Fermi surface instabilities and quantum order at high pressure

20世纪世纪最大的科学惊喜之一是超导性的发现，即某些金属可以携带电流而绝对没有能量损失。在经典物理学中，电子的无扰流动似乎是不可能的，但事实上，在量子力学系统中，如原子或分子，确实发生了。通过将我们通常与量子物理学的不寻常微观世界相关联的现象带入电缆和开关的实际宏观世界，超导性的发现为新设备和应用铺平了道路，如磁共振成像（MRI）扫描仪，高电流故障限制开关，高频滤波器和基于约瑟夫森效应的超灵敏测量设备。超导性等基本发现催生了高科技产业和相关的熟练劳动力需求。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。真实的材料含有电子量子液体。由于电子的质量很小，密度很高，量子物理的效应在高温下仍然存在，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态，近年来发现了越来越多的有序态-包括各种形式的超导性。有效的相互作用取决于特定材料的细节，从而变得高度可调：它们可以通过改变材料成分，通过施加磁场或电场，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的到新的有序相的转变被称为量子相变。量子相变附近是一片肥沃的土壤，可以带来意想不到的、往往是壮观的发现。例子包括铁-磷属元素化物材料中的高温超导性、Sr 3Ru 2 O 7中的量子超导态以及铁磁体中的非常规超导性。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性附近运行的机制。在这个项目中，我们将研究所选材料的电子结构接近量子相变，这可以在压力下最好地访问。在某些方面，这类似于推导晶体结构，但由于电子总是在运动，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。这是通过观察电阻率、磁化率或其他性质的磁场依赖性的振荡来实现的。这些量子振荡测量是检查当前感兴趣的各种材料的电子结构的强大工具。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，但我们小组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，并将通过所产生的好处来证明。我们将研究相关金属态对接近金属-绝缘体转变，从密度波阶到正常金属态的转变，重费米子系统中的局部矩到巡回电子交叉，以及其他及时和特别的理论和实践兴趣的主题。我们还将使用高精度的压力下的热容测量来检查量子相变附近的电子态密度，并确定在某些高调的情况下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金刚石光源同步加速器设施的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.