Heavy Fermion Superconductivity in Non-magnetic Cage Compound PrV₂Al₂₀

PrT₂Al₂₀ (T = Ti, V) are ideal systems to study the quadrupole Kondo effect and quantum criticality arising from orbital degrees of freedom. Both systems have the nonmagnetic cubic Γ₃ crystal electric field ground doublet with the well separated excited state. In particular, PrV₂Al₂₀ exhibits anomalous metallic behavior above and below the multipolar ordering temperatures, reflecting the even stronger hybridization between f and conduction electrons possibly due to a proximity to a orbital quantum critical point. Here, we discuss the heavy fermion superconductivity (SC) of PrV₂Al₂₀ in detail. The SC appears at T_c = 0.05 K with the highly enhanced effective mass (m* /m₀ ∼ 140) estimated using the temperature dependence of the upper critical field. In addition, large electronic specific heat coefficient of γ ∼ 0.9 J/mol K² above T_c and the large specific heat jump at T_c of ΔC/T_c ∼ 0.3 J/mol K² provide direct evidences of the heavy fermion SC. This observation indicates the first realization of the novel SC arising from the orbital fluctuation of the f electrons at ambient pressure. (paper)