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

Abstract

We have performed ¹³⁹La,³¹P-NMR studies on a filled skutterudite superconductor LaFe₄P₁₂ with the critical temperature T_c=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 is observed just below T_c, and is easily suppressed by applied fields. An exponential decrease of 1⁄T₁ is observed, suggestive of the isotropic SC gap with 2Δ⁄(k_BT_c)=3.8. Besides, we observed the decrease of the Knight shift, indicative of the singlet-pair formation below T_c. These results are clear evidence that LaFe₄P₁₂ is a rare s-wave superconductor with significant AFM fluctuations.