Low-Temperature Phase Transition Detected by ESR in ����-(BEDT-TTF)2IBr2 Single Crystals

Hysteresis of the electron spin resonance (ESR) spectra of localized charge carriers has been observed in ????-(BEDT-TTF)₂IBr₂ single crystals in the temperature range of 15?C30?K. The first-order phase transition has been detected by ESR components. Two lines observed in the ESR spectrum correspond to low-temperature and high-temperature phases.     

Composition,temperature and radiation induced changes in gamma irradiated AAAMPS copolymer

Effect of composition, temperature and radiation dose in gamma irradiated acrylamide-2-acrylamido-2-methyl propane sulphonic acid (AA) copolymer has been investigated by electron spin resonance (ESR) and fourier transform infrared (FTIR) techniques. ESR spectra of gamma irradiated AA copolymer have been recorded under different conditions. The observed ESR spectra are analysed by computer simulation techniques, to separate the constituent component spectra. Magnetic parameters employed to simulate the component spectra enabled the identification of corresponding free radicals. The AA copolymer with low acrylamide content composed of macroradicals of the type ?CH₂?CH?CH₂? and methyl radicals (CH₃) whereas the copolymer with high acryl amide content possess methyl radicals and radicals of the type ?CH₂?C(CONH₂)?CH₂?/CH₃?C?CH₃. Reasons for the variation in the formation of free radicals have been explained. The observed changes in ESR spectra of irradiated AA copolymer at higher temperatures are thought to be due to the recombination of free radicals. Formation of free radicals found to be enhanced with the increase in dose of irradiation. FTIR spectra of pure and irradiated copolymers have also confirmed the previous results.     

Gd3+ electron spin resonance spectroscopy on LaO1 ? x F x FeAs superconductors

We report an electron spin resonance (ESR) spectroscopy study on polycrystalline samples of the LaO_{1 ? x} F _x FeAs (x = 0 and 0.1) compound with small levels of Gd doping (2% and 5%). The Gd ESR signal is found to be sensitive to the magnetic phase transition from the paramagnetic to the spin density wave (SDW) state occurring in the parent LaO_{1 ? x} F _x FeAs compounds at T _SDW ?? 130 K. Interestingly, the analysis of the low-temperature ESR spectra of the c-axis oriented Gd_{1 ? y} La _y OFeAs samples gives evidence for the magnetically nonequivalent Gd sites and also for sites having a different local charge environment. The analysis of the temperature dependence of the ESR linewidths gives evidence for a coupling of the localized 4f electrons of Gd to the conduction electrons in the FeAs layers. The ESR data reveal that the fluorine substitution, which provides electron doping, suppresses the SDW order and enhances the density of states in the electronic bands stemming from the xz and yz orbital states of Fe to which the 4f electrons are most strongly coupled.     

An electron spin resonance study of the soy bean phosphatidylcholine-based reversed micelles

The ESR spectra of l-palmitoyl-2-stearoyl-(n-doxyl)-glycero-3-phosphocholine spin label positional isomers (n = 5, 7, 10, 12 and 16) have been studied in soy bean phosphatidylcholine (SPC)-based microemulsions with various volume fractions of disperse phase over the wide temperature range. The maximum hyperfine splitting 2A _max and the order parametersS were taken as indices of the rotational mobility and the motion spatial restrictions of the labeled lipid chain segments. It is found that the temperaturesT _tr at which sharp enhancements of2A _max andS occur depend on concentration and size of the reversed micelles in solutions. To explain this, a plausible model, taking into account capability of the SPC molecule hydrocarbon chains to change a tilt angle with respect to the surface of a polar head group as temperature varies, is proposed. The estimations of the correlation times τ_sl obtained from the lineshape characteristics of the ESR spectra provided the possibility to suggest that these correlation times characterize the reorientations of the SPC chain axis about the normal to the surface of a polar head group of a reversed micelle.     

Evaluation of Conformational dynamics properties of spin-labelled proteins at different degrees of nitroxide radicals immobilization

Spin-labelling has found wide applications in elucidation of the dynamic behaviour of biological macromolecules in aqueous media and biomembranes. Most of the proposed methods aimed at estimation of macromolecular correlation times (τ_c) assume, however, spin label molecules rigidly bound within the protein matrix. To avoid this limitation theoretical models which involve additional dynamic parameters to characterize the spin label motion should be considered. We have used ESR spectra analysis technique which permits quantitative separation of slow macromolecular rotation (described by the rotational correlation time, τ_c) and fast anisotropic relative to protein nitroxyl radical motion (described by the “order parameter”,S). This method was applied to study: i) conformational dynamics of covalently and non-covalently spin-labelled human serum albumin (HSA) in solution; ii) protein-protein (antigen-antibody) interactions in a model system containing spin-labelled bovine serum albumin (BSA) and anti-BSA immunoglobulin (IgG) in solution; and iii) dynamic properties of membrane-bound proteins: H⁺-ATPase (CF₁-CF₀ coupling factor of photophosphorylation) and Photosystem I pigment-protein reaction centre complex (PSI RC) isolated from spinach chloroplasts and reconstituted in proteoliposomes.     

Poly(ethylene glycol) and phospholipid packing in the structure of reverse micelles

The influence of water substitution by a substance with a different polarity on the structure of phospholipid monolayer interface in water-in-oil microemulsion has been studied by the Fourier-transform pulsed-gradient spin-echo (FT PGSE)¹H nuclear magnetic resonance (NMR) and electron spin resonance (ESR) spin-label methods. For this purpose the soybean phosphatidylcholine-based microemulsion and water soluble poly(ethylene glycol) with molecular weight 400 (PEG400) were used. Self-diffusion coefficients of all microemulsion components obtained by the FT PGSE NMR technique provided information about both the size of reverse micelles and distribution of components between different microemulsion compartments. The maximum hyperfine splitting, 2A _max, in the ESR spectra was used to characterize the degree of the phospholipid hydrocarbon chain mobility. It was shown that PEG400 alters significantly the size of the reverse micelles and the motion of the labeled segments of the lipid tails. A mechanism of PEG400 acting in solution of the phospholipid-based reverse micelles on the basis of the rough decrease of the micelle core polarity was suggested.     

Electron spin resonance study of Fe doping effect in La0.67Ca0.33MnO3

Electron spin resonance (ESR) study was carried out on La_0.67Ca_0.33Mn_1−xFe_xO₃ (x=0.0, 0.04) samples. The temperature dependence of the ESR spectra indicates the presence of phase separation above and below T_C in x=0.0 and 0.04 sample, respectively. The increase of the g-value in the high-temperature region indicates the existence of local spin correlations even in the paramagnetic state. The activation energy obtained from both the temperature dependence of the ESR intensity and linewidth exhibits a smaller value in the Fe-doped sample. Our study suggests that the ferromagnetic spin correlations would be significantly weakened by a slight doping of Fe ions on Mn sites.     

Azurin-Solvent interaction: an ESR spin labeling investigation

We have investigated by means of electron spin resonance (ESR) spectroscopy using two spin labels, Iodoacetamido-proxyl and 3-Maleimido-proxyl, the dynamics of two different regious around the active site of azurin, a copper containing blue protein. The ESR measurements of spin labeled azurin have been carried out in the 110–300 K temperature range on wet (H₂O, D₂O and ethanol/water mixtures) and lyophilized samples. The behaviours of the outer hyperfine splitting separation, 2A′ _zz , of the ESR spectravs temperature of the lyophilized, and fully hydrated azurin in H₂O and D₂O suggest that the two spin labels are located in regions of the protein surface with different dynamics and polarity. Moreover, all differences in the 2A′ _zz values shown by the spin labeled azurin in normal and heavy water as well as the temperature behaviour disappear when azurin is in ethanol/water mixtures. The results are discussed in terms of a close correlation between the molecular dynamics of the protein fragments to which the two spin labels are bound and the properties of the solvent used.     

Polaron motional narrowing of electron spin resonance in organic field-effect transistors

Polaron states in organic thin-film transistors (TFTs) were investigated by the electron spin resonance (ESR) technique. Gate-field-dependent and temperature-dependent single-Lorentzian ESR spectra were observed for field-induced polarons in pentacene TFTs, demonstrating the effect of motional narrowing due to polaron diffusion. Analyses of the ESR linewidth revealed a considerably long trapping time (tau_(C) approximately 0.7 ns), the variation of which is discussed in terms of the multiple trap-and-release model.     

Electron spin resonance study of the dynamic magnetic susceptibility of CuO, Cu1−x Zn x O, and Cu1−x Li x O single crystals

Results are presented of studies of the dynamic magnetic susceptibility of CuO, Cu_1?x Zn _x O (x ≈ 1.5%), and Cu_1?x Li _x O (x ≈ 1%) single crystals. The orientational dependence of the ESR spectra was investigated at room temperature. The results for CuO are analyzed using a model of a quasi-one-dimensional antiferromagnet (S = 1/2) with anisotropic exchange interaction between Cu²⁺ spins in the chains and exchange coupling between the chains allowing for one-dimensional spin diffusion and spinon excitations. The estimated line width is of the same order of magnitude as the experimental data. Substituting Cu with Zn scarcely alters the spin dynamics of the Cu²⁺ ions, as in weakly diluted magnets. Lithium doping substantially increases the ESR line width and this is attributed to excess holes forming rapidly relaxing spin complexes with copper ions.     

Effect ofV K centers on positron annihilation in KCl and Ag-doped KCl

Positron lifetime spectra have been measured at 77 K for KCl and Ag-doped KCl before and after x-irradiation at 77 K and after annealing at room temperature. Radiation at 77 K reduces the intensity of the intermediate lifetime (τ₂) component. Radiation-induced defects were monitored optically and by ESR. The experiment shows that the changes observed in the positron decay are associated with the presence ofV _K centers.     

Multipulse sequences for explosives detection by NQR under conditions of magnetoacoustic and piezoelectric ringing

A number of methods for cancelling magnetoacoustic and piezoelectric ringing signals in the spectroscopy of the nuclear quadrupole resonance are presented. The suggested methods include using the sequence (?₀)_?-(τ-?_x-2τ-? _y -2τ-?_?x -2τ-?_?y -τ) _n and a multipulse analog of the two-pulse Hahn sequence with the first pulse replaced by a short steady-state sequence. Another method presented is the method of orthogonal effective fields for a fast saturation of the quadrupole spin system which can be used for subtracting the magnetoacoustic and piezoelectric components from the signal. The suggested methods can be used for the practical purposes of detecting explosive substances and narcotics.     

Structural and electronic properties of the LiNiPO4 orthophosphate

Microcrystalline LiNiPO₄ powders have been prepared by solid-state reaction using various precursors. Characterization of the structure and morphology of powders was performed using XRD, SEM, HRTEM, Raman, and FTIR. The electronic properties of materials were investigated by SQUID and ESR. The LiNiPO₄ material adopts the olivine-like structure (Pnma S.G.). Analysis of the Raman and FTIR spectra figures out, with the aid of a molecular vibration model, the bonding between NiO₆ octahedral and (PO₄)^3? tetrahedral groups. The electronic configuration and the local cationic arrangement are confirmed by magnetic susceptibility and electron spin resonance spectroscopy.     

Structural and electrochemical properties of LiNi1/3Co1/3Mn1/3O2 material prepared by a two-step synthesis via oxalate precursor

LiNi_1/3Co_1/3Mn_1/3O₂ (LNMCO) powders were formed by a two-step synthesis including preparation of an oxalate precursor by ??chimie douce?? followed by a solid-state reaction with lithium hydroxide. The product was characterized by TG-DTA, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FTIR), Raman spectroscopy, electron spin resonance (ESR), and SQUID magnetometry. XRD data revealed well-crystallized layered LNMCO with ??-NaFeO₂-type structure (R-3?m space group). Morphology studied by SEM and TEM shows submicronic particles of 400?C800?nm with a tendency to agglomerate. The local structure investigated by vibrational spectroscopy (FTIR, Raman), ESR, and SQUID measurements confirms the well-crystallized lattice with a cation disorder of 2.6% Ni²⁺ ions in Li(3b) sites. Electrochemical tests were carried out in the potential range 2.5?C4.5?V vs. lithium metal on samples heated at 900?°C for 12?h. Initial discharge capacity is 154 mAh/g at C/5, while a capacity of 82 mAh/g is still delivered at 10 C by the two-step synthesized LiNi_1/3Co_1/3Mn_1/3O₂ as cathode material.     

Electron Spin Resonance of Free Radicals Produced by Ultraviolet Photolysis in Some Vegetables and Their Juices

Abstract

Paramagnetic species produced by ultraviolet photolysis in various vegetables of domestic origin have been investigated by electron spin resonance technique. The ESR spectra of the small cut pieces of the vegetable fleshes and their frozen juices have been investigated before and under UV photolysis. The samples of potato and carrod exhibit too complex ESR spectra, but parsley, dill, white radish and green pepper do not exhibit any ESR signal before UV photolysis at 113 K. However both the small cut pieces of fleshes of potato, carrot, parsley, dill, white radish and green pepper and their juices exhibit ESR spectra during UV photolysis that we attributed to the H?O and CO^? ₂ radicals. The ESR parameters of these radicals have been obtained and the origins of these radicals have been shown to be the glucose molecules in the carbohydrate chains of these substances.     

Submillimeter ESR spectra of Fe<Superscript>2+</Superscript> ions in synthetic and natural beryl crystals

Electron spin resonance spectra of non-Kramers bivalent iron (Fe²⁺) ions have been detected in synthetic and natural beryl crystals with an iron impurity. The observed ESR spectra have been attributed to resonance transitions of Fe²⁺ ions from the ground (singlet) state to excited (doublet) levels with the splitting Δ = 12.7 cm^–1 between the levels. The experimental angular and frequency dependences of the resonance field of the ESR signal have been described by the spin Hamiltonian with the effective spin S = 1. The analysis of the ESR data and optical absorption spectra indicates that the Fe²⁺ ions are situated in tetrahedral positions and substitute Be²⁺ cations in the beryl structure.     

High-spin states in the N = 84 nucleus 152Er

High-spin states in ¹⁵²Er have been populated through the ¹⁴⁴Sm(¹²C, 4n)¹⁵²Er reaction. Excitation functions, angular distributions, prompt and delayed coincidences against the beam burst were measured. Three isomeric states at 2183 keV (τ = 1.8 ns), 4519 keV (τ = 1.2 ns) and 4915 keV (τ = 6.6 ns) have been observed. The decay scheme is developed up to 7 MeV and is discussed and compared with that of ¹⁵⁰Dy.     

Chemical quenching of positronium in Fe2O3/Al2O3 catalysts

Fe₂O₃/Al₂O₃ catalysts were prepared by solid state reaction method using α-Fe₂O₃ and γ-Al₂O₃ nano powders. The microstructure and surface properties of the catalyst were studied using positron lifetime and coincidence Doppler broadening annihilation radiation measurements. The positron lifetime spectrum shows four components. The two long lifetimes τ₃ and τ₄ are attributed to positronium annihilation in two types of pores distributed inside Al₂O₃ grain and between the grains, respectively. With increasing Fe₂O₃ content from 3 wt% to 40 wt%, the lifetime τ₃ keeps nearly unchanged, while the longest lifetime τ₄ shows decrease from 96 ns to 64 ns. Its intensity decreases drastically from 24% to less than 8%. The Doppler broadening S parameter shows also a continuous decrease. Further analysis of the Doppler broadening spectra reveals a decrease in the p-Ps intensity with increasing Fe₂O₃ content, which rules out the possibility of spin-conversion of positronium. Therefore the decrease of τ₄ is most probably due to the chemical quenching reaction of positronium with Fe ions on the surface of the large pores.     

Why could electron spin resonance be observed in a heavy fermion Kondo lattice?

We develop a theoretical basis for understanding the spin relaxation processes in Kondo lattice systems with heavy fermions as experimentally observed by electron spin resonance (ESR). The Kondo effect leads to a common energy scale that regulates a logarithmic divergence of different spin kinetic coefficients and supports a collective spin motion of the Kondo ions with conduction electrons. We find that the relaxation rate of a collective spin mode is greatly reduced due to a mutual cancellation of all the divergent contributions even in the case of the strongly anisotropic Kondo interaction. The contribution to the ESR linewidth caused by the local magnetic field distribution is subject to motional narrowing supported by ferromagnetic correlations. The developed theoretical model successfully explains the ESR data of YbRh₂Si₂ in terms of their dependence on temperature and magnetic field.     

X-Band ESR and51V NMR study of the Haldane system PbNi2−xMgxV2O8

The temperature and angular dependence of the X-band electron spin resonance (ESR) and⁵¹V nuclear magnetic resonance (NMR) spectra have been measured in a recently discovered Haldenegap system, PbNi_2-xMg_xV₂O₈ (0≤x≤0.24). The angular dependence of the ESR signal suggests that both the spin diffusion as well as the magnetic anisotropy determine the electronic spin correlation functions. However, in doped samples the magnetic anisotropy increasingly dominates the spin dynamics on cooling. The huge broadening of the⁵¹V NMR spectra in doped samples at low temperatures provides evidence for localized magnetic moments in the vicinity of the Mg impurities. Locally distorted structure around each Mg impurity may slightly modify the magnetic interactions and be potentially responsible for the antiferromagnetic ordering (belowT _N≈ 3.5K) in doped compositions.