Kakali Santra

Humboldt Fellow as of December 2024

Spin directed charge transport is at the frontier of research, spanning from spintronics to chemical reactions. In chemical systems controlling spin has traditionally been confined to radical pair reactions. However, this landscape shifts dramatically when chiral systems are involved. The revolutionary discovery of the Chirality Induced Spin Selectivity (CISS) effect in 1999 established that electron transport through homochiral molecules is inherently linked to the electron’s spin. This effect reveals that the electron's linear momentum and spin become strongly coupled, significantly suppressing electron backscattering. As a result, for a given handedness and velocity of an electron, only one spin state is energetically favored. This breakthrough opened new pathways for enantioselective chemistry, where chemical reactivity depends on molecular chirality. The CISS effect shares intriguing parallels with the electron transport behavior observed in topological materials. These materials have become a focal point in condensed matter physics due to their unique properties, such as high electron mobility, robust boundary states, and quantized bulk responses. The most striking feature is spin-momentum locking, where an electron's spin and momentum are tightly interlinked in all directions. This makes them particularly exciting for surface reactions, as they offer a stable electron bath, despite their bulk states being insulating (like Bi₂Te₃ and SnTe) or semimetallic (such as NbP, PtSn₄, Co₃Sn₂S₂). A recent development in the field is the discovery of chiral topological materials, which combine both structural and electronic chirality. These materials are distinguished by their momentum-space chirality and the presence of long Fermi arcs that traverse the entire Brillouin zone. They provide a combination of both structural and electronic chirality. Noble metal-containing chiral crystals, such as PtGa, have demonstrated exceptional catalytic activity, notably outperforming the commercial Pt/C electrocatalysts in electrochemical hydrogen evolution reactions (HER). This indicates that chiral topological crystals hold great promise as catalysts for spin-directed asymmetric reactions, an area that remains largely unexplored till date.

In essence, the intersection of topology and chirality opens new avenues for manipulating electron spin, with vast implications for chemistry, spintronics, and beyond.

As a Humboldt Fellow, my research will focus on controlling spin states in chiral topological materials to drive asymmetric reactions. The enantiospecificity of these reactions is determined by the relative stability of different enantiomeric intermediates. To optimize the reaction pathways, I will carefully select materials with appropriate compositions and electronic band topology as key tuning parameters. Additionally, I will investigate the spin transport properties of these chiral topological materials, aiming to understand their intrinsic spin and orbital angular momentum polarization.

My research will be conducted in the group of Prof. Claudia Felser at the Max Planck Institute for Chemical Physics of Solids in Dresden. Prof. Felser's expertise in topological quantum materials, along with the group's extensive proficiency in crystal growth and solid-state physics, will provide a strong foundation for the successful execution of my work.