Topological superfluids under engineered nanoscale confinement: New phases and the search for Majoranas
Experiments on liquid helium-three near the absolute zero of
temperature have played a key role in the development of many central concepts
in condensed matter physics. The discovery of superfluid 3He gave us the first
p-wave superfluid, a model for unconventional superconductivity, in which the
pairing breaks the symmetry of the parent normal metal. Since that time the
international programme of materials discovery has thrown up many new
unconventional superconductors. And formally the phases of superfluid 3He can
be regarded as quantum vacua, with parallels in particle physics and cosmology
[see "The Universe in a Helium Droplet", G.E.Volovik].
Recently the topology (in momentum space) of condensed matter systems has been
widely applied as a powerful scheme for their classification, alongside the
concept of broken symmetry. The simple truths of topology (eg a sphere's
surface cannot be continuously deformed into that of a torus) have a powerful
impact when applied to complex interacting quantum systems, by pointing to
phenomena that must be there, independent of microscopic details; robust protection
is conferred by the inviolable constraints of topology.
In particular it is predicted that at the surface of the quantum
vacuum that is the B-phase of superfluid 3He, there exist excitations that are
Majorana fermions. This system is also predicted to exhibit “supersymmetry”. Thus the unique condensed matter system that
is superfluid 3He potentially unites condensed matter physics and particle
physics “in the laboratory”.
(Contact Prof. John Saunders or Dr. Andrew Casey for more information)
In this project, with EPSRC support, we study the topological superfluidity
of helium-three confined in regular nanofabricated geometries, as a model
system to further our understanding of topological quantum matter. Our
experiments exploit the recent technical breakthroughs we have made in quantum
nanofluidics, and the development of sensitive NMR
techniques based on the detection of the precessing magnetic signal by SQUIDs
(Superconducting Quantum Interference Devices). See references below.
Confinement of superfluid 3He in a slab-like cavity of thickness of order the
diameter of the Cooper pairs, has a profound effect on the superfluid order and
is expected to stabilize new superfluid states of matter. The compressibility
of 3He allows the pair diameter to be pressure-tuned, varying the effective confinement.
Regular geometries are fabricated with well-characterized surfaces, which can
be tuned in situ by plating with a helium-4 film. This exquisite geometrical
control and tuneability, coupled to the ideal material qualities of superfluid
3He, and highly developed microscopic models provide a rigorous
Phases with different topologies are expected to be stable under different
conditions, and we will map the effect of our new control parameter,
confinement, on these phases. We will quantify the role of disorder, arising
from surface roughness, and the importance of quantum size effects. These
topological superfluids support novel excitations at the faces or edges of the
cavity, at domain walls and vortices. The precise character of these excitations
depends on whether the superfluid ground state preserves or breaks time
At the surface of the B-phase the excitations are propagating
Majorana fermions. Majorana fermions are predicted particles which are their
own antiparticle, and are yet to be discovered, and we will search for these as
part of the project.
This project has a strong international collaborative dimension, both
experimental and theoretical, closely partnering with Cornell and Northwestern
in the USA, and PTB (Berlin) in Germany, and exploiting our membership of the
European Microkelvin Platform www.emplatform.eu. We will connect with other programmes on
topological quantum matter in the UK and internationally, enhanced by the
Hubbard Theory Consortium https://www.royalholloway.ac.uk/cmt/htc.aspx
, through its visitors programmes and workshops.
The project is expected to lead to fundamental insights into topological
quantum matter and topological superfluidity/superconductivity in particular.
It will drive the innovation of new instrumentation at the new frontier
combining ultra-low temperatures and nanoscience, and new SQUID NMR techniques of broad applicability.
EP/J022004/1 £1,140,435. PI J Saunders co-Is: A Casey,
B. Cowan, M. Eschrig. Topological superfluids under engineered nanofluidic
confinement: new order parameters and exotic excitations
Collaborators: Northwestern University (USA), Cornell University
to some of our published work
Phase Diagram of
the Topological Superfluid 3He Confined in a Nanoscale Slab Geometry. L.
Levitin, R. Bennett, A. Casey, B. Cowan, J. Saunders, D. Drung, T. Schurig, J.
340, 841 (2013)
Order Parameter Distortion in Superfluid 3He-B Measured by Non-Linear NMR, L. Levitin, R. Bennett, E.
Surovtsev, J. Parpia, B. Cowan, A. Casey, J. Saunders. Phys. Rev. Lett. 111, 235304 (2013)
in mesoscopic 3He films: experimental study of the interference of
bulk and boundary scattering. P. Sharma, A Corcoles, RG Bennett,
JM Parpia, B Cowan, A Casey, J Saunders.
Lett. 107, 196805 (2011)
submicron microfluidic chambers. S
Dimov, RG Bennett, A Corcoles, LV
Levitin, B Ilic, SS Verbridge, J Saunders, A Casey and JM Parpia. Rev. Sci. Inst. 81,
A nuclear magnetic
resonance spectrometer for operation around 1 MHz with a sub-10 mK noise
temperature, based on a two stage DC SQUID sensor. LV
Levitin, RG Bennett, A Casey, BP Cowan, CP Lusher, J Saunders, D Drung, Th
Schürig. Applied Physics Letters 91, 262507
Study of superfluid
3He under nanoscale confinement: A new approach to the investigation of
superfluid 3He films. L. Levitin, R. Bennett, A. Casey, B.
Cowan, J. Saunders, D. Drung, Th. Schurig, J. Parpia, B. Ilic, N. Zhelev. J. Low Temp. Phys. 175, 667-680 (2014)
associated with this work
Levitin (PDRA RHUL)
Bennett (PDRA Cornell now Isentropic Ltd)
Corcoles (PDRA Cambridge,
Dyball (Managing Editor, Electronics Letters)
Li (now Oxford
Casey (EPSRC Advanced Research Fellow, now Lecturer RHUL).