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In this book, the author investigates hidden-order phase transition at T0 = 17.5 K in the heavy-fermion URu2Si2. The author also studies the superconducting state of URu2Si2 below Tc = 1.4 K, which coexists with the hidden-order phase.

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In this thesis, the author investigates hidden-order phase transition at T 0 = 17.5 K in the heavy-fermion URu 2 Si 2 . 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 URu 2 Si 2 indicates a ubiquitous nature among the strongly correlated electron systems.

The author also studies the superconducting state of URu 2 Si 2 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.

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Introduction.- Heavy-Fermion Superconductor URu 2 Si 2 .- Magnetic torque Study on the Hidden-Order Phase.- Lower Critical Field Study on the Superconducting Phase.- Vortex Lattice Melting Transition.- Conclusion.

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Dr.Ryuji Okazaki
Department of Physics, Nagoya University,
Chikusa, Nagoya 464-8602, Japan.

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