Hidden Orbital Order in $URu_{2}Si_{2}$

TL;DR

This paper proposes that hidden orbital antiferromagnetism, involving circulating currents between uranium ions, explains the mysterious order in URu2Si2 responsible for its specific heat anomaly at 17 K.

Contribution

It introduces a detailed theoretical model suggesting incommensurate orbital antiferromagnetism as the hidden order in URu2Si2, supported by predictions for neutron scattering.

Findings

Orbital antiferromagnetism accounts for local magnetic fields.

Model explains entropy loss at transition.

Predictions match experimental neutron scattering data.

Abstract

When matter is cooled from high temperatures, collective instabilities develop amongst 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 $URu_2Si_2$ 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 in $URu_2Si_2$ leads to an almost incompressible heavy electron fluid, where anisotropically paired quasiparticle states are energetically favored. In this paper we use these insights to develop a detailed proposal for the hidden order in $URu_2Si_2$. 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; furthermore we make detailed predictions for neutron scattering measurements.