High Pressure Transport Measurements on Strongly Correlated Electron Systems

传统的相变，例如加热时固体向液体的熔化，涉及到温度升高时从有序的低熵状态向无序的高熵状态的转变。量子相变的不同之处在于，在零开尔文时，相的变化不会带来熵的变化，因此我们有效地从一个（常规的）有序状态移动到另一个（微妙的或隐藏的）有序状态。这种转变可以在固定温度下发生，并作为一些其他控制参数的函数，如静水压力或化学成分。当跃迁发生时，有序态的性质会发生显著改变，产生非常规的超导性和其他新形式的新量子秩序，这偏离了标准的物质低温理论，即朗道费米液体理论。这种有序状态可以发生，例如，在磁性长程有序边界的系统中。目的是研究目前一些接近磁不稳定性的材料的开放问题，这些材料可能通过使用高压和低温技术表现出新的量子秩序形式和非常规的超导形式。这项工作还将导致高压金刚石砧细胞技术的进步，特别是在电阻率和交流敏感性方法方面。这笔拨款还将允许改进低温方法，完成一种新的双级绝热消磁制冷机，该制冷机能够达到2-5 mK的基本温度。可能需要低于50 mK的温度来观察感兴趣的新量子态。

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.