Superconductivity mediated by a soft phonon mode: specific heat, resistivity, thermal expansion and magnetization of YB6

TL;DR

This study investigates the superconducting properties of YB6, revealing that a dominant low-energy phonon mode mediates superconductivity, with detailed measurements of thermodynamic and magnetic properties supporting a medium-strong coupling mechanism.

Contribution

It provides a comprehensive experimental analysis linking a specific phonon mode to superconductivity in YB6, highlighting the role of yttrium vibrations in the pairing mechanism.

Findings

Superconducting gap indicates medium-strong coupling.

A dominant phonon mode at ~8 meV drives superconductivity.

High-frequency lattice vibrations extend beyond 100 meV.

Abstract

The superconductor YB6 has the second highest critical temperature Tc among the boride family MBn. We report measurements of the specific heat, resistivity, magnetic susceptibility and thermal expansion from 2 to 300 K, using a single crystal with Tc = 7.2 K. The superconducting gap is characteristic of medium-strong coupling. The specific heat, resistivity and expansivity curves are deconvolved to yield approximations of the phonon density of states, the spectral electron-phonon scattering function and the phonon density of states weighted by the frequency-dependent Grueneisen parameter respectively. Lattice vibrations extend to high frequencies >100 meV, but a dominant Einstein-like mode at ~8 meV, associated with the vibrations of yttrium ions in oversized boron cages, appears to provide most of the superconducting coupling and gives rise to an unusual temperature behavior of several observable quantities. A surface critical field Hc3 is also observed.