Evidence for <i>s</i>-Wave Superconductivity with Antiferromagnetic Fluctuations in Filled Skutterudite LaFe₄P₁₂: ¹³⁹La and ³¹P-NMR Studies

Abstract

We have performed $^{139}$La,$^{31}$P-NMR studies on a filled skutterudite\nsuperconductor LaFe$_{4}$P$_{12}$ with a critical temperature $T_c=4.1$ K. In\nthe normal state, the presence of antiferromagnetic (AFM) fluctuations at\nfinite wave vectors is suggested from the relation between the nuclear\nspin-lattice relaxation rate $1/T_1$ and the Knight shift. In the\nsuperconducting (SC) state, the distinct coherence peak was observed just below\n$T_c$, and is easily suppressed by the applied field. An exponential decrease\nof $1/T_1$ was observed, suggestive of the isotropic superconducting gap with\n$2\\Delta/k_{\\rm B}T_c =3.8$. Besides, we observed the decrease of the Knight\nshift, indicative of the singlet-pair formation below $T_c$. These results are\nclear evidence that LaFe$_{4}$P$_{12}$ is a rare s-wave superconductor with\nsignificant AFM fluctuations.\n