Topological mesoscopic superfluidity of 3He

Lead Research Organisation: Royal Holloway, University of London
Department Name: Physics

Abstract

Helium remains liquid down to the absolute zero of temperature through a combination of relatively weak interatomic interactions and quantum zero point motion. It provides models for studying systems of strongly interacting bosons (4He) and fermions (3He). These materials have played a key role in the development of concepts central to condensed matter physics: Bose-Einstein condensation; macroscopic quantum physics; topological phase transitions; the Landau Fermi liquid theory of electrons in metals; unconventional superconductivity; topological quantum matter. The richest system is superfluid 3He (recognized in the 1996 and 2003 Nobel Prizes), the conceptual reach of which is described by Volovik in terms of a "3He-centric universe", conceptually linked to most major fields of physics.

Condensed matter systems feature in every modern technology. The development of this technology has relied on the discovery and creation of new materials with new phases and often unanticipated properties, as well as hybrid devices which combine these materials, increasingly on the nanoscale.

Recently the understanding of new phases solely in terms of symmetry breaking has been shown to be inadequate in some cases. This is the notion of topological quantum matter (2016 Nobel Prize). Two important classes of quantum matter are at the centre of attention: topological insulators and topological superconductors. Usual insulators do not conduct electricity, because of an energy gap between filled and empty energy bands. But in a topological insulator the momentum space topology of the band structure necessarily gives rise to conducting surface excitations. On the other hand, as a metal is cooled into its superconducting state, a gap emerges. Topological superconductors also support surface/edge excitations, and there are substantial efforts to identify materials that are bulk topological superconductors.

This project will exploit superfluid 3He, known to support two distinct topological superfluid phases in bulk, establishing the new research direction of topological mesoscopic superfluidity. Under nanoscale confinement, this material provides a unique model for topological superconductivity. The subtle interplay between symmetry and topology in these materials is an open question. Our approach will be to confine 3He in precisely engineered geometries to create hybrid nanostructures, allowing a degree of control that is unprecedented. Confinement and periodic structures, with liquid pressure as a tuning parameter of Cooper pair diameter, will induce new superfluid phases, for which the order parameter symmetry will be inferred from nuclear magnetic resonance. These materials will be building blocks for hybrid mesoscopic superfluid systems.

Excitations emerge at surfaces/edges/interfaces of the topological superfluid. As well as the interface with inert matter, where we can tune surface scattering in situ, stepped confinement in hybrid structures will create intra-fluid interfaces of the highest quality. Surface and edge spin currents in time reversal invariant superfluid 3He-B will be investigated by NMR and their coupling to confined Anderson-Higgs order parameter collective modes, as well as nano-wires of diameter similar to that of Cooper pairs. Our ambition is to detect non-local response of the surface Majorana modes. Edge states in chiral superfluid 3He-A will be investigated by a predicted anomalous Hall effect in mass and thermal transport. Interface states will be investigated by thermal transport.

This project has a strong international collaborative dimension, including partnerships with Cornell, NIST and PTB (Berlin). Partnerships with theorists from the USA, Europe and Japan are central to the design and interpretation of experiments. Partnerships within the scientific instruments industry will deliver short-medium term impact from the technical developments central to this project.

Planned Impact

Who will benefit?
Topologically protected quantum computing is an ambitious but important long term challenge in quantum technology. The fundamental understanding of topological superconductivity is key to establishing the feasibility of routes to topologically protected quantum computing using such materials. With materials science advances in unconventional superconductivity we can also anticipate future devices based on thin films of these materials with potentially new functionality.

Our fundamental research drives forward the development of state-of-the-art research capacity. These developments in instrumentation measurement systems directly benefit the scientific instruments industry in the short-medium term, especially those focussed on cryogenics, nanoscience and superconducting technologies, widening its customer base. This is fostered by the global reach of UK industry in this sector. In addition new experimental methods, for example the measurement of temperature and new NMR methods, impact on society through National Measurement Institutes.

Collaboration between the UK, mainland Europe and leading groups in the USA, both theoretical and experimental, as well as strong links with Asia increases the opportunity for international impact. We contribute to the scientific society through work for international funding agencies, scientific academies, and globally via IUPAP.

The general public, including young people tend to be excited and motivated by laboratory-based fundamental science at one of Nature's frontiers, and the technical challenges we meet.

How will they benefit?
Advances in the fundamental understanding of topological superconductivity will be underpinned by the research on topological mesoscopic superfluid 3He as a model system. The propagation of these ideas by theorists across discipline boundaries through networking, workshops, will play a key role. The topological classification of condensed matter is a universal scheme, so advances in understanding on model systems will enter the canon of the subject, and are essential for long-term technological progress.

In the short term we expect to continue to export highly trained manpower to industry. This research, at the frontier of the subject, combines new techniques in ultralow temperature physics new measurement techniques, advances in nanofluidics, and crucially a robust confrontation between theory and experiment. Individuals trained in these skills and expertise, equipped with rigorous scientific methodology, are an important resource enhancing economic competitiveness.

Instrumentation development plays a central role in the proposed research. We will work towards short-term impact in: cryogen-free nuclear demagnetization platforms; advances in noise thermometry; definition of the Kelvin; improved heat exchangers; applications of novel superconducting devices to NMR (with potential future applications in medicine).

What will be done to ensure that they benefit?
This proposal includes a number of Project Partnerships. Collaborations are well established with the scientific instruments industry (Oxford Instruments Nanoscience), and here we establish a new partnership with York Instruments.

We have long-term partnerships with National Measurement Institutes, including: strategic partnership with NPL; Project Partnership with PTB. Membership of European Microkelvin Platform with arrangements for technology exchange, innovation management and public engagement both supports our impact agenda, and helps us target areas where we can make a unique contribution. Nationally, impact on Quantum Technology will be enhanced by the new UK Centre of Excellence for superconducting and hybrid quantum systems at Royal Holloway.

Finally, we will develop our public engagement strategy, supported by our dedicated Physics Outreach Officer, through talks, videos, posters, news stories, and enhanced on-line presence.

Publications

10 25 50
 
Description In hybrid mesoscopic devices made from topological superfluid 3He, confinement is the key control parameter which determines the superfluid order parameter. In experiments on 3He confined in a 200 nm cavity we have demonstrated that it is possible to tune the surface scattering from diffuse to specular, by coating the surface with a superfluid 4He film. We show, with our theory collaborators, that both the suppression of Tc and of the superfluid gap as a function of surface specularity can be accounted for using quasiclassical theory. We discovered that with a surface boundary layer of localised magnetic 3He, stronger Tc suppression is observed. Our theory collaborator (Vorontsov) has recently developed a new model for the impact of surface spin-slip scattering on superfluidity to account for our results. This has wide implications in topological superconductivity and its applications, including the feasibility of topological quantum computing. This work has been published in Nature Communications.
The experimentally demonstrated elimination of Tc suppression for specular walls opened the door to the study of superfluid 3He in much thinner cavities, entering the quasi-two-dimensional limit. However we encountered a number of problems with silicon surfaces in the first generation of such cells. These now appear to have been resolved; we used a spectrum of surface probe techniques through collaborators at NPL. Better control over silicon surface passivation is now yielding good results. A further process shift was to use the Southampton Microfabrication Facility for direct wafer bonding of the silicon cells, and to generate precise optical profiles of cavity height across the wafer, which has up to 20 cavities. The opportunity was taken to upgrade the wiring of SQUID amplifier probes on ND2, to comply with best practice evolved mainly on ND3.
Experiments have been performed on a 80 nm cavity. This cavity height is close to the coherence length at zero pressure, so that complete suppression of superfluidity at this pressure will arise with diffuse surface scattering conditions. Our SQUID NMR experiment finds that, as expected, we can create close to specular scattering conditions, and eliminate both Tc and gap suppression. The superfluid A phase is stabilized. This has two implications: we will be able to study thin films of chiral superfluid in which a multiplicity of new phenomena are predicted; the possibility remains open that a magnetic surface boundary layer of 3He will stabilize a new phase. The results are the subject of a paper in preparation for Phys.Rev.Lett.
Analysis of NMR experiments on superfluid 3He confined in a 1.1 micron high cavity has provided evidence for a spatially modulated superfluid phase. Uniquely in superfluid 3He, NMR can clearly fingerprint the order parameter. Such phases, in which spatially modulation of superfluid order arises spontaneously, are sought in a wide range of systems: layered organic superconductors; heavy fermion superconductors; cuprate superconductors, ultracold fermionic gases; astrophysical objects. In our case, while a stripe modulation was predicted by Vorontsov and Sauls, motivating our experiment, we identify a two dimensional modulation, which we refer to as "polka-dot". This work was published in February 2019 in Physical Review Letters. It was selected as an "Editors' Suggestion" and featured in an article in "Physics". Recent, as yet unpublished, theoretical work shows that both 1D and 2D spatially modulated ("pair density wave") phases are topologically stable. This is an exciting result, with significant consequences.
This work was followed up, using different techniques, by a group at Alberta (and their work was published in Phys. Rev. Lett). We believe that the conclusions drawn by these authors are incorrect, and we published a Comment in Phys. Rev. Lett to this effect.
In collaboration with Cornell we made the first study of thermal transport in 3He in strongly confining channels. This work was published in Nature Communications. The fabrication of new designs of thermal transport cells is underway, as well as the development of miniaturised local thermometry. This is a necessary prelude to the planned experiments in hybrid structures. Here we are using nanofabrication facilities at both University of Southampton and Cornell University. The fabrication of superconducting microcoils for local NMR is underway, in collaboration with PTB (Braunschweig).
Exploitation Route These findings are of broad relevance as follows:
The result on surface scattering in topological superfluid 3He (with establish unconventional p-wave pairing) is relevant to future mesoscopic device applications of topological superconductors, of relevance to future quantum technology.
The experimental discovery of spatially modulated superfluidity/superconductivity is of wide interest and relevance, as mentioned in "key findings".
Sectors Manufacturing, including Industrial Biotechology

 
Description This project contributes to the environment supporting a number of activities with technical, and potentially commercial impact. 1. Design and construction at Royal Holloway of a prototype cryogen-free sub-mK ultralow temperature platform with Oxford instruments (ND4). Builds on original proof-of-principle, by us. 2. Development of fast, precise noise thermometry, with significant advances in state-of-the-art over this grant period. 3. Underpinning the 2019 re-definition of the Kelvin, in particular supporting the Mise en Pratique for disseminating the ultra-low temperature regime. Collaboration with European NMIs, in particular, PTB (Berlin) 4. Improved understanding of thermal boundary resistance, leading to better low-temperature heat exchangers. 5. Application of HyQUIDs to NMR. Collaboration with NPL (York Instruments went into administration in Nov 2019). 6. Participation as an access laboratory in European Microkelvin Platform, a European Advanced Infrastructure, funded by European Commission.
Sector Aerospace, Defence and Marine,Digital/Communication/Information Technologies (including Software),Education,Electronics,Environment,Healthcare,Manufacturing, including Industrial Biotechology
Impact Types Economic

 
Description Underpinning the 2019 redefinition of the kelvin, as part of the Mise en Pratique.
Geographic Reach Multiple continents/international 
Policy Influence Type Influenced training of practitioners or researchers
Impact In 2019 the SI unit of temperature, the kelvin, was redefined as part of the most significant overhaul of the seven SI units since their adoption. Our research provides techniques to disseminate the new scale in the ultralow temperature regime, critical for future quantum-enhanced technologies driving the second quantum revolution.
URL https://www.bipm.org/en/publications/mises-en-pratique/
 
Description INFRAIA-01-2018-2019 European Microkelvin Platform
Amount € 9,941,066 (EUR)
Funding ID 824109 
Organisation European Commission 
Sector Public
Country European Union (EU)
Start 01/2019 
End 12/2022
 
Description Cornell University 
Organisation Cornell University
Country United States 
Sector Academic/University 
PI Contribution Collaborative research on superfluid helium-3.
Collaborator Contribution Fabrication of nanofluidic sample chambers.
Impact Joint publications.
 
Description Kyoto University 
Organisation University of Kyoto
Department Department of Physics
Country Japan 
Sector Academic/University 
PI Contribution Access to experimental data
Collaborator Contribution Theoretical support and discussions
Impact Input into papers and talks
Start Year 2018
 
Description Montana State University 
Organisation Montana State University
Department Department of Physics
Country United States 
Sector Academic/University 
PI Contribution Access to experimental data
Collaborator Contribution Theoretical input into project
Impact Input into research talks / papers.
Start Year 2017
 
Description Northwestern_Theory 
Organisation Northwestern University
Country United States 
Sector Academic/University 
PI Contribution Experimental research and discussions on theory. Travel support.
Collaborator Contribution Theoretical collaboration. Facilitating engagement with a wider community of theorists.
Impact Collaboration is ongoing.
Start Year 2011
 
Description Oxford Instruments Nanoscience 
Organisation Oxford Instruments
Country United Kingdom 
Sector Private 
PI Contribution RHUL Low temperature laboratory group have collaborated with OIN on designs of cryogenic platforms. Our cryostat ND2 was the first example of what has become the Kelvinox 400 HA, where the design was influenced by our requirements for nuclear demagnetisation. RHUL/OIN jointly built a prototype of a combined nuclear demagnetisation cryogen free cryostat, the RHUL team demonstrated sub mK performance of a cryogen-free cryostat for the first time. OIN are world leaders in manufacturer of cryostats and we are working with them to replace nuclear orientation thermometers with our current sensing noise thermometer. Through a Memorandum of Understanding the RHUL provide consultation services for the design engineers at OI.
Collaborator Contribution Consultation on specialist magnet designs, Provision of engineered components, Discounts on new cryogenic platforms OIN provided £20,000 of sponsorship for ULT 2008: Frontiers of Low Temperature Physics
Impact RHUL: REF 2014 Impact Case Study London Low Temperature Laboratory http://dx.doi.org/10.1088/1367-2630/15/11/113034 http://dx.doi.org/10.1007/s10909-014-1147-z
 
Description Oxford University 
Organisation University of Oxford
Country United Kingdom 
Sector Academic/University 
PI Contribution Access to experimental data
Collaborator Contribution Theoretical support and discussions
Impact Input into papers and talks
Start Year 2018
 
Description PTB_SQUIDs 
Organisation Physikalisch-Technische Bundesanstalt
Country Germany 
Sector Academic/University 
PI Contribution Development of application of SQUIDs in high precision instrumentation.
Collaborator Contribution Design and fabrication of SQUIDs.
Impact Joint publications. New instrumentation for NMR and noise thermometry.
 
Description University of Jyväskylä 
Organisation University of Jyvaskyla
Department Department of Physics
Country Finland 
Sector Academic/University 
PI Contribution Access to experimental data
Collaborator Contribution Theoretical support
Impact Input into talks an papers
Start Year 2018
 
Description Astrophysics schools residential course 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Schools
Results and Impact 60 A-level students attended a Astrophysics residential course at RHUL organised by the SmallPiece trust, Dr, Andrew Casey gave a lecture at this course on low temperature properties of space, cryogenics and the relationship with the superfluid research at RHUL
Year(s) Of Engagement Activity 2018,2019
 
Description Girls into Physics Residential Course 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Schools
Results and Impact In collaboration with the Smallpiece Trust, we ran a three-day residential course aimed at addressing the underrepresentation of women in physics by providing a mixture of physics activities for year 10/12 female school children from a range of schools. As part of this activity, Dr. Andrew Casey provided a lecture and demonstration about some of the exciting discoveries in low temperature physics. In 2020 this event was delivered online.
Year(s) Of Engagement Activity 2017,2018,2019,2020
URL https://www.royalholloway.ac.uk/physics/events/eventsarchive/girls-into-physics-residential.aspx
 
Description Institute of Physics Low Temperature Techniques Course 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Postgraduate students
Results and Impact The purpose of the meeting is to disseminate best practice and raise awareness of new innovations in low-temperature techniques and thermometry to each new national cohort of PhD students and postdoctoral researchers embarking on a research career at low temperatures. In addition, we raise the awareness of how those skills can be employed in an industrial environment. Each year the event is attended by around 50 delegates (mainly 1st year PhD students, occasionally international), the students report a raised awareness and begin to create a support network with each other and the speakers at the event that will help them during their career.
The event is organised and chaired by: Dr. Andrew Casey (with support from the IOP)
Dr. Andrew Casey and Dr. Jan Nyeki both give presentations at the event.
The event is supported by an annual grant from the IOP Low Temperature group of £1000, which is used to reduce the cost of attendance.
The event is publicised by the IOP through it's website and newsletters.
An e-version of the material presented is distributed to all of the delegates.
The 2020 event was online only.
Year(s) Of Engagement Activity 2009,2010,2011,2012,2013,2014,2015,2016,2017,2018,2019,2020
URL https://www.iopconferences.org/iop/frontend/reg/thome.csp?pageID=407153&eventID=818&eventID=818&CSPC...
 
Description Royal Holloway, Physics Taster Day 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Schools
Results and Impact Royal Holloway Physics Taster Days are one day events held in the Physics department at Royal Holloway where we invite A-level physics students from schools within our region to experience university level physics. Typically each event is attended by between 50-100 students. In some years the event has been targeted at widening participation by inviting schools that do not have a strong track record in physics provision. Dr. Andrew Casey has given a lecture every year at these events.
Surveys of the students before and after the event suggest that students are more to choose physics at degree level after attending the event. In 2020 the event was online.
Year(s) Of Engagement Activity 2010,2011,2012,2013,2014,2015,2016,2017,2018,2019,2020
URL https://www.royalholloway.ac.uk/physics/outreach/a-level/tasteofphysics.aspx
 
Description Royal Holloway, Science Festival 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Our annual science festival is designed to inspire and inform the general public about the research at Royal Holloway. Each year the event is attended by around 5000 members of the general public. Dr. Andrew Casey and Dr. Jan Nyeki provide lectures and demonstrations throughout the event each year. Dr. Andrew Casey runs the low temperature zone, comparing the low temperatures achieved in the Universe with those that can be achieved in the laboratory, and highlights the physics that can be performed at these low temperatures.
Year(s) Of Engagement Activity 2006,2007,2008,2009,2010,2011,2012,2013,2014,2015,2016,2017,2018,2019
URL https://www.royalholloway.ac.uk/science/sciencefestival/home.aspx
 
Description Super Cool Baking (Richmond and Hillcroft Adult Community College) 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact A pilot scheme to reach the low science capital adult audience with funding from the regional IOP branch. (London and South East). The premise was to combine physics discussions and demonstrations with an adult cookery class. Cooking is one of the top pastimes amongst adults and provided a vehicle to connect with an audience that would not have come to a science lecture. The feedback from the evening was good and further funding is being sought to widen the pilot scheme to other venues.
Year(s) Of Engagement Activity 2019
URL https://www.eventbrite.co.uk/e/super-cool-baking-an-evening-of-discovery-tickets-68843088531#
 
Description Widening Participation Event 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Schools
Results and Impact 80 students from West London schools attended a University taster day organized by the physics outreach officer. Dr. Casey provided a lecture with demonstrations about the exciting possibilities of low temperature research. Students, who previously had not considered University as a future option reported that they were now motivated to pursue this.
Year(s) Of Engagement Activity 2020