Quantum Phase Transitions and Quantum Criticality in Helium Films

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


Historically quantum fluids, the helium liquids near absolute zero, have provided simple model systems which have played a crucial role in the development of key concepts in condensed matter physics. The understanding of superfluidity and broken gauge symmetry; the development of the standard model of correlated fermions; the first unconventional superfluid/superconductor; the central role of topological excitations in two dimensional physics: all these discoveries and insights arose from the study of helium. The study of quantum fluids has also fuelled developments in techniques for producing and measuring low temperatures, high magnetic fields, and a host of novel measurement techniques and instrumentation. We propose to study a variety of low dimensional helium model systems to address fundamental issues in the understanding of strongly correlated quantum matter. We will study helium-3 (fermion) films and helium-4 (boson) films. These films grow as atomic layers on the atomically flat surface of graphite, and the lattice potential experienced by a helium layer can give rise to a triangular superlattice structure. The density of these layers can be varied essentially continuously to tune between different quantum mechanical ground states. These may include ground states theoretically proposed but yet to be unambiguously realized. We will study the quantum phase transitions between different ground states in some detail. We will study the Mott transition between a 2D helium-3 Fermi liquid and a 2D quantum spin liquid and the properties of the hole-doped spin liquid on a triangular lattice. We will attempt to stabilise a Mott insulator on a square lattice and perform a comparable experiment. In the corresponding helium-4 film we will study the superfluid-insulator transition, and investigate possible 2D supersolid behaviour. We will develop a highly ordered graphite substrate with a view to optimising conditions under which to search for the holy grail of unconventional superfluidity in a helium-3 fluid monolayer. We will investigate quantum criticality in the helium-3 bilayer heavy fermion system recently discovered by us. And we will study helium-3 in nano-channels as a one dimensional fermion system, and a possible realization of a Luttinger liquid. These experiments on fermionic and bosonic cold atoms are performed on uniform low dimensional systems in thermodynamic equilibrium at precisely measured temperatures in the range 200 microKelvin to 4K. The lowest temperatures will be produced by nuclear adiabatic demagnetization cryostats in our laboratory. A range of high precision experimental probes will be employed to study these systems. Sensitive NMR techniques developed in our laboratory, based on the detection of the precessing magnetic signal by SQUIDs (Superconducting Quantum Interference Devices), will be used to measure magnetic susceptibility, magnetization and spin dynamics. We will extend measurements of the heat capacity to the lowest temperatures in order to access system entropy and probe the elementary excitations. The superfluid density, and any dissipative component of the response, will be measured by high quality torsional mechanical resonators. We will collaborate on developing graphene based nano-mechanical resonators with wide-bandwidth SQUID amplifier detection. The project is expected to lead to fundamental insights into some of the most central issues in the physics of strongly correlated matter, and impact on the understanding of more complex materials of potential technological relevance. The project will drive innovation of new instrumentation and measurement techniques at an important scientific frontier; the low temperature frontier. As in any frontier science we may encounter the unexpected.

Planned Impact

Who will benefit? The research proposed here exploits helium to confront fundamental issues of wide significance and relevance in condensed matter physics. It underpins work on technologically relevant materials. It ultimately benefits manufacturers of new devices of novel functionality based on new materials, their users and society. In addition, the proposed research involves the development of cryogenic instrumentation and novel measurement systems involving the use of state-of-the-art superconducting devices as amplifiers. It directly benefits the scientific instruments industry and users of such instrumentation, including cryogenics industry, superconducting technologies industry, medical imaging and diagnostics. Expertise in this frontier area of research allows us to contribute advice nationally and internationally, mostly to funding agencies and scientific academies, and to engage with national laboratories. The wider public is potentially excited by advances in low temperature physics, a visual subject, and a playground for demonstrating exotic quantum phenomena. How will they benefit? The simplicity of helium provides a test-bed for the new and evolving theories of strongly correlated condensed matter, and the effects of low dimensionality. Strong networking with theorists, enhanced by the establishment of a new theory of condensed matter activity, will ensure that ideas developed in helium physics propagate, to influence the materials discovery agenda. Advances in fundamental understanding enter the canon of the subject, contribute to scientific progress and enhance the quality of life through unanticipated channels. Our understanding of condensed matter is at a crossroads; it is hard to predict the timescale on which the paradigm shift will occur. The highest quality fundamental research acts as a magnet for the best students and young talent. A key impact of our research is the rigorous training of highly skilled manpower. Our experimental work revolves around custom design of equipment and instrumentation, the mastery of design skills and construction techniques, developing high precision measurement techniques and associated data acquisition schemes, data analysis, modelling; it involves managing the interaction with theorists. The training of such manpower enhances our economic competitiveness. The cryogenic industry will benefit by contributions to the development of new refrigerator products, or products with enhanced capabilities. Our expertise will provide necessary credible thermometry for the mK temperature range. Developments in the applications of superconducting devices to NMR, made through this research, will be further advanced by scientific collaborations with end users. Our fundamental research will continue to be the driver for improving sensitivity and technique. The major benefit will be novel MRI modalities, based around compact systems, new techniques for NMR spectroscopy and biodiagnostics. New developments in nano-mechanical resonators are also planned. The wider public, including school children, will benefit through a portfolio of outreach activities designed to encourage interest in science and to promote scientific endeavour and its societal benefits. What will be done to ensure that they benefit? Collaboration arrangements include: an established network of strong collaborations with scientific instruments industry and standards laboratories; an enhanced European network through membership of the European MicroKelvin Collaboration, funded by FP7, with dedicated activities for interaction and knowledge exchange Export of trained manpower; exploitation of novel instrumentation through national knowledge exchange schemes, such as HEIF, and the people pipeline. Wider communication and engagement activities will centre on outreach activities organised through our dedicated physics outreach officer; international conference and workshop participation and organisation.


10 25 50
Description Discovery of a new phase of quantum matter with entwined superfluid and density wave order (a two dimensional supersolid). Significant progress in understanding the density driven Mott transition in two dimensional helium-3 films; a model for strongly correlated quantum matter. Indications of the realization of a striking new state of topological quantum matter, a quantum spin liquid. This is a magnet that does not order, even at absolute zero, and is of great theoretical interest. Characterization of two dimensional ferromagnetism using a helium-3 film. Text book example of a two dimensional Heisenberg magnet. Development of a new high quality exfoliated graphite substrate, by a novel potassium intercalation technique. First demonstration of feasibility of point contact spectroscopy in pulsed magnetic fields up to 60T. Conclusive identification of density wave instabilities in graphite in the ultra-quantum limit, a subject under investigation for more than 30 years. Implies existence of new topologically protected chiral edge states. Systematic study of noise thermometry, to optimise speed/precision. First demonstration of the feasibility of nuclear demagnetization cryostat based on a cryogen-free dilution free platform.
Since the end of the grant a number of further important results have been obtained which capitalize of the upgrade of the ultralow temperature nuclear demagnetization cryostat (ND1) completed during the grant period. These have been enabled by the installation on this cryostat of the broadband SQUID NMR instrumentation we have developed in our laboratory. This has led to fundamental insights into interactions in two dimensional 3He, adsorbed on a superfluid 4He film, which provides an ideal model 2D fermi system. We show that the Landau Fermi liquid survives in 2D, but backwards scattering dominates. This system is highly tuneable, and we can vary the number of 4He layers. For three layers there is a regime in which a uniform 2D 3He system is unstable, and we find evidence for 3He dimer formation. 2D 3He is a paradigm for understanding interactions in 2D electron systems, the basis of a large number of important functional materials (such as high Tc superconductors).
Exploitation Route Further research projects. Collaboration with industry.
By studying a simple model system we provide key insights for theoretical frameworks to understand more complex materials, with important functionality.
Sectors Education,Manufacturing, including Industrial Biotechology,Other

Description The primary purpose of this project was fundamental research on two dimensional helium. As a model system for strongly correlated quantum matter, the anticipated intellectual impact is high, given the well-documented importance of such materials (as evidenced by the award of many Nobel Prizes). A significant and wide impact in the shorter term derives from the recognised importance of research on matter under extreme conditions, in particular low temperatures. Developing capability in this sector, and enhancing measurement technique, metrology and instrumentation, is a key feature of this research, and we have contributed in several ways. This impact has been achieved through close collaboration with the industrial sector (primarily Oxford Instruments Nanoscience (MoU)) and National Measurement Institutes (including funding by European Measurement Research Programme), and was the subject of an Impact Case Study to REF2014, which feature in the IoP document "Inspirational Physics for a Modern Economy". A further instrument for promoting impact was originally our membership of the European Microkelvin Consortium, funded under the EC Infrastructures programme (FP7). The application for Horizon 2020 funding for the European Microkelvin Platform www.emplatform.eu was successful and EMP is now funded as an Advanced Infrastructure, with the London Low Temperature Laboratory at Royal Holloway as an access facility. Engagement with a wide community of researchers and industrial partners is a key feature of this Platform, to further research on Quantum Materials and Quantum Technology. Impact through outreach/public engagement through evening lectures. John Saunders was Chair of IUPAP (International Union of Pure and Applied Physics) Commission C5, and a Vice-President on the IUPAP Executive Council. His term of office ended in 2017. He is a member of the executive board of the European Microkelvin Platform. Research on new routes to develop exfoliated graphite, in collaboration with Dr Chris Howard (UCL), have fed into his work on solutions of 2D materials, which has secure impact funding. A new collaboration is under development with NPL (Prof Tzalenchuk) on applications of large area graphene sheets.
First Year Of Impact 2013
Sector Manufacturing, including Industrial Biotechology,Other
Impact Types Economic,Policy & public services

Description FP7 Capacities Specific Programme, Research Infrastructures
Amount € 300,000 (EUR)
Funding ID 228464 
Organisation European Commission 
Sector Public
Country European Union (EU)
Start 04/2008 
End 09/2013
Title Cryogen free ultralow temperature platforms 
Description A prototype platform was constructed with minimum temperature below 1 mK, extending the range available using cryogen-free dilution refrigerator technology. 
Type Of Material Improvements to research infrastructure 
Year Produced 2013 
Provided To Others? Yes  
Impact This technique is being disseminated by collaboration with Oxford Instruments Nanoscience. It will extend the accessibility of ultralow temperatures to a wider community of users. 
Title Noise thermometry using dc SQUIDs 
Description Enables the precise measurement of absolute temperature below 1K. 
Type Of Material Improvements to research infrastructure 
Year Produced 2014 
Provided To Others? Yes  
Impact The definition of measurement of temperature is central to the ever widening application of low temperature platforms. The dissemination of this technology is via published research and collaboration with Oxford Instruments Nanoscience, and National Measurement Institutes. 
Description The use of SQUIDS (superconducting quantum interference devices) as detectors of NMR has two distinct realizations: broadband and tuned. 
Type Of Material Improvements to research infrastructure 
Year Produced 2011 
Provided To Others? Yes  
Impact These methods extend the capability of the NMR technique in several different ways: enhanced sensitivity; to ultralow low measurement fields; enabling pulsed NMR at ultralow temperatures. Applications in fundamental science, and potential applications in medical and chemical diagnostics. This impact has been realized by: membership of European Microkelvin Platform; skills transfer by movement of trained personnel. 
Description Cornell_NEMS 
Organisation Cornell University
Country United States 
Sector Academic/University 
PI Contribution Application of graphene NEMS.
Collaborator Contribution Fabrication of graphene NEMS. Design and fabrication of torsional oscillator.
Impact Joint publications.
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 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 UCL_Graphite 
Organisation University College London
Country United Kingdom 
Sector Academic/University 
PI Contribution Collaboration on development of new exfoliated graphite substrates, and their application.
Collaborator Contribution Provision of equipment and expertise.
Impact Joint publications are in draft. Potential applications to production of multilayer graphene.
Start Year 2010
Description British Science Association Festival 2009 
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 In 2009 RHUL Physics ran a one day satellite event to the British Science Association Festival held that year in Guildford. At the even Prof. John Saunders and Dr. Andrew Casey gave a talk entitled "Superfluids and Supersolids: Quantum Mechanics of Large System" to an audience of around 100 members of the general public mainly consisting of school aged children.
The event was designed to promote physics as a career choice and to raise awareness of the field of Quantum Fluids and Solids.
Year(s) Of Engagement Activity 2009
URL http://www.britishscienceassociation.org/
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 Talk. Ultracold: The Quantum World near Absolute Zero 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Schools
Results and Impact Schools talk.
Year(s) Of Engagement Activity 2015
Description The coldest place in the universe 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Schools
Results and Impact Abstract: What is the coldest place in the Universe? Is it somewhere in deep space, in a Galaxy far away? Could it be in Neptune, 4.5 billion km away from the sun? Could it be in our own moon, in a crater so deep that light can never reach?

Or is it in physics labs, right here, on planet Earth? One of them is actually in a walking distance from the venue of our Christmas evening lecture! Find out about exotic phenomena like super conductivity and super fluidity, discuss Nobel Prize 2016 and participate in a lecture on the coolest topic ever literally. Join Dr. Andrew Casey, Royal Holloway Physicist, as he explains the amazing, large-scale quantum phenomena that take place in very low temperatures, on the 8th of December.

The main outcome of this talk was to demonstrate the potentials for discovery that exist at the ultra low temperature frontier.
Year(s) Of Engagement Activity 2016
URL https://www.royalholloway.ac.uk/physics/outreach/eveninglectures/eveninglectures.aspx