Topological Faraday Effect

The Hall effect, with its different variations, remains a focus of solid state research in last decades. For example, the quantum Hall effect is a widely recognized manifestation of relevance and importance of the topological numbers and, generally, topological effects for condensed matter physics. Spin and charge Hall effects are extremely appealing subjects for basic research and applications (spintronics). In magnetic materials, the charge Hall effect can typically be separated into the ordinary Hall effect proportional to the external magnetic field and the anomalous Hall effect, which scales with magnetization. The mechanisms leading to the dc Hall effects also influence the electron electrodynamics of charge carriers at finite frequencies. This can be detected via measurements of polarization-plane rotation of probing electro-magnetic radiation, the Faraday effect. These measurements not only provide access to the ac counterparts of dc Hall effects, but also reveal novel properties, potentially interesting for applications. Recently, a new type of contribution to Hall conductivity, which is proportional neither to the external magnetic field nor to the magnetization, is actively studied, the so-called topological Hall effect (THE). The THE was originally proposed to originate from the real-space Berry curvature arising in compounds with finite scalar spin chirality due to non-coplanar magnetic texture, for instance, in skyrmion systems. Experimental observations of the (dc) THE have recently been reported in magnetic Heusler compounds. In this project, we aim at Faraday-rotation experiments on these compounds at (sub)THz frequencies in order to reveal the characteristic frequency and time scales of this novel phenomenon.