Multifrequency Electron Paramagnetic Resonance Research Articles

Nature of electrically detected magnetic resonance in highly nitrogen-doped 6H -SiC single crystals

This work focuses on unraveling electron paramagnetic resonance (EPR) and electrically detected magnetic resonance (EDMR) properties of n-type 6H silicon carbide (SiC) single crystals with high concentrations of uncompensated nitrogen (N) donors, which is essential for fundamental understanding of spin-related phenomena, developing spin-based devices, optimizing materials and devices, and advancing research in quantum information and spintronics. Utilizing low-temperature multifrequency EPR spectroscopy (9.4–395.12 GHz), we identified two intense signals labeled as S line and S1 line in the 6H-SiC crystals with ND–NA≈8×1018 and 4×1019cm−3. In addition, in 6H-SiC crystals with ND–NA≈8×1018cm−3, a low-intensity triplet from N donors substituting the quasicubic “k2” nonequivalent position (Nk2) was observed. The S line [g⊥=2.0029(2),g∥=2.0038(2)] was assigned to the exchange interaction of conduction electrons and Nk2, while the S1 line [g⊥=2.0030(2),g∥=2.0040(2)] is caused by the exchange spin coupling of localized N donors at the “k1” and “k2” positions. The S1 line was observed in high-frequency EDMR spectra of 6H-SiC with ND–NA≈8×1018cm−3, and its emergence was explained by an enhancement of the hopping conductivity due to the EPR-induced temperature increase mechanism. No EDMR spectra were found to occur in the 6H-SiC crystals with ND–NA≈4×1019cm−3, which is close to the critical donor concentration value for a semiconductor-metal transition. Thus it can be concluded that this N donor concentration is too high for the appearance of spin-dependent scattering and too low for the emergence of EPR-induced hopping mechanisms in 6H-SiC. Published by the American Physical Society 2024

Light-Induced Charge Separation in Photosystem I from Different Biological Species Characterized by Multifrequency Electron Paramagnetic Resonance Spectroscopy.

Photosystem I (PSI) serves as a model system for studying fundamental processes such as electron transfer (ET) and energy conversion, which are not only central to photosynthesis but also have broader implications for bioenergy production and biomimetic device design. In this study, we employed electron paramagnetic resonance (EPR) spectroscopy to investigate key light-induced charge separation steps in PSI isolated from several green algal and cyanobacterial species. Following photoexcitation, rapid sequential ET occurs through either of two quasi-symmetric branches of donor/acceptor cofactors embedded within the protein core, termed the A and B branches. Using high-frequency (130 GHz) time-resolved EPR (TR-EPR) and deuteration techniques to enhance spectral resolution, we observed that at low temperatures prokaryotic PSI exhibits reversible ET in the A branch and irreversible ET in the B branch, while PSI from eukaryotic counterparts displays either reversible ET in both branches or exclusively in the B branch. Furthermore, we observed a notable correlation between low-temperature charge separation to the terminal [4Fe-4S] clusters of PSI, termed FA and FB, as reflected in the measured FA/FB ratio. These findings enhance our understanding of the mechanistic diversity of PSI's ET across different species and underscore the importance of experimental design in resolving these differences. Though further research is necessary to elucidate the underlying mechanisms and the evolutionary significance of these variations in PSI charge separation, this study sets the stage for future investigations into the complex interplay between protein structure, ET pathways, and the environmental adaptations of photosynthetic organisms.

Probing structural and dynamic properties of MAPbCl3 hybrid perovskite using Mn2+ EPR.

Hybrid methylammonium (MA) lead halide perovskites have emerged as materials exhibiting excellent photovoltaic performance related to their rich structural and dynamic properties. Here, we use multifrequency (X-, Q-, and W-band) electron paramagnetic resonance (EPR) spectroscopy of Mn2+ impurities in MAPbCl3 to probe the structural and dynamic properties of both the organic and inorganic sublattices of this compound. The temperature dependent continuous-wave (CW) EPR experiments reveal a sudden change of the Mn2+ spin Hamiltonian parameters at the phase transition to the ordered orthorhombic phase indicating its first-order character and significant slowing down of the MA cation reorientation. Pulsed EPR experiments are employed to measure the temperature dependences of the spin-lattice relaxation T1 and decoherence T2 times of the Mn2+ ions in the orthorhombic phase of MAPbCl3 revealing a coupling between the spin center and vibrations of the inorganic framework. Low-temperature electron spin echo envelope modulation (ESEEM) experiments of the protonated and deuterated MAPbCl3 analogues show the presence of quantum rotational tunneling of the ammonium groups, allowing to accurately probe their rotational energy landscape.

Magnitnyy rezonans v kvazidvumernom antiferromagnetike na kvadratnoy reshetke Ba2MnGe2O7

We report results of a multi-frequency (0.8–60 GHz) electron spin resonance study of the spin dynamics in the quasi-2D square lattice antiferromagnet Ba2MnGe2O7 both in antiferromagnetically ordered and paramagnetic phases. We directly observe two zero-field gaps in the excitation spectrum of the ordered phase, the larger one being due to easy-plane anisotropy, and the smaller one indicates the presence of fourth-order in-plane anisotropy probably related to the multiferroic properties of this compound. We observe effects of hyperfine interaction on the electron spin resonance spectra in the antiferromagnetically ordered state, which turns out to be comparable with in-plane anisotropy. The hyperfine field strength is found from the observed low-temperature electron spin resonance data. The spin dynamics of the paramagnetic phase is characterized by strong broadening of the ESR absorption line, which can be ascribed to the vortex dynamics of a 2D magnet.

Magneto-Structural Correlations in a Mixed Porphyrin(Cu2+ )/Trityl Spin System: Magnitude, Sign, and Distribution of the Exchange Coupling Constant.

Tetrathiatriarylmethyl radicals (TAM or trityl) are receiving increasing attention in various fields of magnetic resonance such as imaging, dynamic nuclear polarization, spin labeling, and, more recently, molecular magnetism and quantum information technology. Here, a trityl radical attached via a phenyl bridge to a copper(II)tetraphenylporphyrin was synthesized, and its magnetic properties studied by multi-frequency continuous-wave electron paramagnetic resonance (EPR) spectroscopy and magnetic measurements. EPR revealed that the electron spin-spin coupling constant J between the trityl and Cu2+ spin centers is ferromagnetic with a magnitude of -2.3 GHz (-0.077 cm-1 , convention) and a distribution width of 1.2 GHz (0.040 cm-1 ). With the help of density functional theory (DFT) calculations, the obtained ferromagnetic exchange coupling, which is unusual for para-substituted phenyl-bridged biradicals, could be related to the almost perpendicular orientation of the phenyl linker with respect to the porphyrin and trityl ring planes in the energy minimum, while the J distribution was rationalized by the temperature weighted rotation of the phenyl bridge about the molecular axis connecting both spin centers. This study exemplifies the importance of molecular dynamics for the homogeneity (or heterogeneity) of the magnetic properties of trityl-based systems.

Probing Methyl Group Tunneling in [(CH3)2NH2][Zn(HCOO)3] Hybrid Perovskite Using Co2+ EPR.

At low temperature, methyl groups act as hindered quantum rotors exhibiting rotational quantum tunneling, which is highly sensitive to a local methyl group environment. Recently, we observed this effect using pulsed electron paramagnetic resonance (EPR) in two dimethylammonium-containing hybrid perovskites doped with paramagnetic Mn2+ ions. Here, we investigate the feasibility of using an alternative fast-relaxing Co2+ paramagnetic center to study the methyl group tunneling, and, as a model compound, we use dimethylammonium zinc formate [(CH3)2NH2][Zn(HCOO)3] hybrid perovskite. Our multifrequency (X-, Q- and W-band) EPR experiments reveal a high-spin state of the incorporated Co2+ center, which exhibits fast spin-lattice relaxation and electron spin decoherence. Our pulsed EPR experiments reveal magnetic field independent electron spin echo envelope modulation (ESEEM) signals, which are assigned to the methyl group tunneling. We use density operator simulations to extract the tunnel frequency of 1.84 MHz from the experimental data, which is then used to calculate the rotational barrier of the methyl groups. We compare our results with the previously reported Mn2+ case showing that our approach can detect very small changes in the local methyl group environment in hybrid perovskites and related materials.

Microstructure and Conduction Electron Quantum Properties of Small Diamond Cubic α-Sn Nanocrystals Embedded in Cubic Boron Nitride Crystals.

The morphology, structure, composition, and conduction electron properties of quasi-spherical tin nanocrystals (NCs) of 2.5 nm average diameter, with unstrained, bulk-like α-Sn diamond cubic structure, observed in dark cubic boron nitride (cBN) crystallites, were determined by correlated analytical high-resolution scanning transmission electron microscopy and multifrequency electron spin resonance (ESR) investigations. The narrow Lorentzian ESR line with g = 2.0028 is attributed to the conduction ESR of the α-Sn NCs, consistent with the temperature- and frequency-independent small g-shift and intensity reduction under high temperature (950 °C) vacuum annealing when the α-Sn NCs are thermally dissolved in the host cBN crystallites. The ESR linewidth and line intensity vs temperature dependences recorded in the 20 to 295 K range are quantitatively described considering the presence of discrete, quantum confinement-induced conduction electron energy levels with ΔQC/k B = 125 K separation, close to the theoretical value for conductive α-Sn NCs of 2.5 nm in diameter. The observed properties are tentatively explained with the predicted nanosize induced band-gap opening and change of band ordering from bulk α-Sn to small unstrained α-Sn NCs, resulting in a topological phase transition that also explains the predominantly s-like character of the conduction band electron orbitals.

Spin-Lattice Relaxation Decoherence Suppression in Vanishing Orbital Angular Momentum Qubits.

Multifrequency electron paramagnetic resonance spectroscopy on oriented single crystals of magnetically dilute Gd(III) ions in Gd0.004Y0.996(trensal) is used to determine the Hamiltonian parameters of the ground 8S7/2 term and its phase memory time, Tm, characterizing its coherent spin dynamics. The vanishing orbital angular momentum of the 8S7/2 term makes it relatively insensitive to spin-lattice relaxation mediated by magnetoelastic coupling and leads to a Tm of 12 μs at 3 K, which is not limited by spin-lattice relaxation.

Pulsed Multifrequency Electron Paramagnetic Resonance Spectroscopy Reveals Key Branch Points for One- vs Two-Electron Reactivity in Mn/Fe Proteins.

Traditionally, the ferritin-like superfamily of proteins was thought to exclusively use a diiron active site in catalyzing a diverse array of oxygen-dependent reactions. In recent years, novel redox-active cofactors featuring heterobimetallic Mn/Fe active sites have been discovered in both the radical-generating R2 subunit of class Ic (R2c) ribonucleotide reductases (RNRs) and the related R2-like ligand-binding oxidases (R2lox). However, the protein-specific factors that differentiate the radical reactivity of R2c from the C-H activation reactions of R2lox remain unknown. In this work, multifrequency pulsed electron paramagnetic resonance (EPR) spectroscopy and ligand hyperfine techniques in conjunction with broken-symmetry density functional theory calculations are used to characterize the molecular and electronic structures of two EPR-active intermediates trapped during aerobic assembly of the R2lox Mn/Fe cofactor. A MnIII(μ-O)(μ-OH)FeIII species is identified as the first EPR-active species and represents a common state between the two classes of redox-active Mn/Fe proteins. The species downstream from the MnIII(μ-O)(μ-OH)FeIII state exhibits unique EPR properties, including unprecedented spectral breadth and isotope-dependent g-tensors, which are attributed to a weakly coupled, hydrogen-bonded MnIII(μ-OH)FeIII species. This final intermediate precedes formation of the MnIII/FeIII resting state and is suggested to be relevant to understanding the endogenous reactivity of R2lox.

A mononuclear iron(III) complex with unusual changes of color and magneto-structural properties with temperature: synthesis, structure, magnetization, multi-frequency ESR and DFT study.

From the reaction of 2-hydroxy-6-methylpyridine (L) with iron(II) tetrafluoroborate, a new mononuclear iron(III) octahedral complex [FeL6](BF4)3 has been isolated. The color of the complex reversibly changed from red at room temperature to yellow-orange at the liquid nitrogen temperature. Magnetization measurements indicate that iron(III) in [FeL6](BF4)3 is in a high-spin state S = 5/2, from room temperature to 1.8 K. The high-spin ground state of iron(III) is also confirmed by DFT calculations. Although the spin-crossover of the complex is not observed, X-band and multifrequency high-field/high-frequency electron spin resonance (ESR) spectroscopy shows rather uncommon iron(III) spectra at room temperature and an unusual change with cooling. Spectral simulations reveal that the S = 5/2 ground state multiplet of the complex can be characterized by the temperature independent axial zero-field splitting parameter of |D| = +2 GHz (0.067 cm-1) while the value of the rhombic parameter E of the order of some tenths MHz increases on lowering the temperature. Single crystal X-ray diffraction (SCXRD) shows that the iron(III) coordination geometry does not change with temperature while supramolecular interactions are temperature dependent, influencing the iron(III) rhombicity. Additionally, the DFT calculations show temperature variation of the HOMO-LUMO gap, in agreement with the changes of color and ESR-spectra of the iron(III) complex with temperature.

Identification and characterization of a noncanonical menaquinone-linked formate dehydrogenase

The molybdenum/tungsten—bis-pyranopterin guanine dinucleotide family of formate dehydrogenases (FDHs) plays roles in several metabolic pathways ranging from carbon fixation to energy harvesting because of their reaction with a wide variety of redox partners. Indeed, this metabolic plasticity results from the diverse structures, cofactor content, and substrates used by partner subunits interacting with the catalytic hub. Here, we unveiled two noncanonical FDHs in Bacillus subtilis, which are organized into two-subunit complexes with unique features, ForCE1 and ForCE2. We show that the formate oxidoreductase catalytic subunit interacts with an unprecedented partner subunit, formate oxidoreductase essential subunit, and that its amino acid sequence within the active site deviates from the consensus residues typically associated with FDH activity, as a histidine residue is naturally substituted with a glutamine. The formate oxidoreductase essential subunit mediates the utilization of menaquinone as an electron acceptor as shown by the formate:menadione oxidoreductase activity of both enzymes, their copurification with menaquinone, and the distinctive detection of a protein-bound neutral menasemiquinone radical by multifrequency electron paramagnetic resonance (EPR) experiments on the purified enzymes. Moreover, EPR characterization of both FDHs reveals the presence of several [Fe-S] clusters with distinct relaxation properties and a weakly anisotropic Mo(V) EPR signature, consistent with the characteristic molybdenum/bis-pyranopterin guanine dinucleotide cofactor of this enzyme family. Altogether, this work enlarges our knowledge of the FDH family by identifying a noncanonical FDH, which differs in terms of architecture, amino acid conservation around the molybdenum cofactor, and reactivity.

Radiation-Induced Stable Radicals in Calcium Phosphates: Results of Multifrequency EPR, EDNMR, ESEEM, and ENDOR Studies

This article presents the results of a study of radiation-induced defects in various synthetic calcium phosphate (CP) powder materials (hydroxyapatite—HA and octacalcium phosphate—OCP) by electron paramagnetic resonance (EPR) spectroscopy at the X, Q, and W-bands (9, 34, 95 GHz for the microwave frequencies, respectively). Currently, CP materials are widely used in orthopedics and dentistry owing to their high biocompatibility and physico-chemical similarity with human hard tissue. It is shown that in addition to the classical EPR techniques, other experimental approaches such as ELDOR-detected NMR (EDNMR), electron spin echo envelope modulation (ESEEM), and electron-nuclear double resonance (ENDOR) can be used to analyze the electron–nuclear interactions of CP powders. We demonstrated that the value and angular dependence of the quadrupole interaction for 14N nuclei of a nitrate radical can be determined by the EDNMR method at room temperature. The ESEEM technique has allowed for a rapid analysis of the nuclear environment and estimation of the structural positions of radiation-induced centers in various crystal matrices. ENDOR spectra can provide information about the distribution of the nitrate radicals in the OCP structure.

Field-angle-dependent multi-frequency electron spin resonance spectroscopy in submillimeter wave range based on thermal detection.

We report the thermally detected electron spin resonance (ESR) spectroscopy in the frequency range of millimeter and submillimeter waves. Under high vacuum conditions, a cantilever-shaped device detects ESR absorption of a mounted sample as a temperature difference in its beam direction. Despite the simple experimental setup, the spin sensitivity of the order of 1012 spins/G was achieved at 10K. The developed sample stage is small enough to be used in a 10T split-pair superconducting magnet with a bore of 25mm, enabling precise field-angle-dependent ESR measurements at multi-frequencies above 500GHz. We demonstrate its usefulness by studying the field-angle dependence of the excitation energy of the dimer triplet state in the Shastry-Sutherland magnet SrCu2(BO3)2.

Oxidation State and Local Structure of Chromium Ions in LaOCl.

LaOCl doped with 0–10 mol% Cr was synthesized by thermal decomposition of chlorides. X-ray diffraction (XRD) analysis revealed that incorporation of chromium results in a decrease of the lattice parameter a and a simultaneous increase of the lattice parameter c. The local structure of chromium ions was studied with X-ray photoelectron (XPS), X-ray absorption (XANES), multifrequency electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) spectroscopy techniques. It was determined that synthesis in oxidizing atmosphere promotes the incorporation of chromium ions predominantly in the 5+ oxidation state. Changes of chromium oxidation state and local environment occur after a subsequent treatment in reducing atmosphere. Spin-Hamiltonian (SH) parameters for a Cr5+ and two types of Cr3+ centers in LaOCl were determined from the EPR spectra simulations.

Single-Ion Anisotropy and Intramolecular Interactions in CeIII and NdIII Dimers.

This article reports the syntheses, characterization, structural description, together with magnetic and spectroscopic properties of two isostructural molecular magnets based on the chiral ligand N,N′-bis((1,2-diphenyl-(pyridine-2-yl)methylene)-(R,R/S,S)-ethane-1,2-diamine), L1, of general formula [Ln2(RR-L1)2(Cl6)]·MeOH·1.5H2O, (Ln = Ce (1) or Nd (2)). Multifrequency electron paramagnetic resonance (EPR), cantilever torque magnetometry (CTM) measurements, and ab initio calculations allowed us to determine single-ion magnetic anisotropy and intramolecular magnetic interactions in both compounds, evidencing a more important role of the anisotropic exchange for the NdIII derivative. The comparison of experimental and theoretical data indicates that, in the case of largely rhombic lanthanide ions, ab initio calculations can fail in determining the orientation of the weakest components, while being reliable in determining their principal values. However, they remain of paramount importance to set the analysis of EPR and CTM on sound basis, thus obtaining a very precise picture of the magnetic interactions in these systems. Finally, the electronic structure of the two complexes, as obtained by this approach, is consistent with the absence of zero-field slow relaxation observed in ac susceptibility.

Quantum dynamics of Mn2+ in dimethylammonium magnesium formate.

Dimethylammonium magnesium formate, [(CH3)2NH2][Mg(HCOO)3] or DMAMgF, is a model used to study high temperature hybrid perovskite-like dielectrics. This compound displays an order-disorder phase transition at about 260 K. Using multifrequency electron spin resonance in continuous wave and pulsed modes, we herein present the quantum dynamics of the Mn2+ ion probe in DMAMgF. In the high temperature paraelectric phase, we observe a large distribution of the zero field splitting that is attributed to the high local disorder and further supported by density functional theory computations. In the low temperature ferroelastic phase, a single structure phase is detected and shown to contain two magnetic structures. The complex electron paramagnetic resonance signals were identified by means of the Rabi oscillation method combined with the crystal field kernel density estimation.

Gapped magnetic ground state in quantum spin liquid candidate κ-(BEDT-TTF)2Cu2(CN)3.

Geometrical frustration, quantum entanglement, and disorder may prevent long-range ordering of localized spins with strong exchange interactions, resulting in an exotic state of matter. κ-(BEDT-TTF)2Cu2(CN)3 is considered the prime candidate for this elusive quantum spin liquid state, but its ground-state properties remain puzzling. We present a multifrequency electron spin resonance (ESR) study down to millikelvin temperatures, revealing a rapid drop of the spin susceptibility at 6 kelvin. This opening of a spin gap, accompanied by structural modifications, is consistent with the formation of a valence bond solid ground state. We identify an impurity contribution to the ESR response that becomes dominant when the intrinsic spins form singlets. Probing the electrons directly manifests the pivotal role of defects for the low-energy properties of quantum spin systems without magnetic order.

Spin Excitations of the S = 1/2 One-Dimensional Ising-Like Antiferromagnet BaCo2V2O8 in Transverse Magnetic Fields

The high-field multifrequency electron spin resonance (ESR), the magnetization, and the specific heat of single crystals of BaCo2V2O8 (a quasi-one-dimensional Ising-like antiferromagnet with S = 1/...

Multi-Frequency and Single-Crystal EPR on V4+ in W-Doped \u03b2-Vanadyl(V) Phosphate: Hyperfine Coupling- and g-Tensor Values and Orientation

Different structure types of vanadyl(V) orthophosphate [i.e. (VV ≡O)3+ orthophosphate] have been subjects of research due to their catalytic activity in the oxidation of n-butane to maleic anhydride. Electron paramagnetic resonance (EPR) spectroscopy can be exploited to elucidate the electronic structure of such compounds. When tuning the oxidation state of vanadium in (V1-xWx)OPO4, X-band EPR spectra have confirmed the presence of paramagnetic V4+ ions. However, some of the features in these spectra could not be explained. Here, powder samples of β-(VIV0.01VV.98WVI.01)OPO4 are investigated at S-, X- and Q- band, along with X-band EPR measurements on single crystals. Thereby, the discrepancies between the spectra and their simulations could be resolved. In particular, it could be shown that the g and A tensors are not coaxial. The resulting consistent EPR picture and the refined paramagnetic parameters are reported. The work underlines the indispensability of a multi-frequency approach in EPR for unequivocal conclusions.

Unconventional magnetic excitations and spin dynamics of exotic quantum spin systems BaCo$$_2$$V$$_2$$O$$_8$$ and Ba$$_3$$CuSb$$_2$$O$$_9$$

We review terahertz (THz) electron spin resonance studies of two types of exotic quantum spin systems, namely, the spin(S)-1/2 one-dimensional (1D) Ising-like antiferromagnet BaCo $$_2$$ V $$_2$$ O $$_8$$ and the S=1/2 two-dimensional (2D) honeycomb-like antiferromagnet Ba $$_3$$ CuSb $$_2$$ O $$_9$$ in magnetic fields of up to 50 T. For the former subject, unconventional magnetic excitations were identified below a critical magnetic field $$H_c$$ ( $$\sim $$ 4 T), where the exotic field-induced order-to-disorder transition occurs, and magnetic excitations in a Tomonaga-Luttinger liquid state were observed above $$H_c$$ . The novel magnetic excitations were analyzed with an S=1/2 1D XXZ model by considering the peculiar structure of this compound. For the latter subject, the orbital quantum dynamics of the spin liquid candidate Ba $$_3$$ CuSb $$_2$$ O $$_9$$ was revealed using multifrequency electron spin resonance ranging from 9.3 GHz to 0.73 THz. The g-factor of the hexagonal Ba $$_3$$ CuSb $$_2$$ O $$_9$$ single crystal possesses a weak six-fold symmetry at low frequencies, while two-fold symmetry is manifested at high frequencies. From the critical point between the two frequency regions, the frequency of the dynamic Jahn-Teller distortion is determined to be approximately 10 GHz. This dynamic distortion, accompanied by orbital quantum tunneling, proves the spin-orbital liquid state in Ba $$_3$$ CuSb $$_2$$ O $$_9$$ .