On the importance of the electron-phonon coupling function on the superconducting transition temperature in dodecaboride superconductors: A comparison of LuB12 with ZrB12

TL;DR

This study compares the electron-phonon interactions in LuB12 and ZrB12 superconductors, revealing how differences in phonon coupling influence their distinct superconducting transition temperatures.

Contribution

It provides a detailed analysis of the electron-phonon coupling function and its impact on Tc in two structurally similar boride superconductors, highlighting the role of metal ion vibrations.

Findings

Phonon density of states is similar in both compounds.

Metal ion vibrations contribute differently to electron-phonon coupling.

Differences in Tc are linked to variations in metal ion vibrational contributions.

Abstract

We report a detailed study of specific heat, electrical resistivity and optical spectroscopy in the superconducting boride LuB12 (Tc = 0.4 K) and compare it to the higher Tc compound ZrB12 (Tc = 6 K). Both compounds have the same structure based on enclosed metallic Lu or Zr ions in oversized boron cages. The infrared reflectivity and ellipsometry in the visible range allow us to extract the optical conductivity from 6 meV to 4 eV in the normal state from 20 to 280 K. By extracting the superconducting properties, phonon density of states and electron-phonon coupling function from these measurements we discuss the important factors governing Tc and explain the difference between the two compounds. The phonon density of states seems to be insignificantly modified by substitution of Zr with Lu. However, the soft vibrations of the metal ions in boron cages, responsible for the relatively high Tc in ZrB12, have almost no contribution to the electron-phonon coupling in LuB12.