Momentum-resolved hidden-order gap reveals symmetry breaking and origin of entropy loss in URu2Si2

Abstract

Spontaneous symmetry breaking in physical systems leads to salient phenomena at all scales, from the Higgs mechanism and the emergence of the mass of the elementary particles, to superconductivity and magnetism in solids. The hidden-order state arising below 17.5 K in URu 2 Si 2 is a puzzling example of one of such phase transitions: its associated broken symmetry and gap structure have remained longstanding riddles. Here we directly image how, across the hidden-order transition, the electronic structure of URu 2 Si 2 abruptly reconstructs. We observe an energy gap of 7 meV opening over 70% of a large diamond-like heavy-fermion Fermi surface, resulting in the formation of four small Fermi petals, and a change in the electronic periodicity from body-centred tetragonal to simple tetragonal. Our results explain the large entropy loss in the hidden-order phase, and the similarity between this phase and the high-pressure antiferromagnetic phase found in quantum-oscillation experiments.

Direct bulk-sensitive probe of 5 f symmetry in URu 2 Si 2

“…The order parameter of the HO phase has been the subject of intense investigations for more than 30 years but, so far, has remained hidden, which has been the inspiration for its name. This second-order transition into an electronically ordered state involves a reconstruction of the Fermi surface (9,10) and a change of quasiparticle scattering rate (11). The Fermi surfaces of the HO and high-pressure LMAF phase are very similar (12).…”

“…In URu2Si2 three-energy scales have been identified: a hybridization gap of ∆ hyb ≈ 13 meV [150 K] (13) that opens below 27 K, another gap that opens in the HO phase with ∆HO ≈ 4.1 meV [50 K] in the charge (9,10,14,15) as well as spin channel (16,17), and a resonance mode that appears in the HO gap at ∼ =1.6 meV [18 K], also in both channels (18)(19)(20). Furthermore, with entering the HO phase, the breaking of the fourfold rotational symmetry has been reported from torque experiments (21), high resolution X-ray diffraction on high quality crystals (22), and elastoresistance measurements (23).…”

Collective spin resonance excitation in the gapped itinerant multipole hidden order phase ofURu2Si2

“…The breaking up of the large FS sheets into smaller petals and their downfolding to the Γ point in the HO phase was found in ARPES experiments 4,5 . The reduction of the DOS explains the observed entropy loss and suppressed γ-value in the HO phase 41 .…”

“…The sheets around Γ and Z are nested by the commensurate wave vector Q = (0, 0, 2π c ) which is also the ordering vector of the HO phase. This translational symmetry breaking is suggested by ARPES features that indicate a downfolding of the Fermi surface from bct to a simple tetragonal (st) structure connected with gap opening in the nesting region 4,5 . In addition HO breaks the tetragonal in-plane symmetry from C 4 to C 2 as observed in torque 9 and cyclotron resonance 10 investigations.…”

“…1. In the early theoretical work on thermodynamical properties the HO phase was interpreted in terms of localized and crystalline-electric-field (CEF) split 5f states 2 and their localized multipolar degrees of freedom 3 . However, it has subsequently become evident from comparison of ARPES experiments 4,5 and local density electronic structure calculations 6 that the itinerant 5f electron band picture is a more appropriate starting point. The LDA calculations show that the heavy Fermi surface in URu 2 Si 2 consists mainly of electron sheets around the Γ-point and hole sheets around the body centered tetragonal (bct) Z-point (0,0, 2π c ) consisting mostly of j = 5/2 5f orbitals.…”