Correlated electronic states for cryogenic refrigeration - fundamentals and applications

Lead Research Organisation: University of Cambridge
Department Name: Physics


Low temperature cooling techniques have enabled some of the most dramatic scientific discoveries in condensed matter physics, such as superconductivity, superfluidity, and the quantum Hall effects. These discoveries, like most research at low temperatures, were made by exploiting the high entropy carried by atoms, namely the helium isotopes. Is it possible to formulate solid-state analogues to these techniques, and could they open up new opportunities? Our project combines fundamental research, technology evaluation and instrument development, in order to establish cooling methods which are based on manipulating electrons rather than atoms and which thereby lend themselves to miniaturisation and mass production.

This is important, because access to the sub-Kelvin range is no longer of interest to fundamental research alone: quantum engineering, the use of quantum effects for new technologies, relies on quiet environments, which in solid state devices implies low temperatures. A more diverse arsenal of cooling platforms facilitates the spread of quantum technologies. Solid-state refrigerators can be combined with a mechanical cryocooler to produce small, low-cost, energy-efficient and cryogen-free platforms ideally suited to carrying sensors and other quantum devices. We show that significant miniaturisation is already possible by use of existing correlated electron metals with entropy density changes almost an order of magnitude higher than those of conventional salts for the same low applied magnetic fields, and not requiring encapsulation to retard dehydration nor metallic infrastructures to promote thermal conduction, measures that can limit not only compactness but also long-term reliability.

Fundamental research is needed to provide new insights and to develop new materials for solid-state refrigeration. The cooling methods we consider either exploit the magnetic field dependence of the entropy (magnetocaloric effect), or heat is transported along with a charge current (Peltier effect). We will investigate correlated phenomena which amplify these effects at low temperature by multiprobe measurements over wide ranges of field, temperature and pressure:

(i) In metallic rare earth compounds, the f-orbitals of the rare earth elements can host magnetic moments, which can form a metallic spin liquid. Magnetic moments in these systems are much more densely packed than in conventional refrigerants, in which moments are highly diluted to avoid magnetic order. Because this state is associated with a very high and strongly field-dependent entropy at low temperature, it can be exploited for cooling. We will use a wide range of experimental techniques, including thermal transport, heat capacity and quantum oscillation measurements, to investigate the metallic spin liquid state and its excitations.

(ii) In Kondo insulators, electronic interactions cause semiconducting behaviour at low temperature. Because of their small energy gaps and narrow electronic bands, Kondo insulators are favourable for Peltier cooling. They can, moreover, display further intriguing phenomena, such as topologically protected surface states and quantum oscillations from bulk states in SmB6. We will examine thermal transport in Kondo insulators and explore the nature of the Kondo insulating state by multiprobe measurements, when the gap is varied under applied pressure.

(iii) Structural instabilities are widespread in materials with complex lattice structures, and they can be controlled by varying the composition or the applied pressure. This opens up further options for manipulating the phonon spectrum and for inducing mesoscopic textures which affect the phonon mean free path. We will investigate the consequences for the lattice thermal conductivity and for the material's effectiveness as a Peltier refrigerant.

The insights gained in this project will also help improve solid state refrigeration at elevated temperature.

Planned Impact

The diversity of electronic states in complex materials with strong electronic interactions, their intrinsic quantum nature, their reach into practicable temperature regions, and their tunability by varying the crystal structure or by applying external fields or pressure, can be exploited for next generation quantum innovations. Our project explores the fundamental science underpinning some of the most promising near-term applications of such 'quantum functional materials': cooling and thermal energy harvesting. Both technologies are essential to computing and other high-technology applications, which are increasingly limited by waste heat production. Present cooling and air conditioning methods account for a substantial fraction of our energy consumption, and even modest gains in efficiency would produce major savings.

We concentrate initially on refrigeration at low temperature in the Kelvin and sub-Kelvin range, where we see an increasing demand for compact, efficient and low-maintenance refrigeration in the burgeoning field of quantum technologies. Research on solid-state based quantum sensors and devices at low temperature currently uses cooling technology based on circulating the helium-isotope 3He or on magnetic refrigeration. The former offers excellent performance at the lowest temperatures but is complicated to manufacture, requires gas handling systems and pumping arrangements, and suffers from the high cost of 3He. By contrast, the availability of affordable superconducting magnets has made magnetic refrigerators cheap to build. They can be assembled from mass-produced components, they are reliable and compact, and they are straightforward to operate continuously and routinely over wide temperature ranges.

Current magnetic refrigeration technology relies on electrically insulating refrigerants, which suffer from intrinsic disadvantages, such as their poor thermal conductivity at low temperature, their tendency to degrade over time and their low cooling capacity. Our project will identify and test metallic refrigerants which address these limitations while operating in the same temperature and field range. Importantly, the entropy density of already identified metallic refrigerants is a factor of five to ten higher than that of the best insulating refrigerants, and our research aims to improve on this further. This enables dramatic savings in the size of the magnet required, which in turn helps shrink the system as a whole and brings immediate benefits in applications in which space and weight are critical, such as satellite-borne detectors.

We also investigate the alternative approach of cooling into the Kelvin-range by circulating electrical currents, or Peltier cooling, which enables further miniaturisation. Like magnetic refrigeration, this hinges on the selection of novel materials with very specific properties. Our programme of research will address the underlying scientific questions which determine the usefulness of materials for both approaches, it will examine a series of candidate materials in detail and it will produce demonstrator refrigerators in collaboration with a commercial project partner.

We will evaluate and demonstrate metallic refrigerants in a new miniature demagnetisation cooler, creating a continuously operating, desktop-sized, cryogen-free cooling platform for the sub-Kelvin range. By widening the arsenal of cooling systems, this can assist the spread of previously uneconomic or unrealistic quantum technologies. Moreover, elements of our improved understanding of the fundamentals of solid state refrigeration may be transferred to the problem of efficient solid state refrigeration at elevated temperature. The programme thereby helps to address key concerns of our industrialised economy, in particular with regard to energy and sustainability. Progress in this area benefits the general population as well as specialised companies poised to exploit technological advances.


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Cheung Y (2017) Second-order Structural Transition in (Ca 0.5 Sr 0.5 ) 3 Rh 4 Sn 13 in Journal of Physics: Conference Series

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Friedemann S (2017) Quantum tricritical points in NbFe2 in Nature Physics

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Rauch D (2018) High magnetic field behavior of NbFe 2 in Physica B: Condensed Matter

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Yang YF (2017) Quantum critical scaling and fluctuations in Kondo lattice materials. in Proceedings of the National Academy of Sciences of the United States of America

Description We have identified metallic materials that can be used to cool devices or entire apparatus to temperatures in the sub-Kelvin range by magnetic cooling techniques. We have developed methods for producing these materials in sufficient quantity that they can be used in practice. We have demonstrated the usefulness of this approach in cryogenic experiments.
Exploitation Route This approach can be used by manufacturers of cryogenic equipment to produce small, efficient cooling systems that can be applied in the fields of quantum technology, quantum sensors or satellite-based sensing.
Sectors Aerospace, Defence and Marine,Digital/Communication/Information Technologies (including Software),Energy

Description Initial results of this ongoing project already suggest a number of complex materials that can improve low temperature magnetic cooling equipment by about a factor of five. We have been able to make some of these in sufficient quantity to test their use in a realistic application. We have established contact with one relevant equipment manufacturer and are currently preparing test runs on commercially available equipment. We have furthermore established contact with a group working in surface sensitive low temperature measurements outside the UK, who are now building a new low temperature instrument which will incorporate our cooling technology.
First Year Of Impact 2018
Impact Types Cultural,Economic

Description Isaac Newton Trust
Amount £20,000 (GBP)
Funding ID RG97595 
Organisation University of Cambridge 
Sector Academic/University
Country United Kingdom
Start 08/2018 
End 03/2020
Title Metallic magnetocalorics for low temperature refrigeration 
Description We have identified intermetallic compounds which offer superior refrigeration performance for cryogenic applications. Moreover, we have developed methods for growing sizeable quantities of these materials from constituent elements, and we have designed, built and tested cooling modules which integrate several cooling stages to provide higher cooling power and lower base temperatures than commercially available alternatives. 
Type Of Material Improvements to research infrastructure 
Year Produced 2019 
Provided To Others? No  
Impact The improved cooling performance has immediate impacts on research at low temperatures, but its main impact is expected to result from integrating the new cooling modules in applications which benefit from miniaturisation, such as satellite-born detectors or continuously-cooled multi-stage systems, or for which conventional refrigerants are unsuitable, such as techniques requiring UHV. Continuously-cooled multi-stage systems provide an attractive low temperature platform for upcoming solid-state based quantum technologies. 
Title Research data supporting "Strong coupling superconductivity in a quasiperiodic host-guest structure" 
Description Data underlying the figures shown in the publication 'Strong coupling superconductivity in a quasiperiodic host-guest structure', including resistivity versus temperature at different pressures and magnetic fields, the critical-field curve of high pressure bismuth, the high pressure magnetisation of bismuth, and the results of phonon dispersion calculations. 
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? Yes  
Impact The published datasets allow colleagues to conduct their own analysis and compare new theories for the electronic and vibrational excitations of a quasiperiodic material against experimental data. 
Title Research data supporting 'Quantum Tricritical Points in NbFe2' 
Description Magnetisation, magnetic susceptibility and electrical resistivity data obtained on single crystals of NbFe2 with varying levels of Fe or Nb excess. Analysis in terms of Arrott plots. Resulting phase diagram showing buried ferromagnetic quantum critical point and quantum tricritical points. 
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? Yes  
Description Growth and characterisation of quantum materials 
Organisation University of Central Lancashire
Department Jeremiah Horrocks Institute for Mathematics, Physics and Astronomy
Country United Kingdom 
Sector Academic/University 
PI Contribution We have grown high quality crystals and polycrystals of quantum materials, such as YFe2Ge2, NiS2 and NbSiSb and investigated their properties by a range of transport, thermodynamic, magnetic and spectroscopic probes.
Collaborator Contribution Colleagues at the University of Central Lancashire have carried out high precision structural studies by single crystal and powder x-ray diffraction, in order to resolve details of the crystal structure and its defect concentration.
Impact The collaboration has led to crystals of world-leading quality of both NiS2 and YFe2Ge2. This in turn has enabled us to resolve the electronic structure of pressure-metallised NiS2, the first such measurement near the threshold of Mott localisation, and it has produced clear thermodynamic evidence for the unconventional superconducting state in YFe2Ge2. These outcomes are recorded in a number of joint publications.
Start Year 2015
Description Joint low temperature measurements on high quality crystals 
Organisation Max Planck Society
Country Germany 
Sector Charity/Non Profit 
PI Contribution Provided crystals and suggested aspects of the experiment
Collaborator Contribution Carried out low temperature magnetic torque measurements at the MPI-CPfS in Dresden
Impact Detailed low temperature magnetic measurements in NbFe2 samples of varying stoichiometry have improved our understanding of the origins of magnetic anisotropy in this complex material and will contribute to a future publication.
Start Year 2013
Description Single crystals for low temperature, high field, high pressure measurements 
Organisation University of Warwick
Department Department of Physics
Country United Kingdom 
Sector Academic/University 
PI Contribution Sharing of low temperature, high field, high pressure measurement data. Future joint publications.
Collaborator Contribution Provision of high quality single crystals of two materials of intense current interest.
Impact A series of successful high pressure, low temperature, high magnetic field measurements to elucidate the change in the electronic structure as a material is tuned across a band inversion transition. The data will form part of a PhD thesis, has in parts been presented at conferences, and is due to be published as soon as the measurements are complete.
Start Year 2013