Fermi surface instabilities and quantum order at high pressure

20世纪最大的科学奇迹之一是超导的发现，一些金属可以携带电流而完全没有能量损失。在经典物理学中，电子的无摩擦流动似乎是不可能的，但实际上在原子或分子等量子力学系统中确实存在。通过将我们通常与量子物理不寻常的微观世界联系在一起的现象带入电缆和开关的实际宏观世界，超导的发现为新的设备和应用铺平了道路，这些设备和应用包括磁共振成像(MRI)扫描仪、大电流故障限制开关、高频过滤器和基于约瑟夫森效应的超灵敏测量设备。超导等基础发现催生了高科技产业以及与之相关的对熟练劳动力的需求。未来迫切问题的解决方案，特别是在能源和可持续发展领域，需要具有不同寻常的电子性能的新材料。真实材料中含有电子量子液体。由于电子的质量低，密度高，量子物理的效应一直持续到高温，在许多情况下远远超过室温。电子之间的相互作用使它们的运动相互关联，并可以诱导新的有序状态，近年来发现了越来越多的新有序状态--包括各种形式的超导电性。有效的相互作用取决于特定材料的细节，因此变得高度可调：它们可以通过改变材料成分、施加磁场或电场或通过施加压力改变晶格间距来改变。作为这种形式的量子调谐的函数而转变成新的有序相的过程称为量子相变。量子相变附近是意外发现的沃土，通常也是壮观的发现。例子包括铁多肽材料中的高温超导电性，Sr3Ru2O7中的量子向列态，以及铁磁体中的非传统超导电性。为了为未来的发现铺平道路，我们需要更多地了解在这种电子不稳定附近工作的机制。在这个项目中，我们将研究接近量子相变的选定材料的电子结构，这是最容易在压力下获得的。在某些方面，这类似于推断晶体结构，但因为电子总是在运动，所以我们不确定它们的位置，而是确定它们的速度、能量和有效质量。这是通过观察与电阻率、磁化率或其他性质相关的磁场中的振荡来实现的。这些量子振荡测量是研究各种当前感兴趣的材料的电子结构的有力工具。要在超过100,000个大气压的高压环境中实现所需的超灵敏测量是具有挑战性的，但我们团队和其他地方最近的技术发展表明，这样的实验现在是可能的，并将从所产生的好处中得到证明。我们将研究在接近金属-绝缘体跃迁时的相关金属态，从密度波序到正常金属态的跃迁，重费米子系统中的局域矩到巡回电子交叉，以及其他及时和特别有理论和实践意义的话题。我们还将使用高精度的压力热容测量来检查量子相变附近的电子态密度，并在某些引人注目的情况下识别费米液体击穿的热力学特征。我们的电子结构测量将在新的钻石光源同步加速器设施中得到高压晶格结构测定的补充。

One of the biggest scientific surprises of the twentieth century was the discovery of superconductivity, whereby some metals can carry electrical currents with absolutely no energy loss. Frictionless flow of electrons appears impossible in classical physics, but does in fact occur in quantum mechanical systems such as atoms or molecules. By taking phenomena we normally associate with the unusual micro-world of quantum physics into the practical macro-world of cables and switches, the discovery of superconductivity has paved the way for new devices and applications, in magnetic resonance imaging (MRI) scanners, high current fault limiting switches, high frequency filters and ultrasensitive measurement devices based on the Josephson effect. High-technology industries and the associated need for skilled labour are germinated by fundamental discoveries such as superconductivity. Future solutions for pressing problems, particularly in the fields of energy and sustainability, demand new materials with unusual electronic properties. Real materials contain electronic quantum liquids. Because electrons have a low mass and are present at high density, the effects of quantum physics persist up to high temperatures, in many cases far exceeding room temperature. Interactions between the electrons cause them to correlate their motion and can induce new ordered states, of which an increasing variety - including various forms of superconductivity - have been discovered in recent years. The effective interactions depend on details of the specific material and thereby become highly tunable: they can be varied by changing material composition, by applying magnetic or electric fields, or by changing the lattice spacing through applied pressure. The transition into a new ordered phase as a function of this form of quantum tuning is called a quantum phase transition. The vicinity of quantum phase transitions is a fertile ground for unexpected and often spectacular discoveries. Examples include high temperature superconductivity in the iron-pnictide materials, the quantum nematic state in Sr3Ru2O7, and unconventional superconductivity in ferromagnets. To pave the way for future discoveries, we need to know more about the mechanisms operating near such electronic instabilities. In this project, we will examine the electronic structure of selected materials close to quantum phase transitions, which can best be accessed under pressure. In some ways this is similar to deducing a crystal structure, but because the electrons are always in motion, we do not determine their position but rather their velocity, energy and effective mass. This is achieved by observing oscillations in the magnetic field dependence of the electrical resistivity, the magnetic susceptibility or other properties. These quantum oscillation measurements are a powerful tool for examining the electronic structure of a wide range of materials of current interest. To achieve the required ultra-sensitive measurements in a high pressure environment of more than 100,000 atmospheres is challenging, but recent technical developments in our group and elsewhere suggest that such experiments are now possible and will be justified by the resulting benefits. We will investigate the correlated metallic state on approaching metal-insulator transitions, the transition from density wave order to the normal metallic state, the local moment to itinerant electron cross-over in heavy fermion systems, and other topics which are timely and of particular theoretical and practical interest. We will also use high precision heat capacity measurements under pressure to examine the electronic density of states near quantum phase transitions and to identify thermodynamic signatures of Fermi liquid breakdown in certain high-profile cases. Our electronic structure measurements will be complemented by high pressure lattice structure determination in the new Diamond Light Source synchrotron facility.