Cyclotron Resonance in the Hidden-Order Phase of URu2Si2
  We report the first observation of cyclotron resonance in the hidden-order phase of ultra-clean URu$_2$Si$_2$ crystals, which allows the full determination of angle-dependent electron-mass structure of the main Fermi-surface sheets. We find an anomalous splitting of the sharpest resonance line under in-plane magnetic-field rotation. This is most naturally explained by the domain formation, which breaks the fourfold rotational symmetry of the underlying tetragonal lattice. The results reveal the emergence of an in-plane mass anisotropy with hot spots along the [110] direction, which can account for the anisotropic in-plane magnetic susceptibility reported recently. This is consistent with the `nematic' Fermi liquid state, in which itinerant electrons have unidirectional correlations. 
S. Tonegawa, K. Hashimoto, K. Ikada, Y.-H. Lin, H. Shishido, Y. Haga,   T. D. Matsuda, E. Yamamoto, Y. Onuki, H. Ikeda, Y. M...
cond-mat.str-el cond-mat.supr-con