Revision 1

Superseded · first revision · record

Title: What is the order parameter of hidden order in URu2Si2
Plain: Below 17.5 K, URu2Si2 enters an ordered phase that releases a large amount of entropy, yet nearly forty years of experiments have not identified what orders.
Precise: Identify the order parameter of the phase below $T_{\mathrm{HO}} = 17.5\ \mathrm{K}$ in URu2Si2, which has no ordered magnetic moment of matching size and doubles the unit cell along c (wavevector $(0,0,1)$). Candidates include higher-rank multipoles (hexadecapole, dotriacontapole), hastatic order and chirality density waves. An answer gives the symmetry (irreducible representation) and microscopic character, confirmed by a probe that couples directly to it (resonant x-ray scattering, Raman, NMR).
Settled by: A direct detection of the proposed order parameter whose temperature dependence follows $T_{\mathrm{HO}}$ and vanishes under pressure-induced antiferromagnetism.
Kind: well-posed
Answer type: identification
References: (empty)
Literature status: contested
Status note: Raman data (2015) show an A_2g mode suggesting chirality-density-wave order, while reports of in-plane fourfold symmetry breaking disagree between probes.
Posed since: 1985