Electrical Resistivity and Thermal Expansion Measurements of URu_2Si_2 under Pressure(Condensed matter: electronic structure and electrical, magnetic, and optical properties)

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We carried out simultaneous measurements of electrical resistivity and thermal expansion of the heavy-fermion compound URu_2Si_2 under pressure using a single crystal. We observed a phase transition anomaly between hidden (HO) and antiferromagnetic (AFM) ordered states at T_M in the temperature dependence of both measurements. For the electrical resistivity, the anomaly at T_M was very small compared with the distinct hump anomaly at the phase transition temperature T_0 between the paramagnetic state (PM) and HO, and exhibited only a slight increase and decrease for the I ‖ a-axis and c-axis, respectively. We estimated each excitation gap of HO, Δ_<HO>, and AFM, Δ_<AFM>, from the temperature dependence of electrical resistivity; Δ_<HO> and Δ_<AFM> have different pressure dependences from each other. On the other hand, the temperature dependence of thermal expansion exhibited a small anomaly at T_0 and a large anomaly at T_M. The pressure dependence of the phase boundaries of T_0 and T_M indicates that there is no critical end point and the two phase boundaries meet at the critical point.
2008-12-15