Towards Understanding the Spin Selective Energy Processes in Chiral Systems

In this talk I will report on the outcome of a conference held at NREL last May, concerning prospects and theoretical challenges for the chiral induced spin selectivity effect (CISS). CISS refers to the spin-dependence polarization of electrons observed in chiral systems. Chirality refers to a pair of enantiomers--molecules that are mirror images of each other but cannot be superimposed. These systems show remarkable features with enormous potential implications for energy applications, in transport properties for spintronics and optoelectronics, in ground state properties relevant to chemical reactions, and in electrochemistry. CISS was first discovered in molecular systems, but has seen application in biological systems and solid state perovskites containing an embedded enantiomer. Experimental phenomena can be classed into three main categories. First are transmission and transport measurements where spin polarized currents are observed. Also it has been found spin redistribution accompanies charge redistribution when an enantiomer is polarized, inducing magnetism without a magnetic field. Spin dependent charge reorganization can be used to induce asymmetric chemical reactions, and can greatly enhance desired products in chemical reactions and electrochemical processes.

Even while electronic structure and transport properties of chiral systems have been studied by many groups, the basic principles are poorly understood. Perhaps the most remarkable observation is the apparent violation of Onsager's reciprocal relations, which implies broken time-reversal symmetry. We will discuss speculations on the relation between the enantiomer's curvature and electronic (orbital and spin) angular momentum; what role spin orbit coupling play; how Onsager's relations appear to be violated; what role the electron-nuclear interaction might play; to what extent many-body effects correlations are needed to explain CISS; whether universal principles are operative in different kinds of systems, and what can be learned from short-time behavior.