2002-01-01
W. Ding, Phys. Rev. B 67, 04541 3 (2003) . [13] M. Dresselhaus, G . Dresselhaus, and P. Avouris, eds., Carbon Nanotubes : Synthesis , Structure...Polaritonics System 2 . Carbon Nanotube Yarns and Sheets for Enhanced Thermal Conductivit y Phonon Transistor with Charge Injection Gate I UT D Our...Main Systems and Materials : 1 . New Mechanism of Phonon - Polariton Thermal Conductivity 2 . Carbon Nanotubes with Enhanced K(T) CNT in CNT Yarns and
Phonon engineering for nanostructures.
Aubry, Sylvie; Friedmann, Thomas Aquinas; Sullivan, John Patrick; Peebles, Diane Elaine; Hurley, David H.; Shinde, Subhash L.; Piekos, Edward Stanley; Emerson, John Allen
2010-01-01
Understanding the physics of phonon transport at small length scales is increasingly important for basic research in nanoelectronics, optoelectronics, nanomechanics, and thermoelectrics. We conducted several studies to develop an understanding of phonon behavior in very small structures. This report describes the modeling, experimental, and fabrication activities used to explore phonon transport across and along material interfaces and through nanopatterned structures. Toward the understanding of phonon transport across interfaces, we computed the Kapitza conductance for {Sigma}29(001) and {Sigma}3(111) interfaces in silicon, fabricated the interfaces in single-crystal silicon substrates, and used picosecond laser pulses to image the thermal waves crossing the interfaces. Toward the understanding of phonon transport along interfaces, we designed and fabricated a unique differential test structure that can measure the proportion of specular to diffuse thermal phonon scattering from silicon surfaces. Phonon-scale simulation of the test ligaments, as well as continuum scale modeling of the complete experiment, confirmed its sensitivity to surface scattering. To further our understanding of phonon transport through nanostructures, we fabricated microscale-patterned structures in diamond thin films.
Engineering dissipation with phononic spectral hole burning.
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.
Engineering dissipation with phononic spectral hole burning
NASA Astrophysics Data System (ADS)
Behunin, R. O.; Kharel, P.; Renninger, W. H.; Rakich, P. T.
2016-12-01
Optomechanics, nano-electromechanics, and integrated photonics have brought about a renaissance in phononic device physics and technology. Central to this advance are devices and materials supporting ultra-long-lived photonic and phononic excitations that enable novel regimes of classical and quantum dynamics based on tailorable photon-phonon coupling. Silica-based devices have been at the forefront of such innovations for their ability to support optical excitations persisting for nearly 1 billion cycles, and for their low optical nonlinearity. While acoustic phonon modes can persist for a similar number of cycles in crystalline solids at cryogenic temperatures, it has not been possible to achieve such performance in silica, as silica becomes acoustically opaque at low temperatures. We demonstrate that these intrinsic forms of phonon dissipation are greatly reduced (by >90%) by nonlinear saturation using continuous drive fields of disparate frequencies. The result is a form of steady-state phononic spectral hole burning that produces a wideband transparency window with optically generated phonon fields of modest (nW) powers. We developed a simple model that explains both dissipative and dispersive changes produced by phononic saturation. Our studies, conducted in a microscale device, represent an important step towards engineerable phonon dynamics on demand and the use of glasses as low-loss phononic media.
2005-09-01
Zeolite Encapsulated Nanotubes ………………………………… 76 2.2. Synthesis of Conjugated Polymer-CNT Complexes ……………………………. 84 3. CHARACTERIZATION AND OPTIMIZATION OF... Nanotubes : Synthesis , Structure, Properties and Applications (Springer, Berlin, 2000). [14] D. Cahill, W. Ford, K. Goodson, G. Mahan, A. Majumdar, H. Maris...of vibrational dynamics of “bulk” flat and closed carbon and boron - nitride (BN) nanostructures allows immediate transition to the boundary condition
Scattering Tools for Nanostructure Phonon Engineering
2013-09-25
Arlington, VA 22203 AFOSR The vibrational properties of solids have crucial roles underpinning functional properties ranging from thermal conductivity... thermal diffuse scattering (TDS) techniques to nanoscale systems. With this approach we can probe phonons across the nanomaterials, phonons, x-ray...scattering, nanomembrane fabrication, flatness, large-wavevector vibrational properties, synchrotron x-rays, thermal diffuse scattering, silicon
Research Update: Phonon engineering of nanocrystalline silicon thermoelectrics
NASA Astrophysics Data System (ADS)
Shiomi, Junichiro
2016-10-01
Nanocrystalline silicon thermoelectrics can be a solution to improve the cost-effectiveness of thermoelectric technology from both material and integration viewpoints. While their figure-of-merit is still developing, recent advances in theoretical/numerical calculations, property measurements, and structural synthesis/fabrication have opened up possibilities to develop the materials based on fundamental physics of phonon transport. Here, this is demonstrated by reviewing a series of works on nanocrystalline silicon materials using calculations of multiscale phonon transport, measurements of interfacial heat conduction, and synthesis from nanoparticles. Integration of these approaches allows us to engineer phonon transport to improve the thermoelectric performance by introducing local silicon-oxide structures.
Electron-phonon interaction and scattering in Si and Ge: Implications for phonon engineering
Tandon, Nandan; Albrecht, J. D.; Ram-Mohan, L. R.
2015-07-28
We report ab-initio results for electron-phonon (e-ph) coupling and display the existence of a large variation in the coupling parameter as a function of electron and phonon dispersion. This variation is observed for all phonon modes in Si and Ge, and we show this for representative cases where the initial electron states are at the band gap edges. Using these e-ph matrix elements, which include all possible phonon modes and electron bands within a relevant energy range, we evaluate the imaginary part of the electron self-energy in order to obtain the associated scattering rates. The temperature dependence is seen through calculations of the scattering rates at 0 K and 300 K. The results provide a basis for understanding the impacts of phonon scattering vs. orientation and geometry in the design of devices, and in analysis of transport phenomena. This provides an additional tool for engineering the transfer of energy from carriers to the lattice.
Structural engineering of three-dimensional phononic crystals
NASA Astrophysics Data System (ADS)
Delpero, Tommaso; Schoenwald, Stefan; Zemp, Armin; Bergamini, Andrea
2016-02-01
Artificially-structured materials are attracting the research interest of a growing community of scientists for the possibility to develop novel materials with advantageous properties that arise from the ability to tailor the propagation of elastic waves, and thus energy, through them. In this work, we propose a three-dimensional phononic crystal whose unit cell has been engineered to obtain a strong wave-attenuation band in the middle of the acoustic frequency range. The combination of its acoustic properties with the dimensions of the unit cell and its static mechanical properties makes it an interesting material for possibly several applications in civil and mechanical engineering, for instance as the core of an acoustically insulating sandwich panel. A sample of this crystal has been manufactured and experimentally tested with respect to its acoustic transmissibility. The performance of the phononic crystal core is remarkable both in terms of amplitude reduction in the transmissibility and width of the attenuation band. A parametric study has been finally conducted on selected geometrical parameters of the unit cell and on their effect on the macroscopic properties of the crystal. This work represents an application-oriented example of how the macroscopic properties of an artificially-structured material can be designed, according to specific needs, by a conventional engineering of its unit cell.
Grain-boundary layering transitions and phonon engineering
NASA Astrophysics Data System (ADS)
Rickman, J. M.; Harmer, M. P.; Chan, H. M.
2016-09-01
We employ semi-grand canonical Monte Carlo simulation to investigate layering transitions at grain boundaries in a prototypical binary alloy. We demonstrate the existence of such transitions among various interfacial states and examine the role of elastic fields in dictating state equilibria. The results of these studies are summarized in the form of diagrams that highlight interfacial state coexistence in this system. Finally, we examine the impact of layering transitions on the phononic properties of the system, as given by the specific heat and, by extension, the thermal conductivity. Thus, it is suggested that by inducing interfacial layering transitions via changes in temperature or pressure, one can thereby engineer thermodynamic and transport properties in materials.
Engineering the hypersonic phononic band gap of hybrid Bragg stacks.
Schneider, Dirk; Liaqat, Faroha; El Boudouti, El Houssaine; El Hassouani, Youssef; Djafari-Rouhani, Bahram; Tremel, Wolfgang; Butt, Hans-Jürgen; Fytas, George
2012-06-13
We report on the full control of phononic band diagrams for periodic stacks of alternating layers of poly(methyl methacrylate) and porous silica combining Brillouin light scattering spectroscopy and theoretical calculations. These structures exhibit large and robust on-axis band gaps determined by the longitudinal sound velocities, densities, and spacing ratio. A facile tuning of the gap width is realized at oblique incidence utilizing the vector nature of the elastic wave propagation. Off-axis propagation involves sagittal waves in the individual layers, allowing access to shear moduli at nanoscale. The full theoretical description discerns the most important features of the hypersonic one-dimensional crystals forward to a detailed understanding, a precondition to engineer dispersion relations in such structures.
Engineering thermal conductance using a two-dimensional phononic crystal
Zen, Nobuyuki; Puurtinen, Tuomas A.; Isotalo, Tero J.; Chaudhuri, Saumyadip; Maasilta, Ilari J.
2014-01-01
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. PMID:24647049
Engineering interactions between superconducting qubits and phononic nanostructures
NASA Astrophysics Data System (ADS)
Arrangoiz-Arriola, Patricio; Safavi-Naeini, Amir H.
2016-12-01
Nanomechanical systems can support highly coherent microwave-frequency excitations at cryogenic temperatures. However, generating sufficient coupling between these devices and superconducting quantum circuits is challenging due to the vastly different length scales of acoustic and electromagnetic excitations. Here we demonstrate a general method for calculating piezoelectric interactions between quantum circuits and arbitrary phononic nanostructures. We illustrate our technique by studying the coupling between a transmon qubit and bulk acoustic-wave, Lamb-wave, and phononic crystal resonators, and show that very large coupling rates are possible in all three cases. Our results suggest a route to phononic circuits and systems that are nonlinear at the single-phonon level.
Controlling thermal and electrical properties of graphene by strain-engineering its flexural phonons
NASA Astrophysics Data System (ADS)
Conley, Hiram; Nicholl, Ryan; Bolotin, Kirill
2014-03-01
We explore the effects of flexural phonons on the thermal and electrical properties of graphene. To control the amplitude of flexural phonons, we developed a technique to engineer uniform mechanical strain between 0 and 1% in suspended graphene. We determine the level of strain, thermal conductivity and carrier mobility of graphene through a combination of mechanical resonance and electrical transport measurements. Depending on strain, we find significant changes in the thermal expansion coefficient, thermal conductivity, and carrier mobility of suspended graphene. These changes are consistent with the expected contribution of flexural phonons.
25th Anniversary Article: Ordered Polymer Structures for the Engineering of Photons and Phonons
Lee, Jae-Hwang; Koh, Cheong Yang; Singer, Jonathan P; Jeon, Seog-Jin; Maldovan, Martin; Stein, Ori; Thomas, Edwin L
2014-01-01
The engineering of optical and acoustic material functionalities via construction of ordered local and global architectures on various length scales commensurate with and well below the characteristic length scales of photons and phonons in the material is an indispensable and powerful means to develop novel materials. In the current mature status of photonics, polymers hold a pivotal role in various application areas such as light-emission, sensing, energy, and displays, with exclusive advantages despite their relatively low dielectric constants. Moreover, in the nascent field of phononics, polymers are expected to be a superior material platform due to the ability for readily fabricated complex polymer structures possessing a wide range of mechanical behaviors, complete phononic bandgaps, and resonant architectures. In this review, polymer-centric photonic and phononic crystals and metamaterials are highlighted, and basic concepts, fabrication techniques, selected functional polymers, applications, and emerging ideas are introduced. PMID:24338738
25th anniversary article: ordered polymer structures for the engineering of photons and phonons.
Lee, Jae-Hwang; Koh, Cheong Yang; Singer, Jonathan P; Jeon, Seog-Jin; Maldovan, Martin; Stein, Ori; Thomas, Edwin L
2014-01-01
The engineering of optical and acoustic material functionalities via construction of ordered local and global architectures on various length scales commensurate with and well below the characteristic length scales of photons and phonons in the material is an indispensable and powerful means to develop novel materials. In the current mature status of photonics, polymers hold a pivotal role in various application areas such as light-emission, sensing, energy, and displays, with exclusive advantages despite their relatively low dielectric constants. Moreover, in the nascent field of phononics, polymers are expected to be a superior material platform due to the ability for readily fabricated complex polymer structures possessing a wide range of mechanical behaviors, complete phononic bandgaps, and resonant architectures. In this review, polymer-centric photonic and phononic crystals and metamaterials are highlighted, and basic concepts, fabrication techniques, selected functional polymers, applications, and emerging ideas are introduced.
Cooling phonons with phonons: Acoustic reservoir engineering with silicon-vacancy centers in diamond
NASA Astrophysics Data System (ADS)
Kepesidis, K. V.; Lemonde, M.-A.; Norambuena, A.; Maze, J. R.; Rabl, P.
2016-12-01
We study a setup where a single negatively-charged silicon-vacancy center in diamond is magnetically coupled to a low-frequency mechanical bending mode and via strain to the high-frequency phonon continuum of a semiclamped diamond beam. We show that under appropriate microwave driving conditions, this setup can be used to induce a laser-cooling-like effect for the low-frequency mechanical vibrations, where the high-frequency longitudinal compression modes of the beam serve as an intrinsic low-temperature reservoir. We evaluate the experimental conditions under which cooling close to the quantum ground state can be achieved and describe an extended scheme for the preparation of a stationary entangled state between two mechanical modes. By relying on intrinsic properties of the mechanical beam only, this approach offers an interesting alternative for quantum manipulation schemes of mechanical systems, where otherwise efficient optomechanical interactions are not available.
Modulations of thermal properties of graphene by strain-induced phonon engineering
NASA Astrophysics Data System (ADS)
Tada, Kento; Funatani, Takashi; Konabe, Satoru; Sasaoka, Kenji; Ogawa, Matsuto; Souma, Satofumi; Yamamoto, Takahiro
2017-02-01
Modulation of the thermal properties of graphene due to strain-induced phononic band engineering was theoretically investigated by first-principles calculations based on the density functional theory. The high-energy phonon modes are found to exhibit softening owing to the strain, whereas a low-energy acoustic mode (out-of-plane mode) exhibits hardening. Moreover, the dispersion relation of the out-of-plane mode associated with the strain essentially changes from quadratic (∝ k 2) to linear (∝ k). Accordingly, the temperature dependence of the low-temperature specific heat also changes from linear (∝ T) to quadratic (∝ T 2).
Phonon Spectrum Engineering in Rolled-up Micro- and Nano-Architectures
Fomin, Vladimir M.; Balandin, Alexander A.
2015-10-10
We report on a possibility of efficient engineering of the acoustic phonon energy spectrum in multishell tubular structures produced by a novel high-tech method of self-organization of micro- and nano-architectures. The strain-driven roll-up procedure paved the way for novel classes of metamaterials such as single semiconductor radial micro- and nano-crystals and multi-layer spiral micro- and nano-superlattices. The acoustic phonon dispersion is determined by solving the equations of elastodynamics for InAs and GaAs material systems. It is shown that the number of shells is an important control parameter of the phonon dispersion together with the structure dimensions and acoustic impedance mismatch between the superlattice layers. The obtained results suggest that rolled up nano-architectures are promising for thermoelectric applications owing to a possibility of significant reduction of the thermal conductivity without degradation of the electronic transport.
Nanoscale interface engineering in ZnO twin nanorods for proposed phonon tunnel devices.
Singh, Avanendra; Senapati, Kartik; Satpati, Biswarup; Kumar, Mohit; Sahoo, Pratap K
2015-02-14
Zinc oxide twin nanorods, with two identical crystalline sections connected by an amorphous layer, were reproducibly grown using a simple one-step hydrothermal technique. The thickness of the amorphous layer between the crystalline segments was tunable with growth parameters, as confirmed by high resolution transmission electron microscopy. The photoluminescence spectra of these twin nanorods exhibit strong near band edge emission in the UV range, with convoluted phonon sidebands. De-convolution analyses of these spectra showed that the amorphous interlayers act as effective phonon barriers beyond a certain thickness. Such oriented grown individual crystalline-amorphous-crystalline structures may be a suitable test system for fundamental studies of phonon tunneling in the nanostructure. While physical vapor deposition techniques are seriously constrained in realizing crystalline-amorphous-crystalline structures, our results show the viability of engineering embedded interfaces via chemical routes.
Phonon Spectrum Engineering in Rolled-up Micro- and Nano-Architectures
Fomin, Vladimir M.; Balandin, Alexander A.
2015-10-10
We report on a possibility of efficient engineering of the acoustic phonon energy spectrum in multishell tubular structures produced by a novel high-tech method of self-organization of micro- and nano-architectures. The strain-driven roll-up procedure paved the way for novel classes of metamaterials such as single semiconductor radial micro- and nano-crystals and multi-layer spiral micro- and nano-superlattices. The acoustic phonon dispersion is determined by solving the equations of elastodynamics for InAs and GaAs material systems. It is shown that the number of shells is an important control parameter of the phonon dispersion together with the structure dimensions and acoustic impedance mismatchmore » between the superlattice layers. The obtained results suggest that rolled up nano-architectures are promising for thermoelectric applications owing to a possibility of significant reduction of the thermal conductivity without degradation of the electronic transport.« less
Unusual Exciton-Phonon Interactions at van der Waals Engineered Interfaces.
Chow, Colin M; Yu, Hongyi; Jones, Aaron M; Yan, Jiaqiang; Mandrus, David G; Taniguchi, Takashi; Watanabe, Kenji; Yao, Wang; Xu, Xiaodong
2017-02-08
Raman scattering is a ubiquitous phenomenon in light-matter interactions, which reveals a material's electronic, structural, and thermal properties. Controlling this process would enable new ways of studying and manipulating fundamental material properties. Here, we report a novel Raman scattering process at the interface between different van der Waals (vdW) materials as well as between a monolayer semiconductor and 3D crystalline substrates. We find that interfacing a WSe2 monolayer with materials such as SiO2, sapphire, and hexagonal boron nitride (hBN) enables Raman transitions with phonons that are either traditionally inactive or weak. This Raman scattering can be amplified by nearly 2 orders of magnitude when a foreign phonon mode is resonantly coupled to the A exciton in WSe2 directly or via an A1(') optical phonon from WSe2. We further showed that the interfacial Raman scattering is distinct between hBN-encapsulated and hBN-sandwiched WSe2 sample geometries. This cross-platform electron-phonon coupling, as well as the sensitivity of 2D excitons to their phononic environments, will prove important in the understanding and engineering of optoelectronic devices based on vdW heterostructures.
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.
Cattaneo, R; Zucchelli, G; Garlaschi, F M; Finzi, L; Jennings, R C
1995-11-21
Absorption spectra of the isolated D1/D2/cytochrome b-559 complex have been measured in the temperature range 80-300 K. All spectra were analyzed in terms of a linear combination of Gaussian bands and the thermal broadening data interpreted in terms of a model in which the spectrum of each pigment site is broadened by (a) a homogeneous component due to linear electron-phonon coupling to a low-frequency protein vibration and (b) an inhomogeneous component associated with stochastic fluctuations at each pigment site. In order to obtain a numerically adequate description of the absorption spectra, a minimum number of five sub-bands is required. Further refinement of this sub-band description was achieved by taking into account published data from hole burning and absorption difference spectroscopy. In this way, both a six sub-band description and a seven sub-band description were generated. In arriving at the seven sub-band description, the original five sub-band wavelength positions were essentially unchanged. Thermal broadening analysis of the seven sub-band description yielded data which displayed the closest correspondence with the literature observations. The wavelength positions of the sub-bands were near 661, 667, 670, and 675 nm, with two bands near 680 and 684 nm. The two almost isoenergetic sub-bands near 680 nm, identified as P680 and pheophytin, have optical reorganization energies around 40 and 16 cm-1, respectively. All other sub-bands, identified as accessory pigments, have optical reorganization energies close to 16 cm-1.(ABSTRACT TRUNCATED AT 250 WORDS)
Sub-band/transform compression of video sequences
NASA Technical Reports Server (NTRS)
Sauer, Ken; Bauer, Peter
1992-01-01
The progress on compression of video sequences is discussed. The overall goal of the research was the development of data compression algorithms for high-definition television (HDTV) sequences, but most of our research is general enough to be applicable to much more general problems. We have concentrated on coding algorithms based on both sub-band and transform approaches. Two very fundamental issues arise in designing a sub-band coder. First, the form of the signal decomposition must be chosen to yield band-pass images with characteristics favorable to efficient coding. A second basic consideration, whether coding is to be done in two or three dimensions, is the form of the coders to be applied to each sub-band. Computational simplicity is of essence. We review the first portion of the year, during which we improved and extended some of the previous grant period's results. The pyramid nonrectangular sub-band coder limited to intra-frame application is discussed. Perhaps the most critical component of the sub-band structure is the design of bandsplitting filters. We apply very simple recursive filters, which operate at alternating levels on rectangularly sampled, and quincunx sampled images. We will also cover the techniques we have studied for the coding of the resulting bandpass signals. We discuss adaptive three-dimensional coding which takes advantage of the detection algorithm developed last year. To this point, all the work on this project has been done without the benefit of motion compensation (MC). Motion compensation is included in many proposed codecs, but adds significant computational burden and hardware expense. We have sought to find a lower-cost alternative featuring a simple adaptation to motion in the form of the codec. In sequences of high spatial detail and zooming or panning, it appears that MC will likely be necessary for the proposed quality and bit rates.
Formation of a protected sub-band for conduction in quantum point contacts under extreme biasing.
Lee, J; Han, J E; Xiao, S; Song, J; Reno, J L; Bird, J P
2014-02-01
Managing energy dissipation is critical to the scaling of current microelectronics and to the development of novel devices that use quantum coherence to achieve enhanced functionality. To this end, strategies are needed to tailor the electron-phonon interaction, which is the dominant mechanism for cooling non-equilibrium ('hot') carriers. In experiments aimed at controlling the quantum state, this interaction causes decoherence that fundamentally disrupts device operation. Here, we show a contrasting behaviour, in which strong electron-phonon scattering can instead be used to generate a robust mode for electrical conduction in GaAs quantum point contacts, driven into extreme non-equilibrium by nanosecond voltage pulses. When the amplitude of these pulses is much larger than all other relevant energy scales, strong electron-phonon scattering induces an attraction between electrons in the quantum-point-contact channel, which leads to the spontaneous formation of a narrow current filament and to a renormalization of the electronic states responsible for transport. The lowest of these states coalesce to form a sub-band separated from all others by an energy gap larger than the source voltage. Evidence for this renormalization is provided by a suppression of heating-related signatures in the transient conductance, which becomes pinned near 2e(2)/h (e, electron charge; h, Planck constant) for a broad range of source and gate voltages. This collective non-equilibrium mode is observed over a wide range of temperature (4.2-300 K) and may provide an effective means to manage electron-phonon scattering in nanoscale devices.
Engineering a squeezed phonon reservoir with a bichromatic driving of a quantum dot
NASA Astrophysics Data System (ADS)
Gao, Bo; Li, Gao-xiang; Ficek, Zbigniew
2016-09-01
We demonstrate how an acoustic phonon bath when coupled to a quantum dot with the help of a bichromatic laser field may effectively form a quantum squeezed reservoir. This approach allows one to achieve an arbitrary degree of squeezing of the effective reservoir and it incorporates the properties of the reservoir into two parameters, which can be controlled by varying the ratio of the Rabi frequencies of the bichromatic field. It is found that for unequal Rabi frequencies, the effective reservoir may appear as a quantum squeezed field of ordinary or inverted harmonic oscillators. When the Rabi frequencies are equal the effective reservoir appears as a perfectly squeezed field in which the decay of one of the polarization quadratures of the quantum dot dipole moment is inhibited. The decay of the quantum dot to a stationary state which depends on the initial coherence is predicted. This unusual result is shown to be a consequence of a quantum-nondemolition-type coupling of the quantum dot to the engineered squeezed reservoir. The effect of the initial coherence on the steady-state dressed-state population distribution and the fluorescence spectrum is discussed in detail. The complete polarization of the dressed state population and asymmetric spectra composed of only a single Rabi sideband peak are obtained under strictly resonant excitation.
Engineering single-phonon number states of a mechanical oscillator via photon subtraction
NASA Astrophysics Data System (ADS)
Khan, M. Miskeen; Akram, M. Javed; Paternostro, M.; Saif, F.
2016-12-01
We introduce an optomechanical scheme for the probabilistic preparation of single-phonon Fock states of mechanical modes based on photosubtraction. The quality of the produced mechanical state is confirmed by a number of indicators, including phonon statistics and conditional fidelity. We assess the detrimental effect of parameters such as the temperature of the mechanical system and address the feasibility of the scheme with state-of-the-art technology.
NASA Technical Reports Server (NTRS)
Rippert, Edward D.; Ketterson, John B.; Chen, Jun; Song, Shenian; Lomatch, Susanne; Maglic, Stevan R.; Thomas, Christopher; Cheida, M. A.; Ulmer, Melville P.
1992-01-01
An engineered structure is proposed that can alleviate quasi-particle recombination losses via the existence of a phononic band gap that overlaps the 2-Delta energy of phonons produced during recombination of quasi-particles. Attention is given to a 1D Kronig-Penny model for phonons normally incident to the layers of a multilayered superconducting tunnel junction as an idealized example. A device with a high density of Bragg resonances is identified as desirable; both Nb/Si and NbN/SiN superlattices have been produced, with the latter having generally superior performance.
Phonon manipulation with phononic crystals.
Kim Bongsang; Hopkins, Patrick Edward; Leseman, Zayd C.; Goettler, Drew F.; Su, Mehmet F.; El-Kady, Ihab Fathy; Reinke, Charles M.; Olsson, Roy H., III
2012-01-01
In this work, we demonstrated engineered modification of propagation of thermal phonons, i.e. at THz frequencies, using phononic crystals. This work combined theoretical work at Sandia National Laboratories, the University of New Mexico, the University of Colorado Boulder, and Carnegie Mellon University; the MESA fabrication facilities at Sandia; and the microfabrication facilities at UNM to produce world-leading control of phonon propagation in silicon at frequencies up to 3 THz. These efforts culminated in a dramatic reduction in the thermal conductivity of silicon using phononic crystals by a factor of almost 30 as compared with the bulk value, and about 6 as compared with an unpatterned slab of the same thickness. This work represents a revolutionary advance in the engineering of thermoelectric materials for optimal, high-ZT performance. We have demonstrated the significant reduction of the thermal conductivity of silicon using phononic crystal structuring using MEMS-compatible fabrication techniques and in a planar platform that is amenable to integration with typical microelectronic systems. The measured reduction in thermal conductivity as compared to bulk silicon was about a factor of 20 in the cross-plane direction [26], and a factor of 6 in the in-plane direction. Since the electrical conductivity was only reduced by a corresponding factor of about 3 due to the removal of conductive material (i.e., porosity), and the Seebeck coefficient should remain constant as an intrinsic material property, this corresponds to an effective enhancement in ZT by a factor of 2. Given the number of papers in literature devoted to only a small, incremental change in ZT, the ability to boost the ZT of a material by a factor of 2 simply by reducing thermal conductivity is groundbreaking. The results in this work were obtained using silicon, a material that has benefitted from enormous interest in the microelectronics industry and that has a fairly large thermoelectric power
A Multiple-Channel Sub-Band Transient Detection System
David A. Smith
1998-11-01
We have developed a unique multiple-channel sub-band transient detection system to record transient electromagnetic signals in carrier-dominated radio environments; the system has been used to make unique observations of weak, transient HF signals. The detection system has made these observations possible through improved sensitivity compared to conventional broadband transient detection systems; the sensitivity improvement is estimated to be at least 20 dB. The increase in sensitivity has been achieved through subdivision of the band of interest (an 18 MHz tunable bandwidth) into eight sub-band independent detection channels, each with a 400 kHz bandwidth and its own criteria. The system generates a system trigger signal when a predetermined number of channels (typically five) trigger within a predetermined window of time (typically 100 ~s). Events are recorded with a broadband data acquisition system sampling at 50 or 100 Msample/s, so despite the fact that the detection system operates on portions of the signal confined to narrow bands, data acquisition is broadband. Between May and September of 1994, the system was used to detect and record over six thousand transient events in the frequency band from 3 to 30 MHz. Approximately 500 of the events have been characterized as paired bursts of radio noise with individual durations of 2 to 10 ps and separations between the bursts of 5 to 160 ps. The paired transients are typically 5 to 40 dB brighter than the background electromagnetic spectrum between carrier signals. We have termed these events SubIonospheric Pulse Pairs (SIPPS) and presently have no explanation as to their source. Our observations of SIPPS resemble observations of TransIonospheric Pulse Pairs (TIPPs) recorded by the Blackboard instrument on the ALEXIS satellite; the source of TIPP events is also unknown. Most of the recorded SIPP events do not exhibit frequency dispersion, implying propagation along a line-of-sight (groundwave) path; but seven of
The FORTE receiver and sub-band triggering unit
Enemark, D.C.; Shipley, M.E.
1994-08-01
The FORTE payload receiver and trigger unit represent a significant advance over the currently flying BLACKBEARD payload aboard the ALEXIS satellite. Not only is the polarization sensitive antenna array massive compared to the BLACKBEARD monopole, but the event triggering scheme is completely different. Electromagnetic pulses (EWs) are dispersed when they pass through the ionosphere creating a chirped frequency signal which can be helpful in discriminating between natural and man-made signals. Payloads designed to digitize and store the RF signatures of these signals must include sophisticated triggering circuitry to select events of interest and prevent false alarms from wasting the available memory storage resources. The FORTE wideband receiver tunes from 20 to 320 MHz with eight sub-band trigger channels distributed across the 20 MHz IF bandwidth. The conditions which must be satisfied to generate an event trigger are processor controlled. Early testing of the prototype indicates an ability to reliably trigger on chirped RF signals several dB below the noise level. FORTE is scheduled to be launched with a Pegasus XL vehicle in late 1995.
Streyer, W.; Law, S.; Rosenberg, A.; Wasserman, D.; Roberts, C.; Podolskiy, V. A.; Hoffman, A. J.
2014-03-31
We demonstrate excitation of surface phonon polaritons on patterned gallium phosphide surfaces. Control over the light-polariton coupling frequencies is demonstrated by changing the pattern periodicity and used to experimentally determine the gallium phosphide surface phonon polariton dispersion curve. Selective emission via out-coupling of thermally excited surface phonon polaritons is experimentally demonstrated. Samples are characterized experimentally by Fourier transform infrared reflection and emission spectroscopy, and modeled using finite element techniques and rigorous coupled wave analysis. The use of phonon resonances for control of emissivity and excitation of bound surface waves offers a potential tool for the exploration of long-wavelength Reststrahlen band frequencies.
Giant phonon anomaly associated with superconducting fluctuations in the pseudogap phase of cuprates
Liu, Ye-Hua; Konik, Robert M.; Rice, T. M.; ...
2016-01-20
The pseudogap in underdoped cuprates leads to significant changes in the electronic structure, and was later found to be accompanied by anomalous fluctuations of superconductivity and certain lattice phonons. Here we propose that the Fermi surface breakup due to the pseudogap, leads to a breakup of the pairing order into two weakly coupled sub-band amplitudes, and a concomitant low energy Leggett mode due to phase fluctuations between them. This increases the temperature range of superconducting fluctuations containing an overdamped Leggett mode. In this range inter-sub-band phonons show strong damping due to resonant scattering into an intermediate state with a pairmore » of overdamped Leggett modes. In the ordered state, the Leggett mode develops a finite energy, changing the anomalous phonon damping into an anomaly in the dispersion. Finally, this proposal explains the intrinsic connection between the anomalous pseudogap phase, enhanced superconducting fluctuations and giant anomalies in the phonon spectra.« less
Giant phonon anomaly associated with superconducting fluctuations in the pseudogap phase of cuprates
Liu, Ye-Hua; Konik, Robert M.; Rice, T. M.; Zhang, Fu-Chun
2016-01-01
The pseudogap in underdoped cuprates leads to significant changes in the electronic structure, and was later found to be accompanied by anomalous fluctuations of superconductivity and certain lattice phonons. Here we propose that the Fermi surface breakup due to the pseudogap, leads to a breakup of the pairing order into two weakly coupled sub-band amplitudes, and a concomitant low energy Leggett mode due to phase fluctuations between them. This increases the temperature range of superconducting fluctuations containing an overdamped Leggett mode. In this range inter-sub-band phonons show strong damping due to resonant scattering into an intermediate state with a pair of overdamped Leggett modes. In the ordered state, the Leggett mode develops a finite energy, changing the anomalous phonon damping into an anomaly in the dispersion. This proposal explains the intrinsic connection between the anomalous pseudogap phase, enhanced superconducting fluctuations and giant anomalies in the phonon spectra. PMID:26785835
Giant phonon anomaly associated with superconducting fluctuations in the pseudogap phase of cuprates
Liu, Ye-Hua; Konik, Robert M.; Rice, T. M.; Zhang, Fu-Chun
2016-01-20
The pseudogap in underdoped cuprates leads to significant changes in the electronic structure, and was later found to be accompanied by anomalous fluctuations of superconductivity and certain lattice phonons. Here we propose that the Fermi surface breakup due to the pseudogap, leads to a breakup of the pairing order into two weakly coupled sub-band amplitudes, and a concomitant low energy Leggett mode due to phase fluctuations between them. This increases the temperature range of superconducting fluctuations containing an overdamped Leggett mode. In this range inter-sub-band phonons show strong damping due to resonant scattering into an intermediate state with a pair of overdamped Leggett modes. In the ordered state, the Leggett mode develops a finite energy, changing the anomalous phonon damping into an anomaly in the dispersion. Finally, this proposal explains the intrinsic connection between the anomalous pseudogap phase, enhanced superconducting fluctuations and giant anomalies in the phonon spectra.
Jahidin, A H; Megat Ali, M S A; Taib, M N; Tahir, N Md; Yassin, I M; Lias, S
2014-04-01
This paper elaborates on the novel intelligence assessment method using the brainwave sub-band power ratio features. The study focuses only on the left hemisphere brainwave in its relaxed state. Distinct intelligence quotient groups have been established earlier from the score of the Raven Progressive Matrices. Sub-band power ratios are calculated from energy spectral density of theta, alpha and beta frequency bands. Synthetic data have been generated to increase dataset from 50 to 120. The features are used as input to the artificial neural network. Subsequently, the brain behaviour model has been developed using an artificial neural network that is trained with optimized learning rate, momentum constant and hidden nodes. Findings indicate that the distinct intelligence quotient groups can be classified from the brainwave sub-band power ratios with 100% training and 88.89% testing accuracies.
Nonlinear phononics using atomically thin membranes
NASA Astrophysics Data System (ADS)
Midtvedt, Daniel; Isacsson, Andreas; Croy, Alexander
2014-09-01
Phononic crystals and acoustic metamaterials are used to tailor phonon and sound propagation properties by utilizing artificial, periodic structures. Analogous to photonic crystals, phononic band gaps can be created, which influence wave propagation and, more generally, allow engineering of the acoustic properties of a system. Beyond that, nonlinear phenomena in periodic structures have been extensively studied in photonic crystals and atomic Bose-Einstein condensates in optical lattices. However, creating nonlinear phononic crystals or nonlinear acoustic metamaterials remains challenging and only few examples have been demonstrated. Here, we show that atomically thin and periodically pinned membranes support coupled localized modes with nonlinear dynamics. The proposed system provides a platform for investigating nonlinear phononics.
Tian, Ruoming; Kearley, Gordon J.; Yu, Dehong; Ling, Chris D.; Pham, Anh; Embs, Jan P.; Shoko, Elvis; Li, Sean
2016-01-01
Phonons in condensed matter materials transmit energy through atomic lattices as coherent vibrational waves. Like electronic and photonic properties, an improved understanding of phononic properties is essential for the development of functional materials, including thermoelectric materials. Recently, an Einstein rattling mode was found in thermoelectric material Na0.8CoO2, due to the large displacement of Na between the [CoO2] layers. In this work, we have realized a different type of rattler in another thermoelectric material Ca3Co4O9 by chemical doping, which possesses the same [CoO2] layer as Na0.8CoO2. It remarkably suppressed the thermal conductivity while enhancing its electrical conductivity. This new type of rattler was investigated by inelastic neutron scattering experiments in conjunction with ab-initio molecular dynamics simulations. We found that the large mass of dopant rather than the large displacement is responsible for such rattling in present study, which is fundamentally different from skutterudites, clathrates as well as Na analogue. We have also tentatively studied the phonon band structure of this material by DFT lattice dynamics simulation, showing the relative contribution to phonons in the distinct layers of Ca3Co4O9. PMID:27456817
NASA Astrophysics Data System (ADS)
Tian, Ruoming; Kearley, Gordon J.; Yu, Dehong; Ling, Chris D.; Pham, Anh; Embs, Jan P.; Shoko, Elvis; Li, Sean
2016-07-01
Phonons in condensed matter materials transmit energy through atomic lattices as coherent vibrational waves. Like electronic and photonic properties, an improved understanding of phononic properties is essential for the development of functional materials, including thermoelectric materials. Recently, an Einstein rattling mode was found in thermoelectric material Na0.8CoO2, due to the large displacement of Na between the [CoO2] layers. In this work, we have realized a different type of rattler in another thermoelectric material Ca3Co4O9 by chemical doping, which possesses the same [CoO2] layer as Na0.8CoO2. It remarkably suppressed the thermal conductivity while enhancing its electrical conductivity. This new type of rattler was investigated by inelastic neutron scattering experiments in conjunction with ab-initio molecular dynamics simulations. We found that the large mass of dopant rather than the large displacement is responsible for such rattling in present study, which is fundamentally different from skutterudites, clathrates as well as Na analogue. We have also tentatively studied the phonon band structure of this material by DFT lattice dynamics simulation, showing the relative contribution to phonons in the distinct layers of Ca3Co4O9.
NASA Astrophysics Data System (ADS)
Perrin, Bernard
2007-06-01
phonons can help tracking dark matter. These 328 presentations gave rise to 185 articles published in the present proceedings. The traditional topics of this conference series (phonons in superconductors and new materials, lattice dynamics, phonons in glasses and disordered materials, phase transitions, light, neutrons and x-ray inelastic scattering) were still very important in the scientific program but an increasing number of contributions occurred in the fields of coherent phonon generation, phonons in nanoscaled structures and nano/micro thermal phonon transport, expressing the growing involvement of condensed matter physicists in nanosciences. Areas like acoustic solitons and phononic crystals are now well established. Two noteworthy contributions have been brought in the long term quest for an operational SASER : one by Harold De Wijn's group from Utrecht in the classical ruby system and another one by Anthony Kent's group from Nottingham, who used semiconductor nanodevices to realize both an amplifying medium and a cavity. With these semiconductor devices the possibility for engineering, generation and detection of THz acoustic phonons are now imminent. By tradition, a prize is awarded every three years at the International Conference on Phonon Scattering in Condensed Matter to honour a scientist for his outstanding contributions to the field of phonon physics. For this twelfth edition, Humphrey Maris has been honoured for his numerous breakthroughs in the physics of phonons and quantum fluids. According to the words of James Wolfe 'Humphrey Maris has delighted and innovated the members of our phonon community with an entertaining style and challenging wit'. Prizes were also awarded for the best presentations during the poster sessions. The two winners were Peter van Capel from Utrecht, Netherlands, ('Simulations of acoustic soliton-induced chirping of exciton resonances') and Patrick Emery from Lille, France, ('Acoustic attenuation in silica in the 100-250 GHz
Otelaja, O. O.; Robinson, R. D.
2015-10-26
In this work, the mechanism for enhanced phonon backscattering in silicon is investigated. An understanding of phonon propagation through substrates has implications for engineering heat flow at the nanoscale, for understanding sources of decoherence in quantum systems, and for realizing efficient phonon-mediated particle detectors. In these systems, phonons that backscatter from the bottom of substrates, within the crystal or from interfaces, often contribute to the overall detector signal. We utilize a microscale phonon spectrometer, comprising superconducting tunnel junction emitters and detectors, to specifically probe phonon backscattering in silicon substrates (∼500 μm thick). By etching phonon “enhancers” or deep trenches (∼90 μm) around the detectors, we show that the backscattered signal level increases by a factor of ∼2 for two enhancers versus one enhancer. Using a geometric analysis of the phonon pathways, we show that the mechanism of the backscattered phonon enhancement is due to confinement of the ballistic phonon pathways and increased scattering off the enhancer walls. Our result is applicable to the geometric design and patterning of substrates that are employed in phonon-mediated detection devices.
Electronic Band Structure and Sub-band-gap Absorption of Nitrogen Hyperdoped Silicon.
Zhu, Zhen; Shao, Hezhu; Dong, Xiao; Li, Ning; Ning, Bo-Yuan; Ning, Xi-Jing; Zhao, Li; Zhuang, Jun
2015-05-27
We investigated the atomic geometry, electronic band structure, and optical absorption of nitrogen hyperdoped silicon based on first-principles calculations. The results show that all the paired nitrogen defects we studied do not introduce intermediate band, while most of single nitrogen defects can introduce intermediate band in the gap. Considering the stability of the single defects and the rapid resolidification following the laser melting process in our sample preparation method, we conclude that the substitutional nitrogen defect, whose fraction was tiny and could be neglected before, should have considerable fraction in the hyperdoped silicon and results in the visible sub-band-gap absorption as observed in the experiment. Furthermore, our calculations show that the substitutional nitrogen defect has good stability, which could be one of the reasons why the sub-band-gap absorptance remains almost unchanged after annealing.
Analysis of mammogram images based on texture features of curvelet sub-bands
NASA Astrophysics Data System (ADS)
Gardezi, Syed Jamal Safdar; Faye, Ibrahima; Eltoukhy, Mohamed Meselhy
2014-01-01
Image texture analysis plays an important role in object detection and recognition in image processing. The texture analysis can be used for early detection of breast cancer by classifying the mammogram images into normal and abnormal classes. This study investigates breast cancer detection using texture features obtained from the grey level cooccurrence matrices (GLCM) of curvelet sub-band levels combined with texture feature obtained from the image itself. The GLCM were constructed for each sub-band of three curvelet decomposition levels. The obtained feature vector presented to the classifier to differentiate between normal and abnormal tissues. The proposed method is applied over 305 region of interest (ROI) cropped from MIAS dataset. The simple logistic classifier achieved 86.66% classification accuracy rate with sensitivity 76.53% and specificity 91.3%.
Contextual filtering method applied to sub-bands of interferometric image decomposition
NASA Astrophysics Data System (ADS)
Belhadj-Aissa, S.; Hocine, F.; Boughacha, M. S.; Belhadj-Aissa, M.
2016-10-01
The precision and accuracy of Digital elevation model and deformation measurement, from SAR interferometry (InSAR/DInSAR) depend mainly on the quality of the interferogram. However, the phase noise, which is mainly due to decorrelation between the images and the speckle, makes the step of phase unwrapping most delicate. In this paper, we propose a filtering method that combines the techniques of decomposition into sub-bands and nonlinear local weights. The Spectral / Contextual filter that we propose, inspired from to Goldstein filter is applied to the sub-bands from the wavelet decomposition. To validate the results, we applied to interferometric products tandem pair ERS1/ERS2 taken in the region of Algiers Algeria.
Sub-band processing for grating lobe disambiguation in sparse arrays
NASA Astrophysics Data System (ADS)
Hersey, Ryan K.; Culpepper, Edwin
2014-06-01
Combined synthetic aperture radar (SAR) and ground moving target indication (GMTI) radar modes simultaneously generate SAR and GMTI products from the same radar data. This hybrid mode provides the benefit of combined imaging and moving target displays as well as improved target recognition. However, the differing system, antenna, and waveform requirements between SAR and GMTI modes make implementing the hybrid mode challenging. The Air Force Research Laboratory (AFRL) Gotcha radar has collected wide-bandwidth, multi-channel data that can be used for both SAR and GMTI applications. The spatial channels on the Gotcha array are sparsely separated, which causes spatial grating lobes during the digital beamforming process. Grating lobes have little impact on SAR, which typically uses a single spatial channel. However, grating lobes have a large impact on GMTI, where spatial channels are used to mitigate clutter and estimate the target angle of arrival (AOA). The AOA ambiguity has a significant impact in the Gotcha data, where detections from the sidelobes and skirts of the mainlobe wrap back into the main scene causing a significant number of false alarms. This paper presents a sub-banding method to disambiguate grating lobes in the GMTI processing. This method divides the wideband SAR data into multiple frequency sub-bands. Since each sub-band has a different center frequency, the grating lobes for each sub-band appear at different angles. The method uses this variation to disambiguate target returns and places them at the correct angle of arrival (AOA). Results are presented using AFRL Gotcha radar data.
NASA Astrophysics Data System (ADS)
Eichenfield, Matthew
The dynamic back-action caused by electromagnetic forces (radiation pressure) in optical and microwave cavities is of growing interest. Back-action cooling, for example, is being pursued as a means of achieving the quantum ground state of macroscopic mechanical oscillators. Work in the optical domain has revolved around millimeter- or micrometer-scale structures using the radiation pressure force. By comparison, in microwave devices, low-loss superconducting structures have been used for gradient-force-mediated coupling to a nanomechanical oscillator of picogram mass. In this thesis, two different nanometer-scale structures that use combinations of gradient and radiation pressure optical forces are described theoretically and demonstrated experimentally. These structures merge the fields of cavity optomechanics and nanomechanics into nano-optomechanical systsms (NOMS). The first device, the “Zipper” optomechanical cavity, consists of a pair of doubly-clamped nanoscale beams separated by approximately 100 nanometers, each beam having a mass of 20 picograms and being patterned with a quasi-1D photonic crystal bandgap cavity. The optical mode of the coupled system is exquisitely sensitive to differential motion of the beams, producing optomechanical coupling right at the fundamental limit set by optical diffraction. The mechanical modes of the beam probed with a background sensitivity only a factor of 4 above the standard quantum limit, and the application of less than a milliwatt of optical power is shown to increase the mechanical rigidity of the system by almost an order of magnitude. The second device focuses on just one of the doubly-clamped nanoscale beams of the Zipper. We show that, in addition to a photonic bandgap cavity, the periodic patterning of the beam also produces a phononic bandgap cavity with localized mechanical modes having frequencies in the microwave regime. We call these photonic and phononic crystal bandgap cavities optomechanical crystals
Switchable topological phonon channels
NASA Astrophysics Data System (ADS)
Süsstrunk, Roman; Zimmermann, Philipp; Huber, Sebastian D.
2017-01-01
Guiding energy deliberately is one of the central elements in engineering and information processing. It is often achieved by designing specific transport channels in a suitable material. Topological metamaterials offer a way to construct stable and efficient channels of unprecedented versatility. However, due to their stability it can be tricky to terminate them or to temporarily shut them off without changing the material properties massively. While a lot of effort was put into realizing mechanical topological metamaterials, almost no works deal with manipulating their edge channels in sight of applications. Here, we take a step in this direction, by taking advantage of local symmetry breaking potentials to build a switchable topological phonon channel.
NASA Astrophysics Data System (ADS)
Pourghasemi, Mahyar; Garg, Jivtesh
2015-03-01
There is a huge desire to increase operation speeds in modern integrated circuits as they get more compact. Heat generation in such a submicron devices is a key factor limiting their performances. As a solution, thermoelectric cooling in heterostructures can address heat dissipation issue in submicron devices. Performance of single barrier heterostructures depends strongly on several parameters including barrier height, barrier width and thermal conductivity of barrier. Superlattice structures have been known to have the lowest thermal conductivities reported for crystalline materials. Low thermal conductivity is beneficial for thermoelectric cooling as it reduces the heat flow from hot end to cold junction. Moreover the band offset between the barrier and base material can be easily tuned by changing the superlattice period. By optimizing the conduction band offset (barrier height), it is possible to control the Joule heating and also optimize the amount of heat absorbed due to Peltier cooling. We investigate the feasibility of using PbSe/PbSnSe superlattice in heterostructures using Monte Carlo simulation. The effect of different parameters such as barrier height, barrier width and superlattice thermal conductivity on thermoelectric cooling of such structures will be presented.
NASA Astrophysics Data System (ADS)
Mukherjee, Sushovan; Scarpa, Fabrizio; Gopalakrishnan, S.
2015-04-01
A novel design for the geometric configuration of honeycombs using a seamless combination of auxetic and conventional cores-elements with negative and positive Possion ratios respectively, has been presented. The proposed design has been shown to generate a superior band gap property while retaining all major advantages of a purely conventional or purely auxetic honeycomb structure. Seamless combination ensures that joint cardinality is also retained. Several configurations involving different degree of auxeticity and different proportions auxetic and conventional elements have been analyzed. It has been shown that the preferred configurations open up wide and clean band gap at a significantly lower frequency ranges compared to their pure counterparts. In view of existence of band gaps being desired feature for the phononic applications, reported results might be appealing. Use of such design may enable superior vibration control as well. Proposed configurations can be made isovolumic and iso-weight giving designers a fairer ground of applying such configurations without significantly changing size and weight criteria.
Manipulation of Phonons with Phononic Crystals
Leseman, Zayd Chad
2015-07-09
There were three research goals associated with this project. First, was to experimentally demonstrate phonon spectrum control at THz frequencies using Phononic Crystals (PnCs), i.e. demonstrate coherent phonon scattering with PnCs. Second, was to experimentally demonstrate analog PnC circuitry components at GHz frequencies. The final research goal was to gain a fundamental understanding of phonon interaction using computational methods. As a result of this work, 7 journal papers have been published, 1 patent awarded, 14 conference presentations given, 4 conference publications, and 2 poster presentations given.
NASA Astrophysics Data System (ADS)
Limaye, Mukta V.; Chen, S. C.; Lee, C. Y.; Chen, L. Y.; Singh, Shashi B.; Shao, Y. C.; Wang, Y. F.; Hsieh, S. H.; Hsueh, H. C.; Chiou, J. W.; Chen, C. H.; Jang, L. Y.; Cheng, C. L.; Pong, W. F.; Hu, Y. F.
2015-06-01
The correlation between sub-band gap absorption and the chemical states and electronic and atomic structures of S-hyperdoped Si have been extensively studied, using synchrotron-based x-ray photoelectron spectroscopy (XPS), x-ray absorption near-edge spectroscopy (XANES), extended x-ray absorption fine structure (EXAFS), valence-band photoemission spectroscopy (VB-PES) and first-principles calculation. S 2p XPS spectra reveal that the S-hyperdoped Si with the greatest (~87%) sub-band gap absorption contains the highest concentration of S2- (monosulfide) species. Annealing S-hyperdoped Si reduces the sub-band gap absorptance and the concentration of S2- species, but significantly increases the concentration of larger S clusters [polysulfides (Sn2-, n > 2)]. The Si K-edge XANES spectra show that S hyperdoping in Si increases (decreased) the occupied (unoccupied) electronic density of states at/above the conduction-band-minimum. VB-PES spectra evidently reveal that the S-dopants not only form an impurity band deep within the band gap, giving rise to the sub-band gap absorption, but also cause the insulator-to-metal transition in S-hyperdoped Si samples. Based on the experimental results and the calculations by density functional theory, the chemical state of the S species and the formation of the S-dopant states in the band gap of Si are critical in determining the sub-band gap absorptance of hyperdoped Si samples.
Study of sub band gap absorption of Sn doped CdSe thin films
NASA Astrophysics Data System (ADS)
Kaur, Jagdish; Rani, Mamta; Tripathi, S. K.
2014-04-01
The nanocrystalline thin films of Sn doped CdSe at different dopants concentration are prepared by thermal evaporation technique on glass substrate at room temperature. The effect of Sn doping on the optical properties of CdSe has been studied. A decrease in band gap value is observed with increase in Sn concentration. Constant photocurrent method (CPM) is used to study the absorption coefficient in the sub band gap region. Urbach energy has been obtained from CPM spectra which are found to increase with amount of Sn dopants. The refractive index data calculated from transmittance is used for the identification of oscillator strength and oscillator energy using single oscillator model which is found to be 7.7 and 2.12 eV, 6.7 and 2.5 eV for CdSe:Sn 1% and CdSe:Sn 5% respectively.
Nanoscale pillar hypersonic surface phononic crystals
NASA Astrophysics Data System (ADS)
Yudistira, D.; Boes, A.; Graczykowski, B.; Alzina, F.; Yeo, L. Y.; Sotomayor Torres, C. M.; Mitchell, A.
2016-09-01
We report on nanoscale pillar-based hypersonic phononic crystals in single crystal Z-cut lithium niobate. The phononic crystal is formed by a two-dimensional periodic array of nearly cylindrical nanopillars 240 nm in diameter and 225 nm in height, arranged in a triangular lattice with a 300-nm lattice constant. The nanopillars are fabricated by the recently introduced nanodomain engineering via laser irradiation of patterned chrome followed by wet etching. Numerical simulations and direct measurements using Brillouin light scattering confirm the simultaneous existence of nonradiative complete surface phononic band gaps. The band gaps are found below the sound line at hypersonic frequencies in the range 2-7 GHz, formed from local resonances and Bragg scattering. These hypersonic structures are realized directly in the piezoelectric material lithium niobate enabling phonon manipulation at significantly higher frequencies than previously possible with this platform, opening new opportunities for many applications in plasmonic, optomechanic, microfluidic, and thermal engineering.
Thermally induced effect on sub-band gap absorption in Ag doped CdSe thin films
NASA Astrophysics Data System (ADS)
Kaur, Jagdish; Sharma, Kriti; Bharti, Shivani; Tripathi, S. K.
2015-05-01
Thin films of Ag doped CdSe have been prepared by thermal evaporation using inert gas condensation (IGC) method taking Argon as inert gas. The prepared thin films are annealed at 363 K for one hour. The sub-band gap absorption spectra in the as deposited and annealed thin films have been studied using constant photocurrent method (CPM). The absorption coefficient in the sub-band gap region is described by an Urbach tail in both as deposited and annealed thin films. The value of Urbach energy and number density of trap states have been calculated from the absorption coefficient in the sub-band gap region which have been found to increase after annealing treatment indicating increase in disorderness in the lattice. The energy distribution of the occupied density of states below Fermi level has also been studied using derivative procedure of absorption coefficient.
Phonon Enhancement of Electronic and Optoelectronic Devices
2006-12-01
2 Task #1 Phonon Engineering for the Development of Terahertz Lasers Based On...terms of the MURI program highlights and outcome, the following merit special mention: (a) the project area involving Terahertz lasers (Task #1, led by...design an all-optical experiment on GHz range GaN /AIGaN FETs with considerable input from H. Maris (modulation spectroscopy; coherent phonon generation
47 CFR 15.323 - Specific requirements for devices operating in the 1920-1930 MHz sub-band.
Code of Federal Regulations, 2010 CFR
2010-10-01
... the total power emitted by an intentional radiator shall be at least 50 dB below the transmit power permitted for that radiator; in the bands between 3B and the sub-band edge the total power emitted by an intentional radiator in the measurement bandwidth shall be at least 60 dB below the transmit power...
Acoustic classification of battlefield transient events using wavelet sub-band features
NASA Astrophysics Data System (ADS)
Azimi-Sadjadi, M. R.; Jiang, Y.; Srinivasan, S.
2007-04-01
Detection, localization and classification of battlefield acoustic transient events are of great importance especially for military operations in urban terrain (MOUT). Generally, there can be a wide variety of battlefield acoustic transient events such as different caliber gunshots, artillery fires, and mortar fires. The discrimination of different types of transient sources is plagued by highly non-stationary nature of these signals, which makes the extraction of representative features a challenging task. This is compounded by the variations in the environmental and operating conditions and existence of a wide range of possible interference. This paper presents new approaches for transient signal estimation and feature extraction from acoustic signatures collected by several distributed sensor nodes. A maximum likelihood (ML)-based method is developed to remove noise/interference and fading effects and restore the acoustic transient signals. The estimated transient signals are then represented using wavelets. The multi-resolution property of the wavelets allows for capturing fine details in the transient signals that can be utilized to successfully classify them. Wavelet sub-band higher order moments and energy-based features are used to characterize the transient signals. The discrimination ability of the subband features for transient signal classification has been demonstrated on several different caliber gunshots. Important findings and observations on these results are also presented.
Limaye, Mukta V.; Chen, S. C.; Lee, C. Y.; Chen, L. Y.; Singh, Shashi B.; Shao, Y. C.; Wang, Y. F.; Hsieh, S. H.; Hsueh, H. C.; Chiou, J. W.; Chen, C. H.; Jang, L. Y.; Cheng, C. L.; Pong, W. F.; Hu, Y. F.
2015-01-01
The correlation between sub-band gap absorption and the chemical states and electronic and atomic structures of S-hyperdoped Si have been extensively studied, using synchrotron-based x-ray photoelectron spectroscopy (XPS), x-ray absorption near-edge spectroscopy (XANES), extended x-ray absorption fine structure (EXAFS), valence-band photoemission spectroscopy (VB-PES) and first-principles calculation. S 2p XPS spectra reveal that the S-hyperdoped Si with the greatest (~87%) sub-band gap absorption contains the highest concentration of S2− (monosulfide) species. Annealing S-hyperdoped Si reduces the sub-band gap absorptance and the concentration of S2− species, but significantly increases the concentration of larger S clusters [polysulfides (Sn2−, n > 2)]. The Si K-edge XANES spectra show that S hyperdoping in Si increases (decreased) the occupied (unoccupied) electronic density of states at/above the conduction-band-minimum. VB-PES spectra evidently reveal that the S-dopants not only form an impurity band deep within the band gap, giving rise to the sub-band gap absorption, but also cause the insulator-to-metal transition in S-hyperdoped Si samples. Based on the experimental results and the calculations by density functional theory, the chemical state of the S species and the formation of the S-dopant states in the band gap of Si are critical in determining the sub-band gap absorptance of hyperdoped Si samples. PMID:26098075
NASA Astrophysics Data System (ADS)
Peelaers, H.; Partoens, B.; Peeters, F. M.
2009-09-01
The phonon spectra of thin freestanding, hydrogen passivated, Ge nanowires are calculated by ab initio techniques. The effect of confinement on the phonon modes as caused by the small diameters of the wires is investigated. Confinement causes a hardening of the optical modes and a softening of the longitudinal acoustic modes. The stability of the nanowires, undoped or doped with B or P atoms, is investigated using the obtained phonon spectra. All considered wires were stable, except for highly doped, very thin nanowires.
NASA Astrophysics Data System (ADS)
Robins, Lawrence H.; Bertness, Kris A.; Barker, Joy M.; Sanford, Norman A.; Schlager, John B.
2007-06-01
GaN nanowires with diameters of 50-250 nm, grown by catalyst-free molecular beam epitaxy, were characterized by photoluminescence (PL) and cathodoluminescence (CL) spectroscopy at temperatures from 3 to 297 K. Both as-grown samples and dispersions of the nanowires onto other substrates were examined. The properties of the near-band-edge PL and CL spectra were discussed in Part I of this study by [Robins et al. [L. H. Robins, K. A. Bertness, J. M. Barker, N. A. Sanford, and J. B. Schlager, J. Appl. Phys. 101,113505 (2007)]. Spectral features below the band gap, and the effect of extended electron irradiation on the CL, are discussed in Part II. The observed sub-band-gap PL and CL peaks are identified as phonon replicas of the free-exciton transitions, or excitons bound to structural defects or surface states. The defect-related peaks in the nanowires are correlated with luminescence lines previously reported in GaN films, denoted the Y lines [M. A. Reshchikov and H. Morkoc, J. Appl. Phys. 97, 061301 (2005)]. The CL was partially quenched by electron beam irradiation for an extended time; the quenching was stronger for the free and shallow-donor-bound exciton peaks than for the defect-related peaks. The quenching appeared to saturate at high irradiation dose (with final intensity ≈30% of initial intensity) and was reversible on thermal cycling to room temperature. The electron irradiation-induced quenching of the CL is ascribed to charge injection and trapping phenomena.
Robins, Lawrence H.; Bertness, Kris A.; Barker, Joy M.; Sanford, Norman A.; Schlager, John B.
2007-06-01
GaN nanowires with diameters of 50-250 nm, grown by catalyst-free molecular beam epitaxy, were characterized by photoluminescence (PL) and cathodoluminescence (CL) spectroscopy at temperatures from 3 to 297 K. Both as-grown samples and dispersions of the nanowires onto other substrates were examined. The properties of the near-band-edge PL and CL spectra were discussed in Part I of this study by [Robins et al. [L. H. Robins, K. A. Bertness, J. M. Barker, N. A. Sanford, and J. B. Schlager, J. Appl. Phys. 101,113505 (2007)]. Spectral features below the band gap, and the effect of extended electron irradiation on the CL, are discussed in Part II. The observed sub-band-gap PL and CL peaks are identified as phonon replicas of the free-exciton transitions, or excitons bound to structural defects or surface states. The defect-related peaks in the nanowires are correlated with luminescence lines previously reported in GaN films, denoted the Y lines [M. A. Reshchikov and H. Morkoc, J. Appl. Phys. 97, 061301 (2005)]. The CL was partially quenched by electron beam irradiation for an extended time; the quenching was stronger for the free and shallow-donor-bound exciton peaks than for the defect-related peaks. The quenching appeared to saturate at high irradiation dose (with final intensity {approx_equal}30% of initial intensity) and was reversible on thermal cycling to room temperature. The electron irradiation-induced quenching of the CL is ascribed to charge injection and trapping phenomena.
Toward quantitative modeling of silicon phononic thermocrystals
NASA Astrophysics Data System (ADS)
Lacatena, V.; Haras, M.; Robillard, J.-F.; Monfray, S.; Skotnicki, T.; Dubois, E.
2015-03-01
The wealth of technological patterning technologies of deca-nanometer resolution brings opportunities to artificially modulate thermal transport properties. A promising example is given by the recent concepts of "thermocrystals" or "nanophononic crystals" that introduce regular nano-scale inclusions using a pitch scale in between the thermal phonons mean free path and the electron mean free path. In such structures, the lattice thermal conductivity is reduced down to two orders of magnitude with respect to its bulk value. Beyond the promise held by these materials to overcome the well-known "electron crystal-phonon glass" dilemma faced in thermoelectrics, the quantitative prediction of their thermal conductivity poses a challenge. This work paves the way toward understanding and designing silicon nanophononic membranes by means of molecular dynamics simulation. Several systems are studied in order to distinguish the shape contribution from bulk, ultra-thin membranes (8 to 15 nm), 2D phononic crystals, and finally 2D phononic membranes. After having discussed the equilibrium properties of these structures from 300 K to 400 K, the Green-Kubo methodology is used to quantify the thermal conductivity. The results account for several experimental trends and models. It is confirmed that the thin-film geometry as well as the phononic structure act towards a reduction of the thermal conductivity. The further decrease in the phononic engineered membrane clearly demonstrates that both phenomena are cumulative. Finally, limitations of the model and further perspectives are discussed.
Toward quantitative modeling of silicon phononic thermocrystals
Lacatena, V.; Haras, M.; Robillard, J.-F. Dubois, E.; Monfray, S.; Skotnicki, T.
2015-03-16
The wealth of technological patterning technologies of deca-nanometer resolution brings opportunities to artificially modulate thermal transport properties. A promising example is given by the recent concepts of 'thermocrystals' or 'nanophononic crystals' that introduce regular nano-scale inclusions using a pitch scale in between the thermal phonons mean free path and the electron mean free path. In such structures, the lattice thermal conductivity is reduced down to two orders of magnitude with respect to its bulk value. Beyond the promise held by these materials to overcome the well-known “electron crystal-phonon glass” dilemma faced in thermoelectrics, the quantitative prediction of their thermal conductivity poses a challenge. This work paves the way toward understanding and designing silicon nanophononic membranes by means of molecular dynamics simulation. Several systems are studied in order to distinguish the shape contribution from bulk, ultra-thin membranes (8 to 15 nm), 2D phononic crystals, and finally 2D phononic membranes. After having discussed the equilibrium properties of these structures from 300 K to 400 K, the Green-Kubo methodology is used to quantify the thermal conductivity. The results account for several experimental trends and models. It is confirmed that the thin-film geometry as well as the phononic structure act towards a reduction of the thermal conductivity. The further decrease in the phononic engineered membrane clearly demonstrates that both phenomena are cumulative. Finally, limitations of the model and further perspectives are discussed.
Liu, Yanhong; Li, La; Wang, Song; Gao, Ping; Pan, Lujun; Zhang, Jialiang; Zhou, Peng; Li, Jinhua; Weng, Zhankun
2015-02-09
In this paper, we discuss a model of sub-band in resistive switching nonvolatile memories with a structure of silver/aluminum oxide/p-type silicon (Ag/Al{sub x}O{sub y}/p-Si), in which the sub-band is formed by overlapping of wave functions of electron-occupied oxygen vacancies in Al{sub x}O{sub y} layer deposited by atomic layer deposition technology. The switching processes exhibit the characteristics of the bipolarity, discreteness, and no need of forming process, all of which are discussed deeply based on the model of sub-band. The relationships between the SET voltages and distribution of trap levels are analyzed qualitatively. The semiconductor-like behaviors of ON-state resistance affirm the sub-band transport mechanism instead of the metal filament mechanism.
Temperature dependence of phonon-defect interactions: phonon scattering vs. phonon trapping
Bebek, M. B.; Stanley, C. M.; Gibbons, T. M.; Estreicher, S. K.
2016-01-01
The interactions between thermal phonons and defects are conventionally described as scattering processes, an idea proposed almost a century ago. In this contribution, ab-initio molecular-dynamics simulations provide atomic-level insight into the nature of these interactions. The defect is the Si|X interface in a nanowire containing a δ-layer (X is C or Ge). The phonon-defect interactions are temperature dependent and involve the trapping of phonons for meaningful lengths of time in defect-related, localized, vibrational modes. No phonon scattering occurs and the momentum of the phonons released by the defect is unrelated to the momentum of the phonons that generated the excitation. The results are extended to the interactions involving only bulk phonons and to phonon-defect interactions at high temperatures. These do resemble scattering since phonon trapping occurs for a length of time short enough for the momentum of the incoming phonon to be conserved. PMID:27535463
Phonon squeezed states: quantum noise reduction in solids
NASA Astrophysics Data System (ADS)
Hu, Xuedong; Nori, Franco
1999-03-01
This article discusses quantum fluctuation properties of a crystal lattice, and in particular, phonon squeezed states. Squeezed states of phonons allow a reduction in the quantum fluctuations of the atomic displacements to below the zero-point quantum noise level of coherent phonon states. Here we discuss our studies of both continuous-wave and impulsive second-order Raman scattering mechanisms. The later approach was used to experimentally suppress (by one part in a million) fluctuations in phonons. We calculate the expectation values and fluctuations of both the atomic displacement and the lattice amplitude operators, as well as the effects of the phonon squeezed states on macroscopically measurable quantities, such as changes in the dielectric constant. These results are compared with recent experiments. Further information, including preprints and animations, are available in http://www-personal.engin.umich.edu/∼nori/squeezed.html.
Annealing-induced optical and sub-band-gap absorption parameters of Sn-doped CdSe thin films
NASA Astrophysics Data System (ADS)
Kaur, Jagdish; Tripathi, S. K.
2016-01-01
Thin films of Sn-doped CdSe were prepared by thermal evaporation onto glass substrates in an argon gas atmosphere and annealed at different temperatures. Structural evaluation of the films was carried out using X-ray diffraction and their stoichiometry studied by energy-dispersive X-ray analysis. The films exhibit a preferred orientation along the hexagonal direction of CdSe. The optical transmittance of the films shows a red shift of the absorption edge with annealing. The fundamental absorption edge corresponds to a direct energy gap with a temperature coefficient of 3.34 × 10-3 eV K-1. The refractive index, optical conductivity and real and imaginary parts of the dielectric constants were found to increase after annealing. The sub-band gap absorption coefficient was evaluated using the constant photocurrent method. It varies exponentially with photon energy. The Urbach energy, the density of defect states, and the steepness of the density of localized states were evaluated from the sub-band-gap absorption.
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.
The effect of n- and p-type doping on coherent phonons in GaN.
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.
Phonon waveguides for electromechanical circuits.
Hatanaka, D; Mahboob, I; Onomitsu, K; Yamaguchi, H
2014-07-01
Nanoelectromechanical systems (NEMS), utilizing localized mechanical vibrations, have found application in sensors, signal processors and in the study of macroscopic quantum mechanics. The integration of multiple mechanical elements via electrical or optical means remains a challenge in the realization of NEMS circuits. Here, we develop a phonon waveguide using a one-dimensional array of suspended membranes that offers purely mechanical means to integrate isolated NEMS resonators. We demonstrate that the phonon waveguide can support and guide mechanical vibrations and that the periodic membrane arrangement also creates a phonon bandgap that enables control of the phonon propagation velocity. Furthermore, embedding a phonon cavity into the phonon waveguide allows mobile mechanical vibrations to be dynamically switched or transferred from the waveguide to the cavity, thereby illustrating the viability of waveguide-resonator coupling. These highly functional traits of the phonon waveguide architecture exhibit all the components necessary to permit the realization of all-phononic NEMS circuits.
Surface Phonons and Polaritons.
1976-01-01
for an impurity in the surface of a crystal could be observed in the one phonon cross section for the resonant absorption or e.ission of ,—rays by...localized at the surface. The w5 — dependence has a simple physical origin. It is well known that the cross section for scattering of bulk phonons by a...propagate. In Section II of the present Chapter we present the theory underlying the surface induced vibrational properties of crystals which we have
Phonon dispersion in hypersonic two-dimensional phononic crystal membranes
NASA Astrophysics Data System (ADS)
Graczykowski, B.; Sledzinska, M.; Alzina, F.; Gomis-Bresco, J.; Reparaz, J. S.; Wagner, M. R.; Sotomayor Torres, C. M.
2015-02-01
We investigate experimentally and theoretically the acoustic phonon propagation in two-dimensional phononic crystal membranes. Solid-air and solid-solid phononic crystals were made of square lattices of holes and Au pillars in and on 250 nm thick single crystalline Si membrane, respectively. The hypersonic phonon dispersion was investigated using Brillouin light scattering. Volume reduction (holes) or mass loading (pillars) accompanied with second-order periodicity and local resonances are shown to significantly modify the propagation of thermally activated GHz phonons. We use numerical modeling based on the finite element method to analyze the experimental results and determine polarization, symmetry, or three-dimensional localization of observed modes.
Zarkevich, Nikolai
2014-11-24
ThermoPhonon is a stand-alone code, which can be integrated into other software packages. Typically, it is used together with a density functional theory (DFT) code (such as VASP, Wien2k, AbInit, SIESTA) and a phonon code (such as Phonopy or Phon). The workflow is the following. Molecular dynamics (MD) in a supercell at a given temperature T is performed using another code. After sufficient equilibration, the output in the form of atomic positions and forces for a large number of selected MD steps is recorded into a file. If needed, one can modify this file by applying additional constraints, such as enforced crystal symmetry or subtracted motion of the center of mass. ThermoPhonon reads the file with atomic positions and forces and writes a new file with the force constants. Force constants can be used by another code (such as Phonopy or Phon) to produce phonon spectrum for plotting, in the assumption of known equilibrium atomic positions provided in a separate file.
ERIC Educational Resources Information Center
Reid, John S.
1977-01-01
Discussed are how the thermal vibrations of a solid are described in terms of lattice waves, how these waves interact with other waves, or with themselves, and how one is led from such a description in terms of waves to the concept of a phonon. (Author/MA)
Li, Zenghui; Xu, Bin; Yang, Jian; Song, Jianshe
2015-01-01
This paper focuses on suppressing spectral overlap for sub-band spectral estimation, with which we can greatly decrease the computational complexity of existing spectral estimation algorithms, such as nonlinear least squares spectral analysis and non-quadratic regularized sparse representation. Firstly, our study shows that the nominal ability of the high-order analysis filter to suppress spectral overlap is greatly weakened when filtering a finite-length sequence, because many meaningless zeros are used as samples in convolution operations. Next, an extrapolation-based filtering strategy is proposed to produce a series of estimates as the substitutions of the zeros and to recover the suppression ability. Meanwhile, a steady-state Kalman predictor is applied to perform a linearly-optimal extrapolation. Finally, several typical methods for spectral analysis are applied to demonstrate the effectiveness of the proposed strategy. PMID:25609038
Li, Zenghui; Xu, Bin; Yang, Jian; Song, Jianshe
2014-12-24
This paper focuses on suppressing spectral overlap for sub-band spectral estimation, with which we can greatly decrease the computational complexity of existing spectral estimation algorithms, such as nonlinear least squares spectral analysis and non-quadratic regularized sparse representation. Firstly, our study shows that the nominal ability of the high-order analysis filter to suppress spectral overlap is greatly weakened when filtering a finite-length sequence, because many meaningless zeros are used as samples in convolution operations. Next, an extrapolation-based filtering strategy is proposed to produce a series of estimates as the substitutions of the zeros and to recover the suppression ability. Meanwhile, a steady-state Kalman predictor is applied to perform a linearly-optimal extrapolation. Finally, several typical methods for spectral analysis are applied to demonstrate the effectiveness of the proposed strategy.
Phonon properties of americium phosphide
NASA Astrophysics Data System (ADS)
Arya, B. S.; Aynyas, Mahendra; Sanyal, S. P.
2016-05-01
Phonon properties of AmP have been studied by using breathing shell models (BSM) which includes breathing motion of electrons of the Am atoms due to f-d hybridization. The phonon dispersion curves, specific heat calculated from present model. The calculated phonon dispersion curves of AmP are presented follow the same trend as observed in uranium phosphide. We discuss the significance of this approach in predicting the phonon dispersion curves of these compounds and examine the role of electron-phonon interaction.
Geometrical tuning of thermal phonon spectrum in nanoribbons
NASA Astrophysics Data System (ADS)
Ramiere, Aymeric; Volz, Sebastian; Amrit, Jay
2016-03-01
Phonon spectral energy transmission in silicon nanoribbons is investigated using Monte-Carlo simulations in the boundary scattering regime by changing the length and width geometrical parameters. We show that the transition frequency from specular scattering to diffuse scattering is inversely proportional to the edge roughness σ with a geometry independent factor of proportionality. The increase of the length over width ratio \\zeta leads to a decrease of the energy transmission in the diffuse scattering regime which evolves as {{≤ft(1+{{\\zeta}0.59}\\right)}-1} . This trend is explained by developing a model of phonon energy transmission in the fully diffuse scattering regime which takes into account the probability for a diffusively scattered phonon to be directly transmitted from any position on the edge of the nanoribbon. This model establishes the importance of the solid angles in the energy transmission evolution with \\zeta . The transition from unity energy transmission in the specular scattering regime to reduced transmission in the diffuse scattering regime constitutes a low-pass frequency filter for phonons. Our simulations show an energy rejection rate better than 90% for high \\zeta , which paves the way for potential high performance filters. Filtering out high frequency phonons is of significant interest for phononic crystal applications, which use band engineering of phonons in the wave regime with low frequencies.
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.
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.
Yoctocalorimetry: phonon counting in nanostructures
NASA Astrophysics Data System (ADS)
Roukes, M. L.
1999-03-01
It appears feasible with nanostructures to perform calorimetry at the level of individual thermal phonons. Here I outline an approach employing monocrystalline mesoscopic insulators, which can now be patterned from semiconductor heterostructures into complex geometries with full, three-dimensional relief. Successive application of these techniques also enables definition of integrated nanoscale thermal transducers; coupling these to a dc SQUID readout yields the requisite energy sensitivity and temporal resolution with minimal back action. The prospect of phonon counting opens intriguing experimental possibilities with analogies in quantum optics. These include fluctuation-based phonon spectroscopy, phonon shot noise in the energy relaxation of nanoscale systems, and quantum statistical phenomena such as phonon bunching and anticorrelated electron-phonon exchange.
Chattopadhyay, P.; Karim, B.; Guha Roy, S.
2013-12-28
The sub-band gap optical absorption in chemical bath deposited cadmium sulphide thin films annealed at different temperatures has been critically analyzed with special reference to Urbach relation. It has been found that the absorption co-efficient of the material in the sub-band gap region is nearly constant up to a certain critical value of the photon energy. However, as the photon energy exceeds the critical value, the absorption coefficient increases exponentially indicating the dominance of Urbach rule. The absorption coefficients in the constant absorption region and the Urbach region have been found to be sensitive to annealing temperature. A critical examination of the temperature dependence of the absorption coefficient indicates two different kinds of optical transitions to be operative in the sub-band gap region. After a careful analyses of SEM images, energy dispersive x-ray spectra, and the dc current-voltage characteristics, we conclude that the absorption spectra in the sub-band gap domain is possibly associated with optical transition processes involving deep levels and the grain boundary states of the material.
Estreicher, S. K. Gibbons, T. M.; Kang, By.; Bebek, M. B.
2014-01-07
Defects in semiconductors introduce vibrational modes that are distinct from bulk modes because they are spatially localized in the vicinity of the defect. Light impurities produce high-frequency modes often visible by Fourier-transform infrared absorption or Raman spectroscopy. Their vibrational lifetimes vary by orders of magnitude and sometimes exhibit unexpectedly large isotope effects. Heavy impurities introduce low-frequency modes sometimes visible as phonon replicas in photoluminescence bands. But other defects such as surfaces or interfaces exhibit spatially localized modes (SLMs) as well. All of them can trap phonons, which ultimately decay into lower-frequency bulk phonons. When heat flows through a material containing defects, phonon trapping at localized modes followed by their decay into bulk phonons is usually described in terms of phonon scattering: defects are assumed to be static scattering centers and the properties of the defect-related SLMs modes are ignored. These dynamic properties of defects are important. In this paper, we quantify the concepts of vibrational localization and phonon trapping, distinguish between normal and anomalous decay of localized excitations, discuss the meaning of phonon scattering in real space at the atomic level, and illustrate the importance of phonon trapping in the case of heat flow at Si/Ge and Si/C interfaces.
Phonons in twisted bilayer graphene
NASA Astrophysics Data System (ADS)
Cocemasov, Alexandr I.; Nika, Denis L.; Balandin, Alexander A.
2013-07-01
We theoretically investigate phonon dispersion in AA-stacked, AB-stacked, and twisted bilayer graphene with various rotation angles. The calculations are performed using the Born-von Karman model for the intralayer atomic interactions and the Lennard-Jones potential for the interlayer interactions. It is found that the stacking order affects the out-of-plane acoustic phonon modes the most. The difference in the phonon densities of states in the twisted bilayer graphene and in AA- or AB-stacked bilayer graphene appears in the phonon frequency range 90-110 cm-1. Twisting bilayer graphene leads to the emergence of different phonon branches—termed hybrid folded phonons—which originate from the mixing of phonon modes from different high-symmetry directions in the Brillouin zone. The frequencies of the hybrid folded phonons depend strongly on the rotation angle and can be used for noncontact identification of the twist angles in graphene samples. The obtained results and the tabulated frequencies of phonons in twisted bilayer graphene are important for the interpretation of experimental Raman data and in determining the thermal conductivity of these material systems.
NASA Astrophysics Data System (ADS)
Wong, Joe
2004-03-01
The phonon spectra of plutonium and its alloys have been sought after in the past few decades following the discovery of this actinide element in 1941, but with no success. This was due to a combination of the high neutron absorption cross section of 239Pu, the common isotope, and non-availability of large single crystals of any Pu-bearing materials. We have recent designed a high resolution inelastic x-ray scattering experiment using a bright synchrotron x-ray beam at the European Sychrotron Radiation Facility (ESRF), Grenoble and mapped the full phonon dispersion curves of an fcc delta-phase polycrystalline Pu-Ga alloy (1). Several unusual features including, a large elastic anisotropy, a small shear elastic modulus C', a Kohn-like anomaly in the T1[011] branch, and a pronounced softening of the [111] transverse modes are found. These features can be related to the phase transitions of plutonium and to strong coupling between the lattice structure and the 5f valence instabilities. Our results also provide a critical test for theoretical treatments of highly correlated 5f electron systems as exemplified by recent dynamical mean field theory (DMFT) calculations for d-plutonium.(2) This work was performed in collaboration with Dr. M. Krisch (ESRF)) and Prof. T.-C. Chiang (UIU), and under the auspices of the U. S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48. 1. Joe Wong et al. Science, vol.301, 1078 (2003) 2. X. Dai et al. Science, vol.300, 953 (2003)
Phonon spectra of alkali metals
NASA Astrophysics Data System (ADS)
Zeković, S.; Vukajlović, F.; Veljković, V.
1982-10-01
In this work we used a simple local model pseudopotential which includes screening for the phonon spectra calculations of alkali metals. The results obtained are in very good agreement with experimental data. In some branches of phonon spectra the differences between theoretical and experimental results are within 1-2%, while the maximum error is about 6%. The suggested form of the pseudopotential allows us to describe the phonon spectra of Na, K and Rb with only one, and, at the same time, a unique, parameter. In this case, the maximum disagreements from experiment are 9% for Na, 8% for K and 7% for Rb.
Dynamical Cooper pairing in nonequilibrium electron-phonon systems
NASA Astrophysics Data System (ADS)
Knap, Michael; Babadi, Mehrtash; Refael, Gil; Martin, Ivar; Demler, Eugene
2016-12-01
We analyze Cooper pairing instabilities in strongly driven electron-phonon systems. The light-induced nonequilibrium state of phonons results in a simultaneous increase of the superconducting coupling constant and the electron scattering. We demonstrate that the competition between these effects leads to an enhanced superconducting transition temperature in a broad range of parameters. Our results may explain the observed transient enhancement of superconductivity in several classes of materials upon irradiation with high intensity pulses of terahertz light, and may pave new ways for engineering high-temperature light-induced superconducting states.
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.
Wong, Joe; Krisch, M.; Farber, D.; Occelli, F.; Schwartz, A.; Chiang, T.C.; Wall, M.; Boro, C.; Xu, Ruqing
2010-11-16
Plutonium (Pu) is well known to have complex and unique physico-chemical properties. Notably, the pure metal exhibits six solid-state phase transformations with large volume expansions and contractions along the way to the liquid state: {alpha} {yields} {beta} {yields} {gamma} {yields} {delta} {yields} {delta}{prime} {yields} {var_epsilon} {yields} liquid. Unalloyed Pu melts at a relatively low temperature {approx}640 C to yield a higher density liquid than that of the solid from which it melts, (Figure 1). Detailed understanding of the properties of plutonium and plutonium-based alloys is critical for the safe handling, utilization, and long-term storage of these important, but highly toxic materials. However, both technical and and safety issues have made experimental observations extremely difficult. Phonon dispersion curves (PDCs) are key experimenta l data to the understanding of the basic properties of Pu materials such as: force constants, sound velocities, elastic constants, thermodynamics, phase stability, electron-phonon coupling, structural relaxation, etc. However, phonon dispersion curves (PDCs) in plutonium (Pu) and its alloys have defied measurement for the past few decades since the discovery of this element in 1941. This is due to a combination of the high thermal-neutron absorption cross section of plutonium and the inability to grow the large single crystals (with dimensions of a few millimeters) necessary for inelastic neutron scattering. Theoretical simulations of the Pu PDC continue to be hampered by the lack of suitable inter -atomic potentials. Thus, until recently the PDCs for Pu and its alloys have remained unknown experimentally and theoretically. The experimental limitations have recently been overcome by using a tightly focused undulator x-ray micro-beam scattered from single -grain domains in polycrystalline specimens. This experimental approach has been applied successfully to map the complete PDCs of an fcc d-Pu-Ga alloy using the
Weak Coupling Electron-Phonon for High Tc Superconductors
NASA Astrophysics Data System (ADS)
Labbe, J.
1989-01-01
Our opinion is that, in the high Tc copper oxides, the electronic correlations are not large enough to allow the localization of the electrons of the half-filled d-p sub-band. Thus, we treat them as itinerant electrons, in a bidimensional structure. And we show that, contrary to a widely held opinion, the electron-phonon interaction can induce high Tc superconductivity in these compounds, even in the weak coupling limit. This is due to the fact that, because of the bidimensionality, the electronic density of states is sharply peaked in the neighbourhood of the Fermi energy. A small coupling between nearest neighbouring CuO2 planes is sufficient to prevent a very large reduction of Tc by the critical fluctuations. The calculated isotope effect is much smaller than usually in the BCS theory. And, in our weak coupling theory, the antiferromagnetic (AF) phase is much more rapidly destabilized by dopping or internal charge transfer than the superconducting phase, which takes place when the AF phase has vanished.
Phonon dynamics of neptunium chalcogenides
NASA Astrophysics Data System (ADS)
Aynyas, Mahendra; Rukmangad, Aditi; Arya, Balwant S.; Sanyal, Sankar P.
2012-06-01
We have performed phonon calculations of Neptunium Chalcogenides (NpX) (X= S, Se, Te) based on breathing shell model (BSM) which includes breathing motion of electron of the Np-atoms due to f-d hybridization. The model predicts that the short range breathing phenomenon play a dominant role in the phonon properties. We also report, for the first time specific heat for these compounds.
NASA Astrophysics Data System (ADS)
Hong, Jiawang; Li, Chen W.; May, A. F.; Bansal, D.; Chi, S.; Hong, T.; Ehlers, G.; Delaire, Olivier
The promising thermoelectric material SnSe exhibits ultra-low and strongly anisotropic thermal conductivity. By combining first-principles calculations and inelastic neutron scattering measurements, we have investigated the phonon dispersions and phonon scattering mechanisms, and probed the origin of the large anharmonicity in SnSe. We will discuss the connection between the phonon properties and the high-temperature structural phase transition, and how the electronic structure leads to large anharmonic phonon interactions in SnSe. The present results provide a microscopic picture connecting electronic structure and phonon anharmonicity in SnSe, which could help design materials with ultralow thermal conductivity. Computations were performed using the OLCF at ORNL. Modeling of neutron data was performed in CAMM, measurements were funded by the US DOE, BES, Materials Science and Engineering Division.
Phonons and thermal transport in graphene and graphene-based materials
NASA Astrophysics Data System (ADS)
Nika, Denis L.; Balandin, Alexander A.
2017-03-01
A discovery of the unusual thermal properties of graphene stimulated experimental, theoretical and computational research directed at understanding phonon transport and thermal conduction in two-dimensional material systems. We provide a critical review of recent results in the graphene thermal field focusing on phonon dispersion, specific heat, thermal conductivity, and comparison of different models and computational approaches. The correlation between the phonon spectrum in graphene-based materials and the heat conduction properties is analyzed in details. The effects of the atomic plane rotations in bilayer graphene, isotope engineering, and relative contributions of different phonon dispersion branches are discussed. For readers’ convenience, the summaries of main experimental and theoretical results on thermal conductivity as well as phonon mode contributions to thermal transport are provided in the form of comprehensive annotated tables.
Coherent phonon control via electron-lattice interaction in ferromagnetic Co/Pt multilayers
Kim, Chul Hoon; Shim, Je-Ho; Lee, Kyung Min; Jeong, Jong-Ryul; Kim, Dong-Hyun; Kim, Dong Eon
2016-01-01
The manipulation of coherent phonons in condensed systems has attracted fundamental interest, particularly for its applications to future devices. We demonstrate that a coherent phonon in Co/Pt nano-multilayer can be quantitatively controlled via electron-lattice coupling, specifically by changing the multilayer repeat number. To that end, systematic measurement of the time-resolved reflectivity and magneto-optical Kerr effect in Co/Pt multilayers was performed. The coherent phonon frequency was observed to be shifted with the change of the multilayer repeat number. This shift could be clearly explained based on the two-temperature model. Detailed analysis indicated that the lattice heat capacity and electron-lattice coupling strength are linearly dependent on the repeat number of the periodic multilayer structures. Accessing the control of coherent phonons using nanostructures opens a new avenue for advanced phonon-engineering applications. PMID:26928846
NASA Astrophysics Data System (ADS)
Chen, Gang
In this talk, we will discuss different modes of heat conduction in nanostructures. Ballistic transport happens when phonon mean free path is longer than the characteristic size of the structure. We will discuss how we compute phonon mean free path distributions based on first-principles and measure the distributions with optical pump-probe techniques by exploring ballistic phonon transport processes. In superlattice structures, ballistic phonon transport across the whole thickness of the superlattices implies phase coherence. We observed this coherent transport in GaAs/AlAs superlattices with fixed periodic thickness and varying number of periods. Simulations show that although high frequency phonons are scattering by roughness, remaining long wavelength phonons maintain their phase and traverse the superlattices ballistically. Accessing the coherent heat conduction regime opens a new venue for phonon engineering. We show further that phonon heat conduction localization happens in GaAs/AlAs superlattice by placing ErAs nanodots at interfaces. This heat-conduction localization phenomenon is confirmed by nonequilibrium atomic Green's function simulation. These ballistic and localization effects can be exploited to improve thermoelectric energy conversion materials via reducing their thermal conductivity. In another opposite, we will discuss phonon hydrodynamic transport mode in graphene via first-principle simulations. In this mode, phonons drift with an average velocity under a temperature gradient, similar to fluid flow in a pipe. Conditions for observing such phonon hydrodynamic modes will be discussed. Finally, we will talk about the one-dimensional nature of heat conduction in polymer chains. Such 1D nature can lead to divergent thermal conductivity. Inspired by simulation, we have experimentally demonstrated high thermal conductivity in ultra-drawn polyethylene nanofibers and sheets. Work supported by DOE Office of Basic Energy Sciences under Award Number: DE
Malekiha, Mahdi; Tselniker, Igor; Nazarathy, Moshe; Tolmachev, Alex; Plant, David V
2015-10-05
We experimentally demonstrate a novel digital signal processing (DSP) structure for reduced guard-interval (RGI) OFDM coherent optical systems. The proposed concept is based on digitally slicing optical channel bandwidth into multiple spectrally disjoint sub-bands which are then processed in parallel. Each low bandwidth sub-band has a smaller delay-spread compared to a full-band signal. This enables compensation of both chromatic dispersion (CD) and polarization mode dispersion using a simple timing and one-tap-per-symbol frequency domain equalizer with a small cyclic prefix overhead. In terms of the DSP architecture, this allows for a highly efficient parallelization of DSP tasks performed over the received signal samples by deploying multiple processors running at a lower clock rate. It should be noted that this parallelization is performed in the frequency domain and it allows for flexible optical transceiver schemes. In addition, the resulting optical receiver is simplified due to the removal of the CD compensation equalizer compared to conventional RGI-OFDM systems. In this paper we experimentally demonstrate digital sub-banding of optical bandwidth. We test the system performance for different modulation formats (QPSK, 16QAM and 32QAM) over various transmission distances and optical launch powers using a 1.5% CP overhead in all scenarios. We also compare the proposed RGI-OFDM architecture performance against common single carrier modulation formats. At the same total data rate and signal bandwidth both systems have similar performance and transmission reach whereas the proposed method allows for a significant reduction of computational complexity due to removal of CD pre/post compensation equalizer.
Phonons in active microfluidic crystals
NASA Astrophysics Data System (ADS)
Tsang, Alan Cheng Hou; Kanso, Eva
2016-11-01
One-dimensional crystals of driven particles confined in quasi two-dimensional microfluidic channels have been shown to exhibit propagating sound waves in the form of 'phonons', including both transverse and longitudinal normal modes. Here, we focus on one-dimensional crystals of motile particles in uniform external flows. We study the propagation of phonons in the context of an idealized model that accounts for hydrodynamic interactions among the motile particles. We obtain a closed-form analytical expression for the dispersion relation of the phonons. In the moving frame of reference of the crystals, the traveling directions of the phonons depend on the intensity of the external flow, and are exactly opposite for the transverse and longitudinal modes. We further investigate the stability of the phonons and show that the longitudinal mode is linearly stable, whereas the transverse mode is subject to an instability arising from the activity and orientation dynamics of the motile particles. These findings are important for understanding the propagation of disturbances and instabilities in confined motile particles, and could generate practical insights into the transport of motile cells in microfluidic devices.
NASA Astrophysics Data System (ADS)
Banerjee, Debika; Trudeau, Charles; Gerlein, Luis Felipe; Cloutier, Sylvain G.
2016-03-01
The nanoscale engineering of silicon can significantly change its bulk optoelectronic properties to make it more favorable for device integration. Phonon process engineering is one way to enhance inter-band transitions in silicon's indirect band structure alignment. This paper demonstrates phonon localization at the tip of silicon nanowires fabricated by galvanic displacement using wet electroless chemical etching of a bulk silicon wafer. High-resolution Raman micro-spectroscopy reveals that such arrayed structures of silicon nanowires display phonon localization behaviors, which could help their integration into the future generations of nano-engineered silicon nanowire-based devices such as photodetectors and solar cells.
NASA Astrophysics Data System (ADS)
Yater, J. E.; Shaw, J. L.; Pate, B. B.; Feygelson, T. I.
2016-02-01
Secondary-electron-emission (SEE) current measured from high-purity, single-crystal (100) chemical-vapor-deposited diamond is found to increase when sub-band gap (3.06 eV) photons are incident on the hydrogenated surface. Although the light does not produce photoemission directly, the SEE current increases by more than a factor of 2 before saturating with increasing laser power. In energy distribution curves (EDCs), the emission peak shows a corresponding increase in intensity with increasing laser power. However, the emission-onset energy in the EDCs remains constant, indicating that the bands are pinned at the surface. On the other hand, changes are observed on the high-energy side of the distribution as the laser power increases, with a well-defined shoulder becoming more pronounced. From an analysis of this feature in the EDCs, it is deduced that upward band bending is present in the near-surface region during the SEE measurements and this band bending suppresses the SEE yield. However, sub-band gap photon illumination reduces the band bending and thereby increases the SEE current. Because the bands are pinned at the surface, we conclude that the changes in the band levels occur below the surface in the electron transport region. Sample heating produces similar effects as observed with sub-band gap photon illumination, namely, an increase in SEE current and a reduction in band bending. However, the upward band bending is not fully removed by either increasing laser power or temperature, and a minimum band bending of ˜0.8 eV is established in both cases. The sub-band gap photo-excitation mechanism is under further investigation, although it appears likely at present that defect or gap states play a role in the photo-enhanced SEE process. In the meantime, the study demonstrates the ability of visible light to modify the electronic properties of diamond and enhance the emission capabilities, which may have potential impact for diamond-based vacuum electron
Yater, J. E. Shaw, J. L.; Pate, B. B.; Feygelson, T. I.
2016-02-07
Secondary-electron-emission (SEE) current measured from high-purity, single-crystal (100) chemical-vapor-deposited diamond is found to increase when sub-band gap (3.06 eV) photons are incident on the hydrogenated surface. Although the light does not produce photoemission directly, the SEE current increases by more than a factor of 2 before saturating with increasing laser power. In energy distribution curves (EDCs), the emission peak shows a corresponding increase in intensity with increasing laser power. However, the emission-onset energy in the EDCs remains constant, indicating that the bands are pinned at the surface. On the other hand, changes are observed on the high-energy side of the distribution as the laser power increases, with a well-defined shoulder becoming more pronounced. From an analysis of this feature in the EDCs, it is deduced that upward band bending is present in the near-surface region during the SEE measurements and this band bending suppresses the SEE yield. However, sub-band gap photon illumination reduces the band bending and thereby increases the SEE current. Because the bands are pinned at the surface, we conclude that the changes in the band levels occur below the surface in the electron transport region. Sample heating produces similar effects as observed with sub-band gap photon illumination, namely, an increase in SEE current and a reduction in band bending. However, the upward band bending is not fully removed by either increasing laser power or temperature, and a minimum band bending of ∼0.8 eV is established in both cases. The sub-band gap photo-excitation mechanism is under further investigation, although it appears likely at present that defect or gap states play a role in the photo-enhanced SEE process. In the meantime, the study demonstrates the ability of visible light to modify the electronic properties of diamond and enhance the emission capabilities, which may have potential impact for diamond-based vacuum electron
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.
NASA Astrophysics Data System (ADS)
Yang, Jia-Yue; Qin, Guangzhao; Hu, Ming
2016-12-01
The macroscopic thermal transport is fundamentally determined by the intrinsic interactions among microscopic electrons and phonons. In conventional insulators and semiconductors, phonons dominate the thermal transport, and the contribution of electron-phonon interaction (EPI) is negligible. However, in polar semiconductors, the Fröhlich electron-phonon coupling is strong and its influence on phononic thermal transport is of great significance. In this work, the effect of EPI on phonon dispersion and lattice thermal conductivity of wurtzite gallium nitride (GaN) is comprehensively investigated from the atomistic level by performing first-principles calculations. Due to the existence of relatively large electronegativity difference between Ga and N atoms, the Fröhlich coupling in wurtzite GaN is remarkably strong. Consequently, the lattice thermal conductivity of natural wurtzite GaN at room temperature is reduced by ˜24%-34% when including EPI, and the resulted thermal conductivity value is in better agreement with experiments. Furthermore, the scattering rate of phonons due to EPI, the intrinsic phonon-phonon interaction (PPI) as well as isotope disorder is computed and analyzed. It shows that the EPI scattering rate is comparable to PPI for low-frequency heat-carrying phonons. This work attempts to explore the mechanism of thermal transport beyond intrinsic PPI for polar semiconductors, with a great potential of thermal conductivity engineering for desired performance.
Phonon dynamics of americium telluride
NASA Astrophysics Data System (ADS)
Arya, B. S.; Aynyas, Mahendra; Ahirwar, Ashok K.; Sanyal, S. P.
2013-06-01
We report for the first time the complete phonon dispersion curves for Americium telluride (AmTe) using a breathing shell models (BSM) to establish their predominant ionic nature. The results obtained in the present study show the general features of the phonon spectrum. We could not compare our results with the experimental measurements as they are not available so far. We emphasize the need of neutron scattering measurements to compare our results. We also report, for the first time specific heat for this compound.
Scattering of phonons by vacancies
Ratsifaritana, C.A.; Klemens, P.G.
1987-11-01
The scattering of phonons by vacancies is estimated by a perturbation technique in terms of the missing mass and the missing linkages. An argument is given why distortion effects can be disregarded. The resonance frequency of the defect is sufficiently high so that resonance effects can be disregarded for phonons in the important frequency range for thermal conduction. The theory is applied to the thermal resistance by vacancies in cases where the vacancy concentration is known: potassium chloride with divalent cations, nonstoichiometric zirconium carbide, and tin telluride.
Cavity-type hypersonic phononic crystals
NASA Astrophysics Data System (ADS)
Sato, A.; Pennec, Y.; Yanagishita, T.; Masuda, H.; Knoll, W.; Djafari-Rouhani, B.; Fytas, G.
2012-11-01
We report on the engineering of the phonon dispersion diagram in monodomain anodic porous alumina (APA) films through the porosity and physical state of the material residing in the nanopores. Lattice symmetry and inclusion materials are theoretically identified to be the main factors which control the hypersonic acoustic wave propagation. This involves the interaction between the longitudinal and the transverse modes in the effective medium and a flat band characteristic of the material residing in the cavities. Air and filled nanopores, therefore, display markedly different dispersion relations and the inclusion materials lead to a locally resonant structural behavior uniquely determining their properties under confinement. APA films emerge as a new platform to investigate the rich acoustic phenomena of structured composite matter.
Sound and heat revolutions in phononics.
Maldovan, Martin
2013-11-14
The phonon is the physical particle representing mechanical vibration and is responsible for the transmission of everyday sound and heat. Understanding and controlling the phononic properties of materials provides opportunities to thermally insulate buildings, reduce environmental noise, transform waste heat into electricity and develop earthquake protection. Here I review recent progress and the development of new ideas and devices that make use of phononic properties to control both sound and heat. Advances in sonic and thermal diodes, optomechanical crystals, acoustic and thermal cloaking, hypersonic phononic crystals, thermoelectrics, and thermocrystals herald the next technological revolution in phononics.
Nanoscale control of phonon excitations in graphene
Kim, Hyo Won; Ko, Wonhee; Ku, JiYeon; Jeon, Insu; Kim, Donggyu; Kwon, Hyeokshin; Oh, Youngtek; Ryu, Seunghwa; Kuk, Young; Hwang, Sung Woo; Suh, Hwansoo
2015-01-01
Phonons, which are collective excitations in a lattice of atoms or molecules, play a major role in determining various physical properties of condensed matter, such as thermal and electrical conductivities. In particular, phonons in graphene interact strongly with electrons; however, unlike in usual metals, these interactions between phonons and massless Dirac fermions appear to mirror the rather complicated physics of those between light and relativistic electrons. Therefore, a fundamental understanding of the underlying physics through systematic studies of phonon interactions and excitations in graphene is crucial for realising graphene-based devices. In this study, we demonstrate that the local phonon properties of graphene can be controlled at the nanoscale by tuning the interaction strength between graphene and an underlying Pt substrate. Using scanning probe methods, we determine that the reduced interaction due to embedded Ar atoms facilitates electron–phonon excitations, further influencing phonon-assisted inelastic electron tunnelling. PMID:26109454
Isotope scattering and phonon thermal conductivity in light atom compounds: LiH and LiF
NASA Astrophysics Data System (ADS)
Lindsay, L.
2016-11-01
Engineered isotope variation is a pathway toward modulating lattice thermal conductivity (κ) of a material through changes in phonon-isotope scattering. The effects of isotope variation on intrinsic thermal resistance is little explored, as varying isotopes have relatively small differences in mass and thus do not affect bulk phonon dispersions. However, for light elements, isotope mass variation can be relatively large (e.g., hydrogen and deuterium). Using a first principles Peierls-Boltzmann transport equation approach, the effects of isotope variance on lattice thermal transport in ultra-low-mass compound materials LiH and LiF are characterized. The isotope mass variance modifies the intrinsic thermal resistance via modulation of acoustic and optic phonon frequencies, while phonon-isotope scattering from mass disorder plays only a minor role. This leads to some unusual cases where κ values of isotopically pure systems (6LiH ,7L i2H , and 6LiF ) are lower than the values from their counterparts with naturally occurring isotopes and phonon-isotope scattering. However, these κ differences are relatively small. The effects of temperature-driven lattice expansion on phonon dispersions and calculated κ are also discussed. This paper provides insight into lattice thermal conductivity modulation with mass variation and the interplay of intrinsic phonon-phonon and phonon-isotope scattering in interesting light atom systems.
Isotope scattering and phonon thermal conductivity in light atom compounds: LiH and LiF
Lindsay, Lucas R.
2016-11-08
Engineered isotope variation is a pathway toward modulating lattice thermal conductivity (κ) of a material through changes in phonon-isotope scattering. The effects of isotope variation on intrinsic thermal resistance is little explored, as varying isotopes have relatively small differences in mass and thus do not affect bulk phonon dispersions. However, for light elements isotope mass variation can be relatively large (e.g., hydrogen and deuterium). Using a first principles Peierls-Boltzmann transport equation approach the effects of isotope variance on lattice thermal transport in ultra-low-mass compound materials LiH and LiF are characterized. The isotope mass variance modifies the intrinsic thermal resistance viamore » modulation of acoustic and optic phonon frequencies, while phonon-isotope scattering from mass disorder plays only a minor role. This leads to some unusual cases where values of isotopically pure systems (6LiH, 7Li2H and 6LiF) are lower than the values from their counterparts with naturally occurring isotopes and phonon-isotope scattering. However, these differences are relatively small. The effects of temperature-driven lattice expansion on phonon dispersions and calculated κ are also discussed. This work provides insight into lattice thermal conductivity modulation with mass variation and the interplay of intrinsic phonon-phonon and phonon-isotope scattering in interesting light atom systems.« less
Isotope scattering and phonon thermal conductivity in light atom compounds: LiH and LiF
Lindsay, Lucas R.
2016-11-08
Engineered isotope variation is a pathway toward modulating lattice thermal conductivity (κ) of a material through changes in phonon-isotope scattering. The effects of isotope variation on intrinsic thermal resistance is little explored, as varying isotopes have relatively small differences in mass and thus do not affect bulk phonon dispersions. However, for light elements isotope mass variation can be relatively large (e.g., hydrogen and deuterium). Using a first principles Peierls-Boltzmann transport equation approach the effects of isotope variance on lattice thermal transport in ultra-low-mass compound materials LiH and LiF are characterized. The isotope mass variance modifies the intrinsic thermal resistance via modulation of acoustic and optic phonon frequencies, while phonon-isotope scattering from mass disorder plays only a minor role. This leads to some unusual cases where values of isotopically pure systems (^{6}LiH, ^{7}Li^{2}H and ^{6}LiF) are lower than the values from their counterparts with naturally occurring isotopes and phonon-isotope scattering. However, these differences are relatively small. The effects of temperature-driven lattice expansion on phonon dispersions and calculated κ are also discussed. This work provides insight into lattice thermal conductivity modulation with mass variation and the interplay of intrinsic phonon-phonon and phonon-isotope scattering in interesting light atom systems.
Phonon assisted resonant tunneling and its phonons control
NASA Astrophysics Data System (ADS)
Kusmartsev, F. V.; Krevchik, V. D.; Semenov, M. B.; Filatov, D. O.; Shorokhov, A. V.; Bukharaev, A. A.; Dakhnovsky, Y.; Nikolaev, A. V.; Pyataev, N. A.; Zaytsev, R. V.; Krevchik, P. V.; Egorov, I. A.; Yamamoto, K.; Aringazin, A. K.
2016-09-01
We observe a series of sharp resonant features in the tunneling differential conductance of InAs quantum dots. We found that dissipative quantum tunneling has a strong influence on the operation of nanodevices. Because of such tunneling the current-voltage characteristics of tunnel contact created between atomic force microscope tip and a surface of InAs/GaAs quantum dots display many interesting peaks. We found that the number, position, and heights of these peaks are associated with the phonon modes involved. To describe the found effect we use a quasi-classical approximation. There the tunneling current is related to a creation of a dilute instanton-anti-instanton gas. Our experimental data are well described with exactly solvable model where one charged particle is weakly interacting with two promoting phonon modes associated with external medium. We conclude that the characteristics of the tunnel nanoelectronic devices can thus be controlled by a proper choice of phonons existing in materials, which are involved.
NASA Astrophysics Data System (ADS)
Feng, Tianli; Ruan, Xiulin; Ye, Zhenqiang; Cao, Bingyang
2015-06-01
The spectral phonon properties in defected graphene have been unclear due to the lack of advanced techniques for predicting the phonon-defect scattering rate without fitting parameters. Taking advantage of the extended phonon normal mode analysis, we obtained the spectral phonon relaxation time and mean free path (MFP) in defected graphene and studied the impacts of three common types of defects: Stone-Thrower-Wales (STW) defect, double vacancy (DV), and monovacancy (MV). The phonon-STW defect scattering rate is found to have no significant frequency dependence, and as a result, the relative contribution of long-wavelength phonons sharply decreases. In contrast, the phonon scattering by DVs or MVs exhibits a frequency dependence of τp-d -1˜ω1.1 -1.3 except for a few long-wavelength phonons, revisiting the traditionally used ˜ω4 dependence. We note that although MV-defected graphene has the lowest thermal conductivity as compared to the other two defected graphene samples at the same defect concentration, it has a portion of phonons with the longest MFP. The contribution from the long-MFP and long-wavelength phonons does not decrease much as the vacancy concentration increases. STW defect and MV block more out-of-plane modes than in-plane modes, while DV has less bias for which mode to block. As the MV concentration increases from 0 to 1.1%, the relative contribution from out-of-plane modes decreases from 30% to 18%, while that of the transverse acoustic mode remains at around 30%. These findings of spectral phonon properties can provide more insight than the effective properties and benefit the prospective phononic engineering.
Edge phonons in black phosphorus
Ribeiro, H. B.; Villegas, C. E. P.; Bahamon, D. A.; Muraca, D.; Castro Neto, A. H.; de Souza, E. A. T.; Rocha, A. R.; Pimenta, M. A.; de Matos, C. J. S.
2016-01-01
Black phosphorus has recently emerged as a new layered crystal that, due to its peculiar and anisotropic crystalline and electronic band structures, may have important applications in electronics, optoelectronics and photonics. Despite the fact that the edges of layered crystals host a range of singular properties whose characterization and exploitation are of utmost importance for device development, the edges of black phosphorus remain poorly characterized. In this work, the atomic structure and behaviour of phonons near different black phosphorus edges are experimentally and theoretically studied using Raman spectroscopy and density functional theory calculations. Polarized Raman results show the appearance of new modes at the edges of the sample, and their spectra depend on the atomic structure of the edges (zigzag or armchair). Theoretical simulations confirm that the new modes are due to edge phonon states that are forbidden in the bulk, and originated from the lattice termination rearrangements. PMID:27412813
Edge phonons in black phosphorus
NASA Astrophysics Data System (ADS)
Ribeiro, H. B.; Villegas, C. E. P.; Bahamon, D. A.; Muraca, D.; Castro Neto, A. H.; de Souza, E. A. T.; Rocha, A. R.; Pimenta, M. A.; de Matos, C. J. S.
2016-07-01
Black phosphorus has recently emerged as a new layered crystal that, due to its peculiar and anisotropic crystalline and electronic band structures, may have important applications in electronics, optoelectronics and photonics. Despite the fact that the edges of layered crystals host a range of singular properties whose characterization and exploitation are of utmost importance for device development, the edges of black phosphorus remain poorly characterized. In this work, the atomic structure and behaviour of phonons near different black phosphorus edges are experimentally and theoretically studied using Raman spectroscopy and density functional theory calculations. Polarized Raman results show the appearance of new modes at the edges of the sample, and their spectra depend on the atomic structure of the edges (zigzag or armchair). Theoretical simulations confirm that the new modes are due to edge phonon states that are forbidden in the bulk, and originated from the lattice termination rearrangements.
Phononic crystals of poroelastic spheres
NASA Astrophysics Data System (ADS)
Alevizaki, A.; Sainidou, R.; Rembert, P.; Morvan, B.; Stefanou, N.
2016-11-01
An extension of the layer-multiple-scattering method to phononic crystals of poroelastic spheres immersed in a fluid medium is developed. The applicability of the method is demonstrated on specific examples of close-packed fcc crystals of submerged water-saturated meso- and macroporous silica microspheres. It is shown that, by varying the pore size and/or the porosity, the transmission, reflection, and absorption spectra of finite slabs of these crystals are significantly altered. Strong absorption, driven by the slow waves in the poroelastic material and enhanced by multiple scattering, leads to negligible transmittance over an extended frequency range, which might be useful for practical applications in broadband acoustic shielding. The results are analyzed by reference to relevant phononic dispersion diagrams in the viscous and inertial coupling limits, and a consistent interpretation of the underlying physics is provided.
Measuring phonons in protein crystals
NASA Astrophysics Data System (ADS)
Niessen, Katherine A.; Snell, Edward; Markelz, A. G.
2013-03-01
Using Terahertz near field microscopy we find orientation dependent narrow band absorption features for lysozyme crystals. Here we discuss identification of protein collective modes associated with the observed features. Using normal mode calculations we find good agreement with several of the measured features, suggesting that the modes arise from internal molecular motions and not crystal phonons. Such internal modes have been associated with protein function.
Konstantopoulou, A; Sgouros, A P; Sigalas, M M
2017-03-15
Using molecular dynamics and semi-empirical potentials, large scale transition metal dichalcogenides monolayers (TMDM) were examined. The focus of the study was the modification of the phonon spectrum of TMDMs by engineering substitutional defects to produce phononic resonators and waveguides on the atomic scale. The resonant frequencies of the aforementioned structures can be tuned by applying tensile or compressive stresses. The TMDMs exhibited wide phononic band gaps (PBG) in their phonon spectrum because they consist of atoms with quite different atomic masses. The PBG from the present semi-empirical calculations were found to be in reasonable agreement with previous ab initio calculations. The problem is very broad since many varieties of TMDMs (with or without defects) can be made. The present study focused on MX2 composites with M being Mo or W and X being S or Se. The most interesting behavior was found in WS2 with substitutional defects of either S ↔ Se or W ↔ Mo.
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.
Scanning Tunneling Microscopy Observation of Phonon Condensate.
Altfeder, Igor; Voevodin, Andrey A; Check, Michael H; Eichfeld, Sarah M; Robinson, Joshua A; Balatsky, Alexander V
2017-02-22
Using quantum tunneling of electrons into vibrating surface atoms, phonon oscillations can be observed on the atomic scale. Phonon interference patterns with unusually large signal amplitudes have been revealed by scanning tunneling microscopy in intercalated van der Waals heterostructures. Our results show that the effective radius of these phonon quasi-bound states, the real-space distribution of phonon standing wave amplitudes, the scattering phase shifts, and the nonlinear intermode coupling strongly depend on the presence of defect-induced scattering resonance. The observed coherence of these quasi-bound states most likely arises from phase- and frequency-synchronized dynamics of all phonon modes, and indicates the formation of many-body condensate of optical phonons around resonant defects. We found that increasing the strength of the scattering resonance causes the increase of the condensate droplet radius without affecting the condensate fraction inside it. The condensate can be observed at room temperature.
Scanning Tunneling Microscopy Observation of Phonon Condensate
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
Coherent Phonon Rabi Oscillations with a High-Frequency Carbon Nanotube Phonon Cavity.
Zhu, Dong; Wang, Xin-He; Kong, Wei-Cheng; Deng, Guang-Wei; Wang, Jiang-Tao; Li, Hai-Ou; Cao, Gang; Xiao, Ming; Jiang, Kai-Li; Dai, Xing-Can; Guo, Guang-Can; Nori, Franco; Guo, Guo-Ping
2017-02-08
Phonon-cavity electromechanics allows the manipulation of mechanical oscillations similar to photon-cavity systems. Many advances on this subject have been achieved in various materials. In addition, the coherent phonon transfer (phonon Rabi oscillations) between the phonon cavity mode and another oscillation mode has attracted many interest in nanoscience. Here, we demonstrate coherent phonon transfer in a carbon nanotube phonon-cavity system with two mechanical modes exhibiting strong dynamical coupling. The gate-tunable phonon oscillation modes are manipulated and detected by extending the red-detuned pump idea of photonic cavity electromechanics. The first- and second-order coherent phonon transfers are observed with Rabi frequencies 591 and 125 kHz, respectively. The frequency quality factor product fQm ∼ 2 × 10(12) Hz achieved here is larger than kBTbase/h, which may enable the future realization of Rabi oscillations in the quantum regime.
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.
Electron-phonon interactions from first principles
NASA Astrophysics Data System (ADS)
Giustino, Feliciano
2017-01-01
This article reviews the theory of electron-phonon interactions in solids from the point of view of ab initio calculations. While the electron-phonon interaction has been studied for almost a century, predictive nonempirical calculations have become feasible only during the past two decades. Today it is possible to calculate from first principles many materials properties related to the electron-phonon interaction, including the critical temperature of conventional superconductors, the carrier mobility in semiconductors, the temperature dependence of optical spectra in direct and indirect-gap semiconductors, the relaxation rates of photoexcited carriers, the electron mass renormalization in angle-resolved photoelectron spectra, and the nonadiabatic corrections to phonon dispersion relations. In this article a review of the theoretical and computational framework underlying modern electron-phonon calculations from first principles as well as landmark investigations of the electron-phonon interaction in real materials is given. The first part of the article summarizes the elementary theory of electron-phonon interactions and their calculations based on density-functional theory. The second part discusses a general field-theoretic formulation of the electron-phonon problem and establishes the connection with practical first-principles calculations. The third part reviews a number of recent investigations of electron-phonon interactions in the areas of vibrational spectroscopy, photoelectron spectroscopy, optical spectroscopy, transport, and superconductivity.
Phonon-assisted transient electroluminescence in Si
Cheng, Tzu-Huan; Chu-Su, Yu; Liu, Chien-Sheng; Lin, Chii-Wann
2014-06-30
The phonon-replica infrared emission is observed at room temperature from indirect band gap Si light-emitting diode under forward bias. With increasing injection current density, the broadened electroluminescence spectrum and band gap reduction are observed due to joule heating. The spectral-resolved temporal response of electroluminescence reveals the competitiveness between single (TO) and dual (TO + TA) phonon-assisted indirect band gap transitions. As compared to infrared emission with TO phonon-replica, the retarder of radiative recombination at long wavelength region (∼1.2 μm) indicates lower transition probability of dual phonon-replica before thermal equivalent.
Effect of Rattling Phonons on Sommerfeld Constant
NASA Astrophysics Data System (ADS)
Hotta, Takashi
2008-10-01
By employing a numerical renormalization group technique, we evaluate electronic specific heat coefficient γ of the Anderson model coupled with local anharmonic phonons for the oscillation of a caged atom. For the rattling-type cage potential with a flat and wide region in the bottom, we find that phonon-mediated attraction is largely enhanced. When the potential shape is deformed from the rattling type, there occurs a cancellation between Coulomb repulsion and the phonon-mediated attraction. In such a situation, spin and charge fluctuations are comparable to each other, leading to the realization of exotic electron-phonon complex state with large and magnetically robust γ.
Hypersonic phonon propagation in one-dimensional surface phononic crystal
NASA Astrophysics Data System (ADS)
Graczykowski, B.; Sledzinska, M.; Kehagias, N.; Alzina, F.; Reparaz, J. S.; Sotomayor Torres, C. M.
2014-03-01
Hypersonic, thermally activated surface acoustic waves propagating in the surface of crystalline silicon patterned with periodic stripes were studied by Brillouin light scattering. Two characteristic directions (normal and parallel to the stripes) of surface acoustic waves propagation were examined exhibiting a distinctive propagation behavior. The measured phononic band structure exhibits diverse features, such as zone folding, band gap opening, and hybridization to local resonance for waves propagating normal to the stripes, and a variety of dispersive modes propagating along the stripes. Experimental results were supported by theoretical calculations performed using finite element method.
Splash, pop, sizzle: Information processing with phononic computing
Sklan, Sophia R.
2015-05-15
Phonons, the quanta of mechanical vibration, are important to the transport of heat and sound in solid materials. Recent advances in the fundamental control of phonons (phononics) have brought into prominence the potential role of phonons in information processing. In this review, the many directions of realizing phononic computing and information processing are examined. Given the relative similarity of vibrational transport at different length scales, the related fields of acoustic, phononic, and thermal information processing are all included, as are quantum and classical computer implementations. Connections are made between the fundamental questions in phonon transport and phononic control and the device level approach to diodes, transistors, memory, and logic. .
Microfabricated phononic crystal devices and applications
NASA Astrophysics Data System (ADS)
Olsson, R. H., III; El-Kady, I.
2009-01-01
Phononic crystals are the acoustic wave analogue of photonic crystals. Here a periodic array of scattering inclusions located in a homogeneous host material forbids certain ranges of acoustic frequencies from existence within the crystal, thus creating what are known as acoustic bandgaps. The majority of previously reported phononic crystal devices have been constructed by hand, assembling scattering inclusions in a viscoelastic medium, predominantly air, water or epoxy, resulting in large structures limited to frequencies below 1 MHz. Recently, phononic crystals and devices have been scaled to VHF (30-300 MHz) frequencies and beyond by utilizing microfabrication and micromachining technologies. This paper reviews recent developments in the area of micro-phononic crystals including design techniques, material considerations, microfabrication processes, characterization methods and reported device structures. Micro-phononic crystal devices realized in low-loss solid materials are emphasized along with their potential application in radio frequency communications and acoustic imaging for medical ultrasound and nondestructive testing. The reported advances in batch micro-phononic crystal fabrication and simplified testing promise not only the deployment of phononic crystals in a number of commercial applications but also greater experimentation on a wide variety of phononic crystal structures.
Enhanced electron-phonon coupling for a semiconductor charge qubit in a surface phonon cavity
Chen, J. C. H.; Sato, Y.; Kosaka, R.; Hashisaka, M.; Muraki, K.; Fujisawa, T.
2015-01-01
Electron-phonon coupling is a major decoherence mechanism, which often causes scattering and energy dissipation in semiconductor electronic systems. However, this electron-phonon coupling may be used in a positive way for reaching the strong or ultra-strong coupling regime in an acoustic version of the cavity quantum electrodynamic system. Here we propose and demonstrate a phonon cavity for surface acoustic waves, which is made of periodic metal fingers that constitute Bragg reflectors on a GaAs/AlGaAs heterostructure. Phonon band gap and cavity phonon modes are identified by frequency, time and spatially resolved measurements of the piezoelectric potential. Tunneling spectroscopy on a double quantum dot indicates the enhancement of phonon assisted transitions in a charge qubit. This encourages studying of acoustic cavity quantum electrodynamics with surface phonons. PMID:26469629
Phonon Analysis in Multiphonon Transitions
NASA Astrophysics Data System (ADS)
Huang, Kun; Gu, Zongquan
In the investigation of multiphonon transitions, single-mode or single-frequency models are widely used. In view of the fact that such oversimplified models can be seriously inadequate, the present work bridges the gap between the complexity of the general formal theory and the simplicity required for concrete applications by introducing the concept of multi-frequency models. That is, the theory is so formulated that a general system can be approximated by multi-frequency models of any degree of elaboration. A statistical thermodynamic formalism is developed for treating such multi-frequency models, which, on the one hand, greatly reduces the labour of calculation with such models and, on the other hand, leads directly to a simple statistical distribution law for numbers of phonons of each frequency participating in a multiphonon transition. Applications of the theory to concrete models lead to certain general conclusions on frequency dispersion effects in multiphonon transitions. The use of the theory is further demonstrated by fully accounting for the paradoxical experimental results reported by Jia and Yen that the isotopic substitution of H by D in CsMn Cl3· 2H2O reduces the multiphonon nonradiative transition probability of excited Mn2+ ion by more than ten-fold, and yet leaves the corresponding luminescence phonon sideband little changed. In the last section of the paper, the relation between the statistical thermodynamic formalism and existing multiphonon transition theory is elucidated, thereby the theoretical basis of the statistical formalism becomes clearly defined.
Lattice Boltzmann modeling of phonon transport
NASA Astrophysics Data System (ADS)
Guo, Yangyu; Wang, Moran
2016-06-01
A novel lattice Boltzmann scheme is proposed for phonon transport based on the phonon Boltzmann equation. Through the Chapman-Enskog expansion, the phonon lattice Boltzmann equation under the gray relaxation time approximation recovers the classical Fourier's law in the diffusive limit. The numerical parameters in the lattice Boltzmann model are therefore rigorously correlated to the bulk material properties. The new scheme does not only eliminate the fictitious phonon speed in the diagonal direction of a square lattice system in the previous lattice Boltzmann models, but also displays very robust performances in predicting both temperature and heat flux distributions consistent with analytical solutions for diverse numerical cases, including steady-state and transient, macroscale and microscale, one-dimensional and multi-dimensional phonon heat transport. This method may provide a powerful numerical tool for deep studies of nonlinear and nonlocal heat transports in nanosystems.
Phononic crystals and elastodynamics: Some relevant points
Aravantinos-Zafiris, N.; Sigalas, M. M.; Kafesaki, M.; Economou, E. N.
2014-12-15
In the present paper we review briefly some of the first works on wave propagation in phononic crystals emphasizing the conditions for the creation of acoustic band-gaps and the role of resonances to the band-gap creation. We show that useful conclusions in the analysis of phononic band gap structures can be drawn by considering the mathematical similarities of the basic classical wave equation (Helmholtz equation) with Schrödinger equation and by employing basic solid state physics concepts and conclusions regarding electronic waves. In the second part of the paper we demonstrate the potential of phononic systems to be used as elastic metamaterials. This is done by demonstrating negative refraction in phononic crystals and subwavelength waveguiding in a linear chain of elastic inclusions, and by proposing a novel structure with close to pentamode behavior. Finally the potential of phononic structures to be used in liquid sensor applications is discussed and demonstrated.
Ultrafast Structure Switching through Nonlinear Phononics
NASA Astrophysics Data System (ADS)
Juraschek, D. M.; Fechner, M.; Spaldin, N. A.
2017-02-01
We describe a mechanism by which nonlinear phononics allows ultrafast coherent and directional control of transient structural distortions. With ErFeO3 as a model system, we use density functional theory to calculate the structural properties as input into an anharmonic phonon model that describes the response of the system to a pulsed optical excitation. We find that the trilinear coupling of two orthogonal infrared-active phonons to a Raman-active phonon causes a transient distortion of the lattice. In contrast to the quadratic-linear coupling that has been previously explored, the direction of the distortion is determined by the polarization of the exciting light, introducing a novel mechanism for nonlinear phononic switching. Since the occurrence of the coupling is determined by the symmetry of the system we propose that it is a universal feature of orthorhombic and tetragonal perovskites.
Phononic crystals and elastodynamics: Some relevant points
NASA Astrophysics Data System (ADS)
Aravantinos-Zafiris, N.; Sigalas, M. M.; Kafesaki, M.; Economou, E. N.
2014-12-01
In the present paper we review briefly some of the first works on wave propagation in phononic crystals emphasizing the conditions for the creation of acoustic band-gaps and the role of resonances to the band-gap creation. We show that useful conclusions in the analysis of phononic band gap structures can be drawn by considering the mathematical similarities of the basic classical wave equation (Helmholtz equation) with Schrödinger equation and by employing basic solid state physics concepts and conclusions regarding electronic waves. In the second part of the paper we demonstrate the potential of phononic systems to be used as elastic metamaterials. This is done by demonstrating negative refraction in phononic crystals and subwavelength waveguiding in a linear chain of elastic inclusions, and by proposing a novel structure with close to pentamode behavior. Finally the potential of phononic structures to be used in liquid sensor applications is discussed and demonstrated.
Li, Junqiang; Shan, Xin; Bade, Sri Ganesh R; Geske, Thomas; Jiang, Qinglong; Yang, Xin; Yu, Zhibin
2016-10-03
Charge-carrier injection into an emissive semiconductor thin film can result in electroluminescence and is generally achieved by using a multilayer device structure, which requires an electron-injection layer (EIL) between the cathode and the emissive layer and a hole-injection layer (HIL) between the anode and the emissive layer. The recent advancement of halide perovskite semiconductors opens up a new path to electroluminescent devices with a greatly simplified device structure. We report cesium lead tribromide light-emitting diodes (LEDs) without the aid of an EIL or HIL. These so-called single-layer LEDs have exhibited a sub-band gap turn-on voltage. The devices obtained a brightness of 591 197 cd m(-2) at 4.8 V, with an external quantum efficiency of 5.7% and a power efficiency of 14.1 lm W(-1). Such an advancement demonstrates that very high efficiency of electron and hole injection can be obtained in perovskite LEDs even without using an EIL or HIL.
NASA Astrophysics Data System (ADS)
Sharma, Kriti; Al-Kabbi, A. S.; Singh, Baljinder; Saini, G. S. S.; Tripathi, S. K.
2011-12-01
Nanocrystalline CdSe thin films have been prepared by thermal vaccum evaporation technique using Inert Gas Condensation method using Argon as inert gas. XRD confirms the crystalline cubic nature of nc-CdSe thin films. The optical band gap is calculated for as deposited nc-CdSe and it comes out to be 2.1 eV. CPM has been used to measure sub-band gap absorption in nanocrystalline CdSe thin films. The thin films of nc-CdSe have been annealed at 80 °C for one hour and sub-bandgap absorption in annealed samples has also been calculated. Slope of Urbach tail which is a measure of disorder in both as deposited and annealed samples has been calculated. In the case of as deposited nc-CdSe thin films, Urbach slope is 354 meV. It decreases to the value 198 meV after annealing which shows structural disorder decreases after annealing.
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
2016-10-01
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.
Flow stabilization by subsurface phonons
Hussein, M. I.; Biringen, S.; Bilal, O. R.; Kucala, A.
2015-01-01
The interaction between a fluid and a solid surface in relative motion represents a dynamical process that is central to the problem of laminar-to-turbulent transition (and consequent drag increase) for air, sea and land vehicles, as well as long-range pipelines. This problem may in principle be alleviated via a control stimulus designed to impede the generation and growth of instabilities inherent in the flow. Here, we show that phonon motion underneath a surface may be tuned to passively generate a spatio-temporal elastic deformation profile at the surface that counters these instabilities. We theoretically demonstrate this phenomenon and the underlying mechanism of frequency-dependent destructive interference of the unstable flow waves. The converse process of flow destabilization is illustrated as well. This approach provides a condensed-matter physics treatment to fluid–structure interaction and a new paradigm for flow control. PMID:27547095
Two-Dimensional Phononic Crystals: Disorder Matters.
Wagner, Markus R; Graczykowski, Bartlomiej; Reparaz, Juan Sebastian; El Sachat, Alexandros; Sledzinska, Marianna; Alzina, Francesc; Sotomayor Torres, Clivia M
2016-09-14
The design and fabrication of phononic crystals (PnCs) hold the key to control the propagation of heat and sound at the nanoscale. However, there is a lack of experimental studies addressing the impact of order/disorder on the phononic properties of PnCs. Here, we present a comparative investigation of the influence of disorder on the hypersonic and thermal properties of two-dimensional PnCs. PnCs of ordered and disordered lattices are fabricated of circular holes with equal filling fractions in free-standing Si membranes. Ultrafast pump and probe spectroscopy (asynchronous optical sampling) and Raman thermometry based on a novel two-laser approach are used to study the phononic properties in the gigahertz (GHz) and terahertz (THz) regime, respectively. Finite element method simulations of the phonon dispersion relation and three-dimensional displacement fields furthermore enable the unique identification of the different hypersonic vibrations. The increase of surface roughness and the introduction of short-range disorder are shown to modify the phonon dispersion and phonon coherence in the hypersonic (GHz) range without affecting the room-temperature thermal conductivity. On the basis of these findings, we suggest a criteria for predicting phonon coherence as a function of roughness and disorder.
Acoustic superfocusing by solid phononic crystals
Zhou, Xiaoming; Assouar, M. Badreddine Oudich, Mourad
2014-12-08
We propose a solid phononic crystal lens capable of acoustic superfocusing beyond the diffraction limit. The unit cell of the crystal is formed by four rigid cylinders in a hosting material with a cavity arranged in the center. Theoretical studies reveal that the solid lens produces both negative refraction to focus propagating waves and surface states to amplify evanescent waves. Numerical analyses of the superfocusing effect of the considered solid phononic lens are presented with a separated source excitation to the lens. In this case, acoustic superfocusing beyond the diffraction limit is evidenced. Compared to the fluid phononic lenses, the solid lens is more suitable for ultrasonic imaging applications.
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.
Acoustic superfocusing by solid phononic crystals
NASA Astrophysics Data System (ADS)
Zhou, Xiaoming; Assouar, M. Badreddine; Oudich, Mourad
2014-12-01
We propose a solid phononic crystal lens capable of acoustic superfocusing beyond the diffraction limit. The unit cell of the crystal is formed by four rigid cylinders in a hosting material with a cavity arranged in the center. Theoretical studies reveal that the solid lens produces both negative refraction to focus propagating waves and surface states to amplify evanescent waves. Numerical analyses of the superfocusing effect of the considered solid phononic lens are presented with a separated source excitation to the lens. In this case, acoustic superfocusing beyond the diffraction limit is evidenced. Compared to the fluid phononic lenses, the solid lens is more suitable for ultrasonic imaging applications.
Electrons and Phonons in Semiconductor Multilayers
NASA Astrophysics Data System (ADS)
Ridley, B. K.
2014-08-01
Introduction; 1. Simple models of the electron-phonon interaction; 2. Quantum confinement of carriers; 3. Quasicontinuum theory of lattice vibrations; 4. Bulk vibratory modes in an isotropic continuum; 5. Optical modes in a quantum well; 6. Superlattice modes; 7. Optical modes in various structures; 8. Electron-phonon interaction in a quantum well; 9. Other scattering mechanisms; 10. Quantum screening; 11. The electron distribution function; 12. Spin relaxation; 13. Electrons and phonons in the Wurtzite lattice; 14. Nitride heterostructures; 15. Terahertz sources; References; Index.
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.
Electron-phonon superconductivity in BaSn5
NASA Astrophysics Data System (ADS)
Billington, David; Ernsting, David; Millichamp, Thomas E.; Dugdale, Stephen B.
2015-05-01
First-principles calculations of the electronic structure and phonon dispersion relation of the superconducting compound ? were performed. This has allowed the calculation of the electron-phonon matrix elements from which the electron-phonon coupling constant was found to be ?. Application of the Allen-Dynes formula with ? yielded a superconducting transition temperature of ? K. The calculated ? agrees well with the available experimental data and indicates that ? is an electron-phonon superconductor with intermediate strength electron-phonon coupling.
Stationary Phonon Squeezing by Optical Polaron Excitation
NASA Astrophysics Data System (ADS)
Papenkort, T.; Axt, V. M.; Kuhn, T.
2017-03-01
We demonstrate that a stationary squeezed phonon state can be prepared by a pulsed optical excitation of a semiconductor quantum well. Unlike previously discussed scenarios for generating squeezed phonons, the corresponding uncertainties become stationary after the excitation and do not oscillate in time. The effect is caused by two-phonon correlations within the excited polaron. We demonstrate by quantum kinetic simulations and by a perturbation analysis that the energetically lowest polaron state comprises two-phonon correlations which, after the pulse, result in an uncertainty of the lattice momentum that is continuously lower than in the ground state of the semiconductor. The simulations show the dynamics of the polaron formation process and the resulting time-dependent lattice uncertainties.
Toward stimulated interaction of surface phonon polaritons
Kong, B. D.; Trew, R. J.; Kim, K. W.
2013-12-21
Thermal emission spectra mediated by surface phonon polariton are examined by using a theoretical model that accounts for generation processes. Specifically, the acoustic phonon fusion mechanism is introduced to remedy theoretical deficiencies of the near thermal equilibrium treatments. The model clarifies the thermal excitation mechanism of surface phonon polaritons and the energy transfer path under non-zero energy flow. When applied to GaAs and SiC semi-infinite surfaces, the nonequilibrium model predicts that the temperature dependence of the quasi-monochromatic peak can exhibit distinctly different characteristics of either sharp increase or slow saturation depending on the materials, which is in direct contrast with the estimate made by the near-equilibrium model. The proposed theoretical tool can accurately analyze the nonequilibrium steady states, potentially paving a pathway to demonstrate stimulated interaction/emission of thermally excited surface phonon polaritons.
Toward stimulated interaction of surface phonon polaritons
NASA Astrophysics Data System (ADS)
Kong, B. D.; Trew, R. J.; Kim, K. W.
2013-12-01
Thermal emission spectra mediated by surface phonon polariton are examined by using a theoretical model that accounts for generation processes. Specifically, the acoustic phonon fusion mechanism is introduced to remedy theoretical deficiencies of the near thermal equilibrium treatments. The model clarifies the thermal excitation mechanism of surface phonon polaritons and the energy transfer path under non-zero energy flow. When applied to GaAs and SiC semi-infinite surfaces, the nonequilibrium model predicts that the temperature dependence of the quasi-monochromatic peak can exhibit distinctly different characteristics of either sharp increase or slow saturation depending on the materials, which is in direct contrast with the estimate made by the near-equilibrium model. The proposed theoretical tool can accurately analyze the nonequilibrium steady states, potentially paving a pathway to demonstrate stimulated interaction/emission of thermally excited surface phonon polaritons.
Phonon stop bands in amorphous superlattices
NASA Astrophysics Data System (ADS)
Koblinger, O.; Mebert, J.; Dittrich, E.; Döttinger, S.; Eisenmenger, W.; Santos, P. V.; Ley, L.
1987-06-01
In periodically layered media the phonon-dispersion relation shows energy ranges in which phonon propagation is not possible. The existence of such phonon stop bands in crystalline superlattices has been observed in work by V. Narayanamurti, H. L. Störmer, M. A. Chin, A. C. Gossard, and W. Wiegman [Phys. Rev. Lett. 43, 2012 (1979)]. In this Communication we report the observation of phonon stop bands in amorphous superlattices. The filter characteristic of these amorphous superlattices is much sharper than in the case of the crystalline superlattices studied earlier. The investigated superlattices have been prepared by alternating evaporation of Si and SiO2 layers as well as by plasma-enhanced chemical vapor deposition of a-Si:H/a-SiNx:H films in a glow-discharge reactor.
Phonon transport in silicon nanowires: The reduced group velocity and surface-roughness scattering
NASA Astrophysics Data System (ADS)
Zhu, Liyan; Li, Baowen; Li, Wu
2016-09-01
Using a linear-scaling Kubo simulation approach, we have quantitatively investigated the effects of confinement and surface roughness on phonon transport in silicon nanowires (SiNWs) as thick as 55 nm in diameter R . The confinement effect leads to significant reduction of phonon group velocity v in SiNWs compared to bulk silicon except at extremely low phonon frequencies f , which very likely persists in SiNWs several hundreds of nanometers thick, suggesting the inapplicability of bulk properties, including anharmonic phonon scattering, to SiNWs. For instance, the velocity can be reduced by more than 30% for phonons with f >4.5 THz in 55-nm-thick nanowires. In rough SiNWs Casimir's limit, which is valid in confined macroscopic systems, can underestimate the surface scattering by more than one order of magnitude. For a roughness profile with Lorentzian correlation characterized by root-mean-square roughness σ and correlation length Lr, the frequency-dependent phonon diffusivity D follows power-law dependences D ∝Rασ-βLrγ , where α ˜2 and β ˜1 . On average, γ increases from 0 to 0.5 as R /σ increases. The mean free path and the phonon lifetime essentially follow the same power-law dependences. These dependences are in striking contrast to Casimir's limit, i.e., D ˜v R /3 , and manifest the dominant role of the change in the number of atoms due to roughness. The thermal conductivity κ can vary by one order of magnitude with varying σ and Lr in SiNWs, and increasing σ and shortening Lr can efficiently lower κ below Casimir's limit by one order of magnitude. Our work provides different insights to understand the ultralow thermal conductivity of SiNWs reported experimentally and guidance to manipulate κ via surface roughness engineering.
Phonon Cooling by an Optomechanical Heat Pump.
Dong, Ying; Bariani, F; Meystre, P
2015-11-27
We propose and analyze theoretically a cavity optomechanical analog of a heat pump that uses a polariton fluid to cool mechanical modes coupled to a single precooled phonon mode via external modulation of the substrate of the mechanical resonator. This approach permits us to cool phonon modes of arbitrary frequencies not limited by the cavity-optical field detuning deep into the quantum regime from room temperature.
Slow light and broadband coherent phonon generation
NASA Astrophysics Data System (ADS)
Wang, Zheng; Rakich, Peter; Reinke, Charles; Camacho, Ryan; Davids, Paul
2012-03-01
Recent advance in controlling optical forces using nanostructures suggests that nanoscale optical waveguides are capable of generating coherent acoustic phonons efficiently through a combination of radiation pressure and electrostriction. We discuss the critical roles of group velocity in such processes. This photon-phonon coupling would allow an acoustic intermediary to perform on-chip optical delay with a capacity 105 greater than photonic delay lines of the same size.
The role of anharmonic phonons in under-barrier spin relaxation of single molecule magnets
NASA Astrophysics Data System (ADS)
Lunghi, Alessandro; Totti, Federico; Sessoli, Roberta; Sanvito, Stefano
2017-03-01
The use of single molecule magnets in mainstream electronics requires their magnetic moment to be stable over long times. One can achieve such a goal by designing compounds with spin-reversal barriers exceeding room temperature, namely with large uniaxial anisotropies. Such strategy, however, has been defeated by several recent experiments demonstrating under-barrier relaxation at high temperature, a behaviour today unexplained. Here we propose spin-phonon coupling to be responsible for such anomaly. With a combination of electronic structure theory and master equations we show that, in the presence of phonon dissipation, the relevant energy scale for the spin relaxation is given by the lower-lying phonon modes interacting with the local spins. These open a channel for spin reversal at energies lower than that set by the magnetic anisotropy, producing fast under-barrier spin relaxation. Our findings rationalize a significant body of experimental work and suggest a possible strategy for engineering room temperature single molecule magnets.
Glass-like phonon scattering from a spontaneous nanostructure in AgSbTe2
Ma, J.; Delaire, O.; May, A. F.; Carlton, C. E.; McGuire, M. A.; VanBebber, L. H.; Abernathy, D. L.; Ehlers, G.; Hong, Tao; Huq, A.; Tian, Wei; Keppens, V. M.; Shao-Horn, Y.; Sales, B. C.
2013-06-02
Materials with very low thermal conductivity are of high interest for both thermoelectric and optical phase-change applications. Synthetic nanostructuring is most promising to suppress thermal conductivity by scattering phonons, but challenges remain in producing bulk samples. We show that in crystalline AgSbTe2, a spontaneously-forming nanostructure leads to a suppression of thermal conductivity to a glass-like level. Our mappings of phonon mean-free-paths provide a novel bottom- up microscopic account of thermal conductivity, and also reveal intrinsic anisotropies associated with the nanostructure. Ground-state degeneracy in AgSbTe2 leads to the natural formation of nanoscale domains with different orderings on the cation sublattice, and correlated atomic displacements, which efficiently scatter phonons. This mechanism is general and points to a new avenue in nano- scale engineering of materials, to achieve low thermal conductivities for efficient thermoelectric converters and phase-change memory devices.
Dexterous acoustic trapping and patterning of particles assisted by phononic crystal plate
Wang, Tian; Ke, Manzhu Xu, Shengjun; Feng, Junheng; Qiu, Chunyin; Liu, Zhengyou
2015-04-20
In this letter, we present experimental demonstration of multi-particles trapping and patterning by the artificially engineered acoustic field of phononic crystal plate. Polystyrene particles are precisely trapped and patterned in two dimensional arrays, for example, the square, triangular, or quasi-periodic arrays, depending on the structures of the phononic crystal plates with varying sub-wavelength holes array. Analysis shows that the enhanced acoustic radiation force, induced by the resonant transmission field highly localized near the sub-wavelength apertures, accounts for the particles self-organizing. It can be envisaged that this kind of simple design of phononic crystal plates would pave an alternative route for self-assembly of particles and may be utilized in the lab-on-a-chip devices.
Wang, Jing; Chen, Di; Wallace, Joseph; Gigax, Jonathan; Wang, Xuemei; Shao, Lin
2014-05-12
Through integrated molecular dynamics (MD) simulations and experimental studies, we demonstrated the feasibility of an ion-irradiation-and-annealing based phonon engineering technique to enhance thermal conductivity of carbon nanotube (CNT) films. Upon ion irradiation of CNT films, both inter-tube defects and intra-tube defects are introduced. Our MD simulations show that inter-tube defects created between neighboring tubes are much more stable than intra-tube defects created on tube graphitic planes. Upon thermal annealing, intra-tube defects are preferentially removed but inter-tube defects stay. Consequently, axial phonon transport increases due to reduced phonon scattering and off-axial phonon transport is sustained due to the high stability of inter-tube defects, leading to a conductivity enhancement upon annealing. The modeling predictions agree with experimental observations that thermal conductivities of CNT films were enhanced after 2 MeV hydrogen ion irradiations and conductivities were further enhanced upon post irradiation annealing.
Ab initio phonon limited transport
NASA Astrophysics Data System (ADS)
Verstraete, Matthieu
We revisit the thermoelectric (TE) transport properties of two champion materials, PbTe and SnSe, using fully first principles methods. In both cases the performance of the material is due to subtle combinations of structural effects, scattering, and phase space reduction. In PbTe anharmonic effects are completely opposite to the predicted quasiharmonic evolution of phonon frequencies and to frequently (and incorrectly) cited extrapolations of experiments. This stabilizes the material at high T, but also tends to enhance its thermal conductivity, in a non linear manner, above 600 Kelvin. This explains why PbTe is in practice limited to room temperature applications. SnSe has recently been shown to be the most efficient TE material in bulk form. This is mainly due to a strongly enhanced carrier concentration and electrical conductivity, after going through a phase transition from 600 to 800 K. We calculate the transport coefficients as well as the defect concentrations ab initio, showing excellent agreement with experiment, and elucidating the origin of the double phase transition as well as the new charge carriers. AH Romero, EKU Gross, MJ Verstraete, and O Hellman PRB 91, 214310 (2015) O. Hellman, IA Abrikosov, and SI Simak, PRB 84 180301 (2011)
Squeezed Phonons: Modulating Quantum Fluctuations of Atomic Displacements.
NASA Astrophysics Data System (ADS)
Hu, Xuedong; Nori, Franco
1997-03-01
We have studied phonon squeezed states and also put forward several proposals for their generation(On phonon parametric process, X. Hu and F. Nori, Phys. Rev. Lett. 76), 2294 (1996); on polariton mechanism, X. Hu and F. Nori, Phys. Rev. B 53, 2419 (1996); on second-order Raman scattering, X. Hu and F. Nori, preprint.. Here, we compare the relative merits and limitations of these approaches, including several factors that will limit the amount of phonon squeezing. In particular, we investigate the effect of the initial thermal states on the phonon modes. Using a model for the phonon density matrix, we also study the mixing of the phonon squeezed states with thermal states, which describes the decay of the phonon coherence. Finally, we calculate the maximum possible squeezing from a phonon parametric process limited by phonon decay.
NASA Astrophysics Data System (ADS)
Rury, Aaron S.
2016-06-01
This study reports experimental, computational, and theoretical evidence for a previously unobserved coherent phonon-phonon interaction in an organic solid that can be described by the application of Fano's analysis to a case without the presence of a continuum. Using Raman spectroscopy of the hydrogen-bonded charge-transfer material quinhydrone, two peaks appear near 700 cm-1 we assign as phonons whose position and line-shape asymmetry depend on the sample temperature and light scattering excitation energy. Density functional theory calculations find two nearly degenerate phonons possessing frequencies near the values found in experiment that share similar atomic motion out of the aromatic plane of electron donor and acceptor molecules of quinhydrone. Further analytical modeling of the steady-state light scattering process using the Peierls-Hubbard Hamiltonian and time-dependent perturbation theory motivates assignment of the physical origin of the asymmetric features of each peak's line shape to an interaction between two discrete phonons via nonlinear electron-phonon coupling. In the context of analytical model results, characteristics of the experimental spectra upon 2.33 eV excitation of the Raman scattering process are used to qualify the temperature dependence of the magnitude of this coupling in the valence band of quinhydrone. These results broaden the range of phonon-phonon interactions in materials in general while also highlighting the rich physics and fundamental attributes specific to organic solids that may determine their applicability in next generation electronics and photonics technologies.
Non-equilibrium Phonons in CaWO4: Issues for Phonon Mediated Particle Detectors
NASA Astrophysics Data System (ADS)
Msall, Madeleine; Head, Timothy; Jumper, Daniel
2009-03-01
The CRESST experiment looks for evidence of dark matter particles colliding with nuclei in CaWO4, using cryogenic bolometers sensitive to energy deposition ˜ 10 keV with a few percent accuracy. Calibration of the energy deposited in the phonon system depends upon the details of the evolution of the non-equilibrium energy in the CaWO4 absorber. Our phonon images sensitively measure variations in angular phonon flux, providing key information about the elastic constants and scattering rates that determine the energy evolution. Phonon pulses, created by focused photoexcitation of a 150 nm Cu film, are detected after propagation through 3 mm of CaWO4. The 20 ns Ar-ion laser pulse creates a localized (10-3 mm^2) source of 10-20 K blackbody phonons. The sample is at 2 K. Our images show that the elastic constants derived from ultrasonic velocities along high symmetry axes do not accurately predict the total phonon flux along non-symmetry directions. We present new data on the dependence of phonon flux on excitation level and discuss the influence of isotope and anharmonic decay on the shape of phonon pulses in these ultrapure samples. Thanks to J.P. Wolfe and the Frederick Seitz Materials Research Laboratory, Urbana, IL, for partial support of this work.
Alaie, Seyedhamidreza; Goettler, Drew F; Su, Mehmet; Leseman, Zayd C; Reinke, Charles M; El-Kady, Ihab
2015-06-24
Large reductions in the thermal conductivity of thin silicon membranes have been demonstrated in various porous structures. However, the role of coherent boundary scattering in such structures has become a matter of some debate. Here we report on the first experimental observation of coherent phonon boundary scattering at room temperature in 2D phononic crystals formed by the introduction of air holes in a silicon matrix with minimum feature sizes >100 nm. To delaminate incoherent from coherent boundary scattering, phononic crystals with a fixed minimum feature size, differing only in unit cell geometry, were fabricated. A suspended island technique was used to measure the thermal conductivity. We introduce a hybrid thermal conductivity model that accounts for partially coherent and partially incoherent phonon boundary scattering. We observe excellent agreement between this model and experimental data, and the results suggest that significant room temperature coherent phonon boundary scattering occurs.
Symmetry-adapted phonon analysis of nanotubes
NASA Astrophysics Data System (ADS)
Aghaei, Amin; Dayal, Kaushik; Elliott, Ryan S.
2013-02-01
The characteristics of phonons, i.e. linearized normal modes of vibration, provide important insights into many aspects of crystals, e.g. stability and thermodynamics. In this paper, we use the Objective Structures framework to make concrete analogies between crystalline phonons and normal modes of vibration in non-crystalline but highly symmetric nanostructures. Our strategy is to use an intermediate linear transformation from real-space to an intermediate space in which the Hessian matrix of second derivatives is block-circulant. The block-circulant nature of the Hessian enables us to then follow the procedure to obtain phonons in crystals: namely, we use the Discrete Fourier Transform from this intermediate space to obtain a block-diagonal matrix that is readily diagonalizable. We formulate this for general Objective Structures and then apply it to study carbon nanotubes of various chiralities that are subjected to axial elongation and torsional deformation. We compare the phonon spectra computed in the Objective Framework with spectra computed for armchair and zigzag nanotubes. We also demonstrate the approach by computing the Density of States. In addition to the computational efficiency afforded by Objective Structures in providing the transformations to almost-diagonalize the Hessian, the framework provides an important conceptual simplification to interpret the phonon curves. Our findings include that, first, not all non-optic long-wavelength modes are zero energy and conversely not all zero energy modes are long-wavelength; second, the phonon curves accurately predict both the onset as well as the soft modes for instabilities such as torsional buckling; and third, unlike crystals where phonon stability does not provide information on stability with respect to non-rank-one deformation modes, phonon stability in nanotubes is sufficient to guarantee stability with respect to all perturbations that do not involve structural modes. Our finding of characteristic
Ionizing particle detection based on phononic crystals
Aly, Arafa H. E-mail: arafa.hussien@science.bsu.edu.eg; Mehaney, Ahmed; Eissa, Mostafa F.
2015-08-14
Most conventional radiation detectors are based on electronic or photon collections. In this work, we introduce a new and novel type of ionizing particle detector based on phonon collection. Helium ion radiation treats tumors with better precision. There are nine known isotopes of helium, but only helium-3 and helium-4 are stable. Helium-4 is formed in fusion reactor technology and in enormous quantities during Big Bang nucleo-synthesis. In this study, we introduce a technique for helium-4 ion detection (sensing) based on the innovative properties of the new composite materials known as phononic crystals (PnCs). PnCs can provide an easy and cheap technique for ion detection compared with conventional methods. PnC structures commonly consist of a periodic array of two or more materials with different elastic properties. The two materials are polymethyl-methacrylate and polyethylene polymers. The calculations showed that the energies lost to target phonons are maximized at 1 keV helium-4 ion energy. There is a correlation between the total phonon energies and the transmittance of PnC structures. The maximum transmission for phonons due to the passage of helium-4 ions was found in the case of making polyethylene as a first layer in the PnC structure. Therefore, the concept of ion detection based on PnC structure is achievable.
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.
Ionizing particle detection based on phononic crystals
NASA Astrophysics Data System (ADS)
Aly, Arafa H.; Mehaney, Ahmed; Eissa, Mostafa F.
2015-08-01
Most conventional radiation detectors are based on electronic or photon collections. In this work, we introduce a new and novel type of ionizing particle detector based on phonon collection. Helium ion radiation treats tumors with better precision. There are nine known isotopes of helium, but only helium-3 and helium-4 are stable. Helium-4 is formed in fusion reactor technology and in enormous quantities during Big Bang nucleo-synthesis. In this study, we introduce a technique for helium-4 ion detection (sensing) based on the innovative properties of the new composite materials known as phononic crystals (PnCs). PnCs can provide an easy and cheap technique for ion detection compared with conventional methods. PnC structures commonly consist of a periodic array of two or more materials with different elastic properties. The two materials are polymethyl-methacrylate and polyethylene polymers. The calculations showed that the energies lost to target phonons are maximized at 1 keV helium-4 ion energy. There is a correlation between the total phonon energies and the transmittance of PnC structures. The maximum transmission for phonons due to the passage of helium-4 ions was found in the case of making polyethylene as a first layer in the PnC structure. Therefore, the concept of ion detection based on PnC structure is achievable.
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.
Hierarchical thermoelectrics: crystal grain boundaries as scalable phonon scatterers.
Selli, Daniele; Boulfelfel, Salah Eddine; Schapotschnikow, Philipp; Donadio, Davide; Leoni, Stefano
2016-02-14
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.
NASA Astrophysics Data System (ADS)
Tea, Eric; Hamzeh, Hani; Aniel, Frédéric
2011-12-01
We present a study of the photo-excited charge carriers relaxation dynamics in polar semiconductors comparing calculations to pump probe experiments. Hot carrier densities in the 1018cm-3 range can easily be photo-generated using moderately intense optical excitations. This can lead to known phenomena, namely, hot phonon populations and the coupling of polar optical phonons with plasmon modes. However, these two phenomena can affect the hot carriers relaxation and have never been examined together. This is a problem for the theoretical study of future Hot Carrier Solar Cells, where the conditions allow both of these phenomena to occur. The charge carriers dynamics and the coupling of polar optical phonons with plasmon modes are treated by a Full Band Ensemble Monte Carlo simulation code featuring a self-consistent dielectric function. To take into consideration hot phonon populations and the subsequent phonon bottleneck for the carriers relaxation, the charge carriers simulation code is coupled to a phonon dedicated Ensemble Monte Carlo code. This enables for the first time an accurate study of both the charge carriers and phonon systems dynamics, the latter being most of the time overly simplified in previous studies. The present work explores to which extent the two aforementioned phenomena affect the photo-generated charge carriers relaxation in GaAs and can be easily adapted to other polar semiconductors.
Polaron action for multimode dispersive phonon systems
NASA Astrophysics Data System (ADS)
Kornilovitch, P. E.
2006-03-01
The path-integral approach to the tight-binding polaron is extended to multiple optical phonon modes of arbitrary dispersion and polarization. The nonlinear lattice effects are neglected. Only one electron band is considered. The electron-phonon interaction is of the density-displacement type, but can be of arbitrary spatial range and shape. Feynman’s analytical integration of ion trajectories is performed by transforming the electron-ion forces to the basis in which the phonon dynamical matrix is diagonal. The resulting polaron action is derived for the periodic and shifted boundary conditions in imaginary time. The former can be used for calculating polaron thermodynamics while the latter for the polaron mass and spectrum. The developed formalism is the analytical basis for numerical analysis of such models by path-integral Monte Carlo methods.
Calculating the Phonon Dispersion From First Principles
NASA Astrophysics Data System (ADS)
Ceballos, Frank; O'Hara, Andy; Slepko, Alexander; Demkov, Alexander
2011-10-01
The goal of this project was to construct a user-friendly tool that can compute the phonon dispersion for any solid with a periodic crystal structure. The phonon dispersion describes the crystal's vibrational properties and thermodynamic properties of the solid. Using the Vienna Ab-initio Simulation Package (VASP) we compute the forces between the atoms. Assuming harmonic approximation we numerically evaluate force constant matrix. The lattice Fourier transform of the force constants yields the dynamical matrix, whose eigenvalues and eigenvectors represent the allowed phonon frequencies and displacement patterns for specific k-vectors. Our code then plots the frequencies along high symmetry lines in the Brillouin zone. We will present our results for silicon, GaAs and ZrO2.
Phonon and magnon heat transport and drag effects
NASA Astrophysics Data System (ADS)
Heremans, Joseph P.
2014-03-01
Thermoelectric generators and coolers constitute today's solid-state energy converters. The two goals in thermoelectrics research are to enhance the thermopower while simultaneously maintaining a high electrical conductivity of the same material, and to minimize its lattice thermal conductivity without affecting its electronic properties. Up to now the lattice thermal conductivity has been minimized by using alloy scattering and, more recently, nanostructuring. In the first part of the talk, a new approach to minimize the lattice thermal conductivity is described that affects phonon scattering much more than electron scattering. This can be done by selecting potential thermoelectric materials that have a very high anharmonicity, because this property governs phonon-phonon interaction probability. Several possible types of chemical bonds will be described that exhibit such high anharmonicity, and particular emphasis will be put on solids with highly-polarizable lone-pair electrons, such as the rock salt I-V-VI2 compounds (e.g. NaSbSe2). The second part of the talk will give an introduction to a completely new class of solid-state thermal energy converters based on spin transport. One configuration for such energy converters is based on the recently discovered spin-Seebeck effect (SSE). This quantity is expressed in the same units as the conventional thermopower, and we have recently shown that it can be of the same order of magnitude. The main advantage of SSE converters is that the problem of optimization is now distributed over two different materials, a ferromagnet in which a flux of magnetization is generated by a thermal gradient, and a normal metal where the flux of magnetization is converted into electrical power. The talk will focus on the basic physics behind the spin-Seebeck effect. Recent developments will then be described based on phonon-drag of spin polarized electrons. This mechanism has made it possible to reach magnitudes of SSE that are comparable
Refraction characteristics of phononic crystals
NASA Astrophysics Data System (ADS)
Nemat-Nasser, Sia
2015-08-01
Some of the most interesting refraction properties of phononic crystals are revealed by examining the anti-plane shear waves in doubly periodic elastic composites with unit cells containing rectangular and/or elliptical multi-inclusions. The corresponding band structure, group velocity, and energy-flux vector are calculated using a powerful mixed variational method that accurately and efficiently yields all the field quantities over multiple frequency pass-bands. The background matrix and the inclusions can be anisotropic, each having distinct elastic moduli and mass densities. Equifrequency contours and energy-flux vectors are readily calculated as functions of the wave-vector components. By superimposing the energy-flux vectors on equifrequency contours in the plane of the wave-vector components, and supplementing this with a three-dimensional graph of the corresponding frequency surface, a wealth of information is extracted essentially at a glance. This way it is shown that a composite with even a simple square unit cell containing a central circular inclusion can display negative or positive energy and phase velocity refractions, or simply performs a harmonic vibration (standing wave), depending on the frequency and the wave-vector. Moreover, that the same composite when interfaced with a suitable homogeneous solid can display: (1) negative refraction with negative phase velocity refraction; (2) negative refraction with positive phase velocity refraction; (3) positive refraction with negative phase velocity refraction; (4) positive refraction with positive phase velocity refraction; or even (5) complete reflection with no energy transmission, depending on the frequency, and direction and the wavelength of the plane-wave that is incident from the homogeneous solid to the interface. For elliptical and rectangular inclusion geometries, analytical expressions are given for the key calculation quantities. Expressions for displacement, velocity, linear momentum
NASA Astrophysics Data System (ADS)
Luo, Yixiu; Wang, Jiemin; Li, Yiran; Wang, Jingyang
2016-07-01
Modification of lattice thermal conductivity (κL) of a solid by means of hydrostatic pressure (P) has been a crucially interesting approach that targets a broad range of advanced materials from thermoelectrics and thermal insulators to minerals in mantle. Although it is well documented knowledge that thermal conductivity of bulk materials normally increase upon hydrostatic pressure, such positive relationship is seriously challenged when it comes to ceramics with complex crystal structure and heterogeneous chemical bonds. In this paper, we predict an abnormally negative trend dκL/dP < 0 in Y2Si2O7 silicate using density functional theoretical calculations. The mechanism is disclosed as combined effects of slightly decreased group velocity and significantly augmented scattering of heat-carrying acoustic phonons in pressured lattice, which is originated from pressure-induced downward shift of low-lying optic and acoustic phonons. The structural origin of low-lying optic phonons as well as the induced phonon anharmonicity is also qualitatively elucidated with respect to intrinsic bonding heterogeneity of Y2Si2O7. The present results are expected to bring deeper insights for phonon engineering and modulation of thermal conductivity in complex solids with diverging structural flexibility, enormous bonding heterogeneity, and giant phonon anharmonicity.
Luo, Yixiu; Wang, Jiemin; Li, Yiran; Wang, Jingyang
2016-01-01
Modification of lattice thermal conductivity (κL) of a solid by means of hydrostatic pressure (P) has been a crucially interesting approach that targets a broad range of advanced materials from thermoelectrics and thermal insulators to minerals in mantle. Although it is well documented knowledge that thermal conductivity of bulk materials normally increase upon hydrostatic pressure, such positive relationship is seriously challenged when it comes to ceramics with complex crystal structure and heterogeneous chemical bonds. In this paper, we predict an abnormally negative trend dκL/dP < 0 in Y2Si2O7 silicate using density functional theoretical calculations. The mechanism is disclosed as combined effects of slightly decreased group velocity and significantly augmented scattering of heat-carrying acoustic phonons in pressured lattice, which is originated from pressure-induced downward shift of low-lying optic and acoustic phonons. The structural origin of low-lying optic phonons as well as the induced phonon anharmonicity is also qualitatively elucidated with respect to intrinsic bonding heterogeneity of Y2Si2O7. The present results are expected to bring deeper insights for phonon engineering and modulation of thermal conductivity in complex solids with diverging structural flexibility, enormous bonding heterogeneity, and giant phonon anharmonicity. PMID:27430670
Yan, Zhequan; Chen, Liang; Yoon, Mina; Kumar, Satish
2016-02-21
Hexagonal boron nitride (h-BN) is a promising substrate for graphene based nano-electronic devices. We investigate the ballistic phonon transport at the interface of vertically stacked graphene and h-BN heterostructures using first principles density functional theory and atomistic Green's function simulations considering the influence of lattice stacking. We compute the frequency and wave-vector dependent transmission function and observe distinct stacking-dependent phonon transmission features for the h-BN/graphene/h-BN sandwiched systems. We find that the in-plane acoustic modes have the dominant contributions to the phonon transmission and thermal boundary conductance (TBC) for the interfaces with the carbon atom located directly on top of the boron atom (C-B matched) because of low interfacial spacing. The low interfacial spacing is a consequence of the differences in the effective atomic volume of N and B and the difference in the local electron density around N and B. For the structures with the carbon atom directly on top of the nitrogen atom (C-N matched), the spatial distance increases and the contribution of in-plane modes to the TBC decreases leading to higher contributions by out-of-plane acoustic modes. We find that the C-B matched interfaces have stronger phonon-phonon coupling than the C-N matched interfaces, which results in significantly higher TBC (more than 50%) in the C-B matched interface. The findings in this study will provide insights to understand the mechanism of phonon transport at h-BN/graphene/h-BN interfaces, to better explain the experimental observations and to engineer these interfaces to enhance heat dissipation in graphene based electronic devices.
NASA Astrophysics Data System (ADS)
Yan, Zhequan; Chen, Liang; Yoon, Mina; Kumar, Satish
2016-02-01
Hexagonal boron nitride (h-BN) is a promising substrate for graphene based nano-electronic devices. We investigate the ballistic phonon transport at the interface of vertically stacked graphene and h-BN heterostructures using first principles density functional theory and atomistic Green's function simulations considering the influence of lattice stacking. We compute the frequency and wave-vector dependent transmission function and observe distinct stacking-dependent phonon transmission features for the h-BN/graphene/h-BN sandwiched systems. We find that the in-plane acoustic modes have the dominant contributions to the phonon transmission and thermal boundary conductance (TBC) for the interfaces with the carbon atom located directly on top of the boron atom (C-B matched) because of low interfacial spacing. The low interfacial spacing is a consequence of the differences in the effective atomic volume of N and B and the difference in the local electron density around N and B. For the structures with the carbon atom directly on top of the nitrogen atom (C-N matched), the spatial distance increases and the contribution of in-plane modes to the TBC decreases leading to higher contributions by out-of-plane acoustic modes. We find that the C-B matched interfaces have stronger phonon-phonon coupling than the C-N matched interfaces, which results in significantly higher TBC (more than 50%) in the C-B matched interface. The findings in this study will provide insights to understand the mechanism of phonon transport at h-BN/graphene/h-BN interfaces, to better explain the experimental observations and to engineer these interfaces to enhance heat dissipation in graphene based electronic devices.
Second Harmonic Generation of Nanoscale Phonon Wave Packets.
Bojahr, A; Gohlke, M; Leitenberger, W; Pudell, J; Reinhardt, M; von Reppert, A; Roessle, M; Sander, M; Gaal, P; Bargheer, M
2015-11-06
Phonons are often regarded as delocalized quasiparticles with certain energy and momentum. The anharmonic interaction of phonons determines macroscopic properties of the solid, such as thermal expansion or thermal conductivity, and a detailed understanding becomes increasingly important for functional nanostructures. Although phonon-phonon scattering processes depicted in simple wave-vector diagrams are the basis of theories describing these macroscopic phenomena, experiments directly accessing these coupling channels are scarce. We synthesize monochromatic acoustic phonon wave packets with only a few cycles to introduce nonlinear phononics as the acoustic counterpart to nonlinear optics. Control of the wave vector, bandwidth, and consequently spatial extent of the phonon wave packets allows us to observe nonlinear phonon interaction, in particular, second harmonic generation, in real time by wave-vector-sensitive Brillouin scattering with x-rays and optical photons.
Phononic Phase Conjugation in an Optomechanical System
NASA Astrophysics Data System (ADS)
Buchmann, Lukas; Wright, Ewan; Meystre, Pierre
2013-05-01
We study theoretically the phase conjugation of a phononic field in an optomechanical system with two mechanical modes coupled to a common optical field. Phase conjugation becomes the dominant process for an appropriate choice of driving field parameters, and he effective coupling coefficients between phonon modes can result in amplification and entanglement, phase-conjugation or a mixture thereof. We discuss surprising consequences of mechanical phase-conjugation that could lead to the preparation of mechanical states with negative temperature, the improvement of quantum memories and the study of the quantum-classical transition. Supported by DARPA ORCHID program.
Phonon-Josephson resonances in atomtronic circuits
NASA Astrophysics Data System (ADS)
Bidasyuk, Y. M.; Prikhodko, O. O.; Weyrauch, M.
2016-09-01
We study the resonant excitation of sound modes from Josephson oscillations in Bose-Einstein condensates. From the simulations for various setups using the Gross-Pitaevskii mean-field equations and Josephson equations we observe additional tunneling currents induced by resonant phonons. The proposed experiment may be used for spectroscopy of phonons as well as other low-energy collective excitations in Bose-Einstein condensates. We also argue that the observed effect may mask the observation of Shapiro resonances if not carefully controlled.
THZ Phonon Spectroscopy of Bi-2223 and Bi-2212: Evidence for Phonon Pairing
NASA Astrophysics Data System (ADS)
Ponomarev, Ya. G.; Van, Hoang Hoai
Facts are presented evidencing the strong electron-phonon interaction and the scaling of a superconducting gap and a critical temperature in doped Bi-2212 single crystals. A sharp extra structure in the current-voltage characteristics (CVC's) of Bi-2212 contacts is attributed to the presence of the extended van Hove singularity (EVHS) close to the Fermi level in slightly overdoped and slightly underdoped samples. THZ phonon spectroscopy studies of Bi-2223 and Bi-2212 are overviewed. An observed giant instability in I(V) - characteristics of Bi-2223 nanosteps is probably caused by a resonant emission of 2Δ - optical phonons in a process of recombination of nonequilibrium quasiparticles (Krasnov model).
Manipulating Heat Flow through 3 Dimensional Nanoscale Phononic Crystal Structure
2014-06-02
Nanoscale Phononic Crystal Structure 5a. CONTRACT NUMBER FA23861214047 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 6. AUTHOR(S) Baowen Li 5d...through computer simulation, how the three dimensional (3D) phononic crystal structures can confine phonon and thus reduce thermal conductivity...phononic crystal (PnC) with spherical pores, which can reduce thermal conductivity of bulk Si by a factor up to 10,000 times at room temperature. The
Depth-Dependent Defect Studies Using Coherent Acoustic Phonons
2014-09-29
12211 Research Triangle Park, NC 27709-2211 coherent acoustic phonons, diamond, silicon, photelastic coefficients , refractive index, graphene, Second...attributed to the cooling of the subsystem of hot optical phonons by optical- acoustic phonon scattering . We observe that at different pump energy and...SECURITY CLASSIFICATION OF: Presented is our scientific progress in two areas of research. The first is coherent acoustic phonon (CAP) spectroscopy of
Yi, Kyung-Soo; Kim, Hye-Jung
2017-02-15
We investigate spectral behavior of phonon spectral functions in an interacting multi-component hot carrier plasma. Spectral analysis of various phonon spectral functions is performed considering carrier-phonon channels of polar and nonpolar optical phonons, acoustic deformation-potential, and piezoelectric Coulomb couplings. Effects of phonon self-energy corrections are examined at finite temperature within a random phase approximation extended to include the effects of dynamic screening, plasmon-phonon coupling, and local-field corrections of the plasma species. We provide numerical data for the case of a photo-generated electron-hole plasma formed in a wurtzite GaN. Our result shows the clear significance of the multiplicity of the plasma species in the phonon spectral functions of a multi-component plasma giving rise to a variety of spectral behaviors of carrier-phonon coupled collective modes. A useful sum rule on the plasma-species-resolved dielectric functions is also found.
NASA Astrophysics Data System (ADS)
Yi, Kyung-Soo; Kim, Hye-Jung
2017-02-01
We investigate spectral behavior of phonon spectral functions in an interacting multi-component hot carrier plasma. Spectral analysis of various phonon spectral functions is performed considering carrier-phonon channels of polar and nonpolar optical phonons, acoustic deformation-potential, and piezoelectric Coulomb couplings. Effects of phonon self-energy corrections are examined at finite temperature within a random phase approximation extended to include the effects of dynamic screening, plasmon-phonon coupling, and local-field corrections of the plasma species. We provide numerical data for the case of a photo-generated electron-hole plasma formed in a wurtzite GaN. Our result shows the clear significance of the multiplicity of the plasma species in the phonon spectral functions of a multi-component plasma giving rise to a variety of spectral behaviors of carrier-phonon coupled collective modes. A useful sum rule on the plasma-species-resolved dielectric functions is also found.
NASA Astrophysics Data System (ADS)
Wang, Yan; Lu, Zexi; Ruan, Xiulin
2016-06-01
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 (κL). It is found that p-e scattering plays an important role in determining the κL of Pt and Ni at room temperature, while it has negligible effect on the κL of Cu, Ag, Au, and Al. Specifically, the room temperature κLs of Cu, Ag, Au, and Al predicted from density-functional theory calculations with the local density approximation are 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 κL 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 κL 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, κL is found to be comparable to the electronic thermal conductivity in Ni.
Demonstration of acoustic waveguiding and tight bending in phononic crystals
Ghasemi Baboly, M.; Raza, A.; Brady, J.; ...
2016-10-31
The systematic design, fabrication, and characterization of an isolated, single-mode, 90° bend phononic crystal (PnC) waveguide are presented. A PnC consisting of a 2D square array of circular air holes in an aluminum substrate is used, and waveguides are created by introducing a line defect in the PnC lattice. A high transmission coefficient is observed (–1 dB) for the straight sections of the waveguide, and an overall 2.3 dB transmission loss is observed (a transmission coefficient of 76%) for the 90° bend. Further optimization of the structure may yield higher transmission efficiencies. Lastly, this manuscript shows the complete design processmore » for an engineered 90° bend PnC waveguide from inception to experimental demonstration.« less
Observation of the Phononic Lamb Shift with a Synthetic Vacuum
NASA Astrophysics Data System (ADS)
Rentrop, T.; Trautmann, A.; Olivares, F. A.; Jendrzejewski, F.; Komnik, A.; Oberthaler, M. K.
2016-10-01
In contrast to classical empty space, the quantum vacuum fundamentally alters the properties of embedded particles. This paradigm shift allows one to explain the discovery of the celebrated Lamb shift in the spectrum of the hydrogen atom. Here, we engineer a synthetic vacuum, building on the unique properties of ultracold atomic gas mixtures, offering the ability to switch between empty space and quantum vacuum. Using high-precision spectroscopy, we observe the phononic Lamb shift, an intriguing many-body effect originally conjectured in the context of solid-state physics. We find good agreement with theoretical predictions based on the Fröhlich model. Our observations establish this experimental platform as a new tool for precision benchmarking of open theoretical challenges, especially in the regime of strong coupling between the particles and the quantum vacuum.
Demonstration of acoustic waveguiding and tight bending in phononic crystals
Ghasemi Baboly, M.; Raza, A.; Brady, J.; Reinke, C. M.; Leseman, Z. C.; El-Kady, I.
2016-10-31
The systematic design, fabrication, and characterization of an isolated, single-mode, 90° bend phononic crystal (PnC) waveguide are presented. A PnC consisting of a 2D square array of circular air holes in an aluminum substrate is used, and waveguides are created by introducing a line defect in the PnC lattice. A high transmission coefficient is observed (–1 dB) for the straight sections of the waveguide, and an overall 2.3 dB transmission loss is observed (a transmission coefficient of 76%) for the 90° bend. Further optimization of the structure may yield higher transmission efficiencies. Lastly, this manuscript shows the complete design process for an engineered 90° bend PnC waveguide from inception to experimental demonstration.
Phonon dispersion in austenitic stainless steel Fe18Cr12Ni2Mo
NASA Astrophysics Data System (ADS)
Hoelzel, M.; Danilkin, S. A.; Hoser, A.; Ehrenberg, H.; Wieder, T.; Fuess, H.
The phonon dispersion of Fe18Cr12Ni2Mo austenitic stainless steel was measured along the symmetry directions [001], [110] and [111]. Data were analysed in the frame of the Born-von Karman model of lattice dynamics. The obtained force constants were used to evaluate the elastic constants and the engineering elastic moduli. Our results for the elastic constants confirm empirical relationships between the elastic constants found for FCC FeCrNi alloys.
Hyperbolic phonon polaritons in hexagonal boron nitride
NASA Astrophysics Data System (ADS)
Dai, Siyuan
2015-03-01
Uniaxial materials whose axial and tangential permittivities have opposite signs are referred to as indefinite or hyperbolic media. While hyperbolic responses are normally achieved with metamaterials, hexagonal boron nitride (hBN) naturally possesses this property due to the anisotropic phonons in the mid-infrared. Using scattering-type scanning near-field optical microscopy, we studied polaritonic phenomena in hBN. We performed infrared nano-imaging of highly confined and low-loss hyperbolic phonon polaritons in hBN. The polariton wavelength was shown to be governed by the hBN thickness according to a linear law persisting down to few atomic layers [Science, 343, 1125-1129 (2014)]. Additionally, we carried out the modification of hyperbolic response in heterostructures comprised of a mononlayer graphene deposited on hBN. Electrostatic gating of the top graphene layer allows for the modification of wavelength and intensity of hyperbolic phonon polaritons in bulk hBN. The physics of the modification originates from the plasmon-phonon coupling in the hyperbolic medium. Furthermore, we demonstrated the ``hyperlens'' for subdiffractional imaging and focusing using a slab of hBN.
Kohn anomaly in phonon driven superconductors
NASA Astrophysics Data System (ADS)
Das, M. P.; Chaudhury, R.
2014-08-01
Anomalies often occur in the physical world. Sometimes quite unexpectedly anomalies may give rise to new insight to an unrecognized phenomenon. In this paper we shall discuss about Kohn anomaly in a conventional phonon-driven superconductor by using a microscopic approach. Recently Aynajian et al.'s experiment showed a striking feature; the energy of phonon at a particular wave-vector is almost exactly equal to twice the energy of the superconducting gap. Although the phonon mechanism of superconductivity is well known for many conventional superconductors, as has been noted by Scalapino, the new experimental results reveal a genuine puzzle. In our recent work we have presented a detailed theoretical analysis with the help of microscopic calculations to unravel this mystery. We probe this aspect of phonon behaviour from the properties of electronic polarizability function in the superconducting phase of a Fermi liquid metal, leading to the appearance of a Kohn singularity. We show the crossover to the standard Kohn anomaly of the normal phase for temperatures above the transition temperature. Our analysis provides a nearly complete explanation of this new experimentally discovered phenomenon. This report is a shorter version of our recent work in JPCM.
Phononic subsurface: Flow stabilization by crystals
NASA Astrophysics Data System (ADS)
Hussein, Mahmoud I.; Biringen, Sedat; Bilal, Osama R.; Kucala, Alec
2015-11-01
Flow control is a century-old problem where the goal is to alter a flow's natural state to achieve improved performance, such as delay of laminar-to-turbulent transition or reduction of drag in a fully developed turbulent flow. Meeting this goal promises to significantly reduce the dependence on fossil fuels for global transport. In this work, we show that phonon motion underneath a surface interacting with a flow may be tuned to cause the flow to stabilize, or destabilize, as desired. This concept is demonstrated by simulating a fully developed plane Poiseuille (channel) flow whereby a small portion of an otherwise rigid wall is replaced with a one-dimensional phononic crystal. A Tollmien-Schlichting (TS) wave is introduced to the flow as an evolving disturbance. Upon tuning the frequency-dependent phase and amplitude relations of the surface of the phononic crystal that interfaces with the flow, the TS wave is shown to stabilize, or destabilize, as needed. A theory of subsurface phonons is presented that provides an accurate prediction of this behavior without the need for a flow simulation. This represents an unprecedented capability to passively synchronize wave propagation across a fluid-structure interface and achieve favorable, and predictable, alterations to the flow properties. National Science Foundation, Grant No. 1131802.
Phonon dispersion in red mercuric iodide
Sim, H.; Chang, Y. ); James, R.B. )
1994-02-15
We present theoretical studies of phonon modes of undoped HgI[sub 2] in its red tetragonal form. A rigid-ion model including the Coulomb interaction is used which gives the best fit to the neutron scattering, infrared reflectivity, and Raman scattering data. The calculated sound velocities are also in accord with experiment.
``Forbidden'' phonon in the iron chalcogenide series
NASA Astrophysics Data System (ADS)
Fobes, David M.; Zaliznyak, Igor A.; Xu, Zhijun; Gu, Genda; Tranquada, John M.
2015-03-01
Recently, we uncovered evidence for the formation of a bond-order wave (BOW) leading to ferro-orbital order at low temperature, acting to stabilize the bicollinear AFM order, in the iron-rich parent compound, Fe1+yTe. Investigating the inelastic spectra centered near (100) in Fe1+yTe, a signature peak for the BOW formation in the monoclinic phase, we observed an acoustic phonon dispersion in both tetragonal and monoclinic phases. While a structural Bragg peak accompanies the mode in the monoclinic phase, in the tetragonal phase Bragg scattering at this Q is forbidden by symmetry, and we observed no elastic peak. This phonon mode was also observed in superconducting FeTe0.6Se0.4, where structural and magnetic transitions are suppressed. LDA frozen phonon calculations suggested that this mode could result from a spin imbalance between neighboring Fe atoms, but polarized neutron measurements revealed no additional magnetic scattering. We propose that this ``forbidden'' phonon mode may originate from dynamically broken symmetry, perhaps related to the strong dynamic spin correlations in these materials. Work at BNL was supported by BES, US DOE, under Contract No. DE-AC02-98CH10886. Research at ORNL's HFIR and SNS sponsored by Scientific User Facilities Division, BES, US DOE. We acknowledge the support of NIST, in providing neutron research facilities.
Synthetic thermoelectric materials comprising phononic crystals
El-Kady, Ihab F; Olsson, Roy H; Hopkins, Patrick; Reinke, Charles; Kim, Bongsang
2013-08-13
Synthetic thermoelectric materials comprising phononic crystals can simultaneously have a large Seebeck coefficient, high electrical conductivity, and low thermal conductivity. Such synthetic thermoelectric materials can enable improved thermoelectric devices, such as thermoelectric generators and coolers, with improved performance. Such synthetic thermoelectric materials and devices can be fabricated using techniques that are compatible with standard microelectronics.
Illustrative numerical comparisons between phonon mean free paths and phonon thermal conductivity
NASA Astrophysics Data System (ADS)
MacDonald, W. M.; Anderson, A. C.
Measurements of thermal conductivity are often used as an interrogative technique to learn about phonon scattering processes in solids. In general the relationship between thermal conductivity lambda and a phonon mean free path 1 is complex and it is therefore necessary to make some simplifying assumptions in order to make this relationship tractable. These assumptions may lead to erroneous conclusions, many of which have appeared in the published literature. An intuitive insight is provided to the relationship between lambda and 1.
Controlling thermal emission of phonon by magnetic metasurfaces.
Zhang, X; Liu, H; Zhang, Z G; Wang, Q; Zhu, S N
2017-02-03
Our experiment shows that the thermal emission of phonon can be controlled by magnetic resonance (MR) mode in a metasurface (MTS). Through changing the structural parameter of metasurface, the MR wavelength can be tuned to the phonon resonance wavelength. This introduces a strong coupling between phonon and MR, which results in an anticrossing phonon-plasmons mode. In the process, we can manipulate the polarization and angular radiation of thermal emission of phonon. Such metasurface provides a new kind of thermal emission structures for various thermal management applications.
Controlling thermal emission of phonon by magnetic metasurfaces
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
Phonon-plasmon coupled modes in GaN
NASA Astrophysics Data System (ADS)
Dyson, A.
2009-04-01
The phonon lifetime in GaN is known to exhibit a dependence on electron density. Recent noise measurements have also shown the lifetime to be temperature dependent. The source of these dependences is the coupling of the phonon and plasmon populations through the dielectric function. The effect of this anharmonicity is illustrated by comparing the frequency and wavevector dependent coupled-mode momentum relaxation rate with the phonon momentum relaxation rate obtained by Callen. A simple model that includes the anharmonic interaction and phonon migration yields phonon lifetimes depending on both electron density and temperature.
Controlling thermal emission of phonon by magnetic metasurfaces
NASA Astrophysics Data System (ADS)
Zhang, X.; Liu, H.; Zhang, Z. G.; Wang, Q.; Zhu, S. N.
2017-02-01
Our experiment shows that the thermal emission of phonon can be controlled by magnetic resonance (MR) mode in a metasurface (MTS). Through changing the structural parameter of metasurface, the MR wavelength can be tuned to the phonon resonance wavelength. This introduces a strong coupling between phonon and MR, which results in an anticrossing phonon-plasmons mode. In the process, we can manipulate the polarization and angular radiation of thermal emission of phonon. Such metasurface provides a new kind of thermal emission structures for various thermal management applications.
Acoustic phonon lifetimes and thermal transport in free-standing and strained graphene.
Bonini, Nicola; Garg, Jivtesh; Marzari, Nicola
2012-06-13
We use first-principles methods based on density functional perturbation theory to characterize the lifetimes of the acoustic phonon modes and their consequences on the thermal transport properties of graphene. We show that using a standard perturbative approach, the transverse and longitudinal acoustic phonons in free-standing graphene display finite lifetimes in the long-wavelength limit, making them ill-defined as elementary excitations in samples of dimensions larger than ∼1 μm. This behavior is entirely due to the presence of the quadratic dispersions for the out-of-plane phonon (ZA) flexural modes that appear in free-standing low-dimensional systems. Mechanical strain lifts this anomaly, and all phonons remain well-defined at any wavelength. Thermal transport is dominated by ZA modes, and the thermal conductivity is predicted to diverge with system size for any amount of strain. These findings highlight strain and sample size as key parameters in characterizing or engineering heat transport in graphene.
Large phonon entropy drives the metallization of vanadium dioxide (VO2)
NASA Astrophysics Data System (ADS)
Hong, Jiawang
2015-03-01
Vanadium dioxide (VO2) exhibits a first-order metal-insulator transition (MIT) near room temperature, where conductivity is suppressed and the lattice changes from tetragonal to monoclinic on cooling. This MIT in VO2 has attracted intense interest from both fundamental and technological perspectives. However, most studies performed in the past 50 years have focused on the electronic structure and energetics of the transition, ignoring the role of phonons and their entropic contribution to the phase stability. Much of the reason is that the standard tool of neutron scattering does not yield coherent scattering from V nuclei, and first-principles methods with harmonic approximation cannot capture the stable phonons for the rutile phase. We close this gap by using a combination of ab initio molecular dynamics calculations and neutron/x-ray scattering to establish that the entropy driving the MIT is dominated by soft, anharmonic phonons of the metallic phase. The MIT results from the competition between lower electronic energy in insulating M1 phase due to the Peierls instability, and the higher entropy of the metallic rutile phase resulting from soft anharmonic phonons. This understanding of the role of lattice dynamics and their relationship to electronic structure provides a critical component for developing more complete physical models of phase competition in functional transition metal oxides. Theoretical calculations were performed using the NERSC at LBNL. Modeling of neutron data was performed in CAMM, measurements were funded by the US DOE, BES, Materials Science and Engineering Division.
Tunable evolutions of wave modes and bandgaps in quasi-1D cylindrical phononic crystals
NASA Astrophysics Data System (ADS)
Meidani, Mehrashk; Kim, Eunho; Li, Feng; Yang, Jinkyu; Ngo, Duc
2015-01-01
We investigate the tunable characteristics of mechanical waves propagating in quasi-1D phononic crystals composed of horizontally stacked short cylinders at various contact angles and offsets. According to the Hertzian contact theory, elastic compression of laterally-touching cylindrical bodies exhibits a various range of contact stiffness depending on their alignment angles. In this study, we first assemble cylindrical particles in various combinations of inclination angles and systematically examine their forming mechanisms of frequency bandgaps. We also investigate the effect of the rattling motions of cylindrical particles by introducing asymmetric center-of-mass offsets with respect to their contact points. We find that the frequency responses of these quasi-1D phononic crystals evolve into multiple band structures as we employ higher deviations of contact angles and offsets. We calculate the dispersive behavior of propagating waves using a discrete particle model for simple zero-offset cases, while we use a finite element method for simulating the rattling motions of particles under non-zero offsets. We report branching behavior of frequency band structures and the evolution of their vibration modes as we manipulate the contact angles and offsets of the phononic crystals. This study implies that we can leverage the versatile wave filtering characteristics of quasi-1D phononic crystals to construct tunable wave filtering devices for engineering applications.
Phononic Origins of Friction in Carbon Nanotube Oscillators.
Prasad, Matukumilli V D; Bhattacharya, Baidurya
2017-03-01
Phononic coupling can have a significant role in friction between nanoscale surfaces. We find frictional dissipation per atom in carbon nanotube (CNT) oscillators to depend significantly on interface features such as contact area, commensurability, and by end-capping of the inner core. We perform large-scale phonon wavepacket MD simulations to study phonon coupling between a 250 nm long (10,10) outer tube and inner cores of four different geometries. Five different phonon polarizations known to have dominant roles in thermal transport are selected, and transmission coefficient plots for a range of phonon energies along with phonon scattering dynamics at specific energies are obtained. We find that the length of interface affects friction only through LA phonon scattering and has a significant nonlinear effect on total frictional force. Incommensurate contact does not always give rise to superlubricity: the net effect of two competing interaction mechanisms shown by longitudinal and transverse phonons decides the role of commensurability. Capping of the core has no effect on acoustic phonons but destroys the coherence of transverse optical phonons and creates diffusive scattering. In contrast, the twisting and radial breathing phonon modes have perfect transmission at all energies and can be deemed as the enablers of ultralow friction in CNT oscillators. Our work suggests that tuning of interface geometries can give rise to desirable friction properties in nanoscale devices.
Towards a microscopic understanding of the phonon bottleneck
Garanin, D. A.
2007-03-01
The problem of the phonon bottleneck in the relaxation of two-level systems (spins) to a narrow group of resonant phonons via emission-absorption processes is investigated from first principles. It is shown that the kinetic approach based on the Pauli master equation is invalid because of the narrow distribution of the phonons exchanging their energy with the spins. This results in a long-memory effect that can be best taken into account by introducing an additional dynamical variable corresponding to the nondiagonal matrix elements responsible for spin-phonon correlation. The resulting system of dynamical equations describes the phonon-bottleneck plateau in the spin excitation, as well as a gap in the spin-phonon spectrum, for any finite concentration of spins. On the other hand, it does not accurately render the line shape of emitted phonons and still needs improving.
NASA Astrophysics Data System (ADS)
Dermez, Rasim
2016-10-01
It is solved a time-dependent Hamiltonian using a unitary transformation method which Λ(t) type is used to engineer a cascade Ξ scheme interaction between the vibrational phonons and trapped three-level ion. Quantum entanglement is characterized by comparing concurrence and negativity of the time-dependent ionic-phononic system. In this quantum system, we obtain that the amount of concurrence can be tuned between 0 and 0.99 while the amount of negativity changes between 0 and 0.49.
Frequency stabilization of the zero-phonon line of a quantum dot via phonon-assisted active feedback
Hansom, Jack; Schulte, Carsten H. H.; Matthiesen, Clemens; Stanley, Megan J.; Atatüre, Mete
2014-10-27
We report on the feedback stabilization of the zero-phonon emission frequency of a single InAs quantum dot. The spectral separation of the phonon-assisted component of the resonance fluorescence provides a probe of the detuning between the zero-phonon transition and the resonant driving laser. Using this probe in combination with active feedback, we stabilize the zero-phonon transition frequency against environmental fluctuations. This protocol reduces the zero-phonon fluorescence intensity noise by a factor of 22 by correcting for environmental noise with a bandwidth of 191 Hz, limited by the experimental collection efficiency. The associated sub-Hz fluctuations in the zero-phonon central frequency are reduced by a factor of 7. This technique provides a means of stabilizing the quantum dot emission frequency without requiring access to the zero-phonon emission.
Anyon pairing via phonon-mediated interaction
NASA Astrophysics Data System (ADS)
Kandemir, B. S.
2006-08-01
In this paper, we study the pairing of anyons subjected to an external uniform magnetic field and confined in a two-dimensional parabolic quantum dot within the framework of Fröhlich large bipolaron theory, motivated by the Wilczek’s prescription that treats anyons as composites having both charges and fictitious flux tubes. In this model, electrons bound to Aharanov-Bohm type flux tubes and surrounded by a cloud of virtual LO phonons interact with each other through the long range Coulomb and statistical potentials. In order to discuss the effects of both spatial confinement potential and external uniform magnetic field on the boundaries of the stability region of such a pairing in real space, we perform a self-consistent treatment of the ground-state energies of both an interacting anyon pair and two noninteracting anyons. Our results suggest that two interacting anyons can be bound into a condensate anyon pair through a phonon-mediated interaction.
Magnon rainbows filtered through phonon clouds
NASA Astrophysics Data System (ADS)
Boona, Stephen R.
2016-06-01
The study of heat flow in magnetic insulators is a topic of significant interest in spin caloritronics, especially for understanding the nuanced origins of the spin Seebeck effect (SSE). Recent work by Diniz and Costa (2016 New J. Phys. 18 052002) provides insight into this subject by presenting a microscopic model for the spectral dependence of magnon-phonon interactions in magnetic insulators, which has been a challenging puzzle for decades. Their new paper shows that phonon-mediated magnon-magnon interactions affect the lifetime of magnons differently depending on the magnon wavelength. As a result, low energy magnons transport spin more efficiently, and are more sensitive to applied magnetic fields. These results help explain some unexpected behavior in the SSE recently reported in several experiments.
Phonon dispersion relation of metallic glasses
NASA Astrophysics Data System (ADS)
Crespo, Daniel; Bruna, Pere; Valles, Araceli; Pineda, Eloi
2016-10-01
Experimental data on the phase sound speed of metallic glasses show anomalies in the terahertz range, reflecting an underlying complex behavior of their phonon dispersion spectrum not yet explained. We determine the phonon dispersion curve of metallic glasses by means of massive molecular dynamics simulations, allowing us to obtain the low-q region behavior with unprecedented detail. Results confirm that the sound speed is constant below the THz range, down to the macroscopic limit. On the contrary, a hardening of the sound speed, more notable in the transverse case, is found in the THz range. This behavior is modeled in terms of a relaxation model. The model gives quantitative agreement and allows us to determine a new threshold frequency ωh, at the end of the boson-peak region. Above ωh the shear modulus increases dramatically, reflecting the end of the amorphous-like acoustic propagation region characterized by the excess density of vibrational states.
Phonon arithmetic in a trapped ion system
Um, Mark; Zhang, Junhua; Lv, Dingshun; Lu, Yao; An, Shuoming; Zhang, Jing-Ning; Nha, Hyunchul; Kim, M. S.; Kim, Kihwan
2016-01-01
Single-quantum level operations are important tools to manipulate a quantum state. Annihilation or creation of single particles translates a quantum state to another by adding or subtracting a particle, depending on how many are already in the given state. The operations are probabilistic and the success rate has yet been low in their experimental realization. Here we experimentally demonstrate (near) deterministic addition and subtraction of a bosonic particle, in particular a phonon of ionic motion in a harmonic potential. We realize the operations by coupling phonons to an auxiliary two-level system and applying transitionless adiabatic passage. We show handy repetition of the operations on various initial states and demonstrate by the reconstruction of the density matrices that the operations preserve coherences. We observe the transformation of a classical state to a highly non-classical one and a Gaussian state to a non-Gaussian one by applying a sequence of operations deterministically. PMID:27097897
Phonon arithmetic in a trapped ion system.
Um, Mark; Zhang, Junhua; Lv, Dingshun; Lu, Yao; An, Shuoming; Zhang, Jing-Ning; Nha, Hyunchul; Kim, M S; Kim, Kihwan
2016-04-21
Single-quantum level operations are important tools to manipulate a quantum state. Annihilation or creation of single particles translates a quantum state to another by adding or subtracting a particle, depending on how many are already in the given state. The operations are probabilistic and the success rate has yet been low in their experimental realization. Here we experimentally demonstrate (near) deterministic addition and subtraction of a bosonic particle, in particular a phonon of ionic motion in a harmonic potential. We realize the operations by coupling phonons to an auxiliary two-level system and applying transitionless adiabatic passage. We show handy repetition of the operations on various initial states and demonstrate by the reconstruction of the density matrices that the operations preserve coherences. We observe the transformation of a classical state to a highly non-classical one and a Gaussian state to a non-Gaussian one by applying a sequence of operations deterministically.
Yan, Zhequan; Chen, Liang; Yoon, Mina; ...
2016-01-12
Hexagonal boron nitride (h-BN) is a substrate for graphene based nano-electronic devices. We investigate the ballistic phonon transport at the interface of vertically stacked graphene and h-BN heterostructures using first principles density functional theory and atomistic Green's function simulations considering the influence of lattice stacking. We compute the frequency and wave-vector dependent transmission function and observe distinct stacking-dependent phonon transmission features for the h-BN/graphene/h-BN sandwiched systems. We find that the in-plane acoustic modes have the dominant contributions to the phonon transmission and thermal boundary conductance (TBC) for the interfaces with the carbon atom located directly on top of the boronmore » atom (C–B matched) because of low interfacial spacing. The low interfacial spacing is a consequence of the differences in the effective atomic volume of N and B and the difference in the local electron density around N and B. For the structures with the carbon atom directly on top of the nitrogen atom (C–N matched), the spatial distance increases and the contribution of in-plane modes to the TBC decreases leading to higher contributions by out-of-plane acoustic modes. We find that the C–B matched interfaces have stronger phonon–phonon coupling than the C–N matched interfaces, which results in significantly higher TBC (more than 50%) in the C–B matched interface. The findings in this study will provide insights to understand the mechanism of phonon transport at h-BN/graphene/h-BN interfaces, to better explain the experimental observations and to engineer these interfaces to enhance heat dissipation in graphene based electronic devices.« less
Yan, Zhequan; Chen, Liang; Yoon, Mina; Kumar, Satish
2016-01-12
Hexagonal boron nitride (h-BN) is a substrate for graphene based nano-electronic devices. We investigate the ballistic phonon transport at the interface of vertically stacked graphene and h-BN heterostructures using first principles density functional theory and atomistic Green's function simulations considering the influence of lattice stacking. We compute the frequency and wave-vector dependent transmission function and observe distinct stacking-dependent phonon transmission features for the h-BN/graphene/h-BN sandwiched systems. We find that the in-plane acoustic modes have the dominant contributions to the phonon transmission and thermal boundary conductance (TBC) for the interfaces with the carbon atom located directly on top of the boron atom (C–B matched) because of low interfacial spacing. The low interfacial spacing is a consequence of the differences in the effective atomic volume of N and B and the difference in the local electron density around N and B. For the structures with the carbon atom directly on top of the nitrogen atom (C–N matched), the spatial distance increases and the contribution of in-plane modes to the TBC decreases leading to higher contributions by out-of-plane acoustic modes. We find that the C–B matched interfaces have stronger phonon–phonon coupling than the C–N matched interfaces, which results in significantly higher TBC (more than 50%) in the C–B matched interface. The findings in this study will provide insights to understand the mechanism of phonon transport at h-BN/graphene/h-BN interfaces, to better explain the experimental observations and to engineer these interfaces to enhance heat dissipation in graphene based electronic devices.
Zhou, Changjiang; Sai, Yi; Chen, Jiujiu
2016-09-01
This paper theoretically investigates the band gaps of Lamb mode waves in two-dimensional magnetoelastic phononic crystal slabs by an applied external magnetostatic field. With the assumption of uniformly oriented magnetization, an equivalent piezomagnetic material model is used. The effects of magnetostatic field on phononic crystals are considered carefully in this model. The numerical results indicate that the width of the first band gap is significantly changed by applying the external magnetic field with different amplitude, and the ratio between the maximum and minimum gap widths reaches 228%. Further calculations demonstrate that the orientation of the magnetic field obviously affects the width and location of the first band gap. The contactless tunability of the proposed phononic crystal slabs shows many potential applications of vibration isolation in engineering.
Zeng, Lingping; Collins, Kimberlee C.; Hu, Yongjie; Luckyanova, Maria N.; Maznev, Alexei A.; Huberman, Samuel; Chiloyan, Vazrik; Zhou, Jiawei; Huang, Xiaopeng; Nelson, Keith A.; Chen, Gang
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-domain 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.
Zeng, Lingping; Collins, Kimberlee C.; Hu, Yongjie; Luckyanova, Maria N.; Maznev, Alexei A.; Huberman, Samuel; Chiloyan, Vazrik; Zhou, Jiawei; Huang, Xiaopeng; Nelson, Keith A.; Chen, Gang
2015-01-01
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-domain thermoreflectance measurements and simultaneously act as wire-grid 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. This table-top ultrafast thermal transport spectroscopy technique enables the study of mean free path spectra in a wide range of technologically important materials. PMID:26612032
Zeng, Lingping; Collins, Kimberlee C; Hu, Yongjie; Luckyanova, Maria N; Maznev, Alexei A; Huberman, Samuel; Chiloyan, Vazrik; Zhou, Jiawei; Huang, Xiaopeng; Nelson, Keith A; Chen, Gang
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-domain thermoreflectance measurements and simultaneously act as wire-grid 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. This table-top ultrafast thermal transport spectroscopy technique enables the study of mean free path spectra in a wide range of technologically important materials.
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
Quantum Coherent Electron-Phonon Nanolaboratories
2006-05-31
published as “Single-crystal aluminum nitride nanomechanical resonators”, A.N. Cleland, M. Pophristic and I. Ferguson , Appl. Phys. Lett. 79, 2070 (2001...nanostructured phononic crystals”, Phys. Rev. B 64, 172301 (2001) A.N. Cleland, M. Pophristic and I. Ferguson , “Single-crystal aluminum nitride...Phys. Lett. 79, 1202 ~2001!. 6 M. J. Kelly, R. J. Brown, C. G. Smith, D. A. Wharam, M. Pepper , H. Ahmed, D. G. Hasko, D. C. Peacock, J. E. F. Frost, and
Ignatov, Anatoly A.
2014-08-28
The current (voltage) responsivity of a superlattice-based diode detector has been studied theoretically in the terahertz frequency band that includes the region of the polar-optical phonon frequencies. Within the framework of an equivalent circuit approach, the electro-dynamical model which allows one to analyze the responsivity taking into account the hybridization of the plasma and polar-optical phonon modes both in the substrate and in the cladding layers of the diode has been suggested. It has been shown that the presence of the plasma and polar-optical phonon modes gives rise to strong features in the frequency dependence of the responsivity, i.e., to the resonance dips and peaks at frequencies of hybridized plasmons and polar-optical phonons. It has been suggested that by judicious engineering of the superlattice-based diodes, it would be possible to enhance substantially their responsivity in the terahertz frequency band.
Phonons of the anomalous element cerium
Krisch, Michael; Farber, D. L.; Xu, R.; Antonangeli, Daniele; Aracne, C. M.; Beraud, Alexandre; Chiang, Tai-Chang; Zarestky, J.; Kim, Duck Young; Isaev, Eyvaz I.; Ahuja, Rajeev; Johansson, Börje
2011-01-01
Many physical and chemical properties of the light rare-earths and actinides are governed by the active role of f electrons, and despite intensive efforts the details of the mechanisms of phase stability and transformation are not fully understood. A prominent example which has attracted a lot of interest, both experimentally and theoretically over the years is the isostructural γ - α transition in cerium. We have determined by inelastic X-ray scattering, the complete phonon dispersion scheme of elemental cerium across the γ → α transition, and compared it with theoretical results using ab initio lattice dynamics. Several phonon branches show strong changes in the dispersion shape, indicating large modifications in the interactions between phonons and conduction electrons. This is reflected as well by the lattice Grüneisen parameters, particularly around the X point. We derive a vibrational entropy change , illustrating the importance of the lattice contribution to the transition. Additionally, we compare first principles calculations with the experiments to shed light on the mechanism underlying the isostructural volume collapse in cerium under pressure. PMID:21597000
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.
Phonon-lifetimes in demixing systems
NASA Astrophysics Data System (ADS)
Davaasambuu, J.; Güthoff, F.; Petri, M.; Hradil, K.; Schober, H.; Ollivier, J.; Eckold, G.
2012-06-01
The dynamics of silver-alkali halide mixed single crystals (AgxNa1-xBr, x = 0.23, 0.35, 0.40 and 0.70) were studied by inelastic neutron scattering during the process of spinodal decomposition. Using the thermal three-axes spectrometer PUMA as well as the time-of-flight spectrometer IN5, the time evolution of phonons was observed in time-resolved, stroboscopic measurements. Complementary to the study of long wavelength acoustic phonons, as studied previously, we extended these investigations to Brillouin-zone boundary modes that are particularly sensitive to variations of the local structure. Starting from the homogeneous mixed phase the behaviour of these modes during demixing is observed in real-time. A simple dynamical model based on local structure variants helps to interpret the results. It is shown that the phonon lifetimes vary strongly during the phase separation and increase drastically during the coarsening process. Up to a critical size of precipitates of about 10 nm, zone-boundary modes are found to be strongly damped, while beyond the line widths are reduced to the experimental resolution. This finding leads to the conclusion that the typical mean free path of these modes is of the order of 10 nm, which corresponds to 20 unit cells.
NASA Astrophysics Data System (ADS)
Xiong, Shiyun; Sääskilahti, Kimmo; Kosevich, Yuriy A.; Han, Haoxue; Donadio, Davide; Volz, Sebastian
2016-07-01
Understanding the design rules to obtain materials that enable a tight control of phonon transport over a broad range of frequencies would aid major developments in thermoelectric energy harvesting, heat management in microelectronics, and information and communication technology. Using atomistic simulations we show that the metamaterials approach relying on localized resonances is very promising to engineer heat transport at the nanoscale. Combining designed resonant structures to alloying can lead to extremely low thermal conductivity in silicon nanowires. The hybridization between resonant phonons and propagating modes greatly reduces the group velocities and the phonon mean free paths in the low frequency acoustic range below 4 THz. Concurrently, alloy scattering hinders the propagation of high frequency thermal phonons. Our calculations establish a rationale between the size, shape, and period of the resonant structures, and the thermal conductivity of the nanowire, and demonstrate that this approach is even effective to block phonon transport in wavelengths much longer than the size and period of the surface resonant structures. A further consequence of using resonant structures is that they are not expected to scatter electrons, which is beneficial for thermoelectric applications.
Romano, Giuseppe; Kolpak, Alexie M.
2017-01-01
Boundary-engineering in nanostructures has the potential to dramatically impact the development of materials for high- efficiency conversion of thermal energy directly into electricity. In particular, nanostructuring of semiconductors can lead to strong suppression of heat transport with little degradation of electrical conductivity. Although this combination of material properties is promising for thermoelectric materials, it remains largely unexplored. In this work, we introduce a novel concept, the directional phonon suppression function, to unravel boundary-dominated heat transport in unprecedented detail. Using a combination of density functional theory and the Boltzmann transport equation, we compute this quantity for nanoporous silicon materials. We first compute the thermal conductivity for the case with aligned circular pores, confirming a significant thermal transport degradation with respect to the bulk. Then, by analyzing the information on the directionality of phonon suppression in this system, we identify a new structure of rectangular pores with the same porosity that enables a four-fold decrease in thermal transport with respect to the circular pores. Our results illustrate the utility of the directional phonon suppression function, enabling new avenues for systematic thermal conductivity minimization and potentially accelerating the engineering of next-generation thermoelectric devices. PMID:28338003
NASA Astrophysics Data System (ADS)
Romano, Giuseppe; Kolpak, Alexie M.
2017-03-01
Boundary-engineering in nanostructures has the potential to dramatically impact the development of materials for high- efficiency conversion of thermal energy directly into electricity. In particular, nanostructuring of semiconductors can lead to strong suppression of heat transport with little degradation of electrical conductivity. Although this combination of material properties is promising for thermoelectric materials, it remains largely unexplored. In this work, we introduce a novel concept, the directional phonon suppression function, to unravel boundary-dominated heat transport in unprecedented detail. Using a combination of density functional theory and the Boltzmann transport equation, we compute this quantity for nanoporous silicon materials. We first compute the thermal conductivity for the case with aligned circular pores, confirming a significant thermal transport degradation with respect to the bulk. Then, by analyzing the information on the directionality of phonon suppression in this system, we identify a new structure of rectangular pores with the same porosity that enables a four-fold decrease in thermal transport with respect to the circular pores. Our results illustrate the utility of the directional phonon suppression function, enabling new avenues for systematic thermal conductivity minimization and potentially accelerating the engineering of next-generation thermoelectric devices.
Acoustic phonon transmission spectra in piezoelectric AlN/GaN Fibonacci phononic crystals
NASA Astrophysics Data System (ADS)
Sesion, P. D., Jr.; Albuquerque, E. L.; Chesman, C.; Freire, V. N.
2007-08-01
We study the acoustic-phonon transmission spectra in periodic and quasiperiodic (Fibonacci type) superlattices made up from the III-V nitride materials AlN and GaN. The phonon dynamics is described by a coupled elastic and electromagnetic equations within the static field approximation model, stressing the importance of the piezoelectric polarization field in a strained condition. We use a transfer-matrix treatment to simplify the algebra, which would be otherwise quite complicated, allowing a neat analytical expressions for the phonon transmission coefficients. Numerical results, for the normal incidence case, show a strike self-similar pattern for both hexagonal (class 6 mm) and cubic symmetries crystalizations of the nitrides.
From Modal Mixing to Tunable Functional Switches in Nonlinear Phononic Crystals
NASA Astrophysics Data System (ADS)
Ganesh, R.; Gonella, S.
2015-02-01
We introduce a paradigm for spatial and modal wave manipulation based on nonlinear phononic crystals and explore its potential for engineering wave control systems with tunable, adaptive, and multifunctional characteristics. Our approach exploits nonlinear mechanisms to stretch the frequency signature of the wave response and distribute it over multiple modes, thereby activating a mixture of modal characteristics and enabling functionalities associated with high-frequency optical modes, even while operating in the low-frequency regime. To elucidate the versatility of this approach, we consider different granular crystal configurations that span the available landscape of crystal topologies and wave control functionalities. The ability to switch between complementary functionalities allows rethinking nonlinear phononic crystals as programmable acoustic ports that form the building blocks of a new structural logic framework enabled by nonlinearity.
Topological phononic states of underwater sound based on coupled ring resonators
He, Cheng; Li, Zheng; Ni, Xu; Sun, Xiao-Chen; Yu, Si-Yuan; Lu, Ming-Hui Liu, Xiao-Ping; Chen, Yan-Feng
2016-01-18
We report a design of topological phononic states for underwater sound using arrays of acoustic coupled ring resonators. In each individual ring resonator, two degenerate acoustic modes, corresponding to clockwise and counter-clockwise propagation, are treated as opposite pseudospins. The gapless edge states arise in the bandgap resulting in protected pseudospin-dependent sound transportation, which is a phononic analogue of the quantum spin Hall effect. We also investigate the robustness of the topological sound state, suggesting that the observed pseudospin-dependent sound transportation remains unless the introduced defects facilitate coupling between the clockwise and counter-clockwise modes (in other words, the original mode degeneracy is broken). The topological engineering of sound transportation will certainly promise unique design for next generation of acoustic devices in sound guiding and switching, especially for underwater acoustic devices.
Two-Dimensional Phononic-Photonic Band Gap Optomechanical Crystal Cavity
NASA Astrophysics Data System (ADS)
Safavi-Naeini, Amir H.; Hill, Jeff T.; Meenehan, Seán; Chan, Jasper; Gröblacher, Simon; Painter, Oskar
2014-04-01
We present the fabrication and characterization of an artificial crystal structure formed from a thin film of silicon that has a full phononic band gap for microwave X-band phonons and a two-dimensional pseudo-band gap for near-infrared photons. An engineered defect in the crystal structure is used to localize optical and mechanical resonances in the band gap of the planar crystal. Two-tone optical spectroscopy is used to characterize the cavity system, showing a large coupling (g0/2π≈220 kHz) between the fundamental optical cavity resonance at ωo/2π =195 THz and colocalized mechanical resonances at frequency ωm/2π ≈9.3 GHz.
Interaction of excitons with optical phonons in layer crystals
NASA Astrophysics Data System (ADS)
Nitsovich, Bohdan M.; Zenkova, C. Y.; Kramar, N. K.
2002-02-01
The investigation is concerned with layer crystals of the GaSe, InSe, GaTe, MoS2-type and other inorganic semiconductors, whose phonon spectrum has a great number of peculiarities, among them the availability of low-energy optical phonons. In this case the dispersion of these phonons can be essential and vary in character. The mass operator of the exciton-phonon system and the light absorption coefficient for different dispersion laws of optical phonons have been calculated. The influence of the sign of the phonon 'effective mass' on the exciton absorption band of layer crystals, which causes the opposite in sign dynamics of the absorption maximum shift, and the change of the absorption curve asymmetry have been determined.
Electron-Phonon Coupling in a CdSe Nanowire
NASA Astrophysics Data System (ADS)
Barrett, Christopher; Wang, Lin-Wang
2012-02-01
It is important to calculate the coupling between phonons and electrons in realistic nanostructures, e.g. to understand carrier cooling and dynamics in a nanowire. In this talk, we will present results of phonon spectrum calculations using a customized valence force field (VFF) method. This customized VFF method is developed to be fittable to the results of any ab-initio calculations, with density functional theory (DFT) results being used in this work. By fitting many different DFT calculations on different motifs and their perturbations, we have obtained in the custom VFF a very efficient method that closely reproduces DFT phonons for CdSe nanowires with (10-10) surfaces having Cd-Se dimerization. We have also combined the results of these phonon spectrum calculations with electronic structure calculations to obtain the electron-phonon coupling. We will present this result and and show how the electron-phonon coupling affects the carrier dynamics in the nanowire.
Phonon spectrum in a CdSe nanowire
NASA Astrophysics Data System (ADS)
Barrett, Chris; Wang, Lin-Wang
2011-03-01
It is important to calculate the phonon spectrum of realistic nanowires, e.g. to understand its thermo conductivity or to calculate the electron-phonon interaction. In this talk, we will present results of phonon spectrum calculation using valence force field (VFF) method. An important issue is to construct the VFF to describe the surface atomic displacement. We have developed a general VFF formalism to fit our VFF result with the density functional theory (DFT) calculated surface atom displacement energies. In particular, the (10-10) CdSe surface is modelled with Cd-Se dimerization. We will discuss the quality of such VFF model. The phonon spectrum of the nanowire will be presented, and its implication on the phonon transport and electron-phonon coupling will also be discussed. This work is supported by U.S. Department of Energy BES, office of science, under Contract No. DE-AC02-05CH11231.
Reduction of Thermal Conductivity by Nanoscale 3D Phononic Crystal
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
Single Circuit Parallel Computing with Phonons through Magneto-acoustics
NASA Astrophysics Data System (ADS)
Sklan, Sophia; Grossman, Jeffrey
2013-03-01
Phononic computing - the use of (typically thermal) vibrations for information processing - is a nascent technology; its capabilities are still being discovered. We analyze an alternative form of phononic computing inspired by optical, rather than electronic, computing. Using the acoustic Faraday effect, we design a phonon gyrator and thereby a means of performing computation through the manipulation of polarization in transverse phonon currents. Moreover, we establish that our gyrators act as generalized transistors and can construct digital logic gates. Exploiting the wave nature of phonons and the similarity of our logic gates, we demonstrate parallel computation within a single circuit, an effect presently unique to phonons. Finally, a generic method of designing these parallel circuits is introduced and used to analyze the feasibility of magneto-acoustic materials in realizing these circuits. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. 1122374.
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.
Reduction of thermal conductivity by nanoscale 3D phononic crystal.
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.
Acoustic phonon spectrum and thermal transport in nanoporous alumina arrays
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
Acoustic phonon spectrum and thermal transport in nanoporous alumina arrays
Kargar, Fariborz; Ramirez, Sylvester; Debnath, Bishwajit; Malekpour, Hoda; Lake, Roger; Balandin, Alexander A.
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, analysis 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.
Phonon Scattering Dynamics of Thermophoretic Motion in Carbon Nanotube Oscillators.
Prasad, Matukumilli V D; Bhattacharya, Baidurya
2016-04-13
Using phonon wave packet molecular dynamics simulations, we find that anomalous longitudinal acoustic (LA) mode phonon scattering in low to moderate energy ranges is responsible for initiating thermophoretic motion in carbon nanotube oscillators. The repeated scattering of a single mode LA phonon wave packet near the ends of the inner nanotube provides a net unbalanced force that, if large enough, initiates thermophoresis. By applying a coherent phonon pulse on the outer tube, which generalizes the single mode phonon wave packet, we are able to achieve thermophoresis in a carbon nanotube oscillator. We also find the nature of the unbalanced force on end-atoms to be qualitatively similar to that under an imposed thermal gradient. The thermodiffusion coefficient obtained for a range of thermal gradients and core lengths suggest that LA phonon scattering is the dominant mechanism for thermophoresis in longer cores, whereas for shorter cores, it is the highly diffusive mechanism that provides the effective force.
1985-10-01
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The Seebeck Coefficient and Phonon Drag in Silicon
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.
Weak phonon scattering effect of twin boundaries on thermal transmission
Dong, Huicong; Xiao, Jianwei; Melnik, Roderick; Wen, Bin
2016-01-01
To study the effect of twin boundaries on thermal transmission, thermal conductivities of twinned diamond with different twin thicknesses have been studied by NEMD simulation. Results indicate that twin boundaries show a weak phonon scattering effect on thermal transmission, which is only caused by the additional twin boundaries’ thermal resistance. Moreover, according to phonon kinetic theory, this weak phonon scattering effect of twin boundaries is mainly caused by a slightly reduced average group velocity. PMID:26822675
Weak phonon scattering effect of twin boundaries on thermal transmission.
Dong, Huicong; Xiao, Jianwei; Melnik, Roderick; Wen, Bin
2016-01-29
To study the effect of twin boundaries on thermal transmission, thermal conductivities of twinned diamond with different twin thicknesses have been studied by NEMD simulation. Results indicate that twin boundaries show a weak phonon scattering effect on thermal transmission, which is only caused by the additional twin boundaries' thermal resistance. Moreover, according to phonon kinetic theory, this weak phonon scattering effect of twin boundaries is mainly caused by a slightly reduced average group velocity.
Three material and four material one-dimensional phononic crystals
NASA Astrophysics Data System (ADS)
Kriegel, Ilka; Scotognella, Francesco
2017-01-01
In this work, we studied one-dimensional phononic structures for selective acoustic filtering. The structures are composed of three and four materials which have different elastic properties. We have observed that the phononic band gaps split in two and three transmission valleys for the three-material and the four-material based phononic structures, respectively. Furthermore, the number of transmission peaks between the split gaps is directly related to the number of unit cells composing the phononic structures. The observations of this work can be useful for the fabrication of acoustic filters with the possibility to select the transmission of particular frequencies.
Bloch oscillations in the presence of plasmons and phonons
Ghosh; Jonsson; Wilkins
2000-07-31
The coupling between Bloch oscillating electrons and longitudinal optical phonons in a superlattice leads to resonant phonon excitation but no gap in the Bloch-phonon spectrum. In addition, we predict a sharp transition from plasma to Bloch oscillations at nu(B) = 2nu(P). From a microscopic description with phenomenological dampings, we numerically map out the behavior of coupled Bloch-plasmon-phonon modes for a wide range of parameters, and mimic experimental conditions. Our results are in good agreement with recent experiments by Dekorsy et al. [Phys. Rev. Lett. 85, 1080 (2000)].
Theory of coherent phonon spectroscopy in carbon nanotubes
NASA Astrophysics Data System (ADS)
Sanders, G. D.; Stanton, C. J.; Lim, Y. S.; Yee, K. J.; Kim, J. H.; Haroz, E. H.; Booshehri, L. G.; Kono, J.
2008-03-01
We develop a theory for the generation and detection of coherent phonons in single wall carbon nanotubes. Coherent phonons are generated in the nanotube by ultrafast laser pulses via the deformation potential electron-phonon interaction with the photogenerated carriers. The electronic states are treated in a tight binding formalism which gives a description of the states over the nanotube Brillouin zone while the nanotube phonon modes are treated in a valence force field model that includes bond-stretching, in-plane and out-of-plane bond-bending, and bond-twisting interactions. Equations of motion for the coherent phonon amplitudes are obtained in a density matrix formalism and we find that the coherent phonon amplitudes satisfy driven oscillator equations. In coherent phonon spectroscopy the coherent phonons are detected by ultrafast pump probe differential transmission measurements. We find that for uniform illumination with a 5 fs pump pulse only the q = 0 radial breathing mode and a high frequency G mode are strongly excited. We will discuss excitation strengths for different coherent phonon modes and compare to recent experiments.
Stimulated emission of phonons in an acoustic cavity
NASA Astrophysics Data System (ADS)
Tilstra, Lieuwe Gijsbert
2001-10-01
This thesis will present experiments on stimulated emission of phonons in dilute ruby following complete population inversion of the Zeeman-split E(2E) Kramers doublet by selective pulsed optical pumping into its upper component. The resulting phonon avalanches are detected by use of the R1 luminescence emanating from the inverted zone, located near the end face where the laser beam enters the crystal. The phonons appear to team up into a highly directional phonon beam. The phonon frequency is tunable from, say, 10-100 GHz via the magnetic field splitting of the doublet. Remarkably, the population of the lower doublet component, which is a measure of the number of phonons generated, evolves with a sequence of distinct steps. The time interval in between these steps equals 2L/v, corresponding to the time the phonons need to return to the inverted zone by reflection at the opposite end face at a distance L. The end faces of the ruby crystal thus form an acoustic cavity. The phonon beam passes the inverted zone repeatedly to be amplified further, in a manner similar to light in an optical laser. In other words, the basic ingredients for a phonon laser have been established.
Lifetime of the phonons in the PLT ceramic
Barba-Ortega, J. Joya, M. R.; Londoño, F. A.
2014-11-05
The lifetimes at higher temperatures on lanthanum-modified lead titanate (PLT) are mainly due to the anharmonic decay of optical phonons into low-energy phonons. The temperature-independent contributions from inherent crystal defects and from boundary scattering become comparable to the phonon scattering contribution at lower temperatures. The thermal interaction is large at higher temperatures which decreases the phonon mean free path, and so the decay lifetime decreases as the temperature of the system is increased. This leads to the increased line width at higher temperatures. We made an estimate of the lifetimes for different concentrations and temperatures in PLT.
A new hybrid phononic crystal in low frequencies
NASA Astrophysics Data System (ADS)
Zhang, Z.; Han, X. K.
2016-11-01
A novel hybrid phononic crystal is designed to obtain wider band gaps in low frequency range. The hybrid phononic crystal consists of rubber slab with periodic holes and plumbum stubs. In comparison with the phononic crystal without periodic holes, the new designed phononic crystal can obtain wider band gaps and better vibration damping characteristics. The wider band gap can be attributed to the interaction of local resonance and Bragg scattering. The controlling of the BG is explained by the strain energy of the hybrid PC and the introduced effective mass. The effects of the geometrical parameters and the shapes of the stubs and holes on the controlling of waves are further studied.
Phononic glass: a robust acoustic-absorption material.
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.
Souvatzis, P; Björkman, T; Eriksson, O; Andersson, P; Katsnelson, M I; Rudin, S P
2009-04-29
A recently developed self-consistent ab initio lattice dynamical method has been applied to the high temperature body centered cubic (bcc) phase of La and Th, which are dynamically unstable at low temperatures. The bcc phase of these metals is found to be stabilized by phonon-phonon interactions. The calculated high temperature phonon frequencies for La are found to be in good agreement with the corresponding experimental data.
Coherent phonon optics in a chip with an electrically controlled active device.
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.
Coherent phonon optics in a chip with an electrically controlled active device
Poyser, Caroline L.; Akimov, Andrey V.; Campion, Richard P.; Kent, Anthony J.
2015-01-01
Phonon optics concerns operations with high-frequency acoustic waves in solid media in a similar way to how traditional optics operates with the light beams (i.e. photons). Phonon optics experiments with coherent terahertz and sub-terahertz phonons promise a revolution in various technical applications related to high-frequency acoustics, imaging, and heat transport. Previously, phonon optics used passive methods for manipulations with propagating phonon beams that did not enable their external control. Here we fabricate a phononic chip, which includes a generator of coherent monochromatic phonons with frequency 378 GHz, a sensitive coherent phonon detector, and an active layer: a doped semiconductor superlattice, with electrical contacts, inserted into the phonon propagation path. In the experiments, we demonstrate the modulation of the coherent phonon flux by an external electrical bias applied to the active layer. Phonon optics using external control broadens the spectrum of prospective applications of phononics on the nanometer scale. PMID:25652241
Quantifying electron-phonon coupling in CdTe1-xSex nanocrystals via coherent phonon manipulation
NASA Astrophysics Data System (ADS)
Spann, B. T.; Xu, X.
2014-08-01
We employ ultrafast transient absorption spectroscopy with temporal pulse shaping to manipulate coherent phonon excitation and quantify the strength of electron-phonon coupling in CdTe1-xSex nanocrystals (NCs). Raman active CdSe and CdTe longitudinal optical phonon (LO) modes are excited and probed in the time domain. By temporally controlling pump pulse pairs to coherently excite and cancel coherent phonons in the CdTe1-xSex NCs, we estimate the relative amount of optical energy that is coupled to the coherent CdSe LO mode.
Phonon hydrodynamics and its applications in nanoscale heat transport
NASA Astrophysics Data System (ADS)
Guo, Yangyu; Wang, Moran
2015-09-01
Phonon hydrodynamics is an effective macroscopic method to study heat transport in dielectric solid and semiconductor. It has a clear and intuitive physical picture, transforming the abstract and ambiguous heat transport process into a concrete and evident process of phonon gas flow. Furthermore, with the aid of the abundant models and methods developed in classical hydrodynamics, phonon hydrodynamics becomes much easier to implement in comparison to the current popular approaches based on the first-principle method and kinetic theories involving complicated computations. Therefore, it is a promising tool for studying micro- and nanoscale heat transport in rapidly developing micro and nano science and technology. However, there still lacks a comprehensive account of the theoretical foundations, development and implementation of this approach. This work represents such an attempt in providing a full landscape, from physical fundamental and kinetic theory of phonons to phonon hydrodynamics in view of descriptions of phonon systems at microscopic, mesoscopic and macroscopic levels. Thus a systematical kinetic framework, summing up so far scattered theoretical models and methods in phonon hydrodynamics as individual cases, is established through a frame of a Chapman-Enskog solution to phonon Boltzmann equation. Then the basic tenets and procedures in implementing phonon hydrodynamics in nanoscale heat transport are presented through a review of its recent wide applications in modeling thermal transport properties of nanostructures. Finally, we discuss some pending questions and perspectives highlighted by a novel concept of generalized phonon hydrodynamics and possible applications in micro/nano phononics, which will shed more light on more profound understanding and credible applications of this new approach in micro- and nanoscale heat transport science.
Band gaps in bubble phononic crystals
NASA Astrophysics Data System (ADS)
Leroy, V.; Bretagne, A.; Lanoy, M.; Tourin, A.
2016-12-01
We investigate the interaction between Bragg and hybridization effects on the band gap properties of bubble phononic crystals. These latter consist of air cavities periodically arranged in an elastomer matrix and are fabricated using soft-lithography techniques. Their transmission properties are affected by Bragg effects due to the periodicity of the structure as well as hybridization between the propagating mode of the embedding medium and bubble resonance. The hybridization gap survives disorder while the Bragg gap requires a periodic distribution of bubbles. The distance between two bubble layers can be tuned to make the two gaps overlap or to create a transmission peak in the hybridization gap.
Shear surface waves in phononic crystals.
Kutsenko, A A; Shuvalov, A L
2013-02-01
The existence of shear horizontal (SH) surface waves in two-dimensional periodic phononic crystals with an asymmetric depth-dependent profile is theoretically reported. Examples of dispersion spectra with bandgaps for subsonic and supersonic SH surface waves are demonstrated. The link between the effective (quasistatic) speeds of the SH bulk and surface waves is established. Calculation and analysis is based on the integral form of a projector on the subspace of evanescent modes which means no need for their explicit finding. This method can be extended to the vector waves and the three-dimensional case.
Large scale phononic metamaterials for seismic isolation
Aravantinos-Zafiris, N.; Sigalas, M. M.
2015-08-14
In this work, we numerically examine structures that could be characterized as large scale phononic metamaterials. These novel structures could have band gaps in the frequency spectrum of seismic waves when their dimensions are chosen appropriately, thus raising the belief that they could be serious candidates for seismic isolation structures. Different and easy to fabricate structures were examined made from construction materials such as concrete and steel. The well-known finite difference time domain method is used in our calculations in order to calculate the band structures of the proposed metamaterials.
Temperature dependent phonon properties of thermoelectric materials
NASA Astrophysics Data System (ADS)
Hellman, Olle; Broido, David; Fultz, Brent
2015-03-01
We present recent developments using the temperature dependent effective potential technique (TDEP) to model thermoelectric materials. We use ab initio molecular dynamics to generate an effective Hamiltonian that reproduce neutron scattering spectra, thermal conductivity, phonon self energies, and heat capacities. Results are presented for (among others) SnSe, Bi2Te3, and Cu2Se proving the necessity of careful modelling of finite temperature properties for strongly anharmonic materials. Supported by the Swedish Research Council (VR) Project Number 637-2013-7296.
Phonon scattering in graphene over substrate steps
Sevinçli, H.; Brandbyge, M.
2014-10-13
We calculate the effect on phonon transport of substrate-induced bends in graphene. We consider bending induced by an abrupt kink in the substrate, and provide results for different step-heights and substrate interaction strengths. We find that individual substrate steps reduce thermal conductance in the range between 5% and 47%. We also consider the transmission across linear kinks formed by adsorption of atomic hydrogen at the bends and find that individual kinks suppress thermal conduction substantially, especially at high temperatures. Our analysis show that substrate irregularities can be detrimental for thermal conduction even for small step heights.
Temperature dependence of coherent phonons in TbVO4 crystal probed by ultrafast optical spectroscopy
NASA Astrophysics Data System (ADS)
Jin, Z.; Ma, H.; Li, D.; Wang, L.; Ma, G.; Guo, F.; Chen, J.
2011-07-01
Coherent optical phonons in terbium vanadate (TbVO4) are investigated by using femtosecond time-resolved pump-probe spectroscopy at temperatures from 20 to 300 K. Combined with the Raman spectrum, the coherent phonon mode is attributed to an optical phonon mode of B1g symmetry. The main generation mechanism of the coherent optical phonons is revealed to be the impulsive stimulated Raman scattering. The temperature dependence of the dephasing time reveals that the main mechanism of the coherent phonon population decay is anharmonic phonon-phonon coupling, which causes a redshift of the coherent phonon frequency with increasing temperature.
NASA Astrophysics Data System (ADS)
Bilal, Osama R.
Transmission of everyday sound and heat can be traced back to a physical particle, or wave, called a "phonon". Understanding, analyzing and manipulating phonons across multiple scales/disciplines can be achieved using phononic materials. That is a class of material systems featuring a basic pattern that repeats spatially. Among many qualities, it exhibits distinct frequency characteristics such as band gaps, where vibrational waves of certain frequencies are prohibited from propagation. These properties can benefit a multitude of applications, ranging from vibration isolation and converting waste heat into electricity to exotic concepts like acoustic cloaking. Using unit-cell design and optimization, phononic materials/devices with extraordinary properties may be realized. Since many of these applications are based on band-gap utilization, a critical design objective is to widen band-gap size or precisely synthesize its characteristics. Approaching this problem at the unit cell level is advantageous in many aspects, mostly because it provides a complete picture of the intrinsic local dynamics which is often obscured when analyzing the structure as a whole. Moreover, it is computationally less expensive than designing an entire structure. Unit-cell dispersion engineering is also scale independent; an optimized unit cell may be used to manipulate waves ranging from a few Hz to GHz, or higher, with proper scaling. In order to keep the structure/device size as small as possible, the band-gap central frequency is tuned to be as low as possible. The objective of this thesis is to explore and advance unit-cell design and optimization of phononic materials in one, two and three-dimensions for a broad range of applications. In particular, an application for flow control is investigated where a phononic material is shown to manipulate and alter a flow field in a favorable manner. Results involving unit-cell design and coupled fluid-structure simulations (as part of a
Multiple Quantum Wells for P T -Symmetric Phononic Crystals
NASA Astrophysics Data System (ADS)
Poshakinskiy, A. V.; Poddubny, A. N.; Fainstein, A.
2016-11-01
We demonstrate that the parity-time symmetry for sound is realized in laser-pumped multiple-quantum-well structures. Breaking of the parity-time symmetry for the phonons with wave vectors corresponding to the Bragg condition makes the structure a highly selective acoustic wave amplifier. Single-mode distributed feedback phonon lasing is predicted for structures with realistic parameters.
Topological Phonons and Weyl Lines in Three Dimensions
NASA Astrophysics Data System (ADS)
Stenull, Olaf; Kane, C. L.; Lubensky, T. C.
2016-08-01
Topological mechanics and phononics have recently emerged as an exciting field of study. Here we introduce and study generalizations of the three-dimensional pyrochlore lattice that have topologically protected edge states and Weyl lines in their bulk phonon spectra, which lead to zero surface modes that flip from one edge to the opposite as a function of surface wave number.
Landau-Khalatnikov phonon damping in strongly interacting Fermi gases
NASA Astrophysics Data System (ADS)
Kurkjian, Hadrien; Castin, Yvan; Sinatra, Alice
2016-11-01
We derive the phonon damping rate due to the four-phonon Landau-Khalatnikov process in low-temperature strongly interacting Fermi gases using quantum hydrodynamics, correcting and extending the original calculation of Landau and Khalatnikov (Zh. Eksp. Teor. Fiz., 19 (1949) 637). Our predictions can be tested in state-of-the-art experiments with cold atomic gases in the collisionless regime.
Electron-interface phonon interaction in multiple quantum well structures
NASA Astrophysics Data System (ADS)
Sun, J. P.; Teng, H. B.; Haddad, G. I.; Stroscio, M. A.
1998-08-01
Intersubband relaxation rates due to electron interactions with the interface phonons are evaluated for multiple quantum well structures designed for step quantum well lasers operating at mid-infrared to submillimetre wavelengths. The interface phonon modes and electron-phonon interaction Hamiltonians for the structures are derived using the transfer matrix method, based on the macroscopic dielectric continuum model, whereas the electron wavefunctions are obtained by solving the Schrödinger equation. Fermi's golden rule is employed to calculate the electron relaxation rates between the subbands in these structures. The relaxation rates for two different structures are examined and compared with those calculated using the bulk phonon modes and the Fröhlich interaction Hamiltonian. The sum rule for the relationship between the form factors of the various localized phonon modes and the bulk phonon modes is verified. The results obtained in this work illustrate that the transfer matrix method provides a convenient way for deriving the properties of the interface phonon modes in different structures of current interest and that, for preferential electron relaxation in intersubband laser structures, the effects of the interface phonon modes are significant and should be considered for optimal design of these laser structures.
Remarkable reduction of thermal conductivity in phosphorene phononic crystal
NASA Astrophysics Data System (ADS)
Xu, Wen; Zhang, Gang
2016-05-01
Phosphorene has received much attention due to its interesting physical and chemical properties, and its potential applications such as thermoelectricity. In thermoelectric applications, low thermal conductivity is essential for achieving a high figure of merit. In this work, we propose to reduce the thermal conductivity of phosphorene by adopting the phononic crystal structure, phosphorene nanomesh. With equilibrium molecular dynamics simulations, we find that the thermal conductivity is remarkably reduced in the phononic crystal. Our analysis shows that the reduction is due to the depressed phonon group velocities induced by Brillouin zone folding, and the reduced phonon lifetimes in the phononic crystal. Interestingly, it is found that the anisotropy ratio of thermal conductivity could be tuned by the ‘non-square’ pores in the phononic crystal, as the phonon group velocities in the direction with larger projection of pores is more severely suppressed, leading to greater reduction of thermal conductivity in this direction. Our work provides deep insight into thermal transport in phononic crystals and proposes a new strategy to reduce the thermal conductivity of monolayer phosphorene.
Phonon thermal transport through tilt grain boundaries in strontium titanate
Zheng, Zexi; Chen, Xiang; Yang, Shengfeng; Xiong, Liming; Chen, Youping; Deng, Bowen; Chernatynskiy, Aleksandr
2014-08-21
In this work, we perform nonequilibrium molecular dynamics simulations to study phonon scattering at two tilt grain boundaries (GBs) in SrTiO{sub 3}. Mode-wise energy transmission coefficients are obtained based on phonon wave-packet dynamics simulations. The Kapitza conductance is then quantified using a lattice dynamics approach. The obtained results of the Kapitza conductance of both GBs compare well with those obtained by the direct method, except for the temperature dependence. Contrary to common belief, the results of this work show that the optical modes in SrTiO{sub 3} contribute significantly to phonon thermal transport, accounting for over 50% of the Kapitza conductance. To understand the effect of the GB structural disorder on phonon transport, we compare the local phonon density of states of the atoms in the GB region with that in the single crystalline grain region. Our results show that the excess vibrational modes introduced by the structural disorder do not have a significant effect on phonon scattering at the GBs, but the absence of certain modes in the GB region appears to be responsible for phonon reflections at GBs. This work has also demonstrated phonon mode conversion and simultaneous generation of new modes. Some of the new modes have the same frequency as the initial wave packet, while some have the same wave vector but lower frequencies.
Remarkable reduction of thermal conductivity in phosphorene phononic crystal.
Xu, Wen; Zhang, Gang
2016-05-05
Phosphorene has received much attention due to its interesting physical and chemical properties, and its potential applications such as thermoelectricity. In thermoelectric applications, low thermal conductivity is essential for achieving a high figure of merit. In this work, we propose to reduce the thermal conductivity of phosphorene by adopting the phononic crystal structure, phosphorene nanomesh. With equilibrium molecular dynamics simulations, we find that the thermal conductivity is remarkably reduced in the phononic crystal. Our analysis shows that the reduction is due to the depressed phonon group velocities induced by Brillouin zone folding, and the reduced phonon lifetimes in the phononic crystal. Interestingly, it is found that the anisotropy ratio of thermal conductivity could be tuned by the 'non-square' pores in the phononic crystal, as the phonon group velocities in the direction with larger projection of pores is more severely suppressed, leading to greater reduction of thermal conductivity in this direction. Our work provides deep insight into thermal transport in phononic crystals and proposes a new strategy to reduce the thermal conductivity of monolayer phosphorene.
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.
Phonon analog of topological nodal semimetals
NASA Astrophysics Data System (ADS)
Po, Hoi Chun; Bahri, Yasaman; Vishwanath, Ashvin
2016-05-01
Topological band structures in electronic systems like topological insulators and semimetals give rise to highly unusual physical properties. Analogous topological effects have also been discussed in bosonic systems, but the novel phenomena typically occur only when the system is excited by finite-frequency probes. A mapping recently proposed by C. L. Kane and T. C. Lubensky [Nat. Phys. 10, 39 (2014), 10.1038/nphys2835], however, establishes a closer correspondence. It relates the zero-frequency excitations of mechanical systems to topological zero modes of fermions that appear at the edges of an otherwise gapped system. Here we generalize the mapping to systems with an intrinsically gapless bulk. In particular, we construct mechanical counterparts of topological semimetals. The resulting gapless bulk modes are physically distinct from the usual acoustic Goldstone phonons and appear even in the absence of continuous translation invariance. Moreover, the zero-frequency phonon modes feature adjustable momenta and are topologically protected as long as the lattice coordination is unchanged. Such protected soft modes with tunable wave vector may be useful in designing mechanical structures with fault-tolerant properties.
Size effects in thermal conduction by phonons
NASA Astrophysics Data System (ADS)
Allen, Philip B.
2014-08-01
Heat transport in nanoscale systems is both hard to measure microscopically, and hard to interpret. Ballistic and diffusive heat flow coexist, adding confusion. This paper looks at a very simple case: a nanoscale crystal repeated periodically. This is a popular model for simulation of bulk heat transport using classical molecular dynamics (MD), and is related to transient thermal grating experiments. Nanoscale effects are seen in perhaps their simplest form. The model is solved by an extension of standard quasiparticle gas theory of bulk solids. Both structure and heat flow are constrained by periodic boundary conditions. Diffusive transport is fully included, while ballistic transport by phonons of a long mean free path is diminished in a specific way. Heat current J (x) and temperature gradient ∇T (x') have a nonlocal relationship, via κ (x-x'), over a distance |x-x'| determined by phonon mean free paths. In MD modeling of bulk conductivity, finite computer resources limit system size. Long mean free paths, comparable to the scale of heating and cooling, cause undesired finite-size effects that have to be removed by extrapolation. The present model allows this extrapolation to be quantified. Calculations based on the Peierls-Boltzmann equation, using a generalized Debye model, show that extrapolation involves fractional powers of 1/L. It is also argued that heating and cooling should be distributed sinusoidally [ė∝cos(2πx/L)] to improve convergence of numerics.
Thermoelectric amplification of phonons in graphene
NASA Astrophysics Data System (ADS)
Dompreh, K. A.; Mensah, N. G.; Mensah, S. Y.; Fosuhene, S. K.
2016-06-01
Amplification of acoustic in-plane phonons due to an external temperature gradient (∇T) in single-layer graphene (SLG) was studied theoretically. The threshold temperature gradient (∇ T ) 0 g and the threshold voltage (V T ) 0 g in SLG were evaluated. For T = 77 K , the calculated value for (∇ T ) 0 g = 746.8 K / cm and (V T ) 0 g = 6.6 mV . The calculation was done in the hypersound regime. Further, the dependence of the normalized amplification ( Γ / Γ 0 ) on the frequency ω q and ∇ T / T were evaluated numerically and presented graphically. The calculated threshold temperature gradient (V T ) 0 g for SLG was higher than that obtained for homogeneous semiconductors (n-InSb) (∇ T ) 0 hom ≈ 10 3 K / cm , superlattices (∇ T ) 0 S L ≈ 384 K / cm , and cylindrical quantum wire (∇ T ) 0 c q w ≈ 10 2 K / cm . This makes SLG a much better material for thermoelectric phonon amplification.
Phonon anomalies in some iron telluride materials
C. C. Homes; Dai, Y. M.; Schneeloch, J.; ...
2016-03-21
In this paper, the detailed temperature dependence of the infrared-active mode in Fe1.03Te (TN ≃ 68 K) and Fe1.13Te (TN ≃ 56 K) has been examined, and the position, width, strength, and asymmetry parameter have been determined using an asymmetric Fano profile superimposed on an electronic background. In both materials the frequency of the mode increases as the temperature is reduced; however, there is also a slight asymmetry in the line shape, indicating that the mode is coupled to either spin or charge excitations. Below TN there is an anomalous decrease in frequency, and the mode shows little temperature dependence,more » at the same time becoming more symmetric, suggesting a reduction in spin- or electron-phonon coupling. The frequency of the infrared-active mode and the magnitude of the shift below TN are predicted reasonably well by first-principles calculations; however, the predicted splitting of the mode is not observed. In superconducting FeTe0.55Se0.45 (Tc ≃ 14 K) the infrared-active Eu mode displays asymmetric line shape at all temperatures, which is most pronounced between 100 – 200 K, indicating the presence of either spin- or electron-phonon coupling, which may be a necessary prerequisite for superconductivity in this class of materials.« less
An Artificial Ising System with Phononic Excitations
NASA Astrophysics Data System (ADS)
Ghaffari, Hamed; Griffith, W. Ashley; Benson, Philip; Nasseri, M. H. B.; Young, R. Paul
Many intractable systems and problems can be reduced to a system of interacting spins. Here, we report mapping collective phononic excitations from different sources of crystal vibrations to spin systems. The phononic excitations in our experiments are due to micro and nano cracking (yielding crackling noises due to lattice distortion). We develop real time mapping of the multi-array senores to a network-space and then mapping the excitation- networks to spin-like systems. We show that new mapped system satisfies the quench (impulsive) characteristics of the Ising model in 2D classical spin systems. In particular, we show that our artificial Ising system transits between two ground states and approaching the critical point accompanies with a very short time frozen regime, inducing formation of domains separated by kinks. For a cubic-test under a true triaxial test (3D case), we map the system to a 6-spin ring under a transversal-driving field where using functional multiplex networks, the vector components of the spin are inferred (i.e., XY model). By visualization of spin patterns of the ring per each event, we demonstrate that ``kinks'' (as defects) proliferate when system approach from above to its critical point. We support our observations with employing recorded acoustic excitations during distortion of crystal lattices in nano-indentation tests on different crystals (silicon and graphite), triaxial loading test on rock (poly-crystal) samples and a true 3D triaxial test.
Enhancing phonon flow through one-dimensional interfaces by impedance matching
Polanco, Carlos A. Ghosh, Avik W.
2014-08-28
We extend concepts from microwave engineering to thermal interfaces and explore the principles of impedance matching in 1D. The extension is based on the generalization of acoustic impedance to nonlinear dispersions using the contact broadening matrix Γ(ω), extracted from the phonon self energy. For a single junction, we find that for coherent and incoherent phonons, the optimal thermal conductance occurs when the matching Γ(ω) equals the Geometric Mean of the contact broadenings. This criterion favors the transmission of both low and high frequency phonons by requiring that (1) the low frequency acoustic impedance of the junction matches that of the two contacts by minimizing the sum of interfacial resistances and (2) the cut-off frequency is near the minimum of the two contacts, thereby reducing the spillage of the states into the tunneling regime. For an ultimately scaled single atom/spring junction, the matching criterion transforms to the arithmetic mean for mass and the harmonic mean for spring constant. The matching can be further improved using a composite graded junction with an exponential varying broadening that functions like a broadband antireflection coating. There is, however, a trade off as the increased length of the interface brings in additional intrinsic sources of scattering.
Modulation of the Band Gaps of Phononic Crystals with Thermal Effects
NASA Astrophysics Data System (ADS)
Aly, Arafa H.; Mehaney, Ahmed
2015-11-01
Band gaps of elastic waves, both in-plane and shear waves, propagating through one-dimensional perfect/defect phononic crystals (PnCs) that involve thermal effects are studied in this paper. Based on the transfer matrix method and Bloch theory, the expressions of the reflection coefficients and dispersion relation are presented. Elastic waves localization is obtained by immersing a defect layer through a perfect structure. Compared with the periodic structure, we observed that defected PnCs introduced localized modes or peaks within the phononic band gaps. Hence, Numerical simulations are performed to investigate the influences of the defect layer thickness and type on the number and intensity of the localized modes. Moreover, we have observed that temperature changes have prominent effects on the localized modes and band gaps width, especially at plane wave propagation. Such effects could change thermal properties of the PnCs structure such as thermal conductivity and could control the thermal emission contributed by phonons in many engineering structures.
Lattice dynamics and electron-phonon coupling calculations using nondiagonal supercells
NASA Astrophysics Data System (ADS)
Lloyd-Williams, Jonathan; Monserrat, Bartomeu
Quantities derived from electron-phonon coupling matrix elements require a fine sampling of the vibrational Brillouin zone. Converged results are typically not obtainable using the direct method, in which a perturbation is frozen into the system and the total energy derivatives are calculated using a finite difference approach, because the size of simulation cell needed is prohibitively large. We show that it is possible to determine the response of a periodic system to a perturbation characterized by a wave vector with reduced fractional coordinates (m1 /n1 ,m2 /n2 ,m3 /n3) using a supercell containing a number of primitive cells equal to the least common multiple of n1, n2, and n3. This is accomplished by utilizing supercell matrices containing nonzero off-diagonal elements. We present the results of electron-phonon coupling calculations using the direct method to sample the vibrational Brillouin zone with grids of unprecedented size for a range of systems, including the canonical example of diamond. We also demonstrate that the use of nondiagonal supercells reduces by over an order of magnitude the computational cost of obtaining converged vibrational densities of states and phonon dispersion curves. J.L.-W. is supported by the Engineering and Physical Sciences Research Council (EPSRC). B.M. is supported by Robinson College, Cambridge, and the Cambridge Philosophical Society. This work was supported by EPSRC Grants EP/J017639/1 and EP/K013564/1.
Phonon-mediated quantum spin simulator employing a planar ionic crystal in a Penning trap
NASA Astrophysics Data System (ADS)
Wang, C.-C. Joseph; Keith, Adam C.; Freericks, J. K.
2013-01-01
We derive the normal modes for a rotating Coulomb ion crystal in a Penning trap, quantize the motional degrees of freedom, and illustrate how they can be driven by a spin-dependent optical dipole force to create a quantum spin simulator on a triangular lattice with hundreds of spins. The analysis for the axial modes (oscillations perpendicular to the two-dimensional crystal plane) follow a standard normal-mode analysis, while the remaining planar modes are more complicated to analyze because they have velocity-dependent forces in the rotating frame. After quantizing the normal modes into phonons, we illustrate some of the different spin-spin interactions that can be generated by entangling the motional degrees of freedom with the spin degrees of freedom via a spin-dependent optical dipole force. In addition to the well-known power-law dependence of the spin-spin interactions when driving the axial modes blue of the phonon band, we notice certain parameter regimes in which the level of frustration between the spins can be engineered by driving the axial or planar phonon modes at different frequencies. These systems may allow for the analog simulation of quantum spin glasses with large numbers of spins.
The role of anharmonic phonons in under-barrier spin relaxation of single molecule magnets
Lunghi, Alessandro; Totti, Federico; Sessoli, Roberta; Sanvito, Stefano
2017-01-01
The use of single molecule magnets in mainstream electronics requires their magnetic moment to be stable over long times. One can achieve such a goal by designing compounds with spin-reversal barriers exceeding room temperature, namely with large uniaxial anisotropies. Such strategy, however, has been defeated by several recent experiments demonstrating under-barrier relaxation at high temperature, a behaviour today unexplained. Here we propose spin–phonon coupling to be responsible for such anomaly. With a combination of electronic structure theory and master equations we show that, in the presence of phonon dissipation, the relevant energy scale for the spin relaxation is given by the lower-lying phonon modes interacting with the local spins. These open a channel for spin reversal at energies lower than that set by the magnetic anisotropy, producing fast under-barrier spin relaxation. Our findings rationalize a significant body of experimental work and suggest a possible strategy for engineering room temperature single molecule magnets. PMID:28262663
Quantum noise theory for phonon transport through nanostructures
NASA Astrophysics Data System (ADS)
Wan, Li; Huang, Yunmi; Huang, Changcheng
2017-04-01
We have developed a quantum noise approach to study the phonon transport through nanostructures. The nanostructures acting as phonon channels are attached to two phonon reservoirs. And the temperature drop between the two reservoirs drives the phonon transport through the channels. We have derived a quantum Langevin equation(QLE) to describe the phonon transport with the quantum noise originated from the thermal fluctuation of the reservoirs. Within the Markov approximation, the QLE is used to get the thermal conductivity κ of the nanostructures and the finite size effect of the κ then is studied. In this study, the advantage of the quantum noise approach lays on the fact that no any local temperature needs to be defined for the nanostructures in its non-equilibrium state.
Phononic Frequency Comb via Intrinsic Three-Wave Mixing.
Ganesan, Adarsh; Do, Cuong; Seshia, Ashwin
2017-01-20
Optical frequency combs have resulted in significant advances in optical frequency metrology and found wide applications in precise physical measurements and molecular fingerprinting. A direct analogue of frequency combs in the phononic or acoustic domain has not been reported to date. In this Letter, we report the first clear experimental evidence for a phononic frequency comb. We show that the phononic frequency comb is generated through the intrinsic coupling of a driven phonon mode with an autoparametrically excited subharmonic mode. The experiments depict the comb generation process evidenced by a spectral response consisting of equally spaced discrete and phase coherent comb lines. Through systematic experiments at different drive frequencies and amplitudes, we portray the well-connected process of phononic frequency comb formation and define the attributes to control the features associated with comb formation in such a system. In addition to the demonstration of frequency comb, the interplay between the nonlinear resonances and the well-known Duffing phenomenon is also observed.
Shear viscosity due to phonons in superfluid neutron stars
NASA Astrophysics Data System (ADS)
Manuel, Cristina; Tolos, Laura
2011-12-01
We compute the contribution of phonons to the shear viscosity η in superfluid neutron stars, assuming neutron pairing in a S01 channel. We use a Boltzmann equation amended by a collision term that takes into account the binary collisions of phonons. We use effective field theory techniques to extract the phonon scattering rates, written as a function of the equation of state of the system. Our formulation is rather general, and can be used to extract the shear viscosity due to binary collisions of phonons for other superfluids, such as the cold Fermi gas in the unitarity limit. We find that η∝1/T5, the proportionality factor depending on the equation of state of the system. Our results indicate that the phonon contribution to η cannot be ignored and might have relevant effects in the dynamics of the different oscillation modes of the star.
Suppression of phonon transport in molecular Christmas trees.
Lambert, Colin John; Famili, Marjan; Grace, Iain; Sadeghi, Hatef
2017-02-27
Minimising the phonon thermal conductance of self-assembled molecular films, whilst preserving their electrical properties, is highly desirable, both for thermal management at the nanoscale and for the design of high-efficiency thermoelectric materials. Here we highlight a new strategy for minimising the phonon thermal conductance of Christmas-tree-like molecules composed of a long trunk, along which phonons can propagate, attached to pendant molecular branches. We demonstrate that phonon transport along the trunk is suppressed by Fano resonances associated with internal vibrational modes of the branches and that thermal conductance is suppressed most-effectively in molecules with pendant branches of different lengths. As examples, we use density functional theory to demonstrate the reduction in phonon transport in tree-like molecules formed from alkane or acene trunks with various pendant branches.
Dissipation induced by phonon elastic scattering in crystals
Li, Guolong; Ren, Zhongzhou; Zhang, Xin
2016-01-01
We demonstrate that the phonon elastic scattering leads to a dominant dissipation in crystals at low temperature. The two-level systems (TLSs) should be responsible for the elastic scattering, whereas the dissipation induced by static-point defects (SPDs) can not be neglected. One purpose of this work is to show how the energy splitting distribution of the TLS ensemble affects the dissipation. Besides, this article displays the proportion of phonon-TLS elastic scattering to total phonon dissipation. The coupling coefficient of phonon-SPD scattering and the constant P0 of the TLS distribution are important that we estimate their magnitudes in this paper. Our results is useful to understand the phonon dissipation mechanism, and give some clues to improve the performance of mechanical resonators, apply the desired defects, or reveal the atom configuration in lattice structure of disordered crystals. PMID:27669517
Phonon anharmonicity of monoclinic zirconia and yttrium-stabilized zirconia
Li, Chen W.; Smith, Hillary L.; Lan, Tian; ...
2015-04-13
Inelastic neutron scattering measurements on monoclinic zirconia (ZrO2) and 8 mol% yttrium-stabilized zirconia were performed at temperatures from 300 to 1373 ωK. We reported temperature-dependent phonon densities of states (DOS) and Raman spectra obtained at elevated temperatures. First-principles lattice dynamics calculations with density functional theory gave total and partial phonon DOS curves and mode Grüneisen parameters. These mode Grüneisen parameters were used to predict the experimental temperature dependence of the phonon DOS with partial success. However, substantial anharmonicity was found at elevated temperatures, especially for phonon modes dominated by the motions of oxygen atoms. Yttrium-stabilized zirconia (YSZ) was somewhat moremore » anharmonic and had a broader phonon spectrum at low temperatures, owing in part to defects in its structure. YSZ also has a larger vibrational entropy than monoclinic zirconia.« less
Phononic Frequency Comb via Intrinsic Three-Wave Mixing
NASA Astrophysics Data System (ADS)
Ganesan, Adarsh; Do, Cuong; Seshia, Ashwin
2017-01-01
Optical frequency combs have resulted in significant advances in optical frequency metrology and found wide applications in precise physical measurements and molecular fingerprinting. A direct analogue of frequency combs in the phononic or acoustic domain has not been reported to date. In this Letter, we report the first clear experimental evidence for a phononic frequency comb. We show that the phononic frequency comb is generated through the intrinsic coupling of a driven phonon mode with an autoparametrically excited subharmonic mode. The experiments depict the comb generation process evidenced by a spectral response consisting of equally spaced discrete and phase coherent comb lines. Through systematic experiments at different drive frequencies and amplitudes, we portray the well-connected process of phononic frequency comb formation and define the attributes to control the features associated with comb formation in such a system. In addition to the demonstration of frequency comb, the interplay between the nonlinear resonances and the well-known Duffing phenomenon is also observed.
Phonon localization drives polar nanoregions in a relaxor ferroelectric.
Manley, M E; Lynn, J W; Abernathy, D L; Specht, E D; Delaire, O; Bishop, A R; Sahul, R; Budai, J D
2014-04-10
Relaxor ferroelectrics exemplify a class of functional materials where interplay between disorder and phase instability results in inhomogeneous nanoregions. Although known for about 30 years, there is no definitive explanation for polar nanoregions (PNRs). Here we show that ferroelectric phonon localization drives PNRs in relaxor ferroelectric PMN-30%PT using neutron scattering. At the frequency of a preexisting resonance mode, nanoregions of standing ferroelectric phonons develop with a coherence length equal to one wavelength and the PNR size. Anderson localization of ferroelectric phonons by resonance modes explains our observations and, with nonlinear slowing, the PNRs and relaxor properties. Phonon localization at additional resonances near the zone edges explains competing antiferroelectric distortions known to occur at the zone edges. Our results indicate the size and shape of PNRs that are not dictated by complex structural details, as commonly assumed, but by phonon resonance wave vectors. This discovery could guide the design of next generation relaxor ferroelectrics.
Phonon anharmonicity of monoclinic zirconia and yttrium-stabilized zirconia
Li, Chen W.; Smith, Hillary L.; Lan, Tian; Niedziela, Jennifer L.; Munoz, Jorge A.; Keith, J. Brian; Mauger, L.; Abernathy, Douglas L; Fultz, B.
2015-04-13
Inelastic neutron scattering measurements on monoclinic zirconia (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 somewhat more anharmonic and had a broader phonon spectrum at low temperatures, owing in part to defects in its structure. YSZ also has a larger vibrational entropy than monoclinic zirconia.
Investigation of phonon coherence and backscattering using silicon nanomeshes
NASA Astrophysics Data System (ADS)
Lee, Jaeho; Lee, Woochul; Wehmeyer, Geoff; Dhuey, Scott; Olynick, Deirdre L.; Cabrini, Stefano; Dames, Chris; Urban, Jeffrey J.; Yang, Peidong
2017-01-01
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 with 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.
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.
Interplay between phononic bandgaps and piezoelectric microstructures for energy harvesting
NASA Astrophysics Data System (ADS)
Gonella, Stefano; To, Albert C.; Liu, Wing Kam
2009-03-01
The paper introduces a multifunctional structural design combining superior mechanical wave filtering properties and energy harvesting capabilities. The proposed concept is based on the ability of most periodic structures to forbid elastic waves from propagating within specific frequency ranges known as phononic bandgaps. The bandgap density and the resulting filtering effect are dramatically enhanced through the introduction of a microstructure consisting of stiff inclusions which resonate at specific frequencies and produce significant strain and energy localization. Energy harvesting is achieved as a result of the conversion of the localized kinetic energy into electrical energy through the piezoelectric effect featured by the material in the microstructure. The idea is illustrated through the application to hexagonal truss-core honeycombs featuring periodically distributed stiff cantilever beams provided with piezoelectric electrodes. The multifunctional capability results from the localized oscillatory phenomena exhibited by the cantilevers for excitations falling in the neighborhood of the bending fundamental frequencies of the beams. This application is of particular interest for advanced aerospace and mechanical engineering applications where distinct capabilities are simultaneously pursued and weight containment represents a critical design constraint. The scalability of the analysis suggests the possibility to miniaturize the design to the microscale for microelectromechanical systems (MEMS) applications such as self-powered microsystems and wireless sensors.
Diffusion and Phonon-drag Thermopower in Gated Silicon Nanoribbons
NASA Astrophysics Data System (ADS)
Ryu, Hyuk Ju; Aksamija, Zlatan; Paskiewicz, Deborah; Scott, Shelley; Lagally, Max; Knezevic, Irena; Eriksson, Mark
2010-03-01
Thermoelectric devices are attracting interest for the targeted cooling of local hotspots in integrated circuits and the harvesting of waste heat to generate power. Special interest in silicon as a thermoelectric material arises from the possibility of monolithic integration, significant reduction in thermal conductivity for nanopatterned silicon wires, and the potential to use bandstructure engineering to improve the power factor. We present measurements of the thermopower in gate-tunable silicon nanoribbons. The gate voltage effectively modulates the thermopower by changing both the 2D electron density and the confinement electric field. The thermopower varies by almost a factor of four in the density range studied. We understand much of this variation and its temperature dependence by considering the roles of the carrier diffusion and phonon-drag contributions to the thermopower. The data are well fit by theoretical calculations based on the Boltzmann transport equation and self-consistent modeling of the confinement electrostatics. We discuss the optimization of the power factor and the cooling efficiency in gated silicon nanoribbons. This work is supported by AFOSR, DOE, NSF, and NDSEG.
Phonon spectra of plutonium at high temperatures
NASA Astrophysics Data System (ADS)
Dorado, Boris; Bottin, François; Bouchet, Johann
2017-03-01
Ab initio molecular dynamics calculations are used to investigate the vibrational properties of the high-temperature δ and ɛ phases of plutonium. We combine the local-density approximation (LDA)+U for strong electron correlations and the temperature-dependent effective potential method in order to calculate the phonon spectra of the two phases, as well as their dependence on temperature. Our results show that the ɛ phase can only be stabilized when temperature and correlations are simultaneously accounted for. We are also able to quantify the degree of anharmonicity of the two phases. While the δ phase is fairly harmonic up to 1000 K, we find that the ɛ phase is strongly anharmonic, which explains why this structure dominates the phase diagram at high temperature.
The phonon theory of liquid thermodynamics
Bolmatov, D.; Brazhkin, V. V.; Trachenko, K.
2012-01-01
Heat capacity of matter is considered to be its most important property because it holds information about system's degrees of freedom as well as the regime in which the system operates, classical or quantum. Heat capacity is well understood in gases and solids but not in the third main state of matter, liquids, and is not discussed in physics textbooks as a result. The perceived difficulty is that interactions in a liquid are both strong and system-specific, implying that the energy strongly depends on the liquid type and that, therefore, liquid energy can not be calculated in general form. Here, we develop a phonon theory of liquids where this problem is avoided. The theory covers both classical and quantum regimes. We demonstrate good agreement of calculated and experimental heat capacity of 21 liquids, including noble, metallic, molecular and hydrogen-bonded network liquids in a wide range of temperature and pressure. PMID:22639729
Phonon Overlaps in Molecular Quantum Dot Systems
NASA Astrophysics Data System (ADS)
Chang, Connie; Sethna, James
2004-03-01
We model the amplitudes and frequencies of the vibrational sidebands for the new molecular quantum dot systems. We calculate the Franck-Condon phonon overlaps in the 3N-dimensional configuration sapce. We solve the general case where the vibrational frequencies and eigenmodes change during the transition. We perform PM3 and DFT calculations for the case of the dumb bell-shaped C140 molecule. We find that the strongest amplitudes are associated with the 11 meV stretch mode, in agreement with experiment. The experimental amplitudes vary from molecule to molecule; indicating that the molecular overlaps are environment dependent. We explore overlaps in the presence of external electric fields from image charges and counter ions.
Parity-Time Synthetic Phononic Media
NASA Astrophysics Data System (ADS)
Christensen, J.; Willatzen, M.; Velasco, V. R.; Lu, M.-H.
2016-05-01
Classical systems containing cleverly devised combinations of loss and gain elements constitute extremely rich building units that can mimic non-Hermitian properties, which conventionally are attainable in quantum mechanics only. Parity-time (P T ) symmetric media, also referred to as synthetic media, have been devised in many optical systems with the ground breaking potential to create nonreciprocal structures and one-way cloaks of invisibility. Here we demonstrate a feasible approach for the case of sound where the most important ingredients within synthetic materials, loss and gain, are achieved through electrically biased piezoelectric semiconductors. We study first how wave attenuation and amplification can be tuned, and when combined, can give rise to a phononic P T synthetic media with unidirectional suppressed reflectance, a feature directly applicable to evading sonar detection.
Parity-Time Synthetic Phononic Media.
Christensen, J; Willatzen, M; Velasco, V R; Lu, M-H
2016-05-20
Classical systems containing cleverly devised combinations of loss and gain elements constitute extremely rich building units that can mimic non-Hermitian properties, which conventionally are attainable in quantum mechanics only. Parity-time (PT) symmetric media, also referred to as synthetic media, have been devised in many optical systems with the ground breaking potential to create nonreciprocal structures and one-way cloaks of invisibility. Here we demonstrate a feasible approach for the case of sound where the most important ingredients within synthetic materials, loss and gain, are achieved through electrically biased piezoelectric semiconductors. We study first how wave attenuation and amplification can be tuned, and when combined, can give rise to a phononic PT synthetic media with unidirectional suppressed reflectance, a feature directly applicable to evading sonar detection.
Phonon hydrodynamics in two-dimensional materials
NASA Astrophysics Data System (ADS)
Cepellotti, Andrea; Fugallo, Giorgia; Paulatto, Lorenzo; Lazzeri, Michele; Mauri, Francesco; Marzari, Nicola
2015-03-01
The conduction of heat in two dimensions displays a wealth of fascinating phenomena of key relevance to the scientific understanding and technological applications of graphene and related materials. Here, we use density-functional perturbation theory and an exact, variational solution of the Boltzmann transport equation to study fully from first-principles phonon transport and heat conductivity in graphene, boron nitride, molybdenum disulphide and the functionalized derivatives graphane and fluorographene. In all these materials, and at variance with typical three-dimensional solids, normal processes keep dominating over Umklapp scattering well-above cryogenic conditions, extending to room temperature and more. As a result, novel regimes emerge, with Poiseuille and Ziman hydrodynamics, hitherto typically confined to ultra-low temperatures, characterizing transport at ordinary conditions. Most remarkably, several of these two-dimensional materials admit wave-like heat diffusion, with second sound present at room temperature and above in graphene, boron nitride and graphane.
Basic phononic diagnostic measurements in fluid columns
NASA Astrophysics Data System (ADS)
Hazony, D.; Hazony, Y.
2010-07-01
A pre-selected 21 MHz ultrasonic transducer was used to produce characteristic pulses, arbitrarily similar to the quantum-mechanical concept of a phonon, describable as having a single-frequency modulated Gaussian shape. The propagation of such pulses in water-acoustic channels was studied in conjunction with nonlinear regression analysis and an Erlangian model for size distribution of molecular aggregates. Experimental results obtained distinguish between surface and bulk phenomena and provide quantitative measures of an average molecular cluster size in water. The relevance of the Erlangian model, in studying the near front of the channel, provides a significant distinction between the behavior of pure water and Ringer's solution of water. The inherent consistency between the various results re-enforces the theoretical approach, implying new venues for future research.
Phonon-Mediated Nonclassical Interference in Diamond
NASA Astrophysics Data System (ADS)
England, Duncan G.; Fisher, Kent A. G.; MacLean, Jean-Philippe W.; Bustard, Philip J.; Heshami, Khabat; Resch, Kevin J.; Sussman, Benjamin J.
2016-08-01
Quantum interference of single photons is a fundamental aspect of many photonic quantum processing and communication protocols. Interference requires that the multiple pathways through an interferometer be temporally indistinguishable to within the coherence time of the photon. In this Letter, we use a diamond quantum memory to demonstrate interference between quantum pathways, initially temporally separated by many multiples of the optical coherence time. The quantum memory can be viewed as a light-matter beam splitter, mapping a THz-bandwidth single photon to a variable superposition of the output optical mode and stored phononic mode. Because the memory acts both as a beam splitter and as a buffer, the relevant coherence time for interference is not that of the photon, but rather that of the memory. We use this mechanism to demonstrate nonclassical single-photon and two-photon interference between quantum pathways initially separated by several picoseconds, even though the duration of the photons themselves is just ˜250 fs .
Phonon limited superconducting correlations in metallic nanograins
NASA Astrophysics Data System (ADS)
Croitoru, M. D.; Shanenko, A. A.; Vagov, A.; Milošević, M. V.; Axt, V. M.; Peeters, F. M.
2015-11-01
Conventional superconductivity is inevitably suppressed in ultra-small metallic grains for characteristic sizes smaller than the Anderson limit. Experiments have shown that above the Anderson limit the critical temperature may be either enhanced or reduced when decreasing the particle size, depending on the superconducting material. In addition, there is experimental evidence that whether an enhancement or a reduction is found depends on the strength of the electron-phonon interaction in the bulk. We reveal how the strength of the e-ph interaction interplays with the quantum-size effect and theoretically obtain the critical temperature of the superconducting nanograins in excellent agreement with experimental data. We demonstrate that strong e-ph scattering smears the peak structure in the electronic density-of-states of a metallic grain and enhances the electron mass, and thereby limits the highest Tc achievable by quantum confinement.
Razdolski, Ilya; Chen, Yiguo; Giles, Alexander J; Gewinner, Sandy; Schöllkopf, Wieland; Hong, Minghui; Wolf, Martin; Giannini, Vincenzo; Caldwell, Joshua D; Maier, Stefan A; Paarmann, Alexander
2016-11-09
We report on the strong enhancement of mid-infrared second-harmonic generation (SHG) from SiC nanopillars due to the resonant excitation of localized surface phonon polaritons within the Reststrahlen band. A strong dependence of the SHG enhancement upon the optical mode distribution was observed. One such mode, the monopole, exhibits an enhancement that is beyond what is anticipated from field localization and dispersion of the linear and nonlinear SiC optical properties. Comparing the results for the identical nanostructures made of 4H and 6H SiC polytypes, we demonstrate the interplay of localized surface phonon polaritons with zone-folded weak phonon modes of the anisotropic crystal. Tuning the monopole mode in and out of the region where the zone-folded phonon is excited in 6H-SiC, we observe a further prominent increase of the already enhanced SHG output when the two modes are coupled. Envisioning this interplay as one of the showcase features of mid-infrared nonlinear nanophononics, we discuss its prospects for the effective engineering of nonlinear-optical materials with desired properties in the infrared spectral range.
Thickness-dependent coherent phonon frequency in ultrathin FeSe/SrTiO_{3} films
Yang, Shuolong; Sobota, Jonathan A.; Leuenberger, Dominik; Kemper, Alexander F.; Lee, James J.; Schmitt, Felix T.; Li, Wei; Moore, Rob G.; Kirchmann, Patrick S.; Shen, Zhi -Xun
2015-06-01
Ultrathin FeSe films grown on SrTiO_{3} substrates are a recent milestone in atomic material engineering due to their important role in understanding unconventional superconductivity in Fe-based materials. By using femtosecond time- and angle-resolved photoelectron spectroscopy, we study phonon frequencies in ultrathin FeSe/SrTiO_{3} films grown by molecular beam epitaxy. After optical excitation, we observe periodic modulations of the photoelectron spectrum as a function of pump–probe delay for 1-unit-cell, 3-unit-cell, and 60-unit-cell thick FeSe films. The frequencies of the coherent intensity oscillations increase from 5.00 ± 0.02 to 5.25 ± 0.02 THz with increasing film thickness. By comparing with previous works, we attribute this mode to the Se A_{1g} phonon. The dominant mechanism for the phonon softening in 1-unit-cell thick FeSe films is a substrate-induced lattice strain. Results demonstrate an abrupt phonon renormalization due to a lattice mismatch between the ultrathin film and the substrate.
Phonon interference in crystalline and amorphous confined nanoscopic films
NASA Astrophysics Data System (ADS)
Liang, Zhi; Wilson, Thomas E.; Keblinski, Pawel
2017-02-01
Using molecular dynamics phonon wave packet simulations, we study phonon transmission across hexagonal (h)-BN and amorphous silica (a-SiO2) nanoscopic thin films sandwiched by two crystalline leads. Due to the phonon interference effect, the frequency-dependent phonon transmission coefficient in the case of the crystalline film (Si|h-BN|Al heterostructure) exhibits a strongly oscillatory behavior. In the case of the amorphous film (Si|a-SiO2|Al and Si|a-SiO2|Si heterostructures), in spite of structural disorder, the phonon transmission coefficient also exhibits oscillatory behavior at low frequencies (up to ˜1.2 THz), with a period of oscillation consistent with the prediction from the two-beam interference equation. Above 1.2 THz, however, the phonon interference effect is greatly weakened by the diffuse scattering of higher-frequency phonons within an a-SiO2 thin film and at the two interfaces confining the a-SiO2 thin film.
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.
Variable-Range Hopping through Marginally Localized Phonons.
Banerjee, Sumilan; Altman, Ehud
2016-03-18
We investigate the effect of coupling Anderson localized particles in one dimension to a system of marginally localized phonons having a symmetry protected delocalized mode at zero frequency. This situation is naturally realized for electrons coupled to phonons in a disordered nanowire as well as for ultracold fermions coupled to phonons of a superfluid in a one-dimensional disordered trap. To determine if the coupled system can be many-body localized we analyze the phonon-mediated hopping transport for both the weak and strong coupling regimes. We show that the usual variable-range hopping mechanism involving a low-order phonon process is ineffective at low temperature due to discreteness of the bath at the required energy. Instead, the system thermalizes through a many-body process involving exchange of a diverging number n∝-logT of phonons in the low temperature limit. This effect leads to a highly singular prefactor to Mott's well-known formula and strongly suppresses the variable range hopping rate. Finally, we comment on possible implications of this physics in higher dimensional electron-phonon coupled systems.
Phonon anomalies in some iron telluride materials
C. C. Homes; Dai, Y. M.; Schneeloch, J.; Zhong, R. D.; Gu, G. D.
2016-03-21
In this paper, the detailed temperature dependence of the infrared-active mode in Fe_{1.03}Te (T_{N} ≃ 68 K) and Fe_{1.13}Te (T_{N} ≃ 56 K) has been examined, and the position, width, strength, and asymmetry parameter have been determined using an asymmetric Fano profile superimposed on an electronic background. In both materials the frequency of the mode increases as the temperature is reduced; however, there is also a slight asymmetry in the line shape, indicating that the mode is coupled to either spin or charge excitations. Below T_{N} there is an anomalous decrease in frequency, and the mode shows little temperature dependence, at the same time becoming more symmetric, suggesting a reduction in spin- or electron-phonon coupling. The frequency of the infrared-active mode and the magnitude of the shift below T_{N} are predicted reasonably well by first-principles calculations; however, the predicted splitting of the mode is not observed. In superconducting FeTe_{0.55}Se_{0.45} (T_{c} ≃ 14 K) the infrared-active E_{u} mode displays asymmetric line shape at all temperatures, which is most pronounced between 100 – 200 K, indicating the presence of either spin- or electron-phonon coupling, which may be a necessary prerequisite for superconductivity in this class of materials.
NASA Astrophysics Data System (ADS)
Jin, Jae Sik
2017-03-01
Phonon dynamics in nanostructures is critically important to thermoelectric and optoelectronic devices because it determines the transport and other crucial properties. However, accurately evaluating the phonon lifetimes is extremely difficult. This study reports on the development of a new semi-empirical method to estimate the full-spectrum phonon lifetimes in thin silicon films at room temperature based on the experimental data on the phonon mean-free-path spectrum in bulk silicon and a phenomenological consideration of phonon transport in thin films. The bulk of this work describes the theory and the validation; then, we discuss the trend of the phonon lifetimes in thin silicon films when their thicknesses decrease.
The anharmonic phonon decay rate in group-III nitrides
NASA Astrophysics Data System (ADS)
Srivastava, G. P.
2009-04-01
Measured lifetimes of hot phonons in group-III nitrides have been explained theoretically by considering three-phonon anharmonic interaction processes. The basic ingredients of the theory include full phonon dispersion relations obtained from the application of an adiabatic bond charge model and crystal anharmonic potential within the isotropic elastic continuum model. The role of various decay routes, such as Klemens, Ridley, Vallée-Bogani and Barman-Srivastava channels, in determining the lifetimes of the Raman active zone-centre longitudinal optical (LO) modes in BN (zincblende structure) and A1(LO) modes in AlN, GaN and InN (wurtzite structure) has been quantified.
Analog model for quantum gravity effects: phonons in random fluids.
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.
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.
Vortex-phonon interaction in the Kosterlitz-Thouless theory
Kozik, Evgeny; Prokof'ev, Nikolay; Svistunov, Boris
2006-03-01
The 'canonical' variables of the Kosterlitz-Thouless theory--fields {phi}{sub 0}(r) and {phi}(r), generally believed to stand for vortices and phonons (or their XY equivalents, like spin waves, etc.) turn out to be neither vortices and phonons, nor, strictly speaking, canonical variables. The latter fact explains paradoxes of (i) absence of interaction between {phi}{sub 0} and {phi}, and (ii) nonphysical contribution of small vortex pairs to long-range phase correlations. We resolve the paradoxes by explicitly relating {phi}{sub 0} and {phi} to canonical vortex-pair and phonon variables.
Spin-seebeck effect: a phonon driven spin distribution.
Jaworski, C M; Yang, J; Mack, S; Awschalom, D D; Myers, R C; Heremans, J P
2011-05-06
Here we report on measurements of the spin-Seebeck effect in GaMnAs over an extended temperature range alongside the thermal conductivity, specific heat, magnetization, and thermoelectric power. The amplitude of the spin-Seebeck effect in GaMnAs scales with the thermal conductivity of the GaAs substrate and the phonon-drag contribution to the thermoelectric power of the GaMnAs, demonstrating that phonons drive the spin redistribution. A phenomenological model involving phonon-magnon drag explains the spatial and temperature dependence of the measured spin distribution.
First-Principles Calculation of forces and phonons in solid
NASA Astrophysics Data System (ADS)
Ning, Zhenhua; Shelton, William
We have developed a multiple scattering theory approach to calculate Hellmann-Feynman forces and phonons via the calculation of the force constant and dynamical matrix. To demonstrate the accuracy and validity of our approach we compare with the ELK code, which is a full potential Linear Augmented Plane Wave (FLAPW) based method. As we will show our forces and phonon dispersion curves are in good agreement with the FLAPW code. This work lays the foundation for developing a first principles approach for calculation of phonons in substitutionally disordered materials.
Phonon Quasidiffusion in Cryogenic Dark Matter Search Large Germanium Detectors
Leman, S.W.; Cabrera, B.; McCarthy, K.A.; Pyle, M.; Resch, R.; Sadoulet, B.; Sundqvist, K.M.; Brink, P.L.; Cherry, M.; Do Couto E Silva, E.; Figueroa-Feliciano, E.; Mirabolfathi, N.; Serfass, B.; Tomada, A.; /Stanford U., Phys. Dept.
2012-06-04
We present results on quasidiffusion studies in large, 3 inch diameter, 1 inch thick [100] high purity germanium crystals, cooled to 50 mK in the vacuum of a dilution refrigerator, and exposed with 59.5 keV gamma-rays from an Am-241 calibration source. We compare data obtained in two different detector types, with different phonon sensor area coverage, with results from a Monte Carlo. The Monte Carlo includes phonon quasidiffusion and the generation of phonons created by charge carriers as they are drifted across the detector by ionization readout channels.
Quantum symmetries induced by phonons in the Hubbard model
NASA Astrophysics Data System (ADS)
Montorsi, Arianna; Rasetti, Mario
1994-03-01
We show how the addition of a phonon field to the Hubbard model deforms the superconducting su(2) part of the global symmetry Lie algebra su(2)⊗su(2)/openZ2, holding at half filling for the customary model, into a quantum [su(2)]q symmetry, holding for a filling which depends on the electron-phonon interaction strength. Such symmetry originates in the feature that in the presence of phonons the hopping amplitude turns out to depend on the coupling strength. The states generated by resorting to this q symmetry exhibit both off-diagonal long-range order and pairing.
Observation of Anomalous Phonons in Orthorhombic Rare-earth Manganites
P Gao; H Chen; T Tyson; Z Liu; J Bai; L Wang; Y Chio; S Cheong
2011-12-31
We observe the appearance of a phonon near the lock-in temperature in orthorhombic REMnO{sub 3} (RE denotes rare earth) (RE: Lu and Ho) and anomalous phonon hardening in orthorhombic LuMnO{sub 3}. The anomalous phonon occurs at the onset of spontaneous polarization. No such changes were found in incommensurate orthorhombic DyMnO{sub 3}. These observations directly reveal different electric polarization mechanisms in the E-type and incommensurate-type orthorhombic REMnO{sub 3}.
Modeling and Simulation of Phonon Transport at the Nanoscale for Optimum Thermal Management
NASA Astrophysics Data System (ADS)
Mao, Rui
mixed bonding force between the Pd and C atoms results in incomplete hybridization of Pd and graphene orbital states at the junction, leading effectively to two phonon interfaces and a larger than expected thermal resistance. Comparison with available experimental data shows good agreement. The result clearly suggests the feasibility of phonon engineering for thermal property optimization at the interface. Transition-metal dichalcogenides (TMDs) MX2 (M=Mo,W; X=S,Se), one of the beyond- graphene two-dimensional semiconductor materials, have emerged as promising candidates due to their distinctive electronic and optical properties. Unlike the zero-bandgap graphene, TMDs have intrinsic bandgaps in the range of 1.1--2.2eV, allowing low off-current for field effect transistors. Thermal transport properties at the metal/MoS2 interfaces are then analyzed by using the atomistic phonon transport model. The considered structures include chemisorbed Sc(0001)/MoS2 and Ru(0001)/MoS 2, physisorbed Au(111)/MoS2, as well as Pd(111)/MoS 2 with intermediate characteristics. Calculated results illustrate a distinctive dependence of thermal transfer on the details of interfacial microstructures. More specifically, the chemisorbed case with a stronger bonding exhibits a generally smaller interfacial thermal resistance than the physisorbed. Comparison between metal/MoS2 and metal/graphene systems suggests that metal/MoS 2 is significantly more resistive. Further examination of lattice dynamics identifies the presence of multiple distinct atomic planes and bonding patterns at the interface as the key origin of the observed large thermal resistance. Finally, since the commensurate-incommensurate transitions are ubiquitous in the fabrication of the 2D material based devices, we have extended our investigation to the thermal/phonon transport across the misoriented 2D nanostructures. An analytical model that can incorporate the atomic level detail as well as being time-efficient is developed for
Acoustic-optical phonon up-conversion and hot-phonon bottleneck in lead-halide perovskites
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
NASA Astrophysics Data System (ADS)
Kuleev, I. I.; Bakharev, S. M.; Kuleev, I. G.; Ustinov, V. V.
2017-01-01
Effect of anisotropy of elastic energy on the phonon propagation in single-crystal nanowires made of Fe, Cu, MgO, InSb, and GaAs materials that are used to fabricate spintronics devices in the regime of the Knudsen flow of phonon gas has been studied. A new method of analyzing the focusing of quasi-transverse modes has been suggested, which made it possible to determine the average values of the densities of phonon states in the regions of focusing and defocusing slow and fast quasi-transverse modes. The effect of phonon focusing on the anisotropy of heat conductivity and lengths of the phonon free paths has been analyzed for all acoustic modes that exist in spintronics nanostructures. It has been shown that for all the nanowires investigated the angular dependences of the free paths of fast and slow transverse modes in the {100} and {110} planes correlate with the angular dependences of the densities of phonon states for these modes. Directions of the heat flux that ensure the maximum and minimum phonon heat conductivity in the nanowires have been determined.
Acoustic-optical phonon up-conversion and hot-phonon bottleneck in lead-halide perovskites.
Yang, Jianfeng; Wen, Xiaoming; Xia, Hongze; Sheng, Rui; Ma, Qingshan; Kim, Jincheol; Tapping, Patrick; Harada, Takaaki; Kee, Tak W; Huang, Fuzhi; Cheng, Yi-Bing; Green, Martin; Ho-Baillie, Anita; Huang, Shujuan; Shrestha, Santosh; Patterson, Robert; Conibeer, Gavin
2017-01-20
The hot-phonon bottleneck effect in lead-halide perovskites (APbX3) prolongs the cooling period of hot charge carriers, an effect that could be used in the next-generation photovoltaics devices. Using ultrafast optical characterization and first-principle calculations, four kinds of lead-halide perovskites (A=FA(+)/MA(+)/Cs(+), X=I(-)/Br(-)) are compared in this study to reveal the carrier-phonon dynamics within. Here we show a stronger phonon bottleneck effect in hybrid perovskites than in their inorganic counterparts. Compared with the caesium-based system, a 10 times slower carrier-phonon relaxation rate is observed in FAPbI3. The up-conversion of low-energy phonons is proposed to be responsible for the bottleneck effect. The presence of organic cations introduces overlapping phonon branches and facilitates the up-transition of low-energy modes. The blocking of phonon propagation associated with an ultralow thermal conductivity of the material also increases the overall up-conversion efficiency. This result also suggests a new and general method for achieving long-lived hot carriers in materials.
Acoustic-optical phonon up-conversion and hot-phonon bottleneck in lead-halide perovskites
NASA Astrophysics Data System (ADS)
Yang, Jianfeng; Wen, Xiaoming; Xia, Hongze; Sheng, Rui; Ma, Qingshan; Kim, Jincheol; Tapping, Patrick; Harada, Takaaki; Kee, Tak W.; Huang, Fuzhi; Cheng, Yi-Bing; Green, Martin; Ho-Baillie, Anita; Huang, Shujuan; Shrestha, Santosh; Patterson, Robert; Conibeer, Gavin
2017-01-01
The hot-phonon bottleneck effect in lead-halide perovskites (APbX3) prolongs the cooling period of hot charge carriers, an effect that could be used in the next-generation photovoltaics devices. Using ultrafast optical characterization and first-principle calculations, four kinds of lead-halide perovskites (A=FA+/MA+/Cs+, X=I-/Br-) are compared in this study to reveal the carrier-phonon dynamics within. Here we show a stronger phonon bottleneck effect in hybrid perovskites than in their inorganic counterparts. Compared with the caesium-based system, a 10 times slower carrier-phonon relaxation rate is observed in FAPbI3. The up-conversion of low-energy phonons is proposed to be responsible for the bottleneck effect. The presence of organic cations introduces overlapping phonon branches and facilitates the up-transition of low-energy modes. The blocking of phonon propagation associated with an ultralow thermal conductivity of the material also increases the overall up-conversion efficiency. This result also suggests a new and general method for achieving long-lived hot carriers in materials.
EMRS Spring Meeting 2014 Symposium D: Phonons and fluctuations in low dimensional structures
NASA Astrophysics Data System (ADS)
2014-11-01
The E-MRS 2014 Spring meeting, held from 26-30th May 2014 in Lille included the Symposium D entitled ''Phonons and Fluctuations in Low Dimensional Structures'', the first edition of its kind. The symposium was organised in response to the increasing interest in the study of phonons in the context of advances in condensed matter physics, electronics, experimental methods and theory and, in particular, the transfer of energy across atomic interfaces and the propagation of energy in the nm-scale. Steering heat by light or vice versa and examining nano-scale energy conversion (as in thermoelectricity and harvesting e.g. in biological systems) are two aspects that share the underlying science of energy processes across atomic interfaces and energy propagation in the nanoscale and or in confined systems. The nanometer scale defies several of the bulk relationships as confinement of electrons and phonons, locality and non-equilibrium become increasingly important. The propagation of phonons as energy carriers impacts not only heat transfer, but also the very concept and handling of temperature in non-equilibrium and highly localised conditions. Much of the needed progress depends on the materials studied and this symposium targeted the interface material aspects as well as the emerging concepts to advance in this field. The symposium had its origins in a series of meetings and seminars including: (1) the first Phonon Engineering Workshop, funded by Catalan Institute for Research and Advanced Studies (ICREA), the then MICINN, the CNRS, VTT, and several EU projects, held in Saint Feliu de Guixols (Girona, Spain) from 24th to 27th of May 2010 with 65 participants from Europe, the USA and Japan; (2) the first Phonons and Fluctuations workshop, held in Paris on 8th and 9th November 2010, supported by French, Spanish and Finnish national projects and EU projects, attended by about 50 researchers; (3) the second Phonon and Fluctuations workshop, held in Paris on 8th and 9th
Flexural-Phonon Scattering Induced by Electrostatic Gating in Graphene
NASA Astrophysics Data System (ADS)
Gunst, Tue; Kaasbjerg, Kristen; Brandbyge, Mads
2017-01-01
Graphene has an extremely high carrier mobility partly due to its planar mirror symmetry inhibiting scattering by the highly occupied acoustic flexural phonons. Electrostatic gating of a graphene device can break the planar mirror symmetry, yielding a coupling mechanism to the flexural phonons. We examine the effect of the gate-induced one-phonon scattering on the mobility for several gate geometries and dielectric environments using first-principles calculations based on density functional theory and the Boltzmann equation. We demonstrate that this scattering mechanism can be a mobility-limiting factor, and show how the carrier density and temperature scaling of the mobility depends on the electrostatic environment. Our findings may explain the high deformation potential for in-plane acoustic phonons extracted from experiments and, furthermore, suggest a direct relation between device symmetry and resulting mobility.
Phonon-assisted relaxation in a frustrated antiferromagnet
Ehlers, Georg
2006-01-01
A thermally activated magnetic relaxation is observed using neutron spin-echo in the pyrochlore slab (kagom{acute e} bilayer) compound SrCr{sub 9x}Ga{sub 12-9x}O{sub 19} (x=0.95) in a restricted temperature range, 4K < T < 4K, above a cross-over to a low temperature relaxation regime with a weaker temperature dependence. The activation energy of the thermally activated relaxation, of the order of 7 meV, coincides with the energy of a phonon mode observed with neutron and Raman spectroscopy, indicating a phonon-assisted regime. The experimental observation of phonon-assisted process gives additional insight to the importance of spin-phonon coupling in frustrated magnets with regard to the models mostly based on purely magnetic interactions.
The phononic crystals: An unending quest for tailoring acoustics
NASA Astrophysics Data System (ADS)
Kushwaha, Manvir S.
2016-07-01
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.
Topological Phonon Modes in a Two-Dimensional Wigner Crystal
NASA Astrophysics Data System (ADS)
Ji, Wen-Cheng; Shi, Jun-Ren
2017-03-01
We investigate the spin-orbit coupling effect in a two-dimensional Wigner crystal. We show that sufficiently strong spin-orbit coupling and an appropriate sign of g-factor could transform the Wigner crystal to a topological phonon system. We demonstrate the existence of chiral phonon edge modes in finite size samples, as well as the robustness of the modes in the topological phase. We explore the possibility of realizing the topological phonon system in two-dimensional Wigner crystals confined in semiconductor quantum wells/heterostructure. We find that the spin-orbit coupling is too weak for driving a topological phase transition in these systems. We argue that one may look for the topological phonon system in correlated Wigner crystals with emergent effective spin-orbit coupling.
Planck distribution of phonons in a Bose-Einstein condensate.
Schley, R; Berkovitz, A; Rinott, S; Shammass, I; Blumkin, A; Steinhauer, J
2013-08-02
The Planck distribution of photons emitted by a blackbody led to the development of quantum theory. An analogous distribution of phonons should exist in a Bose-Einstein condensate. We observe this Planck distribution of thermal phonons in a 3D condensate. This observation provides an important confirmation of the basic nature of the condensate's quantized excitations. In contrast to the bunching effect, the density fluctuations are seen to increase with increasing temperature. This is due to the nonconservation of the number of phonons. In the case of rapid cooling, the phonon temperature is out of equilibrium with the surrounding thermal cloud. In this case, a Bose-Einstein condensate is not as cold as previously thought. These measurements are enabled by our in situ k-space technique.
Mapping gigahertz vibrations in a plasmonic-phononic crystal
NASA Astrophysics Data System (ADS)
Kelf, Timothy A.; Hoshii, Wataru; Otsuka, Paul H.; Sakuma, Hirotaka; Veres, Istvan A.; Cole, Robin M.; Mahajan, Sumeet; Baumberg, Jeremy J.; Tomoda, Motonobu; Matsuda, Osamu; Wright, Oliver B.
2013-02-01
We image the gigahertz vibrational modes of a plasmonic-phononic crystal at sub-micron resolution by means of an ultrafast optical technique, using a triangular array of spherical gold nanovoids as a sample. Light is strongly coupled to the plasmonic modes, which interact with the gigahertz phonons by a process akin to surface-enhanced stimulated Brillouin scattering. A marked enhancement in the observed optical reflectivity change at the centre of a void on phononic resonance is likely to be caused by this mechanism. By comparison with numerical simulations of the vibrational field, we identify resonant breathing deformations of the voids and elucidate the corresponding mode shapes. We thus establish scanned optomechanical probing of periodic plasmonic-phononic structures as a new means of investigating their coupled excitations on the nanoscale.
The Electron-Phonon Interaction as Studied by Photoelectron Spectroscopy
D.W. Lynch
2004-09-30
With recent advances in energy and angle resolution, the effects of electron-phonon interactions are manifest in many valence-band photoelectron spectra (PES) for states near the Fermi level in metals.
Molecular Detection and Phonon Filtering in Heat-Transfer Spectroscopy
NASA Astrophysics Data System (ADS)
Walczak, Kamil; Yerkes, Kirk
2014-03-01
We examine heat transport carried by acoustic phonons in the systems composed of nanoscale chains of masses coupled to two thermal baths of different temperatures. Thermal conductance is obtained by using linearized Landauer formula for heat flux with phonon transmission probability calculated within atomistic Green's functions (AGF) method. AGF formalism is extended onto dissipative chains of masses with harmonic coupling beyond nearest-neighbor approximation, while atomistic description of heat reservoirs is also included into computational scheme. The resonant structure of phonon transmission spectrum is analyzed with respect to reservoir-dimensionality effects, molecular damping, and mass-to-mass harmonic coupling. Analysis of transmission zeros (antiresonances) and their accompanied Fano-shape resonances are discussed as a result of interference effects between different vibrational modes. Specifically, we show that the heat-transfer-based characterization method may be used to identify individual molecules or filter out specific phonon modes from the whole frequency spectrum. This work is supported by AFOSR grant.
Dispersive Phonon Imaging in Iii-V Semiconductors.
NASA Astrophysics Data System (ADS)
Hebboul, Saad Eddine
Low-temperature transport properties of high-frequency acoustic phonons are investigated in GaAs, InSb, InP and InAs using the phonon-imaging technique. In this method, a focused laser beam provides a movable heat source on one side of a cooled crystal (<=q2 K). A single small phonon detector on the opposite face records the transmitted heat flux as a function of propagation direction. Ballistic phonons channel along directions in the crystal which are completely determined by the detailed shape of constant-energy surfaces in wavevector space. The resulting focusing patterns are characterized by sharp phonon caustics which are clearly identified from the continuous background due to scattered phonons. In the dispersive regime, where phonon wavelength is comparable to atomic spacing, the angular positions of these caustic lines are very sensitive to phonon frequency, thus providing a novel test for lattice dynamics theories. Experiments are performed with superconducting tunnel junctions and Al bolometers to probe both the high-frequency and low -frequency regimes, respectively. We find that large-k ballistic phonons give rise to distinct focusing patterns in all four types of crystals, with thicknesses varying between 0.4 and 0.8 mm. Due to isotope scattering in the bulk, tunnel-junction experiments yield well-defined caustic patterns with a dominant frequency given by the detector gap 2Delta. In InSb, where zone boundary frequencies are small (nu_ {TA} ~ 1.2 THz), the frequency dependence of the dispersive phonon focusing patterns are measured using PbTl (0.43, 0.59 THz) and PbBi (0.69, 0.73, 0.78, 0.82 THz) tunnel junction detectors. The results are interpreted with Monte Carlo calculations based on rigid, dipole, shell, and bond-charge models. Although each model yields satisfactory fits to the previously measured dispersion curves, the predicted patterns show remarkable differences in the caustic structures. This result underscores the utility of phonon imaging
Correlations of collective observables and the phonon structure of nuclei
Casten, R.F.; Zamfir, N.V. ||
1994-07-01
A ``horizontal`` view of nuclear structures is described in which various observables are correlated over broad mass ranges. This approach leads to a number of remarkable correlations, to new understanding of the evolution of structure, to a challenge to microscopic theories, and to new signatures of structure that will be especially useful with radioactive beam experiments. In particular, this and other evidence suggests a nearly universal and pervasive role of phonon and multi-phonon excitations in nuclei.
Acoustic Bloch oscillations in a two-dimensional phononic crystal
NASA Astrophysics Data System (ADS)
He, Zhaojian; Peng, Shasha; Cai, Feiyan; Ke, Manzhu; Liu, Zhengyou
2007-11-01
We report the observation of acoustic Bloch oscillations at megahertz frequency in a two-dimensional phononic crystal. By creating periodically arrayed cavities with a decreasing gradient in width along one direction in the phononic crystal, acoustic Wannier-Stark ladders are created in the frequency domain. The oscillatory motion of an incident Gaussian pulse inside the sample is demonstrated by both simulation and experiment.
Slow light and slow acoustic phonons in optophononic resonators
NASA Astrophysics Data System (ADS)
Villafañe, V.; Soubelet, P.; Bruchhausen, A. E.; Lanzillotti-Kimura, N. D.; Jusserand, B.; Lemaître, A.; Fainstein, A.
2016-11-01
Slow and confined light have been exploited in optoelectronics to enhance light-matter interactions. Here we describe the GaAs/AlAs semiconductor microcavity as a device that, depending on the excitation conditions, either confines or slows down both light and optically generated acoustic phonons. The localization of photons and phonons in the same place of space amplifies optomechanical processes. Picosecond laser pulses are used to study through time-resolved reflectivity experiments the coupling between photons and both confined and slow acoustic phonons when the laser is tuned either with the cavity (confined) optical mode or with the stop-band edge (slow) optical modes. A model that fully takes into account the modified propagation of the acoustic phonons and light in these resonant structures is used to describe the laser detuning dependence of the coherently generated phonon spectra and amplitude under these different modes of laser excitation. We observe that confined light couples only to confined mechanical vibrations, while slow light can generate both confined and slow coherent vibrations. A strong enhancement of the optomechanical coupling using confined photons and vibrations, and also with properly designed slow photon and phonon modes, is demonstrated. The prospects for the use of these optoelectronic devices in confined and slow optomechanics are addressed.
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.
Magnetic moments induce strong phonon renormalization in FeSi
Krannich, S.; Sidis, Y.; Lamago, D.; Heid, R.; Mignot, J.-M.; Löhneysen, H. v.; Ivanov, A.; Steffens, P.; Keller, T.; Wang, L.; Goering, E.; Weber, F.
2015-01-01
The interactions of electronic, spin and lattice degrees of freedom in solids result in complex phase diagrams, new emergent phenomena and technical applications. While electron–phonon coupling is well understood, and interactions between spin and electronic excitations are intensely investigated, only little is known about the dynamic interactions between spin and lattice excitations. Noncentrosymmetric FeSi is known to undergo with increasing temperature a crossover from insulating to metallic behaviour with concomitant magnetic fluctuations, and exhibits strongly temperature-dependent phonon energies. Here we show by detailed inelastic neutron-scattering measurements and ab initio calculations that the phonon renormalization in FeSi is linked to its unconventional magnetic properties. Electronic states mediating conventional electron–phonon coupling are only activated in the presence of strong magnetic fluctuations. Furthermore, phonons entailing strongly varying Fe–Fe distances are damped via dynamic coupling to the temperature-induced magnetic moments, highlighting FeSi as a material with direct spin–phonon coupling and multiple interaction paths. PMID:26611619
Toward reversing Joule heating with a phonon-absorbing heterobarrier
NASA Astrophysics Data System (ADS)
Shin, Seungha; Kaviany, Massoud
2015-02-01
Using a graded heterobarrier placed along an electron channel, phonons emitted in Joule heating are recycled in situ by increasing the entropy of phonon-absorbing electrons. The asymmetric electric potential distribution created by alloy grading separates the phonon absorption and emission regions, and emission in the larger effective-mass region causes momentum relaxation with smaller electron kinetic energy loss. These lead to smaller overall phonon emission and simultaneous potential-gain and self-cooling effects. Larger potential is gained with lower current and higher optical-phonon temperature. The self-consistent Monte Carlo simulations complying with the lateral momentum conservation combined with the entropy analysis are applied to a GaAs:Al electron channel with a graded heterobarrier, and under ideal lateral thermal isolation from surroundings, the phonon recycling efficiency reaches 25% of the reversible limit at 350 K, and it increases with temperature. The lateral momentum contributes to the transmission across the barrier, so partially nonconserving lateral momentum electron scattering (rough interface) can improve efficiency.
Phonon anharmonicity and negative thermal expansion in SnSe
Bansal, Dipanshu; Hong, Jiawang; Li, Chen W.; May, Andrew F.; Porter, Wallace; Hu, Michael Y.; Abernathy, Douglas L.; Delaire, Olivier
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, 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.
Phonon anharmonicity and negative thermal expansion in SnSe
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
Orbitally-driven giant phonon anharmonicity in SnSe
Li, Chen W.; Hong, Jiawang; May, Andrew F.; Bansal, Dipanshu; Chi, Songxue; Hong, Tao; Ehlers, Georg; Delaire, Olivier A.
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. We 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.
Orbitally-driven giant phonon anharmonicity in SnSe
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
Electron–phonon coupling in hybrid lead halide perovskites
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
Phonon anharmonicity and negative thermal expansion in SnSe
NASA Astrophysics Data System (ADS)
Bansal, Dipanshu; Hong, Jiawang; Li, Chen W.; May, Andrew F.; Porter, Wallace; Hu, Michael Y.; Abernathy, Douglas L.; Delaire, Olivier
2016-08-01
The anharmonic phonon properties of SnSe in the P n m a 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, 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. The origin of the anharmonic phonon thermodynamics is linked to the electronic structure.
Electron-phonon coupling in hybrid lead halide perovskites
NASA Astrophysics Data System (ADS)
Wright, Adam D.; Verdi, Carla; Milot, Rebecca L.; Eperon, Giles E.; Pérez-Osorio, Miguel A.; Snaith, Henry J.; Giustino, Feliciano; Johnston, Michael B.; Herz, Laura M.
2016-05-01
Phonon scattering limits charge-carrier mobilities and governs emission line broadening in hybrid metal halide perovskites. Establishing how charge carriers interact with phonons in these materials is therefore essential for the development of high-efficiency perovskite photovoltaics and low-cost lasers. Here we investigate the temperature dependence of emission line broadening in the four commonly studied formamidinium and methylammonium perovskites, HC(NH2)2PbI3, HC(NH2)2PbBr3, CH3NH3PbI3 and CH3NH3PbBr3, and discover that scattering from longitudinal optical phonons via the Fröhlich interaction is the dominant source of electron-phonon coupling near room temperature, with scattering off acoustic phonons negligible. We determine energies for the interacting longitudinal optical phonon modes to be 11.5 and 15.3 meV, and Fröhlich coupling constants of ~40 and 60 meV for the lead iodide and bromide perovskites, respectively. Our findings correlate well with first-principles calculations based on many-body perturbation theory, which underlines the suitability of an electronic band-structure picture for describing charge carriers in hybrid perovskites.
Electron-phonon coupling in hybrid lead halide perovskites.
Wright, Adam D; Verdi, Carla; Milot, Rebecca L; Eperon, Giles E; Pérez-Osorio, Miguel A; Snaith, Henry J; Giustino, Feliciano; Johnston, Michael B; Herz, Laura M
2016-05-26
Phonon scattering limits charge-carrier mobilities and governs emission line broadening in hybrid metal halide perovskites. Establishing how charge carriers interact with phonons in these materials is therefore essential for the development of high-efficiency perovskite photovoltaics and low-cost lasers. Here we investigate the temperature dependence of emission line broadening in the four commonly studied formamidinium and methylammonium perovskites, HC(NH2)2PbI3, HC(NH2)2PbBr3, CH3NH3PbI3 and CH3NH3PbBr3, and discover that scattering from longitudinal optical phonons via the Fröhlich interaction is the dominant source of electron-phonon coupling near room temperature, with scattering off acoustic phonons negligible. We determine energies for the interacting longitudinal optical phonon modes to be 11.5 and 15.3 meV, and Fröhlich coupling constants of ∼40 and 60 meV for the lead iodide and bromide perovskites, respectively. Our findings correlate well with first-principles calculations based on many-body perturbation theory, which underlines the suitability of an electronic band-structure picture for describing charge carriers in hybrid perovskites.
A moment model for phonon transport at room temperature
NASA Astrophysics Data System (ADS)
Mohammadzadeh, Alireza; Struchtrup, Henning
2017-01-01
Heat transfer in solids is modeled by deriving the macroscopic equations for phonon transport from the phonon-Boltzmann equation. In these equations, the Callaway model with frequency-dependent relaxation time is considered to describe the Resistive and Normal processes in the phonon interactions. Also, the Brillouin zone is considered to be a sphere, and its diameter depends on the temperature of the system. A simple model to describe phonon interaction with crystal boundary is employed to obtain macroscopic boundary conditions, where the reflection kernel is the superposition of diffusive reflection, specular reflection and isotropic scattering. Macroscopic moments are defined using a polynomial of the frequency and wave vector of phonons. As an example, a system of moment equations, consisting of three directional and seven frequency moments, i.e., 63 moments in total, is used to study one-dimensional heat transfer, as well as Poiseuille flow of phonons. Our results show the importance of frequency dependency in relaxation times and macroscopic moments to predict rarefaction effects. Good agreement with data reported in the literature is obtained.
Nonperturbative calculation of phonon effects on spin squeezing
NASA Astrophysics Data System (ADS)
Dylewsky, D.; Freericks, J. K.; Wall, M. L.; Rey, A. M.; Foss-Feig, M.
2016-01-01
Theoretical models of spins coupled to bosons provide a simple setting for studying a broad range of important phenomena in many-body physics, from virtually mediated interactions to decoherence and thermalization. In many atomic, molecular, and optical systems, such models also underlie the most successful attempts to engineer strong, long-ranged interactions for the purpose of entanglement generation. Especially when the coupling between the spins and bosons is strong, such that it cannot be treated perturbatively, the properties of such models are extremely challenging to calculate theoretically. Here, exact analytical expressions for nonequilibrium spin-spin correlation functions are derived for a specific model of spins coupled to bosons. The spatial structure of the coupling between spins and bosons is completely arbitrary, and thus the solution can be applied to systems in any number of dimensions. The explicit and nonperturbative inclusion of the bosons enables the study of entanglement generation (in the form of spin squeezing) even when the bosons are driven strongly and near resonantly, and thus provides a quantitative view of the breakdown of adiabatic elimination that inevitably occurs as one pushes towards the fastest entanglement generation possible. The solution also helps elucidate the effect of finite temperature on spin squeezing. The model considered is relevant to a variety of atomic, molecular, and optical systems, such as atoms in cavities or trapped ions. As an explicit example, the results are used to quantify phonon effects in trapped ion quantum simulators, which are expected to become increasingly important as these experiments push towards larger numbers of ions.
Electrons, Phonons and Excitons at Semiconductor Surfaces
NASA Astrophysics Data System (ADS)
Pollmann, Johannes; Krueger, Peter; Mazur, Albert; Rohlfing, Michael
We briefly address 'state-of-the-art' ab-initio calculations of basic properties of semiconductor surfaces such as their atomic configuration, electronic structure and surface vibrations, as well as, their optical properties and compare exemplary results with experimental data. The surface structure and the electronic ground state are described within the local density approximation (LDA) of density functional theory (DFT). The description of excited electronic states requires to take dynamical correlations in the many electron system into account, which is achieved to a considerable extent within the GW approximation (GWA) leading to the concept of the quasiparticle bandstructure. Surface phonons are treated from first principles within density functional perturbation theory (DFPT). The theory of optical surface properties and surface excitons, in particular, requires to account for the electron-hole Coulomb correlation which is done in the framework of the Bethe-Salpeter equation (BSE). These methods yield results which are in good agreement with experiment and can significantly contribute to an interpretation of experimental data from high-resolution surface microscopy and spectroscopy thus enhancing our current understanding of semiconductor surfaces.
Phononic Crystal Tunable via Ferroelectric Phase Transition
NASA Astrophysics Data System (ADS)
Xu, Chaowei; Cai, Feiyan; Xie, Shuhong; Li, Fei; Sun, Rong; Fu, Xianzhu; Xiong, Rengen; Zhang, Yi; Zheng, Hairong; Li, Jiangyu
2015-09-01
Phononic crystals (PCs) consisting of periodic materials with different acoustic properties have potential applications in functional devices. To realize more smart functions, it is desirable to actively control the properties of PCs on demand, ideally within the same fabricated system. Here, we report a tunable PC made of Ba0.7Sr0.3Ti O3 (BST) ceramics, wherein a 20-K temperature change near room temperature results in a 20% frequency shift in the transmission spectra induced by a ferroelectric phase transition. The tunability phenomenon is attributed to the structure-induced resonant excitation of A0 and A1 Lamb modes that exist intrinsically in the uniform BST plate, while these Lamb modes are sensitive to the elastic properties of the plate and can be modulated by temperature in a BST plate around the Curie temperature. The study finds opportunities for creating tunable PCs and enables smart temperature-tuned devices such as the Lamb wave filter or sensor.
TOPICAL REVIEW: Photonic and phononic quasicrystals
NASA Astrophysics Data System (ADS)
Steurer, Walter; Sutter-Widmer, Daniel
2007-07-01
This review focuses on the peculiarities of quasiperiodic order for the properties of photonic and phononic (sonic) heterostructures. The most beneficial feature of quasiperiodicity is that it can combine perfectly ordered structures with purely point-diffractive spectra of arbitrarily high rotational symmetry. Both are prerequisites for the construction of isotropic band gap composites, in particular from materials with low index contrast, which are required for numerous applications. Another interesting property of quasiperiodic structures is their scaling symmetry, which may be exploited to create spectral gaps in the sub-wavelength regime. This review covers structure/property relationships of heterostructures based on one-dimensional (1D) substitutional sequences such as the Fibonacci, Thue Morse, period-doubling, Rudin Shapiro and Cantor sequence as well as on 1D modulated structures, further on 2D tilings with 8-, 10-, 12- and 14-fold symmetry as well as on the pinwheel tiling, the Sierpinski gasket and on curvilinear tilings and, finally, on the 3D icosahedral Penrose tiling.
Phonon-Mediated Nonclassical Interference in Diamond.
England, Duncan G; Fisher, Kent A G; MacLean, Jean-Philippe W; Bustard, Philip J; Heshami, Khabat; Resch, Kevin J; Sussman, Benjamin J
2016-08-12
Quantum interference of single photons is a fundamental aspect of many photonic quantum processing and communication protocols. Interference requires that the multiple pathways through an interferometer be temporally indistinguishable to within the coherence time of the photon. In this Letter, we use a diamond quantum memory to demonstrate interference between quantum pathways, initially temporally separated by many multiples of the optical coherence time. The quantum memory can be viewed as a light-matter beam splitter, mapping a THz-bandwidth single photon to a variable superposition of the output optical mode and stored phononic mode. Because the memory acts both as a beam splitter and as a buffer, the relevant coherence time for interference is not that of the photon, but rather that of the memory. We use this mechanism to demonstrate nonclassical single-photon and two-photon interference between quantum pathways initially separated by several picoseconds, even though the duration of the photons themselves is just ∼250 fs.
Phonon transport analysis of semiconductor nanocomposites using monte carlo simulations
NASA Astrophysics Data System (ADS)
Malladi, Mayank
Nanocomposites are composite materials which incorporate nanosized particles, platelets or fibers. The addition of nanosized phases into the bulk matrix can lead to significantly different material properties compared to their macrocomposite counterparts. For nanocomposites, thermal conductivity is one of the most important physical properties. Manipulation and control of thermal conductivity in nanocomposites have impacted a variety of applications. In particular, it has been shown that the phonon thermal conductivity can be reduced significantly in nanocomposites due to the increase in phonon interface scattering while the electrical conductivity can be maintained. This extraordinary property of nanocomposites has been used to enhance the energy conversion efficiency of the thermoelectric devices which is proportional to the ratio of electrical to thermal conductivity. This thesis investigates phonon transport and thermal conductivity in Si/Ge semiconductor nanocomposites through numerical analysis. The Boltzmann transport equation (BTE) is adopted for description of phonon thermal transport in the nanocomposites. The BTE employs the particle-like nature of phonons to model heat transfer which accounts for both ballistic and diffusive transport phenomenon. Due to the implementation complexity and computational cost involved, the phonon BTE is difficult to solve in its most generic form. Gray media (frequency independent phonons) is often assumed in the numerical solution of BTE using conventional methods such as finite volume and discrete ordinates methods. This thesis solves the BTE using Monte Carlo (MC) simulation technique which is more convenient and efficient when non-gray media (frequency dependent phonons) is considered. In the MC simulation, phonons are displaced inside the computational domain under the various boundary conditions and scattering effects. In this work, under the relaxation time approximation, thermal transport in the nanocomposites are
NASA Astrophysics Data System (ADS)
Johnston, Steven
2015-03-01
One of the primary goals of superconductivity research is engineering materials that become superconducting at high temperatures. To this end, I will examine forward scattering from phonons as a general means to increase Tc in unconventional superconductors. First I will introduce the mechanisms for generating electron-lattice interactions in quasi-two-dimensional correlated systems that are peaked in the forward scattering direction. Then I will present case studies comparing theory to angle-resolved photoemission spectroscopy data for the Bi-family of cuprate superconductors and the recently discovered FeSe monolayer in SrTiO3 substrates [2]. These studies demonstrate the general principle of using particular electron-phonon interactions for enhancing superconductivity in unconventional pairing channels.
Thermal characterization of nanoscale phononic crystals using supercell lattice dynamics
NASA Astrophysics Data System (ADS)
Davis, Bruce L.; Hussein, Mahmoud I.
2011-12-01
The concept of a phononic crystal can in principle be realized at the nanoscale whenever the conditions for coherent phonon transport exist. Under such conditions, the dispersion characteristics of both the constitutive material lattice (defined by a primitive cell) and the phononic crystal lattice (defined by a supercell) contribute to the value of the thermal conductivity. It is therefore necessary in this emerging class of phononic materials to treat the lattice dynamics at both periodicity levels. Here we demonstrate the utility of using supercell lattice dynamics to investigate the thermal transport behavior of three-dimensional nanoscale phononic crystals formed from silicon and cubic voids of vacuum. The periodicity of the voids follows a simple cubic arrangement with a lattice constant that is around an order of magnitude larger than that of the bulk crystalline silicon primitive cell. We consider an atomic-scale supercell which incorporates all the details of the silicon atomic locations and the void geometry. For this supercell, we compute the phonon band structure and subsequently predict the thermal conductivity following the Callaway-Holland model. Our findings dictate that for an analysis based on supercell lattice dynamics to be representative of the properties of the underlying lattice model, a minimum supercell size is needed along with a minimum wave vector sampling resolution. Below these minimum values, a thermal conductivity prediction of a bulk material based on a supercell will not adequately recover the value obtained based on a primitive cell. Furthermore, our results show that for the relatively small voids and void spacings we consider (where boundary scattering is dominant), dispersion at the phononic crystal unit cell level plays a noticeable role in determining the thermal conductivity.
Skin dominance of the dielectric-electronic-phononic-photonic attribute of nanoscaled silicon
NASA Astrophysics Data System (ADS)
Pan, Likun; Xu, Shiqing; Liu, Xinjuan; Qin, Wei; Sun, Zhuo; Zheng, Weitao; Sun, Chang Q.
2013-11-01
Nanoscaled or porous silicon (p-Si) with and without surface passivation exhibits unusually tunable properties that its parent bulk does never show. Such property tunability amplifies the applicability of Si in the concurrent and upcoming technologies. However, consistent understanding of the fundamental nature of nanoscaled Si remains a high challenge. This article aims to address the recent progress in this regard with focus on reconciling the tunable dielectric, electronic, phononic, and photonic properties of p-Si in terms of skin dominance. We show that the skin-depth bond contraction, local quantum entrapment, and electron localization is responsible for the size-induced property tunability. The shorter and stronger bonds between undercoordinated skin atoms result in the local densification and quantum entrapment of the binding energy and the bonding electrons, which in turn polarizes the dangling bond electrons. Such local entrapment modifies the Hamiltonian and associated properties such as the band gap, core level shift, Stokes shift (electron-phonon interaction), phonon and dielectric relaxation. Therefore, given the known trend of one property change, one is expected to be able to predict the variation of the rest based on the notations of the bond order-length-strength correlation and local bond average approach (BOLS-LBA). Furthermore, skin bond reformation due to Al, Cu, and Ti metallization and O and F passivation adds another freedom to enhance or attenuate the size effect. The developed formulations, spectral analytical methods, and importantly, the established database and knowledge could be of use in engineering p-Si and beyond for desired functions.
Thermoelectric study of dissipative quantum-dot heat engines
NASA Astrophysics Data System (ADS)
De, Bitan; Muralidharan, Bhaskaran
2016-10-01
This paper examines the thermoelectric response of a dissipative quantum-dot heat engine based on the Anderson-Holstein model in two relevant operating limits, (i) when the dot phonon modes are out of equilibrium, and (ii) when the dot phonon modes are strongly coupled to a heat bath. In the first case, a detailed analysis of the physics related to the interplay between the quantum-dot level quantization, the on-site Coulomb interaction, and the electron-phonon coupling on the thermoelectric performance reveals that an n -type heat engine performs better than a p -type heat engine. In the second case, with the aid of the dot temperature estimated by incorporating a thermometer bath, it is shown that the dot temperature deviates from the bath temperature as electron-phonon interaction in the dot becomes stronger. Consequently, it is demonstrated that the dot temperature controls the direction of phonon heat currents, thereby influencing the thermoelectric performance. Finally, the conditions on the maximum efficiency with varying phonon couplings between the dot and all the other macroscopic bodies are analyzed in order to reveal the nature of the optimum junction.
Temperature dependent magnon-phonon coupling in bcc Fe from theory and experiment.
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.
Interaction Induced Electron Self-Interference in a Semiconductor: The Phonon Staircase Effect
NASA Astrophysics Data System (ADS)
Kenrow, J. A.; El Sayed, K.; Stanton, C. J.
1997-06-01
The exact quantum mechanics of a model semiconductor system of an electron with a discrete and equidistant energy spectrum interacting with a single phonon mode is presented. An electron initially excited into a coherent superposition of states interacts with phonons thereby creating a self-interference in time which reduces the emission of phonons to isolated bursts. This self-interference effect gives rise to steplike behavior in the relaxation kinetics of the electron and phonons. We show that this ``phonon staircase'' effect is a consequence of a correlated initial electron distribution and the violation of energy conservation in the electron-phonon interaction on short time scales.
Niu, Zhen-Wei; Zeng, Zhao-Yi; Hu, Cui-E; Cai, Ling-Cang; Chen, Xiang-Rong
2015-01-07
The thermodynamic properties of CeO{sub 2} have been reevaluated by a simple but accurate scheme. All our calculations are based on the self-consistent ab initio lattice dynamical (SCAILD) method that goes beyond the quasiharmonic approximation. Through this method, the effects of phonon-phonon interactions are included. The obtained thermodynamic properties and phonon dispersion relations are in good agreement with experimental data when considering the correction of phonon-phonon interaction. We find that the correction of phonon-phonon interaction is equally important and should not be neglected. At last, by comparing with quasiharmonic approximation, the present scheme based on SCAILD method is probably more suitable for high temperature systems.
Nonlinear Transport and Noise Properties of Acoustic Phonons
NASA Astrophysics Data System (ADS)
Walczak, Kamil
We examine heat transport carried by acoustic phonons in molecular junctions composed of organic molecules coupled to two thermal baths of different temperatures. The phononic heat flux and its dynamical noise properties are analyzed within the scattering (Landauer) formalism with transmission probability function for acoustic phonons calculated within the method of atomistic Green's functions (AGF technique). The perturbative computational scheme is used to determine nonlinear corrections to phononic heat flux and its noise power spectral density with up to the second order terms with respect to temperature difference. Our results show the limited applicability of ballistic Fourier's law and fluctuation-dissipation theorem to heat transport in quantum systems. We also derive several noise-signal relations applicable to nanoscale heat flow carried by phonons, but valid for electrons as well. We also discuss the extension of the perturbative transport theory to higher order terms in order to address a huge variety of problems related to nonlinear thermal effects which may occur at nanoscale and at strongly non-equilibrium conditions with high-intensity heat fluxes. This work was supported by Pace University Start-up Grant.
Phonon properties of graphene derived from molecular dynamics simulations
Koukaras, Emmanuel N.; Kalosakas, George; Galiotis, Costas; Papagelis, Konstantinos
2015-01-01
A method that utilises atomic trajectories and velocities from molecular dynamics simulations has been suitably adapted and employed for the implicit calculation of the phonon dispersion curves of graphene. Classical potentials widely used in the literature were employed. Their performance was assessed for each individual phonon branch and the overall phonon dispersion, using available inelastic x-ray scattering data. The method is promising for systems with large scale periodicity, accounts for anharmonic effects and non-bonding interactions with a general environment, and it is applicable under finite temperatures. The temperature dependence of the phonon dispersion curves has been examined with emphasis on the doubly degenerate Raman active Γ-E2g phonon at the zone centre, where experimental results are available. The potentials used show diverse behaviour. The Tersoff-2010 potential exhibits the most systematic and physically sound behaviour in this regard, and gives a first-order temperature coefficient of χ = −0.05 cm−1/K for the Γ-E2g shift in agreement with reported experimental values. PMID:26316252
Thermally triggered phononic gaps in liquids at THz scale
Bolmatov, Dima; Zhernenkov, Mikhail; Zavyalov, Dmitry; Stoupin, Stanislav; Cunsolo, Alessandro; Cai, Yong Q.
2016-01-14
In this study we present inelastic X-ray scattering experiments in a diamond anvil cell and molecular dynamic simulations to investigate the behavior of phononic excitations in liquid Ar. The spectra calculated using molecular dynamics were found to be in a good agreement with the experimental data. Furthermore, we observe that, upon temperature increases, a low-frequency transverse phononic gap emerges while high-frequency propagating modes become evanescent at the THz scale. The effect of strong localization of a longitudinal phononic mode in the supercritical phase is observed for the first time. The evidence for the high-frequency transverse phononic gap due to the transition from an oscillatory to a ballistic dynamic regimes of motion is presented and supported by molecular dynamics simulations. This transition takes place across the Frenkel line thermodynamic limit which demarcates compressed liquid and non-compressed fluid domains on the phase diagram and is supported by calculations within the Green-Kubo phenomenological formalism. These results are crucial to advance the development of novel terahertz thermal devices, phononic lenses, mirrors, and other THz metamaterials.
Functional crossover in the dispersion relations of magnons and phonons
NASA Astrophysics Data System (ADS)
Hoser, A.; Köbler, U.
2016-09-01
Experimental data are presented showing that the dispersion relations of magnons and acoustic phonons can consist of two sections with different functions of wave vector. In the low wave vector range a power function of wave vector often holds over a finite q-range while dispersions for larger wave vector values better approach the atomistic model predictions. In the magnon spectra ∼⃒qx power functions with exponents x=1.25, 1.5 and 2 are identified. The dispersion of the acoustic phonons can be a linear function of wave vector over a surprisingly large range of energy. Since the slope of the linear section agrees with the known sound velocities it can be concluded that the dispersion of the acoustic phonons has got attracted by the linear dispersion of the mass less Debye bosons (sound waves). Due to the different (translational) symmetries of bosons and atomistic excitations (magnons, phonons) the associated dispersions can attract each other. In the same way the different ∼⃒qx power functions in the magnon dispersions indicate that magnon dispersions are attracted by the dispersion of the bosons of the magnetic continuum (Goldstone bosons). This allows evaluation of the otherwise difficult to obtain dispersions of the Goldstone bosons from the known magnon dispersions. Interestingly, the dispersions of Goldstone bosons (Debye bosons) attract magnon dispersions (phonon dispersions) and not vice versa.
Phonons in Si24 at simultaneously elevated temperature and pressure
NASA Astrophysics Data System (ADS)
Tong, Xiao; Xu, Xiaolin; Fultz, B.; Zhang, Haidong; Strobel, Timothy A.; Kim, Duck Young
2017-03-01
Raman spectroscopy was used to measure the frequencies of phonons in Si24 with an open clathrate structure at temperatures from 80 to 400 K with simultaneous pressures of 0 to 8 GPa. The frequency shifts of the different phonons were substantially different under either temperature or pressure. The quasiharmonic behavior was isolated by varying pressure at low temperatures, and the anharmonic behavior was isolated by varying temperature at low pressures. Phonon modes dominated by bond bending were anomalous, showing stiffening with temperature and softening with pressure. Both the quasiharmonic behavior and the anharmonic behavior changed markedly with simultaneous changes in temperature Δ T and pressure Δ P . With Δ T =320 K and Δ P =8 GPa , some frequency shifts that scaled with the product Δ T Δ P were as large as the shifts from Δ T and Δ P alone. The thermodynamic entropy of this material likely has a dependence on Δ T and Δ P that cannot be obtained by adding effects from quasiharmonicity and phonon-phonon anharmonicity.
Thermally triggered phononic gaps in liquids at THz scale
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
Investigation of phonon coherence and backscattering using silicon nanomeshes
Lee, Jaeho; Lee, Woochul; Wehmeyer, Geoff; Dhuey, Scott; Olynick, Deirdre L.; Cabrini, Stefano; Dames, Chris; Urban, Jeffrey J.; Yang, Peidong
2017-01-01
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 with 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. PMID:28051081
Quantum Phonon Optics: Squeezing Quantum Noise in the Atomic Displacements.
NASA Astrophysics Data System (ADS)
Hu, X.; Nori, F.
1996-03-01
We have investigated(X. Hu and F. Nori, Physical Review B, in press; preprints.) coherent and squeezed quantum states of phonons. Squeezed states are interesting because they allow the possibility of modulating the quantum fluctuations of atomic displacements below the zero-point quantum noise level of phonon vacuum states. We have studiedfootnotemark[1] the possibility of squeezing quantum noise in the atomic displacement using a polariton-based approach and also a method based on the three-phonon anharmonic interaction. Our focus here is on the first approach. We have diagonalized the polariton Hamiltonian and calculated the corresponding expectation values and fluctuations of both the atomic displacement and the lattice amplitude operators (the later is the phonon analog of the electric field operator for photons). Our results shows that squeezing of quantum fluctuations in the atomic displacements can be achieved with appropriate initial states of both photon and phonon fields. The degree of squeezing is directly related to the crystal susceptibility, which is indicative of the interaction strength between the incident light and the crystal.
Thermally triggered phononic gaps in liquids at THz scale
Bolmatov, Dima; Zhernenkov, Mikhail; Zav’yalov, Dmitry; Stoupin, Stanislav; Cunsolo, Alessandro; Cai, Yong Q.
2016-01-01
In this paper we present inelastic X-ray scattering experiments in a diamond anvil cell and molecular dynamic simulations to investigate the behavior of phononic excitations in liquid Ar. The spectra calculated using molecular dynamics were found to be in a good agreement with the experimental data. Furthermore, we observe that, upon temperature increases, a low-frequency transverse phononic gap emerges while high-frequency propagating modes become evanescent at the THz scale. The effect of strong localization of a longitudinal phononic mode in the supercritical phase is observed for the first time. The evidence for the high-frequency transverse phononic gap due to the transition from an oscillatory to a ballistic dynamic regimes of motion is presented and supported by molecular dynamics simulations. This transition takes place across the Frenkel line thermodynamic limit which demarcates compressed liquid and non-compressed fluid domains on the phase diagram and is supported by calculations within the Green-Kubo phenomenological formalism. These results are crucial to advance the development of novel terahertz thermal devices, phononic lenses, mirrors, and other THz metamaterials. PMID:26763899
Thermal transport in phononic crystals: The role of zone folding effect
NASA Astrophysics Data System (ADS)
Dechaumphai, Edward; Chen, Renkun
2012-04-01
Recent experiments [Yu et al., Nature Nanotech 5, 718 (2010); Tang et al., Nano Lett. 10, 4279 (2010); Hopkins etal., Nano Lett. 11, 107(2011)] on silicon based nanoscale phononic crystals demonstrated substantially reduced thermal conductivity compared to bulk Si, which cannot be explained by incoherent phonon boundary scattering within the Boltzmann Transport Equation (BTE). In this paper, partial coherent treatment of phonons, where phonons are regarded as either wave or particles depending on their frequencies, was considered. Phonons with mean free path smaller than the characteristic size of phononic crystals are treated as particles and the transport in this regime is modeled by BTE with phonon boundary scattering taken into account. On the other hand, phonons with mean free path longer than the characteristic size are treated as waves. In this regime, phonon dispersion relations are computed using the Finite Difference Time Domain (FDTD) method and are found to be modified due to the zone folding effect. The new phonon spectra are then used to compute phonon group velocity and density of states for thermal conductivity modeling. Our partial coherent model agrees well with the recent experimental results on in-plane thermal conductivity of phononic crystals. Our study highlights the importance of zone folding effect on thermal transport in phononic crystals.
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.
A new class of tunable hypersonic phononic crystals based on polymer-tethered colloids
Alonso-Redondo, E.; Schmitt, M.; Urbach, Z.; Hui, C. M.; Sainidou, R.; Rembert, P.; Matyjaszewski, K.; Bockstaller, M. R.; Fytas, G.
2015-01-01
The design and engineering of hybrid materials exhibiting tailored phononic band gaps are fundamentally relevant to innovative material technologies in areas ranging from acoustics to thermo-optic devices. Phononic hybridization gaps, originating from the anti-crossing between local resonant and propagating modes, have attracted particular interest because of their relative robustness to structural disorder and the associated benefit to ‘manufacturability'. Although hybridization gap materials are well known, their economic fabrication and efficient control of the gap frequency have remained elusive because of the limited property variability and expensive fabrication methodologies. Here we report a new strategy to realize hybridization gap materials by harnessing the ‘anisotropic elasticity' across the particle–polymer interface in densely polymer-tethered colloidal particles. Theoretical and Brillouin scattering analysis confirm both the robustness to disorder and the tunability of the resulting hybridization gap and provide guidelines for the economic synthesis of new materials with deliberately controlled gap position and width frequencies. PMID:26390851
Janipour, Mohsen; Misirlioglu, Ibrahim Burc; Sendur, Kursat
2016-01-01
Spatial charge distribution for biased semiconductors fundamentally differs from metals since they can allow inhomogeneous charge distributions due to penetration of the electric field, as observed in the classical Schottky junctions. Similarly, the electrostatics of the dielectric/semiconductor interface can lead to a carrier depletion or accumulation in the semiconductor side when under applied bias. In this study, we demonstrate that the inhomogeneous carrier accumulation in a moderately p-doped GaAs–dielectric interface can be tailored for tunable plasmonics by an external voltage. Solving Maxwell’s equations in the doped GaAs-dielectric stack, we investigate the tunability of the surface plasmon and phonon polaritons’ interaction via an external bias. The plasmonic mode analysis of such an interface reveals interesting dispersion curves for surface plasmon and phonon polariton interactions that are not possible in metals. We show that the plasmon dispersion curve can be engineered through an external bias using the inherent properties of the p-doped GaAs– dielectric interface. PMID:27698393
NASA Astrophysics Data System (ADS)
Janipour, Mohsen; Misirlioglu, Ibrahim Burc; Sendur, Kursat
2016-10-01
Spatial charge distribution for biased semiconductors fundamentally differs from metals since they can allow inhomogeneous charge distributions due to penetration of the electric field, as observed in the classical Schottky junctions. Similarly, the electrostatics of the dielectric/semiconductor interface can lead to a carrier depletion or accumulation in the semiconductor side when under applied bias. In this study, we demonstrate that the inhomogeneous carrier accumulation in a moderately p-doped GaAs–dielectric interface can be tailored for tunable plasmonics by an external voltage. Solving Maxwell’s equations in the doped GaAs-dielectric stack, we investigate the tunability of the surface plasmon and phonon polaritons’ interaction via an external bias. The plasmonic mode analysis of such an interface reveals interesting dispersion curves for surface plasmon and phonon polariton interactions that are not possible in metals. We show that the plasmon dispersion curve can be engineered through an external bias using the inherent properties of the p-doped GaAs– dielectric interface.
Polar optical phonons in semiconducting CdS nanocrystals
Belogorokhov, A. I. Belogorokhov, I. A.; Miranda, R. P.; Vasilevskii, M. I.; Gavrilov, S. A.
2007-02-15
We have experimentally and theoretically studied IR-active optical phonons, which are spatially confined in the volume of semiconducting CdS nanocrystals of various shapes synthesized in a dielectric matrix (porous aluminum oxide). Within an approach admitting the mixing of all expected types of vibrations, the complete sets of phonon modes are determined for a spherical quantum dot (QD) and a cylindrical quantum wire (QW) in this matrix. Based on these results, the polarizability spectra of QDs and QWs, as well as the effective dielectric function of a composite material containing such nanoparticles, are calculated for the far-IR wavelength range. It is established that the spectrum of the dielectric function exhibits specific features in the region between the transverse and longitudinal optical phonon frequencies of the massive semiconductor material. These features explain the rather unusual structure of the IR spectra of the composite samples studied.
Oxyfluoroborate host glass for upconversion application: phonon energy calculation
NASA Astrophysics Data System (ADS)
Abdel-Baki, Manal; El-Diasty, Fouad
2016-04-01
Reducing the glass phonon energy is an essential procedure to achieve high efficient radiative upconversion process. The degree of covalence of chemical bonds is responsible for the high oscillator strength of intracenter transitions in rare-earth ions. So, conversion covalent to ionic glass character is proposed as a structure-sensitive criterion that controls the phonon energy of the glasses. A series of oxyfluoro aluminum-borate host glasses used for upconversion application is prepared by the conventional melt-quenching technique. Through lithium oxide substitution by lithium fluoride, the ionic-covalent property of Li+ ion successes to regulate the band gap energies of the studied glasses. Furthermore, a new method to determine the glass phonon energy is offered.
Coulomb and electron-phonon interactions in metals
NASA Astrophysics Data System (ADS)
Tupitsyn, Igor S.; Mishchenko, Andrey S.; Nagaosa, Naoto; Prokof'ev, Nikolay
2016-10-01
An accurate and consistent theory of phonons in metals requires that all long-range Coulomb interactions between charged particles (electrons and ions) be treated on equal footing. So far, all attempts to deal with this nonperturbative system were relying on uncontrolled approximations in the absence of small parameters. In this paper, we develop the diagrammatic Monte Carlo approach for a two-component Coulomb system that obtains the solution to this fundamental problem in an approximation-free way by computing vertex corrections from higher-order skeleton graphs. The feasibility of the method is demonstrated by calculating the spectrum of longitudinal acoustic phonons in a simple cubic lattice, determining their sound velocity, and obtaining the phonon spectral densities by analytic continuation of the Matsubara-Green's functions. Final results are checked against the lowest-order fully self-consistent G W approximation in both adiabatic and nonadiabatic regimes.
Probing confined acoustic phonons in free standing small gold nanoparticles
Mankad, Venu; Jha, Prafulla K.; Ravindran, T. R.
2013-02-21
Polarized and depolarized spectra from gold (Au) nanoparticles of different sizes are investigated in the small size range, between 3 and 7 nm, using low frequency Raman spectroscopy. Acoustic vibrations of the free-standing Au nanoparticles are demonstrated with frequencies ranging from 5 to 35 cm{sup -1}, opening the way to the development of the acoustic resonators. A blue shift in the phonon peaks along with the broadening is observed with a decrease in particle size. Comparison of the measured frequencies with vibrational dynamics calculation and an examination as from the transmission electron microscopy results ascertain that the low frequency phonon modes are due to acoustic phonon quantization. Our results show that the observed low frequency Raman scattering originates from the spherical (l = 0) and quadrupolar (l = 2) vibrations of the spheroidal mode due to plasmon mediated acoustic vibrations in Au nanoparticles.
Terahertz reflection response measurement using a phonon polariton wave
NASA Astrophysics Data System (ADS)
Inoue, Hayato; Katayama, Kenji; Shen, Qing; Toyoda, Taro; Nelson, Keith A.
2009-03-01
We developed a new technique for the measurement of terahertz reflection responses utilizing a propagating phonon polariton wave. Frequency tunable phonon polariton waves were generated by the recently developed continuously variable spatial frequency transient grating method [K. Katayama, H. Inoue, H. Sugiya, Q. Shen, T. Taro, and K. A. Nelson, Appl. Phys. Lett. 92, 031906 (2008)]. The phonon polariton wave traveled in a ferroelectric crystal in an in-plane direction with an inclined angle of 26°, and the wave reflected at the crystal edge where a sample was positioned. The reflected polariton wave was detected by the same method as that used for the generation of the polariton waves. By comparing the reflection intensities in the presence and absence of the sample, reflectivity of the polariton wave was calculated, and the refractive index and absorption in the terahertz region were obtained.
Phonon thermal conduction in novel 2D materials
NASA Astrophysics Data System (ADS)
Xu, Xiangfan; Chen, Jie; Li, Baowen
2016-12-01
Recently, there has been increasing interest in phonon thermal transport in low-dimensional materials, due to the crucial importance of dissipating and managing heat in micro- and nano-electronic devices. Significant progress has been achieved for one-dimensional (1D) systems, both theoretically and experimentally. However, the study of heat conduction in two-dimensional (2D) systems is still in its infancy due to the limited availability of 2D materials and the technical challenges of fabricating suspended samples that are suitable for thermal measurements. In this review, we outline different experimental techniques and theoretical approaches for phonon thermal transport in 2D materials, discuss the problems and challenges of phonon thermal transport measurements and provide a comparison between existing experimental data. Special attention will be given to the effects of size, dimensionality, anisotropy and mode contributions in novel 2D systems, including graphene, boron nitride, MoS2, black phosphorous and silicene.
A GaAs phononic crystal with shallow noncylindrical holes.
Petrus, Joseph A; Mathew, Reuble; Stotz, James A H
2014-02-01
A square lattice of shallow, noncylindrical holes in GaAs is shown to act as a phononic crystal (PnC) reflector. The holes are produced by wet-etching a GaAs substrate using a citric acid:H2O2 etching procedure and a photolithographed array pattern. Although nonuniform and asymmetric etch rates limit the depth and shape of the phononic crystal holes, the matrix acts as a PnC, as demonstrated by insertion loss measurements together with interferometric imaging of surface acoustic waves propagating on the GaAs surface. The measured vertical displacement induced by surface phonons compares favorably with finite-difference time-domain simulations of a PnC with rounded-square holes.
Phonon thermal conduction in novel 2D materials.
Xu, Xiangfan; Chen, Jie; Li, Baowen
2016-12-07
Recently, there has been increasing interest in phonon thermal transport in low-dimensional materials, due to the crucial importance of dissipating and managing heat in micro- and nano-electronic devices. Significant progress has been achieved for one-dimensional (1D) systems, both theoretically and experimentally. However, the study of heat conduction in two-dimensional (2D) systems is still in its infancy due to the limited availability of 2D materials and the technical challenges of fabricating suspended samples that are suitable for thermal measurements. In this review, we outline different experimental techniques and theoretical approaches for phonon thermal transport in 2D materials, discuss the problems and challenges of phonon thermal transport measurements and provide a comparison between existing experimental data. Special attention will be given to the effects of size, dimensionality, anisotropy and mode contributions in novel 2D systems, including graphene, boron nitride, MoS2, black phosphorous and silicene.
Classification of topological phonons in linear mechanical metamaterials
Süsstrunk, Roman
2016-01-01
Topological phononic crystals, alike their electronic counterparts, are characterized by a bulk–edge correspondence where the interior of a material dictates the existence of stable surface or boundary modes. In the mechanical setup, such surface modes can be used for various applications such as wave guiding, vibration isolation, or the design of static properties such as stable floppy modes where parts of a system move freely. Here, we provide a classification scheme of topological phonons based on local symmetries. We import and adapt the classification of noninteracting electron systems and embed it into the mechanical setup. Moreover, we provide an extensive set of examples that illustrate our scheme and can be used to generate models in unexplored symmetry classes. Our work unifies the vast recent literature on topological phonons and paves the way to future applications of topological surface modes in mechanical metamaterials. PMID:27482105
Phononic crystal plate with hollow pillars connected by thin bars
NASA Astrophysics Data System (ADS)
Jin, Yabin; Pennec, Yan; Pan, Yongdong; Djafari-Rouhani, Bahram
2017-01-01
A new type of phononic crystal plate consisting of hollow pillars on a bar-connected plate is proposed. With respect to usual pillar based phononic crystal plates, the Bragg band gap can be tuned to be much wider and extended to a sub-wavelength region, and the low frequency gap can be moved to an extremely low frequency range. Such a structure can generate quadrapolar, hexapolar and octopolar whispering-gallery modes (WGMs) inside the band gaps with very high confinement and quality factors. By filling the hollow pillars with a liquid, these WGMs, together with additional localized compressional and solid-liquid coupling modes, can be tuned either by varying the inner radius of the pillars or controlling the height of the liquid. We discuss some possible functionalities of these phononic crystals for the purpose of sensing the acoustic properties of liquids, multiplexer and wireless communication.
High temperature phonon dispersion in graphene using classical molecular dynamics
Anees, P. Panigrahi, B. K.; Valsakumar, M. C.
2014-04-24
Phonon dispersion and phonon density of states of graphene are calculated using classical molecular dynamics simulations. In this method, the dynamical matrix is constructed based on linear response theory by computing the displacement of atoms during the simulations. The computed phonon dispersions show excellent agreement with experiments. The simulations are done in both NVT and NPT ensembles at 300 K and found that the LO/TO modes are getting hardened at the Γ point. The NPT ensemble simulations capture the anharmonicity of the crystal accurately and the hardening of LO/TO modes is more pronounced. We also found that at 300 K the C-C bond length reduces below the equilibrium value and the ZA bending mode frequency becomes imaginary close to Γ along K-Γ direction, which indicates instability of the flat 2D graphene sheets.
Structure of crystalline oxide ceramics studied by phonon spectroscopy
Kaminskii, Alexandr A; Taranov, A V; Khazanov, E N
2013-03-31
This paper describes a method for gaining detailed insight into the structure and phonon spectrum of polycrystalline oxide ceramics. We examine how the diffusion coefficient of subterahertz phonons is related to the properties of a system of grain boundaries and to the grain size and structure and demonstrate that the temperature dependence of the phonon diffusion coefficient at liquid-helium temperatures is determined by the spectral properties of the intergranular layer, which allows one to estimate the volumeaveraged intergranular layer thickness and acoustic impedance. We also analyse the effect of plastic deformation via twinning on the formation of the structure of grains and intergranular layers, which determine the thermophysical, acoustic and optical properties of ceramic materials. (extreme light fields and their applications)
Evolution of molecular crystal optical phonons near structural phase transitions
NASA Astrophysics Data System (ADS)
Michki, Nigel; Niessen, Katherine; Xu, Mengyang; Markelz, Andrea
Molecular crystals are increasingly important photonic and electronic materials. For example organic semiconductors are lightweight compared to inorganic semiconductors and have inexpensive scale up processing with roll to roll printing. However their implementation is limited by their environmental sensitivity, in part arising from the weak intermolecular interactions of the crystal. These weak interactions result in optical phonons in the terahertz frequency range. We examine the evolution of intermolecular interactions near structural phase transitions by measuring the optical phonons as a function of temperature and crystal orientation using terahertz time-domain spectroscopy. The measured orientation dependence of the resonances provides an additional constraint for comparison of the observed spectra with the density functional calculations, enabling us to follow specific phonon modes. We observe crystal reorganization near 350 K for oxalic acid as it transforms from dihydrate to anhydrous form. We also report the first THz spectra for the molecular crystal fructose through its melting point.
Phonon anharmonicity in silicon from 100 to 1500 K
Kim, D. S.; Smith, Hillary L.; Niedziela, Jennifer L.; ...
2015-01-21
Inelastic neutron scattering was performed on silicon powder to measure the phonon density of states (DOS) from 100 to 1500 K. The mean fractional energy shifts with temperature of the modes weremore » $$\\langle$$Δεi/εiΔT$$\\rangle$$=₋0.07, giving a mean isobaric Grüneisen parameter of +6.95±0.67, which is significantly different from the isothermal parameter of +0.98. These large effects are beyond the predictions from quasiharmonic models using density functional theory or experimental data, demonstrating large effects from phonon anharmonicity. At 1500 K the anharmonicity contributes 0.15kB/atom to the vibrational entropy, compared to 0.03kB/atom from quasiharmonicity. Lastly, excellent agreement was found between the entropy from phonon DOS measurements and the reference NIST-JANAF thermodynamic entropy from calorimetric measurements.« less
Phonon anharmonicity in silicon from 100 to 1500 K
NASA Astrophysics Data System (ADS)
Kim, D. S.; Smith, H. L.; Niedziela, J. L.; Li, C. W.; Abernathy, D. L.; Fultz, B.
2015-01-01
Inelastic neutron scattering was performed on silicon powder to measure the phonon density of states (DOS) from 100 to 1500 K. The mean fractional energy shifts with temperature of the modes were <Δ ɛi/ɛiΔ T >=-0.07 , giving a mean isobaric Grüneisen parameter of +6.95 ±0.67 , which is significantly different from the isothermal parameter of +0.98. These large effects are beyond the predictions from quasiharmonic models using density functional theory or experimental data, demonstrating large effects from phonon anharmonicity. At 1500 K the anharmonicity contributes 0.15 kB /atom to the vibrational entropy, compared to 0.03 kB /atom from quasiharmonicity. Excellent agreement was found between the entropy from phonon DOS measurements and the reference NIST-JANAF thermodynamic entropy from calorimetric measurements.
Dynamics of Phononic Dissipation at the Atomic Scale
NASA Astrophysics Data System (ADS)
Sevincli, Haldun; Mukhopadhay, Soma; Tugrul Senger, R.; Ciraci, Salim
2007-03-01
Dynamics of dissipation of a local phonon distribution to the bulk is a key issue in boundary lubrication and friction between sliding surfaces. We consider a highly excited molecule which interacts weakly with the substrate surface. We study different types of coupling and substrates having different types of dimensionality and phonon densities of states. We propose three different methods to solve the dynamics of the combined system, namely the equation of mation technique, Fano-Anderson method and the Green's function method. Using this theoretical framework we present an analysis of transient properties of energy dissipation via phonon discharge at the microscopic level. The methods allow the theoretical calculations to be extended to include any density of states for the substrate including experimental ones and any type of molecule that represent the lubricant or the asperity.
Anharmonic phonons and magnons in BiFeO3
Delaire, Olivier A; Ma, Jie; Stone, Matthew B; Huq, Ashfia; Gout, Delphine J; Brown, Craig; Wang, Kefeng; Ren, Zhifeng
2012-01-01
The phonon density of states (DOS) and magnetic excitation spectrum of polycrystalline BiFeO3 were measured for temperatures 200 < T < 750K , using inelastic neutron scattering (INS). Our results indicate that the magnetic spectrum of BiFeO3 closely resembles that of similar Fe perovskites, such as LaFeO3, despite the cycloid modulation in BiFeO3. We do not find any evidence for a spin gap. A strong T-dependence of the phonon DOS was found, with a marked broadening of the whole spectrum, providing evidence of strong anharmonicity. This anharmonicity is corroborated by large amplitude motions of Bi and O ions observed with neutron diffraction. These results highlight the importance of spin-phonon coupling in this material.
Tunable polaritons from plasmon-phonon coupling in hyperbolic media
NASA Astrophysics Data System (ADS)
Dai, Siyuan; Ma, Qiong; Zhu, Shou-En; Liu, Mengkun; Andersen, Trond; Fei, Zhe; Goldflam, Michael; Wagner, Martin; Watanabe, Kenji; Taniguchi, Takashi; Thiemens, Mark; Keilmann, Fritz; Janssen, G. C. A. M.; Jarillo-Herrero, Pablo; Fogler, Michael; Basov, D. N.
2015-03-01
Using infrared nano-imaging and nano-spectroscopy, we report on the tunable hyperbolic response in heterostructures comprised of a monolayer graphene deposited on hexagonal boron nitride (G-hBN). Electrostatic gating of the top graphene layer allows for modification of the wavelength and intensity of hyperbolic phonon polaritons in bulk hBN. When compared with the pristine hBN polaritons, the graphene modified ones exhibit a longer wavelength in the Type II hyperbolic region and shorter wavelength in the Type I region. Because of this modification, we achieve a 90% increase of the propagation length for Type II polaritons in hBN. The physical origin of the modification is attributed to plasmon-phonon coupling in the G-hBN heterostructure. Our work provides a comprehensive study of plasmon-phonon coupling in a hyperbolic medium with the exploration of graphene's potential for modification of collective modes in other materials.
Phonon anharmonicity in silicon from 100 to 1500 K
Kim, D. S.; Smith, Hillary L.; Niedziela, Jennifer L.; Li, Chen W.; Abernathy, Douglas L.; Fultz, B.
2015-01-21
Inelastic neutron scattering was performed on silicon powder to measure the phonon density of states (DOS) from 100 to 1500 K. The mean fractional energy shifts with temperature of the modes were $\\langle$Δε_{i}/ε_{i}ΔT$\\rangle$=₋0.07, giving a mean isobaric Grüneisen parameter of +6.95±0.67, which is significantly different from the isothermal parameter of +0.98. These large effects are beyond the predictions from quasiharmonic models using density functional theory or experimental data, demonstrating large effects from phonon anharmonicity. At 1500 K the anharmonicity contributes 0.15k_{B}/atom to the vibrational entropy, compared to 0.03k_{B}/atom from quasiharmonicity. Lastly, excellent agreement was found between the entropy from phonon DOS measurements and the reference NIST-JANAF thermodynamic entropy from calorimetric measurements.
Electron-phonon coupling in perovskites studied by Raman Scattering
NASA Astrophysics Data System (ADS)
Sathe, V. G.; Tyagi, S.; Sharma, G.
2016-10-01
Raman scattering is an unique technique for characterization and quantification of electron-phonon, spin-phonon and spin-lattice coupling in many of the currently prominent compounds like multiferroics and manganites. In manganites, it is understood now that a phase separated landscape with coexisting metallic and insulating regions exist in most of the compounds and application of small external perturbation causes an alteration in this landscape. In such scenario, local metallic regions grow suddenly at the expense of insulating regions below the magnetic ordering temperature. Such regions can be characterized effectively using Raman scattering measurements where delocalized electrons couple with the adjacent phonon peaks giving a Fano resonance in the form of asymmetric line shape.
Iglesias, J. M.; Martín, M. J.; Pascual, E.; Rengel, R.
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 carrier 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.
Photons, phonons, and plasmons with orbital angular momentum in plasmas
Chen, Qiang; Qin, Hong; Liu, Jian
2017-02-06
Exact eigen modes with orbital angular momentum (OAM) in the complex media of unmagnetized homogeneous plasmas are studied. Three exact eigen modes with OAM are derived, i.e., photons, phonons, and plasmons. The OAM of different plasma components are closely related to the charge polarities. For photons, the OAM of electrons and ions are of the same magnitude but opposite direction, and the total OAM is carried by the field. For the phonons and plasmons, their OAM are carried by the electrons and ions. Lastly, the OAM modes in plasmas and their characteristics can be explored for potential applications in plasmamore » physics and accelerator physics.« less
Calculated phonon spectra of plutonium at high temperatures.
Dai, X; Savrasov, S Y; Kotliar, G; Migliori, A; Ledbetter, H; Abrahams, E
2003-05-09
We constructed computer-based simulations of the lattice dynamical properties of plutonium using an electronic structure method, which incorporates correlation effects among the f-shell electrons and calculates phonon spectra at arbitrary wavelengths. Our predicted spectrum for the face-centered cubic delta phase agrees well with experiments in the elastic limit and explains unusually large shear anisotropy of this material. The spectrum of the body-centered cubic phase shows an instability at zero temperature over a broad region of the wave vectors, indicating that this phase is highly anharmonic and can be stabilized at high temperatures by its phonon entropy.
Time Resolved Phonon Spectroscopy, Version 1.0
Goett, Johnny; Zhu, Brian
2016-12-22
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.
Electron-phonon equilibration in laser-heated gold films
NASA Astrophysics Data System (ADS)
White, T. G.; Mabey, P.; Gericke, D. O.; Hartley, N. J.; Doyle, H. W.; McGonegle, D.; Rackstraw, D. S.; Higginbotham, A.; Gregori, G.
2014-07-01
By irradiating a thin metal foil with an intense short-pulse laser, we have created a uniform system far from equilibrium. The deposited energy is initially deposited only within the electronic subsystem, and the subsequent evolution of the system is determined by the details of the electron-phonon coupling. Here, we measure the time evolution of the lattice parameter through multilayer Bragg diffraction and compare the result to classical molecular dynamic simulations to determine the lattice temperature. The electron-ion coupling constant for gold is then determined by comparison with the evolution of a two-temperature electron-phonon system.
Photons, phonons, and plasmons with orbital angular momentum in plasmas
Chen, Qiang; Qin, Hong; Liu, Jian
2017-01-01
Exact eigen modes with orbital angular momentum (OAM) in the complex media of unmagnetized homogeneous plasmas are studied. Three exact eigen modes with OAM are derived, i.e., photons, phonons, and plasmons. The OAM of different plasma components are closely related to the charge polarities. For photons, the OAM of electrons and ions are of the same magnitude but opposite direction, and the total OAM is carried by the field. For the phonons and plasmons, their OAM are carried by the electrons and ions. The OAM modes in plasmas and their characteristics can be explored for potential applications in plasma physics and accelerator physics. PMID:28164998
Enhanced electron?phonon coupling at metal surfaces
NASA Astrophysics Data System (ADS)
Plummer, E. W.; Shi, Junren; Tang, S.-J.; Rotenberg, Eli; Kevan, S. D.
2003-12-01
Recent advances in experimental techniques and theoretical capabilities associated with the study of surfaces show promise for producing in unprecedented detail a picture of electron-phonon coupling. These investigations on surfaces of relatively simple metals can be the platform for understanding functionality in complex materials associated with the coupling between charge and the lattice. In this article, we present an introduction to electron-phonon coupling, especially in systems with reduced dimensionality, and the recent experimental and theoretical achievements. Then, we try to anticipate the exciting future created by advances in surface physics.
YPHON: A package for calculating phonons of polar materials
NASA Astrophysics Data System (ADS)
Wang, Yi; Chen, Long-Qing; Liu, Zi-Kui
2014-11-01
In our recent works, we have developed a mixed-space approach within the framework of direct method for the first-principle calculation of phonon properties. It makes full use of the accuracy of the force constants calculated in the real space and the dipole-dipole interactions in the reciprocal space, making the accurate phonon calculation possible with the direct method for polar materials. In this paper, an efficient C++ implementation of the mixed-space approach, YPHON, is provided as open source, including demos and Linux scripts for extracting input data to YPHON from the output of VASP.5. The functions of the current package include the calculations of: (1) the phonon dispersions; (2) the phonon density of states; (3) the neutron scattering section weighted phonon density of state; (4) the phonons of the high symmetry structure using the force constants from low symmetry structure; (5) the phonon dispersions of random alloys; and (6) the analysis of the vibrational modes using the point group theory. Catalogue identifier: AETS_v1_0 Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AETS_v1_0.html Program obtainable from: CPC Program Library, Queen’s University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 567815 No. of bytes in distributed program, including test data, etc.: 9763594 Distribution format: tar.gz Programming language: C++, Linux scripts. Computer: Linux systems with a g++ or C++ compiler. Operating system: Linux. RAM: Ranges from a few Mbytes to a few Gbytes, dynamically depending on the system size. Classification: 7.8. External routines: GSL-the GNU Scientific Library (GSL) is a numerical library for C and C++ programmers. VASP.5 or later for the calculations of force constants and dielectric constants and Born effective charge for polar materials. Nature of problem: This package has the purpose of computing
Photons, phonons, and plasmons with orbital angular momentum in plasmas
NASA Astrophysics Data System (ADS)
Chen, Qiang; Qin, Hong; Liu, Jian
2017-02-01
Exact eigen modes with orbital angular momentum (OAM) in the complex media of unmagnetized homogeneous plasmas are studied. Three exact eigen modes with OAM are derived, i.e., photons, phonons, and plasmons. The OAM of different plasma components are closely related to the charge polarities. For photons, the OAM of electrons and ions are of the same magnitude but opposite direction, and the total OAM is carried by the field. For the phonons and plasmons, their OAM are carried by the electrons and ions. The OAM modes in plasmas and their characteristics can be explored for potential applications in plasma physics and accelerator physics.
Band structures and localization properties of aperiodic layered phononic crystals
NASA Astrophysics Data System (ADS)
Yan, Zhi-Zhong; Zhang, Chuanzeng
2012-03-01
The band structures and localization properties of in-plane elastic waves with coupling of longitudinal and transverse modes oblique propagating in aperiodic phononic crystals based on Thue-Morse and Rudin-Shapiro sequences are studied. Using transfer matrix method, the concept of the localization factor is introduced and the correctness is testified through the Rytov dispersion relation. For comparison, the perfect periodic structure and the quasi-periodic Fibonacci system are also considered. In addition, the influences of the random disorder, local resonance, translational and/or mirror symmetries on the band structures of the aperiodic phononic crystals are analyzed in this paper.
Electron-phonon interaction model and prediction of thermal energy transport in SOI transistor.
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).
Temperature dependence of the Raman-active phonon frequencies in indium sulfide
NASA Astrophysics Data System (ADS)
Gasanly, N. M.; Özkan, H.; Aydinli, A.; Yilmaz, İ.
1999-03-01
The temperature dependence of the Raman-active mode frequencies in indium sulfide was measured in the range from 10 to 300 K. The analysis of the temperature dependence of the A g intralayer optical modes show that Raman frequency shift results from the change of harmonic frequency with volume expansion and anharmonic coupling to phonons of other branches. The pure-temperature contribution (phonon-phonon coupling) is due to three- and four-phonon processes.
Dean M. P.; Howard, C.A.; Withers, F.
2011-12-19
Graphene phonons are measured as a function of electron doping via the addition of potassium adatoms. In the low doping regime, the in-plane carbon G peak hardens and narrows with increasing doping, analogous to the trend seen in graphene doped via the field effect. At high dopings, beyond those accessible by the field effect, the G peak strongly softens and broadens. This is interpreted as a dynamic, nonadiabatic renormalization of the phonon self-energy. At dopings between the light and heavily doped regimes, we find a robust inhomogeneous phase where the potassium coverage is segregated into regions of high and low density. The phonon energies, linewidths, and tunability are notably very similar for one- to four-layer potassium-doped graphene, but significantly different to bulk potassium-doped graphite.
The Influence of Surface Phonons on Polaron States in Quantum Dots
NASA Astrophysics Data System (ADS)
Maslov, A. Yu.; Proshina, O. V.; Rusina, A. N.
2007-04-01
The influence of the surface phonons on the polaron effect in a quantum dot is investigated. We consider the polar quantum dot embedded into the polar matrix. The polaron energy shift for the electron and hole ground states is calculated. It is shown that the contribution of the surface phonons may exceed the bulk phonon contribution.
Atxitia, U.; Ostler, T. A.; Chantrell, R. W.; Chubykalo-Fesenko, O.
2015-11-09
Using large-scale computer simulations, we thoroughly study the minimum energy required to thermally induced magnetization switching (TIMS) after the application of a femtosecond heat pulse in transition metal-rare earth ferrimagnetic alloys. We find that for an energy efficient TIMS, a low ferrimagnetic net magnetization with a strong temperature dependence is the relevant factor for the magnetic system. For the lattice and electron systems, the key physics for efficient TIMS is a large electron-phonon relaxation time. Importantly, we show that as the cooling time of the heated electrons is increased, the minimum power required to produce TIMS can be reduced by an order of magnitude. Our results show the way to low power TIMS by appropriate engineering of magnetic heterostructures.
A structural mechanics approach for the phonon dispersion analysis of graphene
NASA Astrophysics Data System (ADS)
Hou, X. H.; Deng, Z. C.; Zhang, K.
2017-04-01
A molecular structural mechanics model for the numerical simulation of phonon dispersion relations of graphene is developed by relating the C-C bond molecular potential energy to the strain energy of the equivalent beam-truss space frame. With the stiffness matrix known and further based on the periodic structure characteristics, the Bloch theorem is introduced to develop the dispersion relation of graphene sheet. Being different from the existing structural mechanics model, interactions between the fourth-nearest neighbor atoms are further simulated with beam elements to compensate the reduced stretching stiffness, where as a result not only the dispersion relations in the low frequency field are accurately achieved, but results in the high frequency field are also reasonably obtained. This work is expected to provide new opportunities for the dynamic properties analysis of graphene and future application in the engineering sector.
Emission and propagation of hyperbolic phonon polaritons in hexagonal boron nitride
NASA Astrophysics Data System (ADS)
Dai, Siyuan; Ma, Qiong; Yang, Yafang; Rosenfeld, Jeremy; Goldflam, Michael; McLeod, Alex; Andersen, Trond; Fei, Zhe; Liu, Mengkun; Sun, Zhiyuan; Shao, Yinming; Watanabe, Kenji; Taniguchi, Takashi; Thiemens, Mark; Keilmann, Fritz; Jarillo-Herrero, Pablo; Fogler, Michael; Basov, D. N.
Using scattering-type scanning near-field optical microscope (s-SNOM), we studied various kinds of emission and propagation of hyperbolic phonon polaritons (HP2s) in hexagonal boron nitride (hBN). The systematic study via real-space nano-imaging reveals the emission mechanisms and propagating properties of HP2s excited by crystal edges, artificial structures, surface defects and impurities. Compared with traditional s-SNOM tip emitter, the polaritons from new emitters reported in this work possess longer propagation length and can be artificially manipulated on the hBN surface. Our work may benefit the future applications and engineering of HP2s using convenient emitters which are analogous to collective modes in other materials.
Phonon-enhanced crystal growth and lattice healing
Buonassisi, Anthony; Bertoni, Mariana; Newman, Bonna
2013-05-28
A system for modifying dislocation distributions in semiconductor materials is provided. The system includes one or more vibrational sources for producing at least one excitation of vibrational mode having phonon frequencies so as to enhance dislocation motion through a crystal lattice.
Phononic crystal structures for acoustically driven microfluidic manipulations.
Wilson, Rab; Reboud, Julien; Bourquin, Yannyk; Neale, Steven L; Zhang, Yi; Cooper, Jonathan M
2011-01-21
The development of microfluidic systems is often constrained both by difficulties associated with the chip interconnection to other instruments and by limitations imposed by the mechanisms that can enable fluid movement and processing. Surface acoustic wave (SAW) devices have shown promise in allowing samples to be manipulated, although designing complex fluid operations involves using multiple electrode transducers. We now demonstrate a simple interface between a piezoelectric SAW device and a disposable microfluidic chip, patterned with phononic structures to control the acoustic wave propagation. The surface wave is coupled from the piezoelectric substrate into the disposable chip where it interacts with the phononic lattice. By implementing both a phononic filter and an acoustic waveguide, we illustrate the potential of the technique by demonstrating microcentrifugation for particle and cell concentration in microlitre droplets. We show for the first time that the interaction of the fluid within this metamaterial phononic lattice is dependent upon the frequency of the acoustic wave, providing a route to programme complex fluidic functions into a microchip (in much the same way, by analogy, that a holographic element would change the phase of a light wave in optical tweezers). A practical realisation of this involves the centrifugation of blood on the chip.
Coherently driven, ultrafast electron-phonon dynamics in transport junctions
Szekely, Joshua E.; Seideman, Tamar
2014-07-28
Although the vast majority of studies of transport via molecular-scale heterojunctions have been conducted in the (static) energy domain, experiments are currently beginning to apply time domain approaches to the nanoscale transport problem, combining spatial with temporal resolution. It is thus an opportune time for theory to develop models to explore both new phenomena in, and new potential applications of, time-domain, coherently driven molecular electronics. In this work, we study the interaction of a molecular phonon with an electronic wavepacket transmitted via a conductance junction within a time-domain model that treats the electron and phonon on equal footing and spans the weak to strong electron-phonon coupling strengths. We explore interference between two coherent energy pathways in the electronic subspace, thus complementing previous studies of coherent phenomena in conduction junctions, where the stationary framework was used to study interference between spatial pathways. Our model provides new insights into phase decoherence and population relaxation within the electronic subspace, which have been conventionally treated by density matrix approaches that often rely on phenomenological parameters. Although the specific case of a transport junction is explored, our results are general, applying also to other instances of coupled electron-phonon systems.
Surface phonons near structural phase transitions of fluoridic perovskites
NASA Astrophysics Data System (ADS)
Prade, J.; Kulkarni, A. D.; De Wette, F. W.; Reiger, R.; Schröder, U.; Kress, W.
1989-04-01
The fluoridic perovskite KMnF 3 exhibits an antiferrodistortive phase transition which goes along with a soft mode at the R-point of the Brillouin zone. We investigate in this paper the surface phonons of the KF(001) surface at temperatures near this phase transition. The calculations are carried out for relaxed and reconstructed (001) slabs.
Phonon Trapping in Pearl-Necklace-Shaped Silicon Nanowires.
Miao, Chunyang; Tai, Guoan; Zhou, Jianxin; Guo, Wanlin
2015-12-22
A pearl-necklace-shaped silicon nanowire, in contrast to a smooth nanowire, presents a much lower thermal conductivity due to the phonon trapping effect. By precisely controlling the pearl size and density, this reduction can be more than 70% for the structures designed in the study, which provides a unique approach for designing high-performance nanoscale thermoelectric devices.
Enhanced Electron-Phonon Coupling at Metal Surfaces
Plummer, Ward E.
2010-08-04
The Born-Oppenheimer approximation (BOA) decouples electronic from nuclear motion, providing a focal point for most quantum mechanics textbooks. However, a multitude of important chemical, physical and biological phenomena are driven by violations of this approximation. Vibronic interactions are a necessary ingredient in any process that makes or breaks a covalent bond, for example, conventional catalysis or enzymatically delivered biological reactions. Metastable phenomena associated with defects and dopants in semiconductors, oxides, and glasses entail violation of the BOA. Charge exchange in inorganic polymers, organic slats and biological systems involves charge- induced distortions of the local structure. A classic example is conventional superconductivity, which is driven by the electron-lattice interaction. High-resolution angle-resolved photoemission experiments are yielding new insight into the microscopic origin of electron-phonon coupling (EPC) in anisotropic two-dimensional systems. Our recent surface phonon measurement on the surface of a high-Tc material clearly indicates an important momentum dependent EPC in these materials. In the last few years we have shifted our research focus from solely looking at electron phonon coupling to examining the structure/functionality relationship at the surface of complex transition metal compounds. The investigation on electron phonon coupling has allowed us to move to systems where there is coupling between the lattice, the electrons and the spin.
Femtosecond electron imaging of defect-modulated phonon dynamics
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
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.
Phonon-mediated negative differential conductance in molecular quantum dots
NASA Astrophysics Data System (ADS)
Zazunov, Alex; Feinberg, Denis; Martin, Thierry
2006-03-01
Transport through a single-molecular conductor is considered, showing negative differential conductance behavior associated with phonon-mediated electron tunneling processes. This theoretical work is motivated by a recent experiment by Leroy using a carbon nanotube contacted by a scanning tunneling microscope tip [Nature 432, 371 (2004)], where negative differential conductance of the breathing-mode phonon side peaks could be observed. A peculiarity of this system is that the tunneling couplings which inject electrons and those which collect them on the substrate are highly asymmetrical. A quantum dot model is used, coupling a single electronic level to a local phonon, forming polaron levels. A “half-shuttle” mechanism is also introduced. A quantum kinetic formulation allows us to derive rate equations. Assuming asymmetric tunneling rates and in the absence of the half-shuttle coupling, negative differential conductance (NDC) is obtained for a wide range of parameters. A detailed explanation of this phenomenon is provided, showing that NDC is maximal for intermediate electron-phonon coupling. In addition, in the absence of a gate, the “floating” level results in two distinct lengths for the current plateaus, related to the capacitive couplings at the two junctions. It is shown that the half-shuttle mechanism tends to reinforce the negative differential regions, but it cannot trigger this behavior on its own.
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.
Optical phonons in PbTe/CdTe multilayer heterostructures
Novikova, N. N.; Yakovlev, V. A.; Kucherenko, I. V.; Karczewski, G.; Aleshchenko, Yu. A.; Muratov, A. V.; Zavaritskaya, T. N.; Melnik, N. N.
2015-05-15
The infrared reflection spectra of PbTe/CdTe multilayer nanostructures grown by molecular-beam epitaxy are measured in the frequency range of 20–5000 cm{sup −1} at room temperature. The thicknesses and high-frequency dielectric constants of the PbTe and CdTe layers and the frequencies of the transverse optical (TO) phonons in these structures are determined from dispersion analysis of the spectra. It is found that the samples under study are characterized by two TO phonon frequencies, equal to 28 and 47 cm{sup −1}. The first frequency is close to that of TO phonons in bulk PbTe, and the second is assigned to the optical mode in structurally distorted interface layers. The Raman-scattering spectra upon excitation with the radiation of an Ar{sup +} laser at 514.5 nm are measured at room and liquid-nitrogen temperatures. The weak line at 106 cm{sup −1} observed in these spectra is attributed to longitudinal optical phonons in the interface layers.
Phonon conduction in GaN-diamond composite substrates
NASA Astrophysics Data System (ADS)
Cho, Jungwan; Francis, Daniel; Altman, David H.; Asheghi, Mehdi; Goodson, Kenneth E.
2017-02-01
The integration of strongly contrasting materials can enable performance benefits for semiconductor devices. One example is composite substrates of gallium nitride (GaN) and diamond, which promise dramatically improved conduction cooling of high-power GaN transistors. Here, we examine phonon conduction in GaN-diamond composite substrates fabricated using a GaN epilayer transfer process through transmission electron microscopy, measurements using time-domain thermoreflectance, and semiclassical transport theory for phonons interacting with interfaces and defects. Thermoreflectance amplitude and ratio signals are analyzed at multiple modulation frequencies to simultaneously extract the thermal conductivity of GaN layers and the thermal boundary resistance across GaN-diamond interfaces at room temperature. Uncertainties in the measurement of these two properties are estimated considering those of parameters, including the thickness of a topmost metal transducer layer, given as an input to a multilayer thermal model, as well as those associated with simultaneously fitting the two properties. The volume resistance of an intermediate, disordered SiN layer between the GaN and diamond, as well as a presence of near-interfacial defects in the GaN and diamond, dominates the measured GaN-diamond thermal boundary resistances as low as 17 m2 K GW-1. The GaN thermal conductivity data are consistent with the semiclassical phonon thermal conductivity integral model that accounts for the size effect as well as phonon scattering on point defects at concentrations near 3 × 1018 cm-3.
Hyperbolic phonon polaritons in hexagonal boron nitride (Conference Presentation)
NASA Astrophysics Data System (ADS)
Dai, Siyuan; Ma, Qiong; Fei, Zhe; Liu, Mengkun; Goldflam, Michael D.; Andersen, Trond; Garnett, William; Regan, Will; Wagner, Martin; McLeod, Alexander S.; Rodin, Alexandr; Zhu, Shou-En; Watanabe, Kenji; Taniguchi, T.; Dominguez, Gerado; Thiemens, Mark; Castro Neto, Antonio H.; Janssen, Guido C. A. M.; Zettl, Alex; Keilmann, Fritz; Jarillo-Herrero, Pablo; Fogler, Michael M.; Basov, Dmitri N.
2016-09-01
Uniaxial materials whose axial and tangential permittivities have opposite signs are referred to as indefinite or hyperbolic media. While hyperbolic responses are normally achieved with metamaterials, hexagonal boron nitride (hBN) naturally possesses this property due to the anisotropic phonons in the mid-infrared. Using scattering-type scanning near-field optical microscopy, we studied polaritonic phenomena in hBN. We performed infrared nano-imaging of highly confined and low-loss hyperbolic phonon polaritons in hBN. The polariton wavelength was shown to be governed by the hBN thickness according to a linear law persisting down to few atomic layers [1]. Additionally, we carried out the modification of hyperbolic response in meta-structures comprised of a mononlayer graphene deposited on hBN [2]. Electrostatic gating of the top graphene layer allows for the modification of wavelength and intensity of hyperbolic phonon polaritons in bulk hBN. The physics of the modification originates from the plasmon-phonon coupling in the hyperbolic medium. Furthermore, we demonstrated the "hyperlens" for subdiffractional focusing and imaging using a slab of hBN [3]. References [1] S. Dai et al., Science, 343, 1125 (2014). [2] S. Dai et al., Nature Nanotechnology, 10, 682 (2015). [3] S. Dai et al., Nature Communications, 6, 6963 (2015).
Helium Atom Scattering as a Probe of Surface Phonons.
NASA Astrophysics Data System (ADS)
Yerkes, Steven Charles
A low energy (15-63 meV) helium beam is scattered from two different crystal surfaces; mechanically cleaved and polished LiF(100), and epitaxially grown Ag(111). Conservation equations for total particle energy and parallel momentum are presented as the basis for understanding the scattering experiment. These equations along with the system response function are used to numerically model the inelastic scattering data. The scattering of helium from LiF(100) at a surface temperature of approximately 140(DEGREES)K is reported along the <010> and <110> directions. Coherent elastic scattering intensities are used for surface characterization. Over 80% of the coherent inelastic scattering can be accounted for by the interaction with single Rayleigh phonons, and is consistent with a calculated rigid-ion Rayleigh phonon dispersion relation of LiF(100). These conclusions are supported by over 85 separate inelastic experiments performed at two incident beam energies (62.5 meV and 20.5 meV) and at several incident angles (50(DEGREES), 60(DEGREES), 65(DEGREES), 70(DEGREES)). The scattering from single Rayleigh phonons is reported as a function of crystal surface temperature, scattering angle, phonon frequency, and azimuthal angle. The inelastic scattering data is compared with the calculated intensities of an analytic inelastic scattering theory. The surface temperature dependent inelastic scattering intensity of the data are qualitatively predicted by the theory, however, the theory is inadequate in predicting the experimental intensity as a function of scattering angle, phonon frequency, and azimuthal angle. Incoherent elastic scattering (due to surface defects) is also reported as a function of scattered angle. No existing theory can account for the observed behavior of this scattering. Preliminary coherent inelastic scattering data are reported for. 23 meV helium from Ag(111) directed 12.5(DEGREES) from the <112>. Single(' ). Rayleigh phonons account for less than 60% of
Phonon-drag thermopower in 3D Dirac semimetals.
Kubakaddi, S S
2015-11-18
A theory of low-temperature phonon-drag thermopower S(g) in three-dimensional (3D) Dirac semimetals has been developed considering screened electron-phonon deformation potential coupling. Numerical investigations of S(g), in the boundary scattering regime for phonons, are made in 3D Dirac semimetal Cd3As2, as a function of temperature T and electron concentration n e. S(g) is found to increase rapidly for about T < 1 K and nearly levels off for higher T. It is also seen that S(g) increases (decreases) with decreasing n e at lower (higher) T (<2 K). A screening effect is found to be very significant, strongly affecting T and n e dependence for about <1 K and becoming negligible at higher temperature. In the Bloch-Gruneisen (BG) regime the power laws S(g) ~ T(8) (T(4)) and S(g) ~ n(e)(-5/3)(n(e)(-1/3) with (without) screening are obtained. These laws with respect to T and n e are, respectively, characteristics of 3D phonons and Dirac 3D electrons. Comparison with diffusion thermopower S(d) shows that S (g) dominates (and is much greater than) S(d) for about T > 0.2 K. Herring's law S(g) μ p ~ T (-1), relating phonon limited mobility μ p and S(g) in the BG regime, is shown to be valid in 3D Dirac semimetals. The results obtained here are compared with those in 3D semiconductors, low-dimensional semiconductor heterojunctions and graphene. We conclude that n e-dependent measurements, rather than T-dependent ones, provide a clearer signature of the 3D Dirac semimetal phase.
Optimal design of tunable phononic bandgap plates under equibiaxial stretch
NASA Astrophysics Data System (ADS)
Hedayatrasa, Saeid; Abhary, Kazem; Uddin, M. S.; Guest, James K.
2016-05-01
Design and application of phononic crystal (PhCr) acoustic metamaterials has been a topic with tremendous growth of interest in the last decade due to their promising capabilities to manipulate acoustic and elastodynamic waves. Phononic controllability of waves through a particular PhCr is limited only to the spectrums located within its fixed bandgap frequency. Hence the ability to tune a PhCr is desired to add functionality over its variable bandgap frequency or for switchability. Deformation induced bandgap tunability of elastomeric PhCr solids and plates with prescribed topology have been studied by other researchers. Principally the internal stress state and distorted geometry of a deformed phononic crystal plate (PhP) changes its effective stiffness and leads to deformation induced tunability of resultant modal band structure. Thus the microstructural topology of a PhP can be altered so that specific tunability features are met through prescribed deformation. In the present study novel tunable PhPs of this kind with optimized bandgap efficiency-tunability of guided waves are computationally explored and evaluated. Low loss transmission of guided waves throughout thin walled structures makes them ideal for fabrication of low loss ultrasound devices and structural health monitoring purposes. Various tunability targets are defined to enhance or degrade complete bandgaps of plate waves through macroscopic tensile deformation. Elastomeric hyperelastic material is considered which enables recoverable micromechanical deformation under tuning finite stretch. Phononic tunability through stable deformation of phononic lattice is specifically required and so any topology showing buckling instability under assumed deformation is disregarded. Nondominated sorting genetic algorithm (GA) NSGA-II is adopted for evolutionary multiobjective topology optimization of hypothesized tunable PhP with square symmetric unit-cell and relevant topologies are analyzed through finite
Phonon-drag thermopower in 3D Dirac semimetals
NASA Astrophysics Data System (ADS)
Kubakaddi, S. S.
2015-11-01
A theory of low-temperature phonon-drag thermopower S g in three-dimensional (3D) Dirac semimetals has been developed considering screened electron-phonon deformation potential coupling. Numerical investigations of S g, in the boundary scattering regime for phonons, are made in 3D Dirac semimetal Cd3As2, as a function of temperature T and electron concentration n e. S g is found to increase rapidly for about T < 1 K and nearly levels off for higher T. It is also seen that S g increases (decreases) with decreasing n e at lower (higher) T (<2 K). A screening effect is found to be very significant, strongly affecting T and n e dependence for about <1 K and becoming negligible at higher temperature. In the Bloch-Gruneisen (BG) regime the power laws S g ~ T 8 (T 4) and S g ~ n\\text{e}-5/3 (n\\text{e}-1/3) with (without) screening are obtained. These laws with respect to T and n e are, respectively, characteristics of 3D phonons and Dirac 3D electrons. Comparison with diffusion thermopower S d shows that S g dominates (and is much greater than) S d for about T > 0.2 K. Herring’s law S g μ p ~ T -1, relating phonon limited mobility μ p and S g in the BG regime, is shown to be valid in 3D Dirac semimetals. The results obtained here are compared with those in 3D semiconductors, low-dimensional semiconductor heterojunctions and graphene. We conclude that n e-dependent measurements, rather than T-dependent ones, provide a clearer signature of the 3D Dirac semimetal phase.
Spin-phonon coupling in multiferroic YbMnO3 studied by Raman scattering
NASA Astrophysics Data System (ADS)
Fukumura, H.; Hasuike, N.; Harima, H.; Kisoda, K.; Fukae, K.; Yoshimura, T.; Fujimura, N.
2009-02-01
Hexagonal YbMnO3 bulk polycrystals were prepared and studied by Raman scattering in the temperature range of 15-300 K. A total of 15 phonon modes of A1, E1 and E2 type were identified. Some E2 phonon modes showed anomalous temperature variations in frequency at TN~80 K, suggesting a coupling between the spin and phonon systems below TN. As another evidence of spin-phonon coupling, softening of an A1-phonon mode for the O-Mn vibration was observed at ~TN. Substitution of Mn by Al suggests this view.
Study of LO-phonon decay in semiconductors for hot carrier solar cell
NASA Astrophysics Data System (ADS)
Levard, Hugo; Vidal, Julien; Laribi, Sana; Guillemoles, Jean-François
2014-03-01
Knowledge of phonon decay is of crucial importance when studying basic properties of semiconductors, since they are closely related to Raman linewidth and non-equilibrium-hot-carriers cooling. The latter indeed cools down to the bottom of the conduction band within a picosecond range because of electron-phonon interaction. The eventual emitted hot phonons then decay in few picoseconds. The hot carriers cooling can be slowed down by considering the decay rate dependence of phonon on conservation rules, whose tuning may reduce the allowed two-phonon final states density. This is of direct interest for the third generation photovoltaic devices that are Hot Carrier Solar Cells (HCSC), in which the photoexcited carriers are extracted at an energy higher than thermal equilibrium. One of the HCSC main challenges then is to find an absorber material in which the hot phonons has a relaxation time longer than the carriers cooling time, so that we can expect the electron to ``reabsorb'' a phonon, slowing down the electronic cooling. HCSC yield is ultimately limited by LO phonon decay, though. In this work, we present theoretical results obtained from ab initio calculations of phonon lifetime in III-V and IV-IV semiconductors through a three-phonon process. Common approximations in the literature are questioned. In particular, we show that the usual ``zone-center approximation'' is not valid in some specific semiconductors. The analysis allows to correctly investigate phonon decay mechanisms in bulk and nanostructured materials.
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.
Phonon anharmonicity and components of the entropy in palladium and platinum
NASA Astrophysics Data System (ADS)
Shen, Yang; Li, Chen W.; Tang, Xiaoli; Smith, Hillary L.; Fultz, B.
2016-06-01
Inelastic neutron scattering was used to measure the phonon density of states in fcc palladium and platinum metal at temperatures from 7 K to 1576 K. Both phonon-phonon interactions and electron-phonon interactions were calculated by methods based on density functional theory (DFT) and were consistent with the measured shifts and broadenings of phonons with temperature. Unlike the longitudinal modes, the characteristic transverse modes had a nonlinear dependence on temperature owing to the requirement for a population of thermal phonons for upscattering. Kohn anomalies were observed in the measurements at low temperature and were reproduced by calculations based on DFT. Contributions to the entropy from phonons and electrons were assessed and summed to obtain excellent agreement with prior calorimetric data. The entropy from thermal expansion is positive for both phonons and electrons but larger for phonons. The anharmonic phonon entropy is negative in Pt, but in Pd it changes from positive to negative with increasing temperature. Owing to the position of the Fermi level on the electronic DOS, the electronic entropy was sensitive to the adiabatic electron-phonon interaction in both Pd and Pt. The adiabatic EPI depended strongly on thermal atom displacements.
Thermal transport and anharmonic phonons in strained monolayer hexagonal boron nitride
Li, Shasha; Chen, Yue
2017-01-01
Thermal transport and phonon-phonon coupling in monolayer hexagonal boron nitride (h-BN) under equibiaxial strains are investigated from first principles. Phonon spectra at elevated temperatures have been calculated from perturbation theory using the third-order anharmonic force constants. The stiffening of the out-of-plane transverse acoustic mode (ZA) near the Brillouin zone center and the increase of acoustic phonon lifetimes are found to contribute to the dramatic increase of thermal transport in strained h-BN. The transverse optical mode (TO) at the K point, which was predicted to lead to mechanical failure of h-BN, is found to shift to lower frequencies at elevated temperatures under equibiaxial strains. The longitudinal and transverse acoustic modes exhibit broad phonon spectra under large strains in sharp contrast to the ZA mode, indicating strong in-plane phonon-phonon coupling. PMID:28262786
Coherent optical phonon oscillation and possible electronic softening in WTe2 crystals.
He, Bin; Zhang, Chunfeng; Zhu, Weida; Li, Yufeng; Liu, Shenghua; Zhu, Xiyu; Wu, Xuewei; Wang, Xiaoyong; Wen, Hai-Hu; Xiao, Min
2016-07-26
A rapidly-growing interest in WTe2 has been triggered by the giant magnetoresistance effect discovered in this unique system. While many efforts have been made towards uncovering the electron- and spin-relevant mechanisms, the role of lattice vibration remains poorly understood. Here, we study the coherent vibrational dynamics in WTe2 crystals by using ultrafast pump-probe spectroscopy. The oscillation signal in time domain in WTe2 has been ascribed as due to the coherent dynamics of the lowest energy A1 optical phonons with polarization- and wavelength-dependent measurements. With increasing temperature, the phonon energy decreases due to anharmonic decay of the optical phonons into acoustic phonons. Moreover, a significant drop (15%) of the phonon energy with increasing pump power is observed which is possibly caused by the lattice anharmonicity induced by electronic excitation and phonon-phonon interaction.
Thermal transport and anharmonic phonons in strained monolayer hexagonal boron nitride.
Li, Shasha; Chen, Yue
2017-03-06
Thermal transport and phonon-phonon coupling in monolayer hexagonal boron nitride (h-BN) under equibiaxial strains are investigated from first principles. Phonon spectra at elevated temperatures have been calculated from perturbation theory using the third-order anharmonic force constants. The stiffening of the out-of-plane transverse acoustic mode (ZA) near the Brillouin zone center and the increase of acoustic phonon lifetimes are found to contribute to the dramatic increase of thermal transport in strained h-BN. The transverse optical mode (TO) at the K point, which was predicted to lead to mechanical failure of h-BN, is found to shift to lower frequencies at elevated temperatures under equibiaxial strains. The longitudinal and transverse acoustic modes exhibit broad phonon spectra under large strains in sharp contrast to the ZA mode, indicating strong in-plane phonon-phonon coupling.
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).
NASA Astrophysics Data System (ADS)
Nomura, Yusuke; Arita, Ryotaro
2015-12-01
We formulate an ab initio downfolding scheme for electron-phonon-coupled systems. In this scheme, we calculate partially renormalized phonon frequencies and electron-phonon coupling, which include the screening effects of high-energy electrons, to construct a realistic Hamiltonian consisting of low-energy electron and phonon degrees of freedom. We show that our scheme can be implemented by slightly modifying the density functional-perturbation theory (DFPT), which is one of the standard methods for calculating phonon properties from first principles. Our scheme, which we call the constrained DFPT, can be applied to various phonon-related problems, such as superconductivity, electron and thermal transport, thermoelectricity, piezoelectricity, dielectricity, and multiferroicity. We believe that the constrained DFPT provides a firm basis for the understanding of the role of phonons in strongly correlated materials. Here, we apply the scheme to fullerene superconductors and discuss how the realistic low-energy Hamiltonian is constructed.
Optical phonon lasing and its detection in transport through semiconduc- tor double quantum dots
NASA Astrophysics Data System (ADS)
Okuyama, Rin; Eto, Mikio; Brandes, Tobias
2014-03-01
We theoretically propose optical phonon lasing for a double quantum dot (DQD) fabricated in a semiconductor substrate. No additional cavity or resonator is required. We show that the DQD couples to only two phonon modes that act as a natural cavity. The pumping to the upper level is realized by an electric current through the DQD under a finite bias. Using the rate equation in the Born-Markov-Secular approximation, we analyze the enhanced phonon emission when the level spacing in the DQD is tuned to the phonon energy. We find the phonon lasing when the pumping rate is much larger than the phonon decay rate, whereas anti-bunching of phonon emission is observed when the pumping rate is smaller.[1] Our theory can be also applicable to DQDs embedded in nanomechanical resonators to control the vibrating modes. We discuss detection of amplified modes using the electric current and its noise through the DQD, and another DQD fabricated nearby.
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.
Coherent optical phonon oscillation and possible electronic softening in WTe2 crystals
He, Bin; Zhang, Chunfeng; Zhu, Weida; Li, Yufeng; Liu, Shenghua; Zhu, Xiyu; Wu, Xuewei; Wang, Xiaoyong; Wen, Hai-hu; Xiao, Min
2016-01-01
A rapidly-growing interest in WTe2 has been triggered by the giant magnetoresistance effect discovered in this unique system. While many efforts have been made towards uncovering the electron- and spin-relevant mechanisms, the role of lattice vibration remains poorly understood. Here, we study the coherent vibrational dynamics in WTe2 crystals by using ultrafast pump-probe spectroscopy. The oscillation signal in time domain in WTe2 has been ascribed as due to the coherent dynamics of the lowest energy A1 optical phonons with polarization- and wavelength-dependent measurements. With increasing temperature, the phonon energy decreases due to anharmonic decay of the optical phonons into acoustic phonons. Moreover, a significant drop (15%) of the phonon energy with increasing pump power is observed which is possibly caused by the lattice anharmonicity induced by electronic excitation and phonon-phonon interaction. PMID:27457385
Ultrafast optical generation of coherent phonons in CdTe1-xSex quantum dots
NASA Astrophysics Data System (ADS)
Bragas, A. V.; Aku-Leh, C.; Costantino, S.; Ingale, Alka; Zhao, J.; Merlin, R.
2004-05-01
We report on the impulsive generation of coherent optical phonons in CdTe0.68Se0.32 nanocrystallites embedded in a glass matrix. Pump-probe experiments using femtosecond laser pulses were performed by tuning the laser central energy to resonate with the absorption edge of the nanocrystals. We identify two longitudinal optical phonons, one longitudinal acoustic phonon and a fourth mode of a mixed longitudinal-transverse nature. The amplitude of the optical phonons as a function of the laser central energy exhibits a resonance that is well described by a model based on impulsive stimulated Raman scattering. The phases of the coherent phonons reveal coupling between different modes. At low power density excitations, the frequency of the optical coherent phonons deviates from values obtained from spontaneous Raman scattering. This behavior is ascribed to the presence of electronic impurity states which modify the nanocrystal dielectric function and, thereby, the frequency of the infrared-active phonons.
Phonon dispersion and quantization tuning of strained carbon nanotubes for flexible electronics
Gautreau, Pierre; Chu, Yanbiao; Basaran, Cemal; Ragab, Tarek
2014-06-28
Graphene and carbon nanotubes are materials with large potentials for applications in flexible electronics. Such devices require a high level of sustainable strain and an understanding of the materials electrical properties under strain. Using supercell theory in conjunction with a comprehensive molecular mechanics model, the full band phonon dispersion of carbon nanotubes under uniaxial strain is studied. The results suggest an overall phonon softening and open up the possibility of phonon quantization tuning with uniaxial strain. The change in phonon quantization and the resulting increase in electron-phonon and phonon-phonon scattering rates offer further explanation and theoretical basis to the experimental observation of electrical properties degradation for carbon nanotubes under uniaxial strain.
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.
Vibron and phonon hybridization in dielectric nanostructures.
Preston, Thomas C; Signorell, Ruth
2011-04-05
Plasmon hybridization theory has been an invaluable tool in advancing our understanding of the optical properties of metallic nanostructures. Through the prism of molecular orbital theory, it allows one to interpret complex structures as "plasmonic molecules" and easily predict and engineer their electromagnetic response. However, this formalism is limited to conducting particles. Here, we present a hybridization scheme for the external and internal vibrations of dielectric nanostructures that provides a straightforward understanding of the infrared signatures of these particles through analogy to existing hybridization models of both molecular orbitals and plasmons extending the range of applications far beyond metallic nanostructures. This method not only provides a qualitative understanding, but also allows for the quantitative prediction of vibrational spectra of complex nanoobjects from well-known spectra of their primitive building blocks. The examples of nanoshells illustrate how spectral features can be understood in terms of symmetry, number of nodal planes, and scale parameters.
The Electron-Phonon Interaction from First Principles
NASA Astrophysics Data System (ADS)
Noffsinger, Jesse Dean
2011-12-01
In this thesis the ground state electronic properties, lattice dynamics, electron-phonon coupling and superconductivity of a variety materials are investigated from first principles. The first chapter provides an introduction to the material and concepts of this thesis as well as motivation for the work done herein. Additionally, an overview is given on the theoretical background governing the calculations of this work. This includes overviews of the topics of density functional theory, the pseudopotential approximation, density functional perturbation theory, and applications of these approaches to the calculations of superconductivity. In the second chapter the mechanics of actually performing calculations within the methodology of chapter one are explained. This is accomplished through a detailed description of the computer software EPW. This software has been developed to allow computationally efficient approaches for calculating the electron-phonon interaction. A description of the software package, the particular quantities which it calculates and example calculations are given. The following two chapters present the results of calculations regarding electron-phonon coupling and superconductivity in bulk carbon compounds. The occurrence or absence of superconductivity is found to be related in these compounds to Fermi surface nesting and carrier concentrations. In chapter five we investigate the role of the fluorine dopant in the recently discovered (1111) Fe-pnictide superconductors. Contrary to the results of the literature published shortly after the discovery of these compounds, the presence of the dopant is found to actually result in a net decrease in the electron concentration on the Fe-plane within the local density approximation to density functional theory. In the two chapters which follow, we investigate the limits of two dimensional superconductivity in the recent experiments on ultra-thin Pb samples. Chapter six details calculations on
Dynamics of vortices in neutral superfluids with noninteracting phonons
NASA Astrophysics Data System (ADS)
Fortin, Jean-Yves
2001-05-01
The transverse force on an isolated and moving vortex in a neutral superfluid at rest is evaluated at finite temperature in the case of noninteracting phonons. Using the Thouless, Ao, Niu (TAN) [Phys. Rev. Lett. 76, 3758 (1996)] general theory, we show that the transverse force is exactly equal to the superfluid Magnus force. We extend this theory in the case of a slowly moving vortex on a circular trajectory, and find an additional contribution coming from the centrifugal reaction. This term gives a negative vortex mass due to the phonons and diverges logarithmically at low frequency. The friction force is also evaluated for zero and finite frequencies, and compared with the scattering theory.
Stochastic Approach to Phonon-Assisted Optical Absorption
NASA Astrophysics Data System (ADS)
Zacharias, Marios; Patrick, Christopher E.; Giustino, Feliciano
2015-10-01
We develop a first-principles theory of phonon-assisted optical absorption in semiconductors and insulators which incorporates the temperature dependence of the electronic structure. We show that the Hall-Bardeen-Blatt theory of indirect optical absorption and the Allen-Heine theory of temperature-dependent band structures can be derived from the present formalism by retaining only one-phonon processes. We demonstrate this method by calculating the optical absorption coefficient of silicon using an importance sampling Monte Carlo scheme, and we obtain temperature-dependent line shapes and band gaps in good agreement with experiment. The present approach opens the way to predictive calculations of the optical properties of solids at finite temperature.
Theory of phonon-driven spin Seebeck effect
NASA Astrophysics Data System (ADS)
Adachi, Hiroto; Ohe, Jun-Ichiro; Takahashi, Saburo; Maekawa, Sadamichi
2012-02-01
Spin Seebeck effect refers to a thermal spin injection occurring over millimeter scales from a ferromagnet into an attached nonmagnetic metal [Uchida et al., Nature 455, 778 (2008)]. We discuss the importance of the phonon-drag process in the spin Seebeck effect. Our theory of phonon-drag spin Seebeck effect [Adachi et al., Appl. Phys. Lett. 97, 252506 (2010)] explains simultaneously the local nature of the spin Seebeck effect [Jaworski et al., Nature Materials 9, 898 (2010); Uchida et al., Nature Materials 10, 737 (2011)] and the signal enhancement at low temperatures [Jaworski et al., Phys. Rev. Lett. 106, 186601 (2011)]. We also discuss the difference between our approach and that developed in Xiao et al., Phys. Rev. B 81, 214418 (2010).
Gigantic enhancement of spin Seebeck effect by phonon drag
NASA Astrophysics Data System (ADS)
Adachi, Hiroto; Uchida, Ken-Ichi; Saitoh, Eiji; Ohe, Jun-Ichiro; Takahashi, Saburo; Maekawa, Sadamichi
2011-03-01
We investigate both theoretically and experimentally a gigantic enhancement of the spin Seebeck effect [K. Uchida et al., Nature 455, 778 (2008); C. M. Jaworski et al., Nature Mater. 9, 898 (2010); K. Uchida et al., Nature Mater. 9, 894 (2010)] in a prototypical magnet La Y2 Fe 5 O12 at low temperatures. Our theoretical analysis sheds light on the important role of phonons; the spin Seebeck effect is enormously enhanced by nonequilibrium phonons that drag the low-lying spin excitations. We further argue that this scenario gives a clue to understand the observation of the spin Seebeck effect that is unaccompanied by a global spin current, and predict that the substrate condition affects the observed signal.
Nonlinear control of high-frequency phonons in spider silk
NASA Astrophysics Data System (ADS)
Schneider, Dirk; Gomopoulos, Nikolaos; Koh, Cheong Y.; Papadopoulos, Periklis; Kremer, Friedrich; Thomas, Edwin L.; Fytas, George
2016-10-01
Spider dragline silk possesses superior mechanical properties compared with synthetic polymers with similar chemical structure due to its hierarchical structure comprised of partially crystalline oriented nanofibrils. To date, silk’s dynamic mechanical properties have been largely unexplored. Here we report an indirect hypersonic phononic bandgap and an anomalous dispersion of the acoustic-like branch from inelastic (Brillouin) light scattering experiments under varying applied elastic strains. We show the mechanical nonlinearity of the silk structure generates a unique region of negative group velocity, that together with the global (mechanical) anisotropy provides novel symmetry conditions for gap formation. The phononic bandgap and dispersion show strong nonlinear strain-dependent behaviour. Exploiting material nonlinearity along with tailored structural anisotropy could be a new design paradigm to access new types of dynamic behaviour.
Electron-phonon coupling in quasi-free-standing graphene
NASA Astrophysics Data System (ADS)
Johannsen, Jens Christian; Ulstrup, Søren; Bianchi, Marco; Hatch, Richard; Guan, Dandan; Mazzola, Federico; Hornekær, Liv; Fromm, Felix; Raidel, Christian; Seyller, Thomas; Hofmann, Philip
2013-03-01
Quasi-free-standing monolayer graphene can be produced by intercalating species like oxygen or hydrogen between epitaxial graphene and the substrate crystal. If the graphene was indeed decoupled from the substrate, one would expect the observation of a similar electronic dispersion and many-body effects, irrespective of the substrate and the material used to achieve the decoupling. Here we investigate the electron-phonon coupling in two different types of quasi-free-standing monolayer graphene: decoupled from SiC via hydrogen intercalation and decoupled from Ir via oxygen intercalation. The two systems show similar overall behaviours of the self-energy and a weak renormalization of the bands near the Fermi energy. The electron-phonon coupling is found to be so weak that it renders the precise determination of the coupling constant λ through renormalization difficult. The estimated value of λ is 0.05(3) for both systems.
Constitutive equation for polymer networks with phonon fluctuations.
Hansen, Rasmus; Skov, Anne Ladegaard; Hassager, Ole
2008-01-01
Recent research by Xing [Phys. Rev. Lett. 98, 075502 (2007)] has provided an expression for the Helmholtz free energy related to phonon fluctuations in polymer networks. We extend this result by constructing the corresponding nonlinear constitutive equation, usable for entirely general, volume conserving deformation fields. Constitutive equations for the sliplink model and the tube model are derived and the three models are examined by comparison with each other and with data from Xu and Mark [Rubber Chem. Technol. 63, 276 (1990)] and Wang and Mark [J. Polym. Sci., Part B: Polym. Phys. 30, 801 (1992)]. Elastic moduli are derived for the three models and compared with the moduli determined from the chemical stoichiometry. We conclude that the sliplink model and the phonon fluctuation model are relatively consistent with each other and with the data. The tube model seems consistent neither with the other models nor with the data.
Phonons on the clean metal surfaces and in adsorption structures
NASA Astrophysics Data System (ADS)
Rusina, Galina G.; Chulkov, Evgenii V.
2013-06-01
The state-of-the-art studies of the vibrational dynamics of clean metal surfaces and metal surface structures formed upon the sub-monolayer adsorption of the atoms of various elements are considered. A brief historical survey of the milestones of investigations of surface phonons is presented. The results of studies of the atomic structure and vibration characteristics of surfaces with low and high Miller indices and adsorption structures are analyzed. It is demonstrated that vicinal surfaces of FCC metals tend to exhibit specific vibrational modes located on the step and polarized along the step. Irrespective of the type and position of adsorption or the substrate structure, the phonon spectra of sub-monolayer adsorption structures always tend to display two modes for combined translational displacements of adatoms and for coupled vibrations of substrate atoms and adatoms polarized in the direction normal to the surface. The bibliography includes 202 references.
Giant Anharmonic Phonon Scattering in PbTe
Delaire, Olivier A; Ma, Jie; Marty, Karol J; May, Andrew F; McGuire, Michael A; Singh, David J; Lumsden, Mark D; Sales, Brian C; Du, Mao-Hua; Ehlers, Georg; Podlesnyak, Andrey A
2011-01-01
Understanding the microscopic processes affecting the bulk thermal conductivity is crucial to develop more efficient thermoelectric materials. PbTe is currently one of the leading thermoelectric materials, largely thanks to its low thermal conductivity. However, the origin of this low thermal conductivity in a simple rocksalt structure has so far been elusive. Using a combination of inelastic neutron scattering measurements and first-principles computations of the phonons, we identify a strong anharmonic coupling between the ferroelectric transverse optic (TO) mode and the longitudinal acoustic (LA) modes in PbTe. This interaction extends over a large portion of reciprocal space, and directly affects the heat-carrying LA phonons. The LA-TO anharmonic coupling is likely to play a central role in explaining the low thermal conductivity of PbTe. The present results provide a microscopic picture of why many good thermoelectric materials are found near a lattice instability of the ferroelectric type.
Role of phonons in the metal-insulator phase transition.
NASA Technical Reports Server (NTRS)
Langer, W. D.
1972-01-01
Review, for the transition series oxides, of the Mattis and Lander model, which is one of electrons interacting with lattice vibrations (electron and phonon interaction). The model displays superconducting, insulating, and metallic phases. Its basic properties evolve from a finite crystallographic distortion associated with a dominant phonon mode and the splitting of the Brillouin zone into two subzones, a property of simple cubic and body centered cubic lattices. The order of the metal-insulator phase transition is examined. The basic model has a second-order phase transition and the effects of additional mechanisms on the model are calculated. The way in which these mechanisms affect the magnetically ordered transition series oxides as described by the Hubbard model is discussed.
Ultrafast modulation of electronic structure by coherent phonon excitations
NASA Astrophysics Data System (ADS)
Weisshaupt, J.; Rouzée, A.; Woerner, M.; Vrakking, M. J. J.; Elsaesser, T.; Shirley, E. L.; Borgschulte, A.
2017-02-01
Femtosecond x-ray absorption spectroscopy with a laser-driven high-harmonic source is used to map ultrafast changes of x-ray absorption by femtometer-scale coherent phonon displacements. In LiBH4, displacements along an Ag phonon mode at 10 THz are induced by impulsive Raman excitation and give rise to oscillatory changes of x-ray absorption at the Li K edge. Electron density maps from femtosecond x-ray diffraction data show that the electric field of the pump pulse induces a charge transfer from the BH4- to neighboring Li+ ions, resulting in a differential Coulomb force that drives lattice vibrations in this virtual transition state.
Photonic Aharonov–Bohm effect in photon–phonon interactions
Li, Enbang; Eggleton, Benjamin J.; Fang, Kejie; Fan, Shanhui
2014-01-01
The Aharonov–Bohm effect is one of the most intriguing phenomena in both classical and quantum physics, and associates with a number of important and fundamental issues in quantum mechanics. The Aharonov–Bohm effects of charged particles have been experimentally demonstrated and found applications in various fields. Recently, attention has also focused on the Aharonov–Bohm effect for neutral particles, such as photons. Here we propose to utilize the photon–phonon interactions to demonstrate that photonic Aharonov–Bohm effects do exist for photons. By introducing nonreciprocal phases for photons, we observe experimentally a gauge potential for photons in the visible range based on the photon–phonon interactions in acousto-optic crystals, and demonstrate the photonic Aharonov–Bohm effect. The results presented here point to new possibilities to control and manipulate photons by designing an effective gauge potential. PMID:24476790
Effect of phonon-blocking at sintered interfaces
NASA Astrophysics Data System (ADS)
Shiomi, Junichiro
2015-03-01
With an aim to develop high figure-of-merit silicon nanocrystalline thermoelectrics, controllability of thermal conductivity is demonstrated by combining computation, measurement, and material synthesis. Direct measurements of interfacial thermal conductance at sintered interfaces using a 2D model interface reveal that the interfacial thermal conductance can be greatly reduced by precipitating silicon oxide crystalline nano-dots at the interface. Furthermore the impact of the reduction in interfacial thermal conductance on the overall thermal conductivity of the bulk nanocrystalline material is identified by multiscale phonon transport calculation using intrinsic phonon properties obtained from first principles. These analyses help us identify the required interfacial structure and grain size (mean value and distribution) for a target thermal conductivity. Attempts to implement this in the actual material development will be also introduced. The work is supported in parts by JST PRESTO and JSPS KAKENHI 26709009.
Photonic Aharonov-Bohm effect in photon-phonon interactions.
Li, Enbang; Eggleton, Benjamin J; Fang, Kejie; Fan, Shanhui
2014-01-01
The Aharonov-Bohm effect is one of the most intriguing phenomena in both classical and quantum physics, and associates with a number of important and fundamental issues in quantum mechanics. The Aharonov-Bohm effects of charged particles have been experimentally demonstrated and found applications in various fields. Recently, attention has also focused on the Aharonov-Bohm effect for neutral particles, such as photons. Here we propose to utilize the photon-phonon interactions to demonstrate that photonic Aharonov-Bohm effects do exist for photons. By introducing nonreciprocal phases for photons, we observe experimentally a gauge potential for photons in the visible range based on the photon-phonon interactions in acousto-optic crystals, and demonstrate the photonic Aharonov-Bohm effect. The results presented here point to new possibilities to control and manipulate photons by designing an effective gauge potential.
Phonon thermal conductivity of a nanowire attached to leads
NASA Astrophysics Data System (ADS)
Hershfield, Selman; Muttalib, Khandker
2015-03-01
There is experimental evidence as well as theoretical proposals that nanowires can be made to have high thermoelectric efficiency by tuning the electronic properties; however, there is always a phonon contribution to the heat transport which reduces the thermoelectric efficiency. In the harmonic approximation we compute the transmission of phonons through a nanowire coupled to large leads. There is a finite thermal conductivity because of the restriction provided by the nanowire. The nanowire reduces the thermal transport because of the mismatch between the leads and wire modes. We examine the effect of disorder in three places: in the wire, in the leads near the wire, and in the leads far way from the wire. In some cases disorder can increase the thermal conduction because of enhanced mode coupling. We will discuss the implications of our results for thermoelectric nanowire devices.
Anomalous phonon characteristics of unconventional novel III-N superlattices
Talwar, Devki N.
2014-03-31
Comprehensive results of atomic vibrations are reported in the unconventional short-period zb BN/GaN superlatices (SLs) by exploiting a rigid-ion-model and taking into account both the short- and long-range Coulomb interactions. Besides anisotropic mode behavior of optical phonons, our study provided evidence of acoustic-mode anti-crossing, mini-gap formation, confinement as well as BN-like modes falling within the gap that separates optical phonon bands of the two materials. A bond-polarizability scheme is employed within the second-nearest-neighbor linear-chain model to simulate the Raman intensity profiles of BN/GaN SLs revealing major expected trends of the vibrational characteristics observed experimentally in many conventional superlattice systems while eliciting some interesting contrasts.
Plasmon-phonon coupling in graphene-hyperbolic bilayer heterostructures
NASA Astrophysics Data System (ADS)
Yin, Ge; Yuan, Jun; Jiang, Wei; Zhu, Jianfei; Ma, Yungui
2016-11-01
Polar dielectrics are important optical materials enabling the subwavelength manipulation of light in infrared due to their capability to excite phonon polaritons. In practice, it is highly desired to actively modify these hyperbolic phonon polaritons (HPPs) to optimize or tune the response of the device. In this work, we investigate the plasmonic material, a monolayer graphene, and study its hybrid structure with three kinds of hyperbolic thin films grown on SiO2 substrate. The inter-mode hybridization and their tunability have been thoroughly clarified from both the band dispersions and the mode patterns numerically calculated through a transfer matrix method. Our results show that these hybrid multilayer structures are of strong potentials for applications in plasmonic waveguides, modulators and detectors in infrared. Project supported by the National Natural Science Foundation of China (Grant No. 61271085) and the Natural Science Foundation of Zhejiang Province, China (Grant No. LR15F050001).
Photo-excited charge carriers suppress sub-terahertz phonon mode in silicon at room temperature
NASA Astrophysics Data System (ADS)
Liao, Bolin; Maznev, A. A.; Nelson, Keith A.; Chen, Gang
2016-10-01
There is a growing interest in the mode-by-mode understanding of electron and phonon transport for improving energy conversion technologies, such as thermoelectrics and photovoltaics. Whereas remarkable progress has been made in probing phonon-phonon interactions, it has been a challenge to directly measure electron-phonon interactions at the single-mode level, especially their effect on phonon transport above cryogenic temperatures. Here we use three-pulse photoacoustic spectroscopy to investigate the damping of a single sub-terahertz coherent phonon mode by free charge carriers in silicon at room temperature. Building on conventional pump-probe photoacoustic spectroscopy, we introduce an additional laser pulse to optically generate charge carriers, and carefully design temporal sequence of the three pulses to unambiguously quantify the scattering rate of a single-phonon mode due to the electron-phonon interaction. Our results confirm predictions from first-principles simulations and indicate the importance of the often-neglected effect of electron-phonon interaction on phonon transport in doped semiconductors.
Structural and phonon transmission study of Ge-Au-Ge eutectically bonded interfaces
Knowlton, W.B. |
1995-07-01
This thesis presents a structural analysis and phonon transparency investigation of the Ge-Au-Ge eutectic bond interface. Interface development was intended to maximize the interfacial ballistic phonon transparency to enhance the detection of the dark matter candidate WIMPs. The process which was developed provides an interface which produces minimal stress, low amounts of impurities, and insures Ge lattice continuity through the interface. For initial Au thicknesses of greater than 1,000 {angstrom} Au per substrate side, eutectic epitaxial growth resulted in a Au dendritic structure with 95% cross sectional and 90% planar Au interfacial area coverages. In sections in which Ge bridged the interface, lattice continuity across the interface was apparent. Epitaxial solidification of the eutectic interface with initial Au thicknesses < 500 A per substrate side produced Au agglomerations thereby reducing the Au planar interfacial area coverage to as little as 30%. The mechanism for Au coalescence was attributed to lateral diffusion of Ge and Au in the liquid phase during solidification. Phonon transmission studies were performed on eutectic interfaces with initial Au thicknesses of 1,000 {angstrom}, 500 {angstrom}, and 300 {angstrom} per substrate side. Phonon imaging of eutectically bonded samples with initial Au thicknesses of 300 {angstrom}/side revealed reproducible interfacial percent phonon transmissions from 60% to 70%. Line scan phonon imaging verified the results. Phonon propagation TOF spectra distinctly showed the predominant phonon propagation mode was ballistic. This was substantiated by phonon focusing effects apparent in the phonon imaging data. The degree of interface transparency to phonons and resulting phonon propagation modes correlate with the structure of the interface following eutectic solidification. Structural studies of samples with initial Au thickness of 1,000 {angstrom}/side appear to correspond with the phonon transmission study.
Diffraction of electrons at intermediate energies: The role of phonons
NASA Astrophysics Data System (ADS)
Ascolani, H.; Zampieri, G.
1996-07-01
The intensity of electrons reflected ``elastically'' from crystalline surfaces presents two regimes: the low-energy or LEED regime (<500 eV), in which the electrons are reflected along the Bragg directions, and the intermediate-energy or XPD/AED regime (>500 eV), in which the maxima of intensity are along the main crystallographic axes. We present a model which explains this transition in terms of the excitation/absorption of phonons during the scattering.
Phonons, defects and optical damage in crystalline acetanilide
NASA Astrophysics Data System (ADS)
Kosic, Thomas J.; Hill, Jeffrey R.; Dlott, Dana D.
1986-04-01
Intense picosecond pulses cause accumulated optical damage in acetanilide crystals at low temperature. Catastrophic damage to the irradiated volume occurs after an incubation period where defects accumulate. The optical damage is monitored with subanosecond time resolution. The generation of defects is studied with damage-detected picosecond spectroscopy. The accumulation of defects is studied by time-resolved coherent Raman scattering, which is used to measure optical phonon scattering from the accumulating defects.
Coherent phonon-grain boundary scattering in silicon inverse opals.
Ma, Jun; Parajuli, Bibek R; Ghossoub, Marc G; Mihi, Agustin; Sadhu, Jyothi; Braun, Paul V; Sinha, Sanjiv
2013-02-13
We report measurements and modeling of thermal conductivity in periodic three-dimensional dielectric nanostructures, silicon inverse opals. Such structures represent a three-dimensional "phononic crystal" but affect heat flow instead of acoustics. Employing the Stober method, we fabricate high quality silica opal templates that on filling with amorphous silicon, etching and recrystallizing produce silicon inverse opals. The periodicities and shell thicknesses are in the range 420-900 and 18-38 nm, respectively. The thermal conductivity of inverse opal films are relatively low, ~0.6-1.4 W/mK at 300 K and arise due to macroscopic bending of heat flow lines in the structure. The corresponding material thermal conductivity is in the range 5-12 W/mK and has an anomalous ~T(1.8) dependence at low temperatures, distinct from the typical ~T(3) behavior of bulk polycrystalline silicon. Using phonon scattering theory, we show such dependence arising from coherent phonon reflections in the intergrain region. This is consistent with an unconfirmed theory proposed in 1955. The low thermal conductivity is significant for applications in photonics where they imply significant temperature rise at relatively low absorption and in thermoelectrics, where they suggest the possibility of enhancement in the figure of merit for polysilicon with optimal doping.
Stress Wave Isolation by Purely Mechanical Topological Phononic Crystals
Chaunsali, Rajesh; Li, Feng; Yang, Jinkyu
2016-01-01
We present an active, purely mechanical stress wave isolator that consists of short cylindrical particles arranged in a helical architecture. This phononic structure allows us to change inter-particle stiffness dynamically by controlling the contact angles of the cylinders. We use torsional travelling waves to control the contact angles, thereby imposing a desired spatio-temporal stiffness variation to the phononic crystal along the longitudinal direction. Such torsional excitation is a form of parametric pumping in the system, which results in the breakage of the time-reversal symmetry. We report that, in quasi-static sense, the system shows topologically non-trivial band-gaps. However, in a dynamic regime where the pumping effect is significant, these band-gaps become asymmetric with respect to the frequency and wavenumber domains in the dispersion relationship. By using numerical simulations, we show that such asymmetry has a direct correspondence to the topological invariant, i.e., Chern number, of the system. We propose that this asymmetry, accompanied by selective inter-band transition, can be utilized for directional isolation of the stress wave propagating along the phononic crystal. PMID:27477236
Electron-phonon interaction and Raman scattering in nanocrystals
NASA Astrophysics Data System (ADS)
Klimin, S. N.; Pokatilov, E. P.; Fomin, V. M.; Devreese, J. T.; Gladilin, V. N.; Balaban, S. N.
1997-03-01
The vibrational eigenmodes of a nanocrystal are derived by diagonalization of the equations of motion for the ionic displacement taking into account a non-parabolic dispersion with electrostatic and mechanical boundary conditions. A finite width of the Brillouin zone leads automatically to a finite basis of vibrational modes. The developed method can be applicable to nanostructures of an arbitrary geometry. For a spherical nanocrystal, a dispersion equation contains the effective multimode dielectric function. The resulting eigenmodes are mixed bulk-like and interface waves, especially in the short-wavelength region. Using the obtained Hamiltonian, the one-phonon and two-phonon resonant Raman scattering spectra are calculated for a spherical CdSe nanocrystal in the borosilicate glass. The valence band mixing dramatically enhances relative intensities of the two-phonon peaks and makes the adiabatic approximation inapplicable. Hence, the Huang-Rhys parameter is not an adequate characteristic of the optical spectra. Using a direct expansion of the evolution operator, a good agreement has been achieved between the calculated and the experimentally observed [1] Raman spectra. [1] M. C. Klein, F. Hache, D. Ricard, and C. Flytzanis, Phys. Rev. B 42, 11123 (1990).
3-D phononic crystals with ultra-wide band gaps
Lu, Yan; Yang, Yang; Guest, James K.; Srivastava, Ankit
2017-01-01
In this paper gradient based topology optimization (TO) is used to discover 3-D phononic structures that exhibit ultra-wide normalized all-angle all-mode band gaps. The challenging computational task of repeated 3-D phononic band-structure evaluations is accomplished by a combination of a fast mixed variational eigenvalue solver and distributed Graphic Processing Unit (GPU) parallel computations. The TO algorithm utilizes the material distribution-based approach and a gradient-based optimizer. The design sensitivity for the mixed variational eigenvalue problem is derived using the adjoint method and is implemented through highly efficient vectorization techniques. We present optimized results for two-material simple cubic (SC), body centered cubic (BCC), and face centered cubic (FCC) crystal structures and show that in each of these cases different initial designs converge to single inclusion network topologies within their corresponding primitive cells. The optimized results show that large phononic stop bands for bulk wave propagation can be achieved at lower than close packed spherical configurations leading to lighter unit cells. For tungsten carbide - epoxy crystals we identify all angle all mode normalized stop bands exceeding 100%, which is larger than what is possible with only spherical inclusions. PMID:28233812
Tunable phonon-induced transparency in bilayer graphene nanoribbons.
Yan, Hugen; Low, Tony; Guinea, Francisco; Xia, Fengnian; Avouris, Phaedon
2014-08-13
In the phenomenon of plasmon-induced transparency, which is a classical analogue of electromagnetically induced transparency (EIT) in atomic gases, the coherent interference between two plasmon modes results in an optical transparency window in a broad absorption spectrum. With the requirement of contrasting lifetimes, typically one of the plasmon modes involved is a dark mode that has limited coupling to the electromagnetic radiation and possesses relatively longer lifetime. Plasmon-induced transparency not only leads to light transmission at otherwise opaque frequency regions but also results in the slowing of light group velocity and enhanced optical nonlinearity. In this article, we report an analogous behavior, denoted as phonon-induced transparency (PIT), in AB-stacked bilayer graphene nanoribbons. Here, light absorption due to the plasmon excitation is suppressed in a narrow window due to the coupling with the infrared active Γ-point optical phonon, whose function here is similar to that of the dark plasmon mode in the plasmon-induced transparency. We further show that PIT in bilayer graphene is actively tunable by electrostatic gating and estimate a maximum slow light factor of around 500 at the phonon frequency of 1580 cm(-1), based on the measured spectra. Our demonstration opens an avenue for the exploration of few-photon nonlinear optics and slow light in this novel two-dimensional material.
Studying the electronic and phononic structure of penta-graphane
Einollahzadeh, Hamideh; Fazeli, Seyed Mahdi; Dariani, Reza Sabet
2016-01-01
Abstract In this paper, we theoretically consider a two dimensional nanomaterial which is a form of hydrogenated penta-graphene; we call it penta-graphane. This structure is obtained by adding hydrogen atoms to the sp2 bonded carbon atoms of penta-graphene. We investigate the thermodynamic and mechanical stability of penta-graphane. We also study the electronic and phononic structure of penta-graphane. Firstly, we use density functional theory with the revised Perdew–Burke–Ernzerhof approximation to compute the band structure. Then one–shot GW (G0W0) approach for estimating accurate band gap is applied. The indirect band gap of penta-graphane is 5.78 eV, which is close to the band gap of diamond. Therefore, this new structure is a good electrical insulator. We also investigate the structural stability of penta-graphane by computing the phonon structure. Finally, we calculate its specific heat capacity from the phonon density of states. Penta-graphane has a high specific heat capacity, and can potentially be used for storing and transferring energy. PMID:27877907
Nonharmonic phonons in α-iron at high temperatures
NASA Astrophysics Data System (ADS)
Mauger, L.; Lucas, M. S.; Muñoz, J. A.; Tracy, S. J.; Kresch, M.; Xiao, Yuming; Chow, Paul; Fultz, B.
2014-08-01
Phonon densities of states (DOS) of bcc α-Fe57 were measured from room temperature through the 1044 K Curie transition and the 1185 K fcc γ-Fe phase transition using nuclear resonant inelastic x-ray scattering. At higher temperatures all phonons shift to lower energies (soften) with thermal expansion, but the low transverse modes soften especially rapidly above 700 K, showing strongly nonharmonic behavior that persists through the magnetic transition. Interatomic force constants for the bcc phase were obtained by iteratively fitting a Born-von Kármán model to the experimental phonon spectra using a genetic algorithm optimization. The second-nearest-neighbor fitted axial force constants weakened significantly at elevated temperatures. An unusually large nonharmonic behavior is reported, which increases the vibrational entropy and accounts for a contribution of 35 meV/atom in the free energy at high temperatures. The nonharmonic contribution to the vibrational entropy follows the thermal trend of the magnetic entropy, and may be coupled to magnetic excitations. A small change in vibrational entropy across the α-γ structural phase transformation is also reported.
Spin-phonon coupling in scandium doped gallium ferrite
Chakraborty, Keka R. E-mail: smyusuf@barc.gov.in; Mukadam, M. D.; Basu, S.; Yusuf, S. M. E-mail: smyusuf@barc.gov.in; Paul, Barnita; Roy, Anushree; Grover, Vinita; Tyagi, A. K.
2015-03-28
We embarked on a study of Scandium (Sc) doped (onto Ga site) gallium ferrite (GaFeO{sub 3}) and found remarkable magnetic properties. In both doped as well as parent compounds, there were three types of Fe{sup 3+} ions (depending on the symmetry) with the structure conforming to space group Pna2{sub 1} (Sp. Grp. No. 33) below room temperature down to 5 K. We also found that all Fe{sup 3+} ions occupy octahedral sites, and carry high spin moment. For the higher Sc substituted sample (Ga{sub 1−x}Sc{sub x}FeO{sub 3}: x = 0.3), a canted magnetic ordered state is found. Spin-phonon coupling below Néel temperature was observed in doped compounds. Our results indicated that Sc doping in octahedral site modifies spin-phonon interactions of the parent compound. The spin-phonon coupling strength was estimated for the first time in these Sc substituted compounds.
Phonon dispersion relation in PbTiO3
NASA Astrophysics Data System (ADS)
Tomeno, Izumi; Fernandez-Baca, Jaime; Marty, Karol; Oka, Kunihiko; Tsunoda, Yorihiko
2013-03-01
The phonon dispersion relations for cubic PbTiO3 (Tc = 763 K) have been determined along the high symmetry directions at T = 793 K using inelastic neutron scattering. A set of the TO branches drops significantly toward the zone center. This is quite different from the soft mode anomaly in the Pb-based relaxors, named as the waterfall phenomenon. The zone-center TO mode energy softens with decreasing temperature from 1173 to 793 K. The TA branch along [ ξ , ξ , ξ ] shows significant softening around ξ = 0.25 and 0.5. These two anomalies persist up to 1173 K and are weakly temperature dependent. Moreover, the TA branches along [1,0,0] and [1,1,0] soften in the entire q range as the temperature approaches Tc. Although the phonon softening occurs simultaneously, the softening of the zone center TO mode plays an important role in the single phase transition. The phonon dispersion relations for cubic and tetragonal PbTiO3 are discussed in connection with BaTiO3, KTaO3, Pb(Zn1/3Nb2/3)O3, and Pb(Mg1/3Nb2/3)O3. U.S.-Japan cooperative program on neutron scattering
Stress Wave Isolation by Purely Mechanical Topological Phononic Crystals
NASA Astrophysics Data System (ADS)
Chaunsali, Rajesh; Li, Feng; Yang, Jinkyu
2016-08-01
We present an active, purely mechanical stress wave isolator that consists of short cylindrical particles arranged in a helical architecture. This phononic structure allows us to change inter-particle stiffness dynamically by controlling the contact angles of the cylinders. We use torsional travelling waves to control the contact angles, thereby imposing a desired spatio-temporal stiffness variation to the phononic crystal along the longitudinal direction. Such torsional excitation is a form of parametric pumping in the system, which results in the breakage of the time-reversal symmetry. We report that, in quasi-static sense, the system shows topologically non-trivial band-gaps. However, in a dynamic regime where the pumping effect is significant, these band-gaps become asymmetric with respect to the frequency and wavenumber domains in the dispersion relationship. By using numerical simulations, we show that such asymmetry has a direct correspondence to the topological invariant, i.e., Chern number, of the system. We propose that this asymmetry, accompanied by selective inter-band transition, can be utilized for directional isolation of the stress wave propagating along the phononic crystal.
3-D phononic crystals with ultra-wide band gaps
NASA Astrophysics Data System (ADS)
Lu, Yan; Yang, Yang; Guest, James K.; Srivastava, Ankit
2017-02-01
In this paper gradient based topology optimization (TO) is used to discover 3-D phononic structures that exhibit ultra-wide normalized all-angle all-mode band gaps. The challenging computational task of repeated 3-D phononic band-structure evaluations is accomplished by a combination of a fast mixed variational eigenvalue solver and distributed Graphic Processing Unit (GPU) parallel computations. The TO algorithm utilizes the material distribution-based approach and a gradient-based optimizer. The design sensitivity for the mixed variational eigenvalue problem is derived using the adjoint method and is implemented through highly efficient vectorization techniques. We present optimized results for two-material simple cubic (SC), body centered cubic (BCC), and face centered cubic (FCC) crystal structures and show that in each of these cases different initial designs converge to single inclusion network topologies within their corresponding primitive cells. The optimized results show that large phononic stop bands for bulk wave propagation can be achieved at lower than close packed spherical configurations leading to lighter unit cells. For tungsten carbide - epoxy crystals we identify all angle all mode normalized stop bands exceeding 100%, which is larger than what is possible with only spherical inclusions.
CO2 Infrared Phonon Modes in Interstellar Ice Mixtures
NASA Astrophysics Data System (ADS)
Cooke, Ilsa R.; Fayolle, Edith C.; Öberg, Karin I.
2016-11-01
CO2 ice is an important reservoir of carbon and oxygen in star- and planet-forming regions. Together with water and CO, CO2 sets the physical and chemical characteristics of interstellar icy grain mantles, including desorption and diffusion energies for other ice constituents. A detailed understanding of CO2 ice spectroscopy is a prerequisite to characterize CO2 interactions with other volatiles both in interstellar ices and in laboratory experiments of interstellar ice analogs. We report laboratory spectra of the CO2 longitudinal optical (LO) phonon mode in pure CO2 ice and in CO2 ice mixtures with H2O, CO, and O2 components. We show that the LO phonon mode position is sensitive to the mixing ratio of various ice components of astronomical interest. In the era of the James Webb Space Telescope, this characteristic could be used to constrain interstellar ice compositions and morphologies. More immediately, LO phonon mode spectroscopy provides a sensitive probe of ice mixing in the laboratory and should thus enable diffusion measurements with higher precision than has been previously possible.
Phonon impedance matching: minimizing interfacial thermal resistance of thin films
NASA Astrophysics Data System (ADS)
Polanco, Carlos; Zhang, Jingjie; Ghosh, Avik
2014-03-01
The challenge to minimize interfacial thermal resistance is to allow a broad band spectrum of phonons, with non-linear dispersion and well defined translational and rotational symmetries, to cross the interface. We explain how to minimize this resistance using a frequency dependent broadening matrix that generalizes the notion of acoustic impedance to the whole phonon spectrum including symmetries. We show how to ``match'' two given materials by joining them with a single atomic layer, with a multilayer material and with a graded superlattice. Atomic layer ``matching'' requires a layer with a mass close to the arithmetic mean (or spring constant close to the harmonic mean) to favor high frequency phonon transmission. For multilayer ``matching,'' we want a material with a broadening close to the geometric mean to maximize transmission peaks. For graded superlattices, a continuous sequence of geometric means translates to an exponentially varying broadening that generates a wide-band antireflection coating for both the coherent and incoherent limits. Our results are supported by ``first principles'' calculations of thermal conductance for GaAs / Gax Al1 - x As / AlAs thin films using the Non-Equilibrium Greens Function formalism coupled with Density Functional Perturbation Theory. NSF-CAREER (QMHP 1028883), NSF-IDR (CBET 1134311), XSEDE.
Electron-phonon mediated heat flow in disordered graphene
NASA Astrophysics Data System (ADS)
Chen, Wei; Clerk, Aashish A.
2012-09-01
We calculate the heat flux and electron-phonon thermal conductance in a disordered graphene sheet, going beyond a Fermi’s golden rule approach to fully account for the modification of the electron-phonon interaction by disorder. Using the Keldysh technique combined with standard impurity averaging methods in the regime kFl≫1 (where kF is the Fermi wave vector and l is the mean free path), we consider both scalar potential (i.e., deformation potential) and vector-potential couplings between electrons and phonons. We also consider the effects of electronic screening at the Thomas-Fermi level. We find that the temperature dependence of the heat flux and thermal conductance is sensitive to the presence of disorder and screening, and reflects the underlying chiral nature of electrons in graphene and the corresponding modification of their diffusive behavior. In the case of weak screening, disorder enhances the low-temperature heat flux over the clean system (changing the associated power law from T4 to T3), and the deformation potential dominates. For strong screening, both the deformation potential and vector-potential couplings make comparable contributions, and the low-temperature heat flux obeys a T5 power law.
Tunable phonon-cavity coupling in graphene membranes
NASA Astrophysics Data System (ADS)
de Alba, R.; Massel, F.; Storch, I. R.; Abhilash, T. S.; Hui, A.; McEuen, P. L.; Craighead, H. G.; Parpia, J. M.
2016-09-01
A major achievement of the past decade has been the realization of macroscopic quantum systems by exploiting the interactions between optical cavities and mechanical resonators. In these systems, phonons are coherently annihilated or created in exchange for photons. Similar phenomena have recently been observed through phonon-cavity coupling—energy exchange between the modes of a single system mediated by intrinsic material nonlinearity. This has so far been demonstrated primarily for bulk crystalline, high-quality-factor (Q > 105) mechanical systems operated at cryogenic temperatures. Here, we propose graphene as an ideal candidate for the study of such nonlinear mechanics. The large elastic modulus of this material and capability for spatial symmetry breaking via electrostatic forces is expected to generate a wealth of nonlinear phenomena, including tunable intermodal coupling. We have fabricated circular graphene membranes and report strong phonon-cavity effects at room temperature, despite the modest Q factor (∼100) of this system. We observe both amplification into parametric instability (mechanical lasing) and the cooling of Brownian motion in the fundamental mode through excitation of cavity sidebands. Furthermore, we characterize the quenching of these parametric effects at large vibrational amplitudes, offering a window on the all-mechanical analogue of cavity optomechanics, where the observation of such effects has proven elusive.
Electrons and phonons in amorphous semiconductors
NASA Astrophysics Data System (ADS)
Prasai, Kiran; Biswas, Parthapratim; Drabold, D. A.
2016-07-01
The coupling between lattice vibrations and electrons is one of the central concepts of condensed matter physics. The subject has been deeply studied for crystalline materials, but far less so for amorphous and glassy materials, which are among the most important for applications. In this paper, we explore the electron-lattice coupling using current tools of a first-principles computer simulation. We choose three materials to illustrate the phenomena: amorphous silicon (a-Si), amorphous selenium (a-Se) and amorphous gallium nitride (a-GaN). In each case, we show that there is a strong correlation between the localization of electron states and the magnitude of thermally induced fluctuations in energy eigenvalues obtained from the density-functional theory (i.e. Kohn-Sham eigenvalues). We provide a heuristic theory to explain these observations. The case of a-GaN, a topologically disordered partly ionic insulator, is distinctive compared to the covalent amorphous examples. Next, we explore the consequences of changing the charge state of a system as a proxy for tracking photo-induced structural changes in the materials. Where transport is concerned, we lend insight into the Meyer-Neldel compensation rule and discuss a thermally averaged Kubo-Greenwood formula as a means to estimate electrical conductivity and especially its temperature dependence. We close by showing how the optical gap of an amorphous semiconductor can be computationally engineered with the judicious use of Hellmann-Feynman forces (associated with a few defect states) using molecular dynamics simulations. These forces can be used to close or open an optical gap, and identify a structure with a prescribed gap. We use the approach with plane-wave density functional methods to identify a low-energy amorphous phase of silicon including several coordination defects, yet with a gap close to that of good quality a-Si models.
Thermal transport across metal–insulator interface via electron–phonon interaction.
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.
Temperature dependence of Brillouin light scattering spectra of acoustic phonons in silicon
Olsson, Kevin S.; Klimovich, Nikita; An, Kyongmo; Sullivan, Sean; Weathers, Annie; Shi, Li E-mail: elaineli@physics.utexas.edu; Li, Xiaoqin E-mail: elaineli@physics.utexas.edu
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 order to evaluate their potential use as temperature sensors for acoustic phonons.
Optical Generation and Detection of Local Nonequilibrium Phonons in Suspended Graphene
NASA Astrophysics Data System (ADS)
Sullivan, Sean; Vallabhaneni, Ajit; Kholmanov, Iskandar; Ruan, Xiulin; Murthy, Jayathi; Shi, Li
2017-03-01
The measured frequencies and intensities of different first- and second- order Raman peaks of suspended graphene are used to show that optical phonons and different acoustic phonon polarizations are driven out of local equilibrium inside a sub-micron laser spot. The experimental results are correlated with a first principles-based multiple temperature model to suggest a considerably lower equivalent local temperature of the flexural phonons than those of other phonon polarizations. The finding reveals weak coupling between the flexural modes with hot electrons and optical phonons. Since the ultrahigh intrinsic thermal conductivity of graphene has been largely attributed to contributions from the flexural phonons, the observed local non-equilibrium phenomena have important implications for understanding energy dissipation processes in graphene-based electronic and optoelectronic devices, as well as in Raman measurements of thermal transport in graphene and other two-dimensional materials.
Sadhu, Jyothi; Tian, Hongxiang; Ma, Jun; Azeredo, Bruno; Kim, Junhwan; Balasundaram, Karthik; Zhang, Chen; Li, Xiuling; Ferreira, P M; Sinha, S
2015-05-13
Existing theory and data cannot quantify the contribution of phonon drag to the Seebeck coefficient (S) in semiconductors at room temperature. We show that this is possible through comparative measurements between nanowires and the bulk. Phonon boundary scattering completely quenches phonon drag in silicon nanowires enabling quantification of its contribution to S in bulk silicon in the range 25-500 K. The contribution is surprisingly large (∼34%) at 300 K even at doping of ∼3 × 10(19) cm(-3). Our results contradict the notion that phonon drag is negligible in degenerate semiconductors at temperatures relevant for thermoelectric energy conversion. A revised theory of electron-phonon momentum exchange that accounts for a phonon mean free path spectrum agrees well with the data.
Khurgin, Jacob B.
2014-06-02
Laser cooling of semiconductors has been an elusive goal for many years, and while attempts to cool the narrow gap semiconductors such as GaAs are yet to succeed, recently, net cooling has been attained in a wider gap CdS. This raises the question of whether wider gap semiconductors with higher phonon energies and stronger electron-phonon coupling are better suitable for laser cooling. In this work, we develop a straightforward theory of phonon-assisted absorption and photoluminescence of semiconductors that involves more than one phonon and use to examine wide gap materials, such as GaN and CdS and compare them with GaAs. The results indicate that while strong electron-phonon coupling in both GaN and CdS definitely improves the prospects of laser cooling, large phonon energy in GaN may be a limitation, which makes CdS a better prospect for laser cooling.
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.
Zhang, Yu; Yam, ChiYung; Chen, GuanHua
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 be 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.
Raman selection rule for surface optical phonons in ZnS nanobelts
NASA Astrophysics Data System (ADS)
Ho, Chih-Hsiang; Varadhan, Purushothaman; Wang, Hsin-Hua; Chen, Cheng-Ying; Fang, Xiaosheng; He-Hau, Jr.
2016-03-01
We report Raman scattering results for high-quality wurtzite ZnS nanobelts (NBs) grown by chemical vapor deposition. In the Raman spectrum, the ensembles of ZnS NBs exhibit first order phonon modes at 274 cm-1 and 350 cm-1, corresponding to A1/E1 transverse optical and A1/E1 longitudinal optical phonons, in addition to a strong surface optical (SO) phonon mode at 329 cm-1. The existence of the SO band is confirmed by its shift with different surrounding dielectric media. Polarization dependent Raman spectra were recorded on a single ZnS NB and for the first time a SO phonon band has been detected on a single nanobelt. Different selection rules for the SO phonon mode are shown from their corresponding E1/A1 phonon modes, and were attributed to the breaking of anisotropic translational symmetry on the NB surface.
Interaction Induced Electron Self-Interference in a Semiconductor: The Phonon Staircase Effect
Kenrow, J.A.; El Sayed, K.; Stanton, C.J.
1997-06-01
The exact quantum mechanics of a model semiconductor system of an electron with a discrete and equidistant energy spectrum interacting with a single phonon mode is presented. An electron initially excited into a coherent superposition of states interacts with phonons thereby creating a self-interference {ital in time} which reduces the emission of phonons to isolated bursts. This self-interference effect gives rise to steplike behavior in the relaxation kinetics of the electron and phonons. We show that this {open_quotes}phonon staircase{close_quotes} effect is a consequence of a correlated initial electron distribution and the violation of energy conservation in the electron-phonon interaction on short time scales. {copyright} {ital 1997} {ital The American Physical Society}
Electron-Phonon Coupling and its implication for the superconducting topological insulators
Zhang, Xiao-Long; Liu, Wu-Ming
2015-01-01
The recent observation of superconductivity in doped topological insulators has sparked a flurry of interest due to the prospect of realizing the long-sought topological superconductors. Yet the understanding of underlying pairing mechanism in these systems is far from complete. Here we investigate this problem by providing robust first-principles calculations of the role of electron-phonon coupling for the superconducting pairing in the prime candidate CuxBi2Se3. Our results show that electron-phonon scattering process in this system is dominated by zone center and boundary optical modes, with coexistence of phonon stiffening and softening. While the calculated electron-phonon coupling constant λ suggests that Tc from electron-phonon coupling is 2 orders smaller than the ones reported on bulk inhomogeneous samples, suggesting that superconductivity may not come from pure electron-phonon coupling. We discuss the possible enhancement of superconducting transition temperature by local inhomogeneity introduced by doping. PMID:25753813
Optical studies of terahertz phonons dynamics in small-grain polycrystalline corundum
NASA Astrophysics Data System (ADS)
Feofilov, S. P.; Kaplyanskii, A. A.; Kulinkin, A. B.; Zakharchenya, R. I.
1999-03-01
The dynamics of terahertz acoustic phonons generated by optical pumping in ceramic-like α-Al 2O 3 with grain size ∼100 nm produced with the help of sol-gel technology was studied with the technique of optical detection of phonons by observation of probe Cr 3+ and Mn 4+ ions fluorescence. The dynamics of phonon distribution is very slow (∼ms) and drastically differs from that in regular α-Al 2O 3 ceramics with micron grain size and is similar to that observed earlier in oxide glasses. The results are discussed in the framework of studies of phonons in different structured and spatially restricted Al 2O 3 materials with different ratios between phonon wavelength λ and crystallite size a. It is shown that the acoustic mismatch model which describes phonon scattering in regular ceramics is not valid for small-grain sol-gel produced ceramics-like material.
Strong Electron-Phonon Coupling Superconductivity Induced by a Low-Lying Phonon in IrGe
Hirai, Daigorou; Ali, Mazhar N.; Cava, Robert J.
2014-02-26
The physical properties of the previously reported superconductor IrGe and the Rh_{1-x}Ir_{x}Ge solid solution are investigated. IrGe has an exceptionally high superconducting transition temperature (T_{c}=4.7 K) among the isostructural 1:1 late-metal germanides MGe (M=Rh, Pd, Ir, and Pt). Specific-heat measurements reveal that IrGe has an anomalously low Debye temperature, originating from a low-lying phonon, compared to the other MGe phases. A large jump at T_{c} in the specific-heat data clearly indicates that IrGe is a strong coupling superconductor. In the Rh_{1-x}Ir_{x}Ge solid solution, a relationship between an anomalous change in lattice constants and the Debye temperature is observed. We conclude that the unusually high T_{c} for IrGe is likely due to strong electron–phonon coupling derived from the presence of a low-lying phonon.
Work measurement in a quantum heat engine
NASA Astrophysics Data System (ADS)
Bariani, Francesco; Zhang, Keye; Dong, Ying; Meystre, Pierre
2015-05-01
We consider an optomechanical quantum heat engine operating on an Otto cycle for photon-phonon polaritons, the working substance of the engine. We discuss both the average value and quantum fluctuations of its work output, concentrating in particular on the effects of quantum non-adiabaticity due to the finite duration of the cycle. We also determine the quantum back-action of both absorptive and dispersive continuous measurements of the work, and quantify their impact on the Curzon-Ahlborn engine efficiency at maximum power and its fluctuations. We ackowledge financial support from National Basic Research Program of China, NSF, ARO and the DARPA QuaSAR programs
Anharmonic effects in light scattering due to optical phonons in silicon
NASA Astrophysics Data System (ADS)
Balkanski, M.; Wallis, R. F.; Haro, E.
1983-08-01
Systematic measurements by light scattering of the linewidth and frequency shift of the q-->=0 optical phonon in silicon over the temperature range of 5-1400 K are presented. Both the linewidth and frequency shift exhibit a quadratic dependence on temperature at high temperatures. This indicates the necessity of including terms in the phonon proper self-energy corresponding to four-phonon anharmonic processes.