We have measured the elastic constant (C11-C12)/2 in URu2Si2 by means of
high-frequency ultrasonic measurements in pulsed magnetic fields H || [001] up
to 61.8 T in a wide temperature range from 1.5 to 116 K. We found a reduction
of (C11-C12)/2 that appears only in the temperature and magnetic field region
in which URu2Si2 exhibits a heavy-electron state and hidden-order. This change
in (C11-C12)/2 appears to be a response of the 5f-electrons to an orthorhombic
and volume conservative strain field \epsilon_xx-\epsilon_yy with
{\Gamma}3-symmetry. This lattice instability is likely related to a
symmetry-breaking band instability that arises due to the hybridization of the
localized f electrons with the conduction electrons, and is probably linked to
the hidden-order parameter of this compound.Comment: 5 pages, 4 figure
