Control and Topology of Driven Magnets

• Applicant: Nina Del Ser, Ph.D.
• Subject Area: Theoretical Condensed Matter Physics
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 527528104

Driven chiral magnets constitute an attractive platform for discovering new fundamental concepts of physics. This is because they can host a wealth of exciting different physical behaviours, including (but not limited to) emergent electromagnetism, activated Goldstone modes, magnon lasers and topological phase transitions, while remaining very easy to realise experimentally. In this project, my two main aims will be to explore the role of disorder and topology in driven magnetic systems. Motivated by recent experiments where the rotational (Goldstone mode) motion of a skyrmion lattice driven by femtosecond laser pulses was orders of magnitude larger than expected theoretically for a clean system, I will look at how disorder can assist the dynamics of magnetic textures. By a combination of engineered and random disorder, analytical calculations and micromagnetic simulations, I will attempt to uncover regimes where disorder can be a "friend" rather than a "foe" and speed up the Goldstone modes activated by the driving field. Such "disorder-assisted ratchet motion", while rare, has been suggested in other driven dissipative systems, including disordered superconducting wires, biological Brownian motors and quantum ratchets in disordered media. The second part of the project will be dedicated to topological laser physics. Topologically non-trivial phases such as the skyrmion lattice can host chiral edge states, localised to the surface of the sample. As magnons are bosons, under periodic drive they macroscopically occupy a single coherent magnon state, forming a magnon laser. My goal will be to pump directly into the surface chiral states by exploiting these magnon laser properties. This should activate giant spin currents with little dissipation at the surface of the sample, which has many interesting experimental applications. I will also try to look for the skin effect – an exotic new phenomenon predicted and observed in systems governed by non-Hermitian physics. The skin effect predicts that all the eigenstates are localised at the sample surface thus if observed in our driven magnet I would expect even more gigantic surface currents to be generated!

• DFG Programme: WBP Fellowship
• International Connection: USA
• Host: Professor Gil Refael, Ph.D.