Yu Yamane

We report on the low-temperature thermal expansion and magnetostriction of the single-impurity quadrupolar Kondo candidate ${\mathrm{Y}}_{1-x}{\mathrm{Pr}}_x{\mathrm{Ir}}_2{\mathrm{Zn}}_{20}$. In the dilute limit, we find a quadrupolar strain that possesses a singular dependence on temperature $T, {\varepsilon }_{\mathrm{u}}\sim H^{2}log(1/T)$, for a small but finite magnetic field $H$. Together with the previously reported anomalous specific heat $C$, this implies a quadrupolar Grüneisen ratio ${\mathrm{\Gamma }}_{\text{u}}={\partial }_T{\varepsilon }_{\mathrm{u}}/C\sim H^2/[T^{2}log(1/T)]$ whose divergence for finite $H$ is consistent with the scenario of a quadrupolar Kondo effect. In addition, we find a singular behavior of the isotropic strain ${\varepsilon }_{\mathrm{B}}$ in zero magnetic field resulting in a divergence of both the volume thermal expansion and the volume Grüneisen parameter. We speculate that this behavior might be also induced by putative Kondo correlations via elastic anharmonicities or static strain disorder.

The cubic neodymium-based compound ${\mathrm{NdCo}}_2{\mathrm{Zn}}_{20}$ exhibits a first-order antiferromagnetic (AFM) transition at $T_{\text{N}}=0.53$ K. Strong magnetic fluctuations at temperatures up to 5 K were suggested by the downward curvature of the electrical resistivity $\rho \left(T\right)$ and the reduced magnetic entropy $S_{\text{m}}$ of $0.5R\ln 2$ at $T_{\text{N}}$. In this study, we measured $\rho \left(T\right)$, the isothermal magnetization $M\left(B\right)$, magnetic susceptibility $\chi \left(T\right)$, and specific heat $C\left(T\right)$ of ${\mathrm{NdCo}}_2{\mathrm{Zn}}_{20-x}{\mathrm{Cd}}_x$ for $x=1$ and ${\mathrm{NdCo}}_2{\mathrm{Zn}}_{20-y}{\mathrm{Ga}}_y$ for $y=1$ and 2. The sharp peak of the magnetic specific heat $C_{\text{m}}\left(T\right)$ at $T_{\text{N}}$ for $x$ = 0 is changed to a weak and broad maximum at 0.55 K for $x$ = 1. This drastic change in $C_{\text{m}}\left(T\right)$ suggests that the isovalent Cd substitution for Zn disorders the exchange interactions between the Nd moments to hinder the first-order AFM transition. On the other hand, $C_{\text{m}}\left(T\right)$ for $y=1$ and 2 exhibits a lambda-shaped anomaly, which is a characteristic of a second-order AFM transition, at elevated temperatures of 0.78 and 1.5 K, respectively. The stabilization of the AFM order by the Ga substitution indicates that $4p$ electron doping strengthens the AFM interaction. We therefore propose that the first-order transition in ${\mathrm{NdCo}}_2{\mathrm{Zn}}_{20}$ is maintained by competitive magnetic interactions inherent in the Nd diamond sublattice.