Isoelectronic perturbations to f-d-electron hybridization and the enhancement of hidden order in URu₂Si₂

Electrical resistivity measurements were performed on single crystals of URu_2-xOs_xSi₂up to x = 0.28 under hydrostatic pressure up to P = 2 GPa. As the Os concentration, x, is increased, 1) the lattice expands, creating an effective negative chemical pressure Pch(x); 2) the hidden-order (HO) phase is enhanced and the system is driven toward a large-moment antiferromagnetic (LMAFM) phase; and 3) less external pressure Pc is required to induce the HO!LMAFM phase transition. We compare the behavior of the T(x, P) phase boundary reported here for the URu_2-xOs_xSi₂system with previous reports of enhanced HO in URu₂Si₂upon tuning with P or similarly in URu_2-xFe_xSi₂upon tuning with positive Pch(x). It is noteworthy that pressure, Fe substitution, and Os substitution are the only known perturbations that enhance the HO phase and induce the first-order transition to the LMAFM phase in URu₂Si₂. We present a scenario in which the application of pressure or the isoelectronic substitution of Fe and Os ions for Ru results in an increase in the hybridization of the U-5f-electron and transition metal d-electron states which leads to electronic instability in the paramagnetic phase and the concurrent formation of HO (and LMAFM) in URu₂Si₂.