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

Abstract

${\mathrm{MoBe}}_{22}$ and ${\mathrm{WBe}}_{22}$ compounds belong to the binary $X{\mathrm{Be}}_{22}$ $(X=4d$ or 5$d$ 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 ${\mathrm{MoBe}}_{22}$ and ${\mathrm{WBe}}_{22}$ at the macro- and microscopic level. A concurrent analysis by means of magnetization and heat-capacity measurements, as well as muon-spin spectroscopy $(\ensuremath{\mu}\mathrm{SR})$ was implemented. At ambient pressure, both compounds enter the superconducting state below $2.6\ifmmode\pm\else\textpm\fi{}0.1$ K $({\mathrm{MoBe}}_{22})$ and $4.1\ifmmode\pm\else\textpm\fi{}0.10$ K $({\mathrm{WBe}}_{22})$ and show modest upper critical fields [$({\ensuremath{\mu}}_{0}{H}_{\text{c2}}(0)=48\ifmmode\pm\else\textpm\fi{}1$ mT and ${\ensuremath{\mu}}_{0}{H}_{\text{c2}}(0)=58\ifmmode\pm\else\textpm\fi{}1$ mT, respectively]. In ${\mathrm{WBe}}_{22}$, 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.