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

For more than two decades, the nonmagnetic anomaly observed around 17.5K in URu2Si2, 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=(0,0,1). The characteristics of the hidden order are discussed based on the local 5f(2) electron picture.