Enhanced Tc and multiband superconductivity in the fully-gapped ReBe22 superconductor

T Shang,A Wang,T Shiroka,M Medarde,W Xie,M Shi,A Amon,D Kasinathan,M Bobnar,H Q Yuan,Y Chen

doi:10.1088/1367-2630/ab307b

Abstract

In search of the origin of superconductivity (SC) in diluted rhenium superconductors and their significantly enhanced Tc compared to pure Be (0.026 K), we investigated the intermetallic ReBe22 compound, mostly by means of muon-spin rotation/relaxation (μSR). At a macroscopic level, its bulk SC (with Tc = 9.4 K) was studied via electrical resistivity, magnetization, and heat-capacity measurements. The superfluid density, as determined from transverse-field μSR and electronic specific-heat measurements, suggest that ReBe22 is a fully-gapped superconductor with some multigap features. The larger gap value, , 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 unusual 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 Tc, as determined from zero-field μSR measurements, indicates a preserved time-reversal symmetry in the superconducting state of ReBe22. 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 Tc value of ReBe22.

Highlights

As one of the lightest elements, beryllium exhibits high-frequency lattice vibrations, a condition for achieving superconductivity (SC) with a sizeable critical temperature
Despite the very small amount of Re, the ReBe22 alloy shows a remarkable increase in Tc compared to its elementary constituents, which we mostly attribute to the significant increase of density of states (DOS) at the Fermi level
We investigated the physical properties of the ReBe22 superconductor by means of electrical resistivity, magnetization, heat capacity, and μSR measurements, as well as by electronic band-structure calculations

Summary

Introduction

As one of the lightest elements, beryllium exhibits high-frequency lattice vibrations, a condition for achieving superconductivity (SC) with a sizeable critical temperature. Tc is affected by the electron–phonon coupling strength (typically large in elements with covalent-bonding tendencies) and the density of states (DOS) at the Fermi level N(òF) (rather low in pure Be) The latter depends on the details of crystal structure and on atomic volume, both effects being nicely illustrated by metal-hydride SCs under pressure That SC is more likely to occur in materials containing metal atoms that are close to populating a new electronic subshell, such as the d1- (Sc, Y, La, and Ac) or p0 (Be, Mg, and Ca) elements In these cases, the electronic structure becomes highly sensitive to the positions of the neighboring atoms [4], resulting in stronger electron–phonon interactions and a higher N(òF). Be-rich alloys may achieve a Tc much higher than elementary beryllium, a prediction which turns out to be true for ReBe22 [5], whose Tc ∼ 9.6 K is almost 400(!)

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Discussion

Conclusion