Point-contact spectroscopy of the phononic mechanism of superconductivity in YB6

TL;DR

This study uses point-contact spectroscopy to directly investigate the electron-phonon interaction in YB6, confirming the role of low-energy phonons in mediating its superconductivity and revealing a strong coupling with a superconducting gap of 4.

Contribution

It provides direct spectroscopic evidence of the phononic mechanism of superconductivity in YB6, supporting previous bulk measurement findings.

Findings

Superconducting gap ratio 2Δ/kBTc = 4 indicating strong coupling.

Identification of electron-phonon interaction features around 8 meV.

Phonon features shift with magnetic field, confirming their relation to the superconducting gap.

Abstract

Lortz et al. [Phys. Rev. B 73, 024512 (2006)] have utilized specific heat and resistivity measurements as "thermal spectroscopies" to deconvolve the spectrum of the electron-phonon interaction in YB$_6$ assuming a major role of the low frequency phonon mode in mediating superconductivity. Here, we present direct point-contact spectroscopy studies of the superconducting interaction in this system. As a result the normalized superconducting gap reveals a strong coupling with $2\Delta/k_BT_c = 4$ and moreover the spectra contain nonlinearities typical of the electron-phonon interaction at energies around 8 meV. The in-magnetic-field measurements evidence that the phonon features found in the second derivative of the current-voltage characteristics are due to the energy dependence of the superconducting energy gap as their energy position shrinks equally as the gap is closing. This is a direct proof that the superconducting coupling in the system is due to the low energy Einstein-like phonon mode associated with the yttrium ion vibrations in a perfect agreement with determinations from bulk measurements.