Long-wavelength LO-phonon generation during hot-electron cooling in polar semiconductors
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In a hot-electron system in polar semiconductors a new mechanism is studied theoretically for long-wavelength LO-phonon generation, based on the fusion of hot acoustic phonons. The anharmonic mechanism of the LO-phonon decay into pairs of acoustic phonons is considered together with the reverse process of the fusion of acoustic phonons into the LO ones. Corresponding kinetic equations for the electronic temperature and for the distribution functions of both kinds of phonons are solved numerically in GaAs and CdSe. The relation of the numerical results on the long-wavelength LO-phonon generation, to the recent experiments by Tsen et al. in ultrathin GaAs/AlAs multiple quantum wells is discussed.Although the quasi-harmonic approximation (QHA) method applies to many materials, it is necessary to study the anharmonic interaction for extremely anharmonic materials. In this work, the unusual negative thermal expansion (NTE) property of CaTiF6 is studied by combing QHA and anharmonic interaction. The improved self-consistent phonon approximation (ISCPA), which treats anharmonic effects in solids nonperturbatively, is employed. The agreement of NTE behavior between the calculation and the experiment can be further promoted from qualitative consistency by QHA to quantitative consistency by the ISCPA. From mode Grüneisen parameters, it is found that the low-frequency phonons, especially acoustic phonons, contribute greatly to the NTE behavior of CaTiF6. The rigid unit modes (RUMs) of low-frequency optical phonons can be identified. The phonon lifetime of CaTiF6 is calculated from three-phonon interactions; thereby, the NTE mechanism can be further explored by phonon lifetimes of phonons with different frequencies on heating. The anomalous lattice thermal conductivity (LTC) is predicted using the Boltzmann transport equation within the relaxation time approximation. The glasslike LTC can occur in crystal CaTiF6.
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Negative Thermal Expansion
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Abstract We report a computational study, using the “moments method” [Y. Gao and M. Daw, Modell. Simul. Mater. Sci. Eng. 23 045002 (2015)], of the anharmonicity of the vibrational modes of single‐walled carbon nanotubes. We find that modes with displacements largely within the wall are more anharmonic than modes with dominantly radial character, except for a set of modes that are related to the radial breathing mode that are the most anharmonic of all. We also find that periodicity of the calculation along the tube length does not strongly affect the anharmonicity of the modes but that the tubes with larger diameter show more anharmonicity. Comparison is made with available experiments and other calculations.
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The theoretical and experimental studies of anharmonic temperature factor in X-ray and neutron diffraction are reviewed since early works in 1963. These studies are in the framework of effective one body particle potential in which atoms are treated as independent oscillators.The experimental works are classified into two major groups : investigations of anti-centrosymmetric anharmonicity and centrosymmetric anharmonicity. It is pointed out that anti-centrosymmetric anharmonicity is well determined from the accurate Bragg intensity data at a certain temperature but centrosymmetric anharmonicity, because of the correlation of harmonic and anharmonic potential parameters, both of which are centrosymmetric. It is more reliable to determine the centrosymmetric anharmoni-city using the temperature dependence of integrated Bragg intensities unless extremely high Q data are collected.It is suggested that the degree of anharmonicity is much betterdescribed by a ratio of anharmonic term to harmonic term in one body particle potential than the magnitude of anharmonic potential parameter itself. The anharmonic effect is more significant for those substances which have smaller anharmonic potential coefficients.The relationships between anharmonicity of temperature factor and structural phase transition is discussed in the case of several perovskite substances.
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The anharmonicity of the Ruddlesden Popper metal-halide lattice, and its consequences on their electronic and optical properties, is paramount in their basic semiconductor physics. It is thus critical to identify specific anharmonic optical phonons that govern their photophysics . Here, we address the nature of phonon-phono scattering probabilities of the resonantly excited optical phonons that dress the electronic transitions in these materials by means of variable-temperature resonant impulsive stimulated Raman measurements. Based on the temperature dependence of the coherent phonon lifetimes, we isolate the dominant anharmonic phonon and quantify its phonon-phonon interaction strength. Intriguingly, we also observe that the anharmonicity is distinct for different phonons, with a few select modes exhibiting temperature-independent coherence lifetimes, indicating their predominantly harmonic nature. However, the population and dephasing dynamics of excitons are dominated by the anharmonic phonon.
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This chapter begins with a discussion of the general theory of transition possibilities. It then discusses anharmonic lattice forces, effects of selection rules, interaction with optical modes, four-phonon processes, elastic anharmonicity, thermal expansion, and the absorption of sound in solids.
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Abstract Group I niobates (KNbO 3 and NaNbO 3 ) are promising lead-free alternatives for high-performance energy storage applications. Despite their potential, their complex phase transitions arising from temperature-dependent phonon softening and anharmonic effects on dielectric properties remain poorly explored. In this study, we employ density-functional theory (DFT) and self-consistent phonon (SCP) calculations to investigate finite-temperature phonons in cubic niobate perovskites. To include explicit anharmonic vibrational effects, SCP frequencies are shifted by the bubble self-energy correction within the quasiparticle (QP) approximation, providing precise descriptions of phonon softening in these strongly anharmonic solids. We further calculate the static dielectric constant of KNbO 3 and NaNbO 3 as a function of temperature using the Lyddane-Sachs-Teller (LST) relation and QP-corrected phonon dispersions. Our theoretical results align with experimental data, offering reliable temperature-dependent phonon dispersions while considering anharmonic self-energies and thermal expansion effects, enhancing our understanding of the complex relations between lattice vibrations and phase transitions in these anharmonic oxides.
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Since last several years, the author has been studying the phonons on functional materials and has established structure-property correlations on flexible framework structure materials, lead free oxides, thin film perovskites, low dimensional 2D materials like graphene nanosheets, TiS3 nanofiber, VSe2 nanosheets, SnO2 quasi nanoparticles etc. In this book chapter, temperature dependent Raman spectroscopic studies on negative thermal expansion framework material H3[Co(CN)6] have been presented in the temperature range 80-300 K to elucidate the phonon anharmonicity of different phonons. No discontinuous or slope changes of phonon mode frequencies, linewidths and their band intensities were noticed suggesting that the compound was stable in the entire temperature range of investigation. Phonon anharmonicity models were used to analyse the temperature dependencies of mode frequencies and their linewidths. It was observed that the three-phonon decay process was dominant over the four-phonon process in this flexible compound. Concisely, the present study demonstrates the anharmonicity of the phonons and their correlation on thermal expansion of H3[Co(CN)6] framework material.
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The anharmonicity of the Ruddlesden Popper metal-halide lattice, and its consequences on their electronic and optical properties, is paramount in their basic semiconductor physics. It is thus critical to identify specific anharmonic optical phonons that govern their photophysics . Here, we address the nature of phonon-phono scattering probabilities of the resonantly excited optical phonons that dress the electronic transitions in these materials by means of variable-temperature resonant impulsive stimulated Raman measurements. Based on the temperature dependence of the coherent phonon lifetimes, we isolate the dominant anharmonic phonon and quantify its phonon-phonon interaction strength. Intriguingly, we also observe that the anharmonicity is distinct for different phonons, with a few select modes exhibiting temperature-independent coherence lifetimes, indicating their predominantly harmonic nature. However, the population and dephasing dynamics of excitons are dominated by the anharmonic phonon.
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An approach to compute the anharmonic peaks of the phonon dispersion curves through the ab initio calculated Hellmann-Feynman forces from a series of supercells with realistic atomic displacements of all atoms, which correspond to a given temperature, is reported. Obtained phonon dispersion bands are able to represent the positions and shapes of the anharmonic peaks. As example, the approach to cubic PbTe and perovskite MgSiO3 crystals is applied.
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