  High resolution longitudinal sound velocity measurements in a magnetic field
performed in the temperature tending to zero limit reveal two distinct
signatures attributable to multiple superconducting phases in URu2Si2. A step
change in the sound velocity, for propagation in the basal plane, is observed
at the critical field, Bc2. This step broadens considerably as T tends to Tc
with a concomitant decrease in magnitude. A second step is observed at a field
~0.5 Bc2 and it's magnitude remains constant at all temperatures. Inductive
measurements of the transitions in a magnetic field, however, exhibit a single
signature which coincides with the lower step with no discernible in-phase
signature at the upper transition. Measurements performed with B oriented at
various angles between the a and c-axes reveal a weaker angular dependence of
the lower step and confirm the rapid fall off of Bc2 close to B||c-axis. An off
axis superconducting phase diagram is proposed.
