Superconductivity and Competing Orders in High Tc Cuprates

超导是一种有可能从根本上改变电力应用的现象，从高场磁铁到电力传输、电机和发电机。使用超导材料可以显著减少我们的能源消耗，并使医用核磁共振扫描仪或核聚变反应堆等技术成为可能。充分发挥这一潜力的关键是开发能够在高温下运行和携带大电流的材料。尽管新材料的发现在某种程度上是偶然的，但我们对新材料物理的基本理解一直指导着我们的探索。目前最有前景发展高温超导应用的材料是铜酸盐，因为它们在常压下具有最高的临界温度。然而，与传统超导体不同的是，对于铜酸盐来说，造成超导电性的物理机制还没有达成共识。虽然对铜酸盐的研究已经有30年了，但最近我们对这些材料的理解发生了巨大的变化，这在很大程度上是由实验推动的，这些实验使用非常高的磁场来抑制超导，使用X射线和中子来探测集体电荷和自旋关联。这些进展使这些材料的电荷掺杂-温度相图变得更加清晰，并在一定程度上确定了所谓的赝隙和电荷密度波相的微观来源。虽然已知这些相共存并与超导性竞争，但尚不清楚与这些相相关的波动是高温超导电性的根本原因，还是竞争和削弱它。在这里，我们计划建立在这些最新进展的基础上，这些进展反过来又与现有实验设备的重大进展相结合，如共振非弹性X射线散射(RIXS)和超强磁场，以期在我们对铜酸盐超导体的理解方面取得重大进展。特别是，我们将使用x射线和中子能谱来研究不同的铜酸盐材料在其掺杂温度相图上调谐时磁和电荷涨落的演变，特别强调伪隙和电荷有序相的出现。我们将寻求开发的另一个主要新角度是结合使用高压技术和强磁场来测量量子振荡和磁传输特性。通过迫使原子与压力结合在一起，性质会以不同于电荷掺杂的方式发生变化。例如，只有在高压下，才能实现铜酸盐的最高转变温度。压力可以用来消除相互竞争的相之间的意外简并，因此有望弄清楚其中哪些对超导电性重要，哪些不重要。

Superconductivity is a phenomenon that has the potential to radically transform applications of electrical power, from high-field magnets, to power transmission, motors and generators. Using superconducting materials can significantly reduce our energy usage and enable technologies, such as medical MRI scanners or nuclear fusion reactors. The key to fully realizing this potential is to develop materials which can be operated at high temperature and carry high currents. Although the discovery of new materials can be somewhat serendipitous, the search has been guided by our fundamental understanding of their physics. The the class of materials with the best current prospects for developing superconducting applications at high temperature are the cuprates, as these have the highest critical temperatures at ambient pressure. However for the cuprates, unlike conventional superconductors, there is as yet no consensus as to the physical mechanism which causes the superconductivity. Although cuprates have been studied for 30 years, recently there has been a step change in our understanding of these materials which has, in large part, been driven by experiments carried out using very high magnetic fields to suppress superconductivity and x-rays and neutrons to probe the collective charge and spin correlations. These developments have brought some clarity to the charge-doping versus temperature phase diagram of these materials and has gone some way to identifying the microscopic origin of the so-called pseudogap and charge density wave phases. Although it is known that these phases coexist and compete with the superconductivity, it is less clear whether fluctuations associated with these phases is the root cause of high temperature superconductivity or rather competes and reduces it.Here we plan to build on these recent developments, which are in turn coupled to major advances in available experimental facilities, such as resonant inelastic x-ray scattering (RIXS) and very high magnetic fields, to make major advances in our understanding of cuprate superconductors. In particular, we will use x-ray and neutron spectroscopies to study the evolution of the magnetic and charge fluctuations as different cuprate materials are tuned across their doping-temperature phase diagram, with particular emphasis on the emergence of the pseudogap and charge ordered phases. Another major new angle we will seek to exploit is the use of high pressure techniques in conjunction with high magnetic fields to measure quantum oscillations and magneto-transport properties. By forcing atoms together with pressure the properties are changed in ways that are different from charge doping. For example, it is only under high pressure that the highest transition temperatures of cuprates are realised. Pressure can be used to remove accidental degeneracies between competing phases and hence hopefully clarify which of these are important for superconductivity and which are not.