\Gamma_{3}-Type Lattice Instability and the Hidden Order of URu2Si2

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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 (C_{11}-C_{12})/2, which is associated with a response to the \Gamma_{3}-type symmetry-breaking (orthorhombic) strain field, shows a three-step increase at H \geq 35 T for H \parallel 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 (C_{11}-C_{12})/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 \parallel c associated with the successive transitions. The present results suggest that the \Gamma_{3}-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 \parallel [100] and [110] enhance the softening of (C_{11}-C_{12})/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.
2013-01-15