Orbital Magnetism and Related Phenomena in Topological Metals

• Applicant: Dr. Tomas Rauch
• Subject Area: Theoretical Condensed Matter Physics
• Term: from 2017 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 370878787

Topological materials have been intensively studied over the past decade, because the properties of their electronic states differ in fundamental ways from those of conventional materials, giving rise to new effects. The band topology can be characterized by topological invariants, whose integer values remain unchanged under smooth deformations, until the system undergoes a topological phase transition associated with discontinuous changes in the bulk band structure. Additionally, topological materials host surface states capable of conducting dissipationless spin polarized currents, making them promising candidates for near-future spintronics applications.In most existing spintronics applications, it is usually the spin part of the magnetization that iscontrolled and manipulated. In recent years, it was realized that large orbital moments can be induced in certain classes of nonmagnetic materials by electrical means, and that such effects can be related to the band topology. In topological insulators, an electric field can induce a purely orbital magnetization, with the linear magnetoelectric (ME) coefficient quantized to a very large value compared to conventional magnetoelectrics. Large orbital moments can also be induced in certain classes of nonmagnetic metals by the passage of an electrical current. A prerequisite for this kinetic ME effect is that the crystal structure should lack a center of inversion. Broken inversion symmetry is also a characteristic feature of the important class of topological Weyl semimetals, making them good candidates for realizing this ME effect. In both above examples, magnetism is induced by electrical means in systems that do not possess magnetic order in their ground state, opening up promising possibilities in the emerging field of spin-orbitronics.The aim of the proposed project is to investigate the phenomenon of orbital magnetism in topological materials, particularly the ways in which orbital moments can be induced by external perturbations, as well as related optical effects that probe the nontrivial band topology. Building on previous successful investigations of the electronic properties of topological insulators, Weyl semimetals and nodal-line semimetals, the orbital part of the magnetization and the optical response functions will be evaluated for these systems. Special emphasis will be placed on the semimetallic phases, which have recently emerged as a major research topic, with their response properties in particular being currently under intense scrutiny. The results of this work will allow to identify realistic materials that could act as switchable components in spin-orbitronics devices.

• DFG Programme: Research Fellowships
• International Connection: Spain
• Host: Professor Ivo Souza