Multiband Superconductivity in Filled-Skutterudite Compounds (Pr_{1-x}La_{x})Os_{4}Sb_{12}: An Sb Nuclear-Quadruple-Resonance (NQR) Study

摘要

We report on Sb nuclear-quadrupole-resonance (NQR) study infilled-skutterudite compounds (Pr_1-xLa_x)Os_4Sb_12. The Sb-NQR spectra havesplit into two sets, arising from different Sb_12 cages containing either Pr orLa, which enables us to measure two kinds of nuclear spin-lattice relaxationtime T_1^Pr and T_1^La. In the normal state, the temperature (T) dependence of1/T^Pr_1T showed almost the same behavior as that for PrOs_4Sb_12 regardlessof. In contrast, 1/T^La_1T markedly decreases with increasing La concentration.In the superconducting state for x=0.05 and 0.2, 1/T_1^Pr exponentiallydecreases down to T=0.7 K with no coherence peak below T_c as well as inPrOs_4Sb_12. A remarkable finding is that the residual density of states at theFermi level below T_c is induced by La substitution for Pr. These results areproposed to be understood in terms of a multiband-superconductivity model thatassumes a full gap for part of the FS and the presence of point nodes for asmall 4f^2-derived FS inherent in PrOs_4Sb_12. For x=0.8 and 1,1/T^La_1exhibits a coherence peak and the nodeless energy gap characteristic forweak-coupling s-wave superconductors. With increasing Pr content, T_c and theenergy gap increases. The novel strong-coupling superconductivity inPrOs_4Sb_12 is inferred to be mediated by the local interaction between4f^2-derived crystal-electric-field states with the electric quadrupole degreeof freedom and conduction electrons. This coupling causes a mass enhancement ofquasi-particles for a part of FS and induces a small FS, which is responsiblefor point nodes in the superconducting gap function. Note that the small FSdoes not play any primary role for the strong-coupling superconductivity inPrOs_4Sb_12.