Interacting Antiferromagnetic Droplets in Quantum Critical ${\mathrm{CeCoIn}}_5$

The heavy fermion superconductor ${\mathrm{CeCoIn}}_5$ can be tuned between superconducting and antiferromagnetic ground states by hole doping with Cd. Nuclear magnetic resonance data indicate that these two orders coexist microscopically with an ordered moment $\sim 0.7{\mu }_B$. As the ground state evolves, there is no change in the low-frequency spin dynamics in the disordered state. These results suggest that the magnetism emerges locally in the vicinity of the Cd dopants.

Figure 1

(a) $T_c$ (blue) and $T_N$ (red) versus nominal doping in $\mathrm{CeCo}({\mathrm{In}}_{1-x}{\mathrm{Cd}}_x{)}_5$ and $\mathrm{CeCo}({\mathrm{In}}_{1-x}{\mathrm{Sn}}_x{)}_5$ [3, 26]. Arrows show the concentrations reported in this study, and crosses show the points corresponding to the spectra in Fig. 2. (b) $T_1^{-1}$ versus temperature in $\mathrm{CeCo}({\mathrm{In}}_{1-x}{\mathrm{Cd}}_x{)}_5$ with $x=0.15$ (blue, ▴), $x=0.10$ (green, ●), $x=0$ ( gray ▪, from 18), and ${\mathrm{CeRhIn}}_5$ (⋄). $T_N(x)$ is indicated by red arrows (at peaks of $T_1^{-1}$) and $T_c(x)$ by black arrows (drops in $T_1^{-1}$). (c) $(T_{1}T{)}^{-1}$ versus temperature.

Figure 2

(a) ${}^{115}\mathrm{In}$ NQR spectra of the In(1) and In(2) sites at 3.9 K ($x=0.15$, gray), at 1.7 K ($x=0.15$, red), and at 475 mK ($x=0.10$, blue). The $A$, $B$, and $C$ lines of the In(1) are shown. The In(2) spectrum is split by the internal field below $T_N$. Dotted lines are Gaussian fits, as described in the text. (b) The central transition of the ${}^{59}\mathrm{Co}$ at $H_0=33.824\mathrm{kG}$ for the $x=0.15$ sample. The first satellite transitions are also seen at $\pm 220\mathrm{G}$.

Figure 3

The Knight shift of the ${}^{115}\mathrm{In}(1)$ (▪, $x=0.15$ and □, $x=0$) and ${}^{59}\mathrm{Co}$ (●, $x=0.15$ and ○, $x=0$) versus temperature. The shift of the $x=0.10$ sample (not shown) is qualitatively similar. The solid (dotted) line is the bulk susceptibility for $x=0.15$ ($x=0$) in the $c$ direction [12]. Inset: The Knight shifts versus the bulk susceptibilities. The dotted lines are fits to the high temperature data.

Figure 4

The linewidths (full width half maxima) of the Co (□, $x=0$ at 38 kOe, and ▪, $x=0.15$ at 33.824 kOe) and In(1) (○, $x=0$ at 38 kOe, and ●, $x=0.15$ at 33.824 kOe) spectra. The linewidth of the $x=0.10$ sample (not shown) is qualitatively similar. Inset: The Co Knight shift distribution ($\mathrm{FWHM}/\gamma H_0$) versus the bulk susceptibility.