Why the Hidden Order in URu2Si2 Is Still Hidden–One Simple Answer

For more than two decades, the nonmagnetic anomaly observed around 17.5 K in URu 2 Si 2 , has been investigated intensively.However, any kind of fingerprint for the lattice anomaly has not been observed in the low-temperature ordered phase.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 require any kind of lattice distortion in this system and allows the NQR frequency at a Ru site unchanged.It is compatible with O xy -type antiferro-quadrupole ordering with Q ¼ ð0; 0; 1Þ.The characteristics of the hidden order are discussed based on the local 5 f 2 electron picture.