Anisotropic Spin Fluctuations in Heavy-Fermion Superconductor NpPd5Al2

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We present 27Al NMR measurements of the Knight shift and nuclear spin–lattice relaxation rate $1/T_{1}$ in the normal state for a single crystal of the heavy-fermion superconductor NpPd5Al2 ($T_{\text{c}} = 4.9$ K). We report here that $1/T_{1}$ shows a large anisotropy with a different temperature dependence for $H\parallel a$ and $H\parallel c$. $1/T_{1}$ for $H\parallel c$ shows a power-law temperature dependence $\sim T^{0.38}$, while $1/T_{1}$ for $H\parallel a$ shows a $T$-linear dependence at low temperatures in fields well above $H_{\text{c2}}$. From the analysis of $1/T_{1}$, $XY$-type anisotropy of AFM spin fluctuations is found to become prominent below 20 K.
2010-05-15