Thermal expansion under uniaxial pressure in URu₂Si₂

The temperature dependence of the thermal expansion coefficient α has been measured in URu₂Si₂ under uniaxial pressure σ along the [100] and [110] directions up to ∼0.6 GPa. For both cases, the hidden-order phase transition temperature T₀ increases with increasing uniaxial pressure. An anomaly Δα of the thermal expansion coefficient at T₀ appears to be enhanced under a small uniaxial pressure (<0.03 GPa). An antiferromagnetically ordered phase appears at T_X above the critical pressure σ_X∼0.25 GPa for uniaxial pressure along the [100] direction, whereas no antiferromagnetic transition is detected up to ∼0.6 GPa for uniaxial pressure along the [110] direction, indicating that the critical pressure is beyond ∼0.6 GPa for the [110] case. In the previously reported hydrostatic case, an antiferromagnetic phase transition appears above the critical pressure P_X∼0.5 GPa. The difference in critical pressure between σ_X and P _X can be understood in terms of the pressure dependence of the U-U distance in the basal (001) plane.