Agenda

Résumé : 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 zero temperature.

PrOs4Sb12 is a superconductor below 1.85 K and 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 phase. This combination of properties makes PrOs4Sb12 an ideal material to study the effects of the hyperfine interaction on multiple ordered phases [1].

I will describe magnetic susceptibility experiments performed at the London Low Temperature Laboratory to study the phase diagram of PrOs4Sb12 at temperatures down to 1 mK and in magnetic fields up to 5.4 T. A big part of the project involved designing, building, and testing the susceptibility set-up, such that it was suitable for these extreme environments, and I will explain our considerations in designing the set-up.

Our experiments show that the phase boundaries in PrOs4Sb12 continuously develop down to temperatures as low as a few mK: superconductivity is suppressed at low temperature, whereas AFQ order is enhanced. We attribute this to the influence of hyperfine interactions. More specifically, we explain our results in terms of a ground state composed of hybrid nuclear-electronic states. That is, the low temperature Pr energy levels can no longer be considered as purely electronic entities, but must be described in terms of both electron and nuclear quantum numbers. In the AFQ phase, this results in hybrid nuclear-electronic order, with an associated nuclear-electronic quantum critical point. In the superconducting phase, we gain new insight into the superconducting pairing mechanism.

 

[1] A. McCollam et al., Physical Review B, 88, 075102 (2013).