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Chiral molecules and the electron spin
Abstract
The electron’s spin is essential to the stability of matter, and control over the spin opens up avenues for manipulating the properties of molecules and materials. The Pauli exclusion principle requires that two electrons in a single spatial eigenstate have opposite spins, and this fact dictates basic features of atomic states and chemical bond formation. The energy associated with interacting electron clouds changes with their relative spin orientation, and by manipulating the spin directions, one can guide chemical transformations. However, controlling the relative spin orientation of electrons located on two reactants (atoms, molecules or surfaces) has proved challenging. Recent developments based on the chiral-induced spin selectivity (CISS) effect show that the spin orientation is linked to molecular symmetry and can be controlled in ways not previously imagined. For example, the combination of chiral molecules and electron spin opens up a new approach to (enantio)selective chemistry. This Review describes the theoretical concepts underlying the CISS effect and illustrates its importance by discussing some of its manifestations in chemistry, biology and physics. Specifically, we discuss how the CISS effect allows for efficient long-range electron transfer in chiral molecules and how it affects biorecognition processes. Several applications of the effect are presented, and the importance of controlling relative spin orientations in multi-electron processes, such as electrochemical water splitting, is emphasized. We describe the enantiospecific interaction between ferromagnetic substrates and chiral molecules and how it enables the separation of enantiomers with ferromagnets. Lastly, we discuss the relevance of CISS effects to biological electron transfer, enantioselectivity and CISS-based spintronics applications.
... Although the original effect was observed in photoelectron spectroscopy experiments [9][10][11][12], CISS has since been observed in a wide range of scenarios, such as electron transfer through chiral molecules [13][14][15], chemical reactions of chiral molecules at metal surfaces [16][17][18], and spin-Hall measurements [19][20][21]. Of particular interest to this article are experiments observing spin polarization of electron transport through chiral molecular junctions, where a chiral molecule is placed between two electrodes, one of which is magnetized and can inject spin-polarized electrons [22][23][24][25][26][27], demonstrating that the CISS effect occurs * samuel.rudge@physik.uni-freiburg.de even at the single-molecule level. ...
... (1),s L,+,ℓ,1;ss . (23) From this, we define the spin polarization in the charge current as the relative difference between ⟨I⟩ ↓ and ⟨I⟩ ↑ , ...
The connection between molecular vibrations and spin polarization in charge transport through molecular junctions is currently a topic of high interest, with important consequences for a variety of phenomena, such as chirality-induced spin selectivity (CISS). In this work, we follow this theme by exploring the relationship between vibronic dynamics and the corresponding spin polarization of the nonequilibrium charge current in a molecular junction. We employ the hierarchical equations of motion (HEOM) approach, which, since it is numerically exact and treats the vibrational degrees of freedom quantum mechanically, extends previous analyses of similar models that relied on approximate transport methods. We find significant spin polarization of the charge current in the off-resonant, low-voltage regime, where the vibrations must be treated quantum mechanically. Furthermore, we are able to connect the spin polarization in the charge transport to a corresponding polarization of the vibrational dynamics, which manifests itself in the vibrational angular momentum and excitation. Our analysis covers multiple molecule-lead couplings, temperatures, orbital energies, and spin-orbit couplings, demonstrating that the vibrationally assisted spin polarization is robust across a broad range of parameters.
... Chirality represents a structure that lacks both inversion and mirror symmetries. When electrons pass through chiral materials, their spins become polarized parallel or antiparallel to the direction of the current, a phenomenon known as the chirality-induced spin selectivity (CISS) effect [1][2][3][4][5][6][7][8] . The CISS effect, initially identified in organic molecules, has now been demonstrated in inorganic materials [9][10][11][12][13][14] . ...
... Finally, we show the coefficient γ defined with D z and D sz z in Eq. (5), which quantifies the spin current under the setup with fixed electric current. The order parameter dependence of γ in the s+p-wave superconducting state is presented in Fig. 9. Notably, the current-induced spin current can be enhanced in strongly parity-mixed superconductors. ...
Chiral materials exhibit a spin filtering effect, so-called chirality-induced spin selectivity (CISS). A recent observation of spin accumulation at the ends of a chiral-structured superconductor has opened up a new pathway for studying the CISS effect in superconductors. In chiral-structured superconductors, the admixture of the spin-singlet and spin-triplet order parameters significantly influences the properties of superconductivity. In this paper, we investigate the interplay between the superconducting order parameter and supercurrent-induced spin current, namely, the superconducting CISS effect. In weakly party-mixed superconductors, the spin current, which is predominantly temperature-independent, is carried by spin-polarized Cooper pairs with finite center-of-mass momentum. In contrast, in strongly party-mixed superconductors the temperature-dependent spin current is also carried by electrons with opposite momentum and antiparallel spins forming a Cooper pair. Chiral-structured superconductors will offer a novel platform for exploring the CISS effect and may provide deeper insights into its underlying mechanisms related to the parity-mixed order parameter.
... We provide the details of the single crystal growth used in our study. Single crystals of Te were produced via the physical vapour transport (PVT) method 5 . A mass of 0.36 g of bulk Te was vacuumsealed in a quartz tube and heated in a three-zone electric furnace. ...
The nonlinear thermoelectric effect is a key factor for realising unconventional thermoelectric phenomena, such as heat rectification and power generation using thermal fluctuations. Recent theoretical advances have indicated that chiral materials can host a variety of exotic nonlinear thermoelectric transport arising from inversion-symmetry breaking. However, experimental demonstration has yet to be achieved. Here, we report the observation of the nonlinear chiral thermoelectric Hall effect in chiral tellurium at room temperature, where a voltage is generated as a cross product of the temperature gradient and electric field. The resulting thermoelectric Hall voltage is on the order of {\mu}V, consistent with the prediction from the ab initio calculation. Furthermore, the sign of the thermoelectric Hall voltage is reversed depending on the crystal chirality, demonstrating a novel functionality of sign control of the thermoelectric effect by the chirality degrees of freedom. Our findings reveal the potential of chiral systems as nonlinear thermoelectric materials for advanced thermal management and energy harvesting.
We investigate quantum transport in a two-dimensional electron system coupled to a chiral molecular potential, demonstrating how molecular chirality and orientation affect charge and spin transport properties. We propose a minimal model for realizing true chiral symmetry breaking on a magnetized surface and show, with a crucial role played by the tilt angle of the molecular dipole with respect to the surface. For non-zero tilting, we show that the Hall response exhibits clear signatures of chirality-induced effects, both in charge and spin-resolved observables. Concerning the former, tilted enantiomers produce asymmetric Hall conductances and, even more remarkably, the persistence of this feature in the absence of spin-orbit coupling (SOC) signals how the enantiospecific charge response results from electron scattering off the molecular potential. Concerning spin-resolved observables where SOC plays a relevant role, we reveal that chiral symmetry breaking is crucial in enabling spin-flipping processes.
Current organic light-emitting diode (OLED) technology uses light-emitting molecules in a molecular host. We report green circularly polarized luminescence (CPL) in a chirally ordered supramolecular assembly, with 24% dissymmetry in a triazatruxene (TAT) system. We found that TAT assembled into helices with a pitch of six molecules, associating angular momentum to the valence and conduction bands and obtaining the observed CPL. Cosublimation of TAT as the “guest” in a structurally mismatched “host” enabled fabrication of thin films in which chiral crystallization was achieved in situ by thermally triggered nanophase segregation of dopant and host while preserving film integrity. The OLEDs showed external quantum efficiencies of up to 16% and electroluminescence dissymmetries ≥10%. Vacuum deposition of chiral superstructures opens new opportunities to explore chiral-driven optical and transport phenomena.
Screw dislocation-driven nanostructures of two-dimensional transition metal dichalcogenides (2D TMDs) can feature chirality that enables prominent asymmetric optical properties. One of the outstanding representatives is WSe2 as it can exhibit...
Significance
Charge transfer through organic molecules is extensively studied theoretically and experimentally owing to its importance in basic biological processes. Long-distance electron transport through such molecules, which are typically insulating, is commonly attributed to thermally activated hopping. We introduce an additional mechanism that supports electron transfer across appreciable distances, even at zero temperature. It originates from the electric potential that is either internally created or externally applied before charge flows. The electronic wavefunctions are dramatically changed by this potential, allowing them to efficiently mediate current. This mechanism can explain recent experiments where electron transfer shows only a weak temperature dependence. Our result is an important contribution to the long-standing challenge of developing a comprehensive theory for charge transfer through organic molecules.
The chiral induced spin selectivity (CISS) effect in which selective transport of electron spins through helical chiral molecules occurs, has attracted a lot of attention in recent years. This effect was used to magnetize ferromagnetic (FM) samples by utilizing adsorbed chiral molecules.
The electron transfer through the molecules was generated optically or electrically. In the optical configuration, circularly polarized light induced efficient magnetization by spin torque transfer (STT), using a hybrid of quantum dots (QDs) and chiral molecule self-assembled monolayer (SAM).
Here, we use X-ray magnetic chiral dichroism (XMCD) spectroscopy in order to probe the optically induced magnetization on thin FM films. The results show differences in the FM magnetization depending on the optical circular polarization, matching previous non-local Hall probe measurements.
This work demonstrates the chiral induced spin selectivity effect for inorganic copper oxide films and exploits it to enhance the chemical selectivity in electrocatalytic water splitting. Chiral CuO films are electrodeposited on a polycrystalline Au substrate, and their spin filtering effect on electrons is demonstrated using Mott-polarimetry analysis of photoelectrons. CuO is known to act as an electrocatalyst for the oxygen evolution reaction; however it also generates side products such as H2O2. We show that chiral CuO is selective for O2; H2O2 generation is strongly suppressed on chiral CuO but is present with achiral CuO. The selectivity is rationalized in terms of the electron spin filtering properties of the chiral CuO and the spin constraints for the generation of triplet oxygen. These findings represent an important step toward the development of all inorganic chiral materials for electron spin filtering and the creation of efficient, spin-selective (photo)electrocatalysts for water-splitting.
Chirality-induced Spin Selectivity (CISS) is a recently discovered effect, whose precise microscopic origin has not yet been fully elucidated; it seems, however, clear that spin-orbit interaction plays a pivotal role. Various model Hamiltonian approaches have been proposed, suggesting a close connection between spin selectivity and filtering and helical symmetry. However, first-principles studies revealing the influence of chirality on the spin polarization are missing. To clearly demonstrate the influence of the helical conformation on the spin polarization properties, we have carried out spin-dependent Density-Functional Theory (DFT) based transport calculations for a model molecular system. It consists of α-helix and β-strand conformations of an oligo-glycine peptide (with nine repeated units), which is bonded to a nickel electrode and to a gold electrode in a two-terminal setup, similar to a molecular junction or a local probe, e.g., in STM or AFM configurations. We have found that the α-helix conformation displays a spin polarization, calculated through the intrinsic magneto-resistance of the junction, about 100-1000 times larger than the linear β-strand, clearly demonstrating the crucial role played by the molecular helical geometry on the enhancement of spin polarization associated with the CISS effect.
The exclusion zone (EZ) of water near the hydrophilic surface can exclude colloid suspension for a certain distance, which is typically several hundreds of micrometers, and approximates the size of the cells. Previous studies have shown that the near-surface the EZ expands extensively in the presence of incident radiant energy, especially the infrared light. Developments of electromagnetic biology and quantum biology indicate that the spin magnetic moment may have a direct impact on the biological process. In this article, a spintronic device, with a nickel–manganese ferrite rotator and spiral magnetic vector potential, was utilized to exert the influence on the EZ. Spin states in the nickel–manganese ferrite were polarized by two different chiral configurations, i.e., right-handed and left-handed, of the spiral magnetic vector potential, through the Aharonov–Bohm (AB) effect. The variation of the the EZ width clearly reflects the presence of different chiralities of the polarization and shows a directionality effect of rotation and irradiation. The spintronic device with a right-handed magnetic vector potential can enlarge the the EZ for 30–40%, while the one with the left-handed magnetic vector potential showed no potentiation but sometimes a depression effect. Therefore, a chiral symmetry-breaking phenomenon was observed, which shows that the formation of the EZ water may evolve certain quantum effects related to the chiral induced spin selectivity effect.
Taking enantiomers for a spin
There are two common ways to distinguish mirror-image molecules, or enantiomers. The first relies on their distinct interactions with circularly polarized light, the second on their interactions with a pure enantiomer of some other molecule. Now Banerjee-Ghosh et al. report a conceptually different approach to chiral resolution. Experiments showed that, depending on the direction of magnetization, chiral oligopeptides, oligonucleotides, and amino acids have enantiospecific differences in initial adsorption rates on ferromagnetic surfaces. This effect is attributed to enantiospecific induced spin polarization.
Science , this issue p. 1331
The interaction of low-energy photoelectrons with well-ordered monolayers of enantiopure helical heptahelicene molecules adsorbed on metal surfaces leads to a preferential transmission of one longitudinally polarized spin component, which is strongly coupled to the helical sense of the molecules. Heptahelicene, comprised only of carbon and hydrogen atoms exhibits only a single helical turn, but shows excess in longitudinal spin polarizations of about PZ = 6 to 8% after transmission of initially balanced left and right-handed spin polarized electrons. Insight into the electronic structure, i.e., the projected density of states, and the spin-dependent electron scattering in the helicene molecule is gained by using spin-resolved density-functional theory calculations and a model Hamiltonian approach, respectively. Our results support the picture of transport along a helical pathway of the electrons under the influence of spin-orbit coupling induced by the electrostatic molecular potential.
