1 Solids with strong electron correlations generally develop exotic phases of electron matter at low temperatures [1,2,3,4,5]. Among such systems, the heavyfermion semi-metal URu 2 Si 2 presents an enigmatic transition at T o = 17.5 K to a 'hidden order' state whose order parameter remains unknown after 23 years of intense research [6,7]. Various experiments point to the reconstruction and partial gapping of the Fermi surface when the hidden-order establishes [8,9,10,11,12,13,14,15,16,17,18]. However, up to now, the question of how this transition affects the electronic spectrum at the Fermi surface has not been directly addressed by a spectroscopic probe. Here we show, using angleresolved photoemission spectroscopy, that a band of heavy quasi-particles drops below the Fermi level upon the transition to the hidden-order state. Our data provide the first direct evidence of a large reorganization of the electronic structure across the Fermi surface of URu 2 Si 2 occurring during this transition, and unveil a new kind of Fermi-surface instability in correlated electron systems.Earlier angle-resolved photoemission spectroscopy (ARPES) experiments mapped the basic band structure of URu 2 Si 2 in the paramagnetic state (above T o ), establishing the existence of hole-pockets at the Γ, Z and X points of the Brillouin zone [19,20,21]. These experiments revealed strong disagreements with the calculations for the electronic structure and Fermi surface of URu 2 Si 2 . It was speculated that this was due to the presence of narrow features from the U-5f states, not taken into account by the calculations, and difficult to characterize experimentally with the resolutions available at the time [21]. To date, no reports exist of high-resolution ARPES experiments below or across T o . The pressing question is to determine experimentally the electronic structure near the Fermi level (E F ), inlcuding the heavy 5f states, above and below T o . Figure 1 summarizes our findings for the temperature evolution of the electronic structure near E F . Figure 1a shows the angle-integrated spectra of electrons with k , the momentum component parallel to the sample surface, along the (110) direction at two temperatures across the transition. At T = 26 K, the only apparent feature is a surface state (SS) at binding energies E B < −35 meV, observed at all the investigated temperatures (see Supple-mentary Material). In contrast, at 13 K a narrow peak at E B ≈ −7 meV appears, signaling the presence of a quasi-particle (QP) band. The temperature dependence of this QP band was systematically studied, and is shown in Figures 1b-d. In these figures we normalized 2 the spectra by the Fermi-Dirac distribution, following a well established procedure [22], to reveal the thermally occupied part of the spectral function up to energies ∼ 5k B T above E F .The angle-integrated data of Fig. 1b shows that at 26 K the QP band lies at E B ≈ 5 meV, at 18 K ≈ T o it appears right at E F , and below T o the band shifts to energies below E F . At 10 K the QP peak is ...

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.

We study, using high-resolution angle-resolved photoemission spectroscopy, the evolution of the electronic structure in URu2Si2 at the Γ, Z and X high-symmetry points from the high-temperature Kondo-screened regime to the low-temperature 'hidden-order' (HO) state. At all temperatures and symmetry points, we find structures resulting from the interaction between heavy and light bands, related to the Kondo lattice formation. At the X point, we directly measure a hybridization gap of 11 meV already open at temperatures above the ordered phase. Strikingly, we find that while the HO induces pronounced changes at Γ and Z, the hybridization gap at X does not change, indicating that the hidden-order parameter is anisotropic. Furthermore, at the Γ and Z points, we observe the opening of a gap in momentum in the HO state, and show that the associated electronic structure results from the hybridization of a light electron band with the Kondo-lattice bands characterizing the paramagnetic state.

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