Phonon thermodynamics and elastic behavior of GaAs at high temperatures and pressures

Abstract

The phonons of wurtzite and zinc blende GaAs were calculated at simultaneously elevated temperature and pressure, and elastic constants were calculated as functions of pressure. Pressure caused instabilities of shorterwavelength transverse acoustic modes in both wurtzite and zinc blende GaAs, causing them to fall to zero at 18 and 20 GPa, respectively. The Born stability criteria, which depend on elastic constants and only long wavelength phonons, therefore overestimated the pressure needed to induce instability at 0 K. At elevated temperatures, explicit anharmonicity pushes the onset of instability to higher pressures in both wurtzite and zinc blende GaAs. Phonon linewidth and densities of states data showed that the quasiharmonic approximation failed to account for temperature-induced phonon frequency shifts, and the quasiharmonic approximation became less reliable at elevated pressure. In general, the number of three-phonon processes increased with pressure, thereby increasing the temperature-driven broadening of phonon spectral lineshapes.

Impact of anharmonic effects on two phases of VO2 thin-film phase transition materials

“…where m i , p i and u i are the mass, momentum and displacement from equilibrium of atom i; α β γ correspond to Cartesian components; and φ and Φ are the second-and third-order effective interatomic force constants, respectively. U 0 is the temperature-dependent ground-state energy of the system [16,[20][21][22][23][24][25][26][27][28][29][30][31]. Before calculating the anharmonicity, it is necessary to run molecular dynamics (MD) [16,[20][21][22][23][24][25][26][27][28][29][30][31] using the project or augmented wave (PAW) [21,32] method, implemented in the Vienna Ab initio Simulation Package (VASP) [16,18,[20][21][22][23][32][33][34][35].…”

“…U 0 is the temperature-dependent ground-state energy of the system [16,[20][21][22][23][24][25][26][27][28][29][30][31]. Before calculating the anharmonicity, it is necessary to run molecular dynamics (MD) [16,[20][21][22][23][24][25][26][27][28][29][30][31] using the project or augmented wave (PAW) [21,32] method, implemented in the Vienna Ab initio Simulation Package (VASP) [16,18,[20][21][22][23][32][33][34][35]. The anharmonic term can be projected onto the superposition of a set of harmonic phonon modes obtained by Phonopy The DFPT is used to calculate the harmonic phonon dispersion.…”

“…[16,18,[20][21][22][23][32][33][34][35]. To ensure the efficiency and the accuracy of the calculation, the NVT (AIMD) ensemble (canonical ensemble) is chosen.…”

Doubling the superconducting transition temperature of ultraclean wafer-scale aluminum nanofilms

“…Earlier works (Supplemental Material [25], Fig. S10) suggest the phonon DOS for GaAs is highest in the low-frequency limit but low and broad for sapphire [43][44][45]. Since λ ∼ α 2 F (ω)/ω, assuming the electronic contribution from Al and the electron-phonon coupling from the substrates are similar, the low-frequency end of the phonon DOS may contribute to the rise in λ and T c with decreasing film thickness, resulting in the observed trend in T c for the 3.5-nm-thick Al grown on different substrates.…”

Improved relative temperature sensitivity of over 10% K−1 in fluoride nanocrystals via engineering the interfacial layer

“…It is well known that phonon numbers increase at higher temperatures. 35 For the phonon-assisted UC process in Yb/Nd codoped systems, the CaF 2 @SrYbF 5 :40Nd NCs with relatively larger phonon energy can promote the energy transfer efficiency from Yb 3+ to Nd 3+ ions at the interface with the elevation of the temperature, resulting in a much more evident negative TQE than those of SrF 2 - and BaF 2 -based core/shell NCs.…”