the Ge-Au-Ge eutectic bond interface. Interface development was intended to maximize the interfacial ballistic phonon transparency to enhance the detection of the dark matter candidate WIMPs. The process which was developed provides an interface which produces minimal stress, low amounts of impurities, and insures Ge lattice continuity through the interface. For initial Au thicknesses of greater than 1,000 {angstrom} Au per substrate side, eutectic epitaxial growth resulted in a Au dendritic structure with 95% cross sectional and 90% planar Au interfacial area coverages. In sections in which Ge bridged the interface, lattice continuity across the interface was apparent. Epitaxial solidification of the eutectic interface with initial Au thicknesses < 500 A per substrate side produced Au agglomerations thereby reducing the Au planar interfacial area coverage to as little as 30%. The mechanism for Au coalescence was attributed to lateral diffusion of Ge and Au in the liquid phase during solidification. Phonon transmission studies were performed on eutectic interfaces with initial Au thicknesses of 1,000 {angstrom}, 500 {angstrom}, and 300 {angstrom} per substrate side. Phonon imaging of eutectically bonded samples with initial Au thicknesses of 300 {angstrom}/side revealed reproducible interfacial percent phonon transmissions from 60% to 70%. Line scan phonon imaging verified the results. Phonon propagation TOF spectra distinctly showed the predominant phonon propagation mode was ballistic. This was substantiated by phonon focusing effects apparent in the phonon imaging data. The degree of interface transparency to phonons and resulting phonon propagation modes correlate with the structure of the interface following eutectic solidification. Structural studies of samples with initial Au thickness of 1,000 {angstrom}/side appear to correspond with the phonon transmission study.

  20. Default-mode-like network activation in awake rodents.

    PubMed

    Upadhyay, Jaymin; Baker, Scott J; Chandran, Prasant; Miller, Loan; Lee, Younglim; Marek, Gerard J; Sakoglu, Unal; Chin, Chih-Liang; Luo, Feng; Fox, Gerard B; Day, Mark

    2011-01-01

    During wakefulness and in absence of performing tasks or sensory processing, the default-mode network (DMN), an intrinsic central nervous system (CNS) network, is in an active state. Non-human primate and human CNS imaging studies have identified the DMN in these two species. Clinical imaging studies have shown that the pattern of activity within the DMN is often modulated in various disease states (e.g., Alzheimer's, schizophrenia or chronic pain). However, whether the DMN exists in awake rodents has not been characterized. The current data provides evidence that awake rodents also possess 'DMN-like' functional connectivity, but only subsequent to habituation to what is initially a novel magnetic resonance imaging (MRI) environment as well as physical restraint. Specifically, the habituation process spanned across four separate scanning sessions (Day 2, 4, 6 and 8). At Day 8, significant (p<0.05) functional connectivity was observed amongst structures such as the anterior cingulate (seed region), retrosplenial, parietal, and hippocampal cortices. Prior to habituation (Day 2), functional connectivity was only detected (p<0.05) amongst CNS structures known to mediate anxiety (i.e., anterior cingulate (seed region), posterior hypothalamic area, amygdala and parabracial nucleus). In relating functional connectivity between cingulate-default-mode and cingulate-anxiety structures across Days 2-8, a significant inverse relationship (r = -0.65, p = 0.0004) was observed between these two functional interactions such that increased cingulate-DMN connectivity corresponded to decreased cingulate anxiety network connectivity. This investigation demonstrates that the cingulate is an important component of both the rodent DMN-like and anxiety networks.

  1. Evidence of superconductivity-induced phonon spectra renormalization in alkali-doped iron selenides

    SciTech Connect

    Opačić, M.; Lazarević, N.; Šćepanović, M.; Ryu, Hyejin; Lei, Hechang; Petrovic, C.; Popović, Z. V.

    2015-11-16

    Polarized Raman scattering spectra of superconducting KxFe2-ySe2 and nonsuperconducting K0.8Fe1.8Co0.2Se2 single crystals were measured in a temperature range from 10 K up to 300 K. Two Raman active modes from the I4/mmm phase and seven from the I4/m phase are observed in frequency range from 150 to 325 cm -1 in both compounds, suggesting that K0.8Fe1.8Co0.2Se2 single crystal also has two-phase nature. Temperature dependence of Raman mode energy is analyzed in terms of lattice thermal expansion and phonon-phonon interaction. Temperature dependence of Raman mode linewidth is considered as temperature-induced anharmonic effects. It is shown that change of Raman mode energy with temperature is dominantly driven by thermal expansion of the crystal lattice. Abrupt change of the A1g mode energy near TC was observed in KxFe2-ySe2 , whereas it is absent in K0.8Fe1.8Co0.2Se2. Phonon energy hardening at low temperatures in the superconducting sample is a consequence of superconductivity-induced redistribution of the electronic states below critical temperature.

  2. Yoctocalorimetry: phonon counting in nanostructures

    NASA Astrophysics Data System (ADS)

    Roukes, M. L.

    1999-03-01

    It appears feasible with nanostructures to perform calorimetry at the level of individual thermal phonons. Here I outline an approach employing monocrystalline mesoscopic insulators, which can now be patterned from semiconductor heterostructures into complex geometries with full, three-dimensional relief. Successive application of these techniques also enables definition of integrated nanoscale thermal transducers; coupling these to a dc SQUID readout yields the requisite energy sensitivity and temporal resolution with minimal back action. The prospect of phonon counting opens intriguing experimental possibilities with analogies in quantum optics. These include fluctuation-based phonon spectroscopy, phonon shot noise in the energy relaxation of nanoscale systems, and quantum statistical phenomena such as phonon bunching and anticorrelated electron-phonon exchange.

  3. Phonon analog of topological nodal semimetals

    NASA Astrophysics Data System (ADS)

    Po, Hoi Chun; Bahri, Yasaman; Vishwanath, Ashvin

    2016-05-01

    Topological band structures in electronic systems like topological insulators and semimetals give rise to highly unusual physical properties. Analogous topological effects have also been discussed in bosonic systems, but the novel phenomena typically occur only when the system is excited by finite-frequency probes. A mapping recently proposed by C. L. Kane and T. C. Lubensky [Nat. Phys. 10, 39 (2014), 10.1038/nphys2835], however, establishes a closer correspondence. It relates the zero-frequency excitations of mechanical systems to topological zero modes of fermions that appear at the edges of an otherwise gapped system. Here we generalize the mapping to systems with an intrinsically gapless bulk. In particular, we construct mechanical counterparts of topological semimetals. The resulting gapless bulk modes are physically distinct from the usual acoustic Goldstone phonons and appear even in the absence of continuous translation invariance. Moreover, the zero-frequency phonon modes feature adjustable momenta and are topologically protected as long as the lattice coordination is unchanged. Such protected soft modes with tunable wave vector may be useful in designing mechanical structures with fault-tolerant properties.

  4. Active noise control using a distributed mode flat panel loudspeaker.

    PubMed

    Zhu, H; Rajamani, R; Dudney, J; Stelson, K A

    2003-07-01

    A flat panel distributed mode loudspeaker (DML) has many advantages over traditional cone speakers in terms of its weight, size, and durability. However, its frequency response is uneven and complex, thus bringing its suitability for active noise control (ANC) under question. This paper presents experimental results demonstrating the effective use of panel DML speakers in an ANC application. Both feedback and feedforward control techniques are considered. Effective feedback control with a flat panel speaker could open up a whole range of new noise control applications and has many advantages over feedforward control. The paper develops a new control algorithm to attenuate tonal noise of a known frequency by feedback control. However, due to the uneven response of the speakers, feedback control is found to be only moderately effective even for this narrow-band application. Feedforward control proves to be most capable for the flat panel speaker. Using feedforward control, the sound pressure level can be significantly reduced in close proximity to an error microphone. The paper demonstrates an interesting application of the flat panel in which the panel is placed in the path of sound and effectively used to block sound transmission using feedforward control. This is a new approach to active noise control enabled by the use of flat panels and can be used to prevent sound from entering into an enclosure in the first place rather than the traditional approach of attempting to cancel sound after it enters the enclosure.

  5. Mode of antibacterial activity of Eclalbasaponin isolated from Eclipta alba.

    PubMed

    Ray, A; Bharali, P; Konwar, B K

    2013-12-01

    The present study was undertaken to evaluate the mode of antibacterial activity of Eclalbasaponin isolated from Eclipta alba, against selected Gram-positive and Gram-negative bacteria. The probable chemical structure was determined by using various spectroscopic techniques such as Fourier transform infrared spectroscopy (FTIR) and mass spectroscopy. The antibacterial activity was evaluated by well diffusion technique, pH sensitivity, chemotaxis, and crystal violet assays. Eclalbasaponin showed clear zone of inhibition against both Bacillus subtilis and Pseudomonas aeruginosa and exhibited growth inhibition at the pH range of 5.5-9.0. The isolated saponin exhibited its positive chemoattractant property for both bacterial strains. Results of crystal violet assay and the presence of UV-sensitive materials in the cell-free supernatant confirmed the cellular damages caused by the treatment of Eclalbasaponin. The release of intracellular proteins due to the membrane damage was determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Changes in the cell surface structure and membrane disruption were further revealed by FTIR and scanning electron microscopy analysis. The present study suggests that the isolated saponin from E. alba causes the disruption of the bacterial cell membrane which leads to the loss of bacterial cell viability.

  6. Temperature dependence of the A1(LO) and E2 (high) phonons in hexagonal InN nanowires

    NASA Astrophysics Data System (ADS)

    Song, B.; Jian, J. K.; Wang, G.; Bao, H. Q.; Chen, X. L.

    2007-06-01

    The frequencies and dampings of the zone-center optical phonon modes of A1(LO) (longitudinal-optical) and E2 (high) in wurtzite InN nanowires have been investigated by micro-Raman scattering in the temperature range from 80 to 300 K. Our results reveal that the phonon frequencies decrease and the linewidths broaden with increasing temperature. The obtained experimental data of the frequencies and linewidths at various temperatures can be well described by an empirical model which takes into account the contribution of the thermal expansion of lattice and symmetric decay of phonons into two and three identical phonons with lower energy. The results show that decay into two phonons is the probable channel for the A1(LO) mode and three-phonon decay dominates the E2 (high) mode.

  7. Probing confined acoustic phonons in free standing small gold nanoparticles

    SciTech Connect

    Mankad, Venu; Jha, Prafulla K.; Ravindran, T. R.

    2013-02-21

    Polarized and depolarized spectra from gold (Au) nanoparticles of different sizes are investigated in the small size range, between 3 and 7 nm, using low frequency Raman spectroscopy. Acoustic vibrations of the free-standing Au nanoparticles are demonstrated with frequencies ranging from 5 to 35 cm{sup -1}, opening the way to the development of the acoustic resonators. A blue shift in the phonon peaks along with the broadening is observed with a decrease in particle size. Comparison of the measured frequencies with vibrational dynamics calculation and an examination as from the transmission electron microscopy results ascertain that the low frequency phonon modes are due to acoustic phonon quantization. Our results show that the observed low frequency Raman scattering originates from the spherical (l = 0) and quadrupolar (l = 2) vibrations of the spheroidal mode due to plasmon mediated acoustic vibrations in Au nanoparticles.

  8. Phononic crystal plate with hollow pillars connected by thin bars

    NASA Astrophysics Data System (ADS)

    Jin, Yabin; Pennec, Yan; Pan, Yongdong; Djafari-Rouhani, Bahram

    2017-01-01

    A new type of phononic crystal plate consisting of hollow pillars on a bar-connected plate is proposed. With respect to usual pillar based phononic crystal plates, the Bragg band gap can be tuned to be much wider and extended to a sub-wavelength region, and the low frequency gap can be moved to an extremely low frequency range. Such a structure can generate quadrapolar, hexapolar and octopolar whispering-gallery modes (WGMs) inside the band gaps with very high confinement and quality factors. By filling the hollow pillars with a liquid, these WGMs, together with additional localized compressional and solid-liquid coupling modes, can be tuned either by varying the inner radius of the pillars or controlling the height of the liquid. We discuss some possible functionalities of these phononic crystals for the purpose of sensing the acoustic properties of liquids, multiplexer and wireless communication.

  9. Exciton-phonon system on a star graph: A perturbative approach.

    PubMed

    Yalouz, Saad; Pouthier, Vincent

    2016-05-01

    Based on the operatorial formulation of the perturbation theory, the properties of an exciton coupled with optical phonons on a star graph are investigated. Within this method, the dynamics is governed by an effective Hamiltonian, which accounts for exciton-phonon entanglement. The exciton is dressed by a virtual phonon cloud whereas the phonons are clothed by virtual excitonic transitions. In spite of the coupling with the phonons, it is shown that the energy spectrum of the dressed exciton resembles that of a bare exciton. The only differences originate in a polaronic mechanism that favors an energy shift and a decay of the exciton hopping constant. By contrast, the motion of the exciton allows the phonons to propagate over the graph so that the dressed normal modes drastically differ from the localized modes associated to bare phonons. They define extended vibrations whose properties depend on the state occupied by the exciton that accompanies the phonons. It is shown that the phonon frequencies, either red shifted or blue shifted, are very sensitive to the model parameter in general, and to the size of the graph in particular.

  10. Edge waves and resonances in two-dimensional phononic crystal plates

    NASA Astrophysics Data System (ADS)

    Hsu, Jin-Chen; Hsu, Chih-Hsun

    2015-05-01

    We present a numerical study on phononic band gaps and resonances occurring at the edge of a semi-infinite two-dimensional (2D) phononic crystal plate. The edge supports localized edge waves coupling to evanescent phononic plate modes that decay exponentially into the semi-infinite phononic crystal plate. The band-gap range and the number of edge-wave eigenmodes can be tailored by tuning the distance between the edge and the semi-infinite 2D phononic lattice. As a result, a phononic band gap for simultaneous edge waves and plate waves is created, and phononic cavities beside the edge can be built to support high-frequency edge resonances. We design an L3 edge cavity and analyze its resonance characteristics. Based on the band gap, high quality factor and strong confinement of resonant edge modes are achieved. The results enable enhanced control over acoustic energy flow in phononic crystal plates, which can be used in designing micro and nanoscale resonant devices and coupling of edge resonances to other types of phononic or photonic crystal cavities.

  11. Exciton-phonon system on a star graph: A perturbative approach

    NASA Astrophysics Data System (ADS)

    Yalouz, Saad; Pouthier, Vincent

    2016-05-01

    Based on the operatorial formulation of the perturbation theory, the properties of an exciton coupled with optical phonons on a star graph are investigated. Within this method, the dynamics is governed by an effective Hamiltonian, which accounts for exciton-phonon entanglement. The exciton is dressed by a virtual phonon cloud whereas the phonons are clothed by virtual excitonic transitions. In spite of the coupling with the phonons, it is shown that the energy spectrum of the dressed exciton resembles that of a bare exciton. The only differences originate in a polaronic mechanism that favors an energy shift and a decay of the exciton hopping constant. By contrast, the motion of the exciton allows the phonons to propagate over the graph so that the dressed normal modes drastically differ from the localized modes associated to bare phonons. They define extended vibrations whose properties depend on the state occupied by the exciton that accompanies the phonons. It is shown that the phonon frequencies, either red shifted or blue shifted, are very sensitive to the model parameter in general, and to the size of the graph in particular.

  12. Optical Generation and Detection of Local Nonequilibrium Phonons in Suspended Graphene

    NASA Astrophysics Data System (ADS)

    Sullivan, Sean; Vallabhaneni, Ajit; Kholmanov, Iskandar; Ruan, Xiulin; Murthy, Jayathi; Shi, Li

    2017-03-01

    The measured frequencies and intensities of different first- and second- order Raman peaks of suspended graphene are used to show that optical phonons and different acoustic phonon polarizations are driven out of local equilibrium inside a sub-micron laser spot. The experimental results are correlated with a first principles-based multiple temperature model to suggest a considerably lower equivalent local temperature of the flexural phonons than those of other phonon polarizations. The finding reveals weak coupling between the flexural modes with hot electrons and optical phonons. Since the ultrahigh intrinsic thermal conductivity of graphene has been largely attributed to contributions from the flexural phonons, the observed local non-equilibrium phenomena have important implications for understanding energy dissipation processes in graphene-based electronic and optoelectronic devices, as well as in Raman measurements of thermal transport in graphene and other two-dimensional materials.

  13. Interaction Induced Electron Self-Interference in a Semiconductor: The Phonon Staircase Effect

    SciTech Connect

    Kenrow, J.A.; El Sayed, K.; Stanton, C.J.

    1997-06-01

    The exact quantum mechanics of a model semiconductor system of an electron with a discrete and equidistant energy spectrum interacting with a single phonon mode is presented. An electron initially excited into a coherent superposition of states interacts with phonons thereby creating a self-interference {ital in time} which reduces the emission of phonons to isolated bursts. This self-interference effect gives rise to steplike behavior in the relaxation kinetics of the electron and phonons. We show that this {open_quotes}phonon staircase{close_quotes} effect is a consequence of a correlated initial electron distribution and the violation of energy conservation in the electron-phonon interaction on short time scales. {copyright} {ital 1997} {ital The American Physical Society}

  14. Electron-Phonon Coupling and its implication for the superconducting topological insulators

    PubMed Central

    Zhang, Xiao-Long; Liu, Wu-Ming

    2015-01-01

    The recent observation of superconductivity in doped topological insulators has sparked a flurry of interest due to the prospect of realizing the long-sought topological superconductors. Yet the understanding of underlying pairing mechanism in these systems is far from complete. Here we investigate this problem by providing robust first-principles calculations of the role of electron-phonon coupling for the superconducting pairing in the prime candidate CuxBi2Se3. Our results show that electron-phonon scattering process in this system is dominated by zone center and boundary optical modes, with coexistence of phonon stiffening and softening. While the calculated electron-phonon coupling constant λ suggests that Tc from electron-phonon coupling is 2 orders smaller than the ones reported on bulk inhomogeneous samples, suggesting that superconductivity may not come from pure electron-phonon coupling. We discuss the possible enhancement of superconducting transition temperature by local inhomogeneity introduced by doping. PMID:25753813

  15. Switchable topological phonon channels

    NASA Astrophysics Data System (ADS)

    Süsstrunk, Roman; Zimmermann, Philipp; Huber, Sebastian D.

    2017-01-01

    Guiding energy deliberately is one of the central elements in engineering and information processing. It is often achieved by designing specific transport channels in a suitable material. Topological metamaterials offer a way to construct stable and efficient channels of unprecedented versatility. However, due to their stability it can be tricky to terminate them or to temporarily shut them off without changing the material properties massively. While a lot of effort was put into realizing mechanical topological metamaterials, almost no works deal with manipulating their edge channels in sight of applications. Here, we take a step in this direction, by taking advantage of local symmetry breaking potentials to build a switchable topological phonon channel.

  16. Electron–phonon coupling in hybrid lead halide perovskites

    PubMed Central

    Wright, Adam D.; Verdi, Carla; Milot, Rebecca L.; Eperon, Giles E.; Pérez-Osorio, Miguel A.; Snaith, Henry J.; Giustino, Feliciano; Johnston, Michael B.; Herz, Laura M.

    2016-01-01

    Phonon scattering limits charge-carrier mobilities and governs emission line broadening in hybrid metal halide perovskites. Establishing how charge carriers interact with phonons in these materials is therefore essential for the development of high-efficiency perovskite photovoltaics and low-cost lasers. Here we investigate the temperature dependence of emission line broadening in the four commonly studied formamidinium and methylammonium perovskites, HC(NH2)2PbI3, HC(NH2)2PbBr3, CH3NH3PbI3 and CH3NH3PbBr3, and discover that scattering from longitudinal optical phonons via the Fröhlich interaction is the dominant source of electron–phonon coupling near room temperature, with scattering off acoustic phonons negligible. We determine energies for the interacting longitudinal optical phonon modes to be 11.5 and 15.3 meV, and Fröhlich coupling constants of ∼40 and 60 meV for the lead iodide and bromide perovskites, respectively. Our findings correlate well with first-principles calculations based on many-body perturbation theory, which underlines the suitability of an electronic band-structure picture for describing charge carriers in hybrid perovskites. PMID:27225329

  17. Electron-phonon coupling in hybrid lead halide perovskites

    NASA Astrophysics Data System (ADS)

    Wright, Adam D.; Verdi, Carla; Milot, Rebecca L.; Eperon, Giles E.; Pérez-Osorio, Miguel A.; Snaith, Henry J.; Giustino, Feliciano; Johnston, Michael B.; Herz, Laura M.

    2016-05-01

    Phonon scattering limits charge-carrier mobilities and governs emission line broadening in hybrid metal halide perovskites. Establishing how charge carriers interact with phonons in these materials is therefore essential for the development of high-efficiency perovskite photovoltaics and low-cost lasers. Here we investigate the temperature dependence of emission line broadening in the four commonly studied formamidinium and methylammonium perovskites, HC(NH2)2PbI3, HC(NH2)2PbBr3, CH3NH3PbI3 and CH3NH3PbBr3, and discover that scattering from longitudinal optical phonons via the Fröhlich interaction is the dominant source of electron-phonon coupling near room temperature, with scattering off acoustic phonons negligible. We determine energies for the interacting longitudinal optical phonon modes to be 11.5 and 15.3 meV, and Fröhlich coupling constants of ~40 and 60 meV for the lead iodide and bromide perovskites, respectively. Our findings correlate well with first-principles calculations based on many-body perturbation theory, which underlines the suitability of an electronic band-structure picture for describing charge carriers in hybrid perovskites.

  18. Electron-phonon coupling in hybrid lead halide perovskites.

    PubMed

    Wright, Adam D; Verdi, Carla; Milot, Rebecca L; Eperon, Giles E; Pérez-Osorio, Miguel A; Snaith, Henry J; Giustino, Feliciano; Johnston, Michael B; Herz, Laura M

    2016-05-26

    Phonon scattering limits charge-carrier mobilities and governs emission line broadening in hybrid metal halide perovskites. Establishing how charge carriers interact with phonons in these materials is therefore essential for the development of high-efficiency perovskite photovoltaics and low-cost lasers. Here we investigate the temperature dependence of emission line broadening in the four commonly studied formamidinium and methylammonium perovskites, HC(NH2)2PbI3, HC(NH2)2PbBr3, CH3NH3PbI3 and CH3NH3PbBr3, and discover that scattering from longitudinal optical phonons via the Fröhlich interaction is the dominant source of electron-phonon coupling near room temperature, with scattering off acoustic phonons negligible. We determine energies for the interacting longitudinal optical phonon modes to be 11.5 and 15.3 meV, and Fröhlich coupling constants of ∼40 and 60 meV for the lead iodide and bromide perovskites, respectively. Our findings correlate well with first-principles calculations based on many-body perturbation theory, which underlines the suitability of an electronic band-structure picture for describing charge carriers in hybrid perovskites.

  19. First-principles prediction of phononic thermal conductivity of silicene: A comparison with graphene

    SciTech Connect

    Gu, Xiaokun; Yang, Ronggui

    2015-01-14

    There has been great interest in two-dimensional materials, beyond graphene, for both fundamental sciences and technological applications. Silicene, a silicon counterpart of graphene, has been shown to possess some better electronic properties than graphene. However, its thermal transport properties have not been fully studied. In this paper, we apply the first-principles-based phonon Boltzmann transport equation to investigate the thermal conductivity of silicene as well as the phonon scattering mechanisms. Although both graphene and silicene are two-dimensional crystals with similar crystal structure, we find that phonon transport in silicene is quite different from that in graphene. The thermal conductivity of silicene shows a logarithmic increase with respect to the sample size due to the small scattering rates of acoustic in-plane phonon modes, while that of graphene is finite. Detailed analysis of phonon scattering channels shows that the linear dispersion of the acoustic out-of-plane (ZA) phonon modes, which is induced by the buckled structure, makes the long-wavelength longitudinal acoustic phonon modes in silicene not as efficiently scattered as that in graphene. Compared with graphene, where most of the heat is carried by the acoustic out-of-plane (ZA) phonon modes, the ZA phonon modes in silicene only have ∼10% contribution to the total thermal conductivity, which can also be attributed to the buckled structure. This systematic comparison of phonon transport and thermal conductivity of silicene and graphene using the first-principle-based calculations shed some light on other two-dimensional materials, such as two-dimensional transition metal dichalcogenides.

  20. Optical phonons in the wurtzstannite Cu2ZnGeS4 semiconductor: Polarized Raman spectroscopy and first-principle calculations

    NASA Astrophysics Data System (ADS)

    Guc, M.; Litvinchuk, A. P.; Levcenko, S.; Izquierdo-Roca, V.; Fontané, X.; Valakh, M. Ya.; Arushanov, E.; Pérez-Rodríguez, A.

    2014-05-01

    The vibrational properties of the wurtzstannite Cu2ZnGeS4 are studied experimentally by polarized Raman scattering in off-resonant and resonant conditions and theoretically by ab initio lattice dynamics calculations. Twenty-nine modes from 45 Raman active theoretically predicted have been experimentally detected and identified, including polar A1(TO),A1(LO), and B1(TO+LO)/B2(TO+LO) and nonpolar A2 symmetry phonon modes from measurements on (2 1 0) and (0 0 1) crystallographic planes of Cu2ZnGeS4 single crystals. The lattice dynamics calculations provide a full picture of the zone center phonon spectrum and allow the assignment of experimentally observed lines to first- and second-order lattice vibrations. Using resonance Raman conditions, a strong enhancement of the A1(LO) modes with the highest longitudinal-transversal spiting is observed.

  1. Electron-phonon coupling and thermal conductance at a metal-semiconductor interface: First-principles analysis

    SciTech Connect

    Sadasivam, Sridhar; Fisher, Timothy S.; Waghmare, Umesh V.

    2015-04-07

    The mechanism of heat transfer and the contribution of electron-phonon coupling to thermal conductance of a metal-semiconductor interface remains unclear in the present literature. We report ab initio simulations of a technologically important titanium silicide (metal)–silicon (semiconductor) interface to estimate the Schottky barrier height, and the strength of electron-phonon and phonon-phonon heat transfer across the interface. The electron and phonon dispersion relations of TiSi{sub 2} with C49 structure and the TiSi{sub 2}-Si interface are obtained using first-principles calculations within the density functional theory framework. These are used to estimate electron-phonon linewidths and the associated Eliashberg function that quantifies coupling. We show that the coupling strength of electrons with interfacial phonon modes is of the same order of magnitude as coupling of electrons to phonon modes in the bulk metal, and its contribution to electron-phonon interfacial conductance is comparable to the harmonic phonon-phonon conductance across the interface.

  2. Spectral phonon mean free path and thermal conductivity accumulation in defected graphene: The effects of defect type and concentration

    NASA Astrophysics Data System (ADS)

    Feng, Tianli; Ruan, Xiulin; Ye, Zhenqiang; Cao, Bingyang

    2015-06-01

    The spectral phonon properties in defected graphene have been unclear due to the lack of advanced techniques for predicting the phonon-defect scattering rate without fitting parameters. Taking advantage of the extended phonon normal mode analysis, we obtained the spectral phonon relaxation time and mean free path (MFP) in defected graphene and studied the impacts of three common types of defects: Stone-Thrower-Wales (STW) defect, double vacancy (DV), and monovacancy (MV). The phonon-STW defect scattering rate is found to have no significant frequency dependence, and as a result, the relative contribution of long-wavelength phonons sharply decreases. In contrast, the phonon scattering by DVs or MVs exhibits a frequency dependence of τp-d -1˜ω1.1 -1.3 except for a few long-wavelength phonons, revisiting the traditionally used ˜ω4 dependence. We note that although MV-defected graphene has the lowest thermal conductivity as compared to the other two defected graphene samples at the same defect concentration, it has a portion of phonons with the longest MFP. The contribution from the long-MFP and long-wavelength phonons does not decrease much as the vacancy concentration increases. STW defect and MV block more out-of-plane modes than in-plane modes, while DV has less bias for which mode to block. As the MV concentration increases from 0 to 1.1%, the relative contribution from out-of-plane modes decreases from 30% to 18%, while that of the transverse acoustic mode remains at around 30%. These findings of spectral phonon properties can provide more insight than the effective properties and benefit the prospective phononic engineering.

  3. Batch Mode Active Learning for Regression With Expected Model Change.

    PubMed

    Cai, Wenbin; Zhang, Muhan; Zhang, Ya

    2016-04-20

    While active learning (AL) has been widely studied for classification problems, limited efforts have been done on AL for regression. In this paper, we introduce a new AL framework for regression, expected model change maximization (EMCM), which aims at choosing the unlabeled data instances that result in the maximum change of the current model once labeled. The model change is quantified as the difference between the current model parameters and the updated parameters after the inclusion of the newly selected examples. In light of the stochastic gradient descent learning rule, we approximate the change as the gradient of the loss function with respect to each single candidate instance. Under the EMCM framework, we propose novel AL algorithms for the linear and nonlinear regression models. In addition, by simulating the behavior of the sequential AL policy when applied for k iterations, we further extend the algorithms to batch mode AL to simultaneously choose a set of k most informative instances at each query time. Extensive experimental results on both UCI and StatLib benchmark data sets have demonstrated that the proposed algorithms are highly effective and efficient.

  4. Anisotropic surface phonon dispersion of the hydrogen-terminated Si(110)-(1×1) surface: One-dimensional phonons propagating along the glide planes

    SciTech Connect

    Matsushita, Stephane Yu; Matsui, Kazuki; Kato, Hiroki; Suto, Shozo; Yamada, Taro

    2014-03-14

    We have measured the surface phonon dispersion curves on the hydrogen-terminated Si(110)-(1×1) surface with the two-dimensional space group of p2mg along the two highly symmetric and rectangular directions of ΓX{sup ¯} and ΓX{sup ′¯} using high-resolution electron-energy-loss spectroscopy. All the essential energy-loss peaks on H:Si(110) were assigned to the vibrational phonon modes by using the selection rules of inelastic electron scattering including the glide-plane symmetry. Actually, the surface phonon modes of even-symmetry to the glide plane (along ΓX{sup ¯}) were observed in the first Brillouin zone, and those of odd-symmetry to the glide plane were in the second Brillouin zone. The detailed assignment was made by referring to theoretical phonon dispersion curves of Gräschus et al. [Phys. Rev. B 56, 6482 (1997)]. We found that the H–Si stretching and bending modes, which exhibit highly anisotropic dispersion, propagate along ΓX{sup ¯} direction as a one-dimensional phonon. Judging from the surface structure as well as our classical and quantum mechanical estimations, the H–Si stretching phonon propagates by a direct repulsive interaction between the nearest neighbor H atoms facing each other along ΓX{sup ¯}, whereas the H–Si bending phonon propagates by indirect interaction through the substrate Si atomic linkage.

  5. Thermally triggered phononic gaps in liquids at THz scale

    SciTech Connect

    Bolmatov, Dima; Zhernenkov, Mikhail; Zavyalov, Dmitry; Stoupin, Stanislav; Cunsolo, Alessandro; Cai, Yong Q.

    2016-01-14

    In this study we present inelastic X-ray scattering experiments in a diamond anvil cell and molecular dynamic simulations to investigate the behavior of phononic excitations in liquid Ar. The spectra calculated using molecular dynamics were found to be in a good agreement with the experimental data. Furthermore, we observe that, upon temperature increases, a low-frequency transverse phononic gap emerges while high-frequency propagating modes become evanescent at the THz scale. The effect of strong localization of a longitudinal phononic mode in the supercritical phase is observed for the first time. The evidence for the high-frequency transverse phononic gap due to the transition from an oscillatory to a ballistic dynamic regimes of motion is presented and supported by molecular dynamics simulations. This transition takes place across the Frenkel line thermodynamic limit which demarcates compressed liquid and non-compressed fluid domains on the phase diagram and is supported by calculations within the Green-Kubo phenomenological formalism. These results are crucial to advance the development of novel terahertz thermal devices, phononic lenses, mirrors, and other THz metamaterials.

  6. Thermally triggered phononic gaps in liquids at THz scale

    DOE PAGES

    Bolmatov, Dima; Zhernenkov, Mikhail; Zavyalov, Dmitry; ...

    2016-01-14

    In this study we present inelastic X-ray scattering experiments in a diamond anvil cell and molecular dynamic simulations to investigate the behavior of phononic excitations in liquid Ar. The spectra calculated using molecular dynamics were found to be in a good agreement with the experimental data. Furthermore, we observe that, upon temperature increases, a low-frequency transverse phononic gap emerges while high-frequency propagating modes become evanescent at the THz scale. The effect of strong localization of a longitudinal phononic mode in the supercritical phase is observed for the first time. The evidence for the high-frequency transverse phononic gap due to themore » transition from an oscillatory to a ballistic dynamic regimes of motion is presented and supported by molecular dynamics simulations. This transition takes place across the Frenkel line thermodynamic limit which demarcates compressed liquid and non-compressed fluid domains on the phase diagram and is supported by calculations within the Green-Kubo phenomenological formalism. These results are crucial to advance the development of novel terahertz thermal devices, phononic lenses, mirrors, and other THz metamaterials.« less

  7. Thermally triggered phononic gaps in liquids at THz scale

    PubMed Central

    Bolmatov, Dima; Zhernenkov, Mikhail; Zav’yalov, Dmitry; Stoupin, Stanislav; Cunsolo, Alessandro; Cai, Yong Q.

    2016-01-01

    In this paper we present inelastic X-ray scattering experiments in a diamond anvil cell and molecular dynamic simulations to investigate the behavior of phononic excitations in liquid Ar. The spectra calculated using molecular dynamics were found to be in a good agreement with the experimental data. Furthermore, we observe that, upon temperature increases, a low-frequency transverse phononic gap emerges while high-frequency propagating modes become evanescent at the THz scale. The effect of strong localization of a longitudinal phononic mode in the supercritical phase is observed for the first time. The evidence for the high-frequency transverse phononic gap due to the transition from an oscillatory to a ballistic dynamic regimes of motion is presented and supported by molecular dynamics simulations. This transition takes place across the Frenkel line thermodynamic limit which demarcates compressed liquid and non-compressed fluid domains on the phase diagram and is supported by calculations within the Green-Kubo phenomenological formalism. These results are crucial to advance the development of novel terahertz thermal devices, phononic lenses, mirrors, and other THz metamaterials. PMID:26763899

  8. Band gaps in bubble phononic crystals

    NASA Astrophysics Data System (ADS)

    Leroy, V.; Bretagne, A.; Lanoy, M.; Tourin, A.

    2016-12-01

    We investigate the interaction between Bragg and hybridization effects on the band gap properties of bubble phononic crystals. These latter consist of air cavities periodically arranged in an elastomer matrix and are fabricated using soft-lithography techniques. Their transmission properties are affected by Bragg effects due to the periodicity of the structure as well as hybridization between the propagating mode of the embedding medium and bubble resonance. The hybridization gap survives disorder while the Bragg gap requires a periodic distribution of bubbles. The distance between two bubble layers can be tuned to make the two gaps overlap or to create a transmission peak in the hybridization gap.

  9. Shear surface waves in phononic crystals.

    PubMed

    Kutsenko, A A; Shuvalov, A L

    2013-02-01

    The existence of shear horizontal (SH) surface waves in two-dimensional periodic phononic crystals with an asymmetric depth-dependent profile is theoretically reported. Examples of dispersion spectra with bandgaps for subsonic and supersonic SH surface waves are demonstrated. The link between the effective (quasistatic) speeds of the SH bulk and surface waves is established. Calculation and analysis is based on the integral form of a projector on the subspace of evanescent modes which means no need for their explicit finding. This method can be extended to the vector waves and the three-dimensional case.

  10. Resonance laser-plasma excitation of coherent terahertz phonons in the bulk of fluorine-bearing crystals under high-intensity femtosecond laser irradiation

    SciTech Connect

    Potemkin, F V; Mareev, E I; Khodakovskii, N G; Mikheev, P M

    2013-08-31

    The dynamics of coherent phonons in fluorine-containing crystals was investigated by pump-probe technique in the plasma production regime. Several phonon modes, whose frequencies are overtones of the 0.38-THz fundamental frequency, were simultaneously observed in a lithium fluoride crystal. Phonons with frequencies of 1 and 0.1 THz were discovered in a calcium fluoride crystal and coherent phonons with frequencies of 1 THz and 67 GHz were observed in a barium fluoride crystal. Furthermore, in the latter case the amplitudes of phonon mode oscillations were found to significantly increase 15 ps after laser irradiation. (interaction of laser radiation with matter)

  11. Phonon-enhanced crystal growth and lattice healing

    DOEpatents

    Buonassisi, Anthony; Bertoni, Mariana; Newman, Bonna

    2013-05-28

    A system for modifying dislocation distributions in semiconductor materials is provided. The system includes one or more vibrational sources for producing at least one excitation of vibrational mode having phonon frequencies so as to enhance dislocation motion through a crystal lattice.

  12. Surface phonons near structural phase transitions of fluoridic perovskites

    NASA Astrophysics Data System (ADS)

    Prade, J.; Kulkarni, A. D.; De Wette, F. W.; Reiger, R.; Schröder, U.; Kress, W.

    1989-04-01

    The fluoridic perovskite KMnF 3 exhibits an antiferrodistortive phase transition which goes along with a soft mode at the R-point of the Brillouin zone. We investigate in this paper the surface phonons of the KF(001) surface at temperatures near this phase transition. The calculations are carried out for relaxed and reconstructed (001) slabs.

  13. Low-frequency spatial wave manipulation via phononic crystals with relaxed cell symmetry

    SciTech Connect

    Celli, Paolo; Gonella, Stefano

    2014-03-14

    Phononic crystals enjoy unique wave manipulation capabilities enabled by their periodic topologies. On one hand, they feature frequency-dependent directivity, which allows directional propagation of selected modes even at low frequencies. However, the stellar nature of the propagation patterns and the inability to induce single-beam focusing represent significant limitations of this functionality. On the other hand, one can realize waveguides by defecting the periodic structure of a crystal operating in bandgap mode along some desired path. Waveguides of this type are only activated in the relatively high and narrow frequency bands corresponding to total bandgaps, which limits their potential technological applications. In this work, we introduce a class of phononic crystals with relaxed cell symmetry and we exploit symmetry relaxation of a population of auxiliary microstructural elements to achieve spatial manipulation of elastic waves at very low frequencies, in the range of existence of the acoustic modes. By this approach, we achieve focusing without modifying the default static properties of the medium and by invoking mechanisms that are well suited to envision adaptive configurations for semi-active wave control.

  14. Giant phonon anomaly associated with superconducting fluctuations in the pseudogap phase of cuprates

    DOE PAGES

    Liu, Ye-Hua; Konik, Robert M.; Rice, T. M.; ...

    2016-01-20

    The pseudogap in underdoped cuprates leads to significant changes in the electronic structure, and was later found to be accompanied by anomalous fluctuations of superconductivity and certain lattice phonons. Here we propose that the Fermi surface breakup due to the pseudogap, leads to a breakup of the pairing order into two weakly coupled sub-band amplitudes, and a concomitant low energy Leggett mode due to phase fluctuations between them. This increases the temperature range of superconducting fluctuations containing an overdamped Leggett mode. In this range inter-sub-band phonons show strong damping due to resonant scattering into an intermediate state with a pairmore » of overdamped Leggett modes. In the ordered state, the Leggett mode develops a finite energy, changing the anomalous phonon damping into an anomaly in the dispersion. Finally, this proposal explains the intrinsic connection between the anomalous pseudogap phase, enhanced superconducting fluctuations and giant anomalies in the phonon spectra.« less

  15. Giant phonon anomaly associated with superconducting fluctuations in the pseudogap phase of cuprates

    PubMed Central

    Liu, Ye-Hua; Konik, Robert M.; Rice, T. M.; Zhang, Fu-Chun

    2016-01-01

    The pseudogap in underdoped cuprates leads to significant changes in the electronic structure, and was later found to be accompanied by anomalous fluctuations of superconductivity and certain lattice phonons. Here we propose that the Fermi surface breakup due to the pseudogap, leads to a breakup of the pairing order into two weakly coupled sub-band amplitudes, and a concomitant low energy Leggett mode due to phase fluctuations between them. This increases the temperature range of superconducting fluctuations containing an overdamped Leggett mode. In this range inter-sub-band phonons show strong damping due to resonant scattering into an intermediate state with a pair of overdamped Leggett modes. In the ordered state, the Leggett mode develops a finite energy, changing the anomalous phonon damping into an anomaly in the dispersion. This proposal explains the intrinsic connection between the anomalous pseudogap phase, enhanced superconducting fluctuations and giant anomalies in the phonon spectra. PMID:26785835

  16. Giant phonon anomaly associated with superconducting fluctuations in the pseudogap phase of cuprates

    SciTech Connect

    Liu, Ye-Hua; Konik, Robert M.; Rice, T. M.; Zhang, Fu-Chun

    2016-01-20

    The pseudogap in underdoped cuprates leads to significant changes in the electronic structure, and was later found to be accompanied by anomalous fluctuations of superconductivity and certain lattice phonons. Here we propose that the Fermi surface breakup due to the pseudogap, leads to a breakup of the pairing order into two weakly coupled sub-band amplitudes, and a concomitant low energy Leggett mode due to phase fluctuations between them. This increases the temperature range of superconducting fluctuations containing an overdamped Leggett mode. In this range inter-sub-band phonons show strong damping due to resonant scattering into an intermediate state with a pair of overdamped Leggett modes. In the ordered state, the Leggett mode develops a finite energy, changing the anomalous phonon damping into an anomaly in the dispersion. Finally, this proposal explains the intrinsic connection between the anomalous pseudogap phase, enhanced superconducting fluctuations and giant anomalies in the phonon spectra.

  17. Temperature dependent phonon shifts in few-layer black phosphorus.

    PubMed

    Late, Dattatray J

    2015-03-18

    Atomically thin two-dimensional (2D) sheets of black phosphorus have attracted much attention due to their potential for future nanoelectronic and photonics device applications. Present investigations deal with the temperature dependent phonon shifts in a few-layer black phosphorus nanosheet sample prepared using micromechanical exfoliation on a 300 nm SiO2/Si substrate. The temperature dependent Raman spectroscopy experiments were carried out on a few-layer black phosphorus sample, which depicts softening of Ag(1), B2g, and Ag(2) modes as temperature increases from 77 to 673 K. The calculated temperature coefficients for Ag(1), B2g, and Ag(2) modes of the few-layer black phosphorus nanosheet sample were observed to be -0.01, -0.013, and -0.014 cm(-1) K(-1), respectively. The temperature dependent softening modes of black phosphorus results were explained on the basis of a double resonance process which is more active in an atomically thin sample. This process can also be fundamentally pertinent in other promising and emerging 2D ultrathin layer and heterostructured materials.

  18. Phonon-magnon interactions in body centered cubic iron: A combined molecular and spin dynamics study

    SciTech Connect

    Perera, Dilina Landau, David P.; Nicholson, Don M.; Malcolm Stocks, G.; Eisenbach, Markus; Yin, Junqi; Brown, Gregory

    2014-05-07

    Combining an atomistic many-body potential with a classical spin Hamiltonian parameterized by first principles calculations, molecular-spin dynamics computer simulations were performed to investigate phonon-magnon interactions in body centered cubic iron. Results obtained for spin-spin and density-density dynamic structure factors show that noticeable softening and damping of magnon modes occur due to the presence of lattice vibrations. Furthermore, as a result of the phonon-magnon coupling, additional longitudinal spin wave excitations are observed, with the same frequencies as the longitudinal phonon modes.

  19. Phonon-magnon interactions in BCC iron: A combined molecular and spin dynamics study

    SciTech Connect

    Perera, Meewanage Dilina N; Landau, David P; Nicholson, Don M; Stocks, George Malcolm; Eisenbach, Markus; Yin, Junqi; Brown, Greg

    2014-01-01

    Combining an atomistic many-body potential with a classical spin Hamiltonian pa- rameterized by first principles calculations, molecular-spin dynamics computer sim- ulations were performed to investigate phonon-magnon interactions in BCC iron. Results obtained for spin-spin and density-density dynamic structure factors show that noticeable softening and damping of magnon modes occur due to the presence of lattice vibrations. Furthermore, as a result of the phonon-magnon coupling, addi- tional longitudinal spin wave excitations are observed, with the same frequencies as the longitudinal phonon modes.

  20. Band structures and localization properties of aperiodic layered phononic crystals

    NASA Astrophysics Data System (ADS)

    Yan, Zhi-Zhong; Zhang, Chuanzeng

    2012-03-01

    The band structures and localization properties of in-plane elastic waves with coupling of longitudinal and transverse modes oblique propagating in aperiodic phononic crystals based on Thue-Morse and Rudin-Shapiro sequences are studied. Using transfer matrix method, the concept of the localization factor is introduced and the correctness is testified through the Rytov dispersion relation. For comparison, the perfect periodic structure and the quasi-periodic Fibonacci system are also considered. In addition, the influences of the random disorder, local resonance, translational and/or mirror symmetries on the band structures of the aperiodic phononic crystals are analyzed in this paper.

  1. Phonon spectra of alkali metals

    NASA Astrophysics Data System (ADS)

    Zeković, S.; Vukajlović, F.; Veljković, V.

    1982-10-01

    In this work we used a simple local model pseudopotential which includes screening for the phonon spectra calculations of alkali metals. The results obtained are in very good agreement with experimental data. In some branches of phonon spectra the differences between theoretical and experimental results are within 1-2%, while the maximum error is about 6%. The suggested form of the pseudopotential allows us to describe the phonon spectra of Na, K and Rb with only one, and, at the same time, a unique, parameter. In this case, the maximum disagreements from experiment are 9% for Na, 8% for K and 7% for Rb.

  2. Large-amplitude chirped coherent phonons in tellurium mediated by ultrafast photoexcited carrier diffusion

    NASA Astrophysics Data System (ADS)

    Kamaraju, N.; Kumar, Sunil; Anija, M.; Sood, A. K.

    2010-11-01

    We report femtosecond time-resolved reflectivity measurements of coherent phonons in tellurium performed over a wide range of temperatures (3-296 K) and pump-laser intensities. A totally symmetric A1 coherent phonon at 3.6 THz responsible for the oscillations in the reflectivity data is observed to be strongly positively chirped (i.e., phonon time period decreases at longer pump-probe delay times) with increasing photoexcited carrier density, more so at lower temperatures. We show that the temperature dependence of the coherent phonon frequency is anomalous (i.e, increasing with increasing temperature) at high photoexcited carrier density due to electron-phonon interaction. At the highest photoexcited carrier density of ˜1.4×1021cm-3 and the sample temperature of 3 K, the lattice displacement of the coherent phonon mode is estimated to be as high as ˜0.24Å . Numerical simulations based on coupled effects of optical absorption and carrier diffusion reveal that the diffusion of carriers dominates the nonoscillatory electronic part of the time-resolved reflectivity. Finally, using the pump-probe experiments at low carrier density of 6×1018cm-3 , we separate the phonon anharmonicity to obtain the electron-phonon coupling contribution to the phonon frequency and linewidth.

  3. Impact of Phonon Surface Scattering on Thermal Energy Distribution of Si and SiGe Nanowires

    PubMed Central

    Malhotra, Abhinav; Maldovan, Martin

    2016-01-01

    Thermal transport in nanostructures has attracted considerable attention in the last decade but the precise effects of surfaces on heat conduction have remained unclear due to a limited accuracy in the treatment of phonon surface scattering phenomena. Here, we investigate the impact of phonon-surface scattering on the distribution of thermal energy across phonon wavelengths and mean free paths in Si and SiGe nanowires. We present a rigorous and accurate description of phonon scattering at surfaces and predict and analyse nanowire heat spectra for different diameters and surface conditions. We show that the decrease in the diameter and increased roughness and correlation lengths makes the heat phonon spectra significantly shift towards short wavelengths and mean free paths. We also investigate the emergence of phonon confinement effects for small diameter nanowires and different surface scattering properties. Computed results for bulk materials show excellent agreement with recent experimentally-based approaches that reconstruct the mean-free-path heat spectra. Our phonon surface scattering model allows for an accurate theoretical extraction of heat spectra in nanowires and contributes to elucidate the development of critical phonon transport modes such as phonon confinement and coherent interference effects. PMID:27174699

  4. Phonon-phonon interactions: First principles theory

    SciTech Connect

    Gibbons, T. M.; Bebek, M. B.; Kang, By.; Stanley, C. M.; Estreicher, S. K.

    2015-08-28

    We present the details of a method to perform molecular-dynamics (MD) simulations without thermostat and with very small temperature fluctuations ±ΔT starting with MD step 1. It involves preparing the supercell at the time t = 0 in physically correct microstates using the eigenvectors of the dynamical matrix. Each initial microstate corresponds to a different distribution of kinetic and potential energies for each vibrational mode (the total energy of each microstate is the same). Averaging the MD runs over many initial microstates further reduces ΔT. The electronic states are obtained using first-principles theory (density-functional theory in periodic supercells). Three applications are discussed: the lifetime and decay of vibrational excitations, the isotope dependence of thermal conductivities, and the flow of heat at an interface.

  5. Phonon thermal conduction in novel 2D materials

    NASA Astrophysics Data System (ADS)

    Xu, Xiangfan; Chen, Jie; Li, Baowen

    2016-12-01

    Recently, there has been increasing interest in phonon thermal transport in low-dimensional materials, due to the crucial importance of dissipating and managing heat in micro- and nano-electronic devices. Significant progress has been achieved for one-dimensional (1D) systems, both theoretically and experimentally. However, the study of heat conduction in two-dimensional (2D) systems is still in its infancy due to the limited availability of 2D materials and the technical challenges of fabricating suspended samples that are suitable for thermal measurements. In this review, we outline different experimental techniques and theoretical approaches for phonon thermal transport in 2D materials, discuss the problems and challenges of phonon thermal transport measurements and provide a comparison between existing experimental data. Special attention will be given to the effects of size, dimensionality, anisotropy and mode contributions in novel 2D systems, including graphene, boron nitride, MoS2, black phosphorous and silicene.

  6. Phonon thermal conduction in novel 2D materials.

    PubMed

    Xu, Xiangfan; Chen, Jie; Li, Baowen

    2016-12-07

    Recently, there has been increasing interest in phonon thermal transport in low-dimensional materials, due to the crucial importance of dissipating and managing heat in micro- and nano-electronic devices. Significant progress has been achieved for one-dimensional (1D) systems, both theoretically and experimentally. However, the study of heat conduction in two-dimensional (2D) systems is still in its infancy due to the limited availability of 2D materials and the technical challenges of fabricating suspended samples that are suitable for thermal measurements. In this review, we outline different experimental techniques and theoretical approaches for phonon thermal transport in 2D materials, discuss the problems and challenges of phonon thermal transport measurements and provide a comparison between existing experimental data. Special attention will be given to the effects of size, dimensionality, anisotropy and mode contributions in novel 2D systems, including graphene, boron nitride, MoS2, black phosphorous and silicene.

  7. Evolution of molecular crystal optical phonons near structural phase transitions

    NASA Astrophysics Data System (ADS)

    Michki, Nigel; Niessen, Katherine; Xu, Mengyang; Markelz, Andrea

    Molecular crystals are increasingly important photonic and electronic materials. For example organic semiconductors are lightweight compared to inorganic semiconductors and have inexpensive scale up processing with roll to roll printing. However their implementation is limited by their environmental sensitivity, in part arising from the weak intermolecular interactions of the crystal. These weak interactions result in optical phonons in the terahertz frequency range. We examine the evolution of intermolecular interactions near structural phase transitions by measuring the optical phonons as a function of temperature and crystal orientation using terahertz time-domain spectroscopy. The measured orientation dependence of the resonances provides an additional constraint for comparison of the observed spectra with the density functional calculations, enabling us to follow specific phonon modes. We observe crystal reorganization near 350 K for oxalic acid as it transforms from dihydrate to anhydrous form. We also report the first THz spectra for the molecular crystal fructose through its melting point.

  8. Phonon anharmonicity in silicon from 100 to 1500 K

    DOE PAGES

    Kim, D. S.; Smith, Hillary L.; Niedziela, Jennifer L.; ...

    2015-01-21

    Inelastic neutron scattering was performed on silicon powder to measure the phonon density of states (DOS) from 100 to 1500 K. The mean fractional energy shifts with temperature of the modes weremore » $$\\langle$$Δεi/εiΔT$$\\rangle$$=₋0.07, giving a mean isobaric Grüneisen parameter of +6.95±0.67, which is significantly different from the isothermal parameter of +0.98. These large effects are beyond the predictions from quasiharmonic models using density functional theory or experimental data, demonstrating large effects from phonon anharmonicity. At 1500 K the anharmonicity contributes 0.15kB/atom to the vibrational entropy, compared to 0.03kB/atom from quasiharmonicity. Lastly, excellent agreement was found between the entropy from phonon DOS measurements and the reference NIST-JANAF thermodynamic entropy from calorimetric measurements.« less

  9. Phonon anharmonicity in silicon from 100 to 1500 K

    NASA Astrophysics Data System (ADS)

    Kim, D. S.; Smith, H. L.; Niedziela, J. L.; Li, C. W.; Abernathy, D. L.; Fultz, B.

    2015-01-01

    Inelastic neutron scattering was performed on silicon powder to measure the phonon density of states (DOS) from 100 to 1500 K. The mean fractional energy shifts with temperature of the modes were <Δ ɛi/ɛiΔ T >=-0.07 , giving a mean isobaric Grüneisen parameter of +6.95 ±0.67 , which is significantly different from the isothermal parameter of +0.98. These large effects are beyond the predictions from quasiharmonic models using density functional theory or experimental data, demonstrating large effects from phonon anharmonicity. At 1500 K the anharmonicity contributes 0.15 kB /atom to the vibrational entropy, compared to 0.03 kB /atom from quasiharmonicity. Excellent agreement was found between the entropy from phonon DOS measurements and the reference NIST-JANAF thermodynamic entropy from calorimetric measurements.

  10. Tunable polaritons from plasmon-phonon coupling in hyperbolic media

    NASA Astrophysics Data System (ADS)

    Dai, Siyuan; Ma, Qiong; Zhu, Shou-En; Liu, Mengkun; Andersen, Trond; Fei, Zhe; Goldflam, Michael; Wagner, Martin; Watanabe, Kenji; Taniguchi, Takashi; Thiemens, Mark; Keilmann, Fritz; Janssen, G. C. A. M.; Jarillo-Herrero, Pablo; Fogler, Michael; Basov, D. N.

    2015-03-01

    Using infrared nano-imaging and nano-spectroscopy, we report on the tunable hyperbolic response in heterostructures comprised of a monolayer graphene deposited on hexagonal boron nitride (G-hBN). Electrostatic gating of the top graphene layer allows for modification of the wavelength and intensity of hyperbolic phonon polaritons in bulk hBN. When compared with the pristine hBN polaritons, the graphene modified ones exhibit a longer wavelength in the Type II hyperbolic region and shorter wavelength in the Type I region. Because of this modification, we achieve a 90% increase of the propagation length for Type II polaritons in hBN. The physical origin of the modification is attributed to plasmon-phonon coupling in the G-hBN heterostructure. Our work provides a comprehensive study of plasmon-phonon coupling in a hyperbolic medium with the exploration of graphene's potential for modification of collective modes in other materials.

  11. Phonon anharmonicity in silicon from 100 to 1500 K

    SciTech Connect

    Kim, D. S.; Smith, Hillary L.; Niedziela, Jennifer L.; Li, Chen W.; Abernathy, Douglas L.; Fultz, B.

    2015-01-21

    Inelastic neutron scattering was performed on silicon powder to measure the phonon density of states (DOS) from 100 to 1500 K. The mean fractional energy shifts with temperature of the modes were $\\langle$ΔεiiΔT$\\rangle$=₋0.07, giving a mean isobaric Grüneisen parameter of +6.95±0.67, which is significantly different from the isothermal parameter of +0.98. These large effects are beyond the predictions from quasiharmonic models using density functional theory or experimental data, demonstrating large effects from phonon anharmonicity. At 1500 K the anharmonicity contributes 0.15kB/atom to the vibrational entropy, compared to 0.03kB/atom from quasiharmonicity. Lastly, excellent agreement was found between the entropy from phonon DOS measurements and the reference NIST-JANAF thermodynamic entropy from calorimetric measurements.

  12. Phonon symmetries in hexagonal boron nitride probed by incoherent light emission

    NASA Astrophysics Data System (ADS)

    Vuong, T. Q. P.; Cassabois, G.; Valvin, P.; Jacques, V.; Van Der Lee, A.; Zobelli, A.; Watanabe, K.; Taniguchi, T.; Gil, B.

    2017-03-01

    Layered compounds are stacks of weakly bound two-dimensional atomic crystals, with a prototypal hexagonal structure in graphene, transition metal dichalcogenides and boron nitride. This crystalline anisotropy results in vibrational modes with specific symmetries depending on the in-plane or out-of-plane atomic displacements. We show that polarization-resolved photoluminescence measurements in hexagonal boron nitride reflect the phonon symmetries in this layered semiconductor. Experiments performed with a detection on the sample edge, perpendicular to the c-axis, reveal the strong polarization-dependence of the emission lines corresponding to the recombination assisted by the three acoustic phonon modes. We elucidate the dipole orientation of the fundamental indirect exciton. We demonstrate evidence of the so-far missing phonon replica due to the optical out-of-plane phonon mode.

  13. Phonon-Mediated Nonclassical Interference in Diamond

    NASA Astrophysics Data System (ADS)

    England, Duncan G.; Fisher, Kent A. G.; MacLean, Jean-Philippe W.; Bustard, Philip J.; Heshami, Khabat; Resch, Kevin J.; Sussman, Benjamin J.

    2016-08-01

    Quantum interference of single photons is a fundamental aspect of many photonic quantum processing and communication protocols. Interference requires that the multiple pathways through an interferometer be temporally indistinguishable to within the coherence time of the photon. In this Letter, we use a diamond quantum memory to demonstrate interference between quantum pathways, initially temporally separated by many multiples of the optical coherence time. The quantum memory can be viewed as a light-matter beam splitter, mapping a THz-bandwidth single photon to a variable superposition of the output optical mode and stored phononic mode. Because the memory acts both as a beam splitter and as a buffer, the relevant coherence time for interference is not that of the photon, but rather that of the memory. We use this mechanism to demonstrate nonclassical single-photon and two-photon interference between quantum pathways initially separated by several picoseconds, even though the duration of the photons themselves is just ˜250 fs .

  14. Phonon dynamics of neptunium chalcogenides

    NASA Astrophysics Data System (ADS)

    Aynyas, Mahendra; Rukmangad, Aditi; Arya, Balwant S.; Sanyal, Sankar P.

    2012-06-01

    We have performed phonon calculations of Neptunium Chalcogenides (NpX) (X= S, Se, Te) based on breathing shell model (BSM) which includes breathing motion of electron of the Np-atoms due to f-d hybridization. The model predicts that the short range breathing phenomenon play a dominant role in the phonon properties. We also report, for the first time specific heat for these compounds.

  15. Phonons in Si24 at simultaneously elevated temperature and pressure

    NASA Astrophysics Data System (ADS)

    Tong, Xiao; Xu, Xiaolin; Fultz, B.; Zhang, Haidong; Strobel, Timothy A.; Kim, Duck Young

    2017-03-01

    Raman spectroscopy was used to measure the frequencies of phonons in Si24 with an open clathrate structure at temperatures from 80 to 400 K with simultaneous pressures of 0 to 8 GPa. The frequency shifts of the different phonons were substantially different under either temperature or pressure. The quasiharmonic behavior was isolated by varying pressure at low temperatures, and the anharmonic behavior was isolated by varying temperature at low pressures. Phonon modes dominated by bond bending were anomalous, showing stiffening with temperature and softening with pressure. Both the quasiharmonic behavior and the anharmonic behavior changed markedly with simultaneous changes in temperature Δ T and pressure Δ P . With Δ T =320 K and Δ P =8 GPa , some frequency shifts that scaled with the product Δ T Δ P were as large as the shifts from Δ T and Δ P alone. The thermodynamic entropy of this material likely has a dependence on Δ T and Δ P that cannot be obtained by adding effects from quasiharmonicity and phonon-phonon anharmonicity.

  16. Anomalous phonon characteristics of unconventional novel III-N superlattices

    SciTech Connect

    Talwar, Devki N.

    2014-03-31

    Comprehensive results of atomic vibrations are reported in the unconventional short-period zb BN/GaN superlatices (SLs) by exploiting a rigid-ion-model and taking into account both the short- and long-range Coulomb interactions. Besides anisotropic mode behavior of optical phonons, our study provided evidence of acoustic-mode anti-crossing, mini-gap formation, confinement as well as BN-like modes falling within the gap that separates optical phonon bands of the two materials. A bond-polarizability scheme is employed within the second-nearest-neighbor linear-chain model to simulate the Raman intensity profiles of BN/GaN SLs revealing major expected trends of the vibrational characteristics observed experimentally in many conventional superlattice systems while eliciting some interesting contrasts.

  17. Magnetic moments induce strong phonon renormalization in FeSi

    PubMed Central

    Krannich, S.; Sidis, Y.; Lamago, D.; Heid, R.; Mignot, J.-M.; Löhneysen, H. v.; Ivanov, A.; Steffens, P.; Keller, T.; Wang, L.; Goering, E.; Weber, F.

    2015-01-01

    The interactions of electronic, spin and lattice degrees of freedom in solids result in complex phase diagrams, new emergent phenomena and technical applications. While electron–phonon coupling is well understood, and interactions between spin and electronic excitations are intensely investigated, only little is known about the dynamic interactions between spin and lattice excitations. Noncentrosymmetric FeSi is known to undergo with increasing temperature a crossover from insulating to metallic behaviour with concomitant magnetic fluctuations, and exhibits strongly temperature-dependent phonon energies. Here we show by detailed inelastic neutron-scattering measurements and ab initio calculations that the phonon renormalization in FeSi is linked to its unconventional magnetic properties. Electronic states mediating conventional electron–phonon coupling are only activated in the presence of strong magnetic fluctuations. Furthermore, phonons entailing strongly varying Fe–Fe distances are damped via dynamic coupling to the temperature-induced magnetic moments, highlighting FeSi as a material with direct spin–phonon coupling and multiple interaction paths. PMID:26611619

  18. Hybrid sliding mode control of semi-active suspension systems

    NASA Astrophysics Data System (ADS)

    Assadsangabi, Babak; Eghtesad, Mohammad; Daneshmand, Farhang; Vahdati, Nader

    2009-12-01

    In order to design a controller which can take both ride comfort and road holding into consideration, a hybrid model reference sliding mode controller (HMRSMC) is proposed. The controller includes two separate model reference sliding mode controllers (MRSMC). One of the controllers is designed so as to force the plant to follow the ideal Sky-hook model and the other is to force the plant to follow the ideal Ground-hook model; then the outputs of these two controllers are linearly combined and applied to the plant as the input. Also, since the designed controller requires a knowledge of the terrain input, this input is approximated by the unsprung mass displacement. Finally, in the simulation section of this study, the effect of the relative ratio between the two MRSMCs and the knowledge of the terrain on the performance of the controller is numerically investigated for both steady-state and transient cases.

  19. Phonons of the anomalous element cerium

    PubMed Central

    Krisch, Michael; Farber, D. L.; Xu, R.; Antonangeli, Daniele; Aracne, C. M.; Beraud, Alexandre; Chiang, Tai-Chang; Zarestky, J.; Kim, Duck Young; Isaev, Eyvaz I.; Ahuja, Rajeev; Johansson, Börje

    2011-01-01

    Many physical and chemical properties of the light rare-earths and actinides are governed by the active role of f electrons, and despite intensive efforts the details of the mechanisms of phase stability and transformation are not fully understood. A prominent example which has attracted a lot of interest, both experimentally and theoretically over the years is the isostructural γ - α transition in cerium. We have determined by inelastic X-ray scattering, the complete phonon dispersion scheme of elemental cerium across the γ → α transition, and compared it with theoretical results using ab initio lattice dynamics. Several phonon branches show strong changes in the dispersion shape, indicating large modifications in the interactions between phonons and conduction electrons. This is reflected as well by the lattice Grüneisen parameters, particularly around the X point. We derive a vibrational entropy change , illustrating the importance of the lattice contribution to the transition. Additionally, we compare first principles calculations with the experiments to shed light on the mechanism underlying the isostructural volume collapse in cerium under pressure. PMID:21597000

  20. Modulations of thermal properties of graphene by strain-induced phonon engineering

    NASA Astrophysics Data System (ADS)

    Tada, Kento; Funatani, Takashi; Konabe, Satoru; Sasaoka, Kenji; Ogawa, Matsuto; Souma, Satofumi; Yamamoto, Takahiro

    2017-02-01

    Modulation of the thermal properties of graphene due to strain-induced phononic band engineering was theoretically investigated by first-principles calculations based on the density functional theory. The high-energy phonon modes are found to exhibit softening owing to the strain, whereas a low-energy acoustic mode (out-of-plane mode) exhibits hardening. Moreover, the dispersion relation of the out-of-plane mode associated with the strain essentially changes from quadratic (∝ k 2) to linear (∝ k). Accordingly, the temperature dependence of the low-temperature specific heat also changes from linear (∝ T) to quadratic (∝ T 2).

  1. Coherent Excitation of Optical Phonons in GaAs by Broadband Terahertz Pulses

    PubMed Central

    Fu, Zhengping; Yamaguchi, Masashi

    2016-01-01

    Coherent excitation and control of lattice motion by electromagnetic radiation in optical frequency range has been reported through variety of indirect interaction mechanisms with phonon modes. However, coherent phonon excitation by direct interaction of electromagnetic radiation and nuclei has not been demonstrated experimentally in terahertz (THz) frequency range mainly due to the lack of THz emitters with broad bandwidth suitable for the purpose. We report the experimental observation of coherent phonon excitation and detection in GaAs using ultrafast THz-pump/optical-probe scheme. From the results of THz pump field dependence, pump/probe polarization dependence, and crystal orientation dependence, we attributed THz wave absorption and linear electro-optic effect to the excitation and detection mechanisms of coherent polar TO phonons. Furthermore, the carrier density dependence of the interaction of coherent phonons and free carriers is reported. PMID:27905563

  2. Phonon-based scalable platform for chip-scale quantum computing

    NASA Astrophysics Data System (ADS)

    Reinke, Charles M.; El-Kady, Ihab

    2016-12-01

    We present a scalable phonon-based quantum computer on a phononic crystal platform. Practical schemes involve selective placement of a single acceptor atom in the peak of the strain field in a high-Q phononic crystal cavity that enables coupling of the phonon modes to the energy levels of the atom. We show theoretical optimization of the cavity design and coupling waveguide, along with estimated performance figures of the coupled system. A qubit can be created by entangling a phonon at the resonance frequency of the cavity with the atom states. Qubits based on this half-sound, half-matter quasi-particle, called a phoniton, may outcompete other quantum architectures in terms of combined emission rate, coherence lifetime, and fabrication demands.

  3. Inelastic x-ray scattering measurements of phonon dynamics in URu2Si2

    DOE PAGES

    Gardner, D. R.; Bonnoit, C. J.; Chisnell, R.; ...

    2016-02-11

    In this paper, we study high-resolution inelastic x-ray scattering measurements of the acoustic phonons of URu2Si2. At all temperatures, the longitudinal acoustic phonon linewidths are anomalously broad at small wave vectors revealing a previously unknown anharmonicity. The phonon modes do not change significantly upon cooling into the hidden order phase. In addition, our data suggest that the increase in thermal conductivity in the hidden order phase cannot be driven by a change in phonon dispersions or lifetimes. Hence, the phonon contribution to the thermal conductivity is likely much less significant compared to that of the magnetic excitations in the lowmore » temperature phase.« less

  4. Hole-interface optical phonon relaxation rates with valence band-mixing effects

    NASA Astrophysics Data System (ADS)

    Kim, Cheol-Hoi

    2004-05-01

    We theoretically investigate the hole-interface optical phonon scattering rates for a InGaAs-AlGaAs quantum well structure, taking into account the valence-band mixing. The dispersion relation and the electrostatic potentials for interface optical phonon modes are obtained based on the macroscopic dielectric continuum model. For the hole dispersion relation, the Luttinger-Kohn Hamiltonian is used. The hole-interface optical phonon interaction is evaluated by the Fermi's golden rule taking into account the Bloch overlap factor. Our results show that the hole-interface phonon scattering rates within the parabolic band approximation are different from those including valence band mixing effects. Especially, in the low energy region, the hole-interface phonon scattering rates within the parabolic band approximation are overestimated very significantly.

  5. Proposal for a phonon laser utilizing quantum-dot spin states

    NASA Astrophysics Data System (ADS)

    Khaetskii, Alexander; Golovach, Vitaly; Hu, Xuedong; Zutic, Igor

    2014-03-01

    We propose a nano-scale realization of a phonon laser utilizing phonon-assisted spin flips in quantum dots to amplify sound. Owing to a long spin relaxation time, the device can be operated in a strong pumping regime, in which the population inversion is close to its maximal value allowed under Fermi statistics. In this regime, the threshold for stimulated emission is unaffected by spontaneous spin flips. Considering a nanowire with quantum dots defined along its length, we show that a further improvement arises from confining the phonons to one dimension, and thus reducing the number of phonon modes available for spontaneous emission. Our work calls for the development of nanowire-based, high-finesse phonon resonators. Supported by the SPINMET Project (FP7-PEOPLE-2009-IRSES), by the ONR, DOE-BES DE-SC0004890, NSF-ECCS, US ARO, NSF-PIF, and by the Project FIS2011-28851-C02-02.

  6. Phonons and thermal transport in graphene and graphene-based materials

    NASA Astrophysics Data System (ADS)

    Nika, Denis L.; Balandin, Alexander A.

    2017-03-01

    A discovery of the unusual thermal properties of graphene stimulated experimental, theoretical and computational research directed at understanding phonon transport and thermal conduction in two-dimensional material systems. We provide a critical review of recent results in the graphene thermal field focusing on phonon dispersion, specific heat, thermal conductivity, and comparison of different models and computational approaches. The correlation between the phonon spectrum in graphene-based materials and the heat conduction properties is analyzed in details. The effects of the atomic plane rotations in bilayer graphene, isotope engineering, and relative contributions of different phonon dispersion branches are discussed. For readers’ convenience, the summaries of main experimental and theoretical results on thermal conductivity as well as phonon mode contributions to thermal transport are provided in the form of comprehensive annotated tables.

  7. Phonon-based scalable platform for chip-scale quantum computing

    SciTech Connect

    Reinke, Charles M.; El-Kady, Ihab

    2016-12-19

    Here, we present a scalable phonon-based quantum computer on a phononic crystal platform. Practical schemes involve selective placement of a single acceptor atom in the peak of the strain field in a high-Q phononic crystal cavity that enables coupling of the phonon modes to the energy levels of the atom. We show theoretical optimization of the cavity design and coupling waveguide, along with estimated performance figures of the coupled system. A qubit can be created by entangling a phonon at the resonance frequency of the cavity with the atom states. Qubits based on this half-sound, half-matter quasi-particle, called a phoniton, may outcompete other quantum architectures in terms of combined emission rate, coherence lifetime, and fabrication demands.

  8. Phonon-based scalable platform for chip-scale quantum computing

    DOE PAGES

    Reinke, Charles M.; El-Kady, Ihab

    2016-12-19

    Here, we present a scalable phonon-based quantum computer on a phononic crystal platform. Practical schemes involve selective placement of a single acceptor atom in the peak of the strain field in a high-Q phononic crystal cavity that enables coupling of the phonon modes to the energy levels of the atom. We show theoretical optimization of the cavity design and coupling waveguide, along with estimated performance figures of the coupled system. A qubit can be created by entangling a phonon at the resonance frequency of the cavity with the atom states. Qubits based on this half-sound, half-matter quasi-particle, called a phoniton,more » may outcompete other quantum architectures in terms of combined emission rate, coherence lifetime, and fabrication demands.« less

  9. New improved reconstruction of solar activity over 3 millennia: Evidence for distinct solar dynamo modes

    NASA Astrophysics Data System (ADS)

    Usoskin, Ilya; Kovaltsov, Gennady; Hulot, Gauthier.; Gallet, Yves; Roth, Raphael; Licht, Alexis; Joos, Fortunat; Th, E.; Khokhlov, A.; Kovaltsov, Gennady A.

    The solar magnetic dynamo can operate in distinct modes - a main general mode, a Grand minimum mode corresponding to an inactive Sun, and a possible Grand maximum mode corresponding to an unusually active Sun, as e.g., observed recently. The reality of such mode separation has recently been the subject of much debate, with different theoretical speculations discussed. Here we present the first adjustment-free physical reconstruction of solar activity over the past three millennia, using the latest carbon cycle, (14) C production and archeomagnetic field models. This new improved reconstruction shows that the solar dynamo process indeed switches between different modes, either corresponding to different regimes of the dynamo or to changes in the driving parameters. These results provide important constraints for both dynamo models of Sun-like stars and investigations of possible solar influence on Earth’s climate.

  10. External kink modes as a model for MHD activity associated with ELMs

    SciTech Connect

    Manickam, J.

    1992-01-01

    Tokamak plasmas in the high confinement mode of operation are known to exhibit edge localized activity referred to as ELMs. A model is proposed for the underlying cause in terms of the external kink mode. The build up of the current density near the plasma edge is shown to decrease the shear in the safety-factor, q, profile and lead to destabilization of the kink mode. The role of the plasma geometry and equilibrium profiles is discussed.

  11. Cooling phonons with phonons: Acoustic reservoir engineering with silicon-vacancy centers in diamond

    NASA Astrophysics Data System (ADS)

    Kepesidis, K. V.; Lemonde, M.-A.; Norambuena, A.; Maze, J. R.; Rabl, P.

    2016-12-01

    We study a setup where a single negatively-charged silicon-vacancy center in diamond is magnetically coupled to a low-frequency mechanical bending mode and via strain to the high-frequency phonon continuum of a semiclamped diamond beam. We show that under appropriate microwave driving conditions, this setup can be used to induce a laser-cooling-like effect for the low-frequency mechanical vibrations, where the high-frequency longitudinal compression modes of the beam serve as an intrinsic low-temperature reservoir. We evaluate the experimental conditions under which cooling close to the quantum ground state can be achieved and describe an extended scheme for the preparation of a stationary entangled state between two mechanical modes. By relying on intrinsic properties of the mechanical beam only, this approach offers an interesting alternative for quantum manipulation schemes of mechanical systems, where otherwise efficient optomechanical interactions are not available.

  12. Disturbance observer based sliding mode control of active suspension systems

    NASA Astrophysics Data System (ADS)

    Deshpande, Vaijayanti S.; Mohan, B.; Shendge, P. D.; Phadke, S. B.

    2014-05-01

    In this paper, a novel scheme to reduce the acceleration of the sprung mass, used in combination with sliding mode control, is proposed. The proposed scheme estimates the effects of the uncertain, nonlinear spring and damper, load variation and the unknown road disturbance. The controller needs the states of sprung mass only, obviating the need to measure the states of the unsprung mass. The ultimate boundedness of the overall suspension system is proved. The efficacy of the method is verified through simulations for three different types of road profiles and load variation and the scheme is validated on an experimental setup. The results are compared with passive suspension system.

  13. Momentum relaxation due to polar optical phonons in AlGaN/GaN heterostructures

    NASA Astrophysics Data System (ADS)

    Zhang, J.-Z.; Dyson, A.; Ridley, B. K.

    2011-10-01

    Using the dielectric continuum (DC) model, momentum relaxation rates are calculated for electrons confined in quasi-two-dimensional (quasi-2D) channels of AlGaN/GaN heterostructures. Particular attention is paid to the effects of half-space and interface modes on the momentum relaxation. The total momentum relaxation rates are compared with those evaluated by the three-dimensional phonon (3DP) model, and also with the Callen results for bulk GaN. In heterostructures with a wide channel (effective channel width >100 Å), the DC and 3DP models yield very close momentum relaxation rates. Only for narrow-channel heterostructures do interface phonons become important in momentum relaxation processes, and an abrupt threshold occurs for emission of interface as well as half-space phonons. For a 30-Å GaN channel, for instance, the 3DP model is found to underestimate rates just below the bulk phonon energy by 70% and overestimate rates just above the bulk phonon energy by 40% compared to the DC model. Owing to the rapid decrease in the electron-phonon interaction with the phonon wave vector, negative momentum relaxation rates are predicted for interface phonon absorption in usual GaN channels. The total rates remain positive due to the dominant half-space phonon scattering. The quasi-2D rates can have substantially higher peak values than the three-dimensional rates near the phonon emission threshold. Analytical expressions for momentum relaxation rates are obtained in the extreme quantum limits (i.e., the threshold emission and the near subband-bottom absorption). All the results are well explained in terms of electron and phonon densities of states.

  14. Hong-Ou-Mandel interference of two phonons in trapped ions

    NASA Astrophysics Data System (ADS)

    Toyoda, Kenji; Hiji, Ryoto; Noguchi, Atsushi; Urabe, Shinji

    2015-11-01

    The quantum statistics of bosons and fermions manifest themselves in the manner in which two indistinguishable particles interfere quantum mechanically. When two photons, which are bosonic particles, enter a beam-splitter with one photon in each input port, they bunch together at either of the two output ports. The corresponding disappearance of the coincidence count is the Hong-Ou-Mandel effect. Here we show the phonon counterpart of this effect in a system of trapped-ion phonons, which are collective excitations derived by quantizing vibrational motions that obey Bose-Einstein statistics. We realize a beam-splitter transformation of the phonons by employing the mutual Coulomb repulsion between ions, and perform a two-phonon quantum interference experiment using that transformation. We observe an almost perfect disappearance of the phonon coincidence between two ion sites, confirming that phonons can be considered indistinguishable bosonic particles. The two-particle interference demonstrated here is purely a quantum effect, without a classical counterpart, hence it should be possible to demonstrate the existence of entanglement on this basis. We attempt to generate an entangled state of phonons at the centre of the Hong-Ou-Mandel dip in the coincidence temporal profile, under the assumption that the entangled phonon state is successfully generated if the fidelity of the analysis pulses is taken into account adequately. Two-phonon interference, as demonstrated here, proves the bosonic nature of phonons in a trapped-ion system. It opens the way to establishing phonon modes as carriers of quantum information in their own right, and could have implications for the quantum simulation of bosonic particles and analogue quantum computation via boson sampling.

  15. Hong-Ou-Mandel interference of two phonons in trapped ions.

    PubMed

    Toyoda, Kenji; Hiji, Ryoto; Noguchi, Atsushi; Urabe, Shinji

    2015-11-05

    The quantum statistics of bosons and fermions manifest themselves in the manner in which two indistinguishable particles interfere quantum mechanically. When two photons, which are bosonic particles, enter a beam-splitter with one photon in each input port, they bunch together at either of the two output ports. The corresponding disappearance of the coincidence count is the Hong-Ou-Mandel effect. Here we show the phonon counterpart of this effect in a system of trapped-ion phonons, which are collective excitations derived by quantizing vibrational motions that obey Bose-Einstein statistics. We realize a beam-splitter transformation of the phonons by employing the mutual Coulomb repulsion between ions, and perform a two-phonon quantum interference experiment using that transformation. We observe an almost perfect disappearance of the phonon coincidence between two ion sites, confirming that phonons can be considered indistinguishable bosonic particles. The two-particle interference demonstrated here is purely a quantum effect, without a classical counterpart, hence it should be possible to demonstrate the existence of entanglement on this basis. We attempt to generate an entangled state of phonons at the centre of the Hong-Ou-Mandel dip in the coincidence temporal profile, under the assumption that the entangled phonon state is successfully generated if the fidelity of the analysis pulses is taken into account adequately. Two-phonon interference, as demonstrated here, proves the bosonic nature of phonons in a trapped-ion system. It opens the way to establishing phonon modes as carriers of quantum information in their own right, and could have implications for the quantum simulation of bosonic particles and analogue quantum computation via boson sampling.

  16. Well width dependence of electron-phonon interaction in ZnSe/ZnS xSe 1- x superlattices determined by micro-raman spectroscopy

    NASA Astrophysics Data System (ADS)

    Spagnolo, V.; Scamarcio, G.; Corvasce, C.; Lugará, M.; Suemune, I.

    1994-07-01

    We report a study of multiphonon resonant Raman scattering in a series of symmetric (ZnSe) d(ZnS 1- xSe x) d superlattices (SL), 20Å ≤ d ≤ 150Å. In addition to confined optical phonons, the energies of interface (IF) modes with in-plane wavevectors have been assessed by means of micro-probe Raman measurements in backscattering from the SL edge. The comparison between one- and two-phonon spectra shows that the electron-phonon interaction is dominated by ZnSe-like IF phonons for d < 50Å, whereas LO 2 phonons prevail for larger well.

  17. Sub-Poissonian phonon statistics in an acoustical resonator coupled to a pumped two-level emitter

    NASA Astrophysics Data System (ADS)

    Ceban, V.; Macovei, M. A.

    2015-11-01

    The concept of an acoustical analog of the optical laser has been developed recently in both theoretical and experimental works. We here discuss a model of a coherent phonon generator with a direct signature of the quantum properties of sound vibrations. The considered setup is made of a laser-driven quantum dot embedded in an acoustical nanocavity. The system dynamics is solved for a single phonon mode in the steady-state and in the strong quantum dot—phonon coupling regime beyond the secular approximation. We demonstrate that the phonon statistics exhibits quantum features, i.e., is sub-Poissonian.

  18. Sub-Poissonian phonon statistics in an acoustical resonator coupled to a pumped two-level emitter

    SciTech Connect

    Ceban, V. Macovei, M. A.

    2015-11-15

    The concept of an acoustical analog of the optical laser has been developed recently in both theoretical and experimental works. We here discuss a model of a coherent phonon generator with a direct signature of the quantum properties of sound vibrations. The considered setup is made of a laser-driven quantum dot embedded in an acoustical nanocavity. The system dynamics is solved for a single phonon mode in the steady-state and in the strong quantum dot—phonon coupling regime beyond the secular approximation. We demonstrate that the phonon statistics exhibits quantum features, i.e., is sub-Poissonian.

  19. Fuzzy-logic-based hybrid locomotion mode classification for an active pelvis orthosis: Preliminary results.

    PubMed

    Yuan, Kebin; Parri, Andrea; Yan, Tingfang; Wang, Long; Munih, Marko; Vitiello, Nicola; Wang, Qining

    2015-01-01

    In this paper, we present a fuzzy-logic-based hybrid locomotion mode classification method for an active pelvis orthosis. Locomotion information measured by the onboard hip joint angle sensors and the pressure insoles is used to classify five locomotion modes, including two static modes (sitting, standing still), and three dynamic modes (level-ground walking, ascending stairs, and descending stairs). The proposed method classifies these two kinds of modes first by monitoring the variation of the relative hip joint angle between the two legs within a specific period. Static states are then classified by the time-based absolute hip joint angle. As for dynamic modes, a fuzzy-logic based method is proposed for the classification. Preliminary experimental results with three able-bodied subjects achieve an off-line classification accuracy higher than 99.49%.

  20. Size-Dependent Coherent-Phonon Plasmon Modulation and Deformation Characterization in Gold Bipyramids and Nanojavelins

    SciTech Connect

    Kirschner, Matthew S.; Lethiec, Clotilde M.; Lin, Xiao-Min; Schatz, George C.; Chen, Lin X.; Schaller, Richard D.

    2016-05-18

    Localized surface plasmon resonances (LSPRs) arising from metallic nanoparticles offer an array of prospective applications that range from chemical sensing to biotherapies. Bipyramidal particles exhibit particularly narrow ensemble LSPR resonances that reflect small dispersity of size and shape but until recently were only synthetically accessible over a limited range of sizes with corresponding aspect ratios. Narrow size dispersion offers the opportunity to examine ensemble dynamical phenomena such as coherent phonons that induce periodic oscillations of the LSPR energy. Here, we characterize transient optical behavior of a large range of gold bipyramid sizes, as well as higher aspect ratio nanojavelin ensembles with specific attention to the lowest-order acoustic phonon mode of these nanoparticles. We report coherent phonon-driven oscillations of the LSPR position for particles with resonances spanning 670 to 1330 nm. Nanojavelins were shown to behave similarly to bipyramids but offer the prospect of separate control over LSPR energy and coherent phonon oscillation period. We develop a new methodology for quantitatively measuring mechanical expansion caused by photogenerated coherent phonons. Using this method, we find an elongation of approximately 1% per photon absorbed per unit cell and that particle expansion along the lowest frequency acoustic phonon mode is linearly proportional to excitation fluence for the fluence range studied. These characterizations provide insight regarding means to manipulate phonon period and transient mechanical deformation.

  1. Anomalous phonon behavior in superconducting CaKFe4As4: An optical study

    DOE PAGES

    Yang, Run; Dai, Yaomin; Xu, Bing; ...

    2017-02-08

    Here, the temperature dependence of ab-plane optical conductivity of CaKFe4As4 has been measured below and above its superconducting transition temperature Tc≃35.5 K. In the normal state, analysis with the two-Drude model reveals a T-linear scattering rate for the coherent response, which suggests strong spin-fluctuation scattering. Below the superconducting transition, the optical conductivity below 120 cm–1 vanishes, indicating nodeless gap(s). The Mattis-Bardeen fitting in the superconducting state gives two gaps of Δ1 ≃ 9 meV and Δ2 ≃ 14 meV, in good agreement with recent angle-resolved photoemission spectroscopy (ARPES) results. In addition, around 255 cm–1, we observe two different infrared-active Fe-Asmore » modes with obvious asymmetric lineshape, originating from strong coupling between lattice vibrations and spin or charge excitations. Considering a moderate Hund's rule coupling determined from spectral weight analysis, we propose that the strong fluctuations induced by the coupling between itinerant carriers and local moments may affect the phonon mode, and the electron-phonon coupling through the spin channel is likely to play an important role in the unconventional pairing in iron-based superconductors.« less

  2. Anomalous phonon behavior in superconducting CaKFe4As4 : An optical study

    NASA Astrophysics Data System (ADS)

    Yang, Run; Dai, Yaomin; Xu, Bing; Zhang, Wei; Qiu, Ziyang; Sui, Qiangtao; Homes, Christopher C.; Qiu, Xianggang

    2017-02-01

    The temperature dependence of a b -plane optical conductivity of CaKFe4As4 has been measured below and above its superconducting transition temperature Tc≃35.5 K. In the normal state, analysis with the two-Drude model reveals a T -linear scattering rate for the coherent response, which suggests strong spin-fluctuation scattering. Below the superconducting transition, the optical conductivity below 120 cm-1 vanishes, indicating nodeless gap(s). The Mattis-Bardeen fitting in the superconducting state gives two gaps of Δ1≃9 meV and Δ2≃14 meV, in good agreement with recent angle-resolved photoemission spectroscopy (ARPES) results. In addition, around 255 cm-1, we observe two different infrared-active Fe-As modes with obvious asymmetric lineshape, originating from strong coupling between lattice vibrations and spin or charge excitations. Considering a moderate Hund's rule coupling determined from spectral weight analysis, we propose that the strong fluctuations induced by the coupling between itinerant carriers and local moments may affect the phonon mode, and the electron-phonon coupling through the spin channel is likely to play an important role in the unconventional pairing in iron-based superconductors.

  3. Phonon effects on the current noise spectra and the ac conductance of a single molecular junction.

    PubMed

    Ding, Guo-Hui; Dong, Bing

    2014-07-30

    By using nonequilibrium Green's functions and the equation of motion method, we formulate a self-consistent field theory for the electron transport through a single-molecule junction (SMJ) coupled with a vibrational mode. We show that the nonequilibrium dynamics of the phonons in a strong electron-phonon coupling regime can be taken into account appropriately in this self-consistent perturbation theory, and the self-energy of the phonons is connected with the current fluctuations in the molecular junction. We calculate the finite-frequency nonsymmetrized noise spectra and the ac conductance, which reveal a wealth of inelastic electron tunneling characteristics on the absorption and emission properties of this SMJ. In the presence of a finite bias voltage and the electron tunneling current, the vibration mode of the molecular junction is heated and driven to an unequilibrated state. The influences of unequilibrated phonons on the current and the noise spectra are investigated.

  4. Phonon effects on the current noise spectra and the ac conductance of a single molecular junction

    NASA Astrophysics Data System (ADS)

    Ding, Guo-Hui; Dong, Bing

    2014-07-01

    By using nonequilibrium Green’s functions and the equation of motion method, we formulate a self-consistent field theory for the electron transport through a single-molecule junction (SMJ) coupled with a vibrational mode. We show that the nonequilibrium dynamics of the phonons in a strong electron-phonon coupling regime can be taken into account appropriately in this self-consistent perturbation theory, and the self-energy of the phonons is connected with the current fluctuations in the molecular junction. We calculate the finite-frequency nonsymmetrized noise spectra and the ac conductance, which reveal a wealth of inelastic electron tunneling characteristics on the absorption and emission properties of this SMJ. In the presence of a finite bias voltage and the electron tunneling current, the vibration mode of the molecular junction is heated and driven to an unequilibrated state. The influences of unequilibrated phonons on the current and the noise spectra are investigated.

  5. Modulation of thermal conductivity in kinked silicon nanowires: phonon interchanging and pinching effects.

    PubMed

    Jiang, Jin-Wu; Yang, Nuo; Wang, Bing-Shen; Rabczuk, Timon

    2013-04-10

    We perform molecular dynamics simulations to investigate the reduction of the thermal conductivity by kinks in silicon nanowires. The reduction percentage can be as high as 70% at room temperature. The temperature dependence of the reduction is also calculated. By calculating phonon polarization vectors, two mechanisms are found to be responsible for the reduced thermal conductivity: (1) the interchanging effect between the longitudinal and transverse phonon modes and (2) the pinching effect, that is, a new type of localization, for the twisting and transverse phonon modes in the kinked silicon nanowires. Our work demonstrates that the phonon interchanging and pinching effects, induced by kinking, are brand-new and effective ways in modulating heat transfer in nanowires, which enables the kinked silicon nanowires to be a promising candidate for thermoelectric materials.

  6. Raman scattering study of anharmonic phonon decay in InN

    NASA Astrophysics Data System (ADS)

    Domènech-Amador, Núria; Cuscó, Ramon; Artús, Luis; Yamaguchi, Tomohiro; Nanishi, Yasushi

    2011-06-01

    We present Raman scattering measurements on wurtzite InN over a temperature range from 80 to 660 K. To investigate all phonon modes of the wurtzite structure, measurements were performed on c and m faces of high-quality InN epilayers. High-resolution measurements of the low-frequency E2 mode reveal a slight anharmonic broadening of such a long-lived phonon due to up-conversion processes and a substantial contribution of background impurity broadening in the determination of its linewidth. An analysis of the anharmonicity and lifetimes of the InN phonons is carried out. Possible decay channels including up-conversion processes and four-phonon processes are discussed on the basis of density functional theory calculations.

  7. Soft Phonons in (delta)-Phase Plutonium Near the (delta)-(alpha)' Transition

    SciTech Connect

    Xu, R; Wong, J; Zshack, P; Hong, H; Chiang, T

    2007-09-13

    Plutonium and its alloys exhibit complex phase diagrams that imply anomalous lattice dynamics near phase stability boundaries. Specifically, the TA [111] phonon branch in Ga-stabilized {delta}-Pu at room temperature shows a pronounced soft mode at the zone boundary, which suggests a possible connection to the martensitic transformation from the fcc {delta}-phase to the monoclinic {alpha}{prime}-phase at low temperatures. This work is a study of the lattice dynamics of this system by x-ray thermal diffuse scattering. The results reveal little temperature dependence of the phonon frequencies, thus indicating that kinetic phonon softening is not responsible for this phase transition.

  8. Phonon densities of states of face-centered-cubic Ni-Fe alloys

    SciTech Connect

    Lucas, Matthew; Mauger, L; Munoz, Jorge A.; Halevy, I; Horwath, J; Semiatin, S L; Leontsev, S. O.; Stone, Matthew B; Abernathy, Douglas L; Xiao, Yuming; Chow, P; Fultz, B.

    2013-01-01

    Inelastic neutron scattering and nuclear resonant inelastic x-ray scattering were used to determine the phonon densities of states of face-centered-cubic Ni-Fe alloys. Increasing Fe concentration results in an average softening of the phonon modes. Chemical ordering of the Ni0.72Fe0.28 alloy results in a reduction of the partial vibrational entropy of the Fe atoms but does not significantly change the partial vibrational entropy of the Ni atoms. Changes in the phonon densities of states with composition and chemical ordering are discussed and analyzed with a cluster expansion method.

  9. Impact of temperature-dependent local and global spin order in RMnO3 compounds for spin-phonon coupling and electromagnon activity

    NASA Astrophysics Data System (ADS)

    Elsässer, S.; Schiebl, M.; Mukhin, A. A.; Balbashov, A. M.; Pimenov, A.; Geurts, J.

    2017-01-01

    The orthorhombic rare-earth manganite compounds RMnO3 show a global magnetic order for T< {T}N, and several representatives are multiferroic with a cycloidal spin ground state order for T< {T}{{cycl}}< {T}N≈ 40 {{K}}. We deduce from the temperature dependence of spin-phonon coupling in Raman spectroscopy for a series of RMnO3 compounds that their spin order locally persists up to about twice T N . Along the same line, our observation of the persistence of the electromagnon in GdMnO3 up to T≈ 100 {{K}} is attributed to a local cycloidal spin order for T> {T}{{cycl}}, in contrast to the hitherto assumed incommensurate sinusoidal phase in the intermediate temperature range. The development of the magnetization pattern can be described in terms of an order-disorder transition at T cycl within a pseudospin model of localized spin cycloids with opposite chirality.

  10. Phonon dynamics of americium telluride

    NASA Astrophysics Data System (ADS)

    Arya, B. S.; Aynyas, Mahendra; Ahirwar, Ashok K.; Sanyal, S. P.

    2013-06-01

    We report for the first time the complete phonon dispersion curves for Americium telluride (AmTe) using a breathing shell models (BSM) to establish their predominant ionic nature. The results obtained in the present study show the general features of the phonon spectrum. We could not compare our results with the experimental measurements as they are not available so far. We emphasize the need of neutron scattering measurements to compare our results. We also report, for the first time specific heat for this compound.

  11. Scattering of phonons by vacancies

    SciTech Connect

    Ratsifaritana, C.A.; Klemens, P.G.

    1987-11-01

    The scattering of phonons by vacancies is estimated by a perturbation technique in terms of the missing mass and the missing linkages. An argument is given why distortion effects can be disregarded. The resonance frequency of the defect is sufficiently high so that resonance effects can be disregarded for phonons in the important frequency range for thermal conduction. The theory is applied to the thermal resistance by vacancies in cases where the vacancy concentration is known: potassium chloride with divalent cations, nonstoichiometric zirconium carbide, and tin telluride.

  12. 2.6 W diode-pumped actively mode-locked Tm:YLF laser

    NASA Astrophysics Data System (ADS)

    Mužík, J.; Jelínek, M., Jr.; Vyhlídal, D.; Kubeček, V.

    2015-03-01

    We have experimentally demonstrated an actively mode-locked laser with a Tm3+-doped yttrium lithium fluoride crystal (YLF). A continuous mode-locked regime was achieved using an acousto-optic modulator and a stable train of pulses with 149.3 MHz repetition rate, 170 ps pulse width and 2.6 W average output power at 1.91 µm in a nearly diffraction-limited beam was obtained. To the best of our knowledge, this is the first report on a diode-pumped actively mode-locked bulk thulium laser with a stable output.

  13. Engineering single-phonon number states of a mechanical oscillator via photon subtraction

    NASA Astrophysics Data System (ADS)

    Khan, M. Miskeen; Akram, M. Javed; Paternostro, M.; Saif, F.

    2016-12-01

    We introduce an optomechanical scheme for the probabilistic preparation of single-phonon Fock states of mechanical modes based on photosubtraction. The quality of the produced mechanical state is confirmed by a number of indicators, including phonon statistics and conditional fidelity. We assess the detrimental effect of parameters such as the temperature of the mechanical system and address the feasibility of the scheme with state-of-the-art technology.

  14. Phonons in the ordered c(2 × 2) phases of Na and Li on Al(001).

    PubMed

    Rusina, G G; Eremeev, S V; Borisova, S D; Sklyadneva, I Yu; Echenique, P M; Chulkov, E V

    2007-07-04

    The vibrational properties of the Al(001)-c(2 × 2)-Na (Li) ordered phases formed by alkali atoms (Na and Li) on the Al(001) surface at low and room temperatures are presented. The equilibrium structural characteristics, phonon dispersions and polarization of vibrational modes as well as the local density of phonon states are calculated using the embedded-atom method. The obtained structural parameters are in close agreement with experimental data.

  15. Phonons in the ordered c(2 × 2) phases of Na and Li on Al(001)

    NASA Astrophysics Data System (ADS)

    Rusina, G. G.; Eremeev, S. V.; Borisova, S. D.; Sklyadneva, I. Yu; Echenique, P. M.; Chulkov, E. V.

    2007-07-01

    The vibrational properties of the Al(001)-c(2 × 2)-Na (Li) ordered phases formed by alkali atoms (Na and Li) on the Al(001) surface at low and room temperatures are presented. The equilibrium structural characteristics, phonon dispersions and polarization of vibrational modes as well as the local density of phonon states are calculated using the embedded-atom method. The obtained structural parameters are in close agreement with experimental data.

  16. Phonons in NaO/sub 2/ near the order-disorder transition

    SciTech Connect

    Wakabayashi, N.; Alefeld, B.; Buhrer, W.; Smith, H.G.

    1982-10-15

    The acoustic phonons in cubic NaO/sub 2/ have been studied above and below the order-disorder transition by inelastic neutron scattering. Above the transition the O/sub 2/ /sup -/ ions are disordered and undergoing rapid reorientation about the body diagonals. This reorientation couples with and affects certain acoustic modes in the crystal. The results are described in terms of a phonon-pseudospin coupling theory.

  17. Phonons on the clean metal surfaces and in adsorption structures

    NASA Astrophysics Data System (ADS)

    Rusina, Galina G.; Chulkov, Evgenii V.

    2013-06-01

    The state-of-the-art studies of the vibrational dynamics of clean metal surfaces and metal surface structures formed upon the sub-monolayer adsorption of the atoms of various elements are considered. A brief historical survey of the milestones of investigations of surface phonons is presented. The results of studies of the atomic structure and vibration characteristics of surfaces with low and high Miller indices and adsorption structures are analyzed. It is demonstrated that vicinal surfaces of FCC metals tend to exhibit specific vibrational modes located on the step and polarized along the step. Irrespective of the type and position of adsorption or the substrate structure, the phonon spectra of sub-monolayer adsorption structures always tend to display two modes for combined translational displacements of adatoms and for coupled vibrations of substrate atoms and adatoms polarized in the direction normal to the surface. The bibliography includes 202 references.

  18. Giant Anharmonic Phonon Scattering in PbTe

    SciTech Connect

    Delaire, Olivier A; Ma, Jie; Marty, Karol J; May, Andrew F; McGuire, Michael A; Singh, David J; Lumsden, Mark D; Sales, Brian C; Du, Mao-Hua; Ehlers, Georg; Podlesnyak, Andrey A

    2011-01-01

    Understanding the microscopic processes affecting the bulk thermal conductivity is crucial to develop more efficient thermoelectric materials. PbTe is currently one of the leading thermoelectric materials, largely thanks to its low thermal conductivity. However, the origin of this low thermal conductivity in a simple rocksalt structure has so far been elusive. Using a combination of inelastic neutron scattering measurements and first-principles computations of the phonons, we identify a strong anharmonic coupling between the ferroelectric transverse optic (TO) mode and the longitudinal acoustic (LA) modes in PbTe. This interaction extends over a large portion of reciprocal space, and directly affects the heat-carrying LA phonons. The LA-TO anharmonic coupling is likely to play a central role in explaining the low thermal conductivity of PbTe. The present results provide a microscopic picture of why many good thermoelectric materials are found near a lattice instability of the ferroelectric type.

  19. Phonon thermal conductivity of a nanowire attached to leads

    NASA Astrophysics Data System (ADS)

    Hershfield, Selman; Muttalib, Khandker

    2015-03-01

    There is experimental evidence as well as theoretical proposals that nanowires can be made to have high thermoelectric efficiency by tuning the electronic properties; however, there is always a phonon contribution to the heat transport which reduces the thermoelectric efficiency. In the harmonic approximation we compute the transmission of phonons through a nanowire coupled to large leads. There is a finite thermal conductivity because of the restriction provided by the nanowire. The nanowire reduces the thermal transport because of the mismatch between the leads and wire modes. We examine the effect of disorder in three places: in the wire, in the leads near the wire, and in the leads far way from the wire. In some cases disorder can increase the thermal conduction because of enhanced mode coupling. We will discuss the implications of our results for thermoelectric nanowire devices.

  20. Phonons and crystal structures of the β -pyrochlore superconductors KOs2O6 and RbOs2O6 from micro-Raman spectroscopy

    NASA Astrophysics Data System (ADS)

    Schoenes, J.; Racu, A.-M.; Doll, K.; Bukowski, Z.; Karpinski, J.

    2008-04-01

    The phonons and the crystal structures of the β -pyrochlore superconductors KOs2O6 and RbOs2O6 are studied on single crystals by using micro-Raman spectroscopy. The symmetry of the modes is determined by polarization dependent measurements for various scattering geometries. It is concluded that the crystal structures are centrosymmetric (Fd3¯m) . The vibrational frequencies have been computed from first principles and are in good agreement with the experimental data. The Raman-active rattling mode of the alkali ion is identified at 75 and 60cm-1 for the K and Rb compounds, respectively.

  1. The phonon-polariton spectrum of one-dimensional Rudin-Shapiro photonic superlattices with uniaxial polar materials

    NASA Astrophysics Data System (ADS)

    Gómez-Urrea, H. A.; Duque, C. A.; Mora-Ramos, M. E.

    2015-11-01

    The properties of the optical-phonon-associated polaritonic modes that appear under oblique light incidence in 1D superlattices made of photonic materials are studied. The investigated systems result from the periodic repetition of quasiregular Rudin-Shapiro (RS) multilayer units. It is assume that the structure consists of both passive non-dispersive layers of constant refraction index and active layers of uniaxial polar materials. In particular, we consider III-V wurtzite nitrides. The optical axis of these polaritonic materials is taken along the growth direction. Maxwell equations are solved using the transfer matrix technique for all admissible values of the incidence angle.

  2. Phonon Overlaps in Molecular Quantum Dot Systems

    NASA Astrophysics Data System (ADS)

    Chang, Connie; Sethna, James

    2004-03-01

    We model the amplitudes and frequencies of the vibrational sidebands for the new molecular quantum dot systems. We calculate the Franck-Condon phonon overlaps in the 3N-dimensional configuration sapce. We solve the general case where the vibrational frequencies and eigenmodes change during the transition. We perform PM3 and DFT calculations for the case of the dumb bell-shaped C140 molecule. We find that the strongest amplitudes are associated with the 11 meV stretch mode, in agreement with experiment. The experimental amplitudes vary from molecule to molecule; indicating that the molecular overlaps are environment dependent. We explore overlaps in the presence of external electric fields from image charges and counter ions.

  3. Cavity-type hypersonic phononic crystals

    NASA Astrophysics Data System (ADS)

    Sato, A.; Pennec, Y.; Yanagishita, T.; Masuda, H.; Knoll, W.; Djafari-Rouhani, B.; Fytas, G.

    2012-11-01

    We report on the engineering of the phonon dispersion diagram in monodomain anodic porous alumina (APA) films through the porosity and physical state of the material residing in the nanopores. Lattice symmetry and inclusion materials are theoretically identified to be the main factors which control the hypersonic acoustic wave propagation. This involves the interaction between the longitudinal and the transverse modes in the effective medium and a flat band characteristic of the material residing in the cavities. Air and filled nanopores, therefore, display markedly different dispersion relations and the inclusion materials lead to a locally resonant structural behavior uniquely determining their properties under confinement. APA films emerge as a new platform to investigate the rich acoustic phenomena of structured composite matter.

  4. Revealing the mechanism of passive transport in lipid bilayers via phonon-mediated nanometre-scale density fluctuations

    DOE PAGES

    Zhernenkov, Mikhail; Bolmatov, Dima; Soloviov, Dmitry; ...

    2016-05-12

    We report the high resolution inelastic x-ray study of the in-plane phonon excitations in dipalmitoyl phosphatidylcholine (DPPC) above and below main transition temperature. In the Lβ' gel phase, we observe high frequency longitudinal phonon mode previously predicted by the molecular dynamics simulations and for the first time, we reveal low frequency weakly dispersive transverse acoustic mode which softens and exhibits a low-frequency phonon gap when the DPPC lipid transitions into the Lα fluid phase. The phonon softening of the high frequency longitudinal excitations and the transformation of the transverse excitations upon the phase transition from the Lβ' to Lα phasemore » is explained within the framework of the phonon theory of liquids. These findings illustrate the importance of the collective dynamics of biomembranes and reveal that hydrocarbon tails can act as an efficient mediator in controlling the passive transport across the bilayer plane.« less

  5. Neutron inelastic scattering measurements of low-energy phonons in the multiferroic BiFeO3

    SciTech Connect

    Schneeloch, John A.; Xu, Zhijun; Wen, Jinsheng; Gehring, P. M.; Stock, C.; Matsuda, Masaaki; Winn, Barry L.; Gu, Genda; Shapiro, Stephen M.; Birgeneau, R. J.; Ushiyama, T.; Yanagisawa, Y.; Tomioka, Y.; Ito, T.; Xu, Guangyong

    2015-02-10

    In this study, we present neutron inelastic scattering measurements of the low-energy phonons in single crystal BiFeO3. The dispersions of the three acoustic phonon modes (LA along [100], TA1 along [010], and TA2 along [110]) and two low-energy optic phonon modes (LO and TO1) have been mapped out between 300 and 700 K. Elastic constants are extracted from the phonon measurements. The energy linewidths of both TA phonons at the zone boundary clearly broaden when the system is warmed toward the magnetic ordering temperature TN=640 K. In conclusion, this suggests that the magnetic order and low-energy lattice dynamics in this multiferroic material are coupled.

  6. Neutron inelastic scattering measurements of low-energy phonons in the multiferroic BiFeO3

    DOE PAGES

    Schneeloch, John A.; Xu, Zhijun; Wen, Jinsheng; ...

    2015-02-10

    In this study, we present neutron inelastic scattering measurements of the low-energy phonons in single crystal BiFeO3. The dispersions of the three acoustic phonon modes (LA along [100], TA1 along [010], and TA2 along [110]) and two low-energy optic phonon modes (LO and TO1) have been mapped out between 300 and 700 K. Elastic constants are extracted from the phonon measurements. The energy linewidths of both TA phonons at the zone boundary clearly broaden when the system is warmed toward the magnetic ordering temperature TN=640 K. In conclusion, this suggests that the magnetic order and low-energy lattice dynamics in thismore » multiferroic material are coupled.« less

  7. Effects of intense optical phonon pumping on the structure and electronic properties of yttrium barium copper oxide

    NASA Astrophysics Data System (ADS)

    Fechner, M.; Spaldin, N. A.

    2016-10-01

    We investigate the structural modulations induced by optical excitation of a polar phonon mode in YBa2Cu3O7 using first-principles calculations based on density functional theory. We focus on the intense-excitation regime in which we expect that fourth-order phonon-phonon coupling terms dominate and model the structural modulations induced by pulses of such intensity. Our calculations of the phonon-phonon anharmonicities confirm that the cubic coupling between modes, shown in earlier work to cause a quasistatic change in the apical O-Cu distance and a buckling of the CuO2 planes, is the leading contribution at moderate pump strengths. At higher pump strengths (˜10 MV /cm ) the previously neglected quartic couplings become relevant and produce an additional shearing of the CuO2 planes. Finally, we analyze the changes in the electronic and magnetic properties associated with the induced structural changes.

  8. Observing backfolded and unfolded acoustic phonons by broadband optical light scattering.

    PubMed

    Maerten, L; Bojahr, A; Bargheer, M

    2015-02-01

    We use broadband time domain Brillouin scattering to observe coherently generated phonon modes in bulk and nanolayered samples. We transform the measured transients into a frequency-wavevector diagram and compare the resulting dispersion relations to calculations. The detected oscillation amplitude depends on the occupation of phonon modes induced by the pump pulse. For nanolayered samples with an appropriately large period, the whole wavevector range of the Brillouin zone becomes observable by broadband optical light scattering. The backfolded modes vanish, when the excitation has passed the nanolayers and propagates through the substrate underneath.

  9. Infrared phonon dynamics of a multiferroic BiFeO3 single crystal

    NASA Astrophysics Data System (ADS)

    Lobo, R. P. S. M.; Moreira, R. L.; Lebeugle, D.; Colson, D.

    2007-11-01

    We discuss the infrared reflectivity measurement on a BiFeO3 single crystal between 5K and room temperature. The nine predicted ab -plane E phonon modes are fully and unambiguously determined. The frequencies of the four A1 c -axis phonons are found. These results settle issues between theory and data on ceramics. Our findings show that the softening of the lowest frequency E mode is responsible for the temperature dependence of the dielectric constant, indicating that the ferroelectric transition in BiFeO3 is soft-mode driven.

  10. Tempo and Mode of Transposable Element Activity in Drosophila

    PubMed Central

    Kofler, Robert; Nolte, Viola; Schlötterer, Christian

    2015-01-01

    The evolutionary dynamics of transposable element (TE) insertions have been of continued interest since TE activity has important implications for genome evolution and adaptation. Here, we infer the transposition dynamics of TEs by comparing their abundance in natural D. melanogaster and D. simulans populations. Sequencing pools of more than 550 South African flies to at least 320-fold coverage, we determined the genome wide TE insertion frequencies in both species. We suggest that the predominance of low frequency insertions in the two species (>80% of the insertions have a frequency <0.2) is probably due to a high activity of more than 58 families in both species. We provide evidence for 50% of the TE families having temporally heterogenous transposition rates with different TE families being affected in the two species. While in D. melanogaster retrotransposons were more active, DNA transposons showed higher activity levels in D. simulans. Moreover, we suggest that LTR insertions are mostly of recent origin in both species, while DNA and non-LTR insertions are older and more frequently vertically transmitted since the split of D. melanogaster and D. simulans. We propose that the high TE activity is of recent origin in both species and a consequence of the demographic history, with habitat expansion triggering a period of rapid evolution. PMID:26186437

  11. Doubly active Q switching and mode locking of an all-fiber laser.

    PubMed

    Cuadrado-Laborde, Christian; Díez, Antonio; Cruz, Jose L; Andrés, Miguel V

    2009-09-15

    Simultaneous and independent active Q switching and active mode locking of an erbium-doped fiber laser is demonstrated using all-fiber modulation techniques. A magnetostrictive rod attached to the output fiber Bragg grating modulates the Q factor of the Fabry-Perot cavity, whereas active mode locking is achieved by amplitude modulation with a Bragg-grating-based acousto-optic device. Fully modulated Q-switched mode-locked trains of optical pulses were obtained for a wide range of pump powers and repetition rates. For a Q-switched repetition rate of 500 Hz and a pump power of 100 mW, the laser generates trains of 12-14 mode-locked pulses of about 1 ns each, within an envelope of 550 ns, an overall energy of 0.65 microJ, and a peak power higher than 250 W for the central pulses of the train.

  12. YPHON: A package for calculating phonons of polar materials

    NASA Astrophysics Data System (ADS)

    Wang, Yi; Chen, Long-Qing; Liu, Zi-Kui

    2014-11-01

    In our recent works, we have developed a mixed-space approach within the framework of direct method for the first-principle calculation of phonon properties. It makes full use of the accuracy of the force constants calculated in the real space and the dipole-dipole interactions in the reciprocal space, making the accurate phonon calculation possible with the direct method for polar materials. In this paper, an efficient C++ implementation of the mixed-space approach, YPHON, is provided as open source, including demos and Linux scripts for extracting input data to YPHON from the output of VASP.5. The functions of the current package include the calculations of: (1) the phonon dispersions; (2) the phonon density of states; (3) the neutron scattering section weighted phonon density of state; (4) the phonons of the high symmetry structure using the force constants from low symmetry structure; (5) the phonon dispersions of random alloys; and (6) the analysis of the vibrational modes using the point group theory. Catalogue identifier: AETS_v1_0 Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AETS_v1_0.html Program obtainable from: CPC Program Library, Queen’s University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 567815 No. of bytes in distributed program, including test data, etc.: 9763594 Distribution format: tar.gz Programming language: C++, Linux scripts. Computer: Linux systems with a g++ or C++ compiler. Operating system: Linux. RAM: Ranges from a few Mbytes to a few Gbytes, dynamically depending on the system size. Classification: 7.8. External routines: GSL-the GNU Scientific Library (GSL) is a numerical library for C and C++ programmers. VASP.5 or later for the calculations of force constants and dielectric constants and Born effective charge for polar materials. Nature of problem: This package has the purpose of computing

  13. Phonon dispersion relation in PbTiO3

    NASA Astrophysics Data System (ADS)

    Tomeno, Izumi; Fernandez-Baca, Jaime; Marty, Karol; Oka, Kunihiko; Tsunoda, Yorihiko

    2013-03-01

    The phonon dispersion relations for cubic PbTiO3 (Tc = 763 K) have been determined along the high symmetry directions at T = 793 K using inelastic neutron scattering. A set of the TO branches drops significantly toward the zone center. This is quite different from the soft mode anomaly in the Pb-based relaxors, named as the waterfall phenomenon. The zone-center TO mode energy softens with decreasing temperature from 1173 to 793 K. The TA branch along [ ξ , ξ , ξ ] shows significant softening around ξ = 0.25 and 0.5. These two anomalies persist up to 1173 K and are weakly temperature dependent. Moreover, the TA branches along [1,0,0] and [1,1,0] soften in the entire q range as the temperature approaches Tc. Although the phonon softening occurs simultaneously, the softening of the zone center TO mode plays an important role in the single phase transition. The phonon dispersion relations for cubic and tetragonal PbTiO3 are discussed in connection with BaTiO3, KTaO3, Pb(Zn1/3Nb2/3)O3, and Pb(Mg1/3Nb2/3)O3. U.S.-Japan cooperative program on neutron scattering

  14. Enhancement of phonon backscattering due to confinement of ballistic phonon pathways in silicon as studied with a microfabricated phonon spectrometer

    SciTech Connect

    Otelaja, O. O.; Robinson, R. D.

    2015-10-26

    In this work, the mechanism for enhanced phonon backscattering in silicon is investigated. An understanding of phonon propagation through substrates has implications for engineering heat flow at the nanoscale, for understanding sources of decoherence in quantum systems, and for realizing efficient phonon-mediated particle detectors. In these systems, phonons that backscatter from the bottom of substrates, within the crystal or from interfaces, often contribute to the overall detector signal. We utilize a microscale phonon spectrometer, comprising superconducting tunnel junction emitters and detectors, to specifically probe phonon backscattering in silicon substrates (∼500 μm thick). By etching phonon “enhancers” or deep trenches (∼90 μm) around the detectors, we show that the backscattered signal level increases by a factor of ∼2 for two enhancers versus one enhancer. Using a geometric analysis of the phonon pathways, we show that the mechanism of the backscattered phonon enhancement is due to confinement of the ballistic phonon pathways and increased scattering off the enhancer walls. Our result is applicable to the geometric design and patterning of substrates that are employed in phonon-mediated detection devices.

  15. Design and control of a prosthetic leg for above-knee amputees operated in semi-active and active modes

    NASA Astrophysics Data System (ADS)

    Park, Jinhyuk; Yoon, Gun-Ha; Kang, Je-Won; Choi, Seung-Bok

    2016-08-01

    This paper proposes a new prosthesis operated in two different modes; the semi-active and active modes. The semi-active mode is achieved from a flow mode magneto-rheological (MR) damper, while the active mode is obtained from an electronically commutated (EC) motor. The knee joint part of the above knee prosthesis is equipped with the MR damper and EC motor. The MR damper generates reaction force by controlling the field-dependent yield stress of the MR fluid, while the EC motor actively controls the knee joint angle during gait cycle. In this work, the MR damper is designed as a two-end type flow mode mechanism without air chamber for compact size. On other hand, in order to predict desired knee joint angle to be controlled by EC motor, a polynomial prediction function using a statistical method is used. A nonlinear proportional-derivative controller integrated with the computed torque method is then designed and applied to both MR damper and EC motor to control the knee joint angle. It is demonstrated that the desired knee joint angle is well achieved in different walking velocities on the ground ground.

  16. Phonon properties of graphene derived from molecular dynamics simulations

    PubMed Central

    Koukaras, Emmanuel N.; Kalosakas, George; Galiotis, Costas; Papagelis, Konstantinos

    2015-01-01

    A method that utilises atomic trajectories and velocities from molecular dynamics simulations has been suitably adapted and employed for the implicit calculation of the phonon dispersion curves of graphene. Classical potentials widely used in the literature were employed. Their performance was assessed for each individual phonon branch and the overall phonon dispersion, using available inelastic x-ray scattering data. The method is promising for systems with large scale periodicity, accounts for anharmonic effects and non-bonding interactions with a general environment, and it is applicable under finite temperatures. The temperature dependence of the phonon dispersion curves has been examined with emphasis on the doubly degenerate Raman active Γ-E2g phonon at the zone centre, where experimental results are available. The potentials used show diverse behaviour. The Tersoff-2010 potential exhibits the most systematic and physically sound behaviour in this regard, and gives a first-order temperature coefficient of χ = −0.05 cm−1/K for the Γ-E2g shift in agreement with reported experimental values. PMID:26316252

  17. Active mode locking of quantum cascade lasers in an external ring cavity

    PubMed Central

    Revin, D. G.; Hemingway, M.; Wang, Y.; Cockburn, J. W.; Belyanin, A.

    2016-01-01

    Stable ultrashort light pulses and frequency combs generated by mode-locked lasers have many important applications including high-resolution spectroscopy, fast chemical detection and identification, studies of ultrafast processes, and laser metrology. While compact mode-locked lasers emitting in the visible and near infrared range have revolutionized photonic technologies, the systems operating in the mid-infrared range where most gases have their strong absorption lines, are bulky and expensive and rely on nonlinear frequency down-conversion. Quantum cascade lasers are the most powerful and versatile compact light sources in the mid-infrared range, yet achieving their mode-locked operation remains a challenge, despite dedicated effort. Here we report the demonstration of active mode locking of an external-cavity quantum cascade laser. The laser operates in the mode-locked regime at room temperature and over the full dynamic range of injection currents. PMID:27147409

  18. Phonon-mediated negative differential conductance in molecular quantum dots

    NASA Astrophysics Data System (ADS)

    Zazunov, Alex; Feinberg, Denis; Martin, Thierry

    2006-03-01

    Transport through a single-molecular conductor is considered, showing negative differential conductance behavior associated with phonon-mediated electron tunneling processes. This theoretical work is motivated by a recent experiment by Leroy using a carbon nanotube contacted by a scanning tunneling microscope tip [Nature 432, 371 (2004)], where negative differential conductance of the breathing-mode phonon side peaks could be observed. A peculiarity of this system is that the tunneling couplings which inject electrons and those which collect them on the substrate are highly asymmetrical. A quantum dot model is used, coupling a single electronic level to a local phonon, forming polaron levels. A “half-shuttle” mechanism is also introduced. A quantum kinetic formulation allows us to derive rate equations. Assuming asymmetric tunneling rates and in the absence of the half-shuttle coupling, negative differential conductance (NDC) is obtained for a wide range of parameters. A detailed explanation of this phenomenon is provided, showing that NDC is maximal for intermediate electron-phonon coupling. In addition, in the absence of a gate, the “floating” level results in two distinct lengths for the current plateaus, related to the capacitive couplings at the two junctions. It is shown that the half-shuttle mechanism tends to reinforce the negative differential regions, but it cannot trigger this behavior on its own.

  19. Optical phonons in PbTe/CdTe multilayer heterostructures

    SciTech Connect

    Novikova, N. N.; Yakovlev, V. A.; Kucherenko, I. V.; Karczewski, G.; Aleshchenko, Yu. A.; Muratov, A. V.; Zavaritskaya, T. N.; Melnik, N. N.

    2015-05-15

    The infrared reflection spectra of PbTe/CdTe multilayer nanostructures grown by molecular-beam epitaxy are measured in the frequency range of 20–5000 cm{sup −1} at room temperature. The thicknesses and high-frequency dielectric constants of the PbTe and CdTe layers and the frequencies of the transverse optical (TO) phonons in these structures are determined from dispersion analysis of the spectra. It is found that the samples under study are characterized by two TO phonon frequencies, equal to 28 and 47 cm{sup −1}. The first frequency is close to that of TO phonons in bulk PbTe, and the second is assigned to the optical mode in structurally distorted interface layers. The Raman-scattering spectra upon excitation with the radiation of an Ar{sup +} laser at 514.5 nm are measured at room and liquid-nitrogen temperatures. The weak line at 106 cm{sup −1} observed in these spectra is attributed to longitudinal optical phonons in the interface layers.

  20. Sound and heat revolutions in phononics.

    PubMed

    Maldovan, Martin

    2013-11-14

    The phonon is the physical particle representing mechanical vibration and is responsible for the transmission of everyday sound and heat. Understanding and controlling the phononic properties of materials provides opportunities to thermally insulate buildings, reduce environmental noise, transform waste heat into electricity and develop earthquake protection. Here I review recent progress and the development of new ideas and devices that make use of phononic properties to control both sound and heat. Advances in sonic and thermal diodes, optomechanical crystals, acoustic and thermal cloaking, hypersonic phononic crystals, thermoelectrics, and thermocrystals herald the next technological revolution in phononics.

  1. Thickness mode EMIS of constrained proof-mass piezoelectric wafer active sensors

    NASA Astrophysics Data System (ADS)

    Kamas, Tuncay; Giurgiutiu, Victor; Lin, Bin

    2015-11-01

    This paper addresses theoretical and experimental work on thickness-mode electromechanical (E/M) impedance spectroscopy (EMIS) of proof-mass piezoelectric wafer active sensors (PMPWAS). The proof-mass (PM) concept was used to develop a new method for tuning the ultrasonic wave modes and for relatively high frequency local modal sensing by the PM affixed on PWAS. In order to develop the theoretical basis of the PMPWAS tuning concept, analytical analyses were conducted by applying the resonator theory to derive the EMIS of a PWAS constrained on one and both surfaces by isotropic elastic materials. The normalized thickness-mode shapes were obtained for the normal mode expansion (NME) method to eventually predict the thickness-mode EMIS using the correlation between PMPWAS and the structural dynamic properties of the substrate. Proof-masses of different sizes and materials were used to tune the system resonance towards an optimal frequency point. The results were verified by coupled-field finite element analyses (CF-FEA) and experimental results. An application of the tuning effect of PM on the standing wave modes was discussed as the increase in PM thickness shifts the excitation frequency of the wave mode toward the surface acoustic wave (SAW) mode.

  2. Actively mode-locked fiber ring laser by intermodal acousto-optic modulation.

    PubMed

    Bello-Jiménez, M; Cuadrado-Laborde, C; Sáez-Rodríguez, D; Diez, A; Cruz, J L; Andrés, M V

    2010-11-15

    We report an actively mode-locked fiber ring laser. A simple and low-insertion-loss acousto-optic modulator driven by standing flexural waves, which couples core-to-cladding modes in a standard single-mode optical fiber, is used as an active mechanism for mode locking. Among the remarkable features of the modulator, we mention its high modulation depth (72%), broad bandwidth (187 GHz), easy tunability in the optical wavelength, and low insertion losses (0.7 dB). The narrowest optical pulses obtained were of 95 ps time width, 21 mW peak power, repetition rate of 4.758 MHz, and 110 mW of pump power.

  3. 4-Alkynylphenylsilatranes: Insecticidal activity, mammalian toxicity, and mode of action

    SciTech Connect

    Horsham, M.A.; Palmer, C.J.; Cole, L.M.; Casida, J.E. )

    1990-08-01

    4-Ethynyl- and 4-(prop-1-ynyl)phenylsilatranes (N(CH{sub 2}CH{sub 2}O){sub 3}SiR, R = C{sub 6}H{sub 4}-4-C{triple bond}CH or C{sub 6}H{sub 4}-4-C{triple bond}CCH{sub 3}) are highly toxic to houseflies (pretreated with piperonyl butoxide) and milkweed bugs (topical LD{sub 50}s 3-14 {mu}g/g) and to mice (intraperitoneal LD{sub 50}s 0.4-0.9 mg/kg), and they are moderately potent inhibitors of the ({sup 35}S)-tert-butylbicyclophosphorothionate or TBPS binding site (GABA-gated chloride channel) of mouse brain membranes. Scatchard analysis indicates noncompetitive interaction of 4-ethynylphenylsilatrane with the TBPS binding site. Phenylsilatrane analogues with 4-substituents of H, CH{sub 3}, Cl, Br, and C{triple bond}CSi(CH{sub 3}){sub 3} are highly toxic to mice but have little or no activity in the insect and receptor assays. Radioligand binding studies with (4-{sup 3}H)phenylsilatrane failed to reveal a specific binding site in mouse brain. Silatranes with R = H, CH{sub 3}, CH{sub 2}Cl, CH{double bond}CH{sub 2}, OCH{sub 2}CH{sub 3}, and C{sub 6}H{sub 4}-4-CH{sub 2}CH{sub 3} are of little or no activity in the insect and mouse toxicity and TBPS binding site assays as are the trithia and monocyclic analogues of phenylsilatrane. 4-Alkynylphenylsilatranes are new probes to examine the GABA receptor-ionophore complex of insects and mammals.

  4. Nanoscale control of phonon excitations in graphene

    PubMed Central

    Kim, Hyo Won; Ko, Wonhee; Ku, JiYeon; Jeon, Insu; Kim, Donggyu; Kwon, Hyeokshin; Oh, Youngtek; Ryu, Seunghwa; Kuk, Young; Hwang, Sung Woo; Suh, Hwansoo

    2015-01-01

    Phonons, which are collective excitations in a lattice of atoms or molecules, play a major role in determining various physical properties of condensed matter, such as thermal and electrical conductivities. In particular, phonons in graphene interact strongly with electrons; however, unlike in usual metals, these interactions between phonons and massless Dirac fermions appear to mirror the rather complicated physics of those between light and relativistic electrons. Therefore, a fundamental understanding of the underlying physics through systematic studies of phonon interactions and excitations in graphene is crucial for realising graphene-based devices. In this study, we demonstrate that the local phonon properties of graphene can be controlled at the nanoscale by tuning the interaction strength between graphene and an underlying Pt substrate. Using scanning probe methods, we determine that the reduced interaction due to embedded Ar atoms facilitates electron–phonon excitations, further influencing phonon-assisted inelastic electron tunnelling. PMID:26109454

  5. Low-degree p-mode parameters evolution with solar activity

    NASA Astrophysics Data System (ADS)

    Lochard, J.; Boumier, P.

    We present updated results on the low-degree p-mode parameters changes with solar activity: frequency, spectral width, rotational splittings and spectral asymmetry. A particular attention is devoted to the l=2 splitting asymmetry. Comparaisons of our frequency shifts (derived from the GOLF observations) with predictions of Pr.Dziembowski (based on an extrapolation from intermediate degree modes observations from MDI), are discussed.

  6. Influence of Activity Mode on Feeling States of High School Physical Education Students

    ERIC Educational Resources Information Center

    Hannon, James C.; Pellet, Tracey L.

    2005-01-01

    The purpose of this study was to determine if changes in positive well-being, psychological distress, fatigue, and enjoyment vary as a function of physical activity mode. Fifty-five senior high school students participated in one of four fitness activities including two defined as traditional (running and step-aerobics) and two defined as…

  7. Localized surface phonon polariton resonances in polar gallium nitride

    SciTech Connect

    Feng, Kaijun Islam, S. M.; Verma, Jai; Hoffman, Anthony J.; Streyer, William; Wasserman, Daniel; Jena, Debdeep

    2015-08-24

    We demonstrate the excitation of localized surface phonon polaritons in an array of sub-diffraction pucks fabricated in an epitaxial layer of gallium nitride (GaN) on a silicon carbide (SiC) substrate. The array is characterized via polarization- and angle-dependent reflection spectroscopy in the mid-infrared, and coupling to several localized modes is observed in the GaN Reststrahlen band (13.4–18.0 μm). The same structure is simulated using finite element methods and the charge density of the modes are studied; transverse dipole modes are identified for the transverse electric and magnetic polarizations and a quadrupole mode is identified for the transverse magnetic polarization. The measured mid-infrared spectrum agrees well with numerically simulated spectra. This work could enable optoelectronic structures and devices that support surface modes at mid- and far-infrared wavelengths.

  8. Phonon blockade in a nanomechanical resonator resonantly coupled to a qubit

    NASA Astrophysics Data System (ADS)

    Xu, Xun-Wei; Chen, Ai-Xi; Liu, Yu-xi

    2016-12-01

    We study phonon statistics in a nanomechanical resonator (NAMR) which is resonantly coupled to a qubit. We find that there are two different mechanisms for phonon blockade in such a resonantly coupled NAMR-qubit system. One is due to the strong anharmonicity of the NAMR-qubit system with large coupling strength; the other one is due to the destructive interference between different paths for two-phonon excitation in the NAMR-qubit system with a moderate coupling strength. We find that the phonon blockade is fragile towards thermal mode occupations and can only be observed for NAMR being at ultracold effective temperature. In order to enlarge the mean phonon number for strong phonon antibunching with a moderate NAMR-qubit coupling strength, we assume that two external driving fields are applied to the NAMR and qubit, respectively. In this case, we find that the phonon blockades under two mechanisms can appear at the same frequency regime by optimizing the strength ratio and phase difference of the two external driving fields.

  9. Surface-phonon dispersion of a NiO(100) thin film

    NASA Astrophysics Data System (ADS)

    Kostov, K. L.; Polzin, S.; Saha, S. K.; Brovko, O.; Stepanyuk, V.; Widdra, W.

    2013-06-01

    A well-ordered 25 ML epitaxial NiO(100) film on Ag(100) as prepared by layer-by-layer growth has been characterized by high-resolution electron energy loss spectroscopy. Six different phonon branches have been identified in the Γ¯X¯ direction of the surface Brillouin zone and are compared with first-principles phonon calculations. Whereas the surface Rayleigh mode shows a strong upward dispersion of 173 cm-1 in agreement with observations for the NiO(100) single crystal, the other surface phonons and surface resonances show only smaller dispersion widths in Γ¯X¯ direction. The Wallis and the Lucas phonons are localized at 425 and 367 cm-1 at the Γ¯ point, respectively. Additionally, two phonons are identified that have stronger weight at the zone boundary at 194 and 285 cm-1 and that become surface resonances at the zone center. The dominant spectral feature is the Fuchs-Kliewer (FK) phonon polariton at 559 cm-1, which is excited by dipole scattering and exhibits a rather broad non-Lorentzian lineshape. The lineshape is explained by a FK splitting resulting from the splitting of bulk optical phonons due to antiferromagnetic order. This view is supported by calculations of the surface-loss function from bulk reflectivity data.

  10. Electron-phonon interactions and the intrinsic electrical resistivity of graphene.

    PubMed

    Park, Cheol-Hwan; Bonini, Nicola; Sohier, Thibault; Samsonidze, Georgy; Kozinsky, Boris; Calandra, Matteo; Mauri, Francesco; Marzari, Nicola

    2014-03-12

    We present a first-principles study of the temperature- and density-dependent intrinsic electrical resistivity of graphene. We use density-functional theory and density-functional perturbation theory together with very accurate Wannier interpolations to compute all electronic and vibrational properties and electron-phonon coupling matrix elements; the phonon-limited resistivity is then calculated within a Boltzmann-transport approach. An effective tight-binding model, validated against first-principles results, is also used to study the role of electron-electron interactions at the level of many-body perturbation theory. The results found are in excellent agreement with recent experimental data on graphene samples at high carrier densities and elucidate the role of the different phonon modes in limiting electron mobility. Moreover, we find that the resistivity arising from scattering with transverse acoustic phonons is 2.5 times higher than that from longitudinal acoustic phonons. Last, high-energy, optical, and zone-boundary phonons contribute as much as acoustic phonons to the intrinsic electrical resistivity even at room temperature and become dominant at higher temperatures.

  11. TAE modes and MHD activity in TFTR DT plasmas

    SciTech Connect

    Fredrickson, E.; Batha, S.; Bell, M.

    1995-03-01

    The high power deuterium and tritium experiments on TFTR have produced fusion a parameters similar to those expected on ITER. The achieved {beta}{sub {alpha}}/{beta} and the R{triangledown}{beta}{sub {alpha}} in TFRR D-T shots are 1/2 to 1/3 those predicted in the ITER EDA. Studies of the initial TFTR D-T plasmas find no evidence that the presence of the fast fusion {alpha} population has affected the stability of MHD, with the possible exception of Toroidal Alfven Eigenmodes (TAE`s). The initial TFTR DT plasmas had MHD activity similar to that commonly seen in deuterium plasmas. Operation of TFTR at plasma currents of 2.0--2.5 MA has greatly reduced the deleterious effects of MHD commonly observed at lower currents. Even at these higher currents, the performance of TFTR is limited by {beta}-limit disruptions. The effects of MHD on D-T fusion {alpha}`s was similar to effects observed on other fusion products in D only plasmas.

  12. Mosquitocidal Activity and Mode of Action of the Isoxazoline Fluralaner

    PubMed Central

    Jiang, Shiyao; Tsikolia, Maia; Bernier, Ulrich R.; Bloomquist, Jeffrey R.

    2017-01-01

    Mosquitoes, such as Aedes aegypti and Anopheles gambiae, are important vectors of human diseases. Fluralaner, a recently introduced parasiticide, was evaluated as a mosquitocide in this study. On Ae. aegypti and An. gambiae fourth-instar larvae, fluralaner had 24-h LC50 (lethal concentration for 50% mortality) values of 1.8 ppb and 0.4 ppb, respectively. Following topical application to adult Ae. aegypti, fluralaner toxicity reached a plateau in about 3 days, with 1- and 3-day LD50 (lethal dose for 50% mortality) values of 1.3 ng/mg and 0.26 ng/mg, suggesting a slowly developing toxicity. Fipronil outperformed fluralaner by up to 100-fold in adult topical, glass contact, and feeding assays on Ae. aegypti. These data show that fluralaner does not have exceptional toxicity to mosquitoes in typical exposure paradigms. In electrophysiological recordings on Drosophila melanogaster larval central nervous system, the effectiveness of fluralaner for restoring nerve firing after gamma-aminobutyric acid (GABA) treatment, a measure of GABA antagonism, was similar in susceptible Oregon-R and cyclodiene-resistant rdl-1675 strains, with EC50 (half maximal effective concentration) values of 0.34 µM and 0.29 µM. Although this finding suggests low cross resistance in the presence of rdl, the moderate potency, low contact activity, and slow action of fluralaner argue against its use as an adult mosquitocide for vector control. PMID:28178191

  13. Mode-selective control of the crystal lattice.

    PubMed

    Först, M; Mankowsky, R; Cavalleri, A

    2015-02-17

    CONSPECTUS: Driving phase changes by selective optical excitation of specific vibrational modes in molecular and condensed phase systems has long been a grand goal for laser science. However, phase control has to date primarily been achieved by using coherent light fields generated by femtosecond pulsed lasers at near-infrared or visible wavelengths. This field is now being advanced by progress in generating intense femtosecond pulses in the mid-infrared, which can be tuned into resonance with infrared-active crystal lattice modes of a solid. Selective vibrational excitation is particularly interesting in complex oxides with strong electronic correlations, where even subtle modulations of the crystallographic structure can lead to colossal changes of the electronic and magnetic properties. In this Account, we summarize recent efforts to control the collective phase state in solids through mode-selective lattice excitation. The key aspect of the underlying physics is the nonlinear coupling of the resonantly driven phonon to other (Raman-active) modes due to lattice anharmonicities, theoretically discussed as ionic Raman scattering in the 1970s. Such nonlinear phononic excitation leads to rectification of a directly excited infrared-active mode and to a net displacement of the crystal along the coordinate of all anharmonically coupled modes. We present the theoretical basis and the experimental demonstration of this phenomenon, using femtosecond optical spectroscopy and ultrafast X-ray diffraction at a free electron laser. The observed nonlinear lattice dynamics is shown to drive electronic and magnetic phase transitions in many complex oxides, including insulator-metal transitions, charge/orbital order melting and magnetic switching in manganites. Furthermore, we show that the selective vibrational excitation can drive high-TC cuprates into a transient structure with enhanced superconductivity. The combination of nonlinear phononics with ultrafast crystallography at

  14. Piezoelectric Response to Coherent Longitudinal and Transverse Acoustic Phonons in a Semiconductor Schottky Diode

    NASA Astrophysics Data System (ADS)

    Srikanthreddy, D.; Glavin, B. A.; Poyser, C. L.; Henini, M.; Lehmann, D.; Jasiukiewicz, Cz.; Akimov, A. V.; Kent, A. J.

    2017-02-01

    We study the generation of microwave electronic signals by pumping a (311) GaAs Schottky diode with compressive and shear acoustic phonons, generated by the femtosecond optical excitation of an Al film transducer and mode conversion at the Al-GaAs interface. They propagate through the substrate and arrive at the Schottky device on the opposite surface, where they induce a microwave electronic signal. The arrival time, the amplitude, and the polarity of the signals depend on the phonon mode. A theoretical analysis is made of the polarity of the experimental signals. This analysis includes the piezoelectric and deformation potential mechanisms of electron-phonon interaction in a Schottky contact and shows that the piezoelectric mechanism is dominant for both transverse and longitudinal modes with frequencies below 250 and 70 GHz, respectively.

  15. Phonon quarticity induced by changes in phonon-tracked hybridization during lattice expansion and its stabilization of rutile TiO2

    DOE PAGES

    Lan, Tian; Li, Chen W.; Hellman, O.; ...

    2015-08-11

    Although the rutile structure of TiO2 is stable at high temperatures, the conventional quasiharmonic approximation predicts that several acoustic phonons decrease anomalously to zero frequency with thermal expansion, incorrectly predicting a structural collapse at temperatures well below 1000 K. In this paper, inelastic neutron scattering was used to measure the temperature dependence of the phonon density of states (DOS) of rutile TiO2 from 300 to 1373 K. Surprisingly, these anomalous acoustic phonons were found to increase in frequency with temperature. First-principles calculations showed that with lattice expansion, the potentials for the anomalous acoustic phonons transform from quadratic to quartic, stabilizingmore » the rutile phase at high temperatures. In these modes, the vibrational displacements of adjacent Ti and O atoms cause variations in hybridization of 3d electrons of Ti and 2p electrons of O atoms. Finally, with thermal expansion, the energy variation in this “phonon-tracked hybridization” flattens the bottom of the interatomic potential well between Ti and O atoms, and induces a quarticity in the phonon potential.« less

  16. Remote phonon scattering in field-effect transistors with a high κ insulating layer

    NASA Astrophysics Data System (ADS)

    Laikhtman, B.; Solomon, P. M.

    2008-01-01

    In this paper a remote phonon scattering of channel electrons in a field-effect transistor (FET) with a high dielectric constant (κ) insulator in between the gate and the channel is studied theoretically. The spectrum of phonons confined in the high κ layer and its modification by the gate screening is investigated. Only two phonon modes of five participate in the remote electron-phonon scattering. The gate suppresses one of the modes but increases scattering by the other. Numerical results for the channel mobility limited only by remote phonon scattering were obtained for a Si FET with a HfO2 layer and a SiO2 layer in between the channel and metallic gate. A surprising result is the reduction of the mobility compared to the case when the gate screening is absent. The dependence of the mobility on the widths of HfO2 and interfacial SiO2 layers on channel concentration and temperature was studied. The accuracy of the calculations based on the Boltzmann equation is discussed. Finally, a comparison of our results with available experimental data leads to the conclusion that the remote phonon scattering is not the dominating scattering mechanism.

  17. A multi-mode sensing system for corrosion detection using piezoelectric wafer active sensors

    NASA Astrophysics Data System (ADS)

    Yu, Lingyu; Giurgiutiu, Victor; Pollock, Patrick

    2008-03-01

    As an emerging technology for in-situ damage detection and nondestructive evaluation, structural health monitoring with active sensors (active SHM) plays as a promising candidate for the pipeline inspection and diagnosis. Piezoelectric wafer active sensor (PWAS), as an active sensing device, can be permanently attached to the structure to interrogate it at will and can operate in propagating wave mode or electromechanical impedance mode. Its small size and low cost (about $10 each) make itself a potential and unique technology for in-situ SHM application. The objective of the research in this paper is to develop a permanently installed in-situ "multi-mode" sensing system for the corrosion monitoring and prediction of critical pipeline systems. Such a system is used during in-service period, recording and monitoring the changes of the pipelines over time, such as corrosion, wall thickness, etc. Having the real-time data available, maintenance strategies based on these data can then be developed to ensure a safe and less expensive operation of the pipeline systems. After a detailed review of PWAS SHM methods, including ultrasonic, impedance, and thickness measurement, we introduce the concept of PWAS-based multi-mode sensing approach for corrosion detection in pipelines. Particularly, we investigate the potential for using PWAS waves for in thickness mode experimentally. Finally, experiments are conducted to verify the corrosion detection ability of the PWAS network in both metallic plate and pipe in a laboratory setting. Results show successful corrosion localization in both tests.

  18. Electron and Phonon Dynamics in Hexagonal Pd Nanosheets and Ag/Pd/Ag Sandwich Nanoplates.

    PubMed

    Wang, Li; Sagaguchi, Takuya; Okuhata, Tomoki; Tsuboi, Motohiro; Tamai, Naoto

    2017-02-28

    Pd and its hybrid nanostructures have attracted considerable attention over the past decade, with both catalytic and plasmonic properties. The electron and phonon properties directly govern conversion efficiencies in applications such as energy collectors and photocatalysts. We report the dynamic processes of electron-phonon coupling and coherent acoustic phonon vibration in hexagonal Pd nanosheets and Ag/Pd/Ag sandwich nanoplates using transient absorption spectroscopy. The electron-phonon coupling constant of Pd nanosheets, GPd-nanosheet (8.7 × 10(17) W/(m(3)·K)) is larger than that of the bulk GPd (5.0 × 10(17) W/(m(3)·K)). The effective coupling constant Geff of Ag/Pd/Ag nanoplates decreases with increasing Ag shell thickness, finally approaching the bulk GAg. The variation of Geff is explained in terms of reduced density of states near Fermi level of Pd nanosheets with 1.8 nm ultrathin thickness. Coherent acoustic phonon vibration in Pd nanosheets is assigned to a fundamental breathing mode, similar to the vibration of benzene. The period increases with increasing Ag shell thickness. For Ag/Pd/Ag nanoplates with 20 nm thick Ag shells, the vibrational mode is ascribed to a quasi-extensional mode. The results show that the modes of the coherent acoustic phonon vibration transform with the geometric variation of Pd nanosheets and Ag/Pd/Ag nanoplates. Our results represent an understanding of quantum-confinement related electron dynamics and bulk-like phonon kinetics in the ultrathin Pd nanosheets and their hybrid nanostructures.

  19. Entanglement of two optically driven quantum dots mediated by phonons in nanomechanical resonator

    NASA Astrophysics Data System (ADS)

    He, Yong; Jiang, Meiping

    2017-01-01

    The exciton-phonon coupling between an optically driven quantum dot (QD) and a mechanical resonator can be described by Jaynes-Cummings model under a certain condition, revealing phonon absorption and emission. When two optically driven QDs share a common phonon mode, it shows the phonon-mediated coupling between the QDs. Based on the effective master equation for the reduced density matrix of the two QDs, the temporal evolution of each state and the concurrence (quantum entanglement) between them are studied. The results suggest that the stationary concurrence depends strongly on the resonator temperature. The non-negligible entanglement in the hybrid system is advantaged to develop solid-state quantum information processing.

  20. Coupling of nitrogen vacancy centres in nanodiamonds by means of phonons

    NASA Astrophysics Data System (ADS)

    Albrecht, A.; Retzker, A.; Jelezko, F.; Plenio, M. B.

    2013-08-01

    Realizing controlled quantum dynamics via the magnetic interactions between colour centres in diamond remains a challenge despite recent demonstrations for nanometre separated pairs. Here we propose to use the intrinsic acoustical phonons in diamond as a data bus for accomplishing this task. We show that for nanodiamonds the electron-phonon coupling can take significant values that together with mode frequencies in the THz range can serve as a resource for conditional gate operations. Based on these results, we analyse how to use this phonon-induced interaction for constructing quantum gates among the electron-spin triplet ground states, introducing the phonon dependence via Raman transitions. Combined with decoupling pulses this offers the possibility for creating entangled states within nanodiamonds on the scale of several tens of nanometres, a promising prerequisite for quantum sensing applications.

  1. 3-D phononic crystals with ultra-wide band gaps

    PubMed Central

    Lu, Yan; Yang, Yang; Guest, James K.; Srivastava, Ankit

    2017-01-01

    In this paper gradient based topology optimization (TO) is used to discover 3-D phononic structures that exhibit ultra-wide normalized all-angle all-mode band gaps. The challenging computational task of repeated 3-D phononic band-structure evaluations is accomplished by a combination of a fast mixed variational eigenvalue solver and distributed Graphic Processing Unit (GPU) parallel computations. The TO algorithm utilizes the material distribution-based approach and a gradient-based optimizer. The design sensitivity for the mixed variational eigenvalue problem is derived using the adjoint method and is implemented through highly efficient vectorization techniques. We present optimized results for two-material simple cubic (SC), body centered cubic (BCC), and face centered cubic (FCC) crystal structures and show that in each of these cases different initial designs converge to single inclusion network topologies within their corresponding primitive cells. The optimized results show that large phononic stop bands for bulk wave propagation can be achieved at lower than close packed spherical configurations leading to lighter unit cells. For tungsten carbide - epoxy crystals we identify all angle all mode normalized stop bands exceeding 100%, which is larger than what is possible with only spherical inclusions. PMID:28233812

  2. 3-D phononic crystals with ultra-wide band gaps

    NASA Astrophysics Data System (ADS)

    Lu, Yan; Yang, Yang; Guest, James K.; Srivastava, Ankit

    2017-02-01

    In this paper gradient based topology optimization (TO) is used to discover 3-D phononic structures that exhibit ultra-wide normalized all-angle all-mode band gaps. The challenging computational task of repeated 3-D phononic band-structure evaluations is accomplished by a combination of a fast mixed variational eigenvalue solver and distributed Graphic Processing Unit (GPU) parallel computations. The TO algorithm utilizes the material distribution-based approach and a gradient-based optimizer. The design sensitivity for the mixed variational eigenvalue problem is derived using the adjoint method and is implemented through highly efficient vectorization techniques. We present optimized results for two-material simple cubic (SC), body centered cubic (BCC), and face centered cubic (FCC) crystal structures and show that in each of these cases different initial designs converge to single inclusion network topologies within their corresponding primitive cells. The optimized results show that large phononic stop bands for bulk wave propagation can be achieved at lower than close packed spherical configurations leading to lighter unit cells. For tungsten carbide - epoxy crystals we identify all angle all mode normalized stop bands exceeding 100%, which is larger than what is possible with only spherical inclusions.

  3. Tunable phonon-cavity coupling in graphene membranes

    NASA Astrophysics Data System (ADS)

    de Alba, R.; Massel, F.; Storch, I. R.; Abhilash, T. S.; Hui, A.; McEuen, P. L.; Craighead, H. G.; Parpia, J. M.

    2016-09-01

    A major achievement of the past decade has been the realization of macroscopic quantum systems by exploiting the interactions between optical cavities and mechanical resonators. In these systems, phonons are coherently annihilated or created in exchange for photons. Similar phenomena have recently been observed through phonon-cavity coupling—energy exchange between the modes of a single system mediated by intrinsic material nonlinearity. This has so far been demonstrated primarily for bulk crystalline, high-quality-factor (Q > 105) mechanical systems operated at cryogenic temperatures. Here, we propose graphene as an ideal candidate for the study of such nonlinear mechanics. The large elastic modulus of this material and capability for spatial symmetry breaking via electrostatic forces is expected to generate a wealth of nonlinear phenomena, including tunable intermodal coupling. We have fabricated circular graphene membranes and report strong phonon-cavity effects at room temperature, despite the modest Q factor (∼100) of this system. We observe both amplification into parametric instability (mechanical lasing) and the cooling of Brownian motion in the fundamental mode through excitation of cavity sidebands. Furthermore, we characterize the quenching of these parametric effects at large vibrational amplitudes, offering a window on the all-mechanical analogue of cavity optomechanics, where the observation of such effects has proven elusive.

  4. Phononic crystals of poroelastic spheres

    NASA Astrophysics Data System (ADS)

    Alevizaki, A.; Sainidou, R.; Rembert, P.; Morvan, B.; Stefanou, N.

    2016-11-01

    An extension of the layer-multiple-scattering method to phononic crystals of poroelastic spheres immersed in a fluid medium is developed. The applicability of the method is demonstrated on specific examples of close-packed fcc crystals of submerged water-saturated meso- and macroporous silica microspheres. It is shown that, by varying the pore size and/or the porosity, the transmission, reflection, and absorption spectra of finite slabs of these crystals are significantly altered. Strong absorption, driven by the slow waves in the poroelastic material and enhanced by multiple scattering, leads to negligible transmittance over an extended frequency range, which might be useful for practical applications in broadband acoustic shielding. The results are analyzed by reference to relevant phononic dispersion diagrams in the viscous and inertial coupling limits, and a consistent interpretation of the underlying physics is provided.

  5. Helium Atom Scattering as a Probe of Surface Phonons.

    NASA Astrophysics Data System (ADS)

    Yerkes, Steven Charles

    the observed inelastic scattering, and evidence for interaction with other modes or with multiphonon processes is prevalent for most of the data. The Rayleigh phonon dispersion relation was obtained from the data and qualitatively agrees with known surface lattice dynamic calculations. No incoherent elastic scattering was observed from Ag(111).

  6. Low-Frequency Mode Activity of Heme: Femtosecond Coherence Spectroscopy of Iron Porphine Halides and Nitrophorin

    PubMed Central

    Kubo, Minoru; Gruia, Flaviu; Benabbas, Abdelkrim; Barabanschikov, Alexander; Montfort, William R.; Maes, Estelle M.; Champion, Paul M.

    2009-01-01

    The low-frequency mode activity of metalloporphyrins has been studied for iron porphine-halides (Fe(P)(X), X = Cl, Br) and nitrophorin 4 (NP4) using femtosecond coherence spectroscopy (FCS) in combination with polarized resonance Raman spectroscopy and density functional theory (DFT). It is confirmed that the mode symmetry selection rules for FCS are the same as for Raman scattering and that both Franck-Condon and Jahn-Teller mode activities are observed for Fe(P)(X) under Soret resonance conditions. The DFT-calculated low-frequency (20-400 cm-1) modes, and their frequency shifts upon halide substitution, are in good agreement with experimental Raman and coherence data, so that mode assignments can be made. The doming mode is located at ~80 cm-1 for Fe(P)(Cl) and at ~60 cm-1 for Fe(P)(Br). NP4 is also studied with coherence techniques, and the NO-bound species of ferric and ferrous NP4 display a mode at ~30-40 cm-1 that is associated with transient heme doming motion following NO photolysis. The coherence spectra of three ferric derivatives of NP4 with different degrees of heme ruffling distortion are also investigated. We find a mode at ~60 cm-1 whose relative intensity in the coherence spectra depends quadratically on the magnitude of the ruffling distortion. To quantitatively account for this correlation, a new “distortion-induced” Raman enhancement mechanism is presented. This mechanism is unique to low-frequency “soft modes” of the molecular framework that can be distorted by environmental forces. These results demonstrate the potential of FCS as a sensitive probe of dynamic and functionally important nonplanar heme vibrational excitations that are induced by the protein environmental forces or by the chemical reactions in the aqueous phase. PMID:18597456

  7. INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY: Polar Mixing Optical Phonon Spectra in Wurtzite GaN Cylindrical Quantum Dots: Quantum Size and Dielectric Effects

    NASA Astrophysics Data System (ADS)

    Zhang, Li; Liao, Jian-Shang

    2010-05-01

    The interface-optical-propagating (IO-PR) mixing phonon modes of a quasi-zero-dimensional (QoD) wurtzite cylindrical quantum dot (QD) structure are derived and studied by employing the macroscopic dielectric continuum model. The analytical phonon states of IO-PR mixing modes are given. It is found that there are two types of IO-PR mixing phonon modes, i.e. ρ-IO/z-PR mixing modes and the z-IO/ρ-PR mixing modes existing in QoD wurtzite QDs. And each IO-PR mixing modes also have symmetrical and antisymmetrical forms. Via a standard procedure of field quantization, the Fröhlich Hamiltonians of electron-(IO-PR) mixing phonons interaction are obtained. Numerical calculations on a wurtzite GaN cylindrical QD are performed. The results reveal that both the radial-direction size and the axial-direction size as well as the dielectric matrix have great influence on the dispersive frequencies of the IO-PR mixing phonon modes. The limiting features of dispersive curves of these phonon modes are discussed in depth. The phonon modes “reducing" behavior of wurtzite quantum confined systems has been observed obviously in the structures. Moreover, the degenerating behaviors of the IO-PR mixing phonon modes in wurtzite QoD QDs to the IO modes and PR modes in wurtzite Q2D QW and Q1D QWR systems are analyzed deeply from both of the viewpoints of physics and mathematics.

  8. Strain induced modification in phonon dispersion curves of monolayer boron pnictides

    SciTech Connect

    Jha, Prafulla K. E-mail: prafullaj@yahoo.com; Soni, Himadri R.

    2014-01-14

    In the frame work of density functional theory, the biaxial strain induced phonon dispersion curves of monolayer boron pnictides (BX, X = N, P, As, and Sb) have been investigated. The electron-ion interactions have been modelled using ultrasoft pseudopotentials while exchange-correlation energies have been approximated by the method of local density approximation in the parameterization of Perdew-Zunger. The longitudinal and transverse acoustic phonon modes of boron pnictide sheets show linear dependency on wave vector k{sup →} while out of plane mode varies as k{sup 2}. The in-plane longitudinal and out of plane transverse optical modes in boron nitride displaying significant dispersion similar to graphene. We have analyzed the biaxial strain dependent behaviour of out of plane acoustic phonon mode which is linked to ripple for four BX sheets using a model equation with shell elasticity theory. The strain induces the hardening of this mode with tendency to become more linear with increase in strain percentage. The strain induced hardening of out of plane acoustic phonon mode indicates the absence of rippling in these compounds. Our band structure calculations for both unstrained and strained 2D h-BX are consistent with previous calculations.

  9. High-wavenumber solar f-mode strengthening prior to active region formation

    NASA Astrophysics Data System (ADS)

    Singh, Nishant; Raichur, Harsha; Brandenburg, Axel

    2016-05-01

    We report a systematic strengthening of the local solar surface mode, i.e. the f-mode, 1-2 days prior to the emergence of an active region (AR) in the same (corotating) location while no indication can yet be seen in the magnetograms. Our study is motivated by earlier numerical findings of Singh et al. (2014) which showed that, in the presence of a nonuniform magnetic field that is concentrated a few scale heights below the surface, the f-mode fans out in the diagnostic kΩ diagram at high wavenumbers. Here we explore this possibility using data from the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory, and show for four ARs 11130, 11158, 11768, and 12051, that at large latitudinal wavenumbers (corresponding to horizontal scales of around 3000 km), the f-mode displays strengthening about two days prior to AR formation and thus provides a new precursor for AR formation. The idea that the f-mode is perturbed days before any visible magnetic activity occurs on the surface can be important in constraining dynamo models aimed at understanding the global magnetic activity of the Sun.

  10. Sustained activity within the default mode network during an implicit memory task

    PubMed Central

    Yang, Jiongjiong; Weng, Xuchu; Zang, Yufeng; Xu, Mingwei; Xu, Xiaohong

    2009-01-01

    Recent neuroimaging studies have shown that several brain regions -- namely, the posterior cingulate cortex (PCC), ventral medial prefrontal cortex (vmPFC), and the bilateral angular gyrus -- are more active during resting states than during cognitive tasks (i.e., default mode network). Although there is evidence showing that the default mode network is associated with unconscious state, it is unclear whether this network is associated with unconscious processing when normal human subjects perform tasks without awareness. We manipulated the level of conscious processing in normal subjects by asking them to perform an implicit and an explicit memory task, and analyzed signal changes in the default mode network for the stimuli versus baseline in both tasks. The fMRI analysis showed that the level of activation in regions within this network during the implicit task was not significantly different from that during the baseline, except in the left angular gyrus and the insula. There was strong deactivation for the explicit task when compared with the implicit task in the default mode regions, except in the left angular gyrus and the left middle temporal gyrus. These data suggest that the activity in the default network is sustained and less disrupted when an implicit memory task is performed, but is suspended when explicit retrieval is required. These results provide evidence that the default mode network is associated with unconscious processing when human subjects perform an implicit memory task. PMID:19552900

  11. Role of phonons in the metal-insulator phase transition.

    NASA Technical Reports Server (NTRS)

    Langer, W. D.

    1972-01-01

    Review, for the transition series oxides, of the Mattis and Lander model, which is one of electrons interacting with lattice vibrations (electron and phonon interaction). The model displays superconducting, insulating, and metallic phases. Its basic properties evolve from a finite crystallographic distortion associated with a dominant phonon mode and the splitting of the Brillouin zone into two subzones, a property of simple cubic and body centered cubic lattices. The order of the metal-insulator phase transition is examined. The basic model has a second-order phase transition and the effects of additional mechanisms on the model are calculated. The way in which these mechanisms affect the magnetically ordered transition series oxides as described by the Hubbard model is discussed.

  12. Ultrafast modulation of electronic structure by coherent phonon excitations

    NASA Astrophysics Data System (ADS)

    Weisshaupt, J.; Rouzée, A.; Woerner, M.; Vrakking, M. J. J.; Elsaesser, T.; Shirley, E. L.; Borgschulte, A.

    2017-02-01

    Femtosecond x-ray absorption spectroscopy with a laser-driven high-harmonic source is used to map ultrafast changes of x-ray absorption by femtometer-scale coherent phonon displacements. In LiBH4, displacements along an Ag phonon mode at 10 THz are induced by impulsive Raman excitation and give rise to oscillatory changes of x-ray absorption at the Li K edge. Electron density maps from femtosecond x-ray diffraction data show that the electric field of the pump pulse induces a charge transfer from the BH4- to neighboring Li+ ions, resulting in a differential Coulomb force that drives lattice vibrations in this virtual transition state.

  13. Phonon-Mediated Nonclassical Interference in Diamond.

    PubMed

    England, Duncan G; Fisher, Kent A G; MacLean, Jean-Philippe W; Bustard, Philip J; Heshami, Khabat; Resch, Kevin J; Sussman, Benjamin J

    2016-08-12

    Quantum interference of single photons is a fundamental aspect of many photonic quantum processing and communication protocols. Interference requires that the multiple pathways through an interferometer be temporally indistinguishable to within the coherence time of the photon. In this Letter, we use a diamond quantum memory to demonstrate interference between quantum pathways, initially temporally separated by many multiples of the optical coherence time. The quantum memory can be viewed as a light-matter beam splitter, mapping a THz-bandwidth single photon to a variable superposition of the output optical mode and stored phononic mode. Because the memory acts both as a beam splitter and as a buffer, the relevant coherence time for interference is not that of the photon, but rather that of the memory. We use this mechanism to demonstrate nonclassical single-photon and two-photon interference between quantum pathways initially separated by several picoseconds, even though the duration of the photons themselves is just ∼250  fs.

  14. Gutzwiller charge phase diagram of cuprates, including electron–phonon coupling effects

    DOE PAGES

    Markiewicz, R. S.; Seibold, G.; Lorenzana, J.; ...

    2015-02-01

    Besides significant electronic correlations, high-temperature superconductors also show a strong coupling of electrons to a number of lattice modes. Combined with the experimental detection of electronic inhomogeneities and ordering phenomena in many high-Tc compounds, these features raise the question as to what extent phonons are involved in the associated instabilities. Here we address this problem based on the Hubbard model including a coupling to phonons in order to capture several salient features of the phase diagram of hole-doped cuprates. Charge degrees of freedom, which are suppressed by the large Hubbard U near half-filling, are found to become active at amore » fairly low doping level. We find that possible charge order is mainly driven by Fermi surface nesting, with competition between a near-(π, π) order at low doping and antinodal nesting at higher doping, very similar to the momentum structure of magnetic fluctuations. The resulting nesting vectors are generally consistent with photoemission and tunneling observations, evidence for charge density wave order in YBa₂Cu₃O7-δ including Kohn anomalies, and suggestions of competition between one- and two-q-vector nesting.« less

  15. Gutzwiller charge phase diagram of cuprates, including electron–phonon coupling effects

    SciTech Connect

    Markiewicz, R. S.; Seibold, G.; Lorenzana, J.; Bansil, A.

    2015-02-01

    Besides significant electronic correlations, high-temperature superconductors also show a strong coupling of electrons to a number of lattice modes. Combined with the experimental detection of electronic inhomogeneities and ordering phenomena in many high-Tc compounds, these features raise the question as to what extent phonons are involved in the associated instabilities. Here we address this problem based on the Hubbard model including a coupling to phonons in order to capture several salient features of the phase diagram of hole-doped cuprates. Charge degrees of freedom, which are suppressed by the large Hubbard U near half-filling, are found to become active at a fairly low doping level. We find that possible charge order is mainly driven by Fermi surface nesting, with competition between a near-(π, π) order at low doping and antinodal nesting at higher doping, very similar to the momentum structure of magnetic fluctuations. The resulting nesting vectors are generally consistent with photoemission and tunneling observations, evidence for charge density wave order in YBa₂Cu₃O7-δ including Kohn anomalies, and suggestions of competition between one- and two-q-vector nesting.

  16. Biradicaloid character of thiophene-based heterophenoquinones: the role of electron-phonon coupling.

    PubMed

    Fazzi, Daniele; Canesi, Eleonora V; Negri, Fabrizia; Bertarelli, Chiara; Castiglioni, Chiara

    2010-12-03

    The quinoidal versus biradicaloid character of the ground state of a series of thiophene-based heterophenoquinones is investigated with quantum-chemical calculations. The role of the ground-state electronic character on molecular structure and vibrational properties is emphasized. The vibrational activities are experimentally determined and their analysis is performed by taking advantage of the definition of a collective vibrational coordinate (the R coordinate) maximizing the electron-phonon coupling, and connecting the quinoid and the aromatic biradicaloid resonance structures. The combined experimental and computational investigation supports the biradicaloid nature of the longer oligomers. The modulation of Raman intensities and frequency dispersion, experimentally observed by increasing the length of the chromophore, is shown to be reproduced well by model calculations on a single chromophore as a function of geometry displacements along the R-mode. These results underline the role of electron-phonon coupling in governing the structure-property relationship of highly conjugated organic compounds, underscoring the similarity of thiophene heterophenoquinone systems with other, more classical, oligophenylene and oligothiophene derivatives.

  17. Shelving-style QND phonon-number detection in quantum optomechanics

    NASA Astrophysics Data System (ADS)

    Yanay, Yariv; Clerk, Aashish A.

    2017-03-01

    We propose a new method for optomechanical quantum non-demolition detection of phonon number, based on a ‘shelving’ style measurement. The scheme uses a two-mode optomechanical system where the frequency splitting of the two photonic modes is near-resonant with the mechanical frequency. The combination of a strong optical drive and the underlying nonlinear optomechanical interaction gives rise to spin-like dynamics which facilitate the measurement. This approach allows phonon number measurement to be accomplished parametrically faster than in other schemes which are restricted to weak driving.

  18. Revealing the mechanism of passive transport in lipid bilayers via phonon-mediated nanometre-scale density fluctuations

    PubMed Central

    Zhernenkov, Mikhail; Bolmatov, Dima; Soloviov, Dmitry; Zhernenkov, Kirill; Toperverg, Boris P.; Cunsolo, Alessandro; Bosak, Alexey; Cai, Yong Q.

    2016-01-01

    The passive transport of molecules through a cell membrane relies on thermal motions of the lipids. However, the nature of transmembrane transport and the precise mechanism remain elusive and call for a comprehensive study of phonon excitations. Here we report a high resolution inelastic X-ray scattering study of the in-plane phonon excitations in 1,2-dipalmitoyl-sn-glycero-3-phosphocholine above and below the main transition temperature. In the gel phase, for the first time, we observe low-frequency transverse modes, which exhibit a phonon gap when the lipid transitions into the fluid phase. We argue that the phonon gap signifies the formation of short-lived nanometre-scale lipid clusters and transient pores, which facilitate the passive molecular transport across the bilayer plane. Our findings suggest that the phononic motion of the hydrocarbon tails provides an effective mechanism of passive transport, and illustrate the importance of the collective dynamics of biomembranes. PMID:27175859

  19. Phonon scattering due to van der Waals forces in the lattice thermal conductivity of Bi2Te3 thin films

    NASA Astrophysics Data System (ADS)

    Park, Kyeong Hyun; Mohamed, Mohamed; Aksamija, Zlatan; Ravaioli, Umberto

    2015-01-01

    In this work, we calculate the thermal conductivity of layered bismuth telluride (Bi2Te3) thin films by solving the Boltzmann transport equation in the relaxation-time approximation using full phonon dispersion and compare our results with recently published experimental data and molecular dynamics simulation. The group velocity of each phonon mode is readily extracted from the full phonon dispersion obtained from first-principle density-functional theory calculation and is used along with the phonon frequency to compute the various scattering terms. Our model incorporates the typical interactions impeding thermal transport (e.g., umklapp, isotope, and boundary scatterings) and introduces a new interaction capturing the reduction of phonon transmission through van der Waals interfaces of adjacent Bi2Te3 quintuple layers forming the virtual superlattice thin film. We find that this novel approach extends the empirical Klemens-Callaway relaxation model in such anisotropic materials and recovers the experimental anisotropy while using a minimal set of parameters.

  20. Inelastic x-ray scattering measurements of phonon dynamics in URu2Si2

    SciTech Connect

    Gardner, D. R.; Bonnoit, C. J.; Chisnell, R.; Said, A. H.; Leu, B. M.; Williams, Travis J.; Luke, G. M.; Lee, Y. S.

    2016-02-11

    In this paper, we study high-resolution inelastic x-ray scattering measurements of the acoustic phonons of URu2Si2. At all temperatures, the longitudinal acoustic phonon linewidths are anomalously broad at small wave vectors revealing a previously unknown anharmonicity. The phonon modes do not change significantly upon cooling into the hidden order phase. In addition, our data suggest that the increase in thermal conductivity in the hidden order phase cannot be driven by a change in phonon dispersions or lifetimes. Hence, the phonon contribution to the thermal conductivity is likely much less significant compared to that of the magnetic excitations in the low temperature phase.

  1. Revealing the mechanism of passive transport in lipid bilayers via phonon-mediated nanometre-scale density fluctuations

    SciTech Connect

    Zhernenkov, Mikhail; Bolmatov, Dima; Soloviov, Dmitry; Zhernenkov, Kirill; Toperverg, Boris P.; Cunsolo, Alessandro; Bosak, Alexey; Cai, Yong Q.

    2016-05-12

    The passive transport of molecules through a cell membrane relies on thermal motions of the lipids. However, the nature of transmembrane transport and the precise mechanism remain elusive and call for a comprehensive study of phonon excitations. Here we report a high resolution inelastic X-ray scattering study of the in-plane phonon excitations in 1,2-dipalmitoyl-sn-glycero-3-phosphocholine above and below the main transition temperature. In the gel phase, for the first time, we observe low-frequency transverse modes, which exhibit a phonon gap when the lipid transitions into the fluid phase. We argue that the phonon gap signifies the formation of short-lived nanometre-scale lipid clusters and transient pores, which facilitate the passive molecular transport across the bilayer plane. Finally, our findings suggest that the phononic motion of the hydrocarbon tails provides an effective mechanism of passive transport, and illustrate the importance of the collective dynamics of biomembranes.

  2. Anomalous Infrared Spectra of Hybridized Phonons in Type-I Clathrate Ba8Ga16Ge30

    NASA Astrophysics Data System (ADS)

    Iwamoto, Kei; Kushibiki, Shunsuke; Honda, Hironori; Kajitani, Shuhei; Mori, Tatsuya; Matsumoto, Hideki; Toyota, Naoki; Suekuni, Koichiro; Avila, Marcos A.; Takabatake, Toshiro

    2013-02-01

    The optical conductivity spectra of the rattling phonons in the clathrate Ba8Ga16Ge30 are investigated in detail by use of the terahertz time-domain spectroscopy. The experiment has revealed that the lowest-lying vibrational mode of Ba(2)2+ ions consist of a sharp Lorentzian peak at 1.2 THz superimposed on a broad tail weighted in the lower frequency regime around 1.0 THz. With decreasing temperature, an unexpected linewidth broadening of the phonon peak is observed, together with monotonic softening of the phonon peak and enhancement of the tail structure. These observed anomalies are discussed in terms of impurity scattering effects on the hybridized phonon system of rattling and acoustic phonons.

  3. Observation of Interlayer Phonons in Transition Metal Dichalcogenide Heterostructures

    NASA Astrophysics Data System (ADS)

    He, Rui; Ye, Zhipeng; Ji, Chao; Means-Shively, Casie; Anderson, Heidi; Kidd, Tim; Chiu, Kuan-Chang; Chou, Cheng-Tse; Wu, Jenn-Ming; Lee, Yi-Hsien; Andersen, Trond; Lui, Chun Hung

    Interlayer phonon modes in transition metal dichalcogenide (TMD) heterostructures are observed for the first time. We measured the low-frequency Raman response of MoS2/WSe2 and MoSe2/MoS2 heterobilayers. We discovered a distinct Raman mode (30 - 35 cm-1) that cannot be found in any individual monolayers. By comparing with Raman spectra of Bernal bilayer (2L) MoS2, 2L MoSe2 and 2L WSe2, we identified the new Raman mode as the layer breathing vibration arising from the vertical displacement of the two TMD layers. The layer breathing mode (LBM) only emerges in bilayer regions with atomically close layer-layer proximity and clean interface. In addition, the LBM frequency exhibits noticeable dependence on the rotational angle between the two TMD layers, which implies a change of interlayer separation and interlayer coupling strength with the layer stacking.

  4. Active lower order mode damping for the four rod LHC crab cavity

    NASA Astrophysics Data System (ADS)

    Dexter, A. C.; Burt, G.; Apsimon, R.

    2017-02-01

    The high luminosity upgrade planned for the LHC requires crab cavities to rotate bunches into alignment at the interaction points. They compensate for a crossing angle near 500 μ r ad . It is anticipated that four crab cavities in succession will be utilized to achieve this rotation on either side of each IP in a local crossing scheme. A crab cavity operates in a dipole mode but always has an accelerating mode that may be above or below the frequency of the operating mode. Crab cavities are given couplers to ensure that unwanted acceleration modes are strongly damped however employing standard practice these unwanted modes will always have some level of excitation. Where this excitation has a random phase it might promote bunch growth and limit beam lifetime. This paper sets out a method for active control of the phase and amplitude of the unwanted lowest accelerating mode in the crab cavities. The paper investigates the level of suppression that can be achieved as a function cavity quality factor and proximity to resonance.

  5. Independent component model of the default-mode brain function: Assessing the impact of active thinking.

    PubMed

    Esposito, Fabrizio; Bertolino, Alessandro; Scarabino, Tommaso; Latorre, Valeria; Blasi, Giuseppe; Popolizio, Teresa; Tedeschi, Gioacchino; Cirillo, Sossio; Goebel, Rainer; Di Salle, Francesco

    2006-10-16

    The "default-mode" network is an ensemble of cortical regions, which are typically deactivated during demanding cognitive tasks in functional magnetic resonance imaging (fMRI) studies. Using functional connectivity, this network can be conceptualized and studied as a "stand-alone" function or system. Regardless of the task, independent component analysis (ICA) produces a picture of the "default-mode" function even when the subject is performing a simple sensori-motor task or just resting in the scanner. This has boosted the use of default-mode fMRI for non-invasive research in brain disorders. Here, we studied the effect of cognitive load modulation of fMRI responses on the ICA-based pictures of the default-mode function. In a standard graded working memory study based on the n-back task, we used group-level ICA to explore the variability of the default-mode network related to the engagement in the task, in 10 healthy volunteers. The analysis of the default-mode components highlighted similarities and differences in the layout under three different cognitive loads. We found a load-related general increase of deactivation in the cortical network. Nonetheless, a variable recruitment of the cingulate regions was evident, with greater extension of the anterior and lesser extension of the posterior clusters when switching from lower to higher working memory loads. A co-activation of the hippocampus was only found under no working memory load. As a generalization of our results, the variability of the default-mode pattern may link the default-mode system as a whole to cognition and may more directly support use of the ICA model for evaluating cognitive decline in brain disorders.

  6. Modes of Engagement in Foreign Language Writing: An Activity Theoretical Perspective

    ERIC Educational Resources Information Center

    Haneda, Mari

    2007-01-01

    This article makes the case for using activity theory to explore the learning and teaching of writing in a foreign language. I illustrate my argument by bringing this theory to bear on a re-examination of the different modes of engagement in writing by university-level students of Japanese as a foreign language that I identified in an earlier…

  7. Active mode locking of a p-Ge hot hole laser

    SciTech Connect

    Hovenier, J.N.; Muravjov, A.V.; Pavlov, S.G.; Shastin, V.N.; Strijbos, R.C.; Wenckebach, W.T.

    1997-07-01

    The generation of 200 picosecond pulses of far-infrared radiation from a p-Ge hot hole laser (50{endash}140 cm{sup {minus}1}) was achieved due to active mode locking by electrical intracavity modulation of the gain. {copyright} {ital 1997 American Institute of Physics.}

  8. Off-axis phonon and photon propagation in porous silicon superlattices studied by Brillouin spectroscopy and optical reflectance

    SciTech Connect

    Parsons, L. C. Andrews, G. T.

    2014-07-21

    Brillouin light scattering experiments and optical reflectance measurements were performed on a pair of porous silicon-based optical Bragg mirrors which had constituent layer porosity ratios close to unity. For off-axis propagation, the phononic and photonic band structures of the samples were modeled as a series of intersecting linear dispersion curves. Zone-folding was observed for the longitudinal bulk acoustic phonon and the frequency of the probed zone-folded longitudinal phonon was shown to be dependent on the propagation direction as well as the folding order of the mode branch. There was no conclusive evidence of coupling between the transverse and the folded longitudinal modes. Two additional observed Brillouin peaks were attributed to the Rayleigh surface mode and a possible pseudo-surface mode. Both of these modes were dispersive, with the velocity increasing as the wavevector decreased.

  9. X-mode reflectometry for magnetohydrodynamic activity associated with q=1 surface measurements on Tore Supra

    SciTech Connect

    Vermare, L.; Clairet, F.; Gabillet, F.; Sabot, R.; Sirinelli, A.; Heuraux, S.; Leclert, G.

    2004-10-01

    Tore Supra is equipped with two 20 {mu}s fast sweep X-mode reflectometers operating between 50-110 GHz dedicated to density profile determination and an X-mode fixed frequency reflectometer operating between 105-155 GHz for density fluctuation measurements. Heterodyne and sine-cosine detection provide measurements of the reflected signal with high sensitivity. Operating profile reflectometer in burst mode (5 {mu}s dead time between two consecutive sweeps) allows quasi-simultaneous measurements at fixed frequency over a broad frequency band. Thus, information on plasma fluctuations, such as magnetohydrodynamic (MHD) activity, up to 20 kHz as well as a radial localization of the modes is accessible. The temporal evolution of the q=1 rational surface during sawtooth crash activity has been recorded in the plasma center with high spatial resolution. In addition, a direct comparison between signals associated with a central MHD mode from both profile and fluctuation reflectometers, positioned at different toroidal angles, allows one to determine the plasma toroidal velocity.

  10. Different pulse pattern generation by frequency detuning in pulse modulated actively mode-locked ytterbium doped fiber laser

    NASA Astrophysics Data System (ADS)

    Chen, He; Chen, Sheng-Ping; Si, Lei; Zhang, Bin; Jiang, Zong-Fu

    2015-10-01

    We report the results of our recent experimental investigation of the modulation frequency detuning effect on the output pulse dynamics in a pulse modulated actively mode-locked ytterbium doped fiber laser. The experimental study shows the existence of five different mode-locking states that mainly depend on the modulation frequency detuning, which are: (a) amplitude-even harmonic/fundamental mode-locking, (b) Q-switched harmonic/fundamental mode-locking, (c) sinusoidal wave modulation mode, (d) pulses bundle state, and (e) noise-like state. A detailed experimental characterization of the output pulses dynamics in each operating mode is presented.

  11. Electron-phonon interactions from first principles

    NASA Astrophysics Data System (ADS)

    Giustino, Feliciano

    2017-01-01

    This article reviews the theory of electron-phonon interactions in solids from the point of view of ab initio calculations. While the electron-phonon interaction has been studied for almost a century, predictive nonempirical calculations have become feasible only during the past two decades. Today it is possible to calculate from first principles many materials properties related to the electron-phonon interaction, including the critical temperature of conventional superconductors, the carrier mobility in semiconductors, the temperature dependence of optical spectra in direct and indirect-gap semiconductors, the relaxation rates of photoexcited carriers, the electron mass renormalization in angle-resolved photoelectron spectra, and the nonadiabatic corrections to phonon dispersion relations. In this article a review of the theoretical and computational framework underlying modern electron-phonon calculations from first principles as well as landmark investigations of the electron-phonon interaction in real materials is given. The first part of the article summarizes the elementary theory of electron-phonon interactions and their calculations based on density-functional theory. The second part discusses a general field-theoretic formulation of the electron-phonon problem and establishes the connection with practical first-principles calculations. The third part reviews a number of recent investigations of electron-phonon interactions in the areas of vibrational spectroscopy, photoelectron spectroscopy, optical spectroscopy, transport, and superconductivity.

  12. Phonon-assisted transient electroluminescence in Si

    SciTech Connect

    Cheng, Tzu-Huan; Chu-Su, Yu; Liu, Chien-Sheng; Lin, Chii-Wann

    2014-06-30

    The phonon-replica infrared emission is observed at room temperature from indirect band gap Si light-emitting diode under forward bias. With increasing injection current density, the broadened electroluminescence spectrum and band gap reduction are observed due to joule heating. The spectral-resolved temporal response of electroluminescence reveals the competitiveness between single (TO) and dual (TO + TA) phonon-assisted indirect band gap transitions. As compared to infrared emission with TO phonon-replica, the retarder of radiative recombination at long wavelength region (∼1.2 μm) indicates lower transition probability of dual phonon-replica before thermal equivalent.

  13. Effect of Rattling Phonons on Sommerfeld Constant

    NASA Astrophysics Data System (ADS)

    Hotta, Takashi

    2008-10-01

    By employing a numerical renormalization group technique, we evaluate electronic specific heat coefficient γ of the Anderson model coupled with local anharmonic phonons for the oscillation of a caged atom. For the rattling-type cage potential with a flat and wide region in the bottom, we find that phonon-mediated attraction is largely enhanced. When the potential shape is deformed from the rattling type, there occurs a cancellation between Coulomb repulsion and the phonon-mediated attraction. In such a situation, spin and charge fluctuations are comparable to each other, leading to the realization of exotic electron-phonon complex state with large and magnetically robust γ.

  14. Blink-related momentary activation of the default mode network while viewing videos.

    PubMed

    Nakano, Tamami; Kato, Makoto; Morito, Yusuke; Itoi, Seishi; Kitazawa, Shigeru

    2013-01-08

    It remains unknown why we generate spontaneous eyeblinks every few seconds, more often than necessary for ocular lubrication. Because eyeblinks tend to occur at implicit breakpoints while viewing videos, we hypothesized that eyeblinks are actively involved in the release of attention. We show that while viewing videos, cortical activity momentarily decreases in the dorsal attention network after blink onset but increases in the default-mode network implicated in internal processing. In contrast, physical blackouts of the video do not elicit such reciprocal changes in brain networks. The results suggest that eyeblinks are actively involved in the process of attentional disengagement during a cognitive behavior by momentarily activating the default-mode network while deactivating the dorsal attention network.

  15. Activated vibrational modes and Fermi resonance in tip-enhanced Raman spectroscopy.

    PubMed

    Sun, Mengtao; Fang, Yurui; Zhang, Zhenyu; Xu, Hongxing

    2013-02-01

    Using p-aminothiophenol (PATP) molecules on a gold substrate and high-vacuum tip-enhanced Raman spectroscopy (HV-TERS), we show that the vibrational spectra of these molecules are distinctly different from those in typical surface-enhanced Raman spectroscopy. Detailed first-principles calculations help to assign the Raman peaks in the TERS measurements as Raman-active and IR-active vibrational modes of dimercaptoazobenzene (DMAB), providing strong spectroscopic evidence for the dimerization of PATP molecules to DMAB under the TERS setup. The activation of the IR-active modes is due to enhanced electromagnetic field gradient effects within the gap region of the highly asymmetric tip-surface geometry. Fermi resonances are also observed in HV-TERS. These findings help to broaden the versatility of TERS as a promising technique for ultrasensitive molecular spectroscopy.

  16. Large spin-phonon coupling and magnetically induced phonon anisotropy in SrMO3 perovskites (M=V,Cr,Mn,Fe,Co)

    NASA Astrophysics Data System (ADS)

    Lee, Jun Hee; Rabe, Karin M.

    2011-09-01

    First-principles calculations reveal large zone-center spin-phonon coupling and magnetically-driven phonon anisotropy in cubic perovskites SrMO3 (M=V,Cr,Mn,Fe,Co). In particular, the frequency and splitting of the polar Slater mode is found to depend strongly upon magnetic ordering. The coupling is parameterized in a crystal-structure-dependent Heisenberg model, and its main features seen to arise from the Goodenough-Kanamori rules. This coupling can be expected to produce distinct low-energy alternative phases, resulting in a rich variety of coupled magnetic, structural, and electronic phase transitions driven by temperature, stress, electric field, and cation substitution.

  17. Actively mode-locked fiber laser using acousto-optic modulator

    NASA Astrophysics Data System (ADS)

    Nikodem, Michal P.; Sergeant, Hendrik; Kaczmarek, Pawel; Abramski, Krzysztof M.

    2008-12-01

    In recent years we have observed growing interest in mode-locked fiber lasers. Development of erbium doped fiber (EDF) amplifiers and WDM technique made 3rd telecommunication window extremely interesting region for ultrafast optics. The main advantages of fiber lasers i.e. narrow linewidth and wide gain bandwidth make them very attractive sources in various applications. In this paper we present an actively mode-locked erbium doped fiber ring laser. Modelocking is obtained using an acousto-optic modulator (AOM) coupled into the laser cavity. The impact of different parameters (e.g. light polarization, modulation frequency) is investigated. We study mechanisms of controlling the wavelength of the laser.

  18. Active Noise Control of Low Speed Fan Rotor-Stator Modes

    NASA Technical Reports Server (NTRS)

    Sutliff, Daniel L.; Hu, Ziqiang; Pla, Frederic G.; Heidelberg, Laurence J.

    1996-01-01

    This report describes the Active Noise Cancellation System designed by General Electric and tested in the NASA Lewis Research Center's 48 inch Active Noise Control Fan. The goal of this study was to assess the feasibility of using wall mounted secondary acoustic sources and sensors within the duct of a high bypass turbofan aircraft engine for active noise cancellation of fan tones. The control system is based on a modal control approach. A known acoustic mode propagating in the fan duct is cancelled using an array of flush-mounted compact sound sources. Controller inputs are signals from a shaft encoder and a microphone array which senses the residual acoustic mode in the duct. The canceling modal signal is generated by a modal controller. The key results are that the (6,0) mode was completely eliminated at 920 Hz and substantially reduced elsewhere. The total tone power was reduced 9.4 dB. Farfield 2BPF SPL reductions of 13 dB were obtained. The (4,0) and (4,1) modes were reduced simultaneously yielding a 15 dB modal PWL decrease. Global attenuation of PWL was obtained using an actuator and sensor system totally contained within the duct.

  19. Wavelength-tunable actively mode-locked erbium-doped fiber ring laser using a distributed feedback semiconductor laser as mode locker and tunable filter

    NASA Astrophysics Data System (ADS)

    Li, Shenping; Chan, K. T.

    1999-07-01

    A wavelength-tunable actively mode-locked erbium fiber ring laser was demonstrated using a distributed feedback semiconductor laser as an intensity mode locker and a tunable optical filter. Very stable optical pulse trains at gigabit repetition rates were generated using harmonica mode locking. The supermode noise was suppressed to 60 dB below the signal level and the root-mean-square timing jitter (0.45 kHz-1 MHz) was found to be about 1% of the pulse duration. A continuous wavelength tuning range of 1.8 nm was achieved by changing the semiconductor laser temperature from 11.4 to 30 °C.

  20. Transport and localization in a topological phononic lattice with correlated disorder

    NASA Astrophysics Data System (ADS)

    Ong, Zhun-Yong; Lee, Ching Hua

    2016-10-01

    Recently proposed classical analogs of topological insulators in phononic lattices have the advantage of much more accessible experimental realization compared to conventional materials. Drawn to their potential practical structural applications, we investigate how disorder, which is generically nonnegligible in macroscopic realization, can attenuate the topologically protected edge (TPE) modes that constitute robust transmitting channels at zero disorder. We simulate the transmission of phonon modes in a quasi-one-dimensional classical lattice waveguide with mass disorder and show that the TPE mode transmission remains highly robust (Ξ ˜1 ) in the presence of uncorrelated disorder but diminishes when disorder is spatially correlated. This reduction in transmittance is attributed to the Anderson localization of states within the mass disorder domains. By contrast, non-TPE channels exhibit qualitatively different behavior, with spatial correlation in the mass disorder leading to significant transmittance reduction (enhancement) at low (high) frequencies. Our results demonstrate how TPE modes drastically modify the effect of spatial correlation on mode localization.

  1. Raman spectroscopy of magneto-phonon resonances in graphene and graphite

    NASA Astrophysics Data System (ADS)

    Goler, Sarah; Yan, Jun; Pellegrini, Vittorio; Pinczuk, Aron

    2012-08-01

    The magneto-phonon resonance or MPR occurs in semiconductor materials when the energy spacing between Landau levels is continuously tuned to cross the energy of an optical phonon mode. MPRs have been largely explored in bulk semiconductors, in two-dimensional systems and in quantum dots. Recently there has been significant interest in the MPR interactions of the Dirac fermion magneto-excitons in graphene, and a rich splitting and anti-crossing phenomena of the even parity E2g long wavelength optical phonon mode have been theoretically proposed and experimentally observed. The MPR has been found to crucially depend on disorder in the graphene layer. This is a feature that creates new venues for the study of interplays between disorder and interactions in the atomic layers. We review here the fundamentals of MRP in graphene and the experimental Raman scattering works that have led to the observation of these phenomena in graphene and graphite.

  2. Topological phase transitions in group IV-VI semiconductors by phonons

    NASA Astrophysics Data System (ADS)

    Kim, Jinwoong; Jhi, Seung-Hoon

    2015-09-01

    The topological insulator has an intriguing electronic structure in that it has nontrivial topology enforcing the helical Dirac fermionic states at interfaces to the band insulators. Protected by the time-reversal symmetry and finite band gaps in the bulk, the topology is immune to external nonmagnetic perturbations. One essential question is whether elementary excitations in solids like phonons can trigger a transition in the topological property of the electronic structures. Here we investigate the development of topological insulating phases in IV-VI compounds under dynamic lattice deformations using first-principles calculations. Unlike the static state of topological phases at equilibrium conditions, we show that nontrivial topological phases are induced in the compounds by the dynamic lattice deformations from selective phonon modes. Calculations of the time-reversal polarization show that the Z2 invariant of the compounds is flipped by the selective phonon modes and that the compounds exhibit oscillating topological phases upon dynamic lattice deformations.

  3. Modes of hurricane activity variability in the eastern Pacific: Implications for the 2016 season

    NASA Astrophysics Data System (ADS)

    Boucharel, Julien; Jin, Fei-Fei; England, Matthew H.; Lin, I. I.

    2016-11-01

    A gridded product of accumulated cyclone energy (ACE) in the eastern Pacific is constructed to assess the dominant mode of tropical cyclone (TC) activity variability. Results of an empirical orthogonal function decomposition and regression analysis of environmental variables indicate that the two dominant modes of ACE variability (40% of the total variance) are related to different flavors of the El Niño-Southern Oscillation (ENSO). The first mode, more active during the later part of the hurricane season (September-November), is linked to the eastern Pacific El Niño through the delayed oceanic control associated with the recharge-discharge mechanism. The second mode, dominant in the early months of the hurricane season, is related to the central Pacific El Niño mode and the associated changes in atmospheric variability. A multilinear regression forecast model of the dominant principal components of ACE variability is then constructed. The wintertime subsurface state of the eastern equatorial Pacific (characterizing ENSO heat discharge), the east-west tilt of the thermocline (describing ENSO phase transition), the anomalous ocean surface conditions in the TC region in spring (portraying atmospheric changes induced by persistence of local surface anomalies), and the intraseasonal atmospheric variability in the western Pacific are found to be good predictors of TC activity. Results complement NOAA's official forecast by providing additional spatial and temporal information. They indicate a more active 2016 season ( 2 times the ACE mean) with a spatial expansion into the central Pacific associated with the heat discharge from the 2015/2016 El Niño.

  4. Inelastic x-ray scattering measurements of phonon dispersion and lifetimes in PbTe1-x Se x alloys

    NASA Astrophysics Data System (ADS)

    Tian, Zhiting; Li, Mingda; Ren, Zhensong; Ma, Hao; Alatas, Ahmet; Wilson, Stephen D.; Li, Ju

    2015-09-01

    PbTe1-x Se x alloys are of special interest to thermoelectric applications. Inelastic x-ray scattering determination of phonon dispersion and lifetimes along the high symmetry directions for PbTe1-x Se x alloys are presented. By comparing with calculated results based on the virtual crystal model calculations combined with ab initio density functional theory, the validity of virtual crystal model is evaluated. The results indicate that the virtual crystal model is overall a good assumption for phonon frequencies and group velocities despite the softening of transverse acoustic phonon modes along [1 1 1] direction, while the treatment of lifetimes warrants caution. In addition, phonons remain a good description of vibrational modes in PbTe1-x Se x alloys.

  5. Inelastic x-ray scattering measurements of phonon dispersion and lifetimes in PbTe1-x Se x alloys.

    PubMed

    Tian, Zhiting; Li, Mingda; Ren, Zhensong; Ma, Hao; Alatas, Ahmet; Wilson, Stephen D; Li, Ju

    2015-09-23

    PbTe1-x Se x alloys are of special interest to thermoelectric applications. Inelastic x-ray scattering determination of phonon dispersion and lifetimes along the high symmetry directions for PbTe1-x Se x alloys are presented. By comparing with calculated results based on the virtual crystal model calculations combined with ab initio density functional theory, the validity of virtual crystal model is evaluated. The results indicate that the virtual crystal model is overall a good assumption for phonon frequencies and group velocities despite the softening of transverse acoustic phonon modes along [1 1 1] direction, while the treatment of lifetimes warrants caution. In addition, phonons remain a good description of vibrational modes in PbTe1-x Se x alloys.

  6. A new type of magnetoresistance oscillations: Interaction of a two-dimensional electron gas with leaky interface phonons

    SciTech Connect

    ZUDOV,M.A.; PONOMAREV,I.V.; EFROS,A.L.; DU,R.R.; SIMMONS,JERRY A.; RENO,JOHN L.

    2000-05-11

    The authors report a new type of oscillations in magnetoresistance observed in high-mobility two-dimensional electron gas (2DEG), in GaAs-AIGaAs heterostructures. Being periodic in 1/B these oscillations appear in weak magnetic field (B < 0.3 T) and only in a narrow temperature range (3 K < T < 7 K). Remarkably, these oscillations can be understood in terms of magneto-phonon resonance originating from the interaction of 2DEG and leaky interface-acoustic phonon modes. The existence of such modes on the GaAs:AIGaAs interface is demonstrated theoretically and their velocities are calculated. It is shown that the electron-phonon scattering matrix element exhibits a peak for the phonons carrying momentum q = 2k{sub F} (k{sub F} is the Fermi wave-vector of 2DEG).

  7. Impacts of human activity modes and climate on heavy metal "spread" in groundwater are biased.

    PubMed

    Chen, Ming; Qin, Xiaosheng; Zeng, Guangming; Li, Jian

    2016-06-01

    Groundwater quality deterioration has attracted world-wide concerns due to its importance for human water supply. Although more and more studies have shown that human activities and climate are changing the groundwater status, an investigation on how different groundwater heavy metals respond to human activity modes (e.g. mining, waste disposal, agriculture, sewage effluent and complex activity) in a varying climate has been lacking. Here, for each of six heavy metals (i.e. Fe, Zn, Mn, Pb, Cd and Cu) in groundwater, we use >330 data points together with mixed-effect models to indicate that (i) human activity modes significantly influence the Cu and Mn but not Zn, Fe, Pb and Cd levels, and (ii) annual mean temperature (AMT) only significantly influences Cu and Pb levels, while annual precipitation (AP) only significantly affects Fe, Cu and Mn levels. Given these differences, we suggest that the impacts of human activity modes and climate on heavy metal "spread" in groundwater are biased.

  8. Nanoscale transport of phonons: Dimensionality, subdiffusion, molecular damping, and interference effects

    SciTech Connect

    Walczak, Kamil; Yerkes, Kirk L.

    2014-05-07

    We examine heat transport carried by acoustic phonons in the systems composed of nanoscale chains of masses coupled to two thermal baths of different temperatures. Thermal conductance is obtained by using linearized Landauer-type formula for heat flux with phonon transmission probability calculated within atomistic Green's functions (AGF) method. AGF formalism is extended onto dissipative chains of masses with harmonic coupling beyond nearest-neighbor approximation, while atomistic description of heat reservoirs is also included into computational scheme. In particular, the phonon lifetimes and the phonon frequency shifts are discussed for harmonic lattices of different dimensions. Further, resonant structure of phonon transmission spectrum is analyzed with respect to reservoir-induced effects, molecular damping, and mass-to-mass harmonic coupling. Analysis of transmission zeros (antiresonances) and their accompanied Fano-shape resonances are discussed as a result of interference effects between different vibrational modes. Finally, we also predict subdiffusive transport regime for low-frequency ballistic phonons propagated along a linear chain of harmonically coupled masses.

  9. Nanoscale transport of phonons: Dimensionality, subdiffusion, molecular damping, and interference effects

    NASA Astrophysics Data System (ADS)

    Walczak, Kamil; Yerkes, Kirk L.

    2014-05-01

    We examine heat transport carried by acoustic phonons in the systems composed of nanoscale chains of masses coupled to two thermal baths of different temperatures. Thermal conductance is obtained by using linearized Landauer-type formula for heat flux with phonon transmission probability calculated within atomistic Green's functions (AGF) method. AGF formalism is extended onto dissipative chains of masses with harmonic coupling beyond nearest-neighbor approximation, while atomistic description of heat reservoirs is also included into computational scheme. In particular, the phonon lifetimes and the phonon frequency shifts are discussed for harmonic lattices of different dimensions. Further, resonant structure of phonon transmission spectrum is analyzed with respect to reservoir-induced effects, molecular damping, and mass-to-mass harmonic coupling. Analysis of transmission zeros (antiresonances) and their accompanied Fano-shape resonances are discussed as a result of interference effects between different vibrational modes. Finally, we also predict subdiffusive transport regime for low-frequency ballistic phonons propagated along a linear chain of harmonically coupled masses.

  10. Non-equilibrium optical phonon dynamics in bulk and low-dimensional semiconductors

    NASA Astrophysics Data System (ADS)

    Srivastava, G. P.

    2007-02-01

    We present theoretical investigations of the intrinsic dynamics of long-wavelength non-equilibrium optical phonons in bulk and low-dimensional semiconductors. The theory is based on the application of Fermi's golden rule formula, with phonon dispersion relations as well as crystal anharmonicity considered in the framework of isotropic continuum model. Contributions to the decay rates of the phonon modes are discussed in terms of four possible channels: Klemens channel (into two acoustic daughter modes), generalised Ridley channel (into one acoustic and one optical mode), generalised Vallee-Bogani channel (into a lower mode of the same branch and an acoustic mode), and Barman-Srivastava channel (into two lower-branch optical modes). The role of crystal structure and cation/anion mass ratio in determining the lifetime of such modes in bulk semiconductors is highlighted. Estimates of lifetimes of such modes in silicon nanowires and carbon nanotubes will also be presented. The results support and explain available experimental data, and make predictions in some cases.

  11. Observation of one magnon and magnon-phonon-electric dipole coupling in multiferroics bismuth ferrite thin films

    NASA Astrophysics Data System (ADS)

    Kumar, Ashok; Murari, N. M.; Katiyar, R. S.

    2008-04-01

    We observed "one magnon," scattering in multiferroic polycrystalline BiFeO3 thin films near 17.2cm-1 at 90K employing Raman spectroscopy. It is seen with a kink in magnon intensity at 150K and with strong anomaly near 210K illustrating spin reorientation transition. The spectral weight of one magnon transferred to the lowest phonon mode near the spin reorientation temperature suggests magnon-phonon coupling. Dielectric constant and dielectric loss as function of temperature showed anomaly at 210K suggesting magnon-phonon-electric dipole coupling. The one magnon becomes overdamped or overcome by elastic scattering at elevated temperatures.

  12. Electron-phonon interaction effect on the energy levels and diamagnetic susceptibility of quantum wires: Parallelogram and triangle cross section

    SciTech Connect

    Khordad, R. Bahramiyan, H.

    2014-03-28

    In this paper, optical phonon modes are studied within the framework of dielectric continuum approach for parallelogram and triangular quantum wires, including the derivation of the electron-phonon interaction Hamiltonian and a discussion on the effects of this interaction on the electronic energy levels. The polaronic energy shift is calculated for both ground-state and excited-state electron energy levels by applying the perturbative approach. The effects of the electron-phonon interaction on the expectation value of r{sup 2} and diamagnetic susceptibility for both quantum wires are discussed.

  13. Direct measurement of lattice dynamics and optical phonon excitation in semiconductor nanocrystals using femtosecond stimulated Raman spectroscopy.

    PubMed

    Hannah, Daniel C; Brown, Kristen E; Young, Ryan M; Wasielewski, Michael R; Schatz, George C; Co, Dick T; Schaller, Richard D

    2013-09-06

    We report femtosecond stimulated Raman spectroscopy measurements of lattice dynamics in semiconductor nanocrystals and characterize longitudinal optical (LO) phonon production during confinement-enhanced, ultrafast intraband relaxation. Stimulated Raman signals from unexcited CdSe nanocrystals produce a spectral shape similar to spontaneous Raman signals. Upon photoexcitation, stimulated Raman amplitude decreases owing to experimentally resolved ultrafast phonon generation rates within the lattice. We find a ∼600  fs, particle-size-independent depletion time attributed to hole cooling, evidence of LO-to-acoustic down-conversion, and LO phonon mode softening.

  14. Electron-phonon interaction and Raman scattering in nanocrystals

    NASA Astrophysics Data System (ADS)

    Klimin, S. N.; Pokatilov, E. P.; Fomin, V. M.; Devreese, J. T.; Gladilin, V. N.; Balaban, S. N.

    1997-03-01

    The vibrational eigenmodes of a nanocrystal are derived by diagonalization of the equations of motion for the ionic displacement taking into account a non-parabolic dispersion with electrostatic and mechanical boundary conditions. A finite width of the Brillouin zone leads automatically to a finite basis of vibrational modes. The developed method can be applicable to nanostructures of an arbitrary geometry. For a spherical nanocrystal, a dispersion equation contains the effective multimode dielectric function. The resulting eigenmodes are mixed bulk-like and interface waves, especially in the short-wavelength region. Using the obtained Hamiltonian, the one-phonon and two-phonon resonant Raman scattering spectra are calculated for a spherical CdSe nanocrystal in the borosilicate glass. The valence band mixing dramatically enhances relative intensities of the two-phonon peaks and makes the adiabatic approximation inapplicable. Hence, the Huang-Rhys parameter is not an adequate characteristic of the optical spectra. Using a direct expansion of the evolution operator, a good agreement has been achieved between the calculated and the experimentally observed [1] Raman spectra. [1] M. C. Klein, F. Hache, D. Ricard, and C. Flytzanis, Phys. Rev. B 42, 11123 (1990).

  15. Nonharmonic phonons in α-iron at high temperatures

    NASA Astrophysics Data System (ADS)

    Mauger, L.; Lucas, M. S.; Muñoz, J. A.; Tracy, S. J.; Kresch, M.; Xiao, Yuming; Chow, Paul; Fultz, B.

    2014-08-01

    Phonon densities of states (DOS) of bcc α-Fe57 were measured from room temperature through the 1044 K Curie transition and the 1185 K fcc γ-Fe phase transition using nuclear resonant inelastic x-ray scattering. At higher temperatures all phonons shift to lower energies (soften) with thermal expansion, but the low transverse modes soften especially rapidly above 700 K, showing strongly nonharmonic behavior that persists through the magnetic transition. Interatomic force constants for the bcc phase were obtained by iteratively fitting a Born-von Kármán model to the experimental phonon spectra using a genetic algorithm optimization. The second-nearest-neighbor fitted axial force constants weakened significantly at elevated temperatures. An unusually large nonharmonic behavior is reported, which increases the vibrational entropy and accounts for a contribution of 35 meV/atom in the free energy at high temperatures. The nonharmonic contribution to the vibrational entropy follows the thermal trend of the magnetic entropy, and may be coupled to magnetic excitations. A small change in vibrational entropy across the α-γ structural phase transformation is also reported.

  16. Splash, pop, sizzle: Information processing with phononic computing

    SciTech Connect

    Sklan, Sophia R.

    2015-05-15

    Phonons, the quanta of mechanical vibration, are important to the transport of heat and sound in solid materials. Recent advances in the fundamental control of phonons (phononics) have brought into prominence the potential role of phonons in information processing. In this review, the many directions of realizing phononic computing and information processing are examined. Given the relative similarity of vibrational transport at different length scales, the related fields of acoustic, phononic, and thermal information processing are all included, as are quantum and classical computer implementations. Connections are made between the fundamental questions in phonon transport and phononic control and the device level approach to diodes, transistors, memory, and logic. .

  17. The salience network causally influences default mode network activity during moral reasoning

    PubMed Central

    Wilson, Stephen M.; D’Esposito, Mark; Kayser, Andrew S.; Grossman, Scott N.; Poorzand, Pardis; Seeley, William W.; Miller, Bruce L.; Rankin, Katherine P.

    2013-01-01

    Large-scale brain networks are integral to the coordination of human behaviour, and their anatomy provides insights into the clinical presentation and progression of neurodegenerative illnesses such as Alzheimer’s disease, which targets the default mode network, and behavioural variant frontotemporal dementia, which targets a more anterior salience network. Although the default mode network is recruited when healthy subjects deliberate about ‘personal’ moral dilemmas, patients with Alzheimer’s disease give normal responses to these dilemmas whereas patients with behavioural variant frontotemporal dementia give abnormal responses to these dilemmas. We hypothesized that this apparent discrepancy between activation- and patient-based studies of moral reasoning might reflect a modulatory role for the salience network in regulating default mode network activation. Using functional magnetic resonance imaging to characterize network activity of patients with behavioural variant frontotemporal dementia and healthy control subjects, we present four converging lines of evidence supporting a causal influence from the salience network to the default mode network during moral reasoning. First, as previously reported, the default mode network is recruited when healthy subjects deliberate about ‘personal’ moral dilemmas, but patients with behavioural variant frontotemporal dementia producing atrophy in the salience network give abnormally utilitarian responses to these dilemmas. Second, patients with behavioural variant frontotemporal dementia have reduced recruitment of the default mode network compared with healthy control subjects when deliberating about these dilemmas. Third, a Granger causality analysis of functional neuroimaging data from healthy control subjects demonstrates directed functional connectivity from nodes of the salience network to nodes of the default mode network during moral reasoning. Fourth, this Granger causal influence is diminished in

  18. Scattering Tools for Nanostructure Phonon Engineering

    DTIC Science & Technology

    2013-09-25

    Arlington, VA 22203 AFOSR The vibrational properties of solids have crucial roles underpinning functional properties ranging from thermal conductivity... thermal diffuse scattering (TDS) techniques to nanoscale systems. With this approach we can probe phonons across the nanomaterials, phonons, x-ray...scattering, nanomembrane fabrication, flatness, large-wavevector vibrational properties, synchrotron x-rays, thermal diffuse scattering, silicon

  19. Microfabricated phononic crystal devices and applications

    NASA Astrophysics Data System (ADS)

    Olsson, R. H., III; El-Kady, I.

    2009-01-01

    Phononic crystals are the acoustic wave analogue of photonic crystals. Here a periodic array of scattering inclusions located in a homogeneous host material forbids certain ranges of acoustic frequencies from existence within the crystal, thus creating what are known as acoustic bandgaps. The majority of previously reported phononic crystal devices have been constructed by hand, assembling scattering inclusions in a viscoelastic medium, predominantly air, water or epoxy, resulting in large structures limited to frequencies below 1 MHz. Recently, phononic crystals and devices have been scaled to VHF (30-300 MHz) frequencies and beyond by utilizing microfabrication and micromachining technologies. This paper reviews recent developments in the area of micro-phononic crystals including design techniques, material considerations, microfabrication processes, characterization methods and reported device structures. Micro-phononic crystal devices realized in low-loss solid materials are emphasized along with their potential application in radio frequency communications and acoustic imaging for medical ultrasound and nondestructive testing. The reported advances in batch micro-phononic crystal fabrication and simplified testing promise not only the deployment of phononic crystals in a number of commercial applications but also greater experimentation on a wide variety of phononic crystal structures.

  20. Dual-frequency plasmon lasing modes in active three-layered bimetallic Ag/Au nanoshells

    NASA Astrophysics Data System (ADS)

    Wu, DaJian; Wu, XueWei; Cheng, Ying; Jin, BiaoBing; Liu, XiaoJun

    2015-11-01

    The optical properties of three-layered silver-gold-silica (SGS) nanoshells with gain have been investigated theoretically by using Mie theory. Surface plasmon amplification by stimulated emission of radiation (spaser) phenomena can be observed at two plasmon modes of the active SGS nanoshell in the visible region. It is found with the decrease in the radius of the inner Ag core that the critical value of ɛg″(ωg ) for the super-resonance of the low-energy mode increases first and then decreases while that for the high-energy mode decreases. An interesting overlap between the two curves for the critical value of ɛg″(ωg ) can be found at a special core radius. At this point, two super-resonances can be achieved concurrently at the low- and high-energy modes of the active SGS nanoshell with the same gain coefficient. This dual-frequency spaser based on the bimetallic Ag/Au nanoshell may be an efficient candidate for designing the nanolaser.

  1. A Multi-Mode Blade Damping Control using Shunted Piezoelectric Transducers with Active Feedback Structure

    NASA Technical Reports Server (NTRS)

    Choi, Benjamin; Morrison, Carlos; Min, James

    2009-01-01

    The Structural Dynamics and. Mechanics branch (RXS) is developing smart adaptive structures to improve fan blade damping at resonances using piezoelectric (PE) transducers. In this presentation, only one shunted PE transducer was used to demonstrate active control of multi-mode blade resonance damping on a titanium alloy (Ti-6A1-4V) flat plate model, regardless of bending, torsion, and 2-stripe modes. This work would have a significant impact on the conventional passive shunt damping world because the standard feedback control design tools can now be used to design and implement electric shunt for vibration control. In other words, the passive shunt circuit components using massive inductors and. resistors for multi-mode resonance control can be replaced with digital codes. Furthermore, this active approach with multi patches can simultaneously control several modes in the engine operating range. Dr. Benjamin Choi presented the analytical and experimental results from this work at the Propulsion-Safety and. Affordable Readiness (P-SAR) Conference in March, 2009.

  2. Active and passive kink mode studies in a tokamak with a movable ferromagnetic wall

    SciTech Connect

    Levesque, J. P.; Hughes, P. E.; Bialek, J.; Byrne, P. J.; Mauel, M. E.; Navratil, G. A.; Peng, Q.; Rhodes, D. J.; Stoafer, C. C.

    2015-05-15

    High-resolution active and passive kink mode studies are conducted in a tokamak with an adjustable ferromagnetic wall near the plasma surface. Ferritic tiles made from 5.6 mm thick Hiperco{sup ®} 50 alloy have been mounted on the plasma-facing side of half of the in-vessel movable wall segments in the High Beta Tokamak-Extended Pulse device [D. A. Maurer et al., Plasma Phys. Controlled Fusion 53, 074016 (2011)] in order to explore ferritic resistive wall mode stability. Low-activation ferritic steels are a candidate for structural components of a fusion reactor, and these experiments examine MHD stability of plasmas with nearby ferromagnetic material. Plasma-wall separation for alternating ferritic and non-ferritic wall segments is adjusted between discharges without opening the vacuum vessel. Amplification of applied resonant magnetic perturbations and plasma disruptivity are observed to increase when the ferromagnetic wall is close to plasma surface instead of the standard stainless steel wall. Rapidly rotating m/n=3/1 external kink modes have higher growth rates with the nearby ferritic wall. Feedback suppression of kinks is still as effective as before the installation of ferritic material in vessel, in spite of increased mode growth rates.

  3. MODE IDENTIFICATION OF MHD WAVES IN AN ACTIVE REGION OBSERVED WITH HINODE/EIS

    SciTech Connect

    Kitagawa, N.; Yokoyama, T.; Imada, S.; Hara, H.

    2010-09-20

    In order to better understand the possibility of coronal heating by MHD waves, we analyze Fe XII 195.12A data observed with the EUV Imaging Spectrometer on board Hinode. We performed a Fourier analysis of EUV intensity and Doppler velocity time series data in the active region corona. Notable intensity and Doppler velocity oscillations were found for two moss regions out of the five studied, while only small oscillations were found for five apexes of loops. The amplitudes of the oscillations were 0.4%-5.7% for intensity and 0.2-1.2 km s{sup -1} for Doppler velocity. In addition, oscillations of only the Doppler velocity were seen relatively less often in the data. We compared the amplitudes of intensity and those of Doppler velocity in order to identify MHD wave modes and calculated the phase delays between Fourier components of intensity and those of Doppler velocity. The results are interpreted in terms of MHD waves as follows: (1) few kink modes or torsional Alfven mode waves were seen in both moss regions and the apexes of loops, (2) upwardly propagating and standing slow mode waves were found in moss regions, and (3) consistent with previous studies, estimated values of energy flux of the waves were several orders of magnitude lower than that required for heating active regions.

  4. Lattice Boltzmann modeling of phonon transport

    NASA Astrophysics Data System (ADS)

    Guo, Yangyu; Wang, Moran

    2016-06-01

    A novel lattice Boltzmann scheme is proposed for phonon transport based on the phonon Boltzmann equation. Through the Chapman-Enskog expansion, the phonon lattice Boltzmann equation under the gray relaxation time approximation recovers the classical Fourier's law in the diffusive limit. The numerical parameters in the lattice Boltzmann model are therefore rigorously correlated to the bulk material properties. The new scheme does not only eliminate the fictitious phonon speed in the diagonal direction of a square lattice system in the previous lattice Boltzmann models, but also displays very robust performances in predicting both temperature and heat flux distributions consistent with analytical solutions for diverse numerical cases, including steady-state and transient, macroscale and microscale, one-dimensional and multi-dimensional phonon heat transport. This method may provide a powerful numerical tool for deep studies of nonlinear and nonlocal heat transports in nanosystems.

  5. Phononic crystals and elastodynamics: Some relevant points

    SciTech Connect

    Aravantinos-Zafiris, N.; Sigalas, M. M.; Kafesaki, M.; Economou, E. N.

    2014-12-15

    In the present paper we review briefly some of the first works on wave propagation in phononic crystals emphasizing the conditions for the creation of acoustic band-gaps and the role of resonances to the band-gap creation. We show that useful conclusions in the analysis of phononic band gap structures can be drawn by considering the mathematical similarities of the basic classical wave equation (Helmholtz equation) with Schrödinger equation and by employing basic solid state physics concepts and conclusions regarding electronic waves. In the second part of the paper we demonstrate the potential of phononic systems to be used as elastic metamaterials. This is done by demonstrating negative refraction in phononic crystals and subwavelength waveguiding in a linear chain of elastic inclusions, and by proposing a novel structure with close to pentamode behavior. Finally the potential of phononic structures to be used in liquid sensor applications is discussed and demonstrated.

  6. Phononic crystals and elastodynamics: Some relevant points

    NASA Astrophysics Data System (ADS)

    Aravantinos-Zafiris, N.; Sigalas, M. M.; Kafesaki, M.; Economou, E. N.

    2014-12-01

    In the present paper we review briefly some of the first works on wave propagation in phononic crystals emphasizing the conditions for the creation of acoustic band-gaps and the role of resonances to the band-gap creation. We show that useful conclusions in the analysis of phononic band gap structures can be drawn by considering the mathematical similarities of the basic classical wave equation (Helmholtz equation) with Schrödinger equation and by employing basic solid state physics concepts and conclusions regarding electronic waves. In the second part of the paper we demonstrate the potential of phononic systems to be used as elastic metamaterials. This is done by demonstrating negative refraction in phononic crystals and subwavelength waveguiding in a linear chain of elastic inclusions, and by proposing a novel structure with close to pentamode behavior. Finally the potential of phononic structures to be used in liquid sensor applications is discussed and demonstrated.

  7. Coherent transverse-optical phonon generation induced by lattice defects in nitrogen-ion-implanted GaAs

    NASA Astrophysics Data System (ADS)

    Kim, J. S.; Ryu, H. C.; Kim, S. H.; Kim, H.; Rho, H.; Kim, Y. J.; Lim, Y. S.; Yee, K. J.

    2013-10-01

    We describe our observation of coherent phonon oscillations of X-point zone-boundary transverse-optical (TO) mode, TO(X), in nitrogen-ion-implanted GaAs that has been annealed at high temperatures. With the TO(X) mode being forbidden from the Raman selection rule in pure zinc-blende GaAs, the lattice defects have provided additional momentum for phonon generation. Annealing-induced structural modifications were demonstrated through X-ray diffraction, transmission electron microscopy, and Raman scattering measurements. The polarization dependence of the TO(X) mode was compared with that of the longitudinal optical mode, and the temperature dependence of the TO(X) phonon dephasing was also investigated.

  8. Heat conduction and phonon localization in disordered harmonic crystals

    NASA Astrophysics Data System (ADS)

    Kundu, A.; Chaudhuri, A.; Roy, D.; Dhar, A.; Lebowitz, J. L.; Spohn, H.

    2010-05-01

    We investigate the steady-state heat current in two- and three-dimensional isotopically disordered harmonic lattices. Using localization theory as well as kinetic theory we estimate the system size dependence of the current. These estimates are compared with numerical results obtained using an exact formula for the current given in terms of a phonon transmission function, as well as by direct nonequilibrium simulations. We find that heat conduction by high frequency modes is suppressed by localization while low frequency modes are strongly affected by boundary conditions. Our heuristic arguments show that Fourier's law is valid in a three-dimensional disordered solid except for special boundary conditions. We also study the pinned case relevant to localization in quantum systems and often used as a model system to study the validity of Fourier's law. Here we provide the first numerical verification of Fourier's law in three dimensions. In the two-dimensional pinned case we find that localization of phonon modes leads to a heat insulator.

  9. Bond-Stretching Phonons in a Stripe-Ordered Nickelate

    NASA Astrophysics Data System (ADS)

    Tranquada, J. M.; Nakajima, K.; Braden, M.; Pintschovius, L.; Reichardt, W.; McQueeney, R.

    2001-03-01

    We have used neutron scattering at the Orphée Reactor, LLB, to study the bond-stretching optical phonons in a stripe-ordered single crystal of La_2-xSr_xNiO4 with x≈0.32. The stripes run along a [110] direction, at 45^circ to the Ni-O bond direction. We have measured the dispersion of the highest-energy Σ1 mode; because of twinning of the stripe domains in the tetragonal structure, we simultaneously sample phonons in directions parallel and perpendicular to the stripes. At zone center, a single strong peak appears at 21 THz (87 meV); however, on moving across the zone the mode appears to split, with half of the weight staying near 20 THz and the other half softening to ~17.5 THz at the zone boundary. The splitting is strong at the ordering wave vector, but changes little from there to the zone boundary. In the [100] direction, the Δ1 mode shows a similar softening but with no splitting. Comparisons with the cuprates will be discussed. This work supported by the U.S. DOE under Contract No. DE-AC02-98CH10886, and the U.S.-Japan Cooperative Research Program on Neutron Scattering.

  10. Phonon probe of local strains in SnSxSe2-x mixed crystals

    NASA Astrophysics Data System (ADS)

    Hadjiev, V. G.; De, D.; Peng, H. B.; Manongdo, J.; Guloy, A. M.

    2013-03-01

    We present a combined Raman spectroscopy and density functional perturbation theory (DFPT) study of phonon variation with composition x in the mixed crystals SnSxSe2-x. The experimentally observed two-mode behavior of the A1g and Eg vibrations involving Se(S) atoms is shown to arise from the lack of overlapping of the corresponding phonon dispersion bands in SnS2 and SnSe2. This offers a unique opportunity to assess local distortions of the trigonal Sn3Se pyramids in SnSxSe2-x as no Se and S mode mixing is involved. The dependence of local height and base length of Sn3Se pyramids with x is derived by a procedure that uses the measured A1g (Se) and Eg (Se) phonons in SnSxSe2-x, those calculated by DFPT for SnSe2 at different hydrostatic pressure, DFPT phonon dispersion, and the contribution from mass-disorder induced phonon self-energy.

  11. Computational Study of In-Plane Phonon Transport in Si Thin Films

    PubMed Central

    Wang, Xinjiang; Huang, Baoling

    2014-01-01

    We have systematically investigated the in-plane thermal transport in Si thin films using an approach based on the first-principles calculations and lattice dynamics. The effects of phonon mode depletion induced by the phonon confinement and the corresponding variation in interphonon scattering, which may be important for the thermal conductivities of ultra-thin films but are often neglected in precedent studies, are considered in this study. The in-plane thermal conductivities of Si thin films with different thicknesses have been predicted over a temperature range from 80 K to 800 K and excellent agreements with experimental results are found. The validities of adopting the bulk phonon properties and gray approximation of surface specularity in thin film studies have been clarified. It is found that in ultra-thin films, while the phonon depletion will reduce the thermal conductivity of Si thin films, its effect is largely offset by the reduction in the interphonon scattering rate. The contributions of different phonon modes to the thermal transport and isotope effects in Si films with different thicknesses under various temperatures are also analyzed. PMID:25228061

  12. Using van Hove singularities of the two-phonon density of states to investigate the intrinsically localized vibrations of NaI crystal.

    NASA Astrophysics Data System (ADS)

    Agyare, Benjamin; Riseborough, Peter

    2017-01-01

    Intrinsically Localized Modes (ILMs) have purportedly been observed in NaI but only for wave-vectors, q at the corner of the 3-D Brillouin Zone. It has been suggested that, for high-symmetry q vectors, several van Hove singularities may converge at one frequency producing a large peak in the two-phonon density of state and giving rise to ILMs with these q values. We fit the experimentally determined acoustic and the optic phonon modes using a nearest neighbor and a next-nearest neighbor force constant. We find that the two-phonon density of states, for fixed q exhibits non-divergent van Hove singularities. The frequencies of these features are found to vary as q is varied. We intend to search for q values at which the two-phonon density of states is enhanced and then examine whether the anharmonic interactions can bind the two-phonon excitations to produce a quantized ILM.

  13. Phonon features in terahertz photoconductivity spectra due to data analysis artifact: A case study on organometallic halide perovskites

    NASA Astrophysics Data System (ADS)

    La-o-vorakiat, Chan; Cheng, Liang; Salim, Teddy; Marcus, Rudolph A.; Michel-Beyerle, Maria-Elisabeth; Lam, Yeng Ming; Chia, Elbert E. M.

    2017-03-01

    We propose a simple scenario where the superimposed phonon modes on the photoconductive spectra are experimental artifacts due to the invalid formula used in data analysis. By use of experimental and simulated data of CH3NH3PbI3 perovskites as a case study, we demonstrate that a correction term must be included in the approximated thin-film formula used in the literature; otherwise, parts of the spectra with high background permittivity near the phonon-mode resonances might interfere with the transient photoconductivity. The implication of this work is not limited to perovskites but other materials with strong vibrational modes within the THz spectral range.

  14. Phonon assisted thermophoretic motion of gold nanoparticles inside carbon nanotubes

    NASA Astrophysics Data System (ADS)

    Schoen, Philipp A. E.; Walther, Jens H.; Poulikakos, Dimos; Koumoutsakos, Petros

    2007-06-01

    The authors investigate the thermally driven mass transport of gold nanoparticles confined inside carbon nanotubes using molecular dynamics simulations. The observed thermophoretic motion of the gold nanoparticles correlates with the phonon dispersion exhibited by a standard carbon nanotube and, in particular, with the breathing mode of the tube. Additionally, the results show an increased static friction for gold nanoparticles confines inside a zig-zag carbon nanotube when increasing the size (length) of the nanoparticles. However, an unexpected, opposite trend is observed for the same nanoparticles inside armchair tubes.

  15. Phonons in iron: from the bulk to an epitaxial monolayer.

    PubMed

    Stankov, S; Röhlsberger, R; Slezak, T; Sladecek, M; Sepiol, B; Vogl, G; Chumakov, A I; Rüffer, R; Spiridis, N; Łazewski, J; Parliński, K; Korecki, J

    2007-11-02

    The confinement of materials in low-dimensional structures has significant impact on propagating excitations like phonons. Using the isotope-specific 57Fe nuclear resonant vibrational spectroscopy we were able to determine elastic and thermodynamic properties of ultrathin Fe films on W(110). With decreasing thickness one observes a significant increase of the mean atomic displacement that goes along with an enhancement of vibrational modes at low energies as compared to the bulk. The analysis reveals that these deviations result from atomic vibrations of the single atomic layers at the two boundaries of the film, while the atoms inside the films vibrate almost bulklike.

  16. Surface phonons on Al(111) surface covered by alkali metals

    NASA Astrophysics Data System (ADS)

    Rusina, G. G.; Eremeev, S. V.; Borisova, S. D.; Sklyadneva, I. Yu.; Chulkov, E. V.

    2005-06-01

    We investigated the vibrational and structural properties of the Al(111)-(3×3)R30°-AM (AM=Na,K,Li) adsorbed systems using interaction potentials from the embedded-atom method. The surface relaxation, surface phonon dispersion, and polarization of vibrational modes for the alkali adatoms and the substrate atoms as well as the local density of states are discussed. Our calculated structural parameters are in close agreement with experimental and ab initio results. The obtained vibrational frequencies compare fairly well with the available experimental data.

  17. Self-sustained coherent phonon generation in optomechanical cavities

    NASA Astrophysics Data System (ADS)

    Navarro-Urrios, D.; Gomis-Bresco, J.; Alzina, F.; Capuj, N. E.; García, P. D.; Colombano, M. F.; Chavez-Angel, E.; Sotomayor-Torres, C. M.

    2016-09-01

    Optical forces can set tiny objects in states of mechanical self-sustained oscillation, spontaneously generating periodic signals by extracting power from steady sources. Miniaturized self-sustained coherent phonon sources are interesting for applications such as mass-force sensing, intra-chip metrology and intra-chip time-keeping among others. In this paper, we review several mechanisms and techniques that can drive a mechanical mode into the lasing regime by exploiting the radiation pressure force in optomechanical cavities, namely stimulated emission, dynamical back-action, forward stimulated Brillouin scattering and self-pulsing.

  18. Phonon-Mediated Detection of Trapped Atomic Ions

    NASA Astrophysics Data System (ADS)

    Hume, David; Rosenband, Till; Wineland, David

    2008-03-01

    Both quantum information processing and quantum-limited metrology require sensitive detection of quantum states. Using trapped atomic ions, we investigate quantum non-demolition measurements in a two-species ion chain composed of Al^+ and Be^+. By mapping information from Al^+ to a shared phonon-mode then to Be^+ and detecting repetitively we have experimentally demonstrated a fidelity for state initialization and detection of 0.9994. We have also shown an increase in measurement efficiency through an adaptive procedure. Here we apply these ideas to the detection of states of multiple Al^+ using a single Be^+ ion, and describe the preparation of entangled states through measurement.

  19. Cysteine Proteases: Modes of Activation and Future Prospects as Pharmacological Targets

    PubMed Central

    Verma, Sonia; Dixit, Rajnikant; Pandey, Kailash C.

    2016-01-01

    Proteolytic enzymes are crucial for a variety of biological processes in organisms ranging from lower (virus, bacteria, and parasite) to the higher organisms (mammals). Proteases cleave proteins into smaller fragments by catalyzing peptide bonds hydrolysis. Proteases are classified according to their catalytic site, and distributed into four major classes: cysteine proteases, serine proteases, aspartic proteases, and metalloproteases. This review will cover only cysteine proteases, papain family enzymes which are involved in multiple functions such as extracellular matrix turnover, antigen presentation, processing events, digestion, immune invasion, hemoglobin hydrolysis, parasite invasion, parasite egress, and processing surface proteins. Therefore, they are promising drug targets for various diseases. For preventing unwanted digestion, cysteine proteases are synthesized as zymogens, and contain a prodomain (regulatory) and a mature domain (catalytic). The prodomain acts as an endogenous inhibitor of the mature enzyme. For activation of the mature enzyme, removal of the prodomain is necessary and achieved by different modes. The pro-mature domain interaction can be categorized as protein–protein interactions (PPIs) and may be targeted in a range of diseases. Cysteine protease inhibitors are available that can block the active site but no such inhibitor available yet that can be targeted to block the pro-mature domain interactions and prevent it activation. This review specifically highlights the modes of activation (processing) of papain family enzymes, which involve auto-activation, trans-activation and also clarifies the future aspects of targeting PPIs to prevent the activation of cysteine proteases. PMID:27199750

  20. Examining the Validity of the Phonon Gas Model in Amorphous Materials

    PubMed Central

    Lv, Wei; Henry, Asegun

    2016-01-01

    The idea of treating phonon transport as equivalent to transport through a gas of particles is termed the phonon gas model (PGM), and it has been used almost ubiquitously to try and understand heat conduction in all solids. However, most of the modes in disordered materials do not propagate and thus may contribute to heat conduction in a fundamentally different way than is described by the PGM. From a practical perspective, the problem with trying to apply the PGM to amorphous materials is the fact that one cannot rigorously define the phonon velocities for non-propagating modes, since there is no periodicity. Here, we tested the validity of the PGM for amorphous materials by assuming the PGM is applicable, and then, using a combination of lattice dynamics, molecular dynamics (MD) and experimental thermal conductivity data, we back-calculated the phonon velocities for the vibrational modes. The results of this approach show that if the PGM was valid, a large number of the mid and high frequency modes would have to have either imaginary or extremely high velocities to reproduce the experimental thermal conductivity data. Furthermore, the results of MD based relaxation time calculations suggest that in amorphous materials there is little, if any, connection between relaxation times and thermal conductivity. This then strongly suggests that the PGM is inapplicable to amorphous solids. PMID:27917868