Superconductivity in the geometrically frustrated pyrochloreRbOs2O6

We report the basic thermodynamic properties of the geometrically frustrated $\beta$-pyrochlore bulk superconductor ${\mathrm{RbOs}}_2{\mathrm{O}}_6$ with a critical temperature $T_c=6.4\mathrm{K}$. Specific heat measurements are performed in magnetic fields up to $12\mathrm{T}$. The electronic density of states at the Fermi level in the normal state results in $\gamma =(33.7\pm 0.2){\mathrm{mJ}∕\mathrm{mol}}_{\mathrm{f.u.}}∕{\mathrm{K}}^2$. In the superconducting state, the specific heat follows conventional BCS-type behavior down to $1\mathrm{K}$, i.e., over three orders of magnitude in specific heat data. The upper critical field slope at $T_c$ is $1.2T∕\mathrm{K}$, corresponding to a Maki-parameter $\alpha =0.64\pm 0.1$. From the upper critical field ${\mu }_0{H}_{c2}\approx 6\mathrm{T}$ at $0\mathrm{K}$, we estimate a Ginzburg-Landau coherence length $\xi \approx 74\mathrm{\AA }$. ${\mathrm{RbOs}}_2{\mathrm{O}}_6$ is the second reported metallic ${AB}_2{\mathrm{O}}_6$ type pyrochlore compound after ${\mathrm{KOs}}_2{\mathrm{O}}_6$, and one of only three pyrochlore superconductors in addition to ${\mathrm{Cd}}_2{\mathrm{Re}}_2{\mathrm{O}}_7$ and ${\mathrm{KOs}}_2{\mathrm{O}}_6$.