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Coherent optical and acoustic phonons generated at lattice-matched GaP/Si(001) heterointerfaces
摘要： Thin GaP films can be grown on an exact Si(001) substrate with nearly perfect lattice match. We perform all-optical pump-probe measurements to investigate the ultrafast electron-phonon coupling at the buried interface of GaP/Si. Above-bandgap excitation with a femtosecond laser pulse can induce coherent longitudinal optical (LO) phonons both in the GaP overlayer and in the Si substrate. The coupling of the GaP LO phonons with photoexcited plasma is reduced significantly with decreasing the GaP layer thickness from 56 to 16 nm due to the quasi-two-dimensional confinement of the plasma. The same laser pulse can also generate coherent longitudinal acoustic (LA) phonons in the form of a strain pulse. The strain pulses induce not only a periodic modulation in the optical reflectivity as they propagate in the semiconductors, but also a sharp spike when it arrives at the GaP layer boundaries. The acoustic pulse induced at the GaP/Si interface is remarkably stronger than that at the Si surface, suggesting a possible application of the GaP/Si heterostructure as an opto-acoustic transducer. The amplitude and the phase of the reflectivity modulation varies with the GaP layer thickness, which can be understood in terms of the interference caused by the multiple acoustic pulses generated at the top surface and at the buried interface.
关键词： opto-acoustic transducer,GaP/Si heterointerfaces,coherent acoustic phonons,ultrafast electron-phonon coupling,coherent optical phonons
更新于2025-09-09 09:28:46
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Phonon thermodynamics and elastic behavior of GaN at high temperatures and pressures
摘要： The effects of temperature and pressure on the phonons of GaN were calculated for both the wurtzite and zinc-blende structures. The quasiharmonic approximation (QHA) gave reasonable results for the temperature dependence of the phonon DOS at zero pressure but unreliably predicted the combined effects of temperature and pressure. Pressure was found to change the explicit anharmonicity, altering the thermal shifts of phonons and more notably qualitatively changing the evolution of phonon lifetimes with increasing temperature. These effects were largest for the optical modes, and phonon frequencies below approximately 5 THz were adequately predicted with the QHA. The elastic anisotropies of GaN in both wurtzite and zinc-blende structures were calculated from the elastic constants as a function of pressure at 0 K. The elastic anisotropy increased with pressure until reaching elastic instabilities at 40 GPa (zinc blende) and 65 GPa (wurtzite). The calculated instabilities are consistent with proposed transformation pathways to rocksalt GaN and place upper bounds on the pressures at which wurtzite and zinc-blende GaN can be metastable.
关键词： phonon thermodynamics,high pressures,elastic anisotropy,anharmonicity,high temperatures,GaN,elastic behavior,quasiharmonic approximation
更新于2025-09-09 09:28:46
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Nonequilibrium electron dynamics in pump-probe spectroscopy: Role of excited phonon populations
摘要： We study the role of excited phonon populations in the relaxation rates of nonequilibrium electrons using a nonequilibrium Green’s function formalism. The transient modifications in the phononic properties are accounted for by self-consistently solving the Dyson equation for the electron and phonon Green’s functions. The pump-induced changes manifest in both the electronic and phononic spectral functions. We find that the excited phonon populations suppress the decay rates of nonequilibrium electrons due to enhanced phonon absorption. The increased phonon occupation also sets the nonequilibrium decay rates and the equilibrium scattering rates apart. The decay rates are found to be time dependent, and this is illustrated in the experimentally observed population decay of photoexcited Bi1.5Sb0.5Te1.7Se1.3.
关键词： relaxation rates,nonequilibrium electron dynamics,excited phonon populations,nonequilibrium Green’s function formalism,pump-probe spectroscopy
更新于2025-09-09 09:28:46
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The Correlation between the Energy Gap and the Pseudogap Temperature in Cuprates: The YCBCZO and LSHCO Case
摘要： The paper analyzes the influence of the hole density, the out-of-plane or in-plane disorder, and the isotopic oxygen mass on the zero temperature energy gap (2Δ(0)) Y1???Ca??Ba2(Cu1???Zn??)3O7??? (YCBCZO) and La1.96???Sr??Ho0.04CuO4 (LSHCO) superconductors. It has been found that the energy gap is visibly correlated with the value of the pseudogap temperature (???). On the other hand, no correlation between 2Δ(0) and the critical temperature (????) has been found. The above results mean that the value of the dimensionless ratio 2Δ(0)/???????? can vary very strongly together with the chemical composition, while the parameter 2Δ(0)/??????? does not change significantly. In the paper, the analytical formula which binds the zero temperature energy gap and the pseudogap temperature has been also presented.
关键词： energy gap,pseudogap temperature,superconductors,cuprates,electron-phonon interaction,electron-electron correlations
更新于2025-09-09 09:28:46
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Demonstration of resonant inelastic x-ray scattering as a probe of exciton-phonon coupling
摘要： Resonant inelastic x-ray scattering (RIXS) is a promising technique for obtaining electron-phonon coupling constants. However, the ability to extract these coupling constants throughout the Brillouin zone for crystalline materials remains limited. To address this need, we developed a Green’s function formalism to capture electron-phonon contributions to core-level spectroscopies without explicitly solving the full vibronic problem. Our approach is based on the cumulant expansion of the Green’s function combined with many-body theory calculated vibrational coupling constants. The methodology is applied to x-ray photoemission spectroscopy, x-ray absorption spectroscopy (XAS), and RIXS. In the case of the XAS and RIXS, we use a two-particle exciton Green’s function, which accounts implicitly for particle-hole interference effects that have previously proved difficult. To demonstrate the methodology and gain a deeper understanding of the experimental technique, we apply our formalism to small molecules, for which unambiguous experimental data exist. This comparison reveals that the vibronic coupling constant probed by RIXS is in fact related to exciton-phonon coupling rather than electron-phonon coupling, challenging the conventional interpretation of the experiment.
关键词： electron-phonon coupling,Green’s function formalism,exciton-phonon coupling,Resonant inelastic x-ray scattering,vibronic coupling
更新于2025-09-04 15:30:14
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Strong Coupling of Folded Phonons with Plasmons in 6H-SiC Micro/Nanocrystals
摘要： Silicon carbide (SiC) has a large number of polytypes of which 3C-, 4H-, 6H-SiC are most common. Since different polytypes have different energy gaps and electrical properties, it is important to identify and characterize various SiC polytypes. Here, Raman scattering is performed on 6H-SiC micro/nanocrystal (MNC) films to investigate all four folded transverse optic (TO) and longitudinal optic (LO) modes. With increasing film thickness, the four folded TO modes exhibit the same frequency downshift, whereas the four folded LO modes show a gradually-reduced downshift. For the same film thickness, all the folded modes show larger frequency downshifts with decreasing MNC size. Based on plasmons on MNCs, these folded modes can be attributed to strong coupling of the folded phonons with plasmons which show different strengths for the different folded modes while changing the film thickness and MNC size. This work provides a useful technique to identify SiC polytypes from Raman scattering.
关键词： folded phonons,SiC micro/nanocrystals,plasmon–phonon coupling
更新于2025-09-04 15:30:14
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Effect of an underdamped vibration with both diagonal and off‐diagonal exciton–phonon interactions on excitation energy transfer
摘要： A numerically exact approach, named as the hierarchical stochastic Schr?dinger equation, is employed to investigate the resonant vibration-assisted excitation energy transfer in a dimer system, where an underdamped vibration with both diagonal and off-diagonal exciton–phonon interactions is incorporated. From a large parameter space over the site-energy difference, excitonic coupling, and reorganization energy, it is found that the promotion effect of the underdamped vibration is significant only when the excitonic coupling is smaller than the site-energy difference. Under the circumstance, there is an optimal strength ratio between diagonal and off-diagonal exciton–phonon interactions for the resonant vibration-assisted excitation energy transfer as the site-energy difference is greater than the reorganization energy, whereas in the opposite situation the most efficient energy transfer occurs as the exciton–phonon interaction is totally off-diagonal.
关键词： underdamped vibration,energy transfer,exciton–phonon interaction,non-Condon effect
更新于2025-09-04 15:30:14
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: <i>Ab initio</i> lattice dynamical perspective
摘要： The hydration and ammonization of ZrW2O8 is known to lead to positive and negative thermal expansion behavior, respectively. We report ab initio calculations to understand this anomalous behavior. We identify the crucial low-energy phonon modes involving translations, rotations, and distortions of WO4 and ZrO6 polyhedra, which lead to negative thermal expansion in ZrW2O8 in pure and ammoniated forms; however, the rotation and distortion motions get inhibited on hydration and lead to positive thermal expansion. We demonstrate that the thermal expansion coef?cient could be tailored by engineering the phonon dynamics of a material.
关键词： ZrW2O8,hydration,phonon dynamics,ab initio calculations,ammonization,thermal expansion
更新于2025-09-04 15:30:14
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Unusual polarization and temperature-dependent Raman response in Weyl semimetal NbIrTe4
摘要： We carry out a systematic study of polarized- and temperature-dependent Raman scattering of layered Weyl semimetal NbIrTe4 on the (002) crystal surface, under parallel and perpendicular polarization Raman spectroscopy geometry in back scattering way, the angle-dependent Raman peaks intensity is recorded by rotating the single crystal from 0° to 360°, in combination with group theory analysis, all the prominent Raman peaks are assigned to A1 and B1 vibrational modes, moreover, we show that the angular dependence of Raman peak intensity of A1 mode is rather unusual and can be explained by complex values for Raman tensor elements, which is traced back to electron-phonon interaction in this material. Meanwhile, lattice anharmonic vibrational behavior is studied by investigating the temperature-dependent Raman scattering spectra, it is observed that all the Raman peaks show linear sti?ening behavior as the temperature decreased from 293 K to 83 K, the temperature coe?cients of four prominent Raman peaks suggest the anharmonicity of the lattice in layered NbIrTe4 crystal.
关键词： Lattice anharmonicity,Electron-phonon coupling,Vibrational modes,Polarized Raman scattering
更新于2025-09-04 15:30:14
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Structural Characterization and Room Temperature Low Frequency Raman Scattering from MAPbI3 Halide Perovskite Films Rigidized by Cesium Incorporation
摘要： The structural instability of organometal halide perovskites (OHP) is one of the major issues concerning commercialization of perovskite solar cells. Probing this intrinsic instability is one of the major milestones and challenging tasks towards enhancing the lifespan of the material. Here we have incorporated Cs ions into methylammonium lead iodide (MAPbI3) films and studied the thin film structural and optical properties. Incorporation of Cs into MAPbI3 leads to formation of both α-CsPbI3 and ?-CsPbI3 phases, black and yellow respectively, as indicated by the evolution of the optical band edge and X-Ray diffraction (XRD) spectrum. At a concentration of 20% Cs ions, we observe existence of a stable α-CsPbI3 phase. Incorporating 59% or more Cs ions yields the yellow phase of CsPbI3, due to alloying of Cs with the MAPbI3 matrix. The structural transformations observed in absorption spectra and XRD are confirmed by Low Frequency Raman Spectroscopy. The thermally induced structural fluctuations in pure MAPbI3 films are damped upon Cs incorporation, thus bringing long range stabilized order to the perovskite structure and enabling for the first time observation of Low Frequency Raman Scattering at room temperature for OHPs. In addition to this, Cs incorporation rigidizes the perovskite film and sharpens all low frequency vibrational peaks.
关键词： Cesium incorporation,Low Frequency Raman,Perovskite Stability,Phonon Damping,Hybrid Perovskite
更新于2025-09-04 15:30:14