Hidden orbital order in the heavy fermion metal URu2Si2

Abstract

When matter is cooled from high temperatures, collective instabilities develop among its constituent particles that lead to new kinds of order. An anomaly in the specific heat is a classic signature of this phenomenon. Usually the associated order is easily identified, but sometimes its nature remains elusive. The heavy fermion metal URu(2)Si(2) is one such example, where the order responsible for the sharp specific heat anomaly at T(0) = 17 K has remained unidentified despite more than seventeen years of effort. In URu(2)Si(2), the coexistence of large electron electron repulsion and antiferromagnetic fluctuations leads to an almost incompressible heavy electron fluid, where anisotropically paired quasiparticle states are energetically favoured. Here we develop a proposal for the nature of the hidden order in URu(2)Si(2). We show that incommensurate orbital antiferromagnetism, associated with circulating currents between the uranium ions, can account for the local fields and entropy loss observed at the 17 K transition. We make detailed predictions for the outcome of neutron scattering measurements based on this proposal, so that it can be tested experimentally.

Group theory and octupolar order inURu2Si2

“…However, many physical properties (foremost the temperature dependence of the susceptibility, but also the specific heat) were fitted with apparent success by making different assumptions about the nature of the zero-field hidden order (either quadrupolar order of 5f 2 shells [17,18,30], or non-conventional density waves [31]). Therefore it is important to show that our work is not in conflict with findings for which alternative explanations had been suggested but offers fits to the results of standard measurements, which are at least comparable, and in some cases better, than previous results.…”

Competition between hidden order and antiferromagnetism inURu2Si2under uniaxial stress studied by neutron scattering

“…So far various ideas for the hidden order parameters have been proposed, including valence transition, 11 uranium dimers, 12 unconventional spin density waves, 13,14 quadrupolar order 15,16,17,18,19 and charge current order. 20,21 All of them involve a magnetic instability such that the dipolar order may be replaced with the majority hidden order. This switching is expected to be driven by lattice distortion, since the proposed hidden order parameters are tightly coupled to the lattice system.…”

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

“…One of the most prominent example is the enigmatic 'hidden order' (HO) state in URu 2 Si 2 , which is characterized by a large loss of entropy at T HO =17.5 K [1,2]. Although a multitude of theoretical scenarios have been proposed to explain the HO transition [3][4][5][6][7][8][9], our lack of knowledge of the complex and still debated electronic structure of URu 2 Si 2 [10][11][12][13][14][15][16] remains the major obstacle to developing a definitive understanding of this phase. Here we disentangle the low-energy electronic structure of URu 2 Si 2 by means of angle-resolved photoemission spectroscopy (ARPES) as a function of both excitation photon energy and temperature.…”