Isoelectronic perturbations to <i>f</i>-<i>d</i>-electron hybridization and the enhancement of hidden order in URu₂Si₂.

Abstract

Electrical resistivity measurements were performed on single crystals of URu_2-<i>x</i> Os _<i>x</i> Si₂ up to <i>x</i> = 0.28 under hydrostatic pressure up to <i>P</i> = 2 GPa. As the Os concentration, <i>x</i>, is increased, 1) the lattice expands, creating an effective negative chemical pressure <i>P</i>_ch(<i>x</i>); 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 <i>P</i>_c is required to induce the HO→LMAFM phase transition. We compare the behavior of the <i>T</i>(<i>x</i>, <i>P</i>) phase boundary reported here for the URu_2-<i>x</i> Os _<i>x</i> Si₂ system with previous reports of enhanced HO in URu₂Si₂ upon tuning with <i>P</i> or similarly in URu_2-<i>x</i> Fe _<i>x</i> Si₂ upon tuning with positive <i>P</i>_ch(<i>x</i>). 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-5<i>f</i>-electron and transition metal <i>d</i>-electron states which leads to electronic instability in the paramagnetic phase and the concurrent formation of HO (and LMAFM) in URu₂Si₂ Calculations in the tight-binding approximation are included to determine the strength of hybridization between the U-5<i>f</i>-electron states and the <i>d</i>-electron states of Ru and its isoelectronic Fe and Os substituents in URu₂Si₂.