Site dependence of the Kondo scale in ${\mathrm{CePd}}_{1-x}{\mathrm{Rh}}_x$ due to Pd-Rh disorder

We present measurements of the thermopower $S\left(T\right)$ on ${\mathrm{CePd}}_{1-x}{\mathrm{Rh}}_x$ between 2 K and 300 K. For low Rh content, the system behaves as a ferromagnetic Kondo system with a Curie temperature $T_{\mathrm{C}}$ of about 6 K, a Kondo scale smaller than $T_{\mathrm{C}}$, and an overall crystal electric field splitting of 210 K. As the Rh content increases $T_{\mathrm{C}}$ is suppressed, while the average Kondo scale gets larger. Simultaneously, the presence of different Ce environments leads to a broad distribution of local Kondo scales ranging from very small values to above 50 K. As a consequence, large thermopower values are observed over an extended temperature range. Close to the critical concentration we find power-law dependencies of $S/T$ vs $T$ down to 2 K. For a Rh content of $x=0.95$ we may show explicitly that the thermopower contains contributions from Ce sites with different local energy scales.

Figure 1

Main plot: Thermopower $S\left(T\right)$ of CeRh and its magnetic contribution $S_{\mathrm{mag}}$. The line is a fit to the configuration-crossover model. Inset: Thermopower of the nonmagnetic reference LaRh. The line is a linear fit $S\propto T$.

Figure 2

Thermopower of ${\mathrm{CePd}}_{1-x}{\mathrm{Rh}}_x$ for $x\ge 0.8$ (top) and $x\le 0.8$ (bottom).

Figure 3

Low-temperature thermopower of the ferromagnetic samples. The main plot shows the data for single crystals with $x=0.2$, 0.4. Data on a polycrystal with $x=0.6$ are plotted in the inset. The Curie temperatures marked by vertical lines are taken from Refs. [7, 9] and determined from specific heat measurements on crystals from the same batches.

Figure 4

Main plot: The Kondo contribution $S_{\mathrm{K}}$ to the thermopower for $x=0.95$ (for definition and determination see text). A shoulder appears at the position of the maximum in $S\left(T\right)$ for $x=0.8$ marked by a vertical line, which is not seen in the thermopower of $x=0.95$. Inset: Position of the maximum $T_{\max }$ in $S\left(T\right)$ in comparison to the Kondo temperature $T_{\mathrm{K}}$ estimated as half the Weiss temperature (taken from Ref. [7]). For $x<0.85,T_{\max }$ is almost constant, while it shifts with the Kondo temperature for higher $x$. The horizontal line marks the same value as the vertical line in the main plot (80 K).

Figure 5

Thermopower divided by temperature. The sample with $x=0.87$ exhibits a power-law dependence $S/T\propto T^{-\lambda }$ with an exponent of 0.72 below 30 K as indicated by a line in the main plot. Similar exponents $\lambda$ are found for $0.7\le x\le 0.87$ (insets). At lower and higher $x$ clear deviations from a power law are observed, although a power law with smaller exponent can describe the data for $x=0.9$ below 10 K (main plot).

Figure 6

Main plot: Unit cell volume vs $x$ taken from Ref. [7]. The line is a fit consisting of a linear contribution and one proportional to the amount of Ce ions without Pd nearby neighbors: $A_{1}x+A_2{c}_{\mathrm{IV}}$. Inset: Fraction of Ce ions with a Pd-free environment.