Electron Spin Research Articles

Spin states of metal centers in electrocatalysis.

Understanding the electronic structure of active sites is crucial in efficient catalyst design. The spin state, spin configurations of d-electrons, has been frequently discussed recently. However, its systematic depiction in electrocatalysis is lacking. In this tutorial review, a comprehensive interpretation of the spin state of metal centers in electrocatalysts and its role in electrocatalysis is provided. This review starts with the basics of spin states, including molecular field theory, crystal field theory, and ligand field theory. It further introduces the differences in low spin, intermediate spin, and high spin, and intrinsic factors affecting the spin state. Popular characterization techniques and modeling approaches that can reveal the spin state, such as X-ray absorption microscopy, electron spin resonance spectroscopy, Mössbauer spectroscopy, and density functional theory (DFT) calculations, are introduced as well with examples from the literature. The examples include the most recent progress in tuning the spin state of metal centers for various reactions, e.g., the oxygen evolution reaction, oxygen reduction reaction, hydrogen evolution reaction, carbon dioxide reduction reaction, nitrogen reduction reaction, nitrate reduction reaction, and urea oxidation reaction. Challenges and potential implications for future research related to the spin state are discussed at the end.

Suppressing Thermal Noise to Sub-Millikelvin Level in a Single-Spin Quantum System Using Realtime Frequency Tracking

A single nitrogen-vacancy (NV) center in a diamond can be used as a nanoscale sensor for magnetic field, electric field or nuclear spins. Due to its low photon detection efficiency, such sensing processes often take a long time, suffering from an electron spin resonance (ESR) frequency fluctuation induced by the time-varying thermal perturbations noise. Thus, suppressing the thermal noise is the fundamental way to enhance single-sensor performance, which is typically achieved by utilizing a thermal control protocol with a complicated and highly costly apparatus if a millikelvin-level stabilization is required. Here, we analyze the real-time thermal drift and utilize an active way to alternately track the single-spin ESR frequency drift in the experiment. Using this method, we achieve a temperature stabilization effect equivalent to sub-millikelvin (0.8 mK) level with no extra environmental thermal control, and the spin-state readout contrast is significantly improved in long-lasting experiments. This method holds broad applicability for NV-based single-spin experiments and harbors the potential for prospective expansion into diverse nanoscale quantum sensing domains.

Identifying Sources of Entanglement Loss in Photodriven Molecular Electron Spin Teleportation.

We report on an electron donor-electron acceptor-stable radical (D-A-R•) molecule in which an electron spin state first prepared on R• is followed by photogeneration of an entangled singlet 1[D•+-A•-] spin pair to produce D•+-A•--R•. Since the A•- and R• spins within D•+-A•--R• are uncorrelated, spin teleportation from R• to D•+ occurs with a maximal 25% efficiency only for the singlet pair 1(A•--R•) by spin-allowed electron transfer from A•- to R•. However, since 1[D•+-A•-] is sufficiently long-lived, coherent spin mixing involving the unreactive 3(A•--R•) population affects entanglement and teleportation within D•+-A•--R•. Pulse electron paramagnetic resonance experiments show a direct correlation between electron spin flip-flops and entanglement loss, providing information for designing molecular materials to serve as nanoscale quantum device interconnects.

Nonadiabatic Tunneling in Chemical Reactions.

Quantum tunneling can have a dramatic effect on chemical reaction rates. In nonadiabatic reactions such as electron transfers or spin crossovers, nuclear tunneling effects can be even stronger than for adiabatic proton transfers. Ring-polymer instanton theory enables molecular simulations of tunneling in full dimensionality and has been shown to be far more reliable than commonly used separable approximations. First-principles instanton calculations predict significant nonadiabatic tunneling of heavy atoms even at room temperature and give excellent agreement with experimental measurements for the intersystem crossing of two nitrenes in cryogenic matrix isolation, the spin-forbidden relaxation of photoexcited thiophosgene in the gas phase, and singlet oxygen deactivation in water at ambient conditions. Finally, an outlook of further theoretical developments is discussed.

Loss and decoherence in superconducting circuits on silicon: Insights from electron spin resonance

Loss and decoherence in superconducting circuits on silicon: Insights from electron spin resonance

Anchoring Atomically Precise Chiral Bismuth Oxido Nanoclusters on Gold: The Role of Amino Acid Linkers.

The adsorption of chiral molecules onto metallic surfaces triggers electron spin polarization at the interface, paving the way for applications in chiral opto-spintronics. However, the spin effects sensitively depend on the binding and ordering of the chiral species on surfaces. This study explores the adsorption of chiral thioether-functionalized atomically precise bismuth oxido nanoclusters (BiO-NCs) on gold (Au) surfaces, extending the conventional approach of using thiol-containing molecules and complexes to nanoclusters. Starting from the precursor [Bi38O45(NO3)20(dmso)28](NO3)4·4dmso (A), chiral BiO-NCs were synthesized by substituting the nitrates with N-(tert-butoxycarbonyl)-l-methionine (Boc-l-Met-O-) ligands to obtain [Bi38O45(Boc-l-Met-O)24] (2). The full exchange of nitrate by the Boc-l-methionine ligand was demonstrated by powder X-ray diffractograms, dynamic light scattering, electrospray ionization mass spectrometry, nuclear magnetic resonance, infrared, circular dichroism, and X-ray photoelectron spectroscopy. Compared to previously reported [Bi38O45(Boc-l-Phe-O)24(dmso)9] (1), BiO-NC 2 shows differences in the growth mode on a Au surface as revealed by scanning electron microscopy, wherefore a stronger binding of BiO-NC 2 is assumed. Anchoring of BiO-NC 2 to the Au surface through thioether groups induced a discernible change in the optical response of the Au surface analyzed by spectroscopic ellipsometry (SE). From the numerical modeling of the SE parameters, a layer thickness of ∼2 nm, corresponding to a monolayer of BiO-NC 2, was estimated for the samples prepared by dip coating. Thus, strong adsorption of BiO-NC 2 to the Au surface is concluded, which is an essential prerequisite for chiral-induced interface spin polarization.

An advance in the arithmetic of the Lie groups as an alternative to the forms of the Campbell-Baker-Hausdorff-Dynkin theorem

The exponential of an operator or matrix is widely used in quantum theory, but it sometimes can be a challenge to evaluate. For non-commutative operators X and Y, according to the Campbell-Baker-Hausdorff-Dynkin theorem, e X+Y is not equivalent to e X e Y , but is instead given by the well-known infinite series formula. For a Lie algebra of a basis of three operators {X, Y, Z}, such that [X, Y] = κ Z for scalar κ and cyclic permutations, here it is proven that e a X+b Y is equivalent to e p Z e q X e−p Z for scalar p and q. Extensions for e a X+b Y+c Z are also provided. This method is useful for the dynamics of atomic and molecular nuclear and electronic spins in constant and oscillatory transverse magnetic and electric fields.

Chirality-Induced Magnet-Free Spin Generation in a Semiconductor.

Electrical generation and transduction of polarized electron spins in semiconductors (SCs) are of central interest in spintronics and quantum information science. While spin generation in SCs is frequently realized via electrical injection from a ferromagnet (FM), there are significant advantages in nonmagnetic pathways of creating spin polarization. One such pathway exploits the interplay of electron spin with chirality in electronic structures or real space. Here, utilizing chirality-induced spin selectivity (CISS), the efficient creation of spin accumulation in n-doped GaAs via electric current injection from a normal metal (Au) electrode through a self-assembled monolayer (SAM) of chiral molecules (α-helix l-polyalanine, AHPA-L), is demonstrated. The resulting spin polarization is detected as a Hanle effect in the n-GaAs, which is found to obey a distinct universal scaling with temperature and bias current consistent with chirality-induced spin accumulation. The experiment constitutes a definitive observation of CISS in a fully nonmagnetic device structure and demonstration of its ability to generate spin accumulation in a conventional SC. The results thus place key constraints on the physical mechanism of CISS and present a new scheme for magnet-free SC spintronics.

Encapsulated Atomic Hydrogen in Octamethyl-POSS cages: a Pulsed EPR Study.

Encapsulated atomic hydrogen in polyhedral oligomeric silsesquioxane (POSS) cages is a promising candidate for spin-based quantum technologies. Key parameters such as spin relaxation times and magnetic interactions with surrounding electron and nuclear spins can be typically probed with advanced electron paramagnetic resonance (EPR) methods. Here we present a detailed pulsed EPR study of the species H@Si8O12R8 with R=CH3, namely encapsulated atomic hydrogen in the octamethyl POSS derivative. The temperature dependence of the spin-lattice relaxation rate 1/T1 is analyzed in terms of a Raman process with a Debye temperature of ΘD=145 K and a thermally activated process with Ea=794 K (552 cm-1), whereas, the phase memory time TM shows the typical shortening behaviour at T<150 K observed for all methyl-containing derivatives. The hyperfine coupling of the cage 29Si nuclei is measured by hyperfine sublevel correlation (HYSCORE) spectroscopy and is found to fulfil the so-called "matching condition" at the low-field EPR transition. The potential of this paramagnetic molecule to perform one-qubit quantum operations is probed by room-temperature Rabi oscillations.

Temporal Separation between Lattice Dynamics and Electronic Spin‐State Switching in Spin‐Crossover Thin Films Evidenced by Time‐Resolved X‐Ray Diffraction

AbstractSpin‐crossover (SCO) complexes have drawn significant attention for the possibility to photoswitch their electronic spin state on a sub‐picosecond timescale at the molecular level. However, the multi‐step mechanism of laser‐pulse‐induced switching in solid state is not yet fully understood. Here, time‐resolved synchrotron X‐ray diffraction is used to follow the dynamics of the crystal lattice in response to a picosecond laser excitation in nanometric thin films of the SCO complex [Fe(HB(1,2,4‐triazol‐1‐yl)3)2]. The observed structural dynamics unambiguously reveal a lattice expansion on the 100 picosecond timescale, which is temporally decoupled both from the ultrafast molecular photoswitching process (occurring within 100 fs) and from the delayed, thermo‐elastic (Arrhenius‐driven) conversion (taking place ≈10 ns). These time‐separated dynamics are also manifested by the observation of damped acoustic oscillations in the time evolution of the lattice volume, whereas no such oscillations are observed in the electronic spin‐state dynamics. Overall, these results suggest the existence of a universal behavior whereby the intramolecular energy barrier between low‐spin and high‐spin states acts as an intrinsic dynamical bottleneck in the out‐of‐equilibrium spin‐state switching dynamics of SCO materials.

Impact of microwave power on equivalent dose (De) evaluation in ESR dating

ESR (electron spin resonance) spectra of quartz samples irradiated with varying gamma (γ) dose were measured at different microwave power levels (0–200 mW). The microwave power saturation characteristics of Al and E′ centers were systematically investigated, and the mechanisms by which the power saturation characteristics affected equivalent dose (De) fitting results in ESR dating were revealed. The experimental results demonstrated that irradiation reduces the spin-lattice relaxation time, resulting in the inability to intrinsically characterize the number of spins in a series of aliquots when the ESR intensity exhibits significant saturation. The simulation results indicated that changes in spin-lattice relaxation characteristics can cause distortion in the dose response curve (DRC) at high power levels, leading to deviations in De values. A method that uses the slope of the approximate linear regime of the power saturation curve (PSC) as an intrinsic metric of ESR intensity is proposed; this method has been preliminarily validated in both experiments and simulations.

Non-metallic Lewis acid sites enhance the activation of peracetic acid by cobalt-iron spinel for micropollutant decontamination: The critical role of Lewis acid sites as electron shuttles

Spinel is a common activator for peroxides, e.g., peracetic acid (PAA), in wastewater treatment, but suffers from insufficient electron transfer capacity at the active site and thus the limited activation capacity. Lewis acids are recognized for their capacity to interact with electrons and activate active sites, playing a crucial role in enhancing oxidant adsorption, shortening catalytic reaction distances, and increasing reaction kinetics. Herein, tellurium (Te), a typical Lewis acid, was introduced into cobalt-iron spinel (TCF) as an electron shuttler to improve the electron transfer capacity and thus boost the PAA activation. Results show that the degradation rate of sulfamethoxazole (SMX) with PAA activated via TCF is 4.0 times higher compared with cobalt-iron spinel (CF). Electron spin resonance (ESR) analysis and quenching experiments indicated acetylperoxyl radicals (CH3C(O)OO·) as the dominant reactive species in SMX degradation. Electrochemical tests revealed that the introduction of Lewis acid improved the charge transfer ability and charge utilization efficiency of the catalysts, exhibiting less electronic resistance and superior redox properties. Density functional theory (DFT) calculation suggests that the introduction of Lewis acid optimizes the electron distribution of the catalyst, and reduces the crucial reaction energy barriers for PAA activation. Additionally, the distinctive structural environment of Lewis acid renders TCF to exhibit excellent stability and resistance to complicated matrices in PAA activation. This work provides valuable insights into the Lewis acid regulating bimetallic catalyst performance and understanding the intrinsic interaction mechanisms, thereby establishing a critical groundwork for addressing prevailing wastewater treatment.

Promoting Polysulfide Redox Reactions through Electronic Spin Manipulation.

Catalytic additives able to accelerate the lithium-sulfur redox reaction are a key component of sulfur cathodes in lithium-sulfur batteries (LSBs). Their design focuses on optimizing the charge distribution within the energy spectra, which involves refinement of the distribution and occupancy of the electronic density of states. Herein, beyond charge distribution, we explore the role of the electronic spin configuration on the polysulfide adsorption properties and catalytic activity of the additive. We showcase the importance of this electronic parameter by generating spin polarization through a defect engineering approach based on the introduction of Co vacancies on the surface of CoSe nanosheets. We show vacancies change the electron spin state distribution, increasing the number of unpaired electrons with aligned spins. This local electronic rearrangement enhances the polysulfide adsorption, reducing the activation energy of the Li-S redox reactions. As a result, more uniform nucleation and growth of Li2S and an accelerated liquid-solid conversion in LSB cathodes are obtained. These translate into LSB cathodes exhibiting capacities up to 1089 mA h g-1 at 1 C with 0.017% average capacity loss after 1500 cycles, and up to 5.2 mA h cm-2, with 0.16% decay per cycle after 200 cycles in high sulfur loading cells.

Promoting Polysulfide Redox Reactions through Electronic Spin Manipulation.

Catalytic additives able to accelerate the lithium-sulfur redox reaction are a key component of sulfur cathodes in lithium-sulfur batteries (LSBs). Their design focuses on optimizing the charge distribution within the energy spectra, which involves refinement of the distribution and occupancy of the electronic density of states. Herein, beyond charge distribution, we explore the role of the electronic spin configuration on the polysulfide adsorption properties and catalytic activity of the additive. We showcase the importance of this electronic parameter by generating spin polarization through a defect engineering approach based on the introduction of Co vacancies on the surface of CoSe nanosheets. We show vacancies change the electron spin state distribution, increasing the number of unpaired electrons with aligned spins. This local electronic rearrangement enhances the polysulfide adsorption, reducing the activation energy of the Li-S redox reactions. As a result, more uniform nucleation and growth of Li2S and an accelerated liquid-solid conversion in LSB cathodes are obtained. These translate into LSB cathodes exhibiting capacities up to 1089 mA h g-1 at 1 C with 0.017% average capacity loss after 1500 cycles, and up to 5.2 mA h cm-2, with 0.16% decay per cycle after 200 cycles in high sulfur loading cells.

Frequency-Chirped Magic Angle Spinning Dynamic Nuclear Polarization Combined with Electron Decoupling.

Magic angle spinning (MAS) dynamic nuclear polarization (DNP) increases the signal intensity of solid-state nuclear magnetic resonance. DNP typically uses continuous wave (CW) microwave irradiation close to the resonance frequency of unpaired electron spins. In this study, we demonstrate that frequency-chirped microwaves improve DNP performance under MAS. By modulating the gyrotron anode potential, we generate a train of microwave chirps with a maximum bandwidth of 310 MHz and a maximum incident power on the spinning sample of 18 W. We characterize the efficiency of chirped DNP using the following polarizing agents: TEMTriPol-1, AsymPolPOK, AMUPol, and Finland trityl. The effects of different chirp widths and periods are analyzed at different MAS frequencies and microwave powers. Furthermore, we show that chirped DNP can be combined with electron decoupling to improve signal intensity by 59%, compared to CW DNP without electron decoupling, using Finland trityl as a polarizing agent.

SrCu(OH)3Cl , an isolated equilateral triangle spin S=1/2 model system

We have investigated the magnetic ground state properties of the quantum spin trimer compound strontium hydroxy copper chloride SrCu(OH)3Cl using bulk magnetization, specific heat measurements, nuclear magnetic resonance (NMR), and electron spin resonance (ESR) spectroscopy. SrCu(OH)3Cl consists of layers with isolated Cu2+ triangles and hence provides an opportunity to understand the magnetic ground state of an isolated system of S = 1/2 arranged on an equilateral triangle. Although magnetization measurements do not exhibit a phase transition to a long-range ordered state down to T = 2 K, they reveal the characteristic behavior of isolated trimers with an exchange of J=154 K. The Curie-Weiss behavior changes around 50–80 K, as is also seen in the NMR spin-lattice relaxation rate. In zero magnetic field, our specific heat data establish a second-order phase transition to an antiferromagnetic ground state below T= 1.2 K. We have drawn a magnetic field-temperature (H−T) phase diagram based on the specific heat measurements. The ESR data show divergence of the linewidth at lower temperatures, which precedes the phase transition to an antiferromagnetic long-range ordered state with unconventional critical exponents. The temperature variation of the g-factor further confirms the antiferromagnetic phase transition and reflects the underlying magnetocrystalline anisotropy of the compound. Published by the American Physical Society 2024

SrCu(OH)3Cl , an isolated equilateral triangle spin S=1/2 model system

We have investigated the magnetic ground state properties of the quantum spin trimer compound strontium hydroxy copper chloride SrCu(OH)3Cl using bulk magnetization, specific heat measurements, nuclear magnetic resonance (NMR), and electron spin resonance (ESR) spectroscopy. SrCu(OH)3Cl consists of layers with isolated Cu2+ triangles and hence provides an opportunity to understand the magnetic ground state of an isolated system of S = 1/2 arranged on an equilateral triangle. Although magnetization measurements do not exhibit a phase transition to a long-range ordered state down to T = 2 K, they reveal the characteristic behavior of isolated trimers with an exchange of J=154 K. The Curie-Weiss behavior changes around 50–80 K, as is also seen in the NMR spin-lattice relaxation rate. In zero magnetic field, our specific heat data establish a second-order phase transition to an antiferromagnetic ground state below T= 1.2 K. We have drawn a magnetic field-temperature (H−T) phase diagram based on the specific heat measurements. The ESR data show divergence of the linewidth at lower temperatures, which precedes the phase transition to an antiferromagnetic long-range ordered state with unconventional critical exponents. The temperature variation of the g-factor further confirms the antiferromagnetic phase transition and reflects the underlying magnetocrystalline anisotropy of the compound. Published by the American Physical Society 2024

Activation of peracetic acid by electrodes using biogenic electrons: A novel energy- and catalyst-free process to eliminate pharmaceuticals

Peracetic acid (PAA) has received increasing attention as an alternative oxidant for wastewater treatment. However, existing processes for PAA activation to generate reactive species typically require external energy input (e.g., electrically and UV-mediated activation) or catalysts (e.g., Co2+), inevitably increasing treatment costs or introducing potential new contaminants that necessitate additional removal. In this work, we developed a catalyst-free, self-sustaining bioelectrochemical approach within a two-chamber bioelectrochemical system (BES), where a cathode electrode in-situ activates PAA using renewable biogenic electrons generated by anodic exoelectrogens (e.g., Geobacter) degrading biodegradable organic matter (e.g., acetic acid) in wastewater at the anode. This innovative BES-PAA technique achieved 98 % and 81 % removal of 2 µM sulfamethoxazole (SMX) in two hours at pH 2 (cation exchange membrane) and pH 6 (bipolar membrane) using 100 μM PAA without external voltage. Mechanistic studies, including radical quenching, molecular probe validation, electron spin resonance (ESR) experiments, and density functional theory (DFT) calculations, revealed that SMX degradation was driven by reactive species generated via biogenic electron-mediated OO cleavage of PAA, with CH3C(O)OO• contributing 68.1 %, •OH of 18.4 %, and CH3C(O)O• of 9.4 %, where initial formation of •OH and CH3C(O)O• rapidly reacts with PAA to produce CH3C(O)OO•. The presence of common water constituents such as anions (e.g., Cl−, NO3−, and H2PO4−) and humic acid (HA) significantly hinders SMX removal via the BES-PAA technique, whereas CO32− and HCO3− ions have a comparatively minor impact. Additionally, the study investigated the removal of various pharmaceuticals present in secondary treated municipal wastewater, attributing differences in removal efficiency to the selective action of CH3C(O)OO•. This research demonstrates a novel PAA activation method that is ecologically benign, inexpensive, and capable of overcoming catalyst deactivation and secondary pollution issues.

Frequency-Chirped Magic Angle Spinning Dynamic Nuclear Polarization Combined with Electron Decoupling.

Magic angle spinning (MAS) dynamic nuclear polarization (DNP) increases the signal intensity of solid-state nuclear magnetic resonance. DNP typically uses continuous wave (CW) microwave irradiation close to the resonance frequency of unpaired electron spins. In this study, we demonstrate that frequency-chirped microwaves improve DNP performance under MAS. By modulating the gyrotron anode potential, we generate a train of microwave chirps with a maximum bandwidth of 310 MHz and a maximum incident power on the spinning sample of 18 W. We characterize the efficiency of chirped DNP using the following polarizing agents: TEMTriPol-1, AsymPolPOK, AMUPol, and Finland trityl. The effects of different chirp widths and periods are analyzed at different MAS frequencies and microwave powers. Furthermore, we show that chirped DNP can be combined with electron decoupling to improve signal intensity by 59%, compared to CW DNP without electron decoupling, using Finland trityl as a polarizing agent.

Impact of Zeeman and hyperfine interactions on the magnetic properties of paramagnetic metal Ions: III. Analysis of the local interactions in a single crystal of 173Yb3+ doped Y5SiO5

The electron spin echo envelope modulation (ESEEM) technique is a direct method to probe the nuclear spin coherences induced by electron spin transitions. Recently, this approach was used to study an isotopically pure Y2SiO5 crystal doped with 173Yb3+ ions, and the presence of the Fermi contact interaction was proposed to explain the frequency comb detected in the two-pulse ESEEM experiment [Solovarov N. K. et al. JETP Letters 115 (6): 362–67]. Here we simulate the Fourier images of the ESEEM data. The numerical analysis shows that the modulation is mainly due to the nuclear spin coherences induced by the dipole–dipole interactions. However, the correlation between the experimental and simulated data is better when the super-hyperfine interactions of the nearby yttrium nuclei have an additional isotropic contribution. The analysis of the rescaled X-band ESEEM spectra shows that for the EPR transitions at magnetic fields > 100 mT, the main contribution to the modulation comes from the oscillations of the individual nuclei and the effect of interference between coherences originating from several nuclei is not strong. Further experiments to distinguish the sources of the echo modulation are discussed.