John Saunders
Royal Holloway University of London
Abstract:
The study of quantum materials, strongly correlated electron and helium systems, at the ultralow temperatures frontier is replete with scientific opportunities, and new technical challenges. Helium in two dimensions can be manipulated to realize: 2D Fermi liquids; Wigner-Mott-Hubbard transition; 2D frustrated magnetism with putative quantum spin liquid; heavy fermion Kondo-breakdown quantum criticality; intertwined density wave and superfluid order; many body localization [1,2]. Furthermore there is a renewed impetus to study p-wave superfluid 3He as the only firmly established topological “superconductor”. Technical innovations have enabled its study under precisely engineered nanoscale confinement, on length scales of order the superfluid coherence length. Using such confinement as a control parameter, allows sculpture of new superfluid states, and the engineering of hybrid nanostructures [2]. This opens the study of topological mesoscopic superfluid 3He and the characterization of the emergent excitations arising from bulk/surface/edge correspondence. New frontiers are also opening up in the area of strongly correlated electron systems. We have cooled a 2D electron gas to 1 mK and below, with the prospect to investigate correlation effects in semiconductor nanostructures, and FQHE. The study of quantum critical metallic systems to lower temperatures than hitherto explored is also of interest. Recent transport measurements on the heavy fermion metal YbRh2Si2 identify a superconductor with multiple phases, in which nuclear spin plays an important role. Ongoing work will establish whether this is a spin-triplet crystalline topological superconductor.
The main focus of this talk will be recent experiments on superfluid 3He confined in a nanofabricated cavity of height comparable to the superfluid coherence length. Such confinement is a powerful tool to modify the p-wave superfluid order parameter, and enables the creation of superfluid 3He hybrid nanostructures, with interfaces between two 3He material phases stabilized by a step in cavity height [2,3]. 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 at the cavity surfaces [4], which can be tuned in situ. 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 with specular surface scattering gap suppression and surface states are eliminated, leaving edge states. The cavity height can then be shrunk to below the coherence length and towards the 2D limit, confirmed by experiments on an 80 nm high cavity, which find that the A-phase continues to be stable. 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 [5]. Near to the AB transition the confined B-phase is predicted to be unstable to spontaneous formation of domains, with a predicted stripe phase. However our NMR measurements find a two-dimensional spatially modulated superfluid (pair density wave) [6]. The future holds: the prospect of new superfluid phases under stronger confinement, or sculpted by symmetry breaking confining geometries; superfluid meta-materials; studies of thermal transport in hybrid nanostructures and under rotation; the quest to identify and manipulate Majorana zero modes in the only firmly establish topological “superconductor”.
[1] J. Saunders, B. P. Cowan, J. Nyeki, J. Low Temp. Phys. DOI: 10.1007/s10909-020-02448-9
[2] J. Saunders arXiv: 1910.01058
[3] L. V. Levitin et al. Science 340, 841 (2013)
[4] P. J. Heikkinen et al. arXiv: 1909.04210
[5] N. Zhelev et al. Nat. Comms. 8, 15963 (2017)
[6] L. V. Levitin et al. Phys. Rev. Lett. 122, 085301 (2019)