Many-Body Effect in Spin Dephasing in n-Type GaAs Quantum Wells

NASA Astrophysics Data System (ADS)

Weng, Ming-Qi; Wu, Ming-Wei

2005-03-01

By constructing and numerically solving the kinetic Bloch equations we perform a many-body study of the spin dephasing due to the D'yakonov-Perel' effect in n-type GaAs (100) quantum wells for high temperatures. In our study, we include the spin-conserving scattering such as the electron-phonon, the electron-nonmagnetic impurity as well as the electron-electron Coulomb scattering into consideration. The dephasing obtained from our theory contains both the single-particle and the many-body contributions with the latter originating from the inhomogeneous broadening introduced by the DP term [J. Supercond.: Incorp. Novel Magn. 14 (2001) 245 Eur. Phys. J. B 18 (2000) 373]. Our result agrees very well with the experimental data [Phys. Rev. B 62 (2000) 13034] of Malinowski et al. We further show that in the case we study, the spin dephasing is dominated by the many-body effect.

Strain broadening of the 1042-nm zero phonon line of the NV- center in diamond: A promising spectroscopic tool for defect tomography

NASA Astrophysics Data System (ADS)

Biktagirov, T. B.; Smirnov, A. N.; Davydov, V. Yu.; Doherty, M. W.; Alkauskas, A.; Gibson, B. C.; Soltamov, V. A.

2017-08-01

The negatively charged nitrogen-vacancy (NV-) center in diamond is a promising candidate for many quantum applications. Here, we examine the splitting and broadening of the center's infrared (IR) zero-phonon line (ZPL). We develop a model for these effects that accounts for the strain induced by photodependent microscopic distributions of defects. We apply this model to interpret observed variations of the IR ZPL shape with temperature and photoexcitation conditions. We identify an anomalous temperature-dependent broadening mechanism and that defects other than the substitutional nitrogen center significantly contribute to strain broadening. The former conclusion suggests the presence of a strong Jahn-Teller effect in the center's singlet levels and the latter indicates that major sources of broadening are yet to be identified. These conclusions have important implications for the understanding of the center and the engineering of diamond quantum devices. Finally, we propose that, once the major sources of broadening are identified, the IR ZPL has the potential to be a sensitive spectroscopic tool for probing microscopic strain fields and performing defect tomography.

Microwave-Induced Magneto-Oscillations and Signatures of Zero-Resistance States in Phonon-Drag Voltage in Two-Dimensional Electron Systems

NASA Astrophysics Data System (ADS)

Levin, A. D.; Momtaz, Z. S.; Gusev, G. M.; Raichev, O. E.; Bakarov, A. K.

2015-11-01

We observe the phonon-drag voltage oscillations correlating with the resistance oscillations under microwave irradiation in a two-dimensional electron gas in perpendicular magnetic field. This phenomenon is explained by the influence of dissipative resistivity modified by microwaves on the phonon-drag voltage perpendicular to the phonon flux. When the lowest-order resistance minima evolve into zero-resistance states, the phonon-drag voltage demonstrates sharp features suggesting that current domains associated with these states can exist in the absence of external dc driving.

Microwave-Induced Magneto-Oscillations and Signatures of Zero-Resistance States in Phonon-Drag Voltage in Two-Dimensional Electron Systems.

PubMed

Levin, A D; Momtaz, Z S; Gusev, G M; Raichev, O E; Bakarov, A K

2015-11-13

We observe the phonon-drag voltage oscillations correlating with the resistance oscillations under microwave irradiation in a two-dimensional electron gas in perpendicular magnetic field. This phenomenon is explained by the influence of dissipative resistivity modified by microwaves on the phonon-drag voltage perpendicular to the phonon flux. When the lowest-order resistance minima evolve into zero-resistance states, the phonon-drag voltage demonstrates sharp features suggesting that current domains associated with these states can exist in the absence of external dc driving.

The effect of dephasing on the thermoelectric efficiency of molecular junctions.

PubMed

Zimbovskaya, Natalya A

2014-07-09

In this work we report the results of theoretical analysis of the effect of the thermal environment on the thermoelectric efficiency of molecular junctions. The environment is represented by two thermal phonon baths associated with the electrodes, which are kept at different temperatures. The analysis is carried out using the Buttiker model within the scattering matrix formalism to compute electron transmission through the system. This approach is further developed so that the dephasing parameters are expressed in terms of relevant energies, including the thermal energy, strengths of coupling between the molecular bridge and the electrodes and characteristic energies of electron-phonon interactions. It is shown that the latter significantly affect thermoelectric efficiency by destroying the coherency of electron transport through the considered system.

Indistinguishable near-infrared single photons from an individual organic molecule

DOE Office of Scientific and Technical Information (OSTI.GOV)

Trebbia, Jean-Baptiste; Tamarat, Philippe; Lounis, Brahim

2010-12-15

By using the zero-phonon line emission of an individual organic molecule, we realized a source of indistinguishable single photons in the near infrared. A Hong-Ou-Mandel interference experiment is performed and a two-photon coalescence probability higher than 50% at 2 K is obtained. The contribution of the temperature-dependent dephasing processes to the two-photon interference contrast is studied. We show that the molecule delivers nearly ideal indistinguishable single photons at the lowest temperatures when the dephasing is nearly lifetime limited. This source is used to generate postselected polarization-entangled photon pairs as a test bench for applications in quantum information.

Line width resonance of the longitudinal optical phonon in GaAs:N

NASA Astrophysics Data System (ADS)

Mialitsin, Aleksej; Mascarenhas, Angelo

2013-03-01

We extend resonant Raman scattering studies of Mascarenhas et al. [PRB68, 233201 (2003)] of GaAs1-xNx to the ultra-dilute nitrogen doping concentrations, whereby we unambiguously resolve the line width resonances of the LO phonon. A discontinuity is observed in the LO phonon line width resonance energy as a function of concentration. With decreasing nitrogen concentration the EW line width resonance energy reduces by ca. 40 meV at x = 0 . 4 % . This value corresponds to the concentration, at which the localized to delocalized transition manifests itself in the electro-reflectance signature line widths.

The temperature dependence of vibronic lineshapes: Linear electron-phonon coupling

NASA Astrophysics Data System (ADS)

Roos, Claudia; Köhn, Andreas; Gauss, Jürgen; Diezemann, Gregor

2014-10-01

We calculate the effect of a linear electron-phonon coupling on vibronic transitions of dye molecules of arbitrary complexity. With the assumption of known vibronic frequencies (for instance from quantum-chemical calculations), we give expressions for the absorption or emission lineshapes in a second-order cumulant expansion. We show that the results coincide with those obtained from generalized Redfield theory if one uses the time-local version of the theory and applies the secular approximation. Furthermore, the theory allows to go beyond the Huang-Rhys approximation and can be used to incorporate Dushinsky effects in the treatment of the temperature dependence of optical spectra. We consider both, a pure electron-phonon coupling independent of the molecular vibrations and a coupling bilinear in the molecular vibrational modes and the phonon coordinates. We discuss the behavior of the vibronic density of states for various models for the spectral density representing the coupling of the vibronic system to the harmonic bath. We recover some of the results that have been derived earlier for the spin-boson model and we show that the behavior of the spectral density at low frequencies determines the dominant features of the spectra. In case of the bilinear coupling between the molecular vibrations and the phonons we give analytical expressions for different spectral densities. The spectra are reminiscent of those obtained from the well known Brownian oscillator model and one finds a zero-phonon line and phonon-side bands located at vibrational frequencies of the dye. The intensity of the phonon-side bands diminishes with increasing vibrational frequencies and with decreasing coupling strength (Huang-Rhys factor). It vanishes completely in the Markovian limit where only a Lorentzian zero-phonon line is observed.

Phonon effects on the radiative recombination of excitons in double quantum dots

NASA Astrophysics Data System (ADS)

Karwat, Paweł; Sitek, Anna; Machnikowski, Paweł

2011-11-01

We study theoretically the radiative recombination of excitons in double quantum dots in the presence of carrier-phonon coupling. We show that the phonon-induced pure dephasing effects and transitions between the exciton states strongly modify the spontaneous emission process and make it sensitive to temperature, which may lead to nonmonotonic temperature dependence of the time-resolved luminescence. We show also that, under specific resonance conditions, the biexcitonic interband polarization can be coherently transferred to the excitonic one, leading to an extended lifetime of the total coherent polarization, which is reflected in the nonlinear optical spectrum of the system. We study the stability of this effect against phonon-induced decoherence.

Laser spectroscopy of phonons and rotons in superfluid helium doped with Dy atoms

NASA Astrophysics Data System (ADS)

Moroshkin, P.; Borel, A.; Kono, K.

2018-03-01

We report the results of a high-resolution laser-spectroscopy study of dysprosium atoms injected into superfluid 4He. A special attention is paid to the transitions between the inner 4 f and 5 d electronic shells of Dy. The characteristic gap is observed between the zero-phonon line and the phonon wing in the experimental excitation spectrum that arises due to the peculiar structure of the phonon-roton spectrum of superfluid He. This observation resolves the longstanding discrepancy between the studies of bulk superfluid He and He nanodroplets.

Temperature dependence of the LO phonon sidebands in free exciton emission of GaN

NASA Astrophysics Data System (ADS)

Xu, S. J.; Li, G. Q.; Xiong, S.-J.; Che, C. M.

2006-04-01

Temperature-dependent radiative recombination of free excitons involving one or two LO phonons in GaN is investigated in detail. It is found that both phonon sidebands possess asymmetric lineshapes and their energy spacings from the zero-phonon line strongly deviate from the characteristic energy of LO phonons as the temperature increases. Furthermore, the deviation rates of one- and two-phonon sidebands are significantly different. Segall-Mahan [Phys. Rev. 171, 935 (1968)] theory, taking the exciton-photon and exciton-phonon interactions into account, is employed to calculate the sidebands of one or two LO phonons for free excitons in a wide temperature range. Excellent agreement between the theory and experiment is achieved by using only one adjustable parameter, which leads to determination of the effective mass of heavy holes (~0.5m0).

Quantum theory of phonon-mediated decoherence and relaxation of two-level systems in a structured electromagnetic reservoir

NASA Astrophysics Data System (ADS)

Roy, Chiranjeeb

In this thesis we study the role of nonradiative degrees of freedom on quantum optical properties of mesoscopic quantum dots placed in the structured electromagnetic reservoir of a photonic crystal. We derive a quantum theory of the role of acoustic and optical phonons in modifying the optical absorption lineshape, polarization dynamics, and population dynamics of a two-level atom (quantum dot) in the "colored" electromagnetic vacuum of a photonic band gap (PBG) material. This is based on a microscopic Hamiltonian describing both radiative and vibrational processes quantum mechanically. Phonon sidebands in an ordinary electromagnetic reservoir are recaptured in a simple model of optical phonons using a mean-field factorization of the atomic and lattice displacement operators. Our formalism is then used to treat the non-Markovian dynamics of the same system within the structured electromagnetic density of states of a photonic crystal. We elucidate the extent to which phonon-assisted decay limits the lifetime of a single photon-atom bound state and derive the modified spontaneous emission dynamics due to coupling to various phonon baths. We demonstrate that coherent interaction with undamped phonons can lead to enhanced lifetime of a photon-atom bound state in a PBG by (i) dephasing and reducing the transition electric dipole moment of the atom and (ii) reducing the quantum mechanical overlap of the state vectors of the excited and ground state (polaronic shift). This results in reduction of the steady-state atomic polarization but an increase in the fractionalized upper state population in the photon-atom bound state. We demonstrate, on the other hand, that the lifetime of the photon-atom bound state in a PBG is limited by the lifetime of phonons due to lattice anharmonicities (break-up of phonons into lower energy phonons) and purely nonradiative decay. We demonstrate how these additional damping effects limit the extent of the polaronic (Franck-Condon) shift of

Vibrational dephasing in matter-wave interferometers

NASA Astrophysics Data System (ADS)

Rembold, A.; Schütz, G.; Röpke, R.; Chang, W. T.; Hwang, I. S.; Günther, A.; Stibor, A.

2017-03-01

Matter-wave interferometry is a highly sensitive tool to measure small perturbations in a quantum system. This property allows the creation of precision sensors for dephasing mechanisms such as mechanical vibrations. They are a challenge for phase measurements under perturbing conditions that cannot be perfectly decoupled from the interferometer, e.g. for mobile interferometric devices or vibrations with a broad frequency range. Here, we demonstrate a method based on second-order correlation theory in combination with Fourier analysis, to use an electron interferometer as a sensor that precisely characterizes the mechanical vibration spectrum of the interferometer. Using the high spatial and temporal single-particle resolution of a delay line detector, the data allows to reveal the original contrast and spatial periodicity of the interference pattern from ‘washed-out’ matter-wave interferograms that have been vibrationally disturbed in the frequency region between 100 and 1000 Hz. Other than with electromagnetic dephasing, due to excitations of higher harmonics and additional frequencies induced from the environment, the parts in the setup oscillate with frequencies that can be different to the applied ones. The developed numerical search algorithm is capable to determine those unknown oscillations and corresponding amplitudes. The technique can identify vibrational dephasing and decrease damping and shielding requirements in electron, ion, neutron, atom and molecule interferometers that generate a spatial fringe pattern on the detector plane.

Statistical Fine Structure in Inhomogeneously Broadened Absorption Lines in Solids.

DTIC Science & Technology

1987-12-22

the inhomogeneously broadened zero-phonon SijSo (0-0) absorption of pentacene molecules in crystals of p-terphenyl at liquid helium temperatures. SFS...structure (SFS) in the inhomogeneously broadened zero-phonon S, +- So (0-0) absorption of pentacene molecules in crystals of p-terphenyl at liquid helium...tile large multiplicity of local environments. Inhomogeneously broadened absorption lines are usually treated as smooth, Gaussian profiles. In recent

Pressure induced increase of the exciton phonon interaction in ZnO/(ZnMg)O quantum wells

DOE Office of Scientific and Technical Information (OSTI.GOV)

Jarosz, D.; Suchocki, A.; Kozanecki, A.

2016-03-15

It is a well-established experimental fact that exciton-phonon coupling is very efficient in ZnO. The intensities of the phonon-replicas in ZnO/(ZnMg)O quantum structures strongly depend on the internal electric field. We performed high-pressure measurements on the single ZnO/(ZnMg)O quantum well. We observed a strong increase of the intensity of the phonon-replicas relative to the zero phonon line. In our opinion this effect is related to pressure induced increase of the strain in quantum structure. As a consequence, an increase of the piezoelectric component of the electric field is observed which leads to an increase of the intensity of the phonon-replicas.

Coherence-generating power of quantum dephasing processes

NASA Astrophysics Data System (ADS)

Styliaris, Georgios; Campos Venuti, Lorenzo; Zanardi, Paolo

2018-03-01

We provide a quantification of the capability of various quantum dephasing processes to generate coherence out of incoherent states. The measures defined, admitting computable expressions for any finite Hilbert-space dimension, are based on probabilistic averages and arise naturally from the viewpoint of coherence as a resource. We investigate how the capability of a dephasing process (e.g., a nonselective orthogonal measurement) to generate coherence depends on the relevant bases of the Hilbert space over which coherence is quantified and the dephasing process occurs, respectively. We extend our analysis to include those Lindblad time evolutions which, in the infinite-time limit, dephase the system under consideration and calculate their coherence-generating power as a function of time. We further identify specific families of such time evolutions that, although dephasing, have optimal (over all quantum processes) coherence-generating power for some intermediate time. Finally, we investigate the coherence-generating capability of random dephasing channels.

Control of dephasing in rotationally hot molecules

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bartram, David; Ivanov, Misha

We consider a rotationally hot diatomic molecule as an example of an open quantum system, where molecular vibrational wave packets are subject to dephasing due to rovibrational coupling. We report analytical and numerical results addressing whether the dephasing rate can be controlled by adjustment of the initial wave packet phases. It appears that over long time scales, phase-only control is not possible, but for earlier time scales the possibility of phase-only control of dephasing remains. In addition, we point out that the time dependence of the dephasing process depends significantly upon the degeneracy of the rotational environment states.

Theory of Raman scattering in coupled electron-phonon systems

NASA Astrophysics Data System (ADS)

Itai, K.

1992-01-01

The Raman spectrum is calculated for a coupled conduction-electron-phonon system in the zero-momentum-transfer limit. The Raman scattering is due to electron-hole excitations and phonons as well. The phonons of those branches that contribute to the electron self-energy and the correction of the electron-phonon vertex are assumed to have flat energy dispersion (the Einstein phonons). The effect of electron-impurity scattering is also incorporated. Both the electron-phonon interaction and the electron-impurity interaction cause the fluctuation of the electron distribution between different parts of the Fermi surface, which results in overdamped zero-sound modes of various symmetries. The scattering cross section is obtained by solving the Bethe-Salpeter equation. The spectrum shows a lower threshold at the smallest Einstein phonon energy when only the electron-phonon interaction is taken into consideration. When impurities are also taken into consideration, the threshold disappears.

Intrinsic coherence time of trions in monolayer MoSe2 measured via two-dimensional coherent spectroscopy

NASA Astrophysics Data System (ADS)

Titze, Michael; Li, Bo; Zhang, Xiang; Ajayan, Pulickel M.; Li, Hebin

2018-05-01

Quantum coherence and its dynamics in monolayer transition metal dichalcogenides (TMDs) are essential information to fully control valley pseudospin for valleytronics applications. Experimental understanding of coherence dephasing dynamics has been limited for excitons and largely unexplored for trions in monolayer TMDs. Here we use optical two-dimensional coherent spectroscopy to measure the trion coherence dephasing time in monolayer MoSe2 by analyzing the homogeneous linewidth. An intrinsic coherence time of 182 fs is extrapolated from the excitation density and temperature dependence measurement. The results show that trion-trion and trion-phonon interactions strongly affect the coherence dephasing time, while the intrinsic coherence time at zero excitation and zero temperature is primarily limited by the pure dephasing due to defect states. Our experiment also confirms optical two-dimensional coherent spectroscopy as a reliable technique for studying valley quantum dynamics in two-dimensional layered materials.

Spin dephasing in a magnetic dipole field.

PubMed

Ziener, C H; Kampf, T; Reents, G; Schlemmer, H-P; Bauer, W R

2012-05-01

Transverse relaxation by dephasing in an inhomogeneous field is a general mechanism in physics, for example, in semiconductor physics, muon spectroscopy, or nuclear magnetic resonance. In magnetic resonance imaging the transverse relaxation provides information on the properties of several biological tissues. Since the dipole field is the most important part of the multipole expansion of the local inhomogeneous field, dephasing in a dipole field is highly important in relaxation theory. However, there have been no analytical solutions which describe the dephasing in a magnetic dipole field. In this work we give a complete analytical solution for the dephasing in a magnetic dipole field which is valid over the whole dynamic range.

Probing lattice dynamics and electron-phonon coupling in the topological nodal-line semimetal ZrSiS

NASA Astrophysics Data System (ADS)

Singha, Ratnadwip; Samanta, Sudeshna; Chatterjee, Swastika; Pariari, Arnab; Majumdar, Dipanwita; Satpati, Biswarup; Wang, Lin; Singha, Achintya; Mandal, Prabhat

2018-03-01

Topological materials provide an exclusive platform to study the dynamics of relativistic particles in table-top experiments and offer the possibility of wide-scale technological applications. ZrSiS is a newly discovered topological nodal-line semimetal and has drawn enormous interests. In this paper, we have investigated the lattice dynamics and electron-phonon interaction in single-crystalline ZrSiS using Raman spectroscopy. Polarization and angle-resolved Raman data have been analyzed using crystal symmetries and theoretically calculated atomic vibrational patterns along with phonon dispersion spectra. Wavelength- and temperature-dependent measurements show the complex interplay of electron and phonon degrees of freedom, resulting in resonant phonon and quasielastic electron scattering through interband transition. Our high-pressure Raman studies reveal vibrational anomalies, which are the signature of structural phase transitions. Further investigations through high-pressure synchrotron x-ray diffraction clearly show pressure-induced structural transitions and coexistence of multiple phases, which also indicate possible electronic topological transitions in ZrSiS. This study not only provides the fundamental information on the phonon subsystem, but also sheds some light in understanding the topological nodal-line phase in ZrSiS and other isostructural systems.

Optimal dephasing for ballistic energy transfer in disordered linear chains

NASA Astrophysics Data System (ADS)

Zhang, Yang; Celardo, G. Luca; Borgonovi, Fausto; Kaplan, Lev

2017-11-01

We study the interplay between dephasing, disorder, and coupling to a sink on transport efficiency in a one-dimensional chain of finite length N , and in particular the beneficial or detrimental effect of dephasing on transport. The excitation moves along the chain by coherent nearest-neighbor hopping Ω , under the action of static disorder W and dephasing γ . The last site is coupled to an external acceptor system (sink), where the excitation can be trapped with a rate Γtrap. While it is known that dephasing can help transport in the localized regime, here we show that dephasing can enhance energy transfer even in the ballistic regime. Specifically, in the localized regime we recover previous results, where the optimal dephasing is independent of the chain length and proportional to W or W2/Ω . In the ballistic regime, the optimal dephasing decreases as 1 /N or 1 /√{N } , respectively, for weak and moderate static disorder. When focusing on the excitation starting at the beginning of the chain, dephasing can help excitation transfer only above a critical value of disorder Wcr, which strongly depends on the sink coupling strength Γtrap. Analytic solutions are obtained for short chains.

Pigment spectra and intermolecular interaction potentials in glasses and proteins.

PubMed

Renge, I; van Grondelle, R; Dekker, J P

2007-10-01

A model is proposed for chromophore optical spectra in solids over a wide range of temperatures and pressures. Inhomogeneous band shapes and their pressure dependence, as well as baric shift coefficients of spectral lines, selected by the frequency, were derived using Lennard-Jones potentials of the ground and excited states. Quadratic electron-phonon coupling constants, describing the thermal shift and broadening of zero-phonon lines, were also calculated. Experimentally, thermal shift and broadening of spectral holes were studied between 5 and 40 K for a synthetic pigment, chlorin, embedded in polymer hosts. The baric effects on holes were determined by applying hydrostatic He gas pressure up to 200 bar, at 6 K. Absorption spectra of pheophytin a, chlorophyll a, and beta-carotene in polymers and plant photosystem II CP47 complex were measured between 5 (or 77) and 300 K, and subject to Voigtian deconvolution. A narrowing of inhomogeneous bandwidth with increasing temperature, predicted on the basis of hole behavior, was observed as the shrinking of Gaussian spectral component. The Lorentzian broadening was ascribed to optical dephasing up to 300 K in transitions with weak to moderate linear electron-phonon coupling strength. The thermal broadening is purely Gaussian in multiphonon transitions (S(2) band of beta-carotene, Soret bands of tetrapyrrolic pigments), and the Lorentz process appears to be suppressed, indicating a lack of exponential dephasing. Density, polarity, polarizability, compressibility, and other local parameters of the pigment binding sites in biologically relevant systems can be deduced from spectroscopic data, provided that sufficient background information is available.

Pigment Spectra and Intermolecular Interaction Potentials in Glasses and Proteins

PubMed Central

Renge, I.; van Grondelle, R.; Dekker, J. P.

2007-01-01

A model is proposed for chromophore optical spectra in solids over a wide range of temperatures and pressures. Inhomogeneous band shapes and their pressure dependence, as well as baric shift coefficients of spectral lines, selected by the frequency, were derived using Lennard-Jones potentials of the ground and excited states. Quadratic electron-phonon coupling constants, describing the thermal shift and broadening of zero-phonon lines, were also calculated. Experimentally, thermal shift and broadening of spectral holes were studied between 5 and 40 K for a synthetic pigment, chlorin, embedded in polymer hosts. The baric effects on holes were determined by applying hydrostatic He gas pressure up to 200 bar, at 6 K. Absorption spectra of pheophytin a, chlorophyll a, and β-carotene in polymers and plant photosystem II CP47 complex were measured between 5 (or 77) and 300 K, and subject to Voigtian deconvolution. A narrowing of inhomogeneous bandwidth with increasing temperature, predicted on the basis of hole behavior, was observed as the shrinking of Gaussian spectral component. The Lorentzian broadening was ascribed to optical dephasing up to 300 K in transitions with weak to moderate linear electron-phonon coupling strength. The thermal broadening is purely Gaussian in multiphonon transitions (S2 band of β-carotene, Soret bands of tetrapyrrolic pigments), and the Lorentz process appears to be suppressed, indicating a lack of exponential dephasing. Density, polarity, polarizability, compressibility, and other local parameters of the pigment binding sites in biologically relevant systems can be deduced from spectroscopic data, provided that sufficient background information is available. PMID:17557783

Blue diode-pumped solid-state-laser based on ytterbium doped laser crystals operating on the resonance zero-phonon transition

DOEpatents

Krupke, William F.; Payne, Stephen A.; Marshall, Christopher D.

2001-01-01

The invention provides an efficient, compact means of generating blue laser light at a wavelength near .about.493+/-3 nm, based on the use of a laser diode-pumped Yb-doped laser crystal emitting on its zero phonon line (ZPL) resonance transition at a wavelength near .about.986+/-6 nm, whose fundamental infrared output radiation is harmonically doubled into the blue spectral region. The invention is applied to the excitation of biofluorescent dyes (in the .about.490-496 nm spectral region) utilized in flow cytometry, immunoassay, DNA sequencing, and other biofluorescence instruments. The preferred host crystals have strong ZPL fluorecence (laser) transitions lying in the spectral range from .about.980 to .about.992 nm (so that when frequency-doubled, they produce output radiation in the spectral range from 490 to 496 nm). Alternate preferred Yb doped tungstate crystals, such as Yb:KY(WO.sub.4).sub.2, may be configured to lase on the resonant ZPL transition near 981 nm (in lieu of the normal 1025 nm transition). The laser light is then doubled in the blue at 490.5 nm.

Control quantum evolution speed of a single dephasing qubit for arbitrary initial states via periodic dynamical decoupling pulses.

PubMed

Song, Ya-Ju; Tan, Qing-Shou; Kuang, Le-Man

2017-03-08

We investigate the possibility to control quantum evolution speed of a single dephasing qubit for arbitrary initial states by the use of periodic dynamical decoupling (PDD) pulses. It is indicated that the quantum speed limit time (QSLT) is determined by initial and final quantum coherence of the qubit, as well as the non-Markovianity of the system under consideration during the evolution when the qubit is subjected to a zero-temperature Ohmic-like dephasing reservoir. It is shown that final quantum coherence of the qubit and the non-Markovianity of the system can be modulated by PDD pulses. Our results show that for arbitrary initial states of the dephasing qubit with non-vanishing quantum coherence, PDD pulses can be used to induce potential acceleration of the quantum evolution in the short-time regime, while PDD pulses can lead to potential speedup and slow down in the long-time regime. We demonstrate that the effect of PDD on the QSLT for the Ohmic or sub-Ohmic spectrum (Markovian reservoir) is much different from that for the super-Ohmic spectrum (non-Markovian reservoir).

Heat guiding and focusing using ballistic phonon transport in phononic nanostructures

NASA Astrophysics Data System (ADS)

Anufriev, Roman; Ramiere, Aymeric; Maire, Jeremie; Nomura, Masahiro

2017-05-01

Unlike classical heat diffusion at macroscale, nanoscale heat conduction can occur without energy dissipation because phonons can ballistically travel in straight lines for hundreds of nanometres. Nevertheless, despite recent experimental evidence of such ballistic phonon transport, control over its directionality, and thus its practical use, remains a challenge, as the directions of individual phonons are chaotic. Here, we show a method to control the directionality of ballistic phonon transport using silicon membranes with arrays of holes. First, we demonstrate that the arrays of holes form fluxes of phonons oriented in the same direction. Next, we use these nanostructures as directional sources of ballistic phonons and couple the emitted phonons into nanowires. Finally, we introduce thermal lens nanostructures, in which the emitted phonons converge at the focal point, thus focusing heat into a spot of a few hundred nanometres. These results motivate the concept of ray-like heat manipulations at the nanoscale.

Heat guiding and focusing using ballistic phonon transport in phononic nanostructures.

PubMed

Anufriev, Roman; Ramiere, Aymeric; Maire, Jeremie; Nomura, Masahiro

2017-05-18

Unlike classical heat diffusion at macroscale, nanoscale heat conduction can occur without energy dissipation because phonons can ballistically travel in straight lines for hundreds of nanometres. Nevertheless, despite recent experimental evidence of such ballistic phonon transport, control over its directionality, and thus its practical use, remains a challenge, as the directions of individual phonons are chaotic. Here, we show a method to control the directionality of ballistic phonon transport using silicon membranes with arrays of holes. First, we demonstrate that the arrays of holes form fluxes of phonons oriented in the same direction. Next, we use these nanostructures as directional sources of ballistic phonons and couple the emitted phonons into nanowires. Finally, we introduce thermal lens nanostructures, in which the emitted phonons converge at the focal point, thus focusing heat into a spot of a few hundred nanometres. These results motivate the concept of ray-like heat manipulations at the nanoscale.

The effect of n- and p-type doping on coherent phonons in GaN.

PubMed

Ishioka, Kunie; Kato, Keiko; Ohashi, Naoki; Haneda, Hajime; Kitajima, Masahiro; Petek, Hrvoje

2013-05-22

The effect of doping on the carrier-phonon interaction in wurtzite GaN is investigated by pump-probe reflectivity measurements using 3.1 eV light in near resonance with the fundamental band gap of 3.39 eV. Coherent modulations of the reflectivity due to the E2 and A1(LO) modes, as well as the 2A1(LO) overtone are observed. Doping of acceptor and donor atoms enhances the dephasing of the polar A1(LO) phonon via coupling with plasmons, with the effect of donors being stronger. Doping also enhances the relative amplitude of the coherent A1(LO) phonon with respect to that of the high-frequency E2 phonon, though it does not affect the relative intensity in Raman spectroscopic measurements. We attribute this enhanced coherent amplitude to the transient depletion field screening (TDFS) excitation mechanism, which, in addition to impulsive stimulated Raman scattering (ISRS), contributes to the generation of coherent polar phonons even for sub-band gap excitation. Because the TDFS mechanism requires photoexcitation of carriers, we argue that the interband transition is made possible at a surface with photon energies below the bulk band gap through the Franz-Keldysh effect.

Dephasing Catastrophe in 4-ε Dimensions: A Possible Instability of the Ergodic (Many-Body-Delocalized) Phase.

PubMed

Liao, Yunxiang; Foster, Matthew S

2018-06-08

In two dimensions, dephasing by a bath cuts off Anderson localization that would otherwise occur at any energy density for fermions with disorder. For an isolated system with short-range interactions, the system can be its own bath, exhibiting diffusive (non-Markovian) thermal density fluctuations. We recast the dephasing of weak localization due to a diffusive bath as a self-interacting polymer loop. We investigate the critical behavior of the loop in d=4-ε dimensions, and find a nontrivial fixed point corresponding to a temperature T^{*}∼ε>0 where the dephasing time diverges. Assuming that this fixed point survives to ε=2, we associate it with a possible instability of the ergodic phase. Our approach may open a new line of attack against the problem of the ergodic to many-body-localized phase transition in d>1 spatial dimensions.

Dephasing Catastrophe in 4 -ɛ Dimensions: A Possible Instability of the Ergodic (Many-Body-Delocalized) Phase

NASA Astrophysics Data System (ADS)

Liao, Yunxiang; Foster, Matthew S.

2018-06-01

In two dimensions, dephasing by a bath cuts off Anderson localization that would otherwise occur at any energy density for fermions with disorder. For an isolated system with short-range interactions, the system can be its own bath, exhibiting diffusive (non-Markovian) thermal density fluctuations. We recast the dephasing of weak localization due to a diffusive bath as a self-interacting polymer loop. We investigate the critical behavior of the loop in d =4 -ɛ dimensions, and find a nontrivial fixed point corresponding to a temperature T*˜ɛ >0 where the dephasing time diverges. Assuming that this fixed point survives to ɛ =2 , we associate it with a possible instability of the ergodic phase. Our approach may open a new line of attack against the problem of the ergodic to many-body-localized phase transition in d >1 spatial dimensions.

Heat guiding and focusing using ballistic phonon transport in phononic nanostructures

PubMed Central

Anufriev, Roman; Ramiere, Aymeric; Maire, Jeremie; Nomura, Masahiro

2017-01-01

Unlike classical heat diffusion at macroscale, nanoscale heat conduction can occur without energy dissipation because phonons can ballistically travel in straight lines for hundreds of nanometres. Nevertheless, despite recent experimental evidence of such ballistic phonon transport, control over its directionality, and thus its practical use, remains a challenge, as the directions of individual phonons are chaotic. Here, we show a method to control the directionality of ballistic phonon transport using silicon membranes with arrays of holes. First, we demonstrate that the arrays of holes form fluxes of phonons oriented in the same direction. Next, we use these nanostructures as directional sources of ballistic phonons and couple the emitted phonons into nanowires. Finally, we introduce thermal lens nanostructures, in which the emitted phonons converge at the focal point, thus focusing heat into a spot of a few hundred nanometres. These results motivate the concept of ray-like heat manipulations at the nanoscale. PMID:28516909

Tunable zero-line modes via magnetic field in bilayer graphene

NASA Astrophysics Data System (ADS)

Wang, Ke; Qiao, Zhenhua

Zero-line modes appear in bilayer graphene at the internal boundary between two opposite vertical electrostatic confinements. These one-dimensional modes are metallic along the boundary and exhibit quantized conductance in the absence of inter-valley scattering. However, experimental results show that the conductance is around 0.5 e2/h rather than quantized. This observation can be explained from our numerical results, which suggest that the scattering between zero-line mode and bound states and the presence of atomic scale disorders that provide inter-valley scattering can effectively reduce the conductance to about 0.5 e2/h. We further find that out-of-plane magnetic field can strongly suppress these scattering mechanisms and gives rise to nearly quantized conductance. On one hand, the presence of magnetic field makes bound states become Landau levels, which reduces the scattering between zero-line mode and bound states. On the other hand, the wave function distributions of oppositely propagating zero-line modes at different valleys are spatially separated, which can strongly suppress the inter-valley scattering. Specifically speaking, the conductance can be increased to 3.2 e2/h at 8 T even when the atomic Anderson type disorders are considered.

Correlated phonons and the Tc-dependent dynamical phonon anomalies

NASA Astrophysics Data System (ADS)

Hakioğlu, T.; Türeci, H.

1997-11-01

Anomalously large low-temperature phonon anharmonicities can lead to static as well as dynamical changes in the low-temperature properties of the electron-phonon system. In this work, we focus our attention on the dynamically generated low-temperature correlations in an interacting electron-phonon system using a self-consistent dynamical approach in the intermediate coupling range. In the context of the model, the polaron correlations are produced by the charge-density fluctuations which are generated dynamically by the electron-phonon coupling. Conversely, the latter is influenced in the presence of the former. The purpose of this work is to examine the dynamics of this dual mechanism between the two using the illustrative Fröhlich model. In particular, the influence of the low-temperature phonon dynamics on the superconducting properties in the intermediate coupling range is investigated. The influence on the Holstein reduction factor as well as the enhancement in the zero-point fluctuations and in the electron-phonon coupling are calculated numerically. We also examine these effects in the presence of superconductivity. Within this model, the contribution of the electron-phonon interaction as one of the important elements in the mechanisms of superconductivity can reach values as high as 15-20% of the characteristic scale of the lattice vibrational energy. The second motivation of this work is to understand the nature of the Tc-dependent temperature anomalies observed in the Debye-Waller factor, dynamical pair correlations, and average atomic vibrational energies for a number of high-temperature superconductors. In our approach we do not claim nor believe that the electron-phonon interaction is the primary mechanism leading to high-temperature superconductivity. Nevertheless, our calculations suggest that the dynamically induced low-temperature phonon correlation model can account for these anomalies and illustrates their possible common origin. Finally, the

Qubit dephasing due to low-frequency noise.

NASA Astrophysics Data System (ADS)

Sverdlov, Victor; Rabenstein, Kristian; Averin, Dmitri

2004-03-01

We have numerically investigated the effects of the classical low-frequency noise on the qubit dynamics beyond the standard lowest-order perturbation theory in coupling. Noise is generated as a random process with a correlation function characterized by two parameters, the amplitude v0 and the cut-off frequency 2π/τ. Time evolution of the density matrix was averaged over up to 10^7 noise realizations. Contrary to the relaxation time T_1, which for v_0<ω, where ω is the qubit oscillation frequency, is always given correctly by the ``golden-rule'' expression, the dephasing time deviates from the perturbation-theory result, when (v_0/ω)^2(ωτ) ≥1. In this regime, even for unbiased qubit for which the pure dephasing vanishes in perturbation theory, the dephasing is much larger than it's perturbation-theory value 1/(2 T_1).

Measuring phonon mean free path distributions by probing quasiballistic phonon transport in grating nanostructures

DOE PAGES

Zeng, Lingping; Collins, Kimberlee C.; Hu, Yongjie; ...

2015-11-27

Heat conduction in semiconductors and dielectrics depends upon their phonon mean free paths that describe the average travelling distance between two consecutive phonon scattering events. Nondiffusive phonon transport is being exploited to extract phonon mean free path distributions. Here, we describe an implementation of a nanoscale thermal conductivity spectroscopy technique that allows for the study of mean free path distributions in optically absorbing materials with relatively simple fabrication and a straightforward analysis scheme. We pattern 1D metallic grating of various line widths but fixed gap size on sample surfaces. The metal lines serve as both heaters and thermometers in time-domainmore » thermoreflectance measurements and simultaneously act as wiregrid polarizers that protect the underlying substrate from direct optical excitation and heating. We demonstrate the viability of this technique by studying length-dependent thermal conductivities of silicon at various temperatures. The thermal conductivities measured with different metal line widths are analyzed using suppression functions calculated from the Boltzmann transport equation to extract the phonon mean free path distributions with no calibration required. Furthermore, this table-top ultrafast thermal transport spectroscopy technique enables the study of mean free path spectra in a wide range of technologically important materials.« less

Spacetime representation of topological phononics

NASA Astrophysics Data System (ADS)

Deymier, Pierre A.; Runge, Keith; Lucas, Pierre; Vasseur, Jérôme O.

2018-05-01

Non-conventional topology of elastic waves arises from breaking symmetry of phononic structures either intrinsically through internal resonances or extrinsically via application of external stimuli. We develop a spacetime representation based on twistor theory of an intrinsic topological elastic structure composed of a harmonic chain attached to a rigid substrate. Elastic waves in this structure obey the Klein–Gordon and Dirac equations and possesses spinorial character. We demonstrate the mapping between straight line trajectories of these elastic waves in spacetime and the twistor complex space. The twistor representation of these Dirac phonons is related to their topological and fermion-like properties. The second topological phononic structure is an extrinsic structure composed of a one-dimensional elastic medium subjected to a moving superlattice. We report an analogy between the elastic behavior of this time-dependent superlattice, the scalar quantum field theory and general relativity of two types of exotic particle excitations, namely temporal Dirac phonons and temporal ghost (tachyonic) phonons. These phonons live on separate sides of a two-dimensional frequency space and are delimited by ghost lines reminiscent of the conventional light cone. Both phonon types exhibit spinorial amplitudes that can be measured by mapping the particle behavior to the band structure of elastic waves.

Mutual interactions of phonons, rotons, and gravity

NASA Astrophysics Data System (ADS)

Nicolis, Alberto; Penco, Riccardo

2018-04-01

We introduce an effective point-particle action for generic particles living in a zero-temperature superfluid. This action describes the motion of the particles in the medium at equilibrium as well as their couplings to sound waves and generic fluid flows. While we place the emphasis on elementary excitations such as phonons and rotons, our formalism applies also to macroscopic objects such as vortex rings and rigid bodies interacting with long-wavelength fluid modes. Within our approach, we reproduce phonon decay and phonon-phonon scattering as predicted using a purely field-theoretic description of phonons. We also correct classic results by Landau and Khalatnikov on roton-phonon scattering. Finally, we discuss how phonons and rotons couple to gravity, and show that the former tend to float while the latter tend to sink but with rather peculiar trajectories. Our formalism can be easily extended to include (general) relativistic effects and couplings to additional matter fields. As such, it can be relevant in contexts as diverse as neutron star physics and light dark matter detection.

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.

Temporal fluctuations after a quantum quench: Many-particle dephasing

NASA Astrophysics Data System (ADS)

Marquardt, Florian; Kiendl, Thomas

After a quantum quench, the expectation values of observables continue to fluctuate in time. In the thermodynamic limit, one expects such fluctuations to decrease to zero, in order for standard statistical physics to hold. However, it is a challenge to determine analytically how the fluctuations decay as a function of system size. So far, there have been analytical predictions for integrable models (which are, naturally, somewhat special), analytical bounds for arbitrary systems, and numerical results for moderate-size systems. We have discovered a dynamical regime where the decrease of fluctuations is driven by many-particle dephasing, instead of a redistribution of occupation numbers. On the basis of this insight, we are able to provide exact analytical expressions for a model with weak integrability breaking (transverse Ising chain with additional terms). These predictions explicitly show how fluctuations are exponentially suppressed with system size.

Effect of hydrogen bonding on the vibrational dephasing time in glycerol

NASA Technical Reports Server (NTRS)

Dorsinville, R.; Franklin, W. M.; Ockman, N.; Alfano, R. R.

1982-01-01

The vibrational dephasing of the methyl CH2 symmetric stretch mode in glycerol was directly measured over an extended temperature range using picosecond coherent Raman pump and probe spectroscopy. The dephasing time was found to increase dramatically as the temperature of the supercooled liquid was lowered. This observation is attributed to the increased hydrogen bonding with decreasing temperature which hinders the dephasing of the CH2 vibration by reducing molecular motions.

Gate-tunable current partition in graphene-based topological zero lines

NASA Astrophysics Data System (ADS)

Wang, Ke; Ren, Yafei; Deng, Xinzhou; Yang, Shengyuan A.; Jung, Jeil; Qiao, Zhenhua

2017-06-01

We demonstrate new mechanisms for gate-tunable current partition at topological zero-line intersections in a graphene-based current splitter. Based on numerical calculations of the nonequilibrium Green's functions and Landauer-Büttiker formula, we show that the presence of a perpendicular magnetic field on the order of a few Teslas allows for carrier sign dependent current routing. In the zero-field limit the control on current routing and partition can be achieved within a range of 10-90 % of the total incoming current by tuning the carrier density at tilted intersections or by modifying the relative magnitude of the bulk band gaps via gate voltage. We discuss the implications of our findings in the design of topological zero-line networks where finite orbital magnetic moments are expected when the current partition is asymmetric.

Dephasing effects on ac-driven triple quantum dot systems

NASA Astrophysics Data System (ADS)

Maldonado, I.; Villavicencio, J.; Contreras-Pulido, L. D.; Cota, E.; Maytorena, J. A.

2018-05-01

We analyze the effect of environmental dephasing on the electrical current in an ac-driven triple quantum dot system in a symmetric Λ configuration. The current is explored by solving the time evolution equation of the density matrix as a function of the frequency and amplitude of the driving field. Two characteristic spectra are observed depending on the field amplitude. At the resonance condition, when the frequency matches the interdot energy difference, one spectrum shows a distinctive Fano-type peak, while the other, occurring at larger values of the field amplitude, exhibits a strong current suppression due to dynamic localization. In the former case we observe that the current maximum is reduced due to dephasing, while in the latter it is shown that dephasing partially alleviates the localization. In both cases, away from resonance, we observe current oscillations which are dephasing-enhanced for a wide range of frequencies. These effects are also discussed using Floquet theory, and analytical expressions for the electrical current are obtained within the rotating wave approximation.

Evidence of charged puddles and induced dephasing in topological insulator thin films

NASA Astrophysics Data System (ADS)

Singh, Sourabh; Gopal, R. K.; Sarkar, Jit; Roy, Subhadip; Mitra, Chiranjib

2018-05-01

We investigate the dephasing mechanism in bulk insulating topological insulator thin films. The phase coherence length is extracted from magnetoresistance measurements at different temperatures. There is a crossover of the phase coherence length as a function of temperature signifying the role of more than one dephasing mechanism in the system. The dephasing rates have been studied systematically and explained.

Cooperative Effects in Closely Packed Quantum Emitters with Collective Dephasing

NASA Astrophysics Data System (ADS)

Prasanna Venkatesh, B.; Juan, M. L.; Romero-Isart, O.

2018-01-01

In a closely packed ensemble of quantum emitters, cooperative effects are typically suppressed due to the dephasing induced by the dipole-dipole interactions. Here, we show that by adding sufficiently strong collective dephasing, cooperative effects can be restored. Specifically, we show that the dipole force on a closely packed ensemble of strongly driven two-level quantum emitters, which collectively dephase, is enhanced in comparison to the dipole force on an independent noninteracting ensemble. Our results are relevant to solid-state systems with embedded quantum emitters such as color centers in diamond and superconducting qubits in microwave cavities and waveguides.

Quantum Noise of Electron-Phonon Heat Current

NASA Astrophysics Data System (ADS)

Pekola, Jukka P.; Karimi, Bayan

2018-06-01

We analyze heat current fluctuations between electrons and phonons in a metal. In equilibrium we recover the standard result consistent with the fluctuation-dissipation theorem. Here we show that heat current noise at finite frequencies remains non-vanishing down to zero temperature. From the experimental point of view, it is a small effect and up to now elusive. We briefly discuss the impact of electron-phonon heat current fluctuations on calorimetry, particularly in the regime of single microwave-photon detection.

Phonon quarticity induced by changes in phonon-tracked hybridization during lattice expansion and its stabilization of rutile TiO 2

DOE PAGES

Lan, Tian; Li, Chen W.; Hellman, O.; ...

2015-08-11

Although the rutile structure of TiO 2 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 TiO 2 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 tomore » quartic, stabilizing 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

Control relaxation via dephasing: A quantum-state-diffusion study

NASA Astrophysics Data System (ADS)

Jing, Jun; Yu, Ting; Lam, Chi-Hang; You, J. Q.; Wu, Lian-Ao

2018-01-01

Dynamical decoupling as a quantum control strategy aims at suppressing quantum decoherence adopting the popular philosophy that the disorder in the unitary evolution of the open quantum system caused by environmental noises should be neutralized by a sequence of ordered or well-designed external operations acting on the system. This work studies the solution of quantum-state-diffusion equations by mixing two channels of environmental noises, i.e., relaxation (dissipation) and dephasing. It is interesting to find in two-level and three-level atomic systems that a non-Markovian relaxation or dissipation process can be suppressed by a Markovian dephasing noise. The discovery results in an anomalous control strategy by coordinating relaxation and dephasing processes. Our approach opens an avenue of noise control strategy with no artificial manipulation over the open quantum systems.

Distillability sudden death in qutrit-qutrit systems under global and multilocal dephasing

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ali, Mazhar

2010-04-15

Recently Song et al. [Phys. Rev. A 80, 012331 (2009)] discovered that certain two-qutrit entangled states interacting with multilocal dephasing undergo distillability sudden death (DSD). We generalized their studies for quantum states under global, collective, and multilocal dephasing. We found that for a specific family of quantum states DSD and entanglement sudden death may happen under global dephasing as well as under multilocal dephasing. Moreover, for these states simple local unitary transformations may not avoid DSD under the current dynamics. As there is no definitive criterion for separability or entanglement of density matrices with dimensions greater than six, therefore wemore » believe that the results obtained in this article are restricted to the specific family of states considered here.« less

Phonon dispersion evolution in uniaxially strained aluminum crystal

NASA Astrophysics Data System (ADS)

Parthasarathy, Ranganathan; Misra, Anil; Aryal, Sitaram; Ouyang, Lizhi

2018-04-01

The influence of loading upon the phonon dispersion of crystalline materials could be highly nonlinear with certain particular trends that depend upon the loading path. In this paper, we have calculated the influence of [100] uniaxial strain on the phonon dispersion and group velocities in fcc aluminum using second moments of position obtained from molecular dynamics (MD) simulation at 300 K. In contrast to nonlinear monotonic variation of both longitudinal and transverse phonon frequencies along the Δ , Λ and Σ lines of the first Brillouin zone under tension, transverse phonon branches along the Λ line show inflection at specific wavevectors when the compressive strain exceeds 5%. Further, the longitudinal group velocities along the high-symmetry Δ line vary non-monotonically with strain, reaching a minimum at 5% compressive strain. Throughout the strain range studied, the equilibrium positions of atoms displace in an affine manner preserving certain static structural symmetry. We attribute the anomalies in the phonon dispersion to the non-affine evolution of second moments of atomic position, and the associated plateauing of force constants under the applied strain path.

Analog model for quantum gravity effects: phonons in random fluids.

PubMed

Krein, G; Menezes, G; Svaiter, N F

2010-09-24

We describe an analog model for quantum gravity effects in condensed matter physics. The situation discussed is that of phonons propagating in a fluid with a random velocity wave equation. We consider that there are random fluctuations in the reciprocal of the bulk modulus of the system and study free phonons in the presence of Gaussian colored noise with zero mean. We show that, in this model, after performing the random averages over the noise function a free conventional scalar quantum field theory describing free phonons becomes a self-interacting model.

On-chip photonic-phononic emitter-receiver apparatus

DOEpatents

Cox, Jonathan Albert; Jarecki, Jr., Robert L.; Rakich, Peter Thomas; Wang, Zheng; Shin, Heedeuk; Siddiqui, Aleem; Starbuck, Andrew Lea

2017-07-04

A radio-frequency photonic devices employs photon-phonon coupling for information transfer. The device includes a membrane in which a two-dimensionally periodic phononic crystal (PnC) structure is patterned. The device also includes at least a first optical waveguide embedded in the membrane. At least a first line-defect region interrupts the PnC structure. The first optical waveguide is embedded within the line-defect region.

Phonon assisted carrier motion on the Wannier-Stark ladder

NASA Astrophysics Data System (ADS)

Cheung, Alfred; Berciu, Mona

2014-03-01

It is well known that at zero temperature and in the absence of electron-phonon coupling, the presence of an electric field leads to localization of carriers residing in a single band of finite bandwidth. In this talk, we will present an implementation of the self-consistent Born approximation (SCBA) to study the effect of weak electron-phonon coupling on the motion of a carrier in a biased system. At moderate and strong electron-phonon coupling, we supplement the SCBA, describing the string of phonons left behind by the carrier, with the momentum average approximation to describe the phonon cloud that accompanies the resulting polaron. We find that coupling to the lattice delocalizes the carrier, as expected, although long-lived resonances resulting from the Wannier-Stark states of the polaron may appear in certain regions of the parameter space. We end with a discussion of how our method can be improved to model disorder, other types of electron-phonon coupling, and electron-hole pair dissociation in a biased system.

Mitigation of near-band balanced steady-state free precession through-plane flow artifacts using partial dephasing.

PubMed

Datta, Anjali; Cheng, Joseph Y; Hargreaves, Brian A; Baron, Corey A; Nishimura, Dwight G

2018-06-01

To mitigate artifacts from through-plane flow at the locations of steady-state stopbands in balanced steady-state free precession (SSFP) using partial dephasing. A 60° range in the phase accrual during a TR was created over the voxel by slightly unbalancing the slice-select dephaser. The spectral profiles of SSFP with partial dephasing for various constant flow rates and during pulsatile flow were simulated to determine if partial dephasing decreases through-plane flow artifacts originating near SSFP dark bands while maintaining on-resonant signal. Simulations were then validated in a flow phantom. Lastly, phase-cycled SSFP cardiac cine images were acquired with and without partial dephasing in six subjects. Partial dephasing decreased the strength and non-linearity of the dependence of the signal at the stopbands on the through-plane flow rate. It thus mitigated hyper-enhancement from out-of-slice signal contributions and transient-related artifacts caused by variable flow both in the phantom and in vivo. In six volunteers, partial dephasing noticeably decreased artifacts in all of the phase-cycled cardiac cine datasets. Partial dephasing can mitigate the flow artifacts seen at the stopbands in balanced SSFP while maintaining the sequence's desired signal. By mitigating hyper-enhancement and transient-related artifacts originating from the stopbands, partial dephasing facilitates robust multiple-acquisition phase-cycled SSFP in the heart. Magn Reson Med 79:2944-2953, 2018. © 2017 International Society for Magnetic Resonance in Medicine. © 2017 International Society for Magnetic Resonance in Medicine.

Electron-phonon effects in graphene and an armchair (10,10) single-wall carbon nanotube

NASA Astrophysics Data System (ADS)

Woods, Lilia Milcheva Rapatinska

New effects due to the electron-phonon interaction in some low-dimensional tight-binding systems are discussed. A sheet of graphite (two-dimensional) and an armchair single wall carbon nanotube (SWNT) (quasi-one dimensional) are taken as examples. The geometrical structure and the linear dispersion of the energy with respect to the electron wave vector are expected to play a significant role. For the ordinary electron-phonon coupling which includes modulated hopping and linear electron-phonon interaction the matrix elements for both systems are derived in the context of a two parameter model for the phonon vibrational spectrum. It is found that they (for both structures) strongly depend on the geometry, display a deformation type of potential and are reduced by a factor of (1 - R), where R depends uniquely on the introduced phonon parameters. Next a new type of interaction is derived; it arises from the phonon modulation of the electron-electron interaction. After writing the matrix elements for the new Hamiltonian, the problem is considered in the context of many body physics. There are two contributions. One of them is the random phase approximation with one phonon line. The electron self-energy for it is calculated. It is shown that one might expect that this is not a large effect. Analytical expressions are obtained for the armchair single wall carbon nanotube. The exchange interaction in the one-phonon approximation is another term that arises and is also considered. One is able to write four new Feynman diagrams and derive an expression for -ImSk⃗ . The contribution from this type of coupling could be large and comparable to the one from the modulated hopping. These results are supported by numerical estimates of some characteristics of graphene and SWNT. The values of the electron-phonon coupling constant, lambda, and the electron lifetime, tau, are compared between the traditional electron-phonon interaction and the phonon modulated electron

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

Reshaping the phonon energy landscape of nanocrystals inside a terahertz plasmonic nanocavity.

PubMed

Jin, Xin; Cerea, Andrea; Messina, Gabriele C; Rovere, Andrea; Piccoli, Riccardo; De Donato, Francesco; Palazon, Francisco; Perucchi, Andrea; Di Pietro, Paola; Morandotti, Roberto; Lupi, Stefano; De Angelis, Francesco; Prato, Mirko; Toma, Andrea; Razzari, Luca

2018-02-22

Phonons (quanta of collective vibrations) are a major source of energy dissipation and drive some of the most relevant properties of materials. In nanotechnology, phonons severely affect light emission and charge transport of nanodevices. While the phonon response is conventionally considered an inherent property of a nanomaterial, here we show that the dipole-active phonon resonance of semiconducting (CdS) nanocrystals can be drastically reshaped inside a terahertz plasmonic nanocavity, via the phonon strong coupling with the cavity vacuum electric field. Such quantum zero-point field can indeed reach extreme values in a plasmonic nanocavity, thanks to a mode volume well below λ 3 /10 7 . Through Raman measurements, we find that the nanocrystals within a nanocavity exhibit two new "hybridized" phonon peaks, whose spectral separation increases with the number of nanocrystals. Our findings open exciting perspectives for engineering the optical phonon response of functional nanomaterials and for implementing a novel platform for nanoscale quantum optomechanics.

Nanoscale Phonon Transport as Probed with a Microfabricated Phonon Spectrometer for the Study of Nanoscale Energy Transport

NASA Astrophysics Data System (ADS)

Robinson, Richard; Otelaja, Obafemi; Hertzberg, Jared; Aksit, Mahmut; Stewart, Derek

2013-03-01

Phonons are the dominant heat carriers in dielectrics and a clear understanding of their behavior at the nanoscale is important for the development of efficient thermoelectric devices. In this work we show how acoustic phonon transport can be directly probed by the generation and detection of non-equilibrium phonons in microscale and nanoscale structures. Our technique employs a scalable method of fabricating phonon generators and detectors by forming Al-AlxOy-Al superconducting tunnel junctions on the sidewalls of a silicon mesa etched with KOH and an operating temperature of 0.3K. In the line-of-sight path along the width of these mesas, phonons with frequency ~100 GHz can propagate ballistically The phonons radiate into the mesa and are observed by the detector after passing through the mesa. We fabricated silicon nanosheets of width 100 to 300 nm along the ballistic path and observe surface scattering effects on phonon transmission when the characteristic length scale of a material is less than the phonon mean free path. We compare our results to the Casimir-Ziman theory. Our methods can be adapted for studying phonon transport in other nanostructures and will improve the understanding of phonon contribution to thermal transport. The work was supported in part by the National Science Foundation under Agreement No. DMR-1149036.

Phonon spectra and the one-phonon and two-phonon densities of states of UO2 and PuO2

NASA Astrophysics Data System (ADS)

Poplavnoi, A. S.; Fedorova, T. P.; Fedorov, I. A.

2017-04-01

The vibrational spectra of uranium dioxide UO2 and plutonium dioxide PuO2, as well as the one-phonon densities of states and thermal occupation number weighted two-phonon densities of states, have been calculated within the framework of the phenomenological rigid ion model. It has been shown that the acoustic and optical branches of the spectra are predominantly determined by vibrations of the metal and oxygen atoms, respectively, because the atomic masses of the metal and oxygen differ from each other by an order of magnitude. On this basis, the vibrational spectra can be represented in two Brillouin zones, i.e., in the Brillouin zone of the crystal and the Brillouin zone of the oxygen sublattice. In this case, the number of optical branches decreases by a factor of two. The two-phonon densities of states consist of two broad structured peaks. The temperature dependences of the upper peak exhibit a thermal broadening of the phonon lines L01 and L02 in the upper part of the optical branches. The lower peak is responsible for the thermal broadening of the lowest two optical (T02, T01) and acoustic (LA, TA) branches.

Electrical detection and analysis of surface acoustic wave in line-defect two-dimensional piezoelectric phononic crystals

NASA Astrophysics Data System (ADS)

Cai, Feida; Li, Honglang; Tian, Yahui; Ke, Yabing; Cheng, Lina; Lou, Wei; He, Shitang

2018-03-01

Line-defect piezoelectric phononic crystals (PCs) show good potential applications in surface acoustic wave (SAW) MEMS devices for RF communication systems. To analyze the SAW characteristics in line-defect two-dimensional (2D) piezoelectric PCs, optical methods are commonly used. However, the optical instruments are complex and expensive, whereas conventional electrical methods can only measure SAW transmission of the whole device and lack spatial resolution. In this paper, we propose a new electrical experimental method with multiple receiving interdigital transducers (IDTs) to detect the SAW field distribution, in which an array of receiving IDTs of equal aperture was used to receive the SAW. For this new method, SAW delay lines with perfect and line-defect 2D Al/128°YXLiNbO3 piezoelectric PCs on the transmitting path were designed and fabricated. The experimental results showed that the SAW distributed mainly in the line-defect region, which agrees with the theoretical results.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Jana, R. N.; Meikap, A. K.

The results of a comprehensive study of weak electron localization (WEL) and electron-electron interaction (EEI) effects in disordered V{sub 75}X{sub 25} (X = Pd, Al) alloys has been reported. The resistivity in absence of magnetic field shows a minimum at temperature T = T{sub m} and follows T{sup 1/2} law within the temperature range 5 K ≤ T ≤ T{sub m}, which suggests predominant EEI effect. Magnetoresistivity is positive due to strong spin-orbit interaction. The dephasing scattering time is dominated by the electron-phonon scattering. The electron-phonon scattering rate shows quadratic temperature dependence behavior, which is explained by the theory ofmore » incomplete dragging at the random scattering potential by phonons. The zero temperature scattering time strongly depends on the disorder and its magnitude decreases with increasing disorder.« less

Coupling the Gaussian Free Fields with Free and with Zero Boundary Conditions via Common Level Lines

NASA Astrophysics Data System (ADS)

Qian, Wei; Werner, Wendelin

2018-06-01

We point out a new simple way to couple the Gaussian Free Field (GFF) with free boundary conditions in a two-dimensional domain with the GFF with zero boundary conditions in the same domain: Starting from the latter, one just has to sample at random all the signs of the height gaps on its boundary-touching zero-level lines (these signs are alternating for the zero-boundary GFF) in order to obtain a free boundary GFF. Constructions and couplings of the free boundary GFF and its level lines via soups of reflected Brownian loops and their clusters are also discussed. Such considerations show for instance that in a domain with an axis of symmetry, if one looks at the overlay of a single usual Conformal Loop Ensemble CLE3 with its own symmetric image, one obtains the CLE4-type collection of level lines of a GFF with mixed zero/free boundary conditions in the half-domain.

Temperature-tunable Fano resonance induced by strong Weyl fermion-phonon coupling in TaAs

NASA Astrophysics Data System (ADS)

Dai, Yaomin; Trugman, S. A.; Zhu, J.-X.; Taylor, A. J.; Yarotski, D. A.; Prasankumar, R. P.; Xu, B.; Zhao, L. X.; Wang, K.; Yang, R.; Zhang, W.; Liu, J. Y.; Xiao, H.; Chen, G. F.; Qiu, X. G.

Strong coupling between discrete phonon and continuous electron-hole pair excitations can give rise to a pronounced asymmetry in the phonon line shape, known as the Fano resonance. We present infrared spectroscopic studies on the recently discovered Weyl semimetal TaAs at different temperatures. Our experimental results reveal strong coupling between an infrared-active A1 phonon and electronic transitions near the Weyl points (Weyl fermions), as evidenced by the conspicuous asymmetry in the phonon line shape. More interestingly, the phonon line shape can be continuously tuned by temperature, which we demonstrate to arise from the suppression of the electronic transitions near the Weyl points due to the decreasing occupation of electronic states below the Fermi level with increasing temperature, as well as Pauli blocking caused by thermally excited electrons above the Fermi level. Supported by LANL LDRD and LANL-UCRP programs.

Phononic crystals of spherical particles: A tight binding approach

NASA Astrophysics Data System (ADS)

Mattarelli, M.; Secchi, M.; Montagna, M.

2013-11-01

The vibrational dynamics of a fcc phononic crystal of spheres is studied and compared with that of a single free sphere, modelled either by a continuous homogeneous medium or by a finite cluster of atoms. For weak interaction among the spheres, the vibrational dynamics of the phononic crystal is described by shallow bands, with low degree of dispersion, corresponding to the acoustic spheroidal and torsional modes of the single sphere. The phonon displacements are therefore related to the vibrations of a sphere, as the electron wave functions in a crystal are related to the atomic wave functions in a tight binding model. Important dispersion is found for the two lowest phonon bands, which correspond to zero frequency free translation and rotation of a free sphere. Brillouin scattering spectra are calculated at some values of the exchanged wavevectors of the light, and compared with those of a single sphere. With weak interaction between particles, given the high acoustic impedance mismatch in dry systems, the density of phonon states consist of sharp bands separated by large gaps, which can be well accounted for by a single particle model. Based on the width of the frequency gaps, tunable with the particle size, and on the small number of dispersive acoustic phonons, such systems may provide excellent materials for application as sound or heat filters.

Quantum random walks on congested lattices and the effect of dephasing.

PubMed

Motes, Keith R; Gilchrist, Alexei; Rohde, Peter P

2016-01-27

We consider quantum random walks on congested lattices and contrast them to classical random walks. Congestion is modelled on lattices that contain static defects which reverse the walker's direction. We implement a dephasing process after each step which allows us to smoothly interpolate between classical and quantum random walks as well as study the effect of dephasing on the quantum walk. Our key results show that a quantum walker escapes a finite boundary dramatically faster than a classical walker and that this advantage remains in the presence of heavily congested lattices.

Quantum random walks on congested lattices and the effect of dephasing

PubMed Central

Motes, Keith R.; Gilchrist, Alexei; Rohde, Peter P.

2016-01-01

We consider quantum random walks on congested lattices and contrast them to classical random walks. Congestion is modelled on lattices that contain static defects which reverse the walker’s direction. We implement a dephasing process after each step which allows us to smoothly interpolate between classical and quantum random walks as well as study the effect of dephasing on the quantum walk. Our key results show that a quantum walker escapes a finite boundary dramatically faster than a classical walker and that this advantage remains in the presence of heavily congested lattices. PMID:26812924

Electron-phonon coupling from finite differences

NASA Astrophysics Data System (ADS)

Monserrat, Bartomeu

2018-02-01

The interaction between electrons and phonons underlies multiple phenomena in physics, chemistry, and materials science. Examples include superconductivity, electronic transport, and the temperature dependence of optical spectra. A first-principles description of electron-phonon coupling enables the study of the above phenomena with accuracy and material specificity, which can be used to understand experiments and to predict novel effects and functionality. In this topical review, we describe the first-principles calculation of electron-phonon coupling from finite differences. The finite differences approach provides several advantages compared to alternative methods, in particular (i) any underlying electronic structure method can be used, and (ii) terms beyond the lowest order in the electron-phonon interaction can be readily incorporated. But these advantages are associated with a large computational cost that has until recently prevented the widespread adoption of this method. We describe some recent advances, including nondiagonal supercells and thermal lines, that resolve these difficulties, and make the calculation of electron-phonon coupling from finite differences a powerful tool. We review multiple applications of the calculation of electron-phonon coupling from finite differences, including the temperature dependence of optical spectra, superconductivity, charge transport, and the role of defects in semiconductors. These examples illustrate the advantages of finite differences, with cases where semilocal density functional theory is not appropriate for the calculation of electron-phonon coupling and many-body methods such as the GW approximation are required, as well as examples in which higher-order terms in the electron-phonon interaction are essential for an accurate description of the relevant phenomena. We expect that the finite difference approach will play a central role in future studies of the electron-phonon interaction.

Anharmonic quantum contribution to vibrational dephasing.

PubMed

Barik, Debashis; Ray, Deb Shankar

2004-07-22

Based on a quantum Langevin equation and its corresponding Hamiltonian within a c-number formalism we calculate the vibrational dephasing rate of a cubic oscillator. It is shown that leading order quantum correction due to anharmonicity of the potential makes a significant contribution to the rate and the frequency shift. We compare our theoretical estimates with those obtained from experiments for small diatomics N(2), O(2), and CO.

Electron Spin Dephasing and Decoherence by Interaction with Nuclear Spins in Self-Assembled Quantum Dots

NASA Technical Reports Server (NTRS)

Lee, Seungwon; vonAllmen, Paul; Oyafuso, Fabiano; Klimeck, Gerhard; Whale, K. Birgitta

2004-01-01

Electron spin dephasing and decoherence by its interaction with nuclear spins in self-assembled quantum dots are investigated in the framework of the empirical tight-binding model. Electron spin dephasing in an ensemble of dots is induced by the inhomogeneous precession frequencies of the electron among dots, while electron spin decoherence in a single dot arises from the inhomogeneous precession frequencies of nuclear spins in the dot. For In(x)Ga(1-x) As self-assembled dots containing 30000 nuclei, the dephasing and decoherence times are predicted to be on the order of 100 ps and 1 (micro)s.

Dephasing-covariant operations enable asymptotic reversibility of quantum resources

NASA Astrophysics Data System (ADS)

Chitambar, Eric

2018-05-01

We study the power of dephasing-covariant operations in the resource theories of coherence and entanglement. These are quantum operations whose actions commute with a projective measurement. In the resource theory of coherence, we find that any two states are asymptotically interconvertible under dephasing-covariant operations. This provides a rare example of a resource theory in which asymptotic reversibility can be attained without needing the maximal set of resource nongenerating operations. When extended to the resource theory of entanglement, the resultant operations share similarities with local operations and classical communication, such as prohibiting the increase of all Rényi α -entropies of entanglement under pure-state transformations. However, we show these operations are still strong enough to enable asymptotic reversibility between any two maximally correlated mixed states, even in the multipartite setting.

Interference of qubits in pure dephasing and almost pure dephasing environments

NASA Astrophysics Data System (ADS)

Łobejko, Marcin; Mierzejewski, Marcin; Dajka, Jerzy

2015-07-01

Two-path interference of quantum particles with internal spin (qubits) interacting on one arm of the interferometer with bosonic environment is studied. It is assumed that the energy exchange between the qubit and its environment is either absent, which is a pure dephasing (decoherence) model, or very weak. Both the amplitude and the position of maximum of an output intensity discussed as a function of a phase shift can serve as a quantifier of parameters describing coupling between qubit and its environment. The time evolution of the qubit-environment system is analyzed in the Schrödinger picture and the output intensity for qubit-environment interaction close to pure decoherence is analyzed by means of perturbation theory. Quality of the applied approximation is verified by comparison with numerical results.

Ca(AlH4)2, CaAlH5, and CaH2+6LiBH4: Calculated dehydrogenation enthalpy, including zero point energy, and the structure of the phonon spectra.

PubMed

Marashdeh, Ali; Frankcombe, Terry J

2008-06-21

The dehydrogenation enthalpies of Ca(AlH(4))(2), CaAlH(5), and CaH(2)+6LiBH(4) have been calculated using density functional theory calculations at the generalized gradient approximation level. Harmonic phonon zero point energy (ZPE) corrections have been included using Parlinski's direct method. The dehydrogenation of Ca(AlH(4))(2) is exothermic, indicating a metastable hydride. Calculations for CaAlH(5) including ZPE effects indicate that it is not stable enough for a hydrogen storage system operating near ambient conditions. The destabilized combination of LiBH(4) with CaH(2) is a promising system after ZPE-corrected enthalpy calculations. The calculations confirm that including ZPE effects in the harmonic approximation for the dehydrogenation of Ca(AlH(4))(2), CaAlH(5), and CaH(2)+6LiBH(4) has a significant effect on the calculated reaction enthalpy. The contribution of ZPE to the dehydrogenation enthalpies of Ca(AlH(4))(2) and CaAlH(5) calculated by the direct method phonon analysis was compared to that calculated by the frozen-phonon method. The crystal structure of CaAlH(5) is presented in the more useful standard setting of P2(1)c symmetry and the phonon density of states of CaAlH(5), significantly different to other common complex metal hydrides, is rationalized.

Ca(AlH4)2, CaAlH5, and CaH2+6LiBH4: Calculated dehydrogenation enthalpy, including zero point energy, and the structure of the phonon spectra

NASA Astrophysics Data System (ADS)

Marashdeh, Ali; Frankcombe, Terry J.

2008-06-01

The dehydrogenation enthalpies of Ca(AlH4)2, CaAlH5, and CaH2+6LiBH4 have been calculated using density functional theory calculations at the generalized gradient approximation level. Harmonic phonon zero point energy (ZPE) corrections have been included using Parlinski's direct method. The dehydrogenation of Ca(AlH4)2 is exothermic, indicating a metastable hydride. Calculations for CaAlH5 including ZPE effects indicate that it is not stable enough for a hydrogen storage system operating near ambient conditions. The destabilized combination of LiBH4 with CaH2 is a promising system after ZPE-corrected enthalpy calculations. The calculations confirm that including ZPE effects in the harmonic approximation for the dehydrogenation of Ca(AlH4)2, CaAlH5, and CaH2+6LiBH4 has a significant effect on the calculated reaction enthalpy. The contribution of ZPE to the dehydrogenation enthalpies of Ca(AlH4)2 and CaAlH5 calculated by the direct method phonon analysis was compared to that calculated by the frozen-phonon method. The crystal structure of CaAlH5 is presented in the more useful standard setting of P21/c symmetry and the phonon density of states of CaAlH5, significantly different to other common complex metal hydrides, is rationalized.

Vibronic dephasing model for coherent-to-incoherent crossover in DNA

NASA Astrophysics Data System (ADS)

Karasch, Patrick; Ryndyk, Dmitry A.; Frauenheim, Thomas

2018-05-01

In this paper, we investigate the interplay between coherent and incoherent charge transport in cytosine-guanine (GC-) rich DNA molecules. Our objective is to introduce a physically grounded approach to dephasing in large molecules and to understand the length-dependent charge transport characteristics, and especially the crossover from the coherent tunneling to incoherent hopping regime at different temperatures. Therefore, we apply the vibronic dephasing model and compare the results to the Büttiker probe model which is commonly used to describe decoherence effects in charge transport. Using the full ladder model and simplified one-dimensional model of DNA, we consider molecular junctions with alternating and stacked GC sequences and compare our results to recent experimental measurements.

Fine Structure of the Low-Frequency Raman Phonon Bands of Single-Wall Carbon Nanotubes

NASA Technical Reports Server (NTRS)

Iliev, M. N.; Litvinchuk, A. P.; Arepalli, S.; Nikolaev, P.; Scott, C. D.

1999-01-01

The Raman spectra of singled-wall carbon nanotubes (SWNT) produced by laser and are process were studied between 5 and 500 kappa. The line width vs. temperature dependence of the low-frequency Raman bands between 150 and 200/ cm deviates from that expected for phonon decay through phonon-phonon scattering mechanism. The experimental results and their analysis provided convincing evidence that each of the low-frequency Raman lines is a superposition of several narrower Raman lines corresponding to tubes of nearly the same diameter. The application of Raman spectroscopy to probe the distribution of SWNT by both diameter and chirality is discussed.

Flexural phonon limited phonon drag thermopower in bilayer graphene

NASA Astrophysics Data System (ADS)

Ansari, Mohd Meenhaz; Ashraf, SSZ

2018-05-01

We investigate the phonon drag thermopower from flexural phonons as a function of electron temperature and carrier concentration in the Bloch-Gruneisen regime in non-strained bilayer graphene using Boltzmann transport equation approach. The flexural phonons are expected to be the major source of intrinsic scattering mechanism in unstrained bilayer graphene due to their large density. The flexural phonon modes dispersion relation is quadratic so these low energy flexural phonons abound at room temperature and as a result deform the bilayer graphene sheet in the out of plane direction and affects the transport properties. We also produce analytical result for phonon-drag thermopower from flexural phonons and find that phonon-drag thermopower depicts T2 dependence on temperature and n-1 on carrier concentration.

A new approach to detecting gravitational waves via the coupling of gravity to the zero-point energy of the phonon modes of a superconductor

NASA Astrophysics Data System (ADS)

Inan, Nader A.

The response of a superconductor to a gravitational wave is shown to obey a London-like constituent equation. The Cooper pairs are described by the Ginzburg-Landau free energy density embedded in curved spacetime. The lattice ions are modeled by quantum harmonic oscillators characterized by quasi-energy eigenvalues. This formulation is shown to predict a dynamical Casimir effect since the zero-point energy of the ionic lattice phonons is modulated by the gravitational wave. It is also shown that the response to a gravitational wave is far less for the Cooper pair density than for the ionic lattice. This predicts a “charge separation effect” which can be used to detect the passage of a gravitational wave.

Quantum Metrology beyond the Classical Limit under the Effect of Dephasing

NASA Astrophysics Data System (ADS)

Matsuzaki, Yuichiro; Benjamin, Simon; Nakayama, Shojun; Saito, Shiro; Munro, William J.

2018-04-01

Quantum sensors have the potential to outperform their classical counterparts. For classical sensing, the uncertainty of the estimation of the target fields scales inversely with the square root of the measurement time T . On the other hand, by using quantum resources, we can reduce this scaling of the uncertainty with time to 1 /T . However, as quantum states are susceptible to dephasing, it has not been clear whether we can achieve sensitivities with a scaling of 1 /T for a measurement time longer than the coherence time. Here, we propose a scheme that estimates the amplitude of globally applied fields with the uncertainty of 1 /T for an arbitrary time scale under the effect of dephasing. We use one-way quantum-computing-based teleportation between qubits to prevent any increase in the correlation between the quantum state and its local environment from building up and have shown that such a teleportation protocol can suppress the local dephasing while the information from the target fields keeps growing. Our method has the potential to realize a quantum sensor with a sensitivity far beyond that of any classical sensor.

Frequency-domain coherent multidimensional spectroscopy when dephasing rivals pulsewidth: Disentangling material and instrument response

DOE PAGES

Kohler, Daniel D.; Thompson, Blaise J.; Wright, John C.

2017-08-31

Ultrafast spectroscopy is often collected in the mixed frequency/time domain, where pulse durations are similar to system dephasing times. In these experiments, expectations derived from the familiar driven and impulsive limits are not valid. This work simulates the mixed-domain four-wave mixing response of a model system to develop expectations for this more complex field-matter interaction. We also explore frequency and delay axes. We show that these line shapes are exquisitely sensitive to excitation pulse widths and delays. Near pulse overlap, the excitation pulses induce correlations that resemble signatures of dynamic inhomogeneity. We describe these line shapes using an intuitive picturemore » that connects to familiar field-matter expressions. We develop strategies for distinguishing pulse-induced correlations from true system inhomogeneity. Our simulations provide a foundation for interpretation of ultrafast experiments in the mixed domain.« less

Phonon induced magnetism in ionic materials

NASA Astrophysics Data System (ADS)

Restrepo, Oscar D.; Antolin, Nikolas; Jin, Hyungyu; Heremans, Joseph P.; Windl, Wolfgang

2014-03-01

Thermoelectric phenomena in magnetic materials create exciting possibilities in future spin caloritronic devices by manipulating spin information using heat. An accurate understanding of the spin-lattice interactions, i.e. the coupling between magnetic excitations (magnons) and lattice vibrations (phonons), holds the key to unraveling their underlying physics. We report ab initio frozen-phonon calculations of CsI that result in non-zero magnetization when the degeneracy between spin-up and spin-down electronic density of states is lifted for certain phonon displacement patterns. For those, the magnetization as a function of atomic displacement shows a sharp resonance due to the electronic states on the displaced Cs atoms, while the electrons on indium form a continuous background magnetization. We relate this resonance to the generation of a two-level system in the spin-polarized Cs partial density of states as a function of displacement, which we propose to be described by a simple resonant-susceptibility model. Current work extends these investigations to semiconductors such as InSb. ODR and WW are supported by the Center for Emergent Materials, an NSF MRSEC at OSU (Grant DMR-0820414).HJ and JPH are supported by AFOSR MURI Cryogenic Peltier Cooling, Contract #FA9550-10-1-0533.

Picosecond coherent anti-Stokes Raman scattering (CARS) study of vibrational dephasing of carbon disulfide and benzene in solution

NASA Technical Reports Server (NTRS)

Perry, Joseph W.; Woodward, Anne M.; Stephenson, John C.

1986-01-01

The vibrational dephasing of the 656/cm mode (nu1, a1g) of CS2 and the 991/cm mode (nu2, a1g) of benzene have been studied as a function of concentration in mixtures with a number of solvents using a ps time-resolved CARS technique. This technique employs two tunable synchronously-pumped mode-locked dye lasers in a stimulated Raman pump, coherent anti-Stokes Raman probe time-resolved experiment. Results are obtained for CS2 in carbon tetrachloride, benzene, nitrobenzene, and ethanol and for benzene nu2 in CS2. The dephasing rates of CS2 nu1 increase on dilution with the polar solvents and decrease or remain constant on dilution with the nonpolar solvents. The CS2/benzene solutions show a contrasting behavior, with the CS2 nu1 dephasing rate being nearly independent of concentration whereas the benzene nu2 dephasing rate decreases on dilution. These results are compared to theoretical models for vibrational dephasing of polyatomic molecules in solution.

Phonon cross-plane transport and thermal boundary resistance: effect of heat source size and thermal boundary resistance on phonon characteristics

NASA Astrophysics Data System (ADS)

Ali, H.; Yilbas, B. S.

2016-09-01

Phonon cross-plane transport across silicon and diamond thin films pair is considered, and thermal boundary resistance across the films pair interface is examined incorporating the cut-off mismatch and diffusive mismatch models. In the cut-off mismatch model, phonon frequency mismatch for each acoustic branch is incorporated across the interface of the silicon and diamond films pair in line with the dispersion relations of both films. The frequency-dependent and transient solution of the Boltzmann transport equation is presented, and the equilibrium phonon intensity ratios at the silicon and diamond film edges are predicted across the interface for each phonon acoustic branch. Temperature disturbance across the edges of the films pair is incorporated to assess the phonon transport characteristics due to cut-off and diffusive mismatch models across the interface. The effect of heat source size, which is allocated at high-temperature (301 K) edge of the silicon film, on the phonon transport characteristics at the films pair interface is also investigated. It is found that cut-off mismatch model predicts higher values of the thermal boundary resistance across the films pair interface as compared to that of the diffusive mismatch model. The ratio of equilibrium phonon intensity due to the cut-off mismatch over the diffusive mismatch models remains >1 at the silicon edge, while it becomes <1 at the diamond edge for all acoustic branches.

Dissipation, dephasing and quantum Darwinism in qubit systems with random unitary interactions

NASA Astrophysics Data System (ADS)

Balaneskovic, Nenad; Mendler, Marc

2016-09-01

We investigate the influence of dissipation and decoherence on quantum Darwinism by generalizing Zurek's original qubit model of decoherence and the establishment of pointer states [W.H. Zurek, Nat. Phys. 5, 181 (2009); see also arXiv: quant-ph/0707.2832v1, pp. 14-19.]. Our model allows for repeated multiple qubit-qubit couplings between system and environment which are described by randomly applied two-qubit quantum operations inducing entanglement, dissipation and dephasing. The resulting stationary qubit states of system and environment are investigated. They exhibit the intricate influence of entanglement generation, dissipation and dephasing on this characteristic quantum phenomenon.

Thermal dephasing in second-harmonic generation of an amplified copper-vapor laser beam in beta barium borate.

PubMed

Prakash, Om; Dixit, Sudhir Kumar; Bhatnagar, Rajiva

2005-03-20

The conversion efficiency in second-harmonic generation of an amplified beam in a master-oscillator power amplifier copper-vapor laser (CVL) is lower than that of the oscillator beam alone. This lower efficiency is often vaguely attributed to wave-front degradation in the amplifier. We investigate the role of wave-front degradation and thermal dephasing in the second-harmonic generation of a CVL from a beta-barium borate crystal. Choosing two beams with constant intrapulse divergence, one from a generalized diffraction filtered resonator master oscillator alone and other obtained by amplifying oscillator by use of a power amplifier, we show that at low flux levels the decrease in efficiency is due to wave-front degradation. At a fundamental power above the critical power for thermal dephasing, the decrease is due to increased UV absorption and consequent thermal dephasing. Thermal dephasing is higher for the beam with the lower coherence width.

Unified phonon-based approach to the thermodynamics of solid, liquid and gas states

NASA Astrophysics Data System (ADS)

Bolmatov, Dima; Zav'yalov, Dmitry; Zhernenkov, Mikhail; Musaev, Edvard T.; Cai, Yong Q.

2015-12-01

We introduce a unified approach to states of matter (solid, liquid and gas) and describe the thermodynamics of the pressure-temperature phase diagram in terms of phonon excitations. We derive the effective Hamiltonian with low-energy cutoff in two transverse phonon polarizations (phononic band gaps) by breaking the symmetry in phonon interactions. Further, we construct the statistical mechanics of states of aggregation employing the Debye approximation. The introduced formalism covers the Debye theory of solids, the phonon theory of liquids, and thermodynamic limits such as the Dulong-Petit thermodynamic limit (cV = 3kB), the ideal gas limit (cV =3/2 kB) and the new thermodynamic limit (cV = 2kB), dubbed here the Frenkel line thermodynamic limit. We discuss the phonon propagation and localization effects in liquids above and below the Frenkel line, and explain the "fast sound" phenomenon. As a test for our theory we calculate velocity-velocity autocorrelation and pair distribution functions within the Green-Kubo formalism. We show the consistency between dynamics of phonons and pair correlations in the framework of the unified approach. New directions towards advancements in phononic band gaps engineering, hypersound manipulation technologies and exploration of exotic behaviour of fluids relevant to geo- and planetary sciences are discussed. The presented results are equally important both for practical implications and for fundamental research.

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.

Equivalence of qubit-environment entanglement and discord generation via pure dephasing interactions and the resulting consequences

NASA Astrophysics Data System (ADS)

Roszak, Katarzyna; Cywiński, Łukasz

2018-01-01

We find that when a qubit initialized in a pure state experiences pure dephasing due to interaction with an environment, separable qubit-environment states generated during the evolution also have zero quantum discord with respect to the environment. What follows is that the set of separable states which can be reached during the evolution has zero volume, and hence, such effects as sudden death of qubit-environment entanglement are very unlikely. In the case of the discord with respect to the qubit, a vast majority of qubit-environment separable states is discordant, but in specific situations zero-discord states are possible. This is conceptually important since there is a connection between the discordance with respect to a given subsystem and the possibility of describing the evolution of this subsystem using completely positive maps. Finally, we use the formalism to find an exemplary evolution of an entangled state of two qubits that is completely positive, and occurs solely due to interaction of only one of the qubits with its environment (so one could guess that it corresponds to a local operation, since it is local in a physical sense), but which nevertheless causes the enhancement of entanglement between the qubits. While this simply means that the considered evolution is completely positive, but does not belong to local operations and classical communication, it shows how much caution has to be exercised when identifying evolution channels that belong to that class.

Electronic properties with and without electron-phonon coupling

NASA Astrophysics Data System (ADS)

Allen, Philip

To decent approximation, electronic properties P of solids have a temperature dependence of the type ΔP(T) = Σ (dP/dωi) [ni(T) +1/2], where ωi is the frequency of the ith vibrational normal mode, and ni is the Bose-Einstein equilibrium occupation of the mode. The coupling constant (dP/dωi) comes from electron-phonon interactions. At T =0, the ``1/2'' gives the zero-point electron-phonon renormalization of the property P, and at T>ΘD, the total shift ΔP becomes linear in T, extrapolating toward ΔP =0 at T =0. This form of T-dependence arises from the adiabatic or Born-Oppenheimer approximation, where electrons essentially ``don't notice'' the time-dependence of thermal lattice fluctuations. In other words, the leading order theory for P is ΔP(T) = Σ (d2P/duiduj), responding to the thermal average mean square lattice displacement, as if it were static. There are two situations where non-adiabatic effects alter things. (1) In metals at low T, the thermal smearing kBT of the sharp Fermi edge gets small (ωi <phonon energy to be included in perturbative denominators. (2) In insulators with polar phonons, Froehlich polaron effects enter, and k-integrals diverge unless phonon energies are kept. Most non-adiabatic effects become unimportant by room temperature, but the low T consequences can be very interesting (e.g. superconductivity.) This talk will discuss the confusing history and predict some future developments in this field. invited session: ''Predictive Modeling of Electron-Phonon Coupling in Condensed-Matter Physics'' My talk will be coordinated with that of Xavier Gonze. It would be best to schedule them back-to-back.

Polariton-acoustic-phonon interaction in a semiconductor microcavity

NASA Astrophysics Data System (ADS)

Cassabois, G.; Triques, A. L. C.; Bogani, F.; Delalande, C.; Roussignol, Ph.; Piermarocchi, C.

2000-01-01

The broadening of polariton lines by acoustic phonons is investigated in a semiconductor microcavity by means of interferometric correlation measurements with subpicosecond resolution. A decrease of the polariton-acoustic phonon coupling is clearly observed for the lower polariton branch as one approaches the resonance between exciton and photon states. This behavior cannot be explained in terms of a semiclassical linear dispersion theory but requires a full quantum description of the microcavity in the strong-coupling regime.

Lattice vibrations in the Frenkel-Kontorova model. I. Phonon dispersion, number density, and energy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Meng, Qingping; Wu, Lijun; Welch, David O.

2015-06-17

We studied the lattice vibrations of two inter-penetrating atomic sublattices via the Frenkel-Kontorova (FK) model of a linear chain of harmonically interacting atoms subjected to an on-site potential, using the technique of thermodynamic Green's functions based on quantum field-theoretical methods. General expressions were deduced for the phonon frequency-wave-vector dispersion relations, number density, and energy of the FK model system. In addition, as the application of the theory, we investigated in detail cases of linear chains with various periods of the on-site potential of the FK model. Some unusual but interesting features for different amplitudes of the on-site potential of themore » FK model are discussed. In the commensurate structure, the phonon spectrum always starts at a finite frequency, and the gaps of the spectrum are true ones with a zero density of modes. In the incommensurate structure, the phonon spectrum starts from zero frequency, but at a non-zero wave vector; there are some modes inside these gap regions, but their density is very low. In our approximation, the energy of a higher-order commensurate state of the one-dimensional system at a finite temperature may become indefinitely close to the energy of an incommensurate state. This finding implies that the higher-order incommensurate-commensurate transitions are continuous ones and that the phase transition may exhibit a “devil's staircase” behavior at a finite temperature.« less

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.

Convergence of Ground and Excited State Properties of Divacancy Defects in 4H-SiC with Computational Cell Size

DTIC Science & Technology

2018-03-01

computational parameters needs to be established. We used density functional theory to compute defect formation energies of the neutral and charged hh... energies for the 3A to 3E transition (absorption, zero phonon lines, and emission), which is essential for optical initialization and read-out. We...PBE, defect formation energy , charge transition levels, absorption, zero phonon lines, emission 16. SECURITY CLASSIFICATION OF: 17. LIMITATION

Bright optical centre in diamond with narrow, highly polarised and nearly phonon-free fluorescence at room temperature

NASA Astrophysics Data System (ADS)

John, Roger; Lehnert, Jan; Mensing, Michael; Spemann, Daniel; Pezzagna, Sébastien; Meijer, Jan

2017-05-01

Using shallow implantation of ions and molecules with masses centred at 27 atomic mass units (amu) in diamond, a new artificial optical centre with unique properties has been created. The centre shows a linearly polarised fluorescence with a main narrow emission line mostly found at 582 nm, together with a weak vibronic sideband at room temperature. The fluorescence lifetime is ∼2 ns and the brightest centres are more than three times brighter than the nitrogen-vacancy centres. A majority of the centres shows stable fluorescence whereas some others present a blinking behaviour, at faster or slower rates. Furthermore, a second kind of optical centre has been simultaneously created in the same diamond sample, within the same ion implantation run. This centre has a narrow zero-phonon line (ZPL) at ∼546 nm and a broad phonon sideband at room temperature. Interestingly, optically detected magnetic resonance (ODMR) has been measured on several single 546 nm centres and two resonance peaks are found at 0.99 and 1.27 GHz. In view of their very similar ODMR and optical spectra, the 546 nm centre is likely to coincide with the ST1 centre, reported once (with a ZPL at 550 nm), but of still unknown nature. These new kinds of centres are promising for quantum information processing, sub-diffraction optical imaging or use as single-photon sources.

Detecting the phonon spin in magnon-phonon conversion experiments

NASA Astrophysics Data System (ADS)

Holanda, J.; Maior, D. S.; Azevedo, A.; Rezende, S. M.

2018-05-01

Recent advances in the emerging field of magnon spintronics have stimulated renewed interest in phenomena involving the interaction between spin waves, the collective excitations of spins in magnetic materials that quantize as magnons, and the elastic waves that arise from excitations in the crystal lattice, which quantize as phonons. In magnetic insulators, owing to the magnetostrictive properties of materials, spin waves can become strongly coupled to elastic waves, forming magnetoelastic waves—a hybridized magnon-phonon excitation. While several aspects of this interaction have been subject to recent scrutiny, it remains unclear whether or not phonons can carry spin. Here we report experiments on a film of the ferrimagnetic insulator yttrium iron garnet under a non-uniform magnetic field demonstrating the conversion of coherent magnons generated by a microwave field into phonons that have spin. While it is well established that photons in circularly polarized light carry a spin, the spin of phonons has had little attention in the literature. By means of wavevector-resolved Brillouin light-scattering measurements, we show that the magnon-phonon conversion occurs with constant energy and varying linear momentum, and that the light scattered by the phonons is circularly polarized, thus demonstrating that the phonons have spin.

Towards homonuclear J solid-state NMR correlation experiments for half-integer quadrupolar nuclei: experimental and simulated 11B MAS spin-echo dephasing and calculated 2J(BB) coupling constants for lithium diborate.

PubMed

Barrow, Nathan S; Yates, Jonathan R; Feller, Steven A; Holland, Diane; Ashbrook, Sharon E; Hodgkinson, Paul; Brown, Steven P

2011-04-07

Magic-angle spinning (MAS) NMR spin-echo dephasing is systematically investigated for the spin I = 3/2 (11)B nucleus in lithium diborate, Li(2)O·2B(2)O(3). A clear dependence on the quadrupolar frequency (ω(Q)(PAS)/2π = 3C(Q)/[4I(2I- 1)]) is observed: the B3 (larger C(Q)) site dephases more slowly than the B4 site at all investigated MAS frequencies (5 to 20 kHz) at 14.1 T. Increasing the MAS frequency leads to markedly slower dephasing for the B3 site, while there is a much less evident effect for the B4 site. Considering samples at 5, 25, 80 (natural abundance) and 100% (11)B isotopic abundance, dephasing becomes faster for both sites as the (11)B isotopic abundance increases. The experimental behaviour is rationalised using density matrix simulations for two and three dipolar-coupled (11)B nuclei. The experimentally observed slower dephasing for the larger C(Q) (B3) site is reproduced in all simulations and is explained by the reintroduction of the dipolar coupling by the so-called "spontaneous quadrupolar-driven recoupling mechanism" having a different dependence on the MAS frequency for different quadrupolar frequencies. Specifically, isolated spin-pair simulations show that the spontaneous quadrupolar-driven recoupling mechanism is most efficient when the quadrupolar frequency is equal to twice the MAS frequency. While for isolated spin-pair simulations, increasing the MAS frequency leads to faster dephasing, agreement with experiment is observed for three-spin simulations which additionally include the homogeneous nature of the homonuclear dipolar coupling network. First-principles calculations, using the GIPAW approach, of the (2)J(11B-11B) couplings in lithium diborate, metaborate and triborate are presented: a clear trend is revealed whereby the (2)J(11B-11B) couplings increase with increasing B-O-B bond angle and B-B distance. However, the calculated (2)J(11B-11B) couplings are small (0.95, 1.20 and 2.65 Hz in lithium diborate), thus explaining why no

Temperature dependence of resonant secondary emission in NaNO 2: Spectral behavior

NASA Astrophysics Data System (ADS)

Kato, Riso; Kawaguchi, Yoshizo; Ashida, Masaaki

1990-05-01

Spectral behavior of resonant secondary emission in NaNO 2 has been investigated in the temperature range from 2 to 30 K under the excitation near the v00 line of the lowest singlet absorption. With increasing temperature, luminescence lines separated from multiple-order Raman lines become detectable even under the excitation with the off-resonance energy Δ c ≳ 13 cm -1. The intensity of the luminescence line IL( T) increases with temperature in proportion to the phonon number n( hvp, T) in the temperature range T ≲ Δ c/ k, while it increases more steeply in the range T ≳ Δ c/ k. The temperature dependence of IL( T) is ascribed to the increase in the luminescence from the v00 level after the one-phonon assisted transition to the level induced by the off-resonant incident light. The intensity of the Raman line IR( T) decreases gradually in 2-12 K range and shows rapid drop above 12 K. The temperature dependence of IR( T) is ascribed to the dephasing of the intermediate state due to the two-phonon interaction with the reservoir.

Vibrational dephasing and frequency shifts of hydrogen-bonded pyridine-water complexes

NASA Astrophysics Data System (ADS)

Kalampounias, A. G.; Tsilomelekis, G.; Boghosian, S.

2015-01-01

In this paper we present the picosecond vibrational dynamics and Raman shifts of hydrogen-bonded pyridine-water complexes present in aqueous solutions in a wide concentration range from dense to extreme dilute solutions. We studied the vibrational dephasing and vibrational frequency modulation by calculating time correlation functions of vibrational relaxation by fits in the frequency domain. The concentration induced variations in bandwidths, band frequencies and characteristic dephasing times have been estimated and interpreted as effects due to solute-solvent interactions. The time-correlation functions of vibrational dephasing were obtained for the ring breathing mode of both "free" and hydrogen-bonded pyridine molecules and it was found that sufficiently deviate from the Kubo model. There is a general agreement in the whole concentration range with the modeling proposed by the Rothschild approach, which applies to complex liquids. The results have shown that the reorientation of pyridine aqueous solutions is very slow and hence in both scattering geometries only vibrational dephasing is probed. It is proposed that the spectral changes depend on the perturbations induced by the dynamics of the water molecules in the first hydration cell and water in bulk, while at extreme dilution conditions, the number of bulk water molecules increases and the interchange between molecules belonging to the first hydration cell may not be the predominant modulation mechanism. The evolution of several parameters, such as the characteristic times, the percentage of Gaussian character in the peak shape and the a parameter are indicative of drastic variations at extreme dilution revealing changes in the vibrational relaxation of the pyridine complexes in the aqueous environment. The higher dilution is correlated to diffusion of water molecules into the reference pyridine system in agreement with the jump diffusion model, while at extreme dilutions, almost all pyridine molecules are

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

Enhancement of phonon backscattering due to confinement of ballistic phonon pathways in silicon as studied with a microfabricated phonon spectrometer

DOE Office of Scientific and Technical Information (OSTI.GOV)

Otelaja, O. O.; Robinson, R. D., E-mail: rdr82@cornell.edu

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) aroundmore » 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.« less

Single-shot gas-phase thermometry by time-to-frequency mapping of coherence dephasing.

PubMed

Yue, Orin; Bremer, Marshall T; Pestov, Dmitry; Gord, James R; Roy, Sukesh; Dantus, Marcos

2012-08-09

We demonstrate a single-beam coherent anti-Stokes Raman scattering (CARS) technique for gas-phase thermometry that assesses the species-specific local gas temperature by single-shot time-to-frequency mapping of Raman-coherence dephasing. The proof-of-principle experiments are performed with air in a temperature-controlled gas cell. Impulsive excitation of molecular vibrations by an ultrashort pump/Stokes pulse is followed by multipulse probing of the 2330 cm(-1) Raman transition of N(2). This sequence of colored probe pulses, delayed in time with respect to each other and corresponding to three isolated spectral bands, imprints the coherence dephasing onto the measured CARS spectrum. For calibration purposes, the dephasing rates are recorded at various gas temperatures, and the relationship is fitted to a linear regression. The calibration data are then used to determine the gas temperature and are shown to provide better than 15 K accuracy. The described approach is insensitive to pulse energy fluctuations and can, in principle, gauge the temperature of multiple chemical species in a single laser shot, which is deemed particularly valuable for temperature profiling of reacting flows in gas-turbine combustors.

Ultrafast atomic-scale visualization of acoustic phonons generated by optically excited quantum dots

PubMed Central

Vanacore, Giovanni M.; Hu, Jianbo; Liang, Wenxi; Bietti, Sergio; Sanguinetti, Stefano; Carbone, Fabrizio; Zewail, Ahmed H.

2017-01-01

Understanding the dynamics of atomic vibrations confined in quasi-zero dimensional systems is crucial from both a fundamental point-of-view and a technological perspective. Using ultrafast electron diffraction, we monitored the lattice dynamics of GaAs quantum dots—grown by Droplet Epitaxy on AlGaAs—with sub-picosecond and sub-picometer resolutions. An ultrafast laser pulse nearly resonantly excites a confined exciton, which efficiently couples to high-energy acoustic phonons through the deformation potential mechanism. The transient behavior of the measured diffraction pattern reveals the nonequilibrium phonon dynamics both within the dots and in the region surrounding them. The experimental results are interpreted within the theoretical framework of a non-Markovian decoherence, according to which the optical excitation creates a localized polaron within the dot and a travelling phonon wavepacket that leaves the dot at the speed of sound. These findings indicate that integration of a phononic emitter in opto-electronic devices based on quantum dots for controlled communication processes can be fundamentally feasible. PMID:28852685

A novel dipolar dephasing method for the slow magic angle turning experiment.

PubMed

Hu, J Z; Taylor, C M; Pugmire, R J; Grant, D M

2001-09-01

Complete suppression of the resonances from protonated carbons in a slow magic angle spinning experiment can be achieved using five dipolar dephasing (Five-DD) periods distributed in one rotor period. This produces a spectrum containing only the spinning sidebands (SSB) from the nonprotonated carbons. It is shown that the SSB patterns corresponding to the nonprotonated carbons are not distorted over a wide range of dipolar dephasing times. Hence, this method can be used to obtain reliable principal values of the chemical shift tensors for each nonprotonated carbon. The Five-DD method can be readily incorporated into isotropic-anisotropic 2D experiments such as FIREMAT and 2D-PASS to facilitate the measurement of the (13)C chemical shift tensors in complex systems. Copyright 2001 Academic Press.

Spin coherence and dephasing of localized electrons in monolayer MoS 2

DOE PAGES

Yang, Luyi; Chen, Weibing; McCreary, Kathleen M.; ...

2015-11-10

Here, we report a systematic study of coherent spin precession and spin dephasing in electron-doped monolayer MoS 2. Using time-resolved Kerr rotation spectroscopy and applied in-plane magnetic fields, a nanosecond time scale Larmor spin precession signal commensurate with g-factor |g 0| ≃ 1.86 is observed in several different MoS 2 samples grown by chemical vapor deposition. The dephasing rate of this oscillatory signal increases linearly with magnetic field, suggesting that the coherence arises from a subensemble of localized electron spins having an inhomogeneously broadened distribution of g-factors, g 0 + Δg. In contrast to g 0, Δg is sample-dependent andmore » ranges from 0.042 to 0.115.« less

Relaxometry and Dephasing Imaging of Superparamagnetic Magnetite Nanoparticles Using a Single Qubit.

PubMed

Schmid-Lorch, Dominik; Häberle, Thomas; Reinhard, Friedemann; Zappe, Andrea; Slota, Michael; Bogani, Lapo; Finkler, Amit; Wrachtrup, Jörg

2015-08-12

To study the magnetic dynamics of superparamagnetic nanoparticles, we use scanning probe relaxometry and dephasing of the nitrogen vacancy (NV) center in diamond, characterizing the spin noise of a single 10 nm magnetite particle. Additionally, we show the anisotropy of the NV sensitivity's dependence on the applied decoherence measurement method. By comparing the change in relaxation (T1) and dephasing (T2) time in the NV center when scanning a nanoparticle over it, we are able to extract the nanoparticle's diameter and distance from the NV center using an Ornstein-Uhlenbeck model for the nanoparticle's fluctuations. This scanning probe technique can be used in the future to characterize different spin label substitutes for both medical applications and basic magnetic nanoparticle behavior.

Relaxometry and Dephasing Imaging of Superparamagnetic Magnetite Nanoparticles Using a Single Qubit

NASA Astrophysics Data System (ADS)

Schmid-Lorch, Dominik; Häberle, Thomas; Reinhard, Friedemann; Zappe, Andrea; Slota, Michael; Bogani, Lapo; Finkler, Amit; Wrachtrup, Jörg

2015-08-01

To study the magnetic dynamics of superparamagnetic nanoparticles we use scanning probe relaxometry and dephasing of the nitrogen-vacancy (NV) center in diamond, characterizing the spin-noise of a single 10-nm magnetite particle. Additionally, we show the anisotropy of the NV sensitivity's dependence on the applied decoherence measurement method. By comparing the change in relaxation (T 1 ) and dephasing (T 2 ) time in the NV center when scanning a nanoparticle over it, we are able to extract the nanoparticle's diameter and distance from the NV center using an Ornstein-Uhlenbeck model for the nanoparticle's fluctuations. This scanning-probe technique can be used in the future to characterize different spin label substitutes for both medical applications and basic magnetic nanoparticle behavior.

Visualizing the Zero-Potential Line of Bipolar Electrodes with Arbitrary Geometry.

PubMed

Li, Meng; Liu, Shasha; Jiang, Yingyan; Wang, Wei

2018-06-05

In a typical bipolar electrochemistry (BPE) configuration, voltage applied between the two driving electrodes induced a potential drop through solution filled in the microchannel, resulting in an interfacial potential difference between solution and BPE varied along the BPE. In the present work, we employed a recently developed plasmonic imaging technique to map the distribution of surface potential of bipolar electrodes with various geometries including round, triangle, hexagon, star, and rhombus shapes under the nonfaradaic charging process, from which the line of zero potential (LZP) was visualized and determined. We further investigated the dependence of LZP on electrode geometry and the distribution of external electric field and explained the experimental results with a charge balance mechanism. The triangular and star-shaped BPEs show quite different LZP features from the other ones with symmetrical geometry. These experimentally obtained potential distributions are all in good agreement with electromagnetic simulations. Finally, the line of zero overpotential (LZO) of the triangular-shaped BPE under faradaic reactions were investigated. The results confirm the shift of LZO when faradaic reactions occurred at the corresponding ends of BPE. The present work demonstrates the first experimental capability to map the potential distribution of BPE with arbitrary geometry under an arbitrary driving field. It is anticipated to help the design and optimization on the geometry of electrodes and microchannels with implications for boosting their applications in chemical sensing and materials synthesis.

Helium induced fine structure in the electronic spectra of anthracene derivatives doped into superfluid helium nanodroplets

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pentlehner, D.; Slenczka, A., E-mail: alkwin.slenczka@chemie.uni-regensburg.de

2015-01-07

Electronic spectra of organic molecules doped into superfluid helium nanodroplets show characteristic features induced by the helium environment. Besides a solvent induced shift of the electronic transition frequency, in many cases, a spectral fine structure can be resolved for electronic and vibronic transitions which goes beyond the expected feature of a zero phonon line accompanied by a phonon wing as known from matrix isolation spectroscopy. The spectral shape of the zero phonon line and the helium induced phonon wing depends strongly on the dopant species. Phonon wings, for example, are reported ranging from single or multiple sharp transitions to broadmore » (Δν > 100 cm{sup −1}) diffuse signals. Despite the large number of example spectra in the literature, a quantitative understanding of the helium induced fine structure of the zero phonon line and the phonon wing is missing. Our approach is a systematic investigation of related molecular compounds, which may help to shed light on this key feature of microsolvation in superfluid helium droplets. This paper is part of a comparative study of the helium induced fine structure observed in electronic spectra of anthracene derivatives with particular emphasis on a spectrally sharp multiplet splitting at the electronic origin. In addition to previously discussed species, 9-cyanoanthracene and 9-chloroanthracene will be presented in this study for the first time.« less

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 fQ m ∼ 2 × 10 12 Hz achieved here is larger than k B T base /h, which may enable the future realization of Rabi oscillations in the quantum regime.

Temperature-tunable Fano resonance induced by strong coupling between Weyl fermions and phonons in TaAs

DOE PAGES

Xu, Bing; Dai, Yaomin M.; Zhao, Lingxiao X.; ...

2017-03-30

Strong coupling between discrete phonon and continuous electron–hole pair excitations can induce a pronounced asymmetry in the phonon line shape, known as the Fano resonance. This effect has been observed in various systems. We reveal explicit evidence for strong coupling between an infrared-active phonon and electronic transitions near the Weyl points through the observation of a Fano resonance in the Weyl semimetal TaAs. The resulting asymmetry in the phonon line shape, conspicuous at low temperatures, diminishes continuously with increasing temperature. Furthermore, this behaviour originates from the suppression of electronic transitions near the Weyl points due to the decreasing occupation ofmore » electronic states below the Fermi level (EF) with increasing temperature, as well as Pauli blocking caused by thermally excited electrons above EF. These findings not only elucidate the mechanism governing the tunable Fano resonance but also open a route for exploring exotic physical phenomena through phonon properties in Weyl semimetals.« less

Phonon spectra, electronic, and thermodynamic properties of WS2 nanotubes.

PubMed

Evarestov, Robert A; Bandura, Andrei V; Porsev, Vitaly V; Kovalenko, Alexey V

2017-11-15

Hybrid density functional theory calculations are performed for the first time on the phonon dispersion and thermodynamic properties of WS 2 -based single-wall nanotubes. Symmetry analysis is presented for phonon modes in nanotubes using the standard (crystallographic) factorization for line groups. Symmetry and the number of infra-red and Raman active modes in achiral WS 2 nanotubes are given for armchair and zigzag chiralities. It is demonstrated that a number of infrared and Raman active modes is independent on the nanotube diameter. The zone-folding approach is applied to find out an impact of curvature on electron and phonon band structure of nanotubes rolled up from the monolayer. Phonon frequencies obtained both for layers and nanotubes are used to compute the thermal contributions to their thermodynamic functions. The temperature dependences of energy, entropy, and heat capacity of nanotubes are estimated with respect to those of the monolayer. The role of phonons in the stability estimation of nanotubes is discussed based on Helmholtz free energy calculations. © 2017 Wiley Periodicals, Inc. © 2017 Wiley Periodicals, Inc.

Improvement in T2* via Cancellation of Spin Bath Induced Dephasing in Solid-State Spins

NASA Astrophysics Data System (ADS)

Bauch, Erik; Hart, Connor; Schloss, Jennifer; Turner, Matthew; Barry, John; Walsworth, Ronald L.

2017-04-01

In measurements using ensembles of nitrogen vacancy (NV) centers in diamond, the magnetic field sensitivity can be improved by increasing the NV spin dephasing time, T2*. For NV ensembles, T2* is limited by dephasing arising from variations in the local environment sensed by individual NVs, such as applied magnetic fields, noise induced by other nearby spins, and strain. Here, we describe a systematic study of parameters influencing the NV ensemble T2*, and efforts to mitigate sources of inhomogeneity with demonstrated T2* improvements exceeding one order of magnitude.

Evidence of an Improper Displacive Phase Transition in Cd2 Re2 O7 via Time-Resolved Coherent Phonon Spectroscopy

NASA Astrophysics Data System (ADS)

Harter, J. W.; Kennes, D. M.; Chu, H.; de la Torre, A.; Zhao, Z. Y.; Yan, J.-Q.; Mandrus, D. G.; Millis, A. J.; Hsieh, D.

2018-01-01

We have used a combination of ultrafast coherent phonon spectroscopy, ultrafast thermometry, and time-dependent Landau theory to study the inversion symmetry breaking phase transition at Tc=200 K in the strongly spin-orbit coupled correlated metal Cd2 Re2 O7 . We establish that the structural distortion at Tc is a secondary effect through the absence of any softening of its associated phonon mode, which supports a purely electronically driven mechanism. However, the phonon lifetime exhibits an anomalously strong temperature dependence that decreases linearly to zero near Tc. We show that this behavior naturally explains the spurious appearance of phonon softening in previous Raman spectroscopy experiments and should be a prevalent feature of correlated electron systems with linearly coupled order parameters.

Lattice vibrations in the Frenkel-Kontorova model. I. Phonon dispersion, number density, and energy

NASA Astrophysics Data System (ADS)

Meng, Qingping; Wu, Lijun; Welch, David O.; Zhu, Yimei

2015-06-01

We studied the lattice vibrations of two interpenetrating atomic sublattices via the Frenkel-Kontorova (FK) model of a linear chain of harmonically interacting atoms subjected to an on-site potential using the technique of thermodynamic Green's functions based on quantum field-theoretical methods. General expressions were deduced for the phonon frequency-wave-vector dispersion relations, number density, and energy of the FK model system. As the application of the theory, we investigated in detail cases of linear chains with various periods of the on-site potential of the FK model. Some unusual but interesting features for different amplitudes of the on-site potential of the FK model are discussed. In the commensurate structure, the phonon spectrum always starts at a finite frequency, and the gaps of the spectrum are true ones with a zero density of modes. In the incommensurate structure, the phonon spectrum starts from zero frequency, but at a nonzero wave vector; there are some modes inside these gap regions, but their density is very low. In our approximation, the energy of a higher-order commensurate state of the one-dimensional system at a finite temperature may become indefinitely close to the energy of an incommensurate state. This finding implies that the higher-order incommensurate-commensurate transitions are continuous ones and that the phase transition may exhibit a "devil's staircase" behavior at a finite temperature.

Observation of chiral phonons

NASA Astrophysics Data System (ADS)

Zhu, Hanyu; Yi, Jun; Li, Ming-Yang; Xiao, Jun; Zhang, Lifa; Yang, Chih-Wen; Kaindl, Robert A.; Li, Lain-Jong; Wang, Yuan; Zhang, Xiang

2018-02-01

Chirality reveals symmetry breaking of the fundamental interaction of elementary particles. In condensed matter, for example, the chirality of electrons governs many unconventional transport phenomena such as the quantum Hall effect. Here we show that phonons can exhibit intrinsic chirality in monolayer tungsten diselenide. The broken inversion symmetry of the lattice lifts the degeneracy of clockwise and counterclockwise phonon modes at the corners of the Brillouin zone. We identified the phonons by the intervalley transfer of holes through hole-phonon interactions during the indirect infrared absorption, and we confirmed their chirality by the infrared circular dichroism arising from pseudoangular momentum conservation. The chiral phonons are important for electron-phonon coupling in solids, phonon-driven topological states, and energy-efficient information processing.

Spatial correlation in matter-wave interference as a measure of decoherence, dephasing, and entropy

NASA Astrophysics Data System (ADS)

Chen, Zilin; Beierle, Peter; Batelaan, Herman

2018-04-01

The loss of contrast in double-slit electron diffraction due to dephasing and decoherence processes is studied. It is shown that the spatial intensity correlation function of diffraction patterns can be used to distinguish between dephasing and decoherence. This establishes a measure of time reversibility that does not require the determination of coherence terms of the density matrix, while von Neumann entropy, another measure of time reversibility, does require coherence terms. This technique is exciting in view of the need to understand and control the detrimental experimental effect of contrast loss and for fundamental studies on the transition from the classical to the quantum regime.

Superlubrication by phonon confinement

NASA Astrophysics Data System (ADS)

Wada, Noriyuki; Ishikawa, Makoto; Shiga, Takuma; Shiomi, Junichiro; Suzuki, Masaru; Miura, Kouji

2018-04-01

The superlubrication described here, involving confined phonons, is easily achievable and very simple because it uses only submicron islands, smaller than the mean free path of the phonons, to confine phonons. We can achieve superlubrication with a friction force of piconewton order at the submicron island. We can call this phononic lubrication or self-lubrication because phonons induced by tip shearing are confined within the submicron islands and decrease the friction during the subsequent sliding. Phonon confinement should make it possible to directly develop applications for lubricants and ultimately to open a novel avenue of tribology.

Many-Particle Dephasing after a Quench

NASA Astrophysics Data System (ADS)

Kiendl, Thomas; Marquardt, Florian

2017-03-01

After a quench in a quantum many-body system, expectation values tend to relax towards long-time averages. However, temporal fluctuations remain in the long-time limit, and it is crucial to study the suppression of these fluctuations with increasing system size. The particularly important case of nonintegrable models has been addressed so far only by numerics and conjectures based on analytical bounds. In this work, we are able to derive analytical predictions for the temporal fluctuations in a nonintegrable model (the transverse Ising chain with extra terms). Our results are based on identifying a dynamical regime of "many-particle dephasing," where quasiparticles do not yet relax but fluctuations are nonetheless suppressed exponentially by weak integrability breaking.

Tracing molecular dephasing in biological tissue

NASA Astrophysics Data System (ADS)

Mokim, M.; Carruba, C.; Ganikhanov, F.

2017-10-01

We demonstrate the quantitative spectroscopic characterization and imaging of biological tissue using coherent time-domain microscopy with a femtosecond resolution. We identify tissue constituents and perform dephasing time (T2) measurements of characteristic Raman active vibrations. This was shown in subcutaneous mouse fat embedded within collagen rich areas of the dermis and the muscle connective tissue. The demonstrated equivalent spectral resolution (<0.3 cm-1) is an order of magnitude better compared to commonly used frequency-domain methods for characterization of biological media. This provides with the important dimensions and parameters in biological media characterization and can become an effective tool in detecting minute changes in the bio-molecular composition and environment that is critical for molecular level diagnosis.

Many-Particle Dephasing after a Quench.

PubMed

Kiendl, Thomas; Marquardt, Florian

2017-03-31

After a quench in a quantum many-body system, expectation values tend to relax towards long-time averages. However, temporal fluctuations remain in the long-time limit, and it is crucial to study the suppression of these fluctuations with increasing system size. The particularly important case of nonintegrable models has been addressed so far only by numerics and conjectures based on analytical bounds. In this work, we are able to derive analytical predictions for the temporal fluctuations in a nonintegrable model (the transverse Ising chain with extra terms). Our results are based on identifying a dynamical regime of "many-particle dephasing," where quasiparticles do not yet relax but fluctuations are nonetheless suppressed exponentially by weak integrability breaking.

Multicoaxial cylindrical inclusions in locally resonant phononic crystals

NASA Astrophysics Data System (ADS)

Larabi, H.; Pennec, Y.; Djafari-Rouhani, B.; Vasseur, J. O.

2007-06-01

It is known that the transmission spectrum of the so-called locally resonant phononic crystal can exhibit absolute sharp dips in the sonic frequency range due to the resonance scattering of elastic waves. In this paper, we study theoretically, using a finite difference time domain method, the propagation of acoustic waves through a two-dimensional locally resonant crystal in which the matrix is a fluid (such as water) instead of being a solid as in most of the previous papers. The transmission is shown to be dependent upon the fluid or solid nature of the matrix as well as upon the nature of the coating material in contact with the matrix. The other main purpose of this paper is to consider inclusions constituted by coaxial cylindrical multilayers consisting of several alternate shells of a soft material (such as a soft rubber) and a hard material (such as steel). With respect to the usual case of a hard core coated with a soft rubber, the transmission spectrum can exhibit in the same frequency range several peaks instead of one. If two or more phononic crystals are associated together, we find that the structure displays all the zeros of transmission resulting from each individual crystal. Moreover, we show that it is possible to overlap the dips by an appropriate combination of phononic crystals and create a larger acoustic stop band.

Tunneling of Massive Flux Lines in a High Tc Superconductor at absolute Zero

NASA Astrophysics Data System (ADS)

Narahari Achar, B. N.; Waleed Gaber, M.

1997-11-01

Our previous study(M. W. Gaber and B. N. N. Achar, Phys. Rev. B52, 1314(1995)) of quantum tunneling of damped flux lines of finite mass has been extended over the temperature range from the crossover temperature T0 to T=0 with a view to investigate further the temperature dependence of the tunneling rate found earlier. It has been found that at T=0, for the case of the cubic pinning potential, the action can be evaluated in a closed form for a flux line of finite length. The only non-zero contribution arises from the dissipation term resulting in a finite action.

Interaction between confined phonons and photons in periodic silicon resonators

NASA Astrophysics Data System (ADS)

Iskandar, A.; Gwiazda, A.; Younes, J.; Kazan, M.; Bruyant, A.; Tabbal, M.; Lerondel, G.

2018-03-01

In this paper, we demonstrate that phonons and photons of different momenta can be confined and interact with each other within the same nanostructure. The interaction between confined phonons and confined photons in silicon resonator arrays is observed by means of Raman scattering. The Raman spectra from large arrays of dielectric silicon resonators exhibited Raman enhancement accompanied with a downshift and broadening. The analysis of the Raman intensity and line shape using finite-difference time-domain simulations and a spatial correlation model demonstrated an interaction between photons confined in the resonators and phonons confined in highly defective regions prompted by the structuring process. It was shown that the Raman enhancement is due to collective lattice resonance inducing field confinement in the resonators, while the spectra downshift and broadening are signatures of the relaxation of the phonon wave vector due to phonon confinement in defective regions located in the surface layer of the Si resonators. We found that as the resonators increase in height and their shape becomes cylindrical, the amplitude of their coherent oscillation increases and hence their ability to confine the incoming electric field increases.

The effect of temperature and dot size on the spectral properties of colloidal InP/ZnS core-shell quantum dots.

PubMed

Narayanaswamy, Arun; Feiner, L F; Meijerink, A; van der Zaag, P J

2009-09-22

Visual color changes between 300 and 510 K were observed in the photoluminescence (PL) of colloidal InP/ZnS core-shell nanocrystals. A subsequent study of PL spectra in the range 2-510 K and fitting the temperature dependent line shift and line width to theoretical models show that the dominant (dephasing) interaction is due to scattering by acoustic phonons of about 23 meV. Low temperature photoluminescence excitation measurements show that the excitonic band gap depends approximately inversely linearly on the quantum dot size d, which is distinctly weaker than the dependence predicted by current theories.

Observation of chiral phonons.

PubMed

Zhu, Hanyu; Yi, Jun; Li, Ming-Yang; Xiao, Jun; Zhang, Lifa; Yang, Chih-Wen; Kaindl, Robert A; Li, Lain-Jong; Wang, Yuan; Zhang, Xiang

2018-02-02

Chirality reveals symmetry breaking of the fundamental interaction of elementary particles. In condensed matter, for example, the chirality of electrons governs many unconventional transport phenomena such as the quantum Hall effect. Here we show that phonons can exhibit intrinsic chirality in monolayer tungsten diselenide. The broken inversion symmetry of the lattice lifts the degeneracy of clockwise and counterclockwise phonon modes at the corners of the Brillouin zone. We identified the phonons by the intervalley transfer of holes through hole-phonon interactions during the indirect infrared absorption, and we confirmed their chirality by the infrared circular dichroism arising from pseudoangular momentum conservation. The chiral phonons are important for electron-phonon coupling in solids, phonon-driven topological states, and energy-efficient information processing. Copyright © 2018 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.

Differential BPFs with Multiple Transmission Zeros Based on Terminated Coupled Lines

NASA Astrophysics Data System (ADS)

Niu, Yiming; Yang, Guo; Wu, Wen

2018-04-01

Differential bandpass filters (BPFs) named Filter A and Filter B based on Terminated Coupled Lines (TCLs) are proposed in this letter. The TCLs contributes to not only three poles in differential-mode (DM) for wideband filtering response but also multiple zeros in both DM and common-mode (CM) offering wide DM out-of-band rejection and good CM suppression. Fabricated filters centred at 3.5 GHz with wide DM passband and wideband CM suppression have been designed and measured. The filters improved the noise suppression capability of the communication and radiometer systems. The simulated and measured results are in good agreement.

Spectral features of LO phonon sidebands in luminescence of free excitons in GaN

NASA Astrophysics Data System (ADS)

Xu, S. J.; Li, G. Q.; Xiong, S.-J.; Tong, S. Y.; Che, C. M.; Liu, W.; Li, M. F.

2005-06-01

In the paper a combined experimental and theoretical investigation of the longitudinal optical phonon sidebands (PSBs) in the luminescence of free excitons in GaN at moderately high temperatures was reported. The spectral features, including line broadening, shift, and asymmetry of the one- and two-phonon PSBs, were revealed both experimentally and theoretically. It is found that the linewidth of the one-phonon PSB is surprisingly always larger than that of the two-phonon PSB in the interested temperature range. Moreover, the thermal broadening rates of the one- and two-phonon PSBs are considerably different. We adopted the Segall-Mahan theory [B. Segall and G. D. Mahan, Phys. Rev. 171, 935 (1968)] to compute the PSB spectra of the free excitons in GaN. Only one adjustable parameter, the effective mass of the holes, was used in the calculations. For the one-phonon PSB, an excellent agreement between theory and experiment is achieved when an adequate effective mass of the holes was used.

Phonon Scattering and Confinement in Crystalline Films

NASA Astrophysics Data System (ADS)

Parrish, Kevin D.

. Removing the isotropic assumption, leading to the formulation of modal ray-tracing, corrects the mean free path distribution. The effect of phonon line-of-sight is investigated in nanoporous silicon films using free path sampling. When the line-of-sight is cut off there is a distinct change in thermal conductivity versus porosity. By analyzing the free paths of an obstructed phonon mode, it is concluded that the trend change is due to a hard upper limit on the free paths that can exist due to the nanopore geometry in the material. The transient grating technique is an optical contact-less laser based experiment for measuring the in-plane thermal diffusivity of thin films and membranes. The theory of operation and physical setup of a transient grating experiment is detailed. The procedure for extracting the thermal diffusivity from the raw experimental signal is improved upon by removing arbitrary user choice in the fitting parameters used and constructing a parameterless error minimizing procedure. The thermal conductivity of ultra-thin argon films modeled with the Lennard-Jones potential is calculated from both the Monte Carlo free path sampling technique and from explicit reduced dimensionality lattice dynamics calculations. In these ultra-thin films, the phonon properties are altered in more than a perturbative manner, referred to as the confinement regime. The free path sampling technique, which is a perturbative method, is compared to a reduced dimensionality lattice dynamics calculation where the entire film thickness is taken as the unit cell. Divergence in thermal conductivity magnitude and trend is found at few unit cell thick argon films. Although the phonon group velocities and lifetimes are affected, it is found that alterations to the phonon density of states are the primary cause of the deviation in thermal conductivity in the confinement regime.

Probing the experimental phonon dispersion of graphene using 12C and 13C isotopes

NASA Astrophysics Data System (ADS)

Bernard, S.; Whiteway, E.; Yu, V.; Austing, D. G.; Hilke, M.

2012-08-01

Using very uniform large-scale chemical vapor deposition grown graphene transferred onto silicon, we were able to identify 15 distinct Raman lines associated with graphene monolayers. This was possible thanks to a combination of different carbon isotopes and different Raman laser energies and extensive averaging without increasing the laser power. This allowed us to obtain a detailed experimental phonon dispersion relation for many points in the Brillouin zone. We further identified a D+D' peak corresponding to a double-phonon process involving both an inter- and intravalley phonon.

Four-phonon scattering reduces intrinsic thermal conductivity of graphene and the contributions from flexural phonons

NASA Astrophysics Data System (ADS)

Feng, Tianli; Ruan, Xiulin

2018-01-01

We have developed a formalism of the exact solution to linearized phonon Boltzmann transport equation (BTE) for thermal conductivity calculation including three- and four-phonon scattering. We find strikingly high four-phonon scattering rates in single-layer graphene (SLG) based on the optimized Tersoff potential. The reflection symmetry in graphene, which forbids the three-ZA (out-of-plane acoustic) scattering, allows the four-ZA processes ZA +ZA ⇌ZA +ZA and ZA ⇌ZA +ZA + ZA. As a result, the large phonon population of the low-energy ZA branch originated from the quadratic phonon dispersion leads to high four-phonon scattering rates, even much higher than the three-phonon scattering rates at room temperature. These four-phonon processes are dominated by the normal processes, which lead to a failure of the single mode relaxation time approximation. Therefore, we have solved the exact phonon BTE using an iterative scheme and then calculated the length- and temperature-dependent thermal conductivities. We find that the predicted thermal conductivity of SLG is lower than the previously predicted value from the three-phonon scattering only. The relative contribution of the ZA branch is reduced from 70% to 30% when four-phonon scattering is included. Furthermore, we have demonstrated that the four-phonon scattering in multilayer graphene and graphite is not strong due to the ZA splitting by interlayer van der Waals interaction. We also demonstrate that the five-phonon process in SLG is not strong due to the restriction of reflection symmetry.

A possible high-mobility signal in bulk MoTe2: Temperature independent weak phonon decay

NASA Astrophysics Data System (ADS)

Li, Titao; Zhang, Zhaojun; Zheng, Wei; Lv, Yangyang; Huang, Feng

2016-11-01

Layered transition metal dichalcogenides (TMDs) have attracted great attention due to their non-zero bandgap for potential application in high carrier mobility devices. Recent studies demonstrate that the carrier mobility of MoTe2 would decrease by orders of magnitude when used for few-layer transistors. As phonon scattering has a significant influence on carrier mobility of layered material, here, we first reported temperature-dependent Raman spectra of bulk 2H-MoTe2 from 80 to 300 K and discovered that the phonon lifetime of both E12g and A1g vibration modes are independent with temperature. These results were explained by the weak phonon decay in MoTe2. Our results imply the existence of a carrier mobility higher than the theoretical value in intrinsic bulk 2H-MoTe2 and the feasibility to obtain MoTe2-based transistors with sufficiently high carrier mobility.

Double simple-harmonic-oscillator formulation of the thermal equilibrium of a fluid interacting with a coherent source of phonons

NASA Technical Reports Server (NTRS)

Defacio, B.; Vannevel, Alan; Brander, O.

1993-01-01

A formulation is given for a collection of phonons (sound) in a fluid at a non-zero temperature which uses the simple harmonic oscillator twice; one to give a stochastic thermal 'noise' process and the other which generates a coherent Glauber state of phonons. Simple thermodynamic observables are calculated and the acoustic two point function, 'contrast' is presented. The role of 'coherence' in an equilibrium system is clarified by these results and the simple harmonic oscillator is a key structure in both the formulation and the calculations.

Mapping momentum-dependent electron-phonon coupling and nonequilibrium phonon dynamics with ultrafast electron diffuse scattering

NASA Astrophysics Data System (ADS)

Stern, Mark J.; René de Cotret, Laurent P.; Otto, Martin R.; Chatelain, Robert P.; Boisvert, Jean-Philippe; Sutton, Mark; Siwick, Bradley J.

2018-04-01

Despite their fundamental role in determining material properties, detailed momentum-dependent information on the strength of electron-phonon and phonon-phonon coupling (EPC and PPC, respectively) across the entire Brillouin zone has remained elusive. Here we demonstrate that ultrafast electron diffuse scattering (UEDS) directly provides such information. By exploiting symmetry-based selection rules and time resolution, scattering from different phonon branches can be distinguished even without energy resolution. Using graphite as a model system, we show that UEDS patterns map the relative EPC and PPC strength through their profound sensitivity to photoinduced changes in phonon populations. We measure strong EPC to the K -point TO phonon of A1' symmetry (K -A1' ) and along the entire TO branch between Γ -K , not only to the Γ -E2 g phonon. We also determine that the subsequent phonon relaxation of these strongly coupled optical phonons involve three stages: decay via several identifiable channels to TA and LA phonons (1 -2 ps), intraband thermalization of the non-equilibrium TA/LA phonon populations (30 -40 ps) and interband relaxation of the TA/LA modes (115 ps). Combining UEDS with ultrafast angle-resolved photoelectron spectroscopy will yield a complete picture of the dynamics within and between electron and phonon subsystems, helping to unravel complex phases in which the intertwined nature of these systems has a strong influence on emergent properties.

Optimizing phonon space in the phonon-coupling model

NASA Astrophysics Data System (ADS)

Tselyaev, V.; Lyutorovich, N.; Speth, J.; Reinhard, P.-G.

2017-08-01

We present a new scheme to select the most relevant phonons in the phonon-coupling model, named here the time-blocking approximation (TBA). The new criterion, based on the phonon-nucleon coupling strengths rather than on B (E L ) values, is more selective and thus produces much smaller phonon spaces in the TBA. This is beneficial in two respects: first, it curbs the computational cost, and second, it reduces the danger of double counting in the expansion basis of the TBA. We use here the TBA in a form where the coupling strength is regularized to keep the given Hartree-Fock ground state stable. The scheme is implemented in a random-phase approximation and TBA code based on the Skyrme energy functional. We first explore carefully the cutoff dependence with the new criterion and can work out a natural (optimal) cutoff parameter. Then we use the freshly developed and tested scheme for a survey of giant resonances and low-lying collective states in six doubly magic nuclei looking also at the dependence of the results when varying the Skyrme parametrization.

The Influence of Phonons and Phonon Decay on the Optical Properties of GaN

NASA Astrophysics Data System (ADS)

Song, D. Y.; Basavaraj, M.; Nikishin, S. A.; Holtz, M.; Soukhoveev, V.; Usikov, A.; Dmitriev, V.

2006-03-01

The temperature dependences of vibrational and optical properties of high-quality GaN are studied using Raman and photoluminescence (PL) spectroscopies in the range 20 to 325 K. The Raman-active A1(LO) phonon has temperature dependence described well by combined two- and three-phonon decay. The temperature dependences of E2^2 phonon are almost entirely dominated by the thermal expansion, and the contribution of three-phonon decay process is very small throughout interested temperature range. The shallow neutral donor-bound exciton (D^0,X) and two free excitons (XA and XB) are observed at low temperatrue PL spectra. Also seen are two A1(LO) phonon sidebands (PSBs), originating from the XB free exciton, with the characteristic asymmetry attributed to interactions between discrete and continuum states. Analysis of the band-edge excitons reveals that energy gap shrinkage and exciton linewidths are completely described based on electron-phonon interactions with phonon properties consistent with the Raman analysis. First and second PSBs have temperature dependence associated with the A1(LO) phonon. The shift, broadening, and asymmetry of the PSBs are explained by Segall-Mahan theory adding the decay mechanism of A1(LO) phonon and the exciton broadening from electron-phonon interactions. Work at Texas Tech University supported by National Science Foundation grant ECS-0323640.

Extensions of the Theory of the Electron-Phonon Interaction in Metals: A Collection.

DTIC Science & Technology

1983-11-03

accepted The measured zero -field susceptibility is given 50 . . . . 26 GENERALIZATION OF THE THEORY OF THE ELECTRON-... 1199 JP by X.P_ IM T V.IM 0... Generalization of the Theory of the Electron-Phonon Inter- action: Thermodynamic Formulation of Superconducting- and Normal-State Properties...A microscopic treatment of the consequences for supercon- ductivity of a nonconstant electronic density of states is presented. Generalized

Electron-phonon interactions in semiconductor nanostructures

NASA Astrophysics Data System (ADS)

Yu, Segi

In this dissertation, electron-phonon interactions are studied theoretically in semiconductor nanoscale heterostructures. Interactions of electrons with interface optical phonons dominate over other electron-phonon interactions in narrow width heterostructures. Hence, a transfer matrix method is used to establish a formalism for determining the dispersion relations and electrostatic potentials of the interface phonons for multiple-interface heterostructure within the macroscopic dielectric continuum model. This method facilitates systematic calculations for complex structures where the conventional method is difficult to implement. Several specific cases are treated to illustrate advantages of the formalism. Electrophonon resonance (EPR) is studied in cylindrical quantum wires using the confined/interface optical phonons representation and bulk phonon representation. It has been found that interface phonon contribution to EPR is small compared with confined phonon. Different selection rules for bulk phonons and confined phonons result in different EPR behaviors as the radius of cylindrical wire changes. Experiment is suggested to test which phonon representation is appropriate for EPR. The effects of phonon confinement on elect ron-acoustic-phonon scattering is studied in cylindrical and rectangular quantum wires. In the macroscopic elastic continuum model, the confined-phonon dispersion relations are obtained for several crystallographic directions with free-surface and clamped-surface boundary conditions in cylindrical wires. The scattering rates due to the deformation potential are obtained for these confined phonons and are compared with those of bulk-like phonons. The results show that the inclusion of acoustic phonon confinement may be crucial for calculating accurate low-energy electron scattering rates. Furthermore, it has been found that there is a scaling rule governing the directional dependence of the scattering rates. The Hamiltonian describing the

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

Effect of gauge-field interaction on fermion transport in two dimensions: Hartree conductivity correction and dephasing

NASA Astrophysics Data System (ADS)

Ludwig, T.; Gornyi, I. V.; Mirlin, A. D.; Wölfle, P.

2008-06-01

We consider the quantum corrections to the conductivity of fermions interacting via a Chern Simons gauge field and concentrate on the Hartree-type contributions. The first-order Hartree approximation is only valid in the limit of weak coupling λ≪g-1/2 to the gauge field ( g≫1 is the dimensionless conductance) and results in an antilocalizing conductivity correction ˜λ2gln2T . In the case of strong coupling, an infinite summation of higher-order terms is necessary, which includes both the virtual (renormalization of the frequency) and real (dephasing) processes. At intermediate temperatures, T0≪T≪gT0 , where T0˜1/g2τ and τ is the elastic scattering time, the T dependence of the conductivity is determined by the Hartree correction, δσH(T)-δσH(gT0)∝g1/2-(T/T0)1/2[1+ln(gT0/T)1/2] , so that σ(T) increases with lowering T . At low temperatures, T≪T0 , the temperature-dependent part of the Hartree correction assumes a logarithmic form with a coefficient of order unity, δσH∝ln(1/T) . As a result, the negative exchange contribution δσex∝-lngln(1/T) becomes dominant, which yields localization in the limit of T→0 . We further discuss dephasing at strong coupling and show that the dephasing rates are of the order of T , owing to the interplay of inelastic scattering and renormalization. On the other hand, the dephasing length is anomalously short, Lφ≪LT , where LT is the thermal length. For the case of composite fermions with long-range Coulomb interaction, the gauge-field propagator is less singular. The resulting Hartree correction has the usual sign and temperature dependence, δσH∝lngln(1/T) , and for realistic g is overcompensated by the negative exchange contribution due to the gauge-boson and scalar parts of the interaction. In this case, the dephasing length Lφ is of the order of LT for not too low temperatures and exceeds LT for T≲gT0 .

Phonon anomalies in FeS

DOE PAGES

Baum, A.; Milosavljevic, A.; Lazarevic, N.; ...

2018-02-12

Here, we present results from light scattering experiments on tetragonal FeS with the focus placed on lattice dynamics. We identify the Raman active A 1g and B 1g phonon modes, a second order scattering process involving two acoustic phonons, and contributions from potentially defect-induced scattering. The temperature dependence between 300 and 20 K of all observed phonon energies is governed by the lattice contraction. Below 20 K the phonon energies increase by 0.5–1 cm -1 , thus indicating putative short range magnetic order. Additionally, along with the experiments we performed lattice-dynamical simulations and a symmetry analysis for the phonons andmore » potential overtones and find good agreement with the experiments. In particular, we argue that the two-phonon excitation observed in a gap between the optical branches becomes observable due to significant electron-phonon interaction.« less

Phonon anomalies in FeS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Baum, A.; Milosavljevic, A.; Lazarevic, N.

Here, we present results from light scattering experiments on tetragonal FeS with the focus placed on lattice dynamics. We identify the Raman active A 1g and B 1g phonon modes, a second order scattering process involving two acoustic phonons, and contributions from potentially defect-induced scattering. The temperature dependence between 300 and 20 K of all observed phonon energies is governed by the lattice contraction. Below 20 K the phonon energies increase by 0.5–1 cm -1 , thus indicating putative short range magnetic order. Additionally, along with the experiments we performed lattice-dynamical simulations and a symmetry analysis for the phonons andmore » potential overtones and find good agreement with the experiments. In particular, we argue that the two-phonon excitation observed in a gap between the optical branches becomes observable due to significant electron-phonon interaction.« less

Phonon Self-Energy Corrections to Nonzero Wave-Vector Phonon Modes in Single-Layer Graphene

NASA Astrophysics Data System (ADS)

Araujo, P. T.; Mafra, D. L.; Sato, K.; Saito, R.; Kong, J.; Dresselhaus, M. S.

2012-07-01

Phonon self-energy corrections have mostly been studied theoretically and experimentally for phonon modes with zone-center (q=0) wave vectors. Here, gate-modulated Raman scattering is used to study phonons of a single layer of graphene originating from a double-resonant Raman process with q≠0. The observed phonon renormalization effects are different from what is observed for the zone-center q=0 case. To explain our experimental findings, we explored the phonon self-energy for the phonons with nonzero wave vectors (q≠0) in single-layer graphene in which the frequencies and decay widths are expected to behave oppositely to the behavior observed in the corresponding zone-center q=0 processes. Within this framework, we resolve the identification of the phonon modes contributing to the G⋆ Raman feature at 2450cm-1 to include the iTO+LA combination modes with q≠0 and also the 2iTO overtone modes with q=0, showing both to be associated with wave vectors near the high symmetry point K in the Brillouin zone.

Strain Control of Exciton-Phonon Coupling in Atomically Thin Semiconductors.

PubMed

Niehues, Iris; Schmidt, Robert; Drüppel, Matthias; Marauhn, Philipp; Christiansen, Dominik; Selig, Malte; Berghäuser, Gunnar; Wigger, Daniel; Schneider, Robert; Braasch, Lisa; Koch, Rouven; Castellanos-Gomez, Andres; Kuhn, Tilmann; Knorr, Andreas; Malic, Ermin; Rohlfing, Michael; Michaelis de Vasconcellos, Steffen; Bratschitsch, Rudolf

2018-03-14

Semiconducting transition metal dichalcogenide (TMDC) monolayers have exceptional physical properties. They show bright photoluminescence due to their unique band structure and absorb more than 10% of the light at their excitonic resonances despite their atomic thickness. At room temperature, the width of the exciton transitions is governed by the exciton-phonon interaction leading to strongly asymmetric line shapes. TMDC monolayers are also extremely flexible, sustaining mechanical strain of about 10% without breaking. The excitonic properties strongly depend on strain. For example, exciton energies of TMDC monolayers significantly redshift under uniaxial tensile strain. Here, we demonstrate that the width and the asymmetric line shape of excitonic resonances in TMDC monolayers can be controlled with applied strain. We measure photoluminescence and absorption spectra of the A exciton in monolayer MoSe 2 , WSe 2 , WS 2 , and MoS 2 under uniaxial tensile strain. We find that the A exciton substantially narrows and becomes more symmetric for the selenium-based monolayer materials, while no change is observed for atomically thin WS 2 . For MoS 2 monolayers, the line width increases. These effects are due to a modified exciton-phonon coupling at increasing strain levels because of changes in the electronic band structure of the respective monolayer materials. This interpretation based on steady-state experiments is corroborated by time-resolved photoluminescence measurements. Our results demonstrate that moderate strain values on the order of only 1% are already sufficient to globally tune the exciton-phonon interaction in TMDC monolayers and hold the promise for controlling the coupling on the nanoscale.

Vacuum phonon tunneling.

PubMed

Altfeder, Igor; Voevodin, Andrey A; Roy, Ajit K

2010-10-15

Field-induced phonon tunneling, a previously unknown mechanism of interfacial thermal transport, has been revealed by ultrahigh vacuum inelastic scanning tunneling microscopy (STM). Using thermally broadened Fermi-Dirac distribution in the STM tip as in situ atomic-scale thermometer we found that thermal vibrations of the last tip atom are effectively transmitted to sample surface despite few angstroms wide vacuum gap. We show that phonon tunneling is driven by interfacial electric field and thermally vibrating image charges, and its rate is enhanced by surface electron-phonon interaction.

Studies of Phonon Anharmonicity in Solids

NASA Astrophysics Data System (ADS)

Lan, Tian

Today our understanding of the vibrational thermodynamics of materials at low temperatures is emerging nicely, based on the harmonic model in which phonons are independent. At high temperatures, however, this understanding must accommodate how phonons interact with other phonons or with other excitations. We shall see that the phonon-phonon interactions give rise to interesting coupling problems, and essentially modify the equilibrium and non-equilibrium properties of materials, e.g., thermodynamic stability, heat capacity, optical properties and thermal transport of materials. Despite its great importance, to date the anharmonic lattice dynamics is poorly understood and most studies on lattice dynamics still rely on the harmonic or quasiharmonic models. There have been very few studies on the pure phonon anharmonicity and phonon-phonon interactions. The work presented in this thesis is devoted to the development of experimental and computational methods on this subject. Modern inelastic scattering techniques with neutrons or photons are ideal for sorting out the anharmonic contribution. Analysis of the experimental data can generate vibrational spectra of the materials, i.e., their phonon densities of states or phonon dispersion relations. We obtained high quality data from laser Raman spectrometer, Fourier transform infrared spectrometer and inelastic neutron spectrometer. With accurate phonon spectra data, we obtained the energy shifts and lifetime broadenings of the interacting phonons, and the vibrational entropies of different materials. The understanding of them then relies on the development of the fundamental theories and the computational methods. We developed an efficient post-processor for analyzing the anharmonic vibrations from the molecular dynamics (MD) calculations. Currently, most first principles methods are not capable of dealing with strong anharmonicity, because the interactions of phonons are ignored at finite temperatures. Our method adopts

Preface: Phonons 2007

NASA Astrophysics Data System (ADS)

Perrin, Bernard

2007-06-01

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

SURFACE PHONONS IN THE ORDERED c(2 × 2) PHASE OF Pd ON Au(100)

NASA Astrophysics Data System (ADS)

Chadli, R.; Khater, A.; Tigrine, R.

2013-03-01

The vibrational properties of the Au(100)-c(2 × 2)-Pd ordered phase, which is a stable system in the temperature range of 500 K to 600 K, are presented. This surface alloy is formed by depositing Pd atoms onto the Au(100) surface, and annealing at higher temperatures. The equilibrium structural characteristics, phonon dispersions as well as the local density of phonon states are calculated using the matching theory associated with Green's function formalism evaluated in the harmonic approximation. New surface modes have been found on the ordered metallic surface alloy along the three directions of high symmetry /line{Γ X}, /line{XM}, and /line{MΓ }, in comparison with the clean surface Au(100). Three of them are observed above the bulk bands spectrum.

Aiming for zero: decreasing central line associated bacteraemia in the intensive care unit.

PubMed

Seddon, Mary E; Hocking, Catherine J; Mead, Pat; Simpson, Catherine

2011-07-29

To eliminate Central Line Associated Bacteraemia (CLAB) in the Critical Care Complex (CCC)-Intensive Care Unit (ICU) and High Dependency Unit (HDU)-Middlemore Hospital. Multifaceted quality improvement programme that included: engagement with ICU leadership and education of ICU staff; the introduction of a CLAB prevention bundle of care through standardised checklists for central line insertion (December 2008) and line maintenance (July 2009); the development of a central line pack; and rapid, visual feedback of results. Absolute numbers of CLAB in the CCC decreased from 14 in 2008, to 4 in 2009 and 1 in the first 6 months of 2010 (despite increase in bed census and a doubling of admissions). The CLAB rate per 1,000 line days decreased from 6.6 to 0.9. The days between CLAB increased from a median of 30 to >100 days, with zero CLAB for 5 of the last 6 months. Mortality for patients with CLAB was 37%, compared with mortality of 13% for all other ICU patients. The conservative cost savings were $200,000 in 2009 and $260,000 in 2010. Using an evidenced-based quality improvement approach, it is possible to significantly decrease Central Line Associated Bacteraemia in the Critical Care Complex. In doing so patient morbidity and mortality are reduced and money is saved for other healthcare needs.

Phonon properties of lutetium pnictides

NASA Astrophysics Data System (ADS)

Arya, Balwant Singh; Aynyas, Mahendra; Sanyal, Sankar P.

2018-05-01

Phonon properties of Lutetium pnictides (LuX : X = P, As) have been studied by using breathing shell model (BSM) which includes breathing motion of electrons of the Lu atoms due to f-d hybridization to establish their predominant ionic nature. The calculated phonon dispersion curves of these compounds are presented follow the same trend as observed in ytterbium pnictides (YbP and YbAs). We also report one phonon density of states and specific heat for these compounds. We discuss the significance of this approach in predicting the phonon dispersion curves and examine the role of electron-phonon interaction.

First-principles method for electron-phonon coupling and electron mobility: Applications to two-dimensional materials

NASA Astrophysics Data System (ADS)

Gunst, Tue; Markussen, Troels; Stokbro, Kurt; Brandbyge, Mads

2016-01-01

We present density functional theory calculations of the phonon-limited mobility in n -type monolayer graphene, silicene, and MoS2. The material properties, including the electron-phonon interaction, are calculated from first principles. We provide a detailed description of the normalized full-band relaxation time approximation for the linearized Boltzmann transport equation (BTE) that includes inelastic scattering processes. The bulk electron-phonon coupling is evaluated by a supercell method. The method employed is fully numerical and does therefore not require a semianalytic treatment of part of the problem and, importantly, it keeps the anisotropy information stored in the coupling as well as the band structure. In addition, we perform calculations of the low-field mobility and its dependence on carrier density and temperature to obtain a better understanding of transport in graphene, silicene, and monolayer MoS2. Unlike graphene, the carriers in silicene show strong interaction with the out-of-plane modes. We find that graphene has more than an order of magnitude higher mobility compared to silicene in the limit where the silicene out-of-plane interaction is reduced to zero (by substrate interaction, clamping, or similar). If the out-of-plane interaction is not actively reduced, the mobility of silicene will essentially be zero. For MoS2, we obtain several orders of magnitude lower mobilities compared to graphene in agreement with other recent theoretical results. The simulations illustrate the predictive capabilities of the newly implemented BTE solver applied in simulation tools based on first-principles and localized basis sets.

Ballistic phonon transport in holey silicon.

PubMed

Lee, Jaeho; Lim, Jongwoo; Yang, Peidong

2015-05-13

When the size of semiconductors is smaller than the phonon mean free path, phonons can carry heat with no internal scattering. Ballistic phonon transport has received attention for both theoretical and practical aspects because Fourier's law of heat conduction breaks down and the heat dissipation in nanoscale transistors becomes unpredictable in the ballistic regime. While recent experiments demonstrate room-temperature evidence of ballistic phonon transport in various nanomaterials, the thermal conductivity data for silicon in the length scale of 10-100 nm is still not available due to experimental challenges. Here we show ballistic phonon transport prevails in the cross-plane direction of holey silicon from 35 to 200 nm. The thermal conductivity scales linearly with the length (thickness) even though the lateral dimension (neck) is as narrow as 20 nm. We assess the impact of long-wavelength phonons and predict a transition from ballistic to diffusive regime using scaling models. Our results support strong persistence of long-wavelength phonons in nanostructures and are useful for controlling phonon transport for thermoelectrics and potential phononic applications.

Dispersion of folded phonons in {Si}/{Si xGe1- x} superlattices

NASA Astrophysics Data System (ADS)

Brugger, H.; Reiner, H.; Abstreiter, G.; Jorke, H.; Herzog, H. J.; Kasper, E.

Zone folding effects on acoustic phonons in {Si}/{Si xGe1- x} strained layer superlattices are studied by Raman spectroscopy. A quantitative explanation of the measured frequencies is given in terms of the elastic continuum theory. The scattering wavevector q s is varied by use of different laser lines to probe directly the phonon dispersion curve in the superlattices. For large period samples q s can be shifted through the new Brillouin zone boundary. We report on observation of a finite doublet splitting corresponding to the first zone-edge gap.

Thermalization dynamics of two correlated bosonic quantum wires after a split

NASA Astrophysics Data System (ADS)

Huber, Sebastian; Buchhold, Michael; Schmiedmayer, Jörg; Diehl, Sebastian

2018-04-01

Cherently splitting a one-dimensional Bose gas provides an attractive, experimentally established platform to investigate many-body quantum dynamics. At short enough times, the dynamics is dominated by the dephasing of single quasiparticles, and well described by the relaxation towards a generalized Gibbs ensemble corresponding to the free Luttinger theory. At later times on the other hand, the approach to a thermal Gibbs ensemble is expected for a generic, interacting quantum system. Here, we go one step beyond the quadratic Luttinger theory and include the leading phonon-phonon interactions. By applying kinetic theory and nonequilibrium Dyson-Schwinger equations, we analyze the full relaxation dynamics beyond dephasing and determine the asymptotic thermalization process in the two-wire system for a symmetric splitting protocol. The major observables are the different phonon occupation functions and the experimentally accessible coherence factor, as well as the phase correlations between the two wires. We demonstrate that, depending on the splitting protocol, the presence of phonon collisions can have significant influence on the asymptotic evolution of these observables, which makes the corresponding thermalization dynamics experimentally accessible.

Electron heating and thermal relaxation of gold nanorods revealed by two-dimensional electronic spectroscopy.

PubMed

Lietard, Aude; Hsieh, Cho-Shuen; Rhee, Hanju; Cho, Minhaeng

2018-03-01

To elucidate the complex interplay between the size and shape of gold nanorods and their electronic, photothermal, and optical properties for molecular imaging, photothermal therapy, and optoelectronic devices, it is a prerequisite to characterize ultrafast electron dynamics in gold nanorods. Time-resolved transient absorption (TA) studies of plasmonic electrons in various nanostructures have revealed the time scales for electron heating, lattice vibrational excitation, and phonon relaxation processes in condensed phases. However, because linear spectroscopic and time-resolved TA signals are vulnerable to inhomogeneous line-broadening, pure dephasing and direct electron heating effects are difficult to observe. Here we show that femtosecond two-dimensional electronic spectroscopy, with its unprecedented time resolution and phase sensitivity, can be used to collect direct experimental evidence for ultrafast electron heating, anomalously strong coherent and transient electronic plasmonic responses, and homogenous dephasing processes resulting from electron-vibration couplings even for polydisperse gold nanorods.

The Effect of Phonons in RbCl Quantum Pseudodot Qubits

NASA Astrophysics Data System (ADS)

Sun, Yong; Ding, Zhao-Hua; Xiao, Jing-Lin

2016-07-01

By employing the Pekar variational method, the eigenenergies and eigenfunctions of the ground and first-excited states are obtained in a strong electron-longitudinal optical (LO) phonon coupling RbCl quantum pseudodot (QPD). A single qubit can be realized in this two-level quantum system. The electron probability density (EPD) oscillates in the RbCl QPD with a certain period. The investigated results show that the EPD rises with raising the chemical potential of the two-dimensional electron gas and the zero point of the pseudoharmonic potential, whereas it decays with increasing the polaron radius. However, the oscillating period (OP) possesses precisely the opposite characteristics. Through the results and analysis above, we find three ways to adjust the EPD and the OP via changing the chemical potential of the two-dimensional electron gas, the zero point of the pseudoharmonic potential, and the polaron radius.

Real-Time Observation of Exciton-Phonon Coupling Dynamics in Self-Assembled Hybrid Perovskite Quantum Wells.

PubMed

Ni, Limeng; Huynh, Uyen; Cheminal, Alexandre; Thomas, Tudor H; Shivanna, Ravichandran; Hinrichsen, Ture F; Ahmad, Shahab; Sadhanala, Aditya; Rao, Akshay

2017-11-28

Self-assembled hybrid perovskite quantum wells have attracted attention due to their tunable emission properties, ease of fabrication, and device integration. However, the dynamics of excitons in these materials, especially how they couple to phonons, remains an open question. Here, we investigate two widely used materials, namely, butylammonium lead iodide (CH 3 (CH 2 ) 3 NH 3 ) 2 PbI 4 and hexylammonium lead iodide (CH 3 (CH 2 ) 5 NH 3 ) 2 PbI 4 , both of which exhibit broad photoluminescence tails at room temperature. We performed femtosecond vibrational spectroscopy to obtain a real-time picture of the exciton-phonon interaction and directly identified the vibrational modes that couple to excitons. We show that the choice of the organic cation controls which vibrational modes the exciton couples to. In butylammonium lead iodide, excitons dominantly couple to a 100 cm -1 phonon mode, whereas in hexylammonium lead iodide, excitons interact with phonons with frequencies of 88 and 137 cm -1 . Using the determined optical phonon energies, we analyzed photoluminescence broadening mechanisms. At low temperatures (<100 K), the broadening is due to acoustic phonon scattering, whereas at high temperatures, LO phonon-exciton coupling is the dominant mechanism. Our results help explain the broad photoluminescence line shape observed in hybrid perovskite quantum wells and provide insights into the mechanism of exciton-phonon coupling in these materials.

Finite element analysis of true and pseudo surface acoustic waves in one-dimensional phononic crystals

NASA Astrophysics Data System (ADS)

Graczykowski, B.; Alzina, F.; Gomis-Bresco, J.; Sotomayor Torres, C. M.

2016-01-01

In this paper, we report a theoretical investigation of surface acoustic waves propagating in one-dimensional phononic crystal. Using finite element method eigenfrequency and frequency response studies, we develop two model geometries suitable to distinguish true and pseudo (or leaky) surface acoustic waves and determine their propagation through finite size phononic crystals, respectively. The novelty of the first model comes from the application of a surface-like criterion and, additionally, functional damping domain. Exemplary calculated band diagrams show sorted branches of true and pseudo surface acoustic waves and their quantified surface confinement. The second model gives a complementary study of transmission, reflection, and surface-to-bulk losses of Rayleigh surface waves in the case of a phononic crystal with a finite number of periods. Here, we demonstrate that a non-zero transmission within non-radiative band gaps can be carried via leaky modes originating from the coupling of local resonances with propagating waves in the substrate. Finally, we show that the transmission, reflection, and surface-to-bulk losses can be effectively optimised by tuning the geometrical properties of a stripe.

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.

Self-consistency in the phonon space of the particle-phonon coupling model

NASA Astrophysics Data System (ADS)

Tselyaev, V.; Lyutorovich, N.; Speth, J.; Reinhard, P.-G.

2018-04-01

In the paper the nonlinear generalization of the time blocking approximation (TBA) is presented. The TBA is one of the versions of the extended random-phase approximation (RPA) developed within the Green-function method and the particle-phonon coupling model. In the generalized version of the TBA the self-consistency principle is extended onto the phonon space of the model. The numerical examples show that this nonlinear version of the TBA leads to the convergence of results with respect to enlarging the phonon space of the model.

Monolithic phononic crystals with a surface acoustic band gap from surface phonon-polariton coupling.

PubMed

Yudistira, D; Boes, A; Djafari-Rouhani, B; Pennec, Y; Yeo, L Y; Mitchell, A; Friend, J R

2014-11-21

We theoretically and experimentally demonstrate the existence of complete surface acoustic wave band gaps in surface phonon-polariton phononic crystals, in a completely monolithic structure formed from a two-dimensional honeycomb array of hexagonal shape domain-inverted inclusions in single crystal piezoelectric Z-cut lithium niobate. The band gaps appear at a frequency of about twice the Bragg band gap at the center of the Brillouin zone, formed through phonon-polariton coupling. The structure is mechanically, electromagnetically, and topographically homogeneous, without any physical alteration of the surface, offering an ideal platform for many acoustic wave applications for photonics, phononics, and microfluidics.

Acoustic phonon dispersion at hypersonic frequencies in Si and Ge

NASA Astrophysics Data System (ADS)

Kuok, M. H.; Ng, S. C.; Rang, Z. L.; Lockwood, D. J.

2000-11-01

Brillouin spectra of the longitudinal acoustic (LA) mode, traveling along the [001] direction, in silicon and germanium have been recorded in 180° backscattering using 457.9-514.5-nm laser lines. The wave velocity of the LA phonon propagating in the [001] direction was determined at hypersonic frequencies, from the measured acoustic phonon dispersion in silicon and germanium. The elastic modulus c11 of the two semiconductors has been calculated from the respective measured hypersonic wave velocities and the results are compared with values determined from lower-frequency ultrasonic and other measurements. Interestingly, the hypersonic velocities are consistently lower by ~1-2 % than the ultrasonic ones, but they generally agree within the present experimental accuracy.

Diffusion-mediated dephasing in the dipole field around a single spherical magnetic object.

PubMed

Buschle, Lukas R; Kurz, Felix T; Kampf, Thomas; Triphan, Simon M F; Schlemmer, Heinz-Peter; Ziener, Christian Herbert

2015-11-01

In this work, the time evolution of the free induction decay caused by the local dipole field of a spherical magnetic perturber is analyzed. The complicated treatment of the diffusion process is replaced by the strong-collision-approximation that allows a determination of the free induction decay in dependence of the underlying microscopic tissue parameters such as diffusion coefficient, sphere radius and susceptibility difference. The interplay between susceptibility- and diffusion-mediated effects yields several dephasing regimes of which, so far, only the classical regimes of motional narrowing and static dephasing for dominant and negligible diffusion, respectively, were extensively examined. Due to the asymmetric form of the dipole field for spherical objects, the free induction decay exhibits a complex component in contradiction to the cylindrical case, where the symmetric local dipole field only causes a purely real induction decay. Knowledge of the shape of the corresponding frequency distribution is necessary for the evaluation of more sophisticated pulse sequences and a detailed understanding of the off-resonance distribution allows improved quantification of transverse relaxation. Copyright © 2015 Elsevier Inc. All rights reserved.

Time-varying phononic crystals

NASA Astrophysics Data System (ADS)

Wright, Derek Warren

The primary objective of this thesis was to gain a deeper understanding of acoustic wave propagation in phononic crystals, particularly those that include materials whose properties can be varied periodically in time. This research was accomplished in three ways. First, a 2D phononic crystal was designed, created, and characterized. Its properties closely matched those determined through simulation. The crystal demonstrated band gaps, dispersion, and negative refraction. It served as a means of elucidating the practicalities of phononic crystal design and construction and as a physical verification of their more interesting properties. Next, the transmission matrix method for analyzing 1D phononic crystals was extended to include the effects of time-varying material parameters. The method was then used to provide a closed-form solution for the case of periodically time-varying material parameters. Some intriguing results from the use of the extended method include dramatically altered transmission properties and parametric amplification. New insights can be gained from the governing equations and have helped to identify the conditions that lead to parametric amplification in these structures. Finally, 2D multiple scattering theory was modified to analyze scatterers with time-varying material parameters. It is shown to be highly compatible with existing multiple scattering theories. It allows the total scattered field from a 2D time-varying phononic crystal to be determined. It was shown that time-varying material parameters significantly affect the phononic crystal transmission spectrum, and this was used to switch an incident monochromatic wave. Parametric amplification can occur under certain circumstances, and this effect was investigated using the closed-form solutions provided by the new 1D method. The complexity of the extended methods grows logarithmically as opposed linearly with existing methods, resulting in superior computational complexity for large

The graphene phonon dispersion with C12 and C13 isotopes

NASA Astrophysics Data System (ADS)

Whiteway, Eric; Bernard, Simon; Yu, Victor; Austing, D. Guy; Hilke, Michael

2013-12-01

Using very uniform large scale chemical vapor deposition grown graphene transferred onto silicon, we were able to identify 15 distinct Raman lines associated with graphene monolayers. This was possible thanks to a combination of different carbon isotopes and different Raman laser energies and extensive averaging without increasing the laser power. This allowed us to obtain a detailed experimental phonon dispersion relation for many points in the Brillouin zone. We further identified a D+D' peak corresponding to a double phonon process involving both an inter- and intra-valley phonon. In order to both eliminate substrate effects and to probe large areas, we undertook to study Raman scattering for large scale chemical vapor deposition (CVD) grown graphene using two different isotopes (C12 and C13) so that we can effectively exclude and subtract the substrate contributions, since a heavier mass downshifts only the vibrational properties, while keeping all other properties the same.

Electron—phonon Coupling and the Superconducting Phase Diagram of the LaAlO3—SrTiO3 Interface

PubMed Central

Boschker, Hans; Richter, Christoph; Fillis-Tsirakis, Evangelos; Schneider, Christof W.; Mannhart, Jochen

2015-01-01

The superconductor at the LaAlO3—SrTiO3 interface provides a model system for the study of two-dimensional superconductivity in the dilute carrier density limit. Here we experimentally address the pairing mechanism in this superconductor. We extract the electron—phonon spectral function from tunneling spectra and conclude, without ruling out contributions of further pairing channels, that electron—phonon mediated pairing is strong enough to account for the superconducting critical temperatures. Furthermore, we discuss the electron—phonon coupling in relation to the superconducting phase diagram. The electron—phonon spectral function is independent of the carrier density, except for a small part of the phase diagram in the underdoped region. The tunneling measurements reveal that the increase of the chemical potential with increasing carrier density levels off and is zero in the overdoped region of the phase diagram. This indicates that the additionally induced carriers do not populate the band that hosts the superconducting state and that the superconducting order parameter therefore is weakened by the presence of charge carriers in another band. PMID:26169351

First principles calculation of lattice thermal conductivity of metals considering phonon-phonon and phonon-electron scattering

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wang, Yan; Lu, Zexi; Ruan, Xiulin, E-mail: ruan@purdue.edu

2016-06-14

The effect of phonon-electron (p-e) scattering on lattice thermal conductivity is investigated for Cu, Ag, Au, Al, Pt, and Ni. We evaluate both phonon-phonon (p-p) and p-e scattering rates from first principles and calculate the lattice thermal conductivity (κ{sub L}). It is found that p-e scattering plays an important role in determining the κ{sub L} of Pt and Ni at room temperature, while it has negligible effect on the κ{sub L} of Cu, Ag, Au, and Al. Specifically, the room temperature κ{sub L}s of Cu, Ag, Au, and Al predicted from density-functional theory calculations with the local density approximation aremore » 16.9, 5.2, 2.6, and 5.8 W/m K, respectively, when only p-p scattering is considered, while it is almost unchanged when p-e scattering is also taken into account. However, the κ{sub L} of Pt and Ni is reduced from 7.1 and 33.2 W/m K to 5.8 and 23.2 W/m K by p-e scattering. Even though Al has quite high electron-phonon coupling constant, a quantity that characterizes the rate of heat transfer from hot electrons to cold phonons in the two-temperature model, p-e scattering is not effective in reducing κ{sub L} owing to the relatively low p-e scattering rates in Al. The difference in the strength of p-e scattering in different metals can be qualitatively understood by checking the amount of electron density of states that is overlapped with the Fermi window. Moreover, κ{sub L} is found to be comparable to the electronic thermal conductivity in Ni.« less

Generalization of soft phonon modes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rudin, Sven P.

Soft phonon modes describe a collective movement of atoms that transform a higher-symmetry crystal structure into a lower-symmetry crystal structure. Such structural transformations occur at finite temperatures, where the phonons (i.e., the low-temperature vibrational modes) and the static perfect crystal structures provide an incomplete picture of the dynamics. In this paper, principal vibrational modes (PVMs) are introduced as descriptors of the dynamics of a material system withmore » $N$ atoms. The PVMs represent the independent collective movements of the atoms at a given temperature. Molecular dynamics (MD) simulations, here in the form of quantum MD using density functional theory calculations, provide both the data describing the atomic motion and the data used to construct the PVMs. The leading mode, $${\\mathrm{PVM}}_{0}$$, represents the $3N$-dimensional direction in which the system moves with greatest amplitude. For structural phase transitions, $${\\mathrm{PVM}}_{0}$$ serves as a generalization of soft phonon modes. At low temperatures, $${\\mathrm{PVM}}_{0}$$ reproduces the soft phonon mode in systems where one phonon dominates the phase transformation. In general, multiple phonon modes combine to describe a transformation, in which case $${\\mathrm{PVM}}_{0}$$ culls these phonon modes. Moreover, while soft phonon modes arise in the higher-symmetry crystal structure, $${\\mathrm{PVM}}_{0}$$ can be equally well calculated on either side of the structural phase transition. Finally, two applications demonstrate these properties: first, transitions into and out of bcc titanium, and, second, the two crystal structures proposed for the $${\\beta}$$ phase of uranium, the higher-symmetry structure of which stabilizes with temperature.« less

Generalization of soft phonon modes

DOE PAGES

Rudin, Sven P.

2018-04-27

Soft phonon modes describe a collective movement of atoms that transform a higher-symmetry crystal structure into a lower-symmetry crystal structure. Such structural transformations occur at finite temperatures, where the phonons (i.e., the low-temperature vibrational modes) and the static perfect crystal structures provide an incomplete picture of the dynamics. In this paper, principal vibrational modes (PVMs) are introduced as descriptors of the dynamics of a material system withmore » $N$ atoms. The PVMs represent the independent collective movements of the atoms at a given temperature. Molecular dynamics (MD) simulations, here in the form of quantum MD using density functional theory calculations, provide both the data describing the atomic motion and the data used to construct the PVMs. The leading mode, $${\\mathrm{PVM}}_{0}$$, represents the $3N$-dimensional direction in which the system moves with greatest amplitude. For structural phase transitions, $${\\mathrm{PVM}}_{0}$$ serves as a generalization of soft phonon modes. At low temperatures, $${\\mathrm{PVM}}_{0}$$ reproduces the soft phonon mode in systems where one phonon dominates the phase transformation. In general, multiple phonon modes combine to describe a transformation, in which case $${\\mathrm{PVM}}_{0}$$ culls these phonon modes. Moreover, while soft phonon modes arise in the higher-symmetry crystal structure, $${\\mathrm{PVM}}_{0}$$ can be equally well calculated on either side of the structural phase transition. Finally, two applications demonstrate these properties: first, transitions into and out of bcc titanium, and, second, the two crystal structures proposed for the $${\\beta}$$ phase of uranium, the higher-symmetry structure of which stabilizes with temperature.« less

Generalization of soft phonon modes

NASA Astrophysics Data System (ADS)

Rudin, Sven P.

2018-04-01

Soft phonon modes describe a collective movement of atoms that transform a higher-symmetry crystal structure into a lower-symmetry crystal structure. Such structural transformations occur at finite temperatures, where the phonons (i.e., the low-temperature vibrational modes) and the static perfect crystal structures provide an incomplete picture of the dynamics. Here, principal vibrational modes (PVMs) are introduced as descriptors of the dynamics of a material system with N atoms. The PVMs represent the independent collective movements of the atoms at a given temperature. Molecular dynamics (MD) simulations, here in the form of quantum MD using density functional theory calculations, provide both the data describing the atomic motion and the data used to construct the PVMs. The leading mode, PVM0, represents the 3 N -dimensional direction in which the system moves with greatest amplitude. For structural phase transitions, PVM0 serves as a generalization of soft phonon modes. At low temperatures, PVM0 reproduces the soft phonon mode in systems where one phonon dominates the phase transformation. In general, multiple phonon modes combine to describe a transformation, in which case PVM0 culls these phonon modes. Moreover, while soft phonon modes arise in the higher-symmetry crystal structure, PVM0 can be equally well calculated on either side of the structural phase transition. Two applications demonstrate these properties: first, transitions into and out of bcc titanium, and, second, the two crystal structures proposed for the β phase of uranium, the higher-symmetry structure of which stabilizes with temperature.

Suppression of exciton dephasing in sidewall-functionalized carbon nanotubes embedded into metallo-dielectric antennas.

PubMed

Shayan, Kamran; He, Xiaowei; Luo, Yue; Rabut, Claire; Li, Xiangzhi; Hartmann, Nicolai F; Blackburn, Jeffrey L; Doorn, Stephen K; Htoon, Han; Strauf, Stefan

2018-06-26

Covalent functionalization of single-walled carbon nanotubes (SWCNTs) is a promising route to enhance the quantum yield of exciton emission and can lead to single-photon emission at room temperature. However, the spectral linewidth of the defect-related E11* emission remains rather broad. Here, we systematically investigate the low-temperature exciton emission of individual SWCNTs that have been dispersed with sodium-deoxycholate (DOC) and polyfluorene (PFO-BPy), are grown by laser vaporization (LV) or by CoMoCat techniques and are functionalized with oxygen as well as 3,5-dichlorobenzene groups. The E11 excitons in oxygen-functionalized SWCNTs remain rather broad with up to 10 meV linewidth while exciton emission from 3,5-dichlorobenzene functionalized SWCNTs is found to be about one order of magnitude narrower. In all cases, wrapping with PFO-BPy provides significantly better protection against pump induced dephasing compared to DOC. To further study the influence of exciton localization on pump-induced dephasing, we have embedded the functionalized SWCNTs into metallo-dielectric antenna cavities to maximize light collection. We show that 0D excitons attributed to the E11* emission of 3,5-dichlorobenzene quantum defects of LV-grown SWCNTs can display near resolution-limited linewidths down to 35 μeV. Interestingly, these 0D excitons give rise to a 3-fold suppressed pump-induced exciton dephasing compared to the E11 excitons in the same SWCNT. These findings provide a foundation to build a unified description of the emergence of novel optical behavior from the interplay of covalently introduced defects, dispersants, and exciton confinement in SWCNTs and might further lead to the realization of indistinguishable photons from carbon nanotubes.

Sound and heat revolutions in phononics

NASA Astrophysics Data System (ADS)

Maldovan, Martin

2013-11-01

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.

Electron mobility limited by optical phonons in wurtzite InGaN/GaN core-shell nanowires

NASA Astrophysics Data System (ADS)

Liu, W. H.; Qu, Y.; Ban, S. L.

2017-09-01

Based on the force-balance and energy-balance equations, the optical phonon-limited electron mobility in InxGa1-xN/GaN core-shell nanowires (CSNWs) is discussed. It is found that the electrons tend to distribute in the core of the CSNWs due to the strong quantum confinement. Thus, the scattering from first kind of the quasi-confined optical (CO) phonons is more important than that from the interface (IF) and propagating (PR) optical phonons. Ternary mixed crystal and size effects on the electron mobility are also investigated. The results show that the PR phonons exist while the IF phonons disappear when the indium composition x < 0.047, and vice versa. Accordingly, the total electron mobility μ first increases and then decreases with indium composition x, and reaches a peak value of approximately 3700 cm2/(V.s) when x = 0.047. The results also show that the mobility μ increases as increasing the core radius of CSNWs due to the weakened interaction between the electrons and CO phonons. The total electron mobility limited by the optical phonons exhibits an obvious enhancement as decreasing temperature or increasing line electron density. Our theoretical results are expected to be helpful to develop electronic devices based on CSNWs.

Undoing measurement-induced dephasing in circuit QED

NASA Astrophysics Data System (ADS)

Frisk Kockum, A.; Tornberg, L.; Johansson, G.

2012-05-01

We analyze the backaction of homodyne detection and photodetection on superconducting qubits in circuit quantum electrodynamics. Although both measurement schemes give rise to backaction in the form of stochastic phase rotations, which leads to dephasing, we show that this can be perfectly undone provided that the measurement signal is fully accounted for. This result improves on an earlier one [Phys. Rev. APLRAAN1050-294710.1103/PhysRevA.82.012329 82, 012329 (2010)], showing that the method suggested can be made to realize a perfect two-qubit parity measurement. We propose a benchmarking experiment on a single qubit to demonstrate the method using homodyne detection. By analyzing the limited measurement efficiency of the detector and bandwidth of the amplifier, we show that the parameter values necessary to see the effect are within the limits of existing technology.

Coherent acoustic phonons in nanostructures

NASA Astrophysics Data System (ADS)

Dekorsy, T.; Taubert, R.; Hudert, F.; Bartels, A.; Habenicht, A.; Merkt, F.; Leiderer, P.; Köhler, K.; Schmitz, J.; Wagner, J.

2008-02-01

Phonons are considered as a most important origin of scattering and dissipation for electronic coherence in nanostructures. The generation of coherent acoustic phonons with femtosecond laser pulses opens the possibility to control phonon dynamics in amplitude and phase. We demonstrate a new experimental technique based on two synchronized femtosecond lasers with GHz repetition rate to study the dynamics of coherently generated acoustic phonons in semiconductor heterostructures with high sensitivity. High-speed synchronous optical sampling (ASOPS) enables to scan a time-delay of 1 ns with 100 fs time resolution with a frequency in the kHz range without a moving part in the set-up. We investigate the dynamics of coherent zone-folded acoustic phonons in semiconductor superlattices (GaAs/AlAs and GaSb/InAs) and of coherent vibration of metallic nanostructures of non-spherical shape using ASOPS.

On the interplay between phonon-boundary scattering and phonon-point-defect scattering in SiGe thin films

NASA Astrophysics Data System (ADS)

Iskandar, A.; Abou-Khalil, A.; Kazan, M.; Kassem, W.; Volz, S.

2015-03-01

This paper provides theoretical understanding of the interplay between the scattering of phonons by the boundaries and point-defects in SiGe thin films. It also provides a tool for the design of SiGe-based high-efficiency thermoelectric devices. The contributions of the alloy composition, grain size, and film thickness to the phonon scattering rate are described by a model for the thermal conductivity based on the single-mode relaxation time approximation. The exact Boltzmann equation including spatial dependence of phonon distribution function is solved to yield an expression for the rate at which phonons scatter by the thin film boundaries in the presence of the other phonon scattering mechanisms. The rates at which phonons scatter via normal and resistive three-phonon processes are calculated by using perturbation theories with taking into account dispersion of confined acoustic phonons in a two dimensional structure. The vibrational parameters of the model are deduced from the dispersion of confined acoustic phonons as functions of temperature and crystallographic direction. The accuracy of the model is demonstrated with reference to recent experimental investigations regarding the thermal conductivity of single-crystal and polycrystalline SiGe films. The paper describes the strength of each of the phonon scattering mechanisms in the full temperature range. Furthermore, it predicts the alloy composition and film thickness that lead to minimum thermal conductivity in a single-crystal SiGe film, and the alloy composition and grain size that lead to minimum thermal conductivity in a polycrystalline SiGe film.

Epsilon-near-zero modes for tailored light-matter interaction

DOE Office of Scientific and Technical Information (OSTI.GOV)

Campione, Salvatore; Liu, Sheng; Benz, Alexander

Epsilon-near-zero (ENZ) modes arising from condensed-matter excitations such as phonons and plasmons are a new path for tailoring light-matter interactions at the nanoscale. Complex spectral shaping can be achieved by creating such modes in nanoscale semiconductor layers and controlling their interaction with multiple, distinct, dipole resonant systems. Examples of this behavior are presented at midinfrared frequencies for ENZ modes that are strongly coupled to metamaterial resonators and simultaneously strongly coupled to semiconductor phonons or quantum-well intersubband transitions (ISTs), resulting in double- and triple-polariton branches in transmission spectra. For the double-polariton branch case, we find that the best strategy to maximizemore » the Rabi splitting is to use a combination of a doped layer supporting an ENZ feature and a layer supporting ISTs, with overlapping ENZ and IST frequencies. As a result, this design flexibility renders this platform attractive for low-voltage tunable filters, light-emitting diodes, and efficient nonlinear composite materials.« less

Epsilon-near-zero modes for tailored light-matter interaction

DOE PAGES

Campione, Salvatore; Liu, Sheng; Benz, Alexander; ...

2015-10-20

Epsilon-near-zero (ENZ) modes arising from condensed-matter excitations such as phonons and plasmons are a new path for tailoring light-matter interactions at the nanoscale. Complex spectral shaping can be achieved by creating such modes in nanoscale semiconductor layers and controlling their interaction with multiple, distinct, dipole resonant systems. Examples of this behavior are presented at midinfrared frequencies for ENZ modes that are strongly coupled to metamaterial resonators and simultaneously strongly coupled to semiconductor phonons or quantum-well intersubband transitions (ISTs), resulting in double- and triple-polariton branches in transmission spectra. For the double-polariton branch case, we find that the best strategy to maximizemore » the Rabi splitting is to use a combination of a doped layer supporting an ENZ feature and a layer supporting ISTs, with overlapping ENZ and IST frequencies. As a result, this design flexibility renders this platform attractive for low-voltage tunable filters, light-emitting diodes, and efficient nonlinear composite materials.« less

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.

Sensing coherent phonons with two-photon interference

NASA Astrophysics Data System (ADS)

Ding, Ding; Yin, Xiaobo; Li, Baowen

2018-02-01

Detecting coherent phonons pose different challenges compared to coherent photons due to the much stronger interaction between phonons and matter. This is especially true for high frequency heat carrying phonons, which are intrinsic lattice vibrations experiencing many decoherence events with the environment, and are thus generally assumed to be incoherent. Two photon interference techniques, especially coherent population trapping (CPT) and electromagnetically induced transparency (EIT), have led to extremely sensitive detection, spectroscopy and metrology. Here, we propose the use of two photon interference in a three-level system to sense coherent phonons. Unlike prior works which have treated phonon coupling as damping, we account for coherent phonon coupling using a full quantum-mechanical treatment. We observe strong asymmetry in absorption spectrum in CPT and negative dispersion in EIT susceptibility in the presence of coherent phonon coupling which cannot be accounted for if only pure phonon damping is considered. Our proposal has application in sensing heat carrying coherent phonons effects and understanding coherent bosonic multi-pathway interference effects in three coupled oscillator systems.

Superradiance-Driven Phonon Laser

NASA Astrophysics Data System (ADS)

Jiang, Ya-Jing; Lü, Hao; Jing, Hui

2018-04-01

We propose to enhance the generation of a phonon laser by exploiting optical superradiance. In our scheme, the optomechanical cavity contains a movable membrane, which supports a mechanical mode, and the superradiance cavity can generate the coherent collective light emissions by applying a transverse pump to an ultracold intracavity atomic gas. The superradiant emission turns out to be capable of enhancing the phonon laser performance. This indicates a new way to operate a phonon laser with the assistance of coherent atomic gases trapped in a cavity or lattice potentials.

Electron-Phonon Systems on a Universal Quantum Computer

DOE Office of Scientific and Technical Information (OSTI.GOV)

Macridin, Alexandru; Spentzouris, Panagiotis; Amundson, James

We present an algorithm that extends existing quantum algorithms forsimulating fermion systems in quantum chemistry and condensed matter physics toinclude phonons. The phonon degrees of freedom are represented with exponentialaccuracy on a truncated Hilbert space with a size that increases linearly withthe cutoff of the maximum phonon number. The additional number of qubitsrequired by the presence of phonons scales linearly with the size of thesystem. The additional circuit depth is constant for systems with finite-rangeelectron-phonon and phonon-phonon interactions and linear for long-rangeelectron-phonon interactions. Our algorithm for a Holstein polaron problem wasimplemented on an Atos Quantum Learning Machine (QLM) quantum simulatoremployingmore » the Quantum Phase Estimation method. The energy and the phonon numberdistribution of the polaron state agree with exact diagonalization results forweak, intermediate and strong electron-phonon coupling regimes.« less

Time-resolved photon echoes from donor-bound excitons in ZnO epitaxial layers

NASA Astrophysics Data System (ADS)

Poltavtsev, S. V.; Kosarev, A. N.; Akimov, I. A.; Yakovlev, D. R.; Sadofev, S.; Puls, J.; Hoffmann, S. P.; Albert, M.; Meier, C.; Meier, T.; Bayer, M.

2017-07-01

The coherent optical response from 140 nm and 65 nm thick ZnO epitaxial layers is studied using four-wave-mixing spectroscopy with picosecond temporal resolution. Resonant excitation of neutral donor-bound excitons results in two-pulse and three-pulse photon echoes. For the donor-bound A exciton (D0XA ) at temperature of 1.8 K we evaluate optical coherence times T2=33 -50 ps corresponding to homogeneous line widths of 13 -19 μ eV , about two orders of magnitude smaller as compared with the inhomogeneous broadening of the optical transitions. The coherent dynamics is determined mainly by the population decay with time T1=30 -40 ps, while pure dephasing is negligible. Temperature increase leads to a significant shortening of T2 due to interaction with acoustic phonons. In contrast, the loss of coherence of the donor-bound B exciton (D0XB ) is significantly faster (T2=3.6 ps ) and governed by pure dephasing processes.

Engineering dissipation with phononic spectral hole burning

NASA Astrophysics Data System (ADS)

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.

Finite element analysis of true and pseudo surface acoustic waves in one-dimensional phononic crystals

DOE Office of Scientific and Technical Information (OSTI.GOV)

Graczykowski, B., E-mail: bartlomiej.graczykowski@icn.cat; Alzina, F.; Gomis-Bresco, J.

In this paper, we report a theoretical investigation of surface acoustic waves propagating in one-dimensional phononic crystal. Using finite element method eigenfrequency and frequency response studies, we develop two model geometries suitable to distinguish true and pseudo (or leaky) surface acoustic waves and determine their propagation through finite size phononic crystals, respectively. The novelty of the first model comes from the application of a surface-like criterion and, additionally, functional damping domain. Exemplary calculated band diagrams show sorted branches of true and pseudo surface acoustic waves and their quantified surface confinement. The second model gives a complementary study of transmission, reflection,more » and surface-to-bulk losses of Rayleigh surface waves in the case of a phononic crystal with a finite number of periods. Here, we demonstrate that a non-zero transmission within non-radiative band gaps can be carried via leaky modes originating from the coupling of local resonances with propagating waves in the substrate. Finally, we show that the transmission, reflection, and surface-to-bulk losses can be effectively optimised by tuning the geometrical properties of a stripe.« less

Splash, pop, sizzle: Information processing with phononic computing

DOE Office of Scientific and Technical Information (OSTI.GOV)

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 themore » device level approach to diodes, transistors, memory, and logic. .« less

Phonon coupling to dynamic short-range polar order in a relaxor ferroelectric near the morphotropic phase boundary

DOE PAGES

John A. Schneeloch; Xu, Zhijun; Winn, B.; ...

2015-12-28

We report neutron inelastic scattering experiments on single-crystal PbMg 1/3Nb 2/3O 3 doped with 32% PbTiO 3, a relaxor ferroelectric that lies close to the morphotropic phase boundary. When cooled under an electric field E∥ [001] into tetragonal and monoclinic phases, the scattering cross section from transverse acoustic (TA) phonons polarized parallel to E weakens and shifts to higher energy relative to that under zero-field-cooled conditions. Likewise, the scattering cross section from transverse optic (TO) phonons polarized parallel to E weakens for energy transfers 4 ≤ ℏω ≤ 9 meV. However, TA and TO phonons polarized perpendicular to E showmore » no change. This anisotropic field response is similar to that of the diffuse scattering cross section, which, as previously reported, is suppressed when polarized parallel to E but not when polarized perpendicular to E. Lastly, our findings suggest that the lattice dynamics and dynamic short-range polar correlations that give rise to the diffuse scattering are coupled.« less

Coupling of phonons with excitons bound to different donors and acceptors in hexagonal GaN

NASA Astrophysics Data System (ADS)

Korona, K. P.; Wysmoek, A.; Kuhl, J.; Kamiska, M.; Baranowski, J. M.; Look, D. C.; Park, S. S.

2006-06-01

Time-resolved measurements of GaN with different donors (oxygen or silicon) and acceptors (zinc or magnesium) showed pronounced bound exciton lines and their phonon replicas. The analysis included three phonon modes characteristic for the wurtzite (hexagonal) phase: A1(LO), E1(TO) and E2H. It was shown that relative amplitudes of replicas depended upon the chemical nature of the defects that the bind excitons. The replicas were stronger for acceptor- than for donor-related features. Huang-Rhys factors S = 0.06 +/- 0.02 and S = 0.025 +/- 0.01, were found for the A0X and the D0X LO replicas, respectively. A significant difference in phonon coupling to silicon and oxygen donor bound excitons has been observed.

Improving the accuracy and efficiency of time-resolved electronic spectra calculations: cellular dephasing representation with a prefactor.

PubMed

Zambrano, Eduardo; Šulc, Miroslav; Vaníček, Jiří

2013-08-07

Time-resolved electronic spectra can be obtained as the Fourier transform of a special type of time correlation function known as fidelity amplitude, which, in turn, can be evaluated approximately and efficiently with the dephasing representation. Here we improve both the accuracy of this approximation-with an amplitude correction derived from the phase-space propagator-and its efficiency-with an improved cellular scheme employing inverse Weierstrass transform and optimal scaling of the cell size. We demonstrate the advantages of the new methodology by computing dispersed time-resolved stimulated emission spectra in the harmonic potential, pyrazine, and the NCO molecule. In contrast, we show that in strongly chaotic systems such as the quartic oscillator the original dephasing representation is more appropriate than either the cellular or prefactor-corrected methods.

Phonon properties of iron-based superconductors

NASA Astrophysics Data System (ADS)

Gupta, Yuhit; Goyal, Megha; Sinha, M. M.

2018-05-01

Earlier, it was thought there is antagonist relationship between superconductivity and ferromagnetic materials, But, a discovery of iron-based superconductors have removed this misconception. It gives an idea to make a review on the superconductivity properties of different materials. The new iron-based superconductors' present symmetry breaking competing phases in the form of tetragonal to orthorhombic transition. It consists of mainly four families [1111], [111], [122], and [11] type. Superconductivity of iron-based superconductors mainly related with the phonons and there is an excellent relation between phonons and superconductivity. Phonons properties are helpful in predicting the superconducting properties of materials. Phonon properties of iron-based superconductors in various phases are summarized in this study. We are presenting the review of phonon properties of iron-based superconductors.

Phonon dispersions, band structures, and dielectric functions of BeO and BeS polymorphs

NASA Astrophysics Data System (ADS)

Wang, Ke-Long; Gao, Shang-Peng

2018-07-01

Structures, phonon dispersions, electronic structures, and dielectric functions of beryllium oxide (BeO) and beryllium sulfide (BeS) polymorphs are investigated by density functional theory and many-body perturbation theory. Phonon calculations indicate that both wurtzite (w-) and zincblende (zb-) structures are dynamically stable for BeO and BeS, whereas rocksalt (rs-) structures for both BeO and BeS have imaginary phonon frequencies and thus are dynamically unstable at zero pressure. Band structures for the 4 dynamically stable phases show that only w-BeO has a direct band gap. Both the one-shot G0W0 and quasiparticle self-consistent GW methods are used to correct band energies at high symmetry k-points. Bethe-Salpeter equation (BSE), which considers Coulomb correlated electron-hole pairs, is employed to deal with the computation of macroscopic dielectric functions. It is shown that BSE calculation, employing scissors operator derived by self-consistent GW method, can give dielectric functions agreeing very well with experimental measurement of w-BeO. Weak anisotropic characters can be observed for w-BeO and w-BeS. Both zb-BeS and w-BeS show high optical transition probabilities within a narrow ultraviolet energy range.

Lattice anharmonicity, phonon dispersion, and thermal conductivity of PbTe studied by the phonon quasiparticle approach

NASA Astrophysics Data System (ADS)

Lu, Yong; Sun, Tao; Zhang, Dong-Bo

2018-05-01

We investigated the vibrational property of lead telluride (PbTe) with a focus on lattice anharmonicity at moderate temperatures (300 phonon quasiparticle approach which combines first-principles molecular dynamics and lattice dynamics. The calculated anharmonic phonon dispersions are strongly temperature dependent and some phonon modes adopt giant frequency shifts, e.g., transverse optical modes in the long-wavlength regime. As a result, we witness the avoided crossing between transverse optical modes and longitudinal acoustic modes at elevated temperature, in good agreement with experimentation and available theoretical studies. These results, together with the large root-mean-square displacements of atoms, reveal a strong anharmonic effect in PbTe. The obtained phonon lifetimes allow studies of transport properties. For considered temperatures, the phonon mean free paths can be shorter than lattice constants at relatively high temperature, especially for optical modes. This finding goes against the widely employed minimal phonon mean free path concept. As such, the calculated lattice thermal conductivity of PbTe, which is indeed relatively small, does not have the prescribed minima at high temperature, showcasing the breakdown of the minimal mean free path theory. Our study provides a basis for delineating vibrational and transport properties of PbTe and other thermoelectric materials within the framework of the phonon gas model.

Enhancement of multiple-phonon resonant Raman scattering in Co-doped ZnO nanorods

NASA Astrophysics Data System (ADS)

Phan, The-Long; Vincent, Roger; Cherns, David; Dan, Nguyen Huy; Yu, Seong-Cho

2008-08-01

We have studied Raman scattering in Co-doped ZnO nanorods prepared by thermal diffusion. Experimental results show that the features of their non-resonant spectra are similar to Raman spectra from Co-doped ZnO materials investigated previously. Under resonant conditions, however, there is a strong enhancement of multiple-phonon Raman scattering processes. Longitudinal optical (LO)-phonon overtones up to eleventh order are observed. The modes become more obvious when the Co concentration diffused into ZnO nanorods goes to an appropriate value. This phenomenon is explained due to the shift of the band-gap energy and also due to the decrease in the intensity of near-band-edge luminescence. Our observation is in agreement with the prediction [J. F. Scott, Phys. Rev. B 2, 1209 (1970)] that the number of LO-phonon lines in ZnO is higher than that observed for CdS.

Electron-phonon Interactions in HTSC Cuprates

DOE Office of Scientific and Technical Information (OSTI.GOV)

Egami, T.; Chung, J.-H.; McQueeny, R. J.

Phonons have been generally considered to be irrelevant to the high-temperature superconductivity in the cuprates. However, such a bias is usually based upon the assumption of conventional electron-phonon coupling, while in the cuprates the coupling can be rather unconventional because of strong electron correlation. We present the results of our recent measurements of phonon dispersion in YBa{sub 2}Cu{sub 3}O{sub 6+x} by inelastic neutron scattering. These suggest certain phonon modes interact strongly with electrons and are closely involved in the superconductivity phenomenon with possible contribution to pairing.

Vibrational frequencies and dephasing times in excited electronic states by femtosecond time-resolved four-wave mixing

NASA Astrophysics Data System (ADS)

Joo, Taiha; Albrecht, A. C.

1993-06-01

Time-resolved degenerate four-wave mixing (TRDFWM) for an electronically resonant system in a phase-matching configuration that measures population decay is reported. Because the spectral width of input light exceeds the vibrational Bohr frequency of a strong Raman active mode, the vibrational coherence produces strong oscillations in the TRDFWM signal together with the usual population decay from the excited electronic state. The data are analyzed in terms of a four-level system: ground and excited electronic states each split by a vibrational quantum of a Raman active mode. Absolute frequencies and their dephasing times of the vibrational modes at ≈590 cm -1 are obtained for the excited as well as the ground electronic state. The vibrational dephasing rate in the excited electronic state is about an order of magnitude faster than that in the ground state, the origin of which is speculated upon.

Topological chiral phonons in center-stacked bilayer triangle lattices

NASA Astrophysics Data System (ADS)

Xu, Xifang; Zhang, Wei; Wang, Jiaojiao; Zhang, Lifa

2018-06-01

Since chiral phonons were found in an asymmetric two-dimensional hexagonal lattice, there has been growing interest in the study of phonon chirality, which were experimentally verified very recently in monolayer tungsten diselenide (2018 Science 359 579). In this work, we find chiral phonons with nontrivial topology in center-stacked bilayer triangle lattices. At the Brillouin-zone corners, (), circularly polarized phonons and nonzero phonon Berry curvature are observed. Moreover, we find that the phonon chirality remain robust with changing sublattice mass ratio and interlayer coupling. The chiral phonons at the valleys are demonstrated in doubler-layer sodium chloride along the [1 1 1] direction. We believe that the findings on topological chiral phonons in triangle lattices will give guidance in the study of chiral phonons in real materials and promote the phononic applications.

Phononic Origins of Friction in Carbon Nanotube Oscillators.

PubMed

Prasad, Matukumilli V D; Bhattacharya, Baidurya

2017-04-12

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.

Topological phonon modes in filamentary structures

NASA Astrophysics Data System (ADS)

Berg, Nina; Joel, Kira; Koolyk, Miriam; Prodan, Emil

2011-02-01

This work describes a class of topological phonon modes, that is, mechanical vibrations localized at the edges of special structures that are robust against the deformations of the structures. A class of topological phonons was recently found in two-dimensional structures similar to that of microtubules. The present work introduces another class of topological phonons, this time occurring in quasi-one-dimensional filamentary structures with inversion symmetry. The phenomenon is exemplified using a structure inspired from that of actin microfilaments, present in most live cells. The system discussed here is probably the simplest structure that supports topological phonon modes, a fact that allows detailed analysis in both time and frequency domains. We advance the hypothesis that the topological phonon modes are ubiquitous in the biological world and that living organisms make use of them during various processes.

Phonon thermodynamics of iron and cementite

NASA Astrophysics Data System (ADS)

Mauger, Lisa Mary

The vibrational properties of materials are essential to understanding material stability and thermodynamics. In this thesis I outline vibrational thermodynamic models and the experimental tools that provide evidence on phonon behavior. The introductory section discusses the history of metallurgy and thermodynamic theory, with an emphasis on the role of iron and cementite, two important components of steels. The thermodynamic framework for understanding vibrational material behavior is provided alongside the growing body of experimental and computational tools that provide physical insight on vibrational properties. The high temperature vibrational behavior of iron and cementite are explored within this context in the final chapters. Body-centered-cubic iron exhibits decreasing phonon energies at elevated temperatures. The observed energy change in not uniform across phonon modes in iron, and specific phonon modes show significant decreases in energy that are not explained by simple vibrational models. This anomalously energy decrease is linked to the second-nearest-neighbor interactions in the bcc structure, through examination of fitted interatomic force constants. The large changes in phonon energy result in a significant increase in the vibrational entropy, called the nonharmonic vibrational entropy, which emulates the temperature behavior of the magnetic entropy across the Curie temperature. The nonharmonic vibrational entropy is attributed to interactions between the vibrations and state of magnetic disorder in the material, which persists above the magnetic transitions and extends the stability region of the bcc phase. Orthorombic cementite, Fe3C, exhibits anisotropic magneto-volume behavior in the ferromagnetic phase including regions very low thermal expansion. The phonon modes of cementite show anomalous temperature dependence, with low energy phonon modes increasing their energy at elevated temperatures in the ferromagnetic phase. This behavior is

Scattering Tools for Nanostructure Phonon Engineering

DTIC Science & Technology

2013-09-25

characterization of phonons in nanomaterials, such as Raman scattering, are sensitive only to phonon modes with wavevectors of extremely small magnitude...Fundamentally the wavevectors that can be probed by Raman scattering are limited by the small momentum of photons in the visible spectrum. Our work...serious characterization challenge because existing experimental techniques for the characterization of phonons in nanomaterials, such as Raman

Determining the nature of excitonic dephasing in high-quality GaN/AlGaN quantum wells through time-resolved and spectrally resolved four-wave mixing spectroscopy

NASA Astrophysics Data System (ADS)

Gallart, M.; Ziegler, M.; Crégut, O.; Feltin, E.; Carlin, J.-F.; Butté, R.; Grandjean, N.; Hönerlage, B.; Gilliot, P.

2017-07-01

Applying four-wave mixing spectroscopy to a high-quality GaN/AlGaN single quantum well, we report on the experimental determination of excitonic dephasing times at different temperatures and exciton densities in III-nitride heterostructures. By comparing the evolution with the temperature of the dephasing and the spin-relaxation rate, we conclude that both processes are related to the rate of excitonic collisions. When spin relaxation occurs in the motional-narrowing regime, it remains constant over a large temperature range as the spin-precession frequency increases linearly with temperature, hence compensating for the observed decrease in the dephasing time. From those measurements, a value of the electron-hole exchange interaction strength of 0.45 meV at T =10 K is inferred.

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.

Giant suppression of phononic heat transport in a quantum magnet BiCu2PO6

NASA Astrophysics Data System (ADS)

Jeon, Byung-Gu; Koteswararao, B.; Park, C. B.; Shu, G. J.; Riggs, S. C.; Moon, E. G.; Chung, S. B.; Chou, F. C.; Kim, Kee Hoon

2016-11-01

Thermal transport of quantum magnets has elucidated the nature of low energy elementary excitations and complex interplay between those excited states via strong scattering of thermal carriers. BiCu2PO6 is a unique frustrated spin-ladder compound exhibiting highly anisotropic spin excitations that contain both itinerant and localized dispersion characters along the b- and a-axes respectively. Here, we investigate thermal conductivity κ of BiCu2PO6 under high magnetic fields (H) of up to 30 tesla. A dip-feature in κ, located at ~15 K at zero-H along all crystallographic directions, moves gradually toward lower temperature (T) with increasing H, thus resulting in giant suppression by a factor of ~30 near the critical magnetic field of Hc ≅ 23.5 tesla. The giant H- and T-dependent suppression of κ can be explained by the combined result of resonant scattering of phononic heat carriers with magnetic energy levels and increased phonon scattering due to enhanced spin fluctuation at Hc, unequivocally revealing the existence of strong spin-phonon coupling. Moreover, we find an experimental indication that the remaining magnetic heat transport along the b-axis becomes almost gapless at the magnetic quantum critical point realized at Hc.

Giant suppression of phononic heat transport in a quantum magnet BiCu2PO6.

PubMed

Jeon, Byung-Gu; Koteswararao, B; Park, C B; Shu, G J; Riggs, S C; Moon, E G; Chung, S B; Chou, F C; Kim, Kee Hoon

2016-11-15

Thermal transport of quantum magnets has elucidated the nature of low energy elementary excitations and complex interplay between those excited states via strong scattering of thermal carriers. BiCu 2 PO 6 is a unique frustrated spin-ladder compound exhibiting highly anisotropic spin excitations that contain both itinerant and localized dispersion characters along the b- and a-axes respectively. Here, we investigate thermal conductivity κ of BiCu 2 PO 6 under high magnetic fields (H) of up to 30 tesla. A dip-feature in κ, located at ~15 K at zero-H along all crystallographic directions, moves gradually toward lower temperature (T) with increasing H, thus resulting in giant suppression by a factor of ~30 near the critical magnetic field of H c  ≅ 23.5 tesla. The giant H- and T-dependent suppression of κ can be explained by the combined result of resonant scattering of phononic heat carriers with magnetic energy levels and increased phonon scattering due to enhanced spin fluctuation at H c , unequivocally revealing the existence of strong spin-phonon coupling. Moreover, we find an experimental indication that the remaining magnetic heat transport along the b-axis becomes almost gapless at the magnetic quantum critical point realized at H c .

Giant suppression of phononic heat transport in a quantum magnet BiCu2PO6

PubMed Central

Jeon, Byung-Gu; Koteswararao, B.; Park, C. B.; Shu, G. J.; Riggs, S. C.; Moon, E. G.; Chung, S. B.; Chou, F. C.; Kim, Kee Hoon

2016-01-01

Thermal transport of quantum magnets has elucidated the nature of low energy elementary excitations and complex interplay between those excited states via strong scattering of thermal carriers. BiCu2PO6 is a unique frustrated spin-ladder compound exhibiting highly anisotropic spin excitations that contain both itinerant and localized dispersion characters along the b- and a-axes respectively. Here, we investigate thermal conductivity κ of BiCu2PO6 under high magnetic fields (H) of up to 30 tesla. A dip-feature in κ, located at ~15 K at zero-H along all crystallographic directions, moves gradually toward lower temperature (T) with increasing H, thus resulting in giant suppression by a factor of ~30 near the critical magnetic field of Hc ≅ 23.5 tesla. The giant H- and T-dependent suppression of κ can be explained by the combined result of resonant scattering of phononic heat carriers with magnetic energy levels and increased phonon scattering due to enhanced spin fluctuation at Hc, unequivocally revealing the existence of strong spin-phonon coupling. Moreover, we find an experimental indication that the remaining magnetic heat transport along the b-axis becomes almost gapless at the magnetic quantum critical point realized at Hc. PMID:27845377

Giant suppression of phononic heat transport in a quantum magnet BiCu 2PO 6

DOE PAGES

Jeon, Byung-Gu; Koteswararao, B.; Park, C. B.; ...

2016-11-15

Thermal transport of quantum magnets has elucidated the nature of low energy elementary excitations and complex interplay between those excited states via strong scattering of thermal carriers. BiCu 2PO 6 is a unique frustrated spin-ladder compound exhibiting highly anisotropic spin excitations that contain both itinerant and localized dispersion characters along the b- and a-axes respectively. Here, we investigate thermal conductivity κ of BiCu 2PO 6 under high magnetic fields (H) of up to 30 tesla. A dip-feature in κ, located at ~15K at zero-H along all crystallographic directions, moves gradually toward lower temperature (T) with increasing H, thus resulting inmore » giant suppression by a factor of ~30 near the critical magnetic field of H c≅23.5 tesla. The giant H- and T-dependent suppression of κ can be explained by the combined result of resonant scattering of phononic heat carriers with magnetic energy levels and increased phonon scattering due to enhanced spin fluctuation at H c, unequivocally revealing the existence of strong spin-phonon coupling. Moreover, we find an experimental indication that the remaining magnetic heat transport along the b-axis becomes almost gapless at the magnetic quantum critical point realized at H c.« less

Effect of solvent polarity on the vibrational dephasing dynamics of the nitrosyl stretch in an Fe(II) complex revealed by 2D IR spectroscopy.

PubMed

Brookes, Jennifer F; Slenkamp, Karla M; Lynch, Michael S; Khalil, Munira

2013-07-25

The vibrational dephasing dynamics of the nitrosyl stretching vibration (ν(NO)) in sodium nitroprusside (SNP, Na2[Fe(CN)5NO]·2H2O) are investigated using two-dimensional infrared (2D IR) spectroscopy. The ν(NO) in SNP acts as a model system for the nitrosyl ligand found in metalloproteins which play an important role in the transportation and detection of nitric oxide (NO) in biological systems. We perform a 2D IR line shape study of the ν(NO) in the following solvents: water, deuterium oxide, methanol, ethanol, ethylene glycol, formamide, and dimethyl sulfoxide. The frequency of the ν(NO) exhibits a large vibrational solvatochromic shift of 52 cm(-1), ranging from 1884 cm(-1) in dimethyl sulfoxide to 1936 cm(-1) in water. The vibrational anharmonicity of the ν(NO) varies from 21 to 28 cm(-1) in the solvents used in this study. The frequency-frequency correlation functions (FFCFs) of the ν(NO) in SNP in each of the seven solvents are obtained by fitting the experimentally obtained 2D IR spectra using nonlinear response theory. The fits to the 2D IR line shape reveal that the spectral diffusion time scale of the ν(NO) in SNP varies from 0.8 to 4 ps and is negatively correlated with the empirical solvent polarity scales. We compare our results with the experimentally determined FFCFs of other charged vibrational probes in polar solvents and in the active sites of heme proteins. Our results suggest that the vibrational dephasing dynamics of the ν(NO) in SNP reflect the fluctuations of the nonhomogeneous electric field created by the polar solvents around the nitrosyl and cyanide ligands. The solute solvent interactions occurring at the trans-CN ligand are sensed through the π-back-bonding network along the Fe-NO bond in SNP.

Transmission eigenchannels for coherent phonon transport

NASA Astrophysics Data System (ADS)

Klöckner, J. C.; Cuevas, J. C.; Pauly, F.

2018-04-01

We present a procedure to determine transmission eigenchannels for coherent phonon transport in nanoscale devices using the framework of nonequilibrium Green's functions. We illustrate our procedure by analyzing a one-dimensional chain, where all steps can be carried out analytically. More importantly, we show how the procedure can be combined with ab initio calculations to provide a better understanding of phonon heat transport in realistic atomic-scale junctions. In particular, we study the phonon eigenchannels in a gold metallic atomic-size contact and different single-molecule junctions based on molecules such as an alkane chain, a brominated benzene-diamine, where destructive phonon interference effects take place, and a C60 junction.

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

Implementation and characterization of a controllable dephasing channel based on coupling polarization and spatial degrees of freedom of light.

PubMed

Urrego, Daniel F; Álvarez, Juan-Rafael; Calderón-Losada, Omar; Svozilík, Jiří; Nuñez, Mayerlin; Valencia, Alejandra

2018-04-30

We present the experimental implementation and theoretical model of a controllable dephasing quantum channel using photonic systems. The channel is implemented by coupling the polarization and the spatial distribution of light that play, in the perspective of open quantum systems, the role of quantum system and environment, respectively. The capability of controlling our channel allows us to visualize its effects in a quantum system. Different from standard dephasing channels, our channel presents an exotic behavior in the sense that the evolution of a state, from a pure to a mixed state, shows an oscillatory behavior if tracked in the Bloch sphere. Additionally, we report the evolution of the purity and perform a quantum process tomography to obtain the χ matrix associated to our channel.

Phononic glass: a robust acoustic-absorption material.

PubMed

Jiang, Heng; Wang, Yuren

2012-08-01

In order to achieve strong wide band acoustic absorption under high hydrostatic pressure, an interpenetrating network structure is introduced into the locally resonant phononic crystal to fabricate a type of phononic composite material called "phononic glass." Underwater acoustic absorption coefficient measurements show that the material owns high underwater sound absorption coefficients over 0.9 in 12-30 kHz. Moreover, the quasi-static compressive behavior shows that the phononic glass has a compressive strength over 5 MPa which is crucial for underwater applications.

Scanning Tunneling Microscopy Observation of Phonon Condensate

DOE Office of Scientific and Technical Information (OSTI.GOV)

Altfeder, Igor; Balatsky, Alexander V.; Voevodin, Andrey A.

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 formationmore » 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.« less

Scanning Tunneling Microscopy Observation of Phonon Condensate

DOE PAGES

Altfeder, Igor; Balatsky, Alexander V.; Voevodin, Andrey A.; ...

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 formationmore » 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.« less

Electron-phonon coupling and phonon subbands in small, electrically heated metal wires

NASA Astrophysics Data System (ADS)

Perrin, N.; Wybourne, M. N.

1996-02-01

The initial work of Perrin and Budd is extended to small metal wires in which the usual bulk phonon spectrum is modified into a series of acoustic subbands at low temperature. We analyze the contribution of the subbands to the lack of equilibrium between the electrons and the phonons in the wire heated by an applied electric field. The resulting electrical behavior of the wire is also considered and compared to experimental results.

Anharmonic phonons and second-order phase-transitions by the stochastic self-consistent harmonic approximation

NASA Astrophysics Data System (ADS)

Mauri, Francesco

Anharmonic effects can generally be treated within perturbation theory. Such an approach breaks down when the harmonic solution is dynamically unstable or when the anharmonic corrections of the phonon energies are larger than the harmonic frequencies themselves. This situation occurs near lattice-related second-order phase-transitions such as charge-density-wave (CDW) or ferroelectric instabilities or in H-containing materials, where the large zero-point motion of the protons results in a violation of the harmonic approximation. Interestingly, even in these cases, phonons can be observed, measured, and used to model transport properties. In order to treat such cases, we developed a stochastic implementation of the self-consistent harmonic approximation valid to treat anharmonicity in the nonperturbative regime and to obtain, from first-principles, the structural, thermodynamic and vibrational properties of strongly anharmonic systems. I will present applications to the ferroelectric transitions in SnTe, to the CWD transitions in NbS2 and NbSe2 (in bulk and monolayer) and to the hydrogen-bond symmetrization transition in the superconducting hydrogen sulfide system, that exhibits the highest Tc reported for any superconductor so far. In all cases we are able to predict the transition temperature (pressure) and the evolution of phonons with temperature (pressure). This project has received funding from the European Union's Horizon 2020 research and innovation programme under Grant agreement No. 696656 GrapheneCore1.

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

Quantum many-body correlations in collective phonon-excitations

NASA Astrophysics Data System (ADS)

Droenner, Leon; Kabuss, Julia; Carmele, Alexander

2018-02-01

We present a theoretical study of a many-emitter phonon laser based on optically driven semiconductor quantum dots placed within an acoustic nanocavity. A transformation of the phonon laser Hamiltonian leads to a Tavis-Cummings type interaction with an unexpected additional many-emitter energy shift. This many-emitter interaction with the cavity mode results in a variety of phonon resonances which dependent strongly on the number of participating emitters. These collective resonances show the highest phonon output. Furthermore, we show that the output can be increased even more via lasing at the two phonon resonance.

Tunable infrared reflectance by phonon modulation

DOEpatents

Ihlefeld, Jon F.; Sinclair, Michael B.; Beechem, III, Thomas E.

2018-03-06

The present invention pertains to the use of mobile coherent interfaces in a ferroelectric material to interact with optical phonons and, ultimately, to affect the material's optical properties. In altering the optical phonon properties, the optical properties of the ferroelectric material in the spectral range near-to the phonon mode frequency can dramatically change. This can result in a facile means to change to the optical response of the ferroelectric material in the infrared.

Heterodyne x-ray diffuse scattering from coherent phonons

DOE PAGES

Kozina, M.; Trigo, M.; Chollet, M.; ...

2017-08-10

Here in this paper, we report Fourier-transform inelastic x-ray scattering measurements of photoexcited GaAs with embedded ErAs nanoparticles. We observe temporal oscillations in the x-ray scattering intensity, which we attribute to inelastic scattering from coherent acoustic phonons. Unlike in thermal equilibrium, where inelastic x-ray scattering is proportional to the phonon occupation, we show that the scattering is proportional to the phonon amplitude for coherent states. The wavevectors of the observed phonons extend beyond the excitation wavevector. The nanoparticles break the discrete translational symmetry of the lattice, enabling the generation of large wavevector coherent phonons. Elastic scattering of x-ray photons frommore » the nanoparticles provides a reference for heterodyne mixing, yielding signals proportional to the phonon amplitude.« less

Nonperturbative Quantum Nature of the Dislocation–Phonon Interaction

DOE PAGES

Li, Mingda; Ding, Zhiwei; Meng, Qingping; ...

2017-01-31

Despite the long history of dislocation–phonon interaction studies, there are many problems that have not been fully resolved during this development. These include an incompatibility between a perturbative approach and the long-range nature of a dislocation, the relation between static and dynamic scattering, and their capability of dealing with thermal transport phenomena for bulk material only. Here in this paper, by utilizing a fully quantized dislocation field, which we called a “dislon”, a phonon interacting with a dislocation is renormalized as a quasi-phonon, with shifted quasi-phonon energy, and accompanied by a finite quasi-phonon lifetime, which are reducible to classical results.more » A series of outstanding legacy issues including those above can be directly explained within this unified phonon renormalization approach. For instance, a renormalized phonon naturally resolves the decade-long debate between dynamic and static dislocation–phonon scattering approaches, as two limiting cases. In particular, at nanoscale, both the dynamic and static approaches break down, while the present renormalization approach remains valid by capturing the size effect, showing good agreement with lattice dynamics simulations.« less

Nonperturbative Quantum Nature of the Dislocation–Phonon Interaction

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, Mingda; Ding, Zhiwei; Meng, Qingping

Despite the long history of dislocation–phonon interaction studies, there are many problems that have not been fully resolved during this development. These include an incompatibility between a perturbative approach and the long-range nature of a dislocation, the relation between static and dynamic scattering, and their capability of dealing with thermal transport phenomena for bulk material only. Here in this paper, by utilizing a fully quantized dislocation field, which we called a “dislon”, a phonon interacting with a dislocation is renormalized as a quasi-phonon, with shifted quasi-phonon energy, and accompanied by a finite quasi-phonon lifetime, which are reducible to classical results.more » A series of outstanding legacy issues including those above can be directly explained within this unified phonon renormalization approach. For instance, a renormalized phonon naturally resolves the decade-long debate between dynamic and static dislocation–phonon scattering approaches, as two limiting cases. In particular, at nanoscale, both the dynamic and static approaches break down, while the present renormalization approach remains valid by capturing the size effect, showing good agreement with lattice dynamics simulations.« less

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.

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.

Surface induced phonon decay rates in thin film nano-structures

NASA Astrophysics Data System (ADS)

Photiadis, D. M.

2007-12-01

Nano-scale structure significantly impacts phonon transport and related phonon relaxation rates, with order of magnitude effects on the thermal conductivity of dielectric thin films and quantum wires, and even larger effects on the lifetimes of ultrasonic phonons of micro- (nano-) oscillators. In both cases, efforts to explain the data have been hampered by our lack of knowledge of the effects of confined dimensionality on phonon-phonon scattering rates. Using a phonon Boltzmann equation with appropriate boundary conditions on the free surfaces to take surface roughness into account, we have obtained an expression yielding phonon lifetimes in 2-D dielectric nanostructures(thin films) resulting from phonon-phonon scattering in conjunction with phonon-surface scattering. We present these theoretical results and, in the limit in which surface induced losses dominate, obtain explicit predictions for the phonon lifetimes. The predicted temperature dependence of the ultrason! ic loss does not explain the observed saturation of the loss at low temperatures(τ(T) → const), but does give results of the order of magnitude of measured ultrasonic lifetimes.

Phonon optimized interatomic potential for aluminum

NASA Astrophysics Data System (ADS)

Muraleedharan, Murali Gopal; Rohskopf, Andrew; Yang, Vigor; Henry, Asegun

2017-12-01

We address the problem of generating a phonon optimized interatomic potential (POP) for aluminum. The POP methodology, which has already been shown to work for semiconductors such as silicon and germanium, uses an evolutionary strategy based on a genetic algorithm (GA) to optimize the free parameters in an empirical interatomic potential (EIP). For aluminum, we used the Vashishta functional form. The training data set was generated ab initio, consisting of forces, energy vs. volume, stresses, and harmonic and cubic force constants obtained from density functional theory (DFT) calculations. Existing potentials for aluminum, such as the embedded atom method (EAM) and charge-optimized many-body (COMB3) potential, show larger errors when the EIP forces are compared with those predicted by DFT, and thus they are not particularly well suited for reproducing phonon properties. Using a comprehensive Vashishta functional form, which involves short and long-ranged interactions, as well as three-body terms, we were able to better capture interactions that reproduce phonon properties accurately. Furthermore, the Vashishta potential is flexible enough to be extended to Al2O3 and the interface between Al-Al2O3, which is technologically important for combustion of solid Al nano powders. The POP developed here is tested for accuracy by comparing phonon thermal conductivity accumulation plots, density of states, and dispersion relations with DFT results. It is shown to perform well in molecular dynamics (MD) simulations as well, where the phonon thermal conductivity is calculated via the Green-Kubo relation. The results are within 10% of the values obtained by solving the Boltzmann transport equation (BTE), employing Fermi's Golden Rule to predict the phonon-phonon relaxation times.

Scattering of phonons by dislocations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Anderson, A. C.

1979-01-01

By 1950, an explicit effort had been launched to use lattice thermal conductivity measurements in the investigation of defect structures in solids. This technique has been highly successful, especially when combined with the measurements of other properties such as optical absorption. One exception has been the study of dislocations. Although dislocations have a profound effect on the phonon thermal conductivity, the mechanisms of the phonon-dislocation interaction are poorly understood. The most basic questions are still debated in the literature. It therefore is pointless to attempt a quantitative comparison between an extensive accumulation of experimental data on the one hand, andmore » the numerous theoretical models on the other. Instead, this chapter will attempt to glean a few qualitative conclusions from the existing experimental data. These results will then be compared with two general models which incorporate, in a qualitative manner, most of the proposed theories of the phonon-dislocation interaction. Until very recently, measurement of thermal conductivity was the only means available to probe the interaction between phonons and defects at phonon frequencies above the standard ultrasonic range of approx. = 10/sup 9/ Hz. The introductory paragraphs provide a brief review of the thermal-conductivity technique and the problems which are encountered in practice. There is also a brief presentation of the theoretical models and the complications that may occur in more realistic situations.« less

Bilayer graphene phonovoltaic-FET: In situ phonon recycling

NASA Astrophysics Data System (ADS)

Melnick, Corey; Kaviany, Massoud

2017-11-01

A new heat harvester, the phonovoltaic (pV) cell, was recently proposed. The device converts optical phonons into power before they become heat. Due to the low entropy of a typical hot optical phonon population, the phonovoltaic can operate at high fractions of the Carnot limit and harvest heat more efficiently than conventional heat harvesting technologies such as the thermoelectric generator. Previously, the optical phonon source was presumed to produce optical phonons with a single polarization and momentum. Here, we examine a realistic optical phonon source in a potential pV application and the effects this has on pV operation. Supplementing this work is our investigation of bilayer graphene as a new pV material. Our ab initio calculations show that bilayer graphene has a figure of merit exceeding 0.9, well above previously investigated materials. This allows a room-temperature pV to recycle 65% of a highly nonequilibrium, minimum entropy population of phonons. However, full-band Monte Carlo simulations of the electron and phonon dynamics in a bilayer graphene field-effect transistor (FET) show that the optical phonons emitted by field-accelerated electrons can only be recycled in situ with an efficiency of 50%, and this efficiency falls as the field strength grows. Still, an appropriately designed FET-pV can recycle the phonons produced therein in situ with a much higher efficiency than a thermoelectric generator can harvest heat produced by a FET ex situ.

Specularity of longitudinal acoustic phonons at rough surfaces

NASA Astrophysics Data System (ADS)

Gelda, Dhruv; Ghossoub, Marc G.; Valavala, Krishna; Ma, Jun; Rajagopal, Manjunath C.; Sinha, Sanjiv

2018-01-01

The specularity of phonons at crystal surfaces is of direct importance to thermal transport in nanostructures and to dissipation in nanomechanical resonators. Wave scattering theory provides a framework for estimating wavelength-dependent specularity, but experimental validation remains elusive. Widely available thermal conductivity data presents poor validation since the involvement of the infinitude of phonon wavelengths in thermal transport presents an underconstrained test for specularity theory. Here, we report phonon specularity by measuring the lifetimes of individual coherent longitudinal acoustic phonon modes excited in ultrathin (36-205 nm) suspended silicon membranes at room temperature over the frequency range ˜20 -118 GHz. Phonon surface scattering dominates intrinsic Akhiezer damping at frequencies ≳60 GHz, enabling measurements of phonon boundary scattering time over wavelengths ˜72 -140 nm . We obtain detailed statistics of the surface roughness at the top and bottom surfaces of membranes using HRTEM imaging. We find that the specularity of the excited modes are in good agreement with solutions of wave scattering only when the TEM statistics are corrected for projection errors. The often-cited Ziman formula for phonon specularity also appears in good agreement with the data, contradicting previous results. This work helps to advance the fundamental understanding of phonon scattering at the surfaces of nanostructures.

Imaginary parts of coupled electron and phonon propagators

NASA Astrophysics Data System (ADS)

Schwartzman, K.; Lawrence, W. E.

1988-01-01

Quasiparticle and phonon damping rates due to the electron-phonon and Coulomb interactions are obtained directly from the self-energy formalism of strong-coupling theory. This accounts for all processes involving phonon or quasiparticle decay into a single particle-hole pair, or quasiparticle decay by emission or absorption of a single real phonon. The two quasiparticle decay modes are treated on a common footing, without ad hoc separation, by accounting fully for the dynamics of the phonon propagator and the Coulomb vertex-the latter by expansion of the four-point Coulomb vertex function. The results are shown to be expressible in terms of only the physical (i.e., fully renormalized) energies and coupling constants, and are written in terms of spectral functions such as α2F(ω) and its generalizations. Expansion of these in powers of a phonon linewidth parameter distinguishes (in lowest orders) between quasiparticle decay modes involving real and virtual phonons. However, the simplest prescription for calculating decay rates involves an effective scattering amplitude in which this distinction is not made.

Electron-Mediated Phonon-Phonon Coupling Drives the Vibrational Relaxation of CO on Cu(100)

NASA Astrophysics Data System (ADS)

Novko, D.; Alducin, M.; Juaristi, J. I.

2018-04-01

We bring forth a consistent theory for the electron-mediated vibrational intermode coupling that clarifies the microscopic mechanism behind the vibrational relaxation of adsorbates on metal surfaces. Our analysis points out the inability of state-of-the-art nonadiabatic theories to quantitatively reproduce the experimental linewidth of the CO internal stretch mode on Cu(100) and it emphasizes the crucial role of the electron-mediated phonon-phonon coupling in this regard. The results demonstrate a strong electron-mediated coupling between the internal stretch and low-energy CO modes, but also a significant role of surface motion. Our nonadiabatic theory is also able to explain the temperature dependence of the internal stretch phonon linewidth, thus far considered a sign of the direct anharmonic coupling.

Quantum Bath Refrigeration towards Absolute Zero: Challenging the Unattainability Principle

NASA Astrophysics Data System (ADS)

Kolář, M.; Gelbwaser-Klimovsky, D.; Alicki, R.; Kurizki, G.

2012-08-01

A minimal model of a quantum refrigerator, i.e., a periodically phase-flipped two-level system permanently coupled to a finite-capacity bath (cold bath) and an infinite heat dump (hot bath), is introduced and used to investigate the cooling of the cold bath towards absolute zero (T=0). Remarkably, the temperature scaling of the cold-bath cooling rate reveals that it does not vanish as T→0 for certain realistic quantized baths, e.g., phonons in strongly disordered media (fractons) or quantized spin waves in ferromagnets (magnons). This result challenges Nernst’s third-law formulation known as the unattainability principle.

Quantum bath refrigeration towards absolute zero: challenging the unattainability principle.

PubMed

Kolář, M; Gelbwaser-Klimovsky, D; Alicki, R; Kurizki, G

2012-08-31

A minimal model of a quantum refrigerator, i.e., a periodically phase-flipped two-level system permanently coupled to a finite-capacity bath (cold bath) and an infinite heat dump (hot bath), is introduced and used to investigate the cooling of the cold bath towards absolute zero (T=0). Remarkably, the temperature scaling of the cold-bath cooling rate reveals that it does not vanish as T→0 for certain realistic quantized baths, e.g., phonons in strongly disordered media (fractons) or quantized spin waves in ferromagnets (magnons). This result challenges Nernst's third-law formulation known as the unattainability principle.

A new phase of disordered phonons modelled by random matrices

NASA Astrophysics Data System (ADS)

Schmittner, Sebastian; Zirnbauer, Martin

2015-03-01

Starting from the clean harmonic crystal and not invoking two-level systems, we propose a model for phonons in a disordered solid. In this model the strength of mass and spring constant disorder can be increased separately. Both types of disorder are modelled by random matrices that couple the degrees of freedom locally. Treated in coherent potential approximation (CPA), the speed of sound decreases with increasing disorder until it reaches zero at finite disorder strength. There, a critical transition to a strong disorder phase occurs. In this novel phase, we find the density of states at zero energy in three dimensions to be finite, leading to a linear temperature dependence of the heat capacity, as observed experimentally for vitreous systems. For any disorder strength, our model is stable, i.e. masses and spring constants are positive, and there are no runaway dynamics. This is ensured by using appropriate probability distributions, inspired by Wishart ensembles, for the random matrices. The CPA self-consistency equations are derived in a very accessible way using planar diagrams. The talk focuses on the model and the results. The first author acknowledges financial support by the Deutsche Telekom Stiftung.

Probing Phonon Dynamics in Individual Single-Walled Carbon Nanotubes.

PubMed

Jiang, Tao; Hong, Hao; Liu, Can; Liu, Wei-Tao; Liu, Kaihui; Wu, Shiwei

2018-04-11

Interactions between elementary excitations, such as carriers, phonons, and plasmons, are critical for understanding the optical and electronic properties of materials. The significance of these interactions is more prominent in low-dimensional materials and can dominate their physical properties due to the enhanced interactions between these excitations. One-dimensional single-walled carbon nanotubes provide an ideal system for studying such interactions due to their perfect physical structures and rich electronic properties. Here we investigated G-mode phonon dynamics in individual suspended chirality-resolved single-walled carbon nanotubes by time-resolved anti-Stokes Raman spectroscopy. The improved technique allowed us to probe the intrinsic phonon information on a single-tube level and exclude the influences of tube-tube and tube-substrate interactions. We found that the G-mode phonon lifetime ranges from 0.75-2.25 ps and critically depends on whether the tube is metallic or semiconducting. In comparison with the phonon lifetimes in graphene and graphite, we revealed structure-dependent carrier-phonon and phonon-phonon interactions in nanotubes. Our results provide new information for optimizing the design of nanotube electronic/optoelectronic devices by better understanding and utilizing their phonon decay channels.

Vibronic spectra of Cu(2+) in ZnTe

NASA Technical Reports Server (NTRS)

Volz, M. P.; Su, C.-H.; Lehoczky, S. L.; Szofran, F. R.

1992-01-01

Infrared-absorption spectra of substitutional Cu(2+) ions in ZnTe have been measured at 4.6 K. Several distinct absorption peaks are observed between 800 and 2000/cm. Absorption peaks at 1002 and 1069/cm are identified as zero-phonon lines arising from 2T2-2E transitions. Between 1069 and 2000/cm, several sets of sharp absorption lines are seen to recur regularly at an interval of 210/cm, corresponding to the LO phonon energy. Within each set distinct vibronic sidebands that cannot be identified with critical-point energies of TA, LA, TO or LO phonon modes are observed. A dynamic Jahn-Teller effect, involving coupling between a single-phonon mode and the electronic states of the 2E level, is proposed to account for the observed spectra.

Electron-phonon superconductivity in YIn3

NASA Astrophysics Data System (ADS)

Billington, D.; Llewellyn-Jones, T. M.; Maroso, G.; Dugdale, S. B.

2013-08-01

First-principles calculations of the electron-phonon coupling were performed on the cubic intermetallic compound YIn3. The electron-phonon coupling constant was found to be λep = 0.42. Using the Allen-Dynes formula with a Coulomb pseudopotential of μ* = 0.10, a Tc of approximately 0.77 K is obtained which is reasonably consistent with the experimentally observed temperature (between 0.8 and 1.1 K). The results indicate that conventional electron-phonon coupling is capable of producing the superconductivity in this compound.

"Zeroing" in on mathematics in the monkey brain.

PubMed

Beran, Michael J

2016-03-01

A new study documented that monkeys showed selective neuronal responding to the concept of zero during a numerical task, and that there were two distinct classes of neurons that coded the absence of stimuli either through a discrete activation pattern (zero or not zero) or a continuous one for which zero was integrated with other numerosities in the relative rate of activity. These data indicate that monkeys, like humans, have a concept of zero that is part of their analog number line but that also may have unique properties compared to other numerosities.

Ultrafast Dephasing and Incoherent Light Photon Echoes in Organic Amorphous Systems

NASA Astrophysics Data System (ADS)

Yano, Ryuzi; Matsumoto, Yoshinori; Tani, Toshiro; Nakatsuka, Hiroki

1989-10-01

Incoherent light photon echoes were observed in organic amorphous systems (cresyl violet in polyvinyl alcohol and 1,4-dihydroxyanthraquinone in polymethacrylic acid) by using temporally-incoherent nanosecond laser pulses. It was found that an echo decay curve of an organic amorphous system is composed of a sharp peak which decays very rapidly and a slowly decaying wing at the tail. We show that the persistent hole burning (PHB) spectra were reproduced by the Fourier-cosine transforms of the echo decay curves. We claim that in general, we must take into account the multi-level feature of the system in order to explain ultrafast dephasing at very low temperatures.

Photon energy conversion by near-zero permittivity nonlinear materials

DOE Office of Scientific and Technical Information (OSTI.GOV)

Luk, Ting S.; Sinclair, Michael B.; Campione, Salvatore

Efficient harmonic light generation can be achieved with ultrathin films by coupling an incident pump wave to an epsilon-near-zero (ENZ) mode of the thin film. As an example, efficient third harmonic generation from an indium tin oxide nanofilm (.lamda./42 thick) on a glass substrate for a pump wavelength of 1.4 .mu.m was demonstrated. A conversion efficiency of 3.3.times.10.sup.-6 was achieved by exploiting the field enhancement properties of the ENZ mode with an enhancement factor of 200. This nanoscale frequency conversion method is applicable to other plasmonic materials and reststrahlen materials in proximity of the longitudinal optical phonon frequencies.

Phonons in self-assembled Ge/Si structures

NASA Astrophysics Data System (ADS)

Milekhin, A. G.; Nikiforov, A. I.; Pchelyakov, O. P.; Schulze, S.; Zahn, D. R. T.

2002-03-01

We present the results of an investigation dealing with fundamental vibrations in periodical Ge/Si structures with small-size Ge quantum dots (QDs) performed using macro- and micro-Raman spectroscopy under resonant and off-resonant conditions. Samples with different number of repetition of Ge and Si layers contain Ge QDs with an average dot base size of 15 nm and a QD height of 2 nm. Periodic oscillations observed in the low-frequency region of the Raman spectra are assigned to folded LA phonons in the Ge QD superlattices. The measured phonon frequencies are in a good agreement with those calculated using the Rytov model. These oscillations are superimposed with a broad continuous emission originating from the whole acoustic dispersion branch due to a breaking up of translational invariance. The Raman spectra of the structure with single Ge QD layer reveal a series of peaks corresponding to LA phonons localized in the Si layer. Using the measured phonon frequencies and corresponding wave vectors the dispersion of the LA phonons in the Si is obtained. The longitudinal-acoustic wave velocity determined from the dispersion is 8365 ms-1 and in excellent agreement with that derived from the Brillouin study. In the optical phonon range, the LO and TO phonons localized in Ge QDs are observed. The position of the LO Ge phonons shifts downwards with increasing excitation energy (from 2.5 to 2.7 eV) indicating the presence of a QD size distribution in Ge dot superlattices. Raman scattering from Ge QDs is size-selectively enhanced by the resonance of the exciting laser energy and the confined excitonic states.

Hot-phonon generation in THz quantum cascade lasers

NASA Astrophysics Data System (ADS)

Spagnolo, V.; Vitiello, M. S.; Scamarcio, G.; Williams, B. S.; Kumar, S.; Hu, Q.; Reno, J. L.

2007-12-01

Observation of non-equilibrium optical phonons population associated with electron transport in THz quantum cascade lasers is reported. The phonon occupation number was measured by using a combination of micro-probe photoluminescence and Stokes/Anti-Stokes Raman spectroscopy. Energy balance analysis allows us to estimate the phonon relaxation rate, that superlinearly increases with the electrical power in the range 1.5 W - 1.95 W, above laser threshold. This observation suggests the occurrence of stimulated emission of optical phonons.

Plasphonics: local hybridization of plasmons and phonons.

PubMed

Marty, Renaud; Mlayah, Adnen; Arbouet, Arnaud; Girard, Christian; Tripathy, Sudhiranjan

2013-02-25

We show that the interaction between localized surface plasmons sustained by a metallic nano-antenna and delocalized phonons lying at the surface of an heteropolar semiconductor can generate a new class of hybrid electromagnetic modes. These plasphonic modes are investigated using an analytical model completed by accurate Green dyadic numerical simulations. When surface plasmon and surface phonon frequencies match, the optical resonances exhibit a large Rabi splitting typical of strongly interacting two-level systems. Based on numerical simulations of the electric near-field maps, we investigate the nature of the plaphonic excitations. In particular, we point out a strong local field enhancement boosted by the phononic surface. This effect is interpreted in terms of light harvesting by the plasmonic antenna from the phononic surface. We thus introduce the concept of active phononic surfaces that may be exploited for far-infared optoelectronic devices and sensors.

Enhanced third harmonic generation from the epsilon-near-zero modes of ultrathin films

DOE Office of Scientific and Technical Information (OSTI.GOV)

Luk, Ting S.; De Ceglia, Domenico; Liu, Sheng

We demonstrate, through our experimentation, efficient third harmonic generation from an indium tin oxide nanofilm (λ/42 thick) on a glass substrate for a pump wavelength of 1.4 μm. A conversion efficiency of 3.3 × 10 -6 is achieved by exploiting the field enhancement properties of the epsilon-near-zero mode with an enhancement factor of 200. Furthermore, this nanoscale frequency conversion method is applicable to other plasmonic materials and reststrahlen materials in proximity of the longitudinal optical phonon frequencies.

Enhanced third harmonic generation from the epsilon-near-zero modes of ultrathin films

DOE Office of Scientific and Technical Information (OSTI.GOV)

Luk, Ting S., E-mail: tsluk@sandia.gov; Liu, Sheng; Campione, Salvatore

We experimentally demonstrate efficient third harmonic generation from an indium tin oxide nanofilm (λ/42 thick) on a glass substrate for a pump wavelength of 1.4 μm. A conversion efficiency of 3.3 × 10{sup −6} is achieved by exploiting the field enhancement properties of the epsilon-near-zero mode with an enhancement factor of 200. This nanoscale frequency conversion method is applicable to other plasmonic materials and reststrahlen materials in proximity of the longitudinal optical phonon frequencies.

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.

Phonon Recycling for Ultrasensitive Kinetic Inductance Detectors

NASA Astrophysics Data System (ADS)

Zmuidzinas, Jonas

Initially proposed (Day et al. 2003; Zmuidzinas 2012) in 1999 by our Caltech/JPL group, and thanks to strong support from NASA, the superconducting (microwave) kinetic inductance detector (MKID or KID) technology continues to develop rapidly as it transitions into applications. The development effort worldwide is intensifying and NASA's continued support of KID development is essential in order to keep pace. Here we propose to investigate and demonstrate a new, low-TRL concept, which we call phonon recycling, that promises to open broad new avenues in KID design and performance. Briefly, phonon recycling allows the detector designer to tailor the responsivity and sensitivity of a KID to match the needs of the application by using geometry to restrict the rate at which recombination phonons are allowed to escape from the detector. In particular, phonon recycling should allow very low noise-equivalent power (NEP) to be achieved without requiring very low operating tem- peratures. Phonon recycling is analogous to the use of micromachined suspension legs to control the flow of heat in a bolometer, as measured by the thermal conductivity G. However, phonon recycling exploits the non-thermal distribution of recombination phonons as well as their very slow decay in crystals at low temperatures. These properties translate to geometrical and mechanical requirements for a phonon-recycled KID that are considerably more relaxed than for a bolometer operating at the same temperature and NEP. Our ultimate goal is to develop detector arrays suitable for a far-infrared (FIR) space mission, which will impose strict requirements on the array sensitivity, yield, uniformity, multiplexing density, etc. Through previous NASA support under the Strategic Astrophysics Technology (SAT) program, we have successfully demonstrated the MAKO submillimeter camera at the Caltech Submillimeter Observatory and have become familiar with these practical issues. If our demonstration of phonon recycling

Phonon localization transition in relaxor ferroelectric PZN-5%PT

DOE Office of Scientific and Technical Information (OSTI.GOV)

Manley, Michael E.; Christianson, Andrew D.; Abernathy, Douglas L.

Relaxor ferroelectric behavior occurs in many disordered ferroelectric materials but is not well understood at the atomic level. Recent experiments and theoretical arguments indicate that Anderson localization of phonons instigates relaxor behavior by driving the formation of polar nanoregions (PNRs). Here, we use inelastic neutron scattering to observe phonon localization in relaxor ferroelectric PZN-5%PT (0.95[Pb(Zn 1/3 Nb 2/3)O 3]–0.05PbTiO 3) and detect additional features of the localization process. In the lead, up to phonon localization on cooling, the local resonant modes that drive phonon localization increase in number. The increase in resonant scattering centers is attributed to a known increasemore » in the number of locally off centered Pb atoms on cooling. The transition to phonon localization occurs when these random scattering centers increase to a concentration where the Ioffe-Regel criterion is satisfied for localizing the phonon. Finally, we also model the effects of damped mode coupling on the observed phonons and phonon localization structure.« less

Phonon localization transition in relaxor ferroelectric PZN-5%PT

DOE PAGES

Manley, Michael E.; Christianson, Andrew D.; Abernathy, Douglas L.; ...

2017-03-27

Relaxor ferroelectric behavior occurs in many disordered ferroelectric materials but is not well understood at the atomic level. Recent experiments and theoretical arguments indicate that Anderson localization of phonons instigates relaxor behavior by driving the formation of polar nanoregions (PNRs). Here, we use inelastic neutron scattering to observe phonon localization in relaxor ferroelectric PZN-5%PT (0.95[Pb(Zn 1/3 Nb 2/3)O 3]–0.05PbTiO 3) and detect additional features of the localization process. In the lead, up to phonon localization on cooling, the local resonant modes that drive phonon localization increase in number. The increase in resonant scattering centers is attributed to a known increasemore » in the number of locally off centered Pb atoms on cooling. The transition to phonon localization occurs when these random scattering centers increase to a concentration where the Ioffe-Regel criterion is satisfied for localizing the phonon. Finally, we also model the effects of damped mode coupling on the observed phonons and phonon localization structure.« less

Quantum correlation of high dimensional system in a dephasing environment

NASA Astrophysics Data System (ADS)

Ji, Yinghua; Ke, Qiang; Hu, Juju

2018-05-01

For a high dimensional spin-S system embedded in a dephasing environment, we theoretically analyze the time evolutions of quantum correlation and entanglement via Frobenius norm and negativity. The quantum correlation dynamics can be considered as a function of the decoherence parameters, including the ratio between the system oscillator frequency ω0 and the reservoir cutoff frequency ωc , and the different environment temperature. It is shown that the quantum correlation can not only measure nonclassical correlation of the considered system, but also perform a better robustness against the dissipation. In addition, the decoherence presents the non-Markovian features and the quantum correlation freeze phenomenon. The former is much weaker than that in the sub-Ohmic or Ohmic thermal reservoir environment.

Selective laser spectroscopy of molecules and ions in solids: a history, fundamentals and applications

NASA Astrophysics Data System (ADS)

Sapozhnikov, Michael

2018-03-01

A history of the development of selective laser spectroscopy is presented, beginning with a pioneering work by Yu. V. Denisov and V. A. Kizel in 1967, who were the first to demonstrate the possibility of removing the inhomogeneous broadening of luminescence spectra of impurity ions in glasses upon monochromatic resonance excitation. Selective excitation of optical centers can be achieved due to existence of zero-phonon transitions corresponding to narrow homogeneous zero-phonon lines in the spectra of impurity centers in solids, which are hidden in broad inhomogeneous optical bands upon usual nonselective excitation. The fundamentals of zero-phonon transition spectroscopy are considered and the mechanism of removing the inhomogeneous broadening of optical spectra of ions and molecules in crystals and amorphous solids under selective laser excitation of luminescence and persistent hole burning in absorption spectra is presented in detail. Various applications of selective laser spectroscopy for fundamental and applied studies are discussed.

Nonequilibrium dynamics of the phonon gas in ultrafast-excited antimony

NASA Astrophysics Data System (ADS)

Krylow, Sergej; Zijlstra, Eeuwe S.; Kabeer, Fairoja Cheenicode; Zier, Tobias; Bauerhenne, Bernd; Garcia, Martin E.

2017-12-01

The ultrafast relaxation dynamics of a nonequilibrium phonon gas towards thermal equilibrium involves many-body collisions that cannot be properly described by perturbative approaches. Here, we develop a nonperturbative method to elucidate the microscopic mechanisms underlying the decay of laser-excited coherent phonons in the presence of electron-hole pairs, which so far are not fully understood. Our theory relies on ab initio molecular dynamics simulations on laser-excited potential-energy surfaces. Those simulations are compared with runs in which the laser-excited coherent phonon is artificially deoccupied. We apply this method to antimony and show that the decay of the A1 g phonon mode at low laser fluences can be accounted mainly to three-body down-conversion processes of an A1 g phonon into acoustic phonons. For higher excitation strengths, however, we see a crossover to a four-phonon process, in which two A1 g phonons decay into two optical phonons.

Engineering thermal conductance using a two-dimensional phononic crystal.

PubMed

Zen, Nobuyuki; Puurtinen, Tuomas A; Isotalo, Tero J; Chaudhuri, Saumyadip; Maasilta, Ilari J

2014-03-19

Controlling thermal transport has become relevant in recent years. Traditionally, this control has been achieved by tuning the scattering of phonons by including various types of scattering centres in the material (nanoparticles, impurities, etc). Here we take another approach and demonstrate that one can also use coherent band structure effects to control phonon thermal conductance, with the help of periodically nanostructured phononic crystals. We perform the experiments at low temperatures below 1 K, which not only leads to negligible bulk phonon scattering, but also increases the wavelength of the dominant thermal phonons by more than two orders of magnitude compared to room temperature. Thus, phononic crystals with lattice constants ≥1 μm are shown to strongly reduce the thermal conduction. The observed effect is in quantitative agreement with the theoretical calculation presented, which accurately determined the ballistic thermal conductance in a phononic crystal device.

AB INITIO STUDY OF PHONON DISPERSION AND ELASTIC PROPERTIES OF L12 INTERMETALLICS Ti3Al AND Y3Al

NASA Astrophysics Data System (ADS)

Arikan, N.; Ersen, M.; Ocak, H. Y.; Iyigör, A.; Candan, A.; UǦUR, Ş.; UǦUR, G.; Khenata, R.; Varshney, D.

2013-12-01

In this paper, the structural, elastic and phonon properties of Ti3Al and Y3Al in L12(Cu3Al) phase are studied by performing first-principles calculations within the generalized gradient approximation. The calculated lattice constants, static bulk moduli, first-order pressure derivative of bulk moduli and elastic constants for both compounds are reported. The phonon dispersion curves along several high-symmetry lines at the Brillouin zone, together with the corresponding phonon density of states, are determined using the first-principles linear-response approach of the density functional perturbation theory. Temperature variations of specific heat in the range of 0-500 K are obtained using the quasi-harmonic model.

Quantum Properties of Dichroic Silicon Vacancies in Silicon Carbide

NASA Astrophysics Data System (ADS)

Nagy, Roland; Widmann, Matthias; Niethammer, Matthias; Dasari, Durga B. R.; Gerhardt, Ilja; Soykal, Öney O.; Radulaski, Marina; Ohshima, Takeshi; Vučković, Jelena; Son, Nguyen Tien; Ivanov, Ivan G.; Economou, Sophia E.; Bonato, Cristian; Lee, Sang-Yun; Wrachtrup, Jörg

2018-03-01

Although various defect centers have displayed promise as either quantum sensors, single photon emitters, or light-matter interfaces, the search for an ideal defect with multifunctional ability remains open. In this spirit, we study the dichroic silicon vacancies in silicon carbide that feature two well-distinguishable zero-phonon lines and analyze the quantum properties in their optical emission and spin control. We demonstrate that this center combines 40% optical emission into the zero-phonon lines showing the contrasting difference in optical properties with varying temperature and polarization, and a 100% increase in the fluorescence intensity upon the spin resonance, and long spin coherence time of their spin-3 /2 ground states up to 0.6 ms. These results single out this defect center as a promising system for spin-based quantum technologies.

Ternary mixed crystal effects on interface optical phonon and electron-phonon coupling in zinc-blende GaN/AlxGa1-xN spherical quantum dots

NASA Astrophysics Data System (ADS)

Huang, Wen Deng; Chen, Guang De; Yuan, Zhao Lin; Yang, Chuang Hua; Ye, Hong Gang; Wu, Ye Long

2016-02-01

The theoretical investigations of the interface optical phonons, electron-phonon couplings and its ternary mixed effects in zinc-blende spherical quantum dots are obtained by using the dielectric continuum model and modified random-element isodisplacement model. The features of dispersion curves, electron-phonon coupling strengths, and its ternary mixed effects for interface optical phonons in a single zinc-blende GaN/AlxGa1-xN spherical quantum dot are calculated and discussed in detail. The numerical results show that there are three branches of interface optical phonons. One branch exists in low frequency region; another two branches exist in high frequency region. The interface optical phonons with small quantum number l have more important contributions to the electron-phonon interactions. It is also found that ternary mixed effects have important influences on the interface optical phonon properties in a single zinc-blende GaN/AlxGa1-xN quantum dot. With the increase of Al component, the interface optical phonon frequencies appear linear changes, and the electron-phonon coupling strengths appear non-linear changes in high frequency region. But in low frequency region, the frequencies appear non-linear changes, and the electron-phonon coupling strengths appear linear changes.

Hole dephasing caused by hole-hole interaction in a multilayered black phosphorus.

PubMed

Li, Lijun; Khan, Muhammad Atif; Lee, Yoontae; Lee, Inyeal; Yun, Sun Jin; Youn, Doo-Hyeb; Kim, Gil-Ho

2017-11-01

We study the magnetotransport of holes in a multilayered black phosphorus in a temperature range of 1.9 to 21.5 K. We observed a negative magnetoresistance at magnetic fields up to 1.5 T. This negative magetoresistance was analyzed by weak localization theory in diffusive regime. At the lowest temperature and the highest carrier density we found a phase coherence length of 48 nm. The linear temperature dependence of the dephasing rate shows that the hole-hole scattering processes with small energy transfer are the dominant contribution in breaking the carrier phase coherence.

Pressure-enabled phonon engineering in metals

PubMed Central

Lanzillo, Nicholas A.; Thomas, Jay B.; Watson, Bruce; Washington, Morris; Nayak, Saroj K.

2014-01-01

We present a combined first-principles and experimental study of the electrical resistivity in aluminum and copper samples under pressures up to 2 GPa. The calculations are based on first-principles density functional perturbation theory, whereas the experimental setup uses a solid media piston–cylinder apparatus at room temperature. We find that upon pressurizing each metal, the phonon spectra are blue-shifted and the net electron–phonon interaction is suppressed relative to the unstrained crystal. This reduction in electron–phonon scattering results in a decrease in the electrical resistivity under pressure, which is more pronounced for aluminum than for copper. We show that density functional perturbation theory can be used to accurately predict the pressure response of the electrical resistivity in these metals. This work demonstrates how the phonon spectra in metals can be engineered through pressure to achieve more attractive electrical properties. PMID:24889627

Seeing the invisible plasma with transient phonons in cuprous oxide

DOE PAGES

Frazer, Laszlo; Schaller, Richard D.; Chang, Kelvin B.; ...

2016-12-12

Here, the emission of phonons from electron–hole plasma is the primary limit on the efficiency of photovoltaic devices operating above the bandgap. In cuprous oxide (Cu 2O) there is no luminescence from electron–hole plasma. Therefore, we searched for optical phonons emitted by energetic charge carriers using phonon-to-exciton upconversion transitions. We found 14 meV phonons with a lifetime of 0.916 ± 0.008 ps and 79 meV phonons that are longer lived and overrepresented. It is surprising that the higher energy phonon has a longer lifetime.

Temperature dependence of the Urbach optical absorption edge: A theory of multiple phonon absorption and emission sidebands

NASA Astrophysics Data System (ADS)

Grein, C. H.; John, Sajeev

1989-01-01

The optical absorption coefficient for subgap electronic transitions in crystalline and disordered semiconductors is calculated by first-principles means with use of a variational principle based on the Feynman path-integral representation of the transition amplitude. This incorporates the synergetic interplay of static disorder and the nonadiabatic quantum dynamics of the coupled electron-phonon system. Over photon-energy ranges of experimental interest, this method predicts accurate linear exponential Urbach behavior of the absorption coefficient. At finite temperatures the nonlinear electron-phonon interaction gives rise to multiple phonon emission and absorption sidebands which accompany the optically induced electronic transition. These sidebands dominate the absorption in the Urbach regime and account for the temperature dependence of the Urbach slope and energy gap. The physical picture which emerges is that the phonons absorbed from the heat bath are then reemitted into a dynamical polaronlike potential well which localizes the electron. At zero temperature we recover the usual polaron theory. At high temperatures the calculated tail is qualitatively similar to that of a static Gaussian random potential. This leads to a linear relationship between the Urbach slope and the downshift of the extrapolated continuum band edge as well as a temperature-independent Urbach focus. At very low temperatures, deviations from these rules are predicted arising from the true quantum dynamics of the lattice. Excellent agreement is found with experimental data on c-Si, a-Si:H, a-As2Se3, and a-As2S3. Results are compared with a simple physical argument based on the most-probable-potential-well method.

Self-consistent description of a system of interacting phonons

NASA Astrophysics Data System (ADS)

Poluektov, Yu. M.

2015-11-01

A proposal for a method of self-consistent description of phonon systems. This method generalizes the Debye model to account for phonon-phonon interaction. The idea of "self-consistent" phonons is introduced; their speed depends on the temperature and is determined by solving a non-linear equation. The Debye energy is also a function of the temperature within the framework of the proposed approach. The thermodynamics of "self-consistent" phonon gas are built. It is shown that at low temperatures the cubic law temperature dependence of specific heat acquires an additional term that is proportional to the seventh power of the temperature. This seems to explain the reason why the cubic law for specific heat is observed only at relatively low temperatures. At high temperatures, the theory predicts a linear deviation with respect to temperature from the Dulong-Petit law, which is observed experimentally. A modification to the melting criteria is considered, to account for the phonon-phonon interaction.

Edge waves and resonances in two-dimensional phononic crystal plates

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hsu, Jin-Chen, E-mail: hsujc@yuntech.edu.tw; Hsu, Chih-Hsun

2015-05-07

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. Wemore » 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.« less

The effect of hot phonons and coupled phonon-plasmon modes on scattering-induced NDR in quantum wells

NASA Astrophysics Data System (ADS)

Ridley, B. K.; Al-Mudares, M.

1988-04-01

We have extended our Monte Carlo simulation of scattering-induced NDR in Al. 8Ga 2As/GaAs quantum wells by including (a) the effect of hot phonons (b) coupled phonon-plasmon modes (c) degeneracy. Hot phonons were modelled using a phenomenological lifetime which we ranged from 3ps to 10ps. Coupled modes were modelled in the antiscreening approximation. Bulk-like modes were assumed in both cases. NDR is quenched if the phonon lifetime exceeds 7ps, but is little affected if the lifetime is 3ps. The effect of coupled modes is appreciable at a doping density of 10 18cm -3, virtually eliminating NDR, but at 10 17cm -3 the effect is much smaller. Including degeneracy has only a small effect on the results. We conclude that NDR is still possible at electron densities around 10 17cm -3.

Coupling of Excitons and Discrete Acoustic Phonons in Vibrationally Isolated Quantum Emitters.

PubMed

Werschler, Florian; Hinz, Christopher; Froning, Florian; Gumbsheimer, Pascal; Haase, Johannes; Negele, Carla; de Roo, Tjaard; Mecking, Stefan; Leitenstorfer, Alfred; Seletskiy, Denis V

2016-09-14

The photoluminescence emission by mesoscopic condensed matter is ultimately dictated by the fine-structure splitting of the fundamental exciton into optically allowed and dipole-forbidden states. In epitaxially grown semiconductor quantum dots, nonradiative equilibration between the fine-structure levels is mediated by bulk acoustic phonons, resulting in asymmetric spectral broadening of the excitonic luminescence. In isolated colloidal quantum dots, spatial confinement of the vibrational motion is expected to give rise to an interplay between the quantized electronic and phononic degrees of freedom. In most cases, however, zero-dimensional colloidal nanocrystals are strongly coupled to the substrate such that the charge relaxation processes are still effectively governed by the bulk properties. Here we show that encapsulation of single colloidal CdSe/CdS nanocrystals into individual organic polymer shells allows for systematic vibrational decoupling of the semiconductor nanospheres from the surroundings. In contrast to epitaxially grown quantum dots, simultaneous quantization of both electronic and vibrational degrees of freedom results in a series of strong and narrow acoustic phonon sidebands observed in the photoluminescence. Furthermore, an individual analysis of more than 200 compound particles reveals that enhancement or suppression of the radiative properties of the fundamental exciton is controlled by the interaction between fine-structure states via the discrete vibrational modes. For the first time, pronounced resonances in the scattering rate between the fine-structure states are directly observed, in good agreement with a quantum mechanical model. The unambiguous assignment of mediating acoustic modes to the observed scattering resonances complements the experimental findings. Thus, our results form an attractive basis for future studies on subterahertz quantum opto-mechanics and efficient laser cooling at the nanoscale.

Four-phonon scattering significantly reduces intrinsic thermal conductivity of solids

NASA Astrophysics Data System (ADS)

Feng, Tianli; Lindsay, Lucas; Ruan, Xiulin

2017-10-01

For decades, the three-phonon scattering process has been considered to govern thermal transport in solids, while the role of higher-order four-phonon scattering has been persistently unclear and so ignored. However, recent quantitative calculations of three-phonon scattering have often shown a significant overestimation of thermal conductivity as compared to experimental values. In this Rapid Communication we show that four-phonon scattering is generally important in solids and can remedy such discrepancies. For silicon and diamond, the predicted thermal conductivity is reduced by 30% at 1000 K after including four-phonon scattering, bringing predictions in excellent agreement with measurements. For the projected ultrahigh-thermal conductivity material, zinc-blende BAs, a competitor of diamond as a heat sink material, four-phonon scattering is found to be strikingly strong as three-phonon processes have an extremely limited phase space for scattering. The four-phonon scattering reduces the predicted thermal conductivity from 2200 to 1400 W/m K at room temperature. The reduction at 1000 K is 60%. We also find that optical phonon scattering rates are largely affected, being important in applications such as phonon bottlenecks in equilibrating electronic excitations. Recognizing that four-phonon scattering is expensive to calculate, in the end we provide some guidelines on how to quickly assess the significance of four-phonon scattering, based on energy surface anharmonicity and the scattering phase space. Our work clears the decades-long fundamental question of the significance of higher-order scattering, and points out ways to improve thermoelectrics, thermal barrier coatings, nuclear materials, and radiative heat transfer.

Polarization-controlled coherent phonon generation in acoustoplasmonic metasurfaces

NASA Astrophysics Data System (ADS)

Lanzillotti-Kimura, Norberto D.; O'Brien, Kevin P.; Rho, Junsuk; Suchowski, Haim; Yin, Xiaobo; Zhang, Xiang

2018-06-01

Acoustic vibrations at the nanoscale (GHz-THz frequencies) and their interactions with electrons, photons, and other excitations are the heart of an emerging field in physics: nanophononics. The design of ultrahigh frequency acoustic-phonon transducers, with tunable frequency, and easy to integrate in complex systems is still an open and challenging problem for the development of acoustic nanoscopies and phonon lasers. Here we show how an optimized plasmonic metasurface can act as a high-frequency phonon transducer. We report pump-probe experiments in metasurfaces composed of an array of gold nanostructures, revealing that such arrays can act as efficient and tunable photon-phonon transducers, with a strong spectral dependence on the excitation rate and laser polarization. We anticipate our work to be the starting point for the engineering of phononic metasurfaces based on plasmonic nanostructures.

Understanding photon sideband statistics and correlation for determining phonon coherence

NASA Astrophysics Data System (ADS)

Ding, Ding; Yin, Xiaobo; Li, Baowen

2018-01-01

Generating and detecting coherent high-frequency heat-carrying phonons have been topics of great interest in recent years. Although there have been successful attempts in generating and observing coherent phonons, rigorous techniques to characterize and detect phonon coherence in a crystalline material have been lagging compared to what has been achieved for photons. One main challenge is a lack of detailed understanding of how detection signals for phonons can be related to coherence. The quantum theory of photoelectric detection has greatly advanced the ability to characterize photon coherence in the past century, and a similar theory for phonon detection is necessary. Here, we reexamine the optical sideband fluorescence technique that has been used to detect high-frequency phonons in materials with optically active defects. We propose a quantum theory of phonon detection using the sideband technique and found that there are distinct differences in sideband counting statistics between thermal and coherent phonons. We further propose a second-order correlation function unique to sideband signals that allows for a rigorous distinction between thermal and coherent phonons. Our theory is relevant to a correlation measurement with nontrivial response functions at the quantum level and can potentially bridge the gap of experimentally determining phonon coherence to be on par with that of photons.

Chiral zero energy modes in two-dimensional disordered Dirac semimetals

NASA Astrophysics Data System (ADS)

Liu, Lei; Yu, Yan; Wu, Hai-Bin; Zhang, Yan-Yang; Liu, Jian-Jun; Li, Shu-Shen

2018-04-01

The vacancy-induced chiral zero energy modes (CZEMs) of chiral-unitary-class (AIII) and chiral-symplectic-class (CII) two-dimensional (2 D ) disordered Dirac semimetals realized on a square bipartite lattice are investigated numerically by using the Kubo-Greenwood formula with the kernel polynomial method. The results show that, for both systems, the CZEMs exhibit the critical delocalization. The CZEM conductivity remains a robust constant (i.e., σ CZEM≈1.05 e2/h ), which is insensitive to the sample sizes, the vacancy concentrations, and the numbers of moments of Chebyshev polynomials, i.e., the dephasing strength. For both kinds of chiral systems, the CZEM conductivities are almost identical. However, they are not equal to that of graphene (i.e., 4 e2/π h ), which belongs to the chiral orthogonal class (BDI) semimetal on a 2 D hexagonal bipartite lattice. In addition, for the case that the vacancy concentrations are different in the two sublattices, the CZEM conductivity vanishes, and thus both systems exhibit localization at the Dirac point. Moreover, a band gap and a mobility gap open around zero energy. The widths of the energy gaps and mobility gaps are increasing with larger vacancy concentration difference. The width of the mobility gap is greater than that of the band gap, and a δ -function-like peak of density of states emerges at the Dirac point within the band gap, implying the existence of numerous localized states.

Orbitally-driven giant phonon anharmonicity in SnSe

DOE PAGES

Li, Chen W.; Hong, Jiawang; May, Andrew F.; ...

2015-10-19

We understand that elementary excitations and their couplings in condensed matter systems is critical to develop better energy-conversion devices. In thermoelectric materials, the heat-to-electricity conversion efficiency is directly improved by suppressing the propagation of phonon quasiparticles responsible for macroscopic thermal transport. The material with the current record for thermoelectric conversion efficiency, SnSe, achieves an ultra-low thermal conductivity, but the mechanism enabling this strong phonon scattering remains largely unknown. Using inelastic neutron scattering measurements and first-principles simulations, we mapped the four-dimensional phonon dispersion surfaces of SnSe, and revealed the origin of ionic-potential anharmonicity responsible for the unique properties of SnSe. Wemore » show that the giant phonon scattering arises from an unstable electronic structure, with orbital interactions leading to a ferroelectric-like lattice instability. Our results provide a microscopic picture connecting electronic structure and phonon anharmonicity in SnSe, and offers precious insights on how electron-phonon and phononphonon interactions may lead to the realization of ultra-low thermal conductivity.« less

Orbitally driven giant phonon anharmonicity in SnSe

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, C. W.; Hong, J.; May, A. F.

Understanding elementary excitations and their couplings in condensed matter systems is critical for developing better energy-conversion devices. In thermoelectric materials, the heat-to-electricity conversion efficiency is directly improved by suppressing the propagation of phonon quasiparticles responsible for macroscopic thermal transport. The current record material for thermoelectric conversion efficiency, SnSe, has an ultralow thermal conductivity, but the mechanism behind the strong phonon scattering remains largely unknown. From inelastic neutron scattering measurements and first-principles simulations, we mapped the four-dimensional phonon dispersion surfaces of SnSe, and found the origin of the ionic-potential anharmonicity responsible for the unique properties of SnSe. We show that themore » giant phonon scattering arises from an unstable electronic structure, with orbital interactions leading to a ferroelectric-like lattice instability. The present results provide a microscopic picture connecting electronic structure and phonon anharmonicity in SnSe, and offers new insights on how electron–phonon and phonon–phonon interactions may lead to the realization of ultralow thermal conductivity.« less

Phonons and elasticity of cementite through the Curie temperature

NASA Astrophysics Data System (ADS)

Mauger, L.; Herriman, J. E.; Hellman, O.; Tracy, S. J.; Lucas, M. S.; Muñoz, J. A.; Xiao, Yuming; Li, J.; Fultz, B.

2017-01-01

Phonon partial densities of states (pDOS) of Fe573C were measured from cryogenic temperatures through the Curie transition at 460 K using nuclear resonant inelastic x-ray scattering. The cementite pDOS reveal that low-energy acoustic phonons shift to higher energies (stiffen) with temperature before the magnetic transition. This unexpected stiffening suggests strongly nonharmonic vibrational behavior that impacts the thermodynamics and elastic properties of cementite. Density functional theory calculations reproduced the anomalous stiffening observed experimentally in cementite by accounting for phonon-phonon interactions at finite temperatures. The calculations show that the low-energy acoustic phonon branches with polarizations along the [010] direction are largely responsible for the anomalous thermal stiffening. The effect was further localized to the motions of the FeII site within the orthorhombic structure, which participates disproportionately in the anomalous phonon stiffening.

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.

Nanomesh phononic structures for low thermal conductivity and thermoelectric energy conversion materials

DOEpatents

Yu, Jen-Kan; Mitrovic, Slobodan; Heath, James R.

2016-08-16

A nanomesh phononic structure includes: a sheet including a first material, the sheet having a plurality of phononic-sized features spaced apart at a phononic pitch, the phononic pitch being smaller than or equal to twice a maximum phonon mean free path of the first material and the phononic size being smaller than or equal to the maximum phonon mean free path of the first material.

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.

Resonant Magnon-Phonon Polaritons in a Ferrimagnet

DTIC Science & Technology

2000-09-29

UNCLASSIFIED Defense Technical Information Center Compilation Part Notice ADPO 11604 TITLE: Resonant Magnon -Phonon Polaritons in a Ferrimagnet...part numbers comprise the compilation report: ADP011588 thru ADP011680 UNCLASSIFIED 75 Resonant Magnon -Phonon Polaritons in a Ferrimagnet I. E...susceptibilities X"aa and X’m << X’m appear, where 77 xem - DPx igEo0 i_ Xxy - hy- C1 (0)2 _ 00t2) 4= -7• 4 3. Phonon and magnon polaritons We solve the

A hybrid phononic crystal for roof application.

PubMed

Wan, Qingmian; Shao, Rong

2017-11-01

Phononic crystal is a type of acoustic material, and the study of phononic crystals has attracted great attention from national research institutions. Meanwhile, noise reduction in the low-frequency range has always encountered difficulties and troubles in the engineering field. In order to obtain a unique and effective low-frequency noise reduction method, in this paper a low frequency noise attenuation system based on phononic crystal structure is proposed and demonstrated. The finite element simulation of the band gap is consistent with the final test results. The effects of structure parameters on the band gaps were studied by changing the structure parameters and the band gaps can be controlled by suitably tuning structure parameters. The structure and results provide a good support for phononic crystal structures engineering application.

Nonbolometric bottleneck in electron-phonon relaxation in ultrathin WSi films

NASA Astrophysics Data System (ADS)

Sidorova, Mariia V.; Kozorezov, A. G.; Semenov, A. V.; Korneeva, Yu. P.; Mikhailov, M. Yu.; Devizenko, A. Yu.; Korneev, A. A.; Chulkova, G. M.; Goltsman, G. N.

2018-05-01

We developed the model of the internal phonon bottleneck to describe the energy exchange between the acoustically soft ultrathin metal film and acoustically rigid substrate. Discriminating phonons in the film into two groups, escaping and nonescaping, we show that electrons and nonescaping phonons may form a unified subsystem, which is cooled down only due to interactions with escaping phonons, either due to direct phonon conversion or indirect sequential interaction with an electronic system. Using an amplitude-modulated absorption of the sub-THz radiation technique, we studied electron-phonon relaxation in ultrathin disordered films of tungsten silicide. We found an experimental proof of the internal phonon bottleneck. The experiment and simulation based on the proposed model agree well, resulting in τe -ph˜14 0 -19 0 ps at TC=3.4 K , supporting the results of earlier measurements by independent techniques.

Phonon Mapping in Flowing Equilibrium

NASA Astrophysics Data System (ADS)

Ruff, J. P. C.

2015-03-01

When a material conducts heat, a modification of the phonon population occurs. The equilibrium Bose-Einstein distribution is perturbed towards flowing-equilibrium, for which the distribution function is not analytically known. Here I argue that the altered phonon population can be efficiently mapped over broad regions of reciprocal space, via diffuse x-ray scattering or time-of-flight neutron scattering, while a thermal gradient is applied across a single crystal sample. When compared to traditional transport measurements, this technique offers a superior, information-rich new perspective on lattice thermal conductivity, wherein the band and momentum dependences of the phonon thermal current are directly resolved. The proposed method is benchmarked using x-ray thermal diffuse scattering measurements of single crystal diamond under transport conditions. CHESS is supported by the NSF & NIH/NIGMS via NSF Award DMR-1332208.

Phonon Effects on Charge Transport Through a Two State Molecule

NASA Astrophysics Data System (ADS)

Ulloa, Sergio E.; Yudiarsah, Efta

2008-03-01

We study the effect of local and non-local phonon on the transport properties of a molecule model described by two- electronic states. The local phonon interaction is tackled by means of a Lang Firsov transformation [1,2]. The interaction with non-local phonons (phonon-assisted hopping) is considered perturbatively up to the first nonzero order in the self energy. The presence of different kinds of electron-phonon interaction open new transmission channels. In addition to the polaron shift and replicas due to local phonons, non-local phonons cause the appearance of new satellite states around the initial states. In the weak coupling regime of non-local phonon and electrons, states are shifted an amount proportional to square of the interaction. However, in the strong coupling regime, the non-linear effects emerge and display more interesting features on transport properties. Additional features on transport properties due to new transmission channel are shown to appear at finite temperatures. [1] G. D. Mahan, Many-particle physics, 3rd ed. (Plenum Publishers, New York, 2000). [2] R. Gutierrez et al., Phys. Rev. B. 74, 235105 (2006).

Thermal transport across metal–insulator interface via electron–phonon interaction.

PubMed

Zhang, Lifa; Lü, Jing-Tao; Wang, Jian-Sheng; Li, Baowen

2013-11-06

The thermal transport across a metal–insulator interface can be characterized by electron–phonon interaction through which an electron lead is coupled to a phonon lead if phonon–phonon coupling at the interface is very weak. We investigate the thermal conductance and rectification between the electron part and the phonon part using the nonequilibrium Green's function method. It is found that the thermal conductance has a nonmonotonic behavior as a function of average temperature or the coupling strength between the phonon leads in the metal part and the insulator part. The metal–insulator interface shows a clear thermal rectification effect, which can be reversed by a change in average temperature or the electron–phonon coupling.

Reduction of Thermal Conductivity by Nanoscale 3D Phononic Crystal

PubMed Central

Yang, Lina; Yang, Nuo; Li, Baowen

2013-01-01

We studied how the period length and the mass ratio affect the thermal conductivity of isotopic nanoscale three-dimensional (3D) phononic crystal of Si. Simulation results by equilibrium molecular dynamics show isotopic nanoscale 3D phononic crystals can significantly reduce the thermal conductivity of bulk Si at high temperature (1000 K), which leads to a larger ZT than unity. The thermal conductivity decreases as the period length and mass ratio increases. The phonon dispersion curves show an obvious decrease of group velocities in 3D phononic crystals. The phonon's localization and band gap is also clearly observed in spectra of normalized inverse participation ratio in nanoscale 3D phononic crystal. PMID:23378898

Quasiparticles and phonon satellites in spectral functions of semiconductors and insulators: Cumulants applied to the full first-principles theory and the Fröhlich polaron

NASA Astrophysics Data System (ADS)

Nery, Jean Paul; Allen, Philip B.; Antonius, Gabriel; Reining, Lucia; Miglio, Anna; Gonze, Xavier

2018-03-01

The electron-phonon interaction causes thermal and zero-point motion shifts of electron quasiparticle (QP) energies ɛk(T ) . Other consequences of interactions, visible in angle-resolved photoemission spectroscopy (ARPES) experiments, are broadening of QP peaks and appearance of sidebands, contained in the electron spectral function A (k ,ω ) =-ℑ m GR(k ,ω ) /π , where GR is the retarded Green's function. Electronic structure codes (e.g., using density-functional theory) are now available that compute the shifts and start to address broadening and sidebands. Here we consider MgO and LiF, and determine their nonadiabatic Migdal self-energy. The spectral function obtained from the Dyson equation makes errors in the weight and energy of the QP peak and the position and weight of the phonon-induced sidebands. Only one phonon satellite appears, with an unphysically large energy difference (larger than the highest phonon energy) with respect to the QP peak. By contrast, the spectral function from a cumulant treatment of the same self-energy is physically better, giving a quite accurate QP energy and several satellites approximately spaced by the LO phonon energy. In particular, the positions of the QP peak and first satellite agree closely with those found for the Fröhlich Hamiltonian by Mishchenko et al. [Phys. Rev. B 62, 6317 (2000), 10.1103/PhysRevB.62.6317] using diagrammatic Monte Carlo. We provide a detailed comparison between the first-principles MgO and LiF results and those of the Fröhlich Hamiltonian. Such an analysis applies widely to materials with infrared(IR)-active phonons.

Topological phononic insulator with robust pseudospin-dependent transport

NASA Astrophysics Data System (ADS)

Xia, Bai-Zhan; Liu, Ting-Ting; Huang, Guo-Liang; Dai, Hong-Qing; Jiao, Jun-Rui; Zang, Xian-Guo; Yu, De-Jie; Zheng, Sheng-Jie; Liu, Jian

2017-09-01

Topological phononic states, which facilitate unique acoustic transport around defects and disorders, have significantly revolutionized our scientific cognition of acoustic systems. Here, by introducing a zone folding mechanism, we realize the topological phase transition in a double Dirac cone of the rotatable triangular phononic crystal with C3 v symmetry. We then investigate the distinct topological edge states on two types of interfaces of our phononic insulators. The first one is a zigzag interface which simultaneously possesses a symmetric mode and an antisymmetric mode. Hybridization of the two modes leads to a robust pseudospin-dependent one-way propagation. The second one is a linear interface with a symmetric mode or an antisymmetric mode. The type of mode is dependent on the topological phase transition of the phononic insulators. Based on the rotatability of triangular phononic crystals, we consider several complicated contours defined by the topological zigzag interfaces. Along these contours, the acoustic waves can unimpededly transmit without backscattering. Our research develops a route for the exploration of the topological phenomena in experiments and provides an excellent framework for freely steering the acoustic backscattering-immune propagation within topological phononic structures.

Magnon and phonon thermometry with inelastic light scattering

NASA Astrophysics Data System (ADS)

Olsson, Kevin S.; An, Kyongmo; Li, Xiaoqin

2018-04-01

Spin caloritronics investigates the interplay between the transport of spin and heat. In the spin Seebeck effect, a thermal gradient across a magnetic material generates a spin current. A temperature difference between the energy carriers of the spin and lattice subsystems, namely the magnons and phonons, is necessary for such thermal nonequilibrium generation of spin current. Inelastic light scattering is a powerful method that can resolve the individual temperatures of magnons and phonons. In this review, we discuss the thermometry capabilities of inelastic light scattering for measuring optical and acoustic phonons, as well as magnons. A scattering spectrum offers three temperature sensitive parameters: frequency shift, linewidth, and integrated intensity. We discuss the temperatures measured via each of these parameters for both phonon and magnons. Finally, we discuss inelastic light scattering experiments that have examined the magnon and phonon temperatures in thermal nonequilibrium which are particularly relevant to spin caloritronic phenomena.

Phonon Dispersion in Amorphous Ni-Alloys

NASA Astrophysics Data System (ADS)

Vora, A. M.

2007-06-01

The well-known model potential is used to investigate the longitudinal and transverse phonon dispersion curves for six Ni-based binary amorphous alloys, viz. Ni31Dy69, Ni33Y67, Ni36Zr64, Ni50Zr50, Ni60 Nb40, and Ni81B19. The thermodynamic and elastic properties are also computed from the elastic limits of the phonon dispersion curves. The theoretical approach given by Hubbard-Beeby is used in the present study to compute the phonon dispersion curves. Five local field correction functions proposed by Hartree, Taylor, Ichimaru-Utsumi, Farid et al. and Sarkar et al. are employed to see the effect of exchange and correlation in the aforesaid properties.

Magnetic moments induce strong phonon renormalization in FeSi.

PubMed

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

2015-11-27

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.

Simple tunnel diode circuit for accurate zero crossing timing

NASA Technical Reports Server (NTRS)

Metz, A. J.

1969-01-01

Tunnel diode circuit, capable of timing the zero crossing point of bipolar pulses, provides effective design for a fast crossing detector. It combines a nonlinear load line with the diode to detect the zero crossing of a wide range of input waveshapes.

Temperature dependence of phonons in photosynthesis proteins

NASA Astrophysics Data System (ADS)

Xu, Mengyang; Myles, Dean; Blankenship, Robert; Markelz, Andrea

Protein long range vibrations are essential to biological function. For many proteins, these vibrations steer functional conformational changes. For photoharvesting proteins, the structural vibrations play an additional critical role in energy transfer to the reaction center by both phonon assisted energy transfer and energy dissipation. The characterization of these vibrations to understand how they are optimized to balance photoharvesting and photoprotection is challenging. To date this characterization has mainly relied on fluorescence line narrowing measurements at cryogenic temperatures. However, protein dynamics has a strong temperature dependence, with an apparent turn on in anharmonicity between 180-220 K. If this transition affects intramolecular vibrations, the low temperature measurements will not represent the phonon spectrum at biological temperatures. Here we use the new technique of anisotropic terahertz microscopy (ATM) to measure the intramolecular vibrations of FMO complex. ATM is uniquely capable of isolating protein vibrations from isotropic background. We find resonances both red and blue shift with temperature above the dynamical transition. The results indicate that the characterization of vibrations must be performed at biologically relevant temperatures to properly understand the energy overlap with the excitation energy transfer. This work was supported by NSF:DBI 1556359, BioXFEL seed Grant funding from NSF:DBI 1231306, DOE: DE-SC0016317, and the Bruce Holm University at Buffalo Research Foundation Grant.

Phonon-based scalable platform for chip-scale quantum computing

DOE Office of Scientific and Technical Information (OSTI.GOV)

Reinke, Charles M.; El-Kady, Ihab

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

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

Demonstration of suppressed phonon tunneling losses in phononic bandgap shielded membrane resonators for high-Q optomechanics.

PubMed

Tsaturyan, Yeghishe; Barg, Andreas; Simonsen, Anders; Villanueva, Luis Guillermo; Schmid, Silvan; Schliesser, Albert; Polzik, Eugene S

2014-03-24

Dielectric membranes with exceptional mechanical and optical properties present one of the most promising platforms in quantum opto-mechanics. The performance of stressed silicon nitride nanomembranes as mechanical resonators notoriously depends on how their frame is clamped to the sample mount, which in practice usually necessitates delicate, and difficult-to-reproduce mounting solutions. Here, we demonstrate that a phononic bandgap shield integrated in the membrane's silicon frame eliminates this dependence, by suppressing dissipation through phonon tunneling. We dry-etch the membrane's frame so that it assumes the form of a cm-sized bridge featuring a 1-dimensional periodic pattern, whose phononic density of states is tailored to exhibit one, or several, full band gaps around the membrane's high-Q modes in the MHz-range. We quantify the effectiveness of this phononic bandgap shield by optical interferometry measuring both the suppressed transmission of vibrations, as well as the influence of frame clamping conditions on the membrane modes. We find suppressions up to 40 dB and, for three different realized phononic structures, consistently observe significant suppression of the dependence of the membrane's modes on sample clamping-if the mode's frequency lies in the bandgap. As a result, we achieve membrane mode quality factors of 5 × 10(6) with samples that are tightly bolted to the 8 K-cold finger of a cryostat. Q × f -products of 6 × 10(12) Hz at 300 K and 14 × 10(12) Hz at 8 K are observed, satisfying one of the main requirements for optical cooling of mechanical vibrations to their quantum ground-state.

One and two-phonon processes of the spin-flip relaxation in quantum dots: Spin-phonon coupling mechanism

NASA Astrophysics Data System (ADS)

Wang, Zi-Wu; Li, Shu-Shen

2012-07-01

We investigate the spin-flip relaxation in quantum dots using a non-radiation transition approach based on the descriptions for the electron-phonon deformation potential and Fröhlich interaction in the Pavlov-Firsov spin-phonon Hamiltonian. We give the comparisons of the electron relaxations with and without spin-flip assisted by one and two-phonon processes. Calculations are performed for the dependence of the relaxation time on the external magnetic field, the temperature and the energy separation between the Zeeman sublevels of the ground and first-excited state. We find that the electron relaxation time of the spin-flip process is more longer by three orders of magnitudes than that of no spin-flip process.

Phonons on fcc (100), (110), and (111) surfaces using Lennard-Jones potentials. II. Temperature dependence of surface phonons studied with molecular dynamics

NASA Astrophysics Data System (ADS)

Koleske, D. D.; Sibener, S. J.

In this paper we present temperature dependent studies of the surface phonon dispersion relations for fcc (100), (110), and (111) faces using molecular dynamics (MD) simulations and Lennard-Jones potentials. This study was conducted in order to investigate how anharmonic potential terms influence the dynamical properties of the surface. This was accomplished by examining the temperature dependence of the Q-resolved phonon spectral density function. All phonon frequencies were found to decrease linearly in T as the temperature was increased, while at low temperatures the phonon linewidths increased linearly with T. At higher temperatures, some of the phonon linewidths changed from having a linear to a quadratic dependence on T. The temperature at which this T to T2 change occurs is surface dependent and occurs at the lowest temperature on the (110) surface. The T2 dependence arises from the increasing importance of higher-order phonon-phonon scattering terms. The phonons which exhibit T2 dependence tend to be modes which propagate perpendicularly or nearly perpendicularly to the direction of maximum root-mean-squared displacement (RMSD). This is especially true for the linewidth of the S 1 mode at overlineX on the (110) surface where, at T ≈ 15-23% of the melting temperature, the RMSD perpendicular to the atomic rows become larger than the RMSD normal to the surface. Our results indicate that the dynamics on the (110) surface may be significantly influenced by anharmonic potential terms at temperatures as low as 15% of the melting temperature.

Phononic properties of superlattices and multi quantum well heterostructures (Conference Presentation)

NASA Astrophysics Data System (ADS)

Wagner, Markus R.; Reparaz, Juan Sebastian; Callsen, Gordon; Nippert, Felix; Kure, Thomas; Hoffmann, Axel; Hugues, Maxime; Teysseire, Monique; Damilano, Benjamin; Chauveau, Jean-Michel

2017-03-01

We address the electronic, phononic, and thermal properties of oxide based superlattices and multi quantum well heterostructures. In the first part, we review the present understanding of phonon coupling and phonon propagation in superlattices and elucidate current research aspects of phonon coherence in these structure. Subsequently, we focus on the experimental study of MBE grown ZnO/ZnMgO multi quantum well heterostructures with varying Mg content, barrier thickness, quantum well thickness, and number of periods. In particular, we discuss how the controlled variation of these parameters affect the phonon dispersion relation and phonon propagation and their impact on the thermal properties.

Fiber optical vibrometer based on a phononic crystal filter

NASA Astrophysics Data System (ADS)

Lin, Sijing; Chai, Quan; Zhang, Jianzhong

2012-02-01

We propose that phononic crystals could be used as a packaging method in a fiber optical vibrometer system to filter the vibration at unwanted frequency range. A simple FBG based vibrometer and a aluminum-silicone rubber based 1D phononic crystal with the designed phononic band gap are built up, and the corresponding experimental results are demonstrated to show the feasibility of our proposal. Our proposal also points out that optical fiber sensors could be an excellent candidate to research the inner acoustic response of more complex phononic crystals.

Anharmonic phonon decay in cubic GaN

NASA Astrophysics Data System (ADS)

Cuscó, R.; Domènech-Amador, N.; Novikov, S.; Foxon, C. T.; Artús, L.

2015-08-01

We present a Raman-scattering study of optical phonons in zinc-blende (cubic) GaN for temperatures ranging from 80 to 750 K. The experiments were performed on high-quality, cubic GaN films grown by molecular-beam epitaxy on GaAs (001) substrates. The observed temperature dependence of the optical phonon frequencies and linewidths is analyzed in the framework of anharmonic decay theory, and possible decay channels are discussed in the light of density-functional-theory calculations. The longitudinal-optical (LO) mode relaxation is found to occur via asymmetric decay into acoustic phonons, with an appreciable contribution of higher-order processes. The transverse-optical mode linewidth shows a weak temperature dependence and its frequency downshift is primarily determined by the lattice thermal expansion. The LO phonon lifetime is derived from the observed Raman linewidth and an excellent agreement with previous theoretical predictions is found.

Numerical investigation of diffraction of acoustic waves by phononic crystals

NASA Astrophysics Data System (ADS)

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

2012-05-01

Diffraction as well as transmission of acoustic waves by two-dimensional phononic crystals (PCs) composed of steel rods in water are investigated in this paper. The finite element simulations were performed in order to compute pressure fields generated by a line source that are incident on a finite size PC. Such field maps are analyzed based on the complex band structure for the infinite periodic PC. Finite size computations indicate that the exponential decrease of the transmission at deaf frequencies is much stronger than that in Bragg band gaps.

Four-phonon scattering significantly reduces intrinsic thermal conductivity of solids

DOE Office of Scientific and Technical Information (OSTI.GOV)

Feng, Tianli; Lindsay, Lucas R.; Ruan, Xiulin

We rigorously calculate intrinsic phonon thermal resistance from four-phonon scattering processesusing rst principles Boltzmann transport methods. Fundamental questions concerning the role ofhigher order scattering at high temperature and in systems with otherwise weak intrinsic scatteringare answered. Using diamond and silicon as benchmark materials, the predicted thermal conductiv-ity including intrinsic four-phonon resistance gives signicantly better agreement with measurementsat high temperatures than previous rst principles calculations. In the predicted ultrahigh thermalconductivity material, zincblende BAs, four-phonon scattering is strikingly strong when comparedto three-phonon processes, even at room temperature, as the latter have an extremely limited phasespace for scattering. Including four-phonon thermal resistance reducesmore » the predicted thermal con-ductivity of BAs from 2200 W/m-K to 1400 W/m-K.« less

Phonon response of some heavy Fermion systems in dynamic limit

NASA Astrophysics Data System (ADS)

Sahoo, Jitendra; Shadangi, Namita; Nayak, Pratibindhya

2017-05-01

The phonon excitation spectrum of some Heavy Fermion (HF) systems in the presence of electron-phonon interaction is studied in the dynamic limit (ω≠0). The renormalized excitation phonon frequencies (ω˜ = ω/ω0) are evaluated through Periodic Anderson Model (PAM) in the presence of electron-phonon interaction using Zubarev-type double time temperature-dependent Green function. The calculated renormalized phonon energy is analyzed through the plots of (ω˜ = ω/ω0) against temperature for different system parameters like effective coupling strength ‘g’ and the position of f-level ‘d’. The observed behavior is analyzed and found to agree with the general features of HF systems found in experiments. Further, it is observed that in finite but small q-values the propagating phonons harden and change to localized peaks.

Four-phonon scattering significantly reduces intrinsic thermal conductivity of solids

DOE PAGES

Feng, Tianli; Lindsay, Lucas R.; Ruan, Xiulin

2017-10-27

We rigorously calculate intrinsic phonon thermal resistance from four-phonon scattering processesusing rst principles Boltzmann transport methods. Fundamental questions concerning the role ofhigher order scattering at high temperature and in systems with otherwise weak intrinsic scatteringare answered. Using diamond and silicon as benchmark materials, the predicted thermal conductiv-ity including intrinsic four-phonon resistance gives signicantly better agreement with measurementsat high temperatures than previous rst principles calculations. In the predicted ultrahigh thermalconductivity material, zincblende BAs, four-phonon scattering is strikingly strong when comparedto three-phonon processes, even at room temperature, as the latter have an extremely limited phasespace for scattering. Including four-phonon thermal resistance reducesmore » the predicted thermal con-ductivity of BAs from 2200 W/m-K to 1400 W/m-K.« less

Electrons and Phonons in Semiconductor Multilayers

NASA Astrophysics Data System (ADS)

Ridley, B. K.

1996-11-01

This book provides a detailed description of the quantum confinement of electrons and phonons in semiconductor wells, superlattices and quantum wires, and shows how this affects their mutual interactions. It discusses the transition from microscopic to continuum models, emphasizing the use of quasi-continuum theory to describe the confinement of optical phonons and electrons. The hybridization of optical phonons and their interactions with electrons are treated, as are other electron scattering mechanisms. The book concludes with an account of the electron distribution function in three-, two- and one-dimensional systems, in the presence of electrical or optical excitation. This text will be of great use to graduate students and researchers investigating low-dimensional semiconductor structures, as well as to those developing new devices based on these systems.

Diamond family of colloidal supercrystals as phononic metamaterials

NASA Astrophysics Data System (ADS)

Aryana, Kiumars; Zanjani, Mehdi B.

2018-05-01

Colloidal crystals provide a versatile platform for designing phononic metamaterials with exciting applications for sound and heat management. New advances in the synthesis and self-assembly of anisotropic building blocks such as colloidal clusters have expanded the library of available micro- and nano-scale ordered multicomponent structures. Diamond-like supercrystals formed by such clusters and spherical particles are notable examples that include a rich family of crystal symmetries such as diamond, double diamond, zinc-blende, and MgCu2. This work investigates the design of phononic supercrystals by predicting and analyzing phonon transport properties. In addition to size variation and structural diversity, these supercrystals encapsulate different sub-lattice types within one structure. Computational models are used to calculate the effect of various parameters on the phononic spectrum of diamond-like supercrystals. The results show that structures with relatively small or large filling factors (f > 0.65 or f < 0.45) include smaller bandgaps compared to those with medium filling factors (0.65 > f > 0.45). The double diamond and zinc-blende structures render the largest bandgap size compared to the other supercrystals studied in this paper. Additionally, this article discusses the effect of incorporating various configurations of sub-lattices by selecting different material compositions for the building blocks. The results suggest that, for the same structure, there exist multiple phononic variants with drastically different band structures. This study provides a valuable insight for evaluating novel colloidal supercrystals for phononic applications and guides the future experimental work for the synthesis of colloidal structures with desired phononic behavior.

Dependence of spin dephasing on initial spin polarization in a high-mobility two-dimensional electron system

NASA Astrophysics Data System (ADS)

Stich, D.; Zhou, J.; Korn, T.; Schulz, R.; Schuh, D.; Wegscheider, W.; Wu, M. W.; Schüller, C.

2007-11-01

We have studied the spin dynamics of a high-mobility two-dimensional electron system in a GaAs/Al0.3Ga0.7As single quantum well by time-resolved Faraday rotation and time-resolved Kerr rotation in dependence on the initial degree of spin polarization, P , of the electrons. By increasing the initial spin polarization from the low- P regime to a significant P of several percent, we find that the spin dephasing time, T2* , increases from about 20to200ps . Moreover, T2* increases with temperature at small spin polarization but decreases with temperature at large spin polarization. All these features are in good agreement with theoretical predictions by Weng and Wu [Phys. Rev. B 68, 075312 (2003)]. Measurements as a function of spin polarization at fixed electron density are performed to further confirm the theory. A fully microscopic calculation is performed by setting up and numerically solving the kinetic spin Bloch equations, including the D’yakonov-Perel’ and the Bir-Aronov-Pikus mechanisms, with all the scattering explicitly included. We reproduce all principal features of the experiments, i.e., a dramatic decrease of spin dephasing with increasing P and the temperature dependences at different spin polarizations.

Dynamics of 4-oxo-TEMPO-d16-15N nitroxide-propylene glycol system studied by ESR and ESE in liquid and glassy state in temperature range 10-295 K

NASA Astrophysics Data System (ADS)

Goslar, Janina; Hoffmann, Stanislaw K.; Lijewski, Stefan

2016-08-01

ESR spectra and electron spin relaxation of nitroxide radical in 4-oxo-TEMPO-d16-15N in propylene glycol were studied at X-band in the temperature range 10-295 K. The spin-lattice relaxation in the liquid viscous state determined from the resonance line shape is governed by three mechanisms occurring during isotropic molecular reorientations. In the glassy state below 200 K the spin-lattice relaxation, phase relaxation and electron spin echo envelope modulations (ESEEM) were studied by pulse spin echo technique using 2-pulse and 3-pulse induced signals. Electron spin-lattice relaxation is governed by a single non-phonon relaxation process produced by localized oscillators of energy 76 cm-1. Electron spin dephasing is dominated by a molecular motion producing a resonance-type peak in the temperature dependence of the dephasing rate around 120 K. The origin of the peak is discussed and a simple method for the peak shape analysis is proposed, which gives the activation energy of a thermally activated motion Ea = 7.8 kJ/mol and correlation time τ0 = 10-8 s. The spin echo amplitude is strongly modulated and FT spectrum contains a doublet of lines centered around the 2D nuclei Zeeman frequency. The splitting into the doublet is discussed as due to a weak hyperfine coupling of nitroxide unpaired electron with deuterium of reorienting CD3 groups.

Phonon anharmonicity and negative thermal expansion in SnSe

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bansal, Dipanshu; Hong, Jiawang; Li, Chen W.

In this paper, the anharmonic phonon properties of SnSe in the Pnma phase were investigated with a combination of experiments and first-principles simulations. Using inelastic neutron scattering (INS) and nuclear resonant inelastic X-ray scattering (NRIXS), we have measured the phonon dispersions and density of states (DOS) and their temperature dependence, which revealed a strong, inhomogeneous shift and broadening of the spectrum on warming. First-principles simulations were performed to rationalize these measurements, and to explain the previously reported anisotropic thermal expansion, in particular the negative thermal expansion within the Sn-Se bilayers. Including the anisotropic strain dependence of the phonon free energy,more » in addition to the electronic ground state energy, is essential to reproduce the negative thermal expansion. From the phonon DOS obtained with INS and additional calorimetry measurements, we quantify the harmonic, dilational, and anharmonic components of the phonon entropy, heat capacity, and free energy. Finally, the origin of the anharmonic phonon thermodynamics is linked to the electronic structure.« less

Phonon anharmonicity and negative thermal expansion in SnSe

DOE PAGES

Bansal, Dipanshu; Hong, Jiawang; Li, Chen W.; ...

2016-08-09

In this paper, the anharmonic phonon properties of SnSe in the Pnma phase were investigated with a combination of experiments and first-principles simulations. Using inelastic neutron scattering (INS) and nuclear resonant inelastic X-ray scattering (NRIXS), we have measured the phonon dispersions and density of states (DOS) and their temperature dependence, which revealed a strong, inhomogeneous shift and broadening of the spectrum on warming. First-principles simulations were performed to rationalize these measurements, and to explain the previously reported anisotropic thermal expansion, in particular the negative thermal expansion within the Sn-Se bilayers. Including the anisotropic strain dependence of the phonon free energy,more » in addition to the electronic ground state energy, is essential to reproduce the negative thermal expansion. From the phonon DOS obtained with INS and additional calorimetry measurements, we quantify the harmonic, dilational, and anharmonic components of the phonon entropy, heat capacity, and free energy. Finally, the origin of the anharmonic phonon thermodynamics is linked to the electronic structure.« less

Phonon-Mediated Tunneling into Graphene

NASA Astrophysics Data System (ADS)

Wehling, T. O.; Grigorenko, I.; Lichtenstein, A. I.; Balatsky, A. V.

2008-11-01

Recent scanning tunneling spectroscopy experiments on graphene reported an unexpected gap of about ±60meV around the Fermi level [V. W. Brar , Appl. Phys. Lett.APPLAB0003-6951 91, 122102 (2007); 10.1063/1.2771084Y. Zhang , Nature Phys.NPAHAX1745-2481 4, 627 (2008)10.1038/nphys1022]. Here we give a theoretical investigation explaining the experimentally observed spectra and confirming the phonon-mediated tunneling as the reason for the gap: We study the real space properties of the wave functions involved in the tunneling process by means of ab initio theory and present a model for the electron-phonon interaction, which couples the graphene’s Dirac electrons with quasifree-electron states at the Brillouin zone center. The self-energy associated with this electron-phonon interaction is calculated, and its effects on tunneling into graphene are discussed. Good agreement of the tunneling density of states within our model and the experimental dI/dU spectra is found.

Phonon mediated tunneling into graphene

NASA Astrophysics Data System (ADS)

Wehling, Tim; Grigorenko, Ilya; Lichtenstein, Alexander; Balatsky, Alexander

2009-03-01

Recent scanning tunneling spectroscopy experiments [V. W. Brar et al., Appl. Phys. Lett. 91, 122102 (2007); Y. Zhang et al., Nature Phys. 4, 627 (2008)] on graphene reported an unexpected gap of about ±60,eV around the Fermi level. Here, we give a theoretical investigation explaining the experimentally observed spectra and confirming the phonon mediated tunneling as the reason for the gap: We study the real space properties of the wave functions involved in the tunneling process by means of ab-initio theory and present a model for the electron-phonon interaction, which couples the graphene's Dirac electrons with quasi free electron states at the Brillouin zone center. The self-energy associated with this electron-phonon interaction is calculated and its effects on tunneling into graphene are discussed. In particular, good agreement of the tunneling density of states within our model and the experimental dI/dU spectra is found.

Acoustic phonon spectrum and thermal transport in nanoporous alumina arrays

DOE PAGES

Kargar, Fariborz; Ramirez, Sylvester; Debnath, Bishwajit; ...

2015-10-28

We report results of a combined investigation of thermal conductivity and acoustic phonon spectra in nanoporous alumina membranes with the pore diameter decreasing from D=180 nm to 25 nm. The samples with the hexagonally arranged pores were selected to have the same porosity Ø ≈13%. The Brillouin-Mandelstam spectroscopy measurements revealed bulk-like phonon spectrum in the samples with D = 180 nm pores and spectral features, which were attributed to spatial confinement, in the samples with 25 nm and 40 nm pores. The velocity of the longitudinal acoustic phonons was reduced in the samples with smaller pores. As a result, analysismore » of the experimental data and calculated phonon dispersion suggests that both phonon-boundary scattering and phonon spatial confinement affect heat conduction in membranes with the feature sizes D < 40 nm.« less

The Seebeck Coefficient and Phonon Drag in Silicon

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mahan, Gerald; Lindsay, Lucas R.; Broido, David

2014-12-29

We present a theory of the phonon-drag Seebeck coe cient in nondegenerate semiconductors, and apply it to silicon for temperatures 30 < T < 300K. Our calculation uses only parameters from the literature, and previous calculations of the phonon lifetime. We nd excellent agreement with the measurements of Geballe and Hull [Phys.Rev. 98, 940 (1955)]. The phonon-drag term dominates at low temperature, and shows an important dependence on the dimensions of the experimental sample.

Vibron Solitons and Soliton-Induced Infrared Spectra of Crystalline Acetanilide

NASA Astrophysics Data System (ADS)

Takeno, S.

1986-01-01

Red-shifted infrared spectra at low temperatures of amide I (C=O stretching) vibrations of crystalline acetanilide measured by Careri et al. are shown to be due to vibron solitons, which are nonlinearity-induced localized modes of vibrons arising from their nonlinear interactions with optic-type phonons. A nonlinear eigenvalue equation giving the eigenfrequency of stationary solitons is solved approximately by introducing lattice Green's functions, and the obtained result is in good agreement with the experimental result. Inclusion of interactions with acoustic phonons yields the Debye-Waller factor in the zero-phonon line spectrum of vibron solitons, in a manner analogous to the case of impurity-induced localized harmonic phonon modes in alkali halides.

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.

Phonon structures of GaN-based random semiconductor alloys

NASA Astrophysics Data System (ADS)

Zhou, Mei; Chen, Xiaobin; Li, Gang; Zheng, Fawei; Zhang, Ping

2017-12-01

Accurate modeling of thermal properties is strikingly important for developing next-generation electronics with high performance. Many thermal properties are closely related to phonon dispersions, such as sound velocity. However, random substituted semiconductor alloys AxB1-x usually lack translational symmetry, and simulation with periodic boundary conditions often requires large supercells, which makes phonon dispersion highly folded and hardly comparable with experimental results. Here, we adopt a large supercell with randomly distributed A and B atoms to investigate substitution effect on the phonon dispersions of semiconductor alloys systematically by using phonon unfolding method [F. Zheng, P. Zhang, Comput. Mater. Sci. 125, 218 (2016)]. The results reveal the extent to which phonon band characteristics in (In,Ga)N and Ga(N,P) are preserved or lost at different compositions and q points. Generally, most characteristics of phonon dispersions can be preserved with indium substitution of gallium in GaN, while substitution of nitrogen with phosphorus strongly perturbs the phonon dispersion of GaN, showing a rapid disintegration of the Bloch characteristics of optical modes and introducing localized impurity modes. In addition, the sound velocities of both (In,Ga)N and Ga(N,P) display a nearly linear behavior as a function of substitution compositions. Supplementary material in the form of one pdf file available from the Journal web page at http://https://doi.org/10.1140/epjb/e2017-80481-0.

Phonons and superconductivity in fcc and dhcp lanthanum

NASA Astrophysics Data System (ADS)

Baǧcı, S.; Tütüncü, H. M.; Duman, S.; Srivastava, G. P.

2010-04-01

We have investigated the structural and electronic properties of lanthanum in the face-centered-cubic (fcc) and double hexagonal-close-packed (dhcp) phases using a generalized gradient approximation of the density functional theory and the ab initio pseudopotential method. It is found that double hexagonal-close-packed is the more stable phase for lanthanum. Differences in the density of states at the Fermi level between these two phases are pointed out and discussed in detail. Using the calculated lattice constant and electronic band structure for both phases, a linear response approach based on the density functional theory has been applied to study phonon modes, polarization characteristics of phonon modes, and electron-phonon interaction. Our phonon results show a softening behavior of the transverse acoustic branch along the Γ-L direction and the Γ-M direction for face-centered-cubic and double hexagonal-close-packed phases, respectively. Thus, the transverse-phonon linewidth shows a maximum at the zone boundary M(L) for the double hexagonal-close-packed phase (face-centered-cubic phase), where the transverse-phonon branch exhibits a dip. The electron-phonon coupling parameter λ is found to be 0.97 (1.06) for the double hexagonal-close-packed phase (face-centered-cubic phase), and the superconducting critical temperature is estimated to be 4.87 (dhcp) and 5.88 K (fcc), in good agreement with experimental values of around 5.0 (dhcp) and 6.0 K (fcc). A few superconducting parameters for the double hexagonal-close-packed phase have been calculated and compared with available theoretical and experimental results. Furthermore, the calculated superconducting parameters for both phases are compared between each other in detail.

Longitudinal optical phonon-plasmon coupled modes of degenerate Al-doped ZnO films

NASA Astrophysics Data System (ADS)

Ding, K.; Hu, Q. C.; Lin, W. W.; Huang, J. K.; Huang, F.

2012-07-01

We have investigated the interaction between carriers and polar phonons by using Raman scattering spectroscopy in highly conductive Al-doped ZnO films grown by metalorganic chemical vapor deposition. Different from the longitudinal optical phonon-plasmon coupled modes (LOPPCM) observed in nondegenerate ZnO, an A1(LO)-like mode appears at the low frequency side of the uncoupled A1(LO) mode, and it monotonically shifts to higher frequencies and approaches to the uncoupled A1(LO) mode as Al composition increases. Based on line shape calculations, the A1(LO)-like mode is assigned to the large wave-vector LOPPCM arising from nonconserving scattering dominated by the Al impurity-induced Fröhlich mechanism. Benefiting from the nonmonotonic Al composition dependence of the electron density, it is revealed that the LOPPCM depends mainly on the doping level but not the carrier concentration.

Quantum non-demolition phonon counter with a hybrid optomechnical system

NASA Astrophysics Data System (ADS)

Song, Qiao; Zhang, KeYe; Dong, Ying; Zhang, WeiPing

2018-05-01

A phonon counting scheme based on the control of polaritons in an optomechanical system is proposed. This approach permits us to measure the number of phonons in a quantum non-demolition (QND) manner for arbitrary modes not limited by the frequency matching condition as in usual photon-phonon scattering detections. The performance on phonon number transfer and quantum state transfer of the counter are analyzed and simulated numerically by taking into account all relevant sources of noise.

Ballistic phonon transmission in quasiperiodic acoustic nanocavities

NASA Astrophysics Data System (ADS)

Mo, Yuan; Huang, Wei-Qing; Huang, Gui-Fang; Chen, Yuan; Hu, Wangyu; Wang, Ling-Ling; Pan, Anlian

2011-04-01

Ballistic phonon transport is investigated in acoustic nanocavities modulated in a quasiperiodic manner at low temperatures. Two different types of quasiperiodic acoustic nanocavities are considered: the lengths of nanocavities (QPL) and the lengths of the bridges (QPD) connecting two successive nanocavities are modulated according to the Fibonacci rule. We demonstrate that the transmission spectra and thermal conductance in both systems are similar, which is more prominent in QPD than in QPL. The transmission and thermal conductance of QPD are larger than those of QPL due to the fact that constant nanocavity length in QPD would strengthen ballistic phonon resonant transport, while varying nanocavity length in QPL lead to strong phonon scattering.

Anharmonic, dimensionality and size effects in phonon transport

NASA Astrophysics Data System (ADS)

Thomas, Iorwerth O.; Srivastava, G. P.

2017-12-01

We have developed and employed a numerically efficient semi- ab initio theory, based on density-functional and relaxation-time schemes, to examine anharmonic, dimensionality and size effects in phonon transport in three- and two-dimensional solids of different crystal symmetries. Our method uses third- and fourth-order terms in crystal Hamiltonian expressed in terms of a temperature-dependent Grüneisen’s constant. All input to numerical calculations are generated from phonon calculations based on the density-functional perturbation theory. It is found that four-phonon processes make important and measurable contribution to lattice thermal resistivity above the Debye temperature. From our numerical results for bulk Si, bulk Ge, bulk MoS2 and monolayer MoS2 we find that the sample length dependence of phonon conductivity is significantly stronger in low-dimensional solids.

Phonon thermal transport through tilt grain boundaries in strontium titanate

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zheng, Zexi; Chen, Xiang; Yang, Shengfeng

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.more » 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.« less

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).

Research on soundproof properties of cylindrical shells of generalized phononic crystals

NASA Astrophysics Data System (ADS)

Liu, Ru; Shu, Haisheng; Wang, Xingguo

2017-04-01

Based on the previous studies, the concept of generalized phononic crystals (GPCs) is further introduced into the cylindrical shell structures in this paper. And a type of cylindrical shells of generalized phononic crystals (CS-GPCs) is constructed, the structural field and acoustic-structural coupled field of the composite cylindrical shells are examined respectively. For the structural field, the transfer matrix method of mechanical state vector is adopted to build the transfer matrix of radial waves propagating from inside to outside. For the acoustic-structural coupled field, the expressions of the acoustic transmission/reflection coefficients and the sound insulation of acoustic waves with the excitation of center line sound source are set up. And the acoustic transmission coefficient and the frequency response of sound insulation in this mode were numerical calculated. Furthermore, the theoretical analysis results are verified by using the method of combining the numerical calculation and finite element simulation. Finally, the effects of inner and outer fluid parameters on the transmission/reflection coefficients of CS-GPCs are analyzed in detail.

Temperature dependent magnon-phonon coupling in bcc Fe from theory and experiment.

PubMed

Körmann, F; Grabowski, B; Dutta, B; Hickel, T; Mauger, L; Fultz, B; Neugebauer, J

2014-10-17

An ab initio based framework for quantitatively assessing the phonon contribution due to magnon-phonon interactions and lattice expansion is developed. The theoretical results for bcc Fe are in very good agreement with high-quality phonon frequency measurements. For some phonon branches, the magnon-phonon interaction is an order of magnitude larger than the phonon shift due to lattice expansion, demonstrating the strong impact of magnetic short-range order even significantly above the Curie temperature. The framework closes the previous simulation gap between the ferro- and paramagnetic limits.

Phonon anharmonicity of monoclinic zirconia and yttrium-stabilized zirconia

DOE Office of Scientific and Technical Information (OSTI.GOV)

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

2015-04-13

Inelastic neutron scattering measurements on monoclinic zirconia (ZrO 2) 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 somewhatmore » 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.« less

Solid-state electron spin lifetime limited by phononic vacuum modes.

PubMed

Astner, T; Gugler, J; Angerer, A; Wald, S; Putz, S; Mauser, N J; Trupke, M; Sumiya, H; Onoda, S; Isoya, J; Schmiedmayer, J; Mohn, P; Majer, J

2018-04-01

Longitudinal relaxation is the process by which an excited spin ensemble decays into its thermal equilibrium with the environment. In solid-state spin systems, relaxation into the phonon bath usually dominates over the coupling to the electromagnetic vacuum 1-9 . In the quantum limit, the spin lifetime is determined by phononic vacuum fluctuations 10 . However, this limit was not observed in previous studies due to thermal phonon contributions 11-13 or phonon-bottleneck processes 10, 14,15 . Here we use a dispersive detection scheme 16,17 based on cavity quantum electrodynamics 18-21 to observe this quantum limit of spin relaxation of the negatively charged nitrogen vacancy (NV - ) centre 22 in diamond. Diamond possesses high thermal conductivity even at low temperatures 23 , which eliminates phonon-bottleneck processes. We observe exceptionally long longitudinal relaxation times T 1 of up to 8 h. To understand the fundamental mechanism of spin-phonon coupling in this system we develop a theoretical model and calculate the relaxation time ab initio. The calculations confirm that the low phononic density of states at the NV - transition frequency enables the spin polarization to survive over macroscopic timescales.

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.

Zero Quantum Coherence in a Series of Covalent Spin-Correlated Radical Pairs.

PubMed

Nelson, Jordan N; Krzyaniak, Matthew D; Horwitz, Noah E; Rugg, Brandon K; Phelan, Brian T; Wasielewski, Michael R

2017-03-23

Photoinitiated subnanosecond electron transfer within covalently linked electron donor-acceptor molecules can result in the formation of a spin-correlated radical pair (SCRP) with a well-defined initial singlet spin configuration. Subsequent coherent mixing between the SCRP singlet and triplet m s = 0 spin states, the so-called zero quantum coherence (ZQC), is of potential interest in quantum information processing applications because the ZQC can be probed using pulse electron paramagnetic resonance (pulse-EPR) techniques. Here, pulse-EPR spectroscopy is utilized to examine the ZQC oscillation frequencies and ZQC dephasing in three structurally well-defined D-A systems. While transitions between the singlet and triplet m s = 0 spin states are formally forbidden (Δm s = 0), they can be addressed using specific microwave pulse turning angles to map information from the ZQC onto observable single quantum coherences. In addition, by using structural variations to tune the singlet-triplet energy gap, the ZQC frequencies determined for this series of molecules indicate a stronger dependence on the electronic g-factor than on electron-nuclear hyperfine interactions.

Study of phonons in irradiated epitaxial thin films of UO2

NASA Astrophysics Data System (ADS)

Rennie, S.; Lawrence Bright, E.; Darnbrough, J. E.; Paolasini, L.; Bosak, A.; Smith, A. D.; Mason, N.; Lander, G. H.; Springell, R.

2018-06-01

We report experiments to determine the effect of radiation damage on the phonon spectra of the most common nuclear fuel, UO2. We irradiated thin (˜300 nm) epitaxial films of UO2 with 2.1 MeV He2 + ions to 0.15 displacements per atom and a lattice swelling of Δ a /a ˜0.6 % and then used grazing-incidence inelastic x-ray scattering to measure the phonon spectrum. We succeeded in observing the acoustic modes, both transverse and longitudinal, across the Brillouin zone. The phonon energies, in both the pristine and irradiated samples, are unchanged from those observed in bulk material. On the other hand, the phonon linewidths (inversely proportional to the phonon lifetimes) show a significant broadening when comparing the pristine and irradiated samples. This effect is shown to increase with phonon energy across the Brillouin zone. The decreases in the phonon lifetimes of the acoustic modes are roughly consistent with a 50% reduction in the thermal conductivity.

Phonon properties and slow organic-to-inorganic sub-lattice thermalization in hybrid perovskites

NASA Astrophysics Data System (ADS)

Chan, Maria; Chang, Angela; Xia, Yi; Sadasivam, Sridhar; Guo, Peijun; Kinaci, Alper; Lin, Hao-Wu; Darancet, Pierre; Schaller, Richard

Organic-inorganic hybrid perovskite halide compounds have been investigated extensively for photovoltaics (PVs) and related applications. The thermal transport properties of hybrid perovskites, including phonon-carrier and phonon-phonon interactions, are of significance for their PV and solar thermoelectric applications. The interlocking organic and inorganic sublattices can be thought of as an extreme form of nanostructuring. A result of this nanostructuring is the large gap in phonon frequencies between the organic and inorganic sublattices, which is expected to create bottlenecks in phonon equilibration. In this work, we use a combination of ultrafast spectroscopy including photoluminescence and transient absorption, as well as first principles density functional theory (DFT), ab initio molecular dynamics calculations, phonon lifetimes derived from DFT force constants, and non-equilibrium phonon dynamics accounting for phonon lifetimes, to determine the phonon and charge interaction processes. We find evidence that thermalization of carriers occur at an atypically slow 50-100 ps time scale owing to the complex interplay between electronic and phonon excitations.

Development of phonon-mediated cryogenic particle detectors with electron and nuclear recoil discrimination

NASA Astrophysics Data System (ADS)

Nam, Sae Woo

1999-10-01

Observations have shown that galaxies, including our own, are surrounded by halos of ``dark matter''. One possibility is that this may be an undiscovered form of matter, weakly interacting massive particles (WIMPs). This thesis describes the development of silicon based cryogenic particle detectors designed to directly detect interactions with these WIMPs. These detectors are part of a new class of detectors which are able to reject background events by simultaneously measuring energy deposited into phonons versus electron hole pairs. By using the phonon sensors with the ionization sensors to compare the partitioning of energy between phonons and ionizations we can discriminate between electron recoil events (background radiation) and nuclear recoil events (dark matter events). These detectors with built-in background rejection are a major advance in background rejection over previous searches. Much of this thesis will describe work in scaling the detectors from / g prototype devices to a fully functional prototype 100g dark matter detector. In particular, many sensors were fabricated and tested to understand the behavior of our phonon sensors, Quasipartice trapping assisted Electrothermal feedback Transition edge sensors (QETs). The QET sensors utilize aluminum quasiparticle traps attached to tungsten superconducting transition edge sensors patterned on a silicon substrate. The tungsten lines are voltage biased and self-regulate in the transition region. Phonons from particle interactions within the silicon propogate to the surface where they are absorbed by the aluminum generating quasiparticles in the aluminum. The quasiparticles diffuse into the tungsten and couple energy into the tungsten electron system. Consequently, the tungsten increases in resistance and causes a current pulse which is measured with a high bandwidth SQUID system. With this advanced sensor technology, we were able to demonstrate detectors with xy position sensitivity with electron and

Theory of electron-phonon-dislon interacting system—toward a quantized theory of dislocations

NASA Astrophysics Data System (ADS)

Li, Mingda; Tsurimaki, Yoichiro; Meng, Qingping; Andrejevic, Nina; Zhu, Yimei; Mahan, Gerald D.; Chen, Gang

2018-02-01

We provide a comprehensive theoretical framework to study how crystal dislocations influence the functional properties of materials, based on the idea of a quantized dislocation, namely a ‘dislon’. In contrast to previous work on dislons which focused on exotic phenomenology, here we focus on their theoretical structure and computational power. We first provide a pedagogical introduction that explains the necessity and benefits of taking the dislon approach and why the dislon Hamiltonian takes its current form. Then, we study the electron-dislocation and phonon-dislocation scattering problems using the dislon formalism. Both the effective electron and phonon theories are derived, from which the role of dislocations on electronic and phononic transport properties is computed. Compared with traditional dislocation scattering studies, which are intrinsically single-particle, low-order perturbation and classical quenched defect in nature, the dislon theory not only allows easy incorporation of quantum many-body effects such as electron correlation, electron-phonon interaction, and higher-order scattering events, but also allows proper consideration of the dislocation’s long-range strain field and dynamic aspects on equal footing for arbitrary types of straight-line dislocations. This means that instead of developing individual models for specific dislocation scattering problems, the dislon theory allows for the calculation of electronic structure and electrical transport, thermal transport, optical and superconducting properties, etc, under one unified theory. Furthermore, the dislon theory has another advantage over empirical models in that it requires no fitting parameters. The dislon theory could serve as a major computational tool to understand the role of dislocations on multiple materials’ functional properties at an unprecedented level of clarity, and may have wide applications in dislocated energy materials.

Universal exchange-driven phonon splitting

NASA Astrophysics Data System (ADS)

Deisenhofer, Joachim; Kant, Christian; Schmidt, Michael; Wang, Zhe; Mayr, Franz; Tsurkan, Vladimir; Loidl, Alois

2012-02-01

We report on a linear dependence of the phonon splitting on the non-dominant exchange coupling Jnd in the antiferromagnetic monoxides MnO, Fe0.92O, CoO and NiO, and in the highly frustrated antiferromagnetic spinels CdCr2O4, MgCr2O4 and ZnCr2O4. For the monoxides our results directly confirm the theoretical prediction of a predominantly exchange induced splitting of the zone-centre optical phonon [1,2]. We find the linear relation δφ= βJndS^2 with slope β = 3.7. This relation also holds for a very different class of systems, namely the highly frustrated chromium spinels. Our finding suggests a universal dependence of the exchange-induced phonon splitting at the antiferromagnetic transition on the non-dominant exchange coupling [3].[4pt] [1] S. Massidda et al., Phys. Rev. Lett. 82, 430 (1999).[0pt] [2] W. Luo et al., Solid State Commun. 142, 504 (2007).[0pt] [3] Ch. Kant et al., arxiv:1109.4809.

Investigation of phonon coherence and backscattering using silicon nanomeshes

DOE PAGES

Lee, Jaeho; Lee, Woochul; Wehmeyer, Geoff; ...

2017-01-04

Phonons can display both wave-like and particle-like behaviour during thermal transport. While thermal transport in silicon nanomeshes has been previously interpreted by phonon wave effects due to interference with periodic structures, as well as phonon particle effects including backscattering, the dominant mechanism responsible for thermal conductivity reductions below classical predictions still remains unclear. Here we isolate the wave-related coherence effects by comparing periodic and aperiodic nanomeshes, and quantify the backscattering effect by comparing variable-pitch nanomeshes. We measure identical (within 6% uncertainty) thermal conductivities for periodic and aperiodic nanomeshes of the same average pitch, and reduced thermal conductivities for nanomeshes withmore » smaller pitches. Ray tracing simulations support the measurement results. We conclude phonon coherence is unimportant for thermal transport in silicon nanomeshes with periodicities of 100 nm and higher and temperatures above 14 K, and phonon backscattering, as manifested in the classical size effect, is responsible for the thermal conductivity reduction.« less

Investigating phonon-mediated interactions with polar molecules

NASA Astrophysics Data System (ADS)

Sous, John; Madison, Kirk; Berciu, Mona; Krems, Roman

2017-04-01

We show that an ensemble of polar molecules in an optical lattice realizes the Peierls polaron model for hard-core particles/ pseudospins. We analyze the quasiparticle spectrum in the one-particle subspace, the two-particle subspace and at finite concentrations. We derive an effective model that describes the low-energy behavior of the system. We show that the Hamiltonian includes phonon-mediated repulsions and phonon-mediated ``pair-hopping'' terms which move the particle pair as a whole. We show that microwave excitations of the system exhibit signatures of these interactions. These results pave the way for the experimental observation of phonon-mediated repulsion. This work was supported by NSERC of Canada and the Stewart Blusson Quantum Matter Institute.

Heat transport by phonons in crystalline materials and nanostructures

NASA Astrophysics Data System (ADS)

Koh, Yee Kan

This dissertation presents experimental studies of heat transport by phonons in crystalline materials and nanostructures, and across solid-solid interfaces. Particularly, this dissertation emphasizes advancing understanding of the mean-free-paths (i.e., the distance phonons propagate without being scattered) of acoustic phonons, which are the dominant heat carriers in most crystalline semiconductor nanostructures. Two primary tools for the studies presented in this dissertation are time-domain thermoreflectance (TDTR) for measurements of thermal conductivity of nanostructures and thermal conductance of interfaces; and frequency-domain thermoreflectance (FDTR), which I developed as a direct probe of the mean-free-paths of dominant heat-carrying phonons in crystalline solids. The foundation of FDTR is the dependence of the apparent thermal conductivity on the frequency of periodic heat sources. I find that the thermal conductivity of semiconductor alloys (InGaP, InGaAs, and SiGe) measured by TDTR depends on the modulation frequency, 0.1 ≤ f ≤ 10 MHz, used in TDTR measurements. Reduction in the thermal conductivity of the semiconductor alloys at high f compares well to the reduction in the thermal conductivity of epitaxial thin films, indicating that frequency dependence and thickness dependence of thermal conductivity are fundamentally equivalent. I developed the frequency dependence of thermal conductivity into a convenient probe of phonon mean-free-paths, a technique which I call frequency-domain thermoreflectance (FDTR). In FDTR, I monitor the changes in the intensity of the reflected probe beam as a function of the modulation frequency. To facilitate the analysis of FDTR measurements, I developed a nonlocal theory for heat conduction by phonons at high heating frequencies. Calculations of the nonlocal theory confirm my experimental findings that phonons with mean-free-paths longer than two times the penetration depth do not contribute to the apparent thermal

New quantum properties of phonons and their detection

NASA Technical Reports Server (NTRS)

Artoni, Maurizo; Birman, Joseph L.

1994-01-01

We present a theoretical investigation on new and interesting properties of the phonon polarization field in solids. In particular, non-classical aspects of the phonon population and an experimental scheme that would enable one to detect them will be discussed.

Hot electron energy relaxation in lattice-matched InAlN/AlN/GaN heterostructures: The sum rules for electron-phonon interactions and hot-phonon effect

NASA Astrophysics Data System (ADS)

Zhang, J.-Z.; Dyson, A.; Ridley, B. K.

2015-01-01

Using the dielectric continuum (DC) and three-dimensional phonon (3DP) models, energy relaxation (ER) of the hot electrons in the quasi-two-dimensional channel of lattice-matched InAlN/AlN/GaN heterostructures is studied theoretically, taking into account non-equilibrium polar optical phonons, electron degeneracy, and screening from the mobile electrons. The electron power dissipation (PD) and ER time due to both half-space and interface phonons are calculated as functions of the electron temperature Te using a variety of phonon lifetime values from experiment, and then compared with those evaluated by the 3DP model. Thereby, particular attention is paid to examination of the 3DP model to use for the hot-electron relaxation study. The 3DP model yields very close results to the DC model: With no hot phonons or screening, the power loss calculated from the 3DP model is 5% smaller than the DC power dissipation, whereas slightly larger 3DP power loss (by less than 4% with a phonon lifetime from 0.1 to 1 ps) is obtained throughout the electron temperature range from room temperature to 2500 K after including both the hot-phonon effect (HPE) and screening. Very close results are obtained also for ER time with the two phonon models (within a 5% of deviation). However, the 3DP model is found to underestimate the HPE by 9%. The Mori-Ando sum rule is restored by which it is proved that the PD values obtained from the DC and 3DP models are in general different in the spontaneous phonon emission process, except when scattering with interface phonons is sufficiently weak, or when the degenerate modes condition is imposed, which is also consistent with Register's scattering rate sum rule. The discrepancy between the DC and 3DP results is found to be caused by how much the high-energy interface phonons contribute to the ER: their contribution is enhanced in the spontaneous emission process but is dramatically reduced after including the HPE. Our calculation with both phonon

Spacecraft intercept guidance using zero effort miss steering

NASA Astrophysics Data System (ADS)

Newman, Brett

The suitability of proportional navigation, or an equivalent zero effort miss formulation, for spacecraft intercepts during midcourse guidance, followed by a ballistic coast to the endgame, is addressed. The problem is formulated in terms of relative motion in a general 3D framework. The proposed guidance law for the commanded thrust vector orientation consists of the sum of two terms: (1) along the line of sight unit direction and (2) along the zero effort miss component perpendicular to the line of sight and proportional to the miss itself and a guidance gain. If the guidance law is to be suitable for longer range targeting applications with significant ballistic coasting after burnout, determination of the zero effort miss must account for the different gravitational accelerations experienced by each vehicle. The proposed miss determination techniques employ approximations for the true differential gravity effect. Theoretical results are applied to a numerical engagement scenario and the resulting performance is evaluated in terms of the miss distances determined from nonlinear simulation.

Single-photon blockade in a hybrid cavity-optomechanical system via third-order nonlinearity

NASA Astrophysics Data System (ADS)

Sarma, Bijita; Sarma, Amarendra K.

2018-04-01

Photon statistics in a weakly driven optomechanical cavity, with Kerr-type nonlinearity, are analyzed both analytically and numerically. The single-photon blockade effect is demonstrated via calculations of the zero-time-delay second-order correlation function g (2)(0). The analytical results obtained by solving the Schrödinger equation are in complete conformity with the results obtained through numerical solution of the quantum master equation. A systematic study on the parameter regime for observing photon blockade in the weak coupling regime is reported. The parameter regime where the photon blockade is not realizable due to the combined effect of nonlinearities owing to the optomechanical coupling and the Kerr-effect is demonstrated. The experimental feasibility with state-of-the-art device parameters is discussed and it is observed that photon blockade could be generated at the telecommunication wavelength. An elaborate analysis of the thermal effects on photon antibunching is presented. The system is found to be robust against pure dephasing-induced decoherences and thermal phonon number fluctuations.

General solution to inhomogeneous dephasing and smooth pulse dynamical decoupling

NASA Astrophysics Data System (ADS)

Zeng, Junkai; Deng, Xiu-Hao; Russo, Antonio; Barnes, Edwin

2018-03-01

In order to achieve the high-fidelity quantum control needed for a broad range of quantum information technologies, reducing the effects of noise and system inhomogeneities is an essential task. It is well known that a system can be decoupled from noise or made insensitive to inhomogeneous dephasing dynamically by using carefully designed pulse sequences based on square or delta-function waveforms such as Hahn spin echo or CPMG. However, such ideal pulses are often challenging to implement experimentally with high fidelity. Here, we uncover a new geometrical framework for visualizing all possible driving fields, which enables one to generate an unlimited number of smooth, experimentally feasible pulses that perform dynamical decoupling or dynamically corrected gates to arbitrarily high order. We demonstrate that this scheme can significantly enhance the fidelity of single-qubit operations in the presence of noise and when realistic limitations on pulse rise times and amplitudes are taken into account.

Phonon-assisted oscillatory exciton dynamics in monolayer MoSe 2

DOE PAGES

Chow, Colin M.; Yu, Hongyi; Jones, Aaron M.; ...

2017-10-13

In monolayer semiconductor transition metal dichalcogenides, the exciton–phonon interaction strongly affects the photocarrier dynamics. Here, we report on an unusual oscillatory enhancement of the neutral exciton photoluminescence with the excitation laser frequency in monolayer MoSe 2. The frequency of oscillation matches that of the M-point longitudinal acoustic phonon, LA(M), suggesting the significance of zone-edge acoustic phonons and hence the deformation potential in exciton-phonon coupling in MoSe 2. Moreover, oscillatory behavior is observed in the steady-state emission linewidth and in time-resolved PLE data, which reveals variation with excitation energy in the exciton lifetime. These results clearly expose the key role playedmore » by phonons in the exciton formation and relaxation dynamics of two-dimensional van der Waals semiconductors.« less

Time Resolved X-Ray Diffraction Study of Acoustoelectrically Amplified Phonons.

NASA Astrophysics Data System (ADS)

Chapman, Leroy Dean

X-rays diffracted by nearly perfect crystals of n-type InSb have been investigated in the presence of intense acoustoelectrically (A.E.) amplified phonons. The fact that these phonons are nearly monochromatic and have a well defined propagation and polarization direction presents an excellent opportunity to investigate the nature of x -ray photon-phonon scattering in a diffracting crystal. The Debye-Waller factor which accounts for the attenuation of diffracted x-ray intensities due to thermal phonons is reflection dependent owing to its sin (theta)/(lamda) dependence. We have performed experiments comparing the (004) and (008) anomalously transmitted intensities as a function of A.E. amplified flux. The attenuation of both reflections due to the amplified phonons was the same in direct contradiction to an expected sin (theta)/(lamda) dependence. Some possible reasons for this failure are discussed. In a Bragg reflection scattering geometry, the intense monochromatic amplified phonons give rise to satellite peaks symmetrically located about the central elastic Brag peak in a rocking profile. We report in this thesis on the first observation of satellites in a thin crystal Laue transmission geometry. We have theoretically simulated the rocking profiles with some success. The A.E. amplification process in InSb is strongly favored for {110} propagation fast transverse (FT) phonons. In earlier experiments it was found that non-{110} FT phonons were also produced during the amplification process. We have developed a time resolved x-ray counting system which, in conjunction with a spatially resolved x-ray beam and a localized, traveling A.E. phonon distribution, allow the time evolution of the amplified distribution to be followed. We report on time resolved measurements for both the symmetric Bragg and Laue geometries from which we can determine when and where non-{110 } FT flux is generated and restrict the possible mechanisms for its generation.

Metric for strong intrinsic fourth-order phonon anharmonicity

NASA Astrophysics Data System (ADS)

Yue, Sheng-Ying; Zhang, Xiaoliang; Qin, Guangzhao; Phillpot, Simon R.; Hu, Ming

2017-05-01

Under the framework of Taylor series expansion for potential energy, we propose a simple and robust metric, dubbed "regular residual analysis," to measure the fourth-order phonon anharmonicity in crystals. The method is verified by studying the intrinsic strong higher-order anharmonic effects in UO2 and CeO2. Comparison of the thermal conductivity results, which calculated by the anharmonic lattice dynamics method coupled with the Boltzmann transport equation and the spectral energy density method coupled with ab initio molecular dynamics simulation further validates our analysis. Analysis of the bulk Si and Ge systems confirms that the fourth-order phonon anharmonicity is enhanced and cannot be neglected at high enough temperatures, which agrees with a previous study where the four-phonon scattering was explicitly determined. This metric will facilitate evaluating and interpreting the lattice thermal conductivity of crystals with strong fourth-order phonon anharmonicity.

Phonon Dispersion and the Competition between Pairing and Charge Order

NASA Astrophysics Data System (ADS)

Costa, N. C.; Blommel, T.; Chiu, W.-T.; Batrouni, G.; Scalettar, R. T.

2018-05-01

The Holstein model describes the interaction between fermions and a collection of local (dispersionless) phonon modes. In the dilute limit, the phonon degrees of freedom dress the fermions, giving rise to polaron and bipolaron formation. At higher densities, the phonons mediate collective superconducting (SC) and charge-density wave (CDW) phases. Quantum Monte Carlo (QMC) simulations have considered both these limits but have not yet focused on the physics of more general phonon spectra. Here we report QMC studies of the role of phonon dispersion on SC and CDW order in such models. We quantify the effect of finite phonon bandwidth and curvature on the critical temperature Tcdw for CDW order and also uncover several novel features of diagonal long-range order in the phase diagram, including a competition between charge patterns at momenta q =(π ,π ) and q =(0 ,π ) which lends insight into the relationship between Fermi surface nesting and the wave vector at which charge order occurs. We also demonstrate SC order at half filling in situations where a nonzero bandwidth sufficiently suppresses Tcdw.

Hybridization and electron-phonon coupling in ferroelectric BaTiO3 probed by resonant inelastic x-ray scattering

NASA Astrophysics Data System (ADS)

Fatale, S.; Moser, S.; Miyawaki, J.; Harada, Y.; Grioni, M.

2016-11-01

We investigated the ferroelectric perovskite material BaTiO3 by resonant inelastic x-ray scattering (RIXS) at the Ti L3 edge. We observe with decreasing temperature a transfer of spectral weight from the elastic to the charge-transfer spectral features, indicative of increasing Ti 3 d -O 2 p hybridization. When the incident photon energy selects transitions to the Ti 3 d eg manifold, the quasielastic RIXS response exhibits a tail indicative of phonon excitations. A fit of the spectral line shape by a theoretical model allows us to estimate the electron-phonon coupling strength M ˜0.25 eV, which places BaTiO3 in the intermediate coupling regime.

Pseudospins and Topological Effects of Phonons in a Kekulé Lattice

NASA Astrophysics Data System (ADS)

Liu, Yizhou; Lian, Chao-Sheng; Li, Yang; Xu, Yong; Duan, Wenhui

2017-12-01

The search for exotic topological effects of phonons has attracted enormous interest for both fundamental science and practical applications. By studying phonons in a Kekulé lattice, we find a new type of pseudospin characterized by quantized Berry phases and pseudoangular momenta, which introduces various novel topological effects, including topologically protected pseudospin-polarized interface states and a phonon pseudospin Hall effect. We further demonstrate a pseudospin-contrasting optical selection rule and a pseudospin Zeeman effect, giving a complete generation-manipulation-detection paradigm of the phonon pseudospin. The pseudospin and topology-related physics revealed for phonons is general and applicable for electrons, photons, and other particles.

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.

Charge tuning of nonresonant magnetoexciton phonon interactions in graphene.

PubMed

Rémi, Sebastian; Goldberg, Bennett B; Swan, Anna K

2014-02-07

Far from resonance, the coupling of the G-band phonon to magnetoexcitons in single layer graphene displays kinks and splittings versus filling factor that are well described by Pauli blocking and unblocking of inter- and intra-Landau level transitions. We explore the nonresonant electron-phonon coupling by high-magnetic field Raman scattering while electrostatic tuning of the carrier density controls the filling factor. We show qualitative and quantitative agreement between spectra and a linearized model of electron-phonon interactions in magnetic fields. The splitting is caused by dichroism of left- and right-handed circular polarized light due to lifting of the G-band phonon degeneracy, and the piecewise linear slopes are caused by the linear occupancy of sequential Landau levels versus ν.

Second-harmonic phonon spectroscopy of α -quartz

NASA Astrophysics Data System (ADS)

Winta, Christopher J.; Gewinner, Sandy; Schöllkopf, Wieland; Wolf, Martin; Paarmann, Alexander

2018-03-01

We demonstrate midinfrared second-harmonic generation as a highly sensitive phonon spectroscopy technique that we exemplify using α -quartz (SiO2) as a model system. A midinfrared free-electron laser provides direct access to optical phonon resonances ranging from 350 to 1400 cm-1 . While the extremely wide tunability and high peak fields of a free-electron laser promote nonlinear spectroscopic studies—complemented by simultaneous linear reflectivity measurements—azimuthal scans reveal crystallographic symmetry information of the sample. Additionally, temperature-dependent measurements show how damping rates increase, phonon modes shift spectrally and in certain cases disappear completely when approaching Tc=846 K where quartz undergoes a structural phase transition from trigonal α -quartz to hexagonal β -quartz, demonstrating the technique's potential for studies of phase transitions.

Soft phononic crystals with deformation-independent band gaps

PubMed Central

2017-01-01

Soft phononic crystals have the advantages over their stiff counterparts of being flexible and reconfigurable. Normally, the band gaps of soft phononic crystals will be modified after deformation due to both geometric and constitutive nonlinearity. Indeed these are important properties that can be exploited to tune the dynamic properties of the material. However, in some instances, it may be that one wishes to deform the medium while retaining the band gap structure. A special class of soft phononic crystals is described here with band gaps that are independent or almost-independent of the imposed mechanical deformation, which enables the design of phononic crystals with robust performance. This remarkable behaviour originates from transformation elasticity theory, which leaves the wave equation and the eigenfrequencies invariant after deformation. The necessary condition to achieve such a property is that the Lagrangian elasticity tensor of the hyperelastic material should be constant, i.e. independent of deformation. It is demonstrated that incompressible neo-Hookean materials exhibit such a unique property. Semilinear materials also possess this property under special loading conditions. Phononic crystals composed of these two materials are studied theoretically and the predictions of invariance, or the manner in which the response deviates from invariance, are confirmed via numerical simulation. PMID:28484331

Reduction of thermal conductivity in phononic nanomesh structures.

PubMed

Yu, Jen-Kan; Mitrovic, Slobodan; Tham, Douglas; Varghese, Joseph; Heath, James R

2010-10-01

Controlling the thermal conductivity of a material independently of its electrical conductivity continues to be a goal for researchers working on thermoelectric materials for use in energy applications and in the cooling of integrated circuits. In principle, the thermal conductivity κ and the electrical conductivity σ may be independently optimized in semiconducting nanostructures because different length scales are associated with phonons (which carry heat) and electric charges (which carry current). Phonons are scattered at surfaces and interfaces, so κ generally decreases as the surface-to-volume ratio increases. In contrast, σ is less sensitive to a decrease in nanostructure size, although at sufficiently small sizes it will degrade through the scattering of charge carriers at interfaces. Here, we demonstrate an approach to independently controlling κ based on altering the phonon band structure of a semiconductor thin film through the formation of a phononic nanomesh film. These films are patterned with periodic spacings that are comparable to, or shorter than, the phonon mean free path. The nanomesh structure exhibits a substantially lower thermal conductivity than an equivalently prepared array of silicon nanowires, even though this array has a significantly higher surface-to-volume ratio. Bulk-like electrical conductivity is preserved. We suggest that this development is a step towards a coherent mechanism for lowering thermal conductivity.

Phonon Lifetime Observation in Epitaxial ScN Film with Inelastic X-Ray Scattering Spectroscopy.

PubMed

Uchiyama, H; Oshima, Y; Patterson, R; Iwamoto, S; Shiomi, J; Shimamura, K

2018-06-08

Phonon-phonon scattering dominates the thermal properties in nonmetallic materials, and it directly influences device performance in applications. The understanding of the scattering has been progressing using computational approaches, and the direct and systematic observation of phonon modes that include momentum dependences is desirable. We report experimental data on the phonon dispersion curves and lifetimes in an epitaxially grown ScN film using inelastic x-ray scattering measurements. The momentum dependence of the optical phonon lifetimes is estimated from the spectral width, and the highest-energy phonon mode around the zone center is found to possess a short lifetime of 0.21 ps. A comparison with first-principles calculations shows that our observed phonon lifetimes are quantitatively explained by three-body phonon-phonon interactions.

Phonon Lifetime Observation in Epitaxial ScN Film with Inelastic X-Ray Scattering Spectroscopy

NASA Astrophysics Data System (ADS)

Uchiyama, H.; Oshima, Y.; Patterson, R.; Iwamoto, S.; Shiomi, J.; Shimamura, K.

2018-06-01

Phonon-phonon scattering dominates the thermal properties in nonmetallic materials, and it directly influences device performance in applications. The understanding of the scattering has been progressing using computational approaches, and the direct and systematic observation of phonon modes that include momentum dependences is desirable. We report experimental data on the phonon dispersion curves and lifetimes in an epitaxially grown ScN film using inelastic x-ray scattering measurements. The momentum dependence of the optical phonon lifetimes is estimated from the spectral width, and the highest-energy phonon mode around the zone center is found to possess a short lifetime of 0.21 ps. A comparison with first-principles calculations shows that our observed phonon lifetimes are quantitatively explained by three-body phonon-phonon interactions.

Temperature dependence of Brillouin light scattering spectra of acoustic phonons in silicon

NASA Astrophysics Data System (ADS)

Olsson, Kevin S.; Klimovich, Nikita; An, Kyongmo; Sullivan, Sean; Weathers, Annie; Shi, Li; Li, Xiaoqin

2015-02-01

Electrons, optical phonons, and acoustic phonons are often driven out of local equilibrium in electronic devices or during laser-material interaction processes. The need for a better understanding of such 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 the measured BLS spectra of silicon at different temperatures. 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.

Temperature dependence of Brillouin light scattering spectra of acoustic phonons in silicon

DOE Office of Scientific and Technical Information (OSTI.GOV)

Olsson, Kevin S.; Klimovich, Nikita; An, Kyongmo

2015-02-02

Electrons, optical phonons, and acoustic phonons are often driven out of local equilibrium in electronic devices or during laser-material interaction processes. The need for a better understanding of such 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 the measured BLS spectra of silicon at different temperatures. The origins of the observed temperature dependence of the BLS peak position, linewidth, and intensity are examined in ordermore » to evaluate their potential use as temperature sensors for acoustic phonons.« less

Structural zeroes and zero-inflated models.

PubMed

He, Hua; Tang, Wan; Wang, Wenjuan; Crits-Christoph, Paul

2014-08-01

In psychosocial and behavioral studies count outcomes recording the frequencies of the occurrence of some health or behavior outcomes (such as the number of unprotected sexual behaviors during a period of time) often contain a preponderance of zeroes because of the presence of 'structural zeroes' that occur when some subjects are not at risk for the behavior of interest. Unlike random zeroes (responses that can be greater than zero, but are zero due to sampling variability), structural zeroes are usually very different, both statistically and clinically. False interpretations of results and study findings may result if differences in the two types of zeroes are ignored. However, in practice, the status of the structural zeroes is often not observed and this latent nature complicates the data analysis. In this article, we focus on one model, the zero-inflated Poisson (ZIP) regression model that is commonly used to address zero-inflated data. We first give a brief overview of the issues of structural zeroes and the ZIP model. We then given an illustration of ZIP with data from a study on HIV-risk sexual behaviors among adolescent girls. Sample codes in SAS and Stata are also included to help perform and explain ZIP analyses.

Electrical modulation and switching of transverse acoustic phonons

NASA Astrophysics Data System (ADS)

Jeong, H.; Jho, Y. D.; Rhim, S. H.; Yee, K. J.; Yoon, S. Y.; Shim, J. P.; Lee, D. S.; Ju, J. W.; Baek, J. H.; Stanton, C. J.

2016-07-01

We report on the electrical manipulation of coherent acoustic phonon waves in GaN-based nanoscale piezoelectric heterostructures which are strained both from the pseudomorphic growth at the interfaces as well as through external electric fields. In such structures, transverse symmetry within the c plane hinders both the generation and detection of the transverse acoustic (TA) modes, and usually only longitudinal acoustic phonons are generated by ultrafast displacive screening of potential gradients. We show that even for c -GaN, the combined application of lateral and vertical electric fields can not only switch on the normally forbidden TA mode, but they can also modulate the amplitudes and frequencies of both modes. By comparing the transient differential reflectivity spectra in structures with and without an asymmetric potential distribution, the role of the electrical controllability of phonons was demonstrated as changes to the propagation velocities, the optical birefringence, the electrically polarized TA waves, and the geometrically varying optical sensitivities of phonons.

Quantum decoherence of phonons in Bose-Einstein condensates

NASA Astrophysics Data System (ADS)

Howl, Richard; Sabín, Carlos; Hackermüller, Lucia; Fuentes, Ivette

2018-01-01

We apply modern techniques from quantum optics and quantum information science to Bose-Einstein condensates (BECs) in order to study, for the first time, the quantum decoherence of phonons of isolated BECs. In the last few years, major advances in the manipulation and control of phonons have highlighted their potential as carriers of quantum information in quantum technologies, particularly in quantum processing and quantum communication. Although most of these studies have focused on trapped ion and crystalline systems, another promising system that has remained relatively unexplored is BECs. The potential benefits in using this system have been emphasized recently with proposals of relativistic quantum devices that exploit quantum states of phonons in BECs to achieve, in principle, superior performance over standard non-relativistic devices. Quantum decoherence is often the limiting factor in the practical realization of quantum technologies, but here we show that quantum decoherence of phonons is not expected to heavily constrain the performance of these proposed relativistic quantum devices.

Spin Qubits in Germanium Structures with Phononic Gap

NASA Technical Reports Server (NTRS)

Smelyanskiy, V. N.; Vasko, F. T.; Hafiychuk, V. V.; Dykman, M. I.; Petukhov, A. G.

2014-01-01

We propose qubits based on shallow donor electron spins in germanium structures with phononic gap. We consider a phononic crystal formed by periodic holes in Ge plate or a rigid cover / Ge layer / rigid substrate structure with gaps approximately a few GHz. The spin relaxation is suppressed dramatically, if the Zeeman frequency omegaZ is in the phononic gap, but an effective coupling between the spins of remote donors via exchange of virtual phonons remains essential. If omegaZ approaches to a gap edge in these structures, a long-range (limited by detuning of omegaZ) resonant exchange interaction takes place. We estimate that ratio of the exchange integral to the longitudinal relaxation rate exceeds 10(exp 5) and lateral scale of resonant exchange 0.1 mm. The exchange contribution can be verified under microwave pumping through oscillations of spin echo signal or through the differential absorption measurements. Efficient manipulation of spins due to the Rabi oscillations opens a new way for quantum information applications.

Femtosecond electron imaging of defect-modulated phonon dynamics

PubMed Central

Cremons, Daniel R.; Plemmons, Dayne A.; Flannigan, David J.

2016-01-01

Precise manipulation and control of coherent lattice oscillations via nanostructuring and phonon-wave interference has the potential to significantly impact a broad array of technologies and research areas. Resolving the dynamics of individual phonons in defect-laden materials presents an enormous challenge, however, owing to the interdependent nanoscale and ultrafast spatiotemporal scales. Here we report direct, real-space imaging of the emergence and evolution of acoustic phonons at individual defects in crystalline WSe2 and Ge. Via bright-field imaging with an ultrafast electron microscope, we are able to image the sub-picosecond nucleation and the launch of wavefronts at step edges and resolve dispersion behaviours during propagation and scattering. We discover that the appearance of speed-of-sound (for example, 6 nm ps−1) wavefronts are influenced by spatially varying nanoscale strain fields, taking on the appearance of static bend contours during propagation. These observations provide unprecedented insight into the roles played by individual atomic and nanoscale features on acoustic-phonon dynamics. PMID:27079790

Theory of parametrically amplified electron-phonon superconductivity

DOE Office of Scientific and Technical Information (OSTI.GOV)

Babadi, Mehrtash; Knap, Michael; Martin, Ivar

2017-07-01

Ultrafast optical manipulation of ordered phases in strongly correlated materials is a topic of significant theoretical, experimental, and technological interest. Inspired by a recent experiment on light-induced superconductivity in fullerenes [M. Mitrano et al., Nature (London) 530, 461 (2016)], we develop a comprehensive theory of light-induced superconductivity in driven electron-phonon systemswith lattice nonlinearities. In analogy with the operation of parametric amplifiers, we show how the interplay between the external drive and lattice nonlinearities lead to significantly enhanced effective electron-phonon couplings. We provide a detailed and unbiased study of the nonequilibrium dynamics of the driven system using the real-time Green's functionmore » technique. To this end, we develop a Floquet generalization of the Migdal-Eliashberg theory and derive a numerically tractable set of quantum Floquet-Boltzmann kinetic equations for the coupled electron-phonon system. We study the role of parametric phonon generation and electronic heating in destroying the transient superconducting state. Finally, we predict the transient formation of electronic Floquet bands in time-and angle-resolved photoemission spectroscopy experiments as a consequence of the proposed mechanism.« less

Phonon transport in single-layer boron nanoribbons

NASA Astrophysics Data System (ADS)

Zhang, Zhongwei; Xie, Yuee; Peng, Qing; Chen, Yuanping

2016-11-01

Inspired by the successful synthesis of three two-dimensional (2D) allotropes, the boron sheet has recently been one of the hottest 2D materials around. However, to date, phonon transport properties of these new materials are still unknown. By using the non-equilibrium Green’s function (NEGF) combined with the first principles method, we study ballistic phonon transport in three types of boron sheets; two of them correspond to the structures reported in the experiments, while the third one is a stable structure that has not been synthesized yet. At room temperature, the highest thermal conductance of the boron nanoribbons is comparable with that of graphene, while the lowest thermal conductance is less than half of graphene’s. Compared with graphene, the three boron sheets exhibit diverse anisotropic transport characteristics. With an analysis of phonon dispersion, bonding charge density, and simplified models of atomic chains, the mechanisms of the diverse phonon properties are discussed. Moreover, we find that many hybrid patterns based on the boron allotropes can be constructed naturally without doping, adsorption, and defects. This provides abundant nanostructures for thermal management and thermoelectric applications.

Temperature dependence of the dynamics of zone boundary phonons in ZnO:Li

NASA Astrophysics Data System (ADS)

Yadav, Harish Kumar; Sreenivas, K.; Gupta, Vinay; Katiyar, R. S.

2008-12-01

Investigations of zone boundary phonons in ZnO:Li system (Li concentration: 10%) and their dynamics with temperature are reported. Additional modes at 127, 157, and 194 cm-1 are observed and assigned to zone boundary phonons at critical point M in the Brillouin zone [J. M. Calleja and M. Cardona, Phys. Rev. B 16, 3753 (1977)] due to breakdown of crystal translational symmetry with Li incorporation in ZnO. Anharmonicity in peak frequency and linewidth of the zone boundary phonons in a temperature range from 100 to 1000 K is also analyzed taking into account the decay of zone boundary phonons into three- and four-phonon modes (cubic and quadratic anharmonicities). The anharmonic behavior of peak frequency is found to be feebly dependent on three-phonon decay process but thermal expansion of lattice together with four-phonon decay process appropriately defines the temperature dependence. Linewidths, however, follow the simple four-phonon decay mechanism. E2(low) mode, on the other hand, shows a linear temperature dependency and therefore follows a three-phonon decay channel. The calculated values of phonon lifetimes at 100 K for the 127, 157, 194 cm-1, and E2(low) modes are 8.23, 6.54, 5.32, and 11.39 ps. Decay of the zone boundary phonon modes compared to E2(low) mode reveals that dopant induced disorder has a strong temperature dependency.

Theory and experimental evidence of phonon domains and their roles in pre-martensitic phenomena

NASA Astrophysics Data System (ADS)

Jin, Yongmei M.; Wang, Yu U.; Ren, Yang

2015-12-01

Pre-martensitic phenomena, also called martensite precursor effects, have been known for decades while yet remain outstanding issues. This paper addresses pre-martensitic phenomena from new theoretical and experimental perspectives. A statistical mechanics-based Grüneisen-type phonon theory is developed. On the basis of deformation-dependent incompletely softened low-energy phonons, the theory predicts a lattice instability and pre-martensitic transition into elastic-phonon domains via 'phonon spinodal decomposition.' The phase transition lifts phonon degeneracy in cubic crystal and has a nature of phonon pseudo-Jahn-Teller lattice instability. The theory and notion of phonon domains consistently explain the ubiquitous pre-martensitic anomalies as natural consequences of incomplete phonon softening. The phonon domains are characterised by broken dynamic symmetry of lattice vibrations and deform through internal phonon relaxation in response to stress (a particular case of Le Chatelier's principle), leading to previously unexplored new domain phenomenon. Experimental evidence of phonon domains is obtained by in situ three-dimensional phonon diffuse scattering and Bragg reflection using high-energy synchrotron X-ray single-crystal diffraction, which observes exotic domain phenomenon fundamentally different from usual ferroelastic domain switching phenomenon. In light of the theory and experimental evidence of phonon domains and their roles in pre-martensitic phenomena, currently existing alternative opinions on martensitic precursor phenomena are revisited.

Coherent Phonon Transport Measurement and Controlled Acoustic Excitations Using Tunable Acoustic Phonon Source in GHz-sub THz Range with Variable Bandwidth.

PubMed

Shen, Xiaohan; Lu, Zonghuan; Timalsina, Yukta P; Lu, Toh-Ming; Washington, Morris; Yamaguchi, Masashi

2018-05-04

We experimentally demonstrated a narrowband acoustic phonon source with simultaneous tunabilities of the centre frequency and the spectral bandwidth in the GHz-sub THz frequency range based on photoacoustic excitation using intensity-modulated optical pulses. The centre frequency and bandwidth are tunable from 65 to 381 GHz and 17 to 73 GHz, respectively. The dispersion of the sound velocity and the attenuation of acoustic phonons in silicon dioxide (SiO 2 ) and indium tin oxide (ITO) thin films were investigated using the acoustic phonon source. The sound velocities of SiO 2 and ITO films were frequency-independent in the measured frequency range. On the other hand, the phonon attenuations of both of SiO 2 and ITO films showed quadratic frequency dependences, and polycrystalline ITO showed several times larger attenuation than those in amorphous SiO 2 . In addition, the selective excitation of mechanical resonance modes was demonstrated in nanoscale tungsten (W) film using acoustic pulses with various centre frequencies and spectral widths.

Broadband All-angle Negative Refraction by Optimized Phononic Crystals.

PubMed

Li, Yang Fan; Meng, Fei; Zhou, Shiwei; Lu, Ming-Hui; Huang, Xiaodong

2017-08-07

All-angle negative refraction (AANR) of phononic crystals and its frequency range are dependent on mechanical properties of constituent materials and their spatial distribution. So far, it is impossible to achieve the maximum operation frequency range of AANR theoretically. In this paper, we will present a numerical approach for designing a two-dimensional phononic crystal with broadband AANR without negative index. Through analyzing the mechanism of AANR, a topology optimization problem aiming at broadband AANR is established and solved by bi-directional evolutionary structural optimization method. The optimal steel/air phononic crystal exhibits a record AANR range over 20% and its refractive properties and focusing effects are further investigated. The results demonstrate the multifunctionality of a flat phononic slab including superlensing effect near upper AANR frequencies and self-collimation at lower AANR frequencies.

Picosecond acoustic phonon dynamics in LaF3:Pr3+

NASA Astrophysics Data System (ADS)

Kirkpatrick, Sean M.; Yang, Ho-Soon; Dennis, W. M.

1998-09-01

A plasma switching technique is used to generate subnanosecond, far-infrared (FIR) pulses with frequency 113 cm-1. The generation of subnanosecond FIR pulses enables us to improve the time resolution of phonon spectroscopic measurements from 50 ns to 350 ps. As an application of this technique, we investigate the subnanosecond dynamics of high-frequency phonons in 0.5% LaF3:Pr3+. In particular, we report on the generation and detection of a subnanosecond nonequilibrium phonon population at 113 cm-1, and the temporal evolution of the resulting decay products. The frequency dependence of the phonon relaxation rates of acoustic phonons in this material is found to deviate from the ω5 frequency dependence predicted by an isotropic model with linear dispersion. A more realistic model based on the actual dispersion curves of the material is presented and compared with the data.

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.

Quantum mechanical prediction of four-phonon scattering rates and reduced thermal conductivity of solids

NASA Astrophysics Data System (ADS)

Feng, Tianli; Ruan, Xiulin

2016-01-01

Recently, first principle-based predictions of lattice thermal conductivity κ from perturbation theory have achieved significant success. However, it only includes three-phonon scattering due to the assumption that four-phonon and higher-order processes are generally unimportant. Also, directly evaluating the scattering rates of four-phonon and higher-order processes has been a long-standing challenge. In this work, however, we have developed a formalism to explicitly determine quantum mechanical scattering probability matrices for four-phonon scattering in the full Brillouin zone, and by mitigating the computational challenge we have directly calculated four-phonon scattering rates. We find that four-phonon scattering rates are comparable to three-phonon scattering rates at medium and high temperatures, and they increase quadratically with temperature. As a consequence, κ of Lennard-Jones argon is reduced by more than 60% at 80 K when four-phonon scattering is included. Also, in less anharmonic materials—diamond, silicon, and germanium—κ is still reduced considerably at high temperature by four-phonon scattering by using the classical Tersoff potentials. Also, the thermal conductivity of optical phonons is dominated by the fourth- and higher-orders phonon scattering even at low temperature.

Infrared-active optical phonons in LiFePO4 single crystals

NASA Astrophysics Data System (ADS)

Stanislavchuk, T. N.; Middlemiss, D. S.; Syzdek, J. S.; Janssen, Y.; Basistyy, R.; Sirenko, A. A.; Khalifah, P. G.; Grey, C. P.; Kostecki, R.

2017-07-01

Infrared-active optical phonons were studied in olivine LiFePO4 oriented single crystals by means of both rotating analyzer and rotating compensator spectroscopic ellipsometry in the spectral range between 50 and 1400 cm-1. The eigenfrequencies, oscillator strengths, and broadenings of the phonon modes were determined from fits of the anisotropic harmonic oscillator model to the data. Optical phonons in a heterosite FePO4 crystal were measured from the delithiated ab-surface of the LiFePO4 crystal and compared with the phonon modes of the latter. Good agreement was found between experimental data and the results of solid-state hybrid density functional theory calculations for the phonon modes in both LiFePO4 and FePO4.

Microstructure-Induced Phonon Focusing Effects and Opportunities for Improved Material Quantification (Postprint)

DTIC Science & Technology

2012-02-01

phonon interactions with electrons , electron -hole pairs, defects, super- lattices, and interfaces [1-4]. As pointed out by Hauser et. al. [3], and...phonon-phonon and electron - phonon scattering processes placed limits on the methods applicability. More recently, the advantages of using lower...texture effects. In particular, the elongated grains result in colonies that are largely cigar -shaped or cylindrical in their form, where elastic

Preface to special topic: Selected articles from phononics 2013: The second international conference on phononic crystals/metamaterials, phonon transport and optomechanics, 2-7 June 2013, Sharm El-Sheikh, Egypt

DOE PAGES

Hussein, Mahmoud I.; El-Kady, Ihab; Li, Baowen; ...

2014-12-31

“Phononics” is an interdisciplinary branch of physics and engineering that deals with the behavior of phonons, and more broadly elastic and acoustic waves in similar context, and their manipulation in solids and/or fluids to benefit technological applications. Compared to resembling disciplines, such as electronics and photonics, phononics is a youthful field. It is growing at a remarkable rate, especially when viewed liberally with no limiting constraints on any particular length scale, discipline or application.

Preface to special topic: Selected articles from phononics 2013: The second international conference on phononic crystals/metamaterials, phonon transport and optomechanics, 2-7 June 2013, Sharm El-Sheikh, Egypt

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hussein, Mahmoud I.; El-Kady, Ihab; Li, Baowen

“Phononics” is an interdisciplinary branch of physics and engineering that deals with the behavior of phonons, and more broadly elastic and acoustic waves in similar context, and their manipulation in solids and/or fluids to benefit technological applications. Compared to resembling disciplines, such as electronics and photonics, phononics is a youthful field. It is growing at a remarkable rate, especially when viewed liberally with no limiting constraints on any particular length scale, discipline or application.

Toward single electron resolution phonon mediated ionization detectors

NASA Astrophysics Data System (ADS)

Mirabolfathi, Nader; Harris, H. Rusty; Mahapatra, Rupak; Sundqvist, Kyle; Jastram, Andrew; Serfass, Bruno; Faiez, Dana; Sadoulet, Bernard

2017-05-01

Experiments seeking to detect rare event interactions such as dark matter or coherent elastic neutrino nucleus scattering are striving for large mass detectors with very low detection threshold. Using Neganov-Luke phonon amplification effect, the Cryogenic Dark Matter Search (CDMS) experiment is reaching unprecedented RMS resolutions of ∼14 eVee. CDMSlite is currently the most sensitive experiment to WIMPs of mass ∼5 GeV/c2 but is limited in achieving higher phonon gains due to an early onset of leakage current into Ge crystals. The contact interface geometry is particularly weak for blocking hole injection from the metal, and thus a new design is demonstrated that allows high voltage bias via vacuum separated electrode. With an increased bias voltage and a×2 Luke phonon gain, world best RMS resolution of sigma ∼7 eVee for 0.25 kg (d=75 mm, h=1 cm) Ge detectors was achieved. Since the leakage current is a function of the field and the phonon gain is a function of the applied voltage, appropriately robust interface blocking material combined with thicker substrate (25 mm) will reach a resolution of ∼2.8 eVee. In order to achieve better resolution of ∼ eV, we are investigating a layer of insulator between the phonon readout surface and the semiconductor crystals.

Relative merits of phononics vs. plasmonics: the energy balance approach

NASA Astrophysics Data System (ADS)

Khurgin, Jacob B.

2018-01-01

The common feature of various plasmonic schemes is their ability to confine optical fields of surface plasmon polaritons (SPPs) into subwavelength volumes and thus achieve a large enhancement of linear and nonlinear optical properties. This ability, however, is severely limited by the large ohmic loss inherent to even the best of metals. However, in the mid- and far-infrared ranges of the spectrum, there exists a viable alternative to metals - polar dielectrics and semiconductors, in which dielectric permittivity (the real part) turns negative in the Reststrahlen region. This feature engenders the so-called surface phonon polaritons, capable of confining the field in a way akin to their plasmonic analogs, the SPPs. Since the damping rate of polar phonons is substantially less than that of free electrons, it is not unreasonable to expect that phononic devices may outperform their plasmonic counterparts. Yet a more rigorous analysis of the comparative merits of phononics and plasmonics reveals a more nuanced answer, namely, that while phononic schemes do exhibit narrower resonances and can achieve a very high degree of energy concentration, most of the energy is contained in the form of lattice vibrations so that enhancement of the electric field and, hence, the Purcell factor is rather small compared to what can be achieved with metal nanoantennas. Still, the sheer narrowness of phononic resonances is expected to make phononics viable in applications where frequency selectivity is important.

Role of hydrodynamic viscosity on phonon transport in suspended graphene

NASA Astrophysics Data System (ADS)

Li, Xun; Lee, Sangyeop

2018-03-01

When phonon transport is in the hydrodynamic regime, the thermal conductivity exhibits peculiar dependences on temperatures (T ) and sample widths (W ). These features were used in the past to experimentally confirm the hydrodynamic phonon transport in three-dimensional bulk materials. Suspended graphene was recently predicted to exhibit strong hydrodynamic features in thermal transport at much higher temperature than the three-dimensional bulk materials, but its experimental confirmation requires quantitative guidance by theory and simulation. Here we quantitatively predict those peculiar dependences using the Monte Carlo solution of the Peierls-Boltzmann equation with an ab initio full three-phonon scattering matrix. Thermal conductivity is found to increase as Tα where α ranges from 1.89 to 2.49 depending on a sample width at low temperatures, much larger than 1.68 of the ballistic case. The thermal conductivity has a width dependence of W1.17 at 100 K, clearly distinguished from the sublinear dependence of the ballistic-diffusive regime. These peculiar features are explained with a phonon viscous damping effect of the hydrodynamic regime. We derive an expression for the phonon hydrodynamic viscosity from the Peierls-Boltzmann equation, and discuss the fact that the phonon viscous damping explains well those peculiar dependences of thermal conductivity at 100 K. The phonon viscous damping still causes significant thermal resistance when a temperature is 300 K and a sample width is around 1 µm, even though the hydrodynamic regime is not dominant over other regimes at this condition.

Direct observation of confined acoustic phonon polarization branches in free-standing semiconductor nanowires

DOE PAGES

Kargar, Fariborz; Debnath, Bishwajit; Kakko, Joona -Pekko; ...

2016-11-10

Similar to electron waves, the phonon states in semiconductors can undergo changes induced by external boundaries. However, despite strong scientific and practical importance, conclusive experimental evidence of confined acoustic phonon polarization branches in individual free-standing nanostructures is lacking. Here we report results of Brillouin-Mandelstam light scattering spectroscopy, which reveal multiple (up to ten) confined acoustic phonon polarization branches in GaAs nanowires with a diameter as large as 128 nm, at a length scale that exceeds the grey phonon mean-free path in this material by almost an order-of-magnitude. The dispersion modification and energy scaling with diameter in individual nanowires are inmore » excellent agreement with theory. The phonon confinement effects result in a decrease in the phonon group velocity along the nanowire axis and changes in the phonon density of states. Furthermore, the obtained results can lead to more efficient nanoscale control of acoustic phonons, with benefits for nanoelectronic, thermoelectric and spintronic devices.« less

Extremely Low Loss Phonon-Trapping Cryogenic Acoustic Cavities for Future Physical Experiments

PubMed Central

Galliou, Serge; Goryachev, Maxim; Bourquin, Roger; Abbé, Philippe; Aubry, Jean Pierre; Tobar, Michael E.

2013-01-01

Low loss Bulk Acoustic Wave devices are considered from the point of view of the solid state approach as phonon-confining cavities. We demonstrate effective design of such acoustic cavities with phonon-trapping techniques exhibiting extremely high quality factors for trapped longitudinally-polarized phonons of various wavelengths. Quality factors of observed modes exceed 1 billion, with a maximum Q-factor of 8 billion and Q × f product of 1.6 · 1018 at liquid helium temperatures. Such high sensitivities allow analysis of intrinsic material losses in resonant phonon systems. Various mechanisms of phonon losses are discussed and estimated. PMID:23823569

Acoustic interference suppression of quartz crystal microbalance sensor arrays utilizing phononic crystals

NASA Astrophysics Data System (ADS)

Chen, Yung-Yu; Huang, Li-Chung; Wang, Wei-Shan; Lin, Yu-Ching; Wu, Tsung-Tsong; Sun, Jia-Hong; Esashi, Masayoshi

2013-04-01

Acoustic interference suppression of quartz crystal microbalance (QCM) sensor arrays utilizing phononic crystals is investigated in this paper. A square-lattice phononic crystal structure is designed to have a complete band gap covering the QCM's resonance frequency. The monolithic sensor array consisting of two QCMs separated by phononic crystals is fabricated by micromachining processes. As a result, 12 rows of phononic crystals with band gap boost insertion loss between the two QCMs by 20 dB and also reduce spurious modes. Accordingly, the phononic crystal is verified to be capable of suppressing the acoustic interference between adjacent QCMs in a sensor array.

Designing broad phononic band gaps for in-plane modes

NASA Astrophysics Data System (ADS)

Li, Yang Fan; Meng, Fei; Li, Shuo; Jia, Baohua; Zhou, Shiwei; Huang, Xiaodong

2018-03-01

Phononic crystals are known as artificial materials that can manipulate the propagation of elastic waves, and one essential feature of phononic crystals is the existence of forbidden frequency range of traveling waves called band gaps. In this paper, we have proposed an easy way to design phononic crystals with large in-plane band gaps. We demonstrated that the gap between two arbitrarily appointed bands of in-plane mode can be formed by employing a certain number of solid or hollow circular rods embedded in a matrix material. Topology optimization has been applied to find the best material distributions within the primitive unit cell with maximal band gap width. Our results reveal that the centroids of optimized rods coincide with the point positions generated by Lloyd's algorithm, which deepens our understandings on the formation mechanism of phononic in-plane band gaps.

Electronic and phononic modulation of MoS2 under biaxial strain

NASA Astrophysics Data System (ADS)

Moghadasi, A.; Roknabadi, M. R.; Ghorbani, S. R.; Modarresi, M.

2017-12-01

Dichalcogenides of transition metals are attractive material due to its unique properties. In this work, it has been investigated the electronic band structure, phonon spectrum and heat capacity of MoS2 under the applied tensile and compressive biaxial strain using the density functional theory. The Molybdenum disulfide under compressive (tensile) strain up to 6% (10%) has stable atomic structure without any negative frequency in the phonon dispersion curves. The tensile biaxial strain reduces the energy gap in the electronic band structure and the optical-acoustic gap in phonon dispersion curves. The tensile biaxial strain also increases the specific heat capacity. On the other hand, the compressive biaxial strain in this material increases phonon gap and reduces the heat capacity and the electronic band gap. The phonon softening/hardening is reported for tensile/compressive biaxial strain in MoS2. We report phonon hardening for out of plane ZA mode in the presence of both tensile and compressive strains. Results show that the linear variation of specific heat with strain (CV ∝ε) and square dependency of specific heat with the temperature (CV ∝T2) for low temperature regime. The results demonstrate that the applied biaxial strain tunes the electronic energy gap and modifies the phonon spectrum of MoS2.

Development of a Massive, Highly Multiplexible, Phonon-Mediated Particle Detector Using Kinetic Inductance Detectors

NASA Astrophysics Data System (ADS)

Chang, Y.-Y.; Cornell, B.; Aralis, T.; Bumble, B.; Golwala, S. R.

2018-04-01

We present a status update on the development of a phonon-mediated particle detector using kinetic inductance detector (KID). The design is intended for O(1) kg substrate, using O(102) KIDs on a single readout line, to image the athermal phonon distribution at < 1 mm position resolution and O(10) eV energy resolution. The design specification is set by the need to improve position reconstruction fidelity while maintaining low energy threshold for future rare-event searches such as for low-mass dark matter. We report on the design, which shows negligible crosstalk and > 95% inductor current uniformity, using the coplanar waveguide feedline, ground shield, and a new class of KIDs with symmetric coplanar stripline (sCPS) inductor. The multiplexing is designed upon the frequency-geometry relation we develop for the sCPS KIDs. We introduce the fabrications of the Nb RF assessment prototypes and the high phonon collection efficiency Al-Nb devices. We achieve ≲ 0.07% frequency displacement on a 80-KID RF assessment prototype, and the result indicates that we may place more than 180 resonances in our 0.4 GHz readout band with minimal frequency misordering. The coupling quality factors are ˜ 105 as designed. Finally, we update our work in progress in fabricating the O(102) KID, bi-material, O(1) kg detectors, and the expected position and energy resolutions.

Level repulsion and band sorting in phononic crystals

NASA Astrophysics Data System (ADS)

Lu, Yan; Srivastava, Ankit

2018-02-01

In this paper we consider the problem of avoided crossings (level repulsion) in phononic crystals and suggest a computationally efficient strategy to distinguish them from normal cross points. This process is essential for the correct sorting of the phononic bands and, subsequently, for the accurate determination of mode continuation, group velocities, and emergent properties which depend on them such as thermal conductivity. Through explicit phononic calculations using generalized Rayleigh quotient, we identify exact locations of exceptional points in the complex wavenumber domain which results in level repulsion in the real domain. We show that in the vicinity of the exceptional point the relevant phononic eigenvalue surfaces resemble the surfaces of a 2 by 2 parameter-dependent matrix. Along a closed loop encircling the exceptional point we show that the phononic eigenvalues are exchanged, just as they are for the 2 by 2 matrix case. However, the behavior of the associated eigenvectors is shown to be more complex in the phononic case. Along a closed loop around an exceptional point, we show that the eigenvectors can flip signs multiple times unlike a 2 by 2 matrix where the flip of sign occurs only once. Finally, we exploit these eigenvector sign flips around exceptional points to propose a simple and efficient method of distinguishing them from normal crosses and of correctly sorting the band-structure. Our proposed method is roughly an order-of-magnitude faster than the zoom-in method and correctly identifies > 96% of the cases considered. Both its speed and accuracy can be further improved and we suggest some ways of achieving this. Our method is general and, as such, would be directly applicable to other eigenvalue problems where the eigenspectrum needs to be correctly sorted.

Impurity and phonon scattering in silicon nanowires

NASA Astrophysics Data System (ADS)

Zhang, W.; Persson, M. P.; Mera, H.; Delerue, C.; Niquet, Y. M.; Allan, G.; Wang, E.

2011-03-01

We model the scattering of electrons by phonons and dopant impurities in ultimate [110]-oriented gate-all-around silicon nanowires with an atomistic valence force field and tight-binding approach. All electron-phonons interactions are included. We show that impurity scattering can reduce with decreasing nanowire diameter due to the enhanced screening by the gate. Donors and acceptors however perform very differently : acceptors behave as tunnel barriers for the electrons, while donors behave as quantum wells which introduce Fano resonances in the conductance. As a consequence the acceptors are much more limiting the mobility than the donors. The resistances of single acceptors are also very dependent on their radial position in the nanowire, which might be a significant source of variability in ultimate silicon nanowire devices. Concerning phonons, we show that, as a result of strong confinement, i) electrons couple to a wide and complex distribution of phonons modes, and ii) the mobility has a non-monotonic variation with wire diameter and is strongly reduced with respect to bulk. French National Research Agency ANR project QUANTAMONDE Contract No. ANR-07-NANO-023-02 and by the Délégation Générale pour l'Armement, French Ministry of Defense under Grant No. 2008.34.0031.

Infrared-active optical phonons in LiFePO 4 single crystals

DOE PAGES

Stanislavchuk, T. N.; Middlemiss, D. S.; Syzdek, J. S.; ...

2017-07-28

Infrared-active optical phonons were studied in olivine LiFePO 4 oriented single crystals by means of both rotating analyzer and rotating compensator spectroscopic ellipsometry in the spectral range between 50 and 1400 cm -1. The eigenfrequencies, oscillator strengths, and broadenings of the phonon modes were determined from fits of the anisotropic harmonic oscillator model to the data. Optical phonons in a heterosite FePO 4 crystal were measured from the delithiated ab-surface of the LiFePO 4 crystal and compared with the phonon modes of the latter. Good agreement was found between experimental data and the results of solid-state hybrid density functional theorymore » calculations for the phonon modes in both LiFePO 4 and FePO 4.« less

Electron-phonon interaction in efficient perovskite blue emitters

NASA Astrophysics Data System (ADS)

Gong, Xiwen; Voznyy, Oleksandr; Jain, Ankit; Liu, Wenjia; Sabatini, Randy; Piontkowski, Zachary; Walters, Grant; Bappi, Golam; Nokhrin, Sergiy; Bushuyev, Oleksandr; Yuan, Mingjian; Comin, Riccardo; McCamant, David; Kelley, Shana O.; Sargent, Edward H.

2018-06-01

Low-dimensional perovskites have—in view of their high radiative recombination rates—shown great promise in achieving high luminescence brightness and colour saturation. Here we investigate the effect of electron-phonon interactions on the luminescence of single crystals of two-dimensional perovskites, showing that reducing these interactions can lead to bright blue emission in two-dimensional perovskites. Resonance Raman spectra and deformation potential analysis show that strong electron-phonon interactions result in fast non-radiative decay, and that this lowers the photoluminescence quantum yield (PLQY). Neutron scattering, solid-state NMR measurements of spin-lattice relaxation, density functional theory simulations and experimental atomic displacement measurements reveal that molecular motion is slowest, and rigidity greatest, in the brightest emitter. By varying the molecular configuration of the ligands, we show that a PLQY up to 79% and linewidth of 20 nm can be reached by controlling crystal rigidity and electron-phonon interactions. Designing crystal structures with electron-phonon interactions in mind offers a previously underexplored avenue to improve optoelectronic materials' performance.

Bragg Coherent Diffractive Imaging of Zinc Oxide Acoustic Phonons at Picosecond Timescales

DOE PAGES

Ulvestad, A.; Cherukara, M. J.; Harder, R.; ...

2017-08-29

Mesoscale thermal transport is of fundamental interest and practical importance in materials such as thermoelectrics. Coherent lattice vibrations (acoustic phonons) govern thermal transport in crystalline solids and are affected by the shape, size, and defect density in nanoscale materials. The advent of hard x-ray free electron lasers (XFELs) capable of producing ultrafast x-ray pulses has significantly impacted the understanding of acoustic phonons by enabling their direct study with x-rays. However, previous studies have reported ensemble-averaged results that cannot distinguish the impact of mesoscale heterogeneity on the phonon dynamics. Here we use Bragg coherent diffractive imaging (BCDI) to resolve the 4Dmore » evolution of the acoustic phonons in a single zinc oxide rod with a spatial resolution of 50 nm and a temporal resolution of 25 picoseconds. We observe homogeneous (lattice breathing/rotation) and inhomogeneous (shear) acoustic phonon modes, which are compared to finite element simulations. We investigate the possibility of changing phonon dynamics by altering the crystal through acid etching. Lastly, we find that the acid heterogeneously dissolves the crystal volume, which will significantly impact the phonon dynamics. In general, our results represent the first step towards understanding the effect of structural properties at the individual crystal level on phonon dynamics.« less

Bragg Coherent Diffractive Imaging of Zinc Oxide Acoustic Phonons at Picosecond Timescales

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ulvestad, A.; Cherukara, M. J.; Harder, R.

Mesoscale thermal transport is of fundamental interest and practical importance in materials such as thermoelectrics. Coherent lattice vibrations (acoustic phonons) govern thermal transport in crystalline solids and are affected by the shape, size, and defect density in nanoscale materials. The advent of hard x-ray free electron lasers (XFELs) capable of producing ultrafast x-ray pulses has significantly impacted the understanding of acoustic phonons by enabling their direct study with x-rays. However, previous studies have reported ensemble-averaged results that cannot distinguish the impact of mesoscale heterogeneity on the phonon dynamics. Here we use Bragg coherent diffractive imaging (BCDI) to resolve the 4Dmore » evolution of the acoustic phonons in a single zinc oxide rod with a spatial resolution of 50 nm and a temporal resolution of 25 picoseconds. We observe homogeneous (lattice breathing/rotation) and inhomogeneous (shear) acoustic phonon modes, which are compared to finite element simulations. We investigate the possibility of changing phonon dynamics by altering the crystal through acid etching. Lastly, we find that the acid heterogeneously dissolves the crystal volume, which will significantly impact the phonon dynamics. In general, our results represent the first step towards understanding the effect of structural properties at the individual crystal level on phonon dynamics.« less

Strong magnon-phonon coupling in NaFeAs studied by neutron scattering

NASA Astrophysics Data System (ADS)

Li, Yu; Yamani, Zahra; Song, Yu; Zhang, Chenglin; Dai, Pengcheng

We carried on inelastic neutron scattering experiment on the triple axis spectrometer in CNBC in Chalk River. We measured both the phonon and magnon in NaFeAs single crystals and their temperature dependence. Since structural transition temperature (TS) and the magnetic transition temperature (T N) are well separated in NaFeAs, it provides us an unique chance to exclude the consequence or magnetic order and focus on the so called nematic phase. As the previous paper on BaFe2As2, we observed the strong phonon softening nearby the structural transition temperature at very small q (q<0.1). This makes the phonon in NaFeAs deviate from the classical linear dispersion relationship for acoustic phonons. Besides the phonon softening, we also observe phonon hardening at a larger q range when the temperature goes down. This is accompanied by the stiffening of the magnons which can be represented by the linewidth of the low energy magnetic peaks. Our results suggest that there is strong coupling between the phonons and magnons in NaFeAs.

Band structures in fractal grading porous phononic crystals

NASA Astrophysics Data System (ADS)

Wang, Kai; Liu, Ying; Liang, Tianshu; Wang, Bin

2018-05-01

In this paper, a new grading porous structure is introduced based on a Sierpinski triangle routine, and wave propagation in this fractal grading porous phononic crystal is investigated. The influences of fractal hierarchy and porosity on the band structures in fractal graidng porous phononic crystals are clarified. Vibration modes of unit cell at absolute band gap edges are given to manifest formation mechanism of absolute band gaps. The results show that absolute band gaps are easy to form in fractal structures comparatively to the normal ones with the same porosity. Structures with higher fractal hierarchies benefit multiple wider absolute band gaps. This work provides useful guidance in design of fractal porous phononic crystals.

THz acoustic phonon spectroscopy and nanoscopy by using piezoelectric semiconductor heterostructures.

PubMed

Mante, Pierre-Adrien; Huang, Yu-Ru; Yang, Szu-Chi; Liu, Tzu-Ming; Maznev, Alexei A; Sheu, Jinn-Kong; Sun, Chi-Kuang

2015-02-01

Thanks to ultrafast acoustics, a better understanding of acoustic dynamics on a short time scale has been obtained and new characterization methods at the nanoscale have been developed. Among the materials that were studied during the development of ultrafast acoustics, nitride based heterostructures play a particular role due to their piezoelectric properties and the possibility to generate phonons with over-THz frequency and bandwidth. Here, we review some of the work performed using this type of structure, with a focus on THz phonon spectroscopy and nanoscopy. First, we present a brief description of the theory of coherent acoustic phonon generation by piezoelectric heterostructure. Then the first experimental observation of coherent acoustic phonon generated by the absorption of ultrashort light pulses in piezoelectric heterostructures is presented. From this starting point, we then present some methods developed to realize customizable phonon generation. Finally we review some more recent applications of these structures, including imaging with a nanometer resolution, broadband attenuation measurements with a frequency up to 1THz and phononic bandgap characterization. Copyright © 2014 Elsevier B.V. All rights reserved.

Analytical Modeling of Acoustic Phonon-Limited Mobility in Strained Graphene Nanoribbons

NASA Astrophysics Data System (ADS)

Yousefvand, Ali; Ahmadi, Mohammad T.; Meshginqalam, Bahar

2017-11-01

Recent advances in graphene nanoribbon-based electronic devices encourage researchers to develop modeling and simulation methods to explore device physics. On the other hand, increasing the operating speed of nanoelectronic devices has recently attracted significant attention, and the modification of acoustic phonon interactions because of their important effect on carrier mobility can be considered as a method for carrier mobility optimization which subsequently enhances the device speed. Moreover, strain has an important influence on the electronic properties of the nanoelectronic devices. In this paper, the acoustic phonons mobility of armchair graphene nanoribbons ( n-AGNRs) under uniaxial strain is modeled analytically. In addition, strain, width and temperature effects on the acoustic phonon mobility of strained n-AGNRs are investigated. An increment in the strained AGNR acoustic phonon mobility by increasing the ribbon width is reported. Additionally, two different behaviors for the acoustic phonon mobility are verified by increasing the applied strain in 3 m, 3 m + 2 and 3 m + 1 AGNRs. Finally, the temperature effect on the modeled AGNR phonon mobility is explored, and mobility reduction by raising the temperature is reported.

Dynamics of monochromatically generated nonequilibrium phonons in LaF3:Pr3+

NASA Astrophysics Data System (ADS)

Tolbert, W. A.; Dennis, W. M.; Yen, W. M.

1990-07-01

The temporal evolution of nonequilibrium phonon populations in LaF3:Pr3+ is investigated at low temperatures (1.8 K) utilizing pulsed, tunable, monochromatic generation and time-resolved, tunable, narrow-band detection. High occupation number, narrow-band phonon populations are generated via far-infrared pumping of defect-induced one-phonon absorption. Time-resolved, frequency-selective detection is provided by optical sideband absorption. Nonequilibrium phonon decay times are measured and attributed to anharmonic decay.

Influence of phonon-phonon coupling on superconducting state in honeycomb-type crystal lattice

NASA Astrophysics Data System (ADS)

Drzazga, E. A.; Szczȩśniak, R.; Domagalska, I. A.

2018-01-01

We have taken into account the superconducting state inducing in the crystal lattice of the honeycomb-type. In the framework of the Eliashberg theory, we have determined the thermodynamic properties of the system. The phonon spectral function, which is the input parameter in the Eliashberg equations, has been calculated by using the thermodynamic Green functions. We have considered the model of the coupled Einstein oscillators with frequency ω0 = 100 meV. We have shown that the increasing inter-phonon coupling constant (f) causes the rapid growth of the critical temperature ([TC]max = 36.2 K) just below the maximum value of f equal to 0.25ω0. Simultaneously, the order parameter and the thermodynamic critical field take the values increasingly distant from the predictions of the BCS theory, which results from the strong-coupling and the retardation effects.

Phononic thermal conductivity in silicene: the role of vacancy defects and boundary scattering

NASA Astrophysics Data System (ADS)

Barati, M.; Vazifehshenas, T.; Salavati-fard, T.; Farmanbar, M.

2018-04-01

We calculate the thermal conductivity of free-standing silicene using the phonon Boltzmann transport equation within the relaxation time approximation. In this calculation, we investigate the effects of sample size and different scattering mechanisms such as phonon–phonon, phonon-boundary, phonon-isotope and phonon-vacancy defect. We obtain some similar results to earlier works using a different model and provide a more detailed analysis of the phonon conduction behavior and various mode contributions. We show that the dominant contribution to the thermal conductivity of silicene, which originates from the in-plane acoustic branches, is about 70% at room temperature and this contribution becomes larger by considering vacancy defects. Our results indicate that while the thermal conductivity of silicene is significantly suppressed by the vacancy defects, the effect of isotopes on the phononic transport is small. Our calculations demonstrate that by removing only one of every 400 silicon atoms, a substantial reduction of about 58% in thermal conductivity is achieved. Furthermore, we find that the phonon-boundary scattering is important in defectless and small-size silicene samples, especially at low temperatures.

Ab initio optimization of phonon drag effect for lower-temperature thermoelectric energy conversion.

PubMed

Zhou, Jiawei; Liao, Bolin; Qiu, Bo; Huberman, Samuel; Esfarjani, Keivan; Dresselhaus, Mildred S; Chen, Gang

2015-12-01

Although the thermoelectric figure of merit zT above 300 K has seen significant improvement recently, the progress at lower temperatures has been slow, mainly limited by the relatively low Seebeck coefficient and high thermal conductivity. Here we report, for the first time to our knowledge, success in first-principles computation of the phonon drag effect--a coupling phenomenon between electrons and nonequilibrium phonons--in heavily doped region and its optimization to enhance the Seebeck coefficient while reducing the phonon thermal conductivity by nanostructuring. Our simulation quantitatively identifies the major phonons contributing to the phonon drag, which are spectrally distinct from those carrying heat, and further reveals that although the phonon drag is reduced in heavily doped samples, a significant contribution to Seebeck coefficient still exists. An ideal phonon filter is proposed to enhance zT of silicon at room temperature by a factor of 20 to ∼ 0.25, and the enhancement can reach 70 times at 100 K. This work opens up a new venue toward better thermoelectrics by harnessing nonequilibrium phonons.

Structural imaging of nanoscale phonon transport in ferroelectrics excited by metamaterial-enhanced terahertz fields

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhu, Yi; Chen, Frank; Park, Joonkyu

Nanoscale phonon transport is a key process that governs thermal conduction in a wide range of materials and devices. Creating controlled phonon populations by resonant excitation at terahertz (THz) frequencies can drastically change the characteristics of nanoscale thermal transport and allow a direct real-space characterization of phonon mean-free paths. Using metamaterial-enhanced terahertz excitation, we tailored a phononic excitation by selectively populating low-frequency phonons within a nanoscale volume in a ferroelectric BaTiO3 thin film. Real-space time-resolved x-ray diffraction microscopy following THz excitation reveals ballistic phonon transport over a distance of hundreds of nm, two orders of magnitude longer than the averagedmore » phonon mean-free path in BaTiO3. On longer length scales, diffusive phonon transport dominates the recovery of the transient strain response, largely due to heat conduction into the substrate. The measured real-space phonon transport can be directly compared with the phonon mean-free path as predicted by molecular dynamics modeling. This time-resolved real-space visualization of THz-matter interactions opens up opportunities to engineer and image nanoscale transient structural states with new functionalities.« less

Structural imaging of nanoscale phonon transport in ferroelectrics excited by metamaterial-enhanced terahertz fields

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhu, Yi; Chen, Frank; Park, Joonkyu

Nanoscale phonon transport is a key process that governs thermal conduction in a wide range of materials and devices. Creating controlled phonon populations by resonant excitation at terahertz (THz) frequencies can drastically change the characteristics of nanoscale thermal transport and allow a direct real-space characterization of phonon mean-free paths. Using metamaterial-enhanced terahertz excitation, we tailored a phononic excitation by selectively populating low-frequency phonons within a nanoscale volume in a ferroelectric BaTiO 3 thin film. Real-space time-resolved x-ray diffraction microscopy following THz excitation reveals ballistic phonon transport over a distance of hundreds of nm, two orders of magnitude longer than themore » averaged phonon mean-free path in BaTiO 3. On longer length scales, diffusive phonon transport dominates the recovery of the transient strain response, largely due to heat conduction into the substrate. The measured real-space phonon transport can be directly compared with the phonon mean-free path as predicted by molecular dynamics modeling. In conclusion, this time-resolved real-space visualization of THz-matter interactions opens up opportunities to engineer and image nanoscale transient structural states with new functionalities.« less

Structural imaging of nanoscale phonon transport in ferroelectrics excited by metamaterial-enhanced terahertz fields

DOE PAGES

Zhu, Yi; Chen, Frank; Park, Joonkyu; ...

2017-11-16

Nanoscale phonon transport is a key process that governs thermal conduction in a wide range of materials and devices. Creating controlled phonon populations by resonant excitation at terahertz (THz) frequencies can drastically change the characteristics of nanoscale thermal transport and allow a direct real-space characterization of phonon mean-free paths. Using metamaterial-enhanced terahertz excitation, we tailored a phononic excitation by selectively populating low-frequency phonons within a nanoscale volume in a ferroelectric BaTiO 3 thin film. Real-space time-resolved x-ray diffraction microscopy following THz excitation reveals ballistic phonon transport over a distance of hundreds of nm, two orders of magnitude longer than themore » averaged phonon mean-free path in BaTiO 3. On longer length scales, diffusive phonon transport dominates the recovery of the transient strain response, largely due to heat conduction into the substrate. The measured real-space phonon transport can be directly compared with the phonon mean-free path as predicted by molecular dynamics modeling. In conclusion, this time-resolved real-space visualization of THz-matter interactions opens up opportunities to engineer and image nanoscale transient structural states with new functionalities.« less

First-principles modeling of resonant Raman scattering for the understanding of phonons and electrons in nanomaterials

NASA Astrophysics Data System (ADS)

Liang, Liangbo; Meunier, Vincent; Yan, Jia-An; Sumpter, Bobby

Raman spectroscopy is a popular tool that can probe both phonons and electrons of the materials. First-principles modeling is important in aiding the understanding of experimental data. Raman modeling is typically based on the classical Placzek approximation and limited to the non-resonant condition, and thus the laser energy dependence of Raman intensities could not be captured. Here we showed that resonant Raman scattering could be captured by upgrading the classical approach, i.e., by calculating the dynamic dielectric tensor at the laser energy instead of the commonly used static value at zero energy. Our method was successfully applied to recently synthesized atomically precise graphene nanoribbons, and revealed the photon-energy-dependent Raman intensity of the radial breathing like mode (RBLM), which explained experimental observations that RBLM can be only observed in certain laser energies. Additionally, we also explored anisotropic 2D material, ReS2, and found that the angle-resolved Raman polarization dependence of its Raman modes is sensitive to the laser energy, as confirmed by recent experiments. The intricate electron-phonon coupling could lead to no simple rule for using Raman polarization dependence to determine the crystalline orientation. LL is supported by Eugene P. Wigner Fellowship at Oak Ridge National Laboratory and CNMS (a DOE Office of Science User Facility).

The phononic crystals: An unending quest for tailoring acoustics

NASA Astrophysics Data System (ADS)

Kushwaha, M.

Periodicity (in time or space) is a part and parcel of every living being: One can see, hear, and feel it. Everyday examples are locomotion, respiration, and heart beat. The reinforced N-dimensional periodicity over two or more crystalline solids results in the so-called phononic band-gap crystals. These can have dramatic consequences on the propagation of phonons, vibrations, and sound. The fundamental physics of cleverly fabricated phononic crystals can offer a systematic route to realize the Anderson localization of sound and vibrations. As to the applications, the phononic crystals are envisaged to find ways in the architecture, acoustic waveguides, designing transducers, elastic/acoustic filters, noise control, ultrasonics, medical imaging, and acoustic cloaking, to mention a few. This review focuses on the brief sketch of the progress made in the field that seems to have prospered even more than was originally imagined in the early nineties.

Nonlocal electron-phonon coupling in the pentacene crystal: Beyond the Γ-point approximation

NASA Astrophysics Data System (ADS)

Yi, Yuanping; Coropceanu, Veaceslav; Brédas, Jean-Luc

2012-10-01

There is currently increasing interest in understanding the impact of the nonlocal (Peierls-type) electron-phonon mechanism on charge transport in organic molecular semiconductors. Most estimates of the non-local coupling constants reported in the literature are based on the Γ-point phonon modes. Here, the influence of phonon modes spanning the entire Brillouin zone (phonon dispersion) on the nonlocal electron-phonon couplings is investigated for the pentacene crystal. The phonon modes are obtained by using a supercell approach. The results underline that the overall nonlocal couplings are substantially underestimated by calculations taking sole account of the phonons at the Γ point of the unit cell. The variance of the transfer integrals based on Γ-point normal-mode calculations at room temperature is underestimated in some cases by 40% for herringbone-type dimers and by over 80% for cofacial dimers. Our calculations show that the overall coupling is somewhat larger for holes than for electrons. The results also suggest that the interactions of charge carriers (both electrons and holes) with acoustic and optical phonons are comparable. Therefore, an adequate description of the charge-transport properties in pentacene and similar systems requires that these two electron-phonon coupling mechanisms be treated on the same footing.

Target CLAB Zero: A national improvement collaborative to reduce central line-associated bacteraemia in New Zealand intensive care units.

PubMed

Gray, Jonathon; Proudfoot, Suzanne; Power, Maxine; Bennett, Brandon; Wells, Sue; Seddon, Mary

2015-09-04

Central line-associated bacteraemia (CLAB) is a preventable cause of patient morbidity and mortality in intensive care units. Target CLAB Zero was a national campaign that ran from October 2011 to March 2013 across all New Zealand ICUs (intensive care units). The campaign aimed to reduce the national CLAB rate to less than one incident per 1,000 line days and to establish a national measurement system for CLAB. We used Institute for Healthcare Improvement (IHI) Breakthrough Series methodology to structure the campaign. IHI bundles of care for catheter insertion and maintenance were implemented across 25 New Zealand ICUs. We collected monthly data on line days, CLAB infections and compliance with the bundles. Data were analysed using run charts. The rate of CLAB per 1,000 line days fell from 3.32 at baseline to an average of 0.28 between April 2012 and March 2013. In the final 3-month period, January to March 2013, average insertion bundle compliance was 80% and average maintenance bundle compliance was 75%. All ICUs participated in the collaborative. Over 90% of those invited attended all three national learning sessions and bi-monthly regional learning sessions. National collaboratives can effect improvement and shared learning in New Zealand. International evidence combined with New Zealand experience, a supportive methodology, partnership, clinical respect and an effective communication plan were keys to successful engagement.

Phonon modes and thermal conductance in carbon nanotubes

NASA Astrophysics Data System (ADS)

Tomanek, David

2001-03-01

The unique electronic transport behavior of quasi-1D carbon nanotubes(Stefano Sanvito, Young-Kyun Kwon, David Tomanek, and Colin J. Lambert, Phys. Rev. Lett. 84), 1974 (2000). finds an unexpected counterpart in their unusually high thermal conductance.(Savas Berber, Young-Kyun Kwon, and David Tomanek, Phys. Rev. Lett. 84), 4613 (2000). The latter is a consequence of the structural rigidity of nanotubes, resulting in a large sound velocity, and their phonon structure. Soft phonon modes, primarily associated with tube bending and twisting, are essentially decoupled from the energy-carrying hard phonon modes which originate in the stretching and bending of interatomic bonds. The absence of an efficient coupling mechanism between these different phonon modes is responsible for their low damping and a long phonon mean free path. With a peak value λ=37,000W/m/K at 100K, the thermal conductance of an isolated (10,10) nanotube, predicted using non-equilibrium molecular dynamics simulations, is comparable to that of isotopically pure diamond. At room temperature, the predicted value λ=6,600W/m/K even exceeds that of this best thermal conductor. Unlike bulk graphite, where coupling between the flexible layers reduces the basal plane thermal conductance by one order of magnitude, we find that the inter-tube coupling in nanotube ropes does not reduce the single-tube conductance significantly.

Sensing Atomic Motion from the Zero Point to Room Temperature with Ultrafast Atom Interferometry.

PubMed

Johnson, K G; Neyenhuis, B; Mizrahi, J; Wong-Campos, J D; Monroe, C

2015-11-20

We sense the motion of a trapped atomic ion using a sequence of state-dependent ultrafast momentum kicks. We use this atom interferometer to characterize a nearly pure quantum state with n=1 phonon and accurately measure thermal states ranging from near the zero-point energy to n[over ¯]~10^{4}, with the possibility of extending at least 100 times higher in energy. The complete energy range of this method spans from the ground state to far outside of the Lamb-Dicke regime, where atomic motion is greater than the optical wavelength. Apart from thermometry, these interferometric techniques are useful for characterizing ultrafast entangling gates between multiple trapped ions.

Phonon and thermodynamical properties of CuSc: A DFT study

NASA Astrophysics Data System (ADS)

Jain, Ekta; Pagare, Gitanjali; Dubey, Shubha; Sanyal, S. P.

2018-05-01

A detailed systematic theoretical investigation of phonon and thermodynamical behavior of CuSc intermetallic compound has been carried out by uing first-principles density functional theory in B2-type (CsCl) crystal structure. Phonon dispersion curve and phonon density of states (PhDOS) are studied which confirm the stability of CuSc intermetallic compound in B2 phase. It is found that PhDOS at high frequencies mostly composed of Sc states. We have also presented some temperature dependent properties such as entropy, free energy, heat capacity, internal energy and thermal displacement, which are computed under PHONON code. The various features of these quantities are discussed in detail. From these results we demonstrate that the particular intermetallic have better ductility and larger thermal expansion.

Ballistic phonon and thermal radiation transport across a minute vacuum gap in between aluminum and silicon thin films: Effect of laser repetitive pulses on transport characteristics

NASA Astrophysics Data System (ADS)

Yilbas, B. S.; Ali, H.

2016-08-01

Short-pulse laser heating of aluminum and silicon thin films pair with presence of a minute vacuum gap in between them is considered and energy transfer across the thin films pair is predicted. The frequency dependent Boltzmann equation is used to predict the phonon intensity distribution along the films pair for three cycles of the repetitive short-pulse laser irradiation on the aluminum film surface. Since the gap size considered is within the Casimir limit, thermal radiation and ballistic phonon contributions to energy transfer across the vacuum gap is incorporated. The laser irradiated field is formulated in line with the Lambert's Beer law and it is considered as the volumetric source in the governing equations of energy transport. In order to assess the phonon intensity distribution in the films pair, equivalent equilibrium temperature is introduced. It is demonstrated that thermal separation of electron and lattice sub-systems in the aluminum film, due to the short-pulse laser irradiation, takes place and electron temperature remains high in the aluminum film while equivalent equilibrium temperature for phonons decays sharply in the close region of the aluminum film interface. This behavior is attributed to the phonon boundary scattering at the interface and the ballistic phonon transfer to the silicon film across the vacuum gap. Energy transfer due to the ballistic phonon contribution is significantly higher than that of the thermal radiation across the vacuum gap.

Phonon-drag magnetothermopower in Rashba spin-split two-dimensional electron systems.

PubMed

Biswas, Tutul; Ghosh, Tarun Kanti

2013-10-16

We study the phonon-drag contribution to the thermoelectric power in a quasi-two-dimensional electron system confined in GaAs/AlGaAs heterostructure in the presence of both Rashba spin-orbit interaction and perpendicular magnetic field at very low temperature. It is observed that the peaks in the phonon-drag thermopower split into two when the Rashba spin-orbit coupling constant is strong. This splitting is a direct consequence of the Rashba spin-orbit interaction. We show the dependence of phonon-drag thermopower on both magnetic field and temperature numerically. A power-law dependence of phonon-drag magnetothermopower on the temperature in the Bloch-Gruneisen regime is found. We also extract the exponent of the temperature dependence of phonon-drag thermopower for different parameters like electron density, magnetic field, and the spin-orbit coupling constant.

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

DOE PAGES

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

2016-10-12

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 in this paper, 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 opticallymore » 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.« less

Dispersion of doppleron-phonon modes in strong coupling regime.

PubMed

Gudkov, V V; Zhevstovskikh, I V

2004-04-01

The dispersion equation for doppleron-phonon modes was constructed and solved analytically in the strong coupling regime. The Fermi surface model proposed previously for calculating the doppleron spectrum in an indium crystal was used. It was shown that in the vicinity of doppleron-phonon resonance, the dispersion curves of coupled modes form a gap qualitatively different from the one observed under helicon-phonon resonance: there is a frequency interval forbidden for existence of waves of definite circular polarization depending upon direction of the external DC magnetic field. The physical reason for it is interaction of the waves which have oppositely directed group velocities.

Designing Phononic Crystals with Wide and Robust Band Gaps

NASA Astrophysics Data System (ADS)

Jia, Zian; Chen, Yanyu; Yang, Haoxiang; Wang, Lifeng

2018-04-01

Phononic crystals (PnCs) engineered to manipulate and control the propagation of mechanical waves have enabled the design of a range of novel devices, such as waveguides, frequency modulators, and acoustic cloaks, for which wide and robust phononic band gaps are highly preferable. While numerous PnCs have been designed in recent decades, to the best of our knowledge, PnCs that possess simultaneous wide and robust band gaps (to randomness and deformations) have not yet been reported. Here, we demonstrate that by combining the band-gap formation mechanisms of Bragg scattering and local resonances (the latter one is dominating), PnCs with wide and robust phononic band gaps can be established. The robustness of the phononic band gaps are then discussed from two aspects: robustness to geometric randomness (manufacture defects) and robustness to deformations (mechanical stimuli). Analytical formulations further predict the optimal design parameters, and an uncertainty analysis quantifies the randomness effect of each designing parameter. Moreover, we show that the deformation robustness originates from a local resonance-dominant mechanism together with the suppression of structural instability. Importantly, the proposed PnCs require only a small number of layers of elements (three unit cells) to obtain broad, robust, and strong attenuation bands, which offer great potential in designing flexible and deformable phononic devices.

Hot carrier and hot phonon coupling during ultrafast relaxation of photoexcited electrons in graphene

DOE Office of Scientific and Technical Information (OSTI.GOV)

Iglesias, J. M.; Martín, M. J.; Pascual, E.

2016-01-25

We study, by means of a Monte Carlo simulator, the hot phonon effect on the relaxation dynamics in photoexcited graphene and its quantitative impact as compared with considering an equilibrium phonon distribution. Our multi-particle approach indicates that neglecting the hot phonon effect significantly underestimates the relaxation times in photoexcited graphene. The hot phonon effect is more important for a higher energy of the excitation pulse and photocarrier densities between 1 and 3 × 10{sup 12 }cm{sup −2}. Acoustic intervalley phonons play a non-negligible role, and emitted phonons with wavelengths limited up by a maximum (determined by the carrier concentration) induce a slower carriermore » cooling rate. Intrinsic phonon heating is damped in graphene on a substrate due to the additional cooling pathways, with the hot phonon effect showing a strong inverse dependence with the carrier density.« less

Interlayer electron-phonon coupling in WSe2/hBN heterostructures

NASA Astrophysics Data System (ADS)

Jin, Chenhao; Kim, Jonghwan; Suh, Joonki; Shi, Zhiwen; Chen, Bin; Fan, Xi; Kam, Matthew; Watanabe, Kenji; Taniguchi, Takashi; Tongay, Sefaattin; Zettl, Alex; Wu, Junqiao; Wang, Feng

2017-02-01

Engineering layer-layer interactions provides a powerful way to realize novel and designable quantum phenomena in van der Waals heterostructures. Interlayer electron-electron interactions, for example, have enabled fascinating physics that is difficult to achieve in a single material, such as the Hofstadter's butterfly in graphene/boron nitride (hBN) heterostructures. In addition to electron-electron interactions, interlayer electron-phonon interactions allow for further control of the physical properties of van der Waals heterostructures. Here we report an interlayer electron-phonon interaction in WSe2/hBN heterostructures, where optically silent hBN phonons emerge in Raman spectra with strong intensities through resonant coupling to WSe2 electronic transitions. Excitation spectroscopy reveals the double-resonance nature of such enhancement, and identifies the two resonant states to be the A exciton transition of monolayer WSe2 and a new hybrid state present only in WSe2/hBN heterostructures. The observation of an interlayer electron-phonon interaction could open up new ways to engineer electrons and phonons for device applications.

Multiphonon: Phonon Density of States tools for Inelastic Neutron Scattering Powder Data

DOE Office of Scientific and Technical Information (OSTI.GOV)

Y. Y. Lin, Jiao; Islam, Fahima; Kresh, Max

The multiphonon python package calculates phonon density of states, a reduced representation of vibrational property of condensed matter (see, for example, Section “Density of Normal Modes” in Chapter 23 “Quantum Theory of the Harmonic Crystal” of (Ashcroft and Mermin 2011)), from inelastic neutron scattering (see, for example (B. Fultz et al. 2006–2016)) spectrum from a powder sample. Inelastic neutron spectroscopy (INS) is a probe of excitations in solids of vibrational or magnetic origins. In INS, neutrons can lose(gain) energy to(from) the solid in the form of quantized lattice vibrations – phonons. Measuring phonon density of states is usually the firstmore » step in determining the phonon properties of a material experimentally. Phonons play a very important role in understanding the physical properties of a solid, including thermal conductivity and electrical conductivity. Hence, INS is an important tool for studying thermoelectric materials (Budai et al. 2014, Li et al. (2015)), where low thermal conductivity and high electrical conductivity are desired. Study of phonon entropy also made important contributions to the research of thermal dynamics and phase stability of materials (B. Fultz 2010, bogdanoff2002phonon, swan2006vibrational).« less

Multiphonon: Phonon Density of States tools for Inelastic Neutron Scattering Powder Data

DOE PAGES

Y. Y. Lin, Jiao; Islam, Fahima; Kresh, Max

2018-01-29

The multiphonon python package calculates phonon density of states, a reduced representation of vibrational property of condensed matter (see, for example, Section “Density of Normal Modes” in Chapter 23 “Quantum Theory of the Harmonic Crystal” of (Ashcroft and Mermin 2011)), from inelastic neutron scattering (see, for example (B. Fultz et al. 2006–2016)) spectrum from a powder sample. Inelastic neutron spectroscopy (INS) is a probe of excitations in solids of vibrational or magnetic origins. In INS, neutrons can lose(gain) energy to(from) the solid in the form of quantized lattice vibrations – phonons. Measuring phonon density of states is usually the firstmore » step in determining the phonon properties of a material experimentally. Phonons play a very important role in understanding the physical properties of a solid, including thermal conductivity and electrical conductivity. Hence, INS is an important tool for studying thermoelectric materials (Budai et al. 2014, Li et al. (2015)), where low thermal conductivity and high electrical conductivity are desired. Study of phonon entropy also made important contributions to the research of thermal dynamics and phase stability of materials (B. Fultz 2010, bogdanoff2002phonon, swan2006vibrational).« less

Amide I SFG Spectral Line Width Probes the Lipid-Peptide and Peptide-Peptide Interactions at Cell Membrane In Situ and in Real Time.

PubMed

Zhang, Baixiong; Tan, Junjun; Li, Chuanzhao; Zhang, Jiahui; Ye, Shuji

2018-06-13

The balance of lipid-peptide and peptide-peptide interactions at cell membrane is essential to a large variety of cellular processes. In this study, we have experimentally demonstrated for the first time that sum frequency generation vibrational spectroscopy can be used to probe the peptide-peptide and lipid-peptide interactions in cell membrane in situ and in real time by determination of the line width of amide I band of protein backbone. Using a "benchmark" model of α-helical WALP23, it is found that the dominated lipid-peptide interaction causes a narrow line width of the amide I band, whereas the peptide-peptide interaction can markedly broaden the line width. When WALP23 molecules insert into the lipid bilayer, a quite narrow line width of the amide I band is observed because of the lipid-peptide interaction. In contrast, when the peptide lies down on the bilayer surface, the line width of amide I band becomes very broad owing to the peptide-peptide interaction. In terms of the real-time change in the line width, the transition from peptide-peptide interaction to lipid-peptide interaction is monitored during the insertion of WALP23 into 1,2-dipalmitoyl- sn-glycero-3-phospho-(1'- rac-glycerol) (DPPG) lipid bilayer. The dephasing time of a pure α-helical WALP23 in 1-palmitoyl-2-oleoyl- sn-glycero-3-phospho-(1'- rac-glycerol) and DPPG bilayer is determined to be 2.2 and 0.64 ps, respectively. The peptide-peptide interaction can largely accelerate the dephasing time.

Unexpectedly Fast Phonon-Assisted Exciton Hopping between Carbon Nanotubes

DOE PAGES

Davoody, A. H.; Karimi, F.; Arnold, M. S.; ...

2017-06-05

Carbon-nanotube (CNT) aggregates are promising light-absorbing materials for photovoltaics. The hopping rate of excitons between CNTs directly affects the efficiency of these devices. We theoretically investigate phonon-assisted exciton hopping, where excitons scatter with phonons into a same-tube transition state, followed by intertube Coulomb scattering into the final state. Second-order hopping between bright excitonic states is as fast as the first-order process (~1 ps). For perpendicular CNTs, the high rate stems from the high density of phononic states; for parallel CNTs, the reason lies in relaxed selection rules. Moreover, second-order exciton transfer between dark and bright states, facilitated by phonons withmore » large angular momentum, has rates comparable to bright-to-bright transfer, so dark excitons provide an additional pathway for energy transfer in CNT composites. Furthermore, as dark excitons are difficult to probe in experiment, predictive theory is critical for understanding exciton dynamics in CNT composites.« less

Unexpectedly Fast Phonon-Assisted Exciton Hopping between Carbon Nanotubes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Davoody, A. H.; Karimi, F.; Arnold, M. S.

Carbon-nanotube (CNT) aggregates are promising light-absorbing materials for photovoltaics. The hopping rate of excitons between CNTs directly affects the efficiency of these devices. We theoretically investigate phonon-assisted exciton hopping, where excitons scatter with phonons into a same-tube transition state, followed by intertube Coulomb scattering into the final state. Second-order hopping between bright excitonic states is as fast as the first-order process (~1 ps). For perpendicular CNTs, the high rate stems from the high density of phononic states; for parallel CNTs, the reason lies in relaxed selection rules. Moreover, second-order exciton transfer between dark and bright states, facilitated by phonons withmore » large angular momentum, has rates comparable to bright-to-bright transfer, so dark excitons provide an additional pathway for energy transfer in CNT composites. Furthermore, as dark excitons are difficult to probe in experiment, predictive theory is critical for understanding exciton dynamics in CNT composites.« less

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

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gu, Xiaokun; Yang, Ronggui, E-mail: Ronggui.Yang@Colorado.Edu

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 silicenemore » 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.« less

Influence of electronic band topology on phonon properties in Dirac materials

NASA Astrophysics Data System (ADS)

Garate, Ion; Saha, Kush; Légaré, Katherine

2015-03-01

In Dirac materials, the interaction between electrons and long-wavelength phonons has been shown to induce and stabilize topological insulation [1-2]. Here report on a theoretical study of the converse effect, namely the influence of band topology on phonon properties. We calculate how electron-phonon interactions change the bulk phonon dispersion as a function of pressure and temperature, in both trivial and topological phases. We find that (i) topological insulators are more prone to lattice instabilities than trivial insulators, and (ii) Raman and neutron scattering measurements can be used to determine the electronic band topology. Research funded by Canada's NSERC and Québec's RQMP.

Enhanced photon-phonon cross-Kerr nonlinearity with two-photon driving.

PubMed

Yin, Tai-Shuang; Lü, Xin-You; Wan, Liang-Liang; Bin, Shang-Wu; Wu, Ying

2018-05-01

We propose a scheme to significantly enhance the cross-Kerr (CK) nonlinearity between photons and phonons in a quadratically coupled optomechanical system (OMS) with two-photon driving. This CK nonlinear enhancement originates from the parametric-driving-induced squeezing and the underlying nonlinear optomechanical interaction. Moreover, the noise of the squeezed mode can be suppressed completely by introducing a squeezed vacuum reservoir. As a result of this dramatic nonlinear enhancement and the suppressed noise, we demonstrate the feasibility of the quantum nondemolition measurement of the phonon number in an originally weak coupled OMS. In addition, the photon-phonon blockade phenomenon is also investigated in this regime, which allows for performing manipulations between photons and phonons. This Letter offers a promising route towards the potential application for the OMS in quantum information processing and quantum networks.

Composite-pulse and partially dipolar dephased multiCP for improved quantitative solid-state 13C NMR

NASA Astrophysics Data System (ADS)

Duan, Pu; Schmidt-Rohr, Klaus

2017-12-01

Improved multiple cross polarization (multiCP) pulse sequences for quickly acquiring quantitative 13C NMR spectra of organic solids are presented. Loss of 13C magnetization due to imperfect read-out and storage pulses in multiCP has been identified as a significant mechanism limiting polarization enhancement for 13C sites with weak couplings to 1H. This problem can be greatly reduced by composite 90° pulses with non-orthogonal phases that flip the magnetization onto the spin-lock field and back to the longitudinal direction for the 1H repolarization period; the observed loss is <3% for over ±10 kHz resonance offset and up to 20% flip-angle error. This composite-pulse multiCP (ComPmultiCP) sequence consistently provides performance superior to that of conventional multiCP, without any trade-off. The longer total CP time enabled by the composite pulses allows for a wider amplitude ramp during CP, which decreases the sensitivity to Hartmann-Hahn mismatch by a factor of two, with a <7% root-mean-square deviation within a 1-dB range for Boc-alanine. In samples with very short T1ρ, under-polarization of non-protonated carbons can be compensated by slight dipolar dephasing of CHn signals resulting from relatively weak decoupling during the Hahn spin echo period before detection. Quantitative spectra have been obtained by ComPmultiCP for low-crystallinity branched polyethylene at 4.5 kHz MAS, and in combination with partial dipolar dephasing for soil organic matter at 14 kHz MAS.

Temperature and frequency dependent mean free paths of renormalized phonons in nonlinear lattices

NASA Astrophysics Data System (ADS)

Li, Nianbei; Liu, Junjie; Wu, Changqin; Li, Baowen

2018-02-01

Unraveling general properties of renormalized phonons are of fundamental relevance to the heat transport in the regime of strong nonlinearity. In this work, we directly study the temperature and frequency dependent mean free path (MFP) of renormalized phonons with the newly developed numerical tuning fork method. The typical 1D nonlinear lattices such as Fermi-Pasta-Ulam β lattice and {φ }4 lattice are investigated in detail. Interestingly, it is found that the MFPs are inversely proportional to the frequencies of renormalized phonons rather than the square of phonon frequencies predicted by existing phonon scattering theory.

Calculated temperature dependence of elastic constants and phonon dispersion of hcp and bcc beryllium

NASA Astrophysics Data System (ADS)

Hahn, Steven; Arapan, Sergiu; Harmon, Bruce; Eriksson, Olle

2011-03-01

Conventional first principle methods for calculating lattice dynamics are unable to calculate high temperature thermophysical properties of materials containing modes that are entropically stabilized. In this presentation we use a relatively new approach called self-consistent ab initio lattice dynamics (SCAILD) to study the hcp to bcc transition (1530 K) in beryllium. The SCAILD method goes beyond the harmonic approximation to include phonon-phonon interactions and produces a temperature-dependent phonon dispersion. In the high temperature bcc structure, phonon-phonon interactions dynamically stabilize the N-point phonon. Fits to the calculated phonon dispersion were used to determine the temperature dependence of the elastic constants in the hcp and bcc phases. Work at the Ames Laboratory was supported by the Department of Energy-Basic Energy Sciences under Contract No. DE-AC02-07CH11358.

Hybrid surface-phonon-plasmon polariton modes in graphene/monolayer h-BN heterostructures.

PubMed

Brar, Victor W; Jang, Min Seok; Sherrott, Michelle; Kim, Seyoon; Lopez, Josue J; Kim, Laura B; Choi, Mansoo; Atwater, Harry

2014-07-09

Infrared transmission measurements reveal the hybridization of graphene plasmons and the phonons in a monolayer hexagonal boron nitride (h-BN) sheet. Frequency-wavevector dispersion relations of the electromagnetically coupled graphene plasmon/h-BN phonon modes are derived from measurement of nanoresonators with widths varying from 30 to 300 nm. It is shown that the graphene plasmon mode is split into two distinct optical modes that display an anticrossing behavior near the energy of the h-BN optical phonon at 1370 cm(-1). We explain this behavior as a classical electromagnetic strong-coupling with the highly confined near fields of the graphene plasmons allowing for hybridization with the phonons of the atomically thin h-BN layer to create two clearly separated new surface-phonon-plasmon-polariton (SPPP) modes.

Ab initio phonon point defect scattering and thermal transport in graphene

DOE Office of Scientific and Technical Information (OSTI.GOV)

Polanco, Carlos A.; Lindsay, Lucas R.

Here, we study the scattering of phonons from point defects and their effect on lattice thermal conductivity κ using a parameter-free ab initio Green's function methodology. Specifically, we focus on the scattering of phonons by boron (B), nitrogen (N), and phosphorus substitutions as well as single- and double-carbon vacancies in graphene. We show that changes of the atomic structure and harmonic interatomic force constants locally near defects govern the strength and frequency trends of the scattering of out-of-plane acoustic (ZA) phonons, the dominant heat carriers in graphene. ZA scattering rates due to N substitutions are nearly an order of magnitudemore » smaller than those for B defects despite having similar mass perturbations. Furthermore, ZA phonon scattering rates from N defects decrease with increasing frequency in the lower-frequency spectrum in stark contrast to expected trends from simple models. ZA phonon-vacancy scattering rates are found to have a significantly softer frequency dependence (~ω 0) in graphene than typically employed in phenomenological models. The rigorous Green's function calculations demonstrate that typical mass-defect models do not adequately describe ZA phonon-defect scattering rates. Our ab initio calculations capture well the trend of κ vs vacancy density from experiments, though not the magnitudes. In conclusion, this work elucidates important insights into phonon-defect scattering and thermal transport in graphene, and demonstrates the applicability of first-principles methods toward describing these properties in imperfect materials.« less

Ab initio phonon point defect scattering and thermal transport in graphene

DOE PAGES

Polanco, Carlos A.; Lindsay, Lucas R.

2018-01-04

Here, we study the scattering of phonons from point defects and their effect on lattice thermal conductivity κ using a parameter-free ab initio Green's function methodology. Specifically, we focus on the scattering of phonons by boron (B), nitrogen (N), and phosphorus substitutions as well as single- and double-carbon vacancies in graphene. We show that changes of the atomic structure and harmonic interatomic force constants locally near defects govern the strength and frequency trends of the scattering of out-of-plane acoustic (ZA) phonons, the dominant heat carriers in graphene. ZA scattering rates due to N substitutions are nearly an order of magnitudemore » smaller than those for B defects despite having similar mass perturbations. Furthermore, ZA phonon scattering rates from N defects decrease with increasing frequency in the lower-frequency spectrum in stark contrast to expected trends from simple models. ZA phonon-vacancy scattering rates are found to have a significantly softer frequency dependence (~ω 0) in graphene than typically employed in phenomenological models. The rigorous Green's function calculations demonstrate that typical mass-defect models do not adequately describe ZA phonon-defect scattering rates. Our ab initio calculations capture well the trend of κ vs vacancy density from experiments, though not the magnitudes. In conclusion, this work elucidates important insights into phonon-defect scattering and thermal transport in graphene, and demonstrates the applicability of first-principles methods toward describing these properties in imperfect materials.« less

Ab initio phonon point defect scattering and thermal transport in graphene

NASA Astrophysics Data System (ADS)

Polanco, Carlos A.; Lindsay, Lucas

2018-01-01

We study the scattering of phonons from point defects and their effect on lattice thermal conductivity κ using a parameter-free ab initio Green's function methodology. Specifically, we focus on the scattering of phonons by boron (B), nitrogen (N), and phosphorus substitutions as well as single- and double-carbon vacancies in graphene. We show that changes of the atomic structure and harmonic interatomic force constants locally near defects govern the strength and frequency trends of the scattering of out-of-plane acoustic (ZA) phonons, the dominant heat carriers in graphene. ZA scattering rates due to N substitutions are nearly an order of magnitude smaller than those for B defects despite having similar mass perturbations. Furthermore, ZA phonon scattering rates from N defects decrease with increasing frequency in the lower-frequency spectrum in stark contrast to expected trends from simple models. ZA phonon-vacancy scattering rates are found to have a significantly softer frequency dependence (˜ω0 ) in graphene than typically employed in phenomenological models. The rigorous Green's function calculations demonstrate that typical mass-defect models do not adequately describe ZA phonon-defect scattering rates. Our ab initio calculations capture well the trend of κ vs vacancy density from experiments, though not the magnitudes. This work elucidates important insights into phonon-defect scattering and thermal transport in graphene, and demonstrates the applicability of first-principles methods toward describing these properties in imperfect materials.

Strain effects on the optical conductivity of gapped graphene in the presence of Holstein phonons beyond the Dirac cone approximation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yarmohammadi, Mohsen, E-mail: m.yarmohammadi69@gmail.com

2016-08-15

In this paper we study the optical conductivity and density of states (DOS) of doped gapped graphene beyond the Dirac cone approximation in the presence of electron-phonon (e-ph) interaction under strain, i.e., within the framework of a full π-band Holstein model, by using the Kubo linear response formalism that is established upon the retarded self-energy. A new peak in the optical conductivity for a large enough e-ph interaction strength is found which is associated to transitions between the midgap states and the Van Hove singularities of the main π-band. Optical conductivity decreases with strain and at large strains, the systemmore » has a zero optical conductivity at low energies due to optically inter-band excitations through the limit of zero doping. As a result, the Drude weight changes with e-ph interaction, temperature and strain. Consequently, DOS and optical conductivity remains stable with temperature at low e-ph coupling strengths.« less

Density Functional Approach to Superfluid Phonon in Inner Crust of Neutron Stars

NASA Astrophysics Data System (ADS)

Inakura, Tsunenori; Matsuo, Masayuki

We investigate superfluid phonon emerging in inner crust of neutron stars by means of the nuclear density functional theory. Adopting the Wigner-Seitz approximation and a single spherical cell, we describe low-lying collective excitation with the dipole multipolarity. It is found that the superfluid phonon standing on the low-density neutron superfluid does not penetrate into the interior of the nuclear cluster. This suggests that the coupling between the superfluid phonon and the lattice phonon could be weak, and it may affect the thermal conductivity of inner crust.

Electron-phonon scattering rates in complex polar crystals

DOE Office of Scientific and Technical Information (OSTI.GOV)

Prange, M. P.; Campbell, L. W.; Kerisit, S.

2017-09-01

The thermalization of fast electrons by phonons is studied in CsI, NaI, SrI2, and YAP. This numerical study uses an improvement to a recently developed ab initio method based on a density functional perturbation theoretical description of the phonon modes that provides a way to go beyond widely used phonon models based on binary crystals. Improvements to this method are described, and scattering rates are presented and discussed. The results here treat polar and nonpolar scattering on equal footing and allow an assessment of the relative importance of the two types of scattering. The relative activity of the numerous phononmore » modes in materials with complicated structures is discussed, and a simple criterion for finding the modes that scatter strongly is presented.« less

Phonon-assisted damping of plasmons in three- and two-dimensional metals

NASA Astrophysics Data System (ADS)

Caruso, Fabio; Novko, Dino; Draxl, Claudia

2018-05-01

We investigate the effects of crystal lattice vibrations on the dispersion of plasmons. The loss function of the homogeneous electron gas (HEG) in two and three dimensions is evaluated numerically in the presence of electronic coupling to an optical phonon mode. Our calculations are based on many-body perturbation theory for the dielectric function as formulated by the Hedin-Baym equations in the Fan-Migdal approximation. The coupling to phonons broadens the spectral signatures of plasmons in the electron-energy loss spectrum (EELS) and it induces the decay of plasmons on timescales shorter than 1 ps. Our results further reveal the formation of a kink in the plasmon dispersion of the two-dimensional HEG, which marks the onset of plasmon-phonon scattering. Overall, these features constitute a fingerprint of plasmon-phonon coupling in EELS of simple metals. It is shown that these effects may be accounted for by resorting to a simplified treatment of the electron-phonon interaction which is amenable to first-principles calculations.

Phonon-coupled ultrafast interlayer charge oscillation at van der Waals heterostructure interfaces

NASA Astrophysics Data System (ADS)

Zheng, Qijing; Xie, Yu; Lan, Zhenggang; Prezhdo, Oleg V.; Saidi, Wissam A.; Zhao, Jin

2018-05-01

Van der Waals (vdW) heterostructures of transition-metal dichalcogenide (TMD) semiconductors are central not only for fundamental science, but also for electro- and optical-device technologies where the interfacial charge transfer is a key factor. Ultrafast interfacial charge dynamics has been intensively studied, however, the atomic scale insights into the effects of the electron-phonon (e-p) coupling are still lacking. In this paper, using time dependent ab initio nonadiabatic molecular dynamics, we study the ultrafast interfacial charge transfer dynamics of two different TMD heterostructures MoS2/WS2 and MoSe2/WSe2 , which have similar band structures but different phonon frequencies. We found that MoSe2/WSe2 has softer phonon modes compared to MoS2/WS2 , and thus phonon-coupled charge oscillation can be excited with sufficient phonon excitations at room temperature. In contrast, for MoS2/WS2 , phonon-coupled interlayer charge oscillations are not easily excitable. Our study provides an atomic level understanding on how the phonon excitation and e-p coupling affect the interlayer charge transfer dynamics, which is valuable for both the fundamental understanding of ultrafast dynamics at vdW hetero-interfaces and the design of novel quasi-two-dimensional devices for optoelectronic and photovoltaic applications.

Designing Phononic Crystals with Wide and Robust Band Gaps

DOE Office of Scientific and Technical Information (OSTI.GOV)

Jia, Zian; Chen, Yanyu; Yang, Haoxiang

Here, phononic crystals (PnCs) engineered to manipulate and control the propagation of mechanical waves have enabled the design of a range of novel devices, such as waveguides, frequency modulators, and acoustic cloaks, for which wide and robust phononic band gaps are highly preferable. While numerous PnCs have been designed in recent decades, to the best of our knowledge, PnCs that possess simultaneous wide and robust band gaps (to randomness and deformations) have not yet been reported. Here, we demonstrate that by combining the band-gap formation mechanisms of Bragg scattering and local resonances (the latter one is dominating), PnCs with widemore » and robust phononic band gaps can be established. The robustness of the phononic band gaps are then discussed from two aspects: robustness to geometric randomness (manufacture defects) and robustness to deformations (mechanical stimuli). Analytical formulations further predict the optimal design parameters, and an uncertainty analysis quantifies the randomness effect of each designing parameter. Moreover, we show that the deformation robustness originates from a local resonance-dominant mechanism together with the suppression of structural instability. Importantly, the proposed PnCs require only a small number of layers of elements (three unit cells) to obtain broad, robust, and strong attenuation bands, which offer great potential in designing flexible and deformable phononic devices.« less

Designing Phononic Crystals with Wide and Robust Band Gaps

DOE PAGES

Jia, Zian; Chen, Yanyu; Yang, Haoxiang; ...

2018-04-16

Here, phononic crystals (PnCs) engineered to manipulate and control the propagation of mechanical waves have enabled the design of a range of novel devices, such as waveguides, frequency modulators, and acoustic cloaks, for which wide and robust phononic band gaps are highly preferable. While numerous PnCs have been designed in recent decades, to the best of our knowledge, PnCs that possess simultaneous wide and robust band gaps (to randomness and deformations) have not yet been reported. Here, we demonstrate that by combining the band-gap formation mechanisms of Bragg scattering and local resonances (the latter one is dominating), PnCs with widemore » and robust phononic band gaps can be established. The robustness of the phononic band gaps are then discussed from two aspects: robustness to geometric randomness (manufacture defects) and robustness to deformations (mechanical stimuli). Analytical formulations further predict the optimal design parameters, and an uncertainty analysis quantifies the randomness effect of each designing parameter. Moreover, we show that the deformation robustness originates from a local resonance-dominant mechanism together with the suppression of structural instability. Importantly, the proposed PnCs require only a small number of layers of elements (three unit cells) to obtain broad, robust, and strong attenuation bands, which offer great potential in designing flexible and deformable phononic devices.« less

Migdal's theorem and electron-phonon vertex corrections in Dirac materials

NASA Astrophysics Data System (ADS)

Roy, Bitan; Sau, Jay D.; Das Sarma, S.

2014-04-01

Migdal's theorem plays a central role in the physics of electron-phonon interactions in metals and semiconductors, and has been extensively studied theoretically for parabolic band electronic systems in three-, two-, and one-dimensional systems over the last fifty years. In the current work, we theoretically study the relevance of Migdal's theorem in graphene and Weyl semimetals which are examples of 2D and 3D Dirac materials, respectively, with linear and chiral band dispersion. Our work also applies to 2D and 3D topological insulator systems. In Fermi liquids, the renormalization of the electron-phonon vertex scales as the ratio of sound (vs) to Fermi (vF) velocity, which is typically a small quantity. In two- and three-dimensional quasirelativistic systems, such as undoped graphene and Weyl semimetals, the one loop electron-phonon vertex renormalization, which also scales as η =vs/vF as η →0, is, however, enhanced by an ultraviolet logarithmic divergent correction, arising from the linear, chiral Dirac band dispersion. Such enhancement of the electron-phonon vertex can be significantly softened due to the logarithmic increment of the Fermi velocity, arising from the long range Coulomb interaction, and therefore, the electron-phonon vertex correction does not have a logarithmic divergence at low energy. Otherwise, the Coulomb interaction does not lead to any additional renormalization of the electron-phonon vertex. Therefore, electron-phonon vertex corrections in two- and three-dimensional Dirac fermionic systems scale as vs/vF0, where vF0 is the bare Fermi velocity, and small when vs≪vF0. These results, although explicitly derived for the intrinsic undoped systems, should hold even when the chemical potential is tuned away from the Dirac points.

Dephasing due to Nuclear Spins in Large-Amplitude Electric Dipole Spin Resonance.

PubMed

Chesi, Stefano; Yang, Li-Ping; Loss, Daniel

2016-02-12

We analyze effects of the hyperfine interaction on electric dipole spin resonance when the amplitude of the quantum-dot motion becomes comparable or larger than the quantum dot's size. Away from the well-known small-drive regime, the important role played by transverse nuclear fluctuations leads to a Gaussian decay with characteristic dependence on drive strength and detuning. A characterization of spin-flip gate fidelity, in the presence of such additional drive-dependent dephasing, shows that vanishingly small errors can still be achieved at sufficiently large amplitudes. Based on our theory, we analyze recent electric dipole spin resonance experiments relying on spin-orbit interactions or the slanting field of a micromagnet. We find that such experiments are already in a regime with significant effects of transverse nuclear fluctuations and the form of decay of the Rabi oscillations can be reproduced well by our theory.

Dissipative time-dependent quantum transport theory: Quantum interference and phonon induced decoherence dynamics

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, Yu, E-mail: zhy@yangtze.hku.hk; Chen, GuanHua, E-mail: ghc@everest.hku.hk; Yam, ChiYung

2015-04-28

A time-dependent inelastic electron transport theory for strong electron-phonon interaction is established via the equations of motion method combined with the small polaron transformation. In this work, the dissipation via electron-phonon coupling is taken into account in the strong coupling regime, which validates the small polaron transformation. The corresponding equations of motion are developed, which are used to study the quantum interference effect and phonon-induced decoherence dynamics in molecular junctions. Numerical studies show clearly quantum interference effect of the transport electrons through two quasi-degenerate states with different couplings to the leads. We also found that the quantum interference can bemore » suppressed by the electron-phonon interaction where the phase coherence is destroyed by phonon scattering. This indicates the importance of electron-phonon interaction in systems with prominent quantum interference effect.« less

Time Resolved Phonon Spectroscopy, Version 1.0

DOE Office of Scientific and Technical Information (OSTI.GOV)

Goett, Johnny; Zhu, Brian

TRPS code was developed for the project "Time Resolved Phonon Spectroscopy". Routines contained in this piece of software were specially created to model phonon generation and tracking within materials that interact with ionizing radiation, particularly applicable to the modeling of cryogenic radiation detectors for dark matter and neutrino research. These routines were created to link seamlessly with the open source Geant4 framework for the modeling of radiation transport in matter, with the explicit intent of open sourcing them for eventual integration into that code base.

Hierarchical thermoelectrics: crystal grain boundaries as scalable phonon scatterers

NASA Astrophysics Data System (ADS)

Selli, Daniele; Boulfelfel, Salah Eddine; Schapotschnikow, Philipp; Donadio, Davide; Leoni, Stefano

2016-02-01

Thermoelectric materials are strategically valuable for sustainable development, as they allow for the generation of electrical energy from wasted heat. In recent years several strategies have demonstrated some efficiency in improving thermoelectric properties. Dopants affect carrier concentration, while thermal conductivity can be influenced by alloying and nanostructuring. Features at the nanoscale positively contribute to scattering phonons, however those with long mean free paths remain difficult to alter. Here we use the concept of hierarchical nano-grains to demonstrate thermal conductivity reduction in rocksalt lead chalcogenides. We demonstrate that grains can be obtained by taking advantage of the reconstructions along the phase transition path that connects the rocksalt structure to its high-pressure form. Since grain features naturally change as a function of size, they impact thermal conductivity over different length scales. To understand this effect we use a combination of advanced molecular dynamics techniques to engineer grains and to evaluate thermal conductivity in PbSe. By affecting grain morphologies only, i.e. at constant chemistry, two distinct effects emerge: the lattice thermal conductivity is significantly lowered with respect to the perfect crystal, and its temperature dependence is markedly suppressed. This is due to an increased scattering of low-frequency phonons by grain boundaries over different size scales. Along this line we propose a viable process to produce hierarchical thermoelectric materials by applying pressure via a mechanical load or a shockwave as a novel paradigm for material design.

Quantum Theory of Conditional Phonon States in a Dual-Pumped Raman Optical Frequency Comb

NASA Astrophysics Data System (ADS)

Mondloch, Erin

In this work, we theoretically and numerically investigate nonclassical phonon states created in the collective vibration of a Raman medium by the generation of a dual-pumped Raman optical frequency comb in an optical cavity. This frequency comb is generated by cascaded Raman scattering driven by two phase-locked pump lasers that are separated in frequency by three times the Raman phonon frequency. We characterize the variety of conditioned phonon states that are created when the number of photons in all optical frequency modes except the pump modes are measured. Almost all of these conditioned phonon states are extremely well approximated as three-phonon-squeezed states or Schrodinger-cat states, depending on the outcomes of the photon number measurements. We show how the combinations of first-, second-, and third-order Raman scattering that correspond to each set of measured photon numbers determine the fidelity of the conditioned phonon state with model three-phonon-squeezed states and Schrodinger-cat states. All of the conditioned phonon states demonstrate preferential growth of the phonon mode along three directions in phase space. That is, there are three preferred phase values that the phonon state takes on as a result of Raman scattering. We show that the combination of Raman processes that produces a given set of measured photon numbers always produces phonons in multiples of three. In the quantum number-state representation, these multiples of three are responsible for the threefold phase-space symmetry seen in the conditioned phonon states. With a semiclassical model, we show how this three-phase preference can also be understood in light of phase correlations that are known to spontaneously arise in single-pumped Raman frequency combs. Additionally, our semiclassical model predicts that the optical modes also grow preferentially along three phases, suggesting that the dual-pumped Raman optical frequency comb is partially phase-stabilized.

Bloch wave deafness and modal conversion at a phononic crystal boundary