A quantitative analysis of recombination data in high magnetic fields

Magnetic field effects on the hydrogen abstraction reaction of 4-methoxybenzophenone with thiophenol in several solvents of different viscosity have been reported, and the observed magnetic field dependence was explained as caused by the Δg and a polarized initial triplet radical pair state. The present work reports a quantitative analysis of the data based on a recently derived general analytical formula. It is found that the observed magnetic field dependence can be explained as originating from an unpolarized triplet state, if both the coherent mixing caused by different g values of the two radicals and the incoherent mixing due to spin relaxation are included. Several different expressions for the magnetic field dependence of the longitudinal and transverse relaxation rates were applied. Rather surprisingly, the different models gave almost identical fits. However, the values obtained of the microscopic parameters depended significantly on the model. Physically sensible parameter values were obtained only when the complete magnetic field dependence of the two relaxation times were used. For this model it was found that both the anisotropy factor of the g tensors and the diffusion coefficient agreed with expectations.     

Trapping excess electrons in molecular charge pockets on surfaces

This work considers the use of theoretical ab initio calculations to design a novel molecular trap for excess electrons. The basic principle is to develop a model by which these electrons can be stabilized by charge pockets on the surface of molecules. Calculations reveal that systems with OH groups can form stable hydrogen-bonded networks on one side of a hydrocarbon surface (i.e. cyclohexane sheets), while the hydrogen atoms on the opposite side of this surface form a pocket of positive charge that attracts the extra negative charge.     

Aromaticity on the edge of chaos: an Ab initio CCSD(T) study of the bimodal balance between aromatic and non-aromatic structures for 10-π-diheterocins and heteronins

CCSD(T) coupled cluster ab initio SCF-MO calculations for 10-π-heteroannulenes reveal a range of potential surface characteristics, ranging from single-minimum aromatic planar species to triple-minimum systems involving both planar-aromatic and the two enantiomers of a C₂ symmetric non-planar non-aromatic species. For the specific case of 1,4-dioxocine, the existence of barriers separating the three minima is attributed to an anti-aromatic Möbius-like transition state connecting the two equilibrium forms.     

Exchange interaction and Jahn—Teller correlations in novel tetranuclear supramolecular Cu(II) grid complexes: an ESR study

Novel tetranuclear Cu(II) complexes in which four Cu(II) ions are arranged in a square planar fashion by four bis(bipyridyl)pyrimidine ligands were investigated by X and Q band ESR in the temperature range 4.2-300 K. The ESR spectra of this grid-like structure were well simulated as rising from triplet species. Analysis of their intensities allowed the spectra to be assigned to the first excited triplet state, and revealed intramolecular antiferromagnetic coupling of the Cu(II) spins. The isotropic exchange interaction J was determined as ?47 K and reducing to about ?5 K on functionalizing the bridging pyrimidine ring at the 2-position by methyl or phenyl. For comparison an ESR investigation was also carried out on the mononuclear analogue Cu(II)—(terpyridine)₂ complexes with substituted terpyridine ligands at the 5′ and 5″ position. Depending on the substitutent, the spectra exhibit a static or dynamic Jahn—Teller effect at room temperature. The temperature dependence of their g-values is examined by a modified Silver—Getz model which includes cooperative Jahn—Teller interactions. There is evidence that both an anisotropic spin exchange contribution to D = 0.0159 cm^?1 and a coupled (static) Jahn—Teller effect are responsible for efficient coupling between the four Cu(II) ions in the grid complexes with non-substituted pyrimidine bridges.     

Factors affecting the dimerization of persistent nitrogen‐centered 1‐phenyl urazole radicals to tetrazanes

1‐Phenyl urazole radicals are persistent air‐stable nitrogen‐centered radicals that engage in an equilibrium with the corresponding N―N tetrazane dimers in solution. While the equilibrium typically weakly favors the dimer form, for some 1‐phenyl urazole radicals bearing substituents at the ortho position of the benzene ring, the equilibrium instead strongly favors the dimer form. With the recent surge of interest in the properties and potential applications of heterocyclic radicals, the factors that affect this equilibrium are important to determine. We examined the effect of the extent of ortho substitution (none, 1, or 2 substituents) on the equilibrium by experimentally using variable temperature ¹H nuclear magnetic resonance and UV‐visible spectroscopy in addition to supporting computational investigations at the (U)B3LYP/6‐311G(d,p) level of theory. We confirmed that the equilibrium generally favored the dimer in all cases. However, the equilibrium was more favorable towards dimer formation for urazole radicals substituted with 1 and 2 ortho substituents on the aromatic ring. The activation enthalpies for dissociation of singly substituted dimers were greater than that for dimers without ortho substituents, but lower than that for doubly substituted dimers. The greater preference for dimer formation for the ortho‐substituted urazole radicals is attributed to a greater enthalpic advantage for N―N bond formation. This advantage may be traced to a higher concentration of spin density on the urazole unit of the radicals and a lesser deformation energy required for N―N bond formation.     

E.S.R. of sulphur-substituted purines and nucleosides; carbon-centred radicals in mercaptopurine crystals at 77 K

Single crystals of the sulphur-containing DNA-base analogue 6-methylmercaptopurine (6MeMP) and its riboside 6-methylmercaptopurine riboside (6MeMPR) have been prepared and irradiated by 4·0 MeV electrons at 77 K. Electron spin resonance techniques have been used to study the radiation-induced radicals at 77 K. The primary carbon-centred radical, common to both molecules, has been identified as a species formed by hydrogen atom abstraction from the methyl group. The principal values of the two α-proton hyperfine coupling tensors and the g-tensors were almost the same for both molecules and for 6MeMPR were: α1, -28·8, -17·9 and -6·4 G; α2, -28·2, -15·7 and -9·8 G; and g, 2·0063, 2·0024 and 2·0018. These data indicate a spin density of 0·77 on the methyl carbon atom. Molecular orbitals determined from CNDO/2 methods were used in calculations of the directions and magnitude of the g-tensor principal values. Comparison of these calculated values and experimental data suggests that contribution of spin density in d-orbitals on the sulphur atom is important in describing the g-tensor. Methyl H-abstraction radicals trapped in pairs were also detected in 6MeMP and the data are consistent with an effective interspin distance of 4·67 Å.     

Low-energy electron spectrum in copper oxides in the multiband p-d model

An exact diagonalization of the Hamiltonian in the p-d model of a CuO₆ cluster was used to obtain dependences on the model parameters of the lowest-energy two-hole terms: the energy difference between the 2p orbitals of planar and apical oxygen Δ(apex)=ε(2p)−ε[2p(apex)], the crystal field parameter , and the ratio of the distances between the copper atom and the apical and planar oxygen atoms d(apex)/d(pl). In the limit of large d(apex)/d(pl) and Δ_d, our model is equivalent to the three-band p-d model and, in this case, large singlet-triplet splitting Δε⩾1 eV is also observed. As the parameters decrease, a singlet-triplet crossover is observed. Two mechanisms are identified for stabilization of the triplet term ³ B _1g (0) as the ground state. It is shown that for realistic values of the parameters, reduction of the p-d model to the three-band model is limited by the low energies of the current excitations because of the presence of the lower excited ³ B _1g and ¹ A _1g cluster states. Intercluster hopping causes strong mixing of singlet and triplet states far from the G point. The results of the calculations are compared with data obtained by angle-resolved photoelectron emission in Sr₂CuO₂Cl₂. Fiz. Tverd. Tela (St. Petersburg) 40, 184–190 (February 1998)     

New high magnetic field phase of the frustrated <Emphasis Type="Italic">S</Emphasis> = 1/2 chain compound LiCuVO<Subscript>4</Subscript>

Magnetization of the frustrated S = 1/2 chain compound LiCuVO₄, focusing on high magnetic field phases, is reported. Besides a spin-flop transition and the transition from a planar spiral to a spin modulated structure observed recently, an additional transition was observed just below the saturation field. This newly observed magnetic phase is considered as a spin nematic phase, which was predicted theoretically but was not observed experimentally. The critical fields of this phase and its dM/dH curve are in good agreement with calculations performed in a microscopic model (M.E. Zhitomirsky and H. Tsunetsugu, Europhys. Lett. 92, 37001 (2010)).     

Role of zinc and nickel impurities in high-temperature superconductors

The local changes produced in the electronic structure and their effect on the physical properties of the superconducting and normal phases when zinc and nickel are substituted for copper are examined on the basis of a multiband p-d model. It is shown that strong electronic correlations suppress the S=1 configuration of Ni²⁺ and cause the superposition of the S=1/2 and S=0 states of nickel. The change in the density of states in p-and n-type systems is studied, and the peculiarity of Zn impurity for p-type systems and Ni impurity for n-type systems is shown. The universal dependence of the T _c on the residual resistance in lightly doped superconductors and deviations from it in optimally doped systems are discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 596–600 (April 1999)     

Influence of the quantum size effect for grazing electrons on the electronic conductivity of metal films

The thickness dependence of the electronic conductivity of thin (5–150 nm) single-crystal (100) films of refractory metals is investigated at different temperatures ranging from 4.2 K to room temperature. Regions of square-root, quasilinear, and quadratic dependences are observed. The quasilinear thickness dependence is explained by the influence of quantum effects on the transverse motion of electrons in the case when electron scattering by the film surfaces dominates. For macroscopic film thicknesses 30–50 nm, much greater than the Fermi wavelength of an electron, quantum corrections to the electronic conductivity reach values of the order of 50%. This is a consequence of the quantum size effect for grazing electrons, which leads to an anomaly in electron scattering by the film surfaces. The region of the quadratic thickness dependence corresponds to the quantum limit, and the square-root region corresponds to the classical limit. The effect is explained in a quasiclassical two-parameter model (the effective angle α* for small-angle electrons and the parameter γ, equal to the ratio of this angle to the diffraction angle) that takes into account the diffraction angular limits for grazing electrons. The effect occurs for parameters α*≪1 and γ∼1 and differs from the “ordinary” quantum size effect. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 11, 693–698 (10 December 1997)     

Cross-section energy dependence of planar and non-planar two-body hypercharge-exchange reactions

The energy dependence of total cross sections for twenty-three two-body hypercharge-exchange reactions is studied. It is found that the planar reactions have in general a less steep fall-off with increasing energy than the non-planar reactions. Explanations of the difference in terms of kinematics and dynamics (baryon exchange and t-channel exchange effects) are discussed and found highly improbable. Implications of the effects observed are discussed.     

Isotope effects on the electronic critical points of germanium: Ellipsometric investigation of the and transitions

Within the past years the optical excitations of electrons have been measured for semiconductor samples of different isotope compositions. The isotope shift observed have been compared with calculations of the effects of electron-phonon interaction on the electronic band structure. While qualitative agreement has been obtained, some discrepancies remain especially concerning the E₁ and transitions. We have remeasured the effect of isotope mass on the E₁ and transitions of germanium with several isotopic compositions. The results, obtained by means of spectroscopic ellipsometry, confirm that the real part of the gap self-energies induced by electron-phonon interaction is larger than found from band structure calculations, while the imaginary part agrees with those calculations, which are based on a pseudopotential band structure and a bond charge model for the lattice dynamics. Our results agree with predictions based on the measured temperature dependence of the gaps. We compare our data for E₁ and with results for the lowest direct (E₀) and indirect (E_g) gaps. The measured values of and increase noticeably with increasing isotope mass. Similar effects have been observed in the temperature dependence of in and . A microscopic explanation for this effect is not available. Received: 6 March 1998 / Revised: 27 April 1998 / Accepted: 15 May 1998     

Coexistence of antiferromagnetic and ferromagnetic phase for ferromagnetic Kondo lattice model

To study the proposed phase separations in doped manganites, we performed Monte-Carlo calculations for the ferromagnetic Kondo lattice model with strong Hund's coupling between conduction electrons and localized spins. For the practical calculations, we adopted a one dimensional lattice and treated the spins of the localized t_2g electrons semi-classically. A direct evidence of the phase separation is observed from a snapshot of the spatial dependence of localized spins. No indication of the canted or spiral phases is found in the results of simulations. Further, the calculated results of the spin structure factor in the phase separation region are well compared with recent experiments. Received: 1st September 1998 / Revised: 30 October 1998 / Accepted: 27 November 1998     

Zeeman effect and magnetic anomalies in narrow-gap semiconductor quantum dots

We present a systematic theoretical study, based on the Kane–Weiler 8×8 k·p model, of the linear Zeeman splitting introduced by the interaction between the angular momentum and the magnetic field which can give a measure of the non-linear Zeeman effect associated with interband coupling and diamagnetic contributions. The conduction and valence bands g-factors are calculated for InSb spherical and semi-spherical quantum dots. The calculations of the g-factors showed an almost linear dependence, for the ground state, on the magnetic field. We have also found that the strong magnetic field dependence as well as the dependence on the dot size of the effective spin splitting can be unambiguously attributed to the strength of the inter-level mixing.     

Crystal field in valence-fluctuating CeNi-based compounds

The effects of the crystal field (CF) on the paramagnetic Pr ion in a number of compounds of the type R_1−x Pr_xNi (R = Ce, La, Y), in which a transition of the cerium ions from an intermediate-valence into a Kondo state occurs as La is substituted for Ce, are investigated. The level schemes of the Pr ion in the CF are reconstructed from inelastic neutron scattering spectra and the temperature dependence of the heat capacity in different magnetic fields (B=0–8 T). The parameters of the low-symmetry CF in the compounds RNi are determined from the experimental data. It is established that in the Kondo regime the hybridization of the f electrons with conduction electrons only gives a proportional increase in all the parameters of the CF potential. At the same time, partial delocalization of the f electrons in the intermediate-valence state results in charge redistribution, which is manifested in different scales for the changes in the different CF parameters. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 12, 947–952 (25 June 1996)     

Study of J-T effect on the self-energy of electrons in manganite systems

We present here a theoretical study of the effect of Jahn-Teller(J-T) distortion on the self-energy of electrons in the CMR manganites. The model consists of the itinerant e _g electrons distorted by J-T effect and the localized t _2g core electrons carrying strong ferromagnetism due to Hund’s rule. The phonon interacts with the e _g electrons as well as the J-T distorted e _g band. The electron Green’s functions are calculated by Zubarev’s technique. The electron self-energy which carries all the information of the model is calculated from the Green’s function. The effect of J-T distortion, magnetism on the frequency and temperature dependent dynamic self-energy is presented in this paper. The results are discussed.     

Transformations of radicals and ions in irradiated sodium sulfate doped with nitrate ions

The structure and properties of the paramagnetic centers formed by γ-irradiation at 77 K in sodium sulfate doped with nitrate ions have been investigated by the EPR method. The NO^2? ₃, NO₂ and SO^? ₄ radicals have been identified. The orientation of NO^2? ₃ relation to crystallographic axes is determined. In the 77-400 K temperature range the transformations of observable radicals have been studied. The mechanisms of their formation and thermal annealing have been discussed. The symmetry of nitrate ions in sodium sulfate was investigated by diffuse reflectance infrared Fourier transform spectroscopy. At the concentration of NO^? ₃ up to 5.5 × 10¹⁸ g^?1 the nitrate ion was supposed to have a planar or pyramidal configuration of the D_3h or C_3V symmetries. At the concentration of the dopant higher than 5.5 × 10¹⁸ g^?1 the nitrate ions with the C_2V symmetry were observed.     

Solution of the Dirichlet problem for the Laplace equation for a multiply connected region with point symmetry

A method is proposed which uses an expansion of the potential in irreducible representations of the symmetry group of the field-defining elements of a system. A boundary-value problem is solved for multipole systems with planar plate electrodes for the C _nv symmetry group. A quadrature expression is obtained for the field potential of these systems. Constraints imposed on the electrode potentials, under which such a solution is possible, are determined. Results of calculations of the potential distribution are presented for various specific systems. Zh. Tekh. Fiz. 69, 1–9 (March 1999)