Correlated electronic states for cryogenic refrigeration - fundamentals and applications

Lead Research Organisation: University of Cambridge
Department Name: Physics

Abstract

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.
 
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. We have developed multi-stage cooling modules that can be deployed on existing and wide-spread low temperature platforms such as the Quantum Design PPMS in order to widen their temperature envelope to extend to about 60 mK above absolute zero. We have also begun collaborations with several cryogenics users abroad, who are to trialling our technology (i) in a large project for cooling a scanning probe microscope to sub-Kelvin temperatures, and (ii) in PPMS measurement platforms.
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. We have initiated a joint development project with a UK instrument maker to produce refrigeration modules that extend the temperature range of existing low temperature measurement platforms to lower temperature.
Sectors Aerospace

Defence and Marine

Digital/Communication/Information Technologies (including Software)

Energy

 
Description The main result of this project has been to 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 UK equipment manufacturer and are now jointly developing refrigeration modules that can extend the performance of their commercially available low temperature measurement platform. We have also used this technology to extend the cooling performance of a US-manufactured cryogenic measurement platform into the <100 mK range. We have furthermore as part of a joint project provided a large quantity of one of our metallic magnetocaloric refrigerant materials to 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. A second important result has been the improvement of growth capabilities in our group, including the development of a horizontal liquid transport growth method, which resulted in ultra-pure crystals of the new unconventional superconductor YFe2Ge2, with purity level exceeding that of the best crystals grown elsewhere by an order of magnitude.
Impact Types Cultural

Economic

 
Description Enhanced Magnetic Cooling through Optimising Local Interactions
Amount £88,922 (GBP)
Funding ID EP/T028033/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 07/2020 
End 08/2023
 
Description Isaac Newton Trust
Amount £20,000 (GBP)
Funding ID RG97595 
Organisation University of Cambridge 
Sector Academic/University
Country United Kingdom
Start 07/2018 
End 03/2020
 
Title Cryogenic refrigeration module for commercial cooling platform 
Description We have developed a cooling module that can be used inside the standard low temperature measurement platform 'Physical Properties Measurement System' (PPMS) by Quantum Design. The module will offer base temperatures of less than 100 mK with a hold time below 1 K of about 10 hours, far outperforming currently available commercial refrigeration modules. The module uses adiabatic demagnetisation cooling driven by the magnetic field provided by the PPMS. 
Type Of Material Improvements to research infrastructure 
Year Produced 2021 
Provided To Others? Yes  
Impact The cooling module extends the useful temperature range of the standard PPMS platform by about a factor of 20 at little extra cost. There are hundreds of PPMS installed world-wide. It is the most popular commercial cooling platform currently available. The module in principle makes milli-Kelvin temperatures available to a large number of researchers who have otherwise little experience with low temperature physics. 
 
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 CSD 2052591: Experimental Crystal Structure Determination 
Description Related Article: Jiasheng Chen, Monika B. Gamza, Jacintha Banda, Keiron Murphy, James Tarrant, Manuel Brando, F. Malte Grosche|2020|Phys.Rev.Lett.|125|237002|doi:10.1103/PhysRevLett.125.237002 
Type Of Material Database/Collection of data 
Year Produced 2020 
Provided To Others? Yes  
URL http://www.ccdc.cam.ac.uk/services/structure_request?id=doi:10.25505/fiz.icsd.cc26wwmm&sid=DataCite
 
Title CSD 2052593: Experimental Crystal Structure Determination 
Description Related Article: Jiasheng Chen, Monika B. Gamza, Jacintha Banda, Keiron Murphy, James Tarrant, Manuel Brando, F. Malte Grosche|2020|Phys.Rev.Lett.|125|237002|doi:10.1103/PhysRevLett.125.237002 
Type Of Material Database/Collection of data 
Year Produced 2020 
Provided To Others? Yes  
URL http://www.ccdc.cam.ac.uk/services/structure_request?id=doi:10.25505/fiz.icsd.cc26wwpp&sid=DataCite
 
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. 
URL https://www.repository.cam.ac.uk/handle/1810/278535
 
Title Research data supporting "Unconventional bulk superconductivity in YFe2Ge2 single crystals" 
Description Low temperature heat capacity, electrical transport and magnetisation data obtained on high quality crystals of YFe2Ge2, showing the dependence of crystalline quality on the growth method and demonstrating the influence of disorder level on the superconducting transition. 
Type Of Material Database/Collection of data 
Year Produced 2020 
Provided To Others? Yes  
URL https://www.repository.cam.ac.uk/handle/1810/313218
 
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  
 
Title Research data supporting 'Single crystal growth and hydrostatic pressure study of charge density wave quantum critical Lu(Pt1-xPdx)2In' 
Description Supporting data for 'Single crystal growth and hydrostatic pressure study of charge density wave quantum critical Lu(Pt1-xPdx)2In' published in Journal of the Physical Society of Japan. The files comprise all raw data included in the paper. That are differential scanning calorimetry, differential thermal analysis, furnace profile, magnetic susceptibility and electrical resistivity data. Furthermore, high quality pictures of Laue pattern and of a single crystal are archived. 
Type Of Material Database/Collection of data 
Year Produced 2021 
Provided To Others? Yes  
URL https://www.repository.cam.ac.uk/handle/1810/320571
 
Title Supporting data for "Composition dependence of bulk superconductivity in YFe2Ge2" 
Description Data underlying the figures shown in the publication 'Composition dependence of bulk superconductivity in YFe2Ge2', including resistivity and heat capacity versus temperature for different sample qualities, in zero and 2.5 T applied field, resistive transition temperature vs. residual resistivity for a large number of samples, transition temperature and residual resistance ratio vs. nominal composition, lattice parameters vs. nominal composition, and EDS-derived phase content of polycrystalline ingots from with varying nominal composition. 
Type Of Material Database/Collection of data 
Year Produced 2019 
Provided To Others? Yes  
 
Title Supporting data for 'Fermi surface and mass renormalization in the iron-based superconductor YFe2Ge2' 
Description This deposit holds all the data underlying the figures shown in the linked publication. Please see readme file for details 
Type Of Material Database/Collection of data 
Year Produced 2022 
Provided To Others? Yes  
URL https://www.repository.cam.ac.uk/handle/1810/342603
 
Description Exchange of high quality materials for characterisation at low temperatures 
Organisation University of Kent
Country United Kingdom 
Sector Academic/University 
PI Contribution We have investigated thermodynamic properties in applied magnetic field of magnetic materials provided by collaborators at the University of Kent, in detailed and comprehensive low temperature measurements of the heat capacity and of the magnetocaloric effect.
Collaborator Contribution Our partners have grown high quality samples of new magnetic materials, which hold significant promise as targeted magnetocalorics for low temperature cooling.
Impact We have so far characterised three new materials and publications are being drafted.
Start Year 2020
 
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. Collaboration is continuing and has recently involved the characterisation of new single crystals of YFe2Ge2.
Start Year 2015
 
Title Refrigeration module for commercial measurement platform 
Description We have developed a cooling module that extends the temperature range of the Quantum Design PPMS platform to <100mK. The module makes use of the PPMS magnetic field to enable adiabatic demagnetisation refrigeration of a paramagnetic material assembly, offering far superior performance than currently available commercially. 
Type Of Technology New/Improved Technique/Technology 
Year Produced 2020 
Impact The module has been beta-tested by various colleagues internationally and is performing well. In-house, we have used it for materials characterisation tasks in the sub-Kelvin range. We are exploring effective routes towards commercialisation.