Coherent acoustic phonons in nanostructures
NASA Astrophysics Data System (ADS)
Dekorsy, T.; Taubert, R.; Hudert, F.; Bartels, A.; Habenicht, A.; Merkt, F.; Leiderer, P.; Köhler, K.; Schmitz, J.; Wagner, J.
2008-02-01
Phonons are considered as a most important origin of scattering and dissipation for electronic coherence in nanostructures. The generation of coherent acoustic phonons with femtosecond laser pulses opens the possibility to control phonon dynamics in amplitude and phase. We demonstrate a new experimental technique based on two synchronized femtosecond lasers with GHz repetition rate to study the dynamics of coherently generated acoustic phonons in semiconductor heterostructures with high sensitivity. High-speed synchronous optical sampling (ASOPS) enables to scan a time-delay of 1 ns with 100 fs time resolution with a frequency in the kHz range without a moving part in the set-up. We investigate the dynamics of coherent zone-folded acoustic phonons in semiconductor superlattices (GaAs/AlAs and GaSb/InAs) and of coherent vibration of metallic nanostructures of non-spherical shape using ASOPS.
Uniaxial strain-induced Kohn anomaly and electron-phonon coupling in acoustic phonons of graphene
NASA Astrophysics Data System (ADS)
Cifuentes-Quintal, M. E.; de la Peña-Seaman, O.; Heid, R.; de Coss, R.; Bohnen, K.-P.
2016-08-01
Recent advances in strain engineering at the nanoscale have shown the feasibility to modulate the properties of graphene. Although the electron-phonon (e-ph) coupling and Kohn anomalies in graphene define the phonon branches contributing to the resonance Raman scattering and are relevant to the electronic and thermal transport as a scattering source, the evolution of the e-ph coupling as a function of strain has been less studied. In this work, the Kohn anomalies and the e-ph coupling in uniaxially strained graphene along armchair and zigzag directions were studied by means of density functional perturbation theory calculations. In addition to the phonon anomaly at the transversal optical (TO) phonon branch in the K point for pristine graphene, we found that uniaxial strain induces a discontinuity in the frequency derivative of the longitudinal acoustic phonon branch. This behavior corresponds to the emergence of a Kohn anomaly, as a consequence of a strain-enhanced e-ph coupling. Thus, the present results for uniaxially strained graphene contrast with the commonly assumed view that the e-ph coupling around the K point is only present in the TO phonon branch.
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.
Phonon Emission from Acoustic Black Hole
NASA Astrophysics Data System (ADS)
Fang, Hengzhong; Zhou, Kaihu; Song, Yuming
2012-08-01
We study the phonon tunneling through the horizon of an acoustic black hole by solving the Hamilton-Jacobi equation. We also make use of the closed-path integral to calculate the tunneling probability, and an improved way to determine the temporal contribution is used. Both the results from the two methods agree with Hawking's initial analysis.
Dispersion of Acoustic Phonons in Quasiperiodic Superlattices
NASA Astrophysics Data System (ADS)
Mishra, R. K.; Misra, K. D.; Tiwari, R. P.
The aim of this work is to present an up-to-date study of acoustic phonon excitations that can propagate in multilayered structure with constituents arranged in quasiperiodic fashion. In this paper, the dispersion relation of acoustic phonons for the quasiperiodic superlattice using different semiconducting materials, with the help of transfer matrix method, is derived at normal angle of incidence. Calculation is presented for (a) Ge/Si and (b) Nb/Cu semiconductor superlattices from 5th to 9th generations and dispersion diagrams are plotted using the famous Kronning-Penny model obtained from the transfer matrix of the structure. The concept of allowed and forbidden bands with the help of these dispersion curves in various generations of Fibonacci superlattices and the relation between imaginary value of propagation vector and the existence of forbidden bands is demonstrated.
Chen, Xin; Parker, David S; Singh, David J
2013-01-01
We present first principles calculations of the phonon dispersions of \\BiTe and discuss these in relation to the acoustic phonon interface scattering in ceramics. The phonon dispersions show agreement with what is known from neutron scattering for the optic modes. We find a difference between the generalized gradient approximation and local density results for the acoustic branches. This is a consequence of an artificial compression of the van der Waals bonded gaps in the \\BiTe structure when using the generalized gradient approximation. As a result local density approximation calculations provide a better description of the phonon dispersions in Bi$_{2}$Te$_{3}$. A key characteristic of the acoustic dispersions is the existence of a strong anisotropy in the velocities. We develop a model for interface scattering in ceramics with acoustic wave anisotropy and apply this to \\BiTe and compare with PbTe and diamond.
Yudistira, D; Boes, A; Djafari-Rouhani, B; Pennec, Y; Yeo, L Y; Mitchell, A; Friend, J R
2014-11-21
We theoretically and experimentally demonstrate the existence of complete surface acoustic wave band gaps in surface phonon-polariton phononic crystals, in a completely monolithic structure formed from a two-dimensional honeycomb array of hexagonal shape domain-inverted inclusions in single crystal piezoelectric Z-cut lithium niobate. The band gaps appear at a frequency of about twice the Bragg band gap at the center of the Brillouin zone, formed through phonon-polariton coupling. The structure is mechanically, electromagnetically, and topographically homogeneous, without any physical alteration of the surface, offering an ideal platform for many acoustic wave applications for photonics, phononics, and microfluidics.
Acoustic wave characterization of silicon phononic crystal plate
NASA Astrophysics Data System (ADS)
Feng, Duan; Jiang, Wanli; Xu, Dehui; Xiong, Bin; Wang, Yuelin
2015-08-01
In this paper, characterization of megahertz Lamb waves in a silicon phononic crystal based asymmetry filter by laser Doppler vibrometer is demonstrated. The acoustic power from a piezoelectric substrate was transmitted into the silicon superstrate by fluid coupling method, and measured results show that the displacement amplitude of the acoustic wave in the superstrate was approximately one fifth of that in the piezoelectric substrate. Effect of the phononic bandgap on the propagation of Lamb wave in the silicon superstrate is also measured, and the result shows that the phononic crystal structure could reflect part of the acoustic waves back.
NASA Astrophysics Data System (ADS)
Pop, Eric; Dutton, Robert W.; Goodson, Kenneth E.
2004-11-01
We describe the implementation of a Monte Carlo model for electron transport in silicon. The model uses analytic, nonparabolic electron energy bands, which are computationally efficient and sufficiently accurate for future low-voltage (<1V) nanoscale device applications. The electron-lattice scattering is incorporated using an isotropic, analytic phonon-dispersion model, which distinguishes between the optical/acoustic and the longitudinal/transverse phonon branches. We show that this approach avoids introducing unphysical thresholds in the electron distribution function, and that it has further applications in computing detailed phonon generation spectra from Joule heating. A set of deformation potentials for electron-phonon scattering is introduced and shown to yield accurate transport simulations in bulk silicon across a wide range of electric fields and temperatures. The shear deformation potential is empirically determined at Ξu=6.8eV, and consequently, the isotropically averaged scattering potentials with longitudinal and transverse acoustic phonons are DLA=6.39eV and DTA=3.01eV, respectively, in reasonable agreement with previous studies. The room-temperature electron mobility in strained silicon is also computed and shown to be in better agreement with the most recent phonon-limited data available. As a result, we find that electron coupling with g-type phonons is about 40% lower, and the coupling with f-type phonons is almost twice as strong as previously reported.
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.
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.
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, 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 Bloch oscillations in a two-dimensional phononic crystal.
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.
Electrical modulation and switching of transverse acoustic phonons
NASA Astrophysics Data System (ADS)
Jeong, H.; Jho, Y. D.; Rhim, S. H.; Yee, K. J.; Yoon, S. Y.; Shim, J. P.; Lee, D. S.; Ju, J. W.; Baek, J. H.; Stanton, C. J.
2016-07-01
We report on the electrical manipulation of coherent acoustic phonon waves in GaN-based nanoscale piezoelectric heterostructures which are strained both from the pseudomorphic growth at the interfaces as well as through external electric fields. In such structures, transverse symmetry within the c plane hinders both the generation and detection of the transverse acoustic (TA) modes, and usually only longitudinal acoustic phonons are generated by ultrafast displacive screening of potential gradients. We show that even for c -GaN, the combined application of lateral and vertical electric fields can not only switch on the normally forbidden TA mode, but they can also modulate the amplitudes and frequencies of both modes. By comparing the transient differential reflectivity spectra in structures with and without an asymmetric potential distribution, the role of the electrical controllability of phonons was demonstrated as changes to the propagation velocities, the optical birefringence, the electrically polarized TA waves, and the geometrically varying optical sensitivities of phonons.
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.
NASA Astrophysics Data System (ADS)
Sapega, V. F.; Belitsky, V. I.; Ruf, T.; Fuchs, H. D.; Cardona, M.; Ploog, K.
1992-12-01
A strong increase of low-frequency Raman scattering has been observed in GaAs/AlxGa1-xAs multiple quantum wells in magnetic fields up to 14 T. The spectra, consisting of background scattering, folded acoustic phonons, and additional features, show resonant behavior with respect to the laser frequency and the strength of the magnetic field. The broad background, usually related to geminate recombination, has its origin in a continuum of Raman processes with the emission of longitudinal-acoustic phonons where crystal momentum is not conserved. Such processes can become dominant when interface fluctuations allow for resonant scattering in individual quantum wells only. Thus phonons with all possible energies contribute to the background scattering efficiency. The observed folded longitudinal-acoustic phonons are in good agreement with calculated frequencies. Additional features, detected in all samples measured, are attributed to local vibrational modes tied to the gaps at the folded Brillouin-zone center and edge. Other peculiarities observed correspond to modes localized at crossings of the folded longitudinal- and transverse-acoustic branches inside the Brillouin zone. The appearance of these local modes is attributed to fluctuations in the well and barrier thicknesses of the quantum wells.
Bloch oscillations of THz acoustic phonons in coupled nanocavity structures.
Lanzillotti-Kimura, N D; Fainstein, A; Perrin, B; Jusserand, B; Mauguin, O; Largeau, L; Lemaître, A
2010-05-14
Nanophononic Bloch oscillations and Wannier-Stark ladders have been recently predicted to exist in specifically tailored structures formed by coupled nanocavities. Using pump-probe coherent phonon generation techniques we demonstrate that Bloch oscillations of terahertz acoustic phonons can be directly generated and probed in these complex nanostructures. In addition, by Fourier transforming the time traces we had access to the proper eigenmodes in the frequency domain, thus evidencing the related Wannier-Stark ladder. The observed Bloch oscillation dynamics are compared with simulations based on a model description of the coherent phonon generation and photoelastic detection processes.
Resonant raman scattering and dispersion of polar optical and acoustic phonons in hexagonal inn
Davydov, V. Yu. Klochikhin, A. A.; Smirnov, A. N.; Strashkova, I. Yu.; Krylov, A. S.; Lu Hai; Schaff, William J.; Lee, H.-M.; Hong, Y.-L.; Gwo, S.
2010-02-15
It is shown that a study of the dependence of impurity-related resonant first-order Raman scattering on the frequency of excitation light makes it possible to observe the dispersion of polar optical and acoustic branches of vibrational spectrum in hexagonal InN within a wide range of wave vectors. It is established that the wave vectors of excited phonons are uniquely related to the energy of excitation photon. Frequencies of longitudinal optical phonons E{sub 1}(LO) and A{sub 1}(LO) in hexagonal InN were measured in the range of excitation-photon energies from 2.81 to 1.17 eV and the frequencies of longitudinal acoustic phonons were measured in the range 2.81-1.83 eV of excitation-photon energies. The obtained dependences made it possible to extrapolate the dispersion of phonons A{sub 1}(LO) and E{sub 1}(LO) to as far as the point {Gamma} in the Brillouin zone and estimate the center-band energies of these phonons (these energies have not been uniquely determined so far).
Generation mechanism of terahertz coherent acoustic phonons in Fe
NASA Astrophysics Data System (ADS)
Henighan, T.; Trigo, M.; Bonetti, S.; Granitzka, P.; Higley, D.; Chen, Z.; Jiang, M. P.; Kukreja, R.; Gray, A.; Reid, A. H.; Jal, E.; Hoffmann, M. C.; Kozina, M.; Song, S.; Chollet, M.; Zhu, D.; Xu, P. F.; Jeong, J.; Carva, K.; Maldonado, P.; Oppeneer, P. M.; Samant, M. G.; Parkin, S. S. P.; Reis, D. A.; Dürr, H. A.
2016-06-01
We use femtosecond time-resolved hard x-ray scattering to detect coherent acoustic phonons generated during ultrafast laser excitation of ferromagnetic bcc Fe films grown on MgO(001). We observe the coherent longitudinal-acoustic phonons as a function of wave vector through analysis of the temporal oscillations in the x-ray scattering signal. The width of the extracted strain wave front associated with this coherent motion is ˜100 fs. An effective electronic Grüneisen parameter is extracted within a two-temperature model. However, ab initio calculations show that the phonons are nonthermal on the time scale of the experiment, which calls into question the validity of extracting physical constants by fitting such a two-temperature model.
Direct acoustic phonon excitation by intense and ultrashort terahertz pulses
NASA Astrophysics Data System (ADS)
Manceau, J.-M.; Loukakos, P. A.; Tzortzakis, S.
2010-12-01
We report on the direct and resonant excitation of acoustic phonons in an AlGaAs intrinsic semiconductor using intense coherent and single cycle terahertz pulses created by two-color femtosecond laser pulse filamentation in air. While the electrons are left unperturbed, we follow the lattice dynamics with time-delayed optical photons tuned to the interband transition.
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.
Interface acoustic waves at the interface between two semi-infinite phononic crystals
NASA Astrophysics Data System (ADS)
Tanaka, Yukihiro; Okashiwa, Nobuharu; Nishiguchi, Norihiko
2016-10-01
We have investigated the band structures of interface acoustic waves (IAWs) in a system composed of two dissimilar semi-infinite two-dimensional (2D) phononic crystals (PCs), which is referred to as a dual 2D PC system. We suggest a method by which we can estimate roughly the constituent elements (for example, filling fraction and type of substance) of each PC in a dual 2D PC system. We find that, for a specific set of filling fractions of the constituent PCs, an IAW branch exists below the lowest bulk transverse-wave branch in dispersion relations, and is subject to Bragg reflection in the vicinity of the Brillouin-zone boundary, which reduces the group velocity of IAWs. The findings of this work suggest the possibility of new acoustic devices utilizing IAWs.
Nonlinear propagation and control of acoustic waves in phononic superlattices
NASA Astrophysics Data System (ADS)
Jiménez, Noé; Mehrem, Ahmed; Picó, Rubén; García-Raffi, Lluís M.; Sánchez-Morcillo, Víctor J.
2016-05-01
The propagation of intense acoustic waves in a one-dimensional phononic crystal is studied. The medium consists in a structured fluid, formed by a periodic array of fluid layers with alternating linear acoustic properties and quadratic nonlinearity coefficient. The spacing between layers is of the order of the wavelength, therefore Bragg effects such as band gaps appear. We show that the interplay between strong dispersion and nonlinearity leads to new scenarios of wave propagation. The classical waveform distortion process typical of intense acoustic waves in homogeneous media can be strongly altered when nonlinearly generated harmonics lie inside or close to band gaps. This allows the possibility of engineer a medium in order to get a particular waveform. Examples of this include the design of media with effective (e.g., cubic) nonlinearities, or extremely linear media (where distortion can be canceled). The presented ideas open a way towards the control of acoustic wave propagation in nonlinear regime. xml:lang="fr"
Femtosecond optical excitation of coherent acoustic phonons in a piezoelectric p-n junction
NASA Astrophysics Data System (ADS)
Wen, Yu-Chieh; Chern, Gia-Wei; Lin, Kung-Hsuan; Yeh, Jeffrey Jarren; Sun, Chi-Kuang
2011-11-01
We present a theoretical model for the photogeneration of coherent acoustic phonons in a piezoelectric p-n junction. In our model, the transport of photoexcited carriers is governed by the drift-diffusion equation, whereas the dynamics of acoustic phonons obeys a loaded string equation. Among various mechanisms, the piezoelectric coupling is found to dominate the acoustic-phonon generation process. The waveform of the photogenerated acoustic pulse is strongly influenced by the various dynamics of the photoexcited carriers, especially the picosecond hole drifting. Our calculation also confirms the crucial role of the built-in electric field in the formation of coherent acoustic phonons under optical excitations.
Acoustic waves switch based on meta-fluid phononic crystals
NASA Astrophysics Data System (ADS)
Zhu, Xue-Feng
2012-08-01
The acoustic waves switch based on meta-fluid phononic crystals (MEFL PCs) is theoretically investigated. The MEFL PCs consist of fluid matrix and fluid-like inclusions with extremely anisotropic-density. The dispersion relations are calculated via the plane wave expansion method, which are in good agreement with the transmitted sound pressure level spectra obtained by the finite element method. The results show that the width of absolute band gap in MEFL PCs depends sensitively upon the orientation of the extremely anisotropic-density inclusions and reaches maximum at the rotating angle of 45°, with the gap position nearly unchanged. Also, the inter-mode conversion inside anisotropic-density inclusions can be ignored due to large acoustic mismatch. The study gives a possibility to realize greater flexibility and stronger effects in tuning the acoustic band gaps, which is very significant in the enhanced control over sound waves and has potential applications in ultrasonic imaging and therapy.
Piezoelectric surface acoustical phonon amplification in graphene on a GaAs substrate
NASA Astrophysics Data System (ADS)
Nunes, O. A. C.
2014-06-01
We study the interaction of Dirac Fermions in monolayer graphene on a GaAs substrate in an applied electric field by the combined action of the extrinsic potential of piezoelectric surface acoustical phonons of GaAs (piezoelectric acoustical (PA)) and of the intrinsic deformation potential of acoustical phonons in graphene (deformation acoustical (DA)). We find that provided the dc field exceeds a threshold value, emission of piezoelectric (PA) and deformation (DA) acoustical phonons can be obtained in a wide frequency range up to terahertz at low and high temperatures. We found that the phonon amplification rate RPA ,DA scales with TBGS -1 (S =PA,DA), TBGS being the Block -Gru¨neisen temperature. In the high-T Block -Gru¨neisen regime, extrinsic PA phonon scattering is suppressed by intrinsic DA phonon scattering, where the ratio RPA/RDA scales with ≈1/√n , n being the carrier concentration. We found that only for carrier concentration n ≤1010cm-2, RPA/RDA>1. In the low-T Block -Gru¨neisen regime, and for n =1010cm-2, the ratio RPA/RDA scales with TBGDA/TBGPA≈7.5 and RPA/RDA>1. In this regime, PA phonon dominates the electron scattering and RPA/RDA<1 otherwise. This study is relevant to the exploration of the acoustic properties of graphene and to the application of graphene as an acoustical phonon amplifier and a frequency-tunable acoustical phonon device.
Magnetophonon oscillations caused by acoustic phonons in bulk conductors
NASA Astrophysics Data System (ADS)
Raichev, O. E.
2016-09-01
The interaction of electrons with acoustic phonons under a magnetic field leads to a remarkable kind of magnetophonon oscillation of transport coefficients, recently discovered in two-dimensional electron systems. The present study shows that similar oscillations exist in bulk conductors and provides a theory of this phenomenon for the case of spherical Fermi surfaces. The resonance peaks occur when the product of the Fermi surface diameter by the sound velocity is a multiple of the cyclotron frequency. Theoretical predictions may facilitate the experimental observation of the phenomenon.
Numerical investigation of diffraction of acoustic waves by phononic crystals
NASA Astrophysics Data System (ADS)
Moiseyenko, Rayisa P.; Declercq, Nico F.; Laude, Vincent
2012-05-01
Diffraction as well as transmission of acoustic waves by two-dimensional phononic crystals (PCs) composed of steel rods in water are investigated in this paper. The finite element simulations were performed in order to compute pressure fields generated by a line source that are incident on a finite size PC. Such field maps are analyzed based on the complex band structure for the infinite periodic PC. Finite size computations indicate that the exponential decrease of the transmission at deaf frequencies is much stronger than that in Bragg band gaps.
Extremely low loss phonon-trapping cryogenic acoustic cavities for future physical experiments.
Galliou, Serge; Goryachev, Maxim; Bourquin, Roger; Abbé, Philippe; Aubry, Jean Pierre; Tobar, Michael E
2013-01-01
Low loss Bulk Acoustic Wave devices are considered from the point of view of the solid state approach as phonon-confining cavities. We demonstrate effective design of such acoustic cavities with phonon-trapping techniques exhibiting extremely high quality factors for trapped longitudinally-polarized phonons of various wavelengths. Quality factors of observed modes exceed 1 billion, with a maximum Q-factor of 8 billion and Q × f product of 1.6 · 10(18) at liquid helium temperatures. Such high sensitivities allow analysis of intrinsic material losses in resonant phonon systems. Various mechanisms of phonon losses are discussed and estimated.
Extremely Low Loss Phonon-Trapping Cryogenic Acoustic Cavities for Future Physical Experiments
Galliou, Serge; Goryachev, Maxim; Bourquin, Roger; Abbé, Philippe; Aubry, Jean Pierre; Tobar, Michael E.
2013-01-01
Low loss Bulk Acoustic Wave devices are considered from the point of view of the solid state approach as phonon-confining cavities. We demonstrate effective design of such acoustic cavities with phonon-trapping techniques exhibiting extremely high quality factors for trapped longitudinally-polarized phonons of various wavelengths. Quality factors of observed modes exceed 1 billion, with a maximum Q-factor of 8 billion and Q × f product of 1.6 · 1018 at liquid helium temperatures. Such high sensitivities allow analysis of intrinsic material losses in resonant phonon systems. Various mechanisms of phonon losses are discussed and estimated. PMID:23823569
Extremely low loss phonon-trapping cryogenic acoustic cavities for future physical experiments.
Galliou, Serge; Goryachev, Maxim; Bourquin, Roger; Abbé, Philippe; Aubry, Jean Pierre; Tobar, Michael E
2013-01-01
Low loss Bulk Acoustic Wave devices are considered from the point of view of the solid state approach as phonon-confining cavities. We demonstrate effective design of such acoustic cavities with phonon-trapping techniques exhibiting extremely high quality factors for trapped longitudinally-polarized phonons of various wavelengths. Quality factors of observed modes exceed 1 billion, with a maximum Q-factor of 8 billion and Q × f product of 1.6 · 10(18) at liquid helium temperatures. Such high sensitivities allow analysis of intrinsic material losses in resonant phonon systems. Various mechanisms of phonon losses are discussed and estimated. PMID:23823569
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.
NASA Astrophysics Data System (ADS)
Pokatilov, E. P.; Nika, D. L.; Balandin, A. A.
2005-09-01
We have theoretically studied acoustic phonon spectra and phonon propagation in rectangular nanowires embedded within elastically dissimilar materials. As example systems, we have considered GaN nanowires with AlN and plastic barrier layers. It has been established that the acoustically mismatched barriers dramatically influence the quantized phonon spectrum of the nanowires. The barriers with lower sound velocity compress the phonon energy spectrum and reduce the phonon group velocities in the nanowire. The barriers with higher sound velocity have an opposite effect. The physical origin of this effect is related to redistribution of the elastic deformations in the acoustically mismatched nanowires. In the case of the “acoustically slow” barriers, the elastic deformation waves are squeezed in the barrier layer. The effect predicted for the nanowires embedded with elastically dissimilar materials could be used for reengineering phonon spectrum in nanostructures.
Material and Phonon Engineering for Next Generation Acoustic Devices
NASA Astrophysics Data System (ADS)
Kuo, Nai-Kuei
This thesis presents the theoretical and experimental work related to micromachining of low intrinsic loss sapphire and phononic crystals for engineering new classes of electroacoustic devices for frequency control applications. For the first time, a low loss sapphire suspended membrane was fabricated and utilized to form the main body of a piezoelectric lateral overtone bulk acoustic resonator (LOBAR). Since the metalized piezoelectric transducer area in a LOBAR is only a small fraction of the overall resonant cavity (made out of sapphire), high quality factor (Q) overtones are attained. The experiment confirms the low intrinsic mechanical loss of the transferred sapphire thin film, and the resonators exhibit the highest Q of 5,440 at 2.8 GHz ( f·Q of 1.53.1013 Hz). This is also the highest f·Q demonstrated for aluminum-nitride-(AIN)-based Lamb wave devices to date. Beyond demonstrating a low loss device, this experimental work has laid the foundation for the future development of new micromechanical devices based on a high Q, high hardness and chemically resilient material. The search for alternative ways to more efficiently perform frequency control functionalities lead to the exploration of Phononic Crystal (PnC) structures in AIN thin films. Four unit cell designs were theoretically and experimentally investigated to explore the behavior of phononic bandgaps (PBGs) in the ultra high frequency (UHF) range: (i) the conventional square lattice with circular air scatterer, (ii) the inverse acoustic bandgap (IABG) structure, (iii) the fractal PnC, and (iv) the X-shaped PnC. Each unit cell has its unique frequency characteristic that was exploited to synthesize either cavity resonators or improve the performance of acoustic delay lines. The PBGs operate in the range of 770 MHz to 1 GHz and exhibit a maximum acoustic rejection of 40 dB. AIN Lamb wave transducers (LWTs) were employed for the experimental demonstration of the PBGs and cavity resonances. Ultra
Phononic thin plates with embedded acoustic black holes
NASA Astrophysics Data System (ADS)
Zhu, Hongfei; Semperlotti, Fabio
2015-03-01
We introduce a class of two-dimensional nonresonant single-phase phononic materials and investigate its peculiar dispersion characteristics. The material consists of a thin platelike structure with an embedded periodic lattice of acoustic black holes. The use of these periodic tapers allows achieving remarkable dispersion properties such as zero group velocity in the fundamental modes, negative group refraction index, birefraction, Dirac-like cones, and mode anisotropy. The dispersion properties are numerically investigated using both a three-dimensional supercell plane wave expansion method and finite element analysis. The effect on the dispersion and propagation characteristics of key geometric parameters of the black hole, such as the taper profile and the residual thickness, are also explored.
Acoustic cloaking by a near-zero-index phononic crystal
NASA Astrophysics Data System (ADS)
Zheng, Li-Yang; Wu, Ying; Ni, Xu; Chen, Ze-Guo; Lu, Ming-Hui; Chen, Yan-Feng
2014-04-01
Zero-refractive-index materials may lead to promising applications in various fields. Here, we design and fabricate a near Zero-Refractive-Index (ZRI) material using a phononic crystal (PC) composed of a square array of densely packed square iron rods in air. The dispersion relation exhibits a nearly flat band across the Brillouin zone at the reduced frequency f = 0.5443c/a, which is due to Fabry-Perot (FP) resonance. By using a retrieval method, we find that both the effective mass density and the reciprocal of the effective bulk modulus are close to zero at frequencies near the flat band. We also propose an equivalent tube network model to explain the mechanisms of the near ZRI effect. This FP-resonance-induced near ZRI material offers intriguing wave manipulation properties. We demonstrate both numerically and experimentally its ability to shield a scattering obstacle and guide acoustic waves through a bent structure.
Piezoelectric surface acoustical phonon amplification in graphene on a GaAs substrate
Nunes, O. A. C.
2014-06-21
We study the interaction of Dirac Fermions in monolayer graphene on a GaAs substrate in an applied electric field by the combined action of the extrinsic potential of piezoelectric surface acoustical phonons of GaAs (piezoelectric acoustical (PA)) and of the intrinsic deformation potential of acoustical phonons in graphene (deformation acoustical (DA)). We find that provided the dc field exceeds a threshold value, emission of piezoelectric (PA) and deformation (DA) acoustical phonons can be obtained in a wide frequency range up to terahertz at low and high temperatures. We found that the phonon amplification rate R{sup PA,DA} scales with T{sub BG}{sup S−1} (S=PA,DA), T{sub BG}{sup S} being the Block−Gru{sup ¨}neisen temperature. In the high-T Block−Gru{sup ¨}neisen regime, extrinsic PA phonon scattering is suppressed by intrinsic DA phonon scattering, where the ratio R{sup PA}/R{sup DA} scales with ≈1/√(n), n being the carrier concentration. We found that only for carrier concentration n≤10{sup 10}cm{sup −2}, R{sup PA}/R{sup DA}>1. In the low-T Block−Gru{sup ¨}neisen regime, and for n=10{sup 10}cm{sup −2}, the ratio R{sup PA}/R{sup DA} scales with T{sub BG}{sup DA}/T{sub BG}{sup PA}≈7.5 and R{sup PA}/R{sup DA}>1. In this regime, PA phonon dominates the electron scattering and R{sup PA}/R{sup DA}<1 otherwise. This study is relevant to the exploration of the acoustic properties of graphene and to the application of graphene as an acoustical phonon amplifier and a frequency-tunable acoustical phonon device.
Acoustic scattering from phononic crystals with complex geometry.
Kulpe, Jason A; Sabra, Karim G; Leamy, Michael J
2016-05-01
This work introduces a formalism for computing external acoustic scattering from phononic crystals (PCs) with arbitrary exterior shape using a Bloch wave expansion technique coupled with the Helmholtz-Kirchhoff integral (HKI). Similar to a Kirchhoff approximation, a geometrically complex PC's surface is broken into a set of facets in which the scattering from each facet is calculated as if it was a semi-infinite plane interface in the short wavelength limit. When excited by incident radiation, these facets introduce wave modes into the interior of the PC. Incorporation of these modes in the HKI, summed over all facets, then determines the externally scattered acoustic field. In particular, for frequencies in a complete bandgap (the usual operating frequency regime of many PC-based devices and the requisite operating regime of the presented theory), no need exists to solve for internal reflections from oppositely facing edges and, thus, the total scattered field can be computed without the need to consider internal multiple scattering. Several numerical examples are provided to verify the presented approach. Both harmonic and transient results are considered for spherical and bean-shaped PCs, each containing over 100 000 inclusions. This facet formalism is validated by comparison to an existing self-consistent scattering technique.
NASA Astrophysics Data System (ADS)
Song, Ai-Ling; Chen, Tian-Ning; Wang, Xiao-Peng; Wan, Le-Le
2016-08-01
The waveform distortion happens in most of the unidirectional acoustic transmission (UAT) devices proposed before. In this paper, a novel type of waveform-preserved UAT device composed of an impedance-matched acoustic metasurface (AMS) and a phononic crystal (PC) structure is proposed and numerically investigated. The acoustic pressure field distributions and transmittance are calculated by using the finite element method. The subwavelength AMS that can modulate the wavefront of the transmitted wave at will is designed and the band structure of the PC structure is calculated and analyzed. The sound pressure field distributions demonstrate that the unidirectional acoustic transmission can be realized by the proposed UAT device without changing the waveforms of the output waves, which is the distinctive feature compared with the previous UAT devices. The physical mechanism of the unidirectional acoustic transmission is discussed by analyzing the refraction angle changes and partial band gap map. The calculated transmission spectra show that the UAT device is valid within a relatively broad frequency range. The simulation results agree well with the theoretical predictions. The proposed UAT device provides a good reference for designing waveform-preserved UAT devices and has potential applications in many fields, such as medical ultrasound, acoustic rectifiers, and noise insulation.
NASA Astrophysics Data System (ADS)
Wang, T.; Ke, M.; Qiu, C.; Liu, Z.
2016-06-01
We present the design for an acoustic system that can achieve particle trapping and transport using the acoustic force field above a phononic crystal plate. The phononic crystal plate comprised a thin brass plate with periodic slits alternately embedded with two kinds of elastic inclusions. Enhanced acoustic transmission and localized acoustic fields were achieved when the structure was excited by external acoustic waves. Because of the different resonant frequencies of the two elastic inclusions, the acoustic field could be controlled via the working frequency. Particles were transported between adjacent traps under the influence of the adjustable acoustic field. This device provides a new and versatile avenue for particle manipulation that would complement other means of particle manipulation.
Mante, Pierre-Adrien; Huang, Yu-Ru; Yang, Szu-Chi; Liu, Tzu-Ming; Maznev, Alexei A; Sheu, Jinn-Kong; Sun, Chi-Kuang
2015-02-01
Thanks to ultrafast acoustics, a better understanding of acoustic dynamics on a short time scale has been obtained and new characterization methods at the nanoscale have been developed. Among the materials that were studied during the development of ultrafast acoustics, nitride based heterostructures play a particular role due to their piezoelectric properties and the possibility to generate phonons with over-THz frequency and bandwidth. Here, we review some of the work performed using this type of structure, with a focus on THz phonon spectroscopy and nanoscopy. First, we present a brief description of the theory of coherent acoustic phonon generation by piezoelectric heterostructure. Then the first experimental observation of coherent acoustic phonon generated by the absorption of ultrashort light pulses in piezoelectric heterostructures is presented. From this starting point, we then present some methods developed to realize customizable phonon generation. Finally we review some more recent applications of these structures, including imaging with a nanometer resolution, broadband attenuation measurements with a frequency up to 1THz and phononic bandgap characterization.
Mante, Pierre-Adrien; Huang, Yu-Ru; Yang, Szu-Chi; Liu, Tzu-Ming; Maznev, Alexei A; Sheu, Jinn-Kong; Sun, Chi-Kuang
2015-02-01
Thanks to ultrafast acoustics, a better understanding of acoustic dynamics on a short time scale has been obtained and new characterization methods at the nanoscale have been developed. Among the materials that were studied during the development of ultrafast acoustics, nitride based heterostructures play a particular role due to their piezoelectric properties and the possibility to generate phonons with over-THz frequency and bandwidth. Here, we review some of the work performed using this type of structure, with a focus on THz phonon spectroscopy and nanoscopy. First, we present a brief description of the theory of coherent acoustic phonon generation by piezoelectric heterostructure. Then the first experimental observation of coherent acoustic phonon generated by the absorption of ultrashort light pulses in piezoelectric heterostructures is presented. From this starting point, we then present some methods developed to realize customizable phonon generation. Finally we review some more recent applications of these structures, including imaging with a nanometer resolution, broadband attenuation measurements with a frequency up to 1THz and phononic bandgap characterization. PMID:25455189
Interface nano-confined acoustic waves in polymeric surface phononic crystals
Travagliati, Marco; Nardi, Damiano; Giannetti, Claudio; Ferrini, Gabriele; Banfi, Francesco; Gusev, Vitalyi; Pingue, Pasqualantonio; Piazza, Vincenzo
2015-01-12
The impulsive acoustic dynamics of soft polymeric surface phononic crystals is investigated here in the hypersonic frequency range by near-IR time-resolved optical diffraction. The acoustic response is analysed by means of wavelet spectral methods and finite element modeling. An unprecedented class of acoustic modes propagating within the polymer surface phononic crystal and confined within 100 nm of the nano-patterned interface is revealed. The present finding opens the path to an alternative paradigm for characterizing the mechanical properties of soft polymers at interfaces and for sensing schemes exploiting polymers as embedding materials.
Acoustic-phonon-limited mobility and giant phonon-drag thermopower in MgZnO/ZnO heterostructures
Tsaousidou, M.
2013-12-04
We present numerical simulations for the acoustic-phonon-limited mobility, μ{sub ac}, in two-dimensional electron gases (2DEGs) confined in MgZnO/ZnO heterostructures for temperatures 0.4–20 K. The calculations are based on the semiclassical Boltzmann equation. We examine two 2DEGs with sheet densities 1.4 and 7×10{sup 15} m{sup −2}. Good agreement is found with recent experimental data without any adjustable parameter. We also calculate the contribution to thermopower that arises due to the phonon wind set up by a temperature gradient (the so-called phonon-drag thermopower, S{sup g}). A giant magnitude of S{sup g} is predicted that exceeds 50–100 mV/K at 5 K depending on the sheet density. Our findings suggest that the ZnO based heterostructures could be promising materials for thermoelectric applications at low temperatures.
Coherent control of optically generated and detected picosecond surface acoustic phonons
David Hurley
2007-07-01
Coherent control of electronic and phononic excitations in solids, as well as chemical and biological systems on ultrafast time scales is of current research interest. In semiconductors, coherent control of phonons has been demonstrated for acoustic and optical phonons generated in superlattice structures. The bandwidth of these approaches is typically fully utilized by employing a 1-D geometry where the laser spot size is much larger than the superlattice repeat length. In this article we demonstrate coherent control of optically generated picosecond surface acoustic phonons using sub-optical wavelength absorption gratings. The generation and detection characteristics of two material systems are investigated (aluminum absorption gratings on Si and GaAs substrates). Constructive and complete destructive interference conditions are demonstrated using two pump pulses derived from a single Michelson interferometer.
Enhancement of coherent acoustic phonons in InGaN multiple quantum wells
NASA Astrophysics Data System (ADS)
Hafiz, Shopan D.; Zhang, Fan; Monavarian, Morteza; Avrutin, Vitaliy; Morkoç, Hadis; Özgür, Ümit
2015-03-01
Enhancement of coherent zone folded longitudinal acoustic phonon (ZFLAP) oscillations at terahertz frequencies was demonstrated in InGaN multiple quantum wells (MQWs) by using wavelength degenerate time resolved differential transmission spectroscopy. Screening of the piezoelectric field in InGaN MQWs by photogenerated carriers upon femtosecond pulse excitation gave rise to terahertz ZFLAPs, which were monitored at the Brillouin zone center in the transmission geometry. MQWs composed of 10 pairs InxGa1-xN wells and In0.03Ga0.97N barriers provided coherent phonon frequencies of 0.69-0.80 THz depending on the period of MQWs. Dependences of ZFLAP amplitude on excitation density and wavelength were also investigated. Possibility of achieving phonon cavity, incorporating a MQW placed between two AlN/GaN phonon mirrors designed to exhibit large acoustic gaps at the zone center, was also explored.
Direct measurement of coherent subterahertz acoustic phonons mean free path in GaAs
NASA Astrophysics Data System (ADS)
Legrand, R.; Huynh, A.; Jusserand, B.; Perrin, B.; Lemaître, A.
2016-05-01
The phonon mean free path is generally inferred from the measurement of thermal conductivity and we are still lacking precise information on this quantity. Recent advances in the field of high-frequency phonons transduction using semiconductor superlattices give the opportunity to fill this gap. We present experimental results on the attenuation of longitudinal acoustic phonons in GaAs in the frequency and temperature ranges 0.2-1 THz and 10-80 K respectively. Surprisingly, we observe a plateau in the frequency dependence of the attenuation above 0.7 THz, that we ascribe to a breakdown of Herring processes.
Bleustein-Gulyaev-Shimizu surface acoustic waves in two-dimensional piezoelectric phononic crystals.
Hsu, Jin-Chen; Wu, Tsung-Tsong
2006-06-01
In this paper, we present a study on the existence of Bleustein-Gulyaev-Shimizu piezoelectric surface acoustic waves in a two-dimensional piezoelectric phononic crystal (zinc oxide, ZnO, and cadmium-sulfide, CdS) using the plane wave expansion method. In the configuration of ZnO (100)/CdS(100) phononic crystal, the calculated results show that this type of surface waves has higher acoustic wave velocities, high electromechanical coupling coefficients, and larger band gap width than those of the Rayleigh surface waves and pseudosurface waves. In addition, we find that the folded modes of the Bleustein-Gulyaev-Shimizu surface waves have higher coupling coefficients.
Dexterous acoustic trapping and patterning of particles assisted by phononic crystal plate
NASA Astrophysics Data System (ADS)
Wang, Tian; Ke, Manzhu; Xu, Shengjun; Feng, Junheng; Qiu, Chunyin; Liu, Zhengyou
2015-04-01
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.
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.
Acting on Lessons Learned: A NASA Glenn Acoustics Branch Perspective
NASA Technical Reports Server (NTRS)
Koch, L. Danielle
2008-01-01
Lessons learned from the International Space Station have indicated that early attention to acoustics will be key to achieving safer, more productive environments for new long duration missions. Fans are known to be dominant noise sources, and reducing fan noise poses challenges for fan manufacturers and systems engineers. The NASA Glenn Acoustics Branch has considered ways in which expertise and capabilities traditionally used to understand and mitigate aircraft engine noise can be used to address small fan noise issues in Exploration and Information Technology applications. Many could benefit if NASA can capture what is known about small fan aero and acoustic performance in a "Guide for the Design, Selection, and Installation of Fans for Spaceflight Applications." A draft outline for this document will be offered as a useful starting point for brainstorming ideas for the various smaller, near-term research projects that would need to be addressed first.
Sub-Poissonian phonon statistics in an acoustical resonator coupled to a pumped two-level emitter
Ceban, V. Macovei, M. A.
2015-11-15
The concept of an acoustical analog of the optical laser has been developed recently in both theoretical and experimental works. We here discuss a model of a coherent phonon generator with a direct signature of the quantum properties of sound vibrations. The considered setup is made of a laser-driven quantum dot embedded in an acoustical nanocavity. The system dynamics is solved for a single phonon mode in the steady-state and in the strong quantum dot—phonon coupling regime beyond the secular approximation. We demonstrate that the phonon statistics exhibits quantum features, i.e., is sub-Poissonian.
Persson, A. I. H.; Andreasson, B. P.; Enquist, H.; Jurgilaitis, A.; Larsson, J.
2015-11-14
The spectrum of laser-generated acoustic phonons in indium antimonide coated with a thin nickel film has been studied using time-resolved x-ray diffraction. Strain pulses that can be considered to be built up from coherent phonons were generated in the nickel film by absorption of short laser pulses. Acoustic reflections at the Ni–InSb interface leads to interference that strongly modifies the resulting phonon spectrum. The study was performed with high momentum transfer resolution together with high time resolution. This was achieved by using a third-generation synchrotron radiation source that provided a high-brightness beam and an ultrafast x-ray streak camera to obtain a temporal resolution of 10 ps. We also carried out simulations, using commercial finite element software packages and on-line dynamic diffraction tools. Using these tools, it is possible to calculate the time-resolved x-ray reflectivity from these complicated strain shapes. The acoustic pulses have a peak strain amplitude close to 1%, and we investigated the possibility to use this device as an x-ray switch. At a bright source optimized for hard x-ray generation, the low reflectivity may be an acceptable trade-off to obtain a pulse duration that is more than an order of magnitude shorter.
Acoustic resonance in MEMS scale cylindrical tubes with side branches
NASA Astrophysics Data System (ADS)
Schill, John F.; Holthoff, Ellen L.; Pellegrino, Paul M.; Marcus, Logan S.
2014-05-01
Photoacoustic spectroscopy (PAS) is a useful monitoring technique that is well suited for trace gas detection. This method routinely exhibits detection limits at the parts-per-million (ppm) or parts-per-billion (ppb) level for gaseous samples. PAS also possesses favorable detection characteristics when the system dimensions are scaled to a microelectromechanical system (MEMS) design. One of the central issues related to sensor miniaturization is optimization of the photoacoustic cell geometry, especially in relationship to high acoustical amplification and reduced system noise. Previous work relied on a multiphysics approach to analyze the resonance structures of the MEMS scale photo acoustic cell. This technique was unable to provide an accurate model of the acoustic structure. In this paper we describe a method that relies on techniques developed from musical instrument theory and electronic transmission line matrix methods to describe cylindrical acoustic resonant cells with side branches of various configurations. Experimental results are presented that demonstrate the ease and accuracy of this method. All experimental results were within 2% of those predicted by this theory.
Second Harmonic Generation and Confined Acoustic Phonons in HighlyExcited Semiconductor Nanocrystals
Son, Dong Hee; Wittenberg, Joshua S.; Banin, Uri; Alivisatos, A.Paul
2006-03-30
The photo-induced enhancement of second harmonic generation, and the effect of nanocrystal shape and pump intensity on confined acoustic phonons in semiconductor nanocrystals, has been investigated with time-resolved scattering and absorption measurements. The second harmonic signal showed a sublinear increase of the second order susceptibility with respect to the pump pulse energy, indicating a reduction of the effective one-electron second-order nonlinearity with increasing electron-hole density in the nanocrystals. The coherent acoustic phonons in spherical and rod-shaped semiconductor nanocrystals were detected in a time-resolved absorption measurement. Both nanocrystal morphologies exhibited oscillatory modulation of the absorption cross section, the frequency of which corresponded to their coherent radial breathing modes. The amplitude of the oscillation also increased with the level of photoexcitation, suggesting an increase in the amplitude of the lattice displacement as well.
Acoustic beam splitting in two-dimensional phononic crystals using self-collimation effect
Li, Jing; Wu, Fugen Zhong, Huilin; Yao, Yuanwei; Zhang, Xin
2015-10-14
We propose two models of self-collimation-based beam splitters in phononic crystals. The finite element method is used to investigate the propagation properties of acoustic waves in two-dimensional phononic crystals. The calculated results show that the efficiency of the beam splitter can be controlled systematically by varying the radius of the rods or by changing the orientation of the square rods in the line defect. The effect of changing the side length of the square rods on acoustic wave propagation is discussed. The results show that the total transmission/reflection range decreases/increases as the side length increases. We also find that the relationship between the orientation of the transflective point and the side length of the square rods is quasi-linear.
Electron-acoustic phonon interaction and mobility in stressed rectangular silicon nanowires
NASA Astrophysics Data System (ADS)
Zhu, Lin-Li
2015-01-01
We investigate the effects of pre-stress and surface tension on the electron-acoustic phonon scattering rate and the mobility of rectangular silicon nanowires. With the elastic theory and the interaction Hamiltonian for the deformation potential, which considers both the surface energy and the acoustoelastic effects, the phonon dispersion relation for a stressed nanowire under spatial confinement is derived. The subsequent analysis indicates that both surface tension and pre-stress can dramatically change the electron-acoustic phonon interaction. Under a negative (positive) surface tension and a tensile (compressive) pre-stress, the electron mobility is reduced (enhanced) due to the decrease (increase) of the phonon energy as well as the deformation-potential scattering rate. This study suggests an alternative approach based on the strain engineering to tune the speed and the drive current of low-dimensional electronic devices. Project supported by the National Natural Science Foundation of China (Grant Nos. 11472243, 11302189, and 11321202), the Doctoral Fund of Ministry of Education of China (Grant No. 20130101120175), the Zhejiang Provincial Qianjiang Talent Program, China (Grant No. QJD1202012), and the Educational Commission of Zhejiang Province, China (Grant No. Y201223476).
Polarization transport of transverse acoustic waves: Berry phase and spin Hall effect of phonons
NASA Astrophysics Data System (ADS)
Bliokh, K. Yu.; Freilikher, V. D.
2006-11-01
We carry out a detailed analysis of the short-wave (semiclassical) approximation for the linear equations of the elasticity in a smoothly inhomogeneous isotropic medium. It is shown that the polarization properties of the transverse waves are completely analogous to those of electromagnetic waves and can be considered as spin properties of optical phonons. In particular, the Hamiltonian of the transverse waves contains an additional term of the phonon spin-orbit interaction arising from the Berry gauge potential in the momentum space. This potential is diagonal in the basis of the circularly polarized waves and corresponds to the field of two “magnetic monopoles” of opposite signs for phonons of opposite helicities. This leads to the appearance of the Berry phase in the equation for the polarization evolution and an additional “anomalous velocity” term in the ray equations. The anomalous velocity has the form of the “Lorentz force” caused by the Berry gauge field in momentum space and gives rise to the transverse transport of waves of opposite helicities in opposite directions. This is a manifestation of the spin Hall effect of optical phonons. The effect directly relates to the conservation of total angular momentum of phonons and also influences reflection from a sharp boundary (acoustic analog of the transverse Ferdorov-Imbert shift).
Phonon-electron interactions in piezoelectric semiconductor bulk acoustic wave resonators.
Gokhale, Vikrant J; Rais-Zadeh, Mina
2014-07-08
This work presents the first comprehensive investigation of phonon-electron interactions in bulk acoustic standing wave (BAW) resonators made from piezoelectric semiconductor (PS) materials. We show that these interactions constitute a significant energy loss mechanism and can set practical loss limits lower than anharmonic phonon scattering limits or thermoelastic damping limits. Secondly, we theoretically and experimentally demonstrate that phonon-electron interactions, under appropriate conditions, can result in a significant acoustic gain manifested as an improved quality factor (Q). Measurements on GaN resonators are consistent with the presented interaction model and demonstrate up to 35% dynamic improvement in Q. The strong dependencies of electron-mediated acoustic loss/gain on resonance frequency and material properties are investigated. Piezoelectric semiconductors are an extremely important class of electromechanical materials, and this work provides crucial insights for material choice, material properties, and device design to achieve low-loss PS-BAW resonators along with the unprecedented ability to dynamically tune resonator Q.
Silicon-based filters, resonators and acoustic channels with phononic crystal structures
NASA Astrophysics Data System (ADS)
Huang, Zi-Gui
2011-06-01
This paper discusses the phenomenon of phononic crystal silicon-based filters, resonators and acoustic channels structured in geometrical periodic arrays created by a single silicon material. Component structured geometrical periodic array refers to a structure of square stubbed rods arranged in repeated arrays on a silicon plate. The study discovered that the band gap of the phononic crystal structure can be modulated under different heights and rotational angles of periodically arrayed square stubbed rods. In addition to band gap modulation, we used the finite element method (FEM) and supercell techniques to analyse the resonance characteristics of defect-containing phononic crystal structures with a larger band gap size design. In addition, the paper also investigated the effects on acoustic channels. Previous studies have already analysed defect-containing resonator and channel phenomenon by the plane-wave expansion method with supercell techniques. However, the FEM can solve numerical issues of extreme difficulty to reach convergence. The results of this study elaborated on the manufacturing feasibility of silicon-based acoustic resonance and filter devices under a complementary metal-oxide-semiconductor synchronization process.
Phonon-Electron Interactions in Piezoelectric Semiconductor Bulk Acoustic Wave Resonators
Gokhale, Vikrant J.; Rais-Zadeh, Mina
2014-01-01
This work presents the first comprehensive investigation of phonon-electron interactions in bulk acoustic standing wave (BAW) resonators made from piezoelectric semiconductor (PS) materials. We show that these interactions constitute a significant energy loss mechanism and can set practical loss limits lower than anharmonic phonon scattering limits or thermoelastic damping limits. Secondly, we theoretically and experimentally demonstrate that phonon-electron interactions, under appropriate conditions, can result in a significant acoustic gain manifested as an improved quality factor (Q). Measurements on GaN resonators are consistent with the presented interaction model and demonstrate up to 35% dynamic improvement in Q. The strong dependencies of electron-mediated acoustic loss/gain on resonance frequency and material properties are investigated. Piezoelectric semiconductors are an extremely important class of electromechanical materials, and this work provides crucial insights for material choice, material properties, and device design to achieve low-loss PS-BAW resonators along with the unprecedented ability to dynamically tune resonator Q. PMID:25001100
Phonon-Electron Interactions in Piezoelectric Semiconductor Bulk Acoustic Wave Resonators
NASA Astrophysics Data System (ADS)
Gokhale, Vikrant J.; Rais-Zadeh, Mina
2014-07-01
This work presents the first comprehensive investigation of phonon-electron interactions in bulk acoustic standing wave (BAW) resonators made from piezoelectric semiconductor (PS) materials. We show that these interactions constitute a significant energy loss mechanism and can set practical loss limits lower than anharmonic phonon scattering limits or thermoelastic damping limits. Secondly, we theoretically and experimentally demonstrate that phonon-electron interactions, under appropriate conditions, can result in a significant acoustic gain manifested as an improved quality factor (Q). Measurements on GaN resonators are consistent with the presented interaction model and demonstrate up to 35% dynamic improvement in Q. The strong dependencies of electron-mediated acoustic loss/gain on resonance frequency and material properties are investigated. Piezoelectric semiconductors are an extremely important class of electromechanical materials, and this work provides crucial insights for material choice, material properties, and device design to achieve low-loss PS-BAW resonators along with the unprecedented ability to dynamically tune resonator Q.
NASA Astrophysics Data System (ADS)
Wang, Mingchao; Lin, Shangchao
2015-12-01
The elastic modulus of carbyne, a one-dimensional carbon chain, was recently predicted to be much higher than graphene. Inspired by this discovery and the fundamental correlation between elastic modulus and thermal conductivity, we investigate the intrinsic thermal transport in two carbon allotropes: carbyne and cumulene. Using molecular dynamics simulations, we discover that thermal conductivities of carbyne and cumulene at the quantum-corrected room temperature can exceed 54 and 148 kW/m/K, respectively, much higher than that for graphene. Such conductivity is attributed to high phonon energies and group velocities, as well as reduced scattering from non-overlapped acoustic and optical phonon modes. The prolonged spectral acoustic phonon lifetime of 30-110 ps and mean free path of 0.5-2.5 μm exceed those for graphene, and allow ballistic phonon transport along micron-length carbon chains. Tensile extensions can enhance the thermal conductivity of carbyne due to the increased phonon density of states in the acoustic modes and the increased phonon lifetime from phonon bandgap opening. These findings provide fundamental insights into phonon transport and band structure engineering through tensile deformation in low-dimensional materials, and will inspire studies on carbyne, cumulene, and boron nitride chains for their practical deployments in nano-devices.
Wang, Mingchao; Lin, Shangchao
2015-01-01
The elastic modulus of carbyne, a one-dimensional carbon chain, was recently predicted to be much higher than graphene. Inspired by this discovery and the fundamental correlation between elastic modulus and thermal conductivity, we investigate the intrinsic thermal transport in two carbon allotropes: carbyne and cumulene. Using molecular dynamics simulations, we discover that thermal conductivities of carbyne and cumulene at the quantum-corrected room temperature can exceed 54 and 148 kW/m/K, respectively, much higher than that for graphene. Such conductivity is attributed to high phonon energies and group velocities, as well as reduced scattering from non-overlapped acoustic and optical phonon modes. The prolonged spectral acoustic phonon lifetime of 30–110 ps and mean free path of 0.5–2.5 μm exceed those for graphene, and allow ballistic phonon transport along micron-length carbon chains. Tensile extensions can enhance the thermal conductivity of carbyne due to the increased phonon density of states in the acoustic modes and the increased phonon lifetime from phonon bandgap opening. These findings provide fundamental insights into phonon transport and band structure engineering through tensile deformation in low-dimensional materials, and will inspire studies on carbyne, cumulene, and boron nitride chains for their practical deployments in nano-devices. PMID:26658143
Wang, Mingchao; Lin, Shangchao
2015-01-01
The elastic modulus of carbyne, a one-dimensional carbon chain, was recently predicted to be much higher than graphene. Inspired by this discovery and the fundamental correlation between elastic modulus and thermal conductivity, we investigate the intrinsic thermal transport in two carbon allotropes: carbyne and cumulene. Using molecular dynamics simulations, we discover that thermal conductivities of carbyne and cumulene at the quantum-corrected room temperature can exceed 54 and 148 kW/m/K, respectively, much higher than that for graphene. Such conductivity is attributed to high phonon energies and group velocities, as well as reduced scattering from non-overlapped acoustic and optical phonon modes. The prolonged spectral acoustic phonon lifetime of 30-110 ps and mean free path of 0.5-2.5 μm exceed those for graphene, and allow ballistic phonon transport along micron-length carbon chains. Tensile extensions can enhance the thermal conductivity of carbyne due to the increased phonon density of states in the acoustic modes and the increased phonon lifetime from phonon bandgap opening. These findings provide fundamental insights into phonon transport and band structure engineering through tensile deformation in low-dimensional materials, and will inspire studies on carbyne, cumulene, and boron nitride chains for their practical deployments in nano-devices. PMID:26658143
Nardi, Damiano; Travagliati, Marco; Siemens, Mark E; Li, Qing; Murnane, Margaret M; Kapteyn, Henry C; Ferrini, Gabriele; Parmigiani, Fulvio; Banfi, Francesco
2011-10-12
High-frequency surface acoustic waves can be generated by ultrafast laser excitation of nanoscale patterned surfaces. Here we study this phenomenon in the hypersonic frequency limit. By modeling the thermomechanics from first-principles, we calculate the system's initial heat-driven impulsive response and follow its time evolution. A scheme is introduced to quantitatively access frequencies and lifetimes of the composite system's excited eigenmodes. A spectral decomposition of the calculated response on the eigemodes of the system reveals asymmetric resonances that result from the coupling between surface and bulk acoustic modes. This finding allows evaluation of impulsively excited pseudosurface acoustic wave frequencies and lifetimes and expands our understanding of the scattering of surface waves in mesoscale metamaterials. The model is successfully benchmarked against time-resolved optical diffraction measurements performed on one-dimensional and two-dimensional surface phononic crystals, probed using light at extreme ultraviolet and near-infrared wavelengths.
2011-01-01
High-frequency surface acoustic waves can be generated by ultrafast laser excitation of nanoscale patterned surfaces. Here we study this phenomenon in the hypersonic frequency limit. By modeling the thermomechanics from first-principles, we calculate the system’s initial heat-driven impulsive response and follow its time evolution. A scheme is introduced to quantitatively access frequencies and lifetimes of the composite system’s excited eigenmodes. A spectral decomposition of the calculated response on the eigemodes of the system reveals asymmetric resonances that result from the coupling between surface and bulk acoustic modes. This finding allows evaluation of impulsively excited pseudosurface acoustic wave frequencies and lifetimes and expands our understanding of the scattering of surface waves in mesoscale metamaterials. The model is successfully benchmarked against time-resolved optical diffraction measurements performed on one-dimensional and two-dimensional surface phononic crystals, probed using light at extreme ultraviolet and near-infrared wavelengths. PMID:21910426
Addouche, Mahmoud Al-Lethawe, Mohammed A. Choujaa, Abdelkrim Khelif, Abdelkrim
2014-07-14
We demonstrate super resolution imaging for surface acoustic waves using a phononic structure displaying negative refractive index. This phononic structure is made of a monolithic square lattice of cylindrical pillars standing on a semi-infinite medium. The pillars act as acoustic resonator and induce a surface propagating wave with unusual dispersion. We found, under specific geometrical parameters, one propagating mode that exhibits negative refraction effect with negative effective index close to −1. Furthermore, a flat lens with finite number of pillars is designed to allow the focusing of an acoustic point source into an image with a resolution of (λ)/3 , overcoming the Rayleigh diffraction limit.
Acoustic phonons in chrysotile asbestos probed by high-resolution inelastic x-ray scattering
Mamontov, Eugene; Vakhrushev, S. B.; Kumzerov, Yu. A,; Alatas, A.
2009-01-01
Acoustic phonons in an individual, oriented fiber of chrysotile asbestos (chemical formula Mg{sub 3}Si{sub 2}O{sub 5}(OH){sub 4}) were observed at room temperature in the inelastic x-ray measurement with a very high (meV) resolution. The x-ray scattering vector was aligned along [1 0 0] direction of the reciprocal lattice, nearly parallel to the long axis of the fiber. The latter coincides with [1 0 0] direction of the direct lattice and the axes of the nano-channels. The data were analyzed using a damped harmonic oscillator model. Analysis of the phonon dispersion in the first Brillouin zone yielded the longitudinal sound velocity of (9200 {+-} 600) m/s.
Coherent longitudinal acoustic phonons in three-dimensional supracrystals of cobalt nanocrystals.
Lisiecki, Isabelle; Polli, Dario; Yan, Cong; Soavi, Giancarlo; Duval, Eugène; Cerullo, Giulio; Pileni, Marie-Paule
2013-10-01
We use broadband picosecond acoustics to detect longitudinal acoustic phonons with few-gigahertz frequency in three-dimensional supracrystals (with face-centered cubic lattice) of 7 nm cobalt nanocrystal spheres. In full analogy with atomic crystals, where longitudinal acoustic phonons propagate with the speed of sound through coherent movements of atoms of the lattice out of their equilibrium positions, in these supracrystals atoms are replaced by (uncompressible) nanocrystals and atomic bonds by coating agents (carbon chains) that act like mechanical springs holding together the nanocrystals. By repeating the measurements at different laser angles of incidence it was possible to accurately determine both the index of refraction of the supracrystal (n = 1.26 ± 0.03) and the room-temperature longitudinal speed of sound (v(s)= 1235 ± 12 m/s), which is quite low due to the heavy weight of the spheres (with respect to atoms in a crystal) and the soft carbon chains (with respect to atomic bonds). Interestingly, the speed of sound inside the supracrystal was found to dramatically increase by decreasing the sample temperature due to a change in the stiffness of the dodecanoic acid chains which coat the Co nanocrystals.
Coupling of Excitons and Discrete Acoustic Phonons in Vibrationally Isolated Quantum Emitters.
Werschler, Florian; Hinz, Christopher; Froning, Florian; Gumbsheimer, Pascal; Haase, Johannes; Negele, Carla; de Roo, Tjaard; Mecking, Stefan; Leitenstorfer, Alfred; Seletskiy, Denis V
2016-09-14
The photoluminescence emission by mesoscopic condensed matter is ultimately dictated by the fine-structure splitting of the fundamental exciton into optically allowed and dipole-forbidden states. In epitaxially grown semiconductor quantum dots, nonradiative equilibration between the fine-structure levels is mediated by bulk acoustic phonons, resulting in asymmetric spectral broadening of the excitonic luminescence. In isolated colloidal quantum dots, spatial confinement of the vibrational motion is expected to give rise to an interplay between the quantized electronic and phononic degrees of freedom. In most cases, however, zero-dimensional colloidal nanocrystals are strongly coupled to the substrate such that the charge relaxation processes are still effectively governed by the bulk properties. Here we show that encapsulation of single colloidal CdSe/CdS nanocrystals into individual organic polymer shells allows for systematic vibrational decoupling of the semiconductor nanospheres from the surroundings. In contrast to epitaxially grown quantum dots, simultaneous quantization of both electronic and vibrational degrees of freedom results in a series of strong and narrow acoustic phonon sidebands observed in the photoluminescence. Furthermore, an individual analysis of more than 200 compound particles reveals that enhancement or suppression of the radiative properties of the fundamental exciton is controlled by the interaction between fine-structure states via the discrete vibrational modes. For the first time, pronounced resonances in the scattering rate between the fine-structure states are directly observed, in good agreement with a quantum mechanical model. The unambiguous assignment of mediating acoustic modes to the observed scattering resonances complements the experimental findings. Thus, our results form an attractive basis for future studies on subterahertz quantum opto-mechanics and efficient laser cooling at the nanoscale.
Coupling of Excitons and Discrete Acoustic Phonons in Vibrationally Isolated Quantum Emitters.
Werschler, Florian; Hinz, Christopher; Froning, Florian; Gumbsheimer, Pascal; Haase, Johannes; Negele, Carla; de Roo, Tjaard; Mecking, Stefan; Leitenstorfer, Alfred; Seletskiy, Denis V
2016-09-14
The photoluminescence emission by mesoscopic condensed matter is ultimately dictated by the fine-structure splitting of the fundamental exciton into optically allowed and dipole-forbidden states. In epitaxially grown semiconductor quantum dots, nonradiative equilibration between the fine-structure levels is mediated by bulk acoustic phonons, resulting in asymmetric spectral broadening of the excitonic luminescence. In isolated colloidal quantum dots, spatial confinement of the vibrational motion is expected to give rise to an interplay between the quantized electronic and phononic degrees of freedom. In most cases, however, zero-dimensional colloidal nanocrystals are strongly coupled to the substrate such that the charge relaxation processes are still effectively governed by the bulk properties. Here we show that encapsulation of single colloidal CdSe/CdS nanocrystals into individual organic polymer shells allows for systematic vibrational decoupling of the semiconductor nanospheres from the surroundings. In contrast to epitaxially grown quantum dots, simultaneous quantization of both electronic and vibrational degrees of freedom results in a series of strong and narrow acoustic phonon sidebands observed in the photoluminescence. Furthermore, an individual analysis of more than 200 compound particles reveals that enhancement or suppression of the radiative properties of the fundamental exciton is controlled by the interaction between fine-structure states via the discrete vibrational modes. For the first time, pronounced resonances in the scattering rate between the fine-structure states are directly observed, in good agreement with a quantum mechanical model. The unambiguous assignment of mediating acoustic modes to the observed scattering resonances complements the experimental findings. Thus, our results form an attractive basis for future studies on subterahertz quantum opto-mechanics and efficient laser cooling at the nanoscale. PMID:27550902
Pandya, Ankur; Shinde, Satyam; Jha, Prafulla K.
2015-05-15
In this paper the hot electron transport properties like carrier energy and momentum scattering rates and electron energy loss rates are calculated via interactions of electrons with polar acoustical phonons for Mn doped BN quantum well in BN nanosheets via piezoelectric scattering and deformation potential mechanisms at low temperatures with high electric field. Electron energy loss rate increases with the electric field. It is observed that at low temperatures and for low electric field the phonon absorption is taking place whereas, for sufficient large electric field, phonon emission takes place. Under the piezoelectric (polar acoustical phonon) scattering mechanism, the carrier scattering rate decreases with the reduction of electric field at low temperatures wherein, the scattering rate variation with electric field is limited by a specific temperature beyond which there is no any impact of electric field on such scattering.
Controlled exciton transfer between quantum dots with acoustic phonons taken into account
Golovinski, P. A.
2015-09-15
A system of excitons in two quantum dots coupled by the dipole–dipole interaction is investigated. The excitation transfer process controlled by the optical Stark effect at nonresonant frequencies is considered and the effect of the interaction between excitons and acoustic phonons in a medium on this process is taken into account. The system evolution is described using quantum Heisenberg equations. A truncated set of equations is obtained and the transfer dynamics is numerically simulated. High-efficiency picosecond switching of the excitation transfer by a laser pulse with a rectangular envelope is demonstrated. The dependence of picosecond switching on the quantum-dot parameters and optical-pulse length is presented.
Kumar, Sunil; Kamaraju, N; Karthikeyan, B; Tondusson, M; Freysz, E; Sood, A K
2010-07-01
Terahertz time domain spectroscopy has been used to study low frequency confined acoustic phonons of silver nanoparticles embedded in poly(vinyl alcohol) matrix in the spectral range of 0.1-2.5 THz. The real and imaginary parts of the dielectric function show two bands at 0.60 and 2.12 THz attributed to the spheroidal and toroidal modes of silver nanoparticles, thus demonstrating the usefulness of terahertz time domain spectroscopy as a complementary technique to Raman spectroscopy in characterizing the nanoparticles.
Tunneling times of acoustic phonon packets through a distributed Bragg reflector
2014-01-01
The longwave phenomenological model is used to make simple and precise calculations of various physical quantities such as the vibrational energy density, the vibrational energy, the relative mechanical displacement, and the one-dimensional stress tensor of a porous silicon distributed Bragg reflector. From general principles such as invariance under time reversal, invariance under space reflection, and conservation of energy density flux, the equivalence of the tunneling times for both transmission and reflection is demonstrated. Here, we study the tunneling times of acoustic phonon packets through a distributed Bragg reflector in porous silicon multilayer structures, and we report the possibility that a phenomenon called Hartman effect appears in these structures. PMID:25237288
Coherent acoustic phonon generation in GaAs{sub 1−x}Bi{sub x}
Joshya, R. S.; Kini, R. N.; Ptak, A. J.; France, R.; Mascarenhas, A.
2014-03-03
We have used femtosecond laser pulses to generate coherent acoustic phonons in the dilute Bismide alloy, GaAs{sub 1−x}Bi{sub x}. The observed oscillation periods match well with the oscillation periods calculated using the propagating strain pulse model. We attribute the generation process predominantly to electronic stress due to the absorption of the laser pulse at the surface of the GaAs{sub 1−x}Bi{sub x} layer. Our initial estimates suggest that the incorporation of Bi in GaAs causes an enhancement of the hydrostatic deformation potential because of the resonant state in the valence band due to isolated Bi impurities.
Richardson, M.; Bhethanabotla, V. R.; Sankaranarayanan, S. K. R. S.
2014-06-23
Finite element simulations of a phononic shear-horizontal surface acoustic wave (SAW) sensor based on ST 90°-X Quartz reveal a dramatic reduction in power consumption. The phononic sensor is realized by artificially structuring the delay path to form an acoustic meta-material comprised of a periodic microcavity array incorporating high-density materials such as tantalum or tungsten. Constructive interference of the scattered and secondary reflected waves at every microcavity interface leads to acoustic energy confinement in the high-density regions translating into reduced power loss. Tantalum filled cavities show the best performance while tungsten inclusions create a phononic bandgap. Based on our simulation results, SAW devices with tantalum filled microcavities were fabricated and shown to significantly decrease insertion loss. Our findings offer encouraging prospects for designing low power, highly sensitive portable biosensors.
Coherent Control of Optically Generated and Detected Picosecond Surface Acoustic Phonons
David H. Hurley
2006-11-01
Coherent control of elementary optical excitations is a key issue in ultrafast materials science. Manipulation of electronic and vibronic excitations in solids as well as chemical and biological systems on ultrafast time scales has attracted a great deal of attention recently. In semiconductors, coherent control of vibronic excitations has been demonstrated for bulk acoustic and optical phonons generated in superlattice structures. The bandwidth of these approaches is typically fully utilized by employing a 1-D geometry where the laser spot size is much larger than the superlattice repeat length. In this presentation we demonstrate coherent control of optically generated picosecond surface acoustic waves using sub-optical wavelength absorption gratings. The generation and detection characteristics of two material systems are investigated (aluminum absorption gratings on Si and GaAs substrates).
Morvan, B.; Tinel, A.; Sainidou, R.; Rembert, P.; Vasseur, J. O.; Hladky-Hennion, A.-C.; Swinteck, N.; Deymier, P. A.
2014-12-07
Phononic crystals (PC) can be used to control the dispersion properties of acoustic waves, which are essential to direct their propagation. We use a PC-based two-dimensional solid/solid composite to demonstrate experimentally and theoretically the spatial filtering of a monochromatic non-directional wave source and its emission in a surrounding water medium as an ultra-directional beam with narrow angular distribution. The phenomenon relies on square-shaped equifrequency contours (EFC) enabling self-collimation of acoustic waves within the phononic crystal. Additionally, the angular width of collimated beams is controlled via the EFC size-shrinking when increasing frequency.
NASA Astrophysics Data System (ADS)
Gao, Nansha; Wu, Jiu Hui; Yu, Lie; Hou, Hong
2016-06-01
This paper investigates ultralow frequency acoustic properties and energy recovery of tetragonal folding beam phononic crystal (TFBPC) and its complementary structure. The dispersion curve relationships, transmission spectra and displacement fields of the eigenmodes are studied with FEA in detail. Compared with the traditional three layer phononic crystal (PC) structure, this structure proposed in this paper not only unfold bandgaps (BGs) in lower frequency range (below 300 Hz), but also has lighter weight because of beam structural cracks. We analyze the relevant physical mechanism behind this phenomenon, and discuss the effects of the tetragonal folding beam geometric parameters on band structure maps. FEM proves that the multi-cell structures with different arrangements have different acoustic BGs when compared with single cell structure. Harmonic frequency response and piezoelectric properties of TFBPC are specifically analyzed. The results confirm that this structure does have the recovery ability for low frequency vibration energy in environment. These conclusions in this paper could be indispensable to PC practical applications such as BG tuning and could be applied in portable devices, wireless sensor, micro-electro mechanical systems which can recycle energy from vibration environment as its own energy supply.
NASA Astrophysics Data System (ADS)
Bhargavi, K. S.; Kubakaddi, S. S.
2016-09-01
The amplification coefficient α of acoustic phonons is theoretically investigated in a three-dimensional Dirac semimetal (3DDS) driven by a dc electric field E causing the drift of the electrons. It is numerically studied as a function of the frequency ωq, drift velocity vd, electron concentration ne, and temperature T in the Dirac semimetal Cd3As2. We find that the amplification of acoustic phonons (α ˜ hundreds of cm-1) takes place when the electron drift velocity vd is greater than the sound velocity vs. The amplification is found to occur at small E (˜few V/cm) due to large electron mobility. The frequency dependence of α shows amplification in the THz regime with a maximum αm occurring at the same frequency ωqm for different vd. The αm is found to increase with increasing vd. α vs ωq for different ne also shows a maximum, with αm shifting to higher ωq for larger ne. Each maximum is followed by a vanishing α at nearly "2kf cutoff," where kf is the Fermi wave vector. It is found that αm/ne and ωqm/ne1/3 are nearly constant. The αm ˜ ne can be used to identify the 3DDS phase as it differs from αm ˜ ne1/3 dependence in conventional bulk Cd3As2 semiconductor.
NASA Astrophysics Data System (ADS)
Lin, Ja-Hon; Shen, Yu-Kai; Liu, Wei-Rein; Lu, Chia-Hui; Chen, Yao-Hui; Chang, Chun-peng; Lee, Wei-Chin; Hong, Minghwei; Kwo, Jueinai-Raynien; Hsu, Chia-Hung; Hsieh, Wen-Feng
2016-08-01
Unlike coherent acoustic phonons (CAPs) generated from heat induced thermal stress by the coated Au film, we demonstrated the oscillation from c-ZnO epitaxial film on oxide buffered Si through a degenerate pump-probe technique. As the excited photon energy was set below the exciton resonance, the electronic stress that resulted from defect resonance was used to induce acoustic wave. The damped oscillation revealed a superposition of a high frequency and long decay CAP signal with a backward propagating acoustic pulse which was generated by the absorption of the penetrated pump beam at the Si surface and selected by the ZnO layer as the acoustic resonator.
NASA Astrophysics Data System (ADS)
Lin, Ja-Hon; Shen, Yu-Kai; Liu, Wei-Rein; Lu, Chia-Hui; Chen, Yao-Hui; Chang, Chun-peng; Lee, Wei-Chin; Hong, Minghwei; Kwo, Jueinai-Raynien; Hsu, Chia-Hung; Hsieh, Wen-Feng
2016-08-01
Unlike coherent acoustic phonons (CAPs) generated from heat induced thermal stress by the coated Au film, we demonstrated the oscillation from c-ZnO epitaxial film on oxide buffered Si through a degenerate pump–probe technique. As the excited photon energy was set below the exciton resonance, the electronic stress that resulted from defect resonance was used to induce acoustic wave. The damped oscillation revealed a superposition of a high frequency and long decay CAP signal with a backward propagating acoustic pulse which was generated by the absorption of the penetrated pump beam at the Si surface and selected by the ZnO layer as the acoustic resonator.
Acoustic phonon dynamics in thin-films of the topological insulator Bi{sub 2}Se{sub 3}
Glinka, Yuri D.; Babakiray, Sercan; Johnson, Trent A.; Holcomb, Mikel B.; Lederman, David
2015-04-28
Transient reflectivity traces measured for nanometer-sized films (6–40 nm) of the topological insulator Bi{sub 2}Se{sub 3} revealed GHz-range oscillations driven within the relaxation of hot carriers photoexcited with ultrashort (∼100 fs) laser pulses of 1.51 eV photon energy. These oscillations have been suggested to result from acoustic phonon dynamics, including coherent longitudinal acoustic phonons in the form of standing acoustic waves. An increase of oscillation frequency from ∼35 to ∼70 GHz with decreasing film thickness from 40 to 15 nm was attributed to the interplay between two different regimes employing traveling-acoustic-waves for films thicker than 40 nm and the film bulk acoustic wave resonator (FBAWR) modes for films thinner than 40 nm. The amplitude of oscillations decays rapidly for films below 15 nm thick when the indirect intersurface coupling in Bi{sub 2}Se{sub 3} films switches the FBAWR regime to that of the Lamb wave excitation. The frequency range of coherent longitudinal acoustic phonons is in good agreement with elastic properties of Bi{sub 2}Se{sub 3}.
Zhao, J.; Boyko, O.; Bonello, B.
2014-12-15
This work deals with an analytical and numerical study of the focusing of the lowest order anti-symmetric Lamb wave in gradient index phononic crystals. Computing the ray trajectories of the elastic beam allowed us to analyze the lateral dimensions and shape of the focus, either in the inner or behind the phononic crystal-based acoustic lenses, for frequencies within a broad range in the first band. We analyzed and discussed the focusing behaviors inside the acoustic lenses where the focalization at sub-wavelength scale was achieved. The focalization behind the gradient index phononic crystal is shown to be efficient as well: we report on FMHM = 0.63λ at 11MHz.
NASA Astrophysics Data System (ADS)
Harb, Maher; Peng, Weina; Sciaini, Germán; Hebeisen, Christoph T.; Ernstorfer, Ralph; Eriksson, Mark A.; Lagally, Max G.; Kruglik, Sergei G.; Miller, R. J. Dwayne
2009-03-01
Transmission electron diffraction is naturally sensitive to the detection of shear-type deformations in single-crystalline structures due to the effective tilting of the lattice planes characteristic of shear, but in general is insensitive to longitudinal phonon propagation. Here, we report on the generation and detection of both transverse and longitudinal coherent acoustic phonons in 33 nm free-standing (001)-oriented single crystalline Si films using femtosecond electron diffraction (FED) to monitor these laser-induced atomic displacements. The mechanism for excitation of the shear mode that leads to coupling to the longitudinal phonon is attributed to the inhomogeneous lateral profile of the optical-pump pulse and the periodic boundary condition imposed by the supporting grid structure. In this application, the constructive interference in the diffraction process makes FED particularly sensitive to the detection of coherent phonon modes and offers an atomic perspective of the dynamics involving collective motions.
Acoustic add-drop filters based on phononic crystal ring resonators
NASA Astrophysics Data System (ADS)
Rostami-Dogolsara, Babak; Moravvej-Farshi, Mohammad Kazem; Nazari, Fakhroddin
2016-01-01
We report the design procedure for an acoustic add-drop filter (ADF) composed of two line-defect waveguides coupled through a ring resonator cavity (RRC) all based on a phononic crystal (PnC) platform. Using finite difference time domain and plane wave expansion methods, we study the propagation of acoustic waves through the PnC based ADF structures. Numerical results show that the quality factor for the ADF with a quasisquare ring resonator with a frequency band of 95 Hz centered about 75.21 kHz is Q ˜ 800. We show that the addition of an appropriate scatterer at each RRC corner can reduce the scattering loss, enhancing the quality factor and the transmission efficiency. Moreover, it is also shown that by increasing the coupling gaps between the RRC and waveguides the quality factor can be increased by ˜25 times, at the expense of a significant reduction in the transmission efficiency this is attributed to the enhanced selectivity in expense of weakened coupling. Finally, by varying the effective path length of the acoustic wave in the RRC, via selectively varying the inclusions physical and geometrical properties, we show how one can ultra-fine and fine-tune the resonant frequency of the ADF.
NASA Astrophysics Data System (ADS)
Xie, Zhong-Xiang; Liu, Jing-Zhong; Yu, Xia; Wang, Hai-Bin; Deng, Yuan-Xiang; Li, Ke-Min; Zhang, Yong
2015-03-01
We investigate acoustic phonon transmission and thermal conductance in three dimensional (3D) quasi-periodically stubbed waveguides according to the Fibonacci sequence. Results show that the transmission coefficient exhibits the periodic oscillation upon varying the length of stub/waveguide at low frequency, and the period of such oscillation is tunably decreased with increasing the Fibonacci number N. Interestingly, there also exist some anti-resonant dips that gradually develop into wide stop-frequency gaps with increasing N. As the temperature goes up, a transition of the thermal conductance from the decrease to the increase occurs in these systems. When N is increased, the thermal conductance is approximately decreased with a linear trend. Moreover, the decreasing degree sensitively depends on the variation of temperature. A brief analysis of these results is given.
Development of an acoustic filter for parametric loudspeaker using phononic crystals.
Ji, Peifeng; Hu, Wenlin; Yang, Jun
2016-04-01
The spurious signal generated as a result of nonlinearity at the receiving system affects the measurement of the difference-frequency sound in the parametric loudspeaker, especially in the nearfield or near the beam axis. In this paper, an acoustic filter is designed using phononic crystals and its theoretical simulations are carried out by quasi-one- and two-dimensional models with Comsol Multiphysics. According to the simulated transmission loss (TL), an acoustic filter is prototyped consisting of 5×7 aluminum alloy cylinders and its performance is verified experimentally. There is good agreement with the simulation result for TL. After applying our proposed filter in the axial measurement of the parametric loudspeaker, a clear frequency dependence from parametric array effect is detected, which exhibits a good match with the well-known theory described by the Gaussian-beam expansion technique. During the directivity measurement for the parametric loudspeaker, the proposed filter has also proved to be effective and is only needed for small angles.
Development of an acoustic filter for parametric loudspeaker using phononic crystals.
Ji, Peifeng; Hu, Wenlin; Yang, Jun
2016-04-01
The spurious signal generated as a result of nonlinearity at the receiving system affects the measurement of the difference-frequency sound in the parametric loudspeaker, especially in the nearfield or near the beam axis. In this paper, an acoustic filter is designed using phononic crystals and its theoretical simulations are carried out by quasi-one- and two-dimensional models with Comsol Multiphysics. According to the simulated transmission loss (TL), an acoustic filter is prototyped consisting of 5×7 aluminum alloy cylinders and its performance is verified experimentally. There is good agreement with the simulation result for TL. After applying our proposed filter in the axial measurement of the parametric loudspeaker, a clear frequency dependence from parametric array effect is detected, which exhibits a good match with the well-known theory described by the Gaussian-beam expansion technique. During the directivity measurement for the parametric loudspeaker, the proposed filter has also proved to be effective and is only needed for small angles. PMID:26855254
Investigation of phononic crystals for dispersive surface acoustic wave ozone sensors
NASA Astrophysics Data System (ADS)
Westafer, Ryan S.
The object of this research was to investigate dispersion in surface phononic crystals (PnCs) for application to a newly developed passive surface acoustic wave (SAW) ozone sensor. Frequency band gaps and slow sound already have been reported for PnC lattice structures. Such engineered structures are often advertised to reduce loss, increase sensitivity, and reduce device size. However, these advances have not yet been realized in the context of surface acoustic wave sensors. In early work, we computed SAW dispersion in patterned surface structures and we confirmed that our finite element computations of SAW dispersion in thin films and in one dimensional surface PnC structures agree with experimental results obtained by laser probe techniques. We analyzed the computations to guide device design in terms of sensitivity and joint spectral operating point. Next we conducted simulations and experiments to determine sensitivity and limit of detection for more conventional dispersive SAW devices and PnC sensors. Finally, we conducted extensive ozone detection trials on passive reflection mode SAW devices, using distinct components of the time dispersed response to compensate for the effect of temperature. The experimental work revealed that the devices may be used for dosimetry applications over periods of several days.
NASA Astrophysics Data System (ADS)
Plemmons, Dayne; Flannigan, David
Coherent collective lattice oscillations known as phonons dictate a broad range of physical observables in condensed matter and act as primary energy carriers across a wide range of material systems. Despite this omnipresence, analysis of phonon dynamics on their ultrashort native spatiotemporal length scale - that is, the combined nanometer (nm), spatial and femtosecond (fs), temporal length-scales - has largely remained experimentally inaccessible. Here, we employ ultrafast electron microscopy (UEM) to directly image discrete acoustic phonons in real-space with combined nm-fs resolution. By directly probing electron scattering in the image plane (as opposed to the diffraction plane), we retain phase information critical for following the evolution, propagation, scattering, and decay of phonons in relation to morphological features of the specimen (i.e. interfaces, grain boundaries, voids, ripples, etc.). We extract a variety of morphologically-specific quantitative information from the UEM videos including phonon frequencies, phase velocities, and decays times. We expect these direct manifestations of local elastic properties in the vicinity of material defects and interfaces will aide in the understanding and application of phonon-mediated phenomena in nanostructures. Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN, 55455, USA.
NASA Astrophysics Data System (ADS)
Yang, Aichao; Li, Ping; Wen, Yumei; Yang, Chao; Wang, Decai; Zhang, Feng; Zhang, Jiajia
2015-05-01
A high-Q cross-plate phononic crystal resonator (Cr-PCR) coupled with an electromechanical Helmholtz resonator (EMHR) is proposed to improve acoustic wave localization and energy harvesting. Owing to the strongly directional wave-scattering effect of the cross-plate corners, strong confinement of acoustic waves emerges. Consequently, the proposed Cr-PCR structure exhibits ∼353.5 times higher Q value and ∼6.1 times greater maximum pressure amplification than the phononic crystal resonator (Cy-PCR) (consisting of cylindrical scatterers) of the same size. Furthermore, the harvester using the proposed Cr-PCR and the EMHR has ∼22 times greater maximum output-power volume density than the previous harvester using Cy-PCR and EMHR structures.
Flow-excited acoustic resonances of coaxial side-branches in an annular duct
NASA Astrophysics Data System (ADS)
Arthurs, D.; Ziada, S.
2009-01-01
This paper investigates the aeroacoustic response of an annular duct with closed coaxial side-branches, and examines the effect of several passive countermeasures on the resonance intensity. The investigated geometry is inspired by the design of the Roll-Posts in the Rolls-Royce LiftSystem® engine, which is currently being developed for the Lockheed Martin Joint Strike Fighter (JSF®) aircraft. The effects of design parameters, such as diameter ratio, branch length ratio and thickness of the annular flow on the frequency and resonance intensity of the first acoustic mode are studied experimentally. Numerical simulations of the acoustic mode shapes and frequencies are also performed. The annular flow has been found to excite several acoustic modes, the strongest in all cases being the first acoustic mode, which consists of a quarter wavelength along the length of each branch. The ratios of the branch length and diameter, with respect to the main duct diameter, have been found to have strong effects on the frequency of the acoustic modes.
Rury, Aaron S; Sorenson, Shayne; Dawlaty, Jahan M
2016-03-14
Organic materials that produce coherent lattice phonon excitations in response to external stimuli may provide next generation solutions in a wide range of applications. However, for these materials to lead to functional devices in technology, a full understanding of the possible driving forces of coherent lattice phonon generation must be attained. To facilitate the achievement of this goal, we have undertaken an optical spectroscopic study of an organic charge-transfer material formed from the ubiquitous reduction-oxidation pair hydroquinone and p-benzoquinone. Upon pumping this material, known as quinhydrone, on its intermolecular charge transfer resonance as well as an intramolecular resonance of p-benzoquinone, we find sub-cm(-1) oscillations whose dispersion with probe energy resembles that of a coherent acoustic phonon that we argue is coherently excited following changes in the electron density of quinhydrone. Using the dynamical information from these ultrafast pump-probe measurements, we find that the fastest process we can resolve does not change whether we pump quinhydrone at either energy. Electron-phonon coupling from both ultrafast coherent vibrational and steady-state resonance Raman spectroscopies allows us to determine that intramolecular electronic excitation of p-benzoquinone also drives the electron transfer process in quinhydrone. These results demonstrate the wide range of electronic excitations of the parent of molecules found in many functional organic materials that can drive coherent lattice phonon excitations useful for applications in electronics, photonics, and information technology.
Rury, Aaron S; Sorenson, Shayne; Dawlaty, Jahan M
2016-03-14
Organic materials that produce coherent lattice phonon excitations in response to external stimuli may provide next generation solutions in a wide range of applications. However, for these materials to lead to functional devices in technology, a full understanding of the possible driving forces of coherent lattice phonon generation must be attained. To facilitate the achievement of this goal, we have undertaken an optical spectroscopic study of an organic charge-transfer material formed from the ubiquitous reduction-oxidation pair hydroquinone and p-benzoquinone. Upon pumping this material, known as quinhydrone, on its intermolecular charge transfer resonance as well as an intramolecular resonance of p-benzoquinone, we find sub-cm(-1) oscillations whose dispersion with probe energy resembles that of a coherent acoustic phonon that we argue is coherently excited following changes in the electron density of quinhydrone. Using the dynamical information from these ultrafast pump-probe measurements, we find that the fastest process we can resolve does not change whether we pump quinhydrone at either energy. Electron-phonon coupling from both ultrafast coherent vibrational and steady-state resonance Raman spectroscopies allows us to determine that intramolecular electronic excitation of p-benzoquinone also drives the electron transfer process in quinhydrone. These results demonstrate the wide range of electronic excitations of the parent of molecules found in many functional organic materials that can drive coherent lattice phonon excitations useful for applications in electronics, photonics, and information technology. PMID:26979698
NASA Astrophysics Data System (ADS)
Roy-Choudhury, Kaushik; Hughes, Stephen
2015-11-01
Electron-phonon coupling in semiconductor quantum dots plays a significant role in determining the optical properties of excited excitons, especially the spectral nature of emitted photons. This paper presents a comprehensive theory and analysis of emission spectra from artificial atoms or quantum dots coupled to structured photon reservoirs and acoustic phonons, when excited with incoherent pump fields. As specific examples of structured reservoirs, we chose a Lorentzian cavity and a slow-light coupled-cavity waveguide, which have both been explored experimentally. For the case of optical cavities, we directly compare and contrast the spectra from three well-known and distinct theoretical approaches to treat electron-phonon coupling, including a Markovian polaron master equation, a non-Markovian phonon correlation expansion technique, and a semiclassical linear susceptibility approach, and we point out the limitations of these models. For the cavity-QED polaron master equation, which treats the cavity-mode operator at the level of a system operator, we give closed form analytical solutions to the phonon-assisted scattering rates in the weak excitation approximation, fully accounting for temperature, cavity-exciton detuning, and cavity-dot coupling. We also show explicitly why the semiclassical linear susceptibility approach fails to correctly account for phonon-mediated cavity feeding. For weakly coupled cavities, we calculate the optical spectra using a more general photon reservoir polaron master-equation approach, and explain its differences from the above approaches in the low-Q limit of a Lorentzian cavity. We subsequently use this general reservoir approach to calculate the emission spectra from quantum dots coupled to slow-light photonic crystal waveguides, which demonstrate a number of striking photon-phonon coupling effects.
NASA Astrophysics Data System (ADS)
Lee, Kang Il; Kang, Hwi Suk; Yoon, Suk Wang
2016-04-01
The present work reports a combined experimental and theoretical study on the acoustic band gaps in a two-dimensional (2D) phononic crystal (PC) consisting of periodic square arrays of stainless-steel cylinders with diameters of 1.0 mm and a lattice constant of 1.5 mm in water. The theoretical band structure of the 2D PC was calculated along the ΓX direction of the first Brillouin zone. The transmission and the reflection coefficients were obtained both experimentally and theoretically along the ΓX direction of the 2D PC. The 2D PC exhibited 5 band gaps at frequencies below 2.0 MHz, with the first Bragg gap being around a frequency of 0.5 MHz. To understand the band gaps in the 2D PC, we calculated the acoustic pressure fields at specific frequencies of interest for normal incidence, and we explained them from the perspective of acoustic diffraction gratings.
Coherent heat transport in 2D phononic crystals with acoustic impedance mismatch
NASA Astrophysics Data System (ADS)
Arantes, A.; Anjos, V.
2016-03-01
In this work we have calculated the cumulative thermal conductivities of micro-phononic crystals formed by different combinations of inclusions and matrices at a sub-Kelvin temperature regime. The low-frequency phonon spectra (up to tens of GHz) were obtained by solving the generalized wave equation for inhomogeneous media with the plane wave expansion method. The thermal conductivity was calculated from Boltzmann transport theory highlighting the role of the low-frequency thermal phonons and neglecting phonon-phonon scattering. A purely coherent thermal transport regime was assumed throughout the structures. Our findings show that the cumulative thermal conductivity drops dramatically when compared with their bulk counterpart. Depending on the structural composition this reduction may be attributed to the phonon group velocity due to a flattening of the phonon dispersion relation, the extinction of phonon modes in the density of states or due to the presence of complete band gaps. According to the contrast between the inclusions and the matrices, three types of two dimensional phononic crystals were considered: carbon/epoxy, carbon/polyethylene and tungsten/silicon, which correspond respectively to a moderate, strong and very strong mismatch in the mechanical properties of these materials.
Size dependent acoustic phonon dynamics of CdTe0.68Se0.32 nanoparticles in borosilicate glass
NASA Astrophysics Data System (ADS)
Gupta, Sanjeev K.; Jha, Prafulla K.; Arora, A. K.
2008-06-01
Low frequency acoustic vibration and phonon linewidth for CdTe0.68Se0.32 nanoparticle embedded in borosilicate glass are calculated using two different approaches by considering the elastic continuum model and fixed boundary condition. The presence of medium significantly affects the phonon peaks and results into the broadening of the modes. The linewidth is found to depend inversely on the size, similar to that reported experimentally. The damping time and quality factor have also been calculated. The damping time that is of the order of picoseconds decreases with the decrease in size. High value of quality factor for l =2 normal mode suggests the less loss of energy for this mode.
NASA Astrophysics Data System (ADS)
Fernée, Mark J.; Sinito, Chiara; Louyer, Yann; Potzner, Christian; Nguyen, Tich-Lam; Mulvaney, Paul; Tamarat, Philippe; Lounis, Brahim
2012-12-01
Charged quantum dots provide an important platform for a range of emerging quantum technologies. Colloidal quantum dots in particular offer unique advantages for such applications (facile synthesis, manipulation and compatibility with a wide range of environments), especially if stable charged states can be harnessed in these materials. Here we engineer the CdSe nanocrystal core and shell structure to efficiently ionize at cryogenic temperatures, resulting in trion emission with a single sharp zero-phonon line and a mono exponential decay. Magneto-optical spectroscopy enables direct determination of electron and hole g-factors. Spin relaxation is observed in high fields, enabling unambiguous identification of the trion charge. Importantly, we show that spin flips are completely inhibited for Zeeman splittings below the low-energy bound for confined acoustic phonons. This reveals a characteristic unique to colloidal quantum dots that will promote the use of these versatile materials in challenging quantum technological applications.
Surface and confined acoustic waves in finite size 1D solid-fluid phononic crystals
NASA Astrophysics Data System (ADS)
El Hassouani, Y.; El Boudouti, E. H.; Djafari-Rouhani, B.; Rais, R.
2007-12-01
Using a Green's function method, we investigate theoretically the eigenmodes of a finite one-dimensional phononic crystal (superlattice) composed of N alternating layers of an elastic solid and an ideal fluid. If the finite superlattice is free of stress on both sides, we show that there are always N-1 modes in the allowed bands whereas there is one and only one mode corresponding to each band gap. This mode is either a surface mode in the band gap or a constant-frequency confined band-edge mode. If the finite superlattice is bounded from one side by a homogeneous fluid whereas the other surface is kept free, then an incident phonon from the fluid is perfectly reflected, however this reflection takes place with a large delay time if the frequency of the incident phonon coincides with the eigenfrequency of a surface mode
Ultra-wide acoustic band gaps in pillar-based phononic crystal strips
Coffy, Etienne Lavergne, Thomas; Addouche, Mahmoud; Euphrasie, Sébastien; Vairac, Pascal; Khelif, Abdelkrim
2015-12-07
An original approach for designing a one dimensional phononic crystal strip with an ultra-wide band gap is presented. The strip consists of periodic pillars erected on a tailored beam, enabling the generation of a band gap that is due to both Bragg scattering and local resonances. The optimized combination of both effects results in the lowering and the widening of the main band gap, ultimately leading to a gap-to-midgap ratio of 138%. The design method used to improve the band gap width is based on the flattening of phononic bands and relies on the study of the modal energy distribution within the unit cell. The computed transmission through a finite number of periods corroborates the dispersion diagram. The strong attenuation, in excess of 150 dB for only five periods, highlights the interest of such ultra-wide band gap phononic crystal strips.
Acoustic phonon-limited diffusion thermopower in monolayer MoS{sub 2}
Patil, S. B.; Sankeshwar, N. S. Kubakaddi, S. S.
2015-06-24
Diffusion thermopower S{sub d} is investigated, theoretically, as a function of temperature, T and electron concentration, n{sub s} in a n-type monolayer molebdenum disulfide (MoS{sub 2}). Electron scattering due to unscreened deformation potential (DP) coupling of TA phonons, screened DP coupling of LA phonons, and screened piezoelectric (PE) coupling of LA and TA phonons is considered. Total S{sub d} is dominated by electron scattering by TA phonons via unscreened DP coupling. S{sub d} is found to increase (decrease) with increasing T (n{sub s}). At low T and for high n{sub s}, S{sub d} ∼ T and n{sub s}{sup −1} as found from the Mott formula. At a given T and for given ns, S{sub d} in MoS{sub 2} is much larger than that in GaAs, due to the larger electron effective mass in the former.
NASA Astrophysics Data System (ADS)
Ali, H.; Yilbas, B. S.
2016-09-01
Phonon cross-plane transport across silicon and diamond thin films pair is considered, and thermal boundary resistance across the films pair interface is examined incorporating the cut-off mismatch and diffusive mismatch models. In the cut-off mismatch model, phonon frequency mismatch for each acoustic branch is incorporated across the interface of the silicon and diamond films pair in line with the dispersion relations of both films. The frequency-dependent and transient solution of the Boltzmann transport equation is presented, and the equilibrium phonon intensity ratios at the silicon and diamond film edges are predicted across the interface for each phonon acoustic branch. Temperature disturbance across the edges of the films pair is incorporated to assess the phonon transport characteristics due to cut-off and diffusive mismatch models across the interface. The effect of heat source size, which is allocated at high-temperature (301 K) edge of the silicon film, on the phonon transport characteristics at the films pair interface is also investigated. It is found that cut-off mismatch model predicts higher values of the thermal boundary resistance across the films pair interface as compared to that of the diffusive mismatch model. The ratio of equilibrium phonon intensity due to the cut-off mismatch over the diffusive mismatch models remains >1 at the silicon edge, while it becomes <1 at the diamond edge for all acoustic branches.
A branch of energetic-particle driven geodesic acoustic modes due to magnetic drift resonance
NASA Astrophysics Data System (ADS)
Sasaki, M.; Kasuya, N.; Itoh, K.; Hallatschek, K.; Lesur, M.; Kosuga, Y.; Itoh, S.-I.
2016-10-01
Eigenmode analysis of geodesic acoustic modes (GAMs) driven by fast ions is performed, based on a set of gyrokinetic equations. Resonance to the magnetic drift of the fast ions can destabilize GAMs. A new branch is found in the family of GAMs, whose frequency is close to the magnetic drift frequency of the fast ions. The poloidal eigenfunction of this branch has bump structures in the poloidal direction where the resonance of the magnetic drift with the mode is strong. The ion heating rate by the GAMs is evaluated in the framework of quasi-linear theory. The heating is localized poloidally around the resonance locations. Owing to the bumps in the eigenfunction, the magnitude of the heating is much larger than that estimated without the magnetic drift resonance.
Lin, Kung-Hsuan; Wang, Kuan-Jen; Chang, Chung-Chieh; Wen, Yu-Chieh; Lv, Bing; Chu, Ching-Wu; Wu, Maw-Kuen
2016-01-01
We have utilized ultrafast optical spectroscopy to study carrier dynamics in slightly underdoped (BaK)Fe2As2 crystals without magnetic transition. The photoelastic signals due to coherent acoustic phonons have been quantitatively investigated. According to our temperature-dependent results, we found that the relaxation component of superconducting quasiparticles persisted from the superconducting state up to at least 70 K in the normal state. Our findings suggest that the pseudogaplike feature in the normal state is possibly the precursor of superconductivity. We also highlight that the pseudogap feature of K-doped BaFe2As2 is different from that of other iron-based superconductors, including Co-doped or P-doped BaFe2As2. PMID:27180873
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.
NASA Astrophysics Data System (ADS)
Pontecorvo, Emanuele; Ortolani, Michele; Polli, Dario; Ferretti, Marco; Ruocco, Giancarlo; Cerullo, Giulio; Scopigno, Tullio
2011-01-01
Building on a 1 kHz amplified Ti:sapphire laser source, we developed a novel pump-probe setup for broadband picosecond acoustics using a white-light continuum probe coupled to an optical multichannel analyzer. The system allows one to access, in a single measurement, acoustic parameters such as sound velocity and attenuation all over the bandwidth of the acoustic wave-packet launched by the pump pulse. We use the setup to measure the sound attenuation in fused silica and observe a dynamic crossover occurring at ≈170 GHz.
Tunable broadband unidirectional acoustic transmission based on a waveguide with phononic crystal
NASA Astrophysics Data System (ADS)
Song, Ailing; Chen, Tianning; Wang, Xiaopeng; Wan, Lele
2016-08-01
In this paper, a tunable broadband unidirectional acoustic transmission (UAT) device composed of a bended tube and a superlattice with square columns is proposed and numerically investigated by using finite element method. The UAT is realized in the proposed UAT device within two wide frequency ranges. And the effectiveness of the UAT device is demonstrated by analyzing the sound pressure distributions when the acoustic waves are incident from different directions. The unidirectional band gaps can be effectively tuned by mechanically rotating the square columns, which is a highlight of this paper. Besides, a bidirectional acoustic isolation (BAI) device is obtained by placing two superlattices in the bended tube, in which the acoustic waves cannot propagate along any directions. The physical mechanisms of the proposed UAT device and BAI device are simply discussed. The proposed models show potential applications in some areas, such as unidirectional sonic barrier or noise insulation.
Hussein, Mahmoud I.; El-Kady, Ihab; Li, Baowen; Sánchez-Dehesa, José
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.
Phonon Density of States in MgB{sub 2}
Osborn, R.; Goremychkin, E. A.; Kolesnikov, A. I.; Hinks, D. G.
2001-07-02
We report inelastic neutron scattering measurements of the phonon density of states in Mg
Finite element analysis of surface modes in phononic crystal waveguides
NASA Astrophysics Data System (ADS)
Guo, Yuning; Schubert, Martin; Dekorsy, Thomas
2016-03-01
The study of surface modes in phononic crystal waveguides in the hypersonic regime is a burgeoning field with a large number of possible applications. By using the finite element method, the band structure and the corresponding transmission spectrum of surface acoustic waves in phononic crystal waveguides generated by line defects in a silicon pillar-substrate system were calculated and investigated. The bandgaps are caused by the hybridization effect of band branches induced by local resonances and propagating modes in the substrate. By changing the sizes of selected pillars in the phononic crystal waveguides, the corresponding bands shift and localized modes emerge due to the local resonance effect induced by the pillars. This effect offers further possibilities for tailoring the propagation and filtering of elastic waves. The presented results have implications for the engineering of phonon dynamics in phononic nanostructures.
Phononic crystal surface mode coupling and its use in acoustic Doppler velocimetry.
Cicek, Ahmet; Salman, Aysevil; Kaya, Olgun Adem; Ulug, Bulent
2016-02-01
It is numerically shown that surface modes of two-dimensional phononic crystals, which are Bloch modes bound to the interface between the phononic crystal and the surrounding host, can couple back and forth between the surfaces in a length scale determined by the separation of two surfaces and frequency. Supercell band structure computations through the finite-element method reveal that the surface band of an isolated surface splits into two bands which support either symmetric or antisymmetric hybrid modes. When the surface separation is 3.5 times the lattice constant, a coupling length varying between 30 and 48 periods can be obtained which first increases linearly with frequency and, then, decreases rapidly. In the linear regime, variation of coupling length can be used as a means of measuring speeds of objects on the order of 0.1m/s by incorporating the Doppler shift. Speed sensitivity can be improved by increasing surface separation at the cost of larger device sizes.
Goryachev, Maxim; Creedon, Daniel L; Galliou, Serge; Tobar, Michael E
2013-08-23
The confinement of high frequency phonons approaching 1 GHz is demonstrated in phonon-trapping acoustic cavities at cryogenic temperatures using a low-coupled network approach. The frequency range is extended by nearly an order of magnitude, with excitation at greater than the 200th overtone achieved for the first time. Such a high frequency operation reveals Rayleigh-type phonon scattering losses due to highly diluted lattice impurities and corresponding glasslike behavior, with a maximum Q(L)×f product of 8.6×10(17) at 3.8 K and 4×10(17) at 15 mK. This suggests a limit on the Q×f product due to unavoidable crystal disorder. Operation at 15 mK is high enough in frequency that the average phonon occupation number is less than unity, with a loaded quality factor above half a billion. This work represents significant progress towards the utilization of such acoustic cavities for hybrid quantum systems.
Observation of coherent acoustic phonon in titanyl phthalocyanine thin solid films
NASA Astrophysics Data System (ADS)
Ahn, Hyeyoung; Su, Shiu-Ho
2015-08-01
Ultrafast exciton dynamics is investigated in titanyl phthalocyanine (TiOPc) micrograin films in this study. Exponential exciton relaxation and oscillatory responses are observed in transient reflectivity measurements of the films. Laser-induced coherent acoustic waves (LCAW) are proposed to be responsible for this oscillation. Despite their fast attenuation with the increase in temperature, LCAWs are successfully detected at room temperature through probing at a large oblique angle near the low-energy absorption edge of the Q-Band. From the oscillation period of the LCAW, the sound velocity is estimated to be ∼3.1 × 103 m/s, and an acoustic echo arising from the boundary between the films and substrate is also observed.
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.
Vergeynst, Lidewei L.; Sause, Markus G. R.; Hamstad, Marvin A.; Steppe, Kathy
2015-01-01
When drought occurs in plants, acoustic emission (AE) signals can be detected, but the actual causes of these signals are still unknown. By analyzing the waveforms of the measured signals, it should, however, be possible to trace the characteristics of the AE source and get information about the underlying physiological processes. A problem encountered during this analysis is that the waveform changes significantly from source to sensor and lack of knowledge on wave propagation impedes research progress made in this field. We used finite element modeling and the well-known pencil lead break source to investigate wave propagation in a branch. A cylindrical rod of polyvinyl chloride was first used to identify the theoretical propagation modes. Two wave propagation modes could be distinguished and we used the finite element model to interpret their behavior in terms of source position for both the PVC rod and a wooden rod. Both wave propagation modes were also identified in drying-induced signals from woody branches, and we used the obtained insights to provide recommendations for further AE research in plant science. PMID:26191070
Lee, Sooheyong; Williams, G. Jackson; Campana, Maria I.; Walko, Donald A.; Landahl, Eric C.
2016-01-01
Using a strain-rosette, we demonstrate the existence of transverse strain using time-resolved x-ray diffraction from multiple Bragg reflections in laser-excited bulk gallium arsenide. We find that anisotropic strain is responsible for a considerable fraction of the total lattice motion at early times before thermal equilibrium is achieved. Our measurements are described by a new model where the Poisson ratio drives transverse motion, resulting in the creation of shear waves without the need for an indirect process such as mode conversion at an interface. Using the same excitation geometry with the narrow-gap semiconductor indium antimonide, we detected coherent transverse acoustic oscillations at frequencies of several GHz. PMID:26751616
Lee, Sooheyong; Williams, G Jackson; Campana, Maria I; Walko, Donald A; Landahl, Eric C
2016-01-01
Using a strain-rosette, we demonstrate the existence of transverse strain using time-resolved x-ray diffraction from multiple Bragg reflections in laser-excited bulk gallium arsenide. We find that anisotropic strain is responsible for a considerable fraction of the total lattice motion at early times before thermal equilibrium is achieved. Our measurements are described by a new model where the Poisson ratio drives transverse motion, resulting in the creation of shear waves without the need for an indirect process such as mode conversion at an interface. Using the same excitation geometry with the narrow-gap semiconductor indium antimonide, we detected coherent transverse acoustic oscillations at frequencies of several GHz. PMID:26751616
Lee, Sooheyong; Williams, G Jackson; Campana, Maria I; Walko, Donald A; Landahl, Eric C
2016-01-11
Using a strain-rosette, we demonstrate the existence of transverse strain using time-resolved x-ray diffraction from multiple Bragg reflections in laser-excited bulk gallium arsenide. We find that anisotropic strain is responsible for a considerable fraction of the total lattice motion at early times before thermal equilibrium is achieved. Our measurements are described by a new model where the Poisson ratio drives transverse motion, resulting in the creation of shear waves without the need for an indirect process such as mode conversion at an interface. Using the same excitation geometry with the narrow-gap semiconductor indium antimonide, we detected coherent transverse acoustic oscillations at frequencies of several GHz.
NASA Astrophysics Data System (ADS)
Bruggeman, J. C.; Hirschberg, A.; van Dongen, M. E. H.; Wijnands, A. P. J.; Gorter, J.
1991-11-01
A theoretical model is proposed for the aero-acoustic sources responsible for low-frequency self-sustained pulsations in pipes with closed side branches. The theory successfully explains the acoustic and hydrodynamic conditions for resonance in experiments with a single side branch. It also predicts the order of magnitude of the pulsation amplitude and the effect of losses due to friction and radiation. A high pulsation level, with acoustic velocities of the order of magnitude of the main flow, is observed in a double side branch set-up when the edges at the junctions are rounded. When in the double side branch set-up the rounded upstream edge of the second T-joint is replaced by a sharp edge, the pulsation amplitude is reduced by a factor of five. This effect, which can be explained with the theory of vortex sound, leads us to the design of spoilers. Various "spoilers" have been tested in scale model and full scale experiments. Some of these reduce the pulsation level by 40 dB.
Charge carrier trapping and acoustic phonon modes in single CdTe nanowires.
Lo, Shun Shang; Major, Todd A; Petchsang, Nattasamon; Huang, Libai; Kuno, Masaru K; Hartland, Gregory V
2012-06-26
Semiconductor nanostructures produced by wet chemical synthesis are extremely heterogeneous, which makes single particle techniques a useful way to interrogate their properties. In this paper the ultrafast dynamics of single CdTe nanowires are studied by transient absorption microscopy. The wires have lengths of several micrometers and lateral dimensions on the order of 30 nm. The transient absorption traces show very fast decays, which are assigned to charge carrier trapping into surface defects. The time constants vary for different wires due to differences in the energetics and/or density of surface trap sites. Measurements performed at the band edge compared to the near-IR give slightly different time constants, implying that the dynamics for electron and hole trapping are different. The rate of charge carrier trapping was observed to slow down at high carrier densities, which was attributed to trap-state filling. Modulations due to the fundamental and first overtone of the acoustic breathing mode were also observed in the transient absorption traces. The quality factors for these modes were similar to those measured for metal nanostructures, and indicate a complex interaction with the environment.
Clustering reveals cavitation-related acoustic emission signals from dehydrating branches.
Vergeynst, Lidewei L; Sause, Markus G R; De Baerdemaeker, Niels J F; De Roo, Linus; Steppe, Kathy
2016-06-01
The formation of air emboli in the xylem during drought is one of the key processes leading to plant mortality due to loss in hydraulic conductivity, and strongly fuels the interest in quantifying vulnerability to cavitation. The acoustic emission (AE) technique can be used to measure hydraulic conductivity losses and construct vulnerability curves. For years, it has been believed that all the AE signals are produced by the formation of gas emboli in the xylem sap under tension. More recent experiments, however, demonstrate that gas emboli formation cannot explain all the signals detected during drought, suggesting that different sources of AE exist. This complicates the use of the AE technique to measure emboli formation in plants. We therefore analysed AE waveforms measured on branches of grapevine (Vitis vinifera L. 'Chardonnay') during bench dehydration with broadband sensors, and applied an automated clustering algorithm in order to find natural clusters of AE signals. We used AE features and AE activity patterns during consecutive dehydration phases to identify the different AE sources. Based on the frequency spectrum of the signals, we distinguished three different types of AE signals, of which the frequency cluster with high 100-200 kHz frequency content was strongly correlated with cavitation. Our results indicate that cavitation-related AE signals can be filtered from other AE sources, which presents a promising avenue into quantifying xylem embolism in plants in laboratory and field conditions. PMID:27095256
Biswas, Tutul; Ghosh, Tarun Kanti
2013-01-23
We study the interaction between electron and acoustic phonons in a Rashba spin-orbit coupled two-dimensional electron gas using Boltzmann transport theory. Both the deformation potential and piezoelectric scattering mechanisms are considered in the Bloch-Grüneisen (BG) regime as well as in the equipartition (EP) regime. The effect of the Rashba spin-orbit interaction on the temperature dependence of the resistivity in the BG and EP regimes is discussed. We find that the effective exponent of the temperature dependence of the resistivity in the BG regime decreases due to spin-orbit coupling.
Kasuga, Jun; Charrier, Guillaume; Uemura, Matsuo; Améglio, Thierry
2015-04-01
Ultrasonic acoustic emission (UAE) methods have been applied for the detection of freeze-thaw-induced embolism formation in water conduits of tree species. Until now, however, the exact source(s) of UAE has not been identified especially in angiosperm species, in which xylem tissues are composed of diverse types of cells. In this study, UAE was recorded from excised branches of walnut (Juglans regia cv. Franquette) during freeze-thaw cycles, and attempts were made to characterize UAEs generated by cavitation events leading to embolism formation according to their properties. During freeze-thaw cycles, a large number of UAEs were generated from the sample segments. However, the cumulative numbers of total UAE during freeze-thawing were not correlated with the percentage loss of hydraulic conductivity after thawing, suggesting that the sources of UAE were not only cavitation leading to embolism formation in vessels. Among the UAEs, cumulative numbers of UAEs with absolute energy >10.0 fJ strongly correlated with the increase in percentage loss of hydraulic conductivity. The high absolute energy of the UAEs might reflect the formation of large bubbles in the large lumen of vessels. Therefore, UAEs generated by cavitation events in vessels during freeze-thawing might be distinguished from other signals according to their magnitudes of absolute energy. On the other hand, the freezing of xylem parenchyma cells was followed by a certain number of UAEs. These results indicate the possibility that UAE methods can be applied to the detection of both freeze-thaw-induced embolism and supercooling breakdown in parenchyma cells in xylem.
Kasuga, Jun; Charrier, Guillaume; Uemura, Matsuo; Améglio, Thierry
2015-01-01
Ultrasonic acoustic emission (UAE) methods have been applied for the detection of freeze–thaw-induced embolism formation in water conduits of tree species. Until now, however, the exact source(s) of UAE has not been identified especially in angiosperm species, in which xylem tissues are composed of diverse types of cells. In this study, UAE was recorded from excised branches of walnut (Juglans regia cv. Franquette) during freeze–thaw cycles, and attempts were made to characterize UAEs generated by cavitation events leading to embolism formation according to their properties. During freeze–thaw cycles, a large number of UAEs were generated from the sample segments. However, the cumulative numbers of total UAE during freeze–thawing were not correlated with the percentage loss of hydraulic conductivity after thawing, suggesting that the sources of UAE were not only cavitation leading to embolism formation in vessels. Among the UAEs, cumulative numbers of UAEs with absolute energy >10.0 fJ strongly correlated with the increase in percentage loss of hydraulic conductivity. The high absolute energy of the UAEs might reflect the formation of large bubbles in the large lumen of vessels. Therefore, UAEs generated by cavitation events in vessels during freeze–thawing might be distinguished from other signals according to their magnitudes of absolute energy. On the other hand, the freezing of xylem parenchyma cells was followed by a certain number of UAEs. These results indicate the possibility that UAE methods can be applied to the detection of both freeze–thaw-induced embolism and supercooling breakdown in parenchyma cells in xylem. PMID:25662846
Three-Phonon Phase Space as an Indicator of the Lattice Thermal Conductivity in Semiconductors
NASA Astrophysics Data System (ADS)
Lindsay, L.; Broido, D. A.
2007-03-01
The room temperature lattice thermal conductivity of many semiconductors is limited primarily by three-phonon scattering processes arising from the anharmonicity of the interatomic potential. We employ an adiabatic bond charge model [1,2] for the phonon dispersions to calculate the phase space for three-phonon scattering events of several group IV and III-V semiconductors. We find that the amount of phase space available for this scattering in materials varies inversely with their measured thermal conductivities. Anomalous behavior occurs in III-V materials having large mass differences between cation and anion, which we explain in terms of the severely restricted three-phonon phase space arising from the large gap between acoustic and optic phonon branches. [1] W. Weber, Physical Review B 15, 4789 (1977). [2] K. C. Rustagi and W. Weber, Solid State Communications 18, 673 (1976).
Thermal conductivity and spectral phonon properties of freestanding and supported silicene
Wang, Zuyuan; Feng, Tianli; Ruan, Xiulin
2015-02-28
We conduct molecular dynamics (MD) simulations to study the thermal conductivity of freestanding silicene and silicene supported on an amorphous silicon dioxide (SiO{sub 2}) substrate in the temperature range from 300 to 900 K. The results show that the thermal conductivity decreases with increasing temperature and that the presence of the SiO{sub 2} substrate results in a great reduction, up to 78% at 300 K, to the thermal conductivity of silicene. With atomic trajectories from equilibrium MD simulations, we perform spectral energy density analysis to compute the thermal conductivities, spectral phonon relaxation times, and spectral phonon mean free paths (MFPs) of freestanding and supported silicene at 300 K. When silicene is put on a SiO{sub 2} substrate, the phonon relaxation times are decreased from 1–13 ps to less than 1 ps, and the phonon MFPs are reduced from 10–120 nm to 0–20 nm. We also calculate the thermal conductivity contributions from all phonon branches and find that the thermal conductivities of freestanding and supported silicene are mainly (>85%) contributed by the longitudinal and transverse acoustic phonons, while the out-of-plane acoustic phonons have a contribution less than 3%. Our study predicts the reduction of the thermal conductivity of silicene due to substrate effects and provides a fundamental understanding of the reduction in terms of the spectral phonon relaxation times and MFPs.
Birefringent phononic structures
Psarobas, I. E. Exarchos, D. A.; Matikas, T. E.
2014-12-15
Within the framework of elastic anisotropy, caused in a phononic crystal due to low crystallographic symmetry, we adopt a model structure, already introduced in the case of photonic metamaterials, and by analogy, we study the effect of birefringence and acoustical activity in a phononic crystal. In particular, we investigate its low-frequency behavior and comment on the factors which determine chirality by reference to this model.
Electron-Phonon Coupling and Energy Flow in a Simple Metal beyond the Two-Temperature Approximation
NASA Astrophysics Data System (ADS)
Waldecker, Lutz; Bertoni, Roman; Ernstorfer, Ralph; Vorberger, Jan
2016-04-01
The electron-phonon coupling and the corresponding energy exchange are investigated experimentally and by ab initio theory in nonequilibrium states of the free-electron metal aluminium. The temporal evolution of the atomic mean-squared displacement in laser-excited thin freestanding films is monitored by femtosecond electron diffraction. The electron-phonon coupling strength is obtained for a range of electronic and lattice temperatures from density functional theory molecular dynamics simulations. The electron-phonon coupling parameter extracted from the experimental data in the framework of a two-temperature model (TTM) deviates significantly from the ab initio values. We introduce a nonthermal lattice model (NLM) for describing nonthermal phonon distributions as a sum of thermal distributions of the three phonon branches. The contributions of individual phonon branches to the electron-phonon coupling are considered independently and found to be dominated by longitudinal acoustic phonons. Using all material parameters from first-principles calculations except the phonon-phonon coupling strength, the prediction of the energy transfer from electrons to phonons by the NLM is in excellent agreement with time-resolved diffraction data. Our results suggest that the TTM is insufficient for describing the microscopic energy flow even for simple metals like aluminium and that the determination of the electron-phonon coupling constant from time-resolved experiments by means of the TTM leads to incorrect values. In contrast, the NLM describing transient phonon populations by three parameters appears to be a sufficient model for quantitatively describing electron-lattice equilibration in aluminium. We discuss the general applicability of the NLM and provide a criterion for the suitability of the two-temperature approximation for other metals.
Nakayama, Masaaki Ohno, Tatsuya; Furukawa, Yoshiaki
2015-04-07
We have systematically investigated the photoluminescence (PL) dynamics of free excitons in GaAs/Al{sub 0.3}Ga{sub 0.7}As single quantum wells, focusing on the energy relaxation process due to exciton–acoustic-phonon scattering under non-resonant and weak excitation conditions as a function of GaAs-layer thickness from 3.6 to 12.0 nm and temperature from 30 to 50 K. The free exciton characteristics were confirmed by observation that the PL decay time has a linear dependence with temperature. We found that the free exciton PL rise rate, which is the reciprocal of the rise time, is inversely linear with the GaAs-layer thickness and linear with temperature. This is consistent with a reported theoretical study of the exciton–acoustic-phonon scattering rate in the energy relaxation process in quantum wells. Consequently, it is conclusively verified that the PL rise rate is dominated by the exciton–acoustic-phonon scattering rate. In addition, from quantitative analysis of the GaAs-layer thickness and temperature dependences, we suggest that the PL rise rate reflects the number of exciton–acoustic-phonon scattering events.
NASA Astrophysics Data System (ADS)
Baydin, Andrey; Krzyzanowska, Halina; Dhanunjaya, Munthala; Nageswara Rao, S. V. S.; Davidson, Jimmy L.; Feldman, Leonard C.; Tolk, Norman H.
2016-06-01
Silicon carbide (SiC) is a promising material for new generation electronics including high power/high temperature devices and advanced optical applications such as room temperature spintronics and quantum computing. Both types of applications require the control of defects particularly those created by ion bombardment. In this work, modification of optical constants of 4H-SiC due to hydrogen implantation at 180 keV and at fluences ranging from 1014 to 1016 cm-2 is reported. The depth dependence of the modified optical constants was extracted from coherent acoustic phonon spectra. Implanted spectra show a strong dependence of the 4H-SiC complex refractive index depth profile on H+ fluence. These studies provide basic insight into the dependence of optical properties of 4H silicon carbide on defect densities created by ion implantation, which is of relevance to the fabrication of SiC-based photonic and optoelectronic devices.
Bhargavi, K. S.; Patil, Sukanya; Kubakaddi, S. S.
2015-07-28
The theory of free-carrier absorption (FCA) is given for monolayers of transition-metal dichalcogenides, particularly for molybdenum disulphide (MoS{sub 2}), when carriers are scattered by phonons. Explicit expressions for the absorption coefficient α are obtained and discussed for acoustic phonon scattering via screened deformation potential and piezoelectric coupling taking polarization of the radiation in the plane of the layer. It is found that α monotonously decreases with the increasing photon frequency Ω, increases with the increasing temperature T, and linearly depends on two-dimensional electron concentration n{sub s}. Effect of screening, which is ignored in all the earlier FCA studies, is found to reduce α significantly, attributing to the larger effective mass of the electrons. Results are also obtained in the classical and quantum limit giving the power laws α ∼ Ω{sup −2} and T. Comparison of the results is made with those in bulk semiconductors and semiconductor quantum wells.
Friedrich, Michael; Riefer, Arthur; Sanna, Simone; Schmidt, W G; Schindlmayr, Arno
2015-09-30
The vibrational properties of stoichiometric LiNbO3 are analyzed within density-functional perturbation theory in order to obtain the complete phonon dispersion of the material. The phonon density of states of the ferroelectric (paraelectric) phase shows two (one) distinct band gaps separating the high-frequency (∼800 cm(-1)) optical branches from the continuum of acoustic and lower optical phonon states. This result leads to specific heat capacites in close agreement with experimental measurements in the range 0-350 K and a Debye temperature of 574 K. The calculated zero-point renormalization of the electronic Kohn-Sham eigenvalues reveals a strong dependence on the phonon wave vectors, especially near [Formula: see text]. Integrated over all phonon modes, our results indicate a vibrational correction of the electronic band gap of 0.41 eV at 0 K, which is in excellent agreement with the extrapolated temperature-dependent measurements.
NASA Astrophysics Data System (ADS)
Krivchikov, A. I.; Yushchenko, A. N.; Manzhelii, V. G.; Korolyuk, O. A.; Bermejo, F. J.; Fernández-Perea, R.; Cabrillo, C.; González, M. A.
2006-08-01
The thermal conductivity of all three disordered solid phases of ethyl alcohol has been measured. That for the orientationally disordered bcc phase is found to be remarkably close to that for the structurally amorphous solid, especially at low temperatures. The results, which emphasize the role of orientational disorder in phonon scattering, are discussed with the aid of computer simulations on single-crystalline models of both bcc and monoclinic crystals.
Phonon anomalies and superconductivity in the Heusler compound YPd₂Sn
Tütüncü, H. M.; Srivastava, G. P.
2014-07-07
We have studied the structural and electronic properties of YPd₂Sn in the Heusler structure using a generalized gradient approximation of the density functional theory and the ab initio pseudopotential method. The electronic results indicate that the density of states at the Fermi level is primarily derived from Pd d states, which hybridize with Y d and Sn p states. Using our structural and electronic results, phonons and electron-phonon interactions have been studied by employing a linear response approach based on the density functional theory. Phonon anomalies have been observed for transverse acoustic branches along the [110] direction. This anomalous dispersion is merely a consequence of the strong coupling. By integrating the Eliashberg spectral function, the average electron-phonon coupling parameter is found to be λ=0.99. Using this value, the superconducting critical temperature is calculated to be 4.12 K, in good accordance with the recent experimental value of 4.7 K.
Phononic Molecules Studied by Raman Scattering
Lanzillotti-Kimura, N. D.; Fainstein, A.; Jusserand, B.; Lemaitre, A.
2010-01-04
An acoustic nanocavity can confine phonons in such a way that they act like electrons in an atom. By combining two of these phononic-atoms, it is possible to form a phononic 'molecule', with acoustic modes that are similar to the electronic states in a hydrogen molecule. We report Raman scattering experiments performed in a monolithic structure formed by a phononic molecule embedded in an optical cavity. The acoustic mode splitting becomes evident through both the amplification and change of selection rules induced by the optical cavity confinement. The results are in perfect agreement with photoelastic model simulations.
Acoustics: A branch of engineering at the Universidad Austral de Chile (UACh)
NASA Astrophysics Data System (ADS)
Poblete, Victor; Arenas, Jorge P.; Sommerhoff, Jorge
2002-11-01
At the end of the 1960s, the first acousticians graduating at UACh had acquired an education in applied physics and musical arts, since there was no College of Engineering at that time. Initially, they had a (rather modest) four-year undergraduate program, and most of the faculty were not specialized teachers. The graduates from such a program received a sound engineering degree and they were skilled for jobs in the musical industry and sound reinforcement companies. In addition, they worked as sound engineers and producers. Later, because of the scientific, industrial and educational changes in Chile during the 1980s, the higher education system had massive changes that affected all of the undergraduate and graduate programs of the 61 universities in Chile. The UACh College of Engineering was officially founded in 1989. Then, acoustics as an area of expertise was included, widened and developed as an interdisciplinary subject. Currently, the undergraduate program in acoustics at UACh offers a degree in engineering sciences and a 6-year professional studies in Civil Engineering (Acoustics), having two main fields: Sound and Image, and Environment and Industry.
Analysis of dispersion characteristics of phononic structures
Parkhomenko, D. A. Kolenov, S. A.; Grigoruk, V. I.; Movchan, N. N.
2011-05-15
A general theory for calculating the dispersion of bulk acoustic waves in 3D and 2D phononic crystals made of anisotropic materials is presented, which is based on the plane-wave expansion method. Two approaches to separating acoustic modes in the dispersion diagrams are proposed. The pattern of the acoustic field distribution is studied as depending on the wavevector direction for various types of modes. Degeneracy of acoustic modes in directions different from the axes of symmetry of the phononic crystal is demonstrated. Possibilities of the proposed method are illustrated by the application to 3D and 2D silicon-based phononic crystal structures.
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.
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. .
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
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
NASA Astrophysics Data System (ADS)
Baydin, Andrey; Krzyzanowska, Halina; Dhanunjaya, M.; Rao, S. V. S. Nageswara; Davidson, Jimmy L.; Feldman, Leonard C.; Tolk, Norman H.
Silicon carbide (SiC) is an ideal material for new electronics, such as high power/high temperature devices, and a candidate for advanced optical applications such as room temperature spintronics and quantum computing. Both types of applications may require the control of defects created by ion bombardment. In this work, we examine depth dependent modification of optical constants of 4H-SiC due to hydrogen implantation at 180keV and low doses ranging from 1014 to 1016 cm-2probed by coherent acoustic phonon (CAP) spectroscopy. For our studies, we used Si-face 10 μm epilayers of n-type 4H-SiC grown by CVD on 4H-SiC substrate. A comprehensive analysis of the reference and implanted spectra shows a strong dependence of 4H-SiC complex refractive index shape versus depth on the H+ fluence. We extract the complex refractive index as a function of depth and ion beam dose. Our results demonstrate that the implantation-modified refractive index is distributed over a greater depth range than Monte Carlo calculation predictions of the implantation induced structural damage. These studies provide insight into the application of hydrogen ion implantation to the fabrication of SiC-based photonic and optoelectronic devices. Work is supported by ARO under Contract No. W911NF-14-1-0290.
Nonlinear control of high-frequency phonons in spider silk.
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.
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.
Sound and heat revolutions in phononics
NASA Astrophysics Data System (ADS)
Maldovan, Martin
2013-11-01
The phonon is the physical particle representing mechanical vibration and is responsible for the transmission of everyday sound and heat. Understanding and controlling the phononic properties of materials provides opportunities to thermally insulate buildings, reduce environmental noise, transform waste heat into electricity and develop earthquake protection. Here I review recent progress and the development of new ideas and devices that make use of phononic properties to control both sound and heat. Advances in sonic and thermal diodes, optomechanical crystals, acoustic and thermal cloaking, hypersonic phononic crystals, thermoelectrics, and thermocrystals herald the next technological revolution in phononics.
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. PMID:24226887
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.
NASA Astrophysics Data System (ADS)
Perrin, Bernard
2007-06-01
The conference PHONONS 2007 was held 15-20 July 2007 in the Conservatoire National des Arts et Métiers (CNAM) Paris, France. CNAM is a college of higher technology for training students in the application of science to industry, founded by Henri Grégoire in 1794. This was the 12th International Conference on Phonon Scattering in Condensed Matter. This international conference series, held every 3 years, started in France at Sainte-Maxime in 1972. It was then followed by meetings at Nottingham (1975), Providence (1979), Stuttgart (1983), Urbana-Champaign (1986), Heidelberg (1989), Ithaca (1992), Sapporo (1995), Lancaster (1998), Dartmouth (2001) and St Petersburg (2004). PHONONS 2007 was attended by 346 delegates from 37 different countries as follows: France 120, Japan 45, Germany 25, USA 25, Russia 21, Italy 13, Poland 9, UK 9, Canada 7, The Netherlands 7, Finland 6, Spain 6, Taiwan 6, Greece 4, India 4, Israel 4, Ukraine 4, Serbia 3, South Africa 3, Argentina 2, Belgium 2, China 2, Iran 2, Korea 2, Romania 2, Switzerland 2, and one each from Belarus, Bosnia-Herzegovina, Brazil, Bulgaria, Egypt, Estonia, Mexico, Moldova, Morocco, Saudi Arabia, Turkey. There were 5 plenary lectures, 14 invited talks and 84 oral contributions; 225 posters were presented during three poster sessions. The first plenary lecture was given by H J Maris who presented fascinating movies featuring the motion of a single electron in liquid helium. Robert Blick gave us a review on the new possibilities afforded by nanotechnology to design nano-electomechanical systems (NEMS) and the way to use them to study elementary and fundamental processes. The growing interest for phonon transport studies in nanostructured materials was demonstrated by Arun Majumdar. Andrey Akimov described how ultrafast acoustic solitons can monitor the optical properties of quantum wells. Finally, Maurice Chapellier told us how
Kinetic description of an electron--LO-phonon system with finite phonon lifetime
Nguyen, V.T.; Mahler, G. )
1992-02-15
We study the cooling of an electron plasma from a kinetic point of view. For this purpose, a quantum theory of fluctuations is applied to derive the kinetic equations for an electron--LO-phonon system from various model Hamiltonians. A polarization approximation is provided that goes beyond perturbation theory of the electron-phonon interaction. The description of electron-phonon energy exchange is shown to be impossible with the interacting Hamiltonian in Froehlich's one-phonon form unless dissipation of the bare LO phonon is included. For a Hamiltonian including effects of the scattering of LO phonons by acoustic phonons, kinetic equations are derived. The equation for LO phonons is shown to describe the collective excitations with finite lifetime, in the limiting case of weak damping of the plasmon-phonon coupled modes. A reduction of the cooling rate similar to the hot-phonon'' effect is shown to occur for the case of weak coupling without assuming a steady state of the LO phonons. Finally, an electron-phonon interaction Hamiltonian in two-phonon form is considered and it is shown that electron-phonon energy exchange may be described in the polarization approximation without introducing a finite phonon lifetime.
Nanocrystalline nanowires: 2. Phonons.
Allen, Philip B
2007-01-01
Nanocrystalline nanowires (NCNW) are fragments of bulk crystals that are infinite in only one direction. A construction is given for calculating eigenstates belonging to the symmetry labels (k,m) (wavevector and rotational quantum number). Vibrational harmonic eigenstates are worked out explicitly for a simple model, illustrating the general results: the LA mode has m=0, while with sufficient rotational symmetry, the TA branch is doubly degenerate, has m=+/-1, and has quadratic dispersion with k for k less than the reciprocal diameter of the NCNW. The twiston branch (a fourth Goldstone boson) is an acoustic m=0 branch, additional to the LA and two TA branches.
Thermal conductivity in large-J two-dimensional antiferromagnets: Role of phonon scattering
Chernyshev, A. L.; Brenig, Wolfram
2015-08-05
Different types of relaxation processes for magnon heat current are discussed, with a particular focus on coupling to three-dimensional phonons. There is thermal conductivity by these in-plane magnetic excitations using two distinct techniques: Boltzmann formalism within the relaxation-time approximation and memory-function approach. Also considered are the scattering of magnons by both acoustic and optical branches of phonons. We demonstrate an accord between the two methods, regarding the asymptotic behavior of the effective relaxation rates. It is strongly suggested that scattering from optical or zone-boundary phonons is important for magnon heat current relaxation in a high-temperature window of ΘD≲T<< J.
High- and low-frequency phonon modes in dipolar quantum gases trapped in deep lattices
NASA Astrophysics Data System (ADS)
Maluckov, Aleksandra; Gligorić, Goran; Hadžievski, Ljupčo; Malomed, Boris A.; Pfau, Tilman
2013-02-01
We study normal modes propagating on top of the stable uniform background in arrays of dipolar Bose-Einstein condensate (BEC) droplets trapped in a deep optical lattice. Both the on-site mean-field dynamics of the droplets and their displacement due to the repulsive dipole-dipole interactions (DDIs) are taken into account. Dispersion relations for two modes, viz., high- and low- frequency counterparts of optical and acoustic phonon modes in condensed matter, are derived analytically and verified by direct simulations, for both cases of the repulsive and attractive contact interactions. The (counterpart of the) optical-phonon branch does not exist without the DDIs. These results are relevant in the connection to emerging experimental techniques enabling real-time imaging of the condensate dynamics and direct experimental measurement of phonon dispersion relations in BECs.
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
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
Second Harmonic Generation of Nanoscale Phonon Wave Packets
NASA Astrophysics Data System (ADS)
Bojahr, A.; Gohlke, M.; Leitenberger, W.; Pudell, J.; Reinhardt, M.; von Reppert, A.; Roessle, M.; Sander, M.; Gaal, P.; Bargheer, M.
2015-11-01
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.
ERIC Educational Resources Information Center
Beyer, Robert
1981-01-01
Surveys 50 years of acoustical studies by discussing selected topics including the ear, nonlinear representations, underwater sound, acoustical diagnostics, absorption, electrolytes, phonons, magnetic interaction, and superfluidity and the five sounds. (JN)
NASA Technical Reports Server (NTRS)
Goodman, Jerry R.; Grosveld, Ferdinand
2007-01-01
The acoustics environment in space operations is important to maintain at manageable levels so that the crewperson can remain safe, functional, effective, and reasonably comfortable. High acoustic levels can produce temporary or permanent hearing loss, or cause other physiological symptoms such as auditory pain, headaches, discomfort, strain in the vocal cords, or fatigue. Noise is defined as undesirable sound. Excessive noise may result in psychological effects such as irritability, inability to concentrate, decrease in productivity, annoyance, errors in judgment, and distraction. A noisy environment can also result in the inability to sleep, or sleep well. Elevated noise levels can affect the ability to communicate, understand what is being said, hear what is going on in the environment, degrade crew performance and operations, and create habitability concerns. Superfluous noise emissions can also create the inability to hear alarms or other important auditory cues such as an equipment malfunctioning. Recent space flight experience, evaluations of the requirements in crew habitable areas, and lessons learned (Goodman 2003; Allen and Goodman 2003; Pilkinton 2003; Grosveld et al. 2003) show the importance of maintaining an acceptable acoustics environment. This is best accomplished by having a high-quality set of limits/requirements early in the program, the "designing in" of acoustics in the development of hardware and systems, and by monitoring, testing and verifying the levels to ensure that they are acceptable.
Parsons, L. C. Andrews, G. T.
2014-07-21
Brillouin light scattering experiments and optical reflectance measurements were performed on a pair of porous silicon-based optical Bragg mirrors which had constituent layer porosity ratios close to unity. For off-axis propagation, the phononic and photonic band structures of the samples were modeled as a series of intersecting linear dispersion curves. Zone-folding was observed for the longitudinal bulk acoustic phonon and the frequency of the probed zone-folded longitudinal phonon was shown to be dependent on the propagation direction as well as the folding order of the mode branch. There was no conclusive evidence of coupling between the transverse and the folded longitudinal modes. Two additional observed Brillouin peaks were attributed to the Rayleigh surface mode and a possible pseudo-surface mode. Both of these modes were dispersive, with the velocity increasing as the wavevector decreased.
Wide-stopband aperiodic phononic filters
NASA Astrophysics Data System (ADS)
Rostem, K.; Chuss, D. T.; Denis, K. L.; Wollack, E. J.
2016-06-01
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.
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.
Phonon localization in ultrathin layered structures
NASA Astrophysics Data System (ADS)
Döring, F.; Eberl, C.; Schlenkrich, S.; Schlenkrich, F.; Hoffmann, S.; Liese, T.; Krebs, H. U.; Pisana, S.; Santos, T.; Schuhmann, H.; Seibt, M.; Mansurova, M.; Ulrichs, H.; Zbarsky, V.; Münzenberg, M.
2015-04-01
An efficient way for minimizing phonon thermal conductivity in solids is to nanostructure them by means of reduced phonon mean free path, phonon scattering and phonon reflection at interfaces. A sophisticated approach toward this lies in the fabrication of thin multilayer films of different materials. In this paper, we show by femtosecond-pump-probe reflectivity measurements that in different multilayer systems with varying acoustic mismatch (consisting of metals, semiconductors, oxides and polymers), oscillations due to phonon localization can be observed. For the growth of multilayer films with well-defined layer thicknesses, we used magnetron sputtering, evaporation and pulsed laser deposition. By altering the material combinations and reducing the layer thicknesses down to 3 nm, we observed different mechanisms of phonon blocking, reaching in the frequency regime up to 360 GHz.
Full-dispersion Monte Carlo simulation of phonon transport in micron-sized graphene nanoribbons
Mei, S. Knezevic, I.; Maurer, L. N.; Aksamija, Z.
2014-10-28
We simulate phonon transport in suspended graphene nanoribbons (GNRs) with real-space edges and experimentally relevant widths and lengths (from submicron to hundreds of microns). The full-dispersion phonon Monte Carlo simulation technique, which we describe in detail, involves a stochastic solution to the phonon Boltzmann transport equation with the relevant scattering mechanisms (edge, three-phonon, isotope, and grain boundary scattering) while accounting for the dispersion of all three acoustic phonon branches, calculated from the fourth-nearest-neighbor dynamical matrix. We accurately reproduce the results of several experimental measurements on pure and isotopically modified samples [S. Chen et al., ACS Nano 5, 321 (2011);S. Chen et al., Nature Mater. 11, 203 (2012); X. Xu et al., Nat. Commun. 5, 3689 (2014)]. We capture the ballistic-to-diffusive crossover in wide GNRs: room-temperature thermal conductivity increases with increasing length up to roughly 100 μm, where it saturates at a value of 5800 W/m K. This finding indicates that most experiments are carried out in the quasiballistic rather than the diffusive regime, and we calculate the diffusive upper-limit thermal conductivities up to 600 K. Furthermore, we demonstrate that calculations with isotropic dispersions overestimate the GNR thermal conductivity. Zigzag GNRs have higher thermal conductivity than same-size armchair GNRs, in agreement with atomistic calculations.
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
Origin of the "waterfall" effect in phonon dispersion of relaxor perovskites.
Hlinka, J; Kamba, S; Petzelt, J; Kulda, J; Randall, C A; Zhang, S J
2003-09-01
We have undertaken an inelastic neutron scattering study of the perovskite relaxor ferroelectric Pb(Zn(1/3)Nb(2/3))O3 with 8% PbTiO3 (PZN-8%PT) in order to elucidate the origin of the previously reported unusual kink on the low frequency transverse phonon dispersion curve (known as the "waterfall effect"). We show that its position (q(wf)) depends on the choice of the Brillouin zone and that the relation of q(wf) to the size of the polar nanoregions is highly improbable. The waterfall phenomenon is explained in the framework of a simple model of coupled damped harmonic oscillators representing the acoustic and optic phonon branches.
Phonon surface mapping of graphite: Disentangling quasi-degenerate phonon dispersions
NASA Astrophysics Data System (ADS)
Grüneis, A.; Serrano, J.; Bosak, A.; Lazzeri, M.; Molodtsov, S. L.; Wirtz, L.; Attaccalite, C.; Krisch, M.; Rubio, A.; Mauri, F.; Pichler, T.
2009-08-01
The two-dimensional mapping of the phonon dispersions around the K point of graphite by inelastic x-ray scattering is provided. The present work resolves the longstanding issue related to the correct assignment of transverse and longitudinal phonon branches at K . We observe an almost degeneracy of the three TO-, LA-, and LO-derived phonon branches and a strong phonon trigonal warping. Correlation effects renormalize the Kohn anomaly of the TO mode, which exhibits a trigonal warping effect opposite to that of the electronic band structure. We determined the electron-phonon coupling constant to be 166(eV/Å)2 in excellent agreement to GW calculations. These results are fundamental for understanding angle-resolved photoemission, double-resonance Raman and transport measurements of graphene-based systems.
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.
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.
NASA Astrophysics Data System (ADS)
Cai, Ling; Toulouse, Jean; Luo, Haosu; Tian, Wei
2014-08-01
The lead free relaxor Na1/2Bi1/2TiO3 (NBT) undergoes a structural cubic-to-tetragonal transition near 800 K which is caused by the cooperative rotations of O6 octahedra. These rotations are also accompanied by the displacements of the cations and the formation of the polar nanodomains (PNDs) that are responsible for the characteristic dielectric dispersion of relaxor ferroelectrics. Because of their intrinsic properties, spontaneous polarization, and lack of inversion symmetry, these PNDs are also piezoelectric and can mediate an interaction between polarization and strain or couple the optic and acoustic phonons. Because PNDs introduce a local tetragonal symmetry, the phonon coupling they mediate is found to be anisotropic. In this paper we present inelastic neutron scattering results on coupled transverse acoustic (TA) and transverse optic (TO) phonons in the [110] and [001] directions and across the cubic-tetragonal phase transition at TC˜800 K. The phonon spectra are analyzed using a mode coupling model. In the [110] direction, as in other relaxors and some ferroelectric perovskites, a precipitous drop of the TO phonon into the TA branch or "waterfall" is observed at a certain qwf˜0.14 r.l.u. In the [001] direction, the highly overdamped line shape can be fitted with closely positioned bare mode energies which are largely overlapping along the dispersion curves. Two competing lattice coupling mechanism are proposed to explain these observations.
Thermal conductivity in $\text{large}-J$ two-dimensional antiferromagnets: Role of phonon scattering
Chernyshev, A. L.; Brenig, Wolfram
2015-08-05
Different types of relaxation processes for magnon heat current are discussed, with a particular focus on coupling to three-dimensional phonons. There is thermal conductivity by these in-plane magnetic excitations using two distinct techniques: Boltzmann formalism within the relaxation-time approximation and memory-function approach. Also considered are the scattering of magnons by both acoustic and optical branches of phonons. We demonstrate an accord between the two methods, regarding the asymptotic behavior of the effective relaxation rates.
It is strongly suggested that scattering from optical or zone-boundary phonons is important for magnon heat current relaxation in a high-temperature window of ΘD≲T<< J.
Morozovska, Anna N.; Vysochanskii, Yulian M.; Varenyk, Oleksandr V.; Silibin, Maxim V.; Kalinin, Sergei V.; Eliseev, Eugene A.
2015-09-29
The impact of the flexoelectric effect on the generalized susceptibility and soft phonon dispersion is not well known in the long-range-ordered phases of ferroics. Within the Landau-Ginzburg-Devonshire approach we obtained analytical expressions for the generalized susceptibility and phonon dispersion relations in the ferroelectric phase. The joint action of the static and dynamic flexoelectric effects induces nondiagonal components of the generalized susceptibility, whose amplitude is proportional to the convolution of the spontaneous polarization with the flexocoupling constants. The flexocoupling essentially broadens the k spectrum of the generalized susceptibility and leads to an additional “pushing away” of the optical and acoustic softmore » mode phonon branches. The degeneracy of the transverse optical and acoustic modes disappears in the ferroelectric phase in comparison with the paraelectric phase due to the joint action of flexoelectric coupling and ferroelectric nonlinearity. Lastly, the results obtained might be mainly important for theoretical analyses of a broad spectrum of experimental data, including neutron and Brillouin scattering.« less
Morozovska, Anna N.; Vysochanskii, Yulian M.; Varenyk, Oleksandr V.; Silibin, Maxim V.; Kalinin, Sergei V.; Eliseev, Eugene A.
2015-09-29
The impact of the flexoelectric effect on the generalized susceptibility and soft phonon dispersion is not well known in the long-range-ordered phases of ferroics. Within the Landau-Ginzburg-Devonshire approach we obtained analytical expressions for the generalized susceptibility and phonon dispersion relations in the ferroelectric phase. The joint action of the static and dynamic flexoelectric effects induces nondiagonal components of the generalized susceptibility, whose amplitude is proportional to the convolution of the spontaneous polarization with the flexocoupling constants. The flexocoupling essentially broadens the k spectrum of the generalized susceptibility and leads to an additional “pushing away” of the optical and acoustic soft mode phonon branches. The degeneracy of the transverse optical and acoustic modes disappears in the ferroelectric phase in comparison with the paraelectric phase due to the joint action of flexoelectric coupling and ferroelectric nonlinearity. Lastly, the results obtained might be mainly important for theoretical analyses of a broad spectrum of experimental data, including neutron and Brillouin scattering.
Coupled bipolarons and optical phonons as a model for high-T(sub c) superconductors
NASA Technical Reports Server (NTRS)
Kasperczyk, J.
1990-01-01
The coherence length of the new high-temperature superconductors reaches very small value which is comparable to the dimensions of unit cell of these compounds. This means that a pair consists of two holes occupying the same site or two adjacent sites. Such a situation seems to be described by a model of the local-pairs (bipolarons). The origin of local-pairs may come not only from strong enough electron or hole-phonon interaction but also from other interactions. Independent of the specific nature of such local-pairs, they can undergo a Bose-like condensation to the superconducting state at a critical temperature which is usually much lower than the temperature of the pair formation. An interplay of ferroelectric and superconducting properties is considered within the model of hole-like local-pairs interacting with optical phonons. Therefore, researchers extend the usual local-pair Hamiltonian by including a direct interaction between the local-pairs and the optical phonons. These optical phonons are known to play an important role in the ferroelectric transition, if any, and they transform into an additional pseudo-acoustic branch at the ferroelectric critical temperature. (This is associated with nonzero electric polarization due to the existence of two separate lattices composed of negative and positive ions, respectively.)
Coupled bipolarons and optical phonons as a model for high-Tc superconductors
NASA Technical Reports Server (NTRS)
Kasperczyk, J.
1991-01-01
The coherence length of the new high-temperature superconductors reaches a small value which is comparable to the dimensions of the unit cell of the compound. This means that a pair consists of two holes occupying the same site or two adjacent sites. Such a situation is described by a model of the local-pairs (bipolarons). The origin of local-pairs may come not only from strong enough electron or hole-phonon interaction but also from other interactions. Independent of the specific nature of such local-pairs, they can undergo a Bose-like condensation to the superconducting state at a critical temperature which is usually much lower than the temperature of the pair formation. An interplay of ferroelectric and superconducting properties is considered within the model of hole-like local-pairs interacting with optical phonons. Therefore, researchers extend the usual local-pair Hamiltonian by including a direct interaction between the local-pairs and the optical phonons. These optical phonons are known to play an important role in the ferroelectric transition and they transform into an additional pseudo-acoustic branch at the ferroelectric critical temperature. (This is associated with nonzero electric polarization due to the existence of two separate lattices composed of negative and positive ions, respectively.)
Effects of vacancies on phonon entropy of B2 FeAl
Lucas, M.S.; Delaire, O.; Winterrose, M.L.; Swan-Wood, T.; Kresch, M.; Halevy, I.; Fultz, B.; Hu, Jingzhu; Lerche, M.; Hu, M.Y.; Somayazulu, M.; CIT; ORNL; CIW; UC
2010-01-12
The phonon density of states (DOS) and phonon entropy of B2 FeAl were determined as functions of the Fe site vacancy concentration using several scattering techniques and were computed from first principles. Measurements at elevated temperature and pressure were performed to explore volume effects, test the usefulness of the quasiharmonic (QH) approximation, and provide comparison for the first-principles calculations. The average temperature and pressure dependencies of phonons were consistent with the QH model. The decrease in specific volume associated with the introduction of vacancies causes a stiffening of the DOS that was captured well with the experimentally determined Grueneisen parameter. Features associated with vacancies in the DOS are not well explained by this model, however, especially in the gap between the acoustic and optic branches. First-principles calculations indicated that these modes are primarily associated with vibrations of Al atoms in the first-nearest-neighbor shell of the vacancy, with some vibration amplitude also involving the second-nearest-neighbor Fe atoms. At the vacancy concentrations of study, the phonon entropy of vacancy formation was found to be approximately -1.7k{sub B}/atom, about half as large and of opposite sign as the configurational entropy of vacancy formation.
El-Kady, Ihab F.; Olsson, Roy H.
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.
Xia, H. Patterson, R.; Feng, Y.; Shrestha, S.; Conibeer, G.
2014-08-11
The rates of charge carrier relaxation by phonon emission are of substantial importance in the field of hot carrier solar cell, primarily in investigation of mechanisms to slow down hot carrier cooling. In this work, energy and momentum resolved deformation potentials relevant to electron-phonon scattering are computed for wurtzite InN and GaN as well as an InN/GaN multiple quantum well (MQW) superlattice using ab-initio methods. These deformation potentials reveal important features such as discontinuities across the electronic bandgap of the materials and variations over tens of eV. The energy dependence of the deformation potential is found to be very similar for wurtzite nitrides despite differences between the In and Ga pseudopotentials and their corresponding electronic band structures. Charge carrier relaxation by this mechanism is expected to be minimal for electrons within a few eV of the conduction band edge. However, hole scattering at energies more accessible to excitation by solar radiation is possible between heavy and light hole states. Moderate reductions in overall scattering rates are observed in MQW relative to the bulk nitride materials.
Emission of Phonons from a Rotating Sonic Black Hole
NASA Astrophysics Data System (ADS)
Fang, Heng-Zhong; Zhou, Kai-Hu
2014-01-01
We investigate the Hawking radiation from a rotating acoustic black hole. The phonon emission is calculated by using two methods and the same results are obtained. The contribution of the time coordinate to the phonon radiation is also discussed, which cannot be ignored for the coordinate systems that are not well-behaved at the horizon.
Phononic filter effect of rattling phonons in the thermoelectric clathrate Ba8Ge40+xNi6-x
NASA Astrophysics Data System (ADS)
Euchner, H.; Pailhès, S.; Nguyen, L. T. K.; Assmus, W.; Ritter, F.; Haghighirad, A.; Grin, Y.; Paschen, S.; de Boissieu, M.
2012-12-01
One of the key requirements for good thermoelectric materials is a low lattice thermal conductivity. Here we present a combined neutron scattering and theoretical investigation of the lattice dynamics in the type I clathrate system Ba-Ge-Ni, which fulfills this requirement. We observe a strong hybridization between phonons of the Ba guest atoms and acoustic phonons of the Ge-Ni host structure over a wide region of the Brillouin zone, which is in contrast with the frequently adopted picture of isolated Ba atoms in Ge-Ni host cages. It occurs without a strong decrease of the acoustic phonon lifetime, which contradicts the usual assumption of strong anharmonic phonon-phonon scattering processes. Within the framework of ab initio density-functional theory calculations we interpret these hybridizations as a series of anticrossings which act as a low-pass filter, preventing the propagation of acoustic phonons. To highlight the effect of such a phononic low-pass filter on the thermal transport, we compute the contribution of acoustic phonons to the thermal conductivity of Ba8Ge40Ni6 and compare it to those of pure Ge and a Ge46 empty-cage model system.
Gudelli, Vijay Kumar; Kanchana, V.; Vaitheeswaran, G.; Singh, David J.; Svane, Axel; Christensen, Niels Egede; Mahanti, Subhendra D.
2015-07-15
Here, we report calculations of the electronic structure, vibrational properties, and transport for the p-type semiconductors, SrAgChF (Ch = S, Se, and Te). We find soft phonons with low frequency optical branches intersecting the acoustic modes below 50 cm–1, indicative of a material with low thermal conductivity. The bands at and near the valence-band maxima are highly two-dimensional, which leads to high thermopowers even at high carrier concentrations, which is a combination that suggests good thermoelectric performance. These materials may be regarded as bulk realizations of superlattice thermoelectrics.
Size and dimensionality dependent phonon conductivity in nanocomposites.
Al-Otaibi, Jawaher; Srivastava, G P
2016-04-13
We have studied size and dimensionality dependent phonon conductivity of PbTe-PbSe nanocomposites by considering three configurations: superlattice, embedded nanowire and embedded nanodot. Calculations have been performed in the framework of an effective medium theory. The required bulk thermal conductivities of PbTe and PbSe are evaluated by using Callaway's effective relaxation-time theory, and by accounting for relevant scattering mechanism including three-phonon Normal and Umklapp interactions involving acoustic as well as optical branches. The thermal interface resistance is computed using the diffuse mismatch theory. It is found that the size (thickness) and volume fraction of PbSe are the two main factors that control the effective thermal conductivity in these nanocomposites. In particular, for PbSe size d = 10 nm and volume fraction Vf = 0.1, our results predict significant reductions over the weighted average of room-temperature bulk results of 9%, 17% and 15% in the conductivity across the interfaces for the superlattice, embedded nanowire, and nanosphere structures, respectively. For a given Vf, an increase in d reduces the interface density and the effective conductivity varies approximately as [Formula: see text]. It is shown that nanocompositing in any of the three configurations can beat the alloy limit for lattice thermal conductivity. PMID:26974428
Two-phonon processes of intraband relaxation in the terahertz regime in quantum dots.
Wang, Zi-Wu; Li, Shu-Shen
2011-06-01
We theoretically investigate the intraband relaxation of quantum dots in the terahertz regime due to two acoustic phonon scattering by applying a lattice relaxation approach based on the deformation potential coupling between electrons and acoustic phonons. In particular, we find that the relaxation time depends strongly on the ratio of two acoustic phonons. The influences of the energy separation between the ground and first excited state, the quantum dot height, and the lattice temperature on the relaxation time are also discussed. Our theoretical results not only give a reasonable explanation for the current experimental measurement but also provide some insight into two-phonon intraband relaxation in quantum dots.
Molecular dynamics study of phonon screening in graphene
NASA Astrophysics Data System (ADS)
Javvaji, Brahmanandam; Roy Mahapatra, D.; Raha, S.
2014-04-01
Phonon interaction with electrons or phonons or with structural defects result in a phonon mode conversion. The mode conversion is governed by the frequency wave-vector dispersion relation. The control over phonon mode or the screening of phonon in graphene is studied using the propagation of amplitude modulated phonon wave-packet. Control over phonon properties like frequency and velocity opens up several wave guiding, energy transport and thermo-electric applications of graphene. One way to achieve this control is with the introduction of nano-structured scattering in the phonon path. Atomistic model of thermal energy transport is developed which is applicable to devices consisting of source, channel and drain parts. Longitudinal acoustic phononmode is excited fromone end of the device. Molecular dynamics based time integration is adopted for the propagation of excited phonon to the other end of the device. The amount of energy transfer is estimated from the relative change of kinetic energy. Increase in the phonon frequency decreases the kinetic energy transmission linearly in the frequency band of interest. Further reduction in transmission is observed with the tuning of channel height of the device by increasing the boundary scattering. Phonon mode selective transmission control have potential application in thermal insulation or thermo-electric application or photo-thermal amplification.
Phonon-Photon Mapping in a Color Center in Hexagonal Boron Nitride.
Vuong, T Q P; Cassabois, G; Valvin, P; Ouerghi, A; Chassagneux, Y; Voisin, C; Gil, B
2016-08-26
We report on the ultraviolet optical response of a color center in hexagonal boron nitride. We demonstrate a mapping between the vibronic spectrum of the color center and the phonon dispersion in hexagonal boron nitride, with a striking suppression of the phonon assisted emission signal at the energy of the phonon gap. By means of nonperturbative calculations of the electron-phonon interaction in a strongly anisotropic phonon dispersion, we reach a quantitative interpretation of the acoustic phonon sidebands from cryogenic temperatures up to room temperature. Our analysis provides an original method for estimating the spatial extension of the electronic wave function in a point defect. PMID:27610882
Phonon-Photon Mapping in a Color Center in Hexagonal Boron Nitride
NASA Astrophysics Data System (ADS)
Vuong, T. Q. P.; Cassabois, G.; Valvin, P.; Ouerghi, A.; Chassagneux, Y.; Voisin, C.; Gil, B.
2016-08-01
We report on the ultraviolet optical response of a color center in hexagonal boron nitride. We demonstrate a mapping between the vibronic spectrum of the color center and the phonon dispersion in hexagonal boron nitride, with a striking suppression of the phonon assisted emission signal at the energy of the phonon gap. By means of nonperturbative calculations of the electron-phonon interaction in a strongly anisotropic phonon dispersion, we reach a quantitative interpretation of the acoustic phonon sidebands from cryogenic temperatures up to room temperature. Our analysis provides an original method for estimating the spatial extension of the electronic wave function in a point defect.
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)
Sundaresan, Sasi Sekaran
Monte Carlo based statistical approach to solve Boltzmann Transport Equation (BTE) has become a norm to investigate heat transport in semiconductors at sub-micron regime, owing mainly to its ability to characterize realistically sized device geometries qualitatively. One of the primary issues with this technique is that the approach predominantly uses empirically fitted phonon dispersion relations as input to determine the properties of phonons so as to predict the thermal conductivity of specified material geometry. The empirically fitted dispersion relations assume harmonic approximation thereby failing to account for thermal expansion, interaction of lattice waves, effect of strain on spring stiffness, and accurate phonon-phonon interaction. To circumvent this problem, in this work, a coupled molecular mechanics-Monte Carlo (MM-MC) platform has been developed and used to solve the phonon Boltzmann Transport Equation (BTE) for the calculation of thermal conductivity of several novel and emerging nanostructures. The use of the quasi-anharmonic MM approach (as implemented in the open source NEMO 3-D software toolkit) not only allows one to capture the true atomicity of the underlying lattice but also enables the simulation of realistically-sized structures containing millions of atoms. As compared to the approach using an empirically fitted phonon dispersion relation, here, a 17% increase in the thermal conductivity for a silicon nanowire due to the incorporation of atomistic corrections in the LA (longitudinal acoustic) branch alone has been reported. The atomistically derived thermal conductivity as calculated from the MM-MC framework is then used in the modular design and analysis of (i) a silicon nanowire based thermoelectric cooler (TEC) unit, and (ii) a GaN/InN based nanostructured light emitting device (LED). It is demonstrated that the use of empirically fitted phonon bandstructure parameters overestimates the temperature difference between the hot and the
Wong, Joe; Krisch, M.; Farber, D.; Occelli, F.; Schwartz, A.; Chiang, T.C.; Wall, M.; Boro, C.; Xu, Ruqing
2010-11-16
high resolution inelastic x-ray scattering (HRIXS) capability on ID28. The complete PDCs for an fcc Pu-0.6 wt% Ga alloy are plotted in Figure 2, and represent the first full set of phonon dispersions ever determined for any Pu-bearing materials. The solid curves (red) are calculated using a standard Born-von Karman (B-vK) force constant model. An adequate fit to the experimental data is obtained if interactions up to the fourth-nearest neighbours are included. The dashed curves (blue) are recent dynamical mean field theory (DMFT) results by Dai et al. The elastic moduli calculated from the slopes of the experimental phonon dispersion curves near the {Lambda} point are: C{sub 11} = 35.3 {+-} 1.4 GPa, C{sub 12} = 25.5 {+-} 1.5 GPa and C{sub 44} = 30.53 {+-} 1.1 GPa. These values are in excellent agreement with those of the only other measurement on a similar alloy (1 wt % Ga) using ultrasonic techniques as well as with those recently calculated from a combined DMFT and linear response theory for pure {delta}-Pu. Several unusual features, including a large elastic anisotropy, a small shear elastic modulus C{prime}, a Kohn-like anomaly in the T{sub 1}[011] branch, and a pronounced softening of the [111] transverse modes are found. These features can be related to the phase transitions of plutonium and to strong coupling between the lattice structure and the 5f valence instabilities. The HRIXS results also provide a critical test for theoretical treatments of highly correlated 5f electron systems as exemplified by recent dynamical mean field theory (DMFT) calculations for {delta}-plutonium. The experimental-theoretical agreements shown in Figure 2 in terms of a low shear elastic modulus C{prime}, a Kohn-like anomaly in the T{sub 1}[011] branch, and a large softening of the T[111] modes give credence to the DMFT approach for the theoretical treatment of 5f electron systems of which {delta}-Pu is a classic example. However, quantitative differences remain. These are the
Maximizing phononic band gaps in piezocomposite materials by means of topology optimization.
Vatanabe, Sandro L; Paulino, Glaucio H; Silva, Emílio C N
2014-08-01
Phononic crystals (PCs) can exhibit phononic band gaps within which sound and vibrations at certain frequencies do not propagate. In fact, PCs with large band gaps are of great interest for many applications, such as transducers, elastic/acoustic filters, noise control, and vibration shields. Previous work in the field concentrated on PCs made of elastic isotropic materials; however, band gaps can be enlarged by using non-isotropic materials, such as piezoelectric materials. Because the main property of PCs is the presence of band gaps, one possible way to design microstructures that have a desired band gap is through topology optimization. Thus in this work, the main objective is to maximize the width of absolute elastic wave band gaps in piezocomposite materials designed by means of topology optimization. For band gap calculation, the finite element analysis is implemented with Bloch-Floquet theory to solve the dynamic behavior of two-dimensional piezocomposite unit cells. Higher order frequency branches are investigated. The results demonstrate that tunable phononic band gaps in piezocomposite materials can be designed by means of the present methodology.
NASA Astrophysics Data System (ADS)
Iyer, Srikanth S.; Candler, Robert N.
2016-03-01
In this work, we determine the intrinsic mechanical energy dissipation limit for single-crystal resonators due to anharmonic phonon-phonon scattering in the Akhiezer (Ω τ ≪1 ) regime. The energy loss is derived using perturbation theory and the linearized Boltzmann transport equation for phonons, and includes the direction- and polarization-dependent mode-Grüneisen parameters in order to capture the strain-induced anharmonicity among phonon branches. This expression reveals the fundamental differences among the internal friction limits for different types of bulk-mode elastic waves. For cubic crystals, 2D-extensional modes have increased dissipation compared to width-extensional modes because the biaxial deformation opposes the natural Poisson contraction of the solid. Additionally, we show that shear-mode vibrations, which preserve volume, have significantly reduced energy loss because dissipative phonon-phonon scattering is restricted to pure-shear phonon branches, indicating that Lamé- or wineglass-mode resonators will have the highest upper limit on mechanical efficiency. Finally, we employ key simplifications to evaluate the quality factor limits for common mode shapes in single-crystal silicon devices, explicitly including the correct effective elastic storage moduli for different vibration modes and crystal orientations. Our expression satisfies the pressing need for a reliable analytical model that can predict the phonon-phonon dissipation limits for modern resonant microelectromechanical systems, where precise manufacturing techniques and accurate finite-element methods can be used to select particular vibrational mode shapes and crystal orientations.
Optic phonon bandwidth and lattice thermal conductivity: The case of L i2X (X =O , S, Se, Te)
NASA Astrophysics Data System (ADS)
Mukhopadhyay, S.; Lindsay, L.; Parker, D. S.
2016-06-01
We examine the lattice thermal conductivities (κl) of L i2X (X =O ,S ,Se ,Te ) using a first-principles Peierls-Boltzmann transport methodology. We find low κl values ranging between 12 and 30 W m-1K-1 despite light Li atoms, a large mass difference between constituent atoms, and tightly bunched acoustic branches, all features that give high κl in other materials including BeSe (630 W m-1K-1 ), BeTe (370 W m-1K-1 ), and cubic BAs (3170 W m-1K-1 ). Together these results suggest a missing ingredient in the basic guidelines commonly used to understand and predict κl. Unlike typical simple systems (e.g., Si, GaAs, SiC), the dominant resistance to heat-carrying acoustic phonons in L i2Se and L i2Te comes from interactions of these modes with two optic phonons. These interactions require significant bandwidth and dispersion of the optic branches, both present in L i2X materials. These considerations are important for the discovery and design of new materials for thermal management applications and give a more comprehensive understanding of thermal transport in crystalline solids.
Optic phonon bandwidth and lattice thermal conductivity: The case of Li2X ( X=O , S, Se, Te)
Mukhopadhyay, S.; Lindsay, L.; Parker, D. S.
2016-06-07
Here, we examine the lattice thermal conductivities ( l) of Li2X (X=O, S, Se, Te) using a first-principles Peierls-Boltzmann transport methodology. We find low l values ranging between 12 and 30 W/m-K despite light Li atoms, a large mass difference between constituent atoms and tightly bunched acoustic branches, all features that give high l in other materials including BeSe (630 W/m-1K-1), BeTe (370 W/m-1K-1) and cubic BAs (3150 W/m-1K-1). Together these results suggest a missing ingredient in the basic guidelines commonly used to understand and predict l. Unlike typical simple systems (e.g., Si, GaAs, SiC), the dominant resistance to heat-carryingmore » acoustic phonons in Li2Se and Li2Te comes from interactions of these modes with two optic phonons. These interactions require significant bandwidth and dispersion of the optic branches, both present in Li2X materials. Finally, these considerations are important for the discovery and design of new materials for thermal management applications, and give a more comprehensive understanding of thermal transport in crystalline solids.« less
Gudelli, Vijay Kumar; Kanchana, V.; Vaitheeswaran, G.; Singh, David J.; Svane, Axel; Christensen, Niels Egede; Mahanti, Subhendra D.
2015-07-15
Here, we report calculations of the electronic structure, vibrational properties, and transport for the p-type semiconductors, SrAgChF (Ch = S, Se, and Te). We find soft phonons with low frequency optical branches intersecting the acoustic modes below 50 cm^{–1}, indicative of a material with low thermal conductivity. The bands at and near the valence-band maxima are highly two-dimensional, which leads to high thermopowers even at high carrier concentrations, which is a combination that suggests good thermoelectric performance. These materials may be regarded as bulk realizations of superlattice thermoelectrics.
Resonant plasmon-phonon coupling and its role in magneto-thermoelectricity in bismuth
NASA Astrophysics Data System (ADS)
Chudzinski, Piotr
2015-12-01
Using diagrammatic methods we derive an effective interaction between a low energy collective movement of fermionic liquid (acoustic plasmon) and acoustic phonon. We show that the coupling between the plasmon and the lattice has a very non-trivial, resonant structure. When real and imaginary parts of the acoustic plasmon's velocity are of the same order as the phonon's velocity, the resonance qualitatively changes the nature of phonon-drag. In the following we study how magneto-thermoelectric properties are affected. Our result suggests that the novel mechanism of energy transfer between electron liquid and crystal lattice can be behind the huge Nernst effect in bismuth.
Phonon anharmonicity in bulk Td-MoTe2
NASA Astrophysics Data System (ADS)
Joshi, Jaydeep; Stone, Iris R.; Beams, Ryan; Krylyuk, Sergiy; Kalish, Irina; Davydov, Albert V.; Vora, Patrick M.
2016-07-01
We examine anharmonic contributions to the optical phonon modes in bulk Td-MoTe2 through temperature-dependent Raman spectroscopy. At temperatures ranging from 100 K to 200 K, we find that all modes redshift linearly with temperature in agreement with the Grüneisen model. However, below 100 K, we observe nonlinear temperature-dependent frequency shifts in some modes. We demonstrate that this anharmonic behavior is consistent with the decay of an optical phonon into multiple acoustic phonons. Furthermore, the highest frequency Raman modes show large changes in intensity and linewidth near T ≈ 250 K that correlate well with the T d → 1 T ' structural phase transition. These results suggest that phonon-phonon interactions can dominate anharmonic contributions at low temperatures in bulk Td-MoTe2, an experimental regime that is currently receiving attention in efforts to understand Weyl semimetals.
Acoustic Faraday rotation in Weyl semimetals
NASA Astrophysics Data System (ADS)
Liu, Donghao; Shi, Junren
We investigate the phonon problems in Weyl semimetals, from which both the phonon Berry curvature and the phonon Damping could be obtained. We show that even without a magnetic field, the degenerate transverse acoustic modes could also be split due to the adiabatic curvature. In three dimensional case, acoustic Faraday rotation shows up. And furthermore, since the attenuation procedure could distinguish the polarized mode, single circularly polarized acoustic wave could be realized. We study the mechanism in the novel time reversal symmetry broken Weyl semimetal. New effects rise because of the linear dispersion, which give enlightenment in the measurement of this new kind of three-dimensional material.
NASA Astrophysics Data System (ADS)
González, David G.; Sols, Fernando; Guinea, Francisco; Zapata, Ivar
2016-08-01
We investigate the interaction between the electrons of a two-dimensional metal and the acoustic phonons of an underlying piezoelectric substrate. Fundamental inequalities can be obtained from general energy arguments. As a result, phonon mediated attraction can be proven to never overcome electron Coulomb repulsion, at least for long phonon wavelengths. We study the influence of these phonons on the possible pairing instabilities of a two-dimensional electron gas such as graphene.
Phononic crystals of spherical particles: A tight binding approach
Mattarelli, M.; Secchi, M.; Montagna, M.
2013-11-07
The vibrational dynamics of a fcc phononic crystal of spheres is studied and compared with that of a single free sphere, modelled either by a continuous homogeneous medium or by a finite cluster of atoms. For weak interaction among the spheres, the vibrational dynamics of the phononic crystal is described by shallow bands, with low degree of dispersion, corresponding to the acoustic spheroidal and torsional modes of the single sphere. The phonon displacements are therefore related to the vibrations of a sphere, as the electron wave functions in a crystal are related to the atomic wave functions in a tight binding model. Important dispersion is found for the two lowest phonon bands, which correspond to zero frequency free translation and rotation of a free sphere. Brillouin scattering spectra are calculated at some values of the exchanged wavevectors of the light, and compared with those of a single sphere. With weak interaction between particles, given the high acoustic impedance mismatch in dry systems, the density of phonon states consist of sharp bands separated by large gaps, which can be well accounted for by a single particle model. Based on the width of the frequency gaps, tunable with the particle size, and on the small number of dispersive acoustic phonons, such systems may provide excellent materials for application as sound or heat filters.
Gorishnyy, T; Ullal, C K; Maldovan, M; Fytas, G; Thomas, E L
2005-03-25
In this Letter we propose the use of hypersonic phononic crystals to control the emission and propagation of high frequency phonons. We report the fabrication of high quality, single crystalline hypersonic crystals using interference lithography and show that direct measurement of their phononic band structure is possible with Brillouin light scattering. Numerical calculations are employed to explain the nature of the observed propagation modes. This work lays the foundation for experimental studies of hypersonic crystals and, more generally, phonon-dependent processes in nanostructures.
Coherent phonon decay and the boron isotope effect for MgB2.
Alarco, Jose A; Talbot, Peter C; Mackinnon, Ian D R
2014-12-14
Ab initio DFT calculations for the phonon dispersion (PD) and the phonon density of states (PDOS) of the two isotopic forms ((10)B and (11)B) of MgB2 demonstrate that use of a reduced symmetry super-lattice provides an improved approximation to the dynamical, phonon-distorted P6/mmm crystal structure. Construction of phonon frequency plots using calculated values for these isotopic forms gives linear trends with integer multiples of a base frequency that change in slope in a manner consistent with the isotope effect (IE). Spectral parameters inferred from this method are similar to that determined experimentally for the pure isotopic forms of MgB2. Comparison with AlB2 demonstrates that a coherent phonon decay down to acoustic modes is not possible for this metal. Coherent acoustic phonon decay may be an important contributor to superconductivity for MgB2.
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.
Research on micro-sized acoustic bandgap structures.
Fleming, James Grant; McCormick, Frederick Bossert; Su, Mehmet F.; El-Kady, Ihab Fathy; Olsson, Roy H., III; Tuck, Melanie R.
2010-01-01
Phononic crystals (or acoustic 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 (or phononic) bandgaps. The vast majority of phononic crystal devices reported prior to this LDRD were constructed by hand assembling scattering inclusions in a lossy viscoelastic medium, predominantly air, water or epoxy, resulting in large structures limited to frequencies below 1 MHz. Under this LDRD, phononic crystals and devices were scaled to very (VHF: 30-300 MHz) and ultra (UHF: 300-3000 MHz) high frequencies utilizing finite difference time domain (FDTD) modeling, microfabrication and micromachining technologies. This LDRD developed key breakthroughs in the areas of micro-phononic crystals including physical origins of phononic crystals, advanced FDTD modeling and design techniques, material considerations, microfabrication processes, characterization methods and device structures. Micro-phononic crystal devices realized in low-loss solid materials were emphasized in this work due to their potential applications in radio frequency communications and acoustic imaging for medical ultrasound and nondestructive testing. The results of the advanced modeling, fabrication and integrated transducer designs were that this LDRD produced the 1st measured phononic crystals and phononic crystal devices (waveguides) operating in the VHF (67 MHz) and UHF (937 MHz) frequency bands and established Sandia as a world leader in the area of micro-phononic crystals.
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.
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
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.
Coherent optical phonon oscillation and possible electronic softening in WTe2 crystals
NASA Astrophysics Data System (ADS)
He, Bin; Zhang, Chunfeng; Zhu, Weida; Li, Yufeng; Liu, Shenghua; Zhu, Xiyu; Wu, Xuewei; Wang, Xiaoyong; Wen, Hai-Hu; Xiao, Min
2016-07-01
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.
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
Sub-wavelength phononic crystal liquid sensor
NASA Astrophysics Data System (ADS)
Ke, Manzhu; Zubtsov, Mikhail; Lucklum, Ralf
2011-07-01
We introduce an acoustic liquid sensor based on phononic crystals consisting of steel plate with an array of holes filled with liquid. We both theoretically and experimentally demonstrate sensor properties considering the mechanism of the extraordinary acoustic transmission as underlying phenomenon. The frequency of this resonant transmission peak is shown to rely on the speed of sound of the liquid, and the resonant frequency can be used as a measure of speed of sound and related properties, like concentration of a component in the liquid mixture. The finite-difference time domain method has been applied for sensor design. Ultrasonic transmission experiments are performed. Good consistency of the resonant frequency shift has been found between theoretical results and experiments. The proposed scheme offers a platform for an acoustic liquid sensor.
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.
Microwave-frequency electromechanical resonators incorporating phononic crystals
NASA Astrophysics Data System (ADS)
Satzinger, K. J.; Peairs, G.; Vainsencher, A.; Cleland, A. N.
Piezoelectric micromechanical resonators at gigahertz frequencies have been operated in the quantum limit, with quantum control and measurement achieved using superconducting qubits. However, experiments to date have been limited by mechanical dissipation, due to a combination of internal and radiative losses. In this talk, we explore the incorporation of phononic crystals into resonator designs. In phononic crystals, periodic patterning manipulates the acoustic band structure of the material. Through appropriately chosen geometries, these periodic patterns lead to full acoustic bandgaps which can be used to greatly reduce radiation losses from resonant structures. Alternatively, the crystal geometry can be manipulated to allow isolated modes within the bandgap, giving fine control over the spatial structure of the resonator modes. In this talk, we will describe the design, fabrication, and measurement of resonators with phononic crystals.
Calculation of Phonon Dispersion and Thermal Conductivity in Carbon Nanotubes
NASA Astrophysics Data System (ADS)
Varshney, Mayank
2005-03-01
Many potential applications of carbon nanotubes in nanoelectronic circuits rely on effective removing of excess heat from the device active area. Heat in carbon nanotubes is mostly carried by acoustic phonons. In this work we have calculated phonon dispersion in carbon nanotubes using atomistic approach. The phonon dispersion was then used to calculate phonon density of states, heat capacitance and thermal conductivity. The thermal conductivity has been determined using the modified Callaway -- Klemens approach, which accounts for the low-dimensional size effects [1]. The results of our calculations are compared with the experimental Raman spectroscopic study of carbon nanotubes and reported values of the thermal conductivity. The authors acknowledge the support of MARCO and its Functional Engineered Nano Architectonics (FENA) Focus Center. [1] A.A. Balandin, Thermal Conductivity of Semiconductor Nanostructures, in Encyclopedia of Nanoscience and Nanotechnology (ASP, Los Angeles, 2004) p. 425.
Ward, Sally P; Leyser, Ottoline
2004-02-01
The mature form of a plant shoot system is an expression of several genetically controlled traits, many of which are also environmentally regulated. A major component of this architectural variation is the degree of shoot branching. Recent results indicate conserved mechanisms for shoot branch development across the monocots and eudicots. The existence of a novel long-range branch-inhibiting signal has been inferred from studies of branching mutants in pea and Arabidopsis. PMID:14732444
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
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
Temperature dependence of phonon-defect interactions: phonon scattering vs. phonon trapping
NASA Astrophysics Data System (ADS)
Bebek, M. B.; Stanley, C. M.; Gibbons, T. M.; Estreicher, S. K.
2016-08-01
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.
Broadband evolution of phononic-crystal-waveguide eigenstates in real- and k-spaces
Otsuka, P. H.; Nanri, K.; Matsuda, O.; Tomoda, M.; Profunser, D. M.; Veres, I. A.; Danworaphong, S.; Khelif, A.; Benchabane, S.; Laude, V.; Wright, O. B.
2013-01-01
Control of sound in phononic band-gap structures promises novel control and guiding mechanisms. Designs in photonic systems were quickly matched in phononics, and rows of defects in phononic crystals were shown to guide sound waves effectively. The vast majority of work in such phononic guiding has been in the frequency domain, because of the importance of the phononic dispersion relation in governing acoustic confinement in waveguides. However, frequency-domain studies miss vital information concerning the phase of the acoustic field and eigenstate coupling. Using a wide range of wavevectors k, we implement an ultrafast technique to probe the wave field evolution in straight and L-shaped phononic crystal surface-phonon waveguides in real- and k-space in two spatial dimensions, thus revealing the eigenstate-energy redistribution processes and the coupling between different frequency-degenerate eigenstates. Such use of k-t space is a first in acoustics, and should have other interesting applications such as acoustic-metamaterial characterization. PMID:24284621
Inelastic x-ray scattering measurements of phonon dynamics in URu2Si2
Gardner, D. R.; Bonnoit, C. J.; Chisnell, R.; Said, A. H.; Leu, B. M.; Williams, Travis J.; Luke, G. M.; Lee, Y. S.
2016-02-11
In this paper, we study high-resolution inelastic x-ray scattering measurements of the acoustic phonons of URu2Si2. At all temperatures, the longitudinal acoustic phonon linewidths are anomalously broad at small wave vectors revealing a previously unknown anharmonicity. The phonon modes do not change significantly upon cooling into the hidden order phase. In addition, our data suggest that the increase in thermal conductivity in the hidden order phase cannot be driven by a change in phonon dispersions or lifetimes. Hence, the phonon contribution to the thermal conductivity is likely much less significant compared to that of the magnetic excitations in the lowmore » temperature phase.« less
First-principles prediction of phononic thermal conductivity of silicene: A comparison with graphene
Gu, Xiaokun; Yang, Ronggui
2015-01-14
There has been great interest in two-dimensional materials, beyond graphene, for both fundamental sciences and technological applications. Silicene, a silicon counterpart of graphene, has been shown to possess some better electronic properties than graphene. However, its thermal transport properties have not been fully studied. In this paper, we apply the first-principles-based phonon Boltzmann transport equation to investigate the thermal conductivity of silicene as well as the phonon scattering mechanisms. Although both graphene and silicene are two-dimensional crystals with similar crystal structure, we find that phonon transport in silicene is quite different from that in graphene. The thermal conductivity of silicene shows a logarithmic increase with respect to the sample size due to the small scattering rates of acoustic in-plane phonon modes, while that of graphene is finite. Detailed analysis of phonon scattering channels shows that the linear dispersion of the acoustic out-of-plane (ZA) phonon modes, which is induced by the buckled structure, makes the long-wavelength longitudinal acoustic phonon modes in silicene not as efficiently scattered as that in graphene. Compared with graphene, where most of the heat is carried by the acoustic out-of-plane (ZA) phonon modes, the ZA phonon modes in silicene only have ∼10% contribution to the total thermal conductivity, which can also be attributed to the buckled structure. This systematic comparison of phonon transport and thermal conductivity of silicene and graphene using the first-principle-based calculations shed some light on other two-dimensional materials, such as two-dimensional transition metal dichalcogenides.
Nebulisation on a disposable array structured with phononic lattices.
Reboud, Julien; Wilson, Rab; Zhang, Yi; Ismail, Mohd H; Bourquin, Yannyk; Cooper, Jonathan M
2012-04-01
We demonstrate the use of a phononic crystal to enable the nebulisation of liquid droplets from low-cost disposable arrays, using surface acoustic waves (SAW). The SAWs were generated using interdigitated transducers (IDT) on a piezoelectric surface (LiNbO(3)) and the acoustic waves were coupled into a disposable phononic crystal structure, referred to as a superstrate. Using its excellent reflecting properties, the phononic structures confined the acoustic field within the superstrate, resulting in the concentration of the acoustic energy, in a manner controllable by the excitation frequency. We show that this capability mitigates against coupling losses incurred by the use of a disposable superstrate, greatly reducing the time needed to nebulise a drop of water with respect to an unstructured superstrate for a given power. We also demonstrate that by changing the excitation frequency, it is possible to change the spatial position at which the acoustic energy is concentrated, providing a means to specifically nebulise drops across an array. These results open up a promising future for the use of phonofluidics in high-throughput sample handling applications, such as drug delivery or the "soft" transfer of samples to a mass spectrometer in the field of proteomics. PMID:22327572
Direct measurement of coherent thermal phonons in Bi2Te3/Sb2Te3 superlattice
NASA Astrophysics Data System (ADS)
He, Feng; Wu, Wenzhi; Wang, Yaguo
2016-08-01
Coherent thermal phonons (CTPs) play an important role in thermal transport in superlattice (SL) structures. To have a profound understanding of CTP transport in SL, direct measurement of CTP properties is necessary. In this study, coherent phonon spectroscopy has been utilized to generate and detect CTP in Bi2Te3/Sb2Te3 SL. Phonon lifetimes have been extracted from experimental data, with which mode-wise thermal conductivities have been calculated. Comparing with bulk Bi2Te3, the estimated mode-wise thermal conductivity of longitudinal acoustic phonons shifts to higher frequencies, due to constructive coherent phonon interference. Our results suggest that it is possible to use SL structure to manipulate coherent phonon propagation and to tailor thermal conductivity.
Phonons and hybrid modes in the high and low temperature far infrared dynamics of hexagonal TmMnO3.
Massa, Néstor E; del Campo, Leire; De Sousa Meneses, Domingos; Echegut, Patrick; Martínez-Lope, María Jesús; Alonso, José Antonio
2014-07-01
We report on temperature dependent TmMnO3 far infrared emissivity and reflectivity spectra from 1910 K to 4 K. At the highest temperature the number of infrared bands is lower than that predicted for centrosymmetric P63/mmc (D(4)(6h)) (Z = 2) space group due to high temperature anharmonicity and possible defect induced bitetrahedra misalignments. On cooling, at ~1600 ± 40 K, TmMnO3 goes from non-polar to an antiferroelectric-ferroelectric polar phase reaching the ferroelectric onset at ~700 K. Room temperature reflectivity is fitted using 19 oscillators and this number of phonons is maintained down to 4 K. A weak phonon anomaly in the band profile at 217 cm(-1) (4 K) suggests subtle Rare Earth magneto-electric couplings at ~TN and below. A low energy collective excitation is identified as a THz instability associated with room temperature eg electrons in a d-orbital fluctuating environment. It condenses into two modes that emerge pinned to the E-type antiferromagnetic order hardening simultaneously down to 4 K. They obey power laws with TN as the critical temperature and match known zone center magnons. The one peaking at 26 cm(-1), with critical exponent β=0.42 as for antiferromagnetic order in a hexagonal lattice, is dependent on the Rare Earth ion. The higher frequency companion at ~50 cm(-1), with β=0.25, splits at ~TN into two peaks. The weaker band of the two is assimilated to the upper branch of the gap opening in the transverse acoustical (TA) phonon branch crossing the magnetic dispersion found in YMnO3. (Petit et al 2007 Phys. Rev. Lett. 99 266604). The stronger second band at ~36 cm(-1) corresponds to the lower branch of the TA gap. We assign both excitations as zone center magneto-electric hybrid quasiparticles, concluding that in NdMnO3 perovskite the equivalent picture corresponds to an instability which may be driven by an external field to transform NdMnO3 into a multiferroic compound by perturbation enhancing the TA
Phonons and hybrid modes in the high and low temperature far infrared dynamics of hexagonal TmMnO3.
Massa, Néstor E; del Campo, Leire; De Sousa Meneses, Domingos; Echegut, Patrick; Martínez-Lope, María Jesús; Alonso, José Antonio
2014-07-01
We report on temperature dependent TmMnO3 far infrared emissivity and reflectivity spectra from 1910 K to 4 K. At the highest temperature the number of infrared bands is lower than that predicted for centrosymmetric P63/mmc (D(4)(6h)) (Z = 2) space group due to high temperature anharmonicity and possible defect induced bitetrahedra misalignments. On cooling, at ~1600 ± 40 K, TmMnO3 goes from non-polar to an antiferroelectric-ferroelectric polar phase reaching the ferroelectric onset at ~700 K. Room temperature reflectivity is fitted using 19 oscillators and this number of phonons is maintained down to 4 K. A weak phonon anomaly in the band profile at 217 cm(-1) (4 K) suggests subtle Rare Earth magneto-electric couplings at ~TN and below. A low energy collective excitation is identified as a THz instability associated with room temperature eg electrons in a d-orbital fluctuating environment. It condenses into two modes that emerge pinned to the E-type antiferromagnetic order hardening simultaneously down to 4 K. They obey power laws with TN as the critical temperature and match known zone center magnons. The one peaking at 26 cm(-1), with critical exponent β=0.42 as for antiferromagnetic order in a hexagonal lattice, is dependent on the Rare Earth ion. The higher frequency companion at ~50 cm(-1), with β=0.25, splits at ~TN into two peaks. The weaker band of the two is assimilated to the upper branch of the gap opening in the transverse acoustical (TA) phonon branch crossing the magnetic dispersion found in YMnO3. (Petit et al 2007 Phys. Rev. Lett. 99 266604). The stronger second band at ~36 cm(-1) corresponds to the lower branch of the TA gap. We assign both excitations as zone center magneto-electric hybrid quasiparticles, concluding that in NdMnO3 perovskite the equivalent picture corresponds to an instability which may be driven by an external field to transform NdMnO3 into a multiferroic compound by perturbation enhancing the TA
Phonons and hybrid modes in the high and low temperature far infrared dynamics of hexagonal TmMnO3
NASA Astrophysics Data System (ADS)
Massa, Néstor E.; del Campo, Leire; De Sousa Meneses, Domingos; Echegut, Patrick; Jesús Martínez-Lope, María; Alonso, José Antonio
2014-07-01
We report on temperature dependent TmMnO3 far infrared emissivity and reflectivity spectra from 1910 K to 4 K. At the highest temperature the number of infrared bands is lower than that predicted for centrosymmetric P63/mmc \\left(\\text{D}_{6\\text{h}}^{4}\\right) (Z = 2) space group due to high temperature anharmonicity and possible defect induced bitetrahedra misalignments. On cooling, at ~1600 ± 40 K, TmMnO3 goes from non-polar to an antiferroelectric-ferroelectric polar phase reaching the ferroelectric onset at ~700 K. Room temperature reflectivity is fitted using 19 oscillators and this number of phonons is maintained down to 4 K. A weak phonon anomaly in the band profile at 217 cm-1 (4 K) suggests subtle Rare Earth magneto-electric couplings at ~TN and below. A low energy collective excitation is identified as a THz instability associated with room temperature eg electrons in a d-orbital fluctuating environment. It condenses into two modes that emerge pinned to the E-type antiferromagnetic order hardening simultaneously down to 4 K. They obey power laws with TN as the critical temperature and match known zone center magnons. The one peaking at 26 cm-1, with critical exponent β=0.42 as for antiferromagnetic order in a hexagonal lattice, is dependent on the Rare Earth ion. The higher frequency companion at ~50 cm-1, with β=0.25, splits at ~TN into two peaks. The weaker band of the two is assimilated to the upper branch of the gap opening in the transverse acoustical (TA) phonon branch crossing the magnetic dispersion found in YMnO3. (Petit et al 2007 Phys. Rev. Lett. 99 266604). The stronger second band at ~36 cm-1 corresponds to the lower branch of the TA gap. We assign both excitations as zone center magneto-electric hybrid quasiparticles, concluding that in NdMnO3 perovskite the equivalent picture corresponds to an instability which may be driven by an external field to transform NdMnO3 into a multiferroic compound by
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.
PHONONS IN INTRINSIC JOSEPHSON SYSTEMS
C. PREIS; K. SCHMALZL; ET AL
2000-10-01
Subgap structures in the I-V curves of layered superconductors are explained by the excitation of phonons by Josephson oscillations. In the presence of a magnetic field applied parallel to the layers additional structures due to fluxon motion appear. Their coupling with phonons is investigated theoretically and a shift of the phonon resonances in strong magnetic fields is predicted.
Electronic structure, phonons, and thermal properties of ScN, ZrN, and HfN: A first-principles study
NASA Astrophysics Data System (ADS)
Saha, Bivas; Acharya, Jagaran; Sands, Timothy D.; Waghmare, Umesh V.
2010-02-01
With a motivation to understand microscopic aspects of ScN, ZrN, and HfN relevant to the thermoelectric properties of nitride metal/semiconductor superlattices, we determine their electronic structure, vibrational spectra and thermal properties using first-principles calculations based on density functional theory with a generalized gradient approximation of the exchange correlation energy. We find a large energy gap in the phonon dispersions of metallic ZrN and HfN, but a gapless phonon spectrum for ScN spanning the same energy range, this suggests that a reduced thermal conductivity, suitable for thermoelectric applications, should arise in superlattices made with ScN and ZrN or ScN and HfN. To obtain an electronic energy band gap of ScN comparable to experiment, we use a Hubbard correction with a parameter U (=3.5 eV). Anomalies in the acoustic branches of the phonon dispersion of ZrN and HfN, manifested as dips in the bands, can be understood through the nesting of Fermi surface determined from our calculations. To connect with transport properties, we have determined effective masses of ScN and determined their dependence on the U parameter. Using the relaxation time approximation in the Boltzmann transport theory, we estimate the temperature dependence of the lattice thermal conductivity and discuss the chemical trends among these nitrides.
Decomposition model for phonon thermal conductivity of a monatomic lattice
NASA Astrophysics Data System (ADS)
Evteev, Alexander V.; Momenzadeh, Leila; Levchenko, Elena V.; Belova, Irina V.; Murch, Graeme E.
2014-12-01
An analytical treatment of decomposition of the phonon thermal conductivity of a crystal with a monatomic unit cell is developed on the basis of a two-stage decay of the heat current autocorrelation function observed in molecular dynamics simulations. It is demonstrated that the contributions from the acoustic short- and long-range phonon modes to the total phonon thermal conductivity can be presented in the form of simple kinetic formulas, consisting of products of the heat capacity and the average relaxation time of the considered phonon modes as well as the square of the average phonon velocity. On the basis of molecular dynamics calculations of the heat current autocorrelation function, this treatment allows for a self-consistent numerical evaluation of the aforementioned variables. In addition, the presented analysis allows, within the Debye approximation, for the identification of the temperature range where classical molecular dynamics simulations can be employed for the prediction of phonon thermal transport properties. As a case example, Cu is considered.
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-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.
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.
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 formore » 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.« less
Lan, Tian; Li, Chen W.; Hellman, O.; Kim, D. S.; Muñoz, Jorge A.; Smith, Hillary; Abernathy, Douglas L.; Fultz, B.
2015-08-11
Although the rutile structure of TiO2 is stable at high temperatures, the conventional quasiharmonic approximation predicts that several acoustic phonons decrease anomalously to zero frequency with thermal expansion, incorrectly predicting a structural collapse at temperatures well below 1000 K. In this paper, inelastic neutron scattering was used to measure the temperature dependence of the phonon density of states (DOS) of rutile TiO2 from 300 to 1373 K. Surprisingly, these anomalous acoustic phonons were found to increase in frequency with temperature. First-principles calculations showed that with lattice expansion, the potentials for the anomalous acoustic phonons transform from quadratic to quartic, stabilizingmore » the rutile phase at high temperatures. In these modes, the vibrational displacements of adjacent Ti and O atoms cause variations in hybridization of 3d electrons of Ti and 2p electrons of O atoms. Finally, with thermal expansion, the energy variation in this “phonon-tracked hybridization” flattens the bottom of the interatomic potential well between Ti and O atoms, and induces a quarticity in the phonon potential.« less
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.
Topological Phononic Crystals with One-Way Elastic Edge Waves.
Wang, Pai; Lu, Ling; Bertoldi, Katia
2015-09-01
We report a new type of phononic crystals with topologically nontrivial band gaps for both longitudinal and transverse polarizations, resulting in protected one-way elastic edge waves. In our design, gyroscopic inertial effects are used to break the time-reversal symmetry and realize the phononic analogue of the electronic quantum (anomalous) Hall effect. We investigate the response of both hexagonal and square gyroscopic lattices and observe bulk Chern numbers of 1 and 2, indicating that these structures support single and multimode edge elastic waves immune to backscattering. These robust one-way phononic waveguides could potentially lead to the design of a novel class of surface wave devices that are widely used in electronics, telecommunication, and acoustic imaging. PMID:26382680
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.
Topological Phononic Crystals with One-Way Elastic Edge Waves.
Wang, Pai; Lu, Ling; Bertoldi, Katia
2015-09-01
We report a new type of phononic crystals with topologically nontrivial band gaps for both longitudinal and transverse polarizations, resulting in protected one-way elastic edge waves. In our design, gyroscopic inertial effects are used to break the time-reversal symmetry and realize the phononic analogue of the electronic quantum (anomalous) Hall effect. We investigate the response of both hexagonal and square gyroscopic lattices and observe bulk Chern numbers of 1 and 2, indicating that these structures support single and multimode edge elastic waves immune to backscattering. These robust one-way phononic waveguides could potentially lead to the design of a novel class of surface wave devices that are widely used in electronics, telecommunication, and acoustic imaging.
Han, Haoxue; Li, Baowen; Volz, Sebastian; Kosevich, Yuriy A
2015-04-10
We introduce a novel ultracompact nanocapacitor of coherent phonons formed by high-finesse interference mirrors based on atomic-scale semiconductor metamaterials. Our molecular dynamics simulations show that the nanocapacitor stores coherent monochromatic terahertz lattice waves, which can be used for phonon lasing-the emission of coherent phonons. Either one- or two-color phonon emission can be realized depending on the geometry of the nanodevice. The two-color regime of the interference phonon nanocapacitor originates from the different incidence-angle dependence of the transmission of longitudinal and transverse phonons at the respective interference antiresonances. Coherent phonon storage can be achieved by an adiabatic cooling the nanocapacitor initially thermalized at room temperature or by the pump-probe optical technique. The linewidth narrowing and the computed relative phonon participation number confirm strong phonon confinement in the ultracompact interference nanocavity by an extremely small amount of resonance defects. The emission of coherent terahertz acoustic beams from the nanocapacitor can be realized by applying a tunable reversible stress, which shifts the frequencies of the interference antiresonances. PMID:25910135
Edge waves and resonances in two-dimensional phononic crystal plates
Hsu, Jin-Chen Hsu, Chih-Hsun
2015-05-07
We present a numerical study on phononic band gaps and resonances occurring at the edge of a semi-infinite two-dimensional (2D) phononic crystal plate. The edge supports localized edge waves coupling to evanescent phononic plate modes that decay exponentially into the semi-infinite phononic crystal plate. The band-gap range and the number of edge-wave eigenmodes can be tailored by tuning the distance between the edge and the semi-infinite 2D phononic lattice. As a result, a phononic band gap for simultaneous edge waves and plate waves is created, and phononic cavities beside the edge can be built to support high-frequency edge resonances. We design an L3 edge cavity and analyze its resonance characteristics. Based on the band gap, high quality factor and strong confinement of resonant edge modes are achieved. The results enable enhanced control over acoustic energy flow in phononic crystal plates, which can be used in designing micro and nanoscale resonant devices and coupling of edge resonances to other types of phononic or photonic crystal cavities.
NASA Astrophysics Data System (ADS)
Han, Haoxue; Li, Baowen; Volz, Sebastian; Kosevich, Yuriy A.
2015-04-01
We introduce a novel ultracompact nanocapacitor of coherent phonons formed by high-finesse interference mirrors based on atomic-scale semiconductor metamaterials. Our molecular dynamics simulations show that the nanocapacitor stores coherent monochromatic terahertz lattice waves, which can be used for phonon lasing—the emission of coherent phonons. Either one- or two-color phonon emission can be realized depending on the geometry of the nanodevice. The two-color regime of the interference phonon nanocapacitor originates from the different incidence-angle dependence of the transmission of longitudinal and transverse phonons at the respective interference antiresonances. Coherent phonon storage can be achieved by an adiabatic cooling the nanocapacitor initially thermalized at room temperature or by the pump-probe optical technique. The linewidth narrowing and the computed relative phonon participation number confirm strong phonon confinement in the ultracompact interference nanocavity by an extremely small amount of resonance defects. The emission of coherent terahertz acoustic beams from the nanocapacitor can be realized by applying a tunable reversible stress, which shifts the frequencies of the interference antiresonances.
Femtosecond electron imaging of defect-modulated phonon dynamics
NASA Astrophysics Data System (ADS)
Cremons, Daniel R.; Plemmons, Dayne A.; Flannigan, David J.
2016-04-01
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.
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
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
Lattice Dynamics of EuO: Evidence for Giant Spin-Phonon Coupling
NASA Astrophysics Data System (ADS)
Pradip, R.; Piekarz, P.; Bosak, A.; Merkel, D. G.; Waller, O.; Seiler, A.; Chumakov, A. I.; Rüffer, R.; Oleś, A. M.; Parlinski, K.; Krisch, M.; Baumbach, T.; Stankov, S.
2016-05-01
Comprehensive studies of lattice dynamics in the ferromagnetic semiconductor EuO have been performed by a combination of inelastic x-ray scattering, nuclear inelastic scattering, and ab initio calculations. A remarkably large broadening of the transverse acoustic phonons was discovered at temperatures above and below the Curie temperature TC=69 K . This result indicates a surprisingly strong momentum-dependent spin-phonon coupling induced by the spin dynamics in EuO.
Heat transport by phonons in crystalline materials and nanostructures
NASA Astrophysics Data System (ADS)
Koh, Yee Kan
This dissertation presents experimental studies of heat transport by phonons in crystalline materials and nanostructures, and across solid-solid interfaces. Particularly, this dissertation emphasizes advancing understanding of the mean-free-paths (i.e., the distance phonons propagate without being scattered) of acoustic phonons, which are the dominant heat carriers in most crystalline semiconductor nanostructures. Two primary tools for the studies presented in this dissertation are time-domain thermoreflectance (TDTR) for measurements of thermal conductivity of nanostructures and thermal conductance of interfaces; and frequency-domain thermoreflectance (FDTR), which I developed as a direct probe of the mean-free-paths of dominant heat-carrying phonons in crystalline solids. The foundation of FDTR is the dependence of the apparent thermal conductivity on the frequency of periodic heat sources. I find that the thermal conductivity of semiconductor alloys (InGaP, InGaAs, and SiGe) measured by TDTR depends on the modulation frequency, 0.1 ≤ f ≤ 10 MHz, used in TDTR measurements. Reduction in the thermal conductivity of the semiconductor alloys at high f compares well to the reduction in the thermal conductivity of epitaxial thin films, indicating that frequency dependence and thickness dependence of thermal conductivity are fundamentally equivalent. I developed the frequency dependence of thermal conductivity into a convenient probe of phonon mean-free-paths, a technique which I call frequency-domain thermoreflectance (FDTR). In FDTR, I monitor the changes in the intensity of the reflected probe beam as a function of the modulation frequency. To facilitate the analysis of FDTR measurements, I developed a nonlocal theory for heat conduction by phonons at high heating frequencies. Calculations of the nonlocal theory confirm my experimental findings that phonons with mean-free-paths longer than two times the penetration depth do not contribute to the apparent thermal
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
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.
Phonon thermoelectric transistors and rectifiers
NASA Astrophysics Data System (ADS)
Jiang, Jian-Hua; Kulkarni, Manas; Segal, Dvira; Imry, Yoseph
2015-07-01
We describe nonlinear phonon-thermoelectric devices where charge current and electronic and phononic heat currents are coupled, driven by voltage and temperature biases, when phonon-assisted inelastic processes dominate the transport. Our thermoelectric transistors and rectifiers can be realized in a gate-tunable double quantum-dot system embedded in a nanowire which is realizable within current technology. The inelastic electron-phonon scattering processes are found to induce pronounced charge, heat, and cross rectification effects, as well as a thermal transistor effect that, remarkably, can appear in the present model even in the linear-response regime without relying on the onset of negative differential thermal conductance.
Geometric tuning of thermal conductivity in three-dimensional anisotropic phononic crystals.
Wei, Zhiyong; Wehmeyer, Geoff; Dames, Chris; Chen, Yunfei
2016-10-01
Molecular dynamics simulations are performed to investigate the thermal transport properties of a three-dimensional (3D) anisotropic phononic crystal consisting of silicon nanowires and films. The calculation shows that the in-plane thermal conductivity is negatively correlated with the out-of-plane thermal conductivity upon making geometric changes, whether varying the nanowire diameter or the film thickness. This enables the anisotropy ratio of thermal conductivity to be tailored over a wide range, in some cases by more than a factor of 20. Similar trends in thermal conductivity are also observed from an independent phonon ray tracing simulation considering only diffuse boundary scattering effects, though the range of anisotropy ratios is smaller than that obtained in MD simulation. By analyzing the phonon dispersion relation with varied geometric parameters, it is found that increasing the nanowire diameter increases the out-of-plane acoustic phonon group velocities, but reduces the in-plane longitudinal and fast transverse acoustic phonon group velocities. The calculated phonon irradiation further verified the negative correlation between the in-plane and the out-of-plane thermal conductivity. The proposed 3D phononic crystal may find potential application in thermoelectrics, energy storage, catalysis and sensing applications owing to its widely tailorable thermal conductivity.
Phononic and magnonic dispersions of surface waves on a permalloy/BARC nanostructured array
2013-01-01
Phononic and magnonic dispersions of a linear array of periodic alternating Ni80Fe20 and bottom anti-reflective coating nanostripes on a Si substrate have been measured using Brillouin light scattering. The observed phononic gaps are considerably larger than those of laterally patterned multi-component crystals previously reported, mainly a consequence of the high elastic and density contrasts between the stripe materials. Additionally, the phonon hybridization bandgap has an unusual origin in the hybridization and avoided crossing of the zone-folded Rayleigh and pseudo-Sezawa waves. The magnonic band structure features near-dispersionless branches, with unusual vortex-like dynamic magnetization profiles, some of which lie below the highly-dispersive fundamental mode branch. Finite element calculations of the phononic and magnonic dispersions of the magphonic crystal accord well with experimental data. PMID:23452555
Electron-phonon coupling and thermal transport in the thermoelectric compound Mo3Sb7–xTex
Bansal, Dipanshu; Li, Chen W.; Said, Ayman H.; Abernathy, Douglas L.; Yan, Jiaqiang; Delaire, Olivier A.
2015-12-07
Phonon properties of Mo3Sb7–xTex (x = 0, 1.5, 1.7), a potential high-temperature thermoelectric material, have been studied with inelastic neutron and x-ray scattering, and with first-principles simulations. The substitution of Te for Sb leads to pronounced changes in the electronic struc- ture, local bonding, phonon density of states (DOS), dispersions, and phonon lifetimes. Alloying with tellurium shifts the Fermi level upward, near the top of the valence band, resulting in a strong suppression of electron-phonon screening, and a large overall stiffening of interatomic force- constants. The suppression in electron-phonon coupling concomitantly increases group velocities and suppresses phonon scattering rates, surpassingmore » the effects of alloy-disorder scattering, and re- sulting in a surprising increased lattice thermal conductivity in the alloy. We also identify that the local bonding environment changes non-uniformly around different atoms, leading to variable perturbation strengths for different optical phonon branches. The respective roles of changes in phonon group velocities and phonon lifetimes on the lattice thermal conductivity are quantified. Lastly, our results highlight the importance of the electron-phonon coupling on phonon mean-free-paths in this compound, and also estimates the contributions from boundary scattering, umklapp scattering, and point-defect scattering.« less
Farber, D; Chiang, T; Krisch, M; Occelli, F; Schwartz, A; Wall, M; Xu, R; Boro, C
2003-12-17
Plutonium (Pu) is well known to have complex and unique physico-chemical properties [1]. 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}' {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. 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 safety issues have made experimental observations extremely difficult. Phonon dispersion curves (PDCs) are key experimental 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 {delta}-Pu-Ga alloy using the high resolution
Imaging carrier and phonon transport in Si using ultrashort optical pulses
David H. Hurley; O. B. Wright; O. Matsuda; B. E. McCandless; S. Shinde
2009-01-01
A series of experiments have been conducted that microscopically image thermal diffusion and surface acoustic phonon propagation within a single crystallite of a polycrystalline Si sample. The experimental approach employs ultrashort optical pulses to generate an electron-hole plasma and a second probe pulse is used to image the evolution of the plasma. By decomposing the signal into a component that varies with delay time and a steady state component that varies with pump modulation frequency, the respective influence of carrier recombination and thermal diffusion are identified. Additionally, the coherent surface acoustic phonon component to the signal is imaged using a Sagnac interferometer to monitor optical phase.
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.
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
NASA Astrophysics Data System (ADS)
Chandel, Surjeet Kumar; Kumar, Arun; Bharti, Ankush; Sharma, Raman
2015-05-01
Using first principles density functional theoretical calculations, the present paper reports a systematic study of phonon dispersion curves in pristine carbon (CNT) and silicon nanotubes (SiNT) having chirality (6,6) in the armchair configuration. Some of the phonon modes are found to have negative frequencies which leads to instability of the systems under study. The number of phonon branches has been found to be thrice as much as the number of atoms. The frequency of the higher optical bands varies from 1690 to 1957 cm-1 for CNT(6,6) while it is 596 to 658 cm-1 for SiNT.
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
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
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.
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.
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.
Compositional Variation of the Phonon Dispersion Curves of bcc Fe-Ga Alloys
Zarestky, Jerel L; Garlea, Vasile O; Lograsso, Tom; Schlagel, D. L.; Stassis, C.
2005-01-01
Inelastic neutron scattering techniques have been used to measure the phonon dispersion curves of bcc Fe1?xGax x=10.8, 13.3, 16.0, 22.5 alloys as a function of Ga concentration. The phonon frequencies of every branch were found to decrease significantly with increasing Ga concentration. The softening was most pronounced for the T2 0 branch and, to a lesser extent, the L branch in the vicinity of = 2 3. The concentration dependence of the shear elastic constant C =1/2 C11?C12 , calculated from the slope of the T2 0 branch, was found to agree with the results of sound velocity measurements. For the higher concentration sample measured, 22.5 at. % Ga, new branches appeared, an effect associated with the increase in the number of atoms per unit cell.
Multi-band asymmetric acoustic transmission in a bended waveguide with multiple mechanisms
NASA Astrophysics Data System (ADS)
Huang, Yu-lei; Sun, Hong-xiang; Xia, Jian-ping; Yuan, Shou-qi; Ding, Xin-lei
2016-07-01
We report the realization of a multi-band device of the asymmetric acoustic transmission by placing a phononic crystal inside a bended waveguide immersed in water, as determined both experimentally and numerically. The asymmetric acoustic transmission exists in three frequency bands below 500 kHz induced by multiple mechanisms. Besides the band gap of the phononic crystal, we also introduce the deaf mode and interaction between the phononic crystal and waveguide. More importantly, this asymmetric transmission can be systematically controlled by mechanically rotating the square rods of the phononic crystal. The device has the advantages of multiple band, broader bandwidth, and adjustable property, showing promising applications in ultrasonic devices.
ERIC Educational Resources Information Center
Hamilton, M. W.
2007-01-01
A nonlinear aspect of the acousto-optic interaction that is analogous to multi-photon absorption is discussed. An experiment is described in which the second-order acousto-optically scattered intensity is measured and found to scale with the square of the acoustic intensity. This experiment using a commercially available acousto-optic modulator is…
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.
Topological phononic states of underwater sound based on coupled ring resonators
NASA Astrophysics Data System (ADS)
He, Cheng; Li, Zheng; Ni, Xu; Sun, Xiao-Chen; Yu, Si-Yuan; Lu, Ming-Hui; Liu, Xiao-Ping; Chen, Yan-Feng
2016-01-01
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.
THz Acoustic Spectroscopy by using Double Quantum Wells and Ultrafast Optical Spectroscopy.
Wei, Fan Jun; Yeh, Yu-Hsiang; Sheu, Jinn-Kong; Lin, Kung-Hsuan
2016-01-01
GaN is a pivotal material for acoustic transducers and acoustic spectroscopy in the THz regime, but its THz phonon properties have not been experimentally and comprehensively studied. In this report, we demonstrate how to use double quantum wells as a THz acoustic transducer for measuring generated acoustic phonons and deriving a broadband acoustic spectrum with continuous frequencies. We experimentally investigated the sub-THz frequency dependence of acoustic attenuation (i.e., phonon mean-free paths) in GaN, in addition to its physical origins such as anharmonic scattering, defect scattering, and boundary scattering. A new upper limit of attenuation caused by anharmonic scattering, which is lower than previously reported values, was obtained. Our results should be noteworthy for THz acoustic spectroscopy and for gaining a fundamental understanding of heat conduction. PMID:27346494
THz Acoustic Spectroscopy by using Double Quantum Wells and Ultrafast Optical Spectroscopy
NASA Astrophysics Data System (ADS)
Wei, Fan Jun; Yeh, Yu-Hsiang; Sheu, Jinn-Kong; Lin, Kung-Hsuan
2016-06-01
GaN is a pivotal material for acoustic transducers and acoustic spectroscopy in the THz regime, but its THz phonon properties have not been experimentally and comprehensively studied. In this report, we demonstrate how to use double quantum wells as a THz acoustic transducer for measuring generated acoustic phonons and deriving a broadband acoustic spectrum with continuous frequencies. We experimentally investigated the sub-THz frequency dependence of acoustic attenuation (i.e., phonon mean-free paths) in GaN, in addition to its physical origins such as anharmonic scattering, defect scattering, and boundary scattering. A new upper limit of attenuation caused by anharmonic scattering, which is lower than previously reported values, was obtained. Our results should be noteworthy for THz acoustic spectroscopy and for gaining a fundamental understanding of heat conduction.
THz Acoustic Spectroscopy by using Double Quantum Wells and Ultrafast Optical Spectroscopy
Wei, Fan Jun; Yeh, Yu-Hsiang; Sheu, Jinn-Kong; Lin, Kung-Hsuan
2016-01-01
GaN is a pivotal material for acoustic transducers and acoustic spectroscopy in the THz regime, but its THz phonon properties have not been experimentally and comprehensively studied. In this report, we demonstrate how to use double quantum wells as a THz acoustic transducer for measuring generated acoustic phonons and deriving a broadband acoustic spectrum with continuous frequencies. We experimentally investigated the sub-THz frequency dependence of acoustic attenuation (i.e., phonon mean-free paths) in GaN, in addition to its physical origins such as anharmonic scattering, defect scattering, and boundary scattering. A new upper limit of attenuation caused by anharmonic scattering, which is lower than previously reported values, was obtained. Our results should be noteworthy for THz acoustic spectroscopy and for gaining a fundamental understanding of heat conduction. PMID:27346494
Confinement of phonon propagation in laser deposited tungsten/polycarbonate multilayers
NASA Astrophysics Data System (ADS)
Döring, Florian; Ulrichs, Henning; Pagel, Sinja; Müller, Markus; Mansurova, Maria; Müller, Matthias; Eberl, Christian; Erichsen, Torben; Huebner, Dennis; Vana, Philipp; Mann, Klaus; Münzenberg, Markus; Krebs, Hans-Ulrich
2016-09-01
Nanoscale multilayer thin films of W and PC (Polycarbonate) show, due to the great difference of the components’ characteristics, fascinating properties for a variety of possible applications and provide an interesting research field, but are hard to fabricate with low layer thicknesses. Because of the great acoustic mismatch between the two materials, such nanoscale structures are promising candidates for new phononic materials, where phonon propagation is strongly reduced. In this article we show for the first time that W/PC-multilayers can indeed be grown with high quality by pulsed laser deposition. We analyzed the polymer properties depending on the laser fluence used for deposition, which enabled us to find best experimental conditions for the fabrication of high-acoustic-mismatch W/PC multilayers. The multilayers were analyzed by fs pump-probe spectroscopy showing that phonon dynamics on the ps time-scale can strongly be tailored by structural design. While already periodic multilayers exhibit strong phonon localization, especially aperiodic structures present outstandingly low phonon propagation properties making such 1D-layered W/PC nano-structures interesting for new phononic applications.
Acoustical Imaging with Negative Refraction
NASA Astrophysics Data System (ADS)
Gan, W. S.
It is well known that the resolution limit of acoustical images is limited by diffraction to λ/2 where λ is the sound wavelength. Negative refraction proposed by Veselago in 1968 shows possibility of defeating the diffraction limit. His work is for electromagnetic waves. Recently it has been shown experimentally that negative refraction can be achieved for both electromagnetic waves and sound waves by using photonic crystals and phononic crystals respectively. John Pendry proposed the concept of `perfect lens' using negative refraction for electromagnetic waves. In this paper, we propose a `perfect lens' for sound waves and an acoustical imaging system incorporating the `perfect lens' is also outlined
Magnon-phonon interconversion in a dynamically reconfigurable magnetic material
NASA Astrophysics Data System (ADS)
Guerreiro, Sergio C.; Rezende, Sergio M.
2015-12-01
The ferrimagnetic insulator yttrium iron garnet (YIG) is an important material in the field of magnon spintronics, mainly because of its low magnetic losses. YIG also has very low acoustic losses, and for this reason the conversion of a state of magnetic excitation (magnons) into a state of lattice vibration (phonons), or vice versa, broadens its possible applications in spintronics. Since the magnetic parameters can be varied by some external action, the magnon-phonon interconversion can be tuned to perform a desired function. We present a quantum theory of the interaction between magnons and phonons in a ferromagnetic material subject to a dynamic variation of the applied magnetic field. It is shown that when the field gradient at the magnetoelastic crossover region is much smaller than a critical value, an initial elastic excitation can be completely converted into a magnetic excitation, or vice versa. This occurs with conservation of linear momentum and spin angular momentum, implying that phonons created by the conversion of magnons have spin angular momentum and carry spin current. It is shown further that if the system is initially in a quantum coherent state, its coherence properties are maintained regardless of the time dependence of the field.
Low-temperature electron-phonon heat transfer in metal films
NASA Astrophysics Data System (ADS)
Cojocaru, S.; Anghel, D. V.
2016-03-01
We consider the deformation potential mechanism of the electron-phonon coupling in metal films and investigate the intensity of the associated heat transfer between the electron and phonon subsystems. The focus is on the temperature region below dimensional crossover T
Transition from near-field thermal radiation to phonon heat conduction at sub-nanometre gaps.
Chiloyan, Vazrik; Garg, Jivtesh; Esfarjani, Keivan; Chen, Gang
2015-01-01
When the separation of two surfaces approaches sub-nanometre scale, the boundary between the two most fundamental heat transfer modes, heat conduction by phonons and radiation by photons, is blurred. Here we develop an atomistic framework based on microscopic Maxwell's equations and lattice dynamics to describe the convergence of these heat transfer modes and the transition from one to the other. For gaps >1 nm, the predicted conductance values are in excellent agreement with the continuum theory of fluctuating electrodynamics. However, for sub-nanometre gaps we find the conductance is enhanced up to four times compared with the continuum approach, while avoiding its prediction of divergent conductance at contact. Furthermore, low-frequency acoustic phonons tunnel through the vacuum gap by coupling to evanescent electric fields, providing additional channels for energy transfer and leading to the observed enhancement. When the two surfaces are in or near contact, acoustic phonons become dominant heat carriers. PMID:25849305
Phonons with orbital angular momentum
Ayub, M. K.; Ali, S.; Mendonca, J. T.
2011-10-15
Ion accoustic waves or phonon modes are studied with orbital angular momentum (OAM) in an unmagnetized collissionless uniform plasma, whose constituents are the Boltzmann electrons and inertial ions. For this purpose, we have employed the fluid equations to obtain a paraxial equation in terms of ion density perturbations and discussed its Gaussian beam and Laguerre-Gauss (LG) beam solutions. Furthermore, an approximate solution for the electrostatic potential problem is presented, allowing to express the components of the electric field in terms of LG potential perturbations. The energy flux due to phonons is also calculated and the corresponding OAM is derived. Numerically, it is shown that the parameters such as azimuthal angle, radial and angular mode numbers, and beam waist, strongly modify the profiles of the phonon LG potential. The present results should be helpful in understanding the phonon mode excitations produced by Brillouin backscattering of laser beams in a uniform plasma.
Reduction in the thermal conductivity of single crystalline silicon by phononic crystal patterning.
Hopkins, Patrick E; Reinke, Charles M; Su, Mehmet F; Olsson, Roy H; Shaner, Eric A; Leseman, Zayd C; Serrano, Justin R; Phinney, Leslie M; El-Kady, Ihab
2011-01-12
Phononic crystals (PnCs) are the acoustic wave equivalent of photonic crystals, where a periodic array of scattering inclusions located in a homogeneous host material causes certain frequencies to be completely reflected by the structure. In conjunction with creating a phononic band gap, anomalous dispersion accompanied by a large reduction in phonon group velocities can lead to a massive reduction in silicon thermal conductivity. We measured the cross plane thermal conductivity of a series of single crystalline silicon PnCs using time domain thermoreflectance. The measured values are over an order of magnitude lower than those obtained for bulk Si (from 148 W m(-1) K(-1) to as low as 6.8 W m(-1) K(-1)). The measured thermal conductivity is much smaller than that predicted by only accounting for boundary scattering at the interfaces of the PnC lattice, indicating that coherent phononic effects are causing an additional reduction to the cross plane thermal conductivity.
Temperature induced phonon behaviour in germanium selenide thin films probed by Raman spectroscopy
NASA Astrophysics Data System (ADS)
Taube, A.; Łapińska, A.; Judek, J.; Wochtman, N.; Zdrojek, M.
2016-08-01
Here we report a detailed study of temperature-dependent phonon properties of exfoliated germanium selenide thin films (several tens of nanometers thick) probed by Raman spectroscopy in the 70–350 K temperature range. The temperature-dependent behavior of the positions and widths of the Raman modes was nonlinear. We concluded that the observed effects arise from anharmonic phonon–phonon interactions and are explained by the phenomenon of optical phonon decay into acoustic phonons. At temperatures above 200 K, the position of the Raman modes tended to be linearly dependent, and the first order temperature coefficients χ were ‑0.0277, ‑0.0197 and ‑0.031 cm‑1 K‑1 for B 3g , A g(1) and A g(2) modes, respectively.
Influence of electron–phonon interactions in single dopant nanowire transistors
Carrillo-Nuñez, H. Bescond, M. Cavassilas, N.; Dib, E.; Lannoo, M.
2014-10-28
Single dopant nanowire transistors can be viewed as the ultimate miniaturization of nano electronic devices. In this work, we theoretically investigate the influence of the electron-phonon coupling on their transport properties using a non-equilibrium Green's function approach in the self-consistent Born approximation. For an impurity located at the center of the wire we find that, at room temperature, acoustic phonons broaden the impurity level so that the bistability predicted in the ballistic regime is suppressed. Optical phonons are found to have a beneficial impact on carrier transport via a phonon-assisted tunneling effect. We discuss the position and temperature dependence of these effects, showing that such systems might be very promising for engineering of ultimate devices.
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
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.
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.
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.
Lan, Tian; Li, Chen W.; Hellman, O.; Kim, D. S.; Muñoz, Jorge A.; Smith, Hillary; Abernathy, Douglas L.; Fultz, B.
2015-08-11
Although the rutile structure of TiO_{2} is stable at high temperatures, the conventional quasiharmonic approximation predicts that several acoustic phonons decrease anomalously to zero frequency with thermal expansion, incorrectly predicting a structural collapse at temperatures well below 1000 K. In this paper, inelastic neutron scattering was used to measure the temperature dependence of the phonon density of states (DOS) of rutile TiO_{2} from 300 to 1373 K. Surprisingly, these anomalous acoustic phonons were found to increase in frequency with temperature. First-principles calculations showed that with lattice expansion, the potentials for the anomalous acoustic phonons transform from quadratic to quartic, stabilizing the rutile phase at high temperatures. In these modes, the vibrational displacements of adjacent Ti and O atoms cause variations in hybridization of 3d electrons of Ti and 2p electrons of O atoms. Finally, with thermal expansion, the energy variation in this “phonon-tracked hybridization” flattens the bottom of the interatomic potential well between Ti and O atoms, and induces a quarticity in the phonon potential.
Neutron inelastic scattering measurements of low-energy phonons in the multiferroic BiFeO_{3}
Schneeloch, John A.; Xu, Zhijun; Wen, Jinsheng; Gehring, P. M.; Stock, C.; Matsuda, Masaaki; Winn, Barry L.; Gu, Genda; Shapiro, Stephen M.; Birgeneau, R. J.; Ushiyama, T.; Yanagisawa, Y.; Tomioka, Y.; Ito, T.; Xu, Guangyong
2015-02-10
In this study, we present neutron inelastic scattering measurements of the low-energy phonons in single crystal BiFeO_{3}. The dispersions of the three acoustic phonon modes (LA along [100], TA_{1} along [010], and TA_{2} along [110]) and two low-energy optic phonon modes (LO and TO_{1}) have been mapped out between 300 and 700 K. Elastic constants are extracted from the phonon measurements. The energy linewidths of both TA phonons at the zone boundary clearly broaden when the system is warmed toward the magnetic ordering temperature T_{N}=640 K. In conclusion, this suggests that the magnetic order and low-energy lattice dynamics in this multiferroic material are coupled.
Neutron inelastic scattering measurements of low-energy phonons in the multiferroic BiFeO3
Schneeloch, John A.; Xu, Zhijun; Wen, Jinsheng; Gehring, P. M.; Stock, C.; Matsuda, Masaaki; Winn, Barry L.; Gu, Genda; Shapiro, Stephen M.; Birgeneau, R. J.; et al
2015-02-10
In this study, we present neutron inelastic scattering measurements of the low-energy phonons in single crystal BiFeO3. The dispersions of the three acoustic phonon modes (LA along [100], TA1 along [010], and TA2 along [110]) and two low-energy optic phonon modes (LO and TO1) have been mapped out between 300 and 700 K. Elastic constants are extracted from the phonon measurements. The energy linewidths of both TA phonons at the zone boundary clearly broaden when the system is warmed toward the magnetic ordering temperature TN=640 K. In conclusion, this suggests that the magnetic order and low-energy lattice dynamics in thismore » multiferroic material are coupled.« less
Anomalous Infrared Spectra of Hybridized Phonons in Type-I Clathrate Ba8Ga16Ge30
NASA Astrophysics Data System (ADS)
Iwamoto, Kei; Kushibiki, Shunsuke; Honda, Hironori; Kajitani, Shuhei; Mori, Tatsuya; Matsumoto, Hideki; Toyota, Naoki; Suekuni, Koichiro; Avila, Marcos A.; Takabatake, Toshiro
2013-02-01
The optical conductivity spectra of the rattling phonons in the clathrate Ba8Ga16Ge30 are investigated in detail by use of the terahertz time-domain spectroscopy. The experiment has revealed that the lowest-lying vibrational mode of Ba(2)2+ ions consist of a sharp Lorentzian peak at 1.2 THz superimposed on a broad tail weighted in the lower frequency regime around 1.0 THz. With decreasing temperature, an unexpected linewidth broadening of the phonon peak is observed, together with monotonic softening of the phonon peak and enhancement of the tail structure. These observed anomalies are discussed in terms of impurity scattering effects on the hybridized phonon system of rattling and acoustic phonons.
Vaks, V. G. Zhuravlev, I. A.; Zabolotskii, A. D.
2012-03-15
The frequencies of the phonon branches that correspond to the vibrations of the close-packed atomic planes in bcc, fcc, and hcp crystals with short-range interatomic interaction are shown to be described by a universal relationship, which only contains two parameters for each branch, for any polarization {lambda}. These phonon branches correspond to the ({xi}, {xi}, 0) direction in bcc crystals, the ({xi}, {xi}, {xi}) direction in fcc crystals, and the (0, 0, {xi}) direction in hcp crystals. This universal relationship can only be violated by long-range interactions, namely, the interactions outside the sixth coordination shell in a bcc crystal, the fifth coordination shell in an fcc crystal, and the eleventh or tenth coordination shell in an hcp crystal. The effect of these long-range interactions for each phonon branch can be quantitatively characterized by certain parameters {Delta}{sub n{lambda}}, which are simply expressed in terms of the frequencies of three phonons of the branch. The values of these parameters are presented for all bcc, fcc, and hcp metals whose phonon spectra are measured. In most cases, the proposed relationships for the frequencies are found to be fulfilled accurate to several percent. In the cases where the {Delta}{sub n{lambda}} parameters are not small, they can give substantial information on the type and scale of long-range interaction effects in various metals.
Hannah, Daniel C; Brown, Kristen E; Young, Ryan M; Wasielewski, Michael R; Schatz, George C; Co, Dick T; Schaller, Richard D
2013-09-01
We report femtosecond stimulated Raman spectroscopy measurements of lattice dynamics in semiconductor nanocrystals and characterize longitudinal optical (LO) phonon production during confinement-enhanced, ultrafast intraband relaxation. Stimulated Raman signals from unexcited CdSe nanocrystals produce a spectral shape similar to spontaneous Raman signals. Upon photoexcitation, stimulated Raman amplitude decreases owing to experimentally resolved ultrafast phonon generation rates within the lattice. We find a ∼600 fs, particle-size-independent depletion time attributed to hole cooling, evidence of LO-to-acoustic down-conversion, and LO phonon mode softening. PMID:25166708
Design of quasi-one-dimensional phononic crystal cavity for efficient photoelastic modulation
NASA Astrophysics Data System (ADS)
Kim, Ingi; Iwamoto, Satoshi; Arakawa, Yasuhiko
2016-08-01
We propose and design a phononic crystal (PnC) cavity for efficient photoelastic modulation. A strongly confined acoustic field in the cavity enhances light-sound interaction, which results in efficient phase modulation of light. As one of the possible configurations, an acoustic cavity formed in a quasi-one-dimensional (quasi-1D) PnC was investigated. By carefully tuning geometrical parameters, we successfully designed a high-Q cavity mode for a longitudinal wave within a complete phononic band gap. The acoustic Q was calculated to be as high as 9.5 × 104. This enables efficient optical modulation by a factor of 2.5 compared with a bar-type structure without PnCs.
Electron-phonon coupling and thermal transport in the thermoelectric compound Mo_{3}Sb_{7–x}Te_{x}
Bansal, Dipanshu; Li, Chen W.; Said, Ayman H.; Abernathy, Douglas L.; Yan, Jiaqiang; Delaire, Olivier A.
2015-12-07
Phonon properties of Mo_{3}Sb_{7–x}Te_{x} (x = 0, 1.5, 1.7), a potential high-temperature thermoelectric material, have been studied with inelastic neutron and x-ray scattering, and with first-principles simulations. The substitution of Te for Sb leads to pronounced changes in the electronic struc- ture, local bonding, phonon density of states (DOS), dispersions, and phonon lifetimes. Alloying with tellurium shifts the Fermi level upward, near the top of the valence band, resulting in a strong suppression of electron-phonon screening, and a large overall stiffening of interatomic force- constants. The suppression in electron-phonon coupling concomitantly increases group velocities and suppresses phonon scattering rates, surpassing the effects of alloy-disorder scattering, and re- sulting in a surprising increased lattice thermal conductivity in the alloy. We also identify that the local bonding environment changes non-uniformly around different atoms, leading to variable perturbation strengths for different optical phonon branches. The respective roles of changes in phonon group velocities and phonon lifetimes on the lattice thermal conductivity are quantified. Lastly, our results highlight the importance of the electron-phonon coupling on phonon mean-free-paths in this compound, and also estimates the contributions from boundary scattering, umklapp scattering, and point-defect scattering.
Phonon and magnon dispersions of incommensurate spin ladder compound Sr14Cu24O41
NASA Astrophysics Data System (ADS)
Chen, Xi; Bansal, Dipanshu; Sullivan, Sean; Zhou, Jianshi; Delaire, Olivier; Shi, Li
There are a variety of compounds consisting of two or more interpenetrating sublattices with lattice periods incommensurate at least along one crystal axis. One example is spin ladder compound Sr14Cu24O41 consisting of incommensurate spin ladder and spin chain sublattices. It has been predicted that unique phonon modes occur in these compounds due to the relative motion of the sublattices. In the low-wavelength limit, there is only one longitudinal acoustic mode due to the rigid translation of both sublattices. In addition, one extra pseudo-acoustic mode is present due to relative sliding motions of the two sublattices. Although the theoretical aspects of the lattice dynamics of incommensurate compounds have been studied, there have been few experimental investigations on their phonon dynamics. In this work, single crystals of Sr14Cu24O41are grown by the traveling solvent floating zone method. The phonon dispersion of Sr14Cu24O41 is studied through inelastic neutron scattering measurements in order to better understand its phonon dynamics. In addition, its magnon dispersion is investigated and correlated to the large directional magnon thermal conductivity. The measurements reveal a wealth of intriguing features on phonons and magnons in the spin ladder compound. This work is supported by ARO MURI program under Award # W911NF-14-1-0016.
Phonon Mapping in Flowing Equilibrium
NASA Astrophysics Data System (ADS)
Ruff, J. P. C.
2015-03-01
When a material conducts heat, a modification of the phonon population occurs. The equilibrium Bose-Einstein distribution is perturbed towards flowing-equilibrium, for which the distribution function is not analytically known. Here I argue that the altered phonon population can be efficiently mapped over broad regions of reciprocal space, via diffuse x-ray scattering or time-of-flight neutron scattering, while a thermal gradient is applied across a single crystal sample. When compared to traditional transport measurements, this technique offers a superior, information-rich new perspective on lattice thermal conductivity, wherein the band and momentum dependences of the phonon thermal current are directly resolved. The proposed method is benchmarked using x-ray thermal diffuse scattering measurements of single crystal diamond under transport conditions. CHESS is supported by the NSF & NIH/NIGMS via NSF Award DMR-1332208.
Strain induced modification in phonon dispersion curves of monolayer boron pnictides
Jha, Prafulla K. E-mail: prafullaj@yahoo.com; Soni, Himadri R.
2014-01-14
In the frame work of density functional theory, the biaxial strain induced phonon dispersion curves of monolayer boron pnictides (BX, X = N, P, As, and Sb) have been investigated. The electron-ion interactions have been modelled using ultrasoft pseudopotentials while exchange-correlation energies have been approximated by the method of local density approximation in the parameterization of Perdew-Zunger. The longitudinal and transverse acoustic phonon modes of boron pnictide sheets show linear dependency on wave vector k{sup →} while out of plane mode varies as k{sup 2}. The in-plane longitudinal and out of plane transverse optical modes in boron nitride displaying significant dispersion similar to graphene. We have analyzed the biaxial strain dependent behaviour of out of plane acoustic phonon mode which is linked to ripple for four BX sheets using a model equation with shell elasticity theory. The strain induces the hardening of this mode with tendency to become more linear with increase in strain percentage. The strain induced hardening of out of plane acoustic phonon mode indicates the absence of rippling in these compounds. Our band structure calculations for both unstrained and strained 2D h-BX are consistent with previous calculations.
Strain induced modification in phonon dispersion curves of monolayer boron pnictides
NASA Astrophysics Data System (ADS)
Jha, Prafulla K.; Soni, Himadri R.
2014-01-01
In the frame work of density functional theory, the biaxial strain induced phonon dispersion curves of monolayer boron pnictides (BX, X = N, P, As, and Sb) have been investigated. The electron-ion interactions have been modelled using ultrasoft pseudopotentials while exchange-correlation energies have been approximated by the method of local density approximation in the parameterization of Perdew-Zunger. The longitudinal and transverse acoustic phonon modes of boron pnictide sheets show linear dependency on wave vector k→ while out of plane mode varies as k2. The in-plane longitudinal and out of plane transverse optical modes in boron nitride displaying significant dispersion similar to graphene. We have analyzed the biaxial strain dependent behaviour of out of plane acoustic phonon mode which is linked to ripple for four BX sheets using a model equation with shell elasticity theory. The strain induces the hardening of this mode with tendency to become more linear with increase in strain percentage. The strain induced hardening of out of plane acoustic phonon mode indicates the absence of rippling in these compounds. Our band structure calculations for both unstrained and strained 2D h-BX are consistent with previous calculations.
Anees, P; Valsakumar, M C; Panigrahi, B K
2016-01-28
The temperature dependent structural stability, frequency shift and linewidth of 2D hexagonal boron nitride (h-BN) are studied using a combination of lattice dynamics (LD) and molecular dynamics (MD) simulations. The in-plane lattice parameter shows a negative thermal expansion in the whole computed temperature range (0-2000 K). When the in-plane lattice parameter falls below the equilibrium value, the quasi-harmonic bending (ZA) mode frequency becomes imaginary along the Γ-M direction in the Brillouin zone, leading to a structural instability of the 2D sheet. The ZA mode is seen to be stabilized in the dispersion obtained from MD simulations, due to the automatic incorporation of higher order phonon scattering processes in MD, which are absent in a quasi-harmonic dispersion. The mode resolved phonon spectra computed with a quasi-harmonic method predict a blueshift of the longitudinal and transverse (LO/TO) optic mode frequencies with an increase in temperature. On the other hand, both canonical (NVT) and isobaric-isothermal (NPT) ensembles predict a redshift with an increase in temperature, which is more prominent in the NVT ensemble. The strong phonon-phonon coupling dominates over the thermal contraction effect and leads to a redshift in LO/TO mode frequency in the NPT ensemble simulations. The out-of-plane (ZO) optic mode quasi-harmonic frequencies are redshifted due to a membrane effect. The phonon-phonon coupling effects in the NVT and NPT ensemble simulations lead to a further reduction in the ZO mode frequencies. The linewidth of the LO/TO and ZO mode frequencies increases in a monotonic fashion. The temperature dependence of acoustic modes is also analyzed. The quasi-harmonic calculations predict a redshift of ZA mode, and at the same time the TA (transverse acoustic) and LA (longitudinal acoustic) mode frequencies are blueshifted. The strong phonon-phonon coupling in MD simulations causes a redshift of the LA and TA mode frequencies, while the ZA mode
Influence of magnetism on phonons in CaFe2As2 as seen via inelastic x-ray scattering
Hahn, S.E.; Lee, Y.; Ni, N.; Canfield, P.C.; Goldman, A.I.; McQueeney, R.J.; Harmon, B.N.; Alatas, A.; Leu, B.M.; Alp, E.E.; Chung, D.Y.; Todorov, I.S.; Kanatzidis, M.G.
2009-06-19
In the iron pnictides, the strong sensitivity of the iron magnetic moment to the arsenic position suggests a significant relationship between phonons and magnetism. We measured the phonon dispersion of several branches in the high-temperature tetragonal phase of CaFe{sub 2}As{sub 2} using inelastic x-ray scattering on single-crystal samples. These measurements were compared to ab initio calculations of the phonons. Spin-polarized calculations imposing the antiferromagnetic order present in the low-temperature orthorhombic phase dramatically improve agreement between theory and experiment. This is discussed in terms of the strong antiferromagnetic correlations that are known to persist in the tetragonal phase.
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.
Optically tunable acoustic wave band-pass filter
Swinteck, N.; Lucas, P.; Deymier, P. A.
2014-12-15
The acoustic properties of a hybrid composite that exhibits both photonic and phononic behavior are investigated numerically with finite-element and finite-difference time-domain simulations. The structure is constituted of a periodic array of photonic resonant cavities embedded in a background superlattice. The resonant cavities contain a photo-elastic chalcogenide glass that undergoes atomic-scale structural reorganization when irradiated with light having energy close to its band-gap. Photo-excitation of the chalcogenide glass changes its elastic properties and, consequently, augments the acoustic transmission spectrum of the composite. By modulating the intensity of light irradiating the hybrid photonic/phononic structure, the position and spectral width of phonon passing-bands can be controlled. This demonstration offers the technological platform for optically-tunable acoustic wave band-pass filters.
Optically tunable acoustic wave band-pass filter
NASA Astrophysics Data System (ADS)
Swinteck, N.; Lucas, P.; Deymier, P. A.
2014-12-01
The acoustic properties of a hybrid composite that exhibits both photonic and phononic behavior are investigated numerically with finite-element and finite-difference time-domain simulations. The structure is constituted of a periodic array of photonic resonant cavities embedded in a background superlattice. The resonant cavities contain a photo-elastic chalcogenide glass that undergoes atomic-scale structural reorganization when irradiated with light having energy close to its band-gap. Photo-excitation of the chalcogenide glass changes its elastic properties and, consequently, augments the acoustic transmission spectrum of the composite. By modulating the intensity of light irradiating the hybrid photonic/phononic structure, the position and spectral width of phonon passing-bands can be controlled. This demonstration offers the technological platform for optically-tunable acoustic wave band-pass filters.
Phonon-phonon interactions and phonon damping for the curvature modes in carbon nanotubes
NASA Astrophysics Data System (ADS)
Li, Guolong; Ren, Zhongzhou
2016-01-01
We focus on the damping of the lowest-lying gapped modes with integer angular-momentum quantum number |l|=2 in carbon nanotubes (CNTs). These modes, called C modes simply, can be predicted within the framework of the continuum elasticity theory with the curvature term. Based on the phonon-phonon interactions due to the anharmonic effect, we obtain the three-phonon coupling coefficients of different damping processes of C modes. Applying perturbation theory, we calculate relaxation rates τ_C-1 and upper bounds of quality factors for the long-wavelength C modes. In addition, we display the wave vector dependence of τC and show the importance of the C mode damping to thermal conductivity.
Hybrid phononic crystals for broad-band frequency noise control by sound blocking and localization.
Yoo, Sungmin; Kim, Yoon Jae; Kim, Yoon Young
2012-11-01
A bandgap cannot be enlarged sufficiently enough to suppress a broad-band noise only with a single type of finite-length phononic crystals. Here, a hybrid phononic crystal consisting of a bi-prism and an inverted bi-prism is proposed for noise control in a broad band; a stop band is formed in a central frequency range while positive-positive and positive-negative refractions occur in lower and higher frequency ranges to concentrate acoustic energy in a central localized zone. Thereby, the remaining zone becomes little affected by the noise. Analysis and numerical simulations are given for the justification of the proposed configuration. PMID:23145703
Lattice Dynamics of EuO: Evidence for Giant Spin-Phonon Coupling.
Pradip, R; Piekarz, P; Bosak, A; Merkel, D G; Waller, O; Seiler, A; Chumakov, A I; Rüffer, R; Oleś, A M; Parlinski, K; Krisch, M; Baumbach, T; Stankov, S
2016-05-01
Comprehensive studies of lattice dynamics in the ferromagnetic semiconductor EuO have been performed by a combination of inelastic x-ray scattering, nuclear inelastic scattering, and ab initio calculations. A remarkably large broadening of the transverse acoustic phonons was discovered at temperatures above and below the Curie temperature T_{C}=69 K. This result indicates a surprisingly strong momentum-dependent spin-phonon coupling induced by the spin dynamics in EuO. PMID:27203332
Microfabricated bulk wave acoustic bandgap device
Olsson, Roy H.; El-Kady, Ihab F.; McCormick, Frederick; Fleming, James G.; Fleming, legal representative, Carol
2010-11-23
A microfabricated bulk wave acoustic bandgap device comprises a periodic two-dimensional array of scatterers embedded within the matrix material membrane, wherein the scatterer material has a density and/or elastic constant that is different than the matrix material and wherein the periodicity of the array causes destructive interference of the acoustic wave within an acoustic bandgap. The membrane can be suspended above a substrate by an air or vacuum gap to provide acoustic isolation from the substrate. The device can be fabricated using microelectromechanical systems (MEMS) technologies. Such microfabricated bulk wave phononic bandgap devices are useful for acoustic isolation in the ultrasonic, VHF, or UHF regime (i.e., frequencies of order 1 MHz to 10 GHz and higher, and lattice constants of order 100 .mu.m or less).
Microfabricated bulk wave acoustic bandgap device
Olsson, Roy H.; El-Kady, Ihab F.; McCormick, Frederick; Fleming, James G.; Fleming, Carol
2010-06-08
A microfabricated bulk wave acoustic bandgap device comprises a periodic two-dimensional array of scatterers embedded within the matrix material membrane, wherein the scatterer material has a density and/or elastic constant that is different than the matrix material and wherein the periodicity of the array causes destructive interference of the acoustic wave within an acoustic bandgap. The membrane can be suspended above a substrate by an air or vacuum gap to provide acoustic isolation from the substrate. The device can be fabricated using microelectromechanical systems (MEMS) technologies. Such microfabricated bulk wave phononic bandgap devices are useful for acoustic isolation in the ultrasonic, VHF, or UHF regime (i.e., frequencies of order 1 MHz to 10 GHz and higher, and lattice constants of order 100 .mu.m or less).
Low temperature phonon-drag thermopower in a monolayer MoS{sub 2}
Bhargavi, K. S. Kubakaddi, S. S.
2015-06-24
Phonon-drag thermopower S{sup g} is studied theoretically in a monolayer MoS{sub 2} as a function of temperature T. Electron-acoustic phonon (el-ap) interaction via deformation potential (DP) coupling of TA (LA) phonons is taken to be unscreened (screened) and piezoelectric (PE) coupling of LA and TA phonons is taken to be screened. S{sup g} due to DP coupling of TA phonons is found to be dominant over all other mechanisms. In the Bloch-Gruneisen (BG) regime power law S{sup g} ∼ T{sup 3} (T {sup 5}) is predicted for unscreened (screened) el-ap interaction a characteristic of two-dimensional phonons with linear dispersion. Screening strongly suppresses S{sup g} due to large effective mass of the electrons. We find that, S{sup g} due to screened DP and PE couplings are nearly same in contrast with the results in GaAs heterojunctions. With the increasing T its exponent decreases and reaches a sublinear value.
Intrinsic to extrinsic phonon lifetime transition in a GaAs-AlAs superlattice.
Hofmann, F; Garg, J; Maznev, A A; Jandl, A; Bulsara, M; Fitzgerald, E A; Chen, G; Nelson, K A
2013-07-24
We have measured the lifetimes of two zone-center longitudinal acoustic phonon modes, at 320 and 640 GHz, in a 14 nm GaAs/2 nm AlAs superlattice structure. By comparing measurements at 296 and 79 K we separate the intrinsic contribution to phonon lifetime determined by phonon-phonon scattering from the extrinsic contribution due to defects and interface roughness. At 296 K, the 320 GHz phonon lifetime has approximately equal contributions from intrinsic and extrinsic scattering, whilst at 640 GHz it is dominated by extrinsic effects. These measurements are compared with intrinsic and extrinsic scattering rates in the superlattice obtained from first-principles lattice dynamics calculations. The calculated room-temperature intrinsic lifetime of longitudinal phonons at 320 GHz is in agreement with the experimentally measured value of 0.9 ns. The model correctly predicts the transition from predominantly intrinsic to predominantly extrinsic scattering; however the predicted transition occurs at higher frequencies. Our analysis indicates that the 'interfacial atomic disorder' model is not entirely adequate and that the observed frequency dependence of the extrinsic scattering rate is likely to be determined by a finite correlation length of interface roughness. PMID:23817884
Negative refraction imaging of acoustic metamaterial lens in the supersonic range
Han, Jianning; Wen, Tingdun; Yang, Peng; Zhang, Lu
2014-05-15
Acoustic metamaterials with negative refraction index is the most promising method to overcome the diffraction limit of acoustic imaging to achieve ultrahigh resolution. In this paper, we use localized resonant phononic crystal as the unit cell to construct the acoustic negative refraction lens. Based on the vibration model of the phononic crystal, negative quality parameters of the lens are obtained while excited near the system resonance frequency. Simulation results show that negative refraction of the acoustic lens can be achieved when a sound wave transmiting through the phononic crystal plate. The patterns of the imaging field agree well with that of the incident wave, while the dispersion is very weak. The unit cell size in the simulation is 0.0005 m and the wavelength of the sound source is 0.02 m, from which we show that acoustic signal can be manipulated through structures with dimensions much smaller than the wavelength of incident wave.
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. PMID:23286238
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.
Zhang, Ruiwen; Chen, Guodong; Sun, Junqiang
2016-06-13
We present the generation of forward stimulated Brillouin scattering (FSBS) in hybrid phononic-photonic waveguides. To confine the optical and acoustic waves simultaneously, a hybrid waveguide is designed by embedding the silicon line defect in the silicon nitride phononic crystal slab. By taking into account three kinds hybrid waveguide, the appropriate structural parameters are obtained to enhance the acousto-optic interaction. We fabricate the honeycomb hybrid waveguide with a CMOS compatible technology. The forward Brillouin frequency shift is measured up to 2.425 GHz and the acoustic Q-factor of the corresponding acoustic mode is 1100. The influences of pump power, acoustic loss, nonlinear optical loss and lattice constant on the acousto-optic interaction in FSBS are analyzed and discussed. The proposed approach has important potential applications in on-chip all-optical signal processing. PMID:27410324
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.
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.
Acoustical standards in engineering acoustics
NASA Astrophysics Data System (ADS)
Burkhard, Mahlon D.
2001-05-01
The Engineering Acoustics Technical Committee is concerned with the evolution and improvement of acoustical techniques and apparatus, and with the promotion of new applications of acoustics. As cited in the Membership Directory and Handbook (2002), the interest areas include transducers and arrays; underwater acoustic systems; acoustical instrumentation and monitoring; applied sonics, promotion of useful effects, information gathering and transmission; audio engineering; acoustic holography and acoustic imaging; acoustic signal processing (equipment and techniques); and ultrasound and infrasound. Evident connections between engineering and standards are needs for calibration, consistent terminology, uniform presentation of data, reference levels, or design targets for product development. Thus for the acoustical engineer standards are both a tool for practices, for communication, and for comparison of his efforts with those of others. Development of many standards depends on knowledge of the way products are put together for the market place and acoustical engineers provide important input to the development of standards. Acoustical engineers and members of the Engineering Acoustics arm of the Society both benefit from and contribute to the Acoustical Standards of the Acoustical Society.
NASA Astrophysics Data System (ADS)
Varshney, Dinesh; Jain, Rajendra K.
Upon considering the three interactions namely, the electron-acoustic phonon, the electron-optical phonon and the Coulomb, the analytical solutions for the energy gap equation allows one to determine the electronic structure parameters to discuss the behavior of superconducting transition temperature (Tc) and isotope effect coefficient (α) for layered structure YNi2 B2C. Tc of 17 K is estimated for YNi2B2C with electron-acoustic phonon (λac) = 0.31, electron-optical phonon (λop) = 0.1 and Coulomb screening parameter (μ*) = 0.126 indicating that the YNi2B2C superconductor is in the intermediate coupling regime. To correlate the Tc with various coupling strengths as λac, λop and μ*, we present curves of Tc with them. The present approach also explains the conditions for the Boron and Carbon isotope effect. The negative pressure coefficient of Tc in this layered material is attributed to the contraction along c-axis under hydrostatic pressure. We suggest from these results that both the acoustic and optical phonons within the framework of a three-square well scheme consistently explains the effective electron-electron interaction leading to superconductivity in layered structure YNi2B2C.
Heavy-impurity resonance, hybridization, and phonon spectral functions in Fe1-xMxSi, M=Ir,Os
Delaire, O.; Al-Qasir, Iyad I.; May, Andrew F.; Sales, Brian C.; Niedziela, Jennifer L.; Ma, Jie; Matsuda, Masaaki; Abernathy, Douglas L.; Berlijn, Tom
2015-03-31
The vibrational behavior of heavy substitutional impurities (M=Ir,Os) in Fe1-xMxSi (x = 0, 0.02, 0.04, 0.1) was investigated with a combination of inelastic neutron scattering (INS), transport measurements, and first-principles simulations. In this paper, our INS measurements on single-crystals mapped the four-dimensional dynamical structure factor, S(Q;E), for several compositions and temperatures. Our results show that both Ir and Os impurities lead to the formation of a weakly dispersive resonance vibrational mode, in the energy range of the acoustic phonon dispersions of the FeSi host. We also show that Ir doping, which introduces free carriers and increases electron-phonon coupling, leads tomore » softened interatomic force-constants compared to doping with Os, which is isoelectronic to Fe. We analyze the phonon S(Q,E) from INS through a Green's function model incorporating the phonon self-energy based on first-principles density functional theory (DFT) simulations. Calculations of the quasiparticle spectral functions in the doped system reveal the hybridization between the resonance and the acoustic phonon modes. Finally, our results demonstrate a strong interaction of the host acoustic dispersions with the resonance mode, likely leading to the large observed suppression in lattice thermal conductivity.« less
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
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
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
An acoustic neuroma is a benign tumor that develops on the nerve that connects the ear to the brain. The tumor ... press against the brain, becoming life-threatening. Acoustic neuroma can be difficult to diagnose, because the symptoms ...
Phonon counting and intensity interferometry of a nanomechanical resonator.
Cohen, Justin D; Meenehan, Seán M; MacCabe, Gregory S; Gröblacher, Simon; Safavi-Naeini, Amir H; Marsili, Francesco; Shaw, Matthew D; Painter, Oskar
2015-04-23
In optics, the ability to measure individual quanta of light (photons) enables a great many applications, ranging from dynamic imaging within living organisms to secure quantum communication. Pioneering photon counting experiments, such as the intensity interferometry performed by Hanbury Brown and Twiss to measure the angular width of visible stars, have played a critical role in our understanding of the full quantum nature of light. As with matter at the atomic scale, the laws of quantum mechanics also govern the properties of macroscopic mechanical objects, providing fundamental quantum limits to the sensitivity of mechanical sensors and transducers. Current research in cavity optomechanics seeks to use light to explore the quantum properties of mechanical systems ranging in size from kilogram-mass mirrors to nanoscale membranes, as well as to develop technologies for precision sensing and quantum information processing. Here we use an optical probe and single-photon detection to study the acoustic emission and absorption processes in a silicon nanomechanical resonator, and perform a measurement similar to that used by Hanbury Brown and Twiss to measure correlations in the emitted phonons as the resonator undergoes a parametric instability formally equivalent to that of a laser. Owing to the cavity-enhanced coupling of light with mechanical motion, this effective phonon counting technique has a noise equivalent phonon sensitivity of 0.89 ± 0.05. With straightforward improvements to this method, a variety of quantum state engineering tasks using mesoscopic mechanical resonators would be enabled, including the generation and heralding of single-phonon Fock states and the quantum entanglement of remote mechanical elements.
Phonon counting and intensity interferometry of a nanomechanical resonator
NASA Astrophysics Data System (ADS)
Cohen, Justin D.; Meenehan, Seán M.; Maccabe, Gregory S.; Gröblacher, Simon; Safavi-Naeini, Amir H.; Marsili, Francesco; Shaw, Matthew D.; Painter, Oskar
2015-04-01
In optics, the ability to measure individual quanta of light (photons) enables a great many applications, ranging from dynamic imaging within living organisms to secure quantum communication. Pioneering photon counting experiments, such as the intensity interferometry performed by Hanbury Brown and Twiss to measure the angular width of visible stars, have played a critical role in our understanding of the full quantum nature of light. As with matter at the atomic scale, the laws of quantum mechanics also govern the properties of macroscopic mechanical objects, providing fundamental quantum limits to the sensitivity of mechanical sensors and transducers. Current research in cavity optomechanics seeks to use light to explore the quantum properties of mechanical systems ranging in size from kilogram-mass mirrors to nanoscale membranes, as well as to develop technologies for precision sensing and quantum information processing. Here we use an optical probe and single-photon detection to study the acoustic emission and absorption processes in a silicon nanomechanical resonator, and perform a measurement similar to that used by Hanbury Brown and Twiss to measure correlations in the emitted phonons as the resonator undergoes a parametric instability formally equivalent to that of a laser. Owing to the cavity-enhanced coupling of light with mechanical motion, this effective phonon counting technique has a noise equivalent phonon sensitivity of 0.89 +/- 0.05. With straightforward improvements to this method, a variety of quantum state engineering tasks using mesoscopic mechanical resonators would be enabled, including the generation and heralding of single-phonon Fock states and the quantum entanglement of remote mechanical elements.
NASA Technical Reports Server (NTRS)
Steinetz, Bruce M. (Inventor)
2006-01-01
The invention relates to a sealing device having an acoustic resonator. The acoustic resonator is adapted to create acoustic waveforms to generate a sealing pressure barrier blocking fluid flow from a high pressure area to a lower pressure area. The sealing device permits noncontacting sealing operation. The sealing device may include a resonant-macrosonic-synthesis (RMS) resonator.
NASA Technical Reports Server (NTRS)
Steinetz, Bruce M. (Inventor)
2006-01-01
The invention relates to a sealing device having an acoustic resonator. The acoustic resonator is adapted to create acoustic waveforms to generate a sealing pressure barrier blocking fluid flow from a high pressure area to a lower pressure area. The sealing device permits noncontacting sealing operation. The sealing device may include a resonant-macrosonic-synthesis (RMS) resonator.
Phonon assisted resonant tunnelling 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 tunnelling differential conductance of InAs quantum dots. We found that dissipative quantum tunnelling has a strong influence on the operation of nano-devices. Because of such tunnelling 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 tunnelling 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.
Phonon modes and Raman intensity profiles in zinc-blende BN/GaN superlattices
NASA Astrophysics Data System (ADS)
Talwar, Devki N.; Lenze, Benjamin A.; Czak, Jason E.; Bensaoula, Abdelhak
2014-01-01
By exploiting both an elastic continuum and a linear-chain model (LCM) we have systematically investigated the phonon characteristics in a series of unconventional short-period zb (BN)m/(GaN)n (0 0 1) superlattices (SLs)—selecting m or n values between 2 and 10. The calculated longitudinal folded-acoustic modes and the doublets near the phonon wavevector \\vert \\vec{{q}}_{s} \\vert are shown to have stronger dependence on the SL periods dSL. In the framework of a bond-polarizability scheme and using a second-nearest neighbour LCM, our simulated results for the phonon dispersions and Raman intensity profiles have revealed not only the major expected trends of the vibrational characteristics observed experimentally in many conventional SLs but also elicited some interesting contrasts. We expect that the outcome of the present study will encourage experimentalists to utilize Raman scattering spectroscopy, perform material characterizations and authenticate our theoretical conjectures.
Level repulsion of GHz phononic surface waves in quartz substrate with finite-depth holes.
Yeh, Sih-Ling; Lin, Yu-Ching; Tsai, Yao-Chuan; Ono, Takahito; Wu, Tsung-Tsong
2016-09-01
This paper presents numerical and experimental results on the level repulsion of gigahertz surface acoustic waves in an air/ST-cut quartz phononic structure with finite-depth holes. The colorful dispersion with the parameter of the in-plane (sagittal plane) ratio of polarization was adopted to determine the Rayleigh wave bandgap induced by the level repulsion. The results of numerical analyses showed that the frequency and width of the bandgap induced by the level repulsion strongly depend on the geometry of the air holes in the phononic structure. In the experiment, a pair of slanted interdigital transducers with frequency in the gigahertz range was designed and fabricated to generate and receive broadband Rayleigh waves, whereas the reactive ion etching process with electron-beam lithography was used to fabricate submicrometer phononic structures. The measured results of the bandgap induced by the level repulsion agreed favorably with the numerical prediction. PMID:27300272
THz elastic dynamics in finite-size CoFeB-MgO phononic superlattices
NASA Astrophysics Data System (ADS)
Ulrichs, Henning; Meyer, Dennis; Müller, Markus; Wittrock, Steffen; Mansurova, Maria; Walowski, Jakob; Münzenberg, Markus
2016-10-01
In this article, we present the observation of coherent elastic dynamics in a nano-scale phononic superlattice, which consists of only 4 bilayers. We demonstrate how ultra-short light pulses with a length of 40 fs can be utilized to excite a coherent elastic wave at 0.535 THz, which persist over about 20 ps. In later steps of the elastic dynamics, modes with frequency of 1.7 THz and above appear. All these modes are related to acoustic band gaps. Thus, the periodicity strongly manifests in the wave physics, although the system under investigation has only a small number of spatial periods. To further illustrate this, we show how by breaking the translational invariance of the superlattice, these features can be suppressed. Discussed in terms of phonon blocking and radiation, we elucidate in how far our structures can be considered as useful building blocks for phononic devices.
Studies of Phonon Anharmonicity in Solids
NASA Astrophysics Data System (ADS)
Lan, Tian
Today our understanding of the vibrational thermodynamics of materials at low temperatures is emerging nicely, based on the harmonic model in which phonons are independent. At high temperatures, however, this understanding must accommodate how phonons interact with other phonons or with other excitations. We shall see that the phonon-phonon interactions give rise to interesting coupling problems, and essentially modify the equilibrium and non-equilibrium properties of materials, e.g., thermodynamic stability, heat capacity, optical properties and thermal transport of materials. Despite its great importance, to date the anharmonic lattice dynamics is poorly understood and most studies on lattice dynamics still rely on the harmonic or quasiharmonic models. There have been very few studies on the pure phonon anharmonicity and phonon-phonon interactions. The work presented in this thesis is devoted to the development of experimental and computational methods on this subject. Modern inelastic scattering techniques with neutrons or photons are ideal for sorting out the anharmonic contribution. Analysis of the experimental data can generate vibrational spectra of the materials, i.e., their phonon densities of states or phonon dispersion relations. We obtained high quality data from laser Raman spectrometer, Fourier transform infrared spectrometer and inelastic neutron spectrometer. With accurate phonon spectra data, we obtained the energy shifts and lifetime broadenings of the interacting phonons, and the vibrational entropies of different materials. The understanding of them then relies on the development of the fundamental theories and the computational methods. We developed an efficient post-processor for analyzing the anharmonic vibrations from the molecular dynamics (MD) calculations. Currently, most first principles methods are not capable of dealing with strong anharmonicity, because the interactions of phonons are ignored at finite temperatures. Our method adopts
Inelastic x-ray scattering measurements of phonon dispersion and lifetimes in PbTe1-x Se x alloys.
Tian, Zhiting; Li, Mingda; Ren, Zhensong; Ma, Hao; Alatas, Ahmet; Wilson, Stephen D; Li, Ju
2015-09-23
PbTe1-x Se x alloys are of special interest to thermoelectric applications. Inelastic x-ray scattering determination of phonon dispersion and lifetimes along the high symmetry directions for PbTe1-x Se x alloys are presented. By comparing with calculated results based on the virtual crystal model calculations combined with ab initio density functional theory, the validity of virtual crystal model is evaluated. The results indicate that the virtual crystal model is overall a good assumption for phonon frequencies and group velocities despite the softening of transverse acoustic phonon modes along [1 1 1] direction, while the treatment of lifetimes warrants caution. In addition, phonons remain a good description of vibrational modes in PbTe1-x Se x alloys.
NASA Astrophysics Data System (ADS)
Taneda, T.; Pepe, G. P.; Parlato, L.; Golubov, A. A.; Sobolewski, Roman
2007-05-01
We present our femtosecond optical pump-probe studies of proximized ferromagnet-superconductor nanobilayers. The weak ferromagnetic nature of a thin NiCu film makes it possible to observe the dynamics of the nonequilibrium carriers through the near-surface optical reflectivity change measurements. The subpicosecond biexponential reflectivity decay has been identified as electron-phonon Debye and acoustic phonon relaxation times, and the decay of Debye phonons versus temperature dependence was used to evaluate the electron-phonon coupling constants for both the pure Nb and proximized Nb/NiCu heterostructures down to low temperatures. We have also demonstrated that the NiCu overlay on top of Nb dramatically reduced the slow, bolometric component of the photoresponse component, making such bilayers attractive for future radiation detector applications.
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.
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
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
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. PMID:26817419
Phonons from neutron powder diffraction
Dimitrov, D.A.; Louca, D.; Roeder, H. )
1999-09-01
The spherically averaged structure function S([vert bar][bold q][vert bar]) obtained from pulsed neutron powder diffraction contains both elastic and inelastic scattering via an integral over energy. The Fourier transformation of S([vert bar][bold q][vert bar]) to real space, as is done in the pair density function (PDF) analysis, regularizes the data, i.e., it accentuates the diffuse scattering. We present a technique which enables the extraction of off-center ([vert bar][bold q][vert bar][ne]0) phonon information from powder diffraction experiments by comparing the experimental PDF with theoretical calculations based on standard interatomic potentials and the crystal symmetry. This procedure [dynamics from powder diffraction] has been [ital successfully] implemented as demonstrated here for two systems, a simple metal fcc Ni and an ionic crystal CaF[sub 2]. Although computationally intensive, this data analysis allows for a phonon based modeling of the PDF, and additionally provides off-center phonon information from neutron powder diffraction. [copyright] [ital 1999] [ital The American Physical Society
NASA Astrophysics Data System (ADS)
Torén, H.; Samuelsson, L.; Hellsing, B.
2016-09-01
The aim of this model study of the electron-phonon coupling in graphene was to find out about the relative importance of the inter- and intraband scattering and which phonon modes are the most active. This was achieved by analyzing the electron-phonon matrix element of the carbon dimer in the unit cell. We found that for the intra molecular orbital matrix elements the longitudinal optical phonon mode is the active phonon mode. The matrix element corresponding to σ → σ is greater than the matrix element for π → π . The inter molecular orbital scattering π → σ is driven by the out-of-plane acoustic phonon mode, while the out-of-plane optical mode does not contribute for symmetry reasons. We found the unexpected result that the magnitude of matrix element of the inter molecular orbital scattering π → σ exceeds the intra molecular orbital scattering π → π . These results indicate that the in general not considered inter-band scattering has to be taken into account when analyzing e.g. photo-hole lifetimes and the electron-phonon coupling constant λ from photoemission data of graphene.
NASA Astrophysics Data System (ADS)
Tsai, Chia-Nien; Chen, Lien-Wen
2016-07-01
Self-collimation is wave propagation in straight path without diffraction. The performance is evaluated by bandwidth, angular collimating range and straightness of equi-frequency contours. The present study aims to manipulate the self-collimated beam in square-array phononic crystals by means of orientated rectangular inclusions. Finite element simulations are performed to investigate the effects of the aspect ratio and orientation angle of rectangular inclusions on the self-collimated beam. The simulation results show that the proposed design successfully achieves all-angle self-collimation phenomenon. In addition, it also shows that the propagation direction of a self-collimated beam can be effectively manipulated by varying the orientation angle of inclusions. Numerical simulation result of the S-shaped bend demonstrates that acoustic collimated beam can be steered with negligible diffraction. Overall, the proposed design has significant potential for the realization of applications such as collimators, acoustic waveguides and other phononic crystals-based systems.
High-speed asynchronous optical sampling for high-sensitivity detection of coherent phonons
NASA Astrophysics Data System (ADS)
Dekorsy, T.; Taubert, R.; Hudert, F.; Schrenk, G.; Bartels, A.; Cerna, R.; Kotaidis, V.; Plech, A.; Köhler, K.; Schmitz, J.; Wagner, J.
2007-12-01
A new optical pump-probe technique is implemented for the investigation of coherent acoustic phonon dynamics in the GHz to THz frequency range which is based on two asynchronously linked femtosecond lasers. Asynchronous optical sampling (ASOPS) provides the performance of on all-optical oscilloscope and allows us to record optically induced lattice dynamics over nanosecond times with femtosecond resolution at scan rates of 10 kHz without any moving part in the set-up. Within 1 minute of data acquisition time signal-to-noise ratios better than 107 are achieved. We present examples of the high-sensitivity detection of coherent phonons in superlattices and of the coherent acoustic vibration of metallic nanoparticles.
Scanning phononic lattices with ultrasound
Vines, R.E.; Wolfe, J.P.; Every, A.V.
1999-11-01
A method for probing the elastic properties of newly developed periodic structures using acoustic waves is introduced. Highly anisotropic transmission of surface acoustic waves is observed by continuously scanning the wave vector angle. Preliminary models of wave propagation through multilayers and two-dimensional lattices explain some of the experimental features, while other features can be attributed to the resonant excitation of interface waves. {copyright} {ital 1999} {ital The American Physical Society}
A short history of bad acoustics.
Wright, M C M
2006-10-01
Every branch of science attracts its share of cranks and pseudoscientists, and acoustics has been no exception. A brief survey of those who touched on acoustics is given with quotations from the more interesting or egregious examples. A contrast is drawn between the nineteenth century contrarian's quarrel with particular theories and the modern new age wholesale rejection of theory. This world-view is traced back to the later scientific writings of Goethe. Examples of pseudoscience applied to biomedical acoustics, architectural acoustics, and audio reproduction are given.
On-chip optical mode conversion based on dynamic grating in photonic-phononic hybrid waveguide.
Chen, Guodong; Zhang, Ruiwen; Sun, Junqiang
2015-01-01
We present a scheme for reversible and tunable on-chip optical mode conversion based on dynamic grating in a hybrid photonic-phononic waveguide. The dynamic grating is built up through the acousto-optic effect and the theoretical model of the optical mode conversion is developed by considering the geometrical deformation and refractive index change. Three kinds of mode conversions are able to be realized using the same hybrid waveguide structure in a large bandwidth by only changing the launched acoustic frequency. The complete mode conversion can be achieved by choosing a proper acoustic power under a given waveguide length. PMID:25996236
Phonon lineshapes in atom-surface scattering.
Martínez-Casado, R; Sanz, A S; Miret-Artés, S
2010-08-01
Phonon lineshapes in atom-surface scattering are obtained from a simple stochastic model based on the so-called Caldeira-Leggett Hamiltonian. In this single-bath model, the excited phonon resulting from a creation or annihilation event is coupled to a thermal bath consisting of an infinite number of harmonic oscillators, namely the bath phonons. The diagonalization of the corresponding Hamiltonian leads to a renormalization of the phonon frequencies in terms of the phonon friction or damping coefficient. Moreover, when there are adsorbates on the surface, this single-bath model can be extended to a two-bath model accounting for the effect induced by the adsorbates on the phonon lineshapes as well as their corresponding lineshapes. PMID:21399349
Topologically protected elastic waves in phononic metamaterials
Mousavi, S. Hossein; Khanikaev, Alexander B.; Wang, Zheng
2015-01-01
Surface waves in topological states of quantum matter exhibit unique protection from backscattering induced by disorders, making them ideal carriers for both classical and quantum information. Topological matters for electrons and photons are largely limited by the range of bulk properties, and the associated performance trade-offs. In contrast, phononic metamaterials provide access to a much wider range of material properties. Here we demonstrate numerically a phononic topological metamaterial in an elastic-wave analogue of the quantum spin Hall effect. A dual-scale phononic crystal slab is used to support two effective spins for phonons over a broad bandwidth, and strong spin–orbit coupling is realized by breaking spatial mirror symmetry. By preserving the spin polarization with an external load or spatial symmetry, phononic edge states are shown to be robust against scattering from discrete defects as well as disorders in the continuum, demonstrating topological protection for phonons in both static and time-dependent regimes. PMID:26530426
Chandel, Surjeet Kumar; Kumar, Arun; Bharti, Ankush; Sharma, Raman
2015-05-15
Using first principles density functional theoretical calculations, the present paper reports a systematic study of phonon dispersion curves in pristine carbon (CNT) and silicon nanotubes (SiNT) having chirality (6,6) in the armchair configuration. Some of the phonon modes are found to have negative frequencies which leads to instability of the systems under study. The number of phonon branches has been found to be thrice as much as the number of atoms. The frequency of the higher optical bands varies from 1690 to 1957 cm{sup −1} for CNT(6,6) while it is 596 to 658 cm{sup −1} for SiNT.
Electron-phonon interaction in quasi-1D ternary mixed crystals of polar semiconductors
NASA Astrophysics Data System (ADS)
Fan, Yunpeng; Hou, Junhua
2015-07-01
The electron-optical-phonon interaction between an electron and two branches of LO-phonon modes in a quasi-one-dimensional ternary mixed crystal (TMC) of polar semiconductors is studied. The new electron-phonon interaction Hamiltonian including the unit-cell volume variation in TMCs is obtained by using the modified random element isodisplacement model and Born-Huang method. The energies of polaron are numerically calculated for several systems of III-V compounds. A group of III-nitride mixed crystals is also taken into numerical calculation in our theory. It is verified theoretically that the obvious nonlinearity of the polaronic energy and effective mass with the composition is essential and the unit-cell volume effects cannot be neglected except the very weak e-p coupling.
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.
NASA Astrophysics Data System (ADS)
Ayria, Pourya; Tanaka, Shin-ichiro; Nugraha, Ahmad R. T.; Dresselhaus, Mildred S.; Saito, Riichiro
2016-08-01
Indirect transitions of electrons in graphene and graphite are investigated by means of angle-resolved photoemission spectroscopy (ARPES) with several different incident photon energies and light polarizations. The theoretical calculations of the indirect transition for graphene and for a single crystal of graphite are compared with the experimental measurements for highly-oriented pyrolytic graphite and a single crystal of graphite. The dispersion relations for the transverse optical (TO) and the out-of-plane longitudinal acoustic (ZA) phonon modes of graphite and the TO phonon mode of graphene can be extracted from the inelastic ARPES intensity. We find that the TO phonon mode for k points along the Γ -K and K -M -K' directions in the Brillouin zone can be observed in the ARPES spectra of graphite and graphene by using a photon energy ≈11.1 eV. The relevant mechanism in the ARPES process for this case is the resonant indirect transition. On the other hand, the ZA phonon mode of graphite can be observed by using a photon energy ≈6.3 eV through a nonresonant indirect transition, while the ZA phonon mode of graphene within the same mechanism should not be observed.
Phonon-induced diamagnetic force and its effect on the lattice thermal conductivity.
Jin, Hyungyu; Restrepo, Oscar D; Antolin, Nikolas; Boona, Stephen R; Windl, Wolfgang; Myers, Roberto C; Heremans, Joseph P
2015-06-01
Phonons are displacements of atoms around their rest positions in a crystalline solid. They carry sound and heat, but are not classically associated with magnetism. Here, we show that phonons are, in fact, sensitive to magnetic fields, even in diamagnetic materials. We do so by demonstrating experimentally that acoustic phonons in a diamagnetic semiconductor (InSb) scatter more strongly from one another when a magnetic field is applied. We attribute this observation to the magnetic-field sensitivity of the anharmonicity of the interatomic bonds that govern the probability of phonon-phonon interactions. The displacements of atoms locally affect the orbital motion of valence band electrons, which, in the presence of an external magnetic field, spatially modulates the orbital diamagnetism around the displaced atoms. The spatial gradient in magnetic moment results in an anharmonic magnetic force exerted on the displaced atom. The process is modelled by ab initio calculations that, without the use of a single adjustable parameter, reproduce the observed 12% decrease in the lattice thermal conductivity under a 7 T magnetic field at a temperature of 5.2 K. PMID:25799325
Ballistic phonon transport in holey silicon.
Lee, Jaeho; Lim, Jongwoo; Yang, Peidong
2015-05-13
When the size of semiconductors is smaller than the phonon mean free path, phonons can carry heat with no internal scattering. Ballistic phonon transport has received attention for both theoretical and practical aspects because Fourier's law of heat conduction breaks down and the heat dissipation in nanoscale transistors becomes unpredictable in the ballistic regime. While recent experiments demonstrate room-temperature evidence of ballistic phonon transport in various nanomaterials, the thermal conductivity data for silicon in the length scale of 10-100 nm is still not available due to experimental challenges. Here we show ballistic phonon transport prevails in the cross-plane direction of holey silicon from 35 to 200 nm. The thermal conductivity scales linearly with the length (thickness) even though the lateral dimension (neck) is as narrow as 20 nm. We assess the impact of long-wavelength phonons and predict a transition from ballistic to diffusive regime using scaling models. Our results support strong persistence of long-wavelength phonons in nanostructures and are useful for controlling phonon transport for thermoelectrics and potential phononic applications.
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.
NASA Astrophysics Data System (ADS)
Kavitha, C.; Narayana, Chandrabhas
2014-10-01
Low-temperature Brillouin scattering studies on Chalcopyrite AgGaS2 have been carried out between the temperatures ranges 20sbnd 300 K. We observe the Brillouin modes for AgGaS2 as follows: for crossed polarization (VH) the two modes are around 5.4 and 18 GHz, labelled as RSW1 (Rayleigh surface mode), LA (longitudinal) modes and for parallel polarization (HH), we see the modes are around 4.5 and 11 GHz, labelled as RSW2, TA (transverse acoustic). According to this acoustic phonon mode behaviour with temperature, we do see phase transition at 80 K for AgGaS2. The phase change observed at 80 K is attributed to a strong electronsbnd phonon coupling between the acoustic modes, which modulates the acoustic mode frequency 10sbnd 15%. The strong electronsbnd phonon coupling between acoustic modes (LA and TA), which is seen from the anomalous behaviour below 80 K, shows the electronic transition to be of the type direct to indirect band gap. The sound velocity and elastic constants have been calculated using Brillouin modes, which are in good agreement with previous reports.
NASA Astrophysics Data System (ADS)
Yang, Zhaoju; Gao, Fei; Shi, Xihang; Lin, Xiao; Gao, Zhen; Chong, Yidong; Zhang, Baile
2015-03-01
The manipulation of acoustic wave propagation in fluids has numerous applications, including some in everyday life. Acoustic technologies frequently develop in tandem with optics, using shared concepts such as waveguiding and metamedia. It is thus noteworthy that an entirely novel class of electromagnetic waves, known as "topological edge states," has recently been demonstrated. These are inspired by the electronic edge states occurring in topological insulators, and possess a striking and technologically promising property: the ability to travel in a single direction along a surface without backscattering, regardless of the existence of defects or disorder. Here, we develop an analogous theory of topological fluid acoustics, and propose a scheme for realizing topological edge states in an acoustic structure containing circulating fluids. The phenomenon of disorder-free one-way sound propagation, which does not occur in ordinary acoustic devices, may have novel applications for acoustic isolators, modulators, and transducers.
Yang, Zhaoju; Gao, Fei; Shi, Xihang; Lin, Xiao; Gao, Zhen; Chong, Yidong; Zhang, Baile
2015-03-20
The manipulation of acoustic wave propagation in fluids has numerous applications, including some in everyday life. Acoustic technologies frequently develop in tandem with optics, using shared concepts such as waveguiding and metamedia. It is thus noteworthy that an entirely novel class of electromagnetic waves, known as "topological edge states," has recently been demonstrated. These are inspired by the electronic edge states occurring in topological insulators, and possess a striking and technologically promising property: the ability to travel in a single direction along a surface without backscattering, regardless of the existence of defects or disorder. Here, we develop an analogous theory of topological fluid acoustics, and propose a scheme for realizing topological edge states in an acoustic structure containing circulating fluids. The phenomenon of disorder-free one-way sound propagation, which does not occur in ordinary acoustic devices, may have novel applications for acoustic isolators, modulators, and transducers.
ERIC Educational Resources Information Center
Perkins, John W.; And Others
Designed for the training of a newly appointed branch librarian as well as for general background on the function of a branch library for the entire staff, this publication was written as a comprehensive guide to the administration of a branch library. Specific chapters focus on: (1) administrative goals and activities, (2) organizational…
Electron-phonon superconductivity in LaO{sub 0.5}F{sub 0.5}BiSe{sub 2}
Feng, Yanqing; Du, Yongping; Wan, Xiangang Wang, Bogen; Ding, Hang-Chen; Savrasov, Sergey Y.; Duan, Chun-Gang
2014-06-21
We report density functional calculations of the electronic structure, Fermi surface, phonon spectrum and electron–phonon coupling for the newly discovered superconductor LaO{sub 0.5}F{sub 0.5}BiSe{sub 2}. It is confirmed that there is a strong Fermi surface nesting at (π,π,0), which results in unstable phonon branches. Combining the frozen phonon total energy calculations and an anharmonic oscillator model, we find that the quantum fluctuation prevents the appearance of static long–range order. The calculation shows that LaO{sub 0.5}F{sub 0.5}BiSe{sub 2} is highly anisotropic, and same as its cousin LaO{sub 0.5}F{sub 0.5}BiS{sub 2}, this compound is also a conventional electron-phonon coupling induced superconductor.
Applications of time-domain spectroscopy to electron-phonon coupling dynamics at surfaces.
Matsumoto, Yoshiyasu
2014-10-01
Photochemistry is one of the most important branches in chemistry to promote and control chemical reactions. In particular, there has been growing interest in photoinduced processes at solid surfaces and interfaces with liquids such as water for developing efficient solar energy conversion. For example, photoinduced charge transfer between adsorbates and semiconductor substrates at the surfaces of metal oxides induced by photogenerated holes and electrons is a core process in photovoltaics and photocatalysis. In these photoinduced processes, electron-phonon coupling plays a central role. This paper describes how time-domain spectroscopy is applied to elucidate electron-phonon coupling dynamics at metal and semiconductor surfaces. Because nuclear dynamics induced by electronic excitation through electron-phonon coupling take place in the femtosecond time domain, the pump-and-probe method with ultrashort pulses used in time-domain spectroscopy is a natural choice for elucidating the electron-phonon coupling at metal and semiconductor surfaces. Starting with a phenomenological theory of coherent phonons generated by impulsive electronic excitation, this paper describes a couple of illustrative examples of the applications of linear and nonlinear time-domain spectroscopy to a simple adsorption system, alkali metal on Cu(111), and more complex photocatalyst systems. PMID:25139240
NASA Astrophysics Data System (ADS)
Drury, Byron; Klales, Anna; Heller, Eric
2014-03-01
Branched flow appears in a variety of physical systems spanning length scales from microns to thousands of kilometers. For instance, it plays an important role in both electron transport in two dimensional electron gases and the propagation of tsunamis in the ocean. Branches have typically been identified with caustics in the theoretical literature, but concentrations of flux recognizable as branches can arise from other mechanisms. We propose a generalized definition of branching based on a local measure of the stability of trajectories. We analytically and numerically study the characteristics of Hamiltonian flow in phase space and characterize the relationship between branch formation and trajectory stability.
Thermal conductivity of a classical one-dimensional spin-phonon system
NASA Astrophysics Data System (ADS)
Savin, A. V.; Tsironis, G. P.; Zotos, X.
2007-06-01
We investigate the thermal conduction properties of a one-dimensional lattice of atoms carrying classical spins and coupled vibrationally. The spin degrees of freedom interact via a classical Heisenberg interaction, while the vibrational degrees of freedom are coupled through nearest-neighbor linear as well as nonlinear forces. The thermal conductivity in spin-phonon systems has both a phononic as well as a magnetic contribution. We use extensive numerical simulations and evaluate the magnetic and phononic thermal current correlation functions as well as the combined thermal conductivity coefficient. We employ two distinct numerical approaches: The first is based on the linear response theory and proceeds through an evaluation of the energy current correlation function using the Green-Kubo formula. The second is through a simulation of the stochastic baths and a subsequent direct numerical evaluation of the magnetic and phononic heat currents. We find an anomalous thermal conductivity when the spins are coupled to a harmonic acoustic phonon chain. However, when the harmonic phonon chain contains, additionally, an optical mode, we find that the thermal conductivity is normal for a certain regime of on-site force parameters, while it becomes anomalous when the on-site frequency becomes larger than a certain value. Coupling thus to a harmonic system with an optical mode provides a case of tunable conductivity that switches from being diffusive to ballistic as a function of structural model parameters or of the temperature. When the spins are coupled to anharmonic chains, we find an anomalous conductivity when the phonon chain is acoustic, for instance, in the Fermi-Past-Ulam case, or a normal one when the nonlinearity is of optic type. For the cases analyzed, we provide quantitative information on the exponent characterizing the power law decay of the energy current correlation function and determine size and temperature dependencies of the conductivity coefficient
Focusing capability of a phononic crystal based on a hollow metallic structure.
Hladky-Hennion, Anne-Christine; Croenne, Charles; Vasseur, Jerome; Haumesser, Lionel; Norris, Andrew
2014-08-01
The dispersion curves of a phononic crystal (PC) based on a hollow metallic structure are presented. They exhibit a negative refraction dispersion branch and perfect refractive index matching with the surrounding water, leading to focusing capability. Numerical and experimental results are reported for a flat PC lens. The characteristics of the focal spot (intensity, dimensions, etc.) are numerically and experimentally investigated with the goal of finding the frequency of the optimal imaging performance. PMID:25073139
Heavy-impurity resonance, hybridization, and phonon spectral functions in Fe1-xMxSi (M =Ir , Os )
NASA Astrophysics Data System (ADS)
Delaire, O.; Al-Qasir, I. I.; May, A. F.; Li, C. W.; Sales, B. C.; Niedziela, J. L.; Ma, J.; Matsuda, M.; Abernathy, D. L.; Berlijn, T.
2015-03-01
The vibrational behavior of heavy substitutional impurities (M = Ir,Os) in Fe1-xMxSi (x =0 ,0.02 ,0.04 ,0.1 ) was investigated with a combination of inelastic neutron scattering (INS), transport measurements, and first-principles simulations. Our INS measurements on single crystals mapped the four-dimensional dynamical structure factor, S (Q ,E ) , for several compositions and temperatures. Our results show that both Ir and Os impurities lead to the formation of a weakly dispersive resonance vibrational mode, in the energy range of the acoustic phonon dispersions of the FeSi host. We also show that Ir doping, which introduces free carriers, leads to softened interatomic force constants compared to doping with Os, which is isoelectronic to Fe. We analyze the phonon S (Q ,E ) from INS through a Green's-function model incorporating the phonon self-energy based on first-principles density functional theory simulations, and we study the disorder-induced lifetimes on large supercells. Calculations of the quasiparticle spectral functions in the doped system reveal the hybridization between the resonance and the acoustic phonon modes. Our results demonstrate a strong interaction of the host acoustic dispersions with the resonance mode, likely leading to the large observed suppression in lattice thermal conductivity.
Influence of magnetism on phonons in CaFe{sub 2}As{sub 2} as seen via inelastic x-ray scattering.
Hahn, S. E.; Lee, Y.; Ni, N.; Canfield, P. C.; Goldman, A. I.; McQueeney, R. J.; Harmon, B. N.; Alatas, A.; Leu, B. M.; Alp, E. E.; Chung, D. Y.; Todorov, I. S.; Kanatzidis, M. G.; Iowa State Univ.; Northwestern Univ.
2009-01-01
In the iron pnictides, the strong sensitivity of the iron magnetic moment to the arsenic position suggests a significant relationship between phonons and magnetism. We measured the phonon dispersion of several branches in the high-temperature tetragonal phase of CaFe{sub 2}As{sub 2} using inelastic x-ray scattering on single-crystal samples. These measurements were compared to ab initio calculations of the phonons. Spin-polarized calculations imposing the antiferromagnetic order present in the low-temperature orthorhombic phase dramatically improve agreement between theory and experiment. This is discussed in terms of the strong antiferromagnetic correlations that are known to persist in the tetragonal phase.
NASA Astrophysics Data System (ADS)
Gough, Colin
This chapter provides an introduction to the physical and psycho-acoustic principles underlying the production and perception of the sounds of musical instruments. The first section introduces generic aspects of musical acoustics and the perception of musical sounds, followed by separate sections on string, wind and percussion instruments.
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.
The phonon Hall effect: theory and application.
Zhang, Lifa; Ren, Jie; Wang, Jian-Sheng; Li, Baowen
2011-08-01
We present a systematic theory of the phonon Hall effect in a ballistic crystal lattice system, and apply it on the kagome lattice which is ubiquitous in various real materials. By proposing a proper second quantization for the non-Hermitian in the polarization-vector space, we obtain a new heat current density operator with two separate contributions: the normal velocity responsible for the longitudinal phonon transport, and the anomalous velocity manifesting itself as the Hall effect of transverse phonon transport. As exemplified in kagome lattices, our theory predicts that the direction of Hall conductivity at low magnetic field can be reversed by tuning the temperatures, which we hope can be verified by experiments in the future. Three phonon-Hall-conductivity singularities induced by phonon-band-topology change are discovered as well, which correspond to the degeneracies at three different symmetric center points, Γ, K, X, in the wavevector space of the kagome lattice.
Phonon Drag Dislocations at High Pressures
Wolfer, W.G.
1999-10-19
Phonon drag on dislocations is the dominant process which determines the flow stress of metals at elevated temperatures and at very high plastic deformation rates. The dependence of the phonon drag on pressure or density is derived using a Mie-Grueneisen equation of state. The phonon drag is shown to increase nearly linearly with temperature but to decrease with density or pressure. Numerical results are presented for its variation for shock-loaded copper and aluminum. In these cases, density and temperature increase simultaneously, resulting in a more modest net increase in the dislocation drag coefficient. Nevertheless, phonon drag increases by more than an order of magnitude during shock deformations which approach melting. Since the dependencies of elastic moduli and of the phonon drag coefficient on pressure and temperature are fundamentally different, the effect of pressure on the constitutive law for plastic deformation can not simply be accounted for by its effect on the elastic shear modulus.
Phonon triggered rhombohedral lattice distortion in vanadium at high pressure.
Antonangeli, Daniele; Farber, Daniel L; Bosak, Alexei; Aracne, Chantel M; Ruddle, David G; Krisch, Michael
2016-01-01
In spite of the simple body-centered-cubic crystal structure, the elements of group V, vanadium, niobium and tantalum, show strong interactions between the electronic properties and lattice dynamics. Further, these interactions can be tuned by external parameters, such as pressure and temperature. We used inelastic x-ray scattering to probe the phonon dispersion of single-crystalline vanadium as a function of pressure to 45 GPa. Our measurements show an anomalous high-pressure behavior of the transverse acoustic mode along the (100) direction and a softening of the elastic modulus C44 that triggers a rhombohedral lattice distortion occurring between 34 and 39 GPa. Our results provide the missing experimental confirmation of the theoretically predicted shear instability arising from the progressive intra-band nesting of the Fermi surface with increasing pressure, a scenario common to all transition metals of group V. PMID:27539662
Phonon triggered rhombohedral lattice distortion in vanadium at high pressure
NASA Astrophysics Data System (ADS)
Antonangeli, Daniele; Farber, Daniel L.; Bosak, Alexei; Aracne, Chantel M.; Ruddle, David G.; Krisch, Michael
2016-08-01
In spite of the simple body-centered-cubic crystal structure, the elements of group V, vanadium, niobium and tantalum, show strong interactions between the electronic properties and lattice dynamics. Further, these interactions can be tuned by external parameters, such as pressure and temperature. We used inelastic x-ray scattering to probe the phonon dispersion of single-crystalline vanadium as a function of pressure to 45 GPa. Our measurements show an anomalous high-pressure behavior of the transverse acoustic mode along the (100) direction and a softening of the elastic modulus C44 that triggers a rhombohedral lattice distortion occurring between 34 and 39 GPa. Our results provide the missing experimental confirmation of the theoretically predicted shear instability arising from the progressive intra-band nesting of the Fermi surface with increasing pressure, a scenario common to all transition metals of group V.
Phonon triggered rhombohedral lattice distortion in vanadium at high pressure
Antonangeli, Daniele; Farber, Daniel L.; Bosak, Alexei; Aracne, Chantel M.; Ruddle, David G.; Krisch, Michael
2016-01-01
In spite of the simple body-centered-cubic crystal structure, the elements of group V, vanadium, niobium and tantalum, show strong interactions between the electronic properties and lattice dynamics. Further, these interactions can be tuned by external parameters, such as pressure and temperature. We used inelastic x-ray scattering to probe the phonon dispersion of single-crystalline vanadium as a function of pressure to 45 GPa. Our measurements show an anomalous high-pressure behavior of the transverse acoustic mode along the (100) direction and a softening of the elastic modulus C44 that triggers a rhombohedral lattice distortion occurring between 34 and 39 GPa. Our results provide the missing experimental confirmation of the theoretically predicted shear instability arising from the progressive intra-band nesting of the Fermi surface with increasing pressure, a scenario common to all transition metals of group V. PMID:27539662
Norris, Andrew N
2009-02-01
Acoustic metafluids are defined as the class of fluids that allow one domain of fluid to acoustically mimic another, as exemplified by acoustic cloaks. It is shown that the most general class of acoustic metafluids are materials with anisotropic inertia and the elastic properties of what are known as pentamode materials. The derivation uses the notion of finite deformation to define the transformation of one region to another. The main result is found by considering energy density in the original and transformed regions. Properties of acoustic metafluids are discussed, and general conditions are found which ensure that the mapped fluid has isotropic inertia, which potentially opens up the possibility of achieving broadband cloaking. PMID:19206861
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 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
NASA Technical Reports Server (NTRS)
Defacio, B.; Vannevel, Alan; Brander, O.
1993-01-01
A formulation is given for a collection of phonons (sound) in a fluid at a non-zero temperature which uses the simple harmonic oscillator twice; one to give a stochastic thermal 'noise' process and the other which generates a coherent Glauber state of phonons. Simple thermodynamic observables are calculated and the acoustic two point function, 'contrast' is presented. The role of 'coherence' in an equilibrium system is clarified by these results and the simple harmonic oscillator is a key structure in both the formulation and the calculations.
Abernathy, Douglas L.; Ma, Jie; Yan, Jiaqiang; Delaire, Olivier A.; Chen, Xi; Weathers, Annie; Mukhopadhyay, Saikat; Shi, Li
2015-04-15
A variety of crystals contain quasi-one-dimensional substructures, which yield distinctive electronic, spintronic, optical and thermoelectric properties. There is a lack of understanding of the lattice dynamics that influences the properties of such complex crystals. Here we employ inelastic neutron scatting measurements and density functional theory calculations to show that numerous low-energy optical vibrational modes exist in higher manganese silicides, an example of such crystals. These optical modes, including unusually low-frequency twisting motions of the Si ladders inside the Mn chimneys, provide a large phase space for scattering acoustic phonons. A hybrid phonon and diffuson model is proposed to explain the low and anisotropic thermal conductivity of higher manganese silicides and to evaluate nanostructuring as an approach to further suppress the thermal conductivity and enhance the thermoelectric energy conversion efficiency. This discovery offers new insights into the structure-property relationships of a broad class of materials with quasi-one-dimensional substructures for various applications.
Acoustically trapped colloidal crystals that are reconfigurable in real time
Caleap, Mihai; Drinkwater, Bruce W.
2014-01-01
Photonic and phononic crystals are metamaterials with repeating unit cells that result in internal resonances leading to a range of wave guiding and filtering properties and are opening up new applications such as hyperlenses and superabsorbers. Here we show the first, to our knowledge, 3D colloidal phononic crystal that is reconfigurable in real time and demonstrate its ability to rapidly alter its frequency filtering characteristics. Our reconfigurable material is assembled from microspheres in aqueous solution, trapped with acoustic radiation forces. The acoustic radiation force is governed by an energy landscape, determined by an applied high-amplitude acoustic standing wave field, in which particles move swiftly to energy minima. This creates a colloidal crystal of several milliliters in volume with spheres arranged in an orthorhombic lattice in which the acoustic wavelength is used to control the lattice spacing. Transmission acoustic spectroscopy shows that the new colloidal crystal behaves as a phononic metamaterial and exhibits clear band-pass and band-stop frequencies which are adjusted in real time. PMID:24706925
Tuning the acoustic frequency of a gold nanodisk through its adhesion layer
NASA Astrophysics Data System (ADS)
Chang, Wei-Shun; Wen, Fangfang; Chakraborty, Debadi; Su, Man-Nung; Zhang, Yue; Shuang, Bo; Nordlander, Peter; Sader, John E.; Halas, Naomi J.; Link, Stephan
2015-05-01
To fabricate robust metallic nanostructures with top-down patterning methods such as electron-beam lithography, an initial nanometer-scale layer of a second metal is deposited to promote adhesion of the metal of interest. However, how this nanoscale layer affects the mechanical properties of the nanostructure and how adhesion layer thickness controls the binding strength to the substrate are still open questions. Here we use ultrafast laser pulses to impulsively launch acoustic phonons in single gold nanodisks with variable titanium layer thicknesses, and observe an increase in phonon frequencies as a thicker adhesion layer facilitates stronger binding to the glass substrate. In addition to an all-optical interrogation of nanoscale mechanical properties, our results show that the adhesion layer can be used to controllably modify the acoustic phonon modes of a gold nanodisk. This direct coupling between optically excited plasmon modes and phonon modes can be exploited for a variety of emerging optomechanical applications.
Acoustic modes in fluid networks
NASA Technical Reports Server (NTRS)
Michalopoulos, C. D.; Clark, Robert W., Jr.; Doiron, Harold H.
1992-01-01
Pressure and flow rate eigenvalue problems for one-dimensional flow of a fluid in a network of pipes are derived from the familiar transmission line equations. These equations are linearized by assuming small velocity and pressure oscillations about mean flow conditions. It is shown that the flow rate eigenvalues are the same as the pressure eigenvalues and the relationship between line pressure modes and flow rate modes is established. A volume at the end of each branch is employed which allows any combination of boundary conditions, from open to closed, to be used. The Jacobi iterative method is used to compute undamped natural frequencies and associated pressure/flow modes. Several numerical examples are presented which include acoustic modes for the Helium Supply System of the Space Shuttle Orbiter Main Propulsion System. It should be noted that the method presented herein can be applied to any one-dimensional acoustic system involving an arbitrary number of branches.
Resonant acousto-optics in the terahertz range: TO-phonon polaritons driven by an ultrasonic wave
NASA Astrophysics Data System (ADS)
Muljarov, E. A.; Poolman, R. H.; Ivanov, A. L.
2011-03-01
The resonant acousto-optic effect is studied both analytically and numerically in the terahertz range where the transverse-optical (TO) phonons play the role of a mediator which strongly couples the ultrasound and light fields. A propagating acoustic wave interacts with the TO phonons via anharmonic channels and opens band gaps in the TO-phonon polariton energy dispersion that results in pronounced Bragg scattering and reflection of the incoming light. The separation in frequency of different Bragg replicas, which is at the heart of acousto-optics, allows us to study the resonant acousto-optic effect in the most simple and efficient geometry of collinear propagation of electromagnetic and ultrasonic waves. The acoustically induced energy gaps, Bragg reflection spectra, and the spatial distribution of the electric field and polarization are calculated for CuCl parameters, in a wide range of frequencies and intensities of the pumping acoustic wave. Our results show drastic changes in terahertz spectra of semiconductor crystals that open the way for efficient and accessible manipulation of their infrared properties by tuning the parameters of the acoustic wave.
Phonon-drag thermopower in a monolayer MoS2.
Bhargavi, K S; Kubakaddi, S S
2014-12-01
The theory of phonon-drag thermopower S(g) is developed in a monolayer MoS(2), considering the electron–acoustic phonon interaction via deformation potential (DP) and piezoelectric (PE) coupling, as a function of temperature T and electron concentration n(s). DP coupling of TA (LA) phonons is taken to be unscreened (screened) and PE coupling of LA and TA phonons is taken to be screened. S(g) due to DP coupling of TA phonons is found to be dominant over all other mechanisms and in the Bloch–Grüneisen regime it gives power law S(g) ~ T3. All other mechanisms give S(g) ~ T(5). These power laws are characteristic of two-dimensional (2D) phonons with linear dispersion. Screening enhances the exponent of T by 2 and strongly suppresses S(g) due to the large effective mass of the electrons. We find that S(g), due to screened DP and PE couplings is nearly the same in contrast to the results in GaAs heterojunctions. Also, we predict that S(g) ~ n(s)(-3/2), a characteristic of 2D electrons with parabolic relation. With the increasing (decreasing) T(n(s)) its exponent decreases. For comparison, we give diffusion thermopower S(d). At very low T and high n(s), S(d) ~ T and n(2)(-1). S(d) is found to be greater than S(g) for about T < 2–3 K. The results are compared with those in conventional 2D electron gas and graphene.
Phonon-drag thermopower in a monolayer MoS2.
Bhargavi, K S; Kubakaddi, S S
2014-12-01
The theory of phonon-drag thermopower S(g) is developed in a monolayer MoS(2), considering the electron–acoustic phonon interaction via deformation potential (DP) and piezoelectric (PE) coupling, as a function of temperature T and electron concentration n(s). DP coupling of TA (LA) phonons is taken to be unscreened (screened) and PE coupling of LA and TA phonons is taken to be screened. S(g) due to DP coupling of TA phonons is found to be dominant over all other mechanisms and in the Bloch–Grüneisen regime it gives power law S(g) ~ T3. All other mechanisms give S(g) ~ T(5). These power laws are characteristic of two-dimensional (2D) phonons with linear dispersion. Screening enhances the exponent of T by 2 and strongly suppresses S(g) due to the large effective mass of the electrons. We find that S(g), due to screened DP and PE couplings is nearly the same in contrast to the results in GaAs heterojunctions. Also, we predict that S(g) ~ n(s)(-3/2), a characteristic of 2D electrons with parabolic relation. With the increasing (decreasing) T(n(s)) its exponent decreases. For comparison, we give diffusion thermopower S(d). At very low T and high n(s), S(d) ~ T and n(2)(-1). S(d) is found to be greater than S(g) for about T < 2–3 K. The results are compared with those in conventional 2D electron gas and graphene. PMID:25388090
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. PMID:27580163
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.
Mode conversion based on the acousto-optical interaction in photonic-phononic waveguide
NASA Astrophysics Data System (ADS)
Chen, Guodong; Zhang, Ruiwen; Xiong, Huang; Xie, Heng; Gao, Ya; Feng, Danqi; Sun, Junqiang
2015-02-01
We present a scheme for on-chip optical mode conversion in a hybrid photonic-phononic waveguide. Both propagating optical and acoustic wave can be tightly confined in the hybrid waveguide, and the acoustooptical interaction can be enhanced to realize optical mode conversion within a chip-scale size. The theoretical model of the acousto-optic interaction is established to explain the mode conversion. The numerical simulation results indicate that the high efficient mode conversion can be achieved by adjusting the intensity of the acoustic wave. We also show that the mode conversion bandwidth can be dramatically broadened to 13 THz by adjusting the frequency of the acoustic wave to match phase condition of the acousto-optic interaction. This mode converter on-chip is promising in order to increase the capacity of silicon data busses for on-chip optical interconnections.
Kakodkar, Rohit R.; Feser, Joseph P.
2015-09-07
We present a numerical approach to the solution of elastic phonon-interface and phonon-nanostructure scattering problems based on a frequency-domain decomposition of the atomistic equations of motion and the use of perfectly matched layer (PML) boundaries. Unlike molecular dynamic wavepacket analysis, the current approach provides the ability to simulate scattering from individual phonon modes, including wavevectors in highly dispersive regimes. Like the atomistic Green's function method, the technique reduces scattering problems to a system of linear algebraic equations via a sparse, tightly banded matrix regardless of dimensionality. However, the use of PML boundaries enables rapid absorption of scattered wave energies at the boundaries and provides a simple and inexpensive interpretation of the scattered phonon energy flux calculated from the energy dissipation rate in the PML. The accuracy of the method is demonstrated on connected monoatomic chains, for which an analytic solution is known. The parameters defining the PML are found to affect the performance and guidelines for selecting optimal parameters are given. The method is used to study the energy transmission coefficient for connected diatomic chains over all available wavevectors for both optical and longitudinal phonons; it is found that when there is discontinuity between sublattices, even connected chains of equivalent acoustic impedance have near-zero transmission coefficient for short wavelengths. The phonon scattering cross section of an embedded nanocylinder is calculated in 2D for a wide range of frequencies to demonstrate the extension of the method to high dimensions. The calculations match continuum theory for long-wavelength phonons and large cylinder radii, but otherwise show complex physics associated with discreteness of the lattice. Examples include Mie oscillations which terminate when incident phonon frequencies exceed the maximum available frequency in the embedded nanocylinder, and
Hawking radiation from an acoustic black hole on an ion ring.
Horstmann, B; Reznik, B; Fagnocchi, S; Cirac, J I
2010-06-25
In this Letter we propose to simulate acoustic black holes with ions in rings. If the ions are rotating with a stationary and inhomogeneous velocity profile, regions can appear where the ion velocity exceeds the group velocity of the phonons. In these regions phonons are trapped like light in black holes, even though we have a discrete field theory and a nonlinear dispersion relation. We study the appearance of Hawking radiation in this setup and propose a scheme to detect it. PMID:20867352
Entanglement entropy of acoustic black hole in Bose-Einstein condensate
NASA Astrophysics Data System (ADS)
Zhang, Lichun; Li, Huaifan; Zhao, Ren; Cai, Ronggen
2013-04-01
We study the entanglement entropy associated to the phonons generated via the Hawking mechanism of acoustic black holes in a Bose-Einsten condensate. The lowest energy allowed for the radiated phonons is found to be a function of space coordinate. Based this, we calculate the entanglement entropy, which contains three parts: a leading term, which is a constant of value 1/6, a logarithmic correction term and some series terms. We discuss the convergence of the series terms.
Heavy-impurity resonance, hybridization, and phonon spectral functions in Fe_{1-x}M_{x}Si, M=Ir,Os
Delaire, O.; Al-Qasir, Iyad I.; May, Andrew F.; Sales, Brian C.; Niedziela, Jennifer L.; Ma, Jie; Matsuda, Masaaki; Abernathy, Douglas L.; Berlijn, Tom
2015-03-31
The vibrational behavior of heavy substitutional impurities (M=Ir,Os) in Fe_{1-x}M_{x}Si (x = 0, 0.02, 0.04, 0.1) was investigated with a combination of inelastic neutron scattering (INS), transport measurements, and first-principles simulations. In this paper, our INS measurements on single-crystals mapped the four-dimensional dynamical structure factor, S(Q;E), for several compositions and temperatures. Our results show that both Ir and Os impurities lead to the formation of a weakly dispersive resonance vibrational mode, in the energy range of the acoustic phonon dispersions of the FeSi host. We also show that Ir doping, which introduces free carriers and increases electron-phonon coupling, leads to softened interatomic force-constants compared to doping with Os, which is isoelectronic to Fe. We analyze the phonon S(Q,E) from INS through a Green's function model incorporating the phonon self-energy based on first-principles density functional theory (DFT) simulations. Calculations of the quasiparticle spectral functions in the doped system reveal the hybridization between the resonance and the acoustic phonon modes. Finally, our results demonstrate a strong interaction of the host acoustic dispersions with the resonance mode, likely leading to the large observed suppression in lattice thermal conductivity.
One-dimensional hypersonic phononic crystals.
Gomopoulos, N; Maschke, D; Koh, C Y; Thomas, E L; Tremel, W; Butt, H-J; Fytas, G
2010-03-10
We report experimental observation of a normal incidence phononic band gap in one-dimensional periodic (SiO(2)/poly(methyl methacrylate)) multilayer film at gigahertz frequencies using Brillouin spectroscopy. The band gap to midgap ratio of 0.30 occurs for elastic wave propagation along the periodicity direction, whereas for inplane propagation the system displays an effective medium behavior. The phononic properties are well captured by numerical simulations. The porosity in the silica layers presents a structural scaffold for the introduction of secondary active media for potential coupling between phonons and other excitations, such as photons and electrons.
Phonons of the cis-polyacetylene chain
NASA Astrophysics Data System (ADS)
Faulques, Eric; Buisson, Jean-Pierre; Lefrant, Serge
1995-12-01
An investigation of the in-plane phonons of the cis-polyacetylene chain (CH)x and isotopic analogs (CD)x and (13CH)x is presented on the basis of a Fourier's dynamical D-matrix formalism. The conjugation is found to be similar to that of the trans-polyacetylene chain. Phonon dispersions have been calculated and follow the shapes predicted by Božović. Finally, the most interesting result is that phonon density of states exhibits van Hove singularities whose energies are close to those determined experimentally with incoherent inelastic neutron scattering.
Acoustic trauma is a common cause of sensory hearing loss . Damage to the hearing mechanisms within the inner ... Symptoms include: Partial hearing loss that most often involves ... The hearing loss may slowly get worse. Noises, ringing in ...
... slow growing tumor which arise primarily from the vestibular portion of the VIII cranial nerve and lie ... you have a "brain tumor" called acoustic neuroma (vestibular schwannoma). You think you are the only one ...
ERIC Educational Resources Information Center
Creasey, D. J.
1981-01-01
Summarizes the history of underwater acoustics and describes related research studies and teaching activities at the University of Birmingham (England). Also includes research studies on transducer design and mathematical techniques. (SK)
NASA Astrophysics Data System (ADS)
Kuttruff, Heinrich; Mommertz, Eckard
The traditional task of room acoustics is to create or formulate conditions which ensure the best possible propagation of sound in a room from a sound source to a listener. Thus, objects of room acoustics are in particular assembly halls of all kinds, such as auditoria and lecture halls, conference rooms, theaters, concert halls or churches. Already at this point, it has to be pointed out that these conditions essentially depend on the question if speech or music should be transmitted; in the first case, the criterion for transmission quality is good speech intelligibility, in the other case, however, the success of room-acoustical efforts depends on other factors that cannot be quantified that easily, not least it also depends on the hearing habits of the listeners. In any case, absolutely "good acoustics" of a room do not exist.
Thermal phonon transport in silicon nanowires and two-dimensional phononic crystal nanostructures
NASA Astrophysics Data System (ADS)
Nomura, Masahiro; Nakagawa, Junki; Kage, Yuta; Maire, Jeremie; Moser, Dominik; Paul, Oliver
2015-04-01
Thermal phonon transport in silicon nanowires (Si NWs) and two-dimensional phononic crystal (2D PnC) nanostructures was investigated by measuring thermal conductivity using a micrometer-scale time-domain thermoreflectance. The impact of nanopatterning on thermal conductivity strongly depends on the geometry, specularity parameter, and thermal phonon mean free path (MFP) distribution. Thermal conductivities for 2D PnC nanostructures were found to be much lower than that for NWs with similar characteristic length and surface-to-volume ratio due to stronger phonon back scattering. In single-crystalline Si, PnC patterning has a stronger impact at 4 K than at room temperature due to a higher specularity parameter and a longer thermal phonon MFP. Nanowire patterning has a stronger impact in polycrystalline Si, where thermal phonon MFP distribution is biased longer by grain boundary scattering.
NASA Astrophysics Data System (ADS)
Jena, D. P.; Panigrahi, S. N.
2016-03-01
Requirement of designing a sophisticated digital band-pass filter in acoustic based condition monitoring has been eliminated by introducing a passive acoustic filter in the present work. So far, no one has attempted to explore the possibility of implementing passive acoustic filters in acoustic based condition monitoring as a pre-conditioner. In order to enhance the acoustic based condition monitoring, a passive acoustic band-pass filter has been designed and deployed. Towards achieving an efficient band-pass acoustic filter, a generalized design methodology has been proposed to design and optimize the desired acoustic filter using multiple filter components in series. An appropriate objective function has been identified for genetic algorithm (GA) based optimization technique with multiple design constraints. In addition, the sturdiness of the proposed method has been demonstrated in designing a band-pass filter by using an n-branch Quincke tube, a high pass filter and multiple Helmholtz resonators. The performance of the designed acoustic band-pass filter has been shown by investigating the piston-bore defect of a motor-bike using engine noise signature. On the introducing a passive acoustic filter in acoustic based condition monitoring reveals the enhancement in machine learning based fault identification practice significantly. This is also a first attempt of its own kind.
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.
Size and temperature dependence of the electron–phonon scattering by donors in nanowire transistors
NASA Astrophysics Data System (ADS)
Bescond, M.; Carrillo-Nuñez, H.; Berrada, S.; Cavassilas, N.; Lannoo, M.
2016-08-01
Due to the constant size reduction, single-donor-based nanowire transistors receive an increasing interest from the semi-conductor industry. In this work we theoretically investigate the coupled influence of electron-phonon scattering, temperature and size (cross-section and channel length) on the properties of such systems. The aim is to determine under what conditions the localized character of the donor has a remarkable impact on the current characteristics. We use a quantum non-equilibrium Green's function approach in which the acoustic electron-phonon scattering is treated through local self-energies. We first show how this widely used approach, valid at high temperatures, can be extended to lower temperatures. Our simulations predict a hysteresis in the current when reducing the temperature down to 150 K. We also find that acoustic phonons degrade the current characteristics while their optical counterparts might have a beneficial impact with an increase of the ON-current. Finally we discuss the influence of nanowire length and cross-section and emphasize the complexity of precisely controlling the dopant level at room temperature.
Size and temperature dependence of the electron-phonon scattering by donors in nanowire transistors
NASA Astrophysics Data System (ADS)
Bescond, M.; Carrillo-Nuñez, H.; Berrada, S.; Cavassilas, N.; Lannoo, M.
2016-08-01
Due to the constant size reduction, single-donor-based nanowire transistors receive an increasing interest from the semi-conductor industry. In this work we theoretically investigate the coupled influence of electron-phonon scattering, temperature and size (cross-section and channel length) on the properties of such systems. The aim is to determine under what conditions the localized character of the donor has a remarkable impact on the current characteristics. We use a quantum non-equilibrium Green's function approach in which the acoustic electron-phonon scattering is treated through local self-energies. We first show how this widely used approach, valid at high temperatures, can be extended to lower temperatures. Our simulations predict a hysteresis in the current when reducing the temperature down to 150 K. We also find that acoustic phonons degrade the current characteristics while their optical counterparts might have a beneficial impact with an increase of the ON-current. Finally we discuss the influence of nanowire length and cross-section and emphasize the complexity of precisely controlling the dopant level at room temperature.
NASA Astrophysics Data System (ADS)
Gurau, Razvan; Ryan, James P.
2014-11-01
Melonic graphs constitute the family of graphs arising at leading order in the 1/N expansion of tensor models. They were shown to lead to a continuum phase, reminiscent of branched polymers. We show here that they are in fact precisely branched polymers, that is, they possess Hausdorff dimension 2 and spectral dimension 4/3.
Phononic thermal resistance due to a finite periodic array of nano-scatterers
NASA Astrophysics Data System (ADS)
Trang Nghiêm, T. T.; Chapuis, Pierre-Olivier
2016-07-01
The wave property of phonons is employed to explore the thermal transport across a finite periodic array of nano-scatterers such as circular and triangular holes. As thermal phonons are generated in all directions, we study their transmission through a single array for both normal and oblique incidences, using a linear dispersionless time-dependent acoustic frame in a two-dimensional system. Roughness effects can be directly considered within the computations without relying on approximate analytical formulae. Analysis by spatio-temporal Fourier transform allows us to observe the diffraction effects and the conversion of polarization. Frequency-dependent energy transmission coefficients are computed for symmetric and asymmetric objects that are both subject to reciprocity. We demonstrate that the phononic array acts as an efficient thermal barrier by applying the theory of thermal boundary (Kapitza) resistances to arrays of smooth scattering holes in silicon for an exemplifying periodicity of 10 nm in the 5-100 K temperature range. It is observed that the associated thermal conductance has the same temperature dependence as that without phononic filtering.
Control of coherent information via on-chip photonic-phononic emitter-receivers.
Shin, Heedeuk; Cox, Jonathan A; Jarecki, Robert; Starbuck, Andrew; Wang, Zheng; Rakich, Peter T
2015-01-01
Rapid progress in integrated photonics has fostered numerous chip-scale sensing, computing and signal processing technologies. However, many crucial filtering and signal delay operations are difficult to perform with all-optical devices. Unlike photons propagating at luminal speeds, GHz-acoustic phonons moving at slower velocities allow information to be stored, filtered and delayed over comparatively smaller length-scales with remarkable fidelity. Hence, controllable and efficient coupling between coherent photons and phonons enables new signal processing technologies that greatly enhance the performance and potential impact of integrated photonics. Here we demonstrate a mechanism for coherent information processing based on travelling-wave photon-phonon transduction, which achieves a phonon emit-and-receive process between distinct nanophotonic waveguides. Using this device, physics--which supports GHz frequencies--we create wavelength-insensitive radiofrequency photonic filters with frequency selectivity, narrow-linewidth and high power-handling in silicon. More generally, this emit-receive concept is the impetus for enabling new signal processing schemes. PMID:25740405
Control of coherent information via on-chip photonic–phononic emitter–receivers
Shin, Heedeuk; Cox, Jonathan A.; Jarecki, Robert; Starbuck, Andrew; Wang, Zheng; Rakich, Peter T.
2015-01-01
Rapid progress in integrated photonics has fostered numerous chip-scale sensing, computing and signal processing technologies. However, many crucial filtering and signal delay operations are difficult to perform with all-optical devices. Unlike photons propagating at luminal speeds, GHz-acoustic phonons moving at slower velocities allow information to be stored, filtered and delayed over comparatively smaller length-scales with remarkable fidelity. Hence, controllable and efficient coupling between coherent photons and phonons enables new signal processing technologies that greatly enhance the performance and potential impact of integrated photonics. Here we demonstrate a mechanism for coherent information processing based on travelling-wave photon–phonon transduction, which achieves a phonon emit-and-receive process between distinct nanophotonic waveguides. Using this device, physics—which supports GHz frequencies—we create wavelength-insensitive radiofrequency photonic filters with frequency selectivity, narrow-linewidth and high power-handling in silicon. More generally, this emit-receive concept is the impetus for enabling new signal processing schemes. PMID:25740405
Effect of magnetized phonons on electrical and thermal conductivity of neutron star crust
NASA Astrophysics Data System (ADS)
Baiko, D. A.
2016-05-01
We study electrical and thermal conductivities of degenerate electrons emitting and absorbing phonons in a strongly magnetized crystalline neutron star crust. We take into account modification of the phonon spectrum of a Coulomb solid of ions caused by a strong magnetic field. Boltzmann transport equation is solved using a generalized variational method. The ensuing 3D integrals over the transferred momenta are evaluated by two different numerical techniques, the Monte Carlo method and a regular integration over the first Brillouin zone. The results of the two numerical approaches are shown to be in a good agreement. An appreciable growth of electrical and thermal resistivities is reported at quantum and intermediate temperatures T ≲ 0.1Tp (Tp is the ion plasma temperature) in a wide range of chemical compositions and mass densities of matter even for moderately magnetized crystals ωB ˜ ωp (ωB and ωp are the ion cyclotron and plasma frequencies). This effect is due to an appearance of a soft (ω ∝ k2) phonon mode in the magnetized ion Coulomb crystal, which turns out to be easier to excite than acoustic phonons characteristic of the field-free case. These results are important for modelling magneto-thermal evolution of neutron stars.
Control of coherent information via on-chip photonic–phononic emitter–receivers
Shin, Heedeuk; Cox, Jonathan A.; Jarecki, Robert; Starbuck, Andrew; Wang, Zheng; Rakich, Peter T.
2015-03-05
We report that rapid progress in integrated photonics has fostered numerous chip-scale sensing, computing and signal processing technologies. However, many crucial filtering and signal delay operations are difficult to perform with all-optical devices. Unlike photons propagating at luminal speeds, GHz-acoustic phonons moving at slower velocities allow information to be stored, filtered and delayed over comparatively smaller length-scales with remarkable fidelity. Hence, controllable and efficient coupling between coherent photons and phonons enables new signal processing technologies that greatly enhance the performance and potential impact of integrated photonics. Here we demonstrate a mechanism for coherent information processing based on travelling-wave photon–phonon transduction, which achieves a phonon emit-and-receive process between distinct nanophotonic waveguides. Using this device, physics—which supports GHz frequencies—we create wavelength-insensitive radiofrequency photonic filters with frequency selectivity, narrow-linewidth and high power-handling in silicon. More generally, this emit-receive concept is the impetus for enabling new signal processing schemes.
Control of coherent information via on-chip photonic–phononic emitter–receivers
Shin, Heedeuk; Cox, Jonathan A.; Jarecki, Robert; Starbuck, Andrew; Wang, Zheng; Rakich, Peter T.
2015-03-05
We report that rapid progress in integrated photonics has fostered numerous chip-scale sensing, computing and signal processing technologies. However, many crucial filtering and signal delay operations are difficult to perform with all-optical devices. Unlike photons propagating at luminal speeds, GHz-acoustic phonons moving at slower velocities allow information to be stored, filtered and delayed over comparatively smaller length-scales with remarkable fidelity. Hence, controllable and efficient coupling between coherent photons and phonons enables new signal processing technologies that greatly enhance the performance and potential impact of integrated photonics. Here we demonstrate a mechanism for coherent information processing based on travelling-wave photon–phonon transduction,more » which achieves a phonon emit-and-receive process between distinct nanophotonic waveguides. Using this device, physics—which supports GHz frequencies—we create wavelength-insensitive radiofrequency photonic filters with frequency selectivity, narrow-linewidth and high power-handling in silicon. More generally, this emit-receive concept is the impetus for enabling new signal processing schemes.« less
Zhao, Feng-Qi; Guo, Zi-Zheng; Zhu, Jun
2014-07-07
An improved Lee-Low-Pines intermediate coupling method is used to study the energies and binding energies of bound polarons in a wurtzite ZnO/Mg{sub x}Zn{sub 1–x}O quantum well. The contributions from different branches of long-wave optical phonons, i.e., confined optical phonons, interface optical phonons, and half-space optical phonons are considered. In addition to electron-phonon interaction, the impurity-phonon interaction, and the anisotropy of material parameters, such as phonon frequency, electron effective mass, and dielectric constant, are also included in our computation. Ground-state energies, binding energies and detailed phonon contributions from various phonons as functions of well width, impurity position and composition are presented. Our result suggests that total phonon contribution to ground state and binding energies in the studied wurtzite ZnO/Mg₀.₃Zn₀.₇O quantum wells varies between 28–23 meV and 62–45 meV, respectively, which are much larger than the corresponding values (about 3.2–1.8 meV and 1.6–0.3 meV) in GaAs/Al₀.₃Ga₀.₇As quantum wells. For a narrower quantum well, the phonon contribution mainly comes from interface and half-space phonons, for a wider quantum well, most of phonon contribution originates from confined phonons. The contribution from all the phonon modes to binding energies increases slowly either when impurity moves far away from the well center in the z direction or with the increase in magnesium composition (x). It is found that different phonons have different influences on the binding energies of bound polarons. Furthermore, the phonon contributions to binding energies as functions of well width, impurity position, and composition are very different from one another. In general, the electron-optical phonon interaction and the impurity center-optical phonon interaction play an important role in electronic states of ZnO-based quantum wells and cannot be neglected.
Predicting the phonon spectra of coupled nonlinear chains using effective phonon theory
NASA Astrophysics Data System (ADS)
Su, Ruixia; Yuan, Zongqiang; Wang, Jun; Zheng, Zhigang
2016-06-01
In general one-dimensional nonlinear lattices, extensive studies have discovered the existence of renormalized phonons due to nonlinear interactions and found these renormalized phonons, as the energy carriers, are responsible for heat transport. Within the framework of renormalized phonons, a generic form of renormalized phonon spectrum has been derived and effective phonon theory (EPT) has been developed to explain the heat transport in general 1D nonlinear lattices. Our attention is dedicated to generalizing the EPT for two-layer nonlinear lattices and deriving the analytic expression of phonon spectra. By calculating the phonon spectra of different coupled models with EPT, it is found that the phonon dispersion relation is in good agreement with the result obtained from the spectral energy density method. It is demonstrated that the EPT of a coupled system can predict the phonon spectra of two-layer nonlinear lattices well. Thus, this finding may shed light on the prediction of heat conduction behavior in a coupled system, qualitatively, and provide a useful guide for designing thermal devices.
Holographic interpretation of acoustic black holes
NASA Astrophysics Data System (ADS)
Ge, Xian-Hui; Sun, Jia-Rui; Tian, Yu; Wu, Xiao-Ning; Zhang, Yun-Long
2015-10-01
With the attempt to find the holographic description of the usual acoustic black holes in fluid, we construct an acoustic black hole formed in the d -dimensional fluid located at the timelike cutoff surface of a neutral black brane in asymptotically AdSd +1 spacetime; the bulk gravitational dual of the acoustic black hole is presented at the first order of the hydrodynamic fluctuation. Moreover, the Hawking-like temperature of the acoustic black hole horizon is showed to be connected to the Hawking temperature of the real anti-de Sitter (AdS) black brane in the bulk, and the duality between the phonon scattering in the acoustic black hole and the sound channel quasinormal mode propagating in the bulk perturbed AdS black brane is extracted. We thus point out that the acoustic black hole appearing in fluid, which was originally proposed as an analogous model to simulate Hawking radiation of the real black hole, is not merely an analogy, it can indeed be used to describe specific properties of the real AdS black holes, in the spirit of the fluid/gravity duality.
NASA Astrophysics Data System (ADS)
Uzunok, H. Y.; Tütüncü, H. M.; Özer, S.; Ugˇur, Ş.; Srivastava, G. P.
2015-03-01
We report on an ab initio study of the BCS-type superconductivity in the intermetallic borocarbides YPd2B2C, YPt2B2C and LaPt2B2C with a layer-like structure. The largest contribution to the electron-phonon coupling constant λ is identified to come from transverse acoustic phonons at a zone-edge, arising from the atomic vibrations in the boron-transition metal layer. A detailed examination of the atomic geometry in the boron-transition metal layer, the electron-phonon coupling constant λ, and the logarithmically averaged phonon frequency ωln helps explain the relatively higher superconducting temperature Tc of YPd2B2C (20.6 K) compared to that of YPt2B2C (11.3 K) and LaPt2B2C (10.40 K).
Parshall, D.; Pintschovius, L.; Niedziela, Jennifer L.; Castellan, J. -P.; Lamago, D.; Mittal, R.; Wolf, Th.; Reznik, Dmitry
2015-04-27
Pmore » arent compounds of Fe-based superconductors undergo a structural phase transition from a tetragonal to an orthorhombic structure. We investigated the temperature dependence of the frequencies of TA phonons that extrapolate to the shear vibrational mode at the zone center, which corresponds to the orthorhombic deformation of the crystal structure at low temperatures in BaFe2As2 and SrFe2As2. We found that acoustic phonons at small wave vectors soften gradually towards the transition from high temperatures, tracking the increase of the size of slowly fluctuating magnetic domains. On cooling below the transition to base temperature the phonons harden, following the square of the magnetic moment (which we find is proportional to the anisotropy gap). Finally, our results provide evidence for close correlation between magnetic and phonon properties in Fe-based superconductors.« less
Phonon coherence in isotopic silicon superlattices
Frieling, R.; Radek, M.; Eon, S.; Bracht, H.; Wolf, D. E.
2014-09-29
Recent experimental and theoretical investigations have confirmed that a reduction in thermal conductivity of silicon is achieved by isotopic silicon superlattices. In the present study, non-equilibrium molecular dynamics simulations are performed to identify the isotope doping and isotope layer ordering with minimum thermal conductivity. Furthermore, the impact of isotopic intermixing at the superlattice interfaces on phonon transport is investigated. Our results reveal that the coherence of phonons in isotopic Si superlattices is prevented if interfacial mixing of isotopes is considered.
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.
A new class of tunable hypersonic phononic crystals based on polymer-tethered colloids
NASA Astrophysics Data System (ADS)
Alonso-Redondo, E.; Schmitt, M.; Urbach, Z.; Hui, C. M.; Sainidou, R.; Rembert, P.; Matyjaszewski, K.; Bockstaller, M. R.; Fytas, G.
2015-09-01
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.
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
2015-09-22
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.
PHYSICS. Observation of phononic helical edge states in a mechanical topological insulator.
Süsstrunk, Roman; Huber, Sebastian D
2015-07-01
A topological insulator, as originally proposed for electrons governed by quantum mechanics, is characterized by a dichotomy between the interior and the edge of a finite system: The bulk has an energy gap, and the edges sustain excitations traversing this gap. However, it has remained an open question whether the same physics can be observed for systems obeying Newton's equations of motion. We conducted experiments to characterize the collective behavior of mechanical oscillators exhibiting the phenomenology of the quantum spin Hall effect. The phononic edge modes are shown to be helical, and we demonstrate their topological protection via the stability of the edge states against imperfections. Our results may enable the design of topological acoustic metamaterials that can capitalize on the stability of the surface phonons as reliable wave guides.
Phonon spectroscopy with sub-meV resolution by femtosecond x-ray diffuse scattering
Zhu, Diling; Robert, Aymeric; Henighan, Tom; Lemke, Henrik T.; Chollet, Matthieu; Glownia, J. Mike; Reis, David A.; Trigo, Mariano
2015-08-10
We present a reconstruction of the transverse acoustic phonon dispersion of germanium from femtosecond time-resolved x-ray diffuse scattering measurements at the Linac Coherent Light Source. We demonstrate an energy resolution of 0.3 meV with a momentum resolution of 0.01 nm-1 using 10-keV x rays with a bandwidth of ~ 1 eV. This high resolution was achieved simultaneously for a large section of reciprocal space including regions closely following three of the principal symmetry directions. The phonon dispersion was reconstructed with less than 3 h of measurement time, during which neither the x-ray energy, the sample orientation, nor the detector positionmore » were scanned. In conclusion, these results demonstrate how time-domain measurements can complement conventional frequency domain inelastic-scattering techniques.« less
Phonon spectroscopy with sub-meV resolution by femtosecond x-ray diffuse scattering
Zhu, Diling; Robert, Aymeric; Lemke, Henrik T.; Trigo, Mariano
2015-08-10
We present a reconstruction of the transverse acoustic phonon dispersion of germanium from femtosecond time-resolved x-ray diffuse scattering measurements at the Linac Coherent Light Source. We demonstrate an energy resolution of 0.3 meV with a momentum resolution of 0.01 nm^{-1} using 10-keV x rays with a bandwidth of ~ 1 eV. This high resolution was achieved simultaneously for a large section of reciprocal space including regions closely following three of the principal symmetry directions. The phonon dispersion was reconstructed with less than 3 h of measurement time, during which neither the x-ray energy, the sample orientation, nor the detector position were scanned. In conclusion, these results demonstrate how time-domain measurements can complement conventional frequency domain inelastic-scattering techniques.
Spin-flip relaxation via optical phonon scattering in quantum dots
Wang, Zi-Wu; Liu, Lei; Li, Shu-Shen
2013-12-14
Based on the spin-orbit coupling admixture mechanism, we theoretically investigate the spin-flip relaxation via optical phonon scattering in quantum dots by considering the effect of lattice relaxation due to the electron-acoustic phonon deformation potential coupling. The relaxation rate displays a cusp-like structure (or a spin hot spot) that becomes more clearly with increasing temperature. We also calculate the relaxation rate of the spin-conserving process, which follows a Gaussian form and is several orders of magnitude larger than that of spin-flip process. Moreover, we find that the relaxation rate displays the oscillatory behavior due to the interplay effects between the magnetic and spatial confinement for the spin-flip process not for the spin-conserving process. The trends of increasing and decreasing temperature dependence of the relaxation rates for two relaxation processes are obtained in the present model.
NASA Astrophysics Data System (ADS)
Li, Jianbao; Wang, Yue-Sheng; Zhang, Chuanzeng
2010-05-01
In this paper, a finite element method based on the ABAQUS code and user subroutine is presented to evaluate the propagation of acoustic waves in the two-dimensional phononic crystals with Archimedean-like tilings. Two systems composed of cylinder scatters embedded in a host in Ladybug and Bathroom lattices are considered. Complete and accurate band structures and transmission spectra are obtained to identify the band gaps and eigenmodes. We found that Archimedean-like structures can have some advantages over the traditional square lattice regarding the completeness of the gap and its position and width. Also, due to the same square primitive unit cell and the first Brillouin zone, the two square-like lattices have similar acoustic response in lower bands. The results indicate that the finite element method is precise for the band structure computation of the complex phononic crystals with Archimedean tilings.
NASA Technical Reports Server (NTRS)
Gordon, Gail
2012-01-01
The Materials Test Branch resides at Marshall Space Flight Center's Materials and Processing laboratory and has a long history of supporting NASA programs from Mercury to the recently retired Space Shuttle. The Materials Test Branch supports its customers by supplying materials testing expertise in a wide range of applications. The Materials Test Branch is divided into three Teams, The Chemistry Team, The Tribology Team and the Mechanical Test Team. Our mission and goal is to provide world-class engineering excellence in materials testing with a special emphasis on customer service.
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)
Fogel, Ronen; Seshia, Ashwin A.
2016-01-01
Resonant and acoustic wave devices have been researched for several decades for application in the gravimetric sensing of a variety of biological and chemical analytes. These devices operate by coupling the measurand (e.g. analyte adsorption) as a modulation in the physical properties of the acoustic wave (e.g. resonant frequency, acoustic velocity, dissipation) that can then be correlated with the amount of adsorbed analyte. These devices can also be miniaturized with advantages in terms of cost, size and scalability, as well as potential additional features including integration with microfluidics and electronics, scaled sensitivities associated with smaller dimensions and higher operational frequencies, the ability to multiplex detection across arrays of hundreds of devices embedded in a single chip, increased throughput and the ability to interrogate a wider range of modes including within the same device. Additionally, device fabrication is often compatible with semiconductor volume batch manufacturing techniques enabling cost scalability and a high degree of precision and reproducibility in the manufacturing process. Integration with microfluidics handling also enables suitable sample pre-processing/separation/purification/amplification steps that could improve selectivity and the overall signal-to-noise ratio. Three device types are reviewed here: (i) bulk acoustic wave sensors, (ii) surface acoustic wave sensors, and (iii) micro/nano-electromechanical system (MEMS/NEMS) sensors. PMID:27365040
Fogel, Ronen; Limson, Janice; Seshia, Ashwin A
2016-06-30
Resonant and acoustic wave devices have been researched for several decades for application in the gravimetric sensing of a variety of biological and chemical analytes. These devices operate by coupling the measurand (e.g. analyte adsorption) as a modulation in the physical properties of the acoustic wave (e.g. resonant frequency, acoustic velocity, dissipation) that can then be correlated with the amount of adsorbed analyte. These devices can also be miniaturized with advantages in terms of cost, size and scalability, as well as potential additional features including integration with microfluidics and electronics, scaled sensitivities associated with smaller dimensions and higher operational frequencies, the ability to multiplex detection across arrays of hundreds of devices embedded in a single chip, increased throughput and the ability to interrogate a wider range of modes including within the same device. Additionally, device fabrication is often compatible with semiconductor volume batch manufacturing techniques enabling cost scalability and a high degree of precision and reproducibility in the manufacturing process. Integration with microfluidics handling also enables suitable sample pre-processing/separation/purification/amplification steps that could improve selectivity and the overall signal-to-noise ratio. Three device types are reviewed here: (i) bulk acoustic wave sensors, (ii) surface acoustic wave sensors, and (iii) micro/nano-electromechanical system (MEMS/NEMS) sensors. PMID:27365040
Experimental measurement of acoustic plasmons in polycrystalline palladium
NASA Astrophysics Data System (ADS)
Garrity, Patrick L.
2013-03-01
An experimental study of collective oscillations in Pd covering the region of very low energy and momentum transfers is reported. Through Dynamic Electron Scattering spectroscopy, structure factor spectra were measured from 80 K to 298 K on a bulk polycrystalline Pd sample. Here we report the first experimental evidence of damped acoustic plasmons and their evolution to the single-particle excitation continuum. The acoustic plasmons follow a linear dispersion and are experimentally shown to be a separate and distinct resonance mode from acoustic surface plasmons. Calculations of the dielectric function employed a model that incorporates complete mixing of two conduction bands with contributions from both interband and intraband transitions. The model was used in computational studies that focused on specific experimental results to aid the characterization and understanding of the plasmon behavior. We found that the Pd acoustic plasmon energy matched the longitudinal phonon anomaly that has sparked numerous theoretical reports on the possible energetic coupling of these modes. Further experimental evidence of plasmon and phonon dynamical processes are found in the linewidth analysis of the data. The primary decay mechanism of the plasmons is interpreted to be strong phonon-assisted interband transitions. Further spectral features and the plasmon velocity are also reported.
NASA Astrophysics Data System (ADS)
Howard, Colin
The following dissertation presents a comprehensive study of the interaction between Dirac fermion quasiparticles (DFQs) and surface phonons on the surfaces of the topological insulators Bi2Se3 and Bi2Te 3. Inelastic helium atom surface scattering (HASS) spectroscopy and time of flight (TOF) techniques were used to measure the surface phonon dispersion of these materials along the two high-symmetry directions of the surface Brillouin zone (SBZ). Two anomalies common to both materials are exhibited in the experimental data. First, there is an absence of Rayleigh acoustic waves on the surface of these materials, pointing to weak coupling between the surface charge density and the surface acoustic phonon modes and potential applications for soundproofing technologies. Secondly, both materials exhibit an out-of-plane polarized optical phonon mode beginning at the SBZ center and dispersing to lower energy with increasing wave vector along both high-symmetry directions of the SBZ. This trend terminates in a V-shaped minimum at a wave vector corresponding to 2 kF for each material, after which the dispersion resumes its upward trend. This phenomenon constitutes a strong Kohn anomaly and can be attributed to the interaction between the surface phonons and DFQs. To quantify the coupling between the optical phonons experiencing strong renormalization and the DFQs at the surface, a phenomenological model was constructed based within the random phase approximation. Fitting the theoretical model to the experimental data allowed for the extraction of the matrix elements of the coupling Hamiltonian and the modifications to the surface phonon propagator encoded in the phonon self energy. This allowed, for the first time, calculation of phonon mode-specific quasiparticle-phonon coupling lambdanu( q) from experimental data. Additionally, an averaged coupling parameter was determined for both materials yielding bar lambdaTe ≈ 2 and bar lambdaSe ≈ 0.7. These values are
ERIC Educational Resources Information Center
Harnack, William
1984-01-01
The first annual Olive Branch Awards, sponsored by the Writers' and Publishers Alliance and the Editors' Organizing Committee, were given to ten magazines, out of 60 that submitted entries. Winning entries are described briefly. (IM)
,
2007-01-01
The mission of Leetown Science Center (LSC), Restoration Technology Branch (RTB) is to conduct research needed to restore or protect the chemical, physical and biological integrity of desirable aquatic systems.
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.
NASA Astrophysics Data System (ADS)
Melkonyan, G. G.; Kröger, H.; Gulian, A. M.
2003-10-01
Nonequilibrium phenomena in thin solid films can result in cooling effects. These types of effects were predicted theoretically a while ago, and only recently were demonstrated experimentally in superconductor-insulator-normal metal (SIN) tunnel junctions. Since then, there is a growing interest in tunneling effects for the purpose to develop on-chip refrigerators. Thin film devices have the advantage of being extremely compact, operate in a continuous mode, dissipate little power, and can easily be integrated in cryogenic detectors. Currently these refrigerators can generate cooling in the order of 100 mK in an environment of 0.3-0.5 K. There are reasons to expect that this performance can be enhanced but a fundamental investigation of underlying principles is required. One of the outcomes of this type of analysis is the phonon deficit effect. In this article we investigate the phonon deficit effect in thin film superconductor-insulator-superconductor and SIN tunnel junctions. Depending on circumstances, the phonon absorption spectra of such tunnel junctions have spectral windows of phonon emission and/or absorption. We demonstrate that the phonon deficit mechanism can stand for the experimental results with SIN tunnel junctions. In addition, application of the theory of this effect allows us to propose using phonon filters to select the phonon absorption windows and thus to enhance the cooling effect. We discuss a particular superlattice design of corresponding phonon filters.
Branch retinal vein occlusion.
Hamid, Sadaf; Mirza, Sajid Ali; Shokh, Ishrat
2008-01-01
Retinal vein occlusions (RVO) are the second commonest sight threatening vascular disorder. Branch retinal vein occlusion (BRVO) and central retinal vein occlusion (CRVO) are the two basic types of vein occlusion. Branch retinal vein occlusion is three times more common than central retinal vein occlusion and- second only to diabetic retinopathy as the most common retinal vascular cause of visual loss. The origin of branch retinal vein occlusion undoubtedly includes both systemic factors such as hypertension and local anatomic factors such as arteriovenous crossings. Branch retinal vein occlusion causes a painless decrease in vision, resulting in misty or distorted vision. Current treatment options don't address the underlying aetiology of branch retinal vein occlusion. Instead they focus on treating sequelae of the occluded venous branch, such as macular oedema, vitreous haemorrhage and traction retinal detachment from neovascularization. Evidences suggest that the pathogenesis of various types of retinal vein occlusion, like many other ocular vascular occlusive disorders, is a multifactorial process and there is no single magic bullet that causes retinal vein occlusion. A comprehensive management of patients with retinal vascular occlusions is necessary to correct associated diseases or predisposing abnormalities that could lead to local recurrences or systemic event. Along with a review of the literature, a practical approach for the management of retinal vascular occlusions is required, which requires collaboration between the ophthalmologist and other physicians: general practitioner, cardiologist, internist etc. as appropriate according to each case. PMID:19385476
Anharmonic effects on Raman-active phonons
NASA Astrophysics Data System (ADS)
Canonico, Michael John
This dissertation explores anharmonic properties of semiconductor materials associated with strain and phonon lifetime using Raman spectroscopy. In recent years, extensive research and development of strain engineered advanced complementary metal-oxide-semiconductor devices utilizing high-k dielectrics and metal gate technology has been conducted to meet the challenges imposed by fundamental limits of device scaling. From a development and manufacturing viewpoint, the metrology required to drive these new technologies is critical to their success. In particular, UV-Raman spectroscopy has been extensively used to measure wafer and device strain due to the high spatial and spectral resolution coupled with an ultra-short optical penetration depth in Si. However, the strain-shift coefficients reported in the literature, which correlate the shift in Raman frequency with strain, have typically been measured in the visible portion of the spectrum and appear to differ from their UV counter-parts. This work presents a detailed measurement of the strain-shift coefficients in the UV at 325 and 364nm for Si, Ge, and Si:C and SiGe alloys. In addition, the temperature dependence of the frequencies and linewidths of the Raman-active longitudinal-optic (LO) phonons in GaAs and AlAs III-V semiconductor compounds is presented. Contrary to early theoretical predictions, the low temperature lifetime of the LO phonon is similar for the two materials with tau = 9.5 ps and 9.7 ps in GaAs and AlAs, respectively. The discrepancy between theory and experiment is caused by the accidental degeneracy between the AlAs LO phonon frequency and a Van Hove singularity in the two-phonon density of states. A new expression, based on the frequency dependence of the phonon self-energy, is derived to model the phonon lifetime.
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. PMID:27447516
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.
A first-principles study on the intrinsic phonon transport of Cu2GeSe3
NASA Astrophysics Data System (ADS)
Shao, Hezhu; Zhang, Hao; Peng, Bo; Tan, Xiaojian; Liu, Guo-Qiang; Jiang, Jun; Jiang, Haochuan
2016-07-01
First-principles calculations are employed to investigate the phonon transport of Cu2GeSe3. The lattice thermal conductivities of Cu2GeSe3 are reproduced. In Cu2GeSe3, the low-frequency phonons lower than 88 cm-1, which comprise of most acoustic modes and a few optical modes, contribute more than 90% to the overall lattice thermal conductivity in Cu2GeSe3. According to the calculations of phonon transport, nanostructuring may be an effective way to reduce the lattice thermal conductivity of Cu2GeSe3. Particularly, at 300 K, the nanostructuring with length scale 100 nm (10 nm) would possibly reduce the thermal conductivity by more than 40% (80%) for Cu2GeSe3. With increase of temperature, the effect of nanostructuring on reducing the lattice thermal conductivity will diminish, due to the decrease in the phonon mean-free path. We further speculate that phonon-band engineering by doping with heavy elements such as Pb may further reduce the lattice thermal conductivity. Our work facilitates deep understanding of the mechanisms of effective reduction of the thermal conductivity by nanostructuring or doping, and helps design better thermoelectric materials.
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.
Zhu, Jia-pei; Huang, Hui; Li, Gao-xiang
2014-01-21
We theoretically propose a scheme for the quadrature squeezing of the cavity field via dissipative processes. The effects of the electron-phonon interaction (EPI) on the squeezing are investigated, where the cavity is off-resonantly coupled with a coherently driven quantum dot (QD) which is allowed to interact with an acoustic-phonon reservoir. Under certain conditions, the participation of the phonon induced by both the EPI and the off-resonant coupling of the cavity with the QD enables some dissipative processes to occur resonantly in the dressed-state basis of the QD. The cavity-mode photons emitted or absorbed during the phonon-mediated dissipative processes are correlated, thus leading to the squeezing of the cavity field. A squeezed vacuum reservoir for the cavity field is built up due to the EPI plus the off-resonant coupling between the cavity and the QD. The numerical results obtained with an effective polaron master equation derived using second-order perturbation theory indicate that, in low temperature limit, the degree of squeezing is maximal but the increasing temperature of the phonon reservoir could hinder the squeezing and degrade the degree of the squeezing of the cavity field. In addition, the presence of the photonic crystal could enhance the quadrature squeezing of the cavity field.
NASA Astrophysics Data System (ADS)
Li, Nianbei; Li, Baowen
2012-12-01
Heat transport in low-dimensional systems has attracted enormous attention from both theoretical and experimental aspects due to its significance to the perception of fundamental energy transport theory and its potential applications in the emerging field of phononics: manipulating heat flow with electronic anologs. We consider the heat conduction of one-dimensional nonlinear lattice models. The energy carriers responsible for the heat transport have been identified as the renormalized phonons. Within the framework of renormalized phonons, a phenomenological theory, effective phonon theory, has been developed to explain the heat transport in general one-dimensional nonlinear lattices. With the help of numerical simulations, it has been verified that this effective phonon theory is able to predict the scaling exponents of temperature-dependent thermal conductivities quantitatively and consistently.
Design of embedded acoustic lenses in plate-like structures based on periodic acoustic black holes
NASA Astrophysics Data System (ADS)
Zhu, Hongfei; Semperlotti, Fabio
2015-03-01
We use a recently developed class of metamaterials based on geometric inhomogeneities to design acoustic lenses embedded in thin-walled structural element. The geometric inhomogeneity is based on the concept of Acoustic Black Hole (ABH) that is an exponential taper fully integrated in the supporting structure. The ABH is an element able to bend and, eventually, trap acoustic waves by creating areas with carefully engineered phase velocity gradients. Periodic lattices of ABHs are first studied in terms of their dispersion characteristics and then embedded in thin-plate structures to create lenses for ultrasonic focusing and collimation. Numerical simulations show the ability of the ABH lens to create focusing and collimation effects in an extended operating range that goes from the metamaterial to the phononic regime.
PHON: A program to calculate phonons using the small displacement method
NASA Astrophysics Data System (ADS)
Alfè, Dario
2009-12-01
The program PHON calculates force constant matrices and phonon frequencies in crystals. From the frequencies it also calculates various thermodynamic quantities, like the Helmholtz free energy, the entropy, the specific heat and the internal energy of the harmonic crystal. The procedure is based on the small displacement method, and can be used in combination with any program capable to calculate forces on the atoms of the crystal. In order to examine the usability of the method, I present here two examples: metallic Al and insulating MgO. The phonons of these two materials are calculated using density functional theory. The small displacement method results are compared with those obtained using the linear response method. In the case of Al the method provides accurate phonon frequencies everywhere in the Brillouin Zone (BZ). In the case of MgO the longitudinal branch of the optical phonons near the centre of the BZ is incorrectly described as degenerate with the two transverse branches, because the non-analytical part of the dynamical matrix is ignored here; however, thermodynamic properties like the Helmholtz free are essentially unaffected. Program summaryProgram title: PHON Catalogue identifier: AEDP_v1_0 Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEDP_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.: 19 580 No. of bytes in distributed program, including test data, etc.: 612 193 Distribution format: tar.gz Programming language: Fortran 90 Computer: Any Unix, Linux Operating system: Unix RAM: Depends on super-cell size, but usually negligible Classification: 7.8 External routines: Subprograms ZHEEV and DSYEV (Lapack); needs BLAS. A tutorial is provided with the distribution which requires the installation of the quantum-espresso package ( http
Universality of the Phonon-Roton Spectrum in Liquids and Superfluidity of 4He
NASA Astrophysics Data System (ADS)
Bobrov, Viktor; Trigger, Sergey; Litinski, Daniel
2016-06-01
Based on numerous experimental data on inelastic neutron and X-ray scattering in liquids, we assert that the phonon-roton spectrum of collective excitations, predicted by Landau for superfluid helium, is a universal property of the liquid state. We show that the existence of the roton minimum in the spectrum of collective excitations is caused by the short-range order in liquids. Using the virial theorem, we assume that one more branch of excitations should exist in He II, whose energy spectrum differs from the phonon-roton spectrum. Such excitations are associated with the pole of single-particle Green function, which can have a gap at small values of momenta.
Dispersion of Phonon Surface Polaritons in ZnGeP2: Anisotropy and Temperature Impacts.
Shportko, K V; Otto, A; Venger, E F
2016-12-01
Zinc germanium diphosphide (ZnGeP2) is an attractive and promising functional material for different devices of the nano- and optoelectronics. In this paper, dispersion of phonon surface polaritons (PSPs) in ZnGeP2 has been studied in the 200-500-cm(-1) spectral range at 4 and 300 K. Dispersion of "real" and "virtual" PSPs were calculated for C-axis being normal and parallel to the surface. Anisotropy in ZnGeP2 leads to the different numbers of PSP dispersion branches for different orientations of the sample. The temperature-dependent phonon contributions in the dielectric permittivity shift dispersion of the surface polaritons in ZnGeP2 to the higher wavenumbers at 4 K. We have shown that experimental dispersion of PSP is in agreement with theory. PMID:26858158
Phonon Recycling for Ultrasensitive Kinetic Inductance Detectors
NASA Astrophysics Data System (ADS)
Zmuidzinas, Jonas
Initially proposed (Day et al. 2003; Zmuidzinas 2012) in 1999 by our Caltech/JPL group, and thanks to strong support from NASA, the superconducting (microwave) kinetic inductance detector (MKID or KID) technology continues to develop rapidly as it transitions into applications. The development effort worldwide is intensifying and NASA's continued support of KID development is essential in order to keep pace. Here we propose to investigate and demonstrate a new, low-TRL concept, which we call phonon recycling, that promises to open broad new avenues in KID design and performance. Briefly, phonon recycling allows the detector designer to tailor the responsivity and sensitivity of a KID to match the needs of the application by using geometry to restrict the rate at which recombination phonons are allowed to escape from the detector. In particular, phonon recycling should allow very low noise-equivalent power (NEP) to be achieved without requiring very low operating tem- peratures. Phonon recycling is analogous to the use of micromachined suspension legs to control the flow of heat in a bolometer, as measured by the thermal conductivity G. However, phonon recycling exploits the non-thermal distribution of recombination phonons as well as their very slow decay in crystals at low temperatures. These properties translate to geometrical and mechanical requirements for a phonon-recycled KID that are considerably more relaxed than for a bolometer operating at the same temperature and NEP. Our ultimate goal is to develop detector arrays suitable for a far-infrared (FIR) space mission, which will impose strict requirements on the array sensitivity, yield, uniformity, multiplexing density, etc. Through previous NASA support under the Strategic Astrophysics Technology (SAT) program, we have successfully demonstrated the MAKO submillimeter camera at the Caltech Submillimeter Observatory and have become familiar with these practical issues. If our demonstration of phonon recycling
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.
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.
Understandng of phonon anharmonicity in thermoelectric clathrates
NASA Astrophysics Data System (ADS)
Tanigaki, Katsumi; Wu, Jiazhen; Shimotani, Hidekazu; Huynh, Khuong; Akagi, Kazuto; AIMR Collaboration; Department of Physics, Graduate School of Science Collaboration
Anharmonicity in phonons, apart from the conventional Einstein- or Debye- mode harmonic phonons, is frequently observed for amorphous or glass-like materials. A frontier topic relating to anharmonic phonons revolves around the fact that they are also observed in a single crystal with a void of cage structure. Although the origin of the phonon anharmonicity has been the center of scientific debate for many years, a clear understanding has not yet been achieved. In the present study, we show that the anharmonic oscillations in thermoelectric clathrates can successfully be rationalized in terms of a single unified exponential line for a variety of clathrates by employing a new parameter associated with the freedom of space. The intrinsic nature of phonon anharmonicity is described based on the unified picture with a help of first principles calculations. Although the origin of the anharmonicity appearing in disordered materials is complex to understand due to the missing information on the real structure, the present unified picture gives important information applicable to other systems.
Nonlinear phononic crystals based on chains of disks alternating with toroidal structures
NASA Astrophysics Data System (ADS)
Spadoni, A.; Daraio, C.; Hurst, W.; Brown, M.
2011-04-01
We study experimentally the acoustic response of a load-bearing, phononic crystal composed of alternating steel disks, and polytetrafluoroethylene o-rings under precompression. The crystal allows for axial, rocking, and shear-polarized wavemodes when excited by a broad-band signal applied off-axis. Finite element analysis is employed to determine the system's wave modes. The nonlinear interaction between disks and o-rings supports a dynamic response that is tunable with variations in static precompression, leading to controllable frequency shifts in a large band gap. A modal analysis reveals that four of the six principal wave modes are susceptible to external precompression while two modes are not.
Lin, Tzy-Rong; Lin, Chiang-Hsin; Hsu, Jin-Chen
2015-01-01
We propose dynamic modulation of a hybrid plasmonic-photonic crystal nanocavity using monochromatic coherent acoustic phonons formed by ultrahigh-frequency surface acoustic waves (SAWs) to achieve strong optomechanical interaction. The crystal nanocavity used in this study consisted of a defective photonic crystal beam coupled to a metal surface with a nanoscale air gap in between and provided hybridization of a highly confined plasmonic-photonic mode with a high quality factor and deep subwavelength mode volume. Efficient photon-phonon interaction occurs in the air gap through the SAW perturbation of the metal surface, strongly coupling the optical and acoustic frequencies. As a result, a large modulation bandwidth and optical resonance wavelength shift for the crystal nanocavity are demonstrated at telecommunication wavelengths. The proposed SAW-based modulation within the hybrid plasmonic-photonic crystal nanocavities beyond the diffraction limit provides opportunities for various applications in enhanced sound-light interaction and fast coherent acoustic control of optomechanical devices. PMID:26346448
Strouhal numbers of flow-excited resonance of closed side branches
Ziada, S.; Shine, S.
1995-12-01
Flow-excited acoustic resonances of piping systems containing closed side-branches are often encountered in engineering applications. They are excited by the unstable shear layer which separates the mean flow in the main pipe from the stagnant fluid in the branch. The object of this paper is to provide design charts which can be used to predict the critical velocity at which an acoustic resonance may be initiated. Model tests were carried out on three different configurations of side-branches (single, tandem and coaxial branches). For each of these pipe configurations, the effects of the diameter ratio (d/D), the distance from an upstream elbow (L) and the acoustic damping are investigated in some detail. The test results are embodied into a design chart to predict the flow velocity at the onset of resonance as a function of the system operational and geometric parameters.
Influence of phonon emission on intersubband lifetimes in wide GaAs/AlGaAs and Si/SiGe quantum wells
Murdin, B.N.; Pidgeon, C.R.; Lee, S.C.
1995-12-31
We have previously used the picosecond far-infrared free electron laser FELIX, at Rijnhuizen, to make the first direct excite-probe determination of the intersubband relaxation rate in wide GaAs/AlGaAs quantum wells with the subband separation smaller than the optical phonon energy. This measurement yielded short (40ps) lifetimes while acoustic phonon emission occurs on a 200ps scale. This is also in contrast with, among others, saturation measurements of swide wells with the UCSB FEL which gave lifetimes of 600ps. We discuss here the interpretation of the range of published results by calculation of the LO-phonon scattering rate, including the effects of finite electron temperature, T{sub e}. We have shown that relaxation can be dominated by LO-phonon emission even in wide wells, through the high energy tail of the distribution. The rate is very sensitive to T{sub e} between 30-70K, and also to carrier concentration, making it possible to account for the wide variety of published results with a single mechanism. We have extended our measurements to wide Si/SiGe quantum Wells, and find similarly short times (20-30ps). However, in non-polar materials such as SiGe the deformation potential scattering is much weaker and acoustic phonon emission (order 10ps in n-silicon) is expected to dominate.
Phonon wave interference and thermal bandgap materials
NASA Astrophysics Data System (ADS)
Maldovan, Martin
2015-07-01
Wave interference modifies phonon velocities and density of states, and in doing so creates forbidden energy bandgaps for thermal phonons. Materials that exhibit wave interference effects allow the flow of thermal energy to be manipulated by controlling the material's thermal conductivity or using heat mirrors to reflect thermal vibrations. The technological potential of these materials, such as enhanced thermoelectric energy conversion and improved thermal insulation, has fuelled the search for highly efficient phonon wave interference and thermal bandgap materials. In this Progress Article, we discuss recent developments in the understanding and manipulation of heat transport. We show that the rational design and fabrication of nanostructures provides unprecedented opportunities for creating wave-like behaviour of heat, leading to a fundamentally new approach for manipulating the transfer of thermal energy.
Phonon wave interference and thermal bandgap materials.
Maldovan, Martin
2015-07-01
Wave interference modifies phonon velocities and density of states, and in doing so creates forbidden energy bandgaps for thermal phonons. Materials that exhibit wave interference effects allow the flow of thermal energy to be manipulated by controlling the material's thermal conductivity or using heat mirrors to reflect thermal vibrations. The technological potential of these materials, such as enhanced thermoelectric energy conversion and improved thermal insulation, has fuelled the search for highly efficient phonon wave interference and thermal bandgap materials. In this Progress Article, we discuss recent developments in the understanding and manipulation of heat transport. We show that the rational design and fabrication of nanostructures provides unprecedented opportunities for creating wave-like behaviour of heat, leading to a fundamentally new approach for manipulating the transfer of thermal energy.
Subwavelength waveguiding of surface phonons in pillars-based phononic crystal
NASA Astrophysics Data System (ADS)
Addouche, Mahmoud; Al-Lethawe, Mohammed A.; Elayouch, Aliyasin; Khelif, Abdelkrim
2014-12-01
In this study, we theoretically analyze the guiding of surface phonons through locally resonant defects in pillars-based phononic crystal. Using finite element method, we simulate the propagation of surface phonons through a periodic array of cylindrical pillars deposited on a semi-infinite substrate. This structure displays several band gaps, some of which are due to local resonances of the pillar. By introducing pillar defects inside the phononic structure, we show the possibility to perform a waveguiding of surface phonons based on two mechanisms that spatially confine the elastic energy in very small waveguide apertures. A careful choice of the height of the defect pillars, allows to shift the frequency position of the defect modes inside or outside the locally resonant band gaps and create two subwavelenght waveguiding mechanisms. The first is a classical mechanism that corresponds to the presence of the defect modes inside the locally resonant band gap. The seconde is due to the hybridation between the phonon resonances of defect modes and the surface phonons of the semi-infinite homogenous medium. We discuss the nature and the difference between both waveguiding phenomena.
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
Acoustic-like dynamics of amorphous drugs in the THz regime
Pogna, E. A. A.; Rodríguez-Tinoco, C.; Krisch, M.; Rodríguez-Viejo, J.; Scopigno, T.
2013-01-01
The high frequency dynamics of Indomethacin and Celecoxib glasses has been investigated by inelastic x-ray scattering, accessing a momentum-energy region still unexplored in amorphous pharmaceuticals. We find evidence of phonon-like acoustic dynamics, and determine the THz behavior of sound velocity and acoustic attenuation. Connections with ordinary sound propagation are discussed, along with the relation between fast and slow degrees of freedom as represented by non-ergodicity factor and kinetic fragility, respectively. PMID:23989304
Statistical Entropy of an Acoustic Black Hole in Bose—Einstein Condensates
NASA Astrophysics Data System (ADS)
Ma, Meng-Sen; Zhao, Hui-Hua; Zhao, Ren
2013-12-01
The entanglement entropy of an acoustic black hole in a Bose—Einstein condensates (BEC) is derived, which is associated with the phonons generated via the Hawking mechanism in a sonic hole. Considering the dispersion relation of a BEC, we recalculate the entanglement entropy of the acoustic black hole by means of statistical method in two limits. We find that the entropy is still proportional to the area of event horizon, but with a coefficient dependent on the infrared cutoff.
Acoustic-like dynamics of amorphous drugs in the THz regime
NASA Astrophysics Data System (ADS)
Pogna, E. A. A.; Rodríguez-Tinoco, C.; Krisch, M.; Rodríguez-Viejo, J.; Scopigno, T.
2013-08-01
The high frequency dynamics of Indomethacin and Celecoxib glasses has been investigated by inelastic x-ray scattering, accessing a momentum-energy region still unexplored in amorphous pharmaceuticals. We find evidence of phonon-like acoustic dynamics, and determine the THz behavior of sound velocity and acoustic attenuation. Connections with ordinary sound propagation are discussed, along with the relation between fast and slow degrees of freedom as represented by non-ergodicity factor and kinetic fragility, respectively.
Acoustic transducer for acoustic microscopy
Khuri-Yakub, B.T.; Chou, C.H.
1990-03-20
A shear acoustic transducer-lens system is described in which a shear polarized piezoelectric material excites shear polarized waves at one end of a buffer rod having a lens at the other end which excites longitudinal waves in a coupling medium by mode conversion at selected locations on the lens. 9 figs.
Acoustic transducer for acoustic microscopy
Khuri-Yakub, Butrus T.; Chou, Ching H.
1990-01-01
A shear acoustic transducer-lens system in which a shear polarized piezoelectric material excites shear polarized waves at one end of a buffer rod having a lens at the other end which excites longitudinal waves in a coupling medium by mode conversion at selected locations on the lens.
Fuzzy branching temporal logic.
Moon, Seong-ick; Lee, Kwang H; Lee, Doheon
2004-04-01
Intelligent systems require a systematic way to represent and handle temporal information containing uncertainty. In particular, a logical framework is needed that can represent uncertain temporal information and its relationships with logical formulae. Fuzzy linear temporal logic (FLTL), a generalization of propositional linear temporal logic (PLTL) with fuzzy temporal events and fuzzy temporal states defined on a linear time model, was previously proposed for this purpose. However, many systems are best represented by branching time models in which each state can have more than one possible future path. In this paper, fuzzy branching temporal logic (FBTL) is proposed to address this problem. FBTL adopts and generalizes concurrent tree logic (CTL*), which is a classical branching temporal logic. The temporal model of FBTL is capable of representing fuzzy temporal events and fuzzy temporal states, and the order relation among them is represented as a directed graph. The utility of FBTL is demonstrated using a fuzzy job shop scheduling problem as an example. PMID:15376850
Nelson, E.A.; Christensen, E.J.; Mackey, H.E.; Sharitz, R.R.; Jensen, J.R.; Hodgson, M.E.
1984-02-01
Since 1954, cooling water discharges from K Reactor ({anti X} = 370 cfs {at} 59 C) to Pen Branch have altered vegetation and deposited sediment in the Savannah River Swamp forming the Pen Branch delta. Currently, the delta covers over 300 acres and continues to expand at a rate of about 16 acres/yr. Examination of delta expansion can provide important information on environmental impacts to wetlands exposed to elevated temperature and flow conditions. To assess the current status and predict future expansion of the Pen Branch delta, historic aerial photographs were analyzed using both basic photo interpretation and computer techniques to provide the following information: (1) past and current expansion rates; (2) location and changes of impacted areas; (3) total acreage presently affected. Delta acreage changes were then compared to historic reactor discharge temperature and flow data to see if expansion rate variations could be related to reactor operations.
Damage Tolerance Assessment Branch
NASA Technical Reports Server (NTRS)
Walker, James L.
2013-01-01
The Damage Tolerance Assessment Branch evaluates the ability of a structure to perform reliably throughout its service life in the presence of a defect, crack, or other form of damage. Such assessment is fundamental to the use of structural materials and requires an integral blend of materials engineering, fracture testing and analysis, and nondestructive evaluation. The vision of the Branch is to increase the safety of manned space flight by improving the fracture control and the associated nondestructive evaluation processes through development and application of standards, guidelines, advanced test and analytical methods. The Branch also strives to assist and solve non-aerospace related NDE and damage tolerance problems, providing consultation, prototyping and inspection services.
LO phonons in La{sub 1.85}Sr{sub .15}CuO{sub 4}
Egami, T.; Petrov, Y.; McQueeney, R.J.; Petrov, Y.; Yethiraj, M.; Shirane, G.; Endoh, Y.
1997-11-01
Dispersion of the highest energy LO phonons in La{sup 1.85}Sr{sub .15}CuO{sub 4} was studied by neutron inelastic scattering. At T = 10 K the dispersion along (h, 0, 0) is anomalous forming dispersion branches with a discontinuity at h = 1/4. A possible relation of this anomaly to the spin-charge stripes with be discussed.
Nonequilibrium phonon effects in midinfrared quantum cascade lasers
Shi, Y. B. Knezevic, I.
2014-09-28
We investigate the effects of nonequilibrium phonon dynamics on the operation of a GaAs-based midinfrared quantum cascade laser over a range of temperatures (77–300 K) via a coupled ensemble Monte Carlo simulation of electron and optical-phonon systems. Nonequilibrium phonon effects are shown to be important below 200 K. At low temperatures, nonequilibrium phonons enhance injection selectivity and efficiency by drastically increasing the rate of interstage electron scattering from the lowest injector state to the next-stage upper lasing level via optical-phonon absorption. As a result, the current density and modal gain at a given field are higher and the threshold current density lower and considerably closer to experiment than results obtained with thermal phonons. By amplifying phonon absorption, nonequilibrium phonons also hinder electron energy relaxation and lead to elevated electronic temperatures.
Phonon interference effects in molecular junctions
Markussen, Troels
2013-12-28
We study coherent phonon transport through organic, π-conjugated molecules. Using first principles calculations and Green's function methods, we find that the phonon transmission function in cross-conjugated molecules, like meta-connected benzene, exhibits destructive quantum interference features very analogous to those observed theoretically and experimentally for electron transport in similar molecules. The destructive interference features observed in four different cross-conjugated molecules significantly reduce the thermal conductance with respect to linear conjugated analogues. Such control of the thermal conductance by chemical modifications could be important for thermoelectric applications of molecular junctions.
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.
Soft surfaces of nanomaterials enable strong phonon interactions
NASA Astrophysics Data System (ADS)
Bozyigit, Deniz; Yazdani, Nuri; Yarema, Maksym; Yarema, Olesya; Lin, Weyde Matteo Mario; Volk, Sebastian; Vuttivorakulchai, Kantawong; Luisier, Mathieu; Juranyi, Fanni; Wood, Vanessa
2016-03-01
Phonons and their interactions with other phonons, electrons or photons drive energy gain, loss and transport in materials. Although the phonon density of states has been measured and calculated in bulk crystalline semiconductors, phonons remain poorly understood in nanomaterials, despite the increasing prevalence of bottom-up fabrication of semiconductors from nanomaterials and the integration of nanometre-sized components into devices. Here we quantify the phononic properties of bottom-up fabricated semiconductors as a function of crystallite size using inelastic neutron scattering measurements and ab initio molecular dynamics simulations. We show that, unlike in microcrystalline semiconductors, the phonon modes of semiconductors with nanocrystalline domains exhibit both reduced symmetry and low energy owing to mechanical softness at the surface of those domains. These properties become important when phonons couple to electrons in semiconductor devices. Although it was initially believed that the coupling between electrons and phonons is suppressed in nanocrystalline materials owing to the scarcity of electronic states and their large energy separation, it has since been shown that the electron-phonon coupling is large and allows high energy-dissipation rates exceeding one electronvolt per picosecond (refs 10, 11, 12, 13). Despite detailed investigations into the role of phonons in exciton dynamics, leading to a variety of suggestions as to the origins of these fast transition rates and including attempts to numerically calculate them, fundamental questions surrounding electron-phonon interactions in nanomaterials remain unresolved. By combining the microscopic and thermodynamic theories of phonons and our findings on the phononic properties of nanomaterials, we are able to explain and then experimentally confirm the strong electron-phonon coupling and fast multi-phonon transition rates of charge carriers to trap states. This improved understanding of phonon
Acoustics of Fluid-Structure Interactions
NASA Astrophysics Data System (ADS)
Howe, M. S.
1998-08-01
Acoustics of Fluid-Structure Interactions addresses an increasingly important branch of fluid mechanics--the absorption of noise and vibration by fluid flow. This subject, which offers numerous challenges to conventional areas of acoustics, is of growing concern in places where the environment is adversely affected by sound. Howe presents useful background material on fluid mechanics and the elementary concepts of classical acoustics and structural vibrations. Using examples, many of which include complete worked solutions, he vividly illustrates the theoretical concepts involved. He provides the basis for all calculations necessary for the determination of sound generation by aircraft, ships, general ventilation and combustion systems, as well as musical instruments. Both a graduate textbook and a reference for researchers, Acoustics of Fluid-Structure Interactions is an important synthesis of information in this field. It will also aid engineers in the theory and practice of noise control.
Phonon dispersion curves determination in (delta)-phase Pu-Ga alloys
Wong, J; Clatterbuck, D; Occelli, F; Farber, D; Schwartz, A; Wall, M; Boro, C; Krisch, M; Beraud, A; Chiang, T; Xu, R; Hong, H; Zschack, P; Tamura, N
2006-02-07
We have designed and successfully employed a novel microbeam on large grain sample concept to conduct high resolution inelastic x-ray scattering (HRIXS) experiments to map the full phonon dispersion curves of an fcc {delta}-phase Pu-Ga alloy. This approach obviates experimental difficulties with conventional inelastic neutron scattering due to the high absorption cross section of the common {sup 239}Pu isotope and the non-availability of large (mm size) single crystal materials for Pu and its alloys. A classical Born von-Karman force constant model was used to model the experimental results, and no less than 4th nearest neighbor interactions had to be included to account for the observation. Several unusual features including, a large elastic anisotropy, a small shear elastic modulus, (C{sub 11}-C{sub 12})/2, a Kohn-like anomaly in the T{sub 1}[011] branch, and a pronounced softening of the T[111] branch towards the L point in the Brillouin are found. These features may be related to the phase transitions of plutonium and to strong coupling between the crystal structure and the 5f valence instabilities. Our results represent the first full phonon dispersions ever obtained for any Pu-bearing material, thus ending a 40-year quest for this fundamental data. The phonon data also provide a critical test for theoretical treatments of highly correlated 5f electron systems as exemplified by recent dynamical mean field theory (DMFT) calculations for {delta}-plutonium. We also conducted thermal diffuse scattering experiments to study the T(111) dispersion at low temperatures with an attempt to gain insight into bending of the T(111) branch in relationship to the {delta} {yields} {alpha}{prime} transformation.
NASA Astrophysics Data System (ADS)
Beach, Kirk W.; Dunmire, Barbrina
Medical acoustics can be subdivided into diagnostics and therapy. Diagnostics are further separated into auditory and ultrasonic methods, and both employ low amplitudes. Therapy (excluding medical advice) uses ultrasound for heating, cooking, permeablizing, activating and fracturing tissues and structures within the body, usually at much higher amplitudes than in diagnostics. Because ultrasound is a wave, linear wave physics are generally applicable, but recently nonlinear effects have become more important, even in low-intensity diagnostic applications.
Radioiodinated branched carbohydrates
Goodman, Mark M.; Knapp, Jr., Furn F.
1989-01-01
A radioiodinated branched carbohydrate for tissue imaging. Iodine-123 is stabilized in the compound by attaching it to a vinyl functional group that is on the carbohydrate. The compound exhibits good uptake and retention and is promising in the development of radiopharmaceuticals for brain, heart and tumor imaging.
NASA Astrophysics Data System (ADS)
The Front Range Branch of AGU has installed officers for 1990: Ray Noble, National Center for Atmospheric Research, chair; Sherry Oaks, U.S. Geological Survey, chair-elect; Howard Garcia, NOAA, treasurer; Catharine Skokan, Colorado School of Mines, secretary. JoAnn Joselyn of NOAA is past chair. Members at large are Wallace Campbell, NOAA; William Neff, USGS; and Stephen Schneider, NCAR.
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.
Physically founded phonon dispersions of few-layer materials and the case of borophene
Carrete, Jesús; Li, Wu; Lindsay, Lucas; Broido, David A.; Gallego, Luis J.; Mingo, Natalio
2016-04-21
By building physically sound interatomic force constants,we offer evidence of the universal presence of a quadratic phonon branch in all unstrained 2D materials, thus contradicting much of the existing literature. Through a reformulation of the interatomic force constants (IFCs) in terms of internal coordinates, we find that a delicate balance between the IFCs is responsible for this quadraticity. We use this approach to predict the thermal conductivity of Pmmn borophene, which is comparable to that of MoS2, and displays a remarkable in-plane anisotropy. Ultimately, these qualities may enable the efficient heat management of borophene devices in potential nanoelectronic applications
Control of elastic wave propagation in one-dimensional piezomagnetic phononic crystals.
Ponge, Marie-Fraise; Croënne, Charles; Vasseur, Jérôme O; Bou Matar, Olivier; Hladky-Hennion, Anne-Christine; Dubus, Bertrand
2016-06-01
Two ways of controlling the acoustic waves propagation by external inductance or capacitance in a one-dimensional (1-D) piezomagnetic phononic crystal are investigated. The structure is made of identical bars, constituted of a piezomagnetic material, surrounded by a coil and connected to an external impedance. A model of propagation of longitudinal elastic waves through the periodic structure is developed and the dispersion equation is obtained. Reflection and transmission coefficients are derived from a 2 × 2 transfer matrix formalism that also allows for the calculation of elastic effective parameters (density, Young modulus, speed of sound, impedance). The effect of shunting impedances is numerically investigated. The results reveal that a connected external inductance tunes the Bragg band gaps of the 1-D phononic crystal. When the elements are connected via a capacitance, a hybridization gap, due to the resonance of the LC circuit made of the piezomagnetic element and the capacitance, coexists with the Bragg band gap. The value of the external capacitance modifies the boundaries of both gaps. Calculation of the effective characteristics of the phononic crystal leads to an analysis of the physical mechanisms involved in the wave propagation. When periodically connected to external capacitances, a homogeneous piezomagnetic stack behaves as a dispersive tunable metamaterial.
Phonon modes of MgB2: super-lattice structures and spectral response.
Alarco, Jose A; Chou, Alison; Talbot, Peter C; Mackinnon, Ian D R
2014-11-28
Micrometre-sized MgB2 crystals of varying quality, synthesized at low temperature and autogenous pressure, are compared using a combination of Raman and infra-red (IR) spectroscopy. These data, which include new peak positions in both spectroscopies for high quality MgB2, are interpreted using DFT calculations on phonon behaviour for symmetry-related structures. Raman and IR activity additional to that predicted by point group analyses of the P6/mmm symmetry are detected. These additional peaks, as well as the overall shapes of calculated phonon dispersion (PD) models are explained by assuming a double super-lattice, consistent with a lower symmetry structure for MgB2. A 2× super-lattice in the c-direction allows a simple correlation of the pair breaking energy and the superconducting gap by activation of corresponding acoustic frequencies. A consistent physical interpretation of these spectra is obtained when the position of a phonon anomaly defines a super-lattice modulation in the a-b plane.
Control of elastic wave propagation in one-dimensional piezomagnetic phononic crystals.
Ponge, Marie-Fraise; Croënne, Charles; Vasseur, Jérôme O; Bou Matar, Olivier; Hladky-Hennion, Anne-Christine; Dubus, Bertrand
2016-06-01
Two ways of controlling the acoustic waves propagation by external inductance or capacitance in a one-dimensional (1-D) piezomagnetic phononic crystal are investigated. The structure is made of identical bars, constituted of a piezomagnetic material, surrounded by a coil and connected to an external impedance. A model of propagation of longitudinal elastic waves through the periodic structure is developed and the dispersion equation is obtained. Reflection and transmission coefficients are derived from a 2 × 2 transfer matrix formalism that also allows for the calculation of elastic effective parameters (density, Young modulus, speed of sound, impedance). The effect of shunting impedances is numerically investigated. The results reveal that a connected external inductance tunes the Bragg band gaps of the 1-D phononic crystal. When the elements are connected via a capacitance, a hybridization gap, due to the resonance of the LC circuit made of the piezomagnetic element and the capacitance, coexists with the Bragg band gap. The value of the external capacitance modifies the boundaries of both gaps. Calculation of the effective characteristics of the phononic crystal leads to an analysis of the physical mechanisms involved in the wave propagation. When periodically connected to external capacitances, a homogeneous piezomagnetic stack behaves as a dispersive tunable metamaterial. PMID:27369153
Disorder-induced absorption of far-infrared waves by acoustic modes in nematic liquid crystals
NASA Astrophysics Data System (ADS)
Nenashev, A. V.; Wiemer, M.; Koch, M.; Dvurechenskii, A. V.; Gebhard, F.; Baranovskii, S. D.
2016-08-01
A mechanism of light absorption at THz frequencies in nematic liquid crystals based on intermolecular dynamics is proposed. In this mechanism, the energy conservation is supplied by acoustic phonons, whereas momentum conservation is provided by static spatial fluctuations of the director field. The mechanism predicts a continuous absorption spectrum in a broad frequency range.
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.
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.
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.
NASA Technical Reports Server (NTRS)
Heyman, J. S.
1984-01-01
Acoustically-energized water jet aids in plaque breakdown. Acoustic Wand includes acoustic transducer 1/4 wave plate, and tapered cone. Together elements energize solution of water containing mild abrasive injected into mouth to help prevent calculous buildup.
Tunable ultrasonic phononic crystal controlled by infrared radiation
Walker, Ezekiel; Neogi, Arup E-mail: arup@unt.edu; Reyes, Delfino; Rojas, Miguel Mayorga; Krokhin, Arkadii; Wang, Zhiming E-mail: arup@unt.edu
2014-10-06
A tunable phononic crystal based ultrasonic filter was designed by stimulating the phase of the polymeric material embedded in a periodic structure using infrared radiation. The acoustic filter can be tuned remotely using thermal stimulation induced by the infrared radiation. The filter is composed of steel cylinder scatterers arranged periodically in a background of bulk poly (N-isopropylacrylamide) polymer hydrogel. The lattice structure creates forbidden bands for certain sets of mechanical waves that cause it to behave as an ultrasonic filter. Since the bandstructure is determined by not only the arrangement of the scatterers but also the physical properties of the materials composing the scatterers and background, modulating either the arrangement or physical properties will alter the effect of the crystal on propagating mechanical waves. Here, the physical properties of the filter are varied by inducing changes in the polymer hydrogel using an electromagnetic thermal stimulus. With particular focus on the k{sub 00}-wave, the transmission of ultrasonic wave changes by as much as 20 dBm, and band widths by 22% for select bands.
Tunable ultrasonic phononic crystal controlled by infrared radiation
NASA Astrophysics Data System (ADS)
Walker, Ezekiel; Reyes, Delfino; Rojas, Miguel Mayorga; Krokhin, Arkadii; Wang, Zhiming; Neogi, Arup
2014-10-01
A tunable phononic crystal based ultrasonic filter was designed by stimulating the phase of the polymeric material embedded in a periodic structure using infrared radiation. The acoustic filter can be tuned remotely using thermal stimulation induced by the infrared radiation. The filter is composed of steel cylinder scatterers arranged periodically in a background of bulk poly (N-isopropylacrylamide) polymer hydrogel. The lattice structure creates forbidden bands for certain sets of mechanical waves that cause it to behave as an ultrasonic filter. Since the bandstructure is determined by not only the arrangement of the scatterers but also the physical properties of the materials composing the scatterers and background, modulating either the arrangement or physical properties will alter the effect of the crystal on propagating mechanical waves. Here, the physical properties of the filter are varied by inducing changes in the polymer hydrogel using an electromagnetic thermal stimulus. With particular focus on the k00-wave, the transmission of ultrasonic wave changes by as much as 20 dBm, and band widths by 22% for select bands.
NASA Astrophysics Data System (ADS)
Christianson, Caleb; Mukhopadhyay, Saikat; Sachse, Wolfgang; Stewart, Derek
2014-03-01
Phononic crystals are two- and three-dimensional structures with a periodic arrangement of two or more materials with different acoustic properties. Depending on the size, structure, and characteristics of the constituent materials, metamaterials with interesting acoustic properties can be formed. These crystals can be used to control the transmission of sound at selected frequencies, focus sound, or serve as waveguides. In this talk, we will focus on the transmission of ultrasonic waves through polydimethylsiloxane (PDMS) films with entrapped air bubbles. Two different theoretical models were used to predict ultrasonic transmission through air-PDMS crystals: (1) a simple scattering model for a series of partially reflective thin films and (2) the code MULTEL, which calculates the transmission using multiple scattering theory. A fabrication process was also developed to stack layers of the crystals with unprecedented alignment. We measured the ultrasonic transmission through the films using the ultrasonic through-transmission mode in a water bath and found an excellent agreement between the measured and calculated transmission. Additionally, we used these models to predict the performance of new phononic structures by scanning a large parameter space and showed how ultrasonic transmission through PDMS layers can be engineered by varying the dimensions, separation, and arrangement of air bubbles. This work was supported by the National Science Foundation.
Multilayer-split-tube resonators with low-frequency band gaps in phononic crystals
NASA Astrophysics Data System (ADS)
Jing, Li; Wu, Jiu Hui; Guan, Dong; Gao, Nansha
2014-09-01
In this paper, low-frequency band gaps in two-dimensional Helmholtz resonant phononic crystals (PCs) composed of multilayer-split-tube resonators are investigated. The band structures, transmission spectra, and pressure field of the acoustic modes of these PCs are calculated by using a finite element method (FEM). The numerical results show that the first band gap of the structure is from 88 to 140 Hz. The transmission spectra are in accordance with those of the dispersion relation calculations. The acoustic modes of the bands are analyzed to reveal the nature of this phenomenon. It is found that the interaction between the local resonance and the traveling wave modes in proposed structure is responsible for the formation of the first band gap. The influences of the structural parameters on the band gaps are investigated by using FEM and the electrical circuit analogy. Numerical results show that the band gaps can be modulated in an even wider frequency range by changing the structural parameters, such as the rotation angle, the number of tubes, and the radius of the outer tube. The structural design results provide an effective way for phononic crystals to obtain the low-frequency band gaps, which have potential application in the low-frequency noise reduction.
Damping of thermal acoustic oscillations in hydrogen systems
NASA Technical Reports Server (NTRS)
Gu, Youfan; Timmerhaus, Klaus D.
1991-01-01
Acoustic waves initiated by a large temperature gradient along a tube are defined as thermal acoustic oscillations (TAOs). These oscillations have been damped by introducing such sound absorbing techniques as acoustic filters, resonators, etc.. These devices serve as an acoustic sink that is used to absorb or dissipate the acoustic energy thereby eliminating or damping such oscillations. Several empirical damping techniques, such as attaching a resonator as a side branch or inserting a wire in the tube, have been developed in the past and have provided reasonable success. However, the effect of connecting tube radius, length, and resonator volume on the damping of thermal acoustic oscillations has not been evaluated quantitatively. Further, these methods have not been effective when the oscillating tube radius was relatively large. Detailed theoretical analyses of these techniques including a newly developed method for damping oscillations in a tube of relatively large radius are provided in this presentation.
Damping of thermal acoustic oscillations in hydrogen systems
NASA Astrophysics Data System (ADS)
Gu, Youfan; Timmerhaus, Klaus D.
Acoustic waves initiated by a large temperature gradient along a tube are defined as thermal acoustic oscillations (TAOs). These oscillations have been damped by introducing such sound absorbing techniques as acoustic filters, resonators, etc.. These devices serve as an acoustic sink that is used to absorb or dissipate the acoustic energy thereby eliminating or damping such oscillations. Several empirical damping techniques, such as attaching a resonator as a side branch or inserting a wire in the tube, have been developed in the past and have provided reasonable success. However, the effect of connecting tube radius, length, and resonator volume on the damping of thermal acoustic oscillations has not been evaluated quantitatively. Further, these methods have not been effective when the oscillating tube radius was relatively large. Detailed theoretical analyses of these techniques including a newly developed method for damping oscillations in a tube of relatively large radius are provided in this presentation.
Critical branching neural networks.
Kello, Christopher T
2013-01-01
It is now well-established that intrinsic variations in human neural and behavioral activity tend to exhibit scaling laws in their fluctuations and distributions. The meaning of these scaling laws is an ongoing matter of debate between isolable causes versus pervasive causes. A spiking neural network model is presented that self-tunes to critical branching and, in doing so, simulates observed scaling laws as pervasive to neural and behavioral activity. These scaling laws are related to neural and cognitive functions, in that critical branching is shown to yield spiking activity with maximal memory and encoding capacities when analyzed using reservoir computing techniques. The model is also shown to account for findings of pervasive 1/f scaling in speech and cued response behaviors that are difficult to explain by isolable causes. Issues and questions raised by the model and its results are discussed from the perspectives of physics, neuroscience, computer and information sciences, and psychological and cognitive sciences.
NASA Astrophysics Data System (ADS)
Estrada, Ernesto; Rodríguez-Velázquez, Juan A.; Randić, Milan
A graph theoretic measure of extended atomic branching is defined that accounts for the effects of all atoms in the molecule, giving higher weight to the nearest neighbors. It is based on the counting of all substructures in which an atom takes part in a molecule. We prove a theorem that permits the exact calculation of this measure based on the eigenvalues and eigenvectors of the adjacency matrix of the graph representing a molecule. The definition of this measure within the context of the Hückel molecular orbital (HMO) and its calculation for benzenoid hydrocarbons are also studied. We show that the extended atomic branching can be defined using any real symmetric matrix, as well as any Hermitian (self-adjoint) matrix, which permits its calculation in topological, geometrical, and quantum chemical contexts.
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 thermal conductance using a two-dimensional phononic crystal.
Zen, Nobuyuki; Puurtinen, Tuomas A; Isotalo, Tero J; Chaudhuri, Saumyadip; Maasilta, Ilari J
2014-03-19
Controlling thermal transport has become relevant in recent years. Traditionally, this control has been achieved by tuning the scattering of phonons by including various types of scattering centres in the material (nanoparticles, impurities, etc). Here we take another approach and demonstrate that one can also use coherent band structure effects to control phonon thermal conductance, with the help of periodically nanostructured phononic crystals. We perform the experiments at low temperatures below 1 K, which not only leads to negligible bulk phonon scattering, but also increases the wavelength of the dominant thermal phonons by more than two orders of magnitude compared to room temperature. Thus, phononic crystals with lattice constants ≥1 μm are shown to strongly reduce the thermal conduction. The observed effect is in quantitative agreement with the theoretical calculation presented, which accurately determined the ballistic thermal conductance in a phononic crystal device.
Electrochemical Energy Storage Branch
NASA Astrophysics Data System (ADS)
1985-01-01
The activities of the Electrochemical Energy Storage Branch are highlighted, including the Technology Base Research and the Exploratory Technology Development and Testing projects within the Electrochemical Energy Storage Program for the 1984 fiscal year. General Headquarters activities are presented first; and then, a summary of the Director Controlled Milestones, followed by other major accomplishments. A listing of the workshops and seminars held during the year is also included.
Axial interface optical phonon modes in a double-nanoshell system.
Kanyinda-Malu, C; Clares, F J; de la Cruz, R M
2008-07-16
Within the framework of the dielectric continuum (DC) model, we analyze the axial interface optical phonon modes in a double system of nanoshells. This system is constituted by two identical equidistant nanoshells which are embedded in an insulating medium. To illustrate our results, typical II-VI semiconductors are used as constitutive polar materials of the nanoshells. Resolution of Laplace's equation in bispherical coordinates for the potentials derived from the interface vibration modes is made. By imposing the usual electrostatic boundary conditions at the surfaces of the two-nanoshell system, recursion relations for the coefficients appearing in the potentials are obtained, which entails infinite matrices. The problem of deriving the interface frequencies is reduced to the eigenvalue problem on infinite matrices. A truncating method for these matrices is used to obtain the interface phonon branches. Dependences of the interface frequencies on the ratio of inter-nanoshell separation to core size are obtained for different systems with several values of nanoshell interdistance. Effects due to the change of shell and embedding materials are also investigated in interface phonon modes.
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
NASA Astrophysics Data System (ADS)
Hu, JiaGuang; Xu, Wen
2014-06-01
We present a detailed theoretical study on the acoustic band structure of two-dimensional (2D) phononic crystal. The 2D phononic crystal with parallelogram lattice structure is considered to be formed by rigid solid rods embedded in air. For the circular rods, some of the extrema of the acoustic bands appear in the usual high-symmetry points and, in contrast, we find that some of them are located in other specific lines. For the case of elliptic rods, our results indicate that it is necessary to study the whole first Brillouin zone to obtain rightly the band structure and corresponding band gaps. Furthermore, we evaluate the first and second band gaps using the plane wave expansion method and find that these gaps can be tuned by adjusting the side lengths ratio R, inclined angle θ and filling fraction F of the parallelogram lattice with circular rods. The results show that the largest value of the first band gap appears at θ=90° and F=0.7854. In contrast, the largest value of the second band gap is at θ=60° and F=0.9068. Our results indicate that the improvement of matching degree between scatterers and lattice pattern, rather than the reduction of structural symmetry, is mainly responsible for the enhancement of the band gaps in the 2D phononic crystal.