Superconducting and normal states in quantum materials

材料的发现促进了科学技术的进步。量子材料承载着违背半经典描述的集体现象，例如，因为它们产生于强相关性或涉及拓扑秩序。这些集体现象的多样性、它们对实际温度区域的影响以及它们的可调性使新技术成为可能。其中最重要的是超导性，这是一种宏观量子现象，具有多种应用，从用于MRI扫描仪，聚变反应堆和粒子加速器的强大磁体到轻型电动机和发电机，低噪声射频滤波器，低功率电子器件以及用于传感或计算的量子设备。在大多数超导体中，所需的电子相互作用由动态晶格畸变产生。或者，这些相互作用可能是由更复杂的量子过程引起的，类似于产生磁性的过程。这种非传统的“无声子超导性”与丰富的性质有关，其中一些是非常理想的，例如对高磁场，电流密度或温度的弹性。在这个项目中，我们研究了四种材料家族中不寻常的超导和正常状态的驱动因素，以我们最近的突破和发现为基础：（i）铁-锗化物超导体YFe 2Ge 2和LuFe 2Ge 2，（ii）中等重费米子化合物CeNi 2Ge 2和Cepd 2Si 2，（iii）高压近藤晶格超导体CeSb 2，（iv）准周期主-客体结构，例如高压Bi、Sb和Ba。族（i）中的YFe 2Ge 2和族（ii）中的CeNi 2Ge 2在低温下形成接近磁性边界但恰好在顺磁侧，而它们的等电子姊妹材料LuFe_2Ge_2和CePd_2Si_2磁性有序。高压CeSb 2（iii）在磁场下显示出强大的超导性，该磁场似乎太高而不允许自旋单重态库珀对。准周期材料（iv）可以拥有一个低频滑动模式，这极大地影响了正常状态的性质，并导致异常强的电子-声子耦合。因为这些材料在许多细节上不同，但也有共同的现象，新的见解将出现在一个连贯的程序中研究它们。在我们最近的晶体生长进展所产生的清洁，高质量样品的推动下，该计划利用了多个项目合作伙伴的强大投入。突出的理论支持将检查实验结果，以回答以下关键研究问题：（a）软模式的作用，无论是振动，磁性或其他方式，（B）非费米液体签名在传输中的起源和相关金属中的普朗克耗散概念，（c）超导配对相互作用的性质和可调谐性，以及（d）超导态本身的性质和能隙结构。这些都是困难但及时的问题：在发现第一个非常规超导体CeCu 2Si 2 40年后，其超导状态的性质再次受到严格审查，而第一个在氧化铜家族之外发现的氧化物超导体Sr 2 RuO 4也同样受到热烈讨论。上面列出的新超导体大大拓宽了可以有效研究这些基本问题的清洁材料的范围。由此产生的见解有助于指导在广阔的材料空间中进一步寻找新的非传统超导体，研究这些新材料反过来又会产生新的见解和更精确的指导原则。通过利用计算机建模来提高这些搜索的成功率是有空间和必要的，随着计划的展开，计算机建模将获得动力，最终导致具有实际有用特性的功能量子材料。

Materials discovery feeds scientific and technological progress. Quantum materials host collective phenomena that defy a semi-classical description, for example because they arise from strong correlations or involve topological order. The diversity of these collective phenomena, their reach into practicable temperature regions and their tunability enable new technologies. Foremost among them is superconductivity, a macroscopic quantum phenomenon with multiple applications ranging from powerful magnets used in MRI scanners, fusion reactors and particle accelerators to lightweight motors and generators, low-noise rf filters, low-power electronics, and quantum devices used in sensing or computing. In most superconductors, the required electronic interactions are produced by dynamic lattice distortions. Alternatively, these interactions can be caused by more complex quantum processes similar to those which give rise to magnetism. Such unconventional 'superconductivity without phonons' is associated with a rich range of properties, some of which are highly desirable, such as resilience to high magnetic fields, current densities or temperatures. In this project, we investigate the drivers of the unusual superconducting and normal states in four material families, building on our recent breakthroughs and discoveries:(i) iron-germanide superconductors YFe2Ge2 and LuFe2Ge2, (ii) moderate heavy fermion compounds CeNi2Ge2 and CePd2Si2,(iii) the high pressure Kondo lattice superconductor CeSb2,(iv) quasiperiodic host-guest structures such as high pressure Bi, Sb and Ba.YFe2Ge2 in family (i) and CeNi2Ge2 in family (ii) form close to the border of magnetism at low temperature but just on the paramagnetic side, whereas their isoelectronic sister materials LuFe2Ge2 and CePd2Si2 order magnetically. High pressure CeSb2 (iii) displays robust superconductivity at magnetic fields that appear too high to allow spin singlet Cooper pairs. The quasiperiodic materials (iv) can host a low frequency sliding mode which dramatically affects normal state properties and causes unusually strong electron-phonon coupling.Because these materials differ in many details but also share common phenomenology, new insights will arise from studying them in one coherent programme. Fuelled by the clean, high quality samples that our recent crystal growth advances have produced, the programme leverages strong input from multiple project partners. These augment our local high field, high pressure measurements with specialised spectroscopic, thermodynamic and transport techniques.Prominent theory support will examine experimental findings to answer key research questions concerning(a) the role of soft modes, whether vibrational, magnetic or otherwise, (b) the origin of non-Fermi liquid signatures in transport and the notion of Planckian dissipation in correlated metals, (c) the nature and tunability of superconducting pairing interactions, and (d) the nature and gap structure of the superconducting state itself. These are hard but timely questions: 40 years after the discovery of the first unconventional superconductor, CeCu2Si2, the nature of its superconducting state is again under intense scrutiny, and the first oxide superconductor to be found outside the copper-oxide family, Sr2RuO4, is likewise hotly debated. The new superconductors listed above significantly widen the range of clean materials in which these fundamental questions can be studied effectively. The resulting insights help guide the search for further new unconventional superconductors in the vast space of materials, and studying these new materials in turn produces new insights and more precise guiding principles. There is scope and need for improving the success rate of these searches by leveraging computer modelling, which will gather momentum as the programme unfolds, eventually leading the way to functional quantum materials with practically useful properties.