Liu Wei; Niu Hanben
2011-02-15
We provide an approach to significantly break the diffraction limit in coherent anti-Stokes Raman scattering (CARS) microscopy via an additional probe-beam-induced photon depletion (APIPD). The additional probe beam, whose profile is doughnut shaped and whose wavelength is different from the Gaussian probe beam, depletes the phonons to yield an unwanted anti-Stokes signal within a certain bandwidth at the rim of the diffraction-limited spot. When the Gaussian probe beam that follows immediately arrives, no anti-Stokes signal is generated in this region, resembling stimulated emission depletion (STED) microscopy, and the spot-generating useful anti-Stokes signals by this beam are substantially suppressed to a much smaller dimension. Scanning the spot renders three-dimensional, label-free, and chemically selective CARS images with subdiffraction resolution. Also, resolution-enhanced images of the molecule, specified by its broadband even-total CARS spectral signals not only by one anti-Stokes signal for its special chemical bond, can be obtained by employing a supercontinuum source.
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
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
Investigating the existence of coherent phonon scattering in silicon using phononic crystals
NASA Astrophysics Data System (ADS)
Goettler, Drew
In silicon the majority of heat energy is transported by phonons, which are discrete lattice vibrations. Phonon scattering due to the presence of voids in silicon can further alter the material's thermal conductivity. There is a question about the possibility of some of this scattering being coherent rather than purely incoherent. Coherent phonon scattering is defined as constructive interference of phonons scattered from the inclusions in the phononic crystal. The intent of this work is to investigate the existence of coherent scattering in Si via phononic crystals. A phononic crystal is a periodic array of inclusions inside a host material. The inclusions could be a second material or a void. In this work five different supercell phononic crystals comprised of holes in silicon will be used to investigate the existence of coherent phonon scattering. Each of the supercells had nearly identical critical lengths in order to keep the amount of incoherent scattering equal among all of the PnCs. Porosity differences among the supercells were also minimized. All of the PnCs were fabricated with a focused ion beam (FIB). During fabrication a protective layer of Ti was used to protect the Si from unintentional Ga doping from the FIB. The Ti layer also helped generate voids with more vertical sidewalls. A set of experiments was performed to measure the thermal conductivity of each PnC. Thermal conductivity measurements were carried out on a silicon nitride suspended island platform with platinum resistance temperature detectors and coated with aluminum nitride. A silicon slab was concurrently measured with each PnC, and relative thermal conductivity values were determined. The addition of the PnC decreased Si's thermal conductivity to less than 22% of its original value. An analysis of the results shows there is a reduction in thermal conductivity beyond the effects of porosity and incoherent scattering. This enhanced reduction in thermal conductivity is due to coherent
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.
Estreicher, S. K. Gibbons, T. M.; Kang, By.; Bebek, M. B.
2014-01-07
Defects in semiconductors introduce vibrational modes that are distinct from bulk modes because they are spatially localized in the vicinity of the defect. Light impurities produce high-frequency modes often visible by Fourier-transform infrared absorption or Raman spectroscopy. Their vibrational lifetimes vary by orders of magnitude and sometimes exhibit unexpectedly large isotope effects. Heavy impurities introduce low-frequency modes sometimes visible as phonon replicas in photoluminescence bands. But other defects such as surfaces or interfaces exhibit spatially localized modes (SLMs) as well. All of them can trap phonons, which ultimately decay into lower-frequency bulk phonons. When heat flows through a material containing defects, phonon trapping at localized modes followed by their decay into bulk phonons is usually described in terms of phonon scattering: defects are assumed to be static scattering centers and the properties of the defect-related SLMs modes are ignored. These dynamic properties of defects are important. In this paper, we quantify the concepts of vibrational localization and phonon trapping, distinguish between normal and anomalous decay of localized excitations, discuss the meaning of phonon scattering in real space at the atomic level, and illustrate the importance of phonon trapping in the case of heat flow at Si/Ge and Si/C interfaces.
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.
"Phonon" scattering beyond perturbation theory
NASA Astrophysics Data System (ADS)
Qiu, WuJie; Ke, XueZhi; Xi, LiLi; Wu, LiHua; Yang, Jiong; Zhang, WenQing
2016-02-01
Searching and designing materials with intrinsically low lattice thermal conductivity (LTC) have attracted extensive consideration in thermoelectrics and thermal management community. The concept of part-crystalline part-liquid state, or even part-crystalline part-amorphous state, has recently been proposed to describe the exotic structure of materials with chemical- bond hierarchy, in which a set of atoms is weakly bonded to the rest species while the other sublattices retain relatively strong rigidity. The whole system inherently manifests the coexistence of rigid crystalline sublattices and fluctuating noncrystalline substructures. Representative materials in the unusual state can be classified into two categories, i.e., caged and non-caged ones. LTCs in both systems deviate from the traditional T -1 relationship ( T, the absolute temperature), which can hardly be described by small-parameter-based perturbation approaches. Beyond the classical perturbation theory, an extra rattling-like scattering should be considered to interpret the liquid-like and sublattice-amorphization-induced heat transport. Such a kind of compounds could be promising high-performance thermoelectric materials, due to the extremely low LTCs. Other physical properties for these part-crystalline substances should also exhibit certain novelty and deserve further exploration.
Influence of the optical-acoustic phonon hybridization on phonon scattering and thermal conductivity
NASA Astrophysics Data System (ADS)
Li, Wu; Carrete, Jesús; Madsen, Georg K. H.; Mingo, Natalio
2016-05-01
We predict a marked effect of optical-acoustic phonon hybridization on phonon scattering and lattice thermal conductivity (κ ), and illustrate it in the case of Fe2Ge3 . This material presents very low-lying optical phonons with an energy of 1.8 meV at the Brillouin zone center, which show avoided crossings with longitudinal acoustic (LA) phonons, due to optical-acoustic phonon polarization hybridization. Because the optical phonons have nonvanishing scattering rates, even a small amount of hybridization with the optical phonon can increase the scattering rates of LA phonons by much more than one order of magnitude, causing the contribution of these phonons to κ to vanish. At low temperatures, the contributions of all LA phonons are eliminated, and thus the avoided crossing leads to a reduction of thermal conductivity by more than half. The scattering rates are very sensitive to the optical-acoustic phonon hybridization strength, characterized by the gap at the avoided crossing point and varied with the wave-vector direction. Our work presents a different reduction mechanism of κ in systems with optical-acoustic phonon hybridization, which can benefit the search for new thermoelectric materials.
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.
Phonon Scattering Dynamics of Thermophoretic Motion in Carbon Nanotube Oscillators.
Prasad, Matukumilli V D; Bhattacharya, Baidurya
2016-04-13
Using phonon wave packet molecular dynamics simulations, we find that anomalous longitudinal acoustic (LA) mode phonon scattering in low to moderate energy ranges is responsible for initiating thermophoretic motion in carbon nanotube oscillators. The repeated scattering of a single mode LA phonon wave packet near the ends of the inner nanotube provides a net unbalanced force that, if large enough, initiates thermophoresis. By applying a coherent phonon pulse on the outer tube, which generalizes the single mode phonon wave packet, we are able to achieve thermophoresis in a carbon nanotube oscillator. We also find the nature of the unbalanced force on end-atoms to be qualitatively similar to that under an imposed thermal gradient. The thermodiffusion coefficient obtained for a range of thermal gradients and core lengths suggest that LA phonon scattering is the dominant mechanism for thermophoresis in longer cores, whereas for shorter cores, it is the highly diffusive mechanism that provides the effective force. PMID:26965789
Observation of induced longitudinal and shear acoustic phonons by Brillouin scattering.
Yoshida, Taisuke; Matsukawa, Mami; Yanagitani, Takahiko
2011-06-01
To improve the accuracy of velocity measurements in the Brillouin scattering technique using weak thermal phonons, we have used induced coherent phonons, which intensify the scattering. To induce phonons in the gigahertz range, we used a c-axis tilted ZnO film transducer that was developed in our laboratory. This allowed us to induce longitudinal and shear acoustic phonons effectively at hypersonic frequencies. As a result, we obtained scattered light in the silica glass sample that was much more intense than that obtained from the thermal phonons. Because the Brillouin scattering from induced phonons was measured, the shift frequency was that of the electric signal applied to the ZnO transducer. Strong peaks lead to a reduction of the measurement time. This is useful for two-dimensional mapping of thin film elasticity using Brillouin scattering. Additionally, Brillouin scattering enables the simultaneous measurement of longitudinal and shear phonon velocities in the sample plane. This opens up a potential new technique for non-destructive elasticity measurements of various materials. PMID:21693407
Phonon effects in the two-magnon Raman scattering in spin-Peierls systems
Zang, J.; Roeder, H.; Gammel, J.T.; Saxena, A.; Bishop, A.R.
1996-10-01
Recently discovered spin-Peierls (SP) transition in the compound CuGeO{sub 3} revived the interest in theoretical studies of low dimensional spin systems and lattice spin coupling effects. Special attention has been paid to the frustration effects and their consequence on the dynamical properties. In fact, it was pointed out earlier by some of the preset authors that there are frustrated SP fluctuations in the MX materials near the crossover region. In the study of the excitations in the CuGeO{sub 3} compound, several groups have conducted two-magnon Raman scattering experiments. In addition, several theoretical studies were carried out which, however, neglected the phonon effects totally. In the present study, the authors show that the phonon effects are important in the two-magnon Raman scattering. The phonon effects enter the Raman scattering process in two ways. First, there is a strong spin phonon coupling; naturally, the photon-magnon coupling has a phonon term, which has strength {partial_derivative}J/{partial_derivative}u, where u is lattice distortion. Second, without frustrated spin-spin coupling J{sub 2}, there is no two-magnon Raman scattering in the high temperature uniform phase, as the scattering term commutes with the Hamiltonian. However, since the lattice fluctuations (quantum or thermal) will introduce disorder in the spin-spin coupling, there is phonon induced two-magnon Raman scattering even without J{sub 2}.
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.
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.
Inelastic x-ray scattering measurements of phonon dynamics in URu2Si2
Gardner, D. R.; Bonnoit, C. J.; Chisnell, R.; Said, A. H.; Leu, B. M.; Williams, Travis J.; Luke, G. M.; Lee, Y. S.
2016-02-11
In this paper, we study high-resolution inelastic x-ray scattering measurements of the acoustic phonons of URu2Si2. At all temperatures, the longitudinal acoustic phonon linewidths are anomalously broad at small wave vectors revealing a previously unknown anharmonicity. The phonon modes do not change significantly upon cooling into the hidden order phase. In addition, our data suggest that the increase in thermal conductivity in the hidden order phase cannot be driven by a change in phonon dispersions or lifetimes. Hence, the phonon contribution to the thermal conductivity is likely much less significant compared to that of the magnetic excitations in the lowmore » temperature phase.« less
Inelastic x-ray scattering from phonons under multibeam conditions
NASA Astrophysics Data System (ADS)
Bosak, Alexey; Krisch, Michael
2007-03-01
We report on an experimental observation of a previously neglected multibeam contribution to the inelastic x-ray scattering cross section. Its manifestation is a substantial modification of the apparent phonon selection rules when two (or more) reciprocal lattice points are simultaneously intercepted by the Ewald sphere. The observed multibeam contributions can be treated semi-quantitatively in the frame of Renninger’s “simplest approach.” A few corollaries, relevant for experimental work on inelastic scattering from phonons, are presented.
Wave packet simulations of phonon boundary scattering at graphene edges
NASA Astrophysics Data System (ADS)
Wei, Zhiyong; Chen, Yunfei; Dames, Chris
2012-07-01
Wave packet dynamics is used to investigate the scattering of longitudinal (LA), transverse (TA), and bending-mode (ZA) phonons at the zigzag and armchair edges of suspended graphene. The interatomic forces are calculated using a linearized Tersoff potential. The strength of a boundary scattering event at impeding energy flow is described by a forward scattering coefficient, similar in spirit to a specularity parameter. For armchair boundaries, this scattering coefficient is found to depend strongly on the magnitude, direction, and polarization of the incident wavevector, while for zigzag boundaries, the forward scattering coefficient is found to always be unity regardless of wavevector and polarization. Wave packet splitting is observed for ZA phonons incident on armchair boundaries, while both splitting and mode conversion are observed for LA and TA phonons incident on both zigzag and armchair boundaries. These simulation results show that armchair boundaries impede the forward propagation of acoustic phonon energy much more strongly than zigzag boundaries do, suggesting that graphene nanoribbons will have substantially lower thermal conductivity in armchair rather than zigzag orientation.
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.
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.
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.
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
Electron-phonon interaction and Raman scattering in nanocrystals
NASA Astrophysics Data System (ADS)
Klimin, S. N.; Pokatilov, E. P.; Fomin, V. M.; Devreese, J. T.; Gladilin, V. N.; Balaban, S. N.
1997-03-01
The vibrational eigenmodes of a nanocrystal are derived by diagonalization of the equations of motion for the ionic displacement taking into account a non-parabolic dispersion with electrostatic and mechanical boundary conditions. A finite width of the Brillouin zone leads automatically to a finite basis of vibrational modes. The developed method can be applicable to nanostructures of an arbitrary geometry. For a spherical nanocrystal, a dispersion equation contains the effective multimode dielectric function. The resulting eigenmodes are mixed bulk-like and interface waves, especially in the short-wavelength region. Using the obtained Hamiltonian, the one-phonon and two-phonon resonant Raman scattering spectra are calculated for a spherical CdSe nanocrystal in the borosilicate glass. The valence band mixing dramatically enhances relative intensities of the two-phonon peaks and makes the adiabatic approximation inapplicable. Hence, the Huang-Rhys parameter is not an adequate characteristic of the optical spectra. Using a direct expansion of the evolution operator, a good agreement has been achieved between the calculated and the experimentally observed [1] Raman spectra. [1] M. C. Klein, F. Hache, D. Ricard, and C. Flytzanis, Phys. Rev. B 42, 11123 (1990).
Acoustic scattering from phononic crystals with complex geometry.
Kulpe, Jason A; Sabra, Karim G; Leamy, Michael J
2016-05-01
This work introduces a formalism for computing external acoustic scattering from phononic crystals (PCs) with arbitrary exterior shape using a Bloch wave expansion technique coupled with the Helmholtz-Kirchhoff integral (HKI). Similar to a Kirchhoff approximation, a geometrically complex PC's surface is broken into a set of facets in which the scattering from each facet is calculated as if it was a semi-infinite plane interface in the short wavelength limit. When excited by incident radiation, these facets introduce wave modes into the interior of the PC. Incorporation of these modes in the HKI, summed over all facets, then determines the externally scattered acoustic field. In particular, for frequencies in a complete bandgap (the usual operating frequency regime of many PC-based devices and the requisite operating regime of the presented theory), no need exists to solve for internal reflections from oppositely facing edges and, thus, the total scattered field can be computed without the need to consider internal multiple scattering. Several numerical examples are provided to verify the presented approach. Both harmonic and transient results are considered for spherical and bean-shaped PCs, each containing over 100 000 inclusions. This facet formalism is validated by comparison to an existing self-consistent scattering technique. PMID:27250192
A study of phonon anisotropic scattering effect on silicon thermal conductivity at nanoscale
Bong, Victor N-S; Wong, Basil T.
2015-08-28
Previous studies have shown that anisotropy in phonon transport exist because of the difference in phonon dispersion relation due to different lattice direction, as observed by a difference in in-plane and cross-plane thermal conductivity. The directional preference (such as forward or backward scattering) in phonon propagation however, remains a relatively unexplored frontier. Our current work adopts a simple scattering probability in radiative transfer, which is called Henyey and Greenstein probability density function, and incorporates it into the phonon Monte Carlo simulation to investigate the effect of directional scattering in phonon transport. In this work, the effect of applying the anisotropy scattering is discussed, as well as its impact on the simulated thermal conductivity of silicon thin films. While the forward and backward scattering will increase and decrease thermal conductivity respectively, the extent of the effect is non-linear such that forward scattering has a more obvious effect than backward scattering.
Hierarchical thermoelectrics: crystal grain boundaries as scalable phonon scatterers
NASA Astrophysics Data System (ADS)
Selli, Daniele; Boulfelfel, Salah Eddine; Schapotschnikow, Philipp; Donadio, Davide; Leoni, Stefano
2016-02-01
Thermoelectric materials are strategically valuable for sustainable development, as they allow for the generation of electrical energy from wasted heat. In recent years several strategies have demonstrated some efficiency in improving thermoelectric properties. Dopants affect carrier concentration, while thermal conductivity can be influenced by alloying and nanostructuring. Features at the nanoscale positively contribute to scattering phonons, however those with long mean free paths remain difficult to alter. Here we use the concept of hierarchical nano-grains to demonstrate thermal conductivity reduction in rocksalt lead chalcogenides. We demonstrate that grains can be obtained by taking advantage of the reconstructions along the phase transition path that connects the rocksalt structure to its high-pressure form. Since grain features naturally change as a function of size, they impact thermal conductivity over different length scales. To understand this effect we use a combination of advanced molecular dynamics techniques to engineer grains and to evaluate thermal conductivity in PbSe. By affecting grain morphologies only, i.e. at constant chemistry, two distinct effects emerge: the lattice thermal conductivity is significantly lowered with respect to the perfect crystal, and its temperature dependence is markedly suppressed. This is due to an increased scattering of low-frequency phonons by grain boundaries over different size scales. Along this line we propose a viable process to produce hierarchical thermoelectric materials by applying pressure via a mechanical load or a shockwave as a novel paradigm for material design.
Hierarchical thermoelectrics: crystal grain boundaries as scalable phonon scatterers.
Selli, Daniele; Boulfelfel, Salah Eddine; Schapotschnikow, Philipp; Donadio, Davide; Leoni, Stefano
2016-02-14
Thermoelectric materials are strategically valuable for sustainable development, as they allow for the generation of electrical energy from wasted heat. In recent years several strategies have demonstrated some efficiency in improving thermoelectric properties. Dopants affect carrier concentration, while thermal conductivity can be influenced by alloying and nanostructuring. Features at the nanoscale positively contribute to scattering phonons, however those with long mean free paths remain difficult to alter. Here we use the concept of hierarchical nano-grains to demonstrate thermal conductivity reduction in rocksalt lead chalcogenides. We demonstrate that grains can be obtained by taking advantage of the reconstructions along the phase transition path that connects the rocksalt structure to its high-pressure form. Since grain features naturally change as a function of size, they impact thermal conductivity over different length scales. To understand this effect we use a combination of advanced molecular dynamics techniques to engineer grains and to evaluate thermal conductivity in PbSe. By affecting grain morphologies only, i.e. at constant chemistry, two distinct effects emerge: the lattice thermal conductivity is significantly lowered with respect to the perfect crystal, and its temperature dependence is markedly suppressed. This is due to an increased scattering of low-frequency phonons by grain boundaries over different size scales. Along this line we propose a viable process to produce hierarchical thermoelectric materials by applying pressure via a mechanical load or a shockwave as a novel paradigm for material design. PMID:26815914
NASA Astrophysics Data System (ADS)
Figarova, S. R.; Hasiyeva, G. N.; Figarov, V. R.
2016-04-01
The effect of phonon scattering on electrical conductivity (EC) of 2D electron gas in quantum well (QW) systems with a complicated potential profile is described. Dependence of QW electrical conductivity on QW parameters (such as QW width, Fermi level positions etc.) when phonon scattering is employed has been calculated. NDC in EC when it varies with width of the QW has been found.
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
Impact of Phonon Surface Scattering on Thermal Energy Distribution of Si and SiGe Nanowires
NASA Astrophysics Data System (ADS)
Malhotra, Abhinav; Maldovan, Martin
2016-05-01
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.
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
NASA Astrophysics Data System (ADS)
Trigo, Mariano; Reis, David
2014-03-01
In a solid, the elementary excitations of the crystalline lattice (phonons) determine the macroscopic properties such as thermal transport and structural stability. The spectrum of these elementary excitations is normally obtained from inelastic neutron and x-ray scattering near equilibrium conditions, which is a Fourier transform of the spatial and temporal correlations of the system. Recent advances in Free Electron Laser sources provide sufficient flux and time-resolution to explore the dynamics of solids at the fundamental time- and length-scales of the atomic motions. In this talk I will show that by probing phonon correlations by femtosecond diffuse scattering in photoexcited germanium, we were able to obtain the phonon dispersion with extreme frequency and momentum resolution without analyzing the energy of the outgoing photon. I will show that time-dependent coherences are generated when an ultrafast laser pulse slightly quenches the phonon frequencies, generating pairs of correlated phonons at equal and opposite momenta. Using this approach we obtain an extremely high-resolution probe of the excited-state phonon dispersion over large sections of momentum space by a simple Fourier transform.
NASA Astrophysics Data System (ADS)
Birt, Daniel R.; An, Kyongmo; Weathers, Annie; Shi, Li; Tsoi, Maxim; Li, Xiaoqin
2013-02-01
We demonstrate the use of the micro-Brillouin light scattering (micro-BLS) technique as a local temperature sensor for magnons in a permalloy (Py) thin film and phonons in the glass substrate. When the Py film is uniformly heated, we observe a systematic shift in the frequencies of two thermally excited perpendicular standing spin wave modes. Fitting the temperature dependent magnon spectra allows us to achieve a temperature resolution better than 2.5 K. In addition, we demonstrate that the micro-BLS spectra can be used to measure the local temperature of magnons and the relative temperature shift of phonons across a thermal gradient. Such local temperature sensors are useful for investigating spin caloritronic and thermal transport phenomena in general.
Inelastic x-ray scattering measurements of phonon dispersion and lifetimes in PbTe1-x Se x alloys
NASA Astrophysics Data System (ADS)
Tian, Zhiting; Li, Mingda; Ren, Zhensong; Ma, Hao; Alatas, Ahmet; Wilson, Stephen D.; Li, Ju
2015-09-01
PbTe1-x Se x alloys are of special interest to thermoelectric applications. Inelastic x-ray scattering determination of phonon dispersion and lifetimes along the high symmetry directions for PbTe1-x Se x alloys are presented. By comparing with calculated results based on the virtual crystal model calculations combined with ab initio density functional theory, the validity of virtual crystal model is evaluated. The results indicate that the virtual crystal model is overall a good assumption for phonon frequencies and group velocities despite the softening of transverse acoustic phonon modes along [1 1 1] direction, while the treatment of lifetimes warrants caution. In addition, phonons remain a good description of vibrational modes in PbTe1-x Se x alloys.
Temperature Dependence of Brillouin Light Scattering Spectra of Acoustic Phonons in Silicon
NASA Astrophysics Data System (ADS)
Somerville, Kevin; Klimovich, Nikita; An, Kyongmo; Sullivan, Sean; Weathers, Annie; Shi, Li; Li, Xiaoqin
2015-03-01
Thermal management represents an outstanding challenge in many areas of technology. Electrons, optical phonons, and acoustic phonons are often driven out of local equilibrium in electronic devices or during laser-material interaction processes. Interest in non-equilibrium transport processes has motivated the development of Raman spectroscopy as a local temperature sensor of optical phonons and intermediate frequency acoustic phonons, whereas Brillouin light scattering (BLS) has recently been explored as a temperature sensor of low-frequency acoustic phonons. Here, we report temperature dependent BLS spectra of silicon, with Raman spectra taken simultaneously for comparison. The origins of the observed temperature dependence of the BLS peak position, linewidth, and intensity are examined in order to evaluate their potential use as temperature sensors for acoustic phonons. We determine that the integrated BLS intensity can be used measure the temperature of specific acoustic phonon modes. This work is supported by National Science Foundation (NSF) Thermal Transport Processes Program under Grant CBET-1336968.
A Bond-order Theory on the Phonon Scattering by Vacancies in Two-dimensional Materials
NASA Astrophysics Data System (ADS)
Xie, Guofeng; Shen, Yulu; Wei, Xiaolin; Yang, Liwen; Xiao, Huaping; Zhong, Jianxin; Zhang, Gang
2014-05-01
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.
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
Determination of phonon dispersion relations by X-ray thermal diffuse scattering
Xu, R.; Chiang, T.-C.
2010-07-20
Thermal diffuse scattering (TDS) of X-rays from crystals contains information on phonons. This paper reviews the general theory of TDS and some recent experiments aimed at further developing TDS into a useful and efficient method for studying phonon dispersion relations.
NASA Astrophysics Data System (ADS)
Trigo, M.; Fuchs, M.; Chen, J.; Jiang, M. P.; Cammarata, M.; Fahy, S.; Fritz, D. M.; Gaffney, K.; Ghimire, S.; Higginbotham, A.; Johnson, S. L.; Kozina, M. E.; Larsson, J.; Lemke, H.; Lindenberg, A. M.; Ndabashimiye, G.; Quirin, F.; Sokolowski-Tinten, K.; Uher, C.; Wang, G.; Wark, J. S.; Zhu, D.; Reis, D. A.
2013-12-01
The macroscopic characteristics of a material are determined by its elementary excitations, which dictate the response of the system to external stimuli. The spectrum of excitations is related to fluctuations in the density-density correlations and is typically measured through frequency-domain neutron or X-ray scattering. Time-domain measurements of these correlations could yield a more direct way to investigate the excitations of solids and their couplings both near to and far from equilibrium. Here we show that we can access large portions of the phonon dispersion of germanium by measuring the diffuse scattering from femtosecond X-ray free-electron laser pulses. A femtosecond optical laser pulse slightly quenches the vibrational frequencies, producing pairs of high-wavevector phonons with opposite momenta. These phonons manifest themselves as time-dependent coherences in the displacement correlations probed by the X-ray scattering. As the coherences are preferentially created in regions of strong electron-phonon coupling, the time-resolved approach is a natural spectroscopic tool for probing low-energy collective excitations in solids, and their microscopic interactions.
Neutron Scattering Investigation of Phonon Scattering Rates in Ag1-xSb1+xTe2+x (x = 0, 0.1, and 0.2)
Abernathy, Douglas L; Budai, John D; Delaire, Olivier A; Ehlers, Georg; Hong, Tao; Karapetrova, Evguenia A.; Ma, Jie; May, Andrew F; McGuire, Michael A; Specht, Eliot D
2014-01-01
The phonon dispersions and scattering rates of the thermoelectric material AgSbTe$_{2}$ were measured as a function of temperature with inelastic neutron scattering. The results show that phonon scattering rates are large and weakly dependent on temperature. The lattice thermal conductivity was calculated from the measured phonon lifetimes and group velocities, providing good agreement with bulk transport measurements. The measured phonon scattering rates and their temperature dependence are compared with models of phonon scattering by anharmonicity and point defect. We find that these processes cannot account for the large total phonon scattering rates observed, and their lack of temperature dependence. Neutron and synchrotron diffraction measurements on single crystals revealed an extensive nanostructure from cation ordering, which is likely responsible for the strong phonon scattering.
Temperature dependence of Brillouin light scattering spectra of acoustic phonons in silicon
NASA Astrophysics Data System (ADS)
Olsson, Kevin S.; Klimovich, Nikita; An, Kyongmo; Sullivan, Sean; Weathers, Annie; Shi, Li; Li, Xiaoqin
2015-02-01
Electrons, optical phonons, and acoustic phonons are often driven out of local equilibrium in electronic devices or during laser-material interaction processes. The need for a better understanding of such non-equilibrium transport processes has motivated the development of Raman spectroscopy as a local temperature sensor of optical phonons and intermediate frequency acoustic phonons, whereas Brillouin light scattering (BLS) has recently been explored as a temperature sensor of low-frequency acoustic phonons. Here, we report the measured BLS spectra of silicon at different temperatures. The origins of the observed temperature dependence of the BLS peak position, linewidth, and intensity are examined in order to evaluate their potential use as temperature sensors for acoustic phonons.
Temperature dependence of Brillouin light scattering spectra of acoustic phonons in silicon
Olsson, Kevin S.; Klimovich, Nikita; An, Kyongmo; Sullivan, Sean; Weathers, Annie; Shi, Li E-mail: elaineli@physics.utexas.edu; Li, Xiaoqin E-mail: elaineli@physics.utexas.edu
2015-02-02
Electrons, optical phonons, and acoustic phonons are often driven out of local equilibrium in electronic devices or during laser-material interaction processes. The need for a better understanding of such non-equilibrium transport processes has motivated the development of Raman spectroscopy as a local temperature sensor of optical phonons and intermediate frequency acoustic phonons, whereas Brillouin light scattering (BLS) has recently been explored as a temperature sensor of low-frequency acoustic phonons. Here, we report the measured BLS spectra of silicon at different temperatures. The origins of the observed temperature dependence of the BLS peak position, linewidth, and intensity are examined in order to evaluate their potential use as temperature sensors for acoustic phonons.
NASA Astrophysics Data System (ADS)
Shen, Yulu; Xie, Guofeng; Wei, Xiaolin; Zhang, Kaiwang; Tang, Minghua; Zhong, Jianxin; Zhang, Gang; Zhang, Yong-Wei
2014-02-01
Although graphene holds great promise in thermal applications owing to its superior thermal conductivity, an intriguing question remains as to which polarizations and frequencies are dominant in its heat conduction. In this work, by incorporating the direction-dependent phonon-boundary scattering and the special selection rule for three-phonon scattering into the linearized phonon Boltzmann transport equation, we systematically investigate the relative contributions from longitudinal-acoustic, transverse-acoustic, and out-of-plane acoustic (ZA) branches to the thermal conductivity of graphene ribbons, focusing on the effects of their size and temperature. We find that the relative contribution from ZA branch to heat conduction increases with decreasing the size, specularity parameter, and temperature of graphene ribbons. Our analysis reveals that this change arises from the huge difference in the phonon dispersion and in the phonon mean free path of Umklapp process between in-plane and out-of-plane branches.
Surface polar optical phonon scattering of carriers in graphene on various substrates
NASA Astrophysics Data System (ADS)
Lin, I.-Tan; Liu, Jia-Ming
2013-08-01
The surface polar optical phonon scattering of carriers in graphene on various substrates is thoroughly studied using Rode's iteration method. The dependence of the scattering rate on carrier energy, temperature, and Fermi energy is investigated. The experimental data in the literature on the temperature dependence of the resistivity can be fitted using only one free fitting parameter. The fitting results show that the coupling strengths of surface optical phonons are much stronger, by at least a factor of 3.5, than the theoretical prediction. The strong coupling also suggests that the deformation potential of acoustic phonons might have been overestimated in the literature.
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. PMID:15231334
Khurgin, Jacob B.; Bajaj, Sanyam; Rajan, Siddharth
2015-12-28
Longitudinal optical (LO) phonons in GaN generated in the channel of high electron mobility transistors (HEMT) are shown to undergo nearly elastic scattering via collisions with hot electrons. The net result of these collisions is the diffusion of LO phonons in the Brillouin zone causing reduction of phonon and electron temperatures. This previously unexplored diffusion mechanism explicates how an increase in electron density causes reduction of the apparent lifetime of LO phonons, obtained from the time resolved Raman studies and microwave noise measurements, while the actual decay rate of the LO phonons remains unaffected by the carrier density. Therefore, the saturation velocity in GaN HEMT steadily declines with increased carrier density, in a qualitative agreement with experimental results.
Phonon Lifetime Measurement by Stimulated Brillouin Scattering Slow Light Technique in Optical Fiber
NASA Astrophysics Data System (ADS)
Chen, Wei; Meng, Zhou; Zhou, Hui-Juan
2013-07-01
Phonon lifetime is a significant parameter in the process of stimulated Brillouin scattering (SBS). In the present study, SBS slow light technique is used to measure phonon lifetime. Brillouin bandwidth is divided into natural, spontaneous and stimulated bandwidth. Compared with the traditional heterodyne-detection and pump-probe techniques, the natural Brillouin bandwidth can be obtained by SBS slow light technique, which equals the reciprocal of phonon lifetime and has no relations with the pump power. Another advantage of this technique is that the effect of polarization can be excluded. The natural Brillouin bandwidth is measured to be ~50 MHz and the phonon lifetime ~3.2 ns in the conventional single-mode fiber (SMF) at room temperature and zero strain. The obtained results are guidable in applications where the phonon lifetime information is a requisite such as phase conjugation and pulse compression.
Parity conservation in electron-phonon scattering in zigzag graphene nanoribbon
Chu, Yanbiao; Gautreau, Pierre; Basaran, Cemal
2014-09-15
In contrast with carbon nanotubes, the absence of translational symmetry (or periodical boundary condition) in the restricted direction of zigzag graphene nanoribbon removes the selection rule of subband number conservation. However, zigzag graphene nanoribbons with even dimers do have the inversion symmetry. We, therefore, propose a selection rule of parity conservation for electron-phonon interactions. The electron-phonon scattering matrix in zigzag graphene nanoribbons is developed using the tight-binging model within the deformation potential approximation.
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.
Interplay between phonon and impurity scattering in two-dimensional hole transport
NASA Astrophysics Data System (ADS)
Min, Hongki; Hwang, E. H.; Das Sarma, S.
2012-08-01
We investigate temperature-dependent transport properties of two-dimensional p-GaAs systems taking into account both hole-phonon and hole-impurity scattering effects. By analyzing the hole mobility data of p-GaAs in the temperature range 10K
Phonon-interface scattering in multilayer graphene on an amorphous support
Sadeghi, Mir Mohammad; Jo, Insun; Shi, Li
2013-01-01
The recent studies of thermal transport in suspended, supported, and encased graphene just began to uncover the richness of two-dimensional phonon physics, which is relevant to the performance and reliability of graphene-based functional materials and devices. Among the outstanding questions are the exact causes of the suppressed basal-plane thermal conductivity measured in graphene in contact with an amorphous material, and the layer thickness needed for supported or embedded multilayer graphene (MLG) to recover the high thermal conductivity of graphite. Here we use sensitive in-plane thermal transport measurements of graphene samples on amorphous silicon dioxide to show that full recovery to the thermal conductivity of the natural graphite source has yet to occur even after the MLG thickness is increased to 34 layers, considerably thicker than previously thought. This seemingly surprising finding is explained by long intrinsic scattering mean free paths of phonons in graphite along both basal-plane and cross-plane directions, as well as partially diffuse scattering of MLG phonons by the MLG-amorphous support interface, which is treated by an interface scattering model developed for highly anisotropic materials. Based on the phonon transmission coefficient calculated from reported experimental thermal interface conductance results, phonons emerging from the interface consist of a large component that is scattered across the interface, making rational choice of the support materials a potential approach to increasing the thermal conductivity of supported MLG. PMID:24067656
Effects of composition and phonon scattering mechanisms on thermal transport in MFI zeolite films
NASA Astrophysics Data System (ADS)
Hudiono, Yeny; Greenstein, Abraham; Saha-Kuete, Carine; Olson, Brandon; Graham, Samuel; Nair, Sankar
2007-09-01
We report a systematic study that reveals the effect of composition (silicon-to-aluminum ratio) and the role of different phonon scattering processes on thermal transport in the nanoporous zeolite MFI. This is accomplished via synthesis of a series of films with graded compositions, thermal property measurements, and lattice dynamical modeling in the framework of the Boltzmann equation. MFI films with different Si/Al ratios (from infinity to 26) and constant (h0l) out-of-plane orientation were successfully synthesized by a seeded hydrothermal process. Three-omega measurements on these films allowed us to obtain comprehensive information on the thermal conductivity of MFI as a function of temperature (150-450 K) and Si/Al ratio. Detailed atomistic simulations (energy minimization and phonon dispersion calculations) were carried out for the MFI crystal structure with different Si/Al ratios and incorporated into a Boltzmann transport model along with approximate theoretical expressions for describing the rate of phonon scattering through umklapp, defect, and boundary scattering processes. The model predicts the observed thermal conductivity behavior very well across the entire range of temperature and composition investigated, with only a small number of fitting parameters of physical significance which allow us to distinguish the contributions of the different phonon scattering mechanisms. In particular, our results strongly suggest that the upper limit of thermal conductivity is defined by boundary-like scattering associated with the pore structure of the material. Below this limit, silicon substitution with aluminum allows considerable suppression of thermal conductivity by point defect scattering and a decrease in phonon velocity. These findings are important from the point of view of developing a robust platform for understanding thermal transport in complex crystalline materials with nanostructural features (such as an ordered nanopore network), which in turn
Semiclassical multi-phonon theory for atom-surface scattering: Application to the Cu(111) system
NASA Astrophysics Data System (ADS)
Daon, Shauli; Pollak, Eli
2015-05-01
The semiclassical perturbation theory of Hubbard and Miller [J. Chem. Phys. 80, 5827 (1984)] is further developed to include the full multi-phonon transitions in atom-surface scattering. A practically applicable expression is developed for the angular scattering distribution by utilising a discretized bath of oscillators, instead of the continuum limit. At sufficiently low surface temperature good agreement is found between the present multi-phonon theory and the previous one-, and two-phonon theory derived in the continuum limit in our previous study [Daon, Pollak, and Miret-Artés, J. Chem. Phys. 137, 201103 (2012)]. The theory is applied to the measured angular distributions of Ne, Ar, and Kr scattered from a Cu(111) surface. We find that the present multi-phonon theory substantially improves the agreement between experiment and theory, especially at the higher surface temperatures. This provides evidence for the importance of multi-phonon transitions in determining the angular distribution as the surface temperature is increased.
Semiclassical multi-phonon theory for atom-surface scattering: Application to the Cu(111) system.
Daon, Shauli; Pollak, Eli
2015-05-01
The semiclassical perturbation theory of Hubbard and Miller [J. Chem. Phys. 80, 5827 (1984)] is further developed to include the full multi-phonon transitions in atom-surface scattering. A practically applicable expression is developed for the angular scattering distribution by utilising a discretized bath of oscillators, instead of the continuum limit. At sufficiently low surface temperature good agreement is found between the present multi-phonon theory and the previous one-, and two-phonon theory derived in the continuum limit in our previous study [Daon, Pollak, and Miret-Artés, J. Chem. Phys. 137, 201103 (2012)]. The theory is applied to the measured angular distributions of Ne, Ar, and Kr scattered from a Cu(111) surface. We find that the present multi-phonon theory substantially improves the agreement between experiment and theory, especially at the higher surface temperatures. This provides evidence for the importance of multi-phonon transitions in determining the angular distribution as the surface temperature is increased. PMID:25956085
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. PMID:24866858
Semiclassical multi-phonon theory for atom-surface scattering: Application to the Cu(111) system
Daon, Shauli; Pollak, Eli
2015-05-07
The semiclassical perturbation theory of Hubbard and Miller [J. Chem. Phys. 80, 5827 (1984)] is further developed to include the full multi-phonon transitions in atom-surface scattering. A practically applicable expression is developed for the angular scattering distribution by utilising a discretized bath of oscillators, instead of the continuum limit. At sufficiently low surface temperature good agreement is found between the present multi-phonon theory and the previous one-, and two-phonon theory derived in the continuum limit in our previous study [Daon, Pollak, and Miret-Artés, J. Chem. Phys. 137, 201103 (2012)]. The theory is applied to the measured angular distributions of Ne, Ar, and Kr scattered from a Cu(111) surface. We find that the present multi-phonon theory substantially improves the agreement between experiment and theory, especially at the higher surface temperatures. This provides evidence for the importance of multi-phonon transitions in determining the angular distribution as the surface temperature is increased.
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.
Electron-interface-phonon scattering in graded quantum wells of Ga1-xAlxAs
NASA Astrophysics Data System (ADS)
Duan, Wenhui; Zhu, Jia-Lin; Gu, Bing-Lin
1994-05-01
Using the method of series expansion, interface-phonon vibrational modes are calculated in the dielectric continuum model for the graded quantum well of Ga1-xAlxAs with a Ga0.6Al0.4As barrier. The intrasubband and intersubband scattering rates are obtained as functions of quantum-well width. The results reveal that the behavior of interface phonon modes is very different from that in a square quantum-well structure. It is found that the electron-interface-phonon scattering rates can be changed remarkably in a graded quantum-well structure compared with those in a square quantum-well structure, which is useful for some device applications.
Thermal conductivity in large-J two-dimensional antiferromagnets: Role of phonon scattering
Chernyshev, A. L.; Brenig, Wolfram
2015-08-05
Different types of relaxation processes for magnon heat current are discussed, with a particular focus on coupling to three-dimensional phonons. There is thermal conductivity by these in-plane magnetic excitations using two distinct techniques: Boltzmann formalism within the relaxation-time approximation and memory-function approach. Also considered are the scattering of magnons by both acoustic and optical branches of phonons. We demonstrate an accord between the two methods, regarding the asymptotic behavior of the effective relaxation rates. It is strongly suggested that scattering from optical or zone-boundary phonons is important for magnon heat current relaxation in a high-temperature window of ΘD≲T<< J.
LA phonons scattering of surface electrons in Bi2Se3
NASA Astrophysics Data System (ADS)
Huang, Lang-Tao; Zhu, Bang-Fen
2013-03-01
Within the Boltzmann equation formalism we evaluate the transport relaxation time of Dirac surface states (SSs) in the typical topological insulator(TI) Bi2Se3 due to the phonon scattering. We find that although the back-scattering of the SSs in TIs is strictly forbidden, the in-plane scattering between SSs in 3-dimensional TIs is allowed, maximum around the right-angle scattering. Thus the topological property of the SSs only reduces the scattering rate to its one half approximately. Besides, the larger LA deformation potential and lower sound velocity of Bi2Se3 enhance the scattering rate significantly. Compared with the Dirac electrons in graphene, we find the scattering rate of SSs in Bi2Se3 are two orders of magnitudes larger, which agree with the recent transport experiments. This work was supported by the NSFC (Grant No. 11074143), and the Program of Basic Research Development of China (Grant No. 2011CB921901).
LA phonons scattering of surface electrons in Bi2Se3
NASA Astrophysics Data System (ADS)
Huang, Lang-Tao; Zhu, Bang-Fen
2013-12-01
Within the Boltzmann equation formalism we evaluate the transport relaxation time of Dirac surface states (SSs) in the typical topological insulator(TI) Bi2Se3 due to the phonon scattering. We find that although the back-scattering of the SSs in TIs is strictly forbidden, the in-plane scattering between SSs in 3-dimensional TIs is allowed, maximum around the right-angle scattering. Thus the topological property of the SSs only reduces the scattering rate to its one half approximately. Besides, the larger LA deformation potential and lower sound velocity of Bi2Se3 enhance the scattering rate significantly. Compared with the Dirac electrons in graphene, we find the scattering rate of SSs in Bi2Se3 are two orders of magnitudes larger, which agree with the recent transport experiments.
Phonon dispersion in uranium measured using inelastic x-ray scattering.
Manley, M. E.; Lander, G. H.; Sinn, H.; Alatas, A.; Hults, W. L.; McQueeney, R. J.; Smith, J. L.; Wilt, J.; XFD
2003-02-01
Phonon-dispersion curves were obtained from inelastic x-ray scattering measurements on high-purity uranium single crystals at room temperature. Modes displacing atoms along [00{zeta}] and propagating in all three high-symmetry directions were measured. Whereas the acoustic modes agree with the neutron measurements, the longitudinal-optic branch is about 10% higher in energy, but consistent with higher cutoff energies observed in phonon density-of-states measurements on polycrystals. The application of this x-ray technique, which requires only very small samples, opens possibilities in actinide science.
NASA Astrophysics Data System (ADS)
Xiong, Qihua; Eklund, P. C.
2005-03-01
Raman scattering from surface optic (SO) phonons has been observed and identified in cylindrical GaP and rectangular cross-section ZnS nanowires. We propose that the symmetry breaking mechanism which activates the SO phonon is a periodic modulation of the cross-sectional area along the nanowires. In the case of cylindrical GaP nanowires, Raman scattering from SO phonons in air at room temperature is observed at 394 cm-1, in between the first order longitudinal optic (LO) (401 cm-1) and transverse optic (TO) (367 cm-1), and downshift to 392 cm-1 in dichloromethane (?m=2.0) and 390 cm-1 in aniline (?m=2.56). Raman scattering from the ZnS nanowires in air at room temperature reveals a strong first-order LO mode at 346 cm-1 and two TO modes at 269 and 282 cm-1. The SO Raman band in ZnS is observed at 335 cm-1 in air, and downshifts to 328 cm-1 in dichloromethane and to 326 cm-1 in aniline. The position of the SO band in GaP and ZnS nanowires is consistent with a dielectric continuum model. Theoretical SO phonon dispersion for both cylindrical and rectangular cross-section nanowires is presented and compared to experiment. This work was supported by the NSF NIRT program (DMR- 0304178).
Effect of an electric field on electron-interface-phonon scattering in a graded quantum well
NASA Astrophysics Data System (ADS)
Zhu, Jia-Lin; Duan, Wenhui; Gu, Bing-Lin; Wu, Jian
1996-02-01
Within the dielectric continuum model, the effect of an applied longitudinal electric field on electron-interface-phonon scattering is studied for the graded quantum well of Ga 1- xAl xAs with a Ga 0.6Al 0.4As barrier, and compared with that in a staircase-like square quantum well structure. The electron subband and interface phonon modes are calculated using the method of series expansion. The intrasubband and intersubband scattering rates are obtained as functions of the applied electric field, and the influence of the composition gradient of a graded quantum well on the scattering rates is shown. It is found that the variation of the interface-phonon scattering rates with the applied electric field in a graded quantum well structure is significantly different from that in a staircase-like square quantum well structure. However, there is much less difference in the variation of the total scattering rates between the two structures.
Phonon density of states of α - and δ -plutonium by inelastic x-ray scattering
NASA Astrophysics Data System (ADS)
Manley, M. E.; Said, A. H.; Fluss, M. J.; Wall, M.; Lashley, J. C.; Alatas, A.; Moore, K. T.; Shvyd'Ko, Yu.
2009-02-01
Inelastic x-ray scattering measurements of the phonon density of states (DOS) were performed on polycrystalline samples of pure α-Pu and δ-Pu0.98Ga0.02 at room temperature. The heat capacity of α-Pu is well reproduced by contributions calculated from the measured phonon DOS plus conventional thermal-expansion and electronic contributions, showing that α-Pu is a “well-behaved” metal in this regard. A comparison of the phonon DOS of the two phases at room temperature showed that the vibrational entropy difference between them is only a quarter of the total entropy difference expected from known thermodynamic measurements. The missing entropy is too large to be accounted for by conventional electronic entropy and evidence from the literature rules out a contribution from spin fluctuations. Possible alternative sources for the missing entropy are discussed.
Choi, B.H.; Poe, R.T.
1985-08-01
We present a systematic formulation of the atom--surface scattering dynamics which includes the vibrational states of the atoms in the solid (phonons). The properties of the total scattering wave function of the system, a representation of the interaction potential matrix, and the characteristics of the independent physical solutions are all derived from the translational invariance of the full Hamiltonian. The scattering equations in the integral forms as well as the related Green functions were also obtained. The configurational representations of the Green functions, in particular, are quite different from those of the conventional scattering theory where the collision partners are spatially localized. Various versions of the integral expression of scattering, transition, and reactance matrices were also obtained. They are useful for introducing approximation schemes. From the present formulation, some specific theoretical schemes which are more realistic compared to those that have been employed so far and at the same time capable of yielding effective ab initio computation are derived in the following paper. The time reversal invariance and the microscopic reversibility of the atom--surface scattering were discussed. The relations between the in and outgoing scattering wave functions which are satisfied in the atom--surface system and important in the transition matrix methods were presented. The phonon annihilation and creation, and the adsorption and desorption of the atom are related through the time reversal invariance, and thus the microscopic reversibility can be tested by the experiment.
Resonant raman scattering and dispersion of polar optical and acoustic phonons in hexagonal inn
Davydov, V. Yu. Klochikhin, A. A.; Smirnov, A. N.; Strashkova, I. Yu.; Krylov, A. S.; Lu Hai; Schaff, William J.; Lee, H.-M.; Hong, Y.-L.; Gwo, S.
2010-02-15
It is shown that a study of the dependence of impurity-related resonant first-order Raman scattering on the frequency of excitation light makes it possible to observe the dispersion of polar optical and acoustic branches of vibrational spectrum in hexagonal InN within a wide range of wave vectors. It is established that the wave vectors of excited phonons are uniquely related to the energy of excitation photon. Frequencies of longitudinal optical phonons E{sub 1}(LO) and A{sub 1}(LO) in hexagonal InN were measured in the range of excitation-photon energies from 2.81 to 1.17 eV and the frequencies of longitudinal acoustic phonons were measured in the range 2.81-1.83 eV of excitation-photon energies. The obtained dependences made it possible to extrapolate the dispersion of phonons A{sub 1}(LO) and E{sub 1}(LO) to as far as the point {Gamma} in the Brillouin zone and estimate the center-band energies of these phonons (these energies have not been uniquely determined so far).
Phononic thermal resistance due to a finite periodic array of nano-scatterers
NASA Astrophysics Data System (ADS)
Trang Nghiêm, T. T.; Chapuis, Pierre-Olivier
2016-07-01
The wave property of phonons is employed to explore the thermal transport across a finite periodic array of nano-scatterers such as circular and triangular holes. As thermal phonons are generated in all directions, we study their transmission through a single array for both normal and oblique incidences, using a linear dispersionless time-dependent acoustic frame in a two-dimensional system. Roughness effects can be directly considered within the computations without relying on approximate analytical formulae. Analysis by spatio-temporal Fourier transform allows us to observe the diffraction effects and the conversion of polarization. Frequency-dependent energy transmission coefficients are computed for symmetric and asymmetric objects that are both subject to reciprocity. We demonstrate that the phononic array acts as an efficient thermal barrier by applying the theory of thermal boundary (Kapitza) resistances to arrays of smooth scattering holes in silicon for an exemplifying periodicity of 10 nm in the 5-100 K temperature range. It is observed that the associated thermal conductance has the same temperature dependence as that without phononic filtering.
Neutron scattering from coupled phonon-impurity modes in KCl1-c(KCN)c
NASA Astrophysics Data System (ADS)
Nicklow, R. M.; Crummett, W. P.; Mostoller, M.; Wood, R. F.
1980-09-01
The hybridization of host-lattice phonons with the internal-energy states of CN- impurities in KCl has been studied by inelastic neutron scattering as a function of temperature between 10 and 100 K for samples with impurity concentrations in the range c=0.4 to 6 at.%. A temperature- and concentration-dependent coupling between phonons with Eg symmetry and the Eg transitions of CN- ions is observed near a frequency of 0.5 THz, a value which is consistent with the energy-level spacings for CN- in KCl as deduced by Beyeler. However, an expected coupling of phonons and CN- transitions with T2g symmetry near the same frequency was not detected. A simple two-level model for the CN- impurity provides a rather good description of the data for the Eg coupled modes for c<2 at.%, but it deviates significantly for larger concentrations. Quasielastic scattering, which has a strong dependence on impurity concentration, temperature, and phonon wave vector, is also observed.
Imaging phonons in a fcc Pu-Ga alloy by thermal diffuse x-ray scattering
NASA Astrophysics Data System (ADS)
Wong, Joe; Wall, M.; Schwartz, A. J.; Xu, R.; Holt, M.; Hong, Hawoong; Zschack, P.; Chiang, T.-C.
2004-05-01
X-ray thermal diffuse scattering intensity patterns from phonons in a fcc δ-Pu-Ga alloy have been recorded using an 18 keV undulator x-ray beam with a beam diameter of 25 μm. The results are consistent with patterns calculated using the Born-von Karman force constant model of lattice dynamics, and support the pronounced softening of the transverse acoustic branch along the [111] direction observed from inelastic x-ray scattering measurements. This work demonstrates the feasibility of using a "large-grain, small beam" approach to study lattice properties, such as phonon dispersion curves, of materials not readily available in the form of large single crystals.
NASA Astrophysics Data System (ADS)
Latour, B.; Volz, S.; Chalopin, Y.
2014-07-01
We demonstrate the existence of a coherent transport of thermal energy in superlattices by introducing a microscopic definition of the phonon coherence length. A criterion is provided to distinguish the coherent transport regime from diffuse interface scattering and discuss how these can be specifically controlled by several physical parameters. Our approach provides a convenient framework for the interpretation of previous thermal conductivity measurements and calculations; it also paves the way for the design of a new class of thermal interface materials.
NASA Astrophysics Data System (ADS)
Essert, Sven; Schneider, Hans Christian
2011-12-01
We theoretically investigate spin-dependent carrier dynamics due to the electron-phonon interaction after ultrafast optical excitation in ferromagnetic metals. We calculate the electron-phonon matrix elements including the spin-orbit interaction in the electronic wave functions and the interaction potential. Using the matrix elements in Boltzmann scattering integrals, the momentum-resolved carrier distributions are obtained by solving their equation of motion numerically. We find that the optical excitation with realistic laser intensities alone leads to a negligible magnetization change, and that the demagnetization due to electron-phonon interaction is mostly due to hole scattering. Importantly, the calculated demagnetization quenching due to this Elliot-Yafet-type depolarization mechanism is not large enough to explain the experimentally observed result. We argue that the ultrafast demagnetization of ferromagnets does not occur exclusively via an Elliott-Yafet type process, i.e., scattering in the presence of the spin-orbit interaction, but is influenced to a large degree by a dynamical change of the band structure, i.e., the exchange splitting.
Chen, Guodong; Zhang, Ruiwen; Sun, Junqiang; Xie, Heng; Gao, Ya; Feng, Danqi; Xiong, Huang
2014-12-29
We propose a scheme for on-chip all optical mode conversion based on forward stimulated Brillouin scattering in a hybrid phononic-photonic waveguide. To describe the mode conversion the theoretical model of the FSBS is established by taking into account the radiation pressure and the electrostriction force simultaneously. The numerical simulation is carried out for the mode conversion from the fundamental mode E11x to the higher-order mode E21x. The results indicate that the mode conversion efficiency is affected by the waveguide length and the input pump light power, and the highest efficiency can reach upto 88% by considering the influence of optical and acoustic absorption losses in the hybrid waveguide. Additionally, the conversion bandwidth with approximate 12.5 THz can be achieved in 1550nm communication band. This mode converter on-chip is a promising device in the integrated optical systems, which can effectively increase the capacity of silicon data busses for on-chip optical interconnections. PMID:25607172
Spin-dependent intravalley and intervalley electron-phonon scatterings in germanium
NASA Astrophysics Data System (ADS)
Liu, Z.; Nestoklon, M. O.; Cheng, J. L.; Ivchenko, E. L.; Wu, M. W.
2013-08-01
The spin-dependent electron-phonon scattering in the L and Γ valleys of germanium crystals has been investigated theoretically. For this purpose, the 16 × 16 k · p Hamiltonian correctly describing the electron dispersion in the vicinity of the L point of the Brillouin zone in germanium in the lowest conduction bands and the highest valence bands has been constructed. This Hamiltonian facilitates the analysis of the spin-dependent properties of conduction electrons. Then, the electron scatterings by phonons in the L and Γ valleys, i.e., intra- L valley, intra-Γ valley, inter- L-Γ valley, and inter- L-L valley scatterings, have been considered successively. The scattering matrix expanded in powers of the electron wave vectors counted from the centers of the valleys has been constructed using the invariant method for each type of processes. The numerical coefficients in these matrices have been found by the pseudopotential method. The partial contributions of the Elliott and Yafet mechanisms to the spin-dependent electron scattering have been analyzed. The obtained results can be used in studying the optical orientation and relaxation of hot electrons in germanium.
Kakodkar, Rohit R.; Feser, Joseph P.
2015-09-07
We present a numerical approach to the solution of elastic phonon-interface and phonon-nanostructure scattering problems based on a frequency-domain decomposition of the atomistic equations of motion and the use of perfectly matched layer (PML) boundaries. Unlike molecular dynamic wavepacket analysis, the current approach provides the ability to simulate scattering from individual phonon modes, including wavevectors in highly dispersive regimes. Like the atomistic Green's function method, the technique reduces scattering problems to a system of linear algebraic equations via a sparse, tightly banded matrix regardless of dimensionality. However, the use of PML boundaries enables rapid absorption of scattered wave energies at the boundaries and provides a simple and inexpensive interpretation of the scattered phonon energy flux calculated from the energy dissipation rate in the PML. The accuracy of the method is demonstrated on connected monoatomic chains, for which an analytic solution is known. The parameters defining the PML are found to affect the performance and guidelines for selecting optimal parameters are given. The method is used to study the energy transmission coefficient for connected diatomic chains over all available wavevectors for both optical and longitudinal phonons; it is found that when there is discontinuity between sublattices, even connected chains of equivalent acoustic impedance have near-zero transmission coefficient for short wavelengths. The phonon scattering cross section of an embedded nanocylinder is calculated in 2D for a wide range of frequencies to demonstrate the extension of the method to high dimensions. The calculations match continuum theory for long-wavelength phonons and large cylinder radii, but otherwise show complex physics associated with discreteness of the lattice. Examples include Mie oscillations which terminate when incident phonon frequencies exceed the maximum available frequency in the embedded nanocylinder, and
Size and temperature dependence of the electron–phonon scattering by donors in nanowire transistors
NASA Astrophysics Data System (ADS)
Bescond, M.; Carrillo-Nuñez, H.; Berrada, S.; Cavassilas, N.; Lannoo, M.
2016-08-01
Due to the constant size reduction, single-donor-based nanowire transistors receive an increasing interest from the semi-conductor industry. In this work we theoretically investigate the coupled influence of electron-phonon scattering, temperature and size (cross-section and channel length) on the properties of such systems. The aim is to determine under what conditions the localized character of the donor has a remarkable impact on the current characteristics. We use a quantum non-equilibrium Green's function approach in which the acoustic electron-phonon scattering is treated through local self-energies. We first show how this widely used approach, valid at high temperatures, can be extended to lower temperatures. Our simulations predict a hysteresis in the current when reducing the temperature down to 150 K. We also find that acoustic phonons degrade the current characteristics while their optical counterparts might have a beneficial impact with an increase of the ON-current. Finally we discuss the influence of nanowire length and cross-section and emphasize the complexity of precisely controlling the dopant level at room temperature.
NASA Astrophysics Data System (ADS)
Lindsay, L.; Broido, D. A.; Reinecke, T. L.
2013-10-01
The interplay between phonon-isotope and phonon-phonon scattering in determining lattice thermal conductivities in semiconductors and insulators is examined using an ab initio Boltzmann transport equation approach. We identify materials with large enhancements to their thermal conductivities with isotopic purification, known as the isotope effect, and we focus in particular on results for beryllium-VI compounds and cubic germanium carbide. We find that germanium carbide and beryllium selenide have very large room temperature isotope effects of 450%, far larger than in any other material. Thus, isotopic purification in these materials gives surprisingly high intrinsic room temperature thermal conductivities, over 1500 Wm-1 K-1 for germanium carbide and over 600 Wm-1 K-1 for beryllium selenide, well above those of the best metals. In compound semiconductors, a large mass ratio of the constituent atoms and large isotope mixture for the heavier atom gives enhanced isotope scattering. A frequency gap between acoustic and optic phonons (also due to a large mass ratio) and bunching of the acoustic phonon branches give weak anharmonic scattering. Combined, weak anharmonic phonon scattering and strong isotope scattering give a large isotope effect in the materials examined here. The physical insights discussed in this work will help guide the efficient manipulation of thermal transport properties of compound semiconductors through isotopic modification.
NASA Astrophysics Data System (ADS)
Holmes, Jesse Curtis
Nuclear data libraries provide fundamental reaction information required by nuclear system simulation codes. The inclusion of data covariances in these libraries allows the user to assess uncertainties in system response parameters as a function of uncertainties in the nuclear data. Formats and procedures are currently established for representing covariances for various types of reaction data in ENDF libraries. This covariance data is typically generated utilizing experimental measurements and empirical models, consistent with the method of parent data production. However, ENDF File 7 thermal neutron scattering library data is, by convention, produced theoretically through fundamental scattering physics model calculations. Currently, there is no published covariance data for ENDF File 7 thermal libraries. Furthermore, no accepted methodology exists for quantifying or representing uncertainty information associated with this thermal library data. The quality of thermal neutron inelastic scattering cross section data can be of high importance in reactor analysis and criticality safety applications. These cross sections depend on the material's structure and dynamics. The double-differential scattering law, S(alpha, beta), tabulated in ENDF File 7 libraries contains this information. For crystalline solids, S(alpha, beta) is primarily a function of the material's phonon density of states (DOS). Published ENDF File 7 libraries are commonly produced by calculation and processing codes, such as the LEAPR module of NJOY, which utilize the phonon DOS as the fundamental input for inelastic scattering calculations to directly output an S(alpha, beta) matrix. To determine covariances for the S(alpha, beta) data generated by this process, information about uncertainties in the DOS is required. The phonon DOS may be viewed as a probability density function of atomic vibrational energy states that exist in a material. Probable variation in the shape of this spectrum may be
Excitons in one-phonon resonant Raman scattering: Fröhlich and interference effects
NASA Astrophysics Data System (ADS)
Cantarero, A.; Trallero-Giner, C.; Cardona, M.
1989-12-01
A theoretical model of resonant Raman scattering including excitons as intermediate states in the process is compared with recent experimental results in some III-V compound semiconductors where the Raman polarizability was obtained in absolute value for several scattering configurations. In particular, Fröhlich (F) interaction and its interference with the deformation potential (DP) one is analyzed in the E0+Δ0 critical point (CP) of GaAs at three different temperatures. Also the E0 and E0+Δ0 CP of GaP and E0+Δ0 of GaSb are analyzed. We show that the inclusion of impurity-induced forbidden LO-phonon Raman scattering is not necessary when excitonic effects are considered. The experimental data of GaAs corresponding to F interaction can be fitted by assuming a Fröhlich constant cF=0.14 eV Aṥ/2. Lifetime broadenings of 12 meV (10 K), 14 meV (100 K), and 28 meV (300 K) are deduced. The lifetime broadening of GaAs and GaSb at 100 K are taken from two-phonon Raman scattering spectra where the incoming and outgoing resonances are well defined. The general features in the comparison with the experiment is that the measured spectra corresponding to F interaction are well fitted; however, the theoretical interference is stronger than the measured one.
NASA Astrophysics Data System (ADS)
Xie, Hong-Yi; Foster, Matthew S.
2016-05-01
We study the electric and thermal transport of the Dirac carriers in monolayer graphene using the Boltzmann-equation approach. Motivated by recent thermopower measurements [F. Ghahari, H.-Y. Xie, T. Taniguchi, K. Watanabe, M. S. Foster, and P. Kim, Phys. Rev. Lett. 116, 136802 (2016), 10.1103/PhysRevLett.116.136802], we consider the effects of quenched disorder, Coulomb interactions, and electron-optical-phonon scattering. Via an unbiased numerical solution to the Boltzmann equation we calculate the electrical conductivity, thermopower, and electronic component of the thermal conductivity, and discuss the validity of Mott's formula and of the Wiedemann-Franz law. An analytical solution for the disorder-only case shows that screened Coulomb impurity scattering, although elastic, violates the Wiedemann-Franz law even at low temperature. For the combination of carrier-carrier Coulomb and short-ranged impurity scattering, we observe the crossover from the interaction-limited (hydrodynamic) regime to the disorder-limited (Fermi-liquid) regime. In the former, the thermopower and the thermal conductivity follow the results anticipated by the relativistic hydrodynamic theory. On the other hand, we find that optical phonons become non-negligible at relatively low temperatures and that the induced electron thermopower violates Mott's formula. Combining all of these scattering mechanisms, we obtain the thermopower that quantitatively coincides with the experimental data.
Using high pressure to study thermal transport and phonon scattering mechanisms
NASA Astrophysics Data System (ADS)
Hohensee, Gregory Thomas
The aerospace industry studies nanocomposites for heat dissipation and moderation of thermal expansion, and the semiconductor industry faces a Joule heating barrier in devices with high power density. My primary experimental tools are the diamond anvil cell (DAC) coupled with time-domain thermoreflectance (TDTR). TDTR is a precise optical method well-suited to measuring thermal conductivities and conductances at the nanoscale and across interfaces. The DAC-TDTR method yields thermal property data as a function of pressure, rather than temperature. This relatively unexplored independent variable can separate the components of thermal conductance and serve as an independent test for phonon-defect scattering models. I studied the effect of non-equilibrium thermal transport at the aluminum-coated surface of an exotic cuprate material Ca9La5Cu 24O41, which boasts a tenfold enhanced thermal conductivity along one crystalline axis where two-leg copper-oxygen spin-ladder structures carry heat in the form of thermalized magnetic excitations. Highly anisotropic materials are of interest for controlled thermal management applications, and the spin-ladder magnetic heat carriers ("magnons") are not well understood. I found that below room temperature, the apparent thermal conductivity of Ca9La5Cu24O41 depends on the frequency of the applied surface heating in TDTR. This occurs because the thermal penetration depth in the TDTR experiment is comparable to the length-scale for the equilibration of the magnons that are the dominant channel for heat conduction and the phonons that dominate the heat capacity. I applied a two-temperature model to analyze the TDTR data and extracted an effective volumetric magnon-phonon coupling parameter g for Ca9La5Cu24O 41 at temperatures from 75 K to 300 K; g varies by approximately two orders of magnitude over this range of temperature and has the value g = 1015 W m-3 K-1 near the peak of the thermal conductivity at T ≈ 180 K. To examine
Spin-flip relaxation via optical phonon scattering in quantum dots
Wang, Zi-Wu; Liu, Lei; Li, Shu-Shen
2013-12-14
Based on the spin-orbit coupling admixture mechanism, we theoretically investigate the spin-flip relaxation via optical phonon scattering in quantum dots by considering the effect of lattice relaxation due to the electron-acoustic phonon deformation potential coupling. The relaxation rate displays a cusp-like structure (or a spin hot spot) that becomes more clearly with increasing temperature. We also calculate the relaxation rate of the spin-conserving process, which follows a Gaussian form and is several orders of magnitude larger than that of spin-flip process. Moreover, we find that the relaxation rate displays the oscillatory behavior due to the interplay effects between the magnetic and spatial confinement for the spin-flip process not for the spin-conserving process. The trends of increasing and decreasing temperature dependence of the relaxation rates for two relaxation processes are obtained in the present model.
Acoustic phonons in chrysotile asbestos probed by high-resolution inelastic x-ray scattering
Mamontov, Eugene; Vakhrushev, S. B.; Kumzerov, Yu. A,; Alatas, A.
2009-01-01
Acoustic phonons in an individual, oriented fiber of chrysotile asbestos (chemical formula Mg{sub 3}Si{sub 2}O{sub 5}(OH){sub 4}) were observed at room temperature in the inelastic x-ray measurement with a very high (meV) resolution. The x-ray scattering vector was aligned along [1 0 0] direction of the reciprocal lattice, nearly parallel to the long axis of the fiber. The latter coincides with [1 0 0] direction of the direct lattice and the axes of the nano-channels. The data were analyzed using a damped harmonic oscillator model. Analysis of the phonon dispersion in the first Brillouin zone yielded the longitudinal sound velocity of (9200 {+-} 600) m/s.
NASA Astrophysics Data System (ADS)
Lü, X.; Schrottke, L.; Grahn, H. T.
2016-06-01
We present scattering rates for electrons at longitudinal optical phonons within a model completely formulated in the Fourier domain. The total intersubband scattering rates are obtained by averaging over the intrasubband electron distributions. The rates consist of the Fourier components of the electron wave functions and a contribution depending only on the intersubband energies and the intrasubband carrier distributions. The energy-dependent part can be reproduced by a rational function, which allows for the separation of the scattering rates into a dipole-like contribution, an overlap-like contribution, and a contribution which can be neglected for low and intermediate carrier densities of the initial subband. For a balance between accuracy and computation time, the number of Fourier components can be adjusted. This approach facilitates an efficient design of complex heterostructures with realistic, temperature- and carrier density-dependent rates.
Phonon scattering limited performance of monolayer MoS{sub 2} and WSe{sub 2} n-MOSFET
Sengupta, Amretashis; Chanana, Anuja; Mahapatra, Santanu
2015-02-15
In this paper we show the effect of electron-phonon scattering on the performance of monolayer (1L) MoS{sub 2} and WSe{sub 2} channel based n-MOSFETs. Electronic properties of the channel materials are evaluated using the local density approximation (LDA) in density functional theory (DFT). For phonon dispersion we employ the small displacement / frozen phonon calculations in DFT. Thereafter using the non-equilibrium Green’s function (NEGF) formalism, we study the effect of electron-phonon scattering and the contribution of various phonon modes on the performance of such devices. It is found that the performance of the WSe{sub 2} device is less impacted by phonon scattering, showing a ballisticity of 83% for 1L-WSe{sub 2} FET for channel length of 10 nm. Though 1L-MoS{sub 2} FET of similar dimension shows a lesser ballisticity of 75%. Also in the presence of scattering there exist a a 21–36% increase in the intrinsic delay time (τ) and a 10–18% reduction in peak transconductance (g{sub m}) for WSe{sub 2} and MoS{sub 2} devices respectively.
NASA Astrophysics Data System (ADS)
Principi, Alessandro; Carrega, Matteo; Lundeberg, Mark B.; Woessner, Achim; Koppens, Frank H. L.; Vignale, Giovanni; Polini, Marco
2014-10-01
Graphene sheets encapsulated between hexagonal boron nitride (hBN) slabs display superb electronic properties due to very limited scattering from extrinsic disorder sources such as Coulomb impurities and corrugations. Such samples are therefore expected to be ideal platforms for highly tunable low-loss plasmonics in a wide spectral range. In this article we present a theory of collective electron density oscillations in a graphene sheet encapsulated between two hBN semi-infinite slabs (hBN/G/hBN). Graphene plasmons hybridize with hBN optical phonons forming hybrid plasmon-phonon modes. We focus on scattering of these modes against graphene's acoustic phonons and hBN optical phonons, two sources of scattering that are expected to play a key role in hBN/G/hBN stacks. We find that at room temperature the scattering against graphene's acoustic phonons is the dominant limiting factor for hBN/G/hBN stacks, yielding theoretical inverse damping ratios of hybrid plasmon-phonon modes of the order of 50-60, with a weak dependence on carrier density and a strong dependence on illumination frequency. We confirm that the plasmon lifetime is not directly correlated with the mobility: In fact, it can be anticorrelated.
Comparing the anomalous phonons in Fe(Te,Se) and (Fe,Ni)(Te,Se) via neutron scattering
NASA Astrophysics Data System (ADS)
Schneeloch, John; Xu, Zhijun; Gu, Genda; Zaliznyak, Igor; Winn, Barry; Rodriguez-Rivera, Jose; Birgeneau, Robert; Xu, Guangyong; Tranquada, John
We studied the anomalous acoustic-type phonons in the Fe(Te,Se) iron-based superconductor family that arise from the (100) Bragg peak, which is forbidden according to the reported crystal structure for these materials. Inelastic neutron scattering was performed on superconducting and non-superconducting crystals of various compositions. The (100) phonons were much weaker in a non-superconducting nickel-doped crystal than in a superconducting crystal with similar selenium fraction, but comparison with another non-superconducting crystal suggests the difference is not simply related to superconductivity. This composition dependence was observed for both transverse and longitudinal phonons. The temperature dependences of the (100) phonons resembled those of conventional phonons. We will discuss these results and possible explanations for the relation between composition and lattice dynamics in this system.
Neutron inelastic scattering measurements of low-energy phonons in the multiferroic BiFeO_{3}
Schneeloch, John A.; Xu, Zhijun; Wen, Jinsheng; Gehring, P. M.; Stock, C.; Matsuda, Masaaki; Winn, Barry L.; Gu, Genda; Shapiro, Stephen M.; Birgeneau, R. J.; Ushiyama, T.; Yanagisawa, Y.; Tomioka, Y.; Ito, T.; Xu, Guangyong
2015-02-10
In this study, we present neutron inelastic scattering measurements of the low-energy phonons in single crystal BiFeO_{3}. The dispersions of the three acoustic phonon modes (LA along [100], TA_{1} along [010], and TA_{2} along [110]) and two low-energy optic phonon modes (LO and TO_{1}) have been mapped out between 300 and 700 K. Elastic constants are extracted from the phonon measurements. The energy linewidths of both TA phonons at the zone boundary clearly broaden when the system is warmed toward the magnetic ordering temperature T_{N}=640 K. In conclusion, this suggests that the magnetic order and low-energy lattice dynamics in this multiferroic material are coupled.
Neutron inelastic scattering measurements of low-energy phonons in the multiferroic BiFeO3
Schneeloch, John A.; Xu, Zhijun; Wen, Jinsheng; Gehring, P. M.; Stock, C.; Matsuda, Masaaki; Winn, Barry L.; Gu, Genda; Shapiro, Stephen M.; Birgeneau, R. J.; et al
2015-02-10
In this study, we present neutron inelastic scattering measurements of the low-energy phonons in single crystal BiFeO3. The dispersions of the three acoustic phonon modes (LA along [100], TA1 along [010], and TA2 along [110]) and two low-energy optic phonon modes (LO and TO1) have been mapped out between 300 and 700 K. Elastic constants are extracted from the phonon measurements. The energy linewidths of both TA phonons at the zone boundary clearly broaden when the system is warmed toward the magnetic ordering temperature TN=640 K. In conclusion, this suggests that the magnetic order and low-energy lattice dynamics in thismore » multiferroic material are coupled.« less
Li, Chen; Ma, Jie; May, Andrew F; Cao, Huibo; Christianson, Andrew D; Ehlers, Georg; Singh, David J; Sales, Brian C; Delaire, Olivier A
2014-01-01
The anharmonic lattice dynamics of rock-salt thermoelectric compounds SnTe and PbTe are investigated with inelastic neutron scattering (INS) and first-principles calculations. The experiments show that, surprisingly, although SnTe is closer to the ferroelectric instability, phonon spectra in PbTe exhibit a more anharmonic character. This behavior is reproduced in first-principles calculations of the temperature-dependent phonon self-energy. Our simulations reveal how the nesting of phonon dispersions induces prominent features in the self-energy, which account for the measured INS spectra and their temperature dependence. We establish that the phase-space for three-phonon scattering processes, rather than just the proximity to the lattice instability, is the mechanism determining the complex spectrum of the transverse-optical ferroelectric mode.
Role of triple phonon excitations in large angle quasi-elastic scattering of very heavy mass systems
NASA Astrophysics Data System (ADS)
Zamrun, Muhammad Firihu
2016-07-01
We study the effect of multi-phonon excitations on large-angle quasi-elastic scattering of massive systems using the full order coupled-channels formalism. We especially investigate the role of triple phonon excitations of the target and projectile nuclei on the quasi-elastic scattering cross-section as well as the barrier distribution for 54Cr, 56Fe, 64Ni and 70Zn + 208Pb systems. It is shown that the calculations taken into account, the triple octupole phonon excitations of the target and triple quadrupole phonon excitations of the projectile for these systems can explain the experimental data of the quasi-elastic cross-section and the quasi-elastic barrier distribution. These results indicate that the coupled-channels formalism is still valid even for the very heavy mass systems.
One-phonon and multiphonon processes in atom-surface scattering
NASA Astrophysics Data System (ADS)
Celli, V.; Himes, D.; Bortolani, V.; Santoro, G.; Toennies, J. P.; Zhang, G.
1991-02-01
We show that previously unexplained features in the TOF spectra of He and Ne scattered from solid surfaces can be attributed to multiphonon processes. We find that a simple Gaussian distribution, obtained by Brako and Newns within the trajectory approximation, accounts rather well for these features. The average energy transfer appearing in this distribution is evaluated by the Baule formula. We obtain an interpolation formula that describes both multiphonon and one-phonon processes, and is consistent with the Distorted Wave Born Approximation for the latter.
Low-temperature lattice-scattering mobility in multiple heterojunctions: Phonon-drag enhancement
Lyo, S.K. )
1991-01-15
The temperature dependence of the low-temperature lattice-scattering mobility (LSM) of a heterojunction as well as a multiple-heterojunction structure is obtained. We show that the LSM of a multiple-heterojunction structure can be significantly enhanced as compared to that of a single heterojunction due to the phonon-drag effect. The high-temperature approximation employed for the LSM in the current literature is found to underestimate significantly the LSM as well as the deformation-potential coefficient.
Density of phonon states in solid parahydrogen from inelastic neutron scattering
NASA Astrophysics Data System (ADS)
Colognesi, D.; Celli, M.; Zoppi, M.
2004-03-01
We have measured the inelastic neutron scattering spectrum of solid parahydrogen (at low pressure and T=13.3 K) using the thermal original spectrometer with cylindrical analyzers spectrometer at the ISIS pulsed neutron source (UK). From the experimental spectrum we have obtained the parahydrogen density of phonon states which has been compared with the estimates available in the literature. The present determination improves substantially the previous experimental scenario from the point of view of both statistics and accuracy. The comparison with the most recent estimate obtained from a quantum mechanical simulation of the molecular dynamics calls for an improvement of the computational methods.
Gutmann, Matthias J.; Graziano, Gabriella; Mukhopadhyay, Sanghamitra; Refson, Keith; von Zimmerman, Martin
2015-01-01
Direct phonon excitation in a neutron time-of-flight single-crystal Laue diffraction experiment has been observed in a single crystal of NaCl. At room temperature both phonon emission and excitation leave characteristic features in the diffuse scattering and these are well reproduced using ab initio phonons from density functional theory (DFT). A measurement at 20 K illustrates the effect of thermal population of the phonons, leaving the features corresponding to phonon excitation and strongly suppressing the phonon annihilation. A recipe is given to compute these effects combining DFT results with the geometry of the neutron experiment. PMID:26306090
Mei, A. B.; Hellman, O.; Schlepuetz, C. M.; Rockett, A.; Chiang, T. -C.; Hultman, L.; Petrov, I.; Greene, J. E.
2015-11-03
Synchrotron reflection x-ray thermal diffuse scattering (TDS) measurements, rather than previously reported transmission TDS, are carried out at room temperature and analyzed using a formalism based upon second-order interatomic force constants and long-range Coulomb interactions to obtain quantitative determinations of MgO phonon dispersion relations (h) over bar omega(j) (q), phonon densities of states g((h) over bar omega), and isochoric temperature-dependent vibrational heat capacities cv (T). We use MgO as a model system for investigating reflection TDS due to its harmonic behavior as well as its mechanical and dynamic stability. Resulting phonon dispersion relations and densities of states are found tomore » be in good agreement with independent reports from inelastic neutron and x-ray scattering experiments. Temperature-dependent isochoric heat capacities cv (T), computed within the harmonic approximation from (h) over bar omega(j) (q) values, increase with temperature from 0.4 x 10-4 eV/atom K at 100 K to 1.4 x 10-4 eV/atom K at 200 K and 1.9 x 10-4 eV/atom K at 300 K, in excellent agreement with isobaric heat capacity values cp (T) between 4 and 300 K. We anticipate that the experimental approach developed here will be valuable for determining vibrational properties of heteroepitaxial thin films since the use of grazing-incidence (θ ≲ θc where θc is the density-dependent critical angle) allows selective tuning of x-ray penetration depths to ≲ 10 nm.« less
NASA Astrophysics Data System (ADS)
Mei, A. B.; Hellman, O.; Schlepütz, C. M.; Rockett, A.; Chiang, T.-C.; Hultman, L.; Petrov, I.; Greene, J. E.
2015-11-01
Synchrotron reflection x-ray thermal diffuse scattering (TDS) measurements, rather than previously reported transmission TDS, are carried out at room temperature and analyzed using a formalism based upon second-order interatomic force constants and long-range Coulomb interactions to obtain quantitative determinations of MgO phonon dispersion relations ℏ ωj (q), phonon densities of states g (ℏ ω ), and isochoric temperature-dependent vibrational heat capacities cv(T ) . We use MgO as a model system for investigating reflection TDS due to its harmonic behavior as well as its mechanical and dynamic stability. Resulting phonon dispersion relations and densities of states are found to be in good agreement with independent reports from inelastic neutron and x-ray scattering experiments. Temperature-dependent isochoric heat capacities cv(T ) , computed within the harmonic approximation from ℏ ωj (q) values, increase with temperature from 0.4 ×10-4eV /atom K at 100 K to 1.4 ×10-4eV /atom K at 200 K and 1.9 ×10-4eV /atom K at 300 K, in excellent agreement with isobaric heat capacity values cp(T ) between 4 and 300 K. We anticipate that the experimental approach developed here will be valuable for determining vibrational properties of heteroepitaxial thin films since the use of grazing-incidence (θ ≲θc , where θc is the density-dependent critical angle) allows selective tuning of x-ray penetration depths to ≲10 nm .
Mei, A. B.; Hellman, O.; Schlepuetz, C. M.; Rockett, A.; Chiang, T. -C.; Hultman, L.; Petrov, I.; Greene, J. E.
2015-11-03
Synchrotron reflection x-ray thermal diffuse scattering (TDS) measurements, rather than previously reported transmission TDS, are carried out at room temperature and analyzed using a formalism based upon second-order interatomic force constants and long-range Coulomb interactions to obtain quantitative determinations of MgO phonon dispersion relations (h) over bar omega(j) (q), phonon densities of states g((h) over bar omega), and isochoric temperature-dependent vibrational heat capacities c_{v} (T). We use MgO as a model system for investigating reflection TDS due to its harmonic behavior as well as its mechanical and dynamic stability. Resulting phonon dispersion relations and densities of states are found to be in good agreement with independent reports from inelastic neutron and x-ray scattering experiments. Temperature-dependent isochoric heat capacities c_{v} (T), computed within the harmonic approximation from (h) over bar omega(j) (q) values, increase with temperature from 0.4 x 10^{-4} eV/atom K at 100 K to 1.4 x 10^{-4} eV/atom K at 200 K and 1.9 x 10^{-4} eV/atom K at 300 K, in excellent agreement with isobaric heat capacity values c_{p} (T) between 4 and 300 K. We anticipate that the experimental approach developed here will be valuable for determining vibrational properties of heteroepitaxial thin films since the use of grazing-incidence (θ ≲ θ_{c} where θ_{c} is the density-dependent critical angle) allows selective tuning of x-ray penetration depths to ≲ 10 nm.
Bulk phonon scattering in perturbed quasi-3D multichannel crystallographic waveguide.
Rabia, M S
2008-11-19
In the present paper, we concentrate on the influence of local defects on scattering properties of elastic waves in perturbed crystalline quasi-three-dimensional nanostructures in the harmonic approximation. Our model consists of three infinite atomic planes, assimilated into a perfect waveguide in which different distributions of scatterers (or defects) are inserted in the bulk. We have investigated phonon transmission and conductance for three bulk defect configurations. The numerical treatment of the problem, based on the Landauer approach, resorts to the matching method initially employed for the study of surface localized phonons and resonances. We present a detailed study of the defect-induced fluctuations in the transmission spectra. These fluctuations can be related to Fano resonances and Fabry-Pérot oscillations. The first is due to the coupling between localized defect states and the perfect waveguide propagating modes whereas the latter results from the interference between incidental and reflected waves. Numerical results reveal the intimate relation between transmission spectra and localized impurity states and provide a basis for the understanding of conductance spectroscopy experiments in disordered mesoscopic systems. PMID:21693856
Khazanov, E. N. Taranov, A. V.; Gainutdinov, R. V.; Akchurin, M. Sh.; Basiev, T. T.; Konyushkin, V. A.; Fedorov, P. P.; Kuznetsov, S. V.; Osiko, V. V.
2010-06-15
The methods of optical, electron, and atomic force microscopy (AFM) are applied to the study of the real structure of optical lithium fluoride ceramic obtained by hot deformation of single crystals. A comparative analysis is carried out of the scattering mechanisms of weakly nonequilibrium thermal phonons at liquid helium temperatures in LiF single crystals and ceramics. It is demonstrated that the phonon scattering in the original single crystals is determined by the forced vibrations of dislocations in the stress field of an elastic plane wave (a phonon), i.e., by the flutter mechanism. As the degree of deformation of the original material increases, the ceramics exhibit a change in the plastic deformation mechanisms, which leads to a decrease in the average size of grains and to an ordered structure. In this case, the dominant scattering is that by intergrain boundaries. The thickness and the acoustic impedance of these boundaries are evaluated.
NASA Astrophysics Data System (ADS)
Tretiak, Sergei
2014-03-01
The exciton scattering (ES) technique is a multiscale approach developed for efficient calculations of excited-state electronic structure and optical spectra in low-dimensional conjugated macromolecules. Within the ES method, the electronic excitations in the molecular structure are attributed to standing waves representing quantum quasi-particles (excitons), which reside on the graph. The exciton propagation on the linear segments is characterized by the exciton dispersion, whereas the exciton scattering on the branching centers is determined by the energy-dependent scattering matrices. Using these ES energetic parameters, the excitation energies are then found by solving a set of generalized ``particle in a box'' problems on the graph that represents the molecule. All parameters can be extracted from quantum-chemical computations of small molecular fragments and tabulated in the ES library for further applications. Subsequently, spectroscopic modeling for any macrostructure within considered molecular family could be performed with negligible numerical effort. The exciton scattering properties of molecular vertices can be further described by tight-binding or equivalently lattice models. The on-site energies and hopping constants are obtained from the exciton dispersion and scattering matrices. Such tight-binding model approach is particularly useful to describe the exciton-phonon coupling, energetic disorder and incoherent energy transfer in large branched conjugated molecules. Overall the ES applications accurately reproduce the optical spectra compared to the reference quantum chemistry results, and make possible to predict spectra of complex macromolecules, where conventional electronic structure calculations are unfeasible.
NASA Astrophysics Data System (ADS)
Shcherbakov, Alexandre S.; Arellanes, Adan Omar
2016-03-01
We study the potentials of a wide-aperture crystalline calomel-made acousto-optical cell. Characterizing this cell is nontrivial due to the chosen regime based on an advanced noncollinear two-phonon light scattering. Recently revealed important features of this phenomenon are essentially exploited in the cell and are investigated in more detail. These features can be observed more easily and simply in tetragonal crystals, e.g., calomel, exhibiting specific acousto-optical nonlinearity caused by the acoustic waves of finite amplitude. This parametric nonlinearity manifests itself at low acoustic powers in calomel possessing linear acoustic attenuation. The formerly identified additional degree of freedom, unique to this regime, is exploited for designing the cell with an eye to doubling the resolution due to two-phonon processes. We clarify the role of varying the central acoustic frequency and acoustic attenuation using that degree of freedom. Then the efficiency of calomel is exploited to expand the cell's bandwidth with a cost of its efficiency. Proof-of-principle experiments confirm the developed approaches and illustrate their applicability to innovative techniques of optical spectrum analysis with the improved resolution. The achieved spectral resolution of 0.205 Å at 405 nm and the resolving power 19,800 are the best for acousto-optical spectrometers dedicated to space or airborne operations to date as far as we know.
Anharmonicity in light scattering by optical phonons in GaAs1-xBix
NASA Astrophysics Data System (ADS)
Joshya, R. S.; Rajaji, V.; Narayana, Chandrabhas; Mascarenhas, A.; Kini, R. N.
2016-05-01
We present a Raman spectroscopic study of GaAs1-xBix epilayers grown by molecular beam epitaxy. We have investigated the anharmonic effect on the GaAs-like longitudinal optical phonon mode ( LOGaAs' ) of GaAs1-xBix for different Bi concentrations at various temperatures. The results are analyzed in terms of the anharmonic damping effect induced by thermal and compositional disorder. We have observed that the anharmonicity increases with Bi concentration in GaAs1-xBix as evident from the increase in the anharmonicity constants. In addition, the anharmonic lifetime of the optical phonon decreases with increasing Bi concentration in GaAs1-xBix.
Dhital, Chetan; Abernathy, Douglas L; Zhu, Gaohua; Ren, Zhifeng; Broido, D.; Wilson, Stephen D
2012-01-01
Inelastic neutron scattering measurements are utilized to explore relative changes in the generalized phonon density of states of nanocrystalline Si1 xGex thermoelectric materials prepared via ball-milling and hot-pressing techniques. Dynamic signatures of Ge clustering can be inferred from the data by referencing the resulting spectra to a density functional theoretical model assuming homogeneous alloying via the virtual-crystal approximation. Comparisons are also presented between as-milled Si nanopowder and bulk, polycrystalline Si where a preferential low-energy enhancement and lifetime broadening of the phonon density of states appear in the nanopowder. Negligible differences are however observed between the phonon spectra of bulk Si andhot-pressed, nanostructured Si samples suggesting that changes to the single-phonon dynamics above 4 meV play only a secondary role in the modified heat conduction of this compound.
Ozawa, Masakuni; Suzuki, Suguru; Loong, C.K.; Nipko, J.C.
1996-12-31
Inelastic neutron scattering was used to study the phonon densities of states of zirconia nanoparticles, the O-H stretch vibrations of physisorbed water molecules, and chemisorbed hydroxyl groups on the surface. Raman scattering was also used to measure the zone-center phonon modes. The observed distinct phonon frequencies and band widths at 10-120 meV reflect the different crystalline symmetries and compositional fluctuations in the small grain and interfacial regions of monoclinic ZrO{sub 2}, tetragonal or mixed cubic and tetragonal rare-earth-modified zirconia. The dynamics of water and hydroxyl groups on varying local structures of these zirconias result in the different frequencies of the O-H stretch vibrations at 400-600 meV.
Raman scattering by confined optical phonons in Si and Ge nanostructures.
Alfaro, Pedro; Cisneros, Rodolfo; Bizarro, Monserrat; Cruz-Irisson, Miguel; Wang, Chumin
2011-03-01
A microscopic theory of the Raman scattering based on the local bond-polarizability model is presented and applied to the analysis of phonon confinement in porous silicon and porous germanium, as well as nanowire structures. Within the linear response approximation, the Raman shift intensity is calculated by means of the displacement-displacement Green's function and the Born model, including central and non-central interatomic forces. For the porous case, the supercell method is used and ordered pores are produced by removing columns of Si or Ge atoms from their crystalline structures. This microscopic theory predicts a remarkable shift of the highest-frequency of first-order Raman peaks towards lower energies, in comparison with the crystalline case. This shift is discussed within the quantum confinement framework and quantitatively compared with the experimental results obtained from porous silicon samples, which were produced by anodizing p--type (001)-oriented crystalline Si wafers in a hydrofluoric acid bath. PMID:21270988
NASA Astrophysics Data System (ADS)
Qing, Lan; Dery, Hanan; Li, Jing; Appelbaum, Ian
2012-02-01
We derive a simple approximate expression of the spin lifetime of drifting electrons in silicon. This expression agrees well with elaborate Monte Carlo simulations of the charge transport and spin relaxation of conduction electrons heated by the electric field. Already at low temperatures, the drifting electrons become hot enough to undergo f-processes (scattering between valleys of different crystal axes following emission of a shortwave phonon). Such a process involves a direct coupling of valence and conduction bands and dominates the spin relaxation. A sharp decrease of spin lifetime can then be expected in intermediate electric fields in between ˜100 V/cm and ˜1 kV/cm. When electrons are transported between a spin injector and a spin-resolved detector, the decrease of both transit time and spin lifetime results in a non-monotonic behavior of the detected spin polarization with the electric field. The theory shows excellent agreement with empirical results.
NASA Technical Reports Server (NTRS)
Hu, Qing (Inventor); Williams, Benjamin S. (Inventor)
2007-01-01
The present invention provides quantum cascade lasers and amplifier that operate in a frequency range of about 1 Terahertz to about 10 Terahertz. In one aspect, a quantum cascade laser of the invention includes a semiconductor heterostructure that provides a plurality of lasing modules connected in series. Each lasing module includes a plurality of quantum well structure that collectively generate at least an upper lasing state, a lower lasing state, and a relaxation state such that the upper and the lower lasing states are separated by an energy corresponding to an optical frequency in a range of about 1 to about 10 Terahertz. The lower lasing state is selectively depopulated via resonant LO-phonon scattering of electrons into the relaxation state.
NASA Technical Reports Server (NTRS)
Hu, Qing (Inventor); Williams, Benjamin S. (Inventor)
2009-01-01
The present invention provides quantum cascade lasers and amplifier that operate in a frequency range of about 1 Terahertz to about 10 Terahertz. In one aspect, a quantum cascade laser of the invention includes a semiconductor heterostructure that provides a plurality of lasing modules connected in series. Each lasing module includes a plurality of quantum well structure that collectively generate at least an upper lasing state, a lower lasing state, and a relaxation state such that the upper and the lower lasing states are separated by an energy corresponding to an optical frequency in a range of about 1 to about 10 Terahertz. The lower lasing state is selectively depopulated via resonant LO-phonon scattering of electrons into the relaxation state.
Carrier scattering processes and low energy phonon spectroscopy in hybrid perovskites crystals
NASA Astrophysics Data System (ADS)
Even, Jacky; Paofai, Serge; Bourges, Philippe; Letoublon, Antoine; Cordier, Stéphane; Durand, Olivier; Katan, Claudine
2016-03-01
Despite the wealth of research conducted the last three years on hybrid organic perovskites (HOP), several questions remain open including: to what extend the organic moiety changes the properties of the material as compared to allinorganic (AIP) related perovskite structures. To ultimately reach an answer to this question, we have recently introduced two approaches that were designed to take the stochastic molecular degrees of freedom into account, and suggested that the high temperature cubic phase of HOP and AIP is an appropriate reference phase to rationalize HOP's properties. In this paper, we recall the main concepts and discuss more specifically the various possible couplings between charge carriers and low energy excitations such as acoustic and optical phonons. As available experimental or simulated data on low energy excitations are limited, we also present preliminary neutron scattering and ultrasonic measurements obtained and freshly prepared single crystals of CH3NH3PbBr3.
NASA Astrophysics Data System (ADS)
Donovan, Brian F.; Sachet, Edward; Maria, Jon-Paul; Hopkins, Patrick E.
2016-01-01
Understanding the impact and complex interaction of thermal carrier scattering centers in functional oxide systems is critical to their progress and application. In this work, we study the interplay among electron and phonon thermal transport, mass-impurity scattering, and phonon-vacancy interactions on the thermal conductivity of cadmium oxide. We use time domain thermoreflectance to measure the thermal conductivity of a set of CdO thin films doped with Dy up to the saturation limit. Using measurements at room temperature and 80 K, our results suggest that the enhancement in thermal conductivity at low Dy concentrations is dominated by an increase in the electron mobility due to a decrease in oxygen vacancy concentration. Furthermore, we find that at intermediate doping concentrations, the subsequent decrease in thermal conductivity can be ascribed to a large reduction in phononic thermal transport due to both point defect and cation-vacancy scattering. With these results, we gain insight into the complex dynamics driving phonon scattering and resulting thermal transport in functional oxides.
NASA Astrophysics Data System (ADS)
Crepaldi, A.; Cilento, F.; Ressel, B.; Cacho, C.; Johannsen, J. C.; Zacchigna, M.; Berger, H.; Bugnon, Ph.; Grazioli, C.; Turcu, I. C. E.; Springate, E.; Kern, K.; Grioni, M.; Parmigiani, F.
2013-09-01
The nature of the Dirac quasiparticles in topological insulators calls for a direct investigation of the electron-phonon scattering at the surface. By comparing time-resolved ARPES measurements of the topological insulator Bi2Se3 with different probing depths, we show that the relaxation dynamics of the electronic temperature of the conduction band is much slower at the surface than in the bulk. This observation suggests that surface phonons are less effective in cooling the electron gas in the conduction band.
Surface defects characterization in a quantum wire by coherent phonons scattering
Rabia, M. S.
2015-03-30
The influence of surface defects on the scattering properties of elastic waves in a quasi-planar crystallographic waveguide is studied in the harmonic approximation using the matching method formalism. The structural model is based on three infinite atomic chains forming a perfect lattice surmounted by an atomic surface defect. Following the Landauer approach, we solve directly the Newton dynamical equation with scattering boundary conditions and taking into account the next nearest neighbour’s interaction. A detailed study of the defect-induced fluctuations in the transmission spectra is presented for different adatom masses. As in the electronic case, the presence of localized defect-induced states leads to Fano-like resonances. In the language of mechanical vibrations, these are called continuum resonances. Numerical results reveal the intimate relation between transmission spectra and localized defect states and provide a basis for the understanding of conductance spectroscopy experiments in disordered mesoscopic systems. The results could be useful for the design of phononic devices.
The phonon density of states of (alpha) and (delta)-Plutonium by inelastic x-ray scattering
Manley, M E; Said, A; Fluss, M J; Wall, M; Lashley, J C; Alatas, A; Moore, K T
2008-10-08
Inelastic x-ray scattering measurements of the phonon density of states (DOS) were performed on polycrystalline samples of pure {alpha}-Pu and {delta}-Pu{sub 0.98}Ga{sub 0.02} at room temperature. The heat capacity of {alpha}-Pu is well reproduced by contributions calculated from the measured phonon DOS plus conventional thermal expansion and electronic contributions, showing that {alpha}-Pu is a 'well-behaved' metal in this regard. A comparison of the phonon DOS of the two phases at room temperature surprised us in that the vibrational entropy difference between them is only a quarter of the total entropy difference expected from known thermodynamic measurements. The missing entropy is too large to be accounted for by conventional electronic entropy and evidence from the literature rules out a contribution from spin fluctuations. Possible alternative sources for the missing entropy are discussed.
Phonon softening near the structural transition in BaFe2As2 observed by inelastic x-ray scattering
Niedziela, Jennifer L; Parshall, D; Lokshin, Konstantin A; Safa-Sefat, Athena; Alatas, A; Egami, Takeshi
2011-01-01
In this work we present the results of an inelastic x-ray scattering experiment detailing the behavior of the transverse acoustic [110] phonon in BaFe{sub 2}As{sub 2} as a function of temperature. When cooling through the structural transition temperature, the transverse acoustic phonon energy is reduced from the value at room temperature, reaching a maximum shift near inelastic momentum transfer q = 0.1. This softening of the lattice results in a change of the symmetry from tetragonal to orthorhombic at the same temperature as the transition to long-range antiferromagnetic order. While the lattice distortion is minor, the anisotropy in the magnetic exchange constants in pnictide parent compounds is large. We suggest mechanisms of electron-phonon coupling to describe the interaction between the lattice softening and the onset of magnetic ordering.
Neutron scattering from coupled phonon-impurity modes in KCl/sub 1-c/(KCN)/sub c/
Nicklow, R.M.; Crummett, W.P.; Mostoller, M.; Wood, R.F.
1980-09-15
The hybridization of host-lattice phonons with the internal-energy states of CN/sup -/ impurities in KCl has been studied by inelastic neutron scattering as a function of temperature between 10 and 100 K for samples with impurity concentrations in the range c=0.4 to 6 at. %. A temperature- and concentration-dependent coupling between phonons with E/sub g/ symmetry and the E/sub g/ transitions of CN/sup -/ ions is observed near a frequency of 0.5 THz, a value which is consistent with the energy-level spacings for CN/sup -/ in KCl as deduced by Beyeler. However, an expected coupling of phonons and CN/sup -/ transitions with T/sub 2g/ symmetry near the same frequency was not detected. A simple two-level model for the CN/sup -/ impurity provides a rather good description of the data for the E/sub g/ coupled modes for c<2 at. %, but it deviates significantly for larger concentrations. Quasielastic scattering, which has a strong dependence on impurity concentration, temperature, and phonon wave vector, is also observed.
NASA Astrophysics Data System (ADS)
Sääskilahti, K.; Oksanen, J.; Tulkki, J.; Volz, S.
2014-10-01
A detailed understanding of the vibrational heat transfer mechanisms between solids is essential for the efficient thermal engineering and control of nanomaterials. We investigate the frequency dependence of anharmonic scattering and interfacial thermal conduction between two acoustically mismatched solids in planar contact by calculating the spectral decomposition of the heat current flowing through an interface between two materials. The calculations are based on analyzing the correlations of atomic vibrations using the data extracted from nonequilibrium molecular dynamics simulations. Inelastic effects arising from anharmonic interactions are shown to significantly facilitate heat transfer between two mass-mismatched face-centered-cubic lattices even at frequencies exceeding the cutoff frequency of the heavier material due to (i) enhanced dissipation of evanescent vibrational modes and (ii) frequency-doubling and frequency-halving three-phonon energy transfer processes at the interface. The results provide substantial insight into interfacial energy transfer mechanisms, especially at high temperatures, where inelastic effects become important and other computational methods are ineffective.
Discrete states and carrier-phonon scattering in quantum dot population dynamics
Man, Minh Tan; Lee, Hong Seok
2015-01-01
The influence of the growth conditions of multilayer CdTe/ZnTe quantum dots (QDs) on Si substrate upon their carrier dynamics is studied using intensity integration and broadening photoluminescence. The unusual temperature dependence of the line broadening is explained using a model for interband transitions that involves a lowest discrete electronic state (1Se) with different discrete hole states (1S3/2 and 2S3/2) and a 1P transition. These transitions are expected to play a critical role in both the thermally activated energy and the line broadening of the QDs. We also demonstrate that a thermally activated transition between two different states occurs with band low-temperature quenching, with values separated by 5.8–16 meV. The main nonradiative process is thermal escape assisted by carrier scattering via emission of longitudinal phonons through the hole states at high temperature, with an average energy of 19.3–20.2 meV. PMID:25652600
Jarlov, C; Wodey, É; Lyasota, A; Calic, M; Gallo, P; Dwir, B; Rudra, A; Kapon, E
2016-08-12
Using site-controlled semiconductor quantum dots (QDs) free of multiexcitonic continuum states, integrated with photonic crystal membrane cavities, we clarify the effects of pure dephasing and phonon scattering on exciton-cavity coupling in the weak-coupling regime. In particular, the observed QD-cavity copolarization and cavity mode feeding versus QD-cavity detuning are explained quantitatively by a model of a two-level system embedded in a solid-state environment. PMID:27563983
NASA Astrophysics Data System (ADS)
Jarlov, C.; Wodey, É.; Lyasota, A.; Calic, M.; Gallo, P.; Dwir, B.; Rudra, A.; Kapon, E.
2016-08-01
Using site-controlled semiconductor quantum dots (QDs) free of multiexcitonic continuum states, integrated with photonic crystal membrane cavities, we clarify the effects of pure dephasing and phonon scattering on exciton-cavity coupling in the weak-coupling regime. In particular, the observed QD-cavity copolarization and cavity mode feeding versus QD-cavity detuning are explained quantitatively by a model of a two-level system embedded in a solid-state environment.
Effect of extended strain fields on point defect phonon scattering in thermoelectric materials.
Ortiz, Brenden R; Peng, Haowei; Lopez, Armando; Parilla, Philip A; Lany, Stephan; Toberer, Eric S
2015-07-15
The design of thermoelectric materials often involves the integration of point defects (alloying) as a route to reduce the lattice thermal conductivity. Classically, the point defect scattering strength follows from simple considerations such as mass contrast and the presence of induced strain fields (e.g. radius contrast, coordination changes). While the mass contrast can be easily calculated, the associated strain fields induced by defect chemistry are not readily predicted and are poorly understood. In this work, we use classical and first principles calculations to provide insight into the strain field component of phonon scattering from isoelectronic point defects. Our results also integrate experimental measurements on bulk samples of SnSe and associated alloys with S, Te, Ge, Sr and Ba. These efforts highlight that the strength and extent of the resulting strain field depends strongly on defect chemistry. Strain fields can have a profound impact on the local structure. For example, in alloys containing Ba, the strain fields have significant spatial extent (1 nm in diameter) and produce large shifts in the atomic equilibrium positions (up to 0.5 Å). Such chemical complexity suggests that computational assessment of point defects for thermal conductivity depression should be hindered. However, in this work, we present and verify several computational descriptors that correlate well with the experimentally measured strain fields. Furthermore, these descriptors are conceptually transparent and computationally inexpensive, allowing computation to provide a pivotal role in the screening of effective alloys. The further development of point defect engineering could complement or replace nanostructuring when optimizing the thermal conductivity, offering the benefits of thermodynamic stability, and providing more clearly defined defect chemistry. PMID:26145414
Multi-Enhanced-Phonon Scattering Modes in Ln-Me-A Sites co-substituted LnMeA11O19 Ceramics
Lu, Haoran; Wang, Chang-An; Huang, Yong; Xie, Huimin
2014-01-01
Authors reported an effective path to decrease the thermal conductivity while to increase the coefficient of thermal expansion, thus enhancing the thermo-physical properties of the LnMeA11O19-type magnetoplumbite LaMgAl11O19 by simultaneously substituting La3+, Mg2+ and Al3+ ions with large ionic radius Ba2+, Zn2+ and Ti4+, respectively. The mechanism behind the lowered thermal conductivity was mainly due to the multi-enhanced-phonon scattering modes in Ln-Me-A sites co-substituted LnMeA11O19 ceramics. These modes involve the following four aspects, namely, point defect mechanism, the intrinsic scattering in the complex crystal cell and materials with stepped surface to localize phonon vibrational modes, as well as nano-platelet-like structure to incorporate additional grain boundary scattering. This study provides novel thoughts for promising candidate materials of even lower thermal conductivity for the next generation thermal barrier coatings. PMID:25351166
NASA Astrophysics Data System (ADS)
Basak, Tista; Rao, Mala N.; Chaplot, S. L.; Salke, Nilesh; Rao, Rekha; Dhanasekaran, R.; Rajarajan, A. K.; Rols, S.; Mittal, R.; Jayakrishnan, V. B.; Sastry, P. U.
2014-01-01
Inelastic neutron scattering, Raman and X-ray diffraction measurements coupled with lattice dynamical calculations (employing a semi-empirical transferable potential model) have been carried out to gain a detailed understanding of the peculiar vibrational spectrum exhibited by the mixed crystal ZnS1-xSex. Raman scattering measurements performed over a varying range of temperature (100-800 K) and pressure (up to 13 GPa) have confirmed that the additional mode observed in the spectra are visible over the entire range of temperature and pressure. Correlation of the individual motions of atoms (obtained from computed total and partial phonon density of states) with the inelastic neutron scattering measurements (carried out over the entire Brillouin zone) have then indicated that the existence of the additional mode in ZnS1-xSex is due to the vibrations of the Se atom being in resonance with that of the S atom. Further, it has been shown that the presence of this additional mode can be tuned by varying the mass of the atom at the Se site. In addition, an analysis of bond-length distribution with increasing Se concentration have elucidated that bond-length spread is not responsible for the presence of the additional mode. An analysis of the peak shifts of the Raman modes with temperature and pressure indicate that the anharmonicity of the vibrational modes increases with increasing compositional disorder. This is attributed to the fact that increasing Se concentration gives rise to a distribution of bond-lengths in ZnS1-xSex, which is responsible for this compositional disorder induced anharmonicity. Our computations have thus revealed that mass of the anion is responsible for the presence of additional mode while bond-length distribution gives rise to the existence of compositional disorder induced anharmonicity in ZnS1-xSex. Further, it is observed that the contribution of explicit anharmonicity to the total anharmonicity becomes dominant at higher temperatures. This
NASA Astrophysics Data System (ADS)
Nag Chowdhury, Basudev; Chattopadhyay, Sanatan
2016-09-01
In the current work, the impact of electron-phonon scattering phenomena on the transport behaviour of silicon nanowire field-effect-transistors with sub-mean free path channel length has been investigated by developing a theoretical model that incorporates the responses of carrier effective mass mismatch between the channel and source/drain. For this purpose, a set of relevant quantum field equations has been solved by non-equilibrium Green's function formalism. The obtained device current for a particular set of biases is found to decrease due to phonon scattering below a certain doping level of source/drain, above which it is observed to enhance anomalously. Analyses of the quantified scattering lifetime and power dissipation at various confinement modes of the device indicates that such unusual enhancement of current is originated from the power served by phonons instead of associated decay processes. The power generation has been observed to improve by using high-k materials as gate insulator. Such results may contribute significantly to the future nano-electronic applications for energy harvesting.
Nakayama, Masaaki Ohno, Tatsuya; Furukawa, Yoshiaki
2015-04-07
We have systematically investigated the photoluminescence (PL) dynamics of free excitons in GaAs/Al{sub 0.3}Ga{sub 0.7}As single quantum wells, focusing on the energy relaxation process due to exciton–acoustic-phonon scattering under non-resonant and weak excitation conditions as a function of GaAs-layer thickness from 3.6 to 12.0 nm and temperature from 30 to 50 K. The free exciton characteristics were confirmed by observation that the PL decay time has a linear dependence with temperature. We found that the free exciton PL rise rate, which is the reciprocal of the rise time, is inversely linear with the GaAs-layer thickness and linear with temperature. This is consistent with a reported theoretical study of the exciton–acoustic-phonon scattering rate in the energy relaxation process in quantum wells. Consequently, it is conclusively verified that the PL rise rate is dominated by the exciton–acoustic-phonon scattering rate. In addition, from quantitative analysis of the GaAs-layer thickness and temperature dependences, we suggest that the PL rise rate reflects the number of exciton–acoustic-phonon scattering events.
NASA Astrophysics Data System (ADS)
Greenstein, Abraham; Hudiono, Yeny; Graham, Samuel; Nair, Sankar
2010-03-01
We present a systematic study to investigate the effects of nonframework cations and the role of phonon scattering mechanisms on the thermal transport properties of zeolite LTA, via experiment and semiempirical lattice dynamics calculations. Our study is motivated by the increasing interest in accurate measurements and mechanistic understanding of the thermal transport properties of zeolite materials. The presence of a nanostructured pore network, extra-framework cations, and tunable framework structure and composition confer interesting thermophysical properties to these materials, making them a good model system to investigate thermal transport in complex materials. Continuous films of zeolite LTA with different nonframework cations (Na+, K+, and Ca+2) were synthesized and characterized. The thermal conductivity was measured using the three-omega method over a wide range of temperature (150-450 K). These are the first thermal conductivity measurements performed on bulk LTA, so they are more accurate than previous measurements, which involved the use of compacted zeolite powders. Our data showed significant dependence of the thermal conductivity on the extra-framework cations as well the temperature. The thermal conductivities of the zeolite LTA samples were modeled with the relaxation time approximation to the Boltzmann transport equation. The full phonon spectra for each type of LTA zeolite were calculated and used in conjunction with semiempirical relaxation time expressions to calculate the thermal conductivity. The results both validated, and suggested the limitations of, this modeling approach. Optical phonons dominated the thermal conductivity and boundarylike scattering was found to be the strongest phonon scattering mechanism, as also observed in MFI zeolite.
NASA Astrophysics Data System (ADS)
Branlund, J. M.; Hofmeister, A.; Dong, J.
2013-12-01
Over the course of several years, we have measured heat transport to high temperatures for a large number (ca. 200) of minerals, rocks, glasses and melts using laser flash analysis which eliminates systematic errors (contact losses and boundary-to-boundary radiative transfer gains) that limit utility of conventional, contact techniques. The database is large enough to elucidate patterns. For most samples and particularly for our >60 non-metallic, large single-crystals, >30 glasses and >12 polycrystals, we show that thermal diffusivity is consistently represented by D(T) =F/T ^G + HT, permitting confident extrapolation from conditions in the laboratory to those in the mantle. The two distinct temperature terms describing D(T) suggest that two microscopic mechanisms of conduction exist in the electrical insulators explored. We propose that phonon scattering (the F/T^G term) sums with radiative diffusion of infrared (IR) light in the form of polaritons (the HT term). Speeds near that of sound over unit cell scale lengths exist for the polariton mechanism due to phonon-photon coupling, thereby distinguishing this proposed mechanism from high frequency diffusive radiative transfer which travels near the speed of light, and only is important following transient heating. For 63 single-crystals and many glasses unaffected by disordering or reconstructive phase transitions, G ranges from 0.3 to 2, depending on structure, and H is ~0.0001/ K, and so HT crosses F/T^G by ~1300 K (for most oxides), meaning that radiative diffusion of IR light is more important than phonon scattering inside the Earth. Importantly, the increase in heat transport due to elevated temperature is augmented by the increase due to high P inside planets, providing stability against convection. The popular view of a vigorously convecting interior needs revisiting, given known feedback in the temperature equation and the large size of the HT term. To understand the microscopic basis of HT term, we re
NASA Astrophysics Data System (ADS)
Jana, R. N.; Meikap, A. K.
2016-05-01
The results of a comprehensive study of weak electron localization (WEL) and electron-electron interaction (EEI) effects in disordered V75X25 (X = Pd, Al) alloys has been reported. The resistivity in absence of magnetic field shows a minimum at temperature T = Tm and follows T1/2 law within the temperature range 5K ≤ T ≤ Tm, which suggests predominant EEI effect. Magnetoresistivity is positive due to strong spin-orbit interaction. The dephasing scattering time is dominated by the electron-phonon scattering. The electron-phonon scattering rate shows quadratic temperature dependence behavior, which is explained by the theory of incomplete dragging at the random scattering potential by phonons. The zero temperature scattering time strongly depends on the disorder and its magnitude decreases with increasing disorder.
NASA Astrophysics Data System (ADS)
Krowne, Clifford M.
1983-05-01
A simple matrix element is used to approximate electron-acoustic phonon scattering between different electron subbands i in the n channel of a (100) surface silicon MOSFET (metal-oxide-semiconductor field-effect transistor) device. This matrix element is used to determine the form of the electron power loss Pij in a i→j intersubband transition. P10 is calculated for TL =4.2 °K lattice temperature and electron temperatures Te between 4.4 °K and 18 °K when the electron inversion density Ninv =(3.76-10.0)×1011 cm-2 and an acceptor density NA =1014/cm3, and compared to Fang and Fowler's experimental data (which is put into the form of an experimental power loss Pexp). This is justified since the total power loss P due to intrasubband scattering as well as other Pij terms besides P10 is small. It is found that good to excellent fits between P10 and Pexp occur by adjusting the separation Δɛ10 between the lowest two circular subband edges. Δɛ10 is between 5.2 and 9.4 meV, and the electron-phonon deformation coupling constant D≊3.5 eV. The values of Δɛ10 obtained in such a manner roughly agree with Stern's theoretical self-consistent results. P10 is very sensitive to both Δɛ10 and to the effective mass for motion parallel to the surface m1 with the results implying that m1≊0.19m0 (m0=free electron rest mass). If one wants to find the contribution of intersubband scattering to P at higher TL, the formalism should still be applicable, although the approach could be much more complicated due to the addition of new Pij terms coming from both higher subbands and new scattering agents such as optical modes.
NASA Astrophysics Data System (ADS)
Romero-Rochín, Víctor; Koehl, Richard M.; Brennan, Ciaran J.; Nelson, Keith A.
1999-08-01
We study theoretically the generation of coherent, anharmonic phonon-polariton responses through impulsive stimulated Raman scattering with intense, crossed ultrafast excitation pulses. We find that the refractive index appears modulated at the stimulated scattering wave vector and the corresponding phonon-polariton frequency, and, due to anharmonicity, at stimulated scattering wave vector overtones and their corresponding frequencies. A realistic model of the soft lattice vibrational mode of the ferroelectric crystal lithium tantalate is considered in detail. Specific predictions for the magnitudes of different wave vector overtone contributions to the lattice displacement are made compared to experimental observations of anharmonic lattice responses.
NASA Astrophysics Data System (ADS)
Savic, Ivana; Murphy, Ronan; Murray, Eamonn; Fahy, Stephen
Efficient thermoelectric energy conversion is highly desirable as 60% of the consumed energy is wasted as heat. Low lattice thermal conductivity is one of the key factors leading to high thermoelectric efficiency of a material. However, the major obstacle in the design of such materials is the difficulty in efficiently scattering phonons across the frequency spectrum. Using first principles calculations, we predict that driving PbTe materials close to a Peierls-like phase transition could be a powerful strategy to solve this problem. We illustrate this concept by applying tensile [001] strain to PbTe and its alloys with another rock-salt IV-VI material, PbSe; and by alloying PbTe with a IV-VI Peierls-distorted material, GeTe. This induces extremely soft optical modes, which increase acoustic-optical phonon coupling and decrease phonon lifetimes at all frequencies. We show that PbTe, Pb(Se,Te) and (Pb,Ge)Te alloys driven near the phase transition in the described manner could have the lattice thermal conductivity considerably lower than that of PbTe. The proposed concept may open new opportunities for the development of more efficient thermoelectric materials. This work was supported by Science Foundation Ireland and the Marie-Curie Action COFUND under Starting Investigator Research Grant 11/SIRG/E2113.
Neumann-Cosel, P. von; Burda, O.; Kuhar, M.; Lenhardt, A.; Ponomarev, V. Yu.; Richter, A.; Wambach, J.; Botha, N. T.; Fearick, R. W.; Carter, J.; Sideras-Haddad, E.; Foertsch, S. V.; Neveling, R.; Smit, F. D.; Fransen, C.; Fujita, H.; Pietralla, N.
2006-03-13
High-resolution inelastic electron (performed at the S-DALINAC) and proton (performed at iThemba LABS) scattering experiments on 92Zr and 94Mo with emphasis on E2 transitions are presented The measured form factors and angular distributions provide a measure for the F-spin purity, respectively the isovector nature, of the proposed one-phonon mixed symmetry states and furthermore provide a sensitive test of a possible two-phonon character of excited 2+ states.
Yoshida, Kyohei; Hachiya, Kan; Okumura, Kensuke; Mishima, Kenta; Inukai, Motoharu; Torgasin, Konstantin; Omer, Mohamed; Sonobe, Taro; Zen, Heishun; Negm, Hani; Kii, Toshiteru; Masuda, Kai; Ohgaki, Hideaki
2013-10-28
Mode-selective phonon excitation by a mid-infrared laser (MIR-FEL) is demonstrated via anti-Stokes Raman scattering measurements of 6H-silicon carbide (SiC). Irradiation of SiC with MIR-FEL and a Nd-YAG laser at 14 K produced a peak where the Raman shift corresponds to a photon energy of 119 meV (10.4 μm). This phenomenon is induced by mode-selective phonon excitation through the irradiation of MIR-FEL, whose photon energy corresponds to the photon-absorption of a particular phonon mode.
Additivity of light-scattering patterns of aggregated biological particles
NASA Astrophysics Data System (ADS)
Moskalensky, Alexander E.; Strokotov, Dmitry I.; Chernyshev, Andrei V.; Maltsev, Valeri P.; Yurkin, Maxim A.
2014-08-01
The paper is focused on light scattering by aggregates of optically soft particles with a size larger than the wavelength, in particular, blood platelets. We conducted a systematic simulation of light scattering by dimers and larger aggregates of blood platelets, each modeled as oblate spheroids, using the discrete dipole approximation. Two-dimensional (2-D) light scattering patterns (LSPs) and internal fields showed that the multiple scattering between constituent particles can be neglected. Additionally, we derived conditions of the scattering angle and orientation of the dimer, under which the averaging of the 2-D LSPs over the azimuthal scattering angle washes out interference in the far field, resulting in averaged LSPs of the aggregate being equal to the sum of that for its constituents. We verified theoretical conclusions using the averaged LSPs of blood platelets measured with the scanning flow cytometer (SFC). Moreover, we obtained similar results for a model system of aggregates of polystyrene beads, studied both experimentally and theoretically. Finally, we discussed the potential of discriminating platelet aggregates from monomers using the SFC.
NASA Astrophysics Data System (ADS)
Shcherbakov, Alexandre S.; Arellanes, Adan O.
2016-03-01
Principally new features of the non-collinear two-phonon light scattering governed by elastic waves of finite amplitude in birefringent bulk crystals are detected and observed. The main goals of our investigations are to reveal novel important details inherent in the nonlinearity of this effect and to study properties of similar parametric nonlinearity both theoretically and experimentally in wide-aperture crystals with moderate linear acoustic attenuation. An additional degree of freedom represented by the dispersive birefringence factor, which can be distinguished within this nonlinear phenomenon, is characterized. This physical degree of freedom gives us a one-of-a-kind opportunity to apply the strongly non-linear two-phonon light scattering in practice for the first time. The local unit-level maxima in the distribution of light scattered into the second order appear periodically as the acoustic power density grows. It makes possible to identify a few transfer function profiles peculiar to these maxima in the isolated planes of angular-frequency mismatches. These maxima give us an opportunity to choose the desirable profile for the transfer function at the fixed angle of incidence for the incoming light beam with a wide spectrum .The needed theoretical analysis is developed and proof-of-principle experiments, performed with a specially designed wide-aperture acousto-optical cell made of the calomel (α-Hg2Cl2) crystal, are presented. The obtained spectral resolution ~0.235 Å at 405 nm (i.e. the resolving power ~17,200) can be compared with the most advanced acousto-optical spectrometers for space/airborne operations. Evidently, our results with the calomel-based acousto-optical cell look like the best we can mention at the moment.
Liu, Jie; Xu, Xu; Anantram, M.P.
2014-09-01
The electron transport through ultra-scaled amorphous phase change material (PCM) GeTe is investigated by using ab initio molecular dynamics, density functional theory, and non-equilibrium Green’s function, and the inelastic electron–phonon scattering is accounted for by using the Born approximation. It is shown that, in ultra-scaled PCM device with 6 nm channel length, < 4 % of the energy carried by the incident electrons from the source is transferred to the atomic lattice before reaching the drain, indicating that the electron transport is largely elastic. Our simulation results show that the inelastic electron–phonon scattering, which plays an important role to excite trapped electrons in bulk PCM devices, exerts very limited influence on the current density value and the shape of current–voltage curve of ultra-scaled PCM devices. The analysis reveals that the Poole–Frenkel law and the Ohm’s law, which are the governing physical mechanisms of the bulk PCM devices, cease to be valid in the ultra-scaled PCM devices.
Muñoz, Jorge A.; Fultz, Brent
2015-07-23
Recent measurements of the phonon spectra of several Au-rich alloys of face-centered-cubic Fe-Au using inelastic neutron scattering and nuclear-resonant inelastic x-ray scattering are summarized. The Wills-Harrison model, accounting for charge transfer upon alloying, is used to explain the observed negative excess vibrational entropy of mixing, which increases the miscibility gap temperature in the system by an estimated maximum of 550 K and we adjudicate to a charge transfer from the Fe to the Au atoms that results in an increase in the electron density in the free-electron-like states and in stronger sd-hybridization. When Au is the solvent, this softens the Fe–Fe bonds but stiffens the Au–Au and Au–Fe bonds which results in a net stiffening relative to the elemental components.
Hong, Min; Chasapis, Thomas C; Chen, Zhi-Gang; Yang, Lei; Kanatzidis, Mercouri G; Snyder, G Jeffrey; Zou, Jin
2016-04-26
Driven by the prospective applications of thermoelectric materials, massive efforts have been dedicated to enhancing the conversion efficiency. The latter is governed by the figure of merit (ZT), which is proportional to the power factor (S(2)σ) and inversely proportional to the thermal conductivity (κ). Here, we demonstrate the synthesis of high-quality ternary Bi2Te3-xSex nanoplates using a microwave-assisted surfactant-free solvothermal method. The obtained n-type Bi2Te2.7Se0.3 nanostructures exhibit a high ZT of 1.23 at 480 K measured from the corresponding sintered pellets, in which a remarkably low κ and a shift of peak S(2)σ to high temperature are observed. By detailed electron microscopy investigations, coupled with theoretical analysis on phonon transports, we propose that the achieved κ reduction is attributed to the strong wide-frequency phonon scatterings. The shifting of peak S(2)σ to high temperature is due to the weakened temperature dependent transport properties governed by the synergistic carrier scatterings and the suppressed bipolar effects by enlarging the band gap. PMID:27058746
Interfacial electron and phonon scattering processes in high-powered nanoscale applications.
Hopkins, Patrick E.
2011-10-01
The overarching goal of this Truman LDRD project was to explore mechanisms of thermal transport at interfaces of nanomaterials, specifically linking the thermal conductivity and thermal boundary conductance to the structures and geometries of interfaces and boundaries. Deposition, fabrication, and post possessing procedures of nanocomposites and devices can give rise to interatomic mixing around interfaces of materials leading to stresses and imperfections that could affect heat transfer. An understanding of the physics of energy carrier scattering processes and their response to interfacial disorder will elucidate the potentials of applying these novel materials to next-generation high powered nanodevices and energy conversion applications. An additional goal of this project was to use the knowledge gained from linking interfacial structure to thermal transport in order to develop avenues to control, or 'tune' the thermal transport in nanosystems.
NASA Astrophysics Data System (ADS)
Shcherbakov, Alexandre S.; Arellanes, Adan O.
2016-03-01
Performances of any system for data processing based on acousto-optical technique are mainly determined by parameters of the acousto-optical cell (AOC) exploited within the schematic arrangement. Here, basic properties of the AOC, involved into a novel processor for precise optical spectrum analysis dedicated to modern astrophysical applications, are considered. Because potential applications of this processor will be focused on investigations in extra-galactic astronomy as well as studies of extra-solar planets, an advanced regime of the non-collinear two-phonon light scattering has been elaborated for spectrum analysis with significantly improved spectral resolution. Under similar uprated requirements, the AOC, based on that specific regime in the calomel (Hg2Cl2) crystal, had been chosen, and its parameters were analyzed theoretically and verified experimentally. Then, the adequate approach to estimating the frequency/spectral bandwidth and spectral resolution had been developed. The bandwidth was calculated and experimentally realized with the additionally involved tilt angle of light incidence, allowing variations for acoustic frequencies. The resolution was characterized taking into account its doubling peculiar to the nonlinear two-phonon mechanism of light scattering. Proof-of-principle experiments were performed with the calomel AOC of 52 mm optical aperture, providing ~94% efficiency in the transmitted light due to the slow-shear acoustic mode of finite amplitude (the acoustic power density ~150 mW/mm2) with the velocity of 0.347×105 cm/s at the radio-wave acoustic frequency ~71 MHz. As a result, we have obtained the spectral resolution <0.235 Å within the spectral bandwidth <290 Å that looks as the best one can mention at the moment in acousto-optics.
Thermal conductivity in $\text{large}-J$ two-dimensional antiferromagnets: Role of phonon scattering
Chernyshev, A. L.; Brenig, Wolfram
2015-08-05
Different types of relaxation processes for magnon heat current are discussed, with a particular focus on coupling to three-dimensional phonons. There is thermal conductivity by these in-plane magnetic excitations using two distinct techniques: Boltzmann formalism within the relaxation-time approximation and memory-function approach. Also considered are the scattering of magnons by both acoustic and optical branches of phonons. We demonstrate an accord between the two methods, regarding the asymptotic behavior of the effective relaxation rates.
It is strongly suggested that scattering from optical or zone-boundary phonons is important for magnon heat current relaxation in a high-temperature window of ΘD≲T<< J.
NASA Astrophysics Data System (ADS)
Murphy, Ronan M.; Murray, Éamonn D.; Fahy, Stephen; Savić, Ivana
2016-03-01
The major obstacle in the design of materials with low lattice thermal conductivity is the difficulty in efficiently scattering phonons across the entire frequency spectrum. Using first-principles calculations, we show that driving PbTe materials to the brink of the ferroelectric phase transition could be a powerful strategy to solve this problem. We illustrate this concept by applying biaxial tensile (001) strain to PbTe and its alloys with another rocksalt IV-VI material, PbSe; and by alloying PbTe with a rhombohedral IV-VI material, GeTe. This induces extremely soft optical modes at the zone center, which increase anharmonic acoustic-optical coupling and decrease phonon lifetimes at all frequencies. We predict that PbTe, Pb(Se,Te), and (Pb,Ge)Te alloys driven close to the phase transition in the described manner will have considerably lower lattice thermal conductivity than that of PbTe (by a factor of 2 -3 ). The proposed concept may open new opportunities for the development of more efficient thermoelectric materials.
Cross-plane heat conduction in thin films with ab-initio phonon dispersions and scattering rates
NASA Astrophysics Data System (ADS)
Vermeersch, Bjorn; Carrete, Jesús; Mingo, Natalio
2016-05-01
We present a first-principles study of the cross-plane thermal conductivity κ ⊥ in a wide variety of semiconductor thin films. We introduce a simple suppression model that matches variance-reduced Monte Carlo simulations with ab-initio phonon dispersions and scattering rates within ≤ 5 % even for anisotropic compounds. This, in turn, enables accurate κ ⊥ reconstruction from tabulated cumulative conductivity curves κ Σ ( Λ ⊥ ) . We furthermore reveal, and explain, a distinct quasiballistic regime characterised by a fractional thickness dependence κ ⊥ ˜ L 2 - α in alloys (where α is the Lévy exponent) and logarithmic dependence κ ⊥ ˜ ln ( L ) in single crystals. These observations culminate in the formulation of two compact parametric forms for κ ⊥ ( L ) that can fit the first-principles curves across the entire ballistic-diffusive range within a few percent for all investigated compounds.
NASA Astrophysics Data System (ADS)
Fukasawa, Ryoichi; Okubo, Yusei; Abe, Osamu; Ohta, Kimihiro
1992-03-01
We report the Raman scattering spectra of the folded longitudinal acoustic phonon of AlxGa1-xAs/GaAs superlattices for various aluminium (Al) mole fractions. The effect of Al mole fraction increases on the Raman intensities and the frequencies was studied.
Bulat, Lev P.; Osvenskii, Vladimir B.; Parkhomenko, Yurii N.; Pshenay-Severin, Dmitry A.
2012-09-15
One of the possible ways to increase the thermoelectric figure of merit is the use of bulk nanostructured materials fabricated by melt spinning with subsequent hot pressing or spark plasma sintering. Among a variety of nanostructure types these materials contain regions of initial solid solution with nanometer sized inclusions of different compositions. In the present work the scattering of holes and phonons on nanoinclusions in such p-Bi{sub x}Sb{sub 1-x}Te{sub 3} based materials is considered. The change of transport coefficients due to this scattering mechanism is theoretically estimated. The estimations showed that the reduction of lattice thermal conductivity (about 12-13%) for nanoinclusions of Bi{sub 2}Te{sub 3}-Sb{sub 2}Te{sub 3} solid solution with different compositions is much greater than the change in power factor. Therefore the corresponding increase of the thermoelectric figure of merit for this case is determined mainly by phonon scattering. Also it is shown that the results of estimations depend on phonon spectrum approximation, e.g. in the case of sine-shaped instead of linear phonon spectrum the estimations give two times higher thermal conductivity reduction. - Graphical abstract: Relative phonon thermal conductivity {kappa}{sub ph} change (black line) due to nanoinclusion scattering versus nanoinclusion radius a, and relative thermoelectric power factor change (red line) due to nanoinclusion scattering versus chemical potential {mu} at nanoinclusion size a=1.5 nm and U{sub 0}=-0.146 eV. Highlights: Black-Right-Pointing-Pointer p-Bi{sub x}Sb{sub 1-x}Te{sub 3} solid solutions with nanosized inclusions were considered. Black-Right-Pointing-Pointer Selective hole scattering can increase power factor at high carrier concentrations. Black-Right-Pointing-Pointer Lattice thermal conductivity estimations depend on phonon spectrum approximation. Black-Right-Pointing-Pointer Phonon scattering can reduce lattice thermal conductivity by about 12-13%. Black
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.
Park, Kyeong Hyun Mohamed, Mohamed; Ravaioli, Umberto; Aksamija, Zlatan
2015-01-07
In this work, we calculate the thermal conductivity of layered bismuth telluride (Bi{sub 2}Te{sub 3}) thin films by solving the Boltzmann transport equation in the relaxation-time approximation using full phonon dispersion and compare our results with recently published experimental data and molecular dynamics simulation. The group velocity of each phonon mode is readily extracted from the full phonon dispersion obtained from first-principle density-functional theory calculation and is used along with the phonon frequency to compute the various scattering terms. Our model incorporates the typical interactions impeding thermal transport (e.g., umklapp, isotope, and boundary scatterings) and introduces a new interaction capturing the reduction of phonon transmission through van der Waals interfaces of adjacent Bi{sub 2}Te{sub 3} quintuple layers forming the virtual superlattice thin film. We find that this novel approach extends the empirical Klemens-Callaway relaxation model in such anisotropic materials and recovers the experimental anisotropy while using a minimal set of parameters.
Photoreflectance investigation of exciton-acoustic phonon scattering in GaN grown by MOVPE
NASA Astrophysics Data System (ADS)
Bouzidi, M.; Soltani, S.; Halidou, I.; Chine, Z.; El Jani, B.
2016-04-01
In this paper, we report a systematic investigation of the near band edge (NBE) excitonic states in GaN using low temperature photoluminescence (PL) and photoreflectance (PR) measurements. For this purpose, GaN films of different thicknesses have been grown on silicon nitride (SiN) treated c-plane sapphire substrates by atmospheric pressure metalorganic vapor phase epitaxy (MOVPE). Low temperature PR spectra exhibit well-defined spectral features related to the A, B and C free excitons denoted by FXA FXB and FXC, respectively. In contrast, PL spectra are essentially dominated by the A free and donor bound excitons. By combining PR spectra and Hall measurements a strong correlation between residual electron concentration and exciton linewidths is observed. From the temperature dependence of the excitonic linewidths, the exciton-acoustic phonon coupling constant is determined for FXA, FXB and FXC. We show that this coupling constant is strongly related to the exciton kinetic energy and to the strain level.
Zamrun, Muhammad F.; Kasim, Hasan Abu
2011-03-30
We study the large angle quasi-elastic scattering of {sup 54}Cr+{sup 208}Pb system in terms of the full-order coupled-channels formalism. We especially investigate the role of single, double and triple phonon excitations on quasi-elastic scattering cross section as well as quasi-elastic barrier distribution of this system for which the experimental data have been measured. It is shown that the triple phonon excitations both in {sup 54}Cr and {sup 208}Pb nuclei seem to be needed by the present coupled-channels calculations in order to reproduce the experimental data of quasi-elastic cross section and barrier distribution for the {sup 54}Cr+{sup 208}Pb system. We also show that the standard value of the surface diffuseness parameter for the nuclear potential a = 0.63 fm, is preferred by the experimental quasi-elastic scattering data for this system.
Sutter, E.; Schafer-Nolte, E.O.; Stoica T.; Gotschke, T.; Limbach, F.A.; Sutter, P.; Grutzmacher, D.; Calarco, R.
2010-08-06
In the literature, there are controversies on the interpretation of the appearance in InN Raman spectra of a strong scattering peak in the energy region of the unscreened longitudinal optical (LO) phonons, although a shift caused by the phonon-plasmon interaction is expected for the high conductance observed in this material. Most measurements on light scattering are performed on ensembles of InN nanowires (NWs). However, it is important to investigate the behavior of individual nanowires and here we report on micro-Raman measurements on single nanowires. When changing the polarization direction of the incident light from parallel to perpendicular to the wire, the expected reduction of the Raman scattering was observed for transversal optical (TO) and E2 phonon scattering modes, while a strong symmetry-forbidden LO mode was observed independently on the laser polarization direction. Single Mg- and Si-doped crystalline InN nanowires were also investigated. Magnesium doping results in a sharpening of the Raman peaks, while silicon doping leads to an asymmetric broadening of the LO peak. The results can be explained based on the influence of the high electron concentration with a strong contribution of the surface accumulation layer and the associated internal electric field.
Price, A. Martinez, A.
2015-04-28
Using quantum transport simulations, the impact of electron-phonon scattering on the transfer characteristic of a gate-all-around nanowire (GaAs) field effect transistor (NWFET) has been thoroughly investigated. The Non-Equilibrium Green's Function formalism in the effective mass approximation using a decoupled mode decomposition has been deployed. NWFETs of different dimensions have been considered, and scattering mechanisms including acoustic, optical and polar optical phonons have been included. The effective masses were extracted from tight binding simulations. High and low drain bias have been considered. We found substantial source to drain tunnelling current and significant impact of phonon scattering on the performance of the NWFET. At low drain bias, for a 2.2 × 2.2 nm{sup 2} cross-section transistor, scattering caused a 72%, 77%, and 81% decrease in the on-current for a 6 nm, 10 nm, and 20 nm channel length, respectively. This reduction in the current due to scattering is influenced by the increase in the tunnelling current. We include the percentage tunnelling for each valley at low and high drain bias. It was also found that the strong quantisation caused the relative position of the valleys to vary with the cross-section. This had a large effect on the overall tunnelling current. The phonon-limited mobility was also calculated, finding a mobility of 950 cm{sup 2}/V s at an inversion charge density of 10{sup 12 }cm{sup −2} for a 4.2 × 4.2 nm{sup 2} cross-section device.
Damped soft phonons and diffuse scattering in (Bi1/2Na1/2)TiO3
NASA Astrophysics Data System (ADS)
Matsuura, M.; Iida, H.; Hirota, K.; Ohwada, K.; Noguchi, Y.; Miyayama, M.
2013-02-01
Neutron-scattering studies of (Bi1/2Na1/2)TiO3 (BNT) have been performed to elucidate the microscopic mechanism of the broad maximum in the temperature dependence of the dielectric constant at Tm˜600 K and the reduction in the piezoelectric properties above the depolarization temperature, 460˜480 K. We observed diffuse scattering near the Γ point below 700 K, which competes with the superlattice peak at the M point of the tetragonal phase but coexists with the superlattice peak at the R point of the rhombohedral phase. The diffuse scattering shows an anisotropic Q shape extending along the <100> direction transverse to the scattering vector Q, which is explained by atomic shifts bridging the tetragonal and rhombohedral structures. We propose that the broad maximum in the dielectric constant is associated with a diffusive first-order transition between the competing tetragonal and rhombohedral phases. In addition, we found that the diffuse scattering is reduced for single crystals grown under high oxygen pressure, which suggests an analogy with the central peak in hydrogen-reduced SrTiO3. Inelastic neutron scattering near the Γ point reveals a heavily overdamped soft mode similar to those reported in lead-based relaxors, the “waterfall” feature. Moreover, a damped soft transverse acoustic mode is observed for the <100> direction as the anisotropic diffuse scattering, indicating phase instabilities with the same origin as that of the diffuse scattering. The recovery of the soft mode is observed near the depolarization temperature, which coincides with the disappearance of the superlattice peak at the M point. These results indicate that the depolarization and the waterfall feature originate in the dynamic nature of ferroelectric clusters in the coexisting tetragonal/rhombohedral phase.
Propagation of large-wavevector acoustic phonons new perspectives from phonon imaging
NASA Astrophysics Data System (ADS)
Wolfe, James P.
Within the last decade a number of attempts have been made to observe the ballistic propagation of large wavevector acoustic phonons in crystals at low temperatures. Time-of-flight heat-pulse methods have difficulty in distinguishing between scattered phonons and ballistic phonons which travel dispersively at subsonic velocities. Fortunately, ballistic phonons can be identified by their highly anisotropic flux, which is observed by phonon imaging techniques. In this paper, several types of phonon imaging experiments are described which reveal the dispersive propagation of large-wavevector phonons and expose interesting details of the phonon scattering processes.
NASA Astrophysics Data System (ADS)
Milekhin, Alexander G.; Sveshnikova, Larisa L.; Duda, Tatyana A.; Rodyakina, Ekaterina E.; Dzhagan, Volodymyr M.; Sheremet, Evgeniya; Gordan, Ovidiu D.; Himcinschi, Cameliu; Latyshev, Alexander V.; Zahn, Dietrich R. T.
2016-05-01
Here we present the results on an investigation of resonant Stokes and anti- Stokes surface-enhanced Raman scattering (SERS) by optical phonons in colloidal CdSe nanocrystals (NCs) homogeneously deposited on arrays of Au nanoclusters using the Langmuir-Blodgett technology. The thickness of deposited NCs, determined by transmission and scanning electron microscopy, amounts to approximately 1 monolayer. Special attention is paid to the determination of the localized surface plasmon resonance (LSPR) energy in the arrays of Au nanoclusters as a function of the nanocluster size by means of micro-ellipsometry. SERS by optical phonons in CdSe NCs shows a significant enhancement factor with a maximal value of 2 × 103 which depends resonantly on the Au nanocluster size and thus on the LSPR energy. The deposition of CdSe NCs on the arrays of Au nanocluster dimers enabled us to study the polarization dependence of SERS. It was found that a maximal SERS signal is observed for the light polarization along the dimer axis. Finally, SERS by optical phonons was observed for CdSe NCs deposited on the structures with a single Au dimer. A difference of the LO phonon energy is observed for CdSe NCs on different single dimers. This effect is explained as the confinement-induced shift which depends on the CdSe nanocrystal size and indicates quasi-single NC Raman spectra being obtained.
NASA Astrophysics Data System (ADS)
Xu, Ruqing; Wong, Joe; Zschack, Paul; Hong, Hawoong; Chiang, Tai-Chang
2007-03-01
The 5f electrons in Pu can be either bonding or localized, depending sensitively on the temperature, pressure, and impurity doping. As a result, Pu displays a rich phase diagram involving a large number of phases with substantially different atomic volumes. In a recent report of the phonon dispersion curves of Ga-stablized δ-Pu at room temperature and ambient pressure, a pronounced deepening of the TA[111] phonon branch near the L point was discovered. This phonon softening was suggested to be related to a lattice shearing mechanism that could lead to the structural phase transition from the fcc δ phase to the monoclinic α' phase at about 170 K. Here we report our measurements of x-ray thermal diffuse scattering from a δ-Pu crystal (with 0.6 wt% Ga) at temperatures from 307K to 200K. The results show no further softening of the phonons near the L point as the sample temperature decreases. The implications regarding the relationship between the soft mode and the phase transition will be discussed.
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.
Liu, Y.; Berti, D.; Baglioni, P.; Chen, S. H.; Alatas, A.; Sinn, H.; Said, A.; Alp, E. E.; Experimental Facilities Division; Massachusetts Inst. of Technology; Univ. of Florence
2005-01-01
Shear-aligned 40 wt% calf-thymus Na-DNA molecules in aqueous solutions are prepared in their liquid crystalline phases and studied by high resolution inelastic X-ray scattering (IXS). Measured IXS spectra are analyzed with the generalized three effective eigenmode (GTEE) theory. The phonon dispersion relations along the axial direction of DNA molecules with different MgCl2 concentrations are constructed and compared. It is found that the sound speed along the axial direction of DNA molecules varies only slightly, but the phonon dampening is greatly affected with the increase amount of MgCl{sub 2} concentration. Using the GTEE theory, we are able to extract the longitudinal viscosity in the hydrodynamic limit from the Q-dependence of a fitted parameter. We make a comprehensive review of the GTEE theory and discuss detailed analyses of IXS spectra taking into account finite energy resolution of the instrument.
Additional and canonical phonon modes in Hg1-xCdxTe(0.06≤x≤0.7)
NASA Astrophysics Data System (ADS)
Polit, J.; Sheregii, E. M.; Cebulski, J.; Kisiel, A.; Robouch, B. V.; Marcelli, A.; Mycielski, A.
2010-07-01
In this experimental work a conception of the phonon spectra of the Hg1-xCdxTe(x=0.06-0.7) solid solution is presented which explains the presence of additional lines in the region 100-115cm-1 . Data of the optical reflectivity measurements obtained in far and middle infrared regions for eleven compositions of these alloys in the temperature range from 20 to 293 K using the synchrotron radiation ( DAΦNE -LIGHT in LNF, Italy) as source are analyzed. Analyses were performed on samples of different types ( n and p type) of conductivity as well as the temperature dependences of the line intensity under consideration in the region from 70 to 118cm-1 . The model of two valley potential of the mercury atom in the Hg1-xCdxTe lattice is used for interpretation of the additional phonon modes.
Phonon localization drives polar nanoregions in a relaxor ferroelectric.
Manley, M E; Lynn, J W; Abernathy, D L; Specht, E D; Delaire, O; Bishop, A R; Sahul, R; Budai, J D
2014-01-01
Relaxor ferroelectrics exemplify a class of functional materials where interplay between disorder and phase instability results in inhomogeneous nanoregions. Although known for about 30 years, there is no definitive explanation for polar nanoregions (PNRs). Here we show that ferroelectric phonon localization drives PNRs in relaxor ferroelectric PMN-30%PT using neutron scattering. At the frequency of a preexisting resonance mode, nanoregions of standing ferroelectric phonons develop with a coherence length equal to one wavelength and the PNR size. Anderson localization of ferroelectric phonons by resonance modes explains our observations and, with nonlinear slowing, the PNRs and relaxor properties. Phonon localization at additional resonances near the zone edges explains competing antiferroelectric distortions known to occur at the zone edges. Our results indicate the size and shape of PNRs that are not dictated by complex structural details, as commonly assumed, but by phonon resonance wave vectors. This discovery could guide the design of next generation relaxor ferroelectrics. PMID:24718289
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
factor. In addition, the techniques and scientific understanding developed in the research can be applied to a wide range of materials, with the caveat that the thermal conductivity of such a material be dominated by phonon, rather than electron, transport. In particular, this includes several thermoelectric materials with attractive properties at elevated temperatures (i.e., greater than room temperature), such as silicon germanium and silicon carbide. It is reasonable that phononic crystal patterning could be used for high-temperature thermoelectric devices using such materials, with applications in energy scavenging via waste-heat recovery and thermoelectric cooling for high-performance microelectronic circuits. The only part of the ZT picture missing in this work was the experimental measurement of the Seebeck coefficient of our phononic crystal devices. While a first-order approximation indicates that the Seebeck coefficient should not change significantly from that of bulk silicon, we were not able to actually verify this assumption within the timeframe of the project. Additionally, with regards to future high-temperature applications of this technology, we plan to measure the thermal conductivity reduction factor of our phononic crystals as elevated temperatures to confirm that it does not diminish, given that the nominal thermal conductivity of most semiconductors, including silicon, decreases with temperature above room temperature. We hope to have the opportunity to address these concerns and further advance the state-of-the-art of thermoelectric materials in future projects.
Xia, H. Patterson, R.; Feng, Y.; Shrestha, S.; Conibeer, G.
2014-08-11
The rates of charge carrier relaxation by phonon emission are of substantial importance in the field of hot carrier solar cell, primarily in investigation of mechanisms to slow down hot carrier cooling. In this work, energy and momentum resolved deformation potentials relevant to electron-phonon scattering are computed for wurtzite InN and GaN as well as an InN/GaN multiple quantum well (MQW) superlattice using ab-initio methods. These deformation potentials reveal important features such as discontinuities across the electronic bandgap of the materials and variations over tens of eV. The energy dependence of the deformation potential is found to be very similar for wurtzite nitrides despite differences between the In and Ga pseudopotentials and their corresponding electronic band structures. Charge carrier relaxation by this mechanism is expected to be minimal for electrons within a few eV of the conduction band edge. However, hole scattering at energies more accessible to excitation by solar radiation is possible between heavy and light hole states. Moderate reductions in overall scattering rates are observed in MQW relative to the bulk nitride materials.
NASA Astrophysics Data System (ADS)
Xia, H.; Patterson, R.; Feng, Y.; Shrestha, S.; Conibeer, G.
2014-08-01
The rates of charge carrier relaxation by phonon emission are of substantial importance in the field of hot carrier solar cell, primarily in investigation of mechanisms to slow down hot carrier cooling. In this work, energy and momentum resolved deformation potentials relevant to electron-phonon scattering are computed for wurtzite InN and GaN as well as an InN/GaN multiple quantum well (MQW) superlattice using ab-initio methods. These deformation potentials reveal important features such as discontinuities across the electronic bandgap of the materials and variations over tens of eV. The energy dependence of the deformation potential is found to be very similar for wurtzite nitrides despite differences between the In and Ga pseudopotentials and their corresponding electronic band structures. Charge carrier relaxation by this mechanism is expected to be minimal for electrons within a few eV of the conduction band edge. However, hole scattering at energies more accessible to excitation by solar radiation is possible between heavy and light hole states. Moderate reductions in overall scattering rates are observed in MQW relative to the bulk nitride materials.
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.
Zhang, Xin; Qiao, Xiao-Fen; Shi, Wei; Wu, Jiang-Bin; Jiang, De-Sheng; Tan, Ping-Heng
2015-05-01
Two-dimensional (2D) transition metal dichalcogenide (TMD) nanosheets exhibit remarkable electronic and optical properties. The 2D features, sizable bandgaps and recent advances in the synthesis, characterization and device fabrication of the representative MoS2, WS2, WSe2 and MoSe2 TMDs make TMDs very attractive in nanoelectronics and optoelectronics. Similar to graphite and graphene, the atoms within each layer in 2D TMDs are joined together by covalent bonds, while van der Waals interactions keep the layers together. This makes the physical and chemical properties of 2D TMDs layer-dependent. In this review, we discuss the basic lattice vibrations of 2D TMDs from monolayer, multilayer to bulk material, including high-frequency optical phonons, interlayer shear and layer breathing phonons, the Raman selection rule, layer-number evolution of phonons, multiple phonon replica and phonons at the edge of the Brillouin zone. The extensive capabilities of Raman spectroscopy in investigating the properties of TMDs are discussed, such as interlayer coupling, spin-orbit splitting and external perturbations. The interlayer vibrational modes are used in rapid and substrate-free characterization of the layer number of multilayer TMDs and in probing interface coupling in TMD heterostructures. The success of Raman spectroscopy in investigating TMD nanosheets paves the way for experiments on other 2D crystals and related van der Waals heterostructures. PMID:25679474
NASA Astrophysics Data System (ADS)
Wang, Y.; Nakatsukasa, K.; Rademaker, L.; Berlijn, T.; Johnston, S.
2016-05-01
Mono- and multilayer FeSe thin films grown on SrTiO3 and BiTiO3 substrates exhibit a greatly enhanced superconductivity over that found in bulk FeSe. A number of proposals have been advanced for the mechanism of this enhancement. One possibility is the introduction of a cross-interface electron–phonon (e–ph) interaction between the FeSe electrons and oxygen phonons in the substrates that is peaked in the forward scattering (small {q}) direction due to the two-dimensional nature of the interface system. Motivated by this, we explore the consequences of such an interaction on the superconducting state and electronic structure of a two-dimensional system using Migdal–Eliashberg (ME) theory. This interaction produces not only deviations from the expectations of conventional phonon-mediated pairing but also replica structures in the spectral function and density of states, as probed by angle-resolved photoemission spectroscopy, scanning tunneling microscopy/spectroscopy, and quasiparticle interference imaging. We also discuss the applicability of ME theory for a situation where the e–ph interaction is peaked at small momentum transfer and in the FeSe/STO system.
Cooper, Michael William D.; Liu, Xiang -Yang; Stanek, Christopher Richard; Andersson, David Anders
2016-07-15
In this study, a new approach for adjusting molecular dynamics results on UO2 thermal conductivity to include phonon-spin scattering has been used to improve calculations on Ux Pu1–x O2 and UxTh1xO2. We demonstrate that by including spin scattering a strong asymmetry as a function of uranium actinide fraction, x, is obtained. Greater degradation is shown for UxTh1–xO2 than UxPu1-xO2. Minimum thermal conductivities are predicted at U0.97Pu0.03O2 and U0.58Th0.42O2, although the degradation in UxPu1–xO2 is negligible relative to pure UO2.
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.
Giefers, H.; Koval, S.; Wortmann, G.; Sturhahn, W.; Alp, E.E.; Hu, M.Y.; X-Ray Science Division; Univ. of Paderborn; Univ. of Nevada at Las Vegas; Univ. Nacional de Rosario
2006-09-29
The local phonon density of states (DOS) at the Sn site in tin monoxide (SnO) is studied at pressures up to 8 GPa with 119Sn nuclear resonant inelastic x-ray scattering (NRIXS) of synchrotron radiation at 23.88 keV. The preferred orientation (texture) of the SnO crystallites in the investigated samples is used to measure NRIXS spectra preferentially parallel and almost perpendicular to the c axis of tetragonal SnO. A subtraction method is applied to these NRIXS spectra to produce projected local Sn DOS spectra as seen parallel and perpendicular to the c axis of SnO. These experimentally obtained local Sn DOS spectra, both in the polycrystalline case as well as projected parallel and perpendicular to the c axis, are compared with corresponding theoretical phonon DOS spectra, derived from dispersion relations calculated with a recently developed shell model. Comparison between the experimental projected Sn DOS spectra and the corresponding theoretical DOS spectra enables us to follow the pressure-induced shifts of several acoustic and optic phonon modes. While the principal spectral features of the experimental and theoretical phonon DOS agree well at energies above 10 meV, the pressure behavior of the low-energy part of the DOS is not well reproduced by the theoretical calculations. In fact, they exhibit, in contrast to the experimental data, a dramatic softening of two low-energy modes, their energies approaching zero around 2.5 GPa, clearly indicating the limitations of the applied shell model. These difficulties are obviously connected with the complex Sn-O and Sn-Sn bindings within and between the Sn-O-Sn layers in the litharge structure of SnO. We derived from the experimental and theoretical DOS spectra a variety of elastic and thermodynamic parameters of the Sn sublattice, such as the Lamb-M{umlt o}ssbauer factor, the mean force constant, and Debye temperatures, as well as the vibrational contributions to the Helmholtz free energy, specific heat, entropy, and
NASA Astrophysics Data System (ADS)
Obeid, A. Scheikh; Burda, O.; Chernykh, M.; Krugmann, A.; von Neumann-Cosel, P.; Pietralla, N.; Poltoratska, I.; Ponomarev, V. Yu.; Walz, C.
2013-01-01
Background: Mixed-symmetry 2+ states in vibrational nuclei are characterized by a sign change between dominant proton and neutron valence-shell components with respect to the fully symmetric 2+ state. The sign can be measured by a decomposition of proton and neutron transition radii with a combination of inelastic electron and hadron scattering [C. Walz , Phys. Rev. Lett.PRLTAO0031-900710.1103/PhysRevLett.106.062501 106, 062501 (2011)]. For the case of 92Zr, a difference could be experimentally established for the neutron components, while about equal proton transition radii were indicated by the data.Purpose: Determination of the ground-state (g.s.) transition strength of the mixed-symmetry 22+ state and verification of the expected vanishing of the proton transition radii difference between the one-phonon 2+ states in 92Zr.Method: Differential cross sections for the excitation of one-phonon 2+ and 3- states in 92Zr have been measured with the (e,e') reaction at the S-DALINAC in a momentum transfer range q≃0.3-0.6 fm-1.Results: Transition strengths B(E2;21+→01+)=6.18(23), B(E2;22+→01+)=3.31(10), and B(E3;31-→01+)=18.4(1.1) Weisskopf units are determined from a comparison of the experimental cross sections to quasiparticle-phonon model (QPM) calculations. It is shown that a model-independent plane wave Born approximation (PWBA) analysis can fix the ratio of B(E2) transition strengths to the 21,2+ states with a precision of about 1%. The method furthermore allows to extract their proton transition radii difference. With the present data ΔR=-0.12(51) fm is obtained.Conclusions: Electron scattering at low momentum transfers can provide information on transition radii differences of one-phonon 2+ states even in heavy nuclei. Proton transition radii for the 21,2+ states in 92Zr are found to be identical within uncertainties. The g.s. transition probability for the mixed-symmetry state can be determined with high precision limited only by the available
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
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
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.
Phonon dynamics of graphene on metals.
Al Taleb, Amjad; Farías, Daniel
2016-03-16
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. PMID:26886508
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.
NASA Astrophysics Data System (ADS)
Bobrov, N. L.
2015-08-01
Nonlinear electrical effects in superconducting S-c-S contacts, including the spectroscopy of electron-phonon interactions (EPI) in these systems, and the recovery of the EPI function from experimental data are discussed. The effect of a magnetic field on the current-voltage characteristics (I-V curves) and their derivatives for ErNi2B2C point contacts (PC) with d ≥ ξ (where d is the diameter of the PC and ξ is the coherence length) is studied. It is found that in zero magnetic fields and in near-critical fields, when the size of the superconducting gap can be neglected, the position of the peaks in dV/dI coincides with the peaks in the Yanson EPI spectra. In low fields the peaks are shifted toward lower energies and in intermediate fields, the peaks split. For PC with diameters greater than or on the order of the coherence length, the relative size of the negative phonon contribution to the excess current is considerably greater than in ballistic contacts. This leads to substantial suppression of the high-frequency peaks in the spectra for the superconducting state. In order to recover the EPI function from these spectra it is necessary to correct their intensities at high energies. For "dirty" NbSe2 and Nb point-contacts with d ≥ ξ, which have no phonon features in the second derivative of the I-V curve in the normal state, the EPI can be reconstructed from the superconducting state.
Phonons in chalcopyrite compounds
NASA Astrophysics Data System (ADS)
Derollez, P.; Laamyem, A.; Fouret, R.; Hennion, B.; Gonzalez, J.
1999-06-01
The phonon dispersion curves along the [100] and [001] directions of CuInSe2 and AgGaSe2 have been measured by inelastic neutron scattering. They are analyzed with different rigid-ion models: Born-von Karman and valence force field models. The calculated dispersion curves are in good agreement with experiments.
NASA Astrophysics Data System (ADS)
Liao, Y. Y.; Li, Y. W.; Hu, Z. G.; Chu, J. H.
2012-02-01
Lattice vibrations of highly a-axis oriented CoFe2O4 (CFO) films have been investigated by Raman scattering in the temperature range of 80-873 K. The five phonon modes T1g(2), T1g(3), Eg, A1g(1), A1g(2), and their evolutions can be uniquely distinguished. It was found that an electron transfer between Co2+ and Fe3+ cations occurs in octahedral sites at about 173 K. The structure disorder in the CFO films appears with increasing the temperature, which indicates the cation migration between tetrahedral and octahedral sites. The phenomena suggest the structural transformation trend from inverse spinel to normal spinel at the elevated temperatures.
Zeier, Wolfgang G; Pei, Yanzhong; Pomrehn, Gregory; Day, Tristan; Heinz, Nicholas; Heinrich, Christophe P; Snyder, G Jeffrey; Tremel, Wolfgang
2013-01-16
Inspired by the promising thermoelectric properties of chalcopyrite-like quaternary chalcogenides, here we describe the synthesis and characterization of the solid solution Cu(2)Zn(1-x)Fe(x)GeSe(4). Upon substitution of Zn with the isoelectronic Fe, no charge carriers are introduced in these intrinsic semiconductors. However, a change in lattice parameters, expressed in an elongation of the c/a lattice parameter ratio with minimal change in unit cell volume, reveals the existence of a three-stage cation restructuring process of Cu, Zn, and Fe. The resulting local anisotropic structural disorder leads to phonon scattering not normally observed, resulting in an effective approach to reduce the lattice thermal conductivity in this class of materials. PMID:23256607
NASA Astrophysics Data System (ADS)
Rowe, Michael P.; Zhou, Li Qin; Banerjee, Debasish; Zhang, Minjuan
2015-01-01
Recovery of waste heat from internal combustion engines is one strategy for meeting the ever increasing demand for more fuel efficient-automobiles. Thermoelectric materials are capable of this, by solid-state conversion of thermal to electrical energy, but the efficiency of this energy conversion requires improvement. In this work the thermoelectric figure of merit ( ZT) was improved by combining phonon scattering with grain boundary modification in a bismuth antimony telluride nanocomposite material with zinc antimony grain boundaries and zinc oxide nanoparticle inclusions. The advantage of including these zinc nanostructures is discussed. By reducing thermal conductivity while increasing the power factor, ZT was been increased from 0.6 to 1.1.
NASA Astrophysics Data System (ADS)
Fainstein, A.; Etchegoin, P.; Chamberlain, M. P.; Cardona, M.; Tötemeyer, K.; Eberl, K.
1995-05-01
We present a detailed experimental study of optical phonon Raman scattering in GaAs/AlAs multiple quantum wells for several in-plane geometries. By exploiting a waveguided structure, we performed 90°, forward, and backscattering experiments with dispersed light propagating along the layers. Using these geometries, phonons with various propagation directions and polarized both parallel and perpendicular to the growth axis can be probed. The 90° data complete and correct earlier results obtained for the same geometry by Zucker et al., bringing them into accord with later experimental and theoretical work. Moreover, in-plane forward scattering data are reportd as a complementary check to these experiments. We discuss selection rules and scattering mechanisms, and compare the results with phonon energies calculated within a continuum model based on linear combinations of LO, TO, and interface modes. We find a very good agreement between the experiment and the predictions of the established theory of phonon modes and Raman scattering in semiconductor heterostructures.
``Forbidden'' phonon in the iron chalcogenide series
NASA Astrophysics Data System (ADS)
Fobes, David M.; Zaliznyak, Igor A.; Xu, Zhijun; Gu, Genda; Tranquada, John M.
2015-03-01
Recently, we uncovered evidence for the formation of a bond-order wave (BOW) leading to ferro-orbital order at low temperature, acting to stabilize the bicollinear AFM order, in the iron-rich parent compound, Fe1+yTe. Investigating the inelastic spectra centered near (100) in Fe1+yTe, a signature peak for the BOW formation in the monoclinic phase, we observed an acoustic phonon dispersion in both tetragonal and monoclinic phases. While a structural Bragg peak accompanies the mode in the monoclinic phase, in the tetragonal phase Bragg scattering at this Q is forbidden by symmetry, and we observed no elastic peak. This phonon mode was also observed in superconducting FeTe0.6Se0.4, where structural and magnetic transitions are suppressed. LDA frozen phonon calculations suggested that this mode could result from a spin imbalance between neighboring Fe atoms, but polarized neutron measurements revealed no additional magnetic scattering. We propose that this ``forbidden'' phonon mode may originate from dynamically broken symmetry, perhaps related to the strong dynamic spin correlations in these materials. Work at BNL was supported by BES, US DOE, under Contract No. DE-AC02-98CH10886. Research at ORNL's HFIR and SNS sponsored by Scientific User Facilities Division, BES, US DOE. We acknowledge the support of NIST, in providing neutron research facilities.
A study of non-equilibrium phonons in GaAs/AlAs quantum wells
Su, Zhenpeng
1996-11-01
In this thesis we have studied the non-equilibrium phonons in GaAs/AlAs quantum wells via Raman scattering. We have demonstrated experimentally that by taking into account the time-reversal symmetry relation between the Stokes and anti-Stokes Raman cross sections, one can successfully measure the non-equilibrium phonon occupancy in quantum wells. Using this technique, we have studied the subject of resonant intersubband scattering of optical phonons. We find that interface roughness plays an important role in resonant Raman scattering in quantum wells. The lateral size of the smooth regions in such interface is estimated to be of the order of 100 {Angstrom}. Through a study of photoluminescence of GaAs/AlAs quantum wells under high intensity laser excitation, we have found that band nonparabolicity has very little effect on the electron subband energies even for subbands as high as a few hundred meV above the lowest one. This finding may require additional theoretical study to understand its origin. We have also studied phonon confinement and propagation in quantum wells. We show that Raman scattering of non-equilibrium phonons in quantum wells can be a sensitive measure of the spatial extent of the longitudinal optical (LO) phonons. We deduce the coherence length of LO phonons in GaAs/Al{sub x}Ga{sub 1-x}As quantum wells as a function of the Al concentration x.
Phonons and their interactions
Nicklow, R.M.
1982-08-01
The phonon energy spectra nu(vector q) of crystalline materials contains key information about the interatomic interactions. However, it is generally not possible to fully understand the phonon spectra without also understanding the influence on phonon energies and lifetimes caused by interactions with defects, electrons and other excitations. The study of several of these types of interactions have grown over the years so as to now constitute subfields of solid state physics and the contributions of neutron scattering research to each has been, if not of paramount importance, at least very significant. In the present review we can merely touch on a few highlights. Perhaps the largest research effort is expended on electron-phonon interactions. These interactions are, of course, fundamental to the properties of metallic solids. They are seen in the phonon nu(vector q) of metals in a wide variety of effects. We shall mention three: the relatively small fine structure produced by Kohn singularities, large anomalies and phonon lifetimes measured in some superconductors and in materials with fluctuating valence.
NASA Astrophysics Data System (ADS)
Zhu, J. J.; Zhang, J. Z.; Xu, G. S.; Zhang, X. L.; Hu, Z. G.; Chu, J. H.
2015-05-01
Optic phonons and lattice vibrations of Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals near the morphotropic phase boundary (MPB) are determined by terahertz (THz) reflectance spectra in a temperature range of 5.5-300 K. Raman scattering is measured with a He-Ne laser with a wavelength of 632.8 nm as the exciting source. On cooling from 300 K, the A1 component of the splitting TO1 mode hardens, which follows the Cochran law with a critical softening temperature of 761-861 K. Moreover, the E component of the soft mode is heavily damped around 0.81 THz (27 cm - 1). Additional polar phonon modes, which are forbidden in the cubic structure, are activated due to the broken cubic symmetry in polar clusters.
NASA Astrophysics Data System (ADS)
Wagman, J. J.; Carlo, J. P.; Gaudet, J.; Van Gastel, G.; Abernathy, D. L.; Stone, M. B.; Granroth, G. E.; Kolesnikov, A. I.; Savici, A. T.; Kim, Y. J.; Zhang, H.; Ellis, D.; Zhao, Y.; Clark, L.; Kallin, A. B.; Mazurek, E.; Dabkowska, H. A.; Gaulin, B. D.
2016-03-01
We present time-of-flight neutron scattering measurements on single crystals of La2-xBaxCuO4 (LBCO) with 0 ≤x ≤0.095 and La2-xSrxCuO4 (LSCO) with x =0.08 and 0.11. This range of dopings spans much of the phase diagram relevant to high-temperature cuprate superconductivity, ranging from insulating, three-dimensional commensurate long-range antiferromagnetic order, for x ≤0.02 , to two-dimensional (2D) incommensurate antiferromagnetism coexisting with superconductivity for x ≥0.05 . Previous work on lightly doped LBCO with x =0.035 showed a clear enhancement of the inelastic scattering coincident with the low-energy crossings of the highly dispersive spin excitations and quasi-2D optic phonons. The present work extends these measurements across the phase diagram and shows this enhancement to be a common feature to this family of layered quantum magnets. Furthermore, we show that the low-temperature, low-energy magnetic spectral weight is substantially larger for samples with nonsuperconducting ground states relative to any of the samples with superconducting ground states. Spin gaps, suppression of low-energy magnetic spectral weight as a function of decreasing temperature, are observed in both superconducting LBCO and LSCO samples, consistent with previous observations for superconducting LSCO.
Wagman, J. J.; Carlo, Jeremy P.; Gaudet, J.; Van Gastel, G. J.; Abernathy, Douglas L.; Stone, Matthew B.; Granroth, Garrett E.; Kolesnikov, Alexander I.; Savici, Andrei T.; Kim, Young -June; et al
2016-03-14
We present time-of-flight neutron-scattering measurements on single crystals of La2-xBaxCuO4 (LBCO) with 0 ≤ x ≤ 0.095 and La2-xSrxCuO4 (LSCO) with x = 0.08 and 0.11. This range of dopings spans much of the phase diagram relevant to high temperature cuprate superconductivity, ranging from insulating, three dimensional commensurate long range antiferromagnetic order for x ≤ 0.02 to two dimensional (2D) incommensurate antiferromagnetism co-existing with superconductivity for x ≥ 0.05. Previous work on lightly doped LBCO with x = 0.035 showed a clear resonant enhancement of the inelastic scattering coincident with the low energy crossings of the highly dispersive spin excitationsmore » and quasi-2D optic phonons. The present work extends these measurements across the phase diagram and shows this enhancement to be a common feature to this family of layered quantum magnets. Furthermore we show that the low temperature, low energy magnetic spectral weight is substantially larger for samples with non-superconducting ground states relative to any of the samples with superconducting ground states. Lastly spin gaps, suppression of low energy magnetic spectral weight, are observed in both superconducting LBCO and LSCO samples, consistent with previous observations for superconducting LSCO« less
Mitri, Farid
2014-11-01
The generalized theory of resonance scattering (GTRS) by an elastic spherical target in acoustics is extended to describe the arbitrary scattering of a finite beam using the addition theorem for the spherical wave functions of the first kind under a translation of the coordinate origin. The advantage of the proposed method over the standard discrete spherical harmonics transform previously used in the GTRS formalism is the computation of the off-axial beam-shape coefficients (BSCs) stemming from a closed-form partial-wave series expansion representing the axial BSCs in spherical coordinates. With this general method, the arbitrary acoustical scattering can be evaluated for any particle shape and size, whether the particle is partially or completely illuminated by the incident beam. Numerical examples for the axial and off-axial resonance scattering from an elastic sphere placed arbitrarily in the field of a finite circular piston transducer with uniform vibration are provided. Moreover, the 3-D resonance directivity patterns illustrate the theory and reveal some properties of the scattering. Numerous applications involving the scattering phenomenon in imaging, particle manipulation, and the characterization of multiphase flows can benefit from the present analysis because all physically realizable beams radiate acoustical waves from finite transducers as opposed to waves of infinite extent. PMID:25389166
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.
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.
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.
Manipulation of thermal phonons
NASA Astrophysics Data System (ADS)
Hsu, Chung-Hao
Developing materials that can conduct electricity easily, but block the motion of phonons is necessary in the applications of thermoelectric devices, which can generate electricity from temperature differences. In converse, a key requirement as chips get faster is to obtain better ways to dissipate heat. Controlling heat transfer in these crystalline materials devices --- such as silicon --- is important. The heat is actually the motion or vibration of atoms known as phonons. Finding ways to manipulate the behavior of phonons is crucial for both energy applications and the cooling of integrated circuits. A novel class of artificially periodic structured materials --- phononic crystals --- might make manipulation of thermal phonons possible. In many fields of physical sciences and engineering, acoustic wave propagation in solids attracts many researchers. Wave propagation phenomena can be analyzed by mathematically solving the acoustic wave equation. However, wave propagation in inhomogeneous media with various geometric structures is too complex to find an exact solution. Hence, the Finite Difference Time Domain method is developed to investigate these complicated problems. In this work, the Finite-Difference Time-Domain formula is derived from acoustic wave equations based on the Taylor's expansion. The numerical dispersion and stability problems are analyzed. In addition, the convergence conditions of numerical acoustic wave are stated. Based on the periodicity of phononic crystal, the Bloch's theorem is applied to fulfill the periodic boundary condition of the FDTD method. Then a wide-band input signal is used to excite various acoustic waves with different frequencies. In the beginning of the calculation process, the wave vector is chosen and fixed. By means of recording the displacement field and taking the Fourier transformation, we can obtain the eigenmodes from the resonance peaks of the spectrum and draw the dispersion relation curve of acoustic waves
Heat transport by phonons in crystalline materials and nanostructures
NASA Astrophysics Data System (ADS)
Koh, Yee Kan
conductivity. I employed FDTR to study the mean-free-paths of acoustic phonons in Si1-xGex. I experimentally demonstrate that 40% of heat is carried in Si1-xGe x alloys by phonons with mean-free-path 0.5 ≤ ℓ ≤ 5 mum, and phonons with > 2 mum do not contribute to the thermal conductivity of Si. I employed TDTR and frequency-dependent TDTR to study scattering of long- and medium-wavelength phonons in two important thermoelectric materials embedded with nanoscale precipitates. I find that the through-thickness lattice thermal conductivity of (PbTe)1-x/(PbSe)x nanodot superlattices (NDSLs) approaches the thermal conductivity of bulk homogenous PbTe1-x Sex alloys with the same average composition. On the other hand, I find that 3% of ErAs nanoparticles embedded in InGaAs is sufficient to scatter most of the phonons in InGaAs that have intermediate mean-free-paths, and thus reduces the thermal conductivity of InGaAs below the alloy limit. I find that scattering by nanoparticles approach the geometrical limit and can be readily accounted for by an additional boundary scattering which depends on the concentration of nanoparticles. Finally, I studied the thermal conductance of Au/Ti/Graphene/SiO 2 interfaces by TDTR. I find that heat transport across the interface is dominated by phonons. Even though graphene is only one atomic layer thick, graphene interfaces should be treated as two discrete interfaces instead of one diffuse interface in thermal analysis, suggesting that direct transmission of phonons from Au to SiO2 is negligible. My study is important for thermal management of graphene devices.
NASA Astrophysics Data System (ADS)
Kuzma, N. N.; Patton, B.; Raman, K.; Happer, W.
2002-03-01
NMR measurements of longitudinal relaxation times T1 in pure solid xenon were carried out using both natural-abundance and isotopically-enriched samples of hyperpolarized ^129Xe. At temperatures below 120 K and fields above 500 Gauss, the relaxation rate 1/T1 is field- and abundance-independent, consistent with the model of ^129Xe spin-flip Raman scattering of phonons(R. J. Fitzgerald et al.), Phys. Rev. B 59, 8795 (1999).. Above 120 K, vacancies invade the xenon lattice(P. R. Granfors et al.) Phys. Rev. B 24, 4753 (1981)., and a dramatic cross-over to the nuclear dipole-dipole relaxation due to the diffusion of vacancies is observed. As a result, the measured relaxation times of xenon near its melting point strongly depend on field and somewhat on ^129Xe abundance, and can be as short as several seconds, leading to potential difficulties in cryogenic applications of hyperpolarized ^129Xe. The data are analyzed using the theory of nuclear relaxation due to spin diffusion in cubic crystals(C. A. Sholl, J. Phys. C 21), 319 (1988)., and some estimates of the vacancy density and jump rates are discussed.
NASA Astrophysics Data System (ADS)
Krowne, Clifford M.
1983-05-01
The electron energy relaxation is studied as a function of the ``electron temperature'' Te in the n channel of a (100) surface silicon MOSFET (metal-oxide-semiconductor field-effect transistor) device by inspecting the phenomenological energy relaxation time τɛ(Te) at 4.2 °K, 77 °K, and 300 °K lattice temperatures. τɛ is theoretically calculated in order to determine the relative contributions of shear horizontal (SH), pressure-shear vertical (P-SV), shear vertical-pressure (SV-P), total reflection shear vertical pressure (TR), and Rayleigh (R) surface acoustic phonon modes to the electron energy relaxation at the interface. Two-dimensional electron transport is assumed and the effects of subbanding near the interface are included. Only electron scatter events within subbands are studied (intrasubband). This exhaustive study finds that surface modes do not dominate the electron energy relaxation at the Si-SiO2 interface at TL =4.2 °K. Some other mechanism(s) must predominate at TL =4.2 °K.
NASA Astrophysics Data System (ADS)
Yang, Fan; Dames, Chris
2015-04-01
The heating-frequency dependence of the apparent thermal conductivity in a semi-infinite body with periodic planar surface heating is explained by an analytical solution to the Boltzmann transport equation. This solution is obtained using a two-flux model and gray mean free time approximation and verified numerically with a lattice Boltzmann method and numerical results from the literature. Extending the gray solution to the nongray regime leads to an integral transform and accumulation-function representation of the phonon scattering spectrum, where the natural variable is mean free time rather than mean free path, as often used in previous work. The derivation leads to an approximate cutoff conduction similar in spirit to that of Koh and Cahill [Phys. Rev. B 76, 075207 (2007), 10.1103/PhysRevB.76.075207] except that the most appropriate criterion involves the heater frequency rather than thermal diffusion length. The nongray calculations are consistent with Koh and Cahill's experimental observation that the apparent thermal conductivity shows a stronger heater-frequency dependence in a SiGe alloy than in natural Si. Finally these results are demonstrated using a virtual experiment, which fits the phase lag between surface temperature and heat flux to obtain the apparent thermal conductivity and accumulation function.
NASA Astrophysics Data System (ADS)
Perrin, Bernard
2007-06-01
The conference PHONONS 2007 was held 15-20 July 2007 in the Conservatoire National des Arts et Métiers (CNAM) Paris, France. CNAM is a college of higher technology for training students in the application of science to industry, founded by Henri Grégoire in 1794. This was the 12th International Conference on Phonon Scattering in Condensed Matter. This international conference series, held every 3 years, started in France at Sainte-Maxime in 1972. It was then followed by meetings at Nottingham (1975), Providence (1979), Stuttgart (1983), Urbana-Champaign (1986), Heidelberg (1989), Ithaca (1992), Sapporo (1995), Lancaster (1998), Dartmouth (2001) and St Petersburg (2004). PHONONS 2007 was attended by 346 delegates from 37 different countries as follows: France 120, Japan 45, Germany 25, USA 25, Russia 21, Italy 13, Poland 9, UK 9, Canada 7, The Netherlands 7, Finland 6, Spain 6, Taiwan 6, Greece 4, India 4, Israel 4, Ukraine 4, Serbia 3, South Africa 3, Argentina 2, Belgium 2, China 2, Iran 2, Korea 2, Romania 2, Switzerland 2, and one each from Belarus, Bosnia-Herzegovina, Brazil, Bulgaria, Egypt, Estonia, Mexico, Moldova, Morocco, Saudi Arabia, Turkey. There were 5 plenary lectures, 14 invited talks and 84 oral contributions; 225 posters were presented during three poster sessions. The first plenary lecture was given by H J Maris who presented fascinating movies featuring the motion of a single electron in liquid helium. Robert Blick gave us a review on the new possibilities afforded by nanotechnology to design nano-electomechanical systems (NEMS) and the way to use them to study elementary and fundamental processes. The growing interest for phonon transport studies in nanostructured materials was demonstrated by Arun Majumdar. Andrey Akimov described how ultrafast acoustic solitons can monitor the optical properties of quantum wells. Finally, Maurice Chapellier told us how
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.
NASA Astrophysics Data System (ADS)
Nikoghossian, A. G.; Kapanadze, N. G.
2016-03-01
A group theoretical approach is developed for solving astrophysical radiative transfer problems described in a previous series of papers. Addition laws for observed radiative intensities are derived for the case in which atmospheres not only absorb and scatter radiation incident on them, but radiate themselves because of energy sources contained within them. As an illustration of the application of these laws, several special radiative transfer problems which we believe are of practical interest are discussed.
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.
NASA Astrophysics Data System (ADS)
Zhou, Huchuan; Kropelnicki, Piotr; Lee, Chengkuo
2014-12-01
Although significantly reducing the thermal conductivity of silicon nanowires has been reported, it remains a challenge to integrate silicon nanowires with structure materials and electrodes in the complementary metal-oxide-semiconductor (CMOS) process. In this paper, we investigated the thermal conductivity of nanometer-thick polycrystalline silicon (poly-Si) theoretically and experimentally. By leveraging the phonon-boundary scattering, the thermal conductivity of 52 nm thick poly-Si was measured as low as around 12 W mK-1 which is only about 10% of the value of bulk single crystalline silicon. The ZT of n-doped and p-doped 52 nm thick poly-Si was measured as 0.067 and 0.024, respectively, while most previously reported data had values of about 0.02 and 0.01 for a poly-Si layer with a thickness of 0.5 μm and above. Thermopile infrared sensors comprising 128 pairs of thermocouples made of either n-doped or p-doped nanometer-thick poly-Si strips in a series connected by an aluminium (Al) metal interconnect layer are fabricated using microelectromechanical system (MEMS) technology. The measured vacuum specific detectivity (D*) of the n-doped and p-doped thermopile infrared (IR) sensors are 3.00 × 108 and 1.83 × 108 cm Hz1/2 W-1 for sensors of 52 nm thick poly-Si, and 5.75 × 107 and 3.95 × 107 cm Hz1/2 W-1 for sensors of 300 nm thick poly-Si, respectively. The outstanding thermoelectric properties indicate our approach is promising for diverse applications using ultrathin poly-Si technology.Although significantly reducing the thermal conductivity of silicon nanowires has been reported, it remains a challenge to integrate silicon nanowires with structure materials and electrodes in the complementary metal-oxide-semiconductor (CMOS) process. In this paper, we investigated the thermal conductivity of nanometer-thick polycrystalline silicon (poly-Si) theoretically and experimentally. By leveraging the phonon-boundary scattering, the thermal conductivity of 52 nm
Vasin, A. S.; Vikhrova, O. V.; Vasilevskiy, M. I.
2014-04-14
Confinement and alloy disorder effects on the lattice dynamics and Raman scattering in Si{sub 1−x}Ge{sub x} nanocrystals (NCs) are investigated numerically employing two different empirical inter-atomic potentials. Relaxed NCs of different compositions (x) were built using the Molecular Dynamics method and applying rigid boundary conditions mimicking the effect of surrounding matrix. The resulting variation of bond lengths with x was checked against Vegard's law and the NC phonon modes were calculated using the same inter-atomic potential. The localization of the principal Raman-active (Si-Si, Si-Ge, and Ge-Ge) modes is investigated by analysing representative eigenvectors and their inverse participation ratio. The dependence of the position and intensity of these modes upon x and NC size is presented and compared to previous calculated results and available experimental data. In particular, it is argued that the composition dependence of the intensity of the Si-Ge and Ge-Ge modes does not follow the fraction of the corresponding nearest-neighbour bonds as it was suggested by some authors. Possible effects of alloy segregation are considered by comparing the calculated properties of random and clustered Si{sub x}Ge{sub 1−x} NCs. It is found that the Si-Si mode and Ge-Ge mode are enhanced and blue-shifted (by several cm{sup −1}for the Si-Si mode), while the intensity of the Si-Ge Raman mode is strongly suppressed by clustering.
Jana, R. N.; Sinha, S.; Meikap, A. K.
2015-05-15
We have reported a comprehensive study on temperature and disorder dependence of inelastic electron dephasing scattering rate in disordered V{sub 82}Al{sub 18-x}Fe{sub x} alloys. The dephasing scattering time has been measured by analysis of low field magnetoresistance using the weak localization theory. In absence of magnetic field the variation of low temperature resistivity rise follows the relation Δρ(T)∝−ρ{sub 0}{sup 5/2}√(T), which is well described by three-dimensional electron-electron interactions. The temperature-independent dephasing rate strongly depends on disorder and follows the relation τ{sub 0}{sup −1}∝l{sub e}, where l{sub e} is the electron elastic mean free path. The inelastic electron-phonon scattering rate obeying the anomalous relation τ{sub e−ph}{sup −1}∝T{sup 2}l{sub e}. This anomalous behavior of τ{sub e−ph}{sup −1} cannot be explained in terms of current theories for electron-phonon scattering in impure dirty conductors.
Jiang, P. P.; Zhang, X. L.; Chang, P.; Hu, Z. G. Bai, W.; Li, Y. W.; Chu, J. H.
2014-04-14
Optical phonons of multiferroic Bi{sub 4}Ti{sub 3}O{sub 12}-BiFeO{sub 3} ceramic have been investigated by low temperature Raman scattering and infrared reflectance spectra. Anomalies at about 85 K can be observed from the temperature dependence of the Raman and infrared modes, which arise from spin-phonon interaction during antiferromagnetic to paramagnetic phase transition. It was found that the change of exchange interaction in magnetic phase transition can be induced by Fe-O-Fe octahedral tilting driven from the A-site atoms. Moreover, ferroelectricity-related displacement of Bismuth atoms suggests the coupling of magnetic and ferroelectric orders.
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
NASA Astrophysics Data System (ADS)
Chen, J. C. H.; Sato, Y.; Kosaka, R.; Hashisaka, M.; Muraki, K.; Fujisawa, T.
2015-10-01
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.
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
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
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
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
Phononic and magnonic dispersions of surface waves on a permalloy/BARC nanostructured array
2013-01-01
Phononic and magnonic dispersions of a linear array of periodic alternating Ni80Fe20 and bottom anti-reflective coating nanostripes on a Si substrate have been measured using Brillouin light scattering. The observed phononic gaps are considerably larger than those of laterally patterned multi-component crystals previously reported, mainly a consequence of the high elastic and density contrasts between the stripe materials. Additionally, the phonon hybridization bandgap has an unusual origin in the hybridization and avoided crossing of the zone-folded Rayleigh and pseudo-Sezawa waves. The magnonic band structure features near-dispersionless branches, with unusual vortex-like dynamic magnetization profiles, some of which lie below the highly-dispersive fundamental mode branch. Finite element calculations of the phononic and magnonic dispersions of the magphonic crystal accord well with experimental data. PMID:23452555
Mao, Jun; Wang, Yumei; Ge, Binghui; Jie, Qing; Liu, Zihang; Saparamadu, Udara; Liu, Weishu; Ren, Zhifeng
2016-07-27
In this study, the thermoelectric properties of Mg2Sn0.98-xPbxSb0.02 were first studied, and then Mg2Sn0.93-xSixPb0.05Sb0.02 and Mg2Sn0.93-xGexPb0.05Sb0.02 were accordingly investigated. The results showed that the formation of Mg2Sn0.98-xPbxSb0.02 solid solutions effectively reduced the lattice thermal conductivity of Mg2Sn. The room temperature lattice thermal conductivity of Mg2Sn0.98Sb0.02 is ∼5.2 W m(-1) K(-1) but only ∼2.5 W m(-1) K(-1) for Mg2Sn0.73Pb0.25Sb0.02, a reduction of ∼52%. Further alloying Mg2Sn0.98-xPbxSb0.02 with Mg2Si or Mg2Ge to form Mg2Sn0.93-xSixPb0.05Sb0.02 or Mg2Sn0.93-xGexPb0.05Sb0.02 reduced the lattice thermal conductivity significantly due to enhanced phonon scattering by point defects as well as nanoparticles. Moreover, bipolar thermal conductivities were suppressed due to the larger bandgap of Mg2Si and Mg2Ge than Mg2Sn. Furthermore, similar to the pseudo-binary Mg2Sn-Mg2Si and Mg2Sn-Mg2Ge systems, band convergence was also observed in pseudo-ternary Mg2Sn0.93-xSixPb0.05Sb0.02 and Mg2Sn0.93-xGexPb0.05Sb0.02 materials. The convergence of conduction bands led to higher PFs at lower temperatures for Mg2Sn0.93-xSixPb0.05Sb0.02 and Mg2Sn0.93-xGexPb0.05Sb0.02 materials. As a result, higher peak ZTs of ∼1.3 for Mg2Sn0.63Si0.3Pb0.05Sb0.02 and ∼1.2 for Mg2Sn0.68Ge0.25Pb0.05Sb0.02 were achieved. PMID:27412367
NASA Astrophysics Data System (ADS)
He, Shixuan; Xie, Wanyi; Zhang, Wei; Zhang, Liqun; Wang, Yunxia; Liu, Xiaoling; Liu, Yulong; Du, Chunlei
2015-02-01
A novel strategy which combines iteratively cubic spline fitting baseline correction method with discriminant partial least squares qualitative analysis is employed to analyze the surface enhanced Raman scattering (SERS) spectroscopy of banned food additives, such as Sudan I dye and Rhodamine B in food, Malachite green residues in aquaculture fish. Multivariate qualitative analysis methods, using the combination of spectra preprocessing iteratively cubic spline fitting (ICSF) baseline correction with principal component analysis (PCA) and discriminant partial least squares (DPLS) classification respectively, are applied to investigate the effectiveness of SERS spectroscopy for predicting the class assignments of unknown banned food additives. PCA cannot be used to predict the class assignments of unknown samples. However, the DPLS classification can discriminate the class assignment of unknown banned additives using the information of differences in relative intensities. The results demonstrate that SERS spectroscopy combined with ICSF baseline correction method and exploratory analysis methodology DPLS classification can be potentially used for distinguishing the banned food additives in field of food safety.
He, Shixuan; Xie, Wanyi; Zhang, Wei; Zhang, Liqun; Wang, Yunxia; Liu, Xiaoling; Liu, Yulong; Du, Chunlei
2015-02-25
A novel strategy which combines iteratively cubic spline fitting baseline correction method with discriminant partial least squares qualitative analysis is employed to analyze the surface enhanced Raman scattering (SERS) spectroscopy of banned food additives, such as Sudan I dye and Rhodamine B in food, Malachite green residues in aquaculture fish. Multivariate qualitative analysis methods, using the combination of spectra preprocessing iteratively cubic spline fitting (ICSF) baseline correction with principal component analysis (PCA) and discriminant partial least squares (DPLS) classification respectively, are applied to investigate the effectiveness of SERS spectroscopy for predicting the class assignments of unknown banned food additives. PCA cannot be used to predict the class assignments of unknown samples. However, the DPLS classification can discriminate the class assignment of unknown banned additives using the information of differences in relative intensities. The results demonstrate that SERS spectroscopy combined with ICSF baseline correction method and exploratory analysis methodology DPLS classification can be potentially used for distinguishing the banned food additives in field of food safety. PMID:25300041
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.
Energy Science and Technology Software Center (ESTSC)
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
A wrinkly phononic crystal slab
NASA Astrophysics Data System (ADS)
Bayat, Alireza; Gordaninejad, Faramarz
2015-03-01
The buckling induced surface instability is employed to propose a tunable phononic crystal slab composed of a stiff thin film bonded on a soft elastomer. Wrinkles formation is used to generate one-dimensional periodic scatterers at the surface of a finitely thick slab. Wrinkles' pattern change and corresponding stress is employed to control wave propagation triggered by a compressive strain. Simulation results show that the periodic wrinkly structure can be used as a transformative phononic crystal which can switch band diagram of the structure in a reversible behavior. Results of this study provide opportunities for the smart design of tunable switch and elastic wave filters at ultrasonic and hypersonic frequency ranges.
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.
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.
Electron-phonon interaction effects in tantalum
Al-Lehaibi, A.; Swihart, J.C.; Butler, W.H.; Pinski, F.J.
1987-09-15
The results of calculations for a number of electron-phonon interaction effects for tantalum are presented. The calculations are based on Korringa-Kohn-Rostoker energy bands, Born--von Karman phonons, and the rigid-muffin-tin approximation for the electron-phonon matrix element. The calculated Eliashberg spectral function ..cap alpha../sup 2/F is compared with the earlier tunneling data of Shen and the proximity tunneling data of Wolf et al. The calculated and tunneling transverse-phonon peaks agree well, but the height of the tunneling longitudinal-phonon peak is smaller than the calculated results. The calculated electron-phonon coupling parameter lambda is 0.88, which is larger than the lambda determined from superconducting tunneling and superconducting T/sub c/ measurements, but is slightly smaller than the lambda determined from electronic specific-heat measurements. Calculated phonon linewidths along various symmetry directions are presented. The temperature dependence of the electrical resistivity due to phonon scattering is calculated in the lowest-order variational approximation and it agrees with experiment. The point-contact spectral function of Kulik, G(..omega..), is determined and compared with ..cap alpha../sup 2/F(..omega..). The agreement between calculated and measured electronic specific heat and high-temperature electrical resistivity gives strong support to the validity of the rigid-muffin-tin approximation for electron-phonon matrix elements.
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
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.
Phonon Spectrum of SrFe2As2 determined by multizone phonon refinement
Parshall, D; Heid, R; Niedziela, Jennifer L; Wolf, Th.; Stone, Matthew B; Abernathy, Douglas L; Reznik, Dmitry
2014-01-01
The ferropnictidesuperconductors exhibit a sensitive interplay between the lattice and magnetic degrees of freedom, including a number of phonon modes that are much softer than predicted by nonmagnetic calculations using density functional theory (DFT). However, it is not known what effect, if any, the long-range magnetic order has on phonon frequencies above 23 meV, where several phonon branches are very closely spaced in energy and it is challenging to isolate them from each other. We measured these phonons using inelastic time-of-flight neutron scattering in 40 Brillouin zones, and developed a technique to determine their frequencies. We find this method capable of determining phonon energies to 0.1 meV accuracy, and that the DFT calculations using the experimental structure yield qualitatively correct energies and eigenvectors. We do not find any effect of the magnetic transition on these phonons.
Chiral phonons at high-symmetry points in monolayer hexagonal lattices.
Zhang, Lifa; Niu, Qian
2015-09-11
In monolayer hexagonal lattices, the intravalley and intervalley scattering of electrons can involve chiral phonons at Brillouin-zone center and corners, respectively. At these high-symmetry points, there is a threefold rotational symmetry endowing phonon eigenmodes with a quantized pseudoangular momentum, which includes orbital and spin parts. Conservation of pseudoangular momentum yields selection rules for intravalley and intervalley scattering of electrons by phonons. Concrete predictions of helicity-resolved optical phenomena are made on monolayer molybdenum disulfide. The chiral phonons at Brillouin-zone corners excited by polarized photons can be detected by a valley phonon Hall effect. The chiral phonons, together with phonon circular polarization, phonon pseudoangular momentum, selection rules, and valley phonon Hall effect will extend the basis for valley-based electronics and phononics applications in the future. PMID:26406841
Chiral Phonons at High-Symmetry Points in Monolayer Hexagonal Lattices
NASA Astrophysics Data System (ADS)
Zhang, Lifa; Niu, Qian
2015-09-01
In monolayer hexagonal lattices, the intravalley and intervalley scattering of electrons can involve chiral phonons at Brillouin-zone center and corners, respectively. At these high-symmetry points, there is a threefold rotational symmetry endowing phonon eigenmodes with a quantized pseudoangular momentum, which includes orbital and spin parts. Conservation of pseudoangular momentum yields selection rules for intravalley and intervalley scattering of electrons by phonons. Concrete predictions of helicity-resolved optical phenomena are made on monolayer molybdenum disulfide. The chiral phonons at Brillouin-zone corners excited by polarized photons can be detected by a valley phonon Hall effect. The chiral phonons, together with phonon circular polarization, phonon pseudoangular momentum, selection rules, and valley phonon Hall effect will extend the basis for valley-based electronics and phononics applications in the future.
Phonon dispersion in red mercuric iodide
Sim, H.; Chang, Y. ); James, R.B. )
1994-02-15
We present theoretical studies of phonon modes of undoped HgI[sub 2] in its red tetragonal form. A rigid-ion model including the Coulomb interaction is used which gives the best fit to the neutron scattering, infrared reflectivity, and Raman scattering data. The calculated sound velocities are also in accord with experiment.
Calculates Thermal Neutron Scattering Kernel.
Energy Science and Technology Software Center (ESTSC)
1989-11-10
Version 00 THRUSH computes the thermal neutron scattering kernel by the phonon expansion method for both coherent and incoherent scattering processes. The calculation of the coherent part is suitable only for calculating the scattering kernel for heavy water.
NASA Astrophysics Data System (ADS)
Sasmal, Kalyan; Hadjiev, Viktor; Chu, C. W.(Paul)
Quaternary CaFeAsF has ZrCuSiAs-type structure,(RO)δ+ layer in RFeAsO replaced by (CaF)δ+ layer,with tetragonal (P4/nmm)-orthorhombic (Cmma) phase transition at 134K,while magnetic order,SDW sets in at 114K. Partial replacement of Fe with Co/Ni is direct electron doping to (FeAs)δ+ layer.Tc ~15K in CaFe0.9Ni0.1AsF.Substitution of rare earth metal for alkaline earth metal suppresses anomaly in resistivity & induces superconductivity.Tc ~52K in Ca0.5Pr0.5FeAsF.Characterized by resistivity, susceptibility,XRD & EDX-SEM.Upper critical field estimated from magneto resistance.Bulk superconductivity proved by DC magnetization. Hall coefficient RH revealed hole-like charge carriers in parent compound CaFeAsF, while electron-type (RH in normal state is -Ve) for Ca0.5Pr0.5FeAsF.Evolution of Raman active phonons of Ca1-xPrxFeAsF measured with polarized Raman spectroscopy at room temperature from absurfaces of impurity-free microcrystals.Spectra exhibit sharp phonon lines on very weak electronic scattering background.Frequency and symmetry of Raman phonons involving out-of-plane atomic vibrations are found at 162.5 cm-1 (A1 g, Pr), 201 cm-1 (A1 g, As), 215.5 cm-1 (B1 g, Fe), 265 cm-1 (Eg, Fe) and 334 cm-1 (B1 g, F) for Ca0.5Pr0.5FeAsF.Observations are compared with RFeAsO unconventional superconductors also possibly related to magnetic fluctuations
Lifetime of the phonons in the PLT ceramic
Barba-Ortega, J. Joya, M. R.; Londoño, F. A.
2014-11-05
The lifetimes at higher temperatures on lanthanum-modified lead titanate (PLT) are mainly due to the anharmonic decay of optical phonons into low-energy phonons. The temperature-independent contributions from inherent crystal defects and from boundary scattering become comparable to the phonon scattering contribution at lower temperatures. The thermal interaction is large at higher temperatures which decreases the phonon mean free path, and so the decay lifetime decreases as the temperature of the system is increased. This leads to the increased line width at higher temperatures. We made an estimate of the lifetimes for different concentrations and temperatures in PLT.
Geometrical tuning of thermal phonon spectrum in nanoribbons
NASA Astrophysics Data System (ADS)
Ramiere, Aymeric; Volz, Sebastian; Amrit, Jay
2016-03-01
Phonon spectral energy transmission in silicon nanoribbons is investigated using Monte-Carlo simulations in the boundary scattering regime by changing the length and width geometrical parameters. We show that the transition frequency from specular scattering to diffuse scattering is inversely proportional to the edge roughness σ with a geometry independent factor of proportionality. The increase of the length over width ratio \\zeta leads to a decrease of the energy transmission in the diffuse scattering regime which evolves as {{≤ft(1+{{\\zeta}0.59}\\right)}-1} . This trend is explained by developing a model of phonon energy transmission in the fully diffuse scattering regime which takes into account the probability for a diffusively scattered phonon to be directly transmitted from any position on the edge of the nanoribbon. This model establishes the importance of the solid angles in the energy transmission evolution with \\zeta . The transition from unity energy transmission in the specular scattering regime to reduced transmission in the diffuse scattering regime constitutes a low-pass frequency filter for phonons. Our simulations show an energy rejection rate better than 90% for high \\zeta , which paves the way for potential high performance filters. Filtering out high frequency phonons is of significant interest for phononic crystal applications, which use band engineering of phonons in the wave regime with low frequencies.
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.
Electrons and Phonons in Semiconductor Multilayers
NASA Astrophysics Data System (ADS)
Ridley, B. K.
2014-08-01
Introduction; 1. Simple models of the electron-phonon interaction; 2. Quantum confinement of carriers; 3. Quasicontinuum theory of lattice vibrations; 4. Bulk vibratory modes in an isotropic continuum; 5. Optical modes in a quantum well; 6. Superlattice modes; 7. Optical modes in various structures; 8. Electron-phonon interaction in a quantum well; 9. Other scattering mechanisms; 10. Quantum screening; 11. The electron distribution function; 12. Spin relaxation; 13. Electrons and phonons in the Wurtzite lattice; 14. Nitride heterostructures; 15. Terahertz sources; References; Index.
NASA Astrophysics Data System (ADS)
Huewe, Florian; Steeger, Alexander; Bauer, Irene; Doerrich, Steffen; Strohriegl, Peter; Pflaum, Jens
2015-10-01
We present a dynamical study on the nonlinear conduction behavior in the commensurate charge-density-wave phase of the quasi-one-dimensional conductor DCNQI2Cu below 75 K. We can accurately simulate magnitude and time dependence of the measured conductivity in response to large voltage pulses by accounting for the energy exchange between the phononic and electronic subsystems by means of an electrothermal model. Our simulations reveal a distinct nonequilibrium population of optical phonon states with an average energy of Eph¯=19 meV, being half the activation energy of about Δ Ea=39 meV observed in dc resistivity measurements. By inelastic scattering, this hot optical phonon bath generates additional charge-carrying excitations, thus providing a multiplication effect while energy transferred to the acoustic phonons is dissipated out of the system via heat conduction. Therefore, in high electric fields a preferred interaction of charge-carrying excitations with optical phonons compared to acoustic phonon modes is considered to be responsible for the nonlinear conduction effects observed in DCNQI2Cu .
Ultrafast coupling of coherent phonons with a nonequilibrium electron-hole plasma in GaAs
NASA Astrophysics Data System (ADS)
Basak, Amlan Kumar; Petek, Hrvoje; Ishioka, Kunie; Thatcher, Evan M.; Stanton, Christopher J.
2015-03-01
We present a joint experimental theoretical study of the coupling of coherent phonons in bulk GaAs with a nonequilibrium electron-hole plasma following photoexcitation at the E1 gap by ultrafast laser pulses. In contrast to prior coherent phonon experiments where photoexcitation across the E0 gap generated electrons in the Γ valley, for the E1 gap excitation, the majority of the electrons are generated in the satellite L valleys. This leads to a drastically different situation from the previous studies because the damping of electrons is now faster due to the higher scattering rates in the L valley, and, in addition, the diffusion of carriers has a significant effect on the plasma response due to the shorter optical absorption depth of the pump-probe light. Reflectivity measurements show coherent phonon-plasmon oscillations, whose frequencies lie between the transverse and longitudinal optical phonon frequencies due to the heavy damping and change with time due to the diffusion of the plasma. We analyze the experimental data with a theoretical model that describes the time and density-dependent coupling of the coherent phonon and the electron-hole plasma as the photoexcited carriers diffuse into the sample on a subpicosecond time scale. The calculated phonon-plasmon dynamics qualitatively reproduce the experimentally observed time-dependent frequency.
Polarized and spatially resolved Raman scattering from composition-graded wurtzite InGaAs nanowires
NASA Astrophysics Data System (ADS)
Kim, H.; Rho, H.; Lee, E. H.; Song, J. D.
2016-05-01
We report Raman scattering from wurtzite single-crystalline InGaAs nanowires (NWs) to probe optical phonon behaviors associated with spatial grading in alloy composition along the NW length. Polarized Raman spectra revealed several optical phonons and their scattering symmetries: (i) InAs-like A 1(LO) and A 1(TO) phonons and (ii) GaAs-like A 1(LO), A 1(TO), and E 2(high) phonons. In addition, strong anisotropic behavior was observed in the Raman tensor elements of the A 1(TO) phonon mode. Interestingly, a spatial mapping of the GaAs-like A 1(TO) phonon along the NW length direction showed a systematic increase in energy from the NW top (~255 cm‑1) to the midpoint (~263 cm‑1), indicating an increase in the Ga mole fraction from about 0.5 to about 0.8. Further toward the NW bottom, the GaAs-like A 1(TO) phonon energy saturated to the peak value at about 264 cm‑1. In the upper half of the NW, the phonon linewidths broadened significantly due to the spatial grading in In/Ga composition along the NW length. When the composition grading was negligible in the bottom half of the NW, the spectral widths were considerably narrowed. The GaAs-like E 2(high) phonon showed similar variations in both energy and spectral width along the NW length.
Localization of phonon polaritons in disordered polar media.
Satanin, Arkady M; Joe, Yong S; Kim, Chang Sub; Vasilevskiy, Mikhail I
2005-12-01
The localization of the hybrid modes of phonons and photons in polar matter is investigated in the presence of random scatterers theoretically. We employ the self-consistent generalized Born-Huang approach to derive effective equations describing the phonon-polariton fields. Based on these equations, the density of states and various localization properties are exploited in two-dimensional systems both analytically and numerically within the framework of the Anderson model with a non-Hermitian effective Hamiltonian. Consequently, it is shown that the disorder effect brings some intriguing features which include the appearance of the localized states in the polariton bottleneck in the energy spectrum and the collapse of the energy gap. In addition, an analysis is given of the polariton level-spacing distribution. PMID:16486089
NASA Technical Reports Server (NTRS)
Smalheer, C. V.
1973-01-01
The chemistry of lubricant additives is discussed to show what the additives are chemically and what functions they perform in the lubrication of various kinds of equipment. Current theories regarding the mode of action of lubricant additives are presented. The additive groups discussed include the following: (1) detergents and dispersants, (2) corrosion inhibitors, (3) antioxidants, (4) viscosity index improvers, (5) pour point depressants, and (6) antifouling agents.
Mishra, S. K.; Gupta, M. K.; Mittal, R.; Kolesnikov, Alexander I.; Chaplot, S. L.
2016-06-22
Here, we report inelastic neutron scattering measurements over 7–1251 K in CaMnO3 covering various phase transitions, and over 6–150 K in PrMnO3 covering the magnetic transition. The excitations around 20 meV in CaMnO3 and at 17 meV in PrMnO3 at low temperatures are found to be associated with magnetic origin. We observe coherent magnetic neutron scattering in localized regions in reciprocal space and show it to arise from long-range correlated magnetic spin-waves below the magnetic transition temperature (TN) and short-range stochastic spin-spin fluctuations above TN. In spite of the similarity of the structure of the two compounds, the neutron inelasticmore » spectrum of PrMnO3 exhibits broad features at 150 K unlike well-defined peaks in the spectrum of CaMnO3. This might result from the difference in the nature of interactions in the two compounds (magnetic and Jahn-Teller distortion). Ab initio phonon calculations have been used to interpret the observed phonon spectra. The ab initio calculations at high pressures show that the variations of Mn-O distances are isotropic for CaMnO3 and highly anisotropic for PrMnO3. The calculation in PrMnO3 shows the suppression of Jahn-Teller distortion and simultaneous insulator-to-metal transition. It appears that this transition may not be associated with the occurrence of the tetragonal phase above 20 GPa as reported in the literature, since the tetragonal phase is found to be dynamically unstable, although it is found to be energetically favored over the orthorhombic phase above 20 GPa. CaMnO3 does not show any phase transition up to 60 GPa.« less
Mitri, F. G.
2015-09-15
The standard Resonance Scattering Theory (RST) of plane waves is extended for the case of any two-dimensional (2D) arbitrarily-shaped monochromatic beam incident upon an elastic cylinder with arbitrary location using an exact methodology based on Graf’s translational addition theorem for the cylindrical wave functions. The analysis is exact as it does not require numerical integration procedures. The formulation is valid for any cylinder of finite size and material that is immersed in a nonviscous fluid. Partial-wave series expansions (PWSEs) for the incident, internal and scattered linear pressure fields are derived, and the analysis is further extended to obtain generalized expressions for the on-axis and off-axis acoustic radiation force components. The wave-fields are expressed using generalized PWSEs involving the beam-shape coefficients (BSCs) and the scattering coefficients of the cylinder. The off-axial BSCs are expressed analytically in terms of an infinite PWSE with emphasis on the translational offset distance d. Numerical computations are considered for a zeroth-order quasi-Gaussian beam chosen as an example to illustrate the analysis. Acoustic resonance scattering directivity diagrams are calculated by subtracting an appropriate background from the expression of the scattered pressure field. In addition, computations for the radiation force exerted on an elastic cylinder centered on the axis of wave propagation of the beam, and shifted off-axially are analyzed and discussed.
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
Soft surfaces of nanomaterials enable strong phonon interactions.
Bozyigit, Deniz; Yazdani, Nuri; Yarema, Maksym; Yarema, Olesya; Lin, Weyde Matteo Mario; Volk, Sebastian; Vuttivorakulchai, Kantawong; Luisier, Mathieu; Juranyi, Fanni; Wood, Vanessa
2016-03-31
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 10-13). 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 processes
Leman, Steven W
2012-09-01
This review discusses detector physics and Monte Carlo techniques for cryogenic, radiation detectors that utilize combined phonon and ionization readout. A general review of cryogenic phonon and charge transport is provided along with specific details of the Cryogenic Dark Matter Search detector instrumentation. In particular, this review covers quasidiffusive phonon transport, which includes phonon focusing, anharmonic decay, and isotope scattering. The interaction of phonons in the detector surface is discussed along with the downconversion of phonons in superconducting films. The charge transport physics include a mass tensor which results from the crystal band structure and is modeled with a Herring-Vogt transformation. Charge scattering processes involve the creation of Neganov-Luke phonons. Transition-edge-sensor (TES) simulations include a full electric circuit description and all thermal processes including Joule heating, cooling to the substrate, and thermal diffusion within the TES, the latter of which is necessary to model normal-superconducting phase separation. Relevant numerical constants are provided for these physical processes in germanium, silicon, aluminum, and tungsten. Random number sampling methods including inverse cumulative distribution function (CDF) and rejection techniques are reviewed. To improve the efficiency of charge transport modeling, an additional second order inverse CDF method is developed here along with an efficient barycentric coordinate sampling method of electric fields. Results are provided in a manner that is convenient for use in Monte Carlo and references are provided for validation of these models. PMID:23020355
Leman, Steven W.
2012-09-15
This review discusses detector physics and Monte Carlo techniques for cryogenic, radiation detectors that utilize combined phonon and ionization readout. A general review of cryogenic phonon and charge transport is provided along with specific details of the Cryogenic Dark Matter Search detector instrumentation. In particular, this review covers quasidiffusive phonon transport, which includes phonon focusing, anharmonic decay, and isotope scattering. The interaction of phonons in the detector surface is discussed along with the downconversion of phonons in superconducting films. The charge transport physics include a mass tensor which results from the crystal band structure and is modeled with a Herring-Vogt transformation. Charge scattering processes involve the creation of Neganov-Luke phonons. Transition-edge-sensor (TES) simulations include a full electric circuit description and all thermal processes including Joule heating, cooling to the substrate, and thermal diffusion within the TES, the latter of which is necessary to model normal-superconducting phase separation. Relevant numerical constants are provided for these physical processes in germanium, silicon, aluminum, and tungsten. Random number sampling methods including inverse cumulative distribution function (CDF) and rejection techniques are reviewed. To improve the efficiency of charge transport modeling, an additional second order inverse CDF method is developed here along with an efficient barycentric coordinate sampling method of electric fields. Results are provided in a manner that is convenient for use in Monte Carlo and references are provided for validation of these models.
NASA Astrophysics Data System (ADS)
Leman, Steven W.
2012-09-01
This review discusses detector physics and Monte Carlo techniques for cryogenic, radiation detectors that utilize combined phonon and ionization readout. A general review of cryogenic phonon and charge transport is provided along with specific details of the Cryogenic Dark Matter Search detector instrumentation. In particular, this review covers quasidiffusive phonon transport, which includes phonon focusing, anharmonic decay, and isotope scattering. The interaction of phonons in the detector surface is discussed along with the downconversion of phonons in superconducting films. The charge transport physics include a mass tensor which results from the crystal band structure and is modeled with a Herring-Vogt transformation. Charge scattering processes involve the creation of Neganov-Luke phonons. Transition-edge-sensor (TES) simulations include a full electric circuit description and all thermal processes including Joule heating, cooling to the substrate, and thermal diffusion within the TES, the latter of which is necessary to model normal-superconducting phase separation. Relevant numerical constants are provided for these physical processes in germanium, silicon, aluminum, and tungsten. Random number sampling methods including inverse cumulative distribution function (CDF) and rejection techniques are reviewed. To improve the efficiency of charge transport modeling, an additional second order inverse CDF method is developed here along with an efficient barycentric coordinate sampling method of electric fields. Results are provided in a manner that is convenient for use in Monte Carlo and references are provided for validation of these models.
Anharmonicity due to Electron-Phonon Coupling in Magnetite
NASA Astrophysics Data System (ADS)
Hoesch, Moritz; Piekarz, Przemysław; Bosak, Alexey; Le Tacon, Mathieu; Krisch, Michael; Kozłowski, Andrzej; Oleś, Andrzej M.; Parlinski, Krzysztof
2013-05-01
We present the results of inelastic x-ray scattering for magnetite and analyze the energies and widths of the phonon modes with different symmetries in a broad range of temperature 125
Phonon limited superconducting correlations in metallic nanograins
NASA Astrophysics Data System (ADS)
Croitoru, M. D.; Shanenko, A. A.; Vagov, A.; Milošević, M. V.; Axt, V. M.; Peeters, F. M.
2015-11-01
Conventional superconductivity is inevitably suppressed in ultra-small metallic grains for characteristic sizes smaller than the Anderson limit. Experiments have shown that above the Anderson limit the critical temperature may be either enhanced or reduced when decreasing the particle size, depending on the superconducting material. In addition, there is experimental evidence that whether an enhancement or a reduction is found depends on the strength of the electron-phonon interaction in the bulk. We reveal how the strength of the e-ph interaction interplays with the quantum-size effect and theoretically obtain the critical temperature of the superconducting nanograins in excellent agreement with experimental data. We demonstrate that strong e-ph scattering smears the peak structure in the electronic density-of-states of a metallic grain and enhances the electron mass, and thereby limits the highest Tc achievable by quantum confinement.
Phonon limited superconducting correlations in metallic nanograins
Croitoru, M. D.; Shanenko, A. A.; Vagov, A.; Milošević, M. V.; Axt, V. M.; Peeters, F. M.
2015-01-01
Conventional superconductivity is inevitably suppressed in ultra-small metallic grains for characteristic sizes smaller than the Anderson limit. Experiments have shown that above the Anderson limit the critical temperature may be either enhanced or reduced when decreasing the particle size, depending on the superconducting material. In addition, there is experimental evidence that whether an enhancement or a reduction is found depends on the strength of the electron-phonon interaction in the bulk. We reveal how the strength of the e-ph interaction interplays with the quantum-size effect and theoretically obtain the critical temperature of the superconducting nanograins in excellent agreement with experimental data. We demonstrate that strong e-ph scattering smears the peak structure in the electronic density-of-states of a metallic grain and enhances the electron mass, and thereby limits the highest Tc achievable by quantum confinement. PMID:26565073
Origin of reduction in phonon thermal conductivity of microporous solids
NASA Astrophysics Data System (ADS)
Hopkins, Patrick E.; Rakich, Peter T.; Olsson, Roy H.; El-kady, Ihab F.; Phinney, Leslie M.
2009-10-01
Porous structures have strong tunable size effects due to increased surface area. Size effects on phonon thermal conductivity have been observed in porous materials with periodic voids on the order of microns. This letter explores the origin of this size effect on phonon thermal conductivity observed in periodic microporous membranes. Pore-edge boundary scattering of low frequency phonons explains the temperature trends in the thermal conductivity; further reduction in thermal conductivity is explained by the porosity.
Phonon populations and electrical power dissipation in carbon nanotube transistors.
Steiner, Mathias; Freitag, Marcus; Perebeinos, Vasili; Tsang, James C; Small, Joshua P; Kinoshita, Megumi; Yuan, Dongning; Liu, Jie; Avouris, Phaedon
2009-05-01
Carbon nanotubes and graphene are candidate materials for nanoscale electronic devices. Both materials show weak acoustic phonon scattering and long mean free paths for low-energy charge carriers. However, high-energy carriers couple strongly to optical phonons, which leads to current saturation and the generation of hot phonons. A non-equilibrium phonon distribution has been invoked to explain the negative differential conductance observed in suspended metallic nanotubes, while Raman studies have shown the electrical generation of hot G-phonons in metallic nanotubes. Here, we present a complete picture of the phonon distribution in a functioning nanotube transistor including the G and the radial breathing modes, the Raman-inactive zone boundary K mode and the intermediate-frequency mode populated by anharmonic decay. The effective temperatures of the high- and intermediate-frequency phonons are considerably higher than those of acoustic phonons, indicating a phonon-decay bottleneck. Most importantly, inclusion of scattering by substrate polar phonons is needed to fully account for the observed electronic transport behaviour. PMID:19421219
Pressure-enabled phonon engineering in metals.
Lanzillo, Nicholas A; Thomas, Jay B; Watson, Bruce; Washington, Morris; Nayak, Saroj K
2014-06-17
We present a combined first-principles and experimental study of the electrical resistivity in aluminum and copper samples under pressures up to 2 GPa. The calculations are based on first-principles density functional perturbation theory, whereas the experimental setup uses a solid media piston-cylinder apparatus at room temperature. We find that upon pressurizing each metal, the phonon spectra are blue-shifted and the net electron-phonon interaction is suppressed relative to the unstrained crystal. This reduction in electron-phonon scattering results in a decrease in the electrical resistivity under pressure, which is more pronounced for aluminum than for copper. We show that density functional perturbation theory can be used to accurately predict the pressure response of the electrical resistivity in these metals. This work demonstrates how the phonon spectra in metals can be engineered through pressure to achieve more attractive electrical properties. PMID:24889627