Topological Superfluid ³He under mesoscopic confinement

Confinement of superfluid ³He in a nanofabricated cavity of height comparable to the superfluid coherence length is a powerful tool to modify the p-wave superfluid order parameter [1]. This enables the creation of superfluid ³He hybrid nanostructures, with interfaces between two ³He material phases stabilized by a step in cavity height [2]. Measurements on the chiral A-phase, stabilized at low pressure in a 200 nm tall cavity, show that the order parameter suppression and the spectrum of surface bound states are fragile with respect to details of quasiparticle scattering [3]. We show that magnetic surface scattering leads to an unexpectedly large suppression of the transition temperature, corresponding to an increased density of low energy bound states. On the other hand specular surface scattering eliminates gap suppression and surface states. In taller cavities an AB transition is observed; the 0.7 and 1.1 micron cavities show a universal phase boundary, with minute super-cooling, which is potential evidence for an intrinsic nucleation mechanism under confinement [4]. Near to the AB transition the confined B-phase is predicted to be unstable to spontaneous formation of domains, with a predicted stripe phase [5]. However our NMR measurements find a two-dimensional spatially modulated superfluid (pair density wave) [6]. The future quest is to identify and manipulate Majorana zero modes in the only firmly establish topological “superconductor”.
[1] L. V. Levitin et al. Science 340, 841 (2013)
[2] J. Saunders arXiv: 1910.01058
[3] P. J. Heikkinen et al. arXiv: 1909.04210
[4] N. Zhelev et al. Nat. Comms. 8, 15963 (2017)
[5] A Vorontsov, J Sauls, Phys. Rev. Lett. 98, 045301 (2007)
[6] L. V. Levitin et al. Phys. Rev. Lett. 122, 085301 (2019)