Elastic response of URu₂Si₂ under high magnetic fields

We have measured the elastic constants, C₄₄, C₆₆, (C₁₁-C₁₂)/2 in URu₂Si₂ by means of high-frequency ultrasonic measurements in pulsed magnetic fields up to 68.7 T in a wide temperature range from 1.5 to ∝120 K. We found a reduction of (C₁₁-C₁₂)/2 for magnetic field H parallel [001] 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 ε_xx-ε_yy with Γ₃ symmetry. The lattice instability is likely related to a symmetry-breaking band instability that arises due to the hybridization of the localized 5f electrons with the conduction electrons and is probably linked to the hidden-order parameter of this compound. Recent progress obtained by our measurements of the transverse ultrasonic modes C₄₄ and C₆₆ will also be discussed.