Emergence of higher order rotational symmetry in the hidden order phase of URu2Si2

Electrical resistivity measurements were performed as functions of temperature, magnetic field, and angle theta between the magnetic field and the c-axis of a URu2Si2 single crystal. The resistivity exhibits a two-fold oscillation as a function of theta at high temperatures, which undergoes a 180 degrees-phase shift (sign change) with decreasing temperature at around 35 K. The hidden order transition is manifested as a minimum in the magnetoresistance and amplitude of the two-fold oscillation. Interestingly, the resistivity also showed four-fold, six-fold, and eight-fold symmetries at the hidden order transition. These higher order symmetries were also detected at low temperatures, which could be a sign of the formation of another pseudogap phase above the superconducting transition, consistent with recent evidence for a pseudogap from point-contact spectroscopy measurements and NMR. Measurements of the magnetisation of single crystalline URu2Si2 with the magnetic field applied parallel and perpendicular to the crystallographic c-axis revealed regions with linear temperature dependencies between the hidden order transition temperature and about 25 K. This T-linear behaviour of the magnetisation may be associated with the formation of a precursor phase or 'pseudogap' in the density of states in the vicinity of 30-35 K.