High Pressure Transport Measurements on Strongly Correlated Electron Systems

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A conventional phase transition, e.g. the melting of a solid to liquid as heat is applied, involves moving from an ordered, low entropy state to a disordered, high entropy state when temperature is increased. Quantum phase transitions are different in that the change of phase brings with it no change in entropy at zero Kelvin and thus we are effectively moving from one (conventional) ordered state, to another (subtle or hidden ) ordered state. The transformation can occur at fixed temperature and as a function of some other control parameter, such a hydrostatic pressure or chemical composition. When the transitions occur the nature of the ordered state can be dramatically altered producing unconventional superconductivity and other new forms of novel quantum order which deviate from the standard low temperature theory of matter known as Landau's Fermi Liquid theory. Such ordered states can occur, for example, in systems that are on the border of magnetic long range order. The aim is to investigate current open questions on a number of materials close to magnetic instabilities which are likely to exhibit novel forms of quantum order and unconventional forms of superconductivity by use of high pressure and low temperature techniques. The work will also lead to advancements in high pressure diamond anvil cell techniques, in particular with respect to resistivity and AC-susceptibility methods. The grant will also allow for improvements in cryogenic methods with the completion of a new double stage adiabatic demagnetisation refrigerator capable of achieving a base temperature of 2-5 mK. Temperatures below 50 mK may be necessary to observe the new quantum states of interest.