Abstract

Many strongly correlated electron systems develop ordered phases at low temperatures that can be well understood in terms of an electronic order parameter. At ultra-low temperatures, however, the hyperfine interaction between nuclei and electrons becomes increasingly important, and we have to consider how this affects ordered phases and phase transitions close to T = 0. PrOs4Sb12 is a superconductor below TC = 1.85 K and HC2 = 2.2 T, and develops antiferroquadrupolar (AFQ) order in magnetic fields between 4 T and 14 T. The hyperfine constant of Pr is relatively large at 52 mK and the Pr crystal electric field levels are closely involved in both the superconducting and AFQ phases. To explore the influence of hyperfine interactions on the low temperature behaviour of PrOs4Sb12, we performed magnetic susceptibility measurements as a function of temperature and magnetic field, to temperatures as low as 1 mK. We find that the phase boundaries in this material anomalously develop down to ~5 mK. AFQ order is enhanced at low temperature, whereas superconductivity is suppressed. We explain our results in terms of a ground state composed of hybrid nuclear-electronic states with novel low energy excitations: the low temperature quadrupole excitations that develop from these nuclear- electronic states, are considerably modified compared to their higher temperature counterparts. I will discuss how this leads to a novel type of nuclear-electronic quantum critical point at the AFQ transition, with a strongly damped region of criticality. I will also explain how the hyperfine-induced suppression of superconductivity gives insight into a highly unconventional superconducting pairing mechanism.