IQIM Postdoctoral and Graduate Student Seminar

Abstract: The heavy fermion compound URu2Si2 is a prime example of systems containing elements with partially filled d- or f -electron shells that exhibit rich and complex low-T phase diagrams. This compound is renown for its enigmatic "hidden order" (HO) phase that occurs below T0 = 17.5 K whose order parameter (OP) has eluded identification for nearly three decades. At T0, a specific heat anomaly that resembles a second-order BCS-like mean field transition is observed; however, the small antiferromagnetic (AFM) moment of only ≈ 0.03µB /U that arises within the HO phase cannot account for the entropy of ≈ 0.2ln(2) that is associated with the specific heat anomaly. A large moment antiferromagnetic (LMAFM) phase arises at a critical pressure Pc reported by various researchers to lie in the range 5 - 15 GPa. There is strong evidence that the HO/LMAFM phase transition is of first order, leading to the widely held view that the magnetic structure in the HO phase is due to a small amount of the LMAFM phase induced by strain. Electrical resistivity, specific heat, and magnetization measurements on URu2−xFexSi2 and URu2−xOsxSi2 reveal an enhancement of the HO/LMAFM phase boundary T0(x). Due to the striking similarity of the T − P phase diagram of URu2Si2 and the T − x phase diagram of URu2−xFexSi2, it has been suggested that the enhancement of the HO/LMAFM phase for Fe substitution is driven by the "chemical pressure" generated by substitution of smaller Fe ions for Ru ions. However, the HO phase is also enhanced by substituting larger Os ions for Ru ions, indicating that the enhancement of the hidden order transition temperature may not be solely due to an increase in chemical pressure.

 - Refreshments follow the seminar.