Evidence for s-wave superconductivity with antiferromagnetic fluctuations in filled skutterudite LaFe${}_4$P${}_{12}$: ${}^{139}$La and ${}^{31}$P-NMR studies

Abstract

We have performed ${}^{139}$La,${}^{31}$P-NMR studies on a filled skutterudite superconductor LaFe${}_4$P${}_{12}$ with a 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 $1/T_1$ and the Knight shift. In the superconducting (SC) state, the distinct coherence peak was observed just below $T_c$, and is easily suppressed by the applied field. An exponential decrease of $1/T_1$ was observed, suggestive of the isotropic superconducting gap with $2\Delta /k_{\mathrm{B}}{T}_c=3.8$. Besides, we observed the decrease of the Knight shift, indicative of the singlet-pair formation below $T_c$. These results are clear evidence that LaFe${}_4$P${}_{12}$ is a rare s-wave superconductor with significant AFM fluctuations.

5 pages, 6 figures, to be published in Journal of the Physical Society of Japan, Vol.74 No.12

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