Phase instability, spin fluctuations, and superconductivity in theC15compoundV2Zr

From low-temperature measurements of the specific heat and the magnetic susceptibility, we determined $T_c$, $\gamma$, and ${\chi }^{\mathrm{spin}}$ for both the cubic and the rhombohedral phase of the $C15\mathrm{}$ compound ${\mathrm{V}}_2$Zr. In addition, neutron-diffraction experiments were performed for a proper identification of these two phases. We found that the martensitic transformation ($T_M\sim 110$ K) from the cubic to the rhombohedral phase leads to a substantial reduction in $\gamma$ and ${\chi }^{\mathrm{spin}}: {\gamma }_{\mathrm{cub}} \left({\gamma }_{\mathrm{rh}}\right)\sim 23 \mathrm{}\left(16\right)\mathrm{}$ mJ/g-at. ${\mathrm{K}}^2$ and ${\chi }_{\mathrm{cub}}^{\mathrm{spin}} \left({\chi }_{\mathrm{rh}}^{\mathrm{spin}}\right)\sim 2.1\times {10}^{-7\mathrm{}} (1.4\mathrm{}\times {10}^{-4\mathrm{}})$ emu/g-at. In contrast, $T_c$ is only slightly changed: $T_c^{\mathrm{cub}}\sim 8.7$ K and $T_c^{\mathrm{rh}}\sim 8.0$ K. Together with the fact that the "Wilson ratio" $\frac{N\left(\chi \right)}{N\left(\gamma \right)}\sim 0.7$ in both phases, we take our findings as clear evidence that in ${\mathrm{V}}_2$Zr $T_c$ is limited by spin fluctuations.