Orthorhombic Lattice Instability in URu2Si2
We have measured the elastic constant (C11-C12)/2 in URu2Si2 by means of high-frequency ultrasonic measurements in pulsed magnetic fields H ||  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 εxx-εyy with Γ3-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.
For more information please see our paper: T. Yanagisawa et al., Phys. Rev. B 88 (2013) 195150.
(also available on arXiv:1311.3748 [cond-mat.str-el])