Polarization of Electron Spin and Orbitals from Structural Chirality—Spin-Selective Transport in Chiral Molecular Junctions on Semiconductors

Electrical generation and transduction of polarized electron spins in semiconductors via <i>nonmagnetic</i> means are of broad interest in spintronics and quantum information science. One such pathway exploits chiral/helical spin textures in electronic structures; a contrasting approach utilizes the interplay of electron spin with chirality in real space. Breaking of spatial inversion symmetry has profound effects on the electronic properties of materials. One prominent manifestation of such effects of much recent interest is chirality-induced spin selectivity (CISS), where real-space structural chirality induces spin polarization of electrons from a nonmagnetic electrode¹. CISS has been reported in a variety of chiral molecules and hybrid chiral crystals, however, definitive understanding of its physical origin remains elusive. We have studied the CISS effect through measurement of spin-selective transport in chiral molecular junctions comprising a nonmagnetic normal metal electrode and a self-assembled monolayer of chiral molecules (α-helix L-polyalanine) on magnetic (GaMnAs) or nonmagnetic (<i>n</i>-GaAs) semiconductors, where the spin polarization is detected via measurements of the spin-valve conductance and Hanle effect, respectively. The pronounced CISS effect in the robust semiconductor-based molecular device platform enabled systematic and rigorous examination of its dependences on the molecular structure, normal metal material, and bias current. The results reveal several key characteristics of the CISS effect: i) nontrivial linear-response magnetoconductance in two-terminal CISS spin valves, in apparent violation of the Onsager reciprocal relation²; ii) crucial role of the spin-orbit coupling in the normal metal electrode, suggesting the importance of orbital polarization in the chiral molecules³; iii) spin generation by CISS in semiconductors⁴. Our experiments have provided significant new insights on CISS and demonstrated its potential for enabling semiconductor spintronics free of any magnetic materials.<br/><br/>*Work supported by NSF grants DMR-1905843 and DMR-2325147<br/>¹B.P. Bloom, <i>et.</i> <i>al</i>., <i>Chem. </i><i>Rev</i>. 124, 1950 (2024).<br/>²T. Liu, <i>et. al</i>., <i>ACS Nano</i> 14, 15983 (2020).<br/>³Y. Adhikari, T. Liu, <i>et. al</i>., <i>Nat. Commun</i>. 14:5163 (2023).<br/>⁴ T. Liu, Y. Adhikari, <i>et. al</i>., arXiv:2403.18964