Anisotropic Spin Fluctuations and Superconductivity in “115” Heavy Fermion Compounds: ${}^{59}\mathrm{Co}$ NMR Study in ${\mathrm{PuCoGa}}_5$

We report results of ${}^{59}\mathrm{Co}$ nuclear magnetic resonance measurements on a single crystal of superconducting ${\mathrm{PuCoGa}}_5$ in its normal state. The nuclear spin-lattice relaxation rates and the Knight shifts as a function of temperature reveal an anisotropy of spin fluctuations with finite wave vector $q$. By comparison with the isostructural members, we conclude that antiferromagnetic $XY$-type anisotropy of spin fluctuations plays an important role in mediating superconductivity in these heavy fermion materials.

Figure 1

(a) and (b) ${}^{59}\mathrm{Co}$ NMR spectra at 19 K obtained by sweeping the external field $H$ at a fixed frequency 32.5 MHz. (c) Knight shifts of the central transition for $H\parallel c$ and $H\perp c$. For $H\perp c$, a second order quadrupole correction was made, which is given by $\Delta \nu =(15/16)({\nu }_Q^2/{\nu }_0)\sim 0.03\mathrm{MHz}$, or $\sim 0.09\%$, where ${\nu }_0$ is the resonance frequency.

Figure 2

Nuclear spin-lattice relaxation rate divided by $T$, $(T_{1}T{)}^{-1}$, as a function of $T$. For comparison, ${}^{59}\mathrm{Co}$ NMR of the nonmagnetic metal ${\mathrm{LuCoGa}}_5$ is presented (filled circle: $H\perp c$; empty circle: $H\parallel c$). Inset: A plot of the in-plane component of fluctuations ($R_a$), which increases rapidly with decreasing $T$, and the out-of-plane component ($R_c$), which is almost independent of $T$.

Figure 3

Ratio of spin fluctuation energy $\rho \equiv {\Gamma }_c/{\Gamma }_a$ as a function of temperature in the normal state. Shown for comparison are results from ${}^{69}\mathrm{Ga}$ NMR in ${\mathrm{PuRhGa}}_5$, ${}^{59}\mathrm{Co}$ NMR in ${\mathrm{CeCoIn}}_5$ [21], ${}^{101}\mathrm{Ru}$ NMR in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ [24], and ${}^{63}\mathrm{Cu}(2)$ NMR in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ [22, 23].

Figure 4

$T_c$ versus ${\Gamma }_c/{\Gamma }_a$ just above $T_c$ for 115 HF superconductors. Data for ${\mathrm{CeIrIn}}_5$ and ${\mathrm{NpPd}}_5{\mathrm{Al}}_2$ are taken from Refs. 27, 28, respectively. The dotted line is a guide to the eye, and the error bar for ${\mathrm{PuCoGa}}_5$ is estimated assuming anisotropy of the static susceptibility is unity $\pm 20\%$.