Heavy Fermion Superconductivity in the Quadrupole Ordered State of PrV2Al20

PrV${}_2$Al${}_{20}$ is a rare example of a heavy fermion system based on strong hybridization between conduction electrons and nonmagnetic quadrupolar moments of the cubic ${\mathrm{\Gamma }}_3$ ground doublet. Here, we report that a high-quality single crystal of PrV${}_2$Al${}_{20}$ exhibits superconductivity at $T_{\mathrm{c}}=$ 50 mK in the antiferroquadrupole-ordered state under ambient pressure. The heavy fermion character of the superconductivity is evident from the specific heat jump of $\mathrm{\Delta }C\mathrm{/}T\sim 0.3$ J/mol K${}^2$ and the effective mass $m^*/m_0 \sim 140$ estimated from the temperature dependence of the upper critical field. Furthermore, the high-quality single crystals exhibit double transitions at $T_{\mathrm{Q}}$ = 0.75 K and $T^{\ast }$ = 0.65 K associated with quadrupole and octapole degrees of freedom of the ${\mathrm{\Gamma }}_3$ doublet. In the ordered state, the specific heat $C\mathrm{/}T$ shows a $T^3$ dependence, indicating the gapless mode associated with the quadrupole and/or octapole order. The strong sensitivity to impurity of the superconductivity suggests unconventional character due to significant quadrupolar fluctuations.