Spin Conservation Research Articles

Dynamics of hydrogen shift reactions between peroxy radicals.

Peroxy radicals are key intermediates in many atmospheric processes. Reactions between such radicals are of particular interest as they can lead to accretion products capable of participating in new particle formation (NPF). These reactions proceed through a tetroxide intermediate, which then decomposes to a complex of two alkoxy radicals and O2, with spin conservation dictating that the complex must be formed in the triplet state. The alkoxy complex can follow different pathways e.g. hydrogen(H)-shift reactions, dissociation reactions etc., but the details of the full processes are not yet fully understood. This paper establishes the microscopic mechanisms of the H-shift and other associated pathways in the context of a self-reaction between methoxy radicals, with focus on the roles of the singlet and triplet states involved. Dynamics in time is explored by two methods: the multireference XMS-CASPT2 and very recently developed mixed reference spin-flip TDDFT (MRSF-TDDFT). The metadynamics method is used to compute energetics. The XMS-CASPT2 and the MRSF-TDDFT dynamics simulations yield similar results. This would be very encouraging for future simulations for large radicals, since MRSF-TDDFT simulations enjoy the advantages of linear response theory. Our calculations demonstrate that the reaction between methoxy radicals, though initiated on the triplet state, leads to products predominantly on the singlet surface, following efficient intersystem crossing (ISC). The computed branching ratio between H-shift and dissociation channels agrees well with experiment.

Magnetization-current-induced E1 transition in the d+t→α+n+γ reaction

The d−t collisions have a minor reaction branch, of the order of 10−5, involving γ emission from the incident state with the spin of 3/2+ to the final 3/2− and 1/2− states of the α−n system. Standard E1 treatment of such a transition results in zero cross sections because of the spin conservation rule. For this reason, the first theoretical work on d+t→α+n+γ [], described the E1 transition as coming from the high-energy d-wave spin-1/2 state of the α−n partition of He5, coupled to the d−t component by tensor interaction. However, the exact E1 operator contains a contribution from the usually neglected magnetization current, represented by the nucleon spin operators, which can allow the E1 d+t→α+n+γ transition to occur without passing via the d-wave spin-1/2 α−n channel. The present paper studies the magnetization-current-induced E1 transition using several potential models of the d−t interaction. It was found that the strength of this transition depends strongly on the choice of the d−t interaction model. With the optical folding potential, correctly capturing the deuteron extended structure, this contribution is suppressed by three orders of magnitude, thus validating its neglect in the previous work. Published by the American Physical Society 2024

Double Quantum Spin Hall Phase in Moiré WSe2.

Quantum spin Hall (QSH) insulators are topologically protected phases of matter in two dimensions that can support a pair of helical edge states surrounding an insulating bulk. A higher (even) number of helical edge state pairs is usually not possible in real materials because spin mixing would gap out the edge states. Here, we report experimental evidence for a QSH phase with one and two pairs of helical edge states in twisted bilayer WSe2 at Moiré hole filling factor ν = 2 and 4, respectively. We observe nearly quantized (within 10%) resistance plateaus of and large nonlocal transport at ν = 2 and 4 while the bulk is insulating. The resistance is independent of the out-of-plane magnetic field and increases under an in-plane magnetic field. The results agree with quantum transport of helical edge states in a material with high spin Chern bands protected by Ising spin conservation symmetry.

Discrete Laplacian thermostat for flocks and swarms: the fully conserved Inertial Spin Model

Abstract Experiments on bird flocks and midge swarms reveal that these natural systems are well described by an active theory in which conservation laws play a crucial role. By building a symplectic structure that couples the particles’ velocities to the generator of their internal rotations (spin), the Inertial Spin Model (ISM) reinstates a second-order temporal dynamics that captures many phenomenological traits of flocks and swarms. The reversible structure of the ISM predicts that the total spin is a constant of motion, the central conservation law responsible for all the novel dynamical features of the model. However, fluctuations and dissipation introduced in the original model to make it relax, violate the spin conservation law, so that the ISM aligns with the biophysical phenomenology only within finite-size regimes, beyond which the overdamped dynamics characteristic of the Vicsek model takes over. Here, we introduce a novel version of the ISM, in which the irreversible terms needed to relax the dynamics strictly respect the conservation of the spin. We perform a numerical investigation of the fully conservative model, exploring both the fixed-network case, which belongs to the equilibrium class of Model G, and the active case, characterized by self-propulsion of the agents and an out-of-equilibrium reshuffling of the underlying interaction network. Our simulations not only capture the correct spin wave phenomenology of the ordered phase, but they also yield dynamical critical exponents in the near-ordering phase that agree very well with the theoretical predictions.

Spin States Matter─from Fundamentals toward Synthetic Methodology Development and Drug Discovery.

ConspectusThe potent reactivity of carbenes and nitrenes has been traditionally harnessed by the employment of a transition-metal catalyst in which the reactivity of the metal carbene/nitrene intermediates can be controlled via the judicious tuning of the metal catalyst. In recent years, progress made in this research area has unveiled novel strategies to directly access free carbenes or nitrenes under visible-light-mediated conditions without the necessity of a metal catalyst for stabilization of the carbene/nitrene intermediate. Such photochemical approaches present new opportunities to leverage orthogonal reactions with classic metal-catalyzed transformations.In this Account, we describe the major contributions from our group over the past years pushing the boundaries of light-mediated carbene and nitrene transfer reactions. In the first section, the development from purely singlet carbene chemistry toward methods that allow access to triplet carbene intermediates will be dissected. We describe how the triplet spin state of reagents provides a rich array of novel synthetic methods that build on the fundamentals of spin conservation. We lay out the different strategies in accessing the triplet spin state of carbenes (i.e., via electronic stabilization, via triplet sensitization with suitable photocatalysts, or via exploitation of geometric features of these intermediates), followed by an analysis of how the triplet spin state can be employed to leverage reactions distinct to the classic singlet carbene chemistry.The second part focuses on free nitrene intermediates, whereby both photochemical and photocatalytic strategies are analyzed and compared. We initiate with a discussion of the reactivity of iminoiodinanes as nitrene precursors in the presence of a photocatalyst or under photochemical conditions and how these two approaches result in fundamentally distinct nitrogen-based intermediates. While a nitrene radical anion is formed under photocatalytic conditions, triplet nitrene is generated under photochemical conditions. We commence with an outline of the basic reactivity of nitrene transfer reactions under both conditions, with a focus on the reaction with substrates containing double bonds. Finally, the latest developments in advanced cycloaddition chemistry beyond classic aziridination reactions are examined, with a special emphasis on the relay of the triplet nitrene reactivity to enable a Pauson-Khand-like (2 + 2 + 1) cycloaddition reaction that offers convenient access to high value bioisosteres in drug discovery.The work from our group on spin-dependent reactivities offers insight into important fundamentals in synthesis, where the spin state of the reactive intermediate will dictate the reaction outcome. We hope this may inspire others to widen the scope of applications of light-mediated carbene and/or nitrene transfer reactions, and furthermore, we anticipate that these understandings may also enable the development of advanced catalytic systems featuring triplet metal carbene/nitrene intermediates.

Modeling the γ spectrum from d−t collisions

Motivated by possible industrial fusion applications of spectroscopy of the γ rays accompanying d−t collisions we develop the first model calculations of the minor branching ratio of the d+t reaction, d+t→α+n+γ. The model exploits the most relevant physics feature—spin conservation in electric dipole transitions—which leads to a peculiar mechanism of this reaction: γ emission via bremsstrahlung from an intermediate α−n state. We highlight that, as a consequence of the bremsstrahlung, the γ spectrum contains nonzero contributions at all energies, thus making inclusive dtγ cross section measurements sensitive to the low-energy cutoff of the detected γ events. Comparison of our predictions to existing d+t→α+n+γ measurements in accelerators, employing cutoffs of 13 and 14 MeV, and inertial confinement fusion facilities, with a low-limit cutoff of 0.4 to 10 MeV, suggests a possible contradiction between results from these two types of experiments. Our predictions agree well with accelerator measurements and corroborate the cutoff dependence observed in inertial confinement experiments. These predictions are sensitive to the wave function details inside the short-range area of the α−n interaction, with uncertainty comparable to that of available experimental data, but become model independent below 4–5 MeV. This part of the γ spectrum features a previously unexpected rise, which below 0.5 MeV surpasses the main 17-MeV γ peak in strength. The reactivity of the d+t→α+n+γ branch was found to be proportional to its cross section. It strongly depends on the d−t plasma temperature, which opens the possibility of not only total d+t reactivity measurements but also advanced plasma temperature diagnostics. Published by the American Physical Society 2024

Strong decays of Pcs(4338) and its high isospin cousin via QCD sum rules

In the present work, the strong decays of the newly observed Pcs(4338) as well as its high isospin cousin Pcs(4460) are studied via the QCD sum rules. According to conservation of isospin, spin, and parity, the hadronic coupling constants in four decay channels are obtained; then, the partial decay widths are obtained. The total width of the Pcs(4338) coincides with the experimental data nicely, while the predictions for the Pcs(4460) can be testified in the future experiment and shed light on the nature of the Pcs(4338). Published by the American Physical Society 2024

Iron‐Catalyzed Allylic C(sp3)−H Silylation: Spin‐Crossover‐Efficiency‐Determined Chemoselectivity

AbstractThe nuanced role of spin effects remains a critical gap in designing proficient open‐shell catalysts. This study elucidates an iron‐catalyzed allylic C(sp3)−H silylation/alkyne hydrosilylation reaction, in which the spin state of the open‐shell iron catalyst dictates the reaction kinetics and pathway. Specifically, spin crossover led to alkyne hydrosilylation, whereas spin conservation resulted in a novel allylic C(sp3)−H silylation reaction. This chemoselectivity, governed by the spin‐crossover efficiency, reveals an unexpected dimension in spin effects and a first in the realm of transition‐metal‐catalyzed in situ silylation of allylic C(sp3)−H bonds, which had been previously inhibited by the heightened reactivity of alkenes in hydrosilylation reactions. Furthermore, this spin crossover can either accelerate or hinder the reaction at different stages within a single catalytic reaction, a phenomenon scarcely documented. Moreover, we identify a substrate‐assisted C−H activation mechanism, a departure from known ligand‐assisted processes, offering a fresh perspective on C−H activation strategies.

Iron-Catalyzed Allylic C(sp3)-H Silylation: Spin-Crossover-Efficiency-Determined Chemoselectivity.

The nuanced role of spin effects remains a critical gap in designing proficient open-shell catalysts. This study elucidates an iron-catalyzed allylic C(sp3)-H silylation/alkyne hydrosilylation reaction, in which the spin state of the open-shell iron catalyst dictates the reaction kinetics and pathway. Specifically, spin crossover led to alkyne hydrosilylation, whereas spin conservation resulted in a novel allylic C(sp3)-H silylation reaction. This chemoselectivity, governed by the spin-crossover efficiency, reveals an unexpected dimension in spin effects and a first in the realm of transition-metal-catalyzed in situ silylation of allylic C(sp3)-H bonds, which had been previously inhibited by the heightened reactivity of alkenes in hydrosilylation reactions. Furthermore, this spin crossover can either accelerate or hinder the reaction at different stages within a single catalytic reaction, a phenomenon scarcely documented. Moreover, we identify a substrate-assisted C-H activation mechanism, a departure from known ligand-assisted processes, offering a fresh perspective on C-H activation strategies.

Emergent Inflation of the Efimov Spectrum under Three-Body Spin-Exchange Interactions.

We resolve the unexpected and long-standing disagreement between experiment and theory in the Efimovian three-body spectrum of ^{7}Li, commonly referred to as the lithium few-body puzzle. Our results show that the discrepancy arises out of the presence of strong nonuniversal three-body spin-exchange interactions, which enact an effective inflation of the universal Efimov spectrum. This conclusion is obtained from a thorough numerical solution of the quantum mechanical three-body problem, including precise interatomic interactions and all spin degrees of freedom for three alkali-metal atoms. Our results show excellent agreement with the experimental data regarding both the Efimov spectrum and the absolute rate constants of three-body recombination, and in addition reveal a general product propensity for such triatomic reactions in the Paschen-Back regime, stemming from Wigner's spin conservation rule.

Conventional and Unconventional Dicke Models: Multistabilities and Nonequilibrium Dynamics.

The Dicke model describes the collective behavior of a subwavelength-size ensemble of two-level atoms (i.e., spin-1/2) interacting identically with a single quantized radiation field of a cavity. Across a critical coupling strength it exhibits a zero-temperature phase transition from the normal state to the superradiant phase where the field is populated and the collective spin acquires a nonzero x component, which can be imagined as ferromagnetic ordering of the atomic spins along x. Here we introduce a variant of this model where two subwavelength-size ensembles of spins interact with a single quantized radiation field with different strengths. Subsequently, we restrict ourselves to a special case where the coupling strengths are opposite (which is unitarily equivalent to equal-coupling strengths). Because of the conservation of the total spin in each ensemble individually, the system supports two distinct superradiant states with x-ferromagnetic and x-ferrimagnetic spin ordering, coexisting with each other in a large parameter regime. The stability and dynamics of the system in the thermodynamic limit are examined using a semiclassical approach, which predicts nonstationary behaviors due to the multistabilities. At the end, we also perform small-scale full quantum-mechanical calculations, with results consistent with the semiclassical ones.

F-Block reactions of metal cations with carbon dioxide studied by inductively coupled plasma tandem mass spectrometry.

f-Block chemistry offers an opportunity to test current knowledge of chemical reactivity. The energy dependence of lanthanide cation (Ln+ = Ce+, Pr+, Nd+-Eu+) and actinide cation (An+ = Th+, U+-Am+) oxidation reactions by CO2, was observed by inductively coupled plasma tandem mass spectrometry. This reaction is commonly spin-unallowed because the neutral reactant (CO2, 1Σ+g) and product (CO, 1Σ+) require the metal and metal oxide cations to have the same spin state. Correlation of the promotion energy (Ep) to the first state with two free d-electrons with the reaction efficiency indicates that spin conservation is not a primary factor in the reaction rate. The Ep likely influences the reaction rate by partially setting the crossing between the ground and reactive states. Comparison of Ln+ and An+ congener reactivity indicates that the 5f-orbitals play a small role in the An+ reactions.

Oxygen Hole Formation Controls Stability in LiNiO2 Cathodes: DFT Studies of Oxygen Loss and Singlet Oxygen Formation in Li-Ion Batteries

Ni-rich cathode materials achieve both high voltages and capacities in Li-ion batteries but are prone to structural instabilities and oxygen loss via the formation of singlet oxygen. Using ab initio molecular dynamics simulations, we observe spontaneous O2 loss from the (012) surface of delithiated LiNiO2, singlet oxygen forming in the process. We find that the origin of the instability lies in the pronounced oxidation of O during delithiation, i.e., O plays a central role in Ni-O redox in LiNiO2. For LiNiO2, NiO2, and the prototype rock salt NiO, density-functional theory and dynamical mean-field theory calculations based on maximally localised Wannier functions yield a Ni charge state of ca. +2, with O varying between –2 (NiO), –1.5 (LiNiO2) and –1 (NiO2). Predicted XAS Ni K and O K-edge spectra are in excellent agreement with experimental XAS spectra, confirming the predicted charge states. The calculations also show that a high-voltage O K-edge feature at 531 eV previously assigned to lattice O-redox processes could alternatively arise from O-redox induced water intercalation and O–O dimer formation with lattice O at high states of charge. The O2 surface loss route observed here consists of 2 surface O•– radicals combining to form a peroxide ion, which is oxidised to O2, leaving behind 2 O vacancies and 2 O2– ions: effectively 4 O•– radicals disproportionate to O2 and 2 O2– ions. The reaction liberates ca. 3 eV per O2 molecule. Singlet oxygen formation is caused by the singlet ground state of the peroxide ion, with spin conservation dictating the preferential release of 1O2, the strongly exergonic reaction providing the free energy required for the formation of 1O2 in its excited state.

Quantitative Aspects of Gas-Phase Metal Ion Chemistry: Conservation of Spin, Participation of f Orbitals, and C-H Activation and C-C Coupling.

In this Featured Article, I reflect on over 40 years of guided ion beam tandem mass spectrometry (GIBMS) studies involving atomic metal cations and their clusters throughout the periodic table. Studies that have considered the role of spin conservation (or lack thereof) are a primary focus with a quantitative assessment of the effects examined. A need for state-specific studies of heavier elements is noted, as is a more quantitative assessment of spin-orbit interactions in reactivity. Because GIBMS experiments explicitly evaluate the kinetic energy dependence of reactions over a wide range, several interesting and unusual observations are highlighted. More detailed studies of such unusual reaction events would be welcome. Activation of C-H bonds and ensuing C-C coupling events are reviewed, with future work encouraged. Finally, studies of lanthanides and actinides are examined with an eye on understanding the role of f orbitals in the chemistry, both as participants (or not) in the bonding and as sources/sinks of electron density. This area seems to be ripe for more quantitative experiments.

A range three elliptic deformation of the Hubbard model

In this paper we present a new integrable deformation of the Hubbard model. Our deformation gives rise to a range 3 interaction term in the Hamiltonian which does not preserve spin or particle number. This is the first non-trivial medium range deformation of the Hubbard model that is integrable. Our model can be mapped to a new integrable nearest-neighbour model via a duality transformation. The resulting nearest-neighbour model also breaks spin conservation. We compute the RR-matrices for our models, and find that there is a very unusual dependence on the spectral parameters in terms of the elliptic amplitude.

Spin-dependent reactivity and spin-flipping dynamics in oxygen atom scattering from graphite

The formation of two-electron chemical bonds requires the alignment of spins. Hence, it is well established for gas-phase reactions that changing a molecule’s electronic spin state can dramatically alter its reactivity. For reactions occurring at surfaces, which are of great interest during, among other processes, heterogeneous catalysis, there is an absence of definitive state-to-state experiments capable of observing spin conservation and therefore the role of electronic spin in surface chemistry remains controversial. Here we use an incoming/outgoing correlation ion imaging technique to perform scattering experiments for O(3P) and O(1D) atoms colliding with a graphite surface, in which the initial spin-state distribution is controlled and the final spin states determined. We demonstrate that O(1D) is more reactive with graphite than O(3P). We also identify electronically nonadiabatic pathways whereby incident O(1D) is quenched to O(3P), which departs from the surface. With the help of molecular dynamics simulations carried out on high-dimensional machine-learning-assisted first-principles potential energy surfaces, we obtain a mechanistic understanding for this system: spin-forbidden transitions do occur, but with low probabilities.

I-SPin: an integrator for multicomponent Schrödinger-Poisson systems with self-interactions

We provide an algorithm and a publicly available code to numerically evolve multicomponent Schrödinger-Poisson (SP) systems with a SO(n) symmetry, including attractive or repulsive self-interactions in addition to gravity. Focusing on the case where the SP system represents the non-relativistic limit of a massive vector field, non-gravitational self-interactions (in particular spin-spin interactions) introduce complexities related to mass and spin conservation which are not present in purely gravitational systems. We address them with an analytical solution for the `kick' step in the algorithm, where we are able to decouple the multicomponent system completely. Equipped with this analytical solution, the full field evolution is second order accurate, preserves spin and mass to machine precision, and is reversible. Our algorithm allows for an expanding universe relevant for cosmology, and the inclusion of external potentials relevant for laboratory settings.

Theory of Excitons in Gated Bilayer Graphene Quantum Dots

We present a theory of excitons in gated bilayer graphene (BLG) quantum dots (QDs). Electrical gating of BLG opens an energy gap, turning this material into an electrically tunable semiconductor. Unlike in laterally gated semiconductor QDs, where electrons are attracted and holes repelled, we show here that lateral structuring of metallic gates results in a gated lateral QD confining both electrons and holes. Using an accurate atomistic approach and exact diagonalization tools, we describe strongly interacting electrons and holes forming an electrically tunable exciton. We find these excitons to be different from those found in semiconductor QDs and nanocrystals, with exciton energy tunable by voltage from the terahertz to far infrared (FIR) range. The conservation of spin, valley, and orbital angular momentum results in an exciton fine structure with a band of dark low-energy states, making this system a promising candidate for storage, detection and emission of photons in the terahertz range.

Magnons, magnon bound pairs, and their hybrid spin-multipolar topology

The fully polarized ferromagnetic ground state is arguably the least quantum of all magnetic ground states. Nonetheless, the authors show that it can support topological quantum spin excitations with exotic spin-multipolar properties if spin conservation is broken. These excitations are superpositions of dipolar magnons and quadrupolar two-magnon bound states. As a result of their topology, such quantum ferromagnets exhibit both chiral edge excitations, akin to edge states in Chern insulators but at finite energy, and an intrinsic thermal Hall effect.

Effect of Anesthesia Gases on the Oxygen Reduction Reaction.

The oxygen reduction reaction (ORR) is of high importance, among others, because of its role in cellular respiration and in the operation of fuel cells. Recently, a possible relation between respiration and general anesthesia has been found. This work aims to explore whether anesthesia related gases affect the ORR. In ORR, oxygen which is in its triplet ground state is reduced to form products that are all in the singlet state. While this process is "in principle" forbidden because of spin conservation, it is known that if the electrons transferred in the ORR are spin-polarized, the reaction occurs efficiently. Here we show, in electrochemical experiments, that the efficiency of the oxygen reduction is reduced by the presence of general anesthetics in solution. We suggest that a spin-orbit coupling to the anesthetics depolarizes the spins. This causes both a reduction in reaction efficiency and a change in the reaction products. The findings may point to a possible relation between ORR efficiency and anesthetic action.