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Nano-scale wave dispersion beyond the First Brillouin Zone simulated with inertia gradient continua
摘要： Nano-scale wave dispersion beyond the First Brillouin Zone is often observed as descending branches and inflection points when plotting frequency or phase velocity against the wave number. Modeling this with discrete chain models is hampered by their restricted resolution. A continuum model equipped with higher-order inertia gradients is here developed as a suitable and versatile alternative. This model can be derived from discrete chain models, thereby providing a lower-scale motivation for the higher-order gradient terms. The derived gradient model is without free parameters, as the material constants are calculated a priori by minimising the error with respect to the discrete chain response. Unlike asymptotic approximations that provide a best fit for vanishing wave numbers, the error is here minimised over the entire range of reduced wave numbers 0 to 1, which leads to a much improved accuracy beyond the First Brillouin Zone. The new gradient model has been validated against (i) phonon dispersion curves measured through neutron scattering experiments in bismuth, aluminum, and nickel and (ii) Molecular Dynamics (MD) flexural wave propagation simulations of carbon nanotubes. The model captures all qualitative aspects of the experimental and MD dispersion curves without requiring a bespoke curve fitting procedure. With the exception of one set of MD results, the accuracy of the gradient model is very good.
关键词： Nano-scale wave dispersion,Molecular Dynamics,First Brillouin Zone,inertia gradient continua,carbon nanotubes,phonon dispersion curves
更新于2025-09-10 09:29:36
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Selective thermal radiation at longitudinal optical phonon energy under geometric condition of metal-semiconductor mesa stripe structures
摘要： Selective thermal emission at approximately 284 cm?1 with a line width of 13 cm–1 at 628 K is obtained by heating the Au-GaAs mesa-type line and space structures. This emission demonstrates a characteristic polar radiation distribution of electric dipoles and a wavelength independent of the structure size or the direction of emission; thus, it is ascribed to thermally generated electric dipoles resonating with the longitudinal optical phonon energy. This dipole is formed by the interface polarization charges of metal-GaAs-metal stripes. These distinct features are different from the radiation properties of interface phonon polaritons or electronic systems such as intersubband transitions of quantum wells. It is estimated that the mesa height and GaAs space width ranging from hundreds of nanometers to several micrometers provide high generation ef?ciency of electric dipoles and resultant radiation at high temperatures.
关键词： metal-semiconductor mesa stripe structures,longitudinal optical phonon,electric dipoles,thermal emission
更新于2025-09-10 09:29:36
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Anharmonic stabilization and lattice heat transport in rocksalt <i>β</i> -GeTe
摘要： Peierls-Boltzmann transport equation, coupled with third-order anharmonic lattice dynamics calculations, has been widely used to model lattice thermal conductivity (jl) in bulk crystals. However, its application to materials with structural phase transition at relatively high temperature is fundamentally challenged by the presence of lattice instabilities (imaginary phonon modes). Additionally, its accuracy suffers from the absence of higher-than-third-order phonon scattering processes, which are important near/above the Debye temperature. In this letter, we present an effective scheme that combines temperature-induced anharmonic phonon renormalization and four-phonon scattering to resolve these two theoretical challenges. We apply this scheme to investigate the lattice dynamics and thermal transport properties of GeTe, which undergoes a second-order ferroelectric phase transition from rhombohedral a-GeTe to rocksalt b-GeTe at about 700 K. Our results on the high-temperature phase b-GeTe at 800 K con?rm the stabilization of b-GeTe by temperature effects. We ?nd that considering only three-phonon scattering leads to signi?cantly overestimated jl of 3.8 W/mK at 800 K, whereas including four-phonon scattering reduces jl to 1.7 W/mK, a value comparable with experiments. To explore the possibility to further suppress jl, we show that alloying b-GeTe with heavy cations such as Pb and Bi can effectively reduce jl to about 1.0 W/mK, whereas particle size needs to be around 10 nm through nanostructuring to achieve a comparable reduction in jl.
关键词： rocksalt β-GeTe,lattice heat transport,thermal conductivity,phonon scattering,anharmonic stabilization
更新于2025-09-10 09:29:36
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Effect of B2O3 Content on Eu3+?Surrounding in The System SrO?x.B2O3 Containing Eu3+ Ions Prepared by Combustion Method
摘要： The change of Eu3+? surrounding in the system SrO?x.B2O3 with various x values (x = 0.33, 0.5, 1) synthesized by combustion method was investigated via photoluminescence (PL) spectra and phonon sideband (PSB) spectra. From the analysis of PSB spectra, it is reasonable to suggest that when increasing B2O3 content the electron?phonon coupling strength increases due to the increase in the number of B?O? non?bridging bond. By using Judd?Ofelt (JO) theory for PL spectra of Eu3+ ion, the ?2 intensity parameter was calculated and it also shows the asymmetry of Eu3+? surrounding increase corresponding to the B2O3 content.
关键词： Eu3+,Judd?Ofelt theory,Combustion method,electron-phonon coupling,B2O3,phonon sideband
更新于2025-09-09 09:28:46
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Direct probing of cross-plane thermal properties of ALD Al <sub/>2</sub> O <sub/>3</sub> /ZnO superlattice films with improved figure of merit and their cross-plane thermoelectric generating performance
摘要： There is a recent interest in semiconducting superlattice films because their low dimensionality can increase the thermal power and phonon scattering at the interface in superlattice films. However, experimental studies in all cross-plane thermoelectric (TE) properties, including thermal conductivity, Seebeck coefficient, and electrical conductivity, has not been performed from these semiconducting superlattice films, because of substantial difficulties in the direct measurement of the Seebeck coefficient and electrical conductivity. Unlike the conventional measurement method, we present technique using a structure of sandwiched superlattice films between two embedded heaters as heating source, and electrodes with two Cu plates, which directly enables the investigation of the Seebeck coefficient and electrical conductivity across the Al2O3/ZnO superlattice films, prepared by atomic layer deposition (ALD) method. Used in combination with the promising cross-plane four-point-probe 3-ω method, our measurements and analysis demonstrate all cross-plane TE properties of Al2O3/ZnO superlattice films in the temperature range from 80 to 500 K. Our experimental methodology and the obtaining results represent a significant advancement in the understating of phonon and electrical transports in nanostructured materials, especially in semiconducting superlattice films in various temperature ranges.
关键词： Thermal conductivity,Cross-plane thermoelectric properties,Seebeck coefficient,Phonon transport,Superlattice films,3-omega measurement,Phonon scattering
更新于2025-09-09 09:28:46
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On the thermal conductivity of AgSbTe <sub/>2</sub> and Ag <sub/>0.82</sub> Sb <sub/>1.18</sub> Te <sub/>2.18</sub>
摘要： The temperature dependencies of the thermal conductivity of AgSbTe2 and Ag0.82Sb1.18Te2.18 were studied in the temperature range of 5–320 K. The obtained results are compared with quantitative calculations based on a theoretical model of generalized conductivity of heterogeneous systems. It was shown that a rather low value of thermal conductivity of AgSbTe2 and Ag0.82Sb1.18Te2.18 is associated with the disordering of the crystal structure, as well as phonon scattering by point defects and structural components. The presence of the second phase of Ag2Te leads to an increase in the thermal conductivity of AgSbTe2.
关键词： crystal structure disordering,AgSbTe2,thermal conductivity,Ag0.82Sb1.18Te2.18,phonon scattering
更新于2025-09-09 09:28:46
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Coherent Acoustic Phonon and Its Chirping in Dirac Semimetal Cd <sub/>3</sub> As <sub/>2</sub>
摘要： Ultrafast optical spectroscopy of a single crystal of a Dirac semimetal Cd3As2 is carried out. An acoustic phonon (AP) mode with central frequency ?? = 0.037 THz (i.e., 1.23 cm?1 or 0.153 meV) is unambiguously generated and detected, which we attribute to laser-induced thermal strain. An AP chirping (i.e., variation of the phonon frequency) is clearly detected, which is ascribed to heat capacity variation with time. By comparing our experimental results and the theoretical model, we obtain a chirping time constant, which is 31.2 ps at 6 K and 19.8 ps at 300 K, respectively. Significantly, we identify an asymmetry in the AP frequency domain peak and find that it is caused by the chirping, instead of a Fano resonance. Moreover, we experimentally demonstrate that the central frequency of AP is extremely stable with varying laser fluence, as well as temperature, which endows Cd3As2 application potentials in thermoelectric devices.
关键词： Cd3As2,chirping,acoustic phonon,Dirac semimetal,thermoelectric devices,thermal strain
更新于2025-09-09 09:28:46
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Raman scattering studies on very thin layers of gallium sulfide (GaS) as a function of sample thickness and temperature.
摘要： Gallium sulfide is a semiconducting material with a layered structure and a characteristic low interlayer interaction. Because of weak van der Waals forces, GaS crystals are relatively easy to exfoliate to very thin layers. In this work nanometric-GaS layers were obtained by a micro-mechanical exfoliation process and were transferred to Si/SiO2 substrate. The thickness of these layers was estimated from AFM measurements. Raman spectra were collected for different layer thicknesses ranging from one layer to bulk crystal. An analytical function fitted to experimental data is proposed to determine layer thickness from Raman measurements. For the first time, the Raman position and the FWHM of the main Raman peaks were measured on very thin GaS layers as a function of temperature in the range from 80 to 470 K. The first order temperature coefficients of the A1g Raman peaks were determined. Phonon decay due to anharmonic processes at temperatures above 300 K in layers of thickness below 4 nm was observed. Contribution of optical phonon scattering processes to thermal properties of very thin GaS layers is discussed.
关键词： GaS monolayer,anharmonic decay,metal III chalcogenides,layered 2d semiconductors,phonon lifetime
更新于2025-09-09 09:28:46
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A Model on Heat Signal of Crystal Detector at Low Temperature
摘要： We present a model to calculate heat signal shapes from low-temperature bolometer attached to a crystal. This model is based on the elementary acoustic wave theory at low temperature and has been developed using modern Monte Carlo techniques. Physical processes in phonon propagation, such as transmission, scattering and re?ection are considered. Using our model, the calculated time dependence of signal agrees with real experimental data. This model has applications in low-temperature rare event particle detectors for dark matter and neutrinos.
关键词： Cryogenic calorimeter,Lattice dynamics,Phonon physics,Simulation,Acoustic wave
更新于2025-09-09 09:28:46
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Prospects for the Detection of Electronic Preturbulence in Graphene
摘要： Based on extensive numerical simulations, accounting for electrostatic interactions and dissipative electron-phonon scattering, we propose experimentally realizable geometries capable of sustaining electronic preturbulence in graphene samples. In particular, preturbulence is predicted to occur at experimentally attainable values of the Reynolds number between 10 and 50, over a broad spectrum of frequencies between 10 and 100 GHz.
关键词： Reynolds number,electron-phonon scattering,graphene,electronic preturbulence
更新于2025-09-09 09:28:46