Possible multigap Type-I superconductivity in the layered Boride RuB${}_2$

Abstract

The structure of the layered transition-metal Borides $A$B${}_2$ ($A=$ Os, Ru) is built up by alternating $T$ and B layers with the B layers forming a puckered honeycomb. Here we report superconducting properties of RuB${}_2$ with a $T_c\approx 1.5$K using measurements of the magnetic susceptibility versus temperature $T$, magnetization $M$ versus magnetic field $H$, resistivity versus $T$, and heat capacity versus $T$ at various $H$. We observe a reduced heat capacity anomaly at $T_c$ given by $\Delta C/\gamma T_c\approx 1.1$ suggesting multi-gap superconductivity. Strong support for this is obtained by the successful fitting of the electronic specific heat data to a two-gap model with gap values ${\Delta }_1/k_B{T}_c\approx 1.88$ and ${\Delta }_2/k_B{T}_c\approx 1.13$. Additionally, $M$ versus $H$ measurements reveal a behaviour consistent with Type-I superconductivity. This is confirmed by estimates of the Ginzburg-Landau parameter $\kappa \approx 0.1$--$0.66$. These results strongly suggest multi-gap Type-I superconductivity in RuB${}_2$. We also calculate the band structure and obtain the Fermi surface for RuB${}_2$. The Fermi surface consists of one quasi-two-dimensional sheet and two nested ellipsoidal sheets very similar to OsB${}_2$. An additional small $4^{\mathrm{th}}$ sheet is also found for RuB${}_2$. RuB${}_2$ could thus be a rare example of a multi-gap Type-I superconductor.

8 pages, 8 figures