Superconducting and normal states in quantum materials

材料的发现是科技进步的动力。量子材料承载的集体现象与半经典描述相悖，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MRI扫描仪中使用的强力磁铁，聚变反应堆和粒子加速器到轻型电机和发电机，低噪声射频滤波器，低功耗电子设备以及用于传感或计算的量子器件。在大多数超导体中，所需的电子相互作用是由动态晶格扭曲产生的。或者，这些相互作用可以由更复杂的量子过程引起，类似于产生磁性的量子过程。这种非常规的“无声子的超导性”与一系列丰富的特性有关，其中一些特性是非常理想的，例如对高磁场、电流密度或温度的弹性。在这个项目中，我们研究了四个材料家族中不寻常的超导和正常状态的驱动因素，基于我们最近的突破和发现：(i)铁锗超导体YFe2Ge2和LuFe2Ge2， （ii）中等重费米子化合物CeNi2Ge2和CePd2Si2，（iii）高压近道晶格超导体CeSb2，（iv）准周期主客体结构，如高压Bi， Sb和Ba。YFe2Ge2族(i)和CeNi2Ge2族（ii）在低温下接近磁性边界形成，但仅在顺磁侧形成，而它们的等电子姐妹材料LuFe2Ge2和CePd2Si2的磁性顺序。高压CeSb2 （iii）在高磁场下表现出强大的超导性，使得单线态库珀对无法自旋。准周期材料（iv）可以承载低频滑动模式，这极大地影响了正常状态特性并导致异常强的电子-声子耦合。由于这些材料在许多细节上有所不同，但也有共同的现象学，因此在一个连贯的计划中研究它们将产生新的见解。在我们最近的晶体生长进展所产生的干净、高质量样品的推动下，该计划利用了多个项目合作伙伴的大力投入。这些仪器通过专门的光谱、热力学和输运技术增强了我们当地的高场高压测量。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，磁性还是其他方式，(b)输运中的非费米液体特征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性，以及(d)超导状态本身的性质和间隙结构。这些都是困难但及时的问题：在发现第一个非常规超导体CeCu2Si2 40年后，其超导状态的性质再次受到严格的审查，而在氧化铜家族之外发现的第一个氧化物超导体Sr2RuO4也同样受到激烈的争论。上面列出的新型超导体大大扩大了清洁材料的范围，这些基本问题可以在其中有效地研究。由此产生的见解有助于指导在广阔的材料空间中寻找更多新的非常规超导体，而研究这些新材料反过来又产生新的见解和更精确的指导原则。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，随着项目的展开，它将获得动力，最终引领具有实际有用特性的功能量子材料的发展。

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.