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 , P͒ phase diagram of the heavy fermion compound URu 2 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 ͑LMAF͒ 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. URu 2 Si 2 is one of the most spectacular examples of the dual itinerant and local character of uranium-based heavy fermion compounds.

Strong correlation between anomalous quasiparticle scattering and unconventional superconductivity in the hidden-order phase of URu2Si2

“…Applying pressure to URu 2 Si 2 induces a first order phase transition from the HO to a large moment antiferromagnetic (LAFM) state at P x = 0.5-0.9 GPa. [4][5][6][7][8] The bulk-SC state exists only below P x . 5,6 Previous studies have reported unusual electrical transport in URu 2 Si 2 .…”

Non-Fermi Liquid Regimes and Superconductivity in the Low Temperature Phase Diagrams of Strongly Correlated d- and f-Electron Materials

“…Despite this change in ground state, the anomalies in the bulk properties associated with the HO and antiferromagnetic phase transitions are similar and changes in the T dependence are subtle between the two phases [93,95,97,98]. For example, it is possible to fit ρ(T ) with a T 2 term to at least 20 kbar [95,97]. Complementary low-T magnetic and calorimetric data are lacking.…”

“…While the application of pressure does suppress superconductivity [92], it also transitions the system from HO into an antiferromagnetic state at 8 kbar [81,96]. Despite this change in ground state, the anomalies in the bulk properties associated with the HO and antiferromagnetic phase transitions are similar and changes in the T dependence are subtle between the two phases [93,95,97,98]. For example, it is possible to fit ρ(T ) with a T 2 term to at least 20 kbar [95,97].…”

“…The electrical resistivity ρ(T ) has a negative slope between room temperature and 75 K, where it reaches a maximum and then decreases dramatically, which may be attributed to Kondo coherence. Well below the HO transition temperature, ρ(T ) has been described both by T 2 Fermi liquid behavior [92][93][94], and T n dependence, where n < 2 [95,96]. Given the presence of the two phase transitions, the electronic specific heat C(T ) is also somewhat complicated.…”

Local suppression of the hidden-order phase by impurities in URu2Si2

“…Moreover recent neutron scattering experiments evidence that this small moment is not an intrinsic feature of the HO, but a spurious effect due to local strains induced by crystal defects in the sample 6 . Nevertheless magnetic ordering is not completely extraneous to URu 2 Si 2 : an antiferromagnetic phase with large moment can be stabilized by applying pressure or strain 5,[7][8][9] . Since 1985 several theories have been proposed to identify the nature of the hidden order parameter.…”