He and the Structure of Anisotropic Aerogel*

Recently, sharp transitions were observed in superfluid ³He imbibed in anisotropic silica aerogel that are analogous to the spin-flop transition seen in antiferromagnets. The Cooper pair orbital angular momentum in both the A- and B-phases spontaneously reorients by 90^o uniformly across the entire system as temperature or pressure is swept^1,2,3. We propose that the anisotropic structure present in the aerogel is the driving mechanism for this transition. To better characterize these aerogels, we perform diffusion-limited cluster aggregation simulations. By biasing the diffusion process, we obtained two distinct classes of globally, anisotropic aerogels which we call "nematic" and "planar". The calculated structure factor from simulated aerogels is compared with small-angle X-ray scattering of lab-grown aerogels allowing us to classify the spatial structure of the lab-grown samples. Unexpectedly, lab-grown compressed aerogel has nematic structure while lab-grown stretched aerogel has planar structure. This surprising classification however produces a consistent model for the reorientation of the orbital angular momentum.

[1] Zimmerman, A. M., et al. PRL 121, 255303 (2018)

[2] Zimmerman, A. M., et al. PRL 124, 025302 (2020)

[3] Li, J.I.A., et al. Nat. Phys. 9, 775-779 (2013)