Photomagnetic-Chiral Anisotropy mediated by Chirality-Driven Asymmetric Spin Splitting

TL;DR

This work reveals that chirality-induced orbital momentum locking combined with spin-orbit coupling enables photomagnetic-chiral anisotropy (PM-ChA) in noble-metal CNACs. Using DFT-NEGF and real-time TDDFT, the authors show that chiral geometries create SOC-enabled spin-split states and enantiomer-specific spin-flip dynamics under optical excitation, generating opposite photomagnetic fields for left- and right-handed structures. A key finding is the critical role of SOC, whose presence amplifies spin polarization and magnetization by several orders of magnitude and whose absence suppresses the effect by roughly six orders of magnitude. The results offer a theoretical framework and design roadmap for chiral spin-photonic devices and chiral optoelectronics with programmable spin-photon coupling.

Abstract

Photo-magnetic effects (PMEs), intrinsic to transition metals, arises from the interaction between light-induced angu-lar momentum and electronic spin. These effects are suppressed in noble metals with high symmetry and electron density. Introducing chiral structures can induce photomagnetic-chiral anisotropy (PM-ChA) of metals by linking chirality and spin dynamics. However, a theoretical explain remains elusive. Here, we investigated the mechanism of PM-ChA in tetrahelix-stacked chiral nanostructured gold chains (CNACs) using first-principles calculations. Non-equilibrium Green's function calculations reveal that chiral potentials enhance spin channel asymmetry by amplify-ing spin-orbit coupling (SOC)-induced spin splitting. Real-time time-dependent density functional theory simulations further identify SOC as the bridge connecting chiral spintronics to PME, where chirality-driven spin flips from asymmetric geometries generate opposing photomagnetic fields in materials of different handedness. These findings are consistent with experimental observations in chiral nanostructured gold films and provide a theoretical instruction for design metallic spintronic devices.

Figures (16)

• Figure 1: Mechanism of PM-ChA. Schematic of spin dynamics in chiral materials under laser irradiation. Left-handed (in blue) and right-handed (in red) helical configurations generate chirality-dependent band splitting through geometric symmetry breaking. (I) Ground-state orbital texture of left-handed system shows antiparallel orbital polarization $L_z$ distribution (red/blue bands) induced by chiral symmetry breaking. (II) SOC-induced band splitting in left-handed system maintains parallel $L_z$ alignment in $\pm\bm{k}$ branches, creating spin population asymmetry ($\uparrow$: spin up, $\downarrow$: spin down). (III) Under laser excitation (orange arrow: $h\nu$ absorption), photon angular momentum triggers preferential $+L_z$ transitions, locking spin-down states and generating net positive spin current ($-\bm{k}$ direction) wan2023anomalous. (IV)-(VI) right-handed system exhibits mirrored behavior with reversed $L_z$ alignment asymmetry and $+\bm{k}$-directed spin current.
• Figure 2: Orbital texture and spin population. a, Top and side views of the atomic structures of ($\text{a}_1$) L-CNAC and ($\text{a}_2$) R-CNAC. Chirality arises from the clockwise (left-handed) or counterclockwise (right-handed) stacking orientation of tetrahedra. b, c, The $ab initio$ band structure of L-CNAC and R-CNAC with orbital texture. The orbital texture refers to the parallel or antiparallel relation between orbital polarization $L_z$ and the momentum. d, e, Spin-resolved band structures of L-CNAC and R-CNAC, respectively. Spin projection ($S_z$) is color-coded: red (blue) indicates spin-up (spin-down).
• Figure 3: Spin-selective transport of CNACs. a, b, simulated current as a function of applied voltage for L-CNAC and R-CNAC device models, respectively. c, total magnetic moment variation in L-CNAC and R-CNAC under a uniform external electric field, calculated over a time-dependent run within 50 fs. d, time-dependent magnetic moment components of ANAC, which show no chirality-induced spin selectivity. e$_1$, e$_2$, schematic illustrations of spin-selective transport in a device model, where CNACs serve as the central region. The left and right electrodes have the same chirality as the central region, and electrons flow from the left to the right electrode.
• Figure 4: Time evolution of chiral photomagnetic responsiveness with SOC. a, c, e, Time-dependent occupations of spin-up and spin-down channels driven by linearly polarized light for L-CNAC, R-CNAC, and ANAC, respectively. b, d, f, Time-dependent magnetic moment components driven by linearly polarized light for L-CNAC, R-CNAC, and ANAC, respectively. The $x$, $y$, and $z$ directions correspond to periodic boundary conditions for CNACs ($z$-axis).
• Figure S1: Schematic representation of the atomic model construction of L-CANC.