Influence of Pressure on the Intramolecular Spin Exchange in a Short Imidazolium-Nitroxide Biradical

A short-chain imidazoline-type nitroxide biradical R ₅ ^NO –CH=N–N=C(CH₃)–R ₅ ^N , B2, with nitroxide rings R ₅ ^N and R ₅ ^NO as 1-oxyl-2,2,5,5-tetramethyl-3-imidazoline and 1-oxyl-2,2,5,5-tetramethyl-3-N-oxide imidazoline, respectively, has been studied using X-band electron paramagnetic resonance (EPR) spectroscopy in CH₃CN solutions at variable temperature T and pressure P. Changes of the solution viscosity on the intramolecular electron spin exchange in B2 is characterized by calculating the value of the exchange integral |J/a|, where a denotes the ¹⁴N hyperfine splitting (hfs) constant. It is revealed that the intramolecular dynamics in B2 do not follow the Debye–Stokes–Einstein law, while the Arrhenius dependence is fulfilled. Probable reasons of such behavior are discussed.     

High-frequency EPR of Cr<Superscript>2+</Superscript> ions in CdGa<Subscript>2</Subscript>S<Subscript>4</Subscript>

High-frequency broad-band (65–240 GHz) EPR is used to study impurity centers of bivalent chromium in a CdGa₂S₄ crystal. It is found that the EPR spectra correspond to tetragonal symmetry. The spin Hamiltonian H = βB · g · S + B ₂ ⁰ O ₂ ⁰ + B ₄ ⁰ O ₄ ⁰ + B ₄ ⁴ O ₄ ⁴ with the parameters B ₂ ⁰ =23659±2 MHz, B ₄ ⁰ =1.9±1 MHz, |B ₄ ⁴ |=54.2±2 MHz, g_‖=1.93±0.02, and g_⊥=1.99±0.02 is used to describe the observed spectra. It is concluded that chromium ions occupy one of the tetrahedrally coordinated cation positions.     

Ordering of the lowest tungsten carbide W<Subscript>2</Subscript>C

Atomic-vacancy ordering of the lowest tungsten carbide W₂C with the basis hexagonal structure of the L’3 type is analyzed by neutron and x-ray diffraction studies. It is found that the trigonal phase ?-W₂C (space group \(P\bar 3\) 1m) is the only ordered phase in the temperature range from ～2700 to 1370 K. The disorder-order phase transition channel associated with the formation of the trigonal ?-W₂C phase is found to include three superstructure vectors k ₁₅ ⁽¹⁾ , k ₁₅ ⁽²⁾ , and k ₁₇ ⁽¹⁾ of two stars {k ₁₅} and {k ₁₇}. The distribution function of the carbon atoms in the trigonal ?-W₂C superstructure is calculated.     

Formation of the incommensurate ordered phase in TaC<Subscript>y</Subscript> carbide

Structural neutron diffraction studies indicate that only one ordered phase arises after the disorder-order transition in nonstoichiometric cubic tantalum carbide TaC_y. This phase arises in the composition range y = 0.79–0.89 due to long-term annealing with a decrease in temperature from 1600 to 300 K. It is incommensurate in the [1–11]_B1 direction, but it is close to commensurate M₆C₅ structures (C2/m and P3₁ space groups) in mutual arrangement of atoms and vacancies in nonmetallic (1–11)_B1 planes. The disorder-order transition channel that is associated with the formation of the incommensurate superstructure in TaC_y carbide includes two arms k ₅ ⁽⁶⁾ ≈ 0.473b₂ and k ₅ ⁽⁵⁾ = ?k ₅ ⁽⁶⁾ of the {k₅} star and arms of the {k₄} and {k₃} stars. The translation period of the incommensurate phase in the [1–11]_B1 direction is 8.9–9.1 nm, which is larger than that in the commensurate phase M₆C₅ by a factor of about 18.     

Vibronic-spin-orbit interactions in heterocyclic analogues of fluorene with the N and O heteroatoms

The transition dipole moments P _0n ^s for the transitions from the electronic triplet state ³ B ²(ππ*) to vibrational sublevels of the vibrational out-of-plane modes n of the carbazole and dibenzofuran molecules are calculated. The values of the radiative deactivation rate constant k _rad ^s of the triplet sublevels T ^s are determined along with the components k _SO ^s and k _VSO ^s of this constant, which depend on the intramolecular spin-orbit (SO) and vibronic-spin-orbit (VSO) interaction. It is ascertained that k _rad ^z > k _rad ^y . For different structural units of the molecules (the heteroatom and the carbon atoms of the dibenzene fragment), the effect of the SO coupling on the constant k _VSO～Σ_{s, n} (P _0n ^s )² is studied. A competition between the effects on k _VSO from the SO coupling in the carbon atoms and in the light N and O heteroatoms is revealed. This competition accounts for the weak influence of the heteroatom on this component of the rate constant k _rad in these molecules. It is ascertained that the intensity distribution among the vibronic lines in the phosphorescence spectra of carbazole and dibenzofuran I _0n ～Σs (P _0n ^s )² is different due to the substantially different influence of the N and O heteroatoms on the deactivation of the triplet sublevel T ^y .     

Measurement of light scalar mesons in many-body decays in the PHENIX experiment at the Relativistic Heavy-Ion Collider

The unique design of the PHENIX detector at the Relativistic Heavy-Ion Collider allows one to detect neutral and charged particles produced in high-energy collisions of heavy ions. This circumstance made it possible to measure many-particle decays of light mesons, such as K _S ⁰ →π⁰π⁰, η→π⁰π^?π⁺, and η→γγ in p + p, d + Au and Au + Au collisions at the energy \(\sqrt {S_{NN} }\) = 200 GeV. The latest results of measuring the differential production cross sections, ratios of particle yields (K _S ⁰ /π⁰ and η/π⁰), and the nuclear modification factors (\(R_{dA}^{K_S }\), R _dA ^η , R _AA ^η ) in a wide range of transverse momenta (from 2 to 12 GeV s^?1) are reported.     

CO<Stack><Subscript>2</Subscript><Superscript>−</Superscript></Stack> radicals in synthetic hydroxyapatite

Powders of the B-type synthetic apatite exposed to gamma or ultraviolet irradiation were investigated using EPR spectroscopy. It was shown that ultraviolet irradiation leads to the appearance of the EPR spectrum near g = 2, which is similar to the spectrum observed upon gamma irradiation. The decomposition of the EPR spectra into components and the simulation of the shape of the experimental EPR signals revealed that these signals are associated primarily with two types of CO ₂ ^? radicals, namely, the axial CO ₂ ^? radicals and the orthorhombic CO ₂ ^? radicals. The differences in the shapes of the EPR spectra of the samples exposed to gamma and ultraviolet irradiation were explained by different ratios between the axial and orthorhombic CO ₂ ^? radicals. It was established that thermal annealing results in an increase in the relative contribution to the total EPR spectrum. This increase was explained by the transformation of the orthorhombic radicals into the axial radicals.     

Ab initio calculations of the geometry and electronic structure of point defects in ferroelectrics with a perovskite structure

Ab initio calculations of the optimized geometry and the electronic structure of lattice defects in incipient perovskite ferroelectrics SrTiO₃ and KTaO₃ are performed in the framework of the density functional theory. The results are presented for the Li⁺ impurity ion at the A site in the KTaO₃ and SrTiO₃ ferroelectrics; the Mn²⁺, Cd²⁺, Ca²⁺, Mg²⁺, and Zn²⁺ ions at the A site and the Mn⁴⁺ and Mg²⁺ ions at the B site in the SrTiO₃ compound; and the MN _Ti ²⁺ -V _O and Mg _Ti ²⁺ -V _O complexes in the SrTiO₃ ferroelectric. The results are obtained by the cluster method with allowance made for the structural relaxation initiated by the defect and, for nonisovalent substitutional impurities, with due regard for the charge and spin states of the defect. It is established that the Ca _Sr ²⁺ , Cd _Sr ²⁺ , Mn _Ti ⁴⁺ , and Mg _Ti ²⁺ ions have a stable central position, whereas the Li _K ⁺ ion in the KTaO₃ compound and the Li _Sr ⁺ , Mn _Sr ²⁺ , and Zn _Sr ²⁺ defects in the SrTiO₃ ferroelectric are off-center ions. The shape of the multiminimum adiabatic potential and the parameters of dielectric relaxators (activation barrier, dipole moment) for polar defects are obtained. The electronic impurity levels are determined for the Li _Sr ⁺ and Mg _Ti ²⁺ neutral defects.     

Analysis of the new data on single-particle structure of nuclei from the middle of the 1<Emphasis Type="Italic">f</Emphasis>–2<Emphasis Type="Italic">p</Emphasis> shell within the dispersive optical model

On the basis of the most reliable experimental information about the single-particle energies of neutron and proton states in ₂₀ ⁴⁸ Ca₂₈, ₂₂ ⁵⁰ Ti₂₈, ₂₄ ⁵² Cr₂₈, and ₂₆ ⁵⁴ Fe₂₈ nuclei, single-particle energies in mirror ₂₈ ⁴⁸ Ni₂₀, ₂₈ ⁵⁰ Ni₂₂, ₂₈ ⁵² Ni₂₄ and ₂₈ ⁵⁴ Ni₂₆ nuclei have been evaluated. The evaluated single-particle energies are compared with the energies calculated within the dispersive optical model. Good agreement between the calculated and experimental single-particle energies is demonstrated.     

Secondary ion emission from a GaAs single crystal upon bombardment with Bi<Stack> <Subscript> <Emphasis Type="Italic">m</Emphasis> </Subscript> <Superscript>+</Superscript> </Stack> cluster ions

Secondary-ion mass spectra and energy distributions upon bombarding a gallium arsenide single crystal using Bi_m⁺(m = 1–5) cluster ions with energies of 2–12 keV are investigated. The gallium cluster ion yield grew nonadditively with the number of atoms in the cluster projectiles. A quasi-thermal component found in the energy spectra of secondary Ga⁺ and Ga₂⁺ ions is indicative of the occurrence of the thermal spike mode upon cluster ion bombardment. The quasi-thermal component in the yield of atomic Ga⁺ ions upon bombardment with Bi₂⁺–Bi₅⁺–ions is 35–75%.     

Synthesis and electrical properties of new perovskite-like <Emphasis Type="Italic">A</Emphasis>Mn<Subscript>3</Subscript>V<Subscript>4</Subscript>O<Subscript>12</Subscript> (A = Ca,Ce, and Sm) compounds

AMn₃V₄O₁₂ (A = Ca, Ce, and Sm) compounds with a perovskite structure are synthesized at high pressures and temperatures. The crystalline structure of these compounds (space group \(Im\bar 3\)Z = 2) is determined via X-ray analysis. If ions in the A sublattice are changed in the order Ca²⁺–Sm³⁺–Ce³⁺, the valence is redistributed from Ca²⁺Mn₃²⁺V₄⁴⁺O₁₂ to Sm³⁺Mn₃²⁺V₄^3.75+O₁₂, and to Ce³⁺Mn₃²⁺V₄^3.75+O₁₂. The temperature dependences of the electrical resistivity are studied.     

Magnetoresistance of granular YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7 − δ</Subscript> HTSC in weak magnetic field: Angular dependence

The magnetoresistance of ceramic YBa₂Cu₃O_～6.5 HTSC samples is studied as a function of the mutual orientation of the current I and external magnetic field H _ext at T = 77.3 K in magnetic fields of up to ～500 Oe. It is found that, if the demagnetization factor D is taken into account, the effective critical field of complete penetration of Josephson vortices into weak links H _c2J ^eff does not depend on the mutual orientation of I and H _ext. The lower critical field H _c1A ^eff associated with the beginning of penetration of Abrikosov vortices into superconducting grains increases substantially with the angle between I and H _ext. The strongest variation with the mutual orientation of I and H _ext is exhibited by the critical field of the Bragg glass-vortex glass first-order phase transition H _BG-VG ^eff and by the magnetoresistance jump at this phase transition.     

Dipole moment functions for electronic transitions from ion-pair states of the second tier of molecular iodine

The emission spectra caused by the transitions from the ion-pair states and f0 _g ⁺ and G1_g of the I₂ molecule are obtained by excitation of individual rovibronic levels of the molecule by the method of optical-optical double resonance. The emission spectra from the state F0 _u ⁺ populated due to collisions I₂(f) + I₂(X) are also measured. By modeling the experimental emission spectra, the dipole moment functions for the electronic transitions f _g ⁺ -B0 _u ⁺ , A0 _u ⁺ , and B″0 _u ⁺ ; G1_g-A0 _u ⁺ and B″0 _u ⁺ ; and F0 _u ⁺ -X0 _g ⁺ and a′0 _g ⁺ of the iodine molecule are reconstructed.     

Hysteresis of the magnetoresistance of granular HTSC YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7 − δ</Subscript> in weak fields

Hysteresis of the magnetoresistance of ceramic YBa₂Cu₃O_～6.95 HTSC samples is studied at T = 77.3 K in an external magnetic field H _ext changing in 0 → H _max → 0 cycles, where H _max is the maximum magnitude of H _ext. Information is obtained about the dependences of the critical fields of Josephson weak links H _c2J , the lower critical fields of superconducting grains H _c1A , and the critical fields H _BG-VG of the Bragg glass-vortex glass phase transition in the vortex matter on transport current I, magnetic field, and the mutual orientation of I and H _ext. It is found that the magnetoresistance δρ⁺/ρ_{273 K} measured with increasing H _ext is significantly higher than Δρ^?/ρ_{273 K} and that H _c2J ⁺ < H _c2J ^? , H _c1A ⁺ < H _c1A ^? , and H _BG-VG ⁺ < H _BG-VG ^? .     

Photon distribution functions of the fluorescence photons from a single nanoparticle in various photon counting methods

Simple expressions have been derived for three photon distribution functions w _N ^M (T), w _N ^Z (T), and w _N ^O (T) corresponding to three different methods for counting fluorescence photons from a single nanoparticle excited by continuous laser radiation. In contrast to the previously derived expressions represented in the form of N multiple integrals, the new expressions contain only single or double integrals of Poisson functions, which makes it possible to easily perform the numerical calculation of the photon distribution. The simplest photon counting method corresponds to the lengthiest function w _N ^M (T); on the contrary, the simplest function w _N ^O (T) corresponds to the most complex photon counting method. The functions w _N ^M (T), w _N ^Z (T), and w _N ^O (T) are noticeably different in short time intervals T; however, the distributions calculated using these functions are almost indistinguishable from each other in long T intervals. This circumstance makes it possible to use the simplest function w _N ^O (T) to consider the photon statistics measured by the simplest method. This possibility is particularly important for investigating the fluorescence photon statistics, where the intensity fluctuates.     

Study of the effects of hydroxyapatite nanocrystal codoping by pulsed electron paramagnetic resonance methods

The effect of codoping of hydroxyapatite (HAP) nanocrystals with average sizes of 35 ± 15 nm during “wet” synthesis by CO₃^2? carbonate anions and Mn²⁺ cations on relaxation characteristics (for the times of electron spin–spin relaxation) of the NO₃^2? nitrate radical anion has been studied. By the example of HAP, it has been demonstrated that the electron paramagnetic resonance (EPR) is an efficient method for studying anion–cation (co)doping of nanoscale particles. It has been shown experimentally and by quantummechanical calculations that simultaneous introduction of several ions can be energetically more favorable than their separate inclusion. Possible codoping models have been proposed, and their energy parameters have been calculated.     

Genesis of the anomalous Hall effect in CeAl<Subscript>2</Subscript>

The Hall coefficient R _H , resistivity ρ, and Seebeck coefficient S of the CeAl₂ compound with fast electron density fluctuations were studied in a wide temperature range (from 1.8 to 300 K). Detailed measurements of the angular dependences R _H (? T, H≤70 kOe) were performed to determine contributions to the anomalous Hall effect and study the behavior of the anomalous magnetic R _H ^am and main R _H ^a components of the Hall signal of this compound with strong electron correlation. The special features of the behavior of the anomalous magnetic component R _H ^am were used to analyze the complex magnetic phase diagram H-T determined by magnetic ordering in the presence of strong spin fluctuations. An analysis of changes in the main contribution R _H ^a (H, T) to the Hall effect made it possible to determine the complex activation behavior of this anomalous component in the CeAl₂ intermetallic compound. The results led us to conclude that taking into account spin-polaron effects was necessary and that the Kondo lattice and skew-scattering models were of very limited applicability as methods for describing the low-temperature transport of charge carriers in cerium-based intermetallic compounds. The effective masses and localization radii of manybody states in the CeAl₂ matrix were estimated to be (55–90)m₀ and 6–10 Å, respectively. The behaviors of the parameters R _H , S, and ρ were jointly analyzed. The results allowed us to consistently describe the transport coefficients of CeAl₂.     

Phenomenological Λ-nuclear interactions

Variational Monte-Carlo calculations have been performed for the ligh s-shell hypernuclei, namely, _Λ ⁴ H, _Λ ⁴ H^*, and _Λ ⁵ He. The main aim of the study has been to give more insight into the Λ-nuclear interactions. Our study shows that the three-body ANN force has a larger share in the splitting energy of _Λ ⁴ H (0⁺–1⁺) compared to the two-body AN force. The analyses on Λ-binding to nuclear matter based on our s-shell results demonstrate that the three-body ANN correlations need to be incorporated suitably to describe these systems.     

Coulomb instability of charged clusters

A model different from the Rayleigh model for Coulomb instability of charged metallic clusters is proposed. The two-component model of a metallic cluster in the quasi-classical approximation offers different critical charges depending on the type of charged particles. For small-sized parallelepiped clusters, the quantization of the electronic spectrum is taken into account. The critical sizes of Ag _N ^2? and Ag _N ^3? clusters are calculated in the framework of the proposed model. The results of calculations are in good agreement with experimental data. The Coulomb explosion of positively charged clusters Na _N ⁿ⁺ at 3≤n≤5 is explained qualitatively.