  High resolution longitudinal sound velocity measurements in a magnetic field
performed at T tending to zero in URu2Si2 reveal a second signature at a field
BLT in addition to the step change in velocity expected at the superconducting
upper critical field, Bc2. Characteristic Fermi surface related
magneto-acoustic quantum oscillations (MAQO) emerge beyond the field BLT at a
frequency ~160 T for B||a-axis. Measurements performed with B oriented at
various angles between the a and c-axes reveal an anisotropy for BLT nearly
identical to that of BC2 suggesting similarity of the electron states involved
in both the transitions. Given the observed frequency of ~160 T in the MAQO the
transition is most likely related to the emergence of the \\eta Fermi surface
in URu2Si2 (Lifshitz transition).
