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.
Phonon Engineering in Isotopically Disordered Silicon Nanowires.
Mukherjee, S; Givan, U; Senz, S; Bergeron, A; Francoeur, S; de la Mata, M; Arbiol, J; Sekiguchi, T; Itoh, K M; Isheim, D; Seidman, D N; Moutanabbir, O
2015-06-10
The introduction of stable isotopes in the fabrication of semiconductor nanowires provides an additional degree of freedom to manipulate their basic properties, design an entirely new class of devices, and highlight subtle but important nanoscale and quantum phenomena. With this perspective, we report on phonon engineering in metal-catalyzed silicon nanowires with tailor-made isotopic compositions grown using isotopically enriched silane precursors (28)SiH4, (29)SiH4, and (30)SiH4 with purity better than 99.9%. More specifically, isotopically mixed nanowires (28)Si(x)(30)Si(1-x) with a composition close to the highest mass disorder (x ∼ 0.5) were investigated. The effect of mass disorder on the phonon behavior was elucidated and compared to that in isotopically pure (29)Si nanowires having a similar reduced mass. We found that the disorder-induced enhancement in phonon scattering in isotopically mixed nanowires is unexpectedly much more significant than in bulk crystals of close isotopic compositions. This effect is explained by a nonuniform distribution of (28)Si and (30)Si isotopes in the grown isotopically mixed nanowires with local compositions ranging from x = ∼0.25 to 0.70. Moreover, we also observed that upon heating, phonons in (28)Si(x)(30)Si(1-x) nanowires behave remarkably differently from those in (29)Si nanowires suggesting a reduced thermal conductivity induced by mass disorder. Using Raman nanothermometry, we found that the thermal conductivity of isotopically mixed (28)Si(x)(30)Si(1-x) nanowires is ∼30% lower than that of isotopically pure (29)Si nanowires in agreement with theoretical predictions.
Research Update: Phonon engineering of nanocrystalline silicon thermoelectrics
NASA Astrophysics Data System (ADS)
Shiomi, Junichiro
2016-10-01
Nanocrystalline silicon thermoelectrics can be a solution to improve the cost-effectiveness of thermoelectric technology from both material and integration viewpoints. While their figure-of-merit is still developing, recent advances in theoretical/numerical calculations, property measurements, and structural synthesis/fabrication have opened up possibilities to develop the materials based on fundamental physics of phonon transport. Here, this is demonstrated by reviewing a series of works on nanocrystalline silicon materials using calculations of multiscale phonon transport, measurements of interfacial heat conduction, and synthesis from nanoparticles. Integration of these approaches allows us to engineer phonon transport to improve the thermoelectric performance by introducing local silicon-oxide structures.
Electron-phonon interaction and scattering in Si and Ge: Implications for phonon engineering
Tandon, Nandan; Albrecht, J. D.; Ram-Mohan, L. R.
2015-07-28
We report ab-initio results for electron-phonon (e-ph) coupling and display the existence of a large variation in the coupling parameter as a function of electron and phonon dispersion. This variation is observed for all phonon modes in Si and Ge, and we show this for representative cases where the initial electron states are at the band gap edges. Using these e-ph matrix elements, which include all possible phonon modes and electron bands within a relevant energy range, we evaluate the imaginary part of the electron self-energy in order to obtain the associated scattering rates. The temperature dependence is seen through calculations of the scattering rates at 0 K and 300 K. The results provide a basis for understanding the impacts of phonon scattering vs. orientation and geometry in the design of devices, and in analysis of transport phenomena. This provides an additional tool for engineering the transfer of energy from carriers to the lattice.
Structural engineering of three-dimensional phononic crystals
NASA Astrophysics Data System (ADS)
Delpero, Tommaso; Schoenwald, Stefan; Zemp, Armin; Bergamini, Andrea
2016-02-01
Artificially-structured materials are attracting the research interest of a growing community of scientists for the possibility to develop novel materials with advantageous properties that arise from the ability to tailor the propagation of elastic waves, and thus energy, through them. In this work, we propose a three-dimensional phononic crystal whose unit cell has been engineered to obtain a strong wave-attenuation band in the middle of the acoustic frequency range. The combination of its acoustic properties with the dimensions of the unit cell and its static mechanical properties makes it an interesting material for possibly several applications in civil and mechanical engineering, for instance as the core of an acoustically insulating sandwich panel. A sample of this crystal has been manufactured and experimentally tested with respect to its acoustic transmissibility. The performance of the phononic crystal core is remarkable both in terms of amplitude reduction in the transmissibility and width of the attenuation band. A parametric study has been finally conducted on selected geometrical parameters of the unit cell and on their effect on the macroscopic properties of the crystal. This work represents an application-oriented example of how the macroscopic properties of an artificially-structured material can be designed, according to specific needs, by a conventional engineering of its unit cell.
Grain-boundary layering transitions and phonon engineering
NASA Astrophysics Data System (ADS)
Rickman, J. M.; Harmer, M. P.; Chan, H. M.
2016-09-01
We employ semi-grand canonical Monte Carlo simulation to investigate layering transitions at grain boundaries in a prototypical binary alloy. We demonstrate the existence of such transitions among various interfacial states and examine the role of elastic fields in dictating state equilibria. The results of these studies are summarized in the form of diagrams that highlight interfacial state coexistence in this system. Finally, we examine the impact of layering transitions on the phononic properties of the system, as given by the specific heat and, by extension, the thermal conductivity. Thus, it is suggested that by inducing interfacial layering transitions via changes in temperature or pressure, one can thereby engineer thermodynamic and transport properties in materials.
Engineering the hypersonic phononic band gap of hybrid Bragg stacks.
Schneider, Dirk; Liaqat, Faroha; El Boudouti, El Houssaine; El Hassouani, Youssef; Djafari-Rouhani, Bahram; Tremel, Wolfgang; Butt, Hans-Jürgen; Fytas, George
2012-06-13
We report on the full control of phononic band diagrams for periodic stacks of alternating layers of poly(methyl methacrylate) and porous silica combining Brillouin light scattering spectroscopy and theoretical calculations. These structures exhibit large and robust on-axis band gaps determined by the longitudinal sound velocities, densities, and spacing ratio. A facile tuning of the gap width is realized at oblique incidence utilizing the vector nature of the elastic wave propagation. Off-axis propagation involves sagittal waves in the individual layers, allowing access to shear moduli at nanoscale. The full theoretical description discerns the most important features of the hypersonic one-dimensional crystals forward to a detailed understanding, a precondition to engineer dispersion relations in such structures.
Engineering thermal conductance using a two-dimensional phononic crystal
Zen, Nobuyuki; Puurtinen, Tuomas A.; Isotalo, Tero J.; Chaudhuri, Saumyadip; Maasilta, Ilari J.
2014-01-01
Controlling thermal transport has become relevant in recent years. Traditionally, this control has been achieved by tuning the scattering of phonons by including various types of scattering centres in the material (nanoparticles, impurities, etc). Here we take another approach and demonstrate that one can also use coherent band structure effects to control phonon thermal conductance, with the help of periodically nanostructured phononic crystals. We perform the experiments at low temperatures below 1 K, which not only leads to negligible bulk phonon scattering, but also increases the wavelength of the dominant thermal phonons by more than two orders of magnitude compared to room temperature. Thus, phononic crystals with lattice constants ≥1 μm are shown to strongly reduce the thermal conduction. The observed effect is in quantitative agreement with the theoretical calculation presented, which accurately determined the ballistic thermal conductance in a phononic crystal device. PMID:24647049
Engineering 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.
Sub-band adaptive noise cancelling
NASA Astrophysics Data System (ADS)
Ryan, James G.; Goubran, Rafik A.
The technique of using sub-band adaptive filters for acoustic noise suppression is examined. Simulation results are presented for experimental systems trained with white noise and colored noise for both decimated and non-decimated sub-band signals. To illustrate the usefulness of the sub-band approach for a real application, the full band and sub-band noise cancellers were applied to car data. Estimates of the input and output power spectra for the different cases are presented. It is shown that the full-band noise canceller is unable to attenuate the noise in the higher frequencies due to the low input signal power in this region. The sub-band decomposition of the input signal can significantly increase the rate as compared to a full-band implementation under certain conditions. This increase in convergence speed is manifested as an increased noise attenuation in those regions of the input spectrum with relatively low amounts of power. In addition, the sub-band processing is seen to eliminate the noise enhancement phenomenon found in acoustic noise cancellers for mobile telephony.
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
Controlling thermal and electrical properties of graphene by strain-engineering its flexural phonons
NASA Astrophysics Data System (ADS)
Conley, Hiram; Nicholl, Ryan; Bolotin, Kirill
2014-03-01
We explore the effects of flexural phonons on the thermal and electrical properties of graphene. To control the amplitude of flexural phonons, we developed a technique to engineer uniform mechanical strain between 0 and 1% in suspended graphene. We determine the level of strain, thermal conductivity and carrier mobility of graphene through a combination of mechanical resonance and electrical transport measurements. Depending on strain, we find significant changes in the thermal expansion coefficient, thermal conductivity, and carrier mobility of suspended graphene. These changes are consistent with the expected contribution of flexural phonons.
25th Anniversary Article: Ordered Polymer Structures for the Engineering of Photons and Phonons
Lee, Jae-Hwang; Koh, Cheong Yang; Singer, Jonathan P; Jeon, Seog-Jin; Maldovan, Martin; Stein, Ori; Thomas, Edwin L
2014-01-01
The engineering of optical and acoustic material functionalities via construction of ordered local and global architectures on various length scales commensurate with and well below the characteristic length scales of photons and phonons in the material is an indispensable and powerful means to develop novel materials. In the current mature status of photonics, polymers hold a pivotal role in various application areas such as light-emission, sensing, energy, and displays, with exclusive advantages despite their relatively low dielectric constants. Moreover, in the nascent field of phononics, polymers are expected to be a superior material platform due to the ability for readily fabricated complex polymer structures possessing a wide range of mechanical behaviors, complete phononic bandgaps, and resonant architectures. In this review, polymer-centric photonic and phononic crystals and metamaterials are highlighted, and basic concepts, fabrication techniques, selected functional polymers, applications, and emerging ideas are introduced. PMID:24338738
Nanoscale interface engineering in ZnO twin nanorods for proposed phonon tunnel devices.
Singh, Avanendra; Senapati, Kartik; Satpati, Biswarup; Kumar, Mohit; Sahoo, Pratap K
2015-02-14
Zinc oxide twin nanorods, with two identical crystalline sections connected by an amorphous layer, were reproducibly grown using a simple one-step hydrothermal technique. The thickness of the amorphous layer between the crystalline segments was tunable with growth parameters, as confirmed by high resolution transmission electron microscopy. The photoluminescence spectra of these twin nanorods exhibit strong near band edge emission in the UV range, with convoluted phonon sidebands. De-convolution analyses of these spectra showed that the amorphous interlayers act as effective phonon barriers beyond a certain thickness. Such oriented grown individual crystalline-amorphous-crystalline structures may be a suitable test system for fundamental studies of phonon tunneling in the nanostructure. While physical vapor deposition techniques are seriously constrained in realizing crystalline-amorphous-crystalline structures, our results show the viability of engineering embedded interfaces via chemical routes. PMID:25572135
Specific heat of twisted bilayer graphene: Engineering phonons by atomic plane rotations
Nika, Denis L.; Cocemasov, Alexandr I.; Balandin, Alexander A.
2014-07-21
We have studied the phonon specific heat in single-layer, bilayer, and twisted bilayer graphene. The calculations were performed using the Born-von Karman model of lattice dynamics for intralayer atomic interactions and spherically symmetric interatomic potential for interlayer interactions. We found that at temperature T < 15 K, specific heat varies with temperature as T{sup n}, where n = 1 for graphene, n = 1.6 for bilayer graphene, and n = 1.3 for the twisted bilayer graphene. The phonon specific heat reveals an intriguing dependence on the twist angle in bilayer graphene, which is particularly pronounced at low temperature. The results suggest a possibility of phonon engineering of thermal properties of layered materials by twisting the atomic planes.
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.
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.
Sub-band/transform compression of video sequences
NASA Technical Reports Server (NTRS)
Sauer, Ken; Bauer, Peter
1992-01-01
The progress on compression of video sequences is discussed. The overall goal of the research was the development of data compression algorithms for high-definition television (HDTV) sequences, but most of our research is general enough to be applicable to much more general problems. We have concentrated on coding algorithms based on both sub-band and transform approaches. Two very fundamental issues arise in designing a sub-band coder. First, the form of the signal decomposition must be chosen to yield band-pass images with characteristics favorable to efficient coding. A second basic consideration, whether coding is to be done in two or three dimensions, is the form of the coders to be applied to each sub-band. Computational simplicity is of essence. We review the first portion of the year, during which we improved and extended some of the previous grant period's results. The pyramid nonrectangular sub-band coder limited to intra-frame application is discussed. Perhaps the most critical component of the sub-band structure is the design of bandsplitting filters. We apply very simple recursive filters, which operate at alternating levels on rectangularly sampled, and quincunx sampled images. We will also cover the techniques we have studied for the coding of the resulting bandpass signals. We discuss adaptive three-dimensional coding which takes advantage of the detection algorithm developed last year. To this point, all the work on this project has been done without the benefit of motion compensation (MC). Motion compensation is included in many proposed codecs, but adds significant computational burden and hardware expense. We have sought to find a lower-cost alternative featuring a simple adaptation to motion in the form of the codec. In sequences of high spatial detail and zooming or panning, it appears that MC will likely be necessary for the proposed quality and bit rates.
Engineering a squeezed phonon reservoir with a bichromatic driving of a quantum dot
NASA Astrophysics Data System (ADS)
Gao, Bo; Li, Gao-xiang; Ficek, Zbigniew
2016-09-01
We demonstrate how an acoustic phonon bath when coupled to a quantum dot with the help of a bichromatic laser field may effectively form a quantum squeezed reservoir. This approach allows one to achieve an arbitrary degree of squeezing of the effective reservoir and it incorporates the properties of the reservoir into two parameters, which can be controlled by varying the ratio of the Rabi frequencies of the bichromatic field. It is found that for unequal Rabi frequencies, the effective reservoir may appear as a quantum squeezed field of ordinary or inverted harmonic oscillators. When the Rabi frequencies are equal the effective reservoir appears as a perfectly squeezed field in which the decay of one of the polarization quadratures of the quantum dot dipole moment is inhibited. The decay of the quantum dot to a stationary state which depends on the initial coherence is predicted. This unusual result is shown to be a consequence of a quantum-nondemolition-type coupling of the quantum dot to the engineered squeezed reservoir. The effect of the initial coherence on the steady-state dressed-state population distribution and the fluorescence spectrum is discussed in detail. The complete polarization of the dressed state population and asymmetric spectra composed of only a single Rabi sideband peak are obtained under strictly resonant excitation.
NASA Technical Reports Server (NTRS)
Rippert, Edward D.; Ketterson, John B.; Chen, Jun; Song, Shenian; Lomatch, Susanne; Maglic, Stevan R.; Thomas, Christopher; Cheida, M. A.; Ulmer, Melville P.
1992-01-01
An engineered structure is proposed that can alleviate quasi-particle recombination losses via the existence of a phononic band gap that overlaps the 2-Delta energy of phonons produced during recombination of quasi-particles. Attention is given to a 1D Kronig-Penny model for phonons normally incident to the layers of a multilayered superconducting tunnel junction as an idealized example. A device with a high density of Bragg resonances is identified as desirable; both Nb/Si and NbN/SiN superlattices have been produced, with the latter having generally superior performance.
Phonon manipulation with phononic crystals.
Kim Bongsang; Hopkins, Patrick Edward; Leseman, Zayd C.; Goettler, Drew F.; Su, Mehmet F.; El-Kady, Ihab Fathy; Reinke, Charles M.; Olsson, Roy H., III
2012-01-01
In this work, we demonstrated engineered modification of propagation of thermal phonons, i.e. at THz frequencies, using phononic crystals. This work combined theoretical work at Sandia National Laboratories, the University of New Mexico, the University of Colorado Boulder, and Carnegie Mellon University; the MESA fabrication facilities at Sandia; and the microfabrication facilities at UNM to produce world-leading control of phonon propagation in silicon at frequencies up to 3 THz. These efforts culminated in a dramatic reduction in the thermal conductivity of silicon using phononic crystals by a factor of almost 30 as compared with the bulk value, and about 6 as compared with an unpatterned slab of the same thickness. This work represents a revolutionary advance in the engineering of thermoelectric materials for optimal, high-ZT performance. We have demonstrated the significant reduction of the thermal conductivity of silicon using phononic crystal structuring using MEMS-compatible fabrication techniques and in a planar platform that is amenable to integration with typical microelectronic systems. The measured reduction in thermal conductivity as compared to bulk silicon was about a factor of 20 in the cross-plane direction [26], and a factor of 6 in the in-plane direction. Since the electrical conductivity was only reduced by a corresponding factor of about 3 due to the removal of conductive material (i.e., porosity), and the Seebeck coefficient should remain constant as an intrinsic material property, this corresponds to an effective enhancement in ZT by a factor of 2. Given the number of papers in literature devoted to only a small, incremental change in ZT, the ability to boost the ZT of a material by a factor of 2 simply by reducing thermal conductivity is groundbreaking. The results in this work were obtained using silicon, a material that has benefitted from enormous interest in the microelectronics industry and that has a fairly large thermoelectric power
A Multiple-Channel Sub-Band Transient Detection System
David A. Smith
1998-11-01
We have developed a unique multiple-channel sub-band transient detection system to record transient electromagnetic signals in carrier-dominated radio environments; the system has been used to make unique observations of weak, transient HF signals. The detection system has made these observations possible through improved sensitivity compared to conventional broadband transient detection systems; the sensitivity improvement is estimated to be at least 20 dB. The increase in sensitivity has been achieved through subdivision of the band of interest (an 18 MHz tunable bandwidth) into eight sub-band independent detection channels, each with a 400 kHz bandwidth and its own criteria. The system generates a system trigger signal when a predetermined number of channels (typically five) trigger within a predetermined window of time (typically 100 ~s). Events are recorded with a broadband data acquisition system sampling at 50 or 100 Msample/s, so despite the fact that the detection system operates on portions of the signal confined to narrow bands, data acquisition is broadband. Between May and September of 1994, the system was used to detect and record over six thousand transient events in the frequency band from 3 to 30 MHz. Approximately 500 of the events have been characterized as paired bursts of radio noise with individual durations of 2 to 10 ps and separations between the bursts of 5 to 160 ps. The paired transients are typically 5 to 40 dB brighter than the background electromagnetic spectrum between carrier signals. We have termed these events SubIonospheric Pulse Pairs (SIPPS) and presently have no explanation as to their source. Our observations of SIPPS resemble observations of TransIonospheric Pulse Pairs (TIPPs) recorded by the Blackboard instrument on the ALEXIS satellite; the source of TIPP events is also unknown. Most of the recorded SIPP events do not exhibit frequency dispersion, implying propagation along a line-of-sight (groundwave) path; but seven of
Streyer, W.; Law, S.; Rosenberg, A.; Wasserman, D.; Roberts, C.; Podolskiy, V. A.; Hoffman, A. J.
2014-03-31
We demonstrate excitation of surface phonon polaritons on patterned gallium phosphide surfaces. Control over the light-polariton coupling frequencies is demonstrated by changing the pattern periodicity and used to experimentally determine the gallium phosphide surface phonon polariton dispersion curve. Selective emission via out-coupling of thermally excited surface phonon polaritons is experimentally demonstrated. Samples are characterized experimentally by Fourier transform infrared reflection and emission spectroscopy, and modeled using finite element techniques and rigorous coupled wave analysis. The use of phonon resonances for control of emissivity and excitation of bound surface waves offers a potential tool for the exploration of long-wavelength Reststrahlen band frequencies.
Phonon Engineering of ZnO nanowires with controlled chemical doping
NASA Astrophysics Data System (ADS)
Bohorquez-Ballen, Jaime; Jayasekera, Thushari
2013-03-01
Using the first principles density functional theory (DFT) calculations, we have investigated electronic and dynamical properties of ZnO nanowires in [001] direction with different diameters in the presence of impurities such as Mg, Al, and Ga. As the impurity concentration is varied, electrical and thermal conductivities of nanowires change. In this way, nanowires can be engineered to reduce the thermal transport, such that their thermoelectric properties can be enhanced.
Sharma, A.; Dhar, S. Singh, B. P.; Nayak, C.; Bhattacharyya, D.; Jha, S. N.
2013-12-07
A compressive hydrostatic strain has been found to develop in the ZnO lattice as a result of accumulation of Tb ions on the surface of the nanoparticles for Tb mole-fraction less than 0.04. This hydrostatic strain can be controlled up to ≈14 GPa by varying the Tb mole-fraction. Here, we have utilized this novel technique of surface strain engineering through Tb doping for introducing hydrostatic compressive strain in the lattice to study the pressure dependent electronic and vibrational properties of ZnO nanoparticles. Our study reveals that when subjected to pressure, nanoparticles of ZnO behave quite differently than bulk in many aspects. Unlike bulk ZnO, which is reported to go through a wurtzite to rock-salt structural phase transition at ≈8 GPa, ZnO nanoparticles do not show such transition and remain in wurtzite phase even at 14 GPa of pressure. Furthermore, the Grüneisen parameters for the optical phonon modes are found to be order of magnitude smaller in ZnO nanoparticles as compared to bulk. Our study also suggests an increase of the dielectric constant with pressure, which is opposite to what has been reported for bulk ZnO. Interestingly, it has also been found that the exciton-phonon interaction depends strongly upon pressure in this system. The exciton-phonon coupling has been found to decrease as pressure increases. A variational technique has been adopted to theoretically calculate the exciton-LO phonon coupling coefficient in ZnO nanoparticles as a function of pressure, which shows a good agreement with the experimental results. These findings imply that surface engineering of ZnO nanoparticles with Tb could indeed be an efficient tool to enhance and control the optical performance of this material.
Giant phonon anomaly associated with superconducting fluctuations in the pseudogap phase of cuprates
Liu, Ye-Hua; Konik, Robert M.; Rice, T. M.; Zhang, Fu-Chun
2016-01-01
The pseudogap in underdoped cuprates leads to significant changes in the electronic structure, and was later found to be accompanied by anomalous fluctuations of superconductivity and certain lattice phonons. Here we propose that the Fermi surface breakup due to the pseudogap, leads to a breakup of the pairing order into two weakly coupled sub-band amplitudes, and a concomitant low energy Leggett mode due to phase fluctuations between them. This increases the temperature range of superconducting fluctuations containing an overdamped Leggett mode. In this range inter-sub-band phonons show strong damping due to resonant scattering into an intermediate state with a pair of overdamped Leggett modes. In the ordered state, the Leggett mode develops a finite energy, changing the anomalous phonon damping into an anomaly in the dispersion. This proposal explains the intrinsic connection between the anomalous pseudogap phase, enhanced superconducting fluctuations and giant anomalies in the phonon spectra. PMID:26785835
Giant phonon anomaly associated with superconducting fluctuations in the pseudogap phase of cuprates
Liu, Ye-Hua; Konik, Robert M.; Rice, T. M.; Zhang, Fu-Chun
2016-01-20
The pseudogap in underdoped cuprates leads to significant changes in the electronic structure, and was later found to be accompanied by anomalous fluctuations of superconductivity and certain lattice phonons. Here we propose that the Fermi surface breakup due to the pseudogap, leads to a breakup of the pairing order into two weakly coupled sub-band amplitudes, and a concomitant low energy Leggett mode due to phase fluctuations between them. This increases the temperature range of superconducting fluctuations containing an overdamped Leggett mode. In this range inter-sub-band phonons show strong damping due to resonant scattering into an intermediate state with a pairmore » of overdamped Leggett modes. In the ordered state, the Leggett mode develops a finite energy, changing the anomalous phonon damping into an anomaly in the dispersion. Finally, this proposal explains the intrinsic connection between the anomalous pseudogap phase, enhanced superconducting fluctuations and giant anomalies in the phonon spectra.« less
Jahidin, A H; Megat Ali, M S A; Taib, M N; Tahir, N Md; Yassin, I M; Lias, S
2014-04-01
This paper elaborates on the novel intelligence assessment method using the brainwave sub-band power ratio features. The study focuses only on the left hemisphere brainwave in its relaxed state. Distinct intelligence quotient groups have been established earlier from the score of the Raven Progressive Matrices. Sub-band power ratios are calculated from energy spectral density of theta, alpha and beta frequency bands. Synthetic data have been generated to increase dataset from 50 to 120. The features are used as input to the artificial neural network. Subsequently, the brain behaviour model has been developed using an artificial neural network that is trained with optimized learning rate, momentum constant and hidden nodes. Findings indicate that the distinct intelligence quotient groups can be classified from the brainwave sub-band power ratios with 100% training and 88.89% testing accuracies.
Tian, Ruoming; Kearley, Gordon J; Yu, Dehong; Ling, Chris D; Pham, Anh; Embs, Jan P; Shoko, Elvis; Li, Sean
2016-01-01
Phonons in condensed matter materials transmit energy through atomic lattices as coherent vibrational waves. Like electronic and photonic properties, an improved understanding of phononic properties is essential for the development of functional materials, including thermoelectric materials. Recently, an Einstein rattling mode was found in thermoelectric material Na0.8CoO2, due to the large displacement of Na between the [CoO2] layers. In this work, we have realized a different type of rattler in another thermoelectric material Ca3Co4O9 by chemical doping, which possesses the same [CoO2] layer as Na0.8CoO2. It remarkably suppressed the thermal conductivity while enhancing its electrical conductivity. This new type of rattler was investigated by inelastic neutron scattering experiments in conjunction with ab-initio molecular dynamics simulations. We found that the large mass of dopant rather than the large displacement is responsible for such rattling in present study, which is fundamentally different from skutterudites, clathrates as well as Na analogue. We have also tentatively studied the phonon band structure of this material by DFT lattice dynamics simulation, showing the relative contribution to phonons in the distinct layers of Ca3Co4O9. PMID:27456817
NASA Astrophysics Data System (ADS)
Tian, Ruoming; Kearley, Gordon J.; Yu, Dehong; Ling, Chris D.; Pham, Anh; Embs, Jan P.; Shoko, Elvis; Li, Sean
2016-07-01
Phonons in condensed matter materials transmit energy through atomic lattices as coherent vibrational waves. Like electronic and photonic properties, an improved understanding of phononic properties is essential for the development of functional materials, including thermoelectric materials. Recently, an Einstein rattling mode was found in thermoelectric material Na0.8CoO2, due to the large displacement of Na between the [CoO2] layers. In this work, we have realized a different type of rattler in another thermoelectric material Ca3Co4O9 by chemical doping, which possesses the same [CoO2] layer as Na0.8CoO2. It remarkably suppressed the thermal conductivity while enhancing its electrical conductivity. This new type of rattler was investigated by inelastic neutron scattering experiments in conjunction with ab-initio molecular dynamics simulations. We found that the large mass of dopant rather than the large displacement is responsible for such rattling in present study, which is fundamentally different from skutterudites, clathrates as well as Na analogue. We have also tentatively studied the phonon band structure of this material by DFT lattice dynamics simulation, showing the relative contribution to phonons in the distinct layers of Ca3Co4O9.
Tian, Ruoming; Kearley, Gordon J; Yu, Dehong; Ling, Chris D; Pham, Anh; Embs, Jan P; Shoko, Elvis; Li, Sean
2016-01-01
Phonons in condensed matter materials transmit energy through atomic lattices as coherent vibrational waves. Like electronic and photonic properties, an improved understanding of phononic properties is essential for the development of functional materials, including thermoelectric materials. Recently, an Einstein rattling mode was found in thermoelectric material Na0.8CoO2, due to the large displacement of Na between the [CoO2] layers. In this work, we have realized a different type of rattler in another thermoelectric material Ca3Co4O9 by chemical doping, which possesses the same [CoO2] layer as Na0.8CoO2. It remarkably suppressed the thermal conductivity while enhancing its electrical conductivity. This new type of rattler was investigated by inelastic neutron scattering experiments in conjunction with ab-initio molecular dynamics simulations. We found that the large mass of dopant rather than the large displacement is responsible for such rattling in present study, which is fundamentally different from skutterudites, clathrates as well as Na analogue. We have also tentatively studied the phonon band structure of this material by DFT lattice dynamics simulation, showing the relative contribution to phonons in the distinct layers of Ca3Co4O9.
Tian, Ruoming; Kearley, Gordon J.; Yu, Dehong; Ling, Chris D.; Pham, Anh; Embs, Jan P.; Shoko, Elvis; Li, Sean
2016-01-01
Phonons in condensed matter materials transmit energy through atomic lattices as coherent vibrational waves. Like electronic and photonic properties, an improved understanding of phononic properties is essential for the development of functional materials, including thermoelectric materials. Recently, an Einstein rattling mode was found in thermoelectric material Na0.8CoO2, due to the large displacement of Na between the [CoO2] layers. In this work, we have realized a different type of rattler in another thermoelectric material Ca3Co4O9 by chemical doping, which possesses the same [CoO2] layer as Na0.8CoO2. It remarkably suppressed the thermal conductivity while enhancing its electrical conductivity. This new type of rattler was investigated by inelastic neutron scattering experiments in conjunction with ab-initio molecular dynamics simulations. We found that the large mass of dopant rather than the large displacement is responsible for such rattling in present study, which is fundamentally different from skutterudites, clathrates as well as Na analogue. We have also tentatively studied the phonon band structure of this material by DFT lattice dynamics simulation, showing the relative contribution to phonons in the distinct layers of Ca3Co4O9. PMID:27456817
Electrical observation of sub-band formation in SnO2 nanobelts.
Viana, E R; González, J C; Ribeiro, G M; de Oliveira, A G
2013-07-21
The electrical observation of energy sub-band formation in the electronic structure, that gives rise to the phenomenon of quantized transport is reported in tin oxide (SnO2) nanobelt back-gate field-effect transistors, at low temperatures. Sub-band formation was observed as current oscillations in the drain current vs. gate voltage characteristics, and was analyzed considering the nanobelt as a "quantum wire" with a rectangular cross-section and hard walls. The lateral quantum confinement in the nanowires created conditions for the successive filling of the first twelve electron energy sub-bands, as the gate voltage increases. When the source-drain voltage is changed, the oscillations are not dislocated with respect to the gate voltage indicating flat-band energies, and that the observations are incompatible with the phenomena of Coulomb blockade and tunnelling oscillations. The separation of the energy sub-bands was found to be in good agreement with the measured cross-section dimensions of the nanobelt and with the threshold temperature, since for T > 60 K the oscillations tend to vanish.
NASA Astrophysics Data System (ADS)
Perrin, Bernard
2007-06-01
phonons can help tracking dark matter. These 328 presentations gave rise to 185 articles published in the present proceedings. The traditional topics of this conference series (phonons in superconductors and new materials, lattice dynamics, phonons in glasses and disordered materials, phase transitions, light, neutrons and x-ray inelastic scattering) were still very important in the scientific program but an increasing number of contributions occurred in the fields of coherent phonon generation, phonons in nanoscaled structures and nano/micro thermal phonon transport, expressing the growing involvement of condensed matter physicists in nanosciences. Areas like acoustic solitons and phononic crystals are now well established. Two noteworthy contributions have been brought in the long term quest for an operational SASER : one by Harold De Wijn's group from Utrecht in the classical ruby system and another one by Anthony Kent's group from Nottingham, who used semiconductor nanodevices to realize both an amplifying medium and a cavity. With these semiconductor devices the possibility for engineering, generation and detection of THz acoustic phonons are now imminent. By tradition, a prize is awarded every three years at the International Conference on Phonon Scattering in Condensed Matter to honour a scientist for his outstanding contributions to the field of phonon physics. For this twelfth edition, Humphrey Maris has been honoured for his numerous breakthroughs in the physics of phonons and quantum fluids. According to the words of James Wolfe 'Humphrey Maris has delighted and innovated the members of our phonon community with an entertaining style and challenging wit'. Prizes were also awarded for the best presentations during the poster sessions. The two winners were Peter van Capel from Utrecht, Netherlands, ('Simulations of acoustic soliton-induced chirping of exciton resonances') and Patrick Emery from Lille, France, ('Acoustic attenuation in silica in the 100-250 GHz
Otelaja, O. O.; Robinson, R. D.
2015-10-26
In this work, the mechanism for enhanced phonon backscattering in silicon is investigated. An understanding of phonon propagation through substrates has implications for engineering heat flow at the nanoscale, for understanding sources of decoherence in quantum systems, and for realizing efficient phonon-mediated particle detectors. In these systems, phonons that backscatter from the bottom of substrates, within the crystal or from interfaces, often contribute to the overall detector signal. We utilize a microscale phonon spectrometer, comprising superconducting tunnel junction emitters and detectors, to specifically probe phonon backscattering in silicon substrates (∼500 μm thick). By etching phonon “enhancers” or deep trenches (∼90 μm) around the detectors, we show that the backscattered signal level increases by a factor of ∼2 for two enhancers versus one enhancer. Using a geometric analysis of the phonon pathways, we show that the mechanism of the backscattered phonon enhancement is due to confinement of the ballistic phonon pathways and increased scattering off the enhancer walls. Our result is applicable to the geometric design and patterning of substrates that are employed in phonon-mediated detection devices.
47 CFR 15.323 - Specific requirements for devices operating in the 1920-1930 MHz sub-band.
Code of Federal Regulations, 2011 CFR
2011-10-01
... the 1920-1930 MHz sub-band. 15.323 Section 15.323 Telecommunication FEDERAL COMMUNICATIONS COMMISSION... requirements for devices operating in the 1920-1930 MHz sub-band. (a) Operation shall be contained within the 1920-1930 MHz band. The emission bandwidth shall be less then 2.5 MHz. The power level shall be...
47 CFR 15.323 - Specific requirements for devices operating in the 1920-1930 MHz sub-band.
Code of Federal Regulations, 2010 CFR
2010-10-01
... the 1920-1930 MHz sub-band. 15.323 Section 15.323 Telecommunication FEDERAL COMMUNICATIONS COMMISSION... requirements for devices operating in the 1920-1930 MHz sub-band. (a) Operation shall be contained within the 1920-1930 MHz band. The emission bandwidth shall be less then 2.5 MHz. The power level shall be...
NASA Astrophysics Data System (ADS)
Eichenfield, Matthew
The dynamic back-action caused by electromagnetic forces (radiation pressure) in optical and microwave cavities is of growing interest. Back-action cooling, for example, is being pursued as a means of achieving the quantum ground state of macroscopic mechanical oscillators. Work in the optical domain has revolved around millimeter- or micrometer-scale structures using the radiation pressure force. By comparison, in microwave devices, low-loss superconducting structures have been used for gradient-force-mediated coupling to a nanomechanical oscillator of picogram mass. In this thesis, two different nanometer-scale structures that use combinations of gradient and radiation pressure optical forces are described theoretically and demonstrated experimentally. These structures merge the fields of cavity optomechanics and nanomechanics into nano-optomechanical systsms (NOMS). The first device, the “Zipper” optomechanical cavity, consists of a pair of doubly-clamped nanoscale beams separated by approximately 100 nanometers, each beam having a mass of 20 picograms and being patterned with a quasi-1D photonic crystal bandgap cavity. The optical mode of the coupled system is exquisitely sensitive to differential motion of the beams, producing optomechanical coupling right at the fundamental limit set by optical diffraction. The mechanical modes of the beam probed with a background sensitivity only a factor of 4 above the standard quantum limit, and the application of less than a milliwatt of optical power is shown to increase the mechanical rigidity of the system by almost an order of magnitude. The second device focuses on just one of the doubly-clamped nanoscale beams of the Zipper. We show that, in addition to a photonic bandgap cavity, the periodic patterning of the beam also produces a phononic bandgap cavity with localized mechanical modes having frequencies in the microwave regime. We call these photonic and phononic crystal bandgap cavities optomechanical crystals
NASA Astrophysics Data System (ADS)
Cocemasov, A. I.; Nika, D. L.; Fomin, V. M.; Grimm, D.; Schmidt, O. G.
2015-07-01
The transition between nanoscale and microscale thermal transport regime at room temperature in silicon wires with constant and periodically modulated cross-section is theoretically investigated. Extrapolating the calculated thermal conductivity from the nano- to micrometer range, we find the characteristic dimensions of the wires where a crossover between nanoscale and microscale thermal transport occurs. This crossover is observed in both generic (smooth) and cross-section-modulated wires. In case of smooth silicon wires, we reveal a strong dependence of the crossing point position on the boundary roughness. For silicon wires with weak boundary roughness, the crossover occurs at cross-sections ˜60 nm × 300 nm, while for very rough boundaries it occurs at cross-sections ˜150 nm × 750 nm. In case of the periodically modulated wires, the crossover between nano- and microscale regimes occurs at typical cross-sections ˜120 nm × 120 nm of the narrow segment, and it is almost independent of boundary roughness. A strong distinction from the case of smooth wires is attributed (i) to the different trends at the nanometer scale, wherefrom the extrapolation was performed, and (ii) to the different phonon-boundary scattering due to the specific geometry. For modulated silicon wires, the influence of modulation thickness, modulation length, and cross-sectional area on the phonon thermal conductivity at the room temperature is analyzed. A possibility of thermal transport engineering in cross-section-modulated wires by resizing them is revealed in both nano- and microscale regimes. The presented results pave the way towards a better understanding of thermal transport reduction in Si nanowires with engineered diameter modulations and shed light on the crossover between nano- and microscale regimes of thermal transport.
NASA Astrophysics Data System (ADS)
Pourghasemi, Mahyar; Garg, Jivtesh
2015-03-01
There is a huge desire to increase operation speeds in modern integrated circuits as they get more compact. Heat generation in such a submicron devices is a key factor limiting their performances. As a solution, thermoelectric cooling in heterostructures can address heat dissipation issue in submicron devices. Performance of single barrier heterostructures depends strongly on several parameters including barrier height, barrier width and thermal conductivity of barrier. Superlattice structures have been known to have the lowest thermal conductivities reported for crystalline materials. Low thermal conductivity is beneficial for thermoelectric cooling as it reduces the heat flow from hot end to cold junction. Moreover the band offset between the barrier and base material can be easily tuned by changing the superlattice period. By optimizing the conduction band offset (barrier height), it is possible to control the Joule heating and also optimize the amount of heat absorbed due to Peltier cooling. We investigate the feasibility of using PbSe/PbSnSe superlattice in heterostructures using Monte Carlo simulation. The effect of different parameters such as barrier height, barrier width and superlattice thermal conductivity on thermoelectric cooling of such structures will be presented.
NASA Astrophysics Data System (ADS)
Mukherjee, Sushovan; Scarpa, Fabrizio; Gopalakrishnan, S.
2015-04-01
A novel design for the geometric configuration of honeycombs using a seamless combination of auxetic and conventional cores-elements with negative and positive Possion ratios respectively, has been presented. The proposed design has been shown to generate a superior band gap property while retaining all major advantages of a purely conventional or purely auxetic honeycomb structure. Seamless combination ensures that joint cardinality is also retained. Several configurations involving different degree of auxeticity and different proportions auxetic and conventional elements have been analyzed. It has been shown that the preferred configurations open up wide and clean band gap at a significantly lower frequency ranges compared to their pure counterparts. In view of existence of band gaps being desired feature for the phononic applications, reported results might be appealing. Use of such design may enable superior vibration control as well. Proposed configurations can be made isovolumic and iso-weight giving designers a fairer ground of applying such configurations without significantly changing size and weight criteria.
Manipulation of Phonons with Phononic Crystals
Leseman, Zayd Chad
2015-07-09
There were three research goals associated with this project. First, was to experimentally demonstrate phonon spectrum control at THz frequencies using Phononic Crystals (PnCs), i.e. demonstrate coherent phonon scattering with PnCs. Second, was to experimentally demonstrate analog PnC circuitry components at GHz frequencies. The final research goal was to gain a fundamental understanding of phonon interaction using computational methods. As a result of this work, 7 journal papers have been published, 1 patent awarded, 14 conference presentations given, 4 conference publications, and 2 poster presentations given.
Malekiha, Mahdi; Tselniker, Igor; Plant, David V
2015-12-14
We propose and experimentally demonstrate a novel sub-band multiplexed data architecture for chromatic dispersion (CD) mitigation. We have demonstrated 32 GBaud multi-sub-band (MSB) dual-polarization (DP) 16QAM transmission over 2400 km. Using this approach, the transmitted signal bandwidth is divided into multiple narrow-bandwidth sub-bands, each operating at a lower baud rate. Within each sub-band bandwidth, the CD frequency response can be approximated as a linear-phase band-pass filter, which can be considered as an analog delay that does not require compensation. Therefore, the resulting receiver digital signal processing (DSP) is simplified due to the removal of the CD compensation equalizer. In addition, this leads to efficient parallelization of DSP tasks by deploying multiple independent sub-band processors running at a lower clock rate. The proposed system reduces receiver computational complexity and offers 1 dB higher Kerr-nonlinearity tolerance and 2% extended transmission reach in comparison to the conventional single carrier systems.
Efficient snoring and breathing detection based on sub-band spectral statistics.
Sun, Xiang; Kim, Jin Young; Won, Yonggwan; Kim, Jung-Ja; Kim, Kyung-Ah
2015-01-01
Snoring, a common symptom in the general population may indicate the presence of obstructive sleep apnea (OSA). In order to detect snoring events in sleep sound recordings, a novel method was proposed in this paper. The proposed method operates by analyzing the acoustic characteristics of the snoring sounds. Based on these acoustic properties, the feature vectors are obtained using the mean and standard deviation of the sub-band spectral energy. A support vector machine is then applied to perform the frame-based classification procedure. This method was demonstrated experimentally to be effective for snoring detection. The database for detection included full-night audio recordings from four individuals who acknowledged having snoring habits. The performance of the proposed method was evaluated by classifying different events (snoring, breathing and silence) from the sleep sound recordings and comparing the classification against ground truth. The proposed algorithm was able to achieve an accuracy of 99.61% for detecting snoring events, 99.16% for breathing, and 99.55% for silence. PMID:26406075
Sub-band denoising and spline curve fitting method for hemodynamic measurement in perfusion MRI
NASA Astrophysics Data System (ADS)
Lin, Hong-Dun; Huang, Hsiao-Ling; Hsu, Yuan-Yu; Chen, Chi-Chen; Chen, Ing-Yi; Wu, Liang-Chi; Liu, Ren-Shyan; Lin, Kang-Ping
2003-05-01
In clinical research, non-invasive MR perfusion imaging is capable of investigating brain perfusion phenomenon via various hemodynamic measurements, such as cerebral blood volume (CBV), cerebral blood flow (CBF), and mean trasnit time (MTT). These hemodynamic parameters are useful in diagnosing brain disorders such as stroke, infarction and periinfarct ischemia by further semi-quantitative analysis. However, the accuracy of quantitative analysis is usually affected by poor signal-to-noise ratio image quality. In this paper, we propose a hemodynamic measurement method based upon sub-band denoising and spline curve fitting processes to improve image quality for better hemodynamic quantitative analysis results. Ten sets of perfusion MRI data and corresponding PET images were used to validate the performance. For quantitative comparison, we evaluate gray/white matter CBF ratio. As a result, the hemodynamic semi-quantitative analysis result of mean gray to white matter CBF ratio is 2.10 +/- 0.34. The evaluated ratio of brain tissues in perfusion MRI is comparable to PET technique is less than 1-% difference in average. Furthermore, the method features excellent noise reduction and boundary preserving in image processing, and short hemodynamic measurement time.
Limaye, Mukta V.; Chen, S. C.; Lee, C. Y.; Chen, L. Y.; Singh, Shashi B.; Shao, Y. C.; Wang, Y. F.; Hsieh, S. H.; Hsueh, H. C.; Chiou, J. W.; Chen, C. H.; Jang, L. Y.; Cheng, C. L.; Pong, W. F.; Hu, Y. F.
2015-01-01
The correlation between sub-band gap absorption and the chemical states and electronic and atomic structures of S-hyperdoped Si have been extensively studied, using synchrotron-based x-ray photoelectron spectroscopy (XPS), x-ray absorption near-edge spectroscopy (XANES), extended x-ray absorption fine structure (EXAFS), valence-band photoemission spectroscopy (VB-PES) and first-principles calculation. S 2p XPS spectra reveal that the S-hyperdoped Si with the greatest (~87%) sub-band gap absorption contains the highest concentration of S2− (monosulfide) species. Annealing S-hyperdoped Si reduces the sub-band gap absorptance and the concentration of S2− species, but significantly increases the concentration of larger S clusters [polysulfides (Sn2−, n > 2)]. The Si K-edge XANES spectra show that S hyperdoping in Si increases (decreased) the occupied (unoccupied) electronic density of states at/above the conduction-band-minimum. VB-PES spectra evidently reveal that the S-dopants not only form an impurity band deep within the band gap, giving rise to the sub-band gap absorption, but also cause the insulator-to-metal transition in S-hyperdoped Si samples. Based on the experimental results and the calculations by density functional theory, the chemical state of the S species and the formation of the S-dopant states in the band gap of Si are critical in determining the sub-band gap absorptance of hyperdoped Si samples. PMID:26098075
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.
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.
NASA Astrophysics Data System (ADS)
Song, Xinfang; Wang, Wenyuan; Fu, Libin
2016-09-01
Oscillating electric field is chosen to investigate the electron-positron pair production process by using a quantum kinetic theory and the effective mass model [Phys. Rev. Lett. 112, 050402 (2014)]. The particle yield exhibits a characteristic oscillatory structure which is related to the multi-photon thresholds. The true peak positions are typically slightly above the naive threshold estimate, which is defined as frequency shift. During the numerical calculations, we find the frequency shift can be affected by the system parameters under adiabatic closing the external field, it is worthwhile to study in detail. In this paper, we investigate the frequency shift and the sub-band effect in electron-positron pair production with oscillating electric field. First, a quantum kinetic theory and the effective mass are presented to obtain the frequency shift, the results are fitted very well. And we find the frequency shift and the sub-band effect can be influenced by pulse duration, photon number, and strength of the external field. The frequency shift becomes evident as increases of photon number and the external field strength. The sub-band width is relatively lower at longer pulse duration, higher photon number region, and weaker external field. The results shown in the paper are helpful for understanding multi-photon pair production process in the strong field.
Shirazinodeh, Alireza; Noubari, Hossein Ahmadi; Rabbani, Hossein; Dehnavi, Alireza Mehri
2015-01-01
Recent studies on wavelet transform and fractal modeling applied on mammograms for the detection of cancerous tissues indicate that microcalcifications and masses can be utilized for the study of the morphology and diagnosis of cancerous cases. It is shown that the use of fractal modeling, as applied to a given image, can clearly discern cancerous zones from noncancerous areas. In this paper, for fractal modeling, the original image is first segmented into appropriate fractal boxes followed by identifying the fractal dimension of each windowed section using a computationally efficient two-dimensional box-counting algorithm. Furthermore, using appropriate wavelet sub-bands and image Reconstruction based on modified wavelet coefficients, it is shown that it is possible to arrive at enhanced features for detection of cancerous zones. In this paper, we have attempted to benefit from the advantages of both fractals and wavelets by introducing a new algorithm. By using a new algorithm named F1W2, the original image is first segmented into appropriate fractal boxes, and the fractal dimension of each windowed section is extracted. Following from that, by applying a maximum level threshold on fractal dimensions matrix, the best-segmented boxes are selected. In the next step, the segmented Cancerous zones which are candidates are then decomposed by utilizing standard orthogonal wavelet transform and db2 wavelet in three different resolution levels, and after nullifying wavelet coefficients of the image at the first scale and low frequency band of the third scale, the modified reconstructed image is successfully utilized for detection of breast cancer regions by applying an appropriate threshold. For detection of cancerous zones, our simulations indicate the accuracy of 90.9% for masses and 88.99% for microcalcifications detection results using the F1W2 method. For classification of detected mictocalcification into benign and malignant cases, eight features are identified and
Liu, Yanhong; Li, La; Wang, Song; Gao, Ping; Pan, Lujun; Zhang, Jialiang; Zhou, Peng; Li, Jinhua; Weng, Zhankun
2015-02-09
In this paper, we discuss a model of sub-band in resistive switching nonvolatile memories with a structure of silver/aluminum oxide/p-type silicon (Ag/Al{sub x}O{sub y}/p-Si), in which the sub-band is formed by overlapping of wave functions of electron-occupied oxygen vacancies in Al{sub x}O{sub y} layer deposited by atomic layer deposition technology. The switching processes exhibit the characteristics of the bipolarity, discreteness, and no need of forming process, all of which are discussed deeply based on the model of sub-band. The relationships between the SET voltages and distribution of trap levels are analyzed qualitatively. The semiconductor-like behaviors of ON-state resistance affirm the sub-band transport mechanism instead of the metal filament mechanism.
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.
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.
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.
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.
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.
Annealing-induced optical and sub-band-gap absorption parameters of Sn-doped CdSe thin films
NASA Astrophysics Data System (ADS)
Kaur, Jagdish; Tripathi, S. K.
2016-01-01
Thin films of Sn-doped CdSe were prepared by thermal evaporation onto glass substrates in an argon gas atmosphere and annealed at different temperatures. Structural evaluation of the films was carried out using X-ray diffraction and their stoichiometry studied by energy-dispersive X-ray analysis. The films exhibit a preferred orientation along the hexagonal direction of CdSe. The optical transmittance of the films shows a red shift of the absorption edge with annealing. The fundamental absorption edge corresponds to a direct energy gap with a temperature coefficient of 3.34 × 10-3 eV K-1. The refractive index, optical conductivity and real and imaginary parts of the dielectric constants were found to increase after annealing. The sub-band gap absorption coefficient was evaluated using the constant photocurrent method. It varies exponentially with photon energy. The Urbach energy, the density of defect states, and the steepness of the density of localized states were evaluated from the sub-band-gap absorption.
Phononic crystal diffraction gratings
NASA Astrophysics Data System (ADS)
Moiseyenko, Rayisa P.; Herbison, Sarah; Declercq, Nico F.; Laude, Vincent
2012-02-01
When a phononic crystal is interrogated by an external source of acoustic waves, there is necessarily a phenomenon of diffraction occurring on the external enclosing surfaces. Indeed, these external surfaces are periodic and the resulting acoustic diffraction grating has a periodicity that depends on the orientation of the phononic crystal. This work presents a combined experimental and theoretical study on the diffraction of bulk ultrasonic waves on the external surfaces of a 2D phononic crystal that consists of a triangular lattice of steel rods in a water matrix. The results of transmission experiments are compared with theoretical band structures obtained with the finite-element method. Angular spectrograms (showing frequency as a function of angle) determined from diffraction experiments are then compared with finite-element simulations of diffraction occurring on the surfaces of the crystal. The experimental results show that the diffraction that occurs on its external surfaces is highly frequency-dependent and has a definite relation with the Bloch modes of the phononic crystal. In particular, a strong influence of the presence of bandgaps and deaf bands on the diffraction efficiency is found. This observation opens perspectives for the design of efficient phononic crystal diffraction gratings.
Spectral Analysis of Surface Controlled Phonon Transport in Nanophononic Metamaterials
NASA Astrophysics Data System (ADS)
Neogi, Sanghamitra; Donadio, Davide
Phonon engineering in nanostructured semiconductors has shown promises to further advance the performance of energy applications beyond the state-of-the-art limit. In nanostructured materials, phonon transport is greatly affected by the surface nanoscale character. The concept of nanophononic metamaterial (NPM) was introduced recently to affect nanoscale thermal transport with the inclusion of local surface resonators. We carried out a systematic investigation of phonon transport in locally resonant silicon-based NPMs. We used classical equilibrium molecular dynamics and a Boltzmann transport equation approach with the relaxation time approximation to investigate the nature of phononic thermal transport in nanopatterned silicon membranes with thicknesses of the order of 10 nm and below. We find the presence of local surface resonators has a significant effect on the phonon dispersion and has a direct consequence of suppression of group velocities of phonons in the NPMs. We completed the investigation by relating nanoscale resonant character (geometry and material composition) with phonon scattering, and consequently, phonon transport in the locally resonant silicon membrane NPMs This project is funded by the program FP7-ENERGY-2012-1-2STAGE under Contract Number 309150.
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.
Li, Zenghui; Xu, Bin; Yang, Jian; Song, Jianshe
2015-01-01
This paper focuses on suppressing spectral overlap for sub-band spectral estimation, with which we can greatly decrease the computational complexity of existing spectral estimation algorithms, such as nonlinear least squares spectral analysis and non-quadratic regularized sparse representation. Firstly, our study shows that the nominal ability of the high-order analysis filter to suppress spectral overlap is greatly weakened when filtering a finite-length sequence, because many meaningless zeros are used as samples in convolution operations. Next, an extrapolation-based filtering strategy is proposed to produce a series of estimates as the substitutions of the zeros and to recover the suppression ability. Meanwhile, a steady-state Kalman predictor is applied to perform a linearly-optimal extrapolation. Finally, several typical methods for spectral analysis are applied to demonstrate the effectiveness of the proposed strategy. PMID:25609038
NASA Astrophysics Data System (ADS)
Gu, Yamei; You, Shanhong
2016-07-01
With the rapid growth of data rate, the optical network is evolving from fixed-grid to flexible-grid to provide spectrum-efficient and scalable transport of 100 Gb/s services and beyond. Also, the deployment of wavelength converter in the existing network can increase the flexibility of routing and wavelength allocation (RWA) and improve blocking performance of the optical networks. In this paper, we present a methodology for computing approximate blocking probabilities of the provision of multiclass services in the flexible-grid optical networks with sub-band spectrum conversion and inverse multiplexing respectively. Numerical calculation results based on the model are compared to the simulation results for the different cases. It is shown that the calculation results match well with the simulation results for the flexible-grid optical networks at different scenarios.
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
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.
NASA Astrophysics Data System (ADS)
Sharma, T. K.; Kumar, Shailendra; Rustagi, K. C.
2002-11-01
Surface photovoltage spectroscopy studies on thick semi-insulating GaAs wafers are reported in the range 850-950 nm using the chopped light geometry. We observed some interesting sharp features in the sub-band-gap of SI-GaAs, which were reported recently [Appl. Phys. Lett. 79, 1715(2001); Rev. Sci. Instrum. 73, 1835 (2002)]. In this article, we present the dependence of these features on the chopping frequency and the source intensity. The intensity variation in the above-band-gap region and for the A peak (898 nm) in the sub-band-gap region could be fitted with single component while it is necessary to consider more than one component to fit the data for the Q peak (887 nm) in the sub-band-gap region. A model consistent with the observed features is also proposed.
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.
Chattopadhyay, P.; Karim, B.; Guha Roy, S.
2013-12-28
The sub-band gap optical absorption in chemical bath deposited cadmium sulphide thin films annealed at different temperatures has been critically analyzed with special reference to Urbach relation. It has been found that the absorption co-efficient of the material in the sub-band gap region is nearly constant up to a certain critical value of the photon energy. However, as the photon energy exceeds the critical value, the absorption coefficient increases exponentially indicating the dominance of Urbach rule. The absorption coefficients in the constant absorption region and the Urbach region have been found to be sensitive to annealing temperature. A critical examination of the temperature dependence of the absorption coefficient indicates two different kinds of optical transitions to be operative in the sub-band gap region. After a careful analyses of SEM images, energy dispersive x-ray spectra, and the dc current-voltage characteristics, we conclude that the absorption spectra in the sub-band gap domain is possibly associated with optical transition processes involving deep levels and the grain boundary states of the material.
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.
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
Effects of pre-stress and surface stress on phonon thermal conductivity of rectangular Si nanowires
NASA Astrophysics Data System (ADS)
Zhu, Linli; Ruan, Haihui
2015-04-01
This work investigates theoretically the phonon property and thermal conductivity of rectangular silicon nanowires under pre-stress and surface stress. In the framework of elasticity theory, the effects of spatial confinement are considered in the phonon dispersion relation of a stressed nanowire. The surface energy, which brings about the variation of the elastic modulus of nanowire and the influence on the phonon property, is then involved. Under a pre-stress field, the acoustoelastic effect gives rise to the change of phonon properties and thermal conductivity. Our numerical results demonstrate that the applied surface stress and pre-stress field can alter the phonon dispersion relation of a silicon nanowire significantly. The phonon energy increases if the surface stress is negative and the pre-stress is positive, and vice versa. The changes of phonon dispersion relation as well as the various phonon scattering rates lead to the variation of phonon thermal conductivity, which is the consequence of the surface stress and pre-stress fields. We further elaborate the size and temperature dependence of phonon thermal conductivity under different applied surface stresses and pre-stress fields and suggest using the strain engineering to tune the thermal performance of semiconductor nanostructures.
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.
Jamaloo, Fatemeh; Mikaeili, Mohammad
2015-01-01
Common spatial pattern (CSP) is a method commonly used to enhance the effects of event-related desynchronization and event-related synchronization present in multichannel electroencephalogram-based brain-computer interface (BCI) systems. In the present study, a novel CSP sub-band feature selection has been proposed based on the discriminative information of the features. Besides, a distinction sensitive learning vector quantization based weighting of the selected features has been considered. Finally, after the classification of the weighted features using a support vector machine classifier, the performance of the suggested method has been compared with the existing methods based on frequency band selection, on the same BCI competitions datasets. The results show that the proposed method yields superior results on “ay” subject dataset compared against existing approaches such as sub-band CSP, filter bank CSP (FBCSP), discriminative FBCSP, and sliding window discriminative CSP. PMID:26284171
High resolution infrared spectroscopy of [1.1.1]propellane: The region of the ν_{9sub}> band
Maki, Arthur; Weber, Alfons; Nibler, Joseph W.; Masiello, Tony; Blake, Thomas A.; Kirkpatrick, Robynne
2010-11-01
The region of the infrared-active band of the ν_{9} CH2 bending mode [1.1.1]propellane has been recorded at a resolution (0.0025 cm^{-1}) sufficient to distinguish individual rovibrational lines. This region includes the partially overlapping bands ν_{9} (e') = 1459 cm^{-1}, 2ν_{18} (l = 2, E') = 1430 cm^{-1}, ν_{6} + ν_{12} (E') = 1489 cm-1, and ν_{4} + ν_{15} (A_{2}") = 1518 cm^{-1}. In addition, the difference band ν_{4} - ν_{15} (A2") was observed in the far infrared near 295 cm^{-1} and analyzed to give good constants for the upper ν_{4} levels. The close proximities of the four bands in the ν_{9} region suggest that Coriolis and Fermi resonance couplings could be significant and theoretical band parameters obtained from Gaussian ab initio calculations were helpful in guiding the band analyses. The analyses of all four bands were accomplished, based on our earlier report of ground state constants determined from combination differences involving more than 4000 pairs of transitions from five fundamental and four combination bands. This paper presents the analyses and the determination of the upper state constants of all four bands in the region of the ν_{9sub> band. Complications were most evident in the 2ν18 (l = 2, E') band, which showed significant perturbations due to mixing with the nearby 2ν18 (l = 0, A1') and ν4 + ν12 (E') levels which are either infrared inactive as transitions from the ground state, or, in the latter case, too weak to observe. Finally, these complications are discussed and a comparison of all molecular constants with those available from the ab initio calculations at the anharmonic level is presented.}
Reconfigurable long-range phonon dynamics in optomechanical arrays.
Xuereb, André; Genes, Claudiu; Pupillo, Guido; Paternostro, Mauro; Dantan, Aurélien
2014-04-01
We investigate periodic optomechanical arrays as reconfigurable platforms for engineering the coupling between multiple mechanical and electromagnetic modes and for exploring many-body phonon dynamics. Exploiting structural resonances in the coupling between light fields and collective motional modes of the array, we show that tunable effective long-range interactions between mechanical modes can be achieved. This paves the way towards the implementation of controlled phononic walks and heat transfer on densely connected graphs as well as the coherent transfer of excitations between distant elements of optomechanical arrays. PMID:24745417
Wong, Joe; Krisch, M.; Farber, D.; Occelli, F.; Schwartz, A.; Chiang, T.C.; Wall, M.; Boro, C.; Xu, Ruqing
2010-11-16
Plutonium (Pu) is well known to have complex and unique physico-chemical properties. Notably, the pure metal exhibits six solid-state phase transformations with large volume expansions and contractions along the way to the liquid state: {alpha} {yields} {beta} {yields} {gamma} {yields} {delta} {yields} {delta}{prime} {yields} {var_epsilon} {yields} liquid. Unalloyed Pu melts at a relatively low temperature {approx}640 C to yield a higher density liquid than that of the solid from which it melts, (Figure 1). Detailed understanding of the properties of plutonium and plutonium-based alloys is critical for the safe handling, utilization, and long-term storage of these important, but highly toxic materials. However, both technical and and safety issues have made experimental observations extremely difficult. Phonon dispersion curves (PDCs) are key experimenta l data to the understanding of the basic properties of Pu materials such as: force constants, sound velocities, elastic constants, thermodynamics, phase stability, electron-phonon coupling, structural relaxation, etc. However, phonon dispersion curves (PDCs) in plutonium (Pu) and its alloys have defied measurement for the past few decades since the discovery of this element in 1941. This is due to a combination of the high thermal-neutron absorption cross section of plutonium and the inability to grow the large single crystals (with dimensions of a few millimeters) necessary for inelastic neutron scattering. Theoretical simulations of the Pu PDC continue to be hampered by the lack of suitable inter -atomic potentials. Thus, until recently the PDCs for Pu and its alloys have remained unknown experimentally and theoretically. The experimental limitations have recently been overcome by using a tightly focused undulator x-ray micro-beam scattered from single -grain domains in polycrystalline specimens. This experimental approach has been applied successfully to map the complete PDCs of an fcc d-Pu-Ga alloy using the
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.
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 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.
A Bond-order Theory on the Phonon Scattering by Vacancies in Two-dimensional Materials
Xie, Guofeng; Shen, Yulu; Wei, Xiaolin; Yang, Liwen; Xiao, Huaping; Zhong, Jianxin; Zhang, Gang
2014-01-01
We theoretically investigate the phonon scattering by vacancies, including the impacts of missing mass and linkages () and the variation of the force constant of bonds associated with vacancies () by the bond-order-length-strength correlation mechanism. We find that in bulk crystals, the phonon scattering rate due to change of force constant is about three orders of magnitude lower than that due to missing mass and linkages . In contrast to the negligible in bulk materials, in two-dimensional materials can be 3–10 folds larger than . Incorporating this phonon scattering mechanism to the Boltzmann transport equation derives that the thermal conductivity of vacancy defective graphene is severely reduced even for very low vacancy density. High-frequency phonon contribution to thermal conductivity reduces substantially. Our findings are helpful not only to understand the severe suppression of thermal conductivity by vacancies, but also to manipulate thermal conductivity in two-dimensional materials by phononic engineering. PMID:24866858
NASA Astrophysics Data System (ADS)
Hong, Jiawang; Li, Chen W.; May, A. F.; Bansal, D.; Chi, S.; Hong, T.; Ehlers, G.; Delaire, Olivier
The promising thermoelectric material SnSe exhibits ultra-low and strongly anisotropic thermal conductivity. By combining first-principles calculations and inelastic neutron scattering measurements, we have investigated the phonon dispersions and phonon scattering mechanisms, and probed the origin of the large anharmonicity in SnSe. We will discuss the connection between the phonon properties and the high-temperature structural phase transition, and how the electronic structure leads to large anharmonic phonon interactions in SnSe. The present results provide a microscopic picture connecting electronic structure and phonon anharmonicity in SnSe, which could help design materials with ultralow thermal conductivity. Computations were performed using the OLCF at ORNL. Modeling of neutron data was performed in CAMM, measurements were funded by the US DOE, BES, Materials Science and Engineering Division.
Modification of phonon processes in nanostructured rare-earth-ion-doped crystals
NASA Astrophysics Data System (ADS)
Lutz, Thomas; Veissier, Lucile; Thiel, Charles W.; Cone, Rufus L.; Barclay, Paul E.; Tittel, Wolfgang
2016-07-01
Nano-structuring impurity-doped crystals affects the phonon density of states and thereby modifies the atomic dynamics induced by interaction with phonons. We propose the use of nano-structured materials in the form of powders or phononic bandgap crystals to enable or improve persistent spectral hole burning and coherence for inhomogeneously broadened absorption lines in rare-earth-ion-doped crystals. This is crucial for applications such as ultra-precise radio-frequency spectrum analyzers and optical quantum memories. As an example, we discuss how phonon engineering can enable spectral hole burning in erbium-doped materials operating in the convenient telecommunication band and present simulations for density of states of nano-sized powders and phononic crystals for the case of Y2SiO5 , a widely used material in current quantum memory research.
Coherent phonon control via electron-lattice interaction in ferromagnetic Co/Pt multilayers
Kim, Chul Hoon; Shim, Je-Ho; Lee, Kyung Min; Jeong, Jong-Ryul; Kim, Dong-Hyun; Kim, Dong Eon
2016-01-01
The manipulation of coherent phonons in condensed systems has attracted fundamental interest, particularly for its applications to future devices. We demonstrate that a coherent phonon in Co/Pt nano-multilayer can be quantitatively controlled via electron-lattice coupling, specifically by changing the multilayer repeat number. To that end, systematic measurement of the time-resolved reflectivity and magneto-optical Kerr effect in Co/Pt multilayers was performed. The coherent phonon frequency was observed to be shifted with the change of the multilayer repeat number. This shift could be clearly explained based on the two-temperature model. Detailed analysis indicated that the lattice heat capacity and electron-lattice coupling strength are linearly dependent on the repeat number of the periodic multilayer structures. Accessing the control of coherent phonons using nanostructures opens a new avenue for advanced phonon-engineering applications. PMID:26928846
Malekiha, Mahdi; Tselniker, Igor; Nazarathy, Moshe; Tolmachev, Alex; Plant, David V
2015-10-01
We experimentally demonstrate a novel digital signal processing (DSP) structure for reduced guard-interval (RGI) OFDM coherent optical systems. The proposed concept is based on digitally slicing optical channel bandwidth into multiple spectrally disjoint sub-bands which are then processed in parallel. Each low bandwidth sub-band has a smaller delay-spread compared to a full-band signal. This enables compensation of both chromatic dispersion (CD) and polarization mode dispersion using a simple timing and one-tap-per-symbol frequency domain equalizer with a small cyclic prefix overhead. In terms of the DSP architecture, this allows for a highly efficient parallelization of DSP tasks performed over the received signal samples by deploying multiple processors running at a lower clock rate. It should be noted that this parallelization is performed in the frequency domain and it allows for flexible optical transceiver schemes. In addition, the resulting optical receiver is simplified due to the removal of the CD compensation equalizer compared to conventional RGI-OFDM systems. In this paper we experimentally demonstrate digital sub-banding of optical bandwidth. We test the system performance for different modulation formats (QPSK, 16QAM and 32QAM) over various transmission distances and optical launch powers using a 1.5% CP overhead in all scenarios. We also compare the proposed RGI-OFDM architecture performance against common single carrier modulation formats. At the same total data rate and signal bandwidth both systems have similar performance and transmission reach whereas the proposed method allows for a significant reduction of computational complexity due to removal of CD pre/post compensation equalizer.
Malekiha, Mahdi; Tselniker, Igor; Nazarathy, Moshe; Tolmachev, Alex; Plant, David V
2015-10-01
We experimentally demonstrate a novel digital signal processing (DSP) structure for reduced guard-interval (RGI) OFDM coherent optical systems. The proposed concept is based on digitally slicing optical channel bandwidth into multiple spectrally disjoint sub-bands which are then processed in parallel. Each low bandwidth sub-band has a smaller delay-spread compared to a full-band signal. This enables compensation of both chromatic dispersion (CD) and polarization mode dispersion using a simple timing and one-tap-per-symbol frequency domain equalizer with a small cyclic prefix overhead. In terms of the DSP architecture, this allows for a highly efficient parallelization of DSP tasks performed over the received signal samples by deploying multiple processors running at a lower clock rate. It should be noted that this parallelization is performed in the frequency domain and it allows for flexible optical transceiver schemes. In addition, the resulting optical receiver is simplified due to the removal of the CD compensation equalizer compared to conventional RGI-OFDM systems. In this paper we experimentally demonstrate digital sub-banding of optical bandwidth. We test the system performance for different modulation formats (QPSK, 16QAM and 32QAM) over various transmission distances and optical launch powers using a 1.5% CP overhead in all scenarios. We also compare the proposed RGI-OFDM architecture performance against common single carrier modulation formats. At the same total data rate and signal bandwidth both systems have similar performance and transmission reach whereas the proposed method allows for a significant reduction of computational complexity due to removal of CD pre/post compensation equalizer. PMID:26480077
NASA Astrophysics Data System (ADS)
Yater, J. E.; Shaw, J. L.; Pate, B. B.; Feygelson, T. I.
2016-02-01
Secondary-electron-emission (SEE) current measured from high-purity, single-crystal (100) chemical-vapor-deposited diamond is found to increase when sub-band gap (3.06 eV) photons are incident on the hydrogenated surface. Although the light does not produce photoemission directly, the SEE current increases by more than a factor of 2 before saturating with increasing laser power. In energy distribution curves (EDCs), the emission peak shows a corresponding increase in intensity with increasing laser power. However, the emission-onset energy in the EDCs remains constant, indicating that the bands are pinned at the surface. On the other hand, changes are observed on the high-energy side of the distribution as the laser power increases, with a well-defined shoulder becoming more pronounced. From an analysis of this feature in the EDCs, it is deduced that upward band bending is present in the near-surface region during the SEE measurements and this band bending suppresses the SEE yield. However, sub-band gap photon illumination reduces the band bending and thereby increases the SEE current. Because the bands are pinned at the surface, we conclude that the changes in the band levels occur below the surface in the electron transport region. Sample heating produces similar effects as observed with sub-band gap photon illumination, namely, an increase in SEE current and a reduction in band bending. However, the upward band bending is not fully removed by either increasing laser power or temperature, and a minimum band bending of ˜0.8 eV is established in both cases. The sub-band gap photo-excitation mechanism is under further investigation, although it appears likely at present that defect or gap states play a role in the photo-enhanced SEE process. In the meantime, the study demonstrates the ability of visible light to modify the electronic properties of diamond and enhance the emission capabilities, which may have potential impact for diamond-based vacuum electron
NASA Astrophysics Data System (ADS)
Chen, Gang
In this talk, we will discuss different modes of heat conduction in nanostructures. Ballistic transport happens when phonon mean free path is longer than the characteristic size of the structure. We will discuss how we compute phonon mean free path distributions based on first-principles and measure the distributions with optical pump-probe techniques by exploring ballistic phonon transport processes. In superlattice structures, ballistic phonon transport across the whole thickness of the superlattices implies phase coherence. We observed this coherent transport in GaAs/AlAs superlattices with fixed periodic thickness and varying number of periods. Simulations show that although high frequency phonons are scattering by roughness, remaining long wavelength phonons maintain their phase and traverse the superlattices ballistically. Accessing the coherent heat conduction regime opens a new venue for phonon engineering. We show further that phonon heat conduction localization happens in GaAs/AlAs superlattice by placing ErAs nanodots at interfaces. This heat-conduction localization phenomenon is confirmed by nonequilibrium atomic Green's function simulation. These ballistic and localization effects can be exploited to improve thermoelectric energy conversion materials via reducing their thermal conductivity. In another opposite, we will discuss phonon hydrodynamic transport mode in graphene via first-principle simulations. In this mode, phonons drift with an average velocity under a temperature gradient, similar to fluid flow in a pipe. Conditions for observing such phonon hydrodynamic modes will be discussed. Finally, we will talk about the one-dimensional nature of heat conduction in polymer chains. Such 1D nature can lead to divergent thermal conductivity. Inspired by simulation, we have experimentally demonstrated high thermal conductivity in ultra-drawn polyethylene nanofibers and sheets. Work supported by DOE Office of Basic Energy Sciences under Award Number: DE
NASA Astrophysics Data System (ADS)
Banerjee, Debika; Trudeau, Charles; Gerlein, Luis Felipe; Cloutier, Sylvain G.
2016-03-01
The nanoscale engineering of silicon can significantly change its bulk optoelectronic properties to make it more favorable for device integration. Phonon process engineering is one way to enhance inter-band transitions in silicon's indirect band structure alignment. This paper demonstrates phonon localization at the tip of silicon nanowires fabricated by galvanic displacement using wet electroless chemical etching of a bulk silicon wafer. High-resolution Raman micro-spectroscopy reveals that such arrayed structures of silicon nanowires display phonon localization behaviors, which could help their integration into the future generations of nano-engineered silicon nanowire-based devices such as photodetectors and solar cells.
Unified phonon-based approach to the thermodynamics of solid, liquid and gas states
NASA Astrophysics Data System (ADS)
Bolmatov, Dima; Zav'yalov, Dmitry; Zhernenkov, Mikhail; Musaev, Edvard T.; Cai, Yong Q.
2015-12-01
We introduce a unified approach to states of matter (solid, liquid and gas) and describe the thermodynamics of the pressure-temperature phase diagram in terms of phonon excitations. We derive the effective Hamiltonian with low-energy cutoff in two transverse phonon polarizations (phononic band gaps) by breaking the symmetry in phonon interactions. Further, we construct the statistical mechanics of states of aggregation employing the Debye approximation. The introduced formalism covers the Debye theory of solids, the phonon theory of liquids, and thermodynamic limits such as the Dulong-Petit thermodynamic limit (cV = 3kB), the ideal gas limit (cV =3/2 kB) and the new thermodynamic limit (cV = 2kB), dubbed here the Frenkel line thermodynamic limit. We discuss the phonon propagation and localization effects in liquids above and below the Frenkel line, and explain the "fast sound" phenomenon. As a test for our theory we calculate velocity-velocity autocorrelation and pair distribution functions within the Green-Kubo formalism. We show the consistency between dynamics of phonons and pair correlations in the framework of the unified approach. New directions towards advancements in phononic band gaps engineering, hypersound manipulation technologies and exploration of exotic behaviour of fluids relevant to geo- and planetary sciences are discussed. The presented results are equally important both for practical implications and for fundamental research.
Phonon Mapping in Flowing Equilibrium
NASA Astrophysics Data System (ADS)
Ruff, J. P. C.
2015-03-01
When a material conducts heat, a modification of the phonon population occurs. The equilibrium Bose-Einstein distribution is perturbed towards flowing-equilibrium, for which the distribution function is not analytically known. Here I argue that the altered phonon population can be efficiently mapped over broad regions of reciprocal space, via diffuse x-ray scattering or time-of-flight neutron scattering, while a thermal gradient is applied across a single crystal sample. When compared to traditional transport measurements, this technique offers a superior, information-rich new perspective on lattice thermal conductivity, wherein the band and momentum dependences of the phonon thermal current are directly resolved. The proposed method is benchmarked using x-ray thermal diffuse scattering measurements of single crystal diamond under transport conditions. CHESS is supported by the NSF & NIH/NIGMS via NSF Award DMR-1332208.
Phonon-tunnelling dissipation in mechanical resonators.
Cole, Garrett D; Wilson-Rae, Ignacio; Werbach, Katharina; Vanner, Michael R; Aspelmeyer, Markus
2011-01-01
Microscale and nanoscale mechanical resonators have recently emerged as ubiquitous devices for use in advanced technological applications, for example, in mobile communications and inertial sensors, and as novel tools for fundamental scientific endeavours. Their performance is in many cases limited by the deleterious effects of mechanical damping. In this study, we report a significant advancement towards understanding and controlling support-induced losses in generic mechanical resonators. We begin by introducing an efficient numerical solver, based on the 'phonon-tunnelling' approach, capable of predicting the design-limited damping of high-quality mechanical resonators. Further, through careful device engineering, we isolate support-induced losses and perform a rigorous experimental test of the strong geometric dependence of this loss mechanism. Our results are in excellent agreement with the theory, demonstrating the predictive power of our approach. In combination with recent progress on complementary dissipation mechanisms, our phonon-tunnelling solver represents a major step towards accurate prediction of the mechanical quality factor.
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.
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.
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.
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.
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.
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
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.
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
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
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
Addition and subtraction of single phonons in a trapped ion system
NASA Astrophysics Data System (ADS)
Lv, Dingshun; An, Shuoming; Um, Mark; Lu, Yao; Zhang, Jingning; Kim, Kihwan
2014-05-01
We introduce an addition and subtraction of single phonons in a trapped ion system. The creation â† and annihilation â operation have been realized with photons and used for the complete engineering of quantum states of light and the probe of fundamental quantum phenomena. The mathematical description of photon is identical to that of phonon. However, phonon is a particle of quantized matter wave, which should be interpreted differently from photon. We implement the addition and the subtraction of phonon by applying an anti-Jaynes-Cummings type of operation on our trapped ion and performing projective measurements. Our realization can be used for the accurate measurement of position and momentum as well as their relation. This work was supported by the National Basic Research Program of China Grant 2011CBA00300, 2011CBA00301, 2011CBA00302, the National Natural Science Foundation of China Grant 61073174, 61033001, 61061130540.
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.
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
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.
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
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.
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.
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.
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)
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
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
Hill,D.; Bartynski, R.; Nguyen, N.; Davydov, A.; Chandler-Horowitz, D.; Frank, M.
2008-01-01
We have measured x-ray absorption spectra (XAS) at the oxygen K edge for hafnium oxide (HfO2) films grown by chemical vapor deposition (CVD) and atomic layer deposition (ALD), as well as for hafnium silicate (HfSiO) films grown by CVD. The XAS results are compared to x-ray diffraction (XRD) and spectroscopic ellipsometry (SE) data from the same films. Features characteristic of crystalline HfO2 are observed in the XAS spectra from all CVD-grown HfO2 films, even for a thickness of 5 nm where XRD is not sensitive. XAS and XRD spectra from the ALD-grown HfO2 films exhibit the signature of crystallinity only for films that are 20 nm or thicker. These characteristic XAS features are absent in all HfSiO films measured, which is consistent with their being amorphous. The appearance of these peaks in XAS and XRD is correlated with sub-band-gap absorption in the SE spectra, which appears to be intrinsic to crystalline HfO2 in the monoclinic phase.
Nonsymmorphic Phononic Metamaterials: shaping waves over multiple length scales
NASA Astrophysics Data System (ADS)
Koh, Cheongyang; Thomas, Edwin
2012-02-01
The vector nature of the phonon makes rational design of phononic metamaterials challenging, despite potential in unique wave propagation behavior, such as negative refraction and hyper-lensing. While most designs to date focus on the ``meta-atom'' (building block) design, their ``spatial arrangement'' (non-locality) is equally instrumental in dispersion engineering. Here, we present a generalized design framework (DF) for PMM design, utilizing both ``global'' and ``local'' symmetry concepts. We demonstrate, utilizing specific properties of nonsymmorphic plane groups, PMMs possessing i) a low-frequency in-plane complete spectral gap (ICSG) of 102% (CSG of 88%), ii) a set of polychromatic ICSGs totaling over 100% in normalized gap size. Within the same DF, we further integrate broken symmetry states (BSS) (edge states, waveguides, etc) with designed polarization, (de)localization and group velocities. In particular, we demonstrate how these BSS may be utilized to elucidate signatures of complex polarization fields through phonon-structure interactions, leading to interesting applications in elastic-wave imaging, as well as information retrieval by probing polarization states of scattering bodies over multiple scales.
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.
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.
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 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.
Phonon number measurements using single photon opto-mechanics
NASA Astrophysics Data System (ADS)
Basiri-Esfahani, S.; Akram, U.; Milburn, G. J.
2012-08-01
We describe a system composed of two coupled optical cavity modes with a coupling modulated by a bulk mechanical resonator. In addition, one of the cavity modes is irreversibly coupled to a single photon source. Our scheme is an opto-mechanical realization of the Jaynes-Cummings model where the qubit is a dual rail optical qubit while the bosonic degree of freedom is a matter degree of freedom realized as the bulk mechanical excitation. We show the possibility of engineering phonon number states of the mechanical oscillator in such a system by computing the conditional state of the mechanics after successive photon counting measurements.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
A first principles method for simulating phonons in strongly disordered materials
NASA Astrophysics Data System (ADS)
Berlijn, Tom; Delaire, Olivier; Larson, Ben
2015-03-01
At the microscopic level the flow of vibrational heat is encoded not only in the energies of phonons but also in their lifetimes. In many functional materials these phonon lifetimes are controlled by strong disorder. Such systems are difficult to understand from conventional perturbation theories or mean field treatments. Here we will present an affordable and accurate first principles method for simulating phonons in strongly disordered materials. The method will be illustrated with applications ranging from thermoelectrics to nuclear fuels. TB was supported as a Wigner Fellow at the Oak Ridge National Laboratory, OD was supported by the US DOE-BES, Materials Science and Engineering Division, and BL was supported by the CMSNF Energy Frontier Research Center.
Ballistic Performance Study of Nanowire FET: Effect of Channel Materials and Phonon Scattering
NASA Astrophysics Data System (ADS)
Iztihad, Hossain Md.; Khan, Touhid; Sufian, Abu; Alam, Md. Nur Kutubul; Mollah, Md. Nurunnabi; Islam, Md. Rafiqul
2016-02-01
The ballistic performance of Si and Ge nanowire (NW) is compared in this study. Current-voltage characteristic is obtained by self-consistently solving the nonequilibrium Green’s function (NEGF) transport equation with Poisson’s equation. The result is obtained at ⟨001⟩ channel orientation. Simulation result shows Ge NW gives higher ON-state current than Si NW, when OFF-state current is made equal by gate metal work function engineering. However, at subthreshold region, performance of NW FET for both material is almost identical. The intravalley and intervalley electron-phonon scattering effect is also calculated using the deformation potential theory and the self-consistent Born approximation. It is found that electron-phonon scattering effect is more pronounced at ON-state of Si NW FET. The ballistic current decreases with the decrease in diameter of the Si NW FET due to electron-phonon scattering.
Wang, Jing; Chen, Di; Wallace, Joseph; Gigax, Jonathan; Wang, Xuemei; Shao, Lin
2014-05-12
Through integrated molecular dynamics (MD) simulations and experimental studies, we demonstrated the feasibility of an ion-irradiation-and-annealing based phonon engineering technique to enhance thermal conductivity of carbon nanotube (CNT) films. Upon ion irradiation of CNT films, both inter-tube defects and intra-tube defects are introduced. Our MD simulations show that inter-tube defects created between neighboring tubes are much more stable than intra-tube defects created on tube graphitic planes. Upon thermal annealing, intra-tube defects are preferentially removed but inter-tube defects stay. Consequently, axial phonon transport increases due to reduced phonon scattering and off-axial phonon transport is sustained due to the high stability of inter-tube defects, leading to a conductivity enhancement upon annealing. The modeling predictions agree with experimental observations that thermal conductivities of CNT films were enhanced after 2 MeV hydrogen ion irradiations and conductivities were further enhanced upon post irradiation annealing.
Ab initio phonon limited transport
NASA Astrophysics Data System (ADS)
Verstraete, Matthieu
We revisit the thermoelectric (TE) transport properties of two champion materials, PbTe and SnSe, using fully first principles methods. In both cases the performance of the material is due to subtle combinations of structural effects, scattering, and phase space reduction. In PbTe anharmonic effects are completely opposite to the predicted quasiharmonic evolution of phonon frequencies and to frequently (and incorrectly) cited extrapolations of experiments. This stabilizes the material at high T, but also tends to enhance its thermal conductivity, in a non linear manner, above 600 Kelvin. This explains why PbTe is in practice limited to room temperature applications. SnSe has recently been shown to be the most efficient TE material in bulk form. This is mainly due to a strongly enhanced carrier concentration and electrical conductivity, after going through a phase transition from 600 to 800 K. We calculate the transport coefficients as well as the defect concentrations ab initio, showing excellent agreement with experiment, and elucidating the origin of the double phase transition as well as the new charge carriers. AH Romero, EKU Gross, MJ Verstraete, and O Hellman PRB 91, 214310 (2015) O. Hellman, IA Abrikosov, and SI Simak, PRB 84 180301 (2011)
A bond-order theory on the phonon scattering by vacancies in two-dimensional materials.
Xie, Guofeng; Shen, Yulu; Wei, Xiaolin; Yang, Liwen; Xiao, Huaping; Zhong, Jianxin; Zhang, Gang
2014-01-01
We theoretically investigate the phonon scattering by vacancies, including the impacts of missing mass and linkages (τ(V)(-1)) and the variation of the force constant of bonds associated with vacancies (τ(A)(-1)) by the bond-order-length-strength correlation mechanism. We find that in bulk crystals, the phonon scattering rate due to change of force constant τ(A)(-1) is about three orders of magnitude lower than that due to missing mass and linkages τ(V)(-1). In contrast to the negligible τ(A)(-1) in bulk materials, τ(A)(-1) in two-dimensional materials can be 3-10 folds larger than τ(V)(-1). Incorporating this phonon scattering mechanism to the Boltzmann transport equation derives that the thermal conductivity of vacancy defective graphene is severely reduced even for very low vacancy density. High-frequency phonon contribution to thermal conductivity reduces substantially. Our findings are helpful not only to understand the severe suppression of thermal conductivity by vacancies, but also to manipulate thermal conductivity in two-dimensional materials by phononic engineering.
Investigation of the Phonon Frequency Shifts in ZnO Quantum Dots
NASA Astrophysics Data System (ADS)
Alim, Khan A.
2005-03-01
Nanostructures made of ZnO have recently attracted attention due to their proposed applications in low-voltage and short-wavelength electro-optical devices. However, the origin of the observed phonon frequency shifts in such nanostructures is not always understood. We carried out both resonant and non-resonant Raman measurements for 20 nm-diameter ZnO quantum dots (QDs) and bulk ZnO reference samples [1]. A comparison with a recently developed theory [2], allowed us to clarify the origin of the phonon frequency shifts in ZnO QDs. It was found that the phonon confinement results in phonon frequency shifts of only few cm-1. At the same time, the UV laser heating of the QD ensemble was found to induce a large red shift of phonon frequencies for up to 14 cm-1. The authors acknowledge the support of MARCO and its Functional Engineered Nano Architectonics (FENA) Focus Center. [1] K.A. Alim, V.A. Fonoberov, and A.A. Balandin, Appl. Phys. Lett., in review (2004). [2] V.A. Fonoberov and A.A. Balandin, Phys. Stat. Solidi C 1, 2650 (2004); cond-mat/0405681; cond-mat/0411742.
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.
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.
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.
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.
Phonon dynamics of graphene on metals
NASA Astrophysics Data System (ADS)
Taleb, Amjad Al; Farías, Daniel
2016-03-01
The study of surface phonon dispersion curves is motivated by the quest for a detailed understanding of the forces between the atoms at the surface and in the bulk. In the case of graphene, additional motivation comes from the fact that thermal conductivity is dominated by contributions from acoustic phonons, while optical phonon properties are essential to understand Raman spectra. In this article, we review recent progress made in the experimental determination of phonon dispersion curves of graphene grown on several single-crystal metal surfaces. The two main experimental techniques usually employed are high-resolution electron energy loss spectroscopy (HREELS) and inelastic helium atom scattering (HAS). The different dispersion branches provide a detailed insight into the graphene-substrate interaction. Softening of optical modes and signatures of the substrate‧s Rayleigh wave are observed for strong graphene-substrate interactions, while acoustic phonon modes resemble those of free-standing graphene for weakly interacting systems. The latter allows determining the bending rigidity and the graphene-substrate coupling strength. A comparison between theory and experiment is discussed for several illustrative examples. Perspectives for future experiments are discussed.
Ionizing particle detection based on phononic crystals
NASA Astrophysics Data System (ADS)
Aly, Arafa H.; Mehaney, Ahmed; Eissa, Mostafa F.
2015-08-01
Most conventional radiation detectors are based on electronic or photon collections. In this work, we introduce a new and novel type of ionizing particle detector based on phonon collection. Helium ion radiation treats tumors with better precision. There are nine known isotopes of helium, but only helium-3 and helium-4 are stable. Helium-4 is formed in fusion reactor technology and in enormous quantities during Big Bang nucleo-synthesis. In this study, we introduce a technique for helium-4 ion detection (sensing) based on the innovative properties of the new composite materials known as phononic crystals (PnCs). PnCs can provide an easy and cheap technique for ion detection compared with conventional methods. PnC structures commonly consist of a periodic array of two or more materials with different elastic properties. The two materials are polymethyl-methacrylate and polyethylene polymers. The calculations showed that the energies lost to target phonons are maximized at 1 keV helium-4 ion energy. There is a correlation between the total phonon energies and the transmittance of PnC structures. The maximum transmission for phonons due to the passage of helium-4 ions was found in the case of making polyethylene as a first layer in the PnC structure. Therefore, the concept of ion detection based on PnC structure is achievable.
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.
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.
Hierarchical thermoelectrics: crystal grain boundaries as scalable phonon scatterers.
Selli, Daniele; Boulfelfel, Salah Eddine; Schapotschnikow, Philipp; Donadio, Davide; Leoni, Stefano
2016-02-14
Thermoelectric materials are strategically valuable for sustainable development, as they allow for the generation of electrical energy from wasted heat. In recent years several strategies have demonstrated some efficiency in improving thermoelectric properties. Dopants affect carrier concentration, while thermal conductivity can be influenced by alloying and nanostructuring. Features at the nanoscale positively contribute to scattering phonons, however those with long mean free paths remain difficult to alter. Here we use the concept of hierarchical nano-grains to demonstrate thermal conductivity reduction in rocksalt lead chalcogenides. We demonstrate that grains can be obtained by taking advantage of the reconstructions along the phase transition path that connects the rocksalt structure to its high-pressure form. Since grain features naturally change as a function of size, they impact thermal conductivity over different length scales. To understand this effect we use a combination of advanced molecular dynamics techniques to engineer grains and to evaluate thermal conductivity in PbSe. By affecting grain morphologies only, i.e. at constant chemistry, two distinct effects emerge: the lattice thermal conductivity is significantly lowered with respect to the perfect crystal, and its temperature dependence is markedly suppressed. This is due to an increased scattering of low-frequency phonons by grain boundaries over different size scales. Along this line we propose a viable process to produce hierarchical thermoelectric materials by applying pressure via a mechanical load or a shockwave as a novel paradigm for material design.
Hierarchical thermoelectrics: crystal grain boundaries as scalable phonon scatterers
NASA Astrophysics Data System (ADS)
Selli, Daniele; Boulfelfel, Salah Eddine; Schapotschnikow, Philipp; Donadio, Davide; Leoni, Stefano
2016-02-01
Thermoelectric materials are strategically valuable for sustainable development, as they allow for the generation of electrical energy from wasted heat. In recent years several strategies have demonstrated some efficiency in improving thermoelectric properties. Dopants affect carrier concentration, while thermal conductivity can be influenced by alloying and nanostructuring. Features at the nanoscale positively contribute to scattering phonons, however those with long mean free paths remain difficult to alter. Here we use the concept of hierarchical nano-grains to demonstrate thermal conductivity reduction in rocksalt lead chalcogenides. We demonstrate that grains can be obtained by taking advantage of the reconstructions along the phase transition path that connects the rocksalt structure to its high-pressure form. Since grain features naturally change as a function of size, they impact thermal conductivity over different length scales. To understand this effect we use a combination of advanced molecular dynamics techniques to engineer grains and to evaluate thermal conductivity in PbSe. By affecting grain morphologies only, i.e. at constant chemistry, two distinct effects emerge: the lattice thermal conductivity is significantly lowered with respect to the perfect crystal, and its temperature dependence is markedly suppressed. This is due to an increased scattering of low-frequency phonons by grain boundaries over different size scales. Along this line we propose a viable process to produce hierarchical thermoelectric materials by applying pressure via a mechanical load or a shockwave as a novel paradigm for material design.
Atomistic modeling of phonon transport in turbostratic graphitic structures
NASA Astrophysics Data System (ADS)
Mao, Rui; Chen, Yifeng; Kim, Ki Wook
2016-05-01
Thermal transport in turbostratic graphitic systems is investigated by using an atomistic analytical model based on the 4th-nearest-neighbor force constant approximation and a registry-dependent interlayer potential. The developed model is shown to produce an excellent agreement with the experimental data and ab initio results in the calculation of bulk properties. Subsequent analysis of phonon transport in combination with the Green's function method illustrates the significant dependence of key characteristics on the misorientation angle, clearly indicating the importance of this degree of freedom in multi-stacked structures. Selecting three angles with the smallest commensurate unit cells, the thermal resistance is evaluated at the twisted interface between two AB stacked graphite. The resulting values in the range of 35 × 10-10 K m2/W to 116 × 10-10 K m2/W are as large as those between two dissimilar material systems such as a metal and graphene. The strong rotational effect on the cross-plane thermal transport may offer an effective means of phonon engineering for applications such as thermoelectric materials.
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
Phonon and magnon heat transport and drag effects
NASA Astrophysics Data System (ADS)
Heremans, Joseph P.
2014-03-01
Thermoelectric generators and coolers constitute today's solid-state energy converters. The two goals in thermoelectrics research are to enhance the thermopower while simultaneously maintaining a high electrical conductivity of the same material, and to minimize its lattice thermal conductivity without affecting its electronic properties. Up to now the lattice thermal conductivity has been minimized by using alloy scattering and, more recently, nanostructuring. In the first part of the talk, a new approach to minimize the lattice thermal conductivity is described that affects phonon scattering much more than electron scattering. This can be done by selecting potential thermoelectric materials that have a very high anharmonicity, because this property governs phonon-phonon interaction probability. Several possible types of chemical bonds will be described that exhibit such high anharmonicity, and particular emphasis will be put on solids with highly-polarizable lone-pair electrons, such as the rock salt I-V-VI2 compounds (e.g. NaSbSe2). The second part of the talk will give an introduction to a completely new class of solid-state thermal energy converters based on spin transport. One configuration for such energy converters is based on the recently discovered spin-Seebeck effect (SSE). This quantity is expressed in the same units as the conventional thermopower, and we have recently shown that it can be of the same order of magnitude. The main advantage of SSE converters is that the problem of optimization is now distributed over two different materials, a ferromagnet in which a flux of magnetization is generated by a thermal gradient, and a normal metal where the flux of magnetization is converted into electrical power. The talk will focus on the basic physics behind the spin-Seebeck effect. Recent developments will then be described based on phonon-drag of spin polarized electrons. This mechanism has made it possible to reach magnitudes of SSE that are comparable
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.
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.
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.
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.
Luo, Yixiu; Wang, Jiemin; Li, Yiran; Wang, Jingyang
2016-01-01
Modification of lattice thermal conductivity (κL) of a solid by means of hydrostatic pressure (P) has been a crucially interesting approach that targets a broad range of advanced materials from thermoelectrics and thermal insulators to minerals in mantle. Although it is well documented knowledge that thermal conductivity of bulk materials normally increase upon hydrostatic pressure, such positive relationship is seriously challenged when it comes to ceramics with complex crystal structure and heterogeneous chemical bonds. In this paper, we predict an abnormally negative trend dκL/dP < 0 in Y2Si2O7 silicate using density functional theoretical calculations. The mechanism is disclosed as combined effects of slightly decreased group velocity and significantly augmented scattering of heat-carrying acoustic phonons in pressured lattice, which is originated from pressure-induced downward shift of low-lying optic and acoustic phonons. The structural origin of low-lying optic phonons as well as the induced phonon anharmonicity is also qualitatively elucidated with respect to intrinsic bonding heterogeneity of Y2Si2O7. The present results are expected to bring deeper insights for phonon engineering and modulation of thermal conductivity in complex solids with diverging structural flexibility, enormous bonding heterogeneity, and giant phonon anharmonicity. PMID:27430670
Yan, Zhequan; Chen, Liang; Yoon, Mina; Kumar, Satish
2016-02-21
Hexagonal boron nitride (h-BN) is a promising substrate for graphene based nano-electronic devices. We investigate the ballistic phonon transport at the interface of vertically stacked graphene and h-BN heterostructures using first principles density functional theory and atomistic Green's function simulations considering the influence of lattice stacking. We compute the frequency and wave-vector dependent transmission function and observe distinct stacking-dependent phonon transmission features for the h-BN/graphene/h-BN sandwiched systems. We find that the in-plane acoustic modes have the dominant contributions to the phonon transmission and thermal boundary conductance (TBC) for the interfaces with the carbon atom located directly on top of the boron atom (C-B matched) because of low interfacial spacing. The low interfacial spacing is a consequence of the differences in the effective atomic volume of N and B and the difference in the local electron density around N and B. For the structures with the carbon atom directly on top of the nitrogen atom (C-N matched), the spatial distance increases and the contribution of in-plane modes to the TBC decreases leading to higher contributions by out-of-plane acoustic modes. We find that the C-B matched interfaces have stronger phonon-phonon coupling than the C-N matched interfaces, which results in significantly higher TBC (more than 50%) in the C-B matched interface. The findings in this study will provide insights to understand the mechanism of phonon transport at h-BN/graphene/h-BN interfaces, to better explain the experimental observations and to engineer these interfaces to enhance heat dissipation in graphene based electronic devices. PMID:26817419
Shin, H.B.
1984-02-28
An internal combustion engine has a piston rack depending from each piston. This rack is connected to a power output shaft through a mechanical rectifier so that the power output shaft rotates in only one direction. A connecting rod is pivotally connected at one end to the rack and at the other end to the crank of a reduced function crankshaft so that the crankshaft rotates at the same angular velocity as the power output shaft and at the same frequency as the pistons. The crankshaft has a size, weight and shape sufficient to return the pistons back into the cylinders in position for the next power stroke.
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
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.
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"
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.
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.
Large phonon entropy drives the metallization of vanadium dioxide (VO2)
NASA Astrophysics Data System (ADS)
Hong, Jiawang
2015-03-01
Vanadium dioxide (VO2) exhibits a first-order metal-insulator transition (MIT) near room temperature, where conductivity is suppressed and the lattice changes from tetragonal to monoclinic on cooling. This MIT in VO2 has attracted intense interest from both fundamental and technological perspectives. However, most studies performed in the past 50 years have focused on the electronic structure and energetics of the transition, ignoring the role of phonons and their entropic contribution to the phase stability. Much of the reason is that the standard tool of neutron scattering does not yield coherent scattering from V nuclei, and first-principles methods with harmonic approximation cannot capture the stable phonons for the rutile phase. We close this gap by using a combination of ab initio molecular dynamics calculations and neutron/x-ray scattering to establish that the entropy driving the MIT is dominated by soft, anharmonic phonons of the metallic phase. The MIT results from the competition between lower electronic energy in insulating M1 phase due to the Peierls instability, and the higher entropy of the metallic rutile phase resulting from soft anharmonic phonons. This understanding of the role of lattice dynamics and their relationship to electronic structure provides a critical component for developing more complete physical models of phase competition in functional transition metal oxides. Theoretical calculations were performed using the NERSC at LBNL. Modeling of neutron data was performed in CAMM, measurements were funded by the US DOE, BES, Materials Science and Engineering Division.
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).
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.
Frequency stabilization of the zero-phonon line of a quantum dot via phonon-assisted active feedback
Hansom, Jack; Schulte, Carsten H. H.; Matthiesen, Clemens; Stanley, Megan J.; Atatüre, Mete
2014-10-27
We report on the feedback stabilization of the zero-phonon emission frequency of a single InAs quantum dot. The spectral separation of the phonon-assisted component of the resonance fluorescence provides a probe of the detuning between the zero-phonon transition and the resonant driving laser. Using this probe in combination with active feedback, we stabilize the zero-phonon transition frequency against environmental fluctuations. This protocol reduces the zero-phonon fluorescence intensity noise by a factor of 22 by correcting for environmental noise with a bandwidth of 191 Hz, limited by the experimental collection efficiency. The associated sub-Hz fluctuations in the zero-phonon central frequency are reduced by a factor of 7. This technique provides a means of stabilizing the quantum dot emission frequency without requiring access to the zero-phonon emission.
Yu, Jen-Kan; Mitrovic, Slobodan; Heath, James R.
2016-08-16
A nanomesh phononic structure includes: a sheet including a first material, the sheet having a plurality of phononic-sized features spaced apart at a phononic pitch, the phononic pitch being smaller than or equal to twice a maximum phonon mean free path of the first material and the phononic size being smaller than or equal to the maximum phonon mean free path of the first material.
Magnon rainbows filtered through phonon clouds
NASA Astrophysics Data System (ADS)
Boona, Stephen R.
2016-06-01
The study of heat flow in magnetic insulators is a topic of significant interest in spin caloritronics, especially for understanding the nuanced origins of the spin Seebeck effect (SSE). Recent work by Diniz and Costa (2016 New J. Phys. 18 052002) provides insight into this subject by presenting a microscopic model for the spectral dependence of magnon-phonon interactions in magnetic insulators, which has been a challenging puzzle for decades. Their new paper shows that phonon-mediated magnon-magnon interactions affect the lifetime of magnons differently depending on the magnon wavelength. As a result, low energy magnons transport spin more efficiently, and are more sensitive to applied magnetic fields. These results help explain some unexpected behavior in the SSE recently reported in several experiments.
Magnon rainbows filtered through phonon clouds
NASA Astrophysics Data System (ADS)
Boona, Stephen R.
2016-06-01
The study of heat flow in magnetic insulators is a topic of significant interest in spin caloritronics, especially for understanding the nuanced origins of the spin Seebeck effect (SSE). Recent work by Diniz and Costa (2016 New J. Phys. 18 052002) provides insight into this subject by presenting a microscopic model for the spectral dependence of magnon–phonon interactions in magnetic insulators, which has been a challenging puzzle for decades. Their new paper shows that phonon-mediated magnon-magnon interactions affect the lifetime of magnons differently depending on the magnon wavelength. As a result, low energy magnons transport spin more efficiently, and are more sensitive to applied magnetic fields. These results help explain some unexpected behavior in the SSE recently reported in several experiments.
Thermodynamics of phonon-modulated tunneling centers
Junker, W.; Wagner, M. )
1989-08-01
In recent years tunneling centers have frequently been used to explain the unusual thermodynamic properties of disordered materials; in these approaches, however, the effect of the tunneling-phonon interaction is neglected. The present study considers the archetype model of phono-assisted tunneling, which is well known from other areas of tunneling physics (quantum diffusion, etc.). It is shown that the full thermodynamic information can be rigorously extracted from a single Green function. An extended factorization procedure beyond Hartree-Fock is introduced, which is checked by sum rules as well as by exact Goldberger-Adams expansions. The phonon-modulated internal energy and specific heat are calculated for different power-law coupling setups.
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.
Phonon arithmetic in a trapped ion system.
Um, Mark; Zhang, Junhua; Lv, Dingshun; Lu, Yao; An, Shuoming; Zhang, Jing-Ning; Nha, Hyunchul; Kim, M S; Kim, Kihwan
2016-01-01
Single-quantum level operations are important tools to manipulate a quantum state. Annihilation or creation of single particles translates a quantum state to another by adding or subtracting a particle, depending on how many are already in the given state. The operations are probabilistic and the success rate has yet been low in their experimental realization. Here we experimentally demonstrate (near) deterministic addition and subtraction of a bosonic particle, in particular a phonon of ionic motion in a harmonic potential. We realize the operations by coupling phonons to an auxiliary two-level system and applying transitionless adiabatic passage. We show handy repetition of the operations on various initial states and demonstrate by the reconstruction of the density matrices that the operations preserve coherences. We observe the transformation of a classical state to a highly non-classical one and a Gaussian state to a non-Gaussian one by applying a sequence of operations deterministically. PMID:27097897
Phonon arithmetic in a trapped ion system
Um, Mark; Zhang, Junhua; Lv, Dingshun; Lu, Yao; An, Shuoming; Zhang, Jing-Ning; Nha, Hyunchul; Kim, M. S.; Kim, Kihwan
2016-01-01
Single-quantum level operations are important tools to manipulate a quantum state. Annihilation or creation of single particles translates a quantum state to another by adding or subtracting a particle, depending on how many are already in the given state. The operations are probabilistic and the success rate has yet been low in their experimental realization. Here we experimentally demonstrate (near) deterministic addition and subtraction of a bosonic particle, in particular a phonon of ionic motion in a harmonic potential. We realize the operations by coupling phonons to an auxiliary two-level system and applying transitionless adiabatic passage. We show handy repetition of the operations on various initial states and demonstrate by the reconstruction of the density matrices that the operations preserve coherences. We observe the transformation of a classical state to a highly non-classical one and a Gaussian state to a non-Gaussian one by applying a sequence of operations deterministically. PMID:27097897
Zeng, Lingping; Collins, Kimberlee C.; Hu, Yongjie; Luckyanova, Maria N.; Maznev, Alexei A.; Huberman, Samuel; Chiloyan, Vazrik; Zhou, Jiawei; Huang, Xiaopeng; Nelson, Keith A.; et al
2015-11-27
Heat conduction in semiconductors and dielectrics depends upon their phonon mean free paths that describe the average travelling distance between two consecutive phonon scattering events. Nondiffusive phonon transport is being exploited to extract phonon mean free path distributions. Here, we describe an implementation of a nanoscale thermal conductivity spectroscopy technique that allows for the study of mean free path distributions in optically absorbing materials with relatively simple fabrication and a straightforward analysis scheme. We pattern 1D metallic grating of various line widths but fixed gap size on sample surfaces. The metal lines serve as both heaters and thermometers in time-domainmore » thermoreflectance measurements and simultaneously act as wiregrid polarizers that protect the underlying substrate from direct optical excitation and heating. We demonstrate the viability of this technique by studying length-dependent thermal conductivities of silicon at various temperatures. The thermal conductivities measured with different metal line widths are analyzed using suppression functions calculated from the Boltzmann transport equation to extract the phonon mean free path distributions with no calibration required. Furthermore, this table-top ultrafast thermal transport spectroscopy technique enables the study of mean free path spectra in a wide range of technologically important materials.« less
NASA Astrophysics Data System (ADS)
Zeng, Lingping; Collins, Kimberlee C.; Hu, Yongjie; Luckyanova, Maria N.; Maznev, Alexei A.; Huberman, Samuel; Chiloyan, Vazrik; Zhou, Jiawei; Huang, Xiaopeng; Nelson, Keith A.; Chen, Gang
2015-11-01
Heat conduction in semiconductors and dielectrics depends upon their phonon mean free paths that describe the average travelling distance between two consecutive phonon scattering events. Nondiffusive phonon transport is being exploited to extract phonon mean free path distributions. Here, we describe an implementation of a nanoscale thermal conductivity spectroscopy technique that allows for the study of mean free path distributions in optically absorbing materials with relatively simple fabrication and a straightforward analysis scheme. We pattern 1D metallic grating of various line widths but fixed gap size on sample surfaces. The metal lines serve as both heaters and thermometers in time-domain thermoreflectance measurements and simultaneously act as wire-grid polarizers that protect the underlying substrate from direct optical excitation and heating. We demonstrate the viability of this technique by studying length-dependent thermal conductivities of silicon at various temperatures. The thermal conductivities measured with different metal line widths are analyzed using suppression functions calculated from the Boltzmann transport equation to extract the phonon mean free path distributions with no calibration required. This table-top ultrafast thermal transport spectroscopy technique enables the study of mean free path spectra in a wide range of technologically important materials.
Zeng, Lingping; Collins, Kimberlee C; Hu, Yongjie; Luckyanova, Maria N; Maznev, Alexei A; Huberman, Samuel; Chiloyan, Vazrik; Zhou, Jiawei; Huang, Xiaopeng; Nelson, Keith A; Chen, Gang
2015-11-27
Heat conduction in semiconductors and dielectrics depends upon their phonon mean free paths that describe the average travelling distance between two consecutive phonon scattering events. Nondiffusive phonon transport is being exploited to extract phonon mean free path distributions. Here, we describe an implementation of a nanoscale thermal conductivity spectroscopy technique that allows for the study of mean free path distributions in optically absorbing materials with relatively simple fabrication and a straightforward analysis scheme. We pattern 1D metallic grating of various line widths but fixed gap size on sample surfaces. The metal lines serve as both heaters and thermometers in time-domain thermoreflectance measurements and simultaneously act as wire-grid polarizers that protect the underlying substrate from direct optical excitation and heating. We demonstrate the viability of this technique by studying length-dependent thermal conductivities of silicon at various temperatures. The thermal conductivities measured with different metal line widths are analyzed using suppression functions calculated from the Boltzmann transport equation to extract the phonon mean free path distributions with no calibration required. This table-top ultrafast thermal transport spectroscopy technique enables the study of mean free path spectra in a wide range of technologically important materials.
Zeng, Lingping; Collins, Kimberlee C.; Hu, Yongjie; Luckyanova, Maria N.; Maznev, Alexei A.; Huberman, Samuel; Chiloyan, Vazrik; Zhou, Jiawei; Huang, Xiaopeng; Nelson, Keith A.; Chen, Gang
2015-11-27
Heat conduction in semiconductors and dielectrics depends upon their phonon mean free paths that describe the average travelling distance between two consecutive phonon scattering events. Nondiffusive phonon transport is being exploited to extract phonon mean free path distributions. Here, we describe an implementation of a nanoscale thermal conductivity spectroscopy technique that allows for the study of mean free path distributions in optically absorbing materials with relatively simple fabrication and a straightforward analysis scheme. We pattern 1D metallic grating of various line widths but fixed gap size on sample surfaces. The metal lines serve as both heaters and thermometers in time-domain thermoreflectance measurements and simultaneously act as wiregrid polarizers that protect the underlying substrate from direct optical excitation and heating. We demonstrate the viability of this technique by studying length-dependent thermal conductivities of silicon at various temperatures. The thermal conductivities measured with different metal line widths are analyzed using suppression functions calculated from the Boltzmann transport equation to extract the phonon mean free path distributions with no calibration required. Furthermore, this table-top ultrafast thermal transport spectroscopy technique enables the study of mean free path spectra in a wide range of technologically important materials.
Zeng, Lingping; Collins, Kimberlee C; Hu, Yongjie; Luckyanova, Maria N; Maznev, Alexei A; Huberman, Samuel; Chiloyan, Vazrik; Zhou, Jiawei; Huang, Xiaopeng; Nelson, Keith A; Chen, Gang
2015-01-01
Heat conduction in semiconductors and dielectrics depends upon their phonon mean free paths that describe the average travelling distance between two consecutive phonon scattering events. Nondiffusive phonon transport is being exploited to extract phonon mean free path distributions. Here, we describe an implementation of a nanoscale thermal conductivity spectroscopy technique that allows for the study of mean free path distributions in optically absorbing materials with relatively simple fabrication and a straightforward analysis scheme. We pattern 1D metallic grating of various line widths but fixed gap size on sample surfaces. The metal lines serve as both heaters and thermometers in time-domain thermoreflectance measurements and simultaneously act as wire-grid polarizers that protect the underlying substrate from direct optical excitation and heating. We demonstrate the viability of this technique by studying length-dependent thermal conductivities of silicon at various temperatures. The thermal conductivities measured with different metal line widths are analyzed using suppression functions calculated from the Boltzmann transport equation to extract the phonon mean free path distributions with no calibration required. This table-top ultrafast thermal transport spectroscopy technique enables the study of mean free path spectra in a wide range of technologically important materials. PMID:26612032
Zeng, Lingping; Collins, Kimberlee C.; Hu, Yongjie; Luckyanova, Maria N.; Maznev, Alexei A.; Huberman, Samuel; Chiloyan, Vazrik; Zhou, Jiawei; Huang, Xiaopeng; Nelson, Keith A.; Chen, Gang
2015-01-01
Heat conduction in semiconductors and dielectrics depends upon their phonon mean free paths that describe the average travelling distance between two consecutive phonon scattering events. Nondiffusive phonon transport is being exploited to extract phonon mean free path distributions. Here, we describe an implementation of a nanoscale thermal conductivity spectroscopy technique that allows for the study of mean free path distributions in optically absorbing materials with relatively simple fabrication and a straightforward analysis scheme. We pattern 1D metallic grating of various line widths but fixed gap size on sample surfaces. The metal lines serve as both heaters and thermometers in time-domain thermoreflectance measurements and simultaneously act as wire-grid polarizers that protect the underlying substrate from direct optical excitation and heating. We demonstrate the viability of this technique by studying length-dependent thermal conductivities of silicon at various temperatures. The thermal conductivities measured with different metal line widths are analyzed using suppression functions calculated from the Boltzmann transport equation to extract the phonon mean free path distributions with no calibration required. This table-top ultrafast thermal transport spectroscopy technique enables the study of mean free path spectra in a wide range of technologically important materials. PMID:26612032
Comments on polaron-phonon scattering theory
NASA Astrophysics Data System (ADS)
Tulub, A. V.
2015-10-01
We use the polaron state function described in terms of coupled classical and quantum fields to calculate the cross section of phonon scattering on a polaron. The value of the resonance momentum is determined by asymptotic values of several integrals. Calculating them with crystal parameters taken into account leads to bounds on the maximum value of the coupling constant. We confirm that the applicability domain of the strong-coupling approximation is near zero.
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
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.
Phonon induced magnetism in ionic materials
NASA Astrophysics Data System (ADS)
Restrepo, Oscar D.; Antolin, Nikolas; Jin, Hyungyu; Heremans, Joseph P.; Windl, Wolfgang
2014-03-01
Thermoelectric phenomena in magnetic materials create exciting possibilities in future spin caloritronic devices by manipulating spin information using heat. An accurate understanding of the spin-lattice interactions, i.e. the coupling between magnetic excitations (magnons) and lattice vibrations (phonons), holds the key to unraveling their underlying physics. We report ab initio frozen-phonon calculations of CsI that result in non-zero magnetization when the degeneracy between spin-up and spin-down electronic density of states is lifted for certain phonon displacement patterns. For those, the magnetization as a function of atomic displacement shows a sharp resonance due to the electronic states on the displaced Cs atoms, while the electrons on indium form a continuous background magnetization. We relate this resonance to the generation of a two-level system in the spin-polarized Cs partial density of states as a function of displacement, which we propose to be described by a simple resonant-susceptibility model. Current work extends these investigations to semiconductors such as InSb. ODR and WW are supported by the Center for Emergent Materials, an NSF MRSEC at OSU (Grant DMR-0820414).HJ and JPH are supported by AFOSR MURI Cryogenic Peltier Cooling, Contract #FA9550-10-1-0533.
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.
Large electron-phonon interactions from FeSe phonons in a monolayer
NASA Astrophysics Data System (ADS)
Coh, Sinisa; Cohen, Marvin L.; Louie, Steven G.
2015-07-01
We show that electron-phonon coupling can induce strong electron pairing in an FeSe monolayer on a SrTiO3 substrate (experimental indications for superconducting {T}{{c}} are between 65 and 109 K). The role of the SrTiO3 substrate in increasing the coupling is two-fold. First, the interaction of the FeSe and TiO2 terminated face of SrTiO3 prevents the FeSe monolayer from undergoing a shear-type (orthorhombic, nematic) structural phase transition. Second, the substrate allows an anti-ferromagnetic ground state of FeSe which opens electron-phonon coupling channels within the monolayer that are prevented by symmetry in the non-magnetic phase. The spectral function for the electron-phonon coupling ({α }2F) in our calculations agrees well with inelastic tunneling data.
Coupled bloch-phonon oscillations in semiconductor superlattices
Dekorsy; Bartels; Kurz; Kohler; Hey; Ploog
2000-07-31
We investigate coherent Bloch oscillations in GaAs/AlxGa1-xAs superlattices with electronic miniband widths larger than the optical phonon energy. In these superlattices the Bloch frequency can be tuned into resonance with the optical phonon. Close to resonance a direct coupling of Bloch oscillations to LO phonons is observed which gives rise to the coherent excitation of LO phonons. The density necessary for driving coherent LO phonons via Bloch oscillations is about 2 orders of magnitude smaller than the density necessary to drive coherent LO phonons in bulk GaAs. The experimental observations are confirmed by the theoretical description of this phenomenon [A.W. Ghosh et al., Phys. Rev. Lett. 85, 1084 (2000)].
Pump pulse duration dependence of coherent phonon amplitudes in antimony
NASA Astrophysics Data System (ADS)
Misochko, O. V.
2016-08-01
Coherent optical phonons of A 1 k and E k symmetry in antimony have been studied using the femtosecond pump-probe technique. By varying the pump-pulse duration and keeping the probe duration constant, it was shown that the amplitude of coherent phonons of both symmetries exponentially decreases with increasing pulse width. It was found that the amplitude decay rate for the fully symmetric phonons with larger frequency is greater than that of the doubly degenerate phonons, whereas the frequency and lifetime for coherent phonons of both symmetries do not depend on the pump-pulse duration. Based on this data, the possibility of separation between dynamic and kinematic contributions to the generation mechanism of coherent phonons is discussed.
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.
Reduction of thermal conductivity by nanoscale 3D phononic crystal.
Yang, Lina; Yang, Nuo; Li, Baowen
2013-01-01
We studied how the period length and the mass ratio affect the thermal conductivity of isotopic nanoscale three-dimensional (3D) phononic crystal of Si. Simulation results by equilibrium molecular dynamics show isotopic nanoscale 3D phononic crystals can significantly reduce the thermal conductivity of bulk Si at high temperature (1000 K), which leads to a larger ZT than unity. The thermal conductivity decreases as the period length and mass ratio increases. The phonon dispersion curves show an obvious decrease of group velocities in 3D phononic crystals. The phonon's localization and band gap is also clearly observed in spectra of normalized inverse participation ratio in nanoscale 3D phononic crystal.
Reduction of Thermal Conductivity by Nanoscale 3D Phononic Crystal
Yang, Lina; Yang, Nuo; Li, Baowen
2013-01-01
We studied how the period length and the mass ratio affect the thermal conductivity of isotopic nanoscale three-dimensional (3D) phononic crystal of Si. Simulation results by equilibrium molecular dynamics show isotopic nanoscale 3D phononic crystals can significantly reduce the thermal conductivity of bulk Si at high temperature (1000 K), which leads to a larger ZT than unity. The thermal conductivity decreases as the period length and mass ratio increases. The phonon dispersion curves show an obvious decrease of group velocities in 3D phononic crystals. The phonon's localization and band gap is also clearly observed in spectra of normalized inverse participation ratio in nanoscale 3D phononic crystal. PMID:23378898
Raman spectra of semiconductor nanoparticles: Disorder-activated phonons
NASA Astrophysics Data System (ADS)
Ingale, Alka; Rustagi, K. C.
1998-09-01
We present Raman spectra of four semiconductor doped glasses and a single crystal of CdS0.55Se0.45 in the range 30-800 cm-1 in the backscattering geometry. This includes the first-order Raman scattering from the disorder-activated zone-edge phonons and the LO phonons. TO phonon modes are not observed, as in bulk CdS, for the excitation well above the lowest gap. We show that the asymmetric line profile of the LO phonon structure can be understood as a composite of two phonon modes: the zone center and the zone edge phonons. Disorder-activated modes in the (30-130)-cm-1 range and the higher-order Raman spectra are also observed and found to be consistent with this assignment.
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
Strong Coupling of Rydberg Atoms and Surface Phonon Polaritons on Piezoelectric Superlattices.
Sheng, Jiteng; Chao, Yuanxi; Shaffer, James P
2016-09-01
We propose a hybrid quantum system where the strong coupling regime can be achieved between a Rydberg atomic ensemble and propagating surface phonon polaritons on a piezoelectric superlattice. By exploiting the large electric dipole moment and long lifetime of Rydberg atoms as well as tightly confined surface phonon polariton modes, it is possible to achieve a coupling constant far exceeding the relevant decay rates. The frequency of the surface mode can be selected so that it is resonant with a Rydberg transition by engineering the piezoelectric superlattice. We describe a way to observe the Rabi splitting associated with the strong coupling regime under realistic experimental conditions. The system can be viewed as a new type of optomechanical system.
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.
Two-dimensional phononic-photonic band gap optomechanical crystal cavity.
Safavi-Naeini, Amir H; Hill, Jeff T; Meenehan, Seán; Chan, Jasper; Gröblacher, Simon; Painter, Oskar
2014-04-18
We present the fabrication and characterization of an artificial crystal structure formed from a thin film of silicon that has a full phononic band gap for microwave X-band phonons and a two-dimensional pseudo-band gap for near-infrared photons. An engineered defect in the crystal structure is used to localize optical and mechanical resonances in the band gap of the planar crystal. Two-tone optical spectroscopy is used to characterize the cavity system, showing a large coupling (g0/2π≈220 kHz) between the fundamental optical cavity resonance at ωo/2π=195 THz and colocalized mechanical resonances at frequency ωm/2π≈9.3 GHz.
Strong Coupling of Rydberg Atoms and Surface Phonon Polaritons on Piezoelectric Superlattices
NASA Astrophysics Data System (ADS)
Sheng, Jiteng; Chao, Yuanxi; Shaffer, James P.
2016-09-01
We propose a hybrid quantum system where the strong coupling regime can be achieved between a Rydberg atomic ensemble and propagating surface phonon polaritons on a piezoelectric superlattice. By exploiting the large electric dipole moment and long lifetime of Rydberg atoms as well as tightly confined surface phonon polariton modes, it is possible to achieve a coupling constant far exceeding the relevant decay rates. The frequency of the surface mode can be selected so that it is resonant with a Rydberg transition by engineering the piezoelectric superlattice. We describe a way to observe the Rabi splitting associated with the strong coupling regime under realistic experimental conditions. The system can be viewed as a new type of optomechanical system.
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.
Strong Coupling of Rydberg Atoms and Surface Phonon Polaritons on Piezoelectric Superlattices.
Sheng, Jiteng; Chao, Yuanxi; Shaffer, James P
2016-09-01
We propose a hybrid quantum system where the strong coupling regime can be achieved between a Rydberg atomic ensemble and propagating surface phonon polaritons on a piezoelectric superlattice. By exploiting the large electric dipole moment and long lifetime of Rydberg atoms as well as tightly confined surface phonon polariton modes, it is possible to achieve a coupling constant far exceeding the relevant decay rates. The frequency of the surface mode can be selected so that it is resonant with a Rydberg transition by engineering the piezoelectric superlattice. We describe a way to observe the Rabi splitting associated with the strong coupling regime under realistic experimental conditions. The system can be viewed as a new type of optomechanical system. PMID:27636473
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.
Weak phonon scattering effect of twin boundaries on thermal transmission
Dong, Huicong; Xiao, Jianwei; Melnik, Roderick; Wen, Bin
2016-01-01
To study the effect of twin boundaries on thermal transmission, thermal conductivities of twinned diamond with different twin thicknesses have been studied by NEMD simulation. Results indicate that twin boundaries show a weak phonon scattering effect on thermal transmission, which is only caused by the additional twin boundaries’ thermal resistance. Moreover, according to phonon kinetic theory, this weak phonon scattering effect of twin boundaries is mainly caused by a slightly reduced average group velocity. PMID:26822675
Isotopic effects on the phonon modes in boron carbide.
Werheit, H; Kuhlmann, U; Rotter, H W; Shalamberidze, S O
2010-10-01
The effect of isotopes ((10)B-(11)B; (12)C-(13)C) on the infrared- and Raman-active phonons of boron carbide has been investigated. For B isotopes, the contributions of the virtual crystal approximation, polarization vector and isotopical disorder are separated. Boron and carbon isotope effects are largely opposite to one another and indicate the share of the particular atoms in the atomic assemblies vibrating in specific phonon modes. Some infrared-active phonons behave as expected for monatomic boron crystals.
The Seebeck Coefficient and Phonon Drag in Silicon
Mahan, Gerald; Lindsay, Lucas R.; Broido, David
2014-12-29
We present a theory of the phonon-drag Seebeck coe cient in nondegenerate semiconductors, and apply it to silicon for temperatures 30 < T < 300K. Our calculation uses only parameters from the literature, and previous calculations of the phonon lifetime. We nd excellent agreement with the measurements of Geballe and Hull [Phys.Rev. 98, 940 (1955)]. The phonon-drag term dominates at low temperature, and shows an important dependence on the dimensions of the experimental sample.
Bloch oscillations in the presence of plasmons and phonons
Ghosh; Jonsson; Wilkins
2000-07-31
The coupling between Bloch oscillating electrons and longitudinal optical phonons in a superlattice leads to resonant phonon excitation but no gap in the Bloch-phonon spectrum. In addition, we predict a sharp transition from plasma to Bloch oscillations at nu(B) = 2nu(P). From a microscopic description with phenomenological dampings, we numerically map out the behavior of coupled Bloch-plasmon-phonon modes for a wide range of parameters, and mimic experimental conditions. Our results are in good agreement with recent experiments by Dekorsy et al. [Phys. Rev. Lett. 85, 1080 (2000)].
Transmission function and thermal conductivity of Si phononic crystals
NASA Astrophysics Data System (ADS)
Oh, Jung Hyun; Jang, Moon-Gyu; Moon, S. E.; Shin, Mincheol
2016-10-01
We investigate phonon transport properties of Si phononic crystals by using an atomistic Green function method. By varying form factors such as thickness, orientation, and pore size of unit cells, mean-free path and associated thermal conductivity of phononic crystals are obtained. We present the empirical formula for thermal conductivity as a function of cell size and, by extrapolating results from it for realistic cell sizes, the thermal conductivity is compared with experimental ones. The formula is found to predict a nearly equal amount of the suppression in thermal conductivity to the experimental values, implying a feasible calculation method for predicting thermal properties of phononic crystals.
Phonon dispersion and lifetimes in MgB2.
Shukla, Abhay; Calandra, Matteo; D'Astuto, Matteo; Lazzeri, Michele; Mauri, Francesco; Bellin, Christophe; Krisch, Michael; Karpinski, J; Kazakov, S M; Jun, J; Daghero, D; Parlinski, K
2003-03-01
We measure phonon dispersion and linewidth in a single crystal of MgB2 along the Gamma-A, Gamma-M, and A-L directions using inelastic x-ray scattering. We use density functional theory to compute the effect of both electron-phonon coupling and anharmonicity on the linewidth, obtaining excellent agreement with experiment. Anomalous broadening of the E(2g) phonon mode is found all along Gamma-A. The dominant contribution to the linewidth is always the electron-phonon coupling.
Theory of the phonon Hall effect in paramagnetic dielectrics.
Sheng, L; Sheng, D N; Ting, C S
2006-04-21
Based upon Raman spin-lattice interaction, we propose a theoretical model for the phonon Hall effect in paramagnetic dielectrics, which was discovered recently in an experiment [C. Strohm, G. L. J. A. Rikken, and P. Wyder, Phys. Rev. Lett. 95, 155901 (2005).]. The phonon Hall effect is revealed to be a phonon analogue to the anomalous Hall effect in electron systems. The thermal Hall conductivity is calculated by using the Kubo formula. Our theory reproduces the essential experimental features of the phonon Hall effect, including the sign, magnitude, and linear magnetic field dependence of the thermal Hall conductivity.
Mi, Xue-Ya; Yu, Xiaoxiang; Yao, Kai-Lun; Huang, Xiaoming; Yang, Nuo; Lü, Jing-Tao
2015-08-12
Low-dimensional electronic and glassy phononic transport are two important ingredients of highly efficient thermoelectric materials, from which two branches of thermoelectric research have emerged. One focuses on controlling electronic transport in the low dimension, while the other focuses on multiscale phonon engineering in the bulk. Recent work has benefited much from combining these two approaches, e.g., phonon engineering in low-dimensional materials. Here we propose to employ the low-dimensional electronic structure in bulk phonon-glass crystals as an alternative way to increase the thermoelectric efficiency. Through first-principles electronic structure calculations and classical molecular dynamics simulations, we show that the π-π-stacking bis(dithienothiophene) molecular crystal is a natural candidate for such an approach. This is determined by the nature of its chemical bonding. Without any optimization of the material parameters, we obtained a maximum room-temperature figure of merit, ZT, of 1.48 at optimal doping, thus validating our idea.
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.
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
Phonon excitations and magnetoelectric coupling in multiferroic RMn2O5
NASA Astrophysics Data System (ADS)
Bahoosh, Safa Golrokh; Wesselinowa, Julia M.; Trimper, Steffen
2013-05-01
Multiferroic rare-earth manganites are theoretically studied by focusing on the coupling to the lattice degrees of freedom. We demonstrate analytically that the phonon excitations in the multiferroic phase are strongly affected by the magnetoelectric coupling, the spin-phonon interaction and the anharmonic phonon-phonon interaction. Based on a microscopic model, the temperature dependence of the phonon dispersion relation is analyzed. It offers an anomaly at both the ferroelectric and the magnetic transition indicating the mutual coupling between multiferroic orders and lattice distortions. Depending on the sign of the spin-phonon coupling the phonon modes become softer or harder in accordance with experimental observations. We show that the phonon spectrum can be also controlled by an external magnetic field. The phonon energy is enhanced by increasing that field. The applied Green's function technique allows the calculation of the macroscopic magnetization depending on both the phonon-phonon and the spin-phonon couplings.
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
Low temperature phonon anomalies in cuprates
Egami, T.; Petrov, Y.; McQueeney, R.J.; Shirane, G.; Endoh, Y.
1998-08-01
The inelastic neutron scattering measurement on La{sub 1.85}Sr{sub .15}CuO{sub 4} single crystals shows that the in-plane LO phonon dispersion at low temperature is incompatible with the current view on the dynamic charge stripes, which for this composition should have the periodicity of 4a. Instead the results are consistent with the dynamic stripes with the periodicity of 2a, half of what is expected and a quarter of the magnetic periodicity. Calculations with the two-band t-t{prime}-J model suggest that such 2a stripe charge ordering may help hole pairing.
Phonon scattering in graphene over substrate steps
Sevinçli, H.; Brandbyge, M.
2014-10-13
We calculate the effect on phonon transport of substrate-induced bends in graphene. We consider bending induced by an abrupt kink in the substrate, and provide results for different step-heights and substrate interaction strengths. We find that individual substrate steps reduce thermal conductance in the range between 5% and 47%. We also consider the transmission across linear kinks formed by adsorption of atomic hydrogen at the bends and find that individual kinks suppress thermal conduction substantially, especially at high temperatures. Our analysis show that substrate irregularities can be detrimental for thermal conduction even for small step heights.
Large scale phononic metamaterials for seismic isolation
Aravantinos-Zafiris, N.; Sigalas, M. M.
2015-08-14
In this work, we numerically examine structures that could be characterized as large scale phononic metamaterials. These novel structures could have band gaps in the frequency spectrum of seismic waves when their dimensions are chosen appropriately, thus raising the belief that they could be serious candidates for seismic isolation structures. Different and easy to fabricate structures were examined made from construction materials such as concrete and steel. The well-known finite difference time domain method is used in our calculations in order to calculate the band structures of the proposed metamaterials.
Phonon thermal transport at the nanoscale
NASA Astrophysics Data System (ADS)
Wilson, Richard Brian
A comprehensive description of how heat and temperature evolve on nanometer to submicron length-scales does not yet exist because of gaps in our fundamental understanding of interfacial thermal transport and nondiffusive thermal transport. In this dissertation, I address these gaps in fundamental understanding. Interfaces often dominate the thermal response in nanoscale systems. However, a microscopic description of how heat is transported across crystal boundaries remains elusive. I present time-domain thermoreflectance (TDTR) experiments that improve our fundamental understanding of interfacial thermal transport. I show that, for clean interfaces between the two crystals, G derived from TDTR data usually lies in the range 0.25 Gmax < G < 0.7G max, where Gmax is the maximum possible conductance predicted by simple theory. Notable exceptions are Al/Si 0.99Ge0.01, and Al/Si0.2Ge0.8, where G < 0.25Gmax. Analyzing TDTR data of Al/SiGe alloys with either a two-channel diffusive model or a two-channel ballistic/diffusive model explains the unusually low thermal conductances. Both models predict a significant reduction in the effective thermal conductivity of semiconductor alloys near an interface as a result of disparate heat flux boundary conditions for different groups of phonons in combination with weak coupling between different groups of phonons in the near interface region of the crystal. While it is well established that Fourier theory can break down in nanoscale thermal transport problems, various theories for how and why Fourier theory breaks down do not adequately describe existing experiments. I characterize the relationship between the failure of Fourier theory, phonon mean-free-paths, important length-scales of the temperature-profile, and interfacial-phonon scattering by TDTR experiments on nonmetallic crystals. When crystals are heated by a laser with a radius of less than two microns, Fourier theory overpredicts the materials ability to carry heat away
Shear surface waves in phononic crystals.
Kutsenko, A A; Shuvalov, A L
2013-02-01
The existence of shear horizontal (SH) surface waves in two-dimensional periodic phononic crystals with an asymmetric depth-dependent profile is theoretically reported. Examples of dispersion spectra with bandgaps for subsonic and supersonic SH surface waves are demonstrated. The link between the effective (quasistatic) speeds of the SH bulk and surface waves is established. Calculation and analysis is based on the integral form of a projector on the subspace of evanescent modes which means no need for their explicit finding. This method can be extended to the vector waves and the three-dimensional case.
Geometric aspects of phonon polarization transport
NASA Astrophysics Data System (ADS)
Mehrafarin, Mohammad; Torabi, Reza
2009-06-01
We study the polarization transport of transverse phonons by adopting a new approach based on the quantum mechanics of spin-orbit interactions. This approach has the advantage of being apt for incorporating fluctuations in the system. The formalism gives rise to Berry effect terms manifested as the Rytov polarization rotation law and the polarization-dependent Hall effect. We derive the distribution of the Rytov rotation angle in the presence of thermal noise and show that the rotation angle is robust against fluctuations.
Phonon hydrodynamics and its applications in nanoscale heat transport
NASA Astrophysics Data System (ADS)
Guo, Yangyu; Wang, Moran
2015-09-01
Phonon hydrodynamics is an effective macroscopic method to study heat transport in dielectric solid and semiconductor. It has a clear and intuitive physical picture, transforming the abstract and ambiguous heat transport process into a concrete and evident process of phonon gas flow. Furthermore, with the aid of the abundant models and methods developed in classical hydrodynamics, phonon hydrodynamics becomes much easier to implement in comparison to the current popular approaches based on the first-principle method and kinetic theories involving complicated computations. Therefore, it is a promising tool for studying micro- and nanoscale heat transport in rapidly developing micro and nano science and technology. However, there still lacks a comprehensive account of the theoretical foundations, development and implementation of this approach. This work represents such an attempt in providing a full landscape, from physical fundamental and kinetic theory of phonons to phonon hydrodynamics in view of descriptions of phonon systems at microscopic, mesoscopic and macroscopic levels. Thus a systematical kinetic framework, summing up so far scattered theoretical models and methods in phonon hydrodynamics as individual cases, is established through a frame of a Chapman-Enskog solution to phonon Boltzmann equation. Then the basic tenets and procedures in implementing phonon hydrodynamics in nanoscale heat transport are presented through a review of its recent wide applications in modeling thermal transport properties of nanostructures. Finally, we discuss some pending questions and perspectives highlighted by a novel concept of generalized phonon hydrodynamics and possible applications in micro/nano phononics, which will shed more light on more profound understanding and credible applications of this new approach in micro- and nanoscale heat transport science.
Remarkable reduction of thermal conductivity in phosphorene phononic crystal.
Xu, Wen; Zhang, Gang
2016-05-01
Phosphorene has received much attention due to its interesting physical and chemical properties, and its potential applications such as thermoelectricity. In thermoelectric applications, low thermal conductivity is essential for achieving a high figure of merit. In this work, we propose to reduce the thermal conductivity of phosphorene by adopting the phononic crystal structure, phosphorene nanomesh. With equilibrium molecular dynamics simulations, we find that the thermal conductivity is remarkably reduced in the phononic crystal. Our analysis shows that the reduction is due to the depressed phonon group velocities induced by Brillouin zone folding, and the reduced phonon lifetimes in the phononic crystal. Interestingly, it is found that the anisotropy ratio of thermal conductivity could be tuned by the 'non-square' pores in the phononic crystal, as the phonon group velocities in the direction with larger projection of pores is more severely suppressed, leading to greater reduction of thermal conductivity in this direction. Our work provides deep insight into thermal transport in phononic crystals and proposes a new strategy to reduce the thermal conductivity of monolayer phosphorene. PMID:27033566
Remarkable reduction of thermal conductivity in phosphorene phononic crystal.
Xu, Wen; Zhang, Gang
2016-05-01
Phosphorene has received much attention due to its interesting physical and chemical properties, and its potential applications such as thermoelectricity. In thermoelectric applications, low thermal conductivity is essential for achieving a high figure of merit. In this work, we propose to reduce the thermal conductivity of phosphorene by adopting the phononic crystal structure, phosphorene nanomesh. With equilibrium molecular dynamics simulations, we find that the thermal conductivity is remarkably reduced in the phononic crystal. Our analysis shows that the reduction is due to the depressed phonon group velocities induced by Brillouin zone folding, and the reduced phonon lifetimes in the phononic crystal. Interestingly, it is found that the anisotropy ratio of thermal conductivity could be tuned by the 'non-square' pores in the phononic crystal, as the phonon group velocities in the direction with larger projection of pores is more severely suppressed, leading to greater reduction of thermal conductivity in this direction. Our work provides deep insight into thermal transport in phononic crystals and proposes a new strategy to reduce the thermal conductivity of monolayer phosphorene.
Remarkable reduction of thermal conductivity in phosphorene phononic crystal
NASA Astrophysics Data System (ADS)
Xu, Wen; Zhang, Gang
2016-05-01
Phosphorene has received much attention due to its interesting physical and chemical properties, and its potential applications such as thermoelectricity. In thermoelectric applications, low thermal conductivity is essential for achieving a high figure of merit. In this work, we propose to reduce the thermal conductivity of phosphorene by adopting the phononic crystal structure, phosphorene nanomesh. With equilibrium molecular dynamics simulations, we find that the thermal conductivity is remarkably reduced in the phononic crystal. Our analysis shows that the reduction is due to the depressed phonon group velocities induced by Brillouin zone folding, and the reduced phonon lifetimes in the phononic crystal. Interestingly, it is found that the anisotropy ratio of thermal conductivity could be tuned by the ‘non-square’ pores in the phononic crystal, as the phonon group velocities in the direction with larger projection of pores is more severely suppressed, leading to greater reduction of thermal conductivity in this direction. Our work provides deep insight into thermal transport in phononic crystals and proposes a new strategy to reduce the thermal conductivity of monolayer phosphorene.
Phonon thermal transport through tilt grain boundaries in strontium titanate
Zheng, Zexi; Chen, Xiang; Yang, Shengfeng; Xiong, Liming; Chen, Youping; Deng, Bowen; Chernatynskiy, Aleksandr
2014-08-21
In this work, we perform nonequilibrium molecular dynamics simulations to study phonon scattering at two tilt grain boundaries (GBs) in SrTiO{sub 3}. Mode-wise energy transmission coefficients are obtained based on phonon wave-packet dynamics simulations. The Kapitza conductance is then quantified using a lattice dynamics approach. The obtained results of the Kapitza conductance of both GBs compare well with those obtained by the direct method, except for the temperature dependence. Contrary to common belief, the results of this work show that the optical modes in SrTiO{sub 3} contribute significantly to phonon thermal transport, accounting for over 50% of the Kapitza conductance. To understand the effect of the GB structural disorder on phonon transport, we compare the local phonon density of states of the atoms in the GB region with that in the single crystalline grain region. Our results show that the excess vibrational modes introduced by the structural disorder do not have a significant effect on phonon scattering at the GBs, but the absence of certain modes in the GB region appears to be responsible for phonon reflections at GBs. This work has also demonstrated phonon mode conversion and simultaneous generation of new modes. Some of the new modes have the same frequency as the initial wave packet, while some have the same wave vector but lower frequencies.
Topological Phonons and Weyl Lines in Three Dimensions.
Stenull, Olaf; Kane, C L; Lubensky, T C
2016-08-01
Topological mechanics and phononics have recently emerged as an exciting field of study. Here we introduce and study generalizations of the three-dimensional pyrochlore lattice that have topologically protected edge states and Weyl lines in their bulk phonon spectra, which lead to zero surface modes that flip from one edge to the opposite as a function of surface wave number. PMID:27541476
Topological Phonons and Weyl Lines in Three Dimensions
NASA Astrophysics Data System (ADS)
Stenull, Olaf; Kane, C. L.; Lubensky, T. C.
2016-08-01
Topological mechanics and phononics have recently emerged as an exciting field of study. Here we introduce and study generalizations of the three-dimensional pyrochlore lattice that have topologically protected edge states and Weyl lines in their bulk phonon spectra, which lead to zero surface modes that flip from one edge to the opposite as a function of surface wave number.
New quantum properties of phonons and their detection
NASA Technical Reports Server (NTRS)
Artoni, Maurizo; Birman, Joseph L.
1994-01-01
We present a theoretical investigation on new and interesting properties of the phonon polarization field in solids. In particular, non-classical aspects of the phonon population and an experimental scheme that would enable one to detect them will be discussed.
Phonon-based scalable quantum computing and sensing (Presentation Video)
NASA Astrophysics Data System (ADS)
El-Kady, Ihab
2015-04-01
Quantum computing fundamentally depends on the ability to concurrently entangle and individually address/control a large number of qubits. In general, the primary inhibitors of large scale entanglement are qubit dependent; for example inhomogeneity in quantum dots, spectral crowding brought about by proximity-based entanglement in ions, weak interactions of neutral atoms, and the fabrication tolerances in the case of Si-vacancies or SQUIDs. We propose an inherently scalable solid-state qubit system with individually addressable qubits based on the coupling of a phonon with an acceptor impurity in a high-Q Phononic Crystal resonant cavity. Due to their unique nonlinear properties, phonons enable new opportunities for quantum devices and physics. We present a phononic crystal-based platform for observing the phonon analogy of cavity quantum electrodynamics, called phonodynamics, in a solid-state system. Practical schemes involve selective placement of a single acceptor atom in the peak of the strain field in a high-Q phononic crystal cavity that enables strong coupling of the phonon modes to the energy levels of the atom. A qubit is then created by entangling a phonon at the resonance frequency of the cavity with the atomic acceptor states. We show theoretical optimization of the cavity design and excitation waveguides, along with estimated performance figures of the phoniton system. Qubits based on this half-sound, half-matter quasi-particle, may outcompete other quantum architectures in terms of combined emission rate, coherence lifetime, and fabrication demands.
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.
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
Size effects in thermal conduction by phonons
NASA Astrophysics Data System (ADS)
Allen, Philip B.
2014-08-01
Heat transport in nanoscale systems is both hard to measure microscopically, and hard to interpret. Ballistic and diffusive heat flow coexist, adding confusion. This paper looks at a very simple case: a nanoscale crystal repeated periodically. This is a popular model for simulation of bulk heat transport using classical molecular dynamics (MD), and is related to transient thermal grating experiments. Nanoscale effects are seen in perhaps their simplest form. The model is solved by an extension of standard quasiparticle gas theory of bulk solids. Both structure and heat flow are constrained by periodic boundary conditions. Diffusive transport is fully included, while ballistic transport by phonons of a long mean free path is diminished in a specific way. Heat current J (x) and temperature gradient ∇T (x') have a nonlocal relationship, via κ (x-x'), over a distance |x-x'| determined by phonon mean free paths. In MD modeling of bulk conductivity, finite computer resources limit system size. Long mean free paths, comparable to the scale of heating and cooling, cause undesired finite-size effects that have to be removed by extrapolation. The present model allows this extrapolation to be quantified. Calculations based on the Peierls-Boltzmann equation, using a generalized Debye model, show that extrapolation involves fractional powers of 1/L. It is also argued that heating and cooling should be distributed sinusoidally [ė∝cos(2πx/L)] to improve convergence of numerics.
Thermoelectric amplification of phonons in graphene
NASA Astrophysics Data System (ADS)
Dompreh, K. A.; Mensah, N. G.; Mensah, S. Y.; Fosuhene, S. K.
2016-06-01
Amplification of acoustic in-plane phonons due to an external temperature gradient (∇T) in single-layer graphene (SLG) was studied theoretically. The threshold temperature gradient (∇ T ) 0 g and the threshold voltage (V T ) 0 g in SLG were evaluated. For T = 77 K , the calculated value for (∇ T ) 0 g = 746.8 K / cm and (V T ) 0 g = 6.6 mV . The calculation was done in the hypersound regime. Further, the dependence of the normalized amplification ( Γ / Γ 0 ) on the frequency ω q and ∇ T / T were evaluated numerically and presented graphically. The calculated threshold temperature gradient (V T ) 0 g for SLG was higher than that obtained for homogeneous semiconductors (n-InSb) (∇ T ) 0 hom ≈ 10 3 K / cm , superlattices (∇ T ) 0 S L ≈ 384 K / cm , and cylindrical quantum wire (∇ T ) 0 c q w ≈ 10 2 K / cm . This makes SLG a much better material for thermoelectric phonon amplification.
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 anomalies in some iron telluride materials
NASA Astrophysics Data System (ADS)
Homes, C. C.; Dai, Y. M.; Schneeloch, J.; Zhong, R. D.; Gu, G. D.
2016-03-01
The detailed temperature dependence of the infrared-active mode in Fe1.03Te (TN≃68 K) and Fe1.13Te (TN≃56 K) has been examined, and the position, width, strength, and asymmetry parameter have been determined using an asymmetric Fano profile superimposed on an electronic background. In both materials the frequency of the mode increases as the temperature is reduced; however, there is also a slight asymmetry in the line shape, indicating that the mode is coupled to either spin or charge excitations. Below TN there is an anomalous decrease in frequency, and the mode shows little temperature dependence, at the same time becoming more symmetric, suggesting a reduction in spin- or electron-phonon coupling. The frequency of the infrared-active mode and the magnitude of the shift below TN are predicted reasonably well by first-principles calculations; however, the predicted splitting of the mode is not observed. In superconducting FeTe0.55Se0.45 (Tc≃14 K) the infrared-active Eu mode displays asymmetric line shape at all temperatures, which is most pronounced between 100 and 200 K, indicating the presence of either spin- or electron-phonon coupling, which may be a necessary prerequisite for superconductivity in this class of materials.
Phonon anomalies in some iron telluride materials
C. C. Homes; Dai, Y. M.; Schneeloch, J.; Zhong, R. D.; Gu, G. D.
2016-03-21
In this paper, the detailed temperature dependence of the infrared-active mode in Fe1.03Te (TN ≃ 68 K) and Fe1.13Te (TN ≃ 56 K) has been examined, and the position, width, strength, and asymmetry parameter have been determined using an asymmetric Fano profile superimposed on an electronic background. In both materials the frequency of the mode increases as the temperature is reduced; however, there is also a slight asymmetry in the line shape, indicating that the mode is coupled to either spin or charge excitations. Below TN there is an anomalous decrease in frequency, and the mode shows little temperature dependence,more » at the same time becoming more symmetric, suggesting a reduction in spin- or electron-phonon coupling. The frequency of the infrared-active mode and the magnitude of the shift below TN are predicted reasonably well by first-principles calculations; however, the predicted splitting of the mode is not observed. In superconducting FeTe0.55Se0.45 (Tc ≃ 14 K) the infrared-active Eu mode displays asymmetric line shape at all temperatures, which is most pronounced between 100 – 200 K, indicating the presence of either spin- or electron-phonon coupling, which may be a necessary prerequisite for superconductivity in this class of materials.« less
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.
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.
Phonon-mediated quantum spin simulator employing a planar ionic crystal in a Penning trap
NASA Astrophysics Data System (ADS)
Wang, C.-C. Joseph; Keith, Adam C.; Freericks, J. K.
2013-01-01
We derive the normal modes for a rotating Coulomb ion crystal in a Penning trap, quantize the motional degrees of freedom, and illustrate how they can be driven by a spin-dependent optical dipole force to create a quantum spin simulator on a triangular lattice with hundreds of spins. The analysis for the axial modes (oscillations perpendicular to the two-dimensional crystal plane) follow a standard normal-mode analysis, while the remaining planar modes are more complicated to analyze because they have velocity-dependent forces in the rotating frame. After quantizing the normal modes into phonons, we illustrate some of the different spin-spin interactions that can be generated by entangling the motional degrees of freedom with the spin degrees of freedom via a spin-dependent optical dipole force. In addition to the well-known power-law dependence of the spin-spin interactions when driving the axial modes blue of the phonon band, we notice certain parameter regimes in which the level of frustration between the spins can be engineered by driving the axial or planar phonon modes at different frequencies. These systems may allow for the analog simulation of quantum spin glasses with large numbers of spins.
Lattice dynamics and electron-phonon coupling calculations using nondiagonal supercells
NASA Astrophysics Data System (ADS)
Lloyd-Williams, Jonathan; Monserrat, Bartomeu
Quantities derived from electron-phonon coupling matrix elements require a fine sampling of the vibrational Brillouin zone. Converged results are typically not obtainable using the direct method, in which a perturbation is frozen into the system and the total energy derivatives are calculated using a finite difference approach, because the size of simulation cell needed is prohibitively large. We show that it is possible to determine the response of a periodic system to a perturbation characterized by a wave vector with reduced fractional coordinates (m1 /n1 ,m2 /n2 ,m3 /n3) using a supercell containing a number of primitive cells equal to the least common multiple of n1, n2, and n3. This is accomplished by utilizing supercell matrices containing nonzero off-diagonal elements. We present the results of electron-phonon coupling calculations using the direct method to sample the vibrational Brillouin zone with grids of unprecedented size for a range of systems, including the canonical example of diamond. We also demonstrate that the use of nondiagonal supercells reduces by over an order of magnitude the computational cost of obtaining converged vibrational densities of states and phonon dispersion curves. J.L.-W. is supported by the Engineering and Physical Sciences Research Council (EPSRC). B.M. is supported by Robinson College, Cambridge, and the Cambridge Philosophical Society. This work was supported by EPSRC Grants EP/J017639/1 and EP/K013564/1.
Phonon 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
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.
Heterobarrier for converting hot-phonon energy to electric potential
NASA Astrophysics Data System (ADS)
Shin, Seungha; Melnick, Corey; Kaviany, Massoud
2013-02-01
We show that hot phonons emitted in energy conversion or resistive processes can be converted to electric potential in heterobarrier structures. Using phonon and electron interaction kinetics and self-consistent ensemble Monte Carlo, we find the favorable conditions for unassisted absorption of hot phonons and design graded heterobarriers for their direct conversion into electric energy. Tandem barriers with nearly optical-phonon height allow for substantial potential gain without current loss. We find that 19% of hot phonons can be harvested with an optimized GaAs/AlxGa1-xAs barrier structure over a range of current and electron densities, thus enhancing the overall energy conversion efficiency and reducing waste heat.
Phonon anharmonicity of monoclinic zirconia and yttrium-stabilized zirconia
Li, Chen W.; Smith, Hillary L.; Lan, Tian; Niedziela, Jennifer L.; Munoz, Jorge A.; Keith, J. Brian; Mauger, L.; Abernathy, Douglas L; Fultz, B.
2015-04-13
Inelastic neutron scattering measurements on monoclinic zirconia (ZrO_{2}) and 8 mol% yttrium-stabilized zirconia were performed at temperatures from 300 to 1373 ωK. We reported temperature-dependent phonon densities of states (DOS) and Raman spectra obtained at elevated temperatures. First-principles lattice dynamics calculations with density functional theory gave total and partial phonon DOS curves and mode Grüneisen parameters. These mode Grüneisen parameters were used to predict the experimental temperature dependence of the phonon DOS with partial success. However, substantial anharmonicity was found at elevated temperatures, especially for phonon modes dominated by the motions of oxygen atoms. Yttrium-stabilized zirconia (YSZ) was somewhat more anharmonic and had a broader phonon spectrum at low temperatures, owing in part to defects in its structure. YSZ also has a larger vibrational entropy than monoclinic zirconia.
Phonon anharmonicity of monoclinic zirconia and yttrium-stabilized zirconia
Li, Chen W.; Smith, Hillary L.; Lan, Tian; Niedziela, Jennifer L.; Munoz, Jorge A.; Keith, J. Brian; Mauger, L.; Abernathy, Douglas L; Fultz, B.
2015-04-13
Inelastic neutron scattering measurements on monoclinic zirconia (ZrO2) and 8 mol% yttrium-stabilized zirconia were performed at temperatures from 300 to 1373 ωK. We reported temperature-dependent phonon densities of states (DOS) and Raman spectra obtained at elevated temperatures. First-principles lattice dynamics calculations with density functional theory gave total and partial phonon DOS curves and mode Grüneisen parameters. These mode Grüneisen parameters were used to predict the experimental temperature dependence of the phonon DOS with partial success. However, substantial anharmonicity was found at elevated temperatures, especially for phonon modes dominated by the motions of oxygen atoms. Yttrium-stabilized zirconia (YSZ) was somewhat moremore » anharmonic and had a broader phonon spectrum at low temperatures, owing in part to defects in its structure. YSZ also has a larger vibrational entropy than monoclinic zirconia.« less
Phonon anharmonicity of monoclinic zirconia and yttrium-stabilized zirconia
NASA Astrophysics Data System (ADS)
Li, C. W.; Smith, H. L.; Lan, T.; Niedziela, J. L.; Muñoz, J. A.; Keith, J. B.; Mauger, L.; Abernathy, D. L.; Fultz, B.
2015-04-01
Inelastic neutron scattering measurements on monoclinic zirconia (ZrO2 ) and 8 mol% yttrium-stabilized zirconia were performed at temperatures from 300 to 1373 w K . Temperature-dependent phonon densities of states (DOS) are reported, as are Raman spectra obtained at elevated temperatures. First-principles lattice dynamics calculations with density functional theory gave total and partial phonon DOS curves and mode Grüneisen parameters. These mode Grüneisen parameters were used to predict the experimental temperature dependence of the phonon DOS with partial success. However, substantial anharmonicity was found at elevated temperatures, especially for phonon modes dominated by the motions of oxygen atoms. Yttrium-stabilized zirconia (YSZ) was somewhat more anharmonic and had a broader phonon spectrum at low temperatures, owing in part to defects in its structure. YSZ also has a larger vibrational entropy than monoclinic zirconia.
Dissipation induced by phonon elastic scattering in crystals
Li, Guolong; Ren, Zhongzhou; Zhang, Xin
2016-01-01
We demonstrate that the phonon elastic scattering leads to a dominant dissipation in crystals at low temperature. The two-level systems (TLSs) should be responsible for the elastic scattering, whereas the dissipation induced by static-point defects (SPDs) can not be neglected. One purpose of this work is to show how the energy splitting distribution of the TLS ensemble affects the dissipation. Besides, this article displays the proportion of phonon-TLS elastic scattering to total phonon dissipation. The coupling coefficient of phonon-SPD scattering and the constant P0 of the TLS distribution are important that we estimate their magnitudes in this paper. Our results is useful to understand the phonon dissipation mechanism, and give some clues to improve the performance of mechanical resonators, apply the desired defects, or reveal the atom configuration in lattice structure of disordered crystals. PMID:27669517
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.
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.
Extremely high electron mobility in a phonon-glass semimetal.
Ishiwata, S; Shiomi, Y; Lee, J S; Bahramy, M S; Suzuki, T; Uchida, M; Arita, R; Taguchi, Y; Tokura, Y
2013-06-01
The electron mobility is one of the key parameters that characterize the charge-carrier transport properties of materials, as exemplified by the quantum Hall effect as well as high-efficiency thermoelectric and solar energy conversions. For thermoelectric applications, introduction of chemical disorder is an important strategy for reducing the phonon-mediated thermal conduction, but is usually accompanied by mobility degradation. Here, we show a multilayered semimetal β-CuAgSe overcoming such a trade-off between disorder and mobility. The polycrystalline ingot shows a giant positive magnetoresistance and Shubnikov de Haas oscillations, indicative of a high-mobility small electron pocket derived from the Ag s-electron band. Ni doping, which introduces chemical and lattice disorder, further enhances the electron mobility up to 90,000 cm(2) V(-1) s(-1) at 10 K, leading not only to a larger magnetoresistance but also a better thermoelectric figure of merit. This Ag-based layered semimetal with a glassy lattice is a new type of promising thermoelectric material suitable for chemical engineering.
The phonon theory of liquid thermodynamics
Bolmatov, D.; Brazhkin, V. V.; Trachenko, K.
2012-01-01
Heat capacity of matter is considered to be its most important property because it holds information about system's degrees of freedom as well as the regime in which the system operates, classical or quantum. Heat capacity is well understood in gases and solids but not in the third main state of matter, liquids, and is not discussed in physics textbooks as a result. The perceived difficulty is that interactions in a liquid are both strong and system-specific, implying that the energy strongly depends on the liquid type and that, therefore, liquid energy can not be calculated in general form. Here, we develop a phonon theory of liquids where this problem is avoided. The theory covers both classical and quantum regimes. We demonstrate good agreement of calculated and experimental heat capacity of 21 liquids, including noble, metallic, molecular and hydrogen-bonded network liquids in a wide range of temperature and pressure. PMID:22639729
Parity-Time Synthetic Phononic Media.
Christensen, J; Willatzen, M; Velasco, V R; Lu, M-H
2016-05-20
Classical systems containing cleverly devised combinations of loss and gain elements constitute extremely rich building units that can mimic non-Hermitian properties, which conventionally are attainable in quantum mechanics only. Parity-time (PT) symmetric media, also referred to as synthetic media, have been devised in many optical systems with the ground breaking potential to create nonreciprocal structures and one-way cloaks of invisibility. Here we demonstrate a feasible approach for the case of sound where the most important ingredients within synthetic materials, loss and gain, are achieved through electrically biased piezoelectric semiconductors. We study first how wave attenuation and amplification can be tuned, and when combined, can give rise to a phononic PT synthetic media with unidirectional suppressed reflectance, a feature directly applicable to evading sonar detection.
Optomechanics with two-phonon driving
NASA Astrophysics Data System (ADS)
Levitan, B. A.; Metelmann, A.; Clerk, A. A.
2016-09-01
We consider the physics of an optomechanical cavity subject to coherent two-phonon driving, i.e. degenerate parametric amplification of the mechanical mode. We show that in such a system, the cavity mode can effectively ‘inherit’ parametric driving from the mechanics, yielding phase-sensitive amplification and squeezing of optical signals reflected from the cavity. We also demonstrate how such a system can be used to perform single-quadrature detection of a near-resonant narrow-band force applied to the mechanics with extremely low added noise from the optics. The system also exhibits strong differences from a conventional degenerate parametric amplifier: in particular, the cavity spectral function can become negative, indicating a negative effective photon temperature.
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.
Parity-Time Synthetic Phononic Media.
Christensen, J; Willatzen, M; Velasco, V R; Lu, M-H
2016-05-20
Classical systems containing cleverly devised combinations of loss and gain elements constitute extremely rich building units that can mimic non-Hermitian properties, which conventionally are attainable in quantum mechanics only. Parity-time (PT) symmetric media, also referred to as synthetic media, have been devised in many optical systems with the ground breaking potential to create nonreciprocal structures and one-way cloaks of invisibility. Here we demonstrate a feasible approach for the case of sound where the most important ingredients within synthetic materials, loss and gain, are achieved through electrically biased piezoelectric semiconductors. We study first how wave attenuation and amplification can be tuned, and when combined, can give rise to a phononic PT synthetic media with unidirectional suppressed reflectance, a feature directly applicable to evading sonar detection. PMID:27258882
Phonon-Mediated Nonclassical Interference in Diamond
NASA Astrophysics Data System (ADS)
England, Duncan G.; Fisher, Kent A. G.; MacLean, Jean-Philippe W.; Bustard, Philip J.; Heshami, Khabat; Resch, Kevin J.; Sussman, Benjamin J.
2016-08-01
Quantum interference of single photons is a fundamental aspect of many photonic quantum processing and communication protocols. Interference requires that the multiple pathways through an interferometer be temporally indistinguishable to within the coherence time of the photon. In this Letter, we use a diamond quantum memory to demonstrate interference between quantum pathways, initially temporally separated by many multiples of the optical coherence time. The quantum memory can be viewed as a light-matter beam splitter, mapping a THz-bandwidth single photon to a variable superposition of the output optical mode and stored phononic mode. Because the memory acts both as a beam splitter and as a buffer, the relevant coherence time for interference is not that of the photon, but rather that of the memory. We use this mechanism to demonstrate nonclassical single-photon and two-photon interference between quantum pathways initially separated by several picoseconds, even though the duration of the photons themselves is just ˜250 fs .
Phonon Overlaps in Molecular Quantum Dot Systems
NASA Astrophysics Data System (ADS)
Chang, Connie; Sethna, James
2004-03-01
We model the amplitudes and frequencies of the vibrational sidebands for the new molecular quantum dot systems. We calculate the Franck-Condon phonon overlaps in the 3N-dimensional configuration sapce. We solve the general case where the vibrational frequencies and eigenmodes change during the transition. We perform PM3 and DFT calculations for the case of the dumb bell-shaped C140 molecule. We find that the strongest amplitudes are associated with the 11 meV stretch mode, in agreement with experiment. The experimental amplitudes vary from molecule to molecule; indicating that the molecular overlaps are environment dependent. We explore overlaps in the presence of external electric fields from image charges and counter ions.
Phonon hydrodynamics in two-dimensional materials
NASA Astrophysics Data System (ADS)
Marzari, Nicola
2015-03-01
The conduction of heat in two dimensions displays a wealth of fascinating phenomena of key relevance to the scientific and technological applications of novel layered materials. Here, we use third order density-functional perturbation theory and an exact, variational solution of the Boltzmann transport equation to study fully from first-principles phonon transport and heat conductivity in graphene and related materials (boron nitride, functionalized derivatives, transition-metal dichalcogenides...). Very good agreement is obtained with experimental data, where available, together with a microscopic understanding of the collective character of heat-carrying excitations, and the unusual length scales involved. Last, and at variance with typical three-dimensional solids, normal processes dominate over Umklapp scattering well above cryogenic conditions, extending to room temperature and more. As a result, novel hydrodynamics regimes, hitherto typically confined to ultra-low temperatures, become readily apparent. Work done in collaboration with Andrea Cepellotti, Giorgia Fugallo, Lorenzo Paulatto, Michele Lazzeri, and Francesco Mauri.
Parity-Time Synthetic Phononic Media
NASA Astrophysics Data System (ADS)
Christensen, J.; Willatzen, M.; Velasco, V. R.; Lu, M.-H.
2016-05-01
Classical systems containing cleverly devised combinations of loss and gain elements constitute extremely rich building units that can mimic non-Hermitian properties, which conventionally are attainable in quantum mechanics only. Parity-time (P T ) symmetric media, also referred to as synthetic media, have been devised in many optical systems with the ground breaking potential to create nonreciprocal structures and one-way cloaks of invisibility. Here we demonstrate a feasible approach for the case of sound where the most important ingredients within synthetic materials, loss and gain, are achieved through electrically biased piezoelectric semiconductors. We study first how wave attenuation and amplification can be tuned, and when combined, can give rise to a phononic P T synthetic media with unidirectional suppressed reflectance, a feature directly applicable to evading sonar detection.
Phonon sidebands of excitons bound to isoelectronic impurities in semiconductors
Zhang, Y.; Ge, W.; Sturge, M.D. ); Zheng, J.; Wu, B. )
1993-03-15
The configuration coordinate (CC) and momentum conservation (MC) models have been widely used to explain the phonon sidebands of impurity spectra in semiconductors. In this paper, the distinction between the CC and MC models is discussed. We conclude that the MC model only applies to shallow Coulombic impurities; in other cases, such as isoelectronic traps, the CC model is more appropriate. We show that the Huang-Rhys parameters for bulk phonon modes coupling to a bound electron or exciton can be calculated from the bound-state wave function in [ital k] space if the phonon-induced intervalley and intravalley electron scattering processes of the pure crystal are known. We study in detail the phonon sidebands of nitrogen-bound excitons in GaP, giving the selection rules for electron-phonon coupling in the CC model, and showing that their strength can be well accounted for by the CC model. The apparently anomalous [ital X]'' peak of the LO-phonon sideband in GaP:N is shown to be associated with intervalley scattering in the conduction band. The MC model, which has been used in an attempt to explain the phonon sidebands of GaP:N in some previous work, is shown to be inapplicable to this case.
Variable-Range Hopping through Marginally Localized Phonons
NASA Astrophysics Data System (ADS)
Banerjee, Sumilan; Altman, Ehud
2016-03-01
We investigate the effect of coupling Anderson localized particles in one dimension to a system of marginally localized phonons having a symmetry protected delocalized mode at zero frequency. This situation is naturally realized for electrons coupled to phonons in a disordered nanowire as well as for ultracold fermions coupled to phonons of a superfluid in a one-dimensional disordered trap. To determine if the coupled system can be many-body localized we analyze the phonon-mediated hopping transport for both the weak and strong coupling regimes. We show that the usual variable-range hopping mechanism involving a low-order phonon process is ineffective at low temperature due to discreteness of the bath at the required energy. Instead, the system thermalizes through a many-body process involving exchange of a diverging number n ∝-log T of phonons in the low temperature limit. This effect leads to a highly singular prefactor to Mott's well-known formula and strongly suppresses the variable range hopping rate. Finally, we comment on possible implications of this physics in higher dimensional electron-phonon coupled systems.
Resonant squeezing and the anharmonic decay of coherent phonons
NASA Astrophysics Data System (ADS)
Fahy, Stephen; Murray, Éamonn D.; Reis, David A.
2016-04-01
We show that the anharmonic decay of large-amplitude coherent phonons in a solid generates strongly enhanced squeezing of the phonon modes near points of the Brillouin zone where energy conservation in the three-phonon decay process is satisfied. The squeezing process leads to temporal oscillations of the mean-square displacement of target modes in resonance with the coherent phonon, which are characteristic of coherent phonon decay and do not occur in the decay of a phonon in a well-defined number state. For realistic material parameters of optically excited group-V semimetals, we predict that this squeezing results in strongly enhanced oscillations of the x-ray diffuse scattering intensity at sharply defined values of the x-ray momentum transfer. Numerical simulations of the phonon dynamics and the x-ray diffuse scattering in optically excited bismuth, using harmonic and anharmonic force parameters calculated with constrained density functional theory, demonstrate oscillations of the diffuse scattering intensity of magnitude 10%-20% of the thermal background at points of the Brillouin zone, where resonance occurs. Such oscillations should be observable using time-resolved optical-pump and x-ray-probe facilities available at current x-ray free-electron laser sources.
Thickness-dependent coherent phonon frequency in ultrathin FeSe/SrTiO_{3} films
Yang, Shuolong; Sobota, Jonathan A.; Leuenberger, Dominik; Kemper, Alexander F.; Lee, James J.; Schmitt, Felix T.; Li, Wei; Moore, Rob G.; Kirchmann, Patrick S.; Shen, Zhi -Xun
2015-06-01
Ultrathin FeSe films grown on SrTiO_{3} substrates are a recent milestone in atomic material engineering due to their important role in understanding unconventional superconductivity in Fe-based materials. By using femtosecond time- and angle-resolved photoelectron spectroscopy, we study phonon frequencies in ultrathin FeSe/SrTiO_{3} films grown by molecular beam epitaxy. After optical excitation, we observe periodic modulations of the photoelectron spectrum as a function of pump–probe delay for 1-unit-cell, 3-unit-cell, and 60-unit-cell thick FeSe films. The frequencies of the coherent intensity oscillations increase from 5.00 ± 0.02 to 5.25 ± 0.02 THz with increasing film thickness. By comparing with previous works, we attribute this mode to the Se A_{1g} phonon. The dominant mechanism for the phonon softening in 1-unit-cell thick FeSe films is a substrate-induced lattice strain. Results demonstrate an abrupt phonon renormalization due to a lattice mismatch between the ultrathin film and the substrate.
Deterministic Single-Phonon Source Triggered by a Single Photon.
Söllner, Immo; Midolo, Leonardo; Lodahl, Peter
2016-06-10
We propose a scheme that enables the deterministic generation of single phonons at gigahertz frequencies triggered by single photons in the near infrared. This process is mediated by a quantum dot embedded on chip in an optomechanical circuit, which allows for the simultaneous control of the relevant photonic and phononic frequencies. We devise new optomechanical circuit elements that constitute the necessary building blocks for the proposed scheme and are readily implementable within the current state-of-the-art of nanofabrication. This will open new avenues for implementing quantum functionalities based on phonons as an on-chip quantum bus.
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.
Deterministic Single-Phonon Source Triggered by a Single Photon
NASA Astrophysics Data System (ADS)
Söllner, Immo; Midolo, Leonardo; Lodahl, Peter
2016-06-01
We propose a scheme that enables the deterministic generation of single phonons at gigahertz frequencies triggered by single photons in the near infrared. This process is mediated by a quantum dot embedded on chip in an optomechanical circuit, which allows for the simultaneous control of the relevant photonic and phononic frequencies. We devise new optomechanical circuit elements that constitute the necessary building blocks for the proposed scheme and are readily implementable within the current state-of-the-art of nanofabrication. This will open new avenues for implementing quantum functionalities based on phonons as an on-chip quantum bus.
Observation of Anomalous Phonons in Orthorhombic Rare-earth Manganites
P Gao; H Chen; T Tyson; Z Liu; J Bai; L Wang; Y Chio; S Cheong
2011-12-31
We observe the appearance of a phonon near the lock-in temperature in orthorhombic REMnO{sub 3} (RE denotes rare earth) (RE: Lu and Ho) and anomalous phonon hardening in orthorhombic LuMnO{sub 3}. The anomalous phonon occurs at the onset of spontaneous polarization. No such changes were found in incommensurate orthorhombic DyMnO{sub 3}. These observations directly reveal different electric polarization mechanisms in the E-type and incommensurate-type orthorhombic REMnO{sub 3}.
Influence of pulse width and detuning on coherent phonon generation
NASA Astrophysics Data System (ADS)
Nakamura, Kazutaka G.; Shikano, Yutaka; Kayanuma, Yosuke
2015-10-01
We investigated the coherent phonon generation mechanism by irradiation of an ultrashort pulse with a simple two-level model. Our derived formulation shows that both impulsive stimulated Raman scattering (ISRS) and impulsive absorption (IA) simultaneously occur, and phonon wave packets are generated in the electronic ground and excited states by ISRS and IA, respectively. We identify the dominant process from the amplitude of the phonon oscillation. For short pulse widths, ISRS is very small and becomes larger as the pulse width increases. We also show that the initial phase is dependent on the pulse width and the detuning.
Strong and Coherent Coupling between Localized and Propagating Phonon Polaritons.
Gubbin, Christopher R; Martini, Francesco; Politi, Alberto; Maier, Stefan A; De Liberato, Simone
2016-06-17
Following the recent observation of localized phonon polaritons in user-defined silicon carbide nanoresonators, here we demonstrate strong and coherent coupling between those localized modes and propagating phonon polaritons bound to the surface of the nanoresonator's substrate. In order to obtain phase matching, the nanoresonators have been fabricated to serve the double function of hosting the localized modes, while also acting as a grating for the propagating ones. The coherent coupling between long lived, optically accessible localized modes, and low-loss propagative ones, opens the way to the design and realization of phonon-polariton based coherent circuits. PMID:27367398
Strong coupling between mid-infrared localized plasmons and phonons.
Wan, Weiwei; Yang, Xiaodong; Gao, Jie
2016-05-30
We numerically and experimentally demonstrate strong coupling between the mid-infrared localized surface plasmon resonances supported by plasmonic metamaterials and the phonon vibrational resonances of polymethyl methacrylate (PMMA) molecules. The plasmonic resonances are tuned across the phonon resonance of PMMA molecules at 52 THz to observe the strong coupling, which manifests itself as an anti-crossing feature with two newly formed plasmon-phonon modes. It is also shown that the forbidden energy gap due to mode splitting is proportional to the overlapped optical power between the plasmonic resonance mode and the PMMA molecules, providing an effective approach for manipulating the coupling strength of light-matter interaction. PMID:27410151
Deterministic Single-Phonon Source Triggered by a Single Photon.
Söllner, Immo; Midolo, Leonardo; Lodahl, Peter
2016-06-10
We propose a scheme that enables the deterministic generation of single phonons at gigahertz frequencies triggered by single photons in the near infrared. This process is mediated by a quantum dot embedded on chip in an optomechanical circuit, which allows for the simultaneous control of the relevant photonic and phononic frequencies. We devise new optomechanical circuit elements that constitute the necessary building blocks for the proposed scheme and are readily implementable within the current state-of-the-art of nanofabrication. This will open new avenues for implementing quantum functionalities based on phonons as an on-chip quantum bus. PMID:27341236
First-Principles Calculation of forces and phonons in solid
NASA Astrophysics Data System (ADS)
Ning, Zhenhua; Shelton, William
We have developed a multiple scattering theory approach to calculate Hellmann-Feynman forces and phonons via the calculation of the force constant and dynamical matrix. To demonstrate the accuracy and validity of our approach we compare with the ELK code, which is a full potential Linear Augmented Plane Wave (FLAPW) based method. As we will show our forces and phonon dispersion curves are in good agreement with the FLAPW code. This work lays the foundation for developing a first principles approach for calculation of phonons in substitutionally disordered materials.
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.
Strong and Coherent Coupling between Localized and Propagating Phonon Polaritons
NASA Astrophysics Data System (ADS)
Gubbin, Christopher R.; Martini, Francesco; Politi, Alberto; Maier, Stefan A.; De Liberato, Simone
2016-06-01
Following the recent observation of localized phonon polaritons in user-defined silicon carbide nanoresonators, here we demonstrate strong and coherent coupling between those localized modes and propagating phonon polaritons bound to the surface of the nanoresonator's substrate. In order to obtain phase matching, the nanoresonators have been fabricated to serve the double function of hosting the localized modes, while also acting as a grating for the propagating ones. The coherent coupling between long lived, optically accessible localized modes, and low-loss propagative ones, opens the way to the design and realization of phonon-polariton based coherent circuits.
Analog model for quantum gravity effects: phonons in random fluids.
Krein, G; Menezes, G; Svaiter, N F
2010-09-24
We describe an analog model for quantum gravity effects in condensed matter physics. The situation discussed is that of phonons propagating in a fluid with a random velocity wave equation. We consider that there are random fluctuations in the reciprocal of the bulk modulus of the system and study free phonons in the presence of Gaussian colored noise with zero mean. We show that, in this model, after performing the random averages over the noise function a free conventional scalar quantum field theory describing free phonons becomes a self-interacting model.
Dispersion of doppleron-phonon modes in strong coupling regime.
Gudkov, V V; Zhevstovskikh, I V
2004-04-01
The dispersion equation for doppleron-phonon modes was constructed and solved analytically in the strong coupling regime. The Fermi surface model proposed previously for calculating the doppleron spectrum in an indium crystal was used. It was shown that in the vicinity of doppleron-phonon resonance, the dispersion curves of coupled modes form a gap qualitatively different from the one observed under helicon-phonon resonance: there is a frequency interval forbidden for existence of waves of definite circular polarization depending upon direction of the external DC magnetic field. The physical reason for it is interaction of the waves which have oppositely directed group velocities. PMID:15047286
Modeling and Simulation of Phonon Transport at the Nanoscale for Optimum Thermal Management
NASA Astrophysics Data System (ADS)
Mao, Rui
mixed bonding force between the Pd and C atoms results in incomplete hybridization of Pd and graphene orbital states at the junction, leading effectively to two phonon interfaces and a larger than expected thermal resistance. Comparison with available experimental data shows good agreement. The result clearly suggests the feasibility of phonon engineering for thermal property optimization at the interface. Transition-metal dichalcogenides (TMDs) MX2 (M=Mo,W; X=S,Se), one of the beyond- graphene two-dimensional semiconductor materials, have emerged as promising candidates due to their distinctive electronic and optical properties. Unlike the zero-bandgap graphene, TMDs have intrinsic bandgaps in the range of 1.1--2.2eV, allowing low off-current for field effect transistors. Thermal transport properties at the metal/MoS2 interfaces are then analyzed by using the atomistic phonon transport model. The considered structures include chemisorbed Sc(0001)/MoS2 and Ru(0001)/MoS 2, physisorbed Au(111)/MoS2, as well as Pd(111)/MoS 2 with intermediate characteristics. Calculated results illustrate a distinctive dependence of thermal transfer on the details of interfacial microstructures. More specifically, the chemisorbed case with a stronger bonding exhibits a generally smaller interfacial thermal resistance than the physisorbed. Comparison between metal/MoS2 and metal/graphene systems suggests that metal/MoS 2 is significantly more resistive. Further examination of lattice dynamics identifies the presence of multiple distinct atomic planes and bonding patterns at the interface as the key origin of the observed large thermal resistance. Finally, since the commensurate-incommensurate transitions are ubiquitous in the fabrication of the 2D material based devices, we have extended our investigation to the thermal/phonon transport across the misoriented 2D nanostructures. An analytical model that can incorporate the atomic level detail as well as being time-efficient is developed for
Influence of anharmonic phonon decay on self-heating in Si nanowire transistors
Rhyner, Reto Luisier, Mathieu
2014-08-11
Anharmonic phonon-phonon scattering is incorporated into an electro-thermal quantum transport approach based on the nonequilibrium Green's function formalism. Electron-phonon and phonon-phonon interactions are taken into account through scattering self-energies solved in the self-consistent Born approximation. While studying self-heating effects in ultra-scaled Si nanowire transistors, it is found that the phonon decay process softens the artificial accumulation of high energy phonons caused by electron relaxations close to the drain region. This leads to an increase of the device current in the ON-state and a reduction of the effective lattice temperature.
EMRS Spring Meeting 2014 Symposium D: Phonons and fluctuations in low dimensional structures
NASA Astrophysics Data System (ADS)
2014-11-01
The E-MRS 2014 Spring meeting, held from 26-30th May 2014 in Lille included the Symposium D entitled ''Phonons and Fluctuations in Low Dimensional Structures'', the first edition of its kind. The symposium was organised in response to the increasing interest in the study of phonons in the context of advances in condensed matter physics, electronics, experimental methods and theory and, in particular, the transfer of energy across atomic interfaces and the propagation of energy in the nm-scale. Steering heat by light or vice versa and examining nano-scale energy conversion (as in thermoelectricity and harvesting e.g. in biological systems) are two aspects that share the underlying science of energy processes across atomic interfaces and energy propagation in the nanoscale and or in confined systems. The nanometer scale defies several of the bulk relationships as confinement of electrons and phonons, locality and non-equilibrium become increasingly important. The propagation of phonons as energy carriers impacts not only heat transfer, but also the very concept and handling of temperature in non-equilibrium and highly localised conditions. Much of the needed progress depends on the materials studied and this symposium targeted the interface material aspects as well as the emerging concepts to advance in this field. The symposium had its origins in a series of meetings and seminars including: (1) the first Phonon Engineering Workshop, funded by Catalan Institute for Research and Advanced Studies (ICREA), the then MICINN, the CNRS, VTT, and several EU projects, held in Saint Feliu de Guixols (Girona, Spain) from 24th to 27th of May 2010 with 65 participants from Europe, the USA and Japan; (2) the first Phonons and Fluctuations workshop, held in Paris on 8th and 9th November 2010, supported by French, Spanish and Finnish national projects and EU projects, attended by about 50 researchers; (3) the second Phonon and Fluctuations workshop, held in Paris on 8th and 9th
Phonon dispersion and electron-phonon coupling in MgB2 and AlB2.
Bohnen, K P; Heid, R; Renker, B
2001-06-18
We present a first principles investigation of the lattice dynamics and electron-phonon coupling of the superconductor MgB2 and the isostructural AlB2 within the framework of density functional perturbation theory using a mixed-basis pseudopotential method. Complete phonon dispersion curves and Eliashberg functions alpha2F are calculated for both systems. The main differences are related to high frequency in-plane boron vibrations, which are strongly softened in MgB2 and exhibit an exceptionally strong electron-phonon coupling. We also report on Raman measurements, which support the theoretical findings. Implications for the superconducting transition temperature are briefly discussed.
Controlling electron-phonon scattering with metamaterial plasmonic structures
NASA Astrophysics Data System (ADS)
Kempa, Krzysztof; Wu, Xueyuan; Kong, Jiantao; Broido, David
Electron-plasmon scattering can be faster than electron-phonon scattering. While in metals plasmons occur in the UV range, phonons dominate behavior at much lower frequencies (far IR range), and this typically decouples these phenomena. In metamaterial plasmonic structures, however, plasma effects can be tuned down to the far IR range, allowing for their interference with phonons. It was recently shown, that such interference can protect hot electron energy induced in a solar cell, from dissipation into heat. In this work we explore the possibility of using such an effect to control the electron-phonon interaction and transport in semiconductors. We demonstrate, that this could lead to a novel path to enhancing the electrical and thermal conductivities and the thermoelectric figure of merit.
Surface phonon-polaritons: To scatter or not to scatter
NASA Astrophysics Data System (ADS)
Staude, Isabelle; Rockstuhl, Carsten
2016-08-01
A rewritable platform for subwavelength optical components is demonstrated by combining surface phonon-polaritons, sustained in a polar dielectric layer, with the switching functionality provided by a phase-change material.
Improved model of optical phonon confinement in silicon nanocrystals
Volodin, V. A.; Sachkov, V. A.
2013-01-15
We develop a model for calculating the Raman scattering spectra from phonons confined in for silicon nanocrystals, which is based on the familiar approach taking into account the uncertainty in the quasi-momentum of phonons localized in the nanocrystals. The model is considerably improved by taking into account dispersion of phonons not only in the magnitude of the quasi-momentum, but also in its direction. A significant refinement of the model is also due to the fact that phonon dispersion is calculated using the widely approved Keating model instead of being approximated by empirical expressions as was done in earlier approaches. The calculations based on this model make it possible to determine the sizes of silicon nanocrystals more precisely from analysis of the experimental Raman spectra.
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.
Phonon Gas Model (PGM) workflow in the VLab Science Gateway
NASA Astrophysics Data System (ADS)
da Silveira, P.; Zhang, D.; Wentzcovitch, R. M.
2013-12-01
This contribution describes a scientific workflow for first principles computations of free energy of crystalline solids using the phonon gas model (PGM). This model was recently implemented as a hybrid method combining molecular dynamics and phonon normal mode analysis to extract temperature dependent phonon frequencies and life times beyond perturbation theory. This is a demanding high throughout workflow and is currently being implemented in VLab Cyberinfrastructure [da Silveira et al., 2008], which has recently been integrated to the XSEDE. First we review the underlying PGM, its practical implementation, and calculation requirements. We then describe the workflow management and its general method for handling actions. We illustrate the PGM application with a calculation of MgSiO3-perovskite's anharmonic phonons. We conclude with an outlook of workflows to compute other material's properties that will use the PGM workflow. Research supported by NSF award EAR-1019853.
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.
Phonon excitations and related thermal properties of aluminum nitride
Nipko, J.C.; Loong, C.
1998-05-01
The phonon density of states of aluminum nitride was determined by time-of-flight neutron spectroscopy using a polycrystalline sample. The observed phonon excitation spectrum consists of a broadband centered at about 35 meV, a small gap in the 75{endash}80 meV region, and two sharp bands at approximately 85 and 92 meV. A rigid-ion model was applied to the interpretation of the data. After optimization, the model provided a satisfactory description of the neutron results as well as the Raman and IR data, sound-velocity measurements, and the lattice specific heat reported in the literature. The partial and total DOS and the phonon-dispersion curves along major symmetry directions of the Brillouin zone were calculated, and the contribution of phonons to the Debye behavior of the low-temperature thermal conductivity was discussed. {copyright} {ital 1998} {ital The American Physical Society}
The Electron-Phonon Interaction as Studied by Photoelectron Spectroscopy
D.W. Lynch
2004-09-30
With recent advances in energy and angle resolution, the effects of electron-phonon interactions are manifest in many valence-band photoelectron spectra (PES) for states near the Fermi level in metals.
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.
Phonon ringing and anharmonicity effects in core spectra
NASA Astrophysics Data System (ADS)
Mansour, A.; Schnatterly, S. E.
1987-08-01
We show that the soft x-ray emission spectrum of the boron K core exciton in B2O3 displays the largest phonon coupling effects yet observed in core spectroscopies. We interpret the observed double-peaked spectrum as the first clear example of phonon ringing. It is necessary to modify the existing theory of this effect by including anharmonicity in order to obtain an accurate description of the experimental results.
Correlations of collective observables and the phonon structure of nuclei
Casten, R.F.; Zamfir, N.V. ||
1994-07-01
A ``horizontal`` view of nuclear structures is described in which various observables are correlated over broad mass ranges. This approach leads to a number of remarkable correlations, to new understanding of the evolution of structure, to a challenge to microscopic theories, and to new signatures of structure that will be especially useful with radioactive beam experiments. In particular, this and other evidence suggests a nearly universal and pervasive role of phonon and multi-phonon excitations in nuclei.
On time-resolved approach for phonon assisted interband transitions
NASA Astrophysics Data System (ADS)
Klotins, E.
2015-03-01
Photoexcited dynamics of electrons and holes in two-band dielectric, with special emphasis on back reaction of phonons are developed by combining the quantum electrodynamics and Baker-Campbell-Hausdorff (BCH) canonical transformation. These methods create an explicit time-domain representation of photoinduced processes and contribute in unifying phonon-assisted description of distribution functions of electron and hole quasiparticles for the description of observable effects of photoinduced processes in dielectrics.
A moment model for phonon transport at room temperature
NASA Astrophysics Data System (ADS)
Mohammadzadeh, Alireza; Struchtrup, Henning
2016-08-01
Heat transfer in solids is modeled by deriving the macroscopic equations for phonon transport from the phonon-Boltzmann equation. In these equations, the Callaway model with frequency-dependent relaxation time is considered to describe the Resistive and Normal processes in the phonon interactions. Also, the Brillouin zone is considered to be a sphere, and its diameter depends on the temperature of the system. A simple model to describe phonon interaction with crystal boundary is employed to obtain macroscopic boundary conditions, where the reflection kernel is the superposition of diffusive reflection, specular reflection and isotropic scattering. Macroscopic moments are defined using a polynomial of the frequency and wave vector of phonons. As an example, a system of moment equations, consisting of three directional and seven frequency moments, i.e., 63 moments in total, is used to study one-dimensional heat transfer, as well as Poiseuille flow of phonons. Our results show the importance of frequency dependency in relaxation times and macroscopic moments to predict rarefaction effects. Good agreement with data reported in the literature is obtained.
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.
Orbitally-driven giant phonon anharmonicity in SnSe
Li, Chen W.; Hong, Jiawang; May, Andrew F.; Bansal, Dipanshu; Chi, Songxue; Hong, Tao; Ehlers, Georg; Delaire, Olivier A.
2015-10-19
We understand that elementary excitations and their couplings in condensed matter systems is critical to develop better energy-conversion devices. In thermoelectric materials, the heat-to-electricity conversion efficiency is directly improved by suppressing the propagation of phonon quasiparticles responsible for macroscopic thermal transport. The material with the current record for thermoelectric conversion efficiency, SnSe, achieves an ultra-low thermal conductivity, but the mechanism enabling this strong phonon scattering remains largely unknown. Using inelastic neutron scattering measurements and first-principles simulations, we mapped the four-dimensional phonon dispersion surfaces of SnSe, and revealed the origin of ionic-potential anharmonicity responsible for the unique properties of SnSe. We show that the giant phonon scattering arises from an unstable electronic structure, with orbital interactions leading to a ferroelectric-like lattice instability. Our results provide a microscopic picture connecting electronic structure and phonon anharmonicity in SnSe, and offers precious insights on how electron-phonon and phononphonon interactions may lead to the realization of ultra-low thermal conductivity.
Orbitally-driven giant phonon anharmonicity in SnSe
Li, Chen W.; Hong, Jiawang; May, Andrew F.; Bansal, Dipanshu; Chi, Songxue; Hong, Tao; Ehlers, Georg; Delaire, Olivier A.
2015-10-19
We understand that elementary excitations and their couplings in condensed matter systems is critical to develop better energy-conversion devices. In thermoelectric materials, the heat-to-electricity conversion efficiency is directly improved by suppressing the propagation of phonon quasiparticles responsible for macroscopic thermal transport. The material with the current record for thermoelectric conversion efficiency, SnSe, achieves an ultra-low thermal conductivity, but the mechanism enabling this strong phonon scattering remains largely unknown. Using inelastic neutron scattering measurements and first-principles simulations, we mapped the four-dimensional phonon dispersion surfaces of SnSe, and revealed the origin of ionic-potential anharmonicity responsible for the unique properties of SnSe. Wemore » show that the giant phonon scattering arises from an unstable electronic structure, with orbital interactions leading to a ferroelectric-like lattice instability. Our results provide a microscopic picture connecting electronic structure and phonon anharmonicity in SnSe, and offers precious insights on how electron-phonon and phononphonon interactions may lead to the realization of ultra-low thermal conductivity.« less
Magnetic moments induce strong phonon renormalization in FeSi
Krannich, S.; Sidis, Y.; Lamago, D.; Heid, R.; Mignot, J.-M.; Löhneysen, H. v.; Ivanov, A.; Steffens, P.; Keller, T.; Wang, L.; Goering, E.; Weber, F.
2015-01-01
The interactions of electronic, spin and lattice degrees of freedom in solids result in complex phase diagrams, new emergent phenomena and technical applications. While electron–phonon coupling is well understood, and interactions between spin and electronic excitations are intensely investigated, only little is known about the dynamic interactions between spin and lattice excitations. Noncentrosymmetric FeSi is known to undergo with increasing temperature a crossover from insulating to metallic behaviour with concomitant magnetic fluctuations, and exhibits strongly temperature-dependent phonon energies. Here we show by detailed inelastic neutron-scattering measurements and ab initio calculations that the phonon renormalization in FeSi is linked to its unconventional magnetic properties. Electronic states mediating conventional electron–phonon coupling are only activated in the presence of strong magnetic fluctuations. Furthermore, phonons entailing strongly varying Fe–Fe distances are damped via dynamic coupling to the temperature-induced magnetic moments, highlighting FeSi as a material with direct spin–phonon coupling and multiple interaction paths. PMID:26611619
Surface phonons and surface reconstruction in calcium doped magnesium oxide
NASA Astrophysics Data System (ADS)
Masri, P.; Tasker, P. W.
1985-01-01
Static lattice calculation of the structure and energy of calcium doped magnesium oxide have indicated that the surface will be heavily segregated with calcium in accordance with experimental measurement. At moderate temperatures the enthalpy of segregation is sufficient to produce a monolayer coverage of impurity at equilibrium. The phonons for the segregated surface reported here show an instability that was not found in the static calculation. A phonon of imaginary frequency over much of the Brillouin zone indicates that a lower energy structure of larger periodicity should exist. We propose a self-consistent static-dynamic procedure that uses the phonon results to suggest a restructuring that is calculated in a static calculation. The phonon calculation is then repeated. When a lowest energy static calculation gives surface phonons with no softening, we can be confident of the predicted structure. In this case we have predicted a segregation induced surface restructuring. The restructured cell is c(√2 × √2)R45° with half the oxygen ions pushed high out of the surface. The small long wavelength phonon anomaly which remains suggests that there may also be a longer range rumpling of surface cations.
Phonon anharmonicity and negative thermal expansion in SnSe
Bansal, Dipanshu; Hong, Jiawang; Li, Chen W.; May, Andrew F.; Porter, Wallace; Hu, Michael Y.; Abernathy, Douglas L.; Delaire, Olivier
2016-08-09
In this paper, the anharmonic phonon properties of SnSe in the Pnma phase were investigated with a combination of experiments and first-principles simulations. Using inelastic neutron scattering (INS) and nuclear resonant inelastic X-ray scattering (NRIXS), we have measured the phonon dispersions and density of states (DOS) and their temperature dependence, which revealed a strong, inhomogeneous shift and broadening of the spectrum on warming. First-principles simulations were performed to rationalize these measurements, and to explain the previously reported anisotropic thermal expansion, in particular the negative thermal expansion within the Sn-Se bilayers. Including the anisotropic strain dependence of the phonon free energy,more » in addition to the electronic ground state energy, is essential to reproduce the negative thermal expansion. From the phonon DOS obtained with INS and additional calorimetry measurements, we quantify the harmonic, dilational, and anharmonic components of the phonon entropy, heat capacity, and free energy. Finally, the origin of the anharmonic phonon thermodynamics is linked to the electronic structure.« less
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 interpretation of the 'boson peak' in supercooled liquids.
Grigera, T S; Martín-Mayor, V; Parisi, G; Verrocchio, P
2003-03-20
Glasses are amorphous solids, in the sense that they display elastic behaviour. In crystalline solids, elasticity is associated with phonons, which are quantized vibrational excitations. Phonon-like excitations also exist in glasses at very high (terahertz; 10(12) Hz) frequencies; surprisingly, these persist in the supercooled liquids. A universal feature of such amorphous systems is the boson peak: the vibrational density of states has an excess compared to the Debye squared-frequency law. Here we investigate the origin of this feature by studying the spectra of inherent structures (local minima of the potential energy) in a realistic glass model. We claim that the peak is the signature of a phase transition in the space of the stationary points of the energy, from a minima-dominated phase (with phonons) at low energy to a saddle-point-dominated phase (without phonons). The boson peak moves to lower frequencies on approaching the phonon-saddle transition, and its height diverges at the critical point. Our numerical results agree with the predictions of euclidean random matrix theory on the existence of a sharp phase transition between an amorphous elastic phase and a phonon-free one. PMID:12646916
Phonon anharmonicity and negative thermal expansion in SnSe
NASA Astrophysics Data System (ADS)
Bansal, Dipanshu; Hong, Jiawang; Li, Chen W.; May, Andrew F.; Porter, Wallace; Hu, Michael Y.; Abernathy, Douglas L.; Delaire, Olivier
2016-08-01
The anharmonic phonon properties of SnSe in the P n m a phase were investigated with a combination of experiments and first-principles simulations. Using inelastic neutron scattering (INS) and nuclear resonant inelastic X-ray scattering (NRIXS), we have measured the phonon dispersions and density of states (DOS) and their temperature dependence, which revealed a strong, inhomogeneous shift and broadening of the spectrum on warming. First-principles simulations were performed to rationalize these measurements, and to explain the previously reported anisotropic thermal expansion, in particular the negative thermal expansion within the Sn-Se bilayers. Including the anisotropic strain dependence of the phonon free energy, in addition to the electronic ground state energy, is essential to reproduce the negative thermal expansion. From the phonon DOS obtained with INS and additional calorimetry measurements, we quantify the harmonic, dilational, and anharmonic components of the phonon entropy, heat capacity, and free energy. The origin of the anharmonic phonon thermodynamics is linked to the electronic structure.
Magnetic moments induce strong phonon renormalization in FeSi.
Krannich, S; Sidis, Y; Lamago, D; Heid, R; Mignot, J-M; Löhneysen, H v; Ivanov, A; Steffens, P; Keller, T; Wang, L; Goering, E; Weber, F
2015-01-01
The interactions of electronic, spin and lattice degrees of freedom in solids result in complex phase diagrams, new emergent phenomena and technical applications. While electron-phonon coupling is well understood, and interactions between spin and electronic excitations are intensely investigated, only little is known about the dynamic interactions between spin and lattice excitations. Noncentrosymmetric FeSi is known to undergo with increasing temperature a crossover from insulating to metallic behaviour with concomitant magnetic fluctuations, and exhibits strongly temperature-dependent phonon energies. Here we show by detailed inelastic neutron-scattering measurements and ab initio calculations that the phonon renormalization in FeSi is linked to its unconventional magnetic properties. Electronic states mediating conventional electron-phonon coupling are only activated in the presence of strong magnetic fluctuations. Furthermore, phonons entailing strongly varying Fe-Fe distances are damped via dynamic coupling to the temperature-induced magnetic moments, highlighting FeSi as a material with direct spin-phonon coupling and multiple interaction paths. PMID:26611619
Phonon-Mediated Nonclassical Interference in Diamond.
England, Duncan G; Fisher, Kent A G; MacLean, Jean-Philippe W; Bustard, Philip J; Heshami, Khabat; Resch, Kevin J; Sussman, Benjamin J
2016-08-12
Quantum interference of single photons is a fundamental aspect of many photonic quantum processing and communication protocols. Interference requires that the multiple pathways through an interferometer be temporally indistinguishable to within the coherence time of the photon. In this Letter, we use a diamond quantum memory to demonstrate interference between quantum pathways, initially temporally separated by many multiples of the optical coherence time. The quantum memory can be viewed as a light-matter beam splitter, mapping a THz-bandwidth single photon to a variable superposition of the output optical mode and stored phononic mode. Because the memory acts both as a beam splitter and as a buffer, the relevant coherence time for interference is not that of the photon, but rather that of the memory. We use this mechanism to demonstrate nonclassical single-photon and two-photon interference between quantum pathways initially separated by several picoseconds, even though the duration of the photons themselves is just ∼250 fs. PMID:27563963
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.
Phonon-Mediated Nonclassical Interference in Diamond.
England, Duncan G; Fisher, Kent A G; MacLean, Jean-Philippe W; Bustard, Philip J; Heshami, Khabat; Resch, Kevin J; Sussman, Benjamin J
2016-08-12
Quantum interference of single photons is a fundamental aspect of many photonic quantum processing and communication protocols. Interference requires that the multiple pathways through an interferometer be temporally indistinguishable to within the coherence time of the photon. In this Letter, we use a diamond quantum memory to demonstrate interference between quantum pathways, initially temporally separated by many multiples of the optical coherence time. The quantum memory can be viewed as a light-matter beam splitter, mapping a THz-bandwidth single photon to a variable superposition of the output optical mode and stored phononic mode. Because the memory acts both as a beam splitter and as a buffer, the relevant coherence time for interference is not that of the photon, but rather that of the memory. We use this mechanism to demonstrate nonclassical single-photon and two-photon interference between quantum pathways initially separated by several picoseconds, even though the duration of the photons themselves is just ∼250 fs.
Nonperturbative calculation of phonon effects on spin squeezing
NASA Astrophysics Data System (ADS)
Dylewsky, D.; Freericks, J. K.; Wall, M. L.; Rey, A. M.; Foss-Feig, M.
2016-01-01
Theoretical models of spins coupled to bosons provide a simple setting for studying a broad range of important phenomena in many-body physics, from virtually mediated interactions to decoherence and thermalization. In many atomic, molecular, and optical systems, such models also underlie the most successful attempts to engineer strong, long-ranged interactions for the purpose of entanglement generation. Especially when the coupling between the spins and bosons is strong, such that it cannot be treated perturbatively, the properties of such models are extremely challenging to calculate theoretically. Here, exact analytical expressions for nonequilibrium spin-spin correlation functions are derived for a specific model of spins coupled to bosons. The spatial structure of the coupling between spins and bosons is completely arbitrary, and thus the solution can be applied to systems in any number of dimensions. The explicit and nonperturbative inclusion of the bosons enables the study of entanglement generation (in the form of spin squeezing) even when the bosons are driven strongly and near resonantly, and thus provides a quantitative view of the breakdown of adiabatic elimination that inevitably occurs as one pushes towards the fastest entanglement generation possible. The solution also helps elucidate the effect of finite temperature on spin squeezing. The model considered is relevant to a variety of atomic, molecular, and optical systems, such as atoms in cavities or trapped ions. As an explicit example, the results are used to quantify phonon effects in trapped ion quantum simulators, which are expected to become increasingly important as these experiments push towards larger numbers of ions.
Phonon transport analysis of semiconductor nanocomposites using monte carlo simulations
NASA Astrophysics Data System (ADS)
Malladi, Mayank
Nanocomposites are composite materials which incorporate nanosized particles, platelets or fibers. The addition of nanosized phases into the bulk matrix can lead to significantly different material properties compared to their macrocomposite counterparts. For nanocomposites, thermal conductivity is one of the most important physical properties. Manipulation and control of thermal conductivity in nanocomposites have impacted a variety of applications. In particular, it has been shown that the phonon thermal conductivity can be reduced significantly in nanocomposites due to the increase in phonon interface scattering while the electrical conductivity can be maintained. This extraordinary property of nanocomposites has been used to enhance the energy conversion efficiency of the thermoelectric devices which is proportional to the ratio of electrical to thermal conductivity. This thesis investigates phonon transport and thermal conductivity in Si/Ge semiconductor nanocomposites through numerical analysis. The Boltzmann transport equation (BTE) is adopted for description of phonon thermal transport in the nanocomposites. The BTE employs the particle-like nature of phonons to model heat transfer which accounts for both ballistic and diffusive transport phenomenon. Due to the implementation complexity and computational cost involved, the phonon BTE is difficult to solve in its most generic form. Gray media (frequency independent phonons) is often assumed in the numerical solution of BTE using conventional methods such as finite volume and discrete ordinates methods. This thesis solves the BTE using Monte Carlo (MC) simulation technique which is more convenient and efficient when non-gray media (frequency dependent phonons) is considered. In the MC simulation, phonons are displaced inside the computational domain under the various boundary conditions and scattering effects. In this work, under the relaxation time approximation, thermal transport in the nanocomposites are
NASA Astrophysics Data System (ADS)
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.
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
Bulk viscosity coefficients due to phonons in superfluid neutron stars
Manuel, Cristina; Tolos, Laura; Tarrús, Jaume E-mail: tarrus@ecm.ub.edu
2013-07-01
We calculate the three bulk viscosity coefficients as arising from the collisions among phonons in superfluid neutron stars. We use effective field theory techniques to extract the allowed phonon collisional processes, written as a function of the equation of state of the system. The solution of the dynamical evolution of the phonon number density allows us to calculate the bulk viscosity coefficients as function of the phonon collisional rate and the phonon dispersion law, which depends on the neutron pairing gap. Our method of computation is rather general, and could be used for different superfluid systems, provided they share the same underlying symmetries. We find that the behavior with temperature of the bulk viscosity coefficients is dominated by the contributions coming from the collinear regime of the 2↔3 phonon processes. For typical star radial pulsation frequencies of ω ∼ 10{sup 4}s{sup −1}, we obtain that the bulk viscosity coefficients at densities n∼>4n{sub 0} are within 10% from its static value for T∼<10{sup 9} K and for the case of strong neutron superfluidity in the core with a maximum value of the {sup 3}P{sub 2} gap above 1 MeV, while, otherwise, the static solution is not a valid approximation to the bulk viscosity coefficients. Compared to previous results from Urca and modified Urca reactions, we conclude that at T ∼ 10{sup 9}K phonon collisions give the leading contribution to the bulk viscosities in the core of the neutron stars, except for n ∼ 2n{sub 0} when the opening of the Urca processes takes place.
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.
Niu, Zhen-Wei; Zeng, Zhao-Yi; Hu, Cui-E; Cai, Ling-Cang; Chen, Xiang-Rong
2015-01-07
The thermodynamic properties of CeO{sub 2} have been reevaluated by a simple but accurate scheme. All our calculations are based on the self-consistent ab initio lattice dynamical (SCAILD) method that goes beyond the quasiharmonic approximation. Through this method, the effects of phonon-phonon interactions are included. The obtained thermodynamic properties and phonon dispersion relations are in good agreement with experimental data when considering the correction of phonon-phonon interaction. We find that the correction of phonon-phonon interaction is equally important and should not be neglected. At last, by comparing with quasiharmonic approximation, the present scheme based on SCAILD method is probably more suitable for high temperature systems.
Thermally triggered phononic gaps in liquids at THz scale
NASA Astrophysics Data System (ADS)
Bolmatov, Dima; Zhernenkov, Mikhail; Zav’Yalov, Dmitry; Stoupin, Stanislav; Cunsolo, Alessandro; Cai, Yong Q.
2016-01-01
In this paper we present inelastic X-ray scattering experiments in a diamond anvil cell and molecular dynamic simulations to investigate the behavior of phononic excitations in liquid Ar. The spectra calculated using molecular dynamics were found to be in a good agreement with the experimental data. Furthermore, we observe that, upon temperature increases, a low-frequency transverse phononic gap emerges while high-frequency propagating modes become evanescent at the THz scale. The effect of strong localization of a longitudinal phononic mode in the supercritical phase is observed for the first time. The evidence for the high-frequency transverse phononic gap due to the transition from an oscillatory to a ballistic dynamic regimes of motion is presented and supported by molecular dynamics simulations. This transition takes place across the Frenkel line thermodynamic limit which demarcates compressed liquid and non-compressed fluid domains on the phase diagram and is supported by calculations within the Green-Kubo phenomenological formalism. These results are crucial to advance the development of novel terahertz thermal devices, phononic lenses, mirrors, and other THz metamaterials.
Nonlinear Transport and Noise Properties of Acoustic Phonons
NASA Astrophysics Data System (ADS)
Walczak, Kamil
We examine heat transport carried by acoustic phonons in molecular junctions composed of organic molecules coupled to two thermal baths of different temperatures. The phononic heat flux and its dynamical noise properties are analyzed within the scattering (Landauer) formalism with transmission probability function for acoustic phonons calculated within the method of atomistic Green's functions (AGF technique). The perturbative computational scheme is used to determine nonlinear corrections to phononic heat flux and its noise power spectral density with up to the second order terms with respect to temperature difference. Our results show the limited applicability of ballistic Fourier's law and fluctuation-dissipation theorem to heat transport in quantum systems. We also derive several noise-signal relations applicable to nanoscale heat flow carried by phonons, but valid for electrons as well. We also discuss the extension of the perturbative transport theory to higher order terms in order to address a huge variety of problems related to nonlinear thermal effects which may occur at nanoscale and at strongly non-equilibrium conditions with high-intensity heat fluxes. This work was supported by Pace University Start-up Grant.
Phonon properties of graphene derived from molecular dynamics simulations.
Koukaras, Emmanuel N; Kalosakas, George; Galiotis, Costas; Papagelis, Konstantinos
2015-01-01
A method that utilises atomic trajectories and velocities from molecular dynamics simulations has been suitably adapted and employed for the implicit calculation of the phonon dispersion curves of graphene. Classical potentials widely used in the literature were employed. Their performance was assessed for each individual phonon branch and the overall phonon dispersion, using available inelastic x-ray scattering data. The method is promising for systems with large scale periodicity, accounts for anharmonic effects and non-bonding interactions with a general environment, and it is applicable under finite temperatures. The temperature dependence of the phonon dispersion curves has been examined with emphasis on the doubly degenerate Raman active Γ-E2g phonon at the zone centre, where experimental results are available. The potentials used show diverse behaviour. The Tersoff-2010 potential exhibits the most systematic and physically sound behaviour in this regard, and gives a first-order temperature coefficient of χ = -0.05 cm(-1)/K for the Γ-E2g shift in agreement with reported experimental values.
Thermally triggered phononic gaps in liquids at THz scale
Bolmatov, Dima; Zhernenkov, Mikhail; Zav’yalov, Dmitry; Stoupin, Stanislav; Cunsolo, Alessandro; Cai, Yong Q.
2016-01-01
In this paper we present inelastic X-ray scattering experiments in a diamond anvil cell and molecular dynamic simulations to investigate the behavior of phononic excitations in liquid Ar. The spectra calculated using molecular dynamics were found to be in a good agreement with the experimental data. Furthermore, we observe that, upon temperature increases, a low-frequency transverse phononic gap emerges while high-frequency propagating modes become evanescent at the THz scale. The effect of strong localization of a longitudinal phononic mode in the supercritical phase is observed for the first time. The evidence for the high-frequency transverse phononic gap due to the transition from an oscillatory to a ballistic dynamic regimes of motion is presented and supported by molecular dynamics simulations. This transition takes place across the Frenkel line thermodynamic limit which demarcates compressed liquid and non-compressed fluid domains on the phase diagram and is supported by calculations within the Green-Kubo phenomenological formalism. These results are crucial to advance the development of novel terahertz thermal devices, phononic lenses, mirrors, and other THz metamaterials. PMID:26763899
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.
Phonon properties of graphene derived from molecular dynamics simulations
Koukaras, Emmanuel N.; Kalosakas, George; Galiotis, Costas; Papagelis, Konstantinos
2015-01-01
A method that utilises atomic trajectories and velocities from molecular dynamics simulations has been suitably adapted and employed for the implicit calculation of the phonon dispersion curves of graphene. Classical potentials widely used in the literature were employed. Their performance was assessed for each individual phonon branch and the overall phonon dispersion, using available inelastic x-ray scattering data. The method is promising for systems with large scale periodicity, accounts for anharmonic effects and non-bonding interactions with a general environment, and it is applicable under finite temperatures. The temperature dependence of the phonon dispersion curves has been examined with emphasis on the doubly degenerate Raman active Γ-E2g phonon at the zone centre, where experimental results are available. The potentials used show diverse behaviour. The Tersoff-2010 potential exhibits the most systematic and physically sound behaviour in this regard, and gives a first-order temperature coefficient of χ = −0.05 cm−1/K for the Γ-E2g shift in agreement with reported experimental values. PMID:26316252
Thermally triggered phononic gaps in liquids at THz scale
Bolmatov, Dima; Zhernenkov, Mikhail; Zavyalov, Dmitry; Stoupin, Stanislav; Cunsolo, Alessandro; Cai, Yong Q.
2016-01-14
In this study we present inelastic X-ray scattering experiments in a diamond anvil cell and molecular dynamic simulations to investigate the behavior of phononic excitations in liquid Ar. The spectra calculated using molecular dynamics were found to be in a good agreement with the experimental data. Furthermore, we observe that, upon temperature increases, a low-frequency transverse phononic gap emerges while high-frequency propagating modes become evanescent at the THz scale. The effect of strong localization of a longitudinal phononic mode in the supercritical phase is observed for the first time. The evidence for the high-frequency transverse phononic gap due to themore » transition from an oscillatory to a ballistic dynamic regimes of motion is presented and supported by molecular dynamics simulations. This transition takes place across the Frenkel line thermodynamic limit which demarcates compressed liquid and non-compressed fluid domains on the phase diagram and is supported by calculations within the Green-Kubo phenomenological formalism. These results are crucial to advance the development of novel terahertz thermal devices, phononic lenses, mirrors, and other THz metamaterials.« less
Thermally triggered phononic gaps in liquids at THz scale.
Bolmatov, Dima; Zhernenkov, Mikhail; Zav'yalov, Dmitry; Stoupin, Stanislav; Cunsolo, Alessandro; Cai, Yong Q
2016-01-01
In this paper we present inelastic X-ray scattering experiments in a diamond anvil cell and molecular dynamic simulations to investigate the behavior of phononic excitations in liquid Ar. The spectra calculated using molecular dynamics were found to be in a good agreement with the experimental data. Furthermore, we observe that, upon temperature increases, a low-frequency transverse phononic gap emerges while high-frequency propagating modes become evanescent at the THz scale. The effect of strong localization of a longitudinal phononic mode in the supercritical phase is observed for the first time. The evidence for the high-frequency transverse phononic gap due to the transition from an oscillatory to a ballistic dynamic regimes of motion is presented and supported by molecular dynamics simulations. This transition takes place across the Frenkel line thermodynamic limit which demarcates compressed liquid and non-compressed fluid domains on the phase diagram and is supported by calculations within the Green-Kubo phenomenological formalism. These results are crucial to advance the development of novel terahertz thermal devices, phononic lenses, mirrors, and other THz metamaterials. PMID:26763899
Bianco, A; Del Zoppo, M; Zerbi, G
2004-01-15
We analyze the infrared and Raman spectra (both experimentally and with the aid of quantum chemical calculations) of a series of polyenals which provide us with the fortunate case of a set of polyene chains with one of the end groups consisting of a C=O group which not only does take part in the conjugation but also pulls electrons from the chain making the whole system highly polar, thus affecting the vibrational transition moments. In the following we show, for the first time, that it is possible to derive experimental phonon dispersion curves and these prove to be different for each chain length. We support our experimental findings with Density Functional Theory quantum chemical calculations which reproduce with sufficient accuracy the IR and Raman spectral pattern and at the same time help in disentangling the assignment of the fine structure observed in the experimental spectra.
Anharmonic phonons and magnons in BiFeO3
Delaire, Olivier A; Ma, Jie; Stone, Matthew B; Huq, Ashfia; Gout, Delphine J; Brown, Craig; Wang, Kefeng; Ren, Zhifeng
2012-01-01
The phonon density of states (DOS) and magnetic excitation spectrum of polycrystalline BiFeO3 were measured for temperatures 200 < T < 750K , using inelastic neutron scattering (INS). Our results indicate that the magnetic spectrum of BiFeO3 closely resembles that of similar Fe perovskites, such as LaFeO3, despite the cycloid modulation in BiFeO3. We do not find any evidence for a spin gap. A strong T-dependence of the phonon DOS was found, with a marked broadening of the whole spectrum, providing evidence of strong anharmonicity. This anharmonicity is corroborated by large amplitude motions of Bi and O ions observed with neutron diffraction. These results highlight the importance of spin-phonon coupling in this material.
Analytical solution for phonon transport across thin films
NASA Astrophysics Data System (ADS)
Yilbas, Bekir S.; Al-Dweik, Ahmed Y.; Mansoor, Saad Bin
2013-12-01
The numerical solution of the Boltzmann equation for the thin film applications requires extensive computational power. An analytical solution to the phonon transport equation is fruitful in order to reduce the computational effort and cost. In the present study, an analytical solution for the phonon radiative transport equation in thin film is carried out. The analytical treatment of the problem reduces the two identical radiative transport equations to Fredholm integral equation of the second kind. The resulting phonon intensity data are presented in terms of the dimensionless temperature across the gray thin films of silicon and diamond. The findings are compared with their counterparts predicted from the numerical simulations. The study is extended to include the effect of the film thickness on the dimensionless temperature distribution. It is found that the analytical solution for temperature agrees well with the numerical predictions. Reducing the film thickness increases the temperature jump at the film edges, which is more pronounced for the diamond film.
Terahertz reflection response measurement using a phonon polariton wave
NASA Astrophysics Data System (ADS)
Inoue, Hayato; Katayama, Kenji; Shen, Qing; Toyoda, Taro; Nelson, Keith A.
2009-03-01
We developed a new technique for the measurement of terahertz reflection responses utilizing a propagating phonon polariton wave. Frequency tunable phonon polariton waves were generated by the recently developed continuously variable spatial frequency transient grating method [K. Katayama, H. Inoue, H. Sugiya, Q. Shen, T. Taro, and K. A. Nelson, Appl. Phys. Lett. 92, 031906 (2008)]. The phonon polariton wave traveled in a ferroelectric crystal in an in-plane direction with an inclined angle of 26°, and the wave reflected at the crystal edge where a sample was positioned. The reflected polariton wave was detected by the same method as that used for the generation of the polariton waves. By comparing the reflection intensities in the presence and absence of the sample, reflectivity of the polariton wave was calculated, and the refractive index and absorption in the terahertz region were obtained.
Phonon anharmonicity in silicon from 100 to 1500 K
Kim, D. S.; Smith, Hillary L.; Niedziela, Jennifer L.; Li, Chen W.; Abernathy, Douglas L.; Fultz, B.
2015-01-21
Inelastic neutron scattering was performed on silicon powder to measure the phonon density of states (DOS) from 100 to 1500 K. The mean fractional energy shifts with temperature of the modes were $\\langle$Δε_{i}/ε_{i}ΔT$\\rangle$=₋0.07, giving a mean isobaric Grüneisen parameter of +6.95±0.67, which is significantly different from the isothermal parameter of +0.98. These large effects are beyond the predictions from quasiharmonic models using density functional theory or experimental data, demonstrating large effects from phonon anharmonicity. At 1500 K the anharmonicity contributes 0.15k_{B}/atom to the vibrational entropy, compared to 0.03k_{B}/atom from quasiharmonicity. Lastly, excellent agreement was found between the entropy from phonon DOS measurements and the reference NIST-JANAF thermodynamic entropy from calorimetric measurements.
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.
Evolution of molecular crystal optical phonons near structural phase transitions
NASA Astrophysics Data System (ADS)
Michki, Nigel; Niessen, Katherine; Xu, Mengyang; Markelz, Andrea
Molecular crystals are increasingly important photonic and electronic materials. For example organic semiconductors are lightweight compared to inorganic semiconductors and have inexpensive scale up processing with roll to roll printing. However their implementation is limited by their environmental sensitivity, in part arising from the weak intermolecular interactions of the crystal. These weak interactions result in optical phonons in the terahertz frequency range. We examine the evolution of intermolecular interactions near structural phase transitions by measuring the optical phonons as a function of temperature and crystal orientation using terahertz time-domain spectroscopy. The measured orientation dependence of the resonances provides an additional constraint for comparison of the observed spectra with the density functional calculations, enabling us to follow specific phonon modes. We observe crystal reorganization near 350 K for oxalic acid as it transforms from dihydrate to anhydrous form. We also report the first THz spectra for the molecular crystal fructose through its melting point.
Classification of topological phonons in linear mechanical metamaterials.
Süsstrunk, Roman; Huber, Sebastian D
2016-08-16
Topological phononic crystals, alike their electronic counterparts, are characterized by a bulk-edge correspondence where the interior of a material dictates the existence of stable surface or boundary modes. In the mechanical setup, such surface modes can be used for various applications such as wave guiding, vibration isolation, or the design of static properties such as stable floppy modes where parts of a system move freely. Here, we provide a classification scheme of topological phonons based on local symmetries. We import and adapt the classification of noninteracting electron systems and embed it into the mechanical setup. Moreover, we provide an extensive set of examples that illustrate our scheme and can be used to generate models in unexplored symmetry classes. Our work unifies the vast recent literature on topological phonons and paves the way to future applications of topological surface modes in mechanical metamaterials.
Classification of topological phonons in linear mechanical metamaterials
NASA Astrophysics Data System (ADS)
Süsstrunk, Roman; Huber, Sebastian D.
2016-08-01
Topological phononic crystals, alike their electronic counterparts, are characterized by a bulk-edge correspondence where the interior of a material dictates the existence of stable surface or boundary modes. In the mechanical setup, such surface modes can be used for various applications such as wave guiding, vibration isolation, or the design of static properties such as stable floppy modes where parts of a system move freely. Here, we provide a classification scheme of topological phonons based on local symmetries. We import and adapt the classification of noninteracting electron systems and embed it into the mechanical setup. Moreover, we provide an extensive set of examples that illustrate our scheme and can be used to generate models in unexplored symmetry classes. Our work unifies the vast recent literature on topological phonons and paves the way to future applications of topological surface modes in mechanical metamaterials.
High temperature phonon dispersion in graphene using classical molecular dynamics
Anees, P. Panigrahi, B. K.; Valsakumar, M. C.
2014-04-24
Phonon dispersion and phonon density of states of graphene are calculated using classical molecular dynamics simulations. In this method, the dynamical matrix is constructed based on linear response theory by computing the displacement of atoms during the simulations. The computed phonon dispersions show excellent agreement with experiments. The simulations are done in both NVT and NPT ensembles at 300 K and found that the LO/TO modes are getting hardened at the Γ point. The NPT ensemble simulations capture the anharmonicity of the crystal accurately and the hardening of LO/TO modes is more pronounced. We also found that at 300 K the C-C bond length reduces below the equilibrium value and the ZA bending mode frequency becomes imaginary close to Γ along K-Γ direction, which indicates instability of the flat 2D graphene sheets.
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.
Reconciling perturbative approaches in phonon-assisted transport junctions.
Agarwalla, Bijay Kumar; Segal, Dvira
2016-02-21
We present consistent results for molecular conduction using two central-complementary approaches: the non-equilibrium Green's function technique and the quantum master equation method. Our model describes electronic conduction in a donor-acceptor junction in which electron transfer is coupled to nuclear motion, modeled by a harmonic vibrational mode. This primary mode is further coupled to secondary phonon modes, a thermal bath. Assuming weak electron-phonon coupling but an arbitrary large molecule-metal hybridization, we compute several non-equilibrium transport quantities: the mean phonon number of the primary mode, charge current statistics. We further present scaling relations for the cumulants valid in the large voltage regime. Our analysis illustrates that the non-equilibrium Green's function technique and the quantum master equation method can be worked out consistently, when taking into account corresponding scattering processes. PMID:26896971
Phonon anharmonicity in silicon from 100 to 1500 K
Kim, D. S.; Smith, Hillary L.; Niedziela, Jennifer L.; Li, Chen W.; Abernathy, Douglas L.; Fultz, B.
2015-01-21
Inelastic neutron scattering was performed on silicon powder to measure the phonon density of states (DOS) from 100 to 1500 K. The mean fractional energy shifts with temperature of the modes weremore » $$\\langle$$Δεi/εiΔT$$\\rangle$$=₋0.07, giving a mean isobaric Grüneisen parameter of +6.95±0.67, which is significantly different from the isothermal parameter of +0.98. These large effects are beyond the predictions from quasiharmonic models using density functional theory or experimental data, demonstrating large effects from phonon anharmonicity. At 1500 K the anharmonicity contributes 0.15kB/atom to the vibrational entropy, compared to 0.03kB/atom from quasiharmonicity. Lastly, excellent agreement was found between the entropy from phonon DOS measurements and the reference NIST-JANAF thermodynamic entropy from calorimetric measurements.« less
Classification of topological phonons in linear mechanical metamaterials.
Süsstrunk, Roman; Huber, Sebastian D
2016-08-16
Topological phononic crystals, alike their electronic counterparts, are characterized by a bulk-edge correspondence where the interior of a material dictates the existence of stable surface or boundary modes. In the mechanical setup, such surface modes can be used for various applications such as wave guiding, vibration isolation, or the design of static properties such as stable floppy modes where parts of a system move freely. Here, we provide a classification scheme of topological phonons based on local symmetries. We import and adapt the classification of noninteracting electron systems and embed it into the mechanical setup. Moreover, we provide an extensive set of examples that illustrate our scheme and can be used to generate models in unexplored symmetry classes. Our work unifies the vast recent literature on topological phonons and paves the way to future applications of topological surface modes in mechanical metamaterials. PMID:27482105
Reprint of : Absorbing/Emitting Phonons with one dimensional MOSFETs
NASA Astrophysics Data System (ADS)
Bosisio, Riccardo; Gorini, Cosimo; Fleury, Geneviève; Pichard, Jean-Louis
2016-08-01
We consider nanowires in the field effect transistor device configuration. Modeling each nanowire as a one dimensional lattice with random site potentials, we study the heat exchanges between the nanowire electrons and the substrate phonons, when electron transport is due to phonon-assisted hops between localized states. Shifting the nanowire conduction band with a metallic gate induces different behaviors. When the Fermi potential is located near the band center, a bias voltage gives rise to small local heat exchanges which fluctuate randomly along the nanowire. When it is located near one of the band edges, the bias voltage yields heat currents which flow mainly from the substrate towards the nanowire near one boundary of the nanowire, and in the opposite direction near the other boundary. This opens interesting perspectives for heat management at submicron scales: arrays of parallel gated nanowires could be used for a field control of phonon emission/absorption.
Overcoming phonon-induced dephasing for indistinguishable photon sources
NASA Astrophysics Data System (ADS)
Close, Tom; Gauger, Erik M.; Lovett, Brendon W.
2012-11-01
Reliable single photon sources constitute the basis of schemes for quantum communication and measurement based quantum computing. Solid state single photon sources based on quantum dots are convenient and versatile but the electronic transitions that generate the photons are subject to interactions with lattice vibrations. Using a microscopic model of electron-phonon interactions and a quantum master equation, we here examine phonon-induced decoherence and assess its impact on the rate of production, and indistinguishability, of single photons emitted from an optically driven quantum dot system. We find that, above a certain threshold of desired indistinguishability, it is possible to mitigate the deleterious effects of phonons by exploiting a three-level Raman process for photon production.
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.
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.
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.
A new class of tunable hypersonic phononic crystals based on polymer-tethered colloids
Alonso-Redondo, E.; Schmitt, M.; Urbach, Z.; Hui, C. M.; Sainidou, R.; Rembert, P.; Matyjaszewski, K.; Bockstaller, M. R.; Fytas, G.
2015-01-01
The design and engineering of hybrid materials exhibiting tailored phononic band gaps are fundamentally relevant to innovative material technologies in areas ranging from acoustics to thermo-optic devices. Phononic hybridization gaps, originating from the anti-crossing between local resonant and propagating modes, have attracted particular interest because of their relative robustness to structural disorder and the associated benefit to ‘manufacturability'. Although hybridization gap materials are well known, their economic fabrication and efficient control of the gap frequency have remained elusive because of the limited property variability and expensive fabrication methodologies. Here we report a new strategy to realize hybridization gap materials by harnessing the ‘anisotropic elasticity' across the particle–polymer interface in densely polymer-tethered colloidal particles. Theoretical and Brillouin scattering analysis confirm both the robustness to disorder and the tunability of the resulting hybridization gap and provide guidelines for the economic synthesis of new materials with deliberately controlled gap position and width frequencies. PMID:26390851
Phonon wave-packet scattering and energy dissipation dynamics in carbon nanotube oscillators
NASA Astrophysics Data System (ADS)
Prasad, Matukumilli V. D.; Bhattacharya, Baidurya
2015-12-01
Friction in carbon nanotube (CNT) oscillators can be explained in terms of the interplay between low frequency mechanical motions and high frequency vibrational modes of the sliding surfaces. We analyze single mode phonon wave packet dynamics of CNT based mechanical oscillators, with cores either stationary or sliding with moderate velocities, and study how various individual phonons travel through the outer CNT, interact with the inner nanostructure, and undergo scattering. Two acoustic modes (longitudinal and transverse) and one optical mode (flexural optical) are found to be responsible for the major portion of friction in these oscillators: the transmission functions display a significant dip in the rather narrow frequency range of 5-15 meV. We also find that the profile of the dip is characteristic of the inner core. In contrast, radial breathing and twisting modes, which are dominant in thermal transport, display ideal transmission at all frequencies. We also observe polarization dependent scattering and find that the scattering dynamics comprises of an oscillating decay of localized energy inside the inner CNT. This work provides a way towards engineering CNT linear oscillators with better tribological properties.
Janipour, Mohsen; Misirlioglu, Ibrahim Burc; Sendur, Kursat
2016-01-01
Spatial charge distribution for biased semiconductors fundamentally differs from metals since they can allow inhomogeneous charge distributions due to penetration of the electric field, as observed in the classical Schottky junctions. Similarly, the electrostatics of the dielectric/semiconductor interface can lead to a carrier depletion or accumulation in the semiconductor side when under applied bias. In this study, we demonstrate that the inhomogeneous carrier accumulation in a moderately p-doped GaAs–dielectric interface can be tailored for tunable plasmonics by an external voltage. Solving Maxwell’s equations in the doped GaAs-dielectric stack, we investigate the tunability of the surface plasmon and phonon polaritons’ interaction via an external bias. The plasmonic mode analysis of such an interface reveals interesting dispersion curves for surface plasmon and phonon polariton interactions that are not possible in metals. We show that the plasmon dispersion curve can be engineered through an external bias using the inherent properties of the p-doped GaAs– dielectric interface. PMID:27698393
NASA Astrophysics Data System (ADS)
Janipour, Mohsen; Misirlioglu, Ibrahim Burc; Sendur, Kursat
2016-10-01
Spatial charge distribution for biased semiconductors fundamentally differs from metals since they can allow inhomogeneous charge distributions due to penetration of the electric field, as observed in the classical Schottky junctions. Similarly, the electrostatics of the dielectric/semiconductor interface can lead to a carrier depletion or accumulation in the semiconductor side when under applied bias. In this study, we demonstrate that the inhomogeneous carrier accumulation in a moderately p-doped GaAs–dielectric interface can be tailored for tunable plasmonics by an external voltage. Solving Maxwell’s equations in the doped GaAs-dielectric stack, we investigate the tunability of the surface plasmon and phonon polaritons’ interaction via an external bias. The plasmonic mode analysis of such an interface reveals interesting dispersion curves for surface plasmon and phonon polariton interactions that are not possible in metals. We show that the plasmon dispersion curve can be engineered through an external bias using the inherent properties of the p-doped GaAs– dielectric interface.
Modification of the phonon spectrum of bulk Si through surface nanostructuring
NASA Astrophysics Data System (ADS)
Iskandar, A.; Gwiazda, A.; Huang, Y.; Kazan, M.; Bruyant, A.; Tabbal, M.; Lerondel, G.
2016-09-01
In this paper, we present experimental evidence on the change of the phonon spectrum and vibrational properties of a bulk material through phonon hybridization mechanisms. The phonon spectrum in a finite material is strongly affected by the presence of free surfaces, which is the addition of a contribution from an essentially two-dimensional crystal. The phonon spectrum of a bulk material can hence be altered by a hybridization mechanism between confined phonon modes in nanostructures introduced on the surface of a bulk material and the underlying bulk phonon modes. We measured the heat capacities of bare and surface-structured silicon substrates originating from the same silicon wafer. Then, we deduced important features of the phonon spectra of the samples investigated through a rigorous analysis of the measured heat capacity curves. The results show that the shape and size of the nanostructures made on the surface of the bulk substrate have a strong effect on the phonon spectrum of the bulk material.
Coherent phonon coupling to individual Bloch states in photoexcited bismuth.
Papalazarou, E; Faure, J; Mauchain, J; Marsi, M; Taleb-Ibrahimi, A; Reshetnyak, I; van Roekeghem, A; Timrov, I; Vast, N; Arnaud, B; Perfetti, L
2012-06-22
We investigate the temporal evolution of the electronic states at the bismuth (111) surface by means of time- and angle-resolved photoelectron spectroscopy. The binding energy of bulklike bands oscillates with the frequency of the A(1g) phonon mode, whereas surface states are insensitive to the coherent displacement of the lattice. A strong dependence of the oscillation amplitude on the electronic wave vector is correctly reproduced by ab initio calculations of electron-phonon coupling. Besides these oscillations, all the electronic states also display a photoinduced shift towards higher binding energy whose dynamics follows the evolution of the electronic temperature.
YPHON: A package for calculating phonons of polar materials
NASA Astrophysics Data System (ADS)
Wang, Yi; Chen, Long-Qing; Liu, Zi-Kui
2014-11-01
In our recent works, we have developed a mixed-space approach within the framework of direct method for the first-principle calculation of phonon properties. It makes full use of the accuracy of the force constants calculated in the real space and the dipole-dipole interactions in the reciprocal space, making the accurate phonon calculation possible with the direct method for polar materials. In this paper, an efficient C++ implementation of the mixed-space approach, YPHON, is provided as open source, including demos and Linux scripts for extracting input data to YPHON from the output of VASP.5. The functions of the current package include the calculations of: (1) the phonon dispersions; (2) the phonon density of states; (3) the neutron scattering section weighted phonon density of state; (4) the phonons of the high symmetry structure using the force constants from low symmetry structure; (5) the phonon dispersions of random alloys; and (6) the analysis of the vibrational modes using the point group theory. Catalogue identifier: AETS_v1_0 Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AETS_v1_0.html Program obtainable from: CPC Program Library, Queen’s University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 567815 No. of bytes in distributed program, including test data, etc.: 9763594 Distribution format: tar.gz Programming language: C++, Linux scripts. Computer: Linux systems with a g++ or C++ compiler. Operating system: Linux. RAM: Ranges from a few Mbytes to a few Gbytes, dynamically depending on the system size. Classification: 7.8. External routines: GSL-the GNU Scientific Library (GSL) is a numerical library for C and C++ programmers. VASP.5 or later for the calculations of force constants and dielectric constants and Born effective charge for polar materials. Nature of problem: This package has the purpose of computing
Spin Hall Effects Due to Phonon Skew Scattering
NASA Astrophysics Data System (ADS)
Gorini, Cosimo; Eckern, Ulrich; Raimondi, Roberto
2015-08-01
A diversity of spin Hall effects in metallic systems is known to rely on Mott skew scattering. In this work its high-temperature counterpart, phonon skew scattering, which is expected to be of foremost experimental relevance, is investigated. In particular, the phonon skew scattering spin Hall conductivity is found to be practically T independent for temperatures above the Debye temperature TD. As a consequence, in Rashba-like systems a high-T linear behavior of the spin Hall angle demonstrates the dominance of extrinsic spin-orbit scattering only if the intrinsic spin splitting is smaller than the temperature.
Dean M. P.; Howard, C.A.; Withers, F.
2011-12-19
Graphene phonons are measured as a function of electron doping via the addition of potassium adatoms. In the low doping regime, the in-plane carbon G peak hardens and narrows with increasing doping, analogous to the trend seen in graphene doped via the field effect. At high dopings, beyond those accessible by the field effect, the G peak strongly softens and broadens. This is interpreted as a dynamic, nonadiabatic renormalization of the phonon self-energy. At dopings between the light and heavily doped regimes, we find a robust inhomogeneous phase where the potassium coverage is segregated into regions of high and low density. The phonon energies, linewidths, and tunability are notably very similar for one- to four-layer potassium-doped graphene, but significantly different to bulk potassium-doped graphite.
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.
Phonon-Assisted Anti-Stokes Lasing in ZnTe Nanoribbons.
Zhang, Qing; Liu, Xinfeng; Utama, M Iqbal Bakti; Xing, Guichuan; Sum, Tze Chien; Xiong, Qihua
2016-01-13
Phonon-assisted anti-Stokes emission and its stimulated emission in polar semiconductor ZnTe are demonstrated via the annihilation of phonons as a result of strong exciton-phonon coupling. The findings are not only important for developing high-power radiation-balanced lasers, but are also promising for manufacturing ultraefficient solid-state laser coolers. PMID:26573758
A note on two-phonon coherent anti-stokes Raman scattering
Shen, Y. R.
1981-01-01
Difference-frequency mixing of two pump waves can in principle excite two coherent phonon waves via the parametric process. Finally, only when the phonon excitation is small can the nonlinear susceptibility of two-phonon coherent anti-Stokes Raman scattering be described as proportional to the product of two Raman tensors.
NASA Astrophysics Data System (ADS)
Bai, Wen-Chao; Lan, Zhong-Jian; Zhang, Han-Zhuang; Zhang, Han; Jiang, Li
2016-09-01
The properties of phonon-polaritons in Czochralski-grown piezoelectric superlattice (CPSL), are studied theoretically. We propose the phonon-polariton mode of CPSL. The mechanism for polariton coupling is analyzed. We discuss the factors that influence the properties of the phonon-polariton. Some potential applications are also discussed.
Enhanced Thermoelectric Performance of Nanostructured Bi2Te3 through Significant Phonon Scattering.
Yang, Lei; Chen, Zhi-Gang; Hong, Min; Han, Guang; Zou, Jin
2015-10-28
N-type Bi2Te3 nanostructures were synthesized using a solvothermal method and in turn sintered using sparking plasma sintering. The sintered n-type Bi2Te3 pellets reserved nanosized grains and showed an ultralow lattice thermal conductivity (∼0.2 W m(-1) K(-1)), which benefits from high-density small-angle grain boundaries accommodated by dislocations. Such a high phonon scattering leads an enhanced ZT of 0.88 at 400 K. This study provides an efficient method to enhance thermoelectric performance of thermoelectric nanomaterials through nanostructure engineering, making the as-prepared n-type nanostructured Bi2Te3 as a promising candidate for room-temperature thermoelectric power generation and Peltier cooling. PMID:26451626
Enhanced Thermoelectric Performance of Nanostructured Bi2Te3 through Significant Phonon Scattering.
Yang, Lei; Chen, Zhi-Gang; Hong, Min; Han, Guang; Zou, Jin
2015-10-28
N-type Bi2Te3 nanostructures were synthesized using a solvothermal method and in turn sintered using sparking plasma sintering. The sintered n-type Bi2Te3 pellets reserved nanosized grains and showed an ultralow lattice thermal conductivity (∼0.2 W m(-1) K(-1)), which benefits from high-density small-angle grain boundaries accommodated by dislocations. Such a high phonon scattering leads an enhanced ZT of 0.88 at 400 K. This study provides an efficient method to enhance thermoelectric performance of thermoelectric nanomaterials through nanostructure engineering, making the as-prepared n-type nanostructured Bi2Te3 as a promising candidate for room-temperature thermoelectric power generation and Peltier cooling.
Atxitia, U.; Ostler, T. A.; Chantrell, R. W.; Chubykalo-Fesenko, O.
2015-11-09
Using large-scale computer simulations, we thoroughly study the minimum energy required to thermally induced magnetization switching (TIMS) after the application of a femtosecond heat pulse in transition metal-rare earth ferrimagnetic alloys. We find that for an energy efficient TIMS, a low ferrimagnetic net magnetization with a strong temperature dependence is the relevant factor for the magnetic system. For the lattice and electron systems, the key physics for efficient TIMS is a large electron-phonon relaxation time. Importantly, we show that as the cooling time of the heated electrons is increased, the minimum power required to produce TIMS can be reduced by an order of magnitude. Our results show the way to low power TIMS by appropriate engineering of magnetic heterostructures.
NASA Astrophysics Data System (ADS)
Atxitia, U.; Ostler, T. A.; Chantrell, R. W.; Chubykalo-Fesenko, O.
2015-11-01
Using large-scale computer simulations, we thoroughly study the minimum energy required to thermally induced magnetization switching (TIMS) after the application of a femtosecond heat pulse in transition metal-rare earth ferrimagnetic alloys. We find that for an energy efficient TIMS, a low ferrimagnetic net magnetization with a strong temperature dependence is the relevant factor for the magnetic system. For the lattice and electron systems, the key physics for efficient TIMS is a large electron-phonon relaxation time. Importantly, we show that as the cooling time of the heated electrons is increased, the minimum power required to produce TIMS can be reduced by an order of magnitude. Our results show the way to low power TIMS by appropriate engineering of magnetic heterostructures.
Phonon Trapping in Pearl-Necklace-Shaped Silicon Nanowires.
Miao, Chunyang; Tai, Guoan; Zhou, Jianxin; Guo, Wanlin
2015-12-22
A pearl-necklace-shaped silicon nanowire, in contrast to a smooth nanowire, presents a much lower thermal conductivity due to the phonon trapping effect. By precisely controlling the pearl size and density, this reduction can be more than 70% for the structures designed in the study, which provides a unique approach for designing high-performance nanoscale thermoelectric devices. PMID:26577864
Phonon spectral densities of Cu surfaces: Application to Cu(211)
NASA Astrophysics Data System (ADS)
Mărinică, M.-C.; Raşeev, G.; Smirnov, K. S.
2001-05-01
Power phonon spectra of vicinal stepped surfaces of Cu(211) have been calculated using a molecular dynamics method combined with a semiempirical potential. The potential is based on an analytic form of inverse powers proposed by Finnis and Sinclair with the parametrization of Sutton and Chen. One of the four independent parameters of the potential was rescaled to reproduce the bulk phonon spectrum of Cu while retaining other properties of the bulk Cu close to the experimental values. Using this potential, we calculated the power surface phonon spectra, projection of the spectra at the high-symmetry points of surface Brillouin zone (SBZ), and the mean square displacements (MSD's) of atoms of the Cu(211) surface. The calculated projected phonon spectra at Γ¯ and at two new SBZ points (at X¯ and Y¯) compare favorably with experiment and theory when available. The MSD of the Cu(211) surface is also well reproduced and its temperature dependence shows that anharmonicity of the atomic motion becomes important above 200 K.
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.
Enhanced Electron-Phonon Coupling at Metal Surfaces
Plummer, Ward E.
2010-08-04
The Born-Oppenheimer approximation (BOA) decouples electronic from nuclear motion, providing a focal point for most quantum mechanics textbooks. However, a multitude of important chemical, physical and biological phenomena are driven by violations of this approximation. Vibronic interactions are a necessary ingredient in any process that makes or breaks a covalent bond, for example, conventional catalysis or enzymatically delivered biological reactions. Metastable phenomena associated with defects and dopants in semiconductors, oxides, and glasses entail violation of the BOA. Charge exchange in inorganic polymers, organic slats and biological systems involves charge- induced distortions of the local structure. A classic example is conventional superconductivity, which is driven by the electron-lattice interaction. High-resolution angle-resolved photoemission experiments are yielding new insight into the microscopic origin of electron-phonon coupling (EPC) in anisotropic two-dimensional systems. Our recent surface phonon measurement on the surface of a high-Tc material clearly indicates an important momentum dependent EPC in these materials. In the last few years we have shifted our research focus from solely looking at electron phonon coupling to examining the structure/functionality relationship at the surface of complex transition metal compounds. The investigation on electron phonon coupling has allowed us to move to systems where there is coupling between the lattice, the electrons and the spin.
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.
Phonon dynamics and inelastic neutron scattering of sodium niobate
NASA Astrophysics Data System (ADS)
Mishra, S. K.; Gupta, M. K.; Mittal, R.; Zbiri, M.; Rols, S.; Schober, H.; Chaplot, S. L.
2014-05-01
Sodium niobate (NaNbO3) exhibits an extremely complex sequence of structural phase transitions in the perovskite family and therefore provides an excellent model system for understanding the mechanism of structural phase transitions. We report temperature dependence of inelastic neutron scattering measurements of phonon densities of states in sodium niobate. The measurements are carried out in various crystallographic phases of this material at various temperatures from 300 to 1048 K. The phonon spectra exhibit peaks centered on 19, 37, 51, 70, and 105 meV. Interestingly, the peak near 70 meV shifts significantly towards lower energy with increasing temperature, while the other peaks do not exhibit any appreciable shift. The phonon spectra at 783 K show prominent change and become more diffusive as compared to those at 303 K. In order to better analyze these features, we have performed first-principles lattice dynamics calculations based on the density functional theory. The computed phonon density of states is found to be in good agreement with the experimental data. Based on our calculation we are able to assign the characteristic Raman modes in the antiferroelectric phase, which are due to the folding of the T (ω = 95 cm-1) and Δ (ω = 129 cm-1) points of the cubic Brillouin zone, to the A1g symmetry.
Phonon coupling to core spectra in homopolar semiconductors
NASA Astrophysics Data System (ADS)
Carson, R. D.; Schnatterly, S. E.
1989-01-01
Using the temperature-dependent broadening in silicon soft-x-ray emission, we have made the first measurements of core-hole-phonon coupling in a nonpolar material. This coupling is significantly weaker than in metals and polar insulators. We propose a simple microscopic model appropriate for nonpolar semiconductors which is able to explain the observed weak coupling.
Phonon-enhanced crystal growth and lattice healing
Buonassisi, Anthony; Bertoni, Mariana; Newman, Bonna
2013-05-28
A system for modifying dislocation distributions in semiconductor materials is provided. The system includes one or more vibrational sources for producing at least one excitation of vibrational mode having phonon frequencies so as to enhance dislocation motion through a crystal lattice.
Electron-phonon coupling using many-body GW theory
NASA Astrophysics Data System (ADS)
Monserrat, Bartomeu; Vanderbilt, David
Electron-phonon coupling drives a plethora of phenomena, such as superconductivity in metals, or the temperature dependence of optical properties in semiconductors. There is increasing evidence that semi-local density functional theory (DFT) is not adequate for the description of electron-phonon coupling, and instead effects such as electronic correlation need to be included. Unfortunately, methods beyond semi-local DFT are computationally demanding, limiting the study of these phenomena. In this talk we will introduce the idea of ``thermal lines'', which can be used to explore the vibrational phase space of solids and molecules at small computational cost. In particular, we will describe how thermal lines can be exploited to calculate the temperature dependence of band structures beyond semi-local DFT, by using many-body GW theory, or by including the effects of spin-orbit coupling. We will present first-principles results showing the effects of electron correlation on the strength of electron-phonon coupling, and the effects of electron-phonon coupling on topological states of matter. Supported by Robinson College, Cambridge, and the Cambridge Philosophical Society.
Coherently driven, ultrafast electron-phonon dynamics in transport junctions
Szekely, Joshua E.; Seideman, Tamar
2014-07-28
Although the vast majority of studies of transport via molecular-scale heterojunctions have been conducted in the (static) energy domain, experiments are currently beginning to apply time domain approaches to the nanoscale transport problem, combining spatial with temporal resolution. It is thus an opportune time for theory to develop models to explore both new phenomena in, and new potential applications of, time-domain, coherently driven molecular electronics. In this work, we study the interaction of a molecular phonon with an electronic wavepacket transmitted via a conductance junction within a time-domain model that treats the electron and phonon on equal footing and spans the weak to strong electron-phonon coupling strengths. We explore interference between two coherent energy pathways in the electronic subspace, thus complementing previous studies of coherent phenomena in conduction junctions, where the stationary framework was used to study interference between spatial pathways. Our model provides new insights into phase decoherence and population relaxation within the electronic subspace, which have been conventionally treated by density matrix approaches that often rely on phenomenological parameters. Although the specific case of a transport junction is explored, our results are general, applying also to other instances of coupled electron-phonon systems.
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.
Phonon localization drives polar nanoregions in a relaxor ferroelectric
Manley, Michael E; Lynn, Jeffrey; Specht, Eliot D; Delaire, Olivier A; Bishop, Alan; Sahul, Raffi; Budai, John D
2014-01-01
Relaxor ferroelectrics1, which are utilized as actuators and sensors2-4, exemplify a class of poorly understood materials where interplay between disorder and phase instability results in inhomogeneous nanoregions. There is no definitive explanation for the onset of relaxor behavior (Burns temperature5, Td) or the origin of polar nanoregions (PNRs). Here we show a vibrational mode that localizes on cooling to Td, remains localized as PNRs form, and then delocalizes as PNRs grow using neutron scattering on relaxor (Pb(Mg1/3Nb2/3)O3)0.69-(PbTiO3)0.31 (PMN-31%PT). Although initially appearing like intrinsic local modes (ILMs)6-10, these modes differ below Td as they form a resonance with the ferroelectric phonon. At the resonance, nanoregions of standing ferroelectric phonons develop with a coherence length matching the PNRs. The size, shape, distribution, and temporal fluctuations of PNRs, and our observations, are explained by ferroelectric phonons trapped by disordered resonance modes via Anderson localization11-13. Our results show the size and shape of PNRs are not dictated by complex structural details, as always assumed, but by a phonon resonance wavevector. This simplification could guide the design of next generation relaxors.
Phonon Trapping in Pearl-Necklace-Shaped Silicon Nanowires.
Miao, Chunyang; Tai, Guoan; Zhou, Jianxin; Guo, Wanlin
2015-12-22
A pearl-necklace-shaped silicon nanowire, in contrast to a smooth nanowire, presents a much lower thermal conductivity due to the phonon trapping effect. By precisely controlling the pearl size and density, this reduction can be more than 70% for the structures designed in the study, which provides a unique approach for designing high-performance nanoscale thermoelectric devices.
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
Optical phonons in PbTe/CdTe multilayer heterostructures
Novikova, N. N.; Yakovlev, V. A.; Kucherenko, I. V.; Karczewski, G.; Aleshchenko, Yu. A.; Muratov, A. V.; Zavaritskaya, T. N.; Melnik, N. N.
2015-05-15
The infrared reflection spectra of PbTe/CdTe multilayer nanostructures grown by molecular-beam epitaxy are measured in the frequency range of 20–5000 cm{sup −1} at room temperature. The thicknesses and high-frequency dielectric constants of the PbTe and CdTe layers and the frequencies of the transverse optical (TO) phonons in these structures are determined from dispersion analysis of the spectra. It is found that the samples under study are characterized by two TO phonon frequencies, equal to 28 and 47 cm{sup −1}. The first frequency is close to that of TO phonons in bulk PbTe, and the second is assigned to the optical mode in structurally distorted interface layers. The Raman-scattering spectra upon excitation with the radiation of an Ar{sup +} laser at 514.5 nm are measured at room and liquid-nitrogen temperatures. The weak line at 106 cm{sup −1} observed in these spectra is attributed to longitudinal optical phonons in the interface layers.
Spin Qubits in Germanium Structures with Phononic Gap
NASA Technical Reports Server (NTRS)
Smelyanskiy, V. N.; Vasko, F. T.; Hafiychuk, V. V.; Dykman, M. I.; Petukhov, A. G.
2014-01-01
We propose qubits based on shallow donor electron spins in germanium structures with phononic gap. We consider a phononic crystal formed by periodic holes in Ge plate or a rigid cover / Ge layer / rigid substrate structure with gaps approximately a few GHz. The spin relaxation is suppressed dramatically, if the Zeeman frequency omegaZ is in the phononic gap, but an effective coupling between the spins of remote donors via exchange of virtual phonons remains essential. If omegaZ approaches to a gap edge in these structures, a long-range (limited by detuning of omegaZ) resonant exchange interaction takes place. We estimate that ratio of the exchange integral to the longitudinal relaxation rate exceeds 10(exp 5) and lateral scale of resonant exchange 0.1 mm. The exchange contribution can be verified under microwave pumping through oscillations of spin echo signal or through the differential absorption measurements. Efficient manipulation of spins due to the Rabi oscillations opens a new way for quantum information applications.
Phonon transport in single-layer boron nanoribbons.
Zhang, Zhongwei; Xie, Yuee; Peng, Qing; Chen, Yuanping
2016-11-01
Inspired by the successful synthesis of three two-dimensional (2D) allotropes, the boron sheet has recently been one of the hottest 2D materials around. However, to date, phonon transport properties of these new materials are still unknown. By using the non-equilibrium Green's function (NEGF) combined with the first principles method, we study ballistic phonon transport in three types of boron sheets; two of them correspond to the structures reported in the experiments, while the third one is a stable structure that has not been synthesized yet. At room temperature, the highest thermal conductance of the boron nanoribbons is comparable with that of graphene, while the lowest thermal conductance is less than half of graphene's. Compared with graphene, the three boron sheets exhibit diverse anisotropic transport characteristics. With an analysis of phonon dispersion, bonding charge density, and simplified models of atomic chains, the mechanisms of the diverse phonon properties are discussed. Moreover, we find that many hybrid patterns based on the boron allotropes can be constructed naturally without doping, adsorption, and defects. This provides abundant nanostructures for thermal management and thermoelectric applications.
Phonon transport in single-layer boron nanoribbons
NASA Astrophysics Data System (ADS)
Zhang, Zhongwei; Xie, Yuee; Peng, Qing; Chen, Yuanping
2016-11-01
Inspired by the successful synthesis of three two-dimensional (2D) allotropes, the boron sheet has recently been one of the hottest 2D materials around. However, to date, phonon transport properties of these new materials are still unknown. By using the non-equilibrium Green’s function (NEGF) combined with the first principles method, we study ballistic phonon transport in three types of boron sheets; two of them correspond to the structures reported in the experiments, while the third one is a stable structure that has not been synthesized yet. At room temperature, the highest thermal conductance of the boron nanoribbons is comparable with that of graphene, while the lowest thermal conductance is less than half of graphene’s. Compared with graphene, the three boron sheets exhibit diverse anisotropic transport characteristics. With an analysis of phonon dispersion, bonding charge density, and simplified models of atomic chains, the mechanisms of the diverse phonon properties are discussed. Moreover, we find that many hybrid patterns based on the boron allotropes can be constructed naturally without doping, adsorption, and defects. This provides abundant nanostructures for thermal management and thermoelectric applications.
Femtosecond electron imaging of defect-modulated phonon dynamics
Cremons, Daniel R.; Plemmons, Dayne A.; Flannigan, David J.
2016-01-01
Precise manipulation and control of coherent lattice oscillations via nanostructuring and phonon-wave interference has the potential to significantly impact a broad array of technologies and research areas. Resolving the dynamics of individual phonons in defect-laden materials presents an enormous challenge, however, owing to the interdependent nanoscale and ultrafast spatiotemporal scales. Here we report direct, real-space imaging of the emergence and evolution of acoustic phonons at individual defects in crystalline WSe2 and Ge. Via bright-field imaging with an ultrafast electron microscope, we are able to image the sub-picosecond nucleation and the launch of wavefronts at step edges and resolve dispersion behaviours during propagation and scattering. We discover that the appearance of speed-of-sound (for example, 6 nm ps−1) wavefronts are influenced by spatially varying nanoscale strain fields, taking on the appearance of static bend contours during propagation. These observations provide unprecedented insight into the roles played by individual atomic and nanoscale features on acoustic-phonon dynamics. PMID:27079790
Phonon transport in single-layer boron nanoribbons.
Zhang, Zhongwei; Xie, Yuee; Peng, Qing; Chen, Yuanping
2016-11-01
Inspired by the successful synthesis of three two-dimensional (2D) allotropes, the boron sheet has recently been one of the hottest 2D materials around. However, to date, phonon transport properties of these new materials are still unknown. By using the non-equilibrium Green's function (NEGF) combined with the first principles method, we study ballistic phonon transport in three types of boron sheets; two of them correspond to the structures reported in the experiments, while the third one is a stable structure that has not been synthesized yet. At room temperature, the highest thermal conductance of the boron nanoribbons is comparable with that of graphene, while the lowest thermal conductance is less than half of graphene's. Compared with graphene, the three boron sheets exhibit diverse anisotropic transport characteristics. With an analysis of phonon dispersion, bonding charge density, and simplified models of atomic chains, the mechanisms of the diverse phonon properties are discussed. Moreover, we find that many hybrid patterns based on the boron allotropes can be constructed naturally without doping, adsorption, and defects. This provides abundant nanostructures for thermal management and thermoelectric applications. PMID:27669055
Phonon-mediated negative differential conductance in molecular quantum dots
NASA Astrophysics Data System (ADS)
Zazunov, Alex; Feinberg, Denis; Martin, Thierry
2006-03-01
Transport through a single-molecular conductor is considered, showing negative differential conductance behavior associated with phonon-mediated electron tunneling processes. This theoretical work is motivated by a recent experiment by Leroy using a carbon nanotube contacted by a scanning tunneling microscope tip [Nature 432, 371 (2004)], where negative differential conductance of the breathing-mode phonon side peaks could be observed. A peculiarity of this system is that the tunneling couplings which inject electrons and those which collect them on the substrate are highly asymmetrical. A quantum dot model is used, coupling a single electronic level to a local phonon, forming polaron levels. A “half-shuttle” mechanism is also introduced. A quantum kinetic formulation allows us to derive rate equations. Assuming asymmetric tunneling rates and in the absence of the half-shuttle coupling, negative differential conductance (NDC) is obtained for a wide range of parameters. A detailed explanation of this phenomenon is provided, showing that NDC is maximal for intermediate electron-phonon coupling. In addition, in the absence of a gate, the “floating” level results in two distinct lengths for the current plateaus, related to the capacitive couplings at the two junctions. It is shown that the half-shuttle mechanism tends to reinforce the negative differential regions, but it cannot trigger this behavior on its own.
Optimal design of tunable phononic bandgap plates under equibiaxial stretch
NASA Astrophysics Data System (ADS)
Hedayatrasa, Saeid; Abhary, Kazem; Uddin, M. S.; Guest, James K.
2016-05-01
Design and application of phononic crystal (PhCr) acoustic metamaterials has been a topic with tremendous growth of interest in the last decade due to their promising capabilities to manipulate acoustic and elastodynamic waves. Phononic controllability of waves through a particular PhCr is limited only to the spectrums located within its fixed bandgap frequency. Hence the ability to tune a PhCr is desired to add functionality over its variable bandgap frequency or for switchability. Deformation induced bandgap tunability of elastomeric PhCr solids and plates with prescribed topology have been studied by other researchers. Principally the internal stress state and distorted geometry of a deformed phononic crystal plate (PhP) changes its effective stiffness and leads to deformation induced tunability of resultant modal band structure. Thus the microstructural topology of a PhP can be altered so that specific tunability features are met through prescribed deformation. In the present study novel tunable PhPs of this kind with optimized bandgap efficiency-tunability of guided waves are computationally explored and evaluated. Low loss transmission of guided waves throughout thin walled structures makes them ideal for fabrication of low loss ultrasound devices and structural health monitoring purposes. Various tunability targets are defined to enhance or degrade complete bandgaps of plate waves through macroscopic tensile deformation. Elastomeric hyperelastic material is considered which enables recoverable micromechanical deformation under tuning finite stretch. Phononic tunability through stable deformation of phononic lattice is specifically required and so any topology showing buckling instability under assumed deformation is disregarded. Nondominated sorting genetic algorithm (GA) NSGA-II is adopted for evolutionary multiobjective topology optimization of hypothesized tunable PhP with square symmetric unit-cell and relevant topologies are analyzed through finite
Phonon-drag thermopower in 3D Dirac semimetals
NASA Astrophysics Data System (ADS)
Kubakaddi, S. S.
2015-11-01
A theory of low-temperature phonon-drag thermopower S g in three-dimensional (3D) Dirac semimetals has been developed considering screened electron-phonon deformation potential coupling. Numerical investigations of S g, in the boundary scattering regime for phonons, are made in 3D Dirac semimetal Cd3As2, as a function of temperature T and electron concentration n e. S g is found to increase rapidly for about T < 1 K and nearly levels off for higher T. It is also seen that S g increases (decreases) with decreasing n e at lower (higher) T (<2 K). A screening effect is found to be very significant, strongly affecting T and n e dependence for about <1 K and becoming negligible at higher temperature. In the Bloch-Gruneisen (BG) regime the power laws S g ~ T 8 (T 4) and S g ~ n\\text{e}-5/3 (n\\text{e}-1/3) with (without) screening are obtained. These laws with respect to T and n e are, respectively, characteristics of 3D phonons and Dirac 3D electrons. Comparison with diffusion thermopower S d shows that S g dominates (and is much greater than) S d for about T > 0.2 K. Herring’s law S g μ p ~ T -1, relating phonon limited mobility μ p and S g in the BG regime, is shown to be valid in 3D Dirac semimetals. The results obtained here are compared with those in 3D semiconductors, low-dimensional semiconductor heterojunctions and graphene. We conclude that n e-dependent measurements, rather than T-dependent ones, provide a clearer signature of the 3D Dirac semimetal phase.
Phonon-drag thermopower in 3D Dirac semimetals.
Kubakaddi, S S
2015-11-18
A theory of low-temperature phonon-drag thermopower S(g) in three-dimensional (3D) Dirac semimetals has been developed considering screened electron-phonon deformation potential coupling. Numerical investigations of S(g), in the boundary scattering regime for phonons, are made in 3D Dirac semimetal Cd3As2, as a function of temperature T and electron concentration n e. S(g) is found to increase rapidly for about T < 1 K and nearly levels off for higher T. It is also seen that S(g) increases (decreases) with decreasing n e at lower (higher) T (<2 K). A screening effect is found to be very significant, strongly affecting T and n e dependence for about <1 K and becoming negligible at higher temperature. In the Bloch-Gruneisen (BG) regime the power laws S(g) ~ T(8) (T(4)) and S(g) ~ n(e)(-5/3)(n(e)(-1/3) with (without) screening are obtained. These laws with respect to T and n e are, respectively, characteristics of 3D phonons and Dirac 3D electrons. Comparison with diffusion thermopower S(d) shows that S (g) dominates (and is much greater than) S(d) for about T > 0.2 K. Herring's law S(g) μ p ~ T (-1), relating phonon limited mobility μ p and S(g) in the BG regime, is shown to be valid in 3D Dirac semimetals. The results obtained here are compared with those in 3D semiconductors, low-dimensional semiconductor heterojunctions and graphene. We conclude that n e-dependent measurements, rather than T-dependent ones, provide a clearer signature of the 3D Dirac semimetal phase. PMID:26490643
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
Electron-phonon interaction and superconductivity in BaIr2P2.
Billington, D
2016-10-01
Detailed calculations of the electronic structure, phonons and electron-phonon coupling of the superconducting compound BaIr2P2 were performed from first-principles. The electronic structure showed excellent agreement with the available experimental data. The total electron-phonon coupling constant was [Formula: see text] and the logarithmically averaged phonon frequency was [Formula: see text] K. From the Allen-Dynes formula, with [Formula: see text], the superconducting critical temperature was estimated to be [Formula: see text] K, which is in excellent agreement with the experiment. These results indicate that the electron-phonon coupling is of moderate strength and is easily capable of supporting the observed superconductivity.
Formation of a Mesa Shaped Phonon Pulse in Superfluid 4He
NASA Astrophysics Data System (ADS)
Adamenko, I. N.; Nemchenko, K. E.; Slipko, V. A.
2010-05-01
We present a theory for the formation of a mesa shaped phonon pulse in superfluid 4He. Starting from the hydrodynamic equations of superfluid helium, we obtain the system of equations which describe the evolution of strongly anisotropic phonon systems. Such systems can be created experimentally. The solution of the equations are simple waves, which correspond to second sound in the moving phonon pulse. Using these exact solutions, we describe the expansion of phonon pulses in superfluid helium at zero temperature. This theory gives an explanation for the mesa shape observed in the measured phonon angular distributions. Almost all dependencies of the mesa shape on the system parameters can be qualitatively understood.
Spann, B. T.; Xu, X.
2014-08-25
We employ ultrafast transient absorption spectroscopy with temporal pulse shaping to manipulate coherent phonon excitation and quantify the strength of electron-phonon coupling in CdTe{sub 1−x}Se{sub x} nanocrystals (NCs). Raman active CdSe and CdTe longitudinal optical phonon (LO) modes are excited and probed in the time domain. By temporally controlling pump pulse pairs to coherently excite and cancel coherent phonons in the CdTe{sub 1−x}Se{sub x} NCs, we estimate the relative amount of optical energy that is coupled to the coherent CdSe LO mode.
Hillenbrand, Rainer
2004-08-01
Diffraction limits the spatial resolution in classical microscopy or the dimensions of optical circuits to about half the illumination wavelength. Scanning near-field microscopy can overcome this limitation by exploiting the evanescent near fields existing close to any illuminated object. We use a scattering-type near-field optical microscope (s-SNOM) that uses the illuminated metal tip of an atomic force microscope (AFM) to act as scattering near-field probe. The presented images are direct evidence that the s-SNOM enables optical imaging at a spatial resolution on a 10nm scale, independent of the wavelength used (lambda=633 nm and 10 microm). Operating the microscope at specific mid-infrared frequencies we found a tip-induced phonon-polariton resonance on flat polar crystals such as SiC and Si3N4. Being a spectral fingerprint of any polar material such phonon-enhanced near-field interaction has enormous applicability in nondestructive, material-specific infrared microscopy at nanoscale resolution. The potential of s-SNOM to study eigenfields of surface polaritons in nanostructures opens the door to the development of phonon photonics-a proposed infrared nanotechnology that uses localized or propagating surface phonon polaritons for probing, manipulating and guiding infrared light in nanoscale devices, analogous to plasmon photonics.
Study of LO-phonon decay in semiconductors for hot carrier solar cell
NASA Astrophysics Data System (ADS)
Levard, Hugo; Vidal, Julien; Laribi, Sana; Guillemoles, Jean-François
2014-03-01
Knowledge of phonon decay is of crucial importance when studying basic properties of semiconductors, since they are closely related to Raman linewidth and non-equilibrium-hot-carriers cooling. The latter indeed cools down to the bottom of the conduction band within a picosecond range because of electron-phonon interaction. The eventual emitted hot phonons then decay in few picoseconds. The hot carriers cooling can be slowed down by considering the decay rate dependence of phonon on conservation rules, whose tuning may reduce the allowed two-phonon final states density. This is of direct interest for the third generation photovoltaic devices that are Hot Carrier Solar Cells (HCSC), in which the photoexcited carriers are extracted at an energy higher than thermal equilibrium. One of the HCSC main challenges then is to find an absorber material in which the hot phonons has a relaxation time longer than the carriers cooling time, so that we can expect the electron to ``reabsorb'' a phonon, slowing down the electronic cooling. HCSC yield is ultimately limited by LO phonon decay, though. In this work, we present theoretical results obtained from ab initio calculations of phonon lifetime in III-V and IV-IV semiconductors through a three-phonon process. Common approximations in the literature are questioned. In particular, we show that the usual ``zone-center approximation'' is not valid in some specific semiconductors. The analysis allows to correctly investigate phonon decay mechanisms in bulk and nanostructured materials.
Phonon anharmonicity and components of the entropy in palladium and platinum
NASA Astrophysics Data System (ADS)
Shen, Yang; Li, Chen W.; Tang, Xiaoli; Smith, Hillary L.; Fultz, B.
2016-06-01
Inelastic neutron scattering was used to measure the phonon density of states in fcc palladium and platinum metal at temperatures from 7 K to 1576 K. Both phonon-phonon interactions and electron-phonon interactions were calculated by methods based on density functional theory (DFT) and were consistent with the measured shifts and broadenings of phonons with temperature. Unlike the longitudinal modes, the characteristic transverse modes had a nonlinear dependence on temperature owing to the requirement for a population of thermal phonons for upscattering. Kohn anomalies were observed in the measurements at low temperature and were reproduced by calculations based on DFT. Contributions to the entropy from phonons and electrons were assessed and summed to obtain excellent agreement with prior calorimetric data. The entropy from thermal expansion is positive for both phonons and electrons but larger for phonons. The anharmonic phonon entropy is negative in Pt, but in Pd it changes from positive to negative with increasing temperature. Owing to the position of the Fermi level on the electronic DOS, the electronic entropy was sensitive to the adiabatic electron-phonon interaction in both Pd and Pt. The adiabatic EPI depended strongly on thermal atom displacements.
Phonovoltaic. I. Harvesting hot optical phonons in a nanoscale p -n junction
NASA Astrophysics Data System (ADS)
Melnick, Corey; Kaviany, Massoud
2016-03-01
The phonovoltaic (pV) cell is similar to the photovoltaic. It harvests nonequilibrium (hot) optical phonons (Ep ,O) more energetic than the band gap (Δ Ee ,g) to generate power in a p-n junction. We examine the theoretical electron-phonon and phonon-phonon scattering rates, the Boltzmann transport of electrons, and the diode equation and hydrodynamic simulations to describe the operation of a pV cell and develop an analytic model predicting its efficiency. Our findings indicate that a pV material with Ep ,O≃Δ Ee ,g≫kBT , where kBT is the thermal energy, and a strong interband electron-phonon coupling surpasses the thermoelectric limit, provided the optical phonon population is excited in a nanoscale cell, enabling the ensuing local nonequilibrium. Finding and tuning a material with these properties is challenging. In Paper II [C. Melnick and M. Kaviany, Phys. Rev. B 93, 125203 (2016), 10.1103/PhysRevB.93.125203], we tune the band gap of graphite within density functional theory through hydrogenation and the application of isotropic strains. The band gap is tuned to resonate with its energetic optical phonon modes and calculate the ab initio electron-phonon and phonon-phonon scattering rates. While hydrogenation degrades the strong electron-phonon coupling in graphene such that the figure of merit vanishes, we outline the methodology for a continued material search.
Surface induced phonon decay rates in thin film nano-structures
NASA Astrophysics Data System (ADS)
Photiadis, D. M.
2007-12-01
Nano-scale structure significantly impacts phonon transport and related phonon relaxation rates, with order of magnitude effects on the thermal conductivity of dielectric thin films and quantum wires, and even larger effects on the lifetimes of ultrasonic phonons of micro- (nano-) oscillators. In both cases, efforts to explain the data have been hampered by our lack of knowledge of the effects of confined dimensionality on phonon-phonon scattering rates. Using a phonon Boltzmann equation with appropriate boundary conditions on the free surfaces to take surface roughness into account, we have obtained an expression yielding phonon lifetimes in 2-D dielectric nanostructures(thin films) resulting from phonon-phonon scattering in conjunction with phonon-surface scattering. We present these theoretical results and, in the limit in which surface induced losses dominate, obtain explicit predictions for the phonon lifetimes. The predicted temperature dependence of the ultrason! ic loss does not explain the observed saturation of the loss at low temperatures(τ(T) → const), but does give results of the order of magnitude of measured ultrasonic lifetimes.
NASA Astrophysics Data System (ADS)
Nomura, Yusuke; Arita, Ryotaro
2015-12-01
We formulate an ab initio downfolding scheme for electron-phonon-coupled systems. In this scheme, we calculate partially renormalized phonon frequencies and electron-phonon coupling, which include the screening effects of high-energy electrons, to construct a realistic Hamiltonian consisting of low-energy electron and phonon degrees of freedom. We show that our scheme can be implemented by slightly modifying the density functional-perturbation theory (DFPT), which is one of the standard methods for calculating phonon properties from first principles. Our scheme, which we call the constrained DFPT, can be applied to various phonon-related problems, such as superconductivity, electron and thermal transport, thermoelectricity, piezoelectricity, dielectricity, and multiferroicity. We believe that the constrained DFPT provides a firm basis for the understanding of the role of phonons in strongly correlated materials. Here, we apply the scheme to fullerene superconductors and discuss how the realistic low-energy Hamiltonian is constructed.
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.
Optical phonon lasing and its detection in transport through semiconduc- tor double quantum dots
NASA Astrophysics Data System (ADS)
Okuyama, Rin; Eto, Mikio; Brandes, Tobias
2014-03-01
We theoretically propose optical phonon lasing for a double quantum dot (DQD) fabricated in a semiconductor substrate. No additional cavity or resonator is required. We show that the DQD couples to only two phonon modes that act as a natural cavity. The pumping to the upper level is realized by an electric current through the DQD under a finite bias. Using the rate equation in the Born-Markov-Secular approximation, we analyze the enhanced phonon emission when the level spacing in the DQD is tuned to the phonon energy. We find the phonon lasing when the pumping rate is much larger than the phonon decay rate, whereas anti-bunching of phonon emission is observed when the pumping rate is smaller.[1] Our theory can be also applicable to DQDs embedded in nanomechanical resonators to control the vibrating modes. We discuss detection of amplified modes using the electric current and its noise through the DQD, and another DQD fabricated nearby.
Phonon dispersion and quantization tuning of strained carbon nanotubes for flexible electronics
Gautreau, Pierre; Chu, Yanbiao; Basaran, Cemal; Ragab, Tarek
2014-06-28
Graphene and carbon nanotubes are materials with large potentials for applications in flexible electronics. Such devices require a high level of sustainable strain and an understanding of the materials electrical properties under strain. Using supercell theory in conjunction with a comprehensive molecular mechanics model, the full band phonon dispersion of carbon nanotubes under uniaxial strain is studied. The results suggest an overall phonon softening and open up the possibility of phonon quantization tuning with uniaxial strain. The change in phonon quantization and the resulting increase in electron-phonon and phonon-phonon scattering rates offer further explanation and theoretical basis to the experimental observation of electrical properties degradation for carbon nanotubes under uniaxial strain.
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
Isotopic phonon effects in β-rhombohedral boron—non-statistical isotope distribution
NASA Astrophysics Data System (ADS)
Werheit, H.; Filipov, V.; Kuhlmann, U.; Schwarz, U.; Armbrüster, M.; Antadze, M.
2012-05-01
On the basis of the spectra of IR- and Raman-active phonons, the isotopic phonon effects in β-rhombohedral boron are analysed for polycrystalline 10B- and 11B-enriched samples of different origin and high-purity natB single crystals. Intra- and inter-icosahedral B-B vibrations are harmonic, hence meeting the virtual crystal approximation (VCA) requirements. Deviations from the phonon shift expected according to the VCA are attributed to the anharmonic share of the lattice vibrations. In the case of icosahedral vibrations, the agreement with calculations on α-rhombohedral boron by Shirai and Katayama-Yoshida is quite satisfactory. Phonon shifts due to isotopic disorder in natB are separated and determined. Some phonon frequencies are sensitive to impurities. The isotopic phonon effects yield valuable specific information on the nature of the different phonon modes. The occupation of regular boron sites by isotopes deviates significantly from the random distribution.
Vibron and phonon hybridization in dielectric nanostructures.
Preston, Thomas C; Signorell, Ruth
2011-04-01
Plasmon hybridization theory has been an invaluable tool in advancing our understanding of the optical properties of metallic nanostructures. Through the prism of molecular orbital theory, it allows one to interpret complex structures as "plasmonic molecules" and easily predict and engineer their electromagnetic response. However, this formalism is limited to conducting particles. Here, we present a hybridization scheme for the external and internal vibrations of dielectric nanostructures that provides a straightforward understanding of the infrared signatures of these particles through analogy to existing hybridization models of both molecular orbitals and plasmons extending the range of applications far beyond metallic nanostructures. This method not only provides a qualitative understanding, but also allows for the quantitative prediction of vibrational spectra of complex nanoobjects from well-known spectra of their primitive building blocks. The examples of nanoshells illustrate how spectral features can be understood in terms of symmetry, number of nodal planes, and scale parameters. PMID:21422288
The negative phonon confinement effect in nanoscopic sodium nitrite.
Koroleva, E Yu; Nuzhnyy, D; Pokorny, J; Kamba, S; Kumzerov, Yu A; Vakhrushev, S B; Petzelt, J
2009-09-30
A nanocomposite of porous glass and a NaNO(2) ferroelectric (channels of approximately 7 nm diameter) was studied using infrared reflectivity, THz transmission and Raman spectroscopy as a function of temperature in the range of 300-500 K, including the ferroelectric transition. From the infrared and THz response the effective dielectric function was calculated and compared with the dielectric functions calculated from the Bruggeman and Lichtenecker models of the effective medium, using the known data on the polar phonon modes of the NaNO(2) single crystals. The results show good qualitative agreement, indicating that the stiffening of the effective modes is due to local depolarization fields on the glass-ferroelectric interfaces. The nonpolar Raman modes show no substantial modification compared to those of the bulk NaNO(2). Some signatures of the ferroelectric transition were even seen. The results indicate that the intrinsic size effect (phonon confinement) is negligible in this nanocomposite.
Phononic heat transport in the transient regime: An analytic solution
NASA Astrophysics Data System (ADS)
Tuovinen, Riku; Säkkinen, Niko; Karlsson, Daniel; Stefanucci, Gianluca; van Leeuwen, Robert
2016-06-01
We investigate the time-resolved quantum transport properties of phonons in arbitrary harmonic systems connected to phonon baths at different temperatures. We obtain a closed analytic expression of the time-dependent one-particle reduced density matrix by explicitly solving the equations of motion for the nonequilibrium Green's function. This is achieved through a well-controlled approximation of the frequency-dependent bath self-energy. Our result allows for exploring transient oscillations and relaxation times of local heat currents, and correctly reduces to an earlier known result in the steady-state limit. We apply the formalism to atomic chains, and benchmark the validity of the approximation against full numerical solutions of the bosonic Kadanoff-Baym equations for the Green's function. We find good agreement between the analytic and numerical solutions for weak contacts and baths with a wide energy dispersion. We further analyze relaxation times from low to high temperature gradients.
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.
Band structures in Sierpinski triangle fractal porous phononic crystals
NASA Astrophysics Data System (ADS)
Wang, Kai; Liu, Ying; Liang, Tianshu
2016-10-01
In this paper, the band structures in Sierpinski triangle fractal porous phononic crystals (FPPCs) are studied with the aim to clarify the effect of fractal hierarchy on the band structures. Firstly, one kind of FPPCs based on Sierpinski triangle routine is proposed. Then the influence of the porosity on the elastic wave dispersion in Sierpinski triangle FPPCs is investigated. The sensitivity of the band structures to the fractal hierarchy is discussed in detail. The results show that the increase of the hierarchy increases the sensitivity of ABG (Absolute band gap) central frequency to the porosity. But further increase of the fractal hierarchy weakens this sensitivity. On the same hierarchy, wider ABGs could be opened in Sierpinski equilateral triangle FPPC; whilst, a lower ABG could be opened at lower porosity in Sierpinski right-angled isosceles FPPCs. These results will provide a meaningful guidance in tuning band structures in porous phononic crystals by fractal design.
Stochastic Approach to Phonon-Assisted Optical Absorption
NASA Astrophysics Data System (ADS)
Zacharias, Marios; Patrick, Christopher E.; Giustino, Feliciano
2015-10-01
We develop a first-principles theory of phonon-assisted optical absorption in semiconductors and insulators which incorporates the temperature dependence of the electronic structure. We show that the Hall-Bardeen-Blatt theory of indirect optical absorption and the Allen-Heine theory of temperature-dependent band structures can be derived from the present formalism by retaining only one-phonon processes. We demonstrate this method by calculating the optical absorption coefficient of silicon using an importance sampling Monte Carlo scheme, and we obtain temperature-dependent line shapes and band gaps in good agreement with experiment. The present approach opens the way to predictive calculations of the optical properties of solids at finite temperature.
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.
Phonon wave interference in graphene and boron nitride superlattice
NASA Astrophysics Data System (ADS)
Chen, Xue-Kun; Xie, Zhong-Xiang; Zhou, Wu-Xing; Tang, Li-Ming; Chen, Ke-Qiu
2016-07-01
The thermal transport properties of the graphene and boron nitride superlattice (CBNSL) are investigated via nonequilibrium molecular dynamics simulations. The simulation results show that a minimum lattice thermal conductivity can be achieved by changing the period length of the superlattice. Additionally, it is found that the period length at the minimum shifts to lower values at higher temperatures, and that the depth of the minimum increases with decreasing temperature. In particular, at 200 K, the thermal conductivities of CBNSLs with certain specific period lengths are nearly equal to the corresponding values at 300 K. A detailed analysis of the phonon spectra shows that this anomalous thermal conductivity behavior is a result of strong phonon wave interference. These observations indicate a promising strategy for manipulation of thermal transport in superlattices.
Phonons in Stabilized B or N doped graphene
NASA Astrophysics Data System (ADS)
Dubey, Girija; Mann, Sarita; Rani, Pooja; Jindal, Vijay
Based on Ab-initio density functional perturbation theory, we have investigated various doped B and N based graphene sheets by raising their concentrations upto 50% of the host carbon. Although the doped structures seem to stabilize in 2-D configurations, but the resulting Phonon frequencies do not confirm the stability as the transverse modes above critical concentrations of B and N tend to result in negative eigenvalues. This essentially requires strained 2-D sheets when doped above such critical concentrations. We find the results of phonons and thermodynamics very interesting and attempt to report these in strained lattices. The motivation to do such a calculation results from our primary goal to address the issue of heat dissipation rate in the devices based on designable electronic and optical properties of such doped graphene reported already.
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.
Phonon drag of electrons in Ag{sub 2}S
Aliev, S. A.; Aliev, F. F. Gasanov, Z. S.; Abdullayev, S. M.; Selim-zade, R. I.
2010-06-15
The temperature dependences of the heat-conductivity coefficient {chi} and the thermopower 6h of Ag{sub 2}S are investigated in the range of 4.2-300 K. It is found that the value of 6h sharply increases (6h {infinity} T{sup -3}) with decreasing T at T < 100 K and passes through a maximum at 16-18 K. The heat-conductivity coefficient passes through a maximum at {approx}30 K. The sharp increase in 6h is found to be caused by the effect of long-wavelength-phonon drag of electrons. It is shown that the shift of the 6h and {chi} peaks, as well as the temperature dependence of the phonon thermopower 6h{sub ph} {infinity} T{sup -3}, agrees with the Herring theory.
Constitutive Equation for Polymer Networks with Phonon Fluctuations
NASA Astrophysics Data System (ADS)
Hansen, Rasmus; Hassager, Ole; Skov, Anne Ladegaard
2008-07-01
Recent research by Xing et al. [Phys. Rev. Lett. 98, 075502, 2007] has provided an expression for the Helmholtz free energy related to phonon fluctuations in polymer networks. We extend this result by constructing the corresponding nonlinear constitutive equation, usable for entirely general, volume conserving deformation fields. Constitutive equations for the sliplink and the tube model are applied. The three models are examined by comparison with each other and with data from Xu and Mark [Rubber Chem. Technol. 63, 276, 1990] and Wang and Mark [J. Polym. Sci. Part B: Polym. Phys. 30, 801, 1992] on end-linked polydimethylsiloxane networks. Elastic moduli are derived for the three models and compared with the moduli determined from the chemical stoichiometry. We conclude that the sliplink model and the phonon fluctuation model are relatively consistent with each other and also with the data. The tube model seems consistent neither with the other models nor with the data.
Phonon dispersion relation of an atomic Bose-Einstein condensate.
Shammass, I; Rinott, S; Berkovitz, A; Schley, R; Steinhauer, J
2012-11-01
We measure the time oscillations of a freely evolving standing wave of phonons in a Bose-Einstein condensate. We present the technique of short Bragg pulses, which stimulates the standing wave. The subsequent oscillations are observed in situ. The frequency of the oscillations gives the dispersion relation, the amplitude gives the static structure factor, and the decay gives the dephasing time. The new technique gives orders of magnitude more sensitivity than Bragg spectroscopy, allowing for the observation of deviations from the local density approximation. Specifically, it is seen that the phonons undergo a transition from three dimensions to one dimension, when their wavelength becomes longer than the transverse radius of the condensate. The one-dimensional regime contains an inflection point in the dispersion relation, a decrease in the superfluid critical velocity, a minimum in the group velocity, and an increase in the lifetime of the standing wave oscillations.
Photo-excited charge carriers suppress sub-terahertz phonon mode in silicon at room temperature
NASA Astrophysics Data System (ADS)
Liao, Bolin; Maznev, A. A.; Nelson, Keith A.; Chen, Gang
2016-10-01
There is a growing interest in the mode-by-mode understanding of electron and phonon transport for improving energy conversion technologies, such as thermoelectrics and photovoltaics. Whereas remarkable progress has been made in probing phonon-phonon interactions, it has been a challenge to directly measure electron-phonon interactions at the single-mode level, especially their effect on phonon transport above cryogenic temperatures. Here we use three-pulse photoacoustic spectroscopy to investigate the damping of a single sub-terahertz coherent phonon mode by free charge carriers in silicon at room temperature. Building on conventional pump-probe photoacoustic spectroscopy, we introduce an additional laser pulse to optically generate charge carriers, and carefully design temporal sequence of the three pulses to unambiguously quantify the scattering rate of a single-phonon mode due to the electron-phonon interaction. Our results confirm predictions from first-principles simulations and indicate the importance of the often-neglected effect of electron-phonon interaction on phonon transport in doped semiconductors.
New inelastic process of phonon scattering by rotons
Pogorelov, L.A.; Sobolev, V.I.
1983-11-01
A new inelastic process has been discovered that determines the kinetics of superfluid helium at elevated pressures. The process involves absorption (emission) of phonons by rotons owing to the dispersion of the roton energy. Rotons moving at maximum velocity are shown to play an essential role in this process. The form of the roton dispersion relation proposed permits a good description of the kinetic processes and the results of direct measurements of the energy spectrum.
Phonon-assisted heat transfer between vacuum-separated surfaces
NASA Astrophysics Data System (ADS)
Pendry, J. B.; Sasihithlu, K.; Craster, R. V.
2016-08-01
With increasing interest in nanotechnology, the question arises of how heat is exchanged between materials separated by only a few nanometers of vacuum. Here, we present calculations of the contribution of phonons to heat transfer mediated by van der Waals forces and compare the results to other mechanisms such as coupling through near field fluctuations. Our results show a more dramatic decay with separation than previous work.
Density matrix embedding theory for interacting electron-phonon systems
NASA Astrophysics Data System (ADS)
Sandhoefer, Barbara; Chan, Garnet Kin-Lic
2016-08-01
We describe the extension of the density matrix embedding theory framework to coupled interacting fermion-boson systems. This provides a frequency-independent, entanglement embedding formalism to treat bulk fermion-boson problems. We illustrate the concepts within the context of the one-dimensional Hubbard-Holstein model, where the phonon bath states are obtained from the Schmidt decomposition of a self-consistently adjusted coherent state. We benchmark our results against accurate density matrix renormalization group calculations.
Phonon spectrum, mechanical and thermophysical properties of thorium carbide
NASA Astrophysics Data System (ADS)
Pérez Daroca, D.; Jaroszewicz, S.; Llois, A. M.; Mosca, H. O.
2013-06-01
In this work, we study, by means of density functional perturbation theory and the pseudopotential method, mechanical and thermophysical properties of thorium carbide. These properties are derived from the lattice dynamics in the quasi-harmonic approximation. The phonon spectrum of ThC presented in this article, to the best authors' knowledge, have not been studied, neither experimentally, nor theoretically. We compare mechanical properties, volume thermal expansion and molar specific capacities with previous results and find a very good agreement.
Phonons, defects and optical damage in crystalline acetanilide
NASA Astrophysics Data System (ADS)
Kosic, Thomas J.; Hill, Jeffrey R.; Dlott, Dana D.
1986-04-01
Intense picosecond pulses cause accumulated optical damage in acetanilide crystals at low temperature. Catastrophic damage to the irradiated volume occurs after an incubation period where defects accumulate. The optical damage is monitored with subanosecond time resolution. The generation of defects is studied with damage-detected picosecond spectroscopy. The accumulation of defects is studied by time-resolved coherent Raman scattering, which is used to measure optical phonon scattering from the accumulating defects.
Structural and phonon transmission study of Ge-Au-Ge eutectically bonded interfaces
Knowlton, W.B. |
1995-07-01
This thesis presents a structural analysis and phonon transparency investigation of the Ge-Au-Ge eutectic bond interface. Interface development was intended to maximize the interfacial ballistic phonon transparency to enhance the detection of the dark matter candidate WIMPs. The process which was developed provides an interface which produces minimal stress, low amounts of impurities, and insures Ge lattice continuity through the interface. For initial Au thicknesses of greater than 1,000 {angstrom} Au per substrate side, eutectic epitaxial growth resulted in a Au dendritic structure with 95% cross sectional and 90% planar Au interfacial area coverages. In sections in which Ge bridged the interface, lattice continuity across the interface was apparent. Epitaxial solidification of the eutectic interface with initial Au thicknesses < 500 A per substrate side produced Au agglomerations thereby reducing the Au planar interfacial area coverage to as little as 30%. The mechanism for Au coalescence was attributed to lateral diffusion of Ge and Au in the liquid phase during solidification. Phonon transmission studies were performed on eutectic interfaces with initial Au thicknesses of 1,000 {angstrom}, 500 {angstrom}, and 300 {angstrom} per substrate side. Phonon imaging of eutectically bonded samples with initial Au thicknesses of 300 {angstrom}/side revealed reproducible interfacial percent phonon transmissions from 60% to 70%. Line scan phonon imaging verified the results. Phonon propagation TOF spectra distinctly showed the predominant phonon propagation mode was ballistic. This was substantiated by phonon focusing effects apparent in the phonon imaging data. The degree of interface transparency to phonons and resulting phonon propagation modes correlate with the structure of the interface following eutectic solidification. Structural studies of samples with initial Au thickness of 1,000 {angstrom}/side appear to correspond with the phonon transmission study.
Phonon effects on the double mass differences in magic nuclei
NASA Astrophysics Data System (ADS)
Saperstein, E. E.; Baldo, M.; Gnezdilov, N. V.; Tolokonnikov, S. V.
2016-03-01
Odd-even double mass differences (DMDs) of magic nuclei are found within an approach starting from the free N N interaction, accounting for particle-phonon coupling (PC) effects. We consider three PC effects: the phonon-induced effective interaction, the renormalization of the "ends" due to the pole PC contribution to the nucleon mass operator, and the change of the single-particle energies. The perturbation theory in gL2, where gL is the vertex of the creation of the L -multipole phonon, is used for PC calculations. PC corrections to single-particle energies are found with an approximate accounting for the tadpole diagram. Results for magic Ca,4840, Ni,7856, Sn,132100, and 208Pb nuclei are presented. For the lighter part of this set of nuclei, from 40Ca to 56Ni, the cases divide approximately in half, between those where the PC corrections to DMD values are in good agreement with the data and the ones with the opposite result. In the major part of the cases of worsening description of DMD, a poor applicability of the perturbation theory for the induced interaction is the most probable reason of the phenomenon. For intermediate nuclei, 78Ni and 100Sn, there are no sufficiently accurate data on masses of nuclei necessary for finding DMD values. Finally, for heavier nuclei, 132Sn and 208Pb, PC corrections always result in better agreement with experiment.
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
Nonharmonic phonons in α-iron at high temperatures
NASA Astrophysics Data System (ADS)
Mauger, L.; Lucas, M. S.; Muñoz, J. A.; Tracy, S. J.; Kresch, M.; Xiao, Yuming; Chow, Paul; Fultz, B.
2014-08-01
Phonon densities of states (DOS) of bcc α-Fe57 were measured from room temperature through the 1044 K Curie transition and the 1185 K fcc γ-Fe phase transition using nuclear resonant inelastic x-ray scattering. At higher temperatures all phonons shift to lower energies (soften) with thermal expansion, but the low transverse modes soften especially rapidly above 700 K, showing strongly nonharmonic behavior that persists through the magnetic transition. Interatomic force constants for the bcc phase were obtained by iteratively fitting a Born-von Kármán model to the experimental phonon spectra using a genetic algorithm optimization. The second-nearest-neighbor fitted axial force constants weakened significantly at elevated temperatures. An unusually large nonharmonic behavior is reported, which increases the vibrational entropy and accounts for a contribution of 35 meV/atom in the free energy at high temperatures. The nonharmonic contribution to the vibrational entropy follows the thermal trend of the magnetic entropy, and may be coupled to magnetic excitations. A small change in vibrational entropy across the α-γ structural phase transformation is also reported.
Sideband Raman Cooling of Optical Phonons in Semiconductors
NASA Astrophysics Data System (ADS)
Zhang, Jun; Kwek, Leong Chuan; Xiong, Qihua
2014-03-01
Last century has witnessed a tremendous success of laser cooling technology from trapped atomic ions to solid-state optical refrigeration. As one of the laser cooling techniques, sideband Raman cooling plays an important role in quantum ground state preparation, coherent quantum-state manipulation and quantum phenomena study. However, those studies still limited in trapped atomic ions and cavity optomechanics, which need be cooled it below than 0.1 Kelvin even tens of nano-Kelvin due to very low frequency of phonons from several kHz to GHz. Here we report sideband Raman cooling and heating experiments of longitudinal optical phonon (LOP) with a 6.23 THz in semiconductor ZnTe nano-ribbons. By using of red-sideband laser, we cool the LOP from 225 to 55 Kelvin, corresponding to an average occupation number reduced from 0.36 to 0.005. We also observe a LOPs heating from 230 to 384 Kelvin with a blue-sideband pumping. Our experiment opens a possibility of all solid state quantum applications using semiconductor optical phonon mediated coupling at room temperature. We gratefully acknowledge funding from Singapore NRF, MOE and NTU.
Tunable phonon-cavity coupling in graphene membranes.
De Alba, R; Massel, F; Storch, I R; Abhilash, T S; Hui, A; McEuen, P L; Craighead, H G; Parpia, J M
2016-09-01
A major achievement of the past decade has been the realization of macroscopic quantum systems by exploiting the interactions between optical cavities and mechanical resonators. In these systems, phonons are coherently annihilated or created in exchange for photons. Similar phenomena have recently been observed through phonon-cavity coupling-energy exchange between the modes of a single system mediated by intrinsic material nonlinearity. This has so far been demonstrated primarily for bulk crystalline, high-quality-factor (Q > 10(5)) mechanical systems operated at cryogenic temperatures. Here, we propose graphene as an ideal candidate for the study of such nonlinear mechanics. The large elastic modulus of this material and capability for spatial symmetry breaking via electrostatic forces is expected to generate a wealth of nonlinear phenomena, including tunable intermodal coupling. We have fabricated circular graphene membranes and report strong phonon-cavity effects at room temperature, despite the modest Q factor (∼100) of this system. We observe both amplification into parametric instability (mechanical lasing) and the cooling of Brownian motion in the fundamental mode through excitation of cavity sidebands. Furthermore, we characterize the quenching of these parametric effects at large vibrational amplitudes, offering a window on the all-mechanical analogue of cavity optomechanics, where the observation of such effects has proven elusive.
Stress Wave Isolation by Purely Mechanical Topological Phononic Crystals
Chaunsali, Rajesh; Li, Feng; Yang, Jinkyu
2016-01-01
We present an active, purely mechanical stress wave isolator that consists of short cylindrical particles arranged in a helical architecture. This phononic structure allows us to change inter-particle stiffness dynamically by controlling the contact angles of the cylinders. We use torsional travelling waves to control the contact angles, thereby imposing a desired spatio-temporal stiffness variation to the phononic crystal along the longitudinal direction. Such torsional excitation is a form of parametric pumping in the system, which results in the breakage of the time-reversal symmetry. We report that, in quasi-static sense, the system shows topologically non-trivial band-gaps. However, in a dynamic regime where the pumping effect is significant, these band-gaps become asymmetric with respect to the frequency and wavenumber domains in the dispersion relationship. By using numerical simulations, we show that such asymmetry has a direct correspondence to the topological invariant, i.e., Chern number, of the system. We propose that this asymmetry, accompanied by selective inter-band transition, can be utilized for directional isolation of the stress wave propagating along the phononic crystal. PMID:27477236
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
Stress Wave Isolation by Purely Mechanical Topological Phononic Crystals.
Chaunsali, Rajesh; Li, Feng; Yang, Jinkyu
2016-01-01
We present an active, purely mechanical stress wave isolator that consists of short cylindrical particles arranged in a helical architecture. This phononic structure allows us to change inter-particle stiffness dynamically by controlling the contact angles of the cylinders. We use torsional travelling waves to control the contact angles, thereby imposing a desired spatio-temporal stiffness variation to the phononic crystal along the longitudinal direction. Such torsional excitation is a form of parametric pumping in the system, which results in the breakage of the time-reversal symmetry. We report that, in quasi-static sense, the system shows topologically non-trivial band-gaps. However, in a dynamic regime where the pumping effect is significant, these band-gaps become asymmetric with respect to the frequency and wavenumber domains in the dispersion relationship. By using numerical simulations, we show that such asymmetry has a direct correspondence to the topological invariant, i.e., Chern number, of the system. We propose that this asymmetry, accompanied by selective inter-band transition, can be utilized for directional isolation of the stress wave propagating along the phononic crystal. PMID:27477236
Stress Wave Isolation by Purely Mechanical Topological Phononic Crystals
NASA Astrophysics Data System (ADS)
Chaunsali, Rajesh; Li, Feng; Yang, Jinkyu
2016-08-01
We present an active, purely mechanical stress wave isolator that consists of short cylindrical particles arranged in a helical architecture. This phononic structure allows us to change inter-particle stiffness dynamically by controlling the contact angles of the cylinders. We use torsional travelling waves to control the contact angles, thereby imposing a desired spatio-temporal stiffness variation to the phononic crystal along the longitudinal direction. Such torsional excitation is a form of parametric pumping in the system, which results in the breakage of the time-reversal symmetry. We report that, in quasi-static sense, the system shows topologically non-trivial band-gaps. However, in a dynamic regime where the pumping effect is significant, these band-gaps become asymmetric with respect to the frequency and wavenumber domains in the dispersion relationship. By using numerical simulations, we show that such asymmetry has a direct correspondence to the topological invariant, i.e., Chern number, of the system. We propose that this asymmetry, accompanied by selective inter-band transition, can be utilized for directional isolation of the stress wave propagating along the phononic crystal.
Spin-phonon coupling in scandium doped gallium ferrite
Chakraborty, Keka R. E-mail: smyusuf@barc.gov.in; Mukadam, M. D.; Basu, S.; Yusuf, S. M. E-mail: smyusuf@barc.gov.in; Paul, Barnita; Roy, Anushree; Grover, Vinita; Tyagi, A. K.
2015-03-28
We embarked on a study of Scandium (Sc) doped (onto Ga site) gallium ferrite (GaFeO{sub 3}) and found remarkable magnetic properties. In both doped as well as parent compounds, there were three types of Fe{sup 3+} ions (depending on the symmetry) with the structure conforming to space group Pna2{sub 1} (Sp. Grp. No. 33) below room temperature down to 5 K. We also found that all Fe{sup 3+} ions occupy octahedral sites, and carry high spin moment. For the higher Sc substituted sample (Ga{sub 1−x}Sc{sub x}FeO{sub 3}: x = 0.3), a canted magnetic ordered state is found. Spin-phonon coupling below Néel temperature was observed in doped compounds. Our results indicated that Sc doping in octahedral site modifies spin-phonon interactions of the parent compound. The spin-phonon coupling strength was estimated for the first time in these Sc substituted compounds.
Electron-phonon mediated heat flow in disordered graphene
NASA Astrophysics Data System (ADS)
Chen, Wei; Clerk, Aashish A.
2012-09-01
We calculate the heat flux and electron-phonon thermal conductance in a disordered graphene sheet, going beyond a Fermi’s golden rule approach to fully account for the modification of the electron-phonon interaction by disorder. Using the Keldysh technique combined with standard impurity averaging methods in the regime kFl≫1 (where kF is the Fermi wave vector and l is the mean free path), we consider both scalar potential (i.e., deformation potential) and vector-potential couplings between electrons and phonons. We also consider the effects of electronic screening at the Thomas-Fermi level. We find that the temperature dependence of the heat flux and thermal conductance is sensitive to the presence of disorder and screening, and reflects the underlying chiral nature of electrons in graphene and the corresponding modification of their diffusive behavior. In the case of weak screening, disorder enhances the low-temperature heat flux over the clean system (changing the associated power law from T4 to T3), and the deformation potential dominates. For strong screening, both the deformation potential and vector-potential couplings make comparable contributions, and the low-temperature heat flux obeys a T5 power law.
Tunable phonon-cavity coupling in graphene membranes
NASA Astrophysics Data System (ADS)
de Alba, R.; Massel, F.; Storch, I. R.; Abhilash, T. S.; Hui, A.; McEuen, P. L.; Craighead, H. G.; Parpia, J. M.
2016-09-01
A major achievement of the past decade has been the realization of macroscopic quantum systems by exploiting the interactions between optical cavities and mechanical resonators. In these systems, phonons are coherently annihilated or created in exchange for photons. Similar phenomena have recently been observed through phonon-cavity coupling—energy exchange between the modes of a single system mediated by intrinsic material nonlinearity. This has so far been demonstrated primarily for bulk crystalline, high-quality-factor (Q > 105) mechanical systems operated at cryogenic temperatures. Here, we propose graphene as an ideal candidate for the study of such nonlinear mechanics. The large elastic modulus of this material and capability for spatial symmetry breaking via electrostatic forces is expected to generate a wealth of nonlinear phenomena, including tunable intermodal coupling. We have fabricated circular graphene membranes and report strong phonon-cavity effects at room temperature, despite the modest Q factor (∼100) of this system. We observe both amplification into parametric instability (mechanical lasing) and the cooling of Brownian motion in the fundamental mode through excitation of cavity sidebands. Furthermore, we characterize the quenching of these parametric effects at large vibrational amplitudes, offering a window on the all-mechanical analogue of cavity optomechanics, where the observation of such effects has proven elusive.
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.
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
NASA Astrophysics Data System (ADS)
Chen, Jianyong
2016-07-01
Using density functional perturbation theory (DFPT) with Born effective charges included, we calculate in detail the phonons of bulk and monolayer HfS2. Our calculated phonon frequencies and LO-TO splitting agree well with experimental data. The variation of phonon frequencies from bulk to monolayer can be accounted for in terms of the delicate balance of the long- and short-range force constants. The ultimate strength under biaxial tensile strain is explored for the first time. Monolayer HfS2 can suffer only no more than 6% tensile strain. Our important finding is that the phonon gap appears when goes to single layer from bulk and can be tuned by biaxial tension. By analysing the Eliashberg function we conclude that HfS2 cannot turn to superconductivity by electron-phonon interaction.
Electrons and phonons in amorphous semiconductors
NASA Astrophysics Data System (ADS)
Prasai, Kiran; Biswas, Parthapratim; Drabold, D. A.
2016-07-01
The coupling between lattice vibrations and electrons is one of the central concepts of condensed matter physics. The subject has been deeply studied for crystalline materials, but far less so for amorphous and glassy materials, which are among the most important for applications. In this paper, we explore the electron-lattice coupling using current tools of a first-principles computer simulation. We choose three materials to illustrate the phenomena: amorphous silicon (a-Si), amorphous selenium (a-Se) and amorphous gallium nitride (a-GaN). In each case, we show that there is a strong correlation between the localization of electron states and the magnitude of thermally induced fluctuations in energy eigenvalues obtained from the density-functional theory (i.e. Kohn–Sham eigenvalues). We provide a heuristic theory to explain these observations. The case of a-GaN, a topologically disordered partly ionic insulator, is distinctive compared to the covalent amorphous examples. Next, we explore the consequences of changing the charge state of a system as a proxy for tracking photo-induced structural changes in the materials. Where transport is concerned, we lend insight into the Meyer–Neldel compensation rule and discuss a thermally averaged Kubo–Greenwood formula as a means to estimate electrical conductivity and especially its temperature dependence. We close by showing how the optical gap of an amorphous semiconductor can be computationally engineered with the judicious use of Hellmann–Feynman forces (associated with a few defect states) using molecular dynamics simulations. These forces can be used to close or open an optical gap, and identify a structure with a prescribed gap. We use the approach with plane-wave density functional methods to identify a low-energy amorphous phase of silicon including several coordination defects, yet with a gap close to that of good quality a-Si models.
Electrons and phonons in amorphous semiconductors
NASA Astrophysics Data System (ADS)
Prasai, Kiran; Biswas, Parthapratim; Drabold, D. A.
2016-07-01
The coupling between lattice vibrations and electrons is one of the central concepts of condensed matter physics. The subject has been deeply studied for crystalline materials, but far less so for amorphous and glassy materials, which are among the most important for applications. In this paper, we explore the electron-lattice coupling using current tools of a first-principles computer simulation. We choose three materials to illustrate the phenomena: amorphous silicon (a-Si), amorphous selenium (a-Se) and amorphous gallium nitride (a-GaN). In each case, we show that there is a strong correlation between the localization of electron states and the magnitude of thermally induced fluctuations in energy eigenvalues obtained from the density-functional theory (i.e. Kohn-Sham eigenvalues). We provide a heuristic theory to explain these observations. The case of a-GaN, a topologically disordered partly ionic insulator, is distinctive compared to the covalent amorphous examples. Next, we explore the consequences of changing the charge state of a system as a proxy for tracking photo-induced structural changes in the materials. Where transport is concerned, we lend insight into the Meyer-Neldel compensation rule and discuss a thermally averaged Kubo-Greenwood formula as a means to estimate electrical conductivity and especially its temperature dependence. We close by showing how the optical gap of an amorphous semiconductor can be computationally engineered with the judicious use of Hellmann-Feynman forces (associated with a few defect states) using molecular dynamics simulations. These forces can be used to close or open an optical gap, and identify a structure with a prescribed gap. We use the approach with plane-wave density functional methods to identify a low-energy amorphous phase of silicon including several coordination defects, yet with a gap close to that of good quality a-Si models.
Evidence of electron-phonon and spin-phonon couplings at the Verwey transition in Fe3O4
NASA Astrophysics Data System (ADS)
Kumar, Ankit; Chaudhary, Sujeet; Pandya, Dinesh K.; Sharma, Shiv K.
2014-07-01
Using low temperature Raman spectra on two-dimensional epitaxial Fe3O4 thin films, the presence of electron-phonon (e-p) and spin-phonon (s-p) couplings is established in the vicinity of Verwey transition (TV). The e-p coupling is linked to A1g and T2g Raman modes and s-p to the T2g mode, below isotropic point (TK). The parabolic dependence of the characteristic spin-correlation eigenstates on the magnetization, Δωs -phn∝
Spin-phonon coupling and ferroelectricity in magnetoelectric gallium ferrite
NASA Astrophysics Data System (ADS)
Mukherjee, Somdutta
2014-03-01
Gallium ferrite (GaFeO3 or GFO) is a low temperature ferrimagnet and room temperature piezoelectric wherein the magnetic transition temperature (TC) could be tailored to room temperature and above by tuning the stoichiometry and processing conditions. Such tunability of the magnetic transition temperature renders GFO a unique perspective in the research of multiferroics to potentially demonstrate room temperature magnetoelectric effect attractive for futuristic digital memory applications. Recent studies in several transition metal oxides highlight the importance of spin-phonon coupling in designing novel multiferroics by means of strain induced phase transition. In the present work, we have systematically studied the evolution of phonons in good quality samples of GFO across the TC using Raman spectroscopy. Using the phonon softening behavior and nearest neighbor spin-spin correlation function below TC we estimated spin-phonon coupling strength in the magnetically ordered state. In the process, we also show, for the first time, the presence of a spin glass phase in GFO where the spin-glass transition has a signature of abrupt change in spin-phonon coupling strength. Though GFO is piezoelectric and crystallizes in polar Pc21n symmetry, its ferroelectric nature remained controversial probably due to the large leakage current in the bulk material. To address this issue, we deposited epitaxial thin film on single crystalline yttria stabilized zirconia (YSZ) substrate using indium tin oxide (ITO) as a bottom conducting layer. We demonstrate clear evidence of room temperature ferroelectricity in the thin films from the 180o phase shift of the piezoresponse upon switching the electric field. Further, suppression of dielectric anomaly in presence of an external magnetic field clearly reveals a pronounced magneto-dielectric coupling across the magnetic transition temperature. In addition, using first principles calculations we elucidate that Fe ions are not only
NASA Astrophysics Data System (ADS)
Silbey, R.; Munn, R. W.
1980-02-01
An improved general theory of electronic transport in molecular crystals with local linear electron-phonon coupling is presented. It is valid for arbitrary electronic and phonon bandwidths and for arbitrary electron-phonon coupling strength, yielding small-polaron theory for narrow electronic bands and strong coupling, and semiconductor theory for wide electronic bands and weak coupling. Detailed results are derived for electronic excitations fully clothed with phonons and having a bandwidth no larger than the phonon frequency; the electronic and phonon densities of states are taken as Gaussian for simplicity. The dependence of the diffusion coefficient on temperature and on the other parameters is analyzed thoroughly. The calculated behavior provides a rational interpretation of observed trends in the magnitude and temperature dependence of charge-carrier drift mobilities in molecular crystals.
Phonon lifetime in SiSn and its suitability for hot-carrier solar cells
Levard, Hugo; Laribi, Sana; Guillemoles, Jean-François
2014-06-02
We present a phononic and electronic study of SiSn in the zinc-blende phase. A detailed description of the longitudinal optical (LO) phonon decay in a three-phonon process is presented together with the corresponding lifetime. The necessity to go beyond the zone center phonon approximation in this case is highlighted as it reveals a steep dependence of the lifetime on the initial phonon wavenumber, which differs from usual semiconductors. The electronic band structure is calculated within the GW formalism and shows a small direct band gap. It is shown that the LO-phonon resulting from electron cooling has a lifetime four to eight orders of magnitude above all the known value in semiconductors for this process. We finally show the suitability of SiSn for hot-carrier solar cells, as it is endowed with ultra-slow cooling of hot carriers.
NASA Astrophysics Data System (ADS)
Khurgin, Jacob B.
2014-06-01
Laser cooling of semiconductors has been an elusive goal for many years, and while attempts to cool the narrow gap semiconductors such as GaAs are yet to succeed, recently, net cooling has been attained in a wider gap CdS. This raises the question of whether wider gap semiconductors with higher phonon energies and stronger electron-phonon coupling are better suitable for laser cooling. In this work, we develop a straightforward theory of phonon-assisted absorption and photoluminescence of semiconductors that involves more than one phonon and use to examine wide gap materials, such as GaN and CdS and compare them with GaAs. The results indicate that while strong electron-phonon coupling in both GaN and CdS definitely improves the prospects of laser cooling, large phonon energy in GaN may be a limitation, which makes CdS a better prospect for laser cooling.
Temperature dependence of Brillouin light scattering spectra of acoustic phonons in silicon
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.
Raman selection rule for surface optical phonons in ZnS nanobelts.
Ho, Chih-Hsiang; Varadhan, Purushothaman; Wang, Hsin-Hua; Chen, Cheng-Ying; Fang, Xiaosheng; He, Jr-Hau
2016-03-21
We report Raman scattering results for high-quality wurtzite ZnS nanobelts (NBs) grown by chemical vapor deposition. In the Raman spectrum, the ensembles of ZnS NBs exhibit first order phonon modes at 274 cm(-1) and 350 cm(-1), corresponding to A1/E1 transverse optical and A1/E1 longitudinal optical phonons, in addition to a strong surface optical (SO) phonon mode at 329 cm(-1). The existence of the SO band is confirmed by its shift with different surrounding dielectric media. Polarization dependent Raman spectra were recorded on a single ZnS NB and for the first time a SO phonon band has been detected on a single nanobelt. Different selection rules for the SO phonon mode are shown from their corresponding E1/A1 phonon modes, and were attributed to the breaking of anisotropic translational symmetry on the NB surface. PMID:26924069
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.
Sadhu, Jyothi; Tian, Hongxiang; Ma, Jun; Azeredo, Bruno; Kim, Junhwan; Balasundaram, Karthik; Zhang, Chen; Li, Xiuling; Ferreira, P M; Sinha, S
2015-05-13
Existing theory and data cannot quantify the contribution of phonon drag to the Seebeck coefficient (S) in semiconductors at room temperature. We show that this is possible through comparative measurements between nanowires and the bulk. Phonon boundary scattering completely quenches phonon drag in silicon nanowires enabling quantification of its contribution to S in bulk silicon in the range 25-500 K. The contribution is surprisingly large (∼34%) at 300 K even at doping of ∼3 × 10(19) cm(-3). Our results contradict the notion that phonon drag is negligible in degenerate semiconductors at temperatures relevant for thermoelectric energy conversion. A revised theory of electron-phonon momentum exchange that accounts for a phonon mean free path spectrum agrees well with the data.
Electron-Phonon Coupling and its implication for the superconducting topological insulators
Zhang, Xiao-Long; Liu, Wu-Ming
2015-01-01
The recent observation of superconductivity in doped topological insulators has sparked a flurry of interest due to the prospect of realizing the long-sought topological superconductors. Yet the understanding of underlying pairing mechanism in these systems is far from complete. Here we investigate this problem by providing robust first-principles calculations of the role of electron-phonon coupling for the superconducting pairing in the prime candidate CuxBi2Se3. Our results show that electron-phonon scattering process in this system is dominated by zone center and boundary optical modes, with coexistence of phonon stiffening and softening. While the calculated electron-phonon coupling constant λ suggests that Tc from electron-phonon coupling is 2 orders smaller than the ones reported on bulk inhomogeneous samples, suggesting that superconductivity may not come from pure electron-phonon coupling. We discuss the possible enhancement of superconducting transition temperature by local inhomogeneity introduced by doping. PMID:25753813
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.
Khurgin, Jacob B.
2014-06-02
Laser cooling of semiconductors has been an elusive goal for many years, and while attempts to cool the narrow gap semiconductors such as GaAs are yet to succeed, recently, net cooling has been attained in a wider gap CdS. This raises the question of whether wider gap semiconductors with higher phonon energies and stronger electron-phonon coupling are better suitable for laser cooling. In this work, we develop a straightforward theory of phonon-assisted absorption and photoluminescence of semiconductors that involves more than one phonon and use to examine wide gap materials, such as GaN and CdS and compare them with GaAs. The results indicate that while strong electron-phonon coupling in both GaN and CdS definitely improves the prospects of laser cooling, large phonon energy in GaN may be a limitation, which makes CdS a better prospect for laser cooling.
Anomalous Hall effect for the phonon heat conductivity in paramagnetic dielectrics.
Kagan, Yu; Maksimov, L A
2008-04-11
The theory of the anomalous Hall effect for the heat transfer in a parmagnetic dielectric, discovered by Strohm, Rikken, and Wyder [Phys. Rev. Lett. 95, 155901 (2005)]10.1103/PhysRevLett.95.155901, is developed. The appearance of the phonon heat flux normal to both the temperature gradient and the magnetic field is connected with the interaction of magnetic ions with the crystal field oscillations. In crystals with an arbitrary phonon spectrum this interaction creates the elliptical polarization of phonons. The kinetics related to phonon scattering induced by the spin-phonon interaction determines an origin of the off-diagonal phonon density matrix. The combination of both factors is decisive for the phenomenon under consideration.
Zhang, Yu; Yam, ChiYung; Chen, GuanHua
2015-04-28
A time-dependent inelastic electron transport theory for strong electron-phonon interaction is established via the equations of motion method combined with the small polaron transformation. In this work, the dissipation via electron-phonon coupling is taken into account in the strong coupling regime, which validates the small polaron transformation. The corresponding equations of motion are developed, which are used to study the quantum interference effect and phonon-induced decoherence dynamics in molecular junctions. Numerical studies show clearly quantum interference effect of the transport electrons through two quasi-degenerate states with different couplings to the leads. We also found that the quantum interference can be suppressed by the electron-phonon interaction where the phase coherence is destroyed by phonon scattering. This indicates the importance of electron-phonon interaction in systems with prominent quantum interference effect.
Zhang, Yu Chen, GuanHua; Yam, ChiYung
2015-04-28
A time-dependent inelastic electron transport theory for strong electron-phonon interaction is established via the equations of motion method combined with the small polaron transformation. In this work, the dissipation via electron-phonon coupling is taken into account in the strong coupling regime, which validates the small polaron transformation. The corresponding equations of motion are developed, which are used to study the quantum interference effect and phonon-induced decoherence dynamics in molecular junctions. Numerical studies show clearly quantum interference effect of the transport electrons through two quasi-degenerate states with different couplings to the leads. We also found that the quantum interference can be suppressed by the electron-phonon interaction where the phase coherence is destroyed by phonon scattering. This indicates the importance of electron-phonon interaction in systems with prominent quantum interference effect.
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.
A qubit coupled with confined phonons: The interplay between true and fake decoherence
Pouthier, Vincent
2013-08-07
The decoherence of a qubit coupled with the phonons of a finite-size lattice is investigated. The confined phonons no longer behave as a reservoir. They remain sensitive to the qubit so that the origin of the decoherence is twofold. First, a qubit-phonon entanglement yields an incomplete true decoherence. Second, the qubit renormalizes the phonon frequency resulting in fake decoherence when a thermal average is performed. To account for the initial thermalization of the lattice, the qua- ntum Langevin theory is applied so that the phonons are viewed as an open system coupled with a thermal bath of harmonic oscillators. Consequently, it is shown that the finite lifetime of the phonons does not modify fake decoherence but strongly affects true decoherence. Depending on the values of the model parameters, the interplay between fake and true decoherence yields a very rich dynamics with various regimes.
Work measurement in a quantum heat engine
NASA Astrophysics Data System (ADS)
Bariani, Francesco; Zhang, Keye; Dong, Ying; Meystre, Pierre
2015-05-01
We consider an optomechanical quantum heat engine operating on an Otto cycle for photon-phonon polaritons, the working substance of the engine. We discuss both the average value and quantum fluctuations of its work output, concentrating in particular on the effects of quantum non-adiabaticity due to the finite duration of the cycle. We also determine the quantum back-action of both absorptive and dispersive continuous measurements of the work, and quantify their impact on the Curzon-Ahlborn engine efficiency at maximum power and its fluctuations. We ackowledge financial support from National Basic Research Program of China, NSF, ARO and the DARPA QuaSAR programs
Blokh, M.D.
1988-01-01
The flux of nonequilibrium phonons excited by light in the near-surface domain of a crystal or a thin plate is investigated. An exact expression is obtained for the phonon energy flux for a crystal with a polar direction and its polarization dependence is analyzed. The magnitude of the energy flux can reach the incident light intensity. The temperature difference produced by the flux of nonequilibrium photo-excited phonons is found.
Interaction of an electron-hole plasma with optical phonons in GaP
NASA Astrophysics Data System (ADS)
Smith, G. O.; Juhasz, T.; Bron, W. E.; Levinson, Y. B.
1992-04-01
The interaction of an optically induced nonstationary electron-hole plasma with coherently excited optical phonons in GaP has been investigated through time-resolved coherent anti-Stokes Raman spectroscopy. Nonexponential optical-phonon dephasing is observed in the presence of the electron-hole plasma. A model describing the interaction between the optical phonons and the electron-hole plasma is presented. A comparison of results from the theoretical model and from the experimental data indicates good agreement.
Photo-excited charge carriers suppress sub-terahertz phonon mode in silicon at room temperature
Liao, Bolin; Maznev, A. A.; Nelson, Keith A.; Chen, Gang
2016-01-01
There is a growing interest in the mode-by-mode understanding of electron and phonon transport for improving energy conversion technologies, such as thermoelectrics and photovoltaics. Whereas remarkable progress has been made in probing phonon–phonon interactions, it has been a challenge to directly measure electron–phonon interactions at the single-mode level, especially their effect on phonon transport above cryogenic temperatures. Here we use three-pulse photoacoustic spectroscopy to investigate the damping of a single sub-terahertz coherent phonon mode by free charge carriers in silicon at room temperature. Building on conventional pump–probe photoacoustic spectroscopy, we introduce an additional laser pulse to optically generate charge carriers, and carefully design temporal sequence of the three pulses to unambiguously quantify the scattering rate of a single-phonon mode due to the electron–phonon interaction. Our results confirm predictions from first-principles simulations and indicate the importance of the often-neglected effect of electron–phonon interaction on phonon transport in doped semiconductors. PMID:27731406
Regner, Keith T; Sellan, Daniel P; Su, Zonghui; Amon, Cristina H; McGaughey, Alan J H; Malen, Jonathan A
2013-01-01
Non-metallic crystalline materials conduct heat by the transport of quantized atomic lattice vibrations called phonons. Thermal conductivity depends on how far phonons travel between scattering events-their mean free paths. Due to the breadth of the phonon mean free path spectrum, nanostructuring materials can reduce thermal conductivity from bulk by scattering long mean free path phonons, whereas short mean free path phonons are unaffected. Here we use a breakdown in diffusive phonon transport generated by high-frequency surface temperature modulation to identify the mean free path-dependent contributions of phonons to thermal conductivity in crystalline and amorphous silicon. Our measurements probe a broad range of mean free paths in crystalline silicon spanning 0.3-8.0 μm at a temperature of 311 K and show that 40±5% of its thermal conductivity comes from phonons with mean free path >1 μm. In a 500 nm thick amorphous silicon film, despite atomic disorder, we identify propagating phonon-like modes that contribute >35±7% to thermal conductivity at a temperature of 306 K.
NASA Astrophysics Data System (ADS)
Regner, Keith T.; Sellan, Daniel P.; Su, Zonghui; Amon, Cristina H.; McGaughey, Alan J. H.; Malen, Jonathan A.
2013-03-01
Non-metallic crystalline materials conduct heat by the transport of quantized atomic lattice vibrations called phonons. Thermal conductivity depends on how far phonons travel between scattering events—their mean free paths. Due to the breadth of the phonon mean free path spectrum, nanostructuring materials can reduce thermal conductivity from bulk by scattering long mean free path phonons, whereas short mean free path phonons are unaffected. Here we use a breakdown in diffusive phonon transport generated by high-frequency surface temperature modulation to identify the mean free path-dependent contributions of phonons to thermal conductivity in crystalline and amorphous silicon. Our measurements probe a broad range of mean free paths in crystalline silicon spanning 0.3-8.0 μm at a temperature of 311 K and show that 40±5% of its thermal conductivity comes from phonons with mean free path >1 μm. In a 500 nm thick amorphous silicon film, despite atomic disorder, we identify propagating phonon-like modes that contribute >35±7% to thermal conductivity at a temperature of 306 K.
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
Impact of Phonon Surface Scattering on Thermal Energy Distribution of Si and SiGe Nanowires
Malhotra, Abhinav; Maldovan, Martin
2016-01-01
Thermal transport in nanostructures has attracted considerable attention in the last decade but the precise effects of surfaces on heat conduction have remained unclear due to a limited accuracy in the treatment of phonon surface scattering phenomena. Here, we investigate the impact of phonon-surface scattering on the distribution of thermal energy across phonon wavelengths and mean free paths in Si and SiGe nanowires. We present a rigorous and accurate description of phonon scattering at surfaces and predict and analyse nanowire heat spectra for different diameters and surface conditions. We show that the decrease in the diameter and increased roughness and correlation lengths makes the heat phonon spectra significantly shift towards short wavelengths and mean free paths. We also investigate the emergence of phonon confinement effects for small diameter nanowires and different surface scattering properties. Computed results for bulk materials show excellent agreement with recent experimentally-based approaches that reconstruct the mean-free-path heat spectra. Our phonon surface scattering model allows for an accurate theoretical extraction of heat spectra in nanowires and contributes to elucidate the development of critical phonon transport modes such as phonon confinement and coherent interference effects. PMID:27174699
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.
Impact of Phonon Surface Scattering on Thermal Energy Distribution of Si and SiGe Nanowires.
Malhotra, Abhinav; Maldovan, Martin
2016-01-01
Thermal transport in nanostructures has attracted considerable attention in the last decade but the precise effects of surfaces on heat conduction have remained unclear due to a limited accuracy in the treatment of phonon surface scattering phenomena. Here, we investigate the impact of phonon-surface scattering on the distribution of thermal energy across phonon wavelengths and mean free paths in Si and SiGe nanowires. We present a rigorous and accurate description of phonon scattering at surfaces and predict and analyse nanowire heat spectra for different diameters and surface conditions. We show that the decrease in the diameter and increased roughness and correlation lengths makes the heat phonon spectra significantly shift towards short wavelengths and mean free paths. We also investigate the emergence of phonon confinement effects for small diameter nanowires and different surface scattering properties. Computed results for bulk materials show excellent agreement with recent experimentally-based approaches that reconstruct the mean-free-path heat spectra. Our phonon surface scattering model allows for an accurate theoretical extraction of heat spectra in nanowires and contributes to elucidate the development of critical phonon transport modes such as phonon confinement and coherent interference effects. PMID:27174699
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.