Why the hidden order in URu2Si2 is still hidden - one simple answer

TL;DR

This paper explains the hidden order in URu2Si2 as an anti-ferro quadrupole order that does not require lattice distortion, based on space group analysis and local electron picture.

Contribution

It provides a simple space group analysis showing the hidden order is an anti-ferro quadrupole order without lattice distortion.

Findings

No lattice distortion needed for the phase transition

Compatible with O_{xy}-type anti-ferro quadrupole ordering

Explains the hidden order without detectable lattice anomalies

Abstract

For more than two decades, the nonmagnetic anomaly observed around 17.5 K in URu2Si2, has been investigated intensively. However, any kind of fingerprint for the lattice anomaly has not been observed. Therefore, the order has been called "the hidden order". One simple answer to why the hidden order is still hidden is presented from the space group analysis. The second order phase transition from I4/mmm (No. 139) to P4_2/mnm (No. 136) does not need any kind of lattice distortion in this system, and allows the NQR frequency at Ru-site unchanged. It is compatible with O_{xy}-type anti-ferro quadrupole ordering with Q=(0, 0, 1). The characteristics of the hidden order are discussed based on the local 5f^2 electron picture.