Search for multipolar instability in URu₂Si₂ studied by ultrasonic measurements under pulsed magnetic field

The elastic properties of URu₂Si₂ in the high magnetic field region above 40 T, over a wide temperature range from 1.5 to 120 K, were systematically investigated by means of high-frequency ultrasonic measurements. The investigation was performed at high magnetic fields to better investigate the innate bare 5f -electron properties, since the unidentified electronic thermodynamic phase of unknown origin, the so-called “hidden order” (HO), and associated hybridization of conduction and f electrons (c-f hybridization) are suppressed at high magnetic fields. From the three different transverse modes we find contrasting results; both the Γ₄(B_2g) and Γ₅(E_g) symmetry modes C₆₆ and C₄₄ show elastic softening that is enhanced above 30 T, while the characteristic softening of the Γ₃/B_1g) symmetry mode (C₁₁ - C₁₂)/2 is suppressed in high magnetic fields. These results underscore the presence of a hybridization-driven Γ₃(B_1g) lattice instability in URu₂Si₂. However, the results from this work cannot be explained by using existing crystalline electric field schemes applied to the quadrupolar susceptibility in a local 5f² configuration. Instead, we present an analysis based on a band Jahn-Teller effect.