Tuning the electronic state in uranium based strongly correlated electron materials

Uranium based materials with the ThCr₂Si₂-type (and related) structure have attracted sustained attention as reservoirs for strongly correlated electron physics and novel ordered states. This includes hidden order and superconductivity in URu₂Si₂ [1], charge ordering in UPt₂Si₂ [2], a structural instability in UCr₂Si₂ [3], and complex magnetism in other examples. Substantial attention has been devoted to understanding what factors lead to these different behaviors, where it has become evident that (i) the hybridization between the f- and conduction electron states is of particular importance and (ii) tuning studies that separately control the unit cell volume and electronic composition are useful for unravelling what factors lead to specific phenomena. In particular, several unifying trends have emerged from a focus on URu₂Si₂. For example, chemical substitution that qualitatively adds electrons (Si → P and Ru → Rh) produces phase diagrams with shared features, including that hidden order is rapidly destroyed even as the underlying Kondo lattice is preserved. Complex magnetism eventually emerges at large substitutions. In contrast, the removal of electrons (Ru → Re) tends to stabilize ferromagnetism. In this talk we will present results for several chemical substitution studies in URu₂Si₂, with a focus on bulk thermodynamic and electrical transport measurements and the influence of high magnetic fields and applied pressure. The results from these measurements will be compared to earlier studies and prospects for understanding what factors differentiate regions in the broader U-based ThCr₂Si₂ family electronic phase diagram will be discussed.

[1] J. A. Mydosh, P. M. Openeer, and P. S. Riseborough, JPCM 32, 132002 (2020).
[2] J. Lee et al., PRB 102, 041112 (2020).
[3] Y. Lai et al., PRM 4, 075003 (2020).