Momentum-Resolved Evolution of the Kondo Lattice into “Hidden Order” inURu2Si2

Abstract

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.

Direct observation of lattice symmetry breaking at the hidden-order transition in URu2Si2

“…This smallness of the lattice change implies that the hidden-order transition is driven by an electronic ordering, and small but finite electron-lattice coupling gives rise to the lattice distortion. It should be noted that in ironpnictides the large orthorhombicity is believed to be associated with the 'ferroic' (Q ¼ 0) orbital ordering, which is in sharp contrast to the URu 2 Si 2 case where the hidden order is most likely an antiferroic order with the wave vector Q ¼ (001) 6,7,[9][10][11][12] . Such an antiferroic ordering is expected to couple only weakly to the ferroic Q ¼ 0 orthorhombic distortion especially for high-rank multipole orders, in which the degree of local electronic distributions near the atoms are much smaller than that of dipoles in the antiferromagnetic case.…”

“…139 in the international tables for crystallography), which has 15 maximal non-isomorphic subgroups. Recent quantum oscillations 9 and angle-resolved photoemission spectroscopy [10][11][12] studies have revealed that the electronic structure in the hidden-order phase is similar to that of antiferromagneitc phase under pressure. This implies that in the hidden-order phase the Brillouin Zone is folded with the antiferroic wave vector Q ¼ (001) and the nested parts of Fermi surface are gapped as in the antiferromagnetic phase 13 .…”

Optical study of hybridization and hidden order in URu2Si2

“…This is strongly suggestive of the opening of a partial gap at 25 K. The PCS measurements of Park et al [32] see a partial gap open below 27 K, also about 10 meV, but they do not see any sharp features at the hidden order transition, and it has been suggested [33] that this may be due to the pressure of the metal tip on the sample. ARPES measurements also see a gap open at the X-point in k-space below 25 K [24]. This gap has a value of approximately 10 meV and is well explained by a simple hybridization model, and furthermore, is associated with a different part of the Fermi surface than the hidden order gap.…”

“…Other techniques yield gaps that have a larger range of variation, in part due to the fact that their resolution is lower than what is common in optics. Both STM [21] and ARPES [22,24] also give gap values of about 4 meV as well as neutron scattering (around 4 meV) [45,50]. Other tunneling data include those of Escadero et al [51] who find ∆ = 5.85 meV.…”

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

“…From an electronic structure standpoint, angle-resolved photoemission spectroscopy (ARPES) experiments detected Fermi-surface pockets at the G, Z and X points of the Brillouin zone in the PM state 8,14 . In addition, high-resolution ARPES and scanning tunnelling microscopy experiments demonstrated that a Fermi-surface instability of itinerant heavy quasi-particles takes place at the transition 14,[16][17][18]22 . However, a complete determination of the heavy-fermion Fermi-surface of URu 2 Si 2 , of its changes across the HO transition and a direct observation of the location and momentum dependence of the HO gap around the Fermi surface are still pressing unsolved questions.…”