Frequency Electron Paramagnetic Resonance Research Articles

High Frequency Electron Spin Resonance in Mn1-xFexSi

High frequency (60 GHz) electron spin resonance (ESR) has been studied for the quantum critical system Mn1−xFexSi (0 < x < 0.24) at temperatures up to 50 K by using the original experimental technique. The analysis of ESR line shape allowed detecting the full set of spectroscopic parameters (oscillating magnetization, line width and g factor). The violation of classical Korringa-type relaxation and scaling behavior of the ESR line width was found to a result from iron doping. Non-Fermi-liquid effects in the temperature dependence of the ESR line width observed at quantum critical points x* = 0.11 and xc = 0.24 are shown to be described quantitatively in the theory by Wölfle and Abrahams.

On-Chip ESR Measurements of DPPH at mK Temperatures

We study electron spin resonance (ESR) of DPPH at mK temperatures. Here we employ a superconducting coplanar microwave resonator that allows convenient implementation in a dilution refrigerator as well as operation at multiple ESR frequencies, in this case 1.5GHz, 3.0GHz, and 4.5GHz. We find a strong temperature and magnetic field dependence of the ESR of DPPH below 1K, which is consistent with an antiferromagnetic transition. Our study documents the potential of this on-chip ESR technique for mK experiments and elucidates the possibility to use DPPH as an ESR reference material in this regime.

Semiconductors with a loop of extrema

Crystals with a Wurtzite-type structure are used as an example to show that owing to spin–orbit coupling a new type of band structure can arise. For this band structure the extrema are reached at a circle, a loop of extrema, rather than in isolated points of the Brillouin zone. Specific properties of such semiconductors are studied theoretically, in particular, the peculiarities of the cyclotron resonance at low temperatures. In strong magnetic fields, spin–orbit coupling results in absorption at the frequency of electron spin resonance but driven by the electric vector of electromagnetic wave (combined resonance).

Connecting lipoxygenase function to structure by electron paramagnetic resonance.

ConspectusLipoxygenase enzymes insert oxygen in a polyunsaturated lipid, yielding a hydroperoxide product. When the acyl chain is arachidonate, with three cis-pentadiene units, 12 positionally and stereochemically different products might result. The plant lipids, linoleate and linolenate, have, respectively, four and eight potential oxygen insertion sites. The puzzle of how specificity is achieved in these reactions grows as more and more protein structures confirm the conservation of a lipoxygenase protein fold in plants, animals, and bacteria. Lipoxygenases are large enough (60–100 kDa) that they provide a protein shell completely surrounding an active site cavity that has the shape of a long acyl chain and contains a catalytic metal (usually iron).This Account summarizes electron paramagnetic resonance (EPR) spectroscopic, and other, experiments designed to bridge the gap between lipid–lipoxygenase interactions in solution and crystal structures. Experiments with spin-labeled lipids give a picture of bound lipids tethered to protein by an acyl chain, but with a polar end emerging from the cavity to solvent exposure, where the headgroup is highly flexible. The location of a spin on the polar end of a lysolecithin was determined by pulsed, dipolar EPR measurements, by representing the protein structure as a five-point grid of spin-labels with coordinates derived from 10 distance determinations between spin pairs. Distances from the lipid spin to each grid site completed a six-point representation of the enzyme with a bound lipid. Insight into the dynamics that allow substrate/product to enter/exit the cavity was obtained with a different set of spin-labeled protein mutants. Once substrate enters the cavity, the rate-limiting step of catalysis involves redox cycling at the metal center. Here, a mononuclear iron cycles between ferric and ferrous (high-spin) forms. Two helices provide pairs of side-chain ligands to the iron, resulting in characteristic EPR signals. Quantitative comparison of EPR spectra of plant and bacterial lipoxygenases has suggested conservation of a unique geometry of lipoxygenase iron centers. High frequency (94 GHz) EPR is consistent with a similar metal center in a manganese version of lipoxygenase. Overall, established and emerging EPR experiments have been developed and applied to the lipoxygenase family of enzymes to elucidate changes in the solution structures that are related to function.

An addressable quantum dot qubit with fault-tolerant control-fidelity.

Exciting progress towards spin-based quantum computing has recently been made with qubits realized using nitrogen-vacancy centres in diamond and phosphorus atoms in silicon. For example, long coherence times were made possible by the presence of spin-free isotopes of carbon and silicon. However, despite promising single-atom nanotechnologies, there remain substantial challenges in coupling such qubits and addressing them individually. Conversely, lithographically defined quantum dots have an exchange coupling that can be precisely engineered, but strong coupling to noise has severely limited their dephasing times and control fidelities. Here, we combine the best aspects of both spin qubit schemes and demonstrate a gate-addressable quantum dot qubit in isotopically engineered silicon with a control fidelity of 99.6%, obtained via Clifford-based randomized benchmarking and consistent with that required for fault-tolerant quantum computing. This qubit has dephasing time T2* = 120 μs and coherence time T2 = 28 ms, both orders of magnitude larger than in other types of semiconductor qubit. By gate-voltage-tuning the electron g*-factor we can Stark shift the electron spin resonance frequency by more than 3,000 times the 2.4 kHz electron spin resonance linewidth, providing a direct route to large-scale arrays of addressable high-fidelity qubits that are compatible with existing manufacturing technologies.

Ferrimagnetism and magnetic phase separation in Nd1−xYxMnO3 studied by magnetization and high frequency electron paramagnetic resonance

Ferrimagnetism and metamagnetic features tunable by composition are observed in the magnetic response of Nd1−xYxMnO3, for x=0.1–0.5. For all values of x in the series, the compound crystallizes in orthorhombic Pbnm space group similar to NdMnO3. Magnetization studies reveal a phase transition of the Mn-sublattice below TNMn≈80K for all compositions, which, decreases up on diluting the Nd-site with Yttrium. For x=0.35, ferrimagnetism is observed. At 5K, metamagnetic transition is observed for all compositions x<0.4. The evolution of magnetic ground states and appearance of ferrimagnetism in Nd1−xYxMnO3 can be accounted for by invoking the scenario of magnetic phase separation. The high frequency electron paramagnetic resonance measurements on x=0.4 sample, which is close to the critical composition for phase separation, revealed complex temperature dependent lineshapes clearly supporting the assumption of magnetic phase separation.

Anomalous spin relaxation and quantum criticality in Mn1 − x Fe x Si solid solutions

We report results of the high frequency (60 GHz) electron spin resonance (ESR) study of the quantum critical metallic system Mn1 − xFexSi. The ESR is observed for the first time in the concentration range 0 < x < 0.24 at temperatures up to 50 K. The application of the original experimental technique allowed carrying out line shape analysis and finding full set of spectroscopic parameters, including oscillating magnetization, line width and g factor. The strongest effect of iron doping consists in influence on the ESR line width and spin relaxation is marked by both violation of the classical Korringa-type relaxation and scaling behavior. Additionally, the non-Fermi-liquid effects in the temperature dependence of the ESR line width, which may be quantitatively described in the theory of Wolfle and Abrahams, are observed at quantum critical points x* ∼ 0.11 and xc ∼ 0.24.

Nitrogen split interstitial center(N−N)Nin GaN: High frequency EPR and ENDOR study

The nitrogen split interstitial defect introduced by high-energy particle irradiation in n-type GaN has been investigated by very high (up to 324 GHz) frequency electron paramagnetic resonance (EPR) and Q-band electron nuclear double resonance (ENDOR) spectroscopy. The increased resolution of the EPR spectra at 324 GHz has allowed us to determine the g-tensor anisotropy, which is not resolved at X or Q band. The good agreement of the principal values g(xx) = 1.9966, g(yy) = 2.0016, and g(zz) = 2.0036 with the theoretically predicted g tensor confirm the (N-N)(N)(0) defect model. The hyperfine interactions of this defect have been studied byQ-band ENDOR. We observed well-resolved ENDOR lines with distant Ga atoms from which the quadrupole coupling constants and the electrical field gradients were determined and discussed with the help of theoretical values. The observation of ENDOR spectra of the central N and Ga atoms predicted in the 20-90-MHz range required the use of field-frequency ENDOR due to the large linewidth of the ENDOR lines. Our results confirm the importance of the nitrogen split interstitial in particle irradiated GaN similar to the case of diamond and silicon carbide in which the stable configuration at room temperature of the carbon interstitials is also the split interstitial configuration.

Simultaneous and spectroscopic redox molecular imaging of multiple free radical intermediates using dynamic nuclear polarization-magnetic resonance imaging.

Redox reactions that generate free radical intermediates are essential to metabolic processes. However, their intermediates can produce reactive oxygen species, which may promote diseases related to oxidative stress. We report here the use of dynamic nuclear polarization-magnetic resonance imaging (DNP-MRI) to conduct redox molecular imaging. Using DNP-MRI, we obtained simultaneous images of free radical intermediates generated from the coenzyme Q10 (CoQ10), flavin mononucleotide (FMN), and flavin adenine dinucleotide (FAD) involved in the mitochondrial electron transport chain as well as the radicals derived from vitamins E and K1. Each of these free radicals was imaged in real time in a phantom comprising a mixture of free radicals localized in either lipophilic or aqueous environments. Changing the frequency of electron spin resonance (ESR) irradiation also allowed each of the radical species to be distinguished in the spectroscopic images. This study is the first to report the spectroscopic DNP-MRI imaging of free radical intermediates that are derived from endogenous species involved in metabolic processes.

The Potential of Low Frequency EPR Spectroscopy in Studying Pottery Artifacts and Pigments.

ABSTRACTNon-destructive investigation, chemically fingerprinting, and authentication of ceramic cultural artifacts is a challenging analytical problem. Electron paramagnetic resonance (EPR) spectroscopy is capable of distinguishing between clays based on the paramagnetic metals present, and firing temperature (TF) based on the complexes of these metals formed at different TF values. Unfortunately, the 9 GHz frequency of conventional X-band EPR restricts sample size to a few mm and limits its applicability to small fragments. Low frequency EPR (LFEPR) is based on an EPR spectrometer operating at a few hundred MHz. LFEPR can utilize larger samples on the order of a few cm, but has a lower sensitivity due to the smaller Boltzmann ratio. Additionally, LFEPR may not be capable of detecting a spectral transition if the LFEPR operating frequency is less then the zero-field splitting of the paramagnetic metal complex. We utilized an LFEPR operating at 300 MHz which scans the applied magnetic field between the local Earth’s magnetic field and 26 mT to determine the feasibility of detecting EPR signals from clays, pigments, and glazes. Various clay samples were studied at 100 &lt; TF &lt; 1200 °C. Spectral differences were seen as a function of both clay type and TF. Differences in the LFEPR spectra of Han, Egyptian, and Ultramarine blue support the ability to distinguish among pigments. Paramagnetic impurities in glass may allow distinction between glaze spectra. We have also explored the utility of LFESR by the use of a radio frequency surface coil rather than an enclosed resonator. Although the active volume of the surface coil is ∼1 cm3, objects as large as 20 cm in diameter might be easily characterized with our spectrometer.

Design and Analysis of a Conduction-Cooled Superconducting Magnet for 30 kW Class Gyrotron

The purpose of this paper is to design and analyze the conduction cooled superconducting magnet for 30 kW class gyrotron. The superconducting magnet for gyrotron provides the central field of 3.57 T to obtain the electron spin resonance frequency of 95 GHz and the high field uniformity so that the electron beam fired from the electron gun can be concentrated on the resonant cavity without scattering of electron beam. For designing the superconducting magnets for gyrotron, the minimum propagation zone and the minimum quench energy of the superconductor composed of NbTi and copper were investigated. Also, the various shapes of superconducting magnet for gyrotron were designed with the given superconductor. The constraint conditions are the central field of magnet should be more than 4.0 T and the field nonhomogeneity should be less than 0.1% at z = ±0.1 m. The objective function is to minimize the amount of the superconductor. The central field and the field non-homogeneity, and the amount of the superconductor of the superconducting magnet for 30 kW class gyrotron were achieved according to the inner and outer diameter, the height of superconducting magnet, and the interval between elementary coils. The characteristics of the designed superconducting magnet were compared with those of the multi-physics analysis using commercial software.

Observation of large frequency shifts of the electron paramagnetic resonance ofRb87atoms due to collisions with cell walls coated with RbH salt

We report an observation of large frequency shifts on the order of several hundred Hz of the electron paramagnetic resonance (EPR) of the gas phase alkali-metal atoms ($^{87}\mathrm{Rb}$) in a millimeter-sized cell coated with RbH salt. We provide convincing evidence that the observed EPR frequency shift is due to collisions with RbH-coated cell walls. The RbH coating is nuclear spin polarized by spin exchange with optically pumped $^{87}\mathrm{Rb}$ vapor, and the observed frequency shift is caused by the Fermi contact interaction between the $s$ electron of the adsorbed $^{87}\mathrm{Rb}$ atom and the polarized nucleus on the salt surface. From the measured EPR frequency shift we calculated the ensemble-averaged phase shift $\ensuremath{\delta}{\ensuremath{\phi}}_{s}$ experienced by the spin polarized $^{87}\mathrm{Rb}$ atoms while adsorbed on the surface. Under our experimental conditions, we found the average phase shift $\ensuremath{\delta}{\ensuremath{\phi}}_{s}\ensuremath{\sim}70\phantom{\rule{0.28em}{0ex}}\mathrm{mrad}$.

Native and induced triplet nitrogen-vacancy centers in nano- and micro-diamonds: Half-field electron paramagnetic resonance fingerprint

Multiple frequency electron paramagnetic resonance (EPR) study of small (4–25 nm) nanodiamonds obtained by various dynamic synthesis techniques reveals systematic presence in the half-field (HF) region a distinctive doublet fingerprint consisting of resolved gHF1 = 4.26 and gHF2 = 4.00 signals. This feature is attributed to “forbidden” ΔMS = 2 transitions in EPR spectra of two native paramagnetic centers of triplet (S = 1) origin designated as TR1 and TR2, characterized by zero field splitting values D1 = 0.0950 ± 0.002 cm−1 and D2 = 0.030 ± 0.005 cm−1. Nanodiamonds of ∼50 nm particle size, obtained by crushing of Ib type nitrogen rich synthetic diamonds, show only HF TR2 signal whereas the same sample undergone high energy (20 MeV) electron irradiation and thermal annealing demonstrates rise of HF TR1 signal. The same HF TR1 signals appear in the process of fabrication of fluorescent nanodiamonds from micron-size synthetic diamond precursors. Results obtained allow unambiguous attribution of the half-field TR1 EPR signals with gHF1 = 4.26, observed in nano- and micron-diamond powders, to triplet negatively charged nitrogen-vacancy centers. These signals are proposed as reliable and convenient fingerprints in both qualitative and quantitative study of fluorescent nano- and micron-diamonds.

Triplet State of the Semiquinone–Rieske Cluster as an Intermediate of Electronic Bifurcation Catalyzed by Cytochrome bc1

Efficient energy conversion often requires stabilization of one-electron intermediates within catalytic sites of redox enzymes. While quinol oxidoreductases are known to stabilize semiquinones, one of the famous exceptions includes the quinol oxidation site of cytochrome bc1 (Qo), for which detection of any intermediate states is extremely difficult. Here we discover a semiquinone at the Qo site (SQo) that is coupled to the reduced Rieske cluster (FeS) via spin–spin exchange interaction. This interaction creates a new electron paramagnetic resonance (EPR) transitions with the most prominent g = 1.94 signal shifting to 1.96 with an increase in the EPR frequency from X- to Q-band. The estimated value of isotropic spin–spin exchange interaction (|J0| = 3500 MHz) indicates that at a lower magnetic field (typical of X-band) the SQo–FeS coupled centers can be described as a triplet state. Concomitantly with the appearance of the SQo–FeS triplet state, we detected a g = 2.0045 radical signal that corresponded to the population of unusually fast-relaxing SQo for which spin–spin exchange does not exist or is too small to be resolved. The g = 1.94 and g = 2.0045 signals reached up to 20% of cytochrome bc1 monomers under aerobic conditions, challenging the paradigm of the high reactivity of SQo toward molecular oxygen. Recognition of stable SQo reflected in g = 1.94 and g = 2.0045 signals offers a new perspective on understanding the mechanism of Qo site catalysis. The frequency-dependent EPR transitions of the SQo–FeS coupled system establish a new spectroscopic approach for the detection of SQo in mitochondria and other bioenergetic systems.

Spin readout and addressability of phosphorus-donor clusters in silicon

The spin states of an electron bound to a single phosphorus donor in silicon show remarkably long coherence and relaxation times, which makes them promising building blocks for the realization of a solid-state quantum computer. Here we demonstrate, by high-fidelity (93%) electrical spin readout, that a long relaxation time T1 of about 2 s, at B=1.2 T and T≈100 mK, is also characteristic of electronic spin states associated with a cluster of few phosphorus donors, suggesting their suitability as hosts for spin qubits. Owing to the difference in the hyperfine coupling, electronic spin transitions of such clusters can be sufficiently distinct from those of a single phosphorus donor. Our atomistic tight-binding calculations reveal that when neighbouring qubits are hosted by a single phosphorus atom and a cluster of two phosphorus donors, the difference in their electron spin resonance frequencies allows qubit rotations with error rates ≈10(-4). These results provide a new approach to achieving individual qubit addressability.

In Situ Determination of Manganese(II) Speciation in Deinococcus radiodurans by High Magnetic Field EPR: DETECTION OF HIGH LEVELS OF Mn(II) BOUND TO PROTEINS

High magnetic field high frequency electron paramagnetic resonance techniques were used to measure in situ Mn(II) speciation in Deinococcus radiodurans, a radiation-resistant bacteria capable of accumulating high concentrations of Mn(II). It was possible to identify and quantify the evolution of Mn(II) species in intact cells at various stages of growth. Aside from water, 95-GHz high field electron nuclear double resonance showed that the Mn(II) ions are bound to histidines and phosphate groups, mostly from fructose-1,6-bisphosphate but also inorganic phosphates and nucleotides. During stationary growth phase, 285-GHz continuous wave EPR measurements showed that histidine is the most common ligand to Mn(II) and that significant amounts of cellular Mn(II) in D. radiodurans are bound to peptides and proteins. As much as 40% of the total Mn(II) was in manganese superoxide dismutase, and it is this protein and not smaller manganese complexes, as has been suggested recently, that is probably the primary defense against superoxide.

Electron spin resonance studies of trityl OX063 at a concentration optimal for DNP.

We have performed temperature-dependent electron spin resonance (ESR) measurements of the stable free radical trityl OX063, an efficient polarizing agent for dissolution dynamic nuclear polarization (DNP), at the optimum DNP concentration (15 mM). We have found that (i) when compared to the W-band electron spin-lattice relaxation rate T1e(-1) of other free radicals used in DNP at the same concentration, trityl OX063 has slower T1e(-1) than BDPA and 4-oxo-TEMPO. At T > 20 K, the T1e(-1)vs. T data of trityl OX063 appears to follow a power law dependence close to the Raman process prediction whereas at T < 10 K, electronic relaxation slows and approaches the direct process behaviour. (ii) Gd(3+) doping, a factor known to enhance DNP, of trityl OX063 samples measured at W-band resulted in monotonic increases of T1e(-1) especially at temperatures below 20-40 K while the ESR lineshapes remained essentially unchanged. (iii) The high frequency ESR spectrum can be fitted with an axial g-tensor with a slight g-anisotropy: g(x) = g(y) = 2.00319(3) and g(z) = 2.00258(3). Although the ESR linewidth D monotonically increases with field, the temperature-dependent T1e(-1) is almost unchanged as the ESR frequency is increased from 9.5 GHz to 95 GHz, but becomes faster at 240 GHz and 336 GHz. The ESR properties of trityl OX063 reported here may provide insights into the efficiency of DNP of low-γ nuclei performed at various magnetic fields, from 0.35 T to 12 T.

NV-Diamond Magnetometer Using Electron Irradiation

ABSTRACTNitrogen-vacancy (NV) center in diamond is an emerging system for quantum-logic device and sensor applications. The key feature of the NV center is the ability of spin manipulation at room temperature. We apply a wide range of electron irradiation to generate the NV centers in nitrogen-rich diamond for creating best sensitivity. The NV0 and NV─ concentrations in electron irradiated diamond are determined from optical spectra. Additionally, electron spin resonance (ESR) has also proven to be an effective method for probing the electron spin transition between |ms=±1&gt; and |ms=0&gt; states of the NV centers. A study of ESR frequency shift and signal broadening and magnetometer sensitivity as a function of electron irradiation dose has been conducted. The research presented herein is a demonstration of minimum detectable magnetic field tailoring required for future-generation high-sensitivity diamond magnetometry.

Slow Magnetic Relaxations in Manganese(III) Tetra(meta-fluorophenyl)porphyrin-tetracyanoethenide. Comparison with the Relative Single Chain MagnetorthoCompound

Mn(III) tetra(meta-fluorophenyl)porphyrin-tetracyanoethenide coordination polymer (abbreviated meta-F) was synthesized and crystallographically and magnetically characterized. The compound crystallizes in the space group C2/c with four equivalent molecules in the unit cell arranged along two symmetry related nonparallel linear chain directions. Magnetic properties were studied by SQUID dc magnetization and ac susceptibility techniques and high field-high frequency electron spin resonance (HF-ESR). Glassy transition to a ferromagnetic-like state is observed at 10 K accompanied by slow magnetic relaxations. The glassiness is interpreted as due to 3D domain wall pinning. In a bias dc magnetic field the width of the relaxation time distribution decreases and the relaxations become similar to the relaxations of the single chain magnet Mn(III) tetra(ortho-fluorophenyl)porphyrin-tetracyanoethenide (abbreviated ortho-F), for which comparative HF-ESR studies were also conducted in this work. Magnetic properties of these two compounds are compared, and the nature of magnetic relaxations in meta-F is discussed.

Theory for cross effect dynamic nuclear polarization under magic-angle spinning in solid state nuclear magnetic resonance: the importance of level crossings.

We present theoretical calculations of dynamic nuclear polarization (DNP) due to the cross effect in nuclear magnetic resonance under magic-angle spinning (MAS). Using a three-spin model (two electrons and one nucleus), cross effect DNP with MAS for electron spins with a large g-anisotropy can be seen as a series of spin transitions at avoided crossings of the energy levels, with varying degrees of adiabaticity. If the electron spin-lattice relaxation time T(1e) is large relative to the MAS rotation period, the cross effect can happen as two separate events: (i) partial saturation of one electron spin by the applied microwaves as one electron spin resonance (ESR) frequency crosses the microwave frequency and (ii) flip of all three spins, when the difference of the two ESR frequencies crosses the nuclear frequency, which transfers polarization to the nuclear spin if the two electron spins have different polarizations. In addition, adiabatic level crossings at which the two ESR frequencies become equal serve to maintain non-uniform saturation across the ESR line. We present analytical results based on the Landau-Zener theory of adiabatic transitions, as well as numerical quantum mechanical calculations for the evolution of the time-dependent three-spin system. These calculations provide insight into the dependence of cross effect DNP on various experimental parameters, including MAS frequency, microwave field strength, spin relaxation rates, hyperfine and electron-electron dipole coupling strengths, and the nature of the biradical dopants.