Speaker
Description
Superfluid 3He is the paradigm for topological superconductors, which require unconventional, odd-parity, spin-triplet pairing states. The order parameter of superfluid 3He can be engineered by confinement in a nanofabricated cavity of height comparable to the superfluid coherence length. An alternative approach has been to embed a network of scattering centres. The surface scattering conditions can be tuned in situ; we can create ideal specular scattering by plating the atomically flat surfaces with a superfluid 4He film. And the 3He itself is pristine, free of impurities. The spin degrees of freedom of the superfluid Cooper pairs are the 3He nuclear spins. Therefore nuclear magnetic resonance (NMR) provides a direct and non-invasive probe to both fingerprint the superfluid order parameter, and the surface bound states. Confinement as a control parameter allows order parameter sculpture, and the engineering of hybrid nanostructures and superfluid meta-materials [1].
The relative stability under confinement of the chiral A-phase and time reversal invariant B-phase has been determined [2,3]. Furthermore, under such conditions new phases emerge; the polar phase [4,5], pair density wave states [6]. The quasi-2D chiral A-phase [7] has already been identified, and the gapless chiral phase is in prospect. This has required the demonstration of the fragility of surface states to surface scattering conditions [8]. The stepped confinement, essential for the engineering of hybrid nanostructure, has been applied to create isolated volumes of superfluid [9]. In conjunction with powerful computer simulations [10], this has led to a “table-top” demonstration of cosmological Kibble-Zurek phase transitions in a system with multi-component order parameter [11].
The talk will also give an overview of the current focus of our research: the investigation of topological surface, edge and interface states, as a benchmark for topological superconductivity. Objectives are: to identify dispersing Majorana fermions, predicted at the surface of the time-reversal-invariant superfluid 3He-B phase; to apply thermal transport in new superfluid devices to investigate interface states, the thermal Hall effect and detect chiral edge currents in the fully gapped quasi-2D chiral state; to create quasi-1D channels of polar phase as analogues of hybrid topological superconductor nanowire structures.
[1] J. Saunders, Realizing quantum materials with Helium: Topological Phase Transitions and New Developments, Ed. Lars Brink, Mike Gunn, Jorge V Jose, John Michael Kosterlitz, Kok Phoo Phua (World Scientific). arXiv: 1910.01058
[2] L.V. Levitin et al., Science 340, 841 (2013).
[3] L.V. Levitin et al., Phys. Rev. Lett. 111, 235304 (2013).
[4] V. Dmitriev et al., Phys. Rev. Lett., 115, 165304 (2015)
[5] S. Autti et al., Phys. Rev. Lett.,117, 255301 (2016)
[6] L.V. Levitin et al., Phys. Rev. Lett. 122, 085301 (2019).
[7] P.J. Heikkinen et al., Phys. Rev. Lett. 134, 136001 (2025)
[8] P.J. Heikkinen et al., Nat. Commun. 12, 1574 (2021).
[9] P.J. Heikkinen et al., J. Low Temp. Phys. 215, 477 (2024).
[10] M. Hindmarsh et al., J. Low Temp. Phys. 215, 495 (2024).
[11] P. J. Heikkinen et al., in preparation.