Why the Hidden Order in URu2Si2 Is Still Hidden - revisited

Orateur : H. HARIMA (Kobe University, Japan)

The phase transition of URu2Si2 at 17 K is known to be the second order from specific heat, but the order parameter at low temperature remains unknown. Now, seven years have passed since narrowing down the space group of low temperature phase ny considering the group theory.[1] However, the order pa rameter consistent with the proposed subgroup has not been observed. According to the recent experiments on Raman scattering,[2] the space group #128 is considered to be promising. Note that the space group #128 is compatible with antiferro hexadecapole (xy(x^2-y^2)) order.[3] However, some anomaly should be observed in NQR measurement at Ru site, breaking 4-hold rotational symmetry, which is lost in #128. By reviewing recent experimental situation, we discuss the hidden order parameter in URu2Si2. As a result, most promising order parameter is considered to be electric antiferro dotriacontapole ; xyz(x^2-y^2), which belongs to the space group #126, where the local inversion symmetry at U-site is lost.

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