Novel Electronic Phases and Competing Interactions in the Correlated f-Electron Compound URu₂Si₂

The correlated f-electron compound URu₂Si₂ undergoes a second-order transition at T_o = 17.5 K into an ordered phase whose identity has eluded researchers for more than three decades. This so-called "hidden order" (HO) phase coexists with unconventional superconductivity (SC) below T_c ≈ 1.5 K. Application of pressure suppresses the T_c of the SC’ing phase and induces a transition from the HO phase to a large-moment antiferromagnetic (LMAFM) phase at a critical pressure P_c ≈ 1.5 GPa. Our research group found that substitution of isoelectronic Fe or Os for Ru suppresses SC and induces a transition from the HO to the LMAFM phase, similar to the behavior of URu₂Si₂ under pressure. The HO-LMAFM phase transition in URu_2−xFe_xSi₂ was attributed to “chemical pressure” associated with the reduction of the unit cell volume upon substitution of smaller Fe atoms for Ru atoms. This allows the HO and LMAFM phases to be studied in single crystals of URu_2−xFe_xSi₂ at atmospheric pressure with techniques that cannot readily be performed on URu₂Si₂ under high pressure (e.g., ARPES, STM, infrared, Raman, neutron scattering, etc.). In this talk, we will review the status of ongoing research on URu_2−xFe_xSi₂ single crystals in the HO and LMAFM phases, such as infrared spectroscopy, elastic and inelastic neutron scattering, electrical resistivity under pressure, and high field (45 T) magnetoresistance measurements. Interestingly, the substitution of Os for Ru in URu₂Si₂, which also induces the HO-LMAFM transition, expands the unit cell and is inconsistent with the "chemical pressure" hypothesis, suggesting the importance of other factors (e.g., spin-orbit coupling). These investigations should be useful in developing an understanding of the underlying physics of URu₂Si₂-based materials and, perhaps, even unmasking the identity of the elusive HO phase!