DFG-RSF: Impact of topology on electronic properties of ordered materials

由于新材料工程对环境、自然资源节约、电信进步和生活质量的潜在影响，新材料工程问题是高度热门的问题。尽管铜酸盐高温超导已有近30年的历史，但层状高温超导材料的相图和性质仍缺乏完整的认识，尤其是在量子临界点附近。自旋和轨道涨落、自旋和电荷有序、许多体效应(在Lifshitz拓扑跃迁附近)以及表面态(平带)的拓扑结构的影响仍然存在争议。显然，为了有目的地寻找和设计新型高温超导材料，必须澄清潜在的有利于高温超导材料的因素的作用。本项目致力于基于层状材料的超导体工程的理论发展和实验实现，研究它们在QCP附近的性质，阐明磁性离子有序、电荷有序和Sc配对的相互作用。该项目的主要思想在于研究和使用平带材料，既有拓扑平凡的，也有非平凡的。前者是由于接近Lifshitz跃迁而导致多体凝聚能对SC能隙的贡献增强，后者是由于非平凡的拓扑(Weyl半金属)。在拓扑非平凡材料中，平带条件有望通过能带工程方法(静压、化学和电掺杂)进行调节。实验和理论研究将在几个相关方向进行：(A)完善理论并收集关于拓扑Lifshitz相变附近和预期QCP附近的Fe-HTS性质的新的实验数据。(B)新的超导过渡金属二卤化物的研究，特别是具有拓扑非平凡光谱的研究。(C)真三维拓扑绝缘体材料的设计、合成和研究，作为研究和利用拓扑保护表面态的平台。材料和结构将通过固相合成技术、熔体生长、物理气相传输以及脉冲激光沉积技术(PLD/MBE)来合成。该项目包括开发新的测量和材料诊断技术，作为对普通运输和光学方法的补充。这项研究将由理论物理、实验化学和物理学家等互补领域的专家组成的团队进行。

The problem of novel material engineering is highly topical due to its potential impact on environment, natural resources saving, progress in telecommunications, and life quality, in total. In spite of almost 30 years history of cuprate HTS, the phase diagram and properties of the layered HTS materials are lacking of complete understanding, particularly in the vicinity of the quantum critical point. There are still debated the effects of the spin and orbital fluctuations, spin and charge ordering, many body effects (in the vicinity of the Lifshitz topological transition), and topology of the surface states (flat bands). Clearly, the role of potentially favorable factors for HTS must be clarified for the purposeful searching and engineering novel HTS materials. The current project aims at theoretical development and experimental implementation of the superconductors engineering based on layered materials (both pnictide, and dichalcogenide-based), studying their properties in the vicinity of QCP, clarifying the role of the interplay of magnetic ions ordering, charge ordering and SC pairing. The main idea of the project consists in studying and use of the flat band materials, both topologically trivial, and non-trivial. The former are to result in the enhanced contribution of the many-body condensation energy to the SC gap due to the proximity to the Lifshitz transition, the latter due to the non-trivial topology (Weyl semimetals). The flat band conditions in the topologically non-trivial materials is expected to be tuned using the band engineering methods (hydrostatic pressure, chemical and electric doping). The experimental and theoretical studies will be carried out in several related directions: (A) refining the theory and collecting new experimental data on the properties of Fe-HTS in the vicinity of the topological Lifshitz transition and in the vicinity of the anticipated QCP. (B) Studies of the new superconducting transition metal dichalcogenides, particularly, with topologically nontrivial spectrum. (C) Design, syntheses and studies of the true 3D topological insulator materials as a platform for studying and utilizing topologically protected surface states. The materials and structures will be synthesized by the solid state synthesis technique, growth from melt, by physical vapor transport, and also by the pulsed laser deposition techniques (PLD/MBE). The project includes development of the novel techniques for measurements and material diagnostics, complemental to the ordinary transport and optical methods. The research will be performed by the team consisting of experts in the complementary fields theoretical physics, and experimental chemistry and physicists.