Entropy Bottlenecks at T$\,\rightarrow \,$0 in Ce-Lattice and Related Compounds

Abstract

A number of specific heat $C_\mathrm{m}$ anomalies are reported in Ce- and Yb-lattice compounds around 1 K which cannot be associated to usual phase transitions despite of their robust magnetic moments. Instead of a $C_\mathrm{m}(T)$ jump, these anomalies show coincident morphology: (i) a significant tail in $C_\mathrm{m}/T$, with similar power law decay above their maxima ($T>T_\mathrm{m}$), (ii) whereas a $C_\mathrm{m}(T^2)$ dependence is observed below $T_\mathrm{m}$. (iii) The comparison of their respective entropy gain $S_\mathrm{m}(T)$ indicates that $\approx 0.7R$ln2 is condensed within the $T>T_\mathrm{m}$ tail, in coincidence with an exemplary spin-ice compound. Such amount of entropy arises from a significant increase of the density of low energy excitations, reflected in a divergent $C_\mathrm{m}(T>T_\mathrm{m})/T$ dependence. (iv) Many of their lattice structures present conditions for magnetic frustration. The origin of these anomalies can be attributed to an interplay between the divergent density of magnetic excitations at $T\rightarrow 0$ and the limited amount of degrees of freedom: $S_\mathrm{m}$ = $R$ln2 for a doublet-ground state. Due to this “entropy bottleneck,” the paramagnetic minimum of energy blurs out and the system slides into an alternative minimum through a continuous transition. A relevant observation in these very heavy fermion systems is the possible existence of an upper limit for $C_\mathrm{m}/T_{\mathrm{Lim} T\rightarrow 0} ~\approx 7~$J/mol K$^2$ observed in four Yb- and Pr-based compounds.