Stability of Superfluid He 3 - B in Compressed Aerogel

In recent work, it was shown that new anisotropic p-wave states of superfluid He3 can be stabilized within high-porosity silica aerogel under uniform positive strain. In contrast, the equilibrium phase in an unstrained aerogel is the isotropic superfluid B phase. Here we report that this phase stability depends on the sign of the strain. For a negative strain of ∼20% achieved by compression, the B phase can be made more stable than the anisotropic A phase, resulting in a tricritical point for A, B, and normal phases with a critical field of ∼100 mT. From pulsed NMR measurements, we identify these phases and the orientation of the angular momentum.