Similarity of the Fermi Surface in the Hidden Order State and in the Antiferromagnetic State ofURu2Si2

Abstract

Shubnikov-de Haas measurements of high quality URu2Si2 single crystals reveal two previously unobserved Fermi surface branches in the so-called hidden order phase. Therefore about 55 % of the enhanced mass is now detected. Under pressure in the antiferromagnetic state, the Shubnikov-de Haas frequencies for magnetic fields applied along the crystalline c axis show little change compared with the zero pressure data. This implies a similar Fermi surface in both the hidden order and antiferromagnetic states, which strongly suggests that the lattice doubling in the antiferromagnetic phase due to the ordering vector QAF = (0 0 1) already occurs in the hidden order. These measurements provide a good test for existing or future theories of the hidden order parameter. PACS numbers:The electronic properties of uranium compounds are determined by the tenuous balance between the localized and itinerant character of the 5f electrons which may lead to the formation of enigmatic ground states [1]. One famous example is the heavy fermion compound URu 2 Si 2 which shows a second order phase transition to a "hidden order" (HO) state at T 0 = 17.5 K. The transition to the HO state is associated with a huge entropy loss of 0.2R ln 2 [2]. Despite intense research for 25 years, the order parameter has not yet been identified. The possible proximity to a 5f 2 configuration of the uranium atoms leads to the possibility of multipolar ordering which is highly debated in Pr 3+ systems in the 4f 2 configuration [3]. Thus the resolution of the HO parameter will have a deep impact on the understanding of heavy fermion materials. A large diversity of theoretical proposals have been given. The most recent ones include multipolar orders [4-6]), dynamical spin density wave [7] or hybridization wave [8].The Fermi surface (FS) properties are directly linked to the itineracy of the 5f electrons and to the change of the symmetry entering into the HO phase. Changes of the FS at T 0 have been observed in various experiments. Optical conductivity [9] and transport measurements [10,11] indicate a gap opening and a drop in the number of charge carriers at T 0 . Recent STM measurements show that a hybridization gap opens suddenly at T 0 [12, 13] while in ARPES measurements abrupt changes of the electronic spectrum are detected [14,15]. Here we focus on the FS determination via Shubnikov-de Haas (SdH) measurements on a new generation of high quality crystals. SdH measurements under pressure provide the great opportunity to study the difference of quantum oscillations between the low pressure HO phase and the high pressure antiferromagnetic (AF) phase with propagation vector Q AF = (0 0 1) and ordered moment m 0 = 0.3 µ B /U. A small pressure of P x ≈ 0.8 GPa is enough to switch the ground state from HO to AF [16][17][18]. The AF phase has been well characterized, notably the change from body centered tetragonal to simple tetragonal crystal structure below T 0 [7,19]. Inelastic neutron scattering experiments under pressure suggest that, due to the disappear...

Unusual Magnetic Field-Dependence of the Electronic Dispersion Relations in a Hidden Ordered Phase.

“…The transition is driven by the spin-flip part of the Hund's rule exchange interaction which produces nesting between two sheets of the Fermi-surface that have different orbital characters. The transition results in a folding of the Brillouin zone through a commensurate vector Q which is consistent with Angle Resolved Photoemission [12], de Haasvan Alphen [13] and neutron scattering measurements [14]. Similar nesting conditions have been found to be satisfied in LDA calculations [15,16] of the electronic structure of URu 2 Si 2 .…”

Optical study of hybridization and hidden order in URu2Si2

“…The gapping of incommensurate magnetic excitations at the hidden order transition has been shown by neutron scattering [44,45], and these account for much of the entropy lost. Band structure calculations [46,47] have yielded a picture of the Fermi surface with strong nesting in the pressure-induced anti-ferromagnetic state, and quantum oscillation measurements [37] demonstrate that there is no significant Fermi surface restructuring between HO and LM-AFM, which implies that the Fermi surface calculated for the latter state applies equally well to the former. Incommensurate nesting will lead to the formation of a spin-density wave gap at F and will be accompanied by a sharp absorption feature in the optical data [6,48], while a commensurate antiferromagnetic order of the localized moments would not be visible in optical measurements because it would not lead to a gap in the excitation spectrum at F .…”

“…The strong suppression of the conductivity indicates that a substantial portion of the Fermi surface is being gapped. We know from quantum oscillations [37] that there are four or five Fermi surface sheets in the hidden order state, so presumably the gaps affect each sheet differently. Optical spectroscopy is not a momentum resolved probe, as it is limited to constant k-vector transitions and averaging over all of k-space for k||E and the individual components of the Fermi surface cannot separated.…”

Formation of the Coherent Heavy Fermion Liquid at the Hidden Order Transition inURu2Si2

“…It is connected, as is shown in our study, to a relatively flat band (feature 4) ostensibly of predominant 5f character. Finally, hole-like states (feature 5) that cross the Fermi level E F at k x ≈ 0.54 π/a form propeller-shaped Fermi surface (FS) sheets, also observed in quantum oscillation measurements [17,18].…”