Correlated electronic states for cryogenic refrigeration - fundamentals and applications

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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.