Superconductivity in the Superhard Boride WB$_{4.2}$
We show that the superhard boride WB$_{4.2}$ is a superconductor with a T$_c$ of 2.05(5) K. Temperature-dependent magnetic susceptibility, electrical resistivity, and specific heat measurements were used to characterize the superconducting transition. The Sommerfeld constant γ for WB$_{4.2}$ is 2.07(3) mJ mol$^{-1}$ K$^{-2}$ and the ΔC/γT$_c$ = 1.56, which is somewhat higher than what is expected for weakly coupled BCS type superconductors. The H$_{c2}$ vs T plot is linear over a wide temperature range but does show signs of flattening by the lowest temperatures studied and therefore the zero-temperature upper critical field (μ$_0$H$_{c2}$(0)) for WB$_{4.2}$ lies somewhere between the linear extrapolation of μ$_0$H$_{c2}$(T) to 0 K and expectations based on the WHH model.