Investigation of thermal transport in polar semiconductors

• Applicant: Dr. Vilmos Kocsis
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 540912241

In the recent decade, the investigation of electric and thermal transport properties has proved to be essential in the research of topologically non-trivial and frustrated magnets. In this research proposal, we will implement a similar experimental approach in the study of multiferroic materials. These compounds also possess strong spin-lattice interaction or topologically non-trivial magnon bands. Besides the detailed insight into the microscopic mechanisms of these complicated magnetic systems, we expect to observe novel transport phenomena. In the future, these new effects may give foundation for electric-field-driven control of heat currents via the magnetoelectric coupling, which functionality has never been demonstrated before. In magnetoelectric (ME) compounds, the spin-orbit and spin-lattice interactions establish a strong coupling among the electric and magnetic degrees of freedoms. As an example, the ME effect has led to the development of electric field driven control over magnetic states, which will be fundamental in future spintronic applications. However, transport of heat in multiferroics is seldomly investigated. In recent years, a giant thermal Hall effect has been observed in the magnetoelectric Fe2Mo3O8. While the strong spin-lattice interaction is suspected in the background of the large Hall effect, there are still numerous open questions, and an interest in to uncover the underlying microscopic mechanisms. Motivated by this, we plan to extend the research of transport effects to other multiferroics by the systematic investigation of Swedenborgites, CaBaFe4O7 and CaBaCo4O7, which compounds show even stronger ME effects, originated from the spin-lattice coupling. Besides the conventional thermal Hall effect, more exotic thermal transport phenomena are expected in multiferroics. In analogue to the recently discovered optical diode effect in LiCoPO4 (in this ME compound light is transmitted in one direction, but absorbed in the opposite way), we expect the appearance of the so-called thermal diode effect, which means that the thermal conductivity for counter propagating heat currents are different. Our research means the first pioneering steps in this direction. Finally, theoretical calculations predict that in ME compounds with strong spin-orbit interaction the magnon bands have non-trivial topology, which is accompanied by emergent topological thermal Hall effect. To reveal this phenomenon, we will study the thermal transport in the topologically non-trivial, skyrmion host multiferroic, GaV4S8, which is a particularly exciting candidate to demonstrate the existence of topological thermal Hall effect.

• DFG Programme: Research Grants