Anomalous Enhancement of Seebeck Coefficient in Pr-Based 1-2-20 System with Non-Kramers Doublet Ground States

Low-temperature Seebeck coefficient S/T measurements have been performed on Pr-based 1-2-20 system, PrTr₂X₂₀ (Tr = Ti, Ta, V, Ir, X = Al, Zn) with non-Kramers doublet ground states. For PrTr₂X₂₀ with X = Al, we find a large S/T, which amounts to those of heavy fermion metals. By contrast, S/T for PrIr₂Zn₂₀ is found to be considerably small as the same order of magnitude as those of ordinary metals, despite the commonly enhanced Sommerfeld coefficient γ throughout the system. A satisfactory of the quasi-universal relation between S/T and γ as well as the Kadowaki-Woods relation demonstrates that the mass enhancement is realized in PrTr₂Al₂₀ due to the hybridization between f-electrons and the conduction electrons. We also find that the small S/T of PrIr₂Zn₂₀ is enhanced at low temperatures under the fields on the verge of quadrupole ordered phase, and in the same regime, the electrical resistivity follows the quadratic temperature dependence with a steep slope as a characteristic of Fermi liquid. The results imply an emergence of a nontrivial coherent state with sizable mass enhancement associated with the quadrupole degree of freedom.