NQR Study of the Heavy-Fermion Pu-115 Superconductors

We present $^{115}$In nuclear quadrupolar resonance (NQR) measurements on the heavy-fermion superconductors Pu$M$In$_5$ ($M$=Co, Rh; $T_c$=2.5K, 1.6K, respectively), in the temperature range \mbox{$0.29{\rm K}\leq T\leq 100{\rm K}$}. From the identified spectral lines, we deduce the quadrupolar parameters for the two inequivalent In sites, which are found to be qualitatively similar to those for other Ce- and Pu-115s. The quadrupolar frequency $\nu_Q$ varies with temperature in the normal state as per the empirical formula for conventional metals. As superconductivity develops, however, $\nu_Q$ exhibits a sharp, albeit small shift, which is a key prediction of the theory of composite superconducting (SC) pairing [1]. The temperature variation of the nuclear spin-lattice relaxation rate $T_1^{-1}$ delineates distinctive regimes of dynamic behavior. An excess of strong in-plane antiferromagnetic spin fluctuations is observed in the vicinity of $T_c$, which are believed to be playing a central role in the formation of the SC condensate. Analysis of the $T_1^{-1}$ data in the SC state suggests that these compounds are strong-coupling $d$-wave superconductors. [1]R. Flint, A. H. Nevidomskyy, and P. Coleman, Phys. Rev. B {\bf 84}, 064514 (2011).