Hidden orbital order in the heavy fermion metal URu2Si2

When matter is cooled from high temperatures, collective instabilities develop among its constituent particles that lead to new kinds of order(1). 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 URu2Si2 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(2). In URu2Si2, 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(3). Here we develop a proposal for the nature of the hidden order in URu2Si2. 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.