Electrical Resistivity and Thermal Expansion Measurements of URu2Si2 under Pressure

We carried out simultaneous measurements of electrical resistivity and thermal expansion of the heavy-fermion Compound URu2Si2 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 parallel to a-axis and c-axis, respectively. We estimated each excitation gap of HO, Delta(HO), and AFM, Delta(AFM), from the temperature dependence of electrical resistivity; Delta(HO) and Delta(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.