We report the first observation of cyclotron resonance in the hidden-order phase of ultra-clean URu2Si2 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.
@article{arxiv.1207.3905,
title = {Cyclotron Resonance in the Hidden-Order Phase of URu2Si2},
author = {S. Tonegawa and K. Hashimoto and K. Ikada and Y. -H. Lin and H. Shishido and Y. Haga and T. D. Matsuda and E. Yamamoto and Y. Onuki and H. Ikeda and Y. Matsuda and T. Shibauchi},
journal= {arXiv preprint arXiv:1207.3905},
year = {2012}
}