Multiband superconductivity in filled-skutterudite compounds (Pr1-xLax)Os4Sb12:: An Sb nuclear-quadrupole-resonance study

We report on the systematic evolution of normal-state properties and superconducting characteristics in filled-skutterudite compounds (Pr1-x,La-x)Os4Sb12 determined using Sb nuclear-quadrupole-resonance (NQR) experiments. The Sb-NQR spectra in these compounds have split into two sets, arising from different Sb-12 cages containing either Pr or La, which enables us to measure two kinds of nuclear spin-lattice relaxation time T-l(Pr) and T-l(La). In the normal state, the temperature (T) dependence of l/(TlT)-T-Pr showed almost the same behavior as that for pure PrOS4Sb12 regardless of the increase in La content. In contrast, l/(TlT)-T-La markedly decreases with increasing La concentration. These results show that 4f(2)- derived magnetic fluctuations are almost localized at the Pr site. In the superconducting state for Pr-rich compounds of x = 0.05 and 0.2, l/T-l(Pr) exponentially decreases down to T = 0.7 K with no coherence peak below T-c as well as in PrOs4Sb12. A remarkable finding is that the residual density of states (RDOS) at the Fermi level below Tc is induced by La substitution for Pr. The impurity effect, usually observed in unconventional superconductors with a line-node gap, may not be the origin of the RDOS induced by the La substitution, since RDOS does not increase and T-c does not decrease with increasing La content. RDOS is more naturally explained if a small part (similar to 5.5%) of the total Fermi surface (FS) becomes gapless for x = 0.05 and 0.2. These results are proposed to be understood in terms of a multiband-superconductivity (MBSC) model that assumes a full gap for part of the FS and the presence of point nodes for a small 4f(2)-derived FS inherent in PrOs4Sb12. The former could be relevant with FS existing in LaOs4Sb12 and with the anisotropic gap with point nodes being markedly suppressed by either applying a magnetic field or substituting La for Pr. For La-rich compounds of x = 0.8 and 1, on the other hand, l/T-l(La) exhibits a coherence peak and the nodeless energy gap characteristic for weak-coupling BCS s-wave superconductors. With increasing Pr content, Tc increases and the energy gap increases from 2 Delta(0)/k(B)T(c) = 3.45 for pure La compounds up to 2 Delta(0)/k(B)T(c) = 4.2 and 5.2 for the 60% Pr and 80% Pr compounds, respectively. The Pr substitution for La enhances the pairing interaction and induces an anisotropy in the energy-gap structure. The novel strong-coupling superconductivity in PrOs4Sb12 is inferred to be mediated by the local interaction between 4f(2)-derived crystal-electric-field states with the electric quadrupole degree of freedom and conduction electrons. This coupling causes a mass enhancement of quasi-particles for a part of FS and induces a small FS, which is responsible for point nodes in the superconducting gap function. Note that the small FS does not play any primary role for the strong-coupling superconductivity in PrOs4Sb12.