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

We have performed ¹³⁹La,³¹P-NMR studies on a filled skutterudite superconductor LaFe₄P₁₂ with a critical temperature Tc=4.1 K. In the normal state, the presence of antiferromagnetic (AFM) fluctuations at finite wave vectors is suggested from the relation between the nuclear spin-lattice relaxation rate 1/T₁ and the Knight shift. In the superconducting (SC) state, the distinct coherence peak was observed just below Tc, and is easily suppressed by the applied field. An exponential decrease of 1/T₁ was observed, suggestive of the isotropic superconducting gap with 2Δ/kBTc =3.8. Besides, we observed the decrease of the Knight shift, indicative of the singlet-pair formation below Tc. These results are clear evidence that LaFe₄P₁₂ is a rare s-wave superconductor with significant AFM fluctuations.