Pressure-induced magnetic ordering effects in correlated-electron uranium monochalcogenides

The isostructural uranium monochalogenides UTe, USe, and US lie in the intermediate range between the localized regime and itinerant regime of f-electron behavior. Among them, UTe is the closest to the localized side; USe is in the middle; and US is to the itinerant side. Applying hydrostatic pressure shifts their position in the localized-delocalized spectrum in the direction of itinerancy with interesting consequences for magnetic ordering. Overall, the application of pressure causes changes in their magnetic behaviors, which resemble the changes along the line of materials, as if an increase in pressure moves a material away from UTe-like toward more US-like. Interestingly, this involves nonmonotonic pressure variation of the Curie temperature of UTe and USe. We have performed ab-initio-based calculations quantifying the above picture. These ab-initio-based calculations are developed from a phenomenological electronic-structure-based theory, described below, and involve no use of experimental data or fitting; i.e., the theory is wholly materially predictive. For UTe and USe, the calculated results capture the experimentally-observed initial increase in T-C with pressure resulting from increased hybridization, as well as the reversal of this initial trend caused by the loss of localized f density; while for US, the ab-initio-based calculations predict a monotonic decrease of T-C with pressure in close agreement with experiment. In providing agreement with the pressure-dependent behavior of the three materials, the results also capture the trend along the material sequence UTe --> USe --> US. This picture of pressure-driven increased 5f itinerancy and the resulting pressure-dependent Curie temperature behavior agrees well with experiment for the uranium monocholcogenides, and is entirely different in nature from the Doniach 'necklace' picture often used to discuss magnetism in Kondo-lattice systems.