Correlated electronic states for cryogenic refrigeration - fundamentals and applications

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.

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.