First-principles study of the lattice dynamics, thermodynamic properties and electron-phonon coupling ofYB6

Abstract

The electronic, vibrational, and superconducting properties of $\mathrm{Y}{\mathrm{B}}_{6}$ with pressure have been extensively studied using the ab initio calculations. The Fermi surface calculation indicates a pressure-induced electronic topological transition under compression. The lattice dynamic study strongly suggests the existence of phase transition in $\mathrm{Y}{\mathrm{B}}_{6}$ under high pressure by evidence of a pressure-induced phonon softening behavior. Moreover, the calculated electron-phonon coupling (EPC) of $\mathrm{Y}{\mathrm{B}}_{6}$ with both the linear response theory and the rigid-muffin-tin approximation suggested that pairing electrons are mainly mediated by the Y low-lying phonon vibrations, and the hardening of Y low-lying phonon frequency with pressure is responsible for the decreased EPC. The current superconducting theory agrees well with the experimental suggestions [R. Lortz et al., Phys. Rev. B 73, 024512 (2006)], but in apparent contrasts to earlier theoretical calculations [G. Schell et al., Phys. Rev. B 25, 1589 (1982)]. Furthermore, the thermodynamic calculations within the quasiharmonic approximation give an accurate description of the pressure-temperature dependence of the linear thermal-expansion coefficient, bulk modulus, specific heat, and overall Gr\uneisen parameters to compare with the experimental measurement.