Temperature-pressure phase diagram ofURu2Si2from resistivity measurements and ac calorimetry: Hidden order and Fermi-surface nesting

Abstract

By performing combined resistivity and calorimetric experiments under pressure, we have determined a precise temperature-pressure $(T,P)$ phase diagram of the heavy fermion compound $\mathrm{U}{\mathrm{Ru}}_{2}{\mathrm{Si}}_{2}$. It will be compared with previous diagrams determined by elastic neutron diffraction and strain gauge techniques. At first glance, the low-pressure ordered phase referred to as hidden order is dominated by Fermi-surface nesting, which has strong consequences on the localized spin dynamics. The high-pressure phase is dominated by large moment antiferromagnetism (LMAF) coexisting with at least dynamical nesting needed to restore on cooling a local moment behavior. ac calorimetry confirms unambiguously that bulk superconductivity does not coexist with LMAF. $\mathrm{U}{\mathrm{Ru}}_{2}{\mathrm{Si}}_{2}$ is one of the most spectacular examples of the dual itinerant and local character of uranium-based heavy fermion compounds.