Isoelectronic perturbations to --electron hybridization and the enhancement of hidden order in URuSi.

Electrical resistivity measurements were performed on single crystals of URu Os Si up to = 0.28 under hydrostatic pressure up to = 2 GPa. As the Os concentration, , is increased, 1) the lattice expands, creating an effective negative chemical pressure (); 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 is required to induce the HO→LMAFM phase transition. We compare the behavior of the (, ) phase boundary reported here for the URu Os Si system with previous reports of enhanced HO in URuSi upon tuning with or similarly in URu Fe Si upon tuning with positive (). 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 URuSi 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-5-electron and transition metal -electron states which leads to electronic instability in the paramagnetic phase and the concurrent formation of HO (and LMAFM) in URuSi Calculations in the tight-binding approximation are included to determine the strength of hybridization between the U-5-electron states and the -electron states of Ru and its isoelectronic Fe and Os substituents in URuSi.