Hidden Order and Exotic Superconductivity in the Heavy-fermion Compound Uru2si2

In this thesis, the author investigates hidden-order phase transition at T₀ = 17.5 K in the heavy-fermion URu₂Si₂. The four-fold rotational symmetry breaking in the hidden order phase, which imposes a strong constraint on the theoretical model, is observed through the magnetic torque measurement. The translationally invariant phase with broken rotational symmetry is interpreted as meaning that the hidden-order phase is an electronic “nematic” phase. The observation of such nematicity in URu₂Si₂ indicates a ubiquitous nature among the strongly correlated electron systems.

The author also studies the superconducting state of URu₂Si₂ below T_c = 1.4 K, which coexists with the hidden-order phase. A peculiar vortex penetration in the superconducting state is found, which may be related to the rotational symmetry breaking in the hidden-order phase. The author also identifies a vortex lattice melting transition. This transport study provides essential clues to the underlying issue of quasiparticle dynamics as to whether a quasiparticle Bloch state is realized in the periodic vortex lattice.