Entropy Bottlenecks at T->0 in Ce-Lattice and Related Compounds

Abstract

A number of specific heat (Cm) anomalies were reported in Ce and related compounds around 1K which cannot be associated to usual phase transitions despite of their trivalent ground state (GS). Instead of a jump in Cm(T) a significant tail in Cm(T)/T, showing a power law thermal dependence, characterizes those systems above the temperature of the maximum (Tmax). The comparison of their respective entropy gains (Sm(T )) indicates that about 70% of Rln2 is condensed at T > Tmax like in exemplary spin-ice systems. Such a large amount of entropy arises from a significant increase of the density of low energy excitations reflected in the divergent Cm(T)/T dependence at low temperature. The origin of these anomalies can be attributed to the interplay between the divergent the excitations density at T → 0 and the limited amount of available degrees of freedom, c.f. Sm = Rln2 for these doublet-GS compounds. Due to this sort of ”entropy bottleneck”, the systems are constrained to search for alternative minima of their free energies. As a consequence, they are driven to exotic magnetic configurations below Tmax in a continuous transition. One of the relevant observations is a possible existence of an upper limit of Cm/TLimT →0 ≈ 7J/mol K2 detected in four Yb- and Pr-based compounds. Its implication on the thermal dependence of Sm(T → 0) derivatives are discussed within the postulates of the third law of thermodynamics.