Chirality is a fundamental property of great importance in physics, chemistry, and biology, and has recently been found to generate unexpected spin polarization for electrons passing through organic molecules, known as chirality-induced spin selectivity (CISS). CISS shows promising application potential in spintronic devices, spin-controlled chemistry, and enantiomer separation. It focuses on organic molecules that exhibit poor electronic conductivity and inherent complexities, such as the debated role of SOC at the molecule-metal interface. In this work, we go beyond organic molecules and study chiral solids with excellent electrical conductivity, intrinsic SOC, and topological electronic structures. We demonstrate that electrons exhibit both spin and orbital polarization as they pass through chiral crystals. Both polarization increases with material thickness before saturating to the bulk values. The spin polarization is proportional to intrinsic SOC while the orbital polarization is insensitive to SOC. The large spin polarization comes with strong electrical magnetochiral anisotropy in the magneto-transport of these chiral crystals (e.g., RhSi). Our work reveals the interplay of chirality, electron spin, and orbital in chiral crystals, paving the way for developing chiral solids for chirality-induced phenomena.
Chirality-induced spin selectivity (CISS) refers to the fact that electrons get spin polarized after passing through organic chiral molecules in a nanoscale device. In CISS, chiral molecules are commonly believed to be a spin filter through which one favored spin transmits and the opposite spin gets reflected, i.e., transmitted and reflected electrons exhibit opposite spin polarization. In this work, we point out that such a spin filter scenario contradicts the principle that equilibrium spin current must vanish. Instead, we find that both transmitted and reflected electrons present the same t…
Real Space Imaging of Spin Scattering in Chirality-Induced Spin Selectivity
Ulsan National Institute of Science and Technology (UNIST)（蔚山科学技术院）；Weizmann Institute of Science（魏茨曼科学研究所）；Pennsylvania State University（宾夕法尼亚州立大学）；Max Planck Institute for the Structure and Dynamics of Matter（物质结构与动力学马普研究所）
Electron exchange and correlations emerging from the coupling between ionic vibrations and electrons are addressed. Spin-dependent electron-phonon coupling originates from the spin-orbit interaction, and it is shown that such electron-phonon coupling introduces exchange splitting between the spin channels in the structure. By application of these results to a model for a chiral molecular structure mounted between metallic leads, the chirality induced spin selectivity is found to become several tens of percents using experimentally feasible parameters.
We report a new type of spin-orbit coupling (SOC) called geometric SOC. Starting from the relativistic theory in curved space, we derive an effective nonrelativistic Hamiltonian in a generic curve embedded into flat three dimensions. The geometric SOC is $O(m^{-1})$, in which $m$ is the electron mass, and hence much larger than the conventional SOC of $O(m^{-2})$. The energy scale is estimated to be a hundred meV for a nanoscale helix. We calculate the current-induced spin polarization in a coupled-helix model as a representative of the chirality-induced spin selectivity. We find that it depen…
The chirality-induced spin selectivity (CISS) effect gives rise to strongly spin-dependent transport through many organic molecules and structures. Its discovery raises fascinating fundamental questions as well as the prospect of possible applications. The basic phenomenology, a strongly asymmetric magnetoresistance despite the absence of magnetism, is now understood to result from the combination of spin-orbit coupling and chiral geometry. However, experimental signatures of electronic helicity were observed at room temperature, i.e., at an energy scale that exceeds the typical spin-orbit cou…
The elucidation of the mechanisms behind chiral-induced spin selectivity remains an outstanding scientific challenge. Here we consider the role of delocalised phonon modes in electron transport in chiral structures and demonstrate that spin selectivity can originate from spin-dependent energy and momentum conservation in electron-phonon scattering events. While this mechanism is robust to the specifical nature of the vibrational modes, the degree of spin polarization depends on environmental factors, such as external driving fields, temperatures and phonon relaxation rates. This dependence is …
The chirality-induced spin selectivity (CISS) effect has been confirmed experimentally for a large class of organic molecules. Adequately modeling the effect remains a challenging task, with both phenomenological models and first-principle simulations yielding inconclusive results. Building upon a previously presented model by K. Michaeli and R. Naaman (J. Phys. Chem C 123, 17043 (2019)) we systematically investigate an effective 1-dimensional model derived as the limit of a 3-dimensional quantum system with strong confinement and including spin-orbit coupling. Having a simple analytic structu…
Symmetry considerations in chirality-induced spin selectivity
Helmholtz-Institut Mainz（亥姆霍兹美因茨研究所）；GSI Helmholtzzentrum für Schwerionenforschung GmbH（GSI重离子研究中心有限公司）；QUANTUM, Institut für Physik, Johannes Gutenberg-Universität（约翰内斯·古腾堡大学量子物理研究所）；Department of Physics, University of California（加州大学物理系）；Institute of Physics, Johannes Gutenberg-University Mainz（约翰内斯·古腾堡大学美因茨分校物理研究所）
Here we propose a mechanism by which spin polarization can be generated dynamically in chiral molecular systems undergoing photo-induced electron transfer. The proposed mechanism explains how spin polarization emerges in systems where charge transport is dominated by incoherent hopping, mediated by spin orbit and electronic exchange couplings through an intermediate charge transfer state. We derive a simple expression for the spin polarization that predicts a non-monotonic temperature dependence consistent with recent experiments. We validate this theory using approximate quantum master equati…
In a magnetic texture, the spin of a conduction electron is forced to be aligned to the localized moment. As a result, the topology of the magnetic texture affects the electron dynamics in nontrivial ways. A representative example is the topological Hall effect in noncoplanar spin textures with finite spin chirality. While propagating in the noncoplanar spin texture, electrons acquire Berry phase, and their motion is deflected as if they were in a magnetic field. Here, we report a distinct Berry phase effect in a coplanar helimagnet: the spin moment of the conduction electron is polarized unde…
Chirality induced spin selectivity, discovered about two decades ago in helical molecules, is a non-equilibrium effect that emerges from the interplay between geometrical helicity and spin-orbit interactions. Several model Hamiltonians building on this interplay have been proposed and while these can yield spin-polarized transport properties that agrees with experimental observations, they simultaneously depend on unrealistic values of the spin-orbit interaction parameters. It is likely, however, that a common deficit originates from the fact that all these models are uncorrelated, or, single-…
Emergent chirality and enantiomeric selectivity in layered NbOX$_2$ crystals
Aalto University（阿尔托大学）；Max Planck Institute for Chemical Physics of Solids（马克斯·普朗克固体化学物理研究所）；Centro de Física de Materiales (CFM-MPC), CSIC-UPV/EHU（材料物理中心，西班牙国家研究委员会-巴斯克国立大学）；University of the Basque Country (UPV/EHU)（巴斯克国立大学）；Donostia International Physics Center (DIPC)（圣塞巴斯蒂安国际物理中心）；Université de Sherbrooke（舍布鲁克大学）；Regroupement Québécois sur les Matériaux de Pointe (RQMP)（魁北克尖端材料联盟）
研究层状NbOX₂晶体的结构和电子性质，揭示其非手性I m m m相与手性C2相之间的联系，通过原理计算发现中间非手性C2/m相，并探讨电场和压力对手性稳定的影响。
Chirality in organic molecules has attracted considerable attention in the fields of chemistry, biology, and spintronics. This paper reports on perpendicular magnetization hysteresis loops of a multilayer consisting of ultrathin Fe (001), chiral phthalocyanine molecule ((P)- or (M)-PbPc-DTBPh), and MgO (001). We find a chirality-dependent shift of the hysteresis loop. Unlike the previously reported bias current induced phenomena, the result shows a chirality-induced effective magnetic field in the phthalocyanine molecule in the absence of a bias current in the system. This study opens up a new…
Chirality-induced spin selectivity due to an emergent molecular orbital moment
The Institute of Physics of the Czech Academy of Sciences（捷克科学院物理研究所）；Institute of Physics, Johannes Gutenberg-University（约翰内斯·古腾堡大学物理研究所）；Peter Grünberg Institut (PGI-6), Forschungszentrum Jülich GmbH（于利希研究中心彼得·格林伯格研究所（PGI-6））
What Is the Real-Time Atomistic Mechanism Behind Chirality-Induced Spin Selectivity in Donor-Chiral Bridge-Acceptor Molecules?
School of Physics and Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology（华中科技大学物理学院与武汉国家强磁场中心）；Institute for Quantum Science and Engineering, Huazhong University of Science and Technology（华中科技大学量子科学与工程研究院）
Anomalous Hall effect arising from non-trivial spin configuration (chirality) is studied based on the $s$-$d$ model. Considering a weak coupling case, the interaction is treated perturbatively. Scattering by normal impurities is included. Chirality is shown to drive locally Hall current and leads to overall Hall effect if there is a finite uniform chirality. This contribution is independent of the conventional spin-orbit contribution and shows distinct low temperature behavior. In mesoscopic spin glasses, chirality-induced anomalous Hall effect is expected below the spin-glass transition tempe…
The chiral induced spin selectivity effect is an excited states phenomenon, which can be probed using photo-spectroscopy as well as transport measurements. On the one hand such measurements represent averaged quantities, on the other hand nearly all theoretical descriptions, with only a few exceptions, have been concerned with energy dependent properties of the pertinent structures. While those properties may or may not be relevant for the chiral induced spin selectivity effect, many of those properties have been attributed as being the, or part of the, origins of the effect. Here, it is demon…
We study theoretically the anomalous Hall effect due to the vector spin chirality carried by the local spins in the $s$-$d$ model. We will show that the vector spin chirality indeed induces local Hall effect in the presence of the electron spin polarization, while the global Hall effect vanishes if electron transport is homogeneous. This anomalous Hall effect can be interpreted in terms of the rotational component of the spin current associated with the vector chirality.
Spin-Orbit Coupling-Driven Chirality Switching of Spin Waves in Altermagnets
Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所凝聚态物理国家实验室）；School of Physical Sciences, University of Chinese Academy of Sciences（中国科学院大学物理科学学院）；School of Physics, Sun Yat-Sen University（中山大学物理学院）；Songshan Lake Materials Laboratory（松山湖材料实验室）
The interplay between magnetic vortex polarity, chirality and the curvature of the underlying surface results in a dependence of the vortex polarity switching efficiency on the vortex chirality. The switching is studied numerically by applying a short Gauss pulse of the external magnetic field to a spherical cap within its cut plane. The minimum field intensity required for the switching essentially depends on the vortex chirality and it does not depend on the initial vortex polarity. This effect decreases with the curvature radius increasing and it vanishes in the planar limit.
Chiral-induced spin selectivity (CISS) is a phenomenon in which electron spins are polarized as they are transported through chiral molecules, and the spin polarization depends on the handedness of the chiral molecule. In this study, we show that spin selectivity can be realized in achiral materials by coupling electrons to a single mode of a chiral optical cavity. By investigating spin-dependent electron transport using the nonequilibrium Green's function approach, the spin polarization in a two-terminal setup is demonstrated to approach unity if the rate of dephasing is sufficiently small an…
We study the non-equilibrium dynamics of electron transmission from a straight waveguide to a helix with spin-orbit coupling. Transmission is found to be spin-selective and can lead to large spin polarizations of the itinerant electrons. The degree of spin selectivity depends on the width of the interface region, and no polarization is found for single-point couplings. We show that this is due to momentum conservation conditions arising from extended interfaces. We therefore identify interface structure and conservation of momentum as crucial ingredients for chiral-induced spin selectivity, an…
The theoretical explanation for the chiral-induced spin selectivity effect, in which electrons' passage through a chiral system depends on their spin and the handedness of the system, remains vague. Although most experimental work was performed at room temperature, most of the proposed theories did not include vibrations. Here, we present temperature-dependent experiments and a theoretical model that captures all observations and provides spin polarization values that are consistent with the experimental results. The model includes vibrational contribution to the spin orbit coupling. It shows …
Surface physics dominated by bulk properties has been one of the central interests in modern condensed matter physics, from electric polarization to bulk-boundary correspondence of topological insulators and superconductors. Here, we extend theory of electric polarization to chirality polarizations, that is, surface charges corresponding to local antisymmetries characterized as a bulk property. Using the notion of chirality polarizations, we prove the recently proposed spectral bulk-boundary correspondence, a generalization of bulk-boundary correspondence in chiral symmetric systems into compl…
We investigate charge pumping in ferromagnet/triplet superconductor junctions where the magnetization of the ferromagnet is inhomogeneous and dynamical. It is shown that charge current is pumped due to the coupling of the localized spin with triplet vector spin chirality, vector spin chirality formed by the triplet vector of Cooper pairing. Physical mechanism of the charge pumping is also discussed.
