Hidden low‐temperature instability in PrOs₄Sb₁₂

Abstract

The low-temperature behavior of many correlated electron systems is characterized by the appearance of macroscopically ordered states that show a dome-shaped evolution of their transition temperatures as a function of non-thermal control parameters. In the heavy-fermion compound PrOs 4 Sb 12 , a fieldinduced phase emerges in a magnetic field range between B % 4:5 and 13.5 T. The phase is characterized by antiferroquadrupolar (AFQ) ordering of the Pr 3þ ions. We measured the specific heat of PrOs 4 Sb 12 single crystals in magnetic fields up to B ¼ 14 T parallel to the crystallographic [100] direction to determine the entropy S as a function of T and B. Above the maximum AFQ transition temperature, the entropy SðT ¼ const; BÞ reveals a pronounced maximum. We show that this maximum can be related to a low-temperature instability which is produced by the degeneracy of the lowest crystalline electric field (CEF) energy levels.