Competing Ordered Phases inURu2Si2: Hydrostatic Pressure and Rhenium Substitution

Abstract

A persistent kink in the pressure dependence of the "hidden order" (HO) transition temperature of URu2−xRexSi2 is observed at a critical pressure Pc=15 kbar for 0 ≤ x ≤ 0.08. In URu2Si2, the kink at Pc is accompanied by the destruction of superconductivity; a change in the magnitude of a spin excitation gap, determined from electrical resistivity measurements; and a complete gapping of a portion of the Fermi surface (FS), inferred from a change in scattering and the competition between the HO state and superconductivity for FS fraction.PACS numbers: 75.30.Mb, 74.70.Tx, 81.30.Bx, 74.62.Fj Since its discovery over 20 years ago [1,2,3], the moderately heavy fermion compound URu 2 Si 2 has been the focus of many theoretical and experimental efforts designed to determine the elusive, hidden order parameter associated with the phase transition occurring at T 0 ≈ 17.5 K. The transition into this "hidden order" (HO) state is characterized by large anomalies (typical of magnetic ordering) in specific heat, electrical resistivity, thermal conductivity, and magnetization measurements [1,2,3,4,5,6,7]; however, only a small antiferromagnetic moment, insufficient to adequately explain the entropy released during the transition, was detected in low-temperature neutron diffraction experiments [8]. In addition to the puzzling order parameter of the HO state, URu 2 Si 2 undergoes a transition into an unconventional superconducting (SC) state, which coexists with weak antiferromagnetism (AFM), at T c ≈ 1.5 K. The potential interplay between the two ordered phases of URu 2 Si 2 as well as the nature of the HO state are underlying problems to our fundamental understanding of the properties of this compound.In an effort to explain the observed properties of URu 2 Si 2 , several microscopic models have been proposed [9,10,11,12,13,14,15]. In addition to the theoretical pursuits, many varied experimental techniques have been employed to confirm and/or constrain the proposed models; however, the experimental results fail to converge upon an encompassing microscopic description of the ordered states of URu 2 Si 2 , but do provide valuable insight when contextually analyzed. Low-temperature neutron diffraction measurements as a function of magnetic field provide evidence that the order parameter of the HO state must break time-reversal symmetry [16], and recent inelastic neutron scattering measurements reveal gapped spin excitations at incommensurate wavevectors [17]. Thermal transport measurements are consistent with the opening of a gap at the Fermi surface (FS), as previously suggested by optical conductivity and specific heat studies [3,18], depleting carriers and reducing electron-phonon scattering [5,6]. These exemplary measurements tend to converge upon a description of the HO state invoking the presence of a FS instability such as a spin density wave (SDW), further suggested by high-field measurements intimating the itinerant nature of the HO state [19].The application of pressure to URu 2 Si 2 further convolutes the discussion...

Symmetry and correlations underlying hidden order inURu2Si2

“…First, Q = 0 phases that are not ferromagnets are generally difficult to detect experimentally. Second, there are incipient Q = 0 correlations in URu 2 Si 2 that maintain strongly correlated behavior [102][103][104] , as evidenced by proximity of the HO phase to a chemically-tuned ferromagnetic (FM) instability 92,[105][106][107] and the presence of a minority-volume FM phase in some samples 108 . Third, a Q = 0 order parameter has no associated change in electronic structure.…”

Electronic structure theory of the hidden-order materialURu2Si2

“…Recent investigations 12,[29][30][31][32][33] on good single crystals have mapped out the phase diagram of URu 2 Si 2 . Apart from the paramagnetic ͑PM͒ phase and the HO phase below 17.5 K at ambient pressure, there is also the large moment antiferromagnetic ͑LMAF͒ phase, which appears with modest pressure of about 0.5 GPa and is characterized by uranium moments of 0.4 B in a type-I AF arrangement.…”

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

“…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.…”