Normal state properties of cuprates superconductors under high pressure

铜氧化物高温超导体超导电性的微观起源是凝聚态物理中尚未解决的一个重要问题。解决这个基本物理问题的关键是理解超导性发展的正常状态。这种正常状态具有许多不同于常规金属的性质，并且其性质随着材料的电子浓度（空穴掺杂）的变化而显著地演变。在一定的掺杂和温度范围内，发现其他相与超导性竞争或共存。这些被称为赝能隙和电荷密度波阶段。确定这些相如何与超导性相互作用一直是一个主要的研究课题。人们认为这些相的涨落可以增强/引起超导电性，或者相反，静态有序相破坏/减弱超导电性。通常使用电荷掺杂来实现铜酸盐材料通过其不同相的调谐。在这个项目中，我们将使用静水压力和单轴应力来调整材料。流体静压可以使超导临界温度发生很大的变化，但其确切机理还不清楚。它的部分效应可能是由电荷转移引起的，但也有一个不太清楚的结构元素。例如，对于大多数铜酸盐，最高超导转变温度只有在高静水压力下才能达到，本文将研究YBa_2Cu_3O_（6 +x）和Tl_2Ba_2CuO_（6 +x）等铜酸盐超导体的正常态输运性质：电阻率、霍尔效应、热电势和Shubnikov de-Haas效应，以了解压力影响超导临界温度的机理。将开发新的高压/应变装置来实现这些目标。实验将在布里斯托（高达16 T）和高磁场设施（奈梅亨和图卢兹的EMFL设施，以及NHMFL塔拉哈西）的超导磁性进行。

The microscopic origin of superconductivity in cuprate high temperature superconductors remains a major unresolved problem in condensed mater physics. Key to solving this fundamental physics problem is to understand the normal state out of which the superconductivity develops. This normal state has many properties which are different from conventional metals, and its nature evolves markedly as the electron concentration (hole-doping) of the material is varied. In certain ranges of doping and temperature, other phases are found to compete or exist alongside the superconductivity. These are known as the pseudogap and charge density wave phases. Determining how these phases interact with the superconductivity has been a major subject of enquiry. It is thought that fluctuations of these phases can enhance/cause superconductivity or instead, statically ordered phases destroy/diminish superconductivity.Tuning of the cuprate materials through their different phases is usually achieved using charge doping. In this project, we will instead use hydrostatic pressure and uniaxial stress to tune the materials. It is known that hydrostatic pressure can make very large changes in the superconducting critical temperature although there is little understanding of its exact mechanism. It is possible that part of its effect results from charge transfer but there is also a less well understood structural element. For example, for most cuprates, the highest superconducting transition temperature is only reach under high hydrostatic pressure.Here we will study the normal state transport properties: electrical resistivity, Hall effect, thermoelectric power and Shubnikov de-Haas effect of cuprate superconductors such as YBa2Cu3O6+x and Tl2Ba2CuO6+x, in order to understand the mechanism of how pressure effects the superconducting critical temperature. New high pressure / strain apparatus will be developed to achieve these aims. Experiments will be conducted in superconducting magnetics in Bristol (up to 16T) and at high magnetic field facilities (EMFL facilities in Nijmegen and Toulouse, and NHMFL Tallahassee).