Γ₃-Type Lattice Instability and the Hidden Order of URu₂Si₂

We have performed ultrasonic measurements on single-crystalline URu₂Si₂ with pulsed magnetic fields, in order to check for possible lattice instabilities due to the hybridized state and the hidden-order state of this compound. The elastic constant (C₁₁ - C₁₂)/2, which is associated with a response to the Γ₃-type symmetry-breaking (orthorhombic) strain field, shows a three-step increase at H ≥ 35 T for H || c at low temperatures, where successive meta-magnetic transitions are observed in the magnetization. We discovered a new fact that the absolute change of the softening of (C₁₁ - C₁₂)/2 in the temperature dependence is quantitatively recovered at the suppression of hybridized-electronic state and the hidden order in high-magnetic field for H || c associated with the successive transitions. The present results suggest that the Γ₃-type lattice instability, is related to both the emergence of the hybridized electronic state and the hidden-order parameter of URu₂Si₂. On the other hand, magnetic fields H || [100] and [110] enhance the softening of (C₁₁ - C₁₂)/2 in the hidden order phase, while no step-like anomaly is observed up to 68.7 T. We discuss the limitation of the localized-electron picture for describing these features of URu₂Si₂ by examination of a crystalline electric field model in terms of mean-field theory.