Search

Chiral molecules can exhibit strong spin–orbit coupling, which can result in a large spin polarization. This is due to the fact that the energy levels of the electrons in a chiral molecule are strongly influenced by the chiral structure of the molecule, which can result in the separation of the energy levels for electrons with different spin orientations. We report a controlled spin-selective transmission of electrons through 20 base-paired poly-cytosine molecules functionalized with MoS2 flakes on ITO glass via the quantum mechanical tunneling effect. A reversion in spin polarization was observed after the silver ions interact with poly-cytosine due to the strong coordination of Ag(I) with cytosine–cytosine (C–C) mismatches, indicating the formation of duplex structural motifs, as confirmed by the circular dichroism spectroscopy at room temperature. Manipulating the spin of an electron through such a small molecule merely controlled by special cations could pave the way for major advances in spin-independent charge transport, advanced bioanalytical system design, and related applications. [ABSTRACT FROM AUTHOR]

In connection to the chiral-induced spin selectivity effect, we theoretically analyze the electronic and spin states of edges of a finite p-orbital helical atomic chain with the intra-atomic spin–orbit interaction. This model can host the spin-filtering state in which two up-spins propagate in one direction and two down-spins propagate in the opposite direction without breaking the time-reversal symmetry (TRS). The enhancement of charge modulations concentrated at the edges due to the evanescent states is induced, although the spin density is absent because of the TRS. A Zeeman field at an edge of the atomic chain, which breaks the TRS, yields a finite spin polarization, whose direction depends on the chirality of the molecule. The chirality change induces a reasonable amount of the energy difference, which may provide an insight into the enantioselective adsorption of chiral molecules on the ferromagnetic surface. [ABSTRACT FROM AUTHOR]

Enantio-discrimination and spin-dependent electrochemistry (SDE), as a manifestation of the chirality-induced spin selectivity (CISS) effect, are important phenomena that can be probed by "chiral" electrochemistry. Here, we prepared chiralized surfaces of gold and nickel, to serve as working electrodes, through effective chemisorption of enantiopure dimethyl-bis(ethylenedithio)-tetrathiafulvalene (DM-BEDT-TTF) 1, tetramethyl-bis(ethylenedithio)-tetrathiafulvalene (TM-BEDT-TTF) 2, and their capped silver nanoparticle (AgNPs) aggregate by simple incubation of the metallic substrates. The effective chemisorption was checked by means of ultrahigh vacuum x-ray photoelectron spectroscopy (XPS) and by electro-desorption experiments, i.e., cyclic voltammetry (CV) scans showing a first electro-desorption peak at about −1.0 V. The Au|1 and Au|2 chiral electrodes were successfully used in CV experiments exploiting chiral redox probes. Finally, the hybrid interfaces Ni|enantiopure 1 or 2|AgNPs served as working electrodes in SDE experiments. In particular, the hybrid chiral interfaces Ni|(R)-2|AgNPs and Ni|(S)-2|AgNPs exhibited a significant spin-filtering ability, as a manifestation of the CISS effect, with average spin polarization values of 15%. [ABSTRACT FROM AUTHOR]

We report on a strategy to indirectly read out the spectrum of an electronic spin via polarization transfer to nuclear spins in its local environment. The nuclear spins are far more abundant and have longer lifetimes, allowing for repeated polarization accumulation in them. Subsequent nuclear interrogation can reveal information about the electronic spectral density of states. We experimentally demonstrate the method by reading out the ESR spectrum of nitrogen vacancy center electrons in diamond via readout of lattice 13C nuclei. Spin-lock control on the 13C nuclei yields a significantly enhanced signal-to-noise ratio for the nuclear readout. Spectrally mapped readout presents operational advantages in being background-free and immune to crystal orientation and optical scattering. We harness these advantages to demonstrate applications in underwater magnetometry. The physical basis for the "one-to-many" spectral map is itself intriguing. To uncover its origin, we develop a theoretical model that maps the system dynamics, involving traversal of a cascaded structure of Landau–Zener anti-crossings, to the operation of a tilted "Galton board." This work points to new opportunities for "ESR-via-NMR" in dilute electronic systems and in hybrid electron–nuclear quantum memories and sensors. [ABSTRACT FROM AUTHOR]

Chiral molecules can act as spin filters, preferentially transmitting electrons with spins polarized along their direction of travel, an effect known as chirality-induced spin selectivity (CISS). In a typical experiment, injected electrons tunnel coherently through a layer of chiral material and emerge spin-polarized. It is also possible that spin polarization arises in radical pairs formed photochemically when electrons hop incoherently between donor and acceptor sites. Here we aim to identify the magnetic properties that would optimise the visibility of CISS polarization in time-resolved electron paramagnetic resonance (EPR) spectra of transient radical pairs without the need to orient or align their precursors. By simulating spectra of actual and model systems, we find that CISS contributions to the polarization should be most obvious when at least one of the radicals has small g-anisotropy and an inhomogeneous linewidth larger than the dipolar coupling of the two radicals. Under these conditions there is extensive cancellation of absorptive and emissive enhancements making the spectrum sensitive to small changes in the individual EPR line intensities. Although these cancellation effects are more pronounced at lower spectrometer frequencies, the spectral changes are easier to appreciate with the enhanced resolution afforded by high-frequency EPR. Consideration of published spectra of light-induced radical pairs in photosynthetic bacterial reaction centres reveals no significant CISS component in the polarization generated by the conventional spin-correlated radical pair mechanism. [ABSTRACT FROM AUTHOR]

The article discusses the chiral induced spin selectivity effect (CISS), which is the finding that the motion of an electron within or through chiral systems depends on the spin of that electron. The article explains that while the effect has been observed experimentally, developing a quantitative theoretical description has been challenging. The article also highlights the different types of experimental measurements that have been used to study CISS, including measurements of spin-dependent transmission, tunneling currents, magnetization, and magnetic domains. The article concludes by discussing the potential applications of CISS in spintronics, electrochemistry, and biology, and expresses optimism that further research will reveal the mechanism behind the CISS effect. [Extracted from the article]

Mastering the manipulation of the electron spin plays a crucial role in comprehending the behavior of organic materials in several applications, such as asymmetric catalysis, chiroptical switches, and electronic devices. A promising avenue for achieving such precise control lies in the Chiral Induced Spin Selectivity (CISS) effect, where electrons with a favored spin exhibit preferential transport through chiral assemblies of specific handedness. Chiral supramolecular polymers emerge as excellent candidates for exploring the CISS effect due to their ability to modulate their helical structure through noncovalent interactions. In this context, systems capable of responding to external stimuli are particularly intriguing, sometimes even displaying chirality inversion. This study unveils spin selectivity in chiral supramolecular polymers, derived from single enantiomers, through scanning tunneling microscopy conducted in scanning tunneling spectroscopy mode. Following two distinct sample preparation protocols for each enantiomer, we generate supramolecular polymers with opposite handedness and specific spin transport characteristics. Our primary focus centers on chiral π-conjugated building blocks, with the aim of advancing novel systems that can inspire the organic spintronics community from a supramolecular chemistry level. [ABSTRACT FROM AUTHOR]

A gold surface is functionalized by chemisorption of the enantiopure N,N′-bis-[2,2′;5′,2″]tert-thiophene-5-yl methylcyclohexane-1,2-diamine (2T3N), a chiral oligothiophene derivative, via overnight incubation in a 2T3N ethanol solution. The Au|2T3N interface is characterized by x-ray photoelectron circular dichroism and comparing x-ray photoemission spectroscopy and electro-desorption results. Charge transmission at the Au|2T3N| solution interface is characterized by recording the cyclic voltammetry of the Fe(III)/Fe(II) reversible redox couple, finding a charge transfer rate constant, k°, variation from 1 × 10−1 to 3.3 × 10−2 cm s−1, when comparing the bare Au and the Au|2T3N interfaces, respectively. The "anomalous" high value of k° found for the chiral Au|2T3N interface can be rationalized on the basis of the chiral-induced spin selectivity effect, as further proved by magnetic–conductive atomic force microscopy measurements at room temperature. A spin polarization of about 30% is found. [ABSTRACT FROM AUTHOR]

Electron transport through a variety of helical structures has been shown to exhibit high-efficiency spin filtering, which is called chirality-induced spin selectivity (CISS). In this paper, we consider a helical chain of atomic p orbitals, which has been employed as a model in exploring the mechanism of CISS in previous theories, and show that the interatomic hopping along the helical chain induces an effective magnetic field (EMF) acting on the atomic orbital angular momentum (OAM). In chains where the curvature and torsion of the helix are small, we find that the EMF on the binormal component of the atomic OAM is created by the curvature, while that on the tangential component is produced by the torsion. We show that such coupling of the atomic OAM and the interatomic hopping leads to current-induced orbital and spin polarizations. We expect that the present coupling, which is expressed locally, can be used to estimate orbital and spin polarizations locally induced in molecules and solids. [ABSTRACT FROM AUTHOR]

Nonlocal spin polarization phenomena are thoroughly investigated in the devices made of chiral metallic single crystals of CrNb3S6 and NbSi2 as well as of polycrystalline NbSi2. We demonstrate that simultaneous injection of charge currents in the opposite ends of the device with the nonlocal setup induces the switching behavior of spin polarization in a controllable manner. Such a nonlocal spin polarization appears regardless of the difference in the materials and device dimensions, implying that the current injection in the nonlocal configuration splits spin-dependent chemical potentials throughout the chiral crystal even though the current is injected into only a part of the crystal. We show that the proposed model of the spin dependent chemical potentials explains the experimental data successfully. The nonlocal double-injection device may offer significant potential to control the spin polarization to large areas because of the nature of long-range nonlocal spin polarization in chiral materials. [ABSTRACT FROM AUTHOR]

The control of electron spin, crucial to the stability of matter, offers new possibilities for manipulating the properties of molecules and materials with potential applications in spintronics and chemical reactions. Recent experiments have demonstrated that electron transmission through chiral molecules depends on the electron spin orientation, a phenomenon known as chiral-induced spin selectivity (CISS). In this study, we show that CISS can be observed in achiral systems driven by an external circularly polarized laser field in the framework of Floquet engineering. By using the Floquet theory for a time-periodically driven system to investigate spin-dependent electron transport in a two-terminal setup, we demonstrate that the spin polarization can approach unity if the light intensity is sufficiently strong, the rate of dephasing is sufficiently low, and the average chemical potential of the two leads is within an appropriate range of values, which is narrow because of the high frequency of the laser field. To obtain a broader range of energies for large spin polarization, a combination of chiral molecules and light–matter interactions is considered, and the spin polarization of electrons transported through a helical molecule driven by a laser field is evaluated. [ABSTRACT FROM AUTHOR]

It has been demonstrated previously that electrons interact differently with chiral molecules depending on their polarization. For enantiomeric pure monolayers of heptahelicene, opposite asymmetries in spin polarization were reported and attributed to the so-called chirality-induced spin selectivity effect. However, these promising proof-of-concept photoemission experiments lack the angular and energy resolution that could provide the necessary insights into the mechanism of this phenomenon. In order to fill in the missing gaps, we provide a detailed spin- and angle-resolved photoemission spectroscopy study of heptahelicene layers on a Cu(111) substrate. Throughout the large accessible energy and angle range, no chirality induced spin asymmetry in photoemission could be observed. Possible reasons for the absence of signatures of the spin-dependent electron transmission through the chiral molecular layer are briefly discussed. [ABSTRACT FROM AUTHOR]

Bio-spinterfaces present numerous opportunities to study spintronics across the biomolecules attached to (ferro)magnetic electrodes. While it offers various exciting phenomena to investigate, it is simultaneously challenging to make stable bio-spinterfaces as biomolecules are sensitive to many factors that it encounters during thin-film growth to device fabrication. The chirality-induced spin-selectivity effect is an exciting discovery, demonstrating an understanding that a specific electron's spin (either up or down) passes through a chiral molecule. The present work utilizes Ustilago maydis Rvb2 protein, an ATP-dependent DNA helicase (also known as Reptin), to fabricate bio-spintronic devices to investigate spin-selective electron transport through the protein. Ferromagnetic materials are well-known for exhibiting spin-polarization, which many chiral and biomolecules can mimic. We report herein spin-selective electron transmission through Rvb2 that exhibits 30% spin polarization at a low bias (+0.5 V) in a device configuration, Ni/Rvb2 protein/indium tin oxide measured under two different magnetic configurations. Our findings demonstrate that biomolecules can be put in circuit components without any expensive vacuum deposition for the top contact. The present study holds a remarkable potential to advance spin-selective electron transport in other biomolecules, such as proteins and peptides, for biomedical applications. [ABSTRACT FROM AUTHOR]

Second-order Møller-Plesset perturbation theory (MP2) often breaks down catastrophically in small-gap systems, leaving much to be desired in its performance for myriad chemical applications such as noncovalent interactions, thermochemistry, and dative bonding in transition metal complexes. This divergence problem has reignited interest in Brillouin-Wigner perturbation theory (BWPT), which is regular at all orders but lacks size consistency and extensivity, severely limiting its application to chemistry. In this work, we propose an alternative partitioning of the Hamiltonian that leads to a regular BWPT perturbation series that, through the second order, is size-extensive, size-consistent (provided its Hartree–Fock reference is also), and orbital invariant. Our second-order size-consistent Brillouin-Wigner (BW-s2) approach can describe the exact dissociation limit of H2 in a minimal basis set, regardless of the spin polarization of the reference orbitals. More broadly, we find that BW-s2 offers improvements relative to MP2 for covalent bond breaking, noncovalent interaction energies, and metal/organic reaction energies, although rivaling coupled-cluster with single and double substitutions for thermochemical properties. [ABSTRACT FROM AUTHOR]

The theory of the magnetic coupling between the localized spins, mediated by the mobile excess electron, is generalized to the case of a trigonal, six-center, four-electron molecule with partial valence delocalization. The combination of the electron transfer occurring within the valence-delocalized subsystem and the interatomic exchange producing coupling of the spin of the mobile electron of valence-delocalized fragment with the three localized spins forming the valence-localized subsystem leads to the appearance of a special kind of double exchange (DE), termed the "external core double exchange" (ECDE), in order to distinguish such DE from the conventional "internal core double exchange" for which the mobile electron is coupled with the spin-cores on the same center via the intra-atomic exchange. The effect of the ECDE on the ground spin state of the considered trigonal molecule is compared with earlier reported effect produced by DE in the four-electron, mixed-valence (MV) trimer. A high diversity of the ground spin states is revealed, depending on the relative magnitudes and signs of the electron transfer and interatomic exchange parameters, with part of these states not appearing to be the ground states in a trigonal trimer exhibiting DE. We briefly discuss some examples of trigonal MV systems from the point of view of the possibility to have different combinations of signs of the transfer and exchange parameters and, accordingly, different ground spin states. The tentative role of the considered systems in molecular electronics and spintronics is also noticed. [ABSTRACT FROM AUTHOR]

In this paper, we outline a physically motivated framework for describing spin-selective recombination processes in chiral systems, from which we derive spin-selective reaction operators for recombination reactions of donor–bridge–acceptor molecules, where the electron transfer is mediated by chirality and spin–orbit coupling. In general, the recombination process is selective only for spin-coherence between singlet and triplet states, and it is not, in general, selective for spin polarization. We find that spin polarization selectivity only arises in hopping-mediated electron transfer. We describe how this effective spin-polarization selectivity is a consequence of spin-polarization generated transiently in the intermediate state. The recombination process also augments the coherent spin dynamics of the charge separated state, which is found to have a significant effect on the recombination dynamics and to destroy any long-lived spin polarization. Although we only consider a simple donor–bridge–acceptor system, the framework we present here can be straightforwardly extended to describe spin-selective recombination processes in more complex systems. [ABSTRACT FROM AUTHOR]

Photoexcitable donor–bridge–acceptor (D–B–A) molecules that support intramolecular charge transfer are ideal platforms to probe the influence of chiral induced spin selectivity (CISS) in electron transfer and resulting radical pairs. In particular, the extent to which CISS influences spin polarization or spin coherence in the initial state of spin-correlated radical pairs following charge transfer through a chiral bridge remains an open question. Here, we introduce a quantum sensing scheme to measure directly the hypothesized spin polarization in radical pairs using shallow nitrogen–vacancy (NV) centers in diamond at the single- to few-molecule level. Importantly, we highlight the perturbative nature of the electron spin–spin dipolar coupling within the radical pair and demonstrate how Lee–Goldburg decoupling can preserve spin polarization in D–B–A molecules for enantioselective detection by a single NV center. The proposed measurements will provide fresh insight into spin selectivity in electron transfer reactions. [ABSTRACT FROM AUTHOR]

Spin polarization in the multiphoton above-threshold ionization of 5p3/2- and 5p1/2-electrons of Xe with intense 395nm, circularly polarized laser pulses, is investigated theoretically. For this purpose, we solve the time-dependent Schrödinger equation on the basis of spherical spinors. We, thus, simultaneously propagate the spin-up and spin-down single-active-electron wave packets, driven by the laser pulses in the ionic potential, which includes the spin–orbit interaction explicitly. The present theoretical results are in good agreement with the recent experimental results [D. Trabert et al., Phys. Rev. Lett. 120, 043202 (2018)]. [ABSTRACT FROM AUTHOR]

High spin-injection-efficiency (SIE) and thermal spin-filter-effect (SFE) from a magnetic material to a barrier material are crucial to the high performance of a spintronic device and a spin caloritronic device, respectively. By performing a nonequilibrium Green's function combined with first-principles calculations, we study the voltage-driven and temperature-driven spin transport properties of a half Heusler alloy RuCrAs based spin valve with different atom-terminated interfaces. The spin valve with a CrAs-top (or Ru-top) interface structure has an ultrahigh equilibrium magnetoresistance (MR) ratio of ∼1.56 × 109% (or ∼5.14 × 108%), ∼100% SIE, a large MR ratio, and high spin current intensity under bias voltage, suggesting that it has a great potential application in spintronic devices. The spin valve with the CrAs-top (or CrAs-bri) interface structure has a perfect SFE due to its very high spin polarization of temperature-driven currents, and it is useful in spin caloritronic devices. [ABSTRACT FROM AUTHOR]

The spintronic properties of cubic ZrO2 (c-ZrO2) nanosheets with intrinsic defects and transition metal (TM) elements doping have been systematically studied by first-principle calculation. The results show that impurity Fe has the lowest formation energy in each monolayer compared to other defects. The most stable (111) nanosheet, coupled with the higher defect formation energy, tends to disintegrate. Only Zr vacancy (VZr) on the (110) surface or O vacancy (VO) on the (111) surface can generate a ferromagnetic ground state, while other intrinsic defects cannot introduce spin polarization. Ni-doped (110) monolayer cannot introduce a local magnetic moment, while Fe and Co can. The magnetic moments produced by Fe, Co, and Ni in the (111) sheet are 2, 4, and 1 µB, respectively. Further investigation revealed that the magnetism was mainly contributed by the d orbitals of the TM atom and the p orbitals of the surrounding O atoms. Magnetic couplings show that only Co–Co doped monolayers can produce macroscopic magnetism. These are predicted to produce TCs Curie temperature above room temperature when Co–Co distances are 5.070 and 6.209 Å on the (110) surface and 7.170 and 9.485 Å on the (111) surface. The research is beneficial to the refinement of the development of spintronics. [ABSTRACT FROM AUTHOR]

Molecular spin systems based on photoexcited triplet pairs formed via singlet fission (SF) are attractive as carriers of quantum information because of their potentially pure and controllable spin polarization, but developing systems that offer optical routes to readout as well as initialization is challenging. Herein, we characterize the electron spin magnetic resonance change in the photoluminescence intensity for a tailored organic molecular crystal while sweeping a microwave drive up to 10 GHz in a broadband loop structure. We observe resonant transitions for both triplet and quintet spin sublevel populations showing their optical sensitivity and revealing the zero-field parameters for each. We map the evolution of these spectra in both microwave frequency and magnetic field, producing a pattern of optically detected magnetic resonance (ODMR) peaks. Fits to these data using a suitable model suggest significant spin polarization in this system with orientation selectivity. Unusual excitation intensity dependence is also observed, which inverts the sign of the ODMR signal for the triplet features, but not for the quintet. These observations demonstrate optical detection of the spin sublevel population dictated by SF and intermolecular geometry, and highlight anisotropic and multi-scale dynamics of triplet pairs. [ABSTRACT FROM AUTHOR]

Physical properties are commonly represented by tensors, such as optical susceptibilities. The conventional approach of deriving non-vanishing tensor elements of symmetric systems relies on the intuitive consideration of positive/negative sign flipping after symmetry operations, which could be tedious and prone to miscalculation. Here, we present a matrix-based approach that gives a physical picture centered on Neumann's principle. The principle states that symmetries in geometric systems are adopted by their physical properties. We mathematically apply the principle to the tensor expressions and show a procedure with clear physical intuition to derive non-vanishing tensor elements based on eigensystems. The validity of the approach is demonstrated by examples of commonly known second and third-order nonlinear susceptibilities of chiral/achiral surfaces, together with complicated scenarios involving symmetries such as D6 and Oh symmetries. We then further applied this method to higher-rank tensors that are useful for 2D and high-order spectroscopy. We also extended our approach to derive nonlinear tensor elements with magnetization, which is critical for measuring spin polarization on surfaces for quantum information technologies. A Mathematica code based on this generalized approach is included that can be applied to any symmetry and higher order nonlinear processes. [ABSTRACT FROM AUTHOR]

The two-spin solid effect (2SSE) is one of the established continuous wave dynamic nuclear polarization mechanisms that enables enhancement of nuclear magnetic resonance signals. It functions via a state-mixing mechanism that mediates the excitation of forbidden transitions in an electron–nuclear spin system. Specifically, microwave irradiation at frequencies ωμw ∼ ω0S ± ω0I, where ω0S and ω0I are electron and nuclear Larmor frequencies, respectively, yields enhanced nuclear spin polarization. Following the recent rediscovery of the three-spin solid effect (3SSE) [Tan et al., Sci. Adv. 5, eaax2743 (2019)], where the matching condition is given by ωμw = ω0S ± 2ω0I, we report here the first direct observation of the four-spin solid effect (4SSE) at ωμw = ω0S ± 3ω0I. The forbidden double- and quadruple-quantum transitions were observed in samples containing trityl radicals dispersed in a glycerol–water mixture at 0.35 T/15 MHz/9.8 GHz and 80 K. We present a derivation of the 4SSE effective Hamiltonian, matching conditions, and transition probabilities. Finally, we show that the experimental observations agree with the results from numerical simulations and analytical theory. [ABSTRACT FROM AUTHOR]

We investigate a spin-boson inspired model of electron transfer, where the diabatic coupling is given by a position-dependent phase, eiWx. We consider both equilibrium and nonequilibrium initial conditions. We show that, for this model, all equilibrium results are completely invariant to the sign of W (to infinite order). However, the nonequilibrium results do depend on the sign of W, suggesting that photo-induced electron transfer dynamics with spin–orbit coupling can exhibit electronic spin polarization (at least for some time). [ABSTRACT FROM AUTHOR]

The chiral molecules are potential generators of high spin-filters due to their inherent inversion asymmetric helical electric field. We report a controlled spin-selective transmission of electrons through self-assembled monolayers of 15 base-paired double-stranded deoxyribonucleic acid functionalized with two enantiomeric cysteine molecules on gold explored through the quantum mechanical tunneling effect. We observed a controlled spin polarization of 33% with dextro-cysteine, whereas a mere 8% was observed with levo-cysteine molecules using these functionalizations at room temperature. The manipulation of electron's spin merely through such a small molecule could lead to significant advancement in the spin-dependent charge transport phenomena and related applications. [ABSTRACT FROM AUTHOR]

We have developed a spin-polarized-hydrogen beam with a hexapole magnet. By combining the beam chopper and pulsed laser ionization detection, the time-of-flight of the hydrogen beam was measured, and the dependence of the beam profile on the velocity was acquired, which was consistent with the beam trajectory simulations. The spin polarization of the beam was analyzed by using the Stern–Gerlach-type magnet in combination with the spatial scan of the detection laser. The spin polarization was about 95% at a focusing condition due to the hexapole magnet. The polarization was, on the other hand, reduced to about 70% for the beam at higher velocities, which is consistent with simulation results. [ABSTRACT FROM AUTHOR]

Coupled pairs of nuclear spin-1/2 support one singlet state and three triplet states. Transitions between the singlet state and one of the triplet states may be driven by an oscillating low-frequency magnetic field, in the presence of couplings to a third nuclear spin, and a weak bias magnetic field. The oscillating field is in the same direction as the bias field and is called a WOLF (Weak Oscillating Low Field) pulse. Application of a WOLF pulse allows for the generation of strong nuclear hyperpolarization of 13C nuclei, starting from the nuclear singlet polarization of a 1H spin pair, associated with the enriched para-spin isomer of hydrogen gas. Hyperpolarization is demonstrated for two molecular systems. [ABSTRACT FROM AUTHOR]

A systematic structure and property investigation of MnGen− (n = 3–14) was conducted by means of density functional theory coupled with mass-selected anion photoelectron spectroscopy. This combined theoretical and experimental study allows global minimum and coexistence structures to be identified. It is found that the pentagonal bipyramid shape is the basic framework for the nascent growth process of MnGen− (n = 3–10), and from n = 10, the endohedral structures can be found. For n = 12, the anion MnGe12− cluster probably includes two isomers: a major isomer with a puckered hexagonal prism geometry and a minor isomer with a distorted icosahedron geometry. Specifically, the puckered hexagonal prism isomer follows the Wade–Mingos rules and can be suggested as a new kind of superatom with the magnetic property. Furthermore, the results of adaptive natural density partitioning and deformation density analyses suggest a polar covalent interaction between Ge and Mn for endohedral clusters of MnGe12−. The spin density and natural population analysis indicate that MnGen− clusters have high magnetic moments localized on Mn. The density of states diagram visually shows the significant spin polarization for endohedral structures and reveals the weak interaction between the Ge 4p orbital and the 4s, 3d orbitals of Mn. [ABSTRACT FROM AUTHOR]

Electronic structure methods emerging from the combination of multiconfigurational wave functions and density functional theory (DFT) aim to take advantage of the strengths of the two nearly antagonistic theories. One of the common strategies employed to merge wave function theory (WFT) with DFT relies on the range separation of the Coulomb operator in which DFT functionals take care of the short-distance part, while long-range inter-electronic interactions are evaluated by using the chosen wave function method (WFT–srDFT). In this work, we uncover the limitations of WFT–srDFT in the characterization of open-shell systems. We show that spin polarization effects have a major impact on the (short-range) DFT exchange energy and are of vital importance in order to provide a balanced description between closed and open-shell configurations. We introduce different strategies to account for spin polarization in the short range based on the definition of a spin polarized electron density and with the use of short-range exact exchange. We test the performance of these approaches in the dissociation of the hydrogen molecule, the calculation of energy gaps in spin-triplet atoms and molecular diradicals, and the characterization of low-lying states of the gallium dimer. Our results indicate that the use of short-range DFT correlation in combination with a (full-range) multiconfigurational wave function might be an excellent approach for the study of open-shell molecules and largely improves the performance of WFT and WFT–srDFT. [ABSTRACT FROM AUTHOR]

We report the implementation of a Fock-operator complete-active space self-consistent field (CAS-SCF) method combined with frozen-density embedding (FDE) into the KOALA quantum-chemistry program. The implementation is based on configuration interaction from an unrestricted reference determinant and is able to treat electronic configurations such as singlet, triplet, or quintet states embedded in a molecular environment. In order to account for possible spin polarization effects, the FDE contribution is extended to the unrestricted case. We assess the convergence obtained with the implementation at the example of a stretched lithium dimer with significant multi-reference character. The efficiency of the implementation enables the orbital optimization for 25 states in a state-average SA[S0–S10,T1–T12,Q1–Q2]-CAS(10,10)-SCF calculation for the retinal molecule using a def2-TZVP basis. The FDE ansatz leads to orbitals localized by definition on the target system, thus facilitating the orbital selection required for CAS methods in complex environments. [ABSTRACT FROM AUTHOR]

The mechanism of the magnetic compass sense of migratory songbirds is thought to involve magnetically sensitive chemical reactions of light-induced radical pairs in cryptochrome proteins located in the birds' eyes. However, it is not yet clear whether this mechanism would be sensitive enough to form the basis of a viable compass. In the present work, we report spin dynamics simulations of models of cryptochrome-based radical pairs to assess whether accumulation of nuclear spin polarization in multiple photocycles could lead to significant enhancements in the sensitivity with which the proteins respond to the direction of the geomagnetic field. Although buildup of nuclear polarization appears to offer sensitivity advantages in the more idealized model systems studied, we find that these enhancements do not carry over to conditions that more closely resemble the situation thought to exist in vivo. On the basis of these simulations, we conclude that buildup of nuclear polarization seems unlikely to be a source of significant improvements in the performance of cryptochrome-based radical pair magnetoreceptors. [ABSTRACT FROM AUTHOR]

Broken-symmetry calculations of diradicals exploit the mean-field energies of determinants that are not eigenfunctions of the Ŝ 2 operator, the mean value of which is close to 1 for the ms = 0 solution. This spin contamination must be corrected. Two different contributions affect ⟨ Ŝ 2 ⟩ , namely, the mixing between neutral and ionic valence bond components, the so-called kinetic exchange, which decreases ⟨ Ŝ 2 ⟩ , and the spin polarization of the supposedly closed shell orbitals, which increases ⟨ Ŝ 2 ⟩. The popular Yamaguchi formula is valid for the first effect but irrelevant for the second one. From a few constrained broken-symmetry calculations, one may treat separately the two contributions and apply their specific spin decontamination correction. This work proposes a consistent spin-decontaminated procedure for the evaluation of singlet–triplet gaps in diradicals. [ABSTRACT FROM AUTHOR]

A perylene (Pery)–phenoxazine (PXZ) compact orthogonal electron donor/acceptor dyad was prepared to study the relationship between the molecular structures and the spin–orbit charge transfer intersystem crossing (SOCT-ISC), as well as the electron spin selectivity of the ISC process. The geometry of Pery–PXZ (80.0°) is different from the previously reported perylene–phenothiazine dyad (Pery–PTZ, 91.5°), although there is only one atom variation for the two dyads. Pery–PXZ shows a high singlet oxygen quantum yield (84%). Femtosecond transient absorption spectra indicate that the charge separation (CS, faster than 120 fs) is faster than the Pery–PTZ analog (CS, 250 fs) and charge recombination (CR, i.e., SOCT-ISC, 5.98 ns) of Pery–PXZ is slower than the Pery–PTZ analog (CR, 0.9 ns). The intrinsic triplet state lifetime of Pery–PXZ is 242 µs vs the lifetime of 181 µs for the Pery–PTZ analog. Moreover, the triplet state lifetime of Pery–PXZ in the solid polymer matrix is extended to 4.45 ms, which indicates that the triplet state of Pery–PXZ in fluid solution is deactivated not only by the triplet–triplet annihilation effect but also by other factors such as vibration coupled relaxation. Interestingly, with pulsed laser excited time-resolved electron paramagnetic resonance spectroscopy, the electron spin polarization (ESP) pattern of the triplet state of the current dyad is opposite to that of Pery–PTZ. These results demonstrated the rich electron spin chemistry of the ISC of compact electron donor/acceptor dyads, e.g., the ESP is dependent on not only the molecular geometry but also the structure of the electron donor (or acceptor). [ABSTRACT FROM AUTHOR]

Intercalation offers a promising way to alter the physical properties of two-dimensional (2D) layered materials. Here, we investigate the electronic and vibrational properties of 2D layered MoSe2 intercalated with atomic manganese at ambient and high pressure up to 7 GPa by Raman scattering and electronic structure calculations. The behavior of optical phonons is studied experimentally with a diamond anvil cell and computationally through density functional theory calculations. Experiment and theory show excellent agreement in optical phonon behavior. The previously Raman inactive A2u mode is activated and enhanced with intercalation and pressure, and a new Raman mode appears upon decompression, indicating a possible onset of a localized structural transition, involving the bonding or trapping of the intercalant in 2D layered materials. Density functional theory calculations reveal a shift of the Fermi level into the conduction band and spin polarization in MnxMoSe2 that increases at low Mn concentrations and low pressure. Our results suggest that intercalation and pressurization of van der Waals materials may allow one to obtain dilute magnetic semiconductors with controllable properties, providing a viable route for the development of new materials for spintronic applications. [ABSTRACT FROM AUTHOR]

The effect of reversible energy hopping between different local environments on the properties of spin-polarized excited states is investigated theoretically using a two-site model. The kinetic equations for the populations of the spin sublevels of the excited state are derived and then used to obtain analytical expressions for the evolution of the spin polarization of excited triplet states under specific conditions. The time dependence of the triplet state polarization patterns is also obtained by numerical solution of the kinetic equations. It is shown that the reversible energy hopping can lead to significant changes in the properties of the triplet state, including changes in the shape of the observed spectrum and, in some cases, the inversion of the sign of the polarization, the generation of the net polarization, and anisotropic spin-lattice relaxation. The relations between the parameters that can be observed experimentally by time-resolved electron paramagnetic resonance spectroscopy and the kinetic and dynamic parameters of the system are discussed. [ABSTRACT FROM AUTHOR]

Spin-polarization effects may play an important role in free radicals and in the magnetic coupling between radical centers. Starting from restricted open-shell calculations, i.e., a closed-shell description of the non-magnetic core electrons, a low-order perturbation expansion identifies the spin-polarization contribution to the energy of mono-radicals and to singlet–triplet energy differences in diradicals. Broken-symmetry (BS) single-determinant calculations introduce only a fraction of spin-polarization effects, and in a biased manner, since BS determinants are not spin eigenfunctions. We propose a simple technique to correctly evaluate spin-polarization energies by taking into account the effect of spin-flip components on one-hole one-particle excited configurations. Spin-decontamination corrections are shown to play a non-negligible role in the BS evaluation of bond energies. The importance of spin decontamination is illustrated in cases for which spin polarization is the leading contribution to the singlet–triplet gap, which characterizes twisted conjugated double bonds and disjoint diradicals. [ABSTRACT FROM AUTHOR]

Taking as an example the simple CH3 radical, this work demonstrates the cooperative character of the spin-polarization phenomenon of the closed-shell core in free radicals. Spin polarization of CH σ bonds is not additive here, as spin polarization of one bond enhances that of the next bond. This cooperativity is demonstrated by a series of configuration interaction calculations converging to the full valence limit and is rationalized by analytic developments. The same phenomenon is shown to take place in those diradicals where spin polarization plays a major role, as illustrated in square planar carbo-cyclobutadiene C12H4. The treatment of cooperativity represents a challenge for usual post-Hatree–Fock methods. [ABSTRACT FROM AUTHOR]

The chirality-induced spin selectivity 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-principles 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 structure, such models can be considered a minimal setup for the description of spin-dependent effects. We use adiabatic perturbation theory to provide a mathematically sound approximation procedure applicable to a large class of spin-dependent continuum models. We take advantage of the simplicity of the models by analyzing its structure to gain a better understanding how the occurrence and magnitude of spin polarization effects relate to the model's parameters and geometry. The obtained spin polarization is not strongly dependent on the strength of the spin–orbit interaction, but it rather results from breaking time-reversal invariance upon selection of a given angular momentum of the incoming electrons. [ABSTRACT FROM AUTHOR]

We extend to strongly correlated molecular systems the recently introduced basis-set incompleteness correction based on density-functional theory (DFT) [E. Giner et al., J. Chem. Phys. 149, 194301 (2018)]. This basis-set correction relies on a mapping between wave-function calculations in a finite basis set and range-separated DFT (RSDFT) through the definition of an effective non-divergent interaction corresponding to the electron–electron Coulomb interaction projected in the finite basis set. This enables the use of RSDFT-type complementary density functionals to recover the dominant part of the short-range correlation effects missing in this finite basis set. To study both weak and strong correlation regimes, we consider the potential energy curves of the H10, N2, O2, and F2 molecules up to the dissociation limit, and we explore various approximations of complementary functionals fulfilling two key properties: spin-multiplet degeneracy (i.e., independence of the energy with respect to the spin projection Sz) and size consistency. Specifically, we investigate the dependence of the functional on different types of on-top pair densities and spin polarizations. The key result of this study is that the explicit dependence on the on-top pair density allows one to completely remove the dependence on any form of spin polarization without any significant loss of accuracy. Quantitatively, we show that the basis-set correction reaches chemical accuracy on atomization energies with triple-ζ quality basis sets for most of the systems studied here. In addition, the present basis-set incompleteness correction provides smooth potential energy curves along the whole range of internuclear distances. [ABSTRACT FROM AUTHOR]

The generation and transfer of electron spin polarization and coherence plays an important role in quantum information technologies and spintronics. In this context, the excited state spin dynamics of molecular systems in which a stable free radical is attached to a chromophore are of particular interest. In such complexes, the coupling between the electron spin on the free radical with those on the chromophore generates excited states referred to as sing-doublet, trip-doublet, and trip-quartet. Here, we study the light-induced electron spin polarization in an aluminum(iii) porphyrin (AlPor) complex in which the nitroxide (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) is covalently bound to the Al center via an ester linkage. In addition to the covalent bond to the TEMPO moiety, the Al center in AlPor also acts as a Lewis acid and can coordinate Lewis bases such as pyridine. Without pyridine bound, the spin polarized transient electron paramagnetic resonance spectrum of the complex at 80 K is unusual and displays a strong absorptive pattern with a Lorentzian lineshape. Coordination of pyridine to AlPor-TEMPO results in a dramatic change in the spin polarization pattern and a spectrum typical of the quartet state with broad wings from the ms = ±1/2 ↔ ±3/2 transitions with emissive/absorptive polarization and a narrow absorptive peak from the ms = ±1/2 ↔ ±1/2 transitions. At later times, the pattern evolves to a purely absorptive spectrum similar to that observed without pyridine. These changes are discussed in terms of a model in which back and forth transitions between the nearly degenerate lowest trip-doublet and trip-quartet states occur. It is argued that these transitions lead to the observed net polarization in AlPor-TEMPO and are fast enough that the outer lines are broadened. Density functional theory computations and the UV/Vis spectra suggest that the exchange interaction between TEMPO and the triplet state of AlPor increases when pyridine is bound, and the near degeneracy of the lowest excited states is lifted. It is argued that this slows the back and forth transitions which results in the strong change in the polarization pattern. [ABSTRACT FROM AUTHOR]

The light-induced electron spin polarization generated in the excited quartet and doublet states of a system consisting of a chromophore with an attached radical is investigated theoretically. Excitation of the chromophore and subsequent relaxation leads to a coupled triplet-doublet spin system. In many such systems, the electronic coupling between the triplet and doublet spins is expected to be strong enough to split the spin system into so-called trip-doublet and trip-quartet states but sufficiently weak that it does not promote significant mixing between the sing-doublet and trip-doublet states. In such moderately coupled systems, the sing-doublet can relax to the trip-doublet and trip-quartet states by spin-orbit coupling mediated intersystem crossing within the chromophore. An analytical expression is derived for the intensity of the polarization generated by this mechanism for the ms = +1/2 ↔ −1/2 electron paramagnetic resonance transitions of the trip-doublet and trip-quartet states. The expression shows that the intensity and sign of the polarization depend strongly on ratio jω = 3J/ω0 between the triplet-doublet exchange interaction J and the Zeeman energy ω0. The polarization becomes undefined when jω = 1 and when jω = 2 because level-anticrossings between the trip-doublet and trip-quartet sublevels occur. The sign of the polarization is also found to change above and below these values. Thus, for such moderately coupled systems, the sign of the polarization and its Zeeman energy dependence can be used to estimate the magnitude of the exchange coupling. [ABSTRACT FROM AUTHOR]

Generation of electron spin polarization (ESP) during the bimolecular quenching of an excited chromophore by a free radical is generally explained by the radical–triplet pair mechanism, which is capable of giving the magnitudes of ESP arising from the quenching of the singlet or the triplet excited chromophore. When the chromophore and the free radical are covalently linked, although there are several mechanisms to explain the observed spin-polarized electron paramagnetic resonance signals under a variety of experimental conditions and in different chromophore–radical systems, there are no schemes that allow quantitative determination of the magnitude of ESP. In this work, we present a phenomenological scheme with this objective. In this scheme, we have incorporated several concepts of the reversed quartet mechanism of Rozenshtein et al. [J. Phys. Chem. A 109, 11144 (2005)] to our phenomenological sequential quenching scheme [V. Rane and R. Das, J. Phys. Chem. A 119, 5515 (2015)] of ESP in covalently linked chromophore–radical systems. This phenomenological reversed quartet scheme is able to explain the observed inversion of ESP with time and can also give a quantitative measure of the absorptive and emissive ESP in such systems. We have applied this scheme to the photophysical quenching of a series of newly synthesized pyrene–TEMPO molecules, where a spacer group of different lengths covalently links the pyrene chromophore and the TEMPO free radical. Given the simplicity of our scheme, reasonable estimates of the magnitudes of the ESP have been obtained in all cases. [ABSTRACT FROM AUTHOR]

Spin polarization transfers among aliphatic 13C nuclei, especially 13Cα–13Cβ transfers, permit correlations of their nuclear magnetic resonance (NMR) frequencies that are essential for signal assignments in multidimensional solid-state NMR of proteins. We derive and demonstrate a new radio-frequency (RF) excitation sequence for homonuclear dipolar recoupling that enhances spin polarization transfers among aliphatic 13C nuclei at moderate magic-angle spinning (MAS) frequencies. The phase-optimized recoupling sequence with five π pulses per MAS rotation period (denoted as PR5) is derived initially from systematic numerical simulations in which only the RF phases are varied. Subsequent theoretical analysis by average Hamiltonian theory explains the favorable properties of numerically optimized phase schemes. The high efficiency of spin polarization transfers in simulations is preserved in experiments, in part because the RF field amplitude in PR5 is only 2.5 times the MAS frequency so that relatively low 1H decoupling powers are required. Experiments on a microcrystalline sample of the β1 immunoglobulin binding domain of protein G demonstrate an average enhancement factor of 1.6 for 13Cα → 13Cβ polarization transfers, compared to the standard 13C–13C spin-diffusion method, implying a two-fold time saving in relevant 2D and 3D experiments. [ABSTRACT FROM AUTHOR]

Two-dimensional SnSe2 has become more and more attractive due to the excellent electronic, optoelectronic, and thermoelectric properties. However, the study on magnetic properties is rare. Inspired by the recent experimental synthesis of SnSe2 monolayer and Ag-doped SnSe2 thin films, we use the first-principles calculations combined with the nonequilibrium Green's function method to investigate the structural, electronic, magnetic, and spin transport properties of an Ag-doped SnSe2 monolayer. It is found that the doped system exhibits half-metallic ferromagnetism with the energy gap of about 0.5 eV in the spin-down channel. The spin-polarized transport properties based on Ag-doped SnSe2 monolayers show an excellent spin filtering effect and a negative differential resistance effect under a bias voltage. Interestingly, under a temperature gradient, the spin Seebeck effect and the temperature-controlled reverse of spin polarization are also observed. These perfect spin transport properties can be understood from the calculated spin-polarized band structure and the spin-polarized transport spectrum. These studies indicate the potential spintronic and spin caloritronic applications for Ag-doped SnSe2 monolayer. [ABSTRACT FROM AUTHOR]

Internally contracted multireference coupled-cluster (icMRCC) theory is extended to the computation of first-order properties (expectation values). We use the previously defined Lagrange formulation of the energy functional to derive the required equations for the Lagrange multipliers and arrive at an expression for first-order properties according to the generalized Hellmann-Feynman theorem, analogous to single-reference coupled-cluster theory. The present formulation does not include orbital relaxation, but in line with previous experience in coupled-cluster theory, the single-excitation cluster operator can recover a significant portion of orbital relaxation. Further aspects of the theory that arise from the internal contraction approach are discussed. Using automated derivation techniques, we have implemented a pilot code for icMRCCSD and icMRCCSDT for testing the method numerically. We find good agreement with full configuration interaction for several properties of boron monohydride and dipole moment curves of hydrogen fluoride and chromium hydride. A particular focus is given to spin-dependent properties: The hyperfine coupling tensors of Σ and Π radicals have been computed and compared to experiment and previous computations. We discuss the problem of describing spin polarization with properly spin-adapted wavefunctions, which requires either including pseudo-triple excitations or employing sufficiently flexible reference functions. [ABSTRACT FROM AUTHOR]

Organic materials with mechanical flexibility, low cost, chemical engineering, and long spin lifetime attract considerable attention for building spintronic devices. Here, a C6H6/Co2MnSi(001) spinterface is investigated by first-principles calculations and spin-polarized scanning tunneling microscopy simulations. Several high symmetry adsorption sites are discussed, together with two possible surface terminations of Co2MnSi(001). An inversion of the spin polarization is induced near EF even in the case of an external electric field, indicating that C66 can act as a spin filter to exploit the spin injection efficiency in organic spintronic devices. Unlike previous studies on molecule/ferromagnet interfaces, this inversion is closely related to the electronic structure of the atoms in the subsurface layer of Co2MnSi according to the orbital symmetry analysis. Furthermore, the magnetic moment and magnetic anisotropic energy (MAE) in the outermost Co2MnSi layer are studied. Particularly, in the most stable configuration, the sign of MAE is inversed due to hybridization between C p and Co dz2 orbitals, which suggests that a greater modification on MAE can be achieved by the use of a highly chemically reactive organic molecule. These findings improve the study on the engineering of magnetic properties at molecule/ferromagnetic interfaces through a single π-conjugated organic molecule. [ABSTRACT FROM AUTHOR]

A method is presented for predicting one-particle energies for a molecule in a junction with one metal electrode, using density functional theory methods. In contrast to previous studies, in which restricted spin configurations were analyzed, we take spin polarization into account. Furthermore, in addition to junctions in which the molecule is weakly coupled, our method is also capable of describing junctions in which the molecule is chemisorbed to the metal contact. We implemented a fully self-consistent scissor operator to correct the highest occupied molecular orbital-lowest unoccupied molecular orbital gap in transport calculations for single molecule junctions. We present results for various systems and compare our results with those obtained by other groups. [ABSTRACT FROM AUTHOR]

We investigate the spin-polarized electron transport through a quantum ring whose arms are embedded by one protein-like single-helical molecule and one quantum dot, respectively. It is found that the inter-arm quantum interference leads to the enhancement of the spin polarization in this structure. Moreover, when local magnetic flux is applied through the ring, the spin polarization in the electron transport process, including the polarization strength and direction, can be further adjusted. Next in the finite-bias case, the spin polarization is also apparent and can be tuned by changing the magnetic flux or the dot level. This work provides a new scheme to manipulate the spin transport based on the single-helical molecule. [ABSTRACT FROM AUTHOR]

Using spin polarized density functional theory based calculations, combined with ab initio molecular dynamics simulation, we carry out a systematic investigation of the bimetallic Ni13-nAgn nanoclusters, for all compositions. This includes prediction of the geometry, mixing behavior, and electronic properties. Our study reveals a tendency towards the formation of a core-shell like structure, following the rule of putting Ni in a high coordination site and Ag in a low coordination site. Our calculations predict negative mixing energies for the entire composition range, indicating mixing to be favored for the bimetallic small sized Ni-Ag clusters, irrespective of the compositions. The magic composition with the highest stability is found for the NiAg12 alloy cluster. We investigate the microscopic origin of a core-shell like structure with negative mixing energy, in which the Ni-Ag inter-facial interaction is found to play a role. We also study the magnetic properties of the Ni-Ag alloy clusters. The Ni dominated magnetism consists of parallel alignment of Ni moments while the tiny moments on Ag align in anti-parallel to Ni moments. The hybridization with the Ag environment causes reduction of Ni moment. [ABSTRACT FROM AUTHOR]