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

The superconductor YB6 has the second highest critical temperature T-c 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 T-c=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 F(omega), the spectral electron-phonon scattering function alpha F-2(tr)(omega), and the phonon density of states weighted by the frequency-dependent Gruneisen parameter gamma(G)(omega)F(omega), respectively. Lattice vibrations extend to high frequencies > 100 meV, but a dominant Einstein-like mode at similar to 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 H-c3 is also observed.