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Abstract

In search of the origin of superconductivity (SC) in diluted rhenium superconductors and their significantly enhanced T-c compared to pure Be (0.026 K), we investigated the intermetallic ReBe22 compound, mostly by means of muon-spin rotation/relaxation (mu SR). At a macroscopic level, its bulk SC (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 ReBe22 is a fully-gapped superconductor with some multigap features. The larger gap value, Delta(l)(0) = 1.78 k(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 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 T-c, as determined from zero-field mu SR 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 T-c value of ReBe22.