Specific Heat of Superconducting HfV$_{2}$: Effect of Magnetic Fields

Specific-heat ($C)$ measurements on a single crystal of HfV$_{2} $ were made from 1 to 300 K in magnetic fields ($H)$ to 14 T along the [110] axis. The HfV$_{2}$ has a martensitic transition at $T_{M}$ = 118 K and becomes superconducting (SC) at $T_{c} \quad \sim $8 K. The $T_{c}$ and $C$ vary following repeated cooling cycles through the $T_{M}$ from room temperature. This variation is probably related to strains or fracturing or both from the cubic-to-orthorhombic transition at $T_{M}$. For zero field, $\Delta C(T_{c})$/\textit{$\gamma $T}$_{c}$ = 2.06, which indicates strong coupling, and is nearly independent of variations in \textit{$\gamma $}$_{n}$, $T_{c}$, and $C$ that result from the thermal cycling. The SC state $C$ can be fitted with the alpha model for strongly-coupled superconductors using an energy gap \textit {$\Delta $}(0)/k$_{B}T_{c}$ = 2.1; the associated electron-phonon coupling constant \textit {$\lambda $} = 1.5. Both of these parameters are similar to those of Pb. In the normal state, the Sommerfeld constant (\textit{$\gamma $}$_{n})$ depends on the thermal history: for $T_{c}$ = 8.0 K, \textit{$\gamma $}$_{n}$ = 42.1 mJ K$^{2}$ mol$^{-1}$ after the first cooling. From fits above $T_{c}$, the Debye theta \textit{$\Theta $}$_{D}$, characterizing the low-temperature lattice $C$ is 177 K following the first cooling through $T_{M}$.