  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
{\Gamma}3-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
\perp 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 || [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.

Strongly Correlated Electrons