Superconductivity of MoBe22 and WBe22 at ambient- and under applied-pressure conditions

Abstract

MoBe22 and WBe22 compounds belong to the binary XBe22 (X = 4d or 5d metal) family of superconductors, whose critical temperature depends strongly on X. Despite the multiphase nature of these samples, it is possible to investigate the superconducting properties of MoBe22 and WBe22 at the macro- and microscopic level. A concurrent analysis by means of magnetization and heat-capacity measurements, as well as muon-spin spectroscopy (mu SR) was implemented. At ambient pressure, both compounds enter the superconducting state below 2.6 +/- 0.1 K (MoBe22) and 4.1 +/- 0.10 K (WBe22) and show modest upper critical fields [(mu H-0(c2)(0) = 48 +/- 1 mT and mu H-0(c2)(0) = 58 +/- 1 mT, respectively]. In WBe22, the temperature-dependent superfluid density suggests a fully gapped superconducting state, well-described by an s-wave model with a single energy gap. Heat-capacity data confirm that such a model applies to both compounds. Finally, ac magnetic susceptibility measurements under applied pressures up to 2.1 GPa reveal a linear suppression of the superconducting temperature, typical of conventional superconducting compounds.