MAGNETOORBITAL · Magnetoelectric couplings in solids and related phenomena: First-principles theory

Magnetoelectric couplings in solids and related phenomena: First-principles theory

This project deals with theoretical and computational studies of the response of crystals to external electric and magnetic fields. It explores the fundamental relation between the symmetry of the crystal structure, and the way in which electrons responde to external perturbations. The behavior is strikingly different depending on whether the material is a metal or an insulator. In conventional materials, an applied magnetic field B induces a magnetization M, and an electric field E induces a polarization P (if an insulator) or a current J (if a metal). The main focus of the project are the cross "magnetoelectric" (ME) responses, whereby B induces P or J, and E induces M. The ME effect occurs in insulators when both spatial inversion and time reversal symmetry are broken. We have developed and implemented first-principles methods to compute this response from, and used them to understand the ME response of Cr2O3, a prototypical ME insulator. Much less understood is the ME response of metals. A major result of the project is the development of a detailed microscopic theory. Contrary to insulators, the ME effect in metals occurs in non-magnetic samples. The symmetry requirements are precisely the same as for the phenomenon of natural optical activity. Thus, spatial inversion must be broken. A sufficient condition is that the structure is either chiral (cannot be superimposed on its mirror image) or polar (has a unique preferred direction). We have obtained a quantum-mechanical expression for the ME response in metals and doped semiconductors, showing that it is controlled by the intrinsic magnetic moment (both spin and orbital) of the conduction electrons. We have carried out a detailed studied on the current-induced ME effect and related effects in a chiral conducting crystal, doped tellurium. In particular, we found that some of these "gyrotropic" effects are strongly influenced by the presence of chiral band touchings ("Weyl points") near the bottom of the conduction band acting as sources and sinks of Berry curvature. Motivated by the study of Weyl points in tellurium, we formulated a systematic classification of the types of Weyl points stabilized by screw-rotational symmetry, with and without time-reversal invariance. We have carried out a systematic survey of the Weyl points in the bandstructure of ferromagnetic bcc Fe, and how they induce a non-trivial topology ("Chern numbers") on the Fermi-surface sheets. We have studied the spontaneous orbital magnetization that occurs in bulk ferromagnets such as iron, due to broken time-reversal symmetry. Moreover, we have studied how shinning circularly-polarized light on two-dimensional samples with broken inversion symmetry (monolayer MoS2) leads to an anomalous Hall effect (AHE) where an in-plane electric field produces a transverse current. Specifically, we investigated how disorder affects this photoinduced anomalous Hall effect. The ME effect in bulk insulators is accompanied by an AHE at the surface, which becomes "half-quantized" in topological insulators. The quantized part of the response depends on the specific surface termination. We have studied how, given an insulator with a specific surface termination, the quantized part of the ME coupling can be determined. This project has been instrumental for my career integration and development. It has taken my research in new directions, allowed me to initiate several new collaborations with colleagues in several countries (Denmark, Italy, United States, and Germany), and to set up my own research group in Spain.

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