Geometric Phases and Spin Pumping in Superconducting Spintronics Devices

• Applicant: Professor Dr. Matthias Eschrig
• Subject Area: Theoretical Condensed Matter Physics
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 530670387

Superconducting spintronics combines superconducting quantum coherence and lossless transport with spin selectivity and spin magnetism. In contrast to conventional superconducting devices, superconducting spintronics deals with spin-polarized electronic pairs that carry a spin. When a ferromagnet is sandwiched between two superconductors, the spin texture of the interface layers (natural or artificial) can lead to non-coplanar spin arrangement with the ferromagnet magnetization vector. The corresponding geometric solid angle defines a spin-geometric phase that spin-selectively modifies the coupling of superconducting condensate phases across a ferromagnet. This property of spin-geometric phases has the potential to lead to an entirely new branch in superconducting spintronics, introducing new types of control of superconducting devices. Developing a theoretical base for describing such devices in equilibrium and non-equilibrium is the goal of this project. It deals (i) with an intertwined relationship between superconducting pair condensate phases and spin-geometric phases in voltage-biased superconducting spintronics structures and (ii) with spin-pumping via ferromagnetic resonance techniques in superconducting spintronics devices. In the former case, the superconducting phase may become time-dependent, and the spin-geometric phase acts as a control parameter. In contrast, in the latter case, the spin-geometric phase may itself become time-dependent. The interplay between spin-geometric phases and superconductivity is the overarching theme across the various sub-projects. This theoretical project orients on several recent experiments on ferromagnet/superconductor spintronics devices, which found unusual behavior in the ferromagnetic resonance linewidth, still lacking a complete explanation. Solving the full, time-dependent and spatially inhomogeneous, non-equilibrium problem enters new ground and has not been attempted so far. This ambitious goal not only is important for applications, but also will add a new direction to the field of superconducting spintronics.

• DFG Programme: Research Grants