Spin States Research Articles

Doublet Spin State Mediated Photoluminescence Upconversion in Organic Radical Donor-Triplet Acceptor Dyads.

Donor-acceptor dyads are promising materials for improving triplet-sensitized photon upconversion due to faster intramolecular energy transfer (ET), which unfortunately competes with charge transfer (CT) dynamics. To circumvent the issue associated with CT, we propose a novel purely organic donor-acceptor dyad, where the CT character is confined within the donor moiety. In this work, we report the synthesis and characterization of a stable organic radical donor-triplet acceptor dyad (TTM-Cz-Per) consisting of the acceptor perylene (Per) linked to the donor (4-N-carbazolyl-2,6-dichlorophenyl)-bis(2,4,6-trichlorophenyl)methyl radical (TTM-Cz). Upon red-excitation of TTM-Cz-Per, the doublet emission of the donor (TTM-Cz) is significantly quenched, and a recorded delayed emission centered at ca. 490 nm was attributed to the fluorescence emission from the Per acceptor. Time-resolved transient absorption spectroscopy suggests doublet-to-triplet energy transfer (DTET) dynamics from the donor to the acceptor as the time constant τ for the donor transient species decreases from 21.47 ns for the TTM-Cz sensitizer to 8.73 ns for TTM-Cz-Per dyad. This process is accompanied by the appearance of a long-lived component with τ = 97.06 ns, which we ascribe to the triplet transient of the acceptor Per. Furthermore, computational results indicate that the DTET is intramolecular as computed spin densities of the quartet state show unpaired electrons of ρ ≈ 1 on the TTM-Cz donor and of ρ ≈ 2 on the acceptor Per. The present study highlights the possibility to employ doublet chromophoric systems for light-harvesting and energy upconversion, which can be further tailored for several optoelectronic applications.

Relationship between Transition-Metal Hydride Bond Lengths and Stretching Wavenumbers.

Here it is demonstrated that there is a linear relationship between the terminal 3d metal hydride stretching wavenumber νMH and the metal hydride distance dMH reported to date: νMH ∼ (-1.05dMH + 3.35) × 103 cm-1. That is, upon moving from Group 3 to Group 10 metals in complexes and binary molecules, νMH (cm-1) increases as the metal-hydrogen distance dMH (Å) decreases as determined by X-ray diffraction, molecular spectroscopy, and, in one case, neutron diffraction. This trend contrasts with the relatively constant bond dissociation free energy (BDFE) of the diamagnetic complexes and the usually smaller BDFE (by as much as -20 kcal/mol) of paramagnetic ones, thus showing that there is no correlation between either νMH or its force constant with the BDFE. If a positive charge is added to the complex by metal-ion oxidation, νMH may increase or decrease depending on the coordination number and spin state of the complex.

Reversibly Modulating the Selectivity of Carbon Dioxide Reduction via Ligand-Driven Spin Crossover.

Selectivity is an essential aspect in catalysis. At present, the improvement of the selectivity for complex reactions with multiple pathways/products, for example the carbon dioxide reduction reaction (CO2RR), can usually be achieved for only one pathway/product. It is still a challenge to reversibly modulate the selectivity between two reaction pathways or products of the CO2RR by one catalyst. Here, we propose the reversible modulation of selectivity between two products via spin crossover. By employing first-principles calculations, six spin crossover molecular catalysts are found among 17 kinds of transition metal embedded porphyrin derivatives (ppy_TM), where the changes in axial ligand configurations can reversibly switch the spin state of catalysts between high spin and low spin. For ppy_Os and ppy_Ru, the alteration in spin state can effectively influence the reduction of CO2 into either formic acid or carbon monoxide by changing the relative stability of the key intermediates *COOH and *HCOO.

Bloch sphere representation for Rabi oscillation driven by Rashba field in the two-dimensional harmonic confinement system

Abstract We studied the dynamical properties of Rabi oscillations driven by an alternating Rashba field applied to a two-dimensional (2D) harmonic confinement system. We solve the time-dependent (TD) Schrödinger equation numerically and rewrite the resulting TD wavefunction onto the Bloch sphere (BS) using two BS parameters of the zenith (θB) and azimuthal (φB) angles, extracting the phase information φB as well as the mixing ratio θB between the two BS-pole states.

We employed a two-state rotating wave (TSRW) approach and studied the fundamental features of θB and φB over time. The TSRW approach reveals a triangular wave formation in θB. Moreover, at each apex of the triangular wave, the TD wavefunction passes through the BS pole, and the state is completely replaced by the opposite spin state. The TSRW approach also elucidates a linear change in φB. The slope of φB vs. time is equal to the difference between the dynamical terms, leading to a confinement potential in the harmonic system. The TSRW approach further demonstrates a jump in the phase difference by π when the wavefunction passes through the BS pole.

The alternating Rashba field causes multiple successive Rabi transitions in the 2D harmonic system. We then introduce the effective BS and transform these complicated transitions into an equivalent "single" Rabi one. Consequently, the effective BS parameters θB eff and φB eff exhibit mixing and phase difference between two spin states α and β, leading to a deep understanding of the TD features of multi-Rabi oscillations. Furthermore, the combination of the BS representation with the TSRW approach successfully reveals the dynamical properties of the Rabi oscillation, even beyond the TSRW approximation.

First evidencing of guest‐induced spin transition for dinuclear Fe(III) complex supported by calix[4]arene Schiff base ligand

For the first time the partial low spin (LS) ‐ high spin (HS) state transition back, caused by trapped guest molecule release from the macrocyclic cavity of calix[4]arene ligand, decorated with salicylideneamine coordinating binding sites, was evidenced for new dinuclear [FeIII2] diamond core cluster, displaying mixed cis‐/trans‐N2O4 coordination environment for metal centers. Such unique spin state transition behavior, triggered by the changing of calix[4]arene backbone conformation, giving rise to a new paradigm of fine‐tuning spin‐dependent functionality in materials design, was unambiguously established by the 57Fe Mössbauer spectroscopy in combination with the single crystal X‐ray diffraction, IR, TGA‐DSC analysis.

Replica symmetry breaking in spin glasses in the replica-free Keldysh formalism

We show that the algebra of Parisi ultrametric matrices is recovered by the real-time, replica-free, Dyson-Keldysh equations of infinite-range quantum spin glasses in the late time glassy limit. This connects to earlier results on classical and quantum systems showing how ultrametricity emerges from the persistent slow aging dynamics of the glass phase. The stationary spin glass state thereby spontaneously breaks thermal symmetry, or the Kubo-Martin-Schwinger relation of a state in global thermal equilibrium. We describe the Keldysh path integral of the infinite-range Ising model in transverse and longitudinal fields, and in the context of the Landau expansion of the action functional, show how the long-time limit connects to the full replica symmetry breaking obtained in the equilibrium formalism. We also illustrate our formalism by applying it to the spherical quantum pp-spin model, which only exhibits one-step replica symmetry breaking.

Modulated Fe Central Spin State in FeCu4 Alloy Nanoparticles for Efficient and Selective Activation of H2O2

AbstractIn advanced oxidation processes, the efficient activation of H2O2 and the selective production of reactive oxygen species are the key steps to achieve the removal of organic pollutants in the water environment. Studies have shown that the precise regulation of catalyst metal active sites can break through the limitation of H2O2 activation and realize the efficient selective conversion of H2O2. Therefore, FeCu bimetallic active sites are developed by constructing FeCu4 alloy nanocluster metal catalyst (FeCu/NC). The introduction of Cu metal sites realized charge redistribution, promoted rapid electron transfer, reduced the spin state of Fe in the catalyst, diminished the interaction between Fe and O, weakened the strong and persistent adsorption between Fe center and *H2O2, lowered the energy barrier of intermediate products in hydroxyl radicals (·OH) generation process, achieved efficient activation of H2O2 and selective generation of ·OH. In addition, a complete electron circulation path is formed between FeCu active sites and the substrate, breaking the contradiction between single metal and H2O2 redox relationship, promoting efficient reduction of Fe3+ to Fe2+. The first‐order kinetic constant (kobs) of FeCu/NC is 0.081 min−1, which is 3.68 and 7.36 times higher than those of single Fe metal species (FeFe/NC) and single Cu metal species (CuCu/NC) catalysts, respectively, and has excellent catalytic performance).

Modulation in Spin State of Co3O4 Decorated Fe Single Atom Enables a Superior Rechargeable Zinc-Air Battery Performance.

High-performance bifunctional electrocatalyst for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is the keystone for the industrialization of rechargeable zinc-air battery (ZAB). In this work, the modulation in the spin state of Fe single atom on nitrogen doped carbon(Fe1-NC) is devised by Co3O4 (Co3O4@Fe1-NC), and a mediate spin state is recorded. Besides, the d band center of Fe is downshifted associated with the increment in eg filling revealing the weakened interaction with OH* moiety, resulting in a boosted ORR performance. The ORR kinetic current density of Co3O4@Fe1-NC is 2.0- and 5.6 times higher than Fe1-NC and commercial Pt/C, respectively. Moreover, high spin state is found for Co in Co3O4@Fe1-NC contributing to the accelerated surface reconstruction of Co3O4 witnessed by operando Raman and electrochemical impedance spectroscopies. A robust OER activity with overpotential of 352mV at 50mA cm-2 is achieved, decreased by 18 and 60mV by comparison with Co3O4@NC and IrO2. The operando Raman reveals a balanced adsorption of OH* species and its deprotonation leading to robust stability. The ZAB performance of Co3O4@Fe1-NC is 193.2mW cm-2 and maintains for 200h. Furthermore, the all-solid-state ZAB shows a promising battery performance of 163.1mW cm-2.

Polytopes of Absolutely Wigner Bounded Spin States

Quasiprobability has become an increasingly popular notion for characterising non-classicality in quantum information, thermodynamics, and metrology. Two important distributions with non-positive quasiprobability are the Wigner function and the Glauber-Sudarshan function. Here we study properties of the spin Wigner function for finite-dimensional quantum systems and draw comparisons with its infinite-dimensional analog, focusing in particular on the relation to the Glauber-Sudarshan function and the existence of absolutely Wigner-bounded states. More precisely, we investigate unitary orbits of mixed spin states that are characterized by Wigner functions lower-bounded by a specified value. To this end, we extend a characterization of the set of absolutely Wigner positive states as a set of linear eigenvalue constraints, which together define a polytope centred on the maximally mixed state in the simplex of spin-j states. The lower bound determines the relative size of such absolutely Wigner bounded (AWB) polytopes and we study their geometric characteristics. In each dimension a Hilbert-Schmidt ball representing a tight purity-based sufficient condition to be AWB is exactly determined, while another ball representing a necessary condition to be AWB is conjectured. Special attention is given to the case where the polytope separates orbits containing only positive Wigner functions from other orbits because of the use of Wigner negativity as a witness of non-classicality. Comparisons are made to absolute symmetric state separability and spin Glauber-Sudarshan positivity, with additional details given for low spin quantum numbers.

Spin-symmetry-enforced solution of the many-body Schrödinger equation with a deep neural network.

The integration of deep neural networks with the variational Monte Carlo (VMC) method has marked a substantial advancement in solving the Schrödinger equation. In this work we enforce spin symmetry in the neural-network-based VMC calculation using a modified optimization target. Our method is designed to solve for the ground state and multiple excited states with target spin symmetry at a low computational cost. It predicts accurate energies while maintaining the correct symmetry in strongly correlated systems, even in cases in which different spin states are nearly degenerate. Our approach also excels at spin-gap calculations, including the singlet-triplet gap in biradical systems, which is of high interest in photochemistry. Overall, this work establishes a robust framework for efficiently calculating various quantum states with specific spin symmetry in correlated systems.

Helical magnetism in poly(aniline-co-ferrocene): structure and magnetism.

This study reports the synthesis and characterisation of a ferrocene-based conjugated polymer and a chiral composite. The precursor copolymer was synthesised from 1,3-phenylenediamine and 1,1'-dibromoferrocene via Buchwald-Hartwig polycondensation. This polymerisation process increased the effective conjugation length and led to magnetic spin interactions along the main chain, resulting in a ground triplet spin state at 25 °C. Furthermore, the polymer was oxidised with m-chloroperoxybenzoic acid and doped with sulfuric acid. A chiral polymer blend was also prepared by combining the copolymer with hydroxypropyl cellulose to form helical polymer liquid crystals. The physical properties of these polymers were analysed using superconducting quantum interference devices, optical spectroscopy, electron spin resonance, and X-ray fluorescence spectroscopy. The resulting polymer blend exhibits helical magnetism. This approach to fabricating a chiral magnetic material from an achiral polymer via supramolecular chirality introduces a new method for preparing chiral magnets.

Spin-blockade and state lifetimes of many-hole spin states in silicon quantum dots

Abstract Hole spins in silicon quantum dots (QDs) are a promising candidate for fault-tolerant quantum computing. To achieve high-fidelity readout and high coherence required for fault tolerance, the evaluation of spin-blockade and state lifetimes is important because they can limit the qubit fidelities. In this study, we characterize these two figures of merit of manyhole qubits in silicon QDs. We report a spin-blockade lifetime of 5.1 µs, which is comparable with the values reported previously, and a spin-state lifetime of 0.91 ms, which is longer than the one measured in a few-hole silicon QD in a previous study. We furthermore provide insights into a many-hole spin state based on the estimated tunneling rate ratio.

Spin-state switching of indium-phthalocyanine on Pb(100).

Indium(iii) phthalocyanine chloride deposited on Pb(100) is studied by scanning tunnelling spectroscopy at cryogenic temperatures. The Cl ions are dissociated and the remaining indium phthalocyanine (InPc) is observed in two states with the metal ion pointing to (↓) or away (↑) from the substrate. Isolated molecules and islands with a superstructure and a unit cell of four inequivalent molecules, namely one InPc↑ and three InPc↓ in different sites, are observed. Using atomic resolution images of the substrate the adsorption sites and azimuthal orientation of InPc are determined and a structure model is proposed. Conductance spectra of the lowest unoccupied molecular orbital reveal differences that depend on the adsorption sites and azimuthal orientations of the complexes. Only InPc↑ molecules exhibit Shiba states, indicating the presence of a localized spin. By electron extraction isolated complexes as well as molecules in islands are converted from InPc↑ to InPc↓. At the same time, their spin state changes, as indicated by the disappearance of the Shiba states.

Difference in Spin Structure of Magnetic Metal-Organic Frameworks between Cluster Model and Periodic System

Abstract We compare the calculated spin structures of magnetic metal-organic frameworks (MOFs) consisting of paddlewheel-type diruthenium complexes and tetracyano-p-quinodimethane derivatives using density functional theory calculations with Gaussian basis sets and DFT+U/plane-wave calculations under periodic conditions. The calculated results indicate that the asymmetricity of the spin density inside the [Ru2] unit disappears in the periodic systems. In addition, based on the partial spin density analysis, the electron conductivity would be switchable by changing the spin states of the MOFs.

Design of spin-orbit controlled plasmonic circuits for selectively transporting photons

Abstract The manipulation of spin-related photons is crucial for developing next-generation integrated circuits. However, the spin-orbit interaction of light is typically very weak at nanoscale due to the diffraction limit. Here, a dielectric-loaded plasmonic nanocircuit is proposed, utilizing a chiral cylindrical structure, to directionally couple and guide plasmonic waves through dual-branch waveguides. Optimized results reveal that the waveguide modes excited by circularly polarized light with different spin states can be selectively coupled to one of the two targeted branch waveguides, with a calculated unidirectional coupling efficiency as high as 0.95. The proposed structure could provide avenues for realizing on-chip integrated nanocircuits and developing advanced multifunctional nanocircuits with enhanced degrees of freedom.

Modulating the electron spin states of Na2FePO4F cathode via high entropy strategy for enhanced sodium storage and ultra-high cycling stability

Modulating the electron spin states of Na2FePO4F cathode via high entropy strategy for enhanced sodium storage and ultra-high cycling stability

Spin state of manganese ions in double manganite Nd0.9Sm0.1BaMn2O6 by X-ray photoemission and X-ray emission spectroscopy

Spin state of manganese ions in double manganite Nd0.9Sm0.1BaMn2O6 by X-ray photoemission and X-ray emission spectroscopy

The limits of copper oxidation states from density functional theory computations: Fluoro-copper complexes, [CuFn]x, where n = 1 through 6 and x = 3+ through 5−

Density functional theory calculations, at the ωB97X-D/6-311+G* level of theory, were performed on homoleptic fluoro-copper complexes [CuFn]x, n = 1 through 6 and x = 3+ through 5−, to determine the highest positive and lowest negative copper oxidation states that can be supported in these complexes. Only singlet and doublet spin states were investigated. All fluoro-copper stoichiometries stabilized copper(III) or greater. However, some stoichiometries stabilized oxidation states up to copper(VI), and the greatest positive copper oxidation state was copper(VIII) in the distorted octahedral [CuF6]2+ cation. Oxidation states as negative as copper(–IV) in the diatomic [CuF]5− anion and copper(–III) in the triatomic [CuF2]5− were also observed as optimized minima, although no negative oxidation states were calculated to exist for fluoro-copper complexes containing more than two fluorine atoms. No singlet or doublet fluoro-copper complexes with charges more positive than 3+, more negative than 5−, or of Cu(VII), could be optimized.

Electron spin states embedded in localized states for opening spin levels effect in silicon nanowires doped with impurities

Electron spin states embedded in localized states for opening spin levels effect in silicon nanowires doped with impurities

Magnetic-Electrical Synergetic Control of Non-Volatile States in Bilayer Graphene-CrOCl Heterostructures.

Anti-ferromagnetic insulator chromium oxychloride (CrOCl) has shown peculiar charge transfer and correlation-enhanced emerging properties when interfaced with other van der Waals conductive channels. However, the influence of its spin states to the channel material remains largely unknown. Here, this issue is addressed by directly measuring the density of states in bilayer graphene (BLG) interfaced with CrOCl via a high-precision capacitance measurement technique and a surprising hysteretic behavior in the charging states of the heterostructure is observed. Such hysteretic behavior depends only on the history of magnetization, but not on the history of electrical gating; it can also be turned off electrically, providing a synergetic control of these non-volatile states. First-principles calculations attribute this observation to magnetic field-controlled charge transfer between BLG and CrOCl during the phase transition of CrOCl from antiferromagnetic (AFM) to ferrimagnetic-like (FiM) states. This magnetic-electrical synergetic control mechanism broadens the scope of proximity effects and opens new possibilities for the design of advanced 2D heterostructures and devices.