Pressure-tuned point-contact spectroscopy of URu2Si2from hidden order to antiferromagnetic states: Similarity of the Fermi surface gapping

Abstract

We report soft point-contact spectroscopy studies of URu 2 Si 2 both in the hidder order (HO) and the large-moment antiferromagnetic (LMAF) states accessed by pressure. In the HO state at ambient pressure, the spectroscopy shows two asymmetric peaks around the Fermi energy that emerge below the hidden order temperature T HO . In the LMAF state at higher pressures, the spectra are remarkably similar to those in the HO state, indicating a similar Fermi-surface gapping in the HO and LMAF states and providing a clue to unraveling the puzzling HO state.Many interesting and exotic ordered states emerge in strongly correlated electronic systems. One example is the hidden order (HO) state of the heavy fermion material URu 2 Si 2 , which has remained mysterious ever since its discovery. Transport and thermodynamic measurements on URu 2 Si 2 at ambient pressure have clearly shown a second-order phase transition at T HO = 17.5 K; 1,2 however, to the best of our knowledge, no experiment has unambiguously identified the origin of the HO state despite considerable effort over the past 25 years. In the HO state, a tiny antiferromagnetic (AFM) moment of ∼0.03μ B has been detected, but now is generally believed to be caused by inhomogeneous stress or strain in samples and parasitic to HO. 3 Interestingly, superconductivity also emerges below T sc ∼ 1.5 K in URu 2 Si 2 , coexisting with HO. Several theoretical models have been proposed to explain the nature of HO: spin-or charge-density wave, multipolar ordering, 4-6 helicity order, 7 dynamical symmetry breaking, 8,9 and hybridization wave, 10 among others. However, as far as we know, no consensus has yet been reached. 11 URu 2 Si 2 displays a rich phase diagram under pressure: 3,12,13 The HO transition temperature T HO slightly increases with pressure while superconductivity is suppressed and finally disappears at a low critical pressure P x ∼ 0.5 GPa. At low temperatures near P x , there is a first-order transition from the HO to a large-moment antiferromagnetic (LMAF) state with a moment of ∼0.4μ B and wave vector Q AF = (0,0,1). The HO and LMAF phase boundary T x (P ) meets the T HO line at a bicritical point (T c ∼ 19 K, P c ∼ 1-1.36 GPa) and, above P c , LMAF order emerges directly from the paramagnetic state below T N . While the HO and LMAF states are different states, they share many remarkable similarities in their transport and thermodynamic properties, 12,14,15 indicating an intimate relationship between them. Furthermore, Shubnikov-de Haas measurements also show that the Fermi surface does not change dramatically between HO and LMAF states. 16 Recent inelastic neutron scattering measurements find that a longitudinal spin fluctuation in the HO is frozen into static AFM moments in the LMAF state and they have the same commensurate wave vector Q AF = (0,0,1). 17,18 It is thus argued that this commensurate spin resonance is a signature of the hidden order state. On the other hand, an incommensurate spin gap with wave vector Q 1 = (1 ± 0.4,0,0) persists from the HO to LMAF...

Symmetry and correlations underlying hidden order inURu2Si2

“…The bigger hole pocket, which is not observed in quantum oscillations 73,[75][76][77][78] , has a carrier density of order 10 21 cm −3 , which agrees well with the larger Hall carrier density at high temperatures 45 . The carrier density decrease and absence of this larger pocket in the HO phase suggests that the large Z FS is gapped at low temperatures, whereas the experimental similarity of the quantum oscillations, bulk properties, and point contact spectroscopy 79 between HO and AFM phases implies that it is gapped in both HO and AFM states. A charge gap is detected at T > T HO 4,7,9 , which suggests that it is not directly responsible for the entropy change at T HO .…”

Two-channel point-contact tunneling theory of superconductors

“…This method has been successfully implemented for the study of the superconducting order parameter of various superconductors at ambient pressure (see the review Ref. 38 and references within), as well as the hidden order and antiferromagnetic phases of URu 2 Si 2 at high pressure, 58,59 with the advantage of reliable stability over a large temperature range.…”

Hidden order behaviour in URu2Si2(A critical review of the status of hidden order in 2014)

“…Such pseudogap was suggested by recent PCS measurements [40] that detected the onset of gap below 27 K, but could hardly observe any notable change at T 0 . Recent PCS studies using a soft contact however detected only a change in the differential conductance with an onset at T 0 [39]. Figure 8 shows the real part of the optical conductivity within the temperature region 4 to 25 K with measured data down to 2 meV (16 cm −1 ) and with the light polarised along a and c axes [31].…”