Microscopic nature of the URu2Si2 hidden-order phase

Determine the microscopic nature of the hidden-order phase that emerges below T0 = 17.5 K in URu2Si2.

Background

URu2Si2 undergoes a second-order transition at 17.5 K into a phase known as hidden order because its order parameter and microscopic origin have not been identified. The paper discusses scanning-tunneling-spectroscopy evidence for a coherent Anderson lattice and a heavy-Fermi-liquid state below the transition, while noting that these observations do not by themselves provide a direct signature of the hidden-order parameter.

Several possible interpretations are mentioned, including a hybridization wave and hastatic order. The paper states that testing the latter would require comparing its predicted differential-conductance lineshapes and quasiparticle-interference spectra with experiment, leaving the microscopic identification unresolved.

References

URu$_2$Si$_2$ \, which undergoes a puzzling second order phase transition at $T_0 =17.5$K into a state with a still unknown, {\it hidden order parameter}.

— Theory of Scanning Tunneling Spectroscopy: from Kondo Impurities to Heavy Fermion Materials  (1701.07574 - Morr, 2017) in Section 1, Introduction; Section 4.1, “The Hidden Order Phase of URu2Si2”