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

In search of the origin of superconductivity in diluted rheniumsuperconductors and their significantly enhanced Talt;subagt;calt;/subagt; compared to pure Be (0.026 K), we investigated the intermetallic ReBealt;subagt;22alt;/subagt; compound, mostly by means of muon-spin rotation/relaxation (?SR). At a macroscopic level, its bulk superconductivity (with Talt;subagt;calt;/subagt; = 9.4 K) was studied via electrical resistivity, magnetization, and heat-capacity measurements. The low-temperature superfluid density, as determined from transverse-field ?SR, and electronic specific-heat measurements, suggest that ReBealt;subagt;22alt;/subagt; is a fully-gapped superconductor with some multigap features. The larger gap value, ?alt;supagt;lalt;/sup alt;subagt;0alt;/subagt; = 1.78 kalt;subagt;Balt;/subagt;Talt;subagt;calt;/subagt;, with a weight of almost 90%, is slightly higher than that expected from the BCS theoryin 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-structurecalculations. The absence of spontaneous magneticfields below Talt;subagt;calt;/subagt;, as determined from zero-field ?SR measurements, indicates a preserved time- reversal symmetryin the superconducting state of ReBealt;subagt;22alt;/subagt;. 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 Talt;subagt;calt;/subagt; value of ReBealt;subagt;22alt;/subagt;.