Cyclotron resonance in the hidden-order phase of URuSi

Tonegawa, Sho*; Hashimoto, Kenichiro*; Ikada, Kisuke*; Lin, Y.-H.*; Shishido, Hiroaki*; Haga, Yoshinori   ; Matsuda, Tatsuma; Yamamoto, Etsuji  ; Onuki, Yoshichika; Ikeda, Hiroaki*; Matsuda, Yuji*; Shibauchi, Takasada*

First observation of cyclotron resonance in the hidden-order phase of ultra clean URuSi is reported. An anomalous splitting of the sharpest resonance line under in-plane magnetic field rotation was found. This is most naturally explained by the domain formation, which breaks the fourfold rotational symmetry of the underlying tetragonal lattice. The result is consistent with the "nematic" Fermi liquid state, in which itinerant electrons have unidirectional correlations.