# Enhanced $T_c$ and multiband superconductivity in the fully-gapped ReBe$_{22}$ superconductor

## TL;DR

This study reveals that ReBe$_{22}$ is a fully-gapped, multiband superconductor with a significantly enhanced $T_c$ of 9.4 K, driven by increased density of states and electron-phonon coupling, with preserved time-reversal symmetry.

## Contribution

The paper provides the first detailed investigation of ReBe$_{22}$'s superconducting properties, demonstrating multiband superconductivity and enhanced $T_c$ in an almost elemental superconductor.

## Key findings

- ReBe$_{22}$ has $T_c=9.4$ K, much higher than pure Be.
- ReBe$_{22}$ exhibits multigap superconductivity with a dominant gap slightly above BCS weak-coupling value.
- No spontaneous magnetic fields below $T_c$, indicating preserved time-reversal symmetry.

## Abstract

In search of the origin of superconductivity in diluted rhenium superconductors and their significantly enhanced $T_c$ compared to pure Be (0.026 K), we investigated the intermetallic ReBe$_{22}$ compound, mostly by means of muon-spin rotation/relaxation ($\mu$SR). At a macroscopic level, its bulk superconductivity (with $T_c=9.4$ K) was studied via electrical resistivity, magnetization, and heat-capacity measurements. The superfluid density, as determined from transverse-field $\mu$SR and electronic specific-heat measurements, suggest that ReBe$_{22}$ is a fully-gapped superconductor with some multigap features. The larger gap value, $\Delta_0^l=1.78$ k$_\mathrm{B}T_c$, with a weight of almost 90\%, is slightly higher than that expected from the BCS theory in the weak-coupling case. The multigap feature, rather unusal for an almost elemental superconductor, is further supported by the field-dependent specific-heat coefficient, the temperature dependence of the upper critical field, as well as by electronic band-structure calculations. The absence of spontaneous magnetic fields below $T_c$, as determined from zero-field $\mu$SR measurements, indicates a preserved time-reversal symmetry in the superconducting state of ReBe$_{22}$. In general, we find that a dramatic increase in the density of states at the Fermi level and an increase in the electron-phonon coupling strength, both contribute to the highly enhanced $T_c$ value of ReBe$_{22}$.