Spatial distribution and formation of nitrate radical NO<Stack><Subscript>3</Subscript><Superscript>2−</Superscript></Stack> in Antarctic calcitic evaporates

Nitrate radical NO₃²⁻ in calcitic evaporate was discovered in Antarctica. The distribution and formation of nitrate radical NO₃²⁻ in the calcite have been studied by pulse and continuous-wave electron spin resonance. In samples that had been annealed to destroy the NO₃²⁻, regeneration of the radical by γ-rays or UV light indicated that the radical was formed by UV light (with wavelengths less than 340 nm) from solar rays, not by environmental radiation. The nonuniform spatial distribution of the nitrate radical, which was deduced from high ratios of local spin density to total spin density, suggests that the nitrate impurity was introduced into the calcium carbonate after carbonate grain formation. Formation of the carbonate-containing nitrate requires the presence of high amounts of nitrate and a dry climate. Formation of the nitrate radical requires sample exposure to UV light. These conditions are satisfied in the environment of Antarctica.     

ESR of V4+ in α-RbTiOPO4 single crystals

We have recorded and investigated the ESR spectrum of vanadium-doped α-RbTiOPO₄ single crystals in the temperature interval 77–300 K. Two types of structurally distinct centers, V1 and V2, with a 4:1 ratio of the peak intensities were observed. The angular dependences of the resonance magnetic fields are described by a spin Hamiltonian corresponding to axial symmetry with the parameters g _∥1=1.9305, g _⊥1=1.9565, A _∥1=−168.2×10⁻⁴cm⁻¹, and A _⊥1=−54.3×10⁻⁴cm⁻¹ for V1 centers and g _∥2=1.9340, g _⊥2=1.9523, A _∥2=−169.0×10⁻⁴cm⁻¹, and A _⊥2=−55.2×10⁻⁴cm⁻¹ for V2 centers. A model of a paramagnetic center is proposed: The vanadium ions replace titanium ions in two structurally distinct positions Ti1 and Ti2 (V1 and V2 centers, respectively). The possibility that a VO²⁺ ion forms when α-RbTiOPO₄ crystals and crystals of the KTP group (KTiOPO₄, NaTiOPO₄, α-and β-LiTiOPO₄), studied earlier, are doped with vanadium is discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 534–536 (March 1998)     

Polarization of leading Λ hyperons produced by neutrons on target nuclei

The polarization of the leading Λ hyperons produced on carbon and lead target nuclei by 4–10 GeV neutrons in the angle range Θ<8.5° with respect to the beam and with only neutral-particle accompaniment is measured: 〈Π〉=−0.096±0.018 for C and 〈Π〉=− 0.128±0.047 for Pb. The dependence of the polarization on the transverse momentum and the Feynman variable is measured. The normalized invariant cross section as a function of p _⊥ ² is found to be approximated by the function A exp(−Bp _⊥ ² ), where the parameter B is independent of the kind of nucleus (B=8.71±0.09 (GeV/c)⁻² for carbon and B=8.83±0.18 (GeV/c)⁻² for lead). Pis’ma Zh. éksp. Teor. Fiz. 64, No. 4, 237–240 (25 August 1996)     

Influence of anisotropy on the dispersion of surface plasmon-phonon polaritons in silicon carbide

Surface plasmon-phonon polaritons (SPPP’s) of types 3 and 4 are investigated in doped anisotropic single crystals of hexagonal silicon carbide (6H-SiC) in the orientation corresponding to K⊥C and xy⊥C. It is shown that a dispersion dependence of the type-3 SPPP’s bounded by K appears in 6H-SiC when the plasmon frequency increases to ν _p⊥⩾350 cm⁻¹. At ν _p⊥⩾400 cm⁻¹, ν _s(K) exists for type-4 SPPP’s in the frequency range Ω _∥ ⁺ <ν< _⊥ ⁺ . When the concentration of free charge carriers is increased, the dispersion curves are displaced toward higher frequencies. The conditions for the existence of type-3 and type-4 SPPP’s in 6H-SiC are determined. Fiz. Tverd. Tela (St. Petersburg) 40, 636–639 (April 1998)     

NO 3 2− and CO 2 − centers in synthetic hydroxyapatite: Features of the formation under γ- and UV-irradiations

The EPR studies of synthetic hydroxyapatite containing carbonate and nitrate ions exposed to γ-ray and UV irradiations have been performed. It has been found that γ irradiation leads to the formation of both NO₃²⁻ and CO₂⁻ paramagnetic centers, while the UV irradiation induces only NO₃²⁻ centers. To explain this fact, the hypothesis has been proposed, according to which in the hydroxyapatites studied, there coexist complexes consisting of nitrate ions and shallow electron traps that serve as sources of secondary electrons during UV irradiation. The EPR spectroscopy parameters (g and A) of the detected centers have been determined and compared with similar centers in hydroxyapatite with a different impurity composition. The study of the thermal stability of the centers has demonstrated that, in the temperature range 20–300°C, the NO₃²⁻ centers formed by UV irradiation are more stable than the same centers created by γ-ray irradiation.     

One-dimensional ionic conduction in solid Ag2 Tl6I10

The ionic and electronic conductivities of Ag₂Tl₆I₁₀ single crystals have been studied as a function of crystallographic orientation and temperature from 20 to 135°C. EMF as well as AC and DC techniques have been employed. The highly anisotropic material is predominantly an Ag⁺-ion conductor parallel toc-direction, with the Ag⁺ ions moving through linear channels that are not interconnected. The conductivity σ_‖c =1.6×10⁻⁷Ω⁻¹cm⁻¹ at 25°C, with an activation enthalpy for σ_‖c of 0.38 eV. The conduction perpendicular toc-direction has been found to be predominantly electronic with a value of σ_⊥c =3×10⁻⁹Ω⁻¹cm⁻¹ at 25°C and an activation enthalpy for σ_⊥c of 0.64 eV. This is the first observation of one-dimensional Ag⁺ conduction and this type of orientation-dependent change from ionic to electronic conduction. On leave from Institute of Physics, Academia Sinica, Peking, China.     

EPR of trivalent iron ions in a LiCaAlF6 crystal

The orientational dependences of the EPR spectra of Fe³⁺-doped LiCaAlF₆ single crystals (space group P31c, Z=2), grown at the Laboratory of Magnetic Radio Spectroscopy at Kazan’ State University, have been investigated in detail. The spectrum is described by a trigonal spin Hamiltonian with the following parameters: B ₂₀=40.072×10⁻⁴ cm⁻¹, B ₄₀=−5.799×10⁻⁴ cm⁻¹, B ₄₃=−4.281×10⁻⁴ cm⁻¹, A _s=24.33±1, A _p=6.13±1, g _∥=g _⊥=2.00217±0.0003. A theoretical calculation of the hyperfine structure parameters shows that they are described quite well when allowance is made for the overlapping of the wave functions of the paramagnetic center and the ligands (F⁻). Fiz. Tverd. Tela (St. Petersburg) 39, 488–490 (March 1997)     

Evidence of large-angle reorientation in supercooledo-terphenyl

Summary We study the reorientation process of a paramagnetic radical dissolved in supercooledo-terphenyl around and belowT _g via the Electron Spin Resonance spectroscopy. The experimental results are at variance with the hypothesis of diffusive reorientation of the radical. Instead, the ESR lineshapes are fairly well reproduced by assuming that the radical reorientates via large angular jumps with a width Δϕ=80° _−5° ^+10° . The results are compared with previous²H NMR studies. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Magnetooptical microwave spectroscopy of the coherent magnetic state in the mixed valence compound SmB6 in the frequency range 40–120 GHz

In undoped pure single crystals of the mixed valence compound SmB₆ anomalous ESR absorption is observed in the frequency range v=40–120 GHz at temperatures of 1.8–4.2 K. The ESR for the case of the coherent ground state consists of two components corresponding to g-factors g ₁=1.907±0.003 and g ₂=1.890±0.003. The amplitude of both ESR lines strongly depends on temperature in the temperature range studied: the amplitude of the first line with g=g ₁ increases and the amplitude of the second line decreases with temperature. A model based on consideration of intrinsic defects in the SmB₆ crystalline lattice, with a densit ∼10¹⁵−10¹⁶ cm⁻³, is suggested as an explanation for the anomalous ESR-behavior. In the frequency range v>70 GHz at T=4.2 K, in addition to the main ESR lines, a new magnetic resonance with a hysteretic field dependence is discovered. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 10, 707–712 (25 November 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Conductivity studies of starch-based polymer electrolytes

A proton-conducting polymer electrolyte based on starch and ammonium nitrate (NH₄NO₃) has been prepared through solution casting method. Ionic conductivity for the system was conducted over a wide range of frequency between 50 Hz and 1 MHz and at temperatures between 303 K and 373 K. Impedance analysis shows that sample with 25 wt.% NH₄NO₃ has a smaller bulk resistance (R _b) compared to that of the pure sample. The amount of NH₄NO₃ was found to influence the proton conduction; the highest obtainable room temperature conductivity was 2.83 × 10⁻⁵ S cm⁻¹, while at 100 °C, the conductivity in found to be 2.09 × 10⁻⁴ S cm⁻¹. The dielectric analysis demonstrates a non-Debye behavior. Transport parameters of the samples were calculated using the Rice and Roth model and thus shows that the increase in conductivity is due to the increase in the number of mobile ions.     

Radiation-induced radical ions in calcium sulfite

We have used EPR to study the effect of γ radiation on calcium sulfite. We have observed and identified the radiation-induced radical ions SO ₂ ⁻ (iso) with g = 2.0055 and SO ₂ ⁻ (orth-1) with g₁ = 2.0093, g₂ = 2.0051, g₃ = 2.0020, identical to the initial and thermally induced SO ₂ ⁻ respectively, SO ₃ ⁻ (iso) with g = 2.0031 and SO ₃ ⁻ (axial) with g_⊥ = 2.0040, g_∥ = 2.0023, identical to mechanically induced SO ₃ ⁻ . We have established the participation of radiation-induced radical ions SO ₃ ⁻ in formation of post-radiation SO ₂ ⁻ . __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 4, pp. 467–472, July–August, 2006.     

Low-temperature mobility of surface electrons and ripplon-phonon interaction in liquid helium

The low-temperature dc mobility of the two-dimensional electron system localized above the surface of superfluid helium is determined by the slowest stage of the longitudinal momentum transfer to the bulk liquid, namely, by the interaction between the surface and volume excitations of liquid helium, which decreases rapidly with the temperature. Thus, the temperature dependence of the low-frequency mobility is μ_dc ≈ 8.4 × 10⁻¹¹ n _e T ^−20/3 cm⁴ K^20/3/(V s), where n _e is the surface electron density. The relation T ^20/3 E_⊥⁻³ ≪ 2 × 10⁻⁷ between the pressing electric field (in kilovolts per centimeter) and temperature (in Kelvins) and the value ω ≲ 10⁸ T ⁵ K⁻⁵ s⁻¹ of the driving-field frequency have been obtained, at which the above effect can be observed. In particular, E _⊥ ≃ 1 kV/cm corresponds to T ≲ 70 mK and ω/2π ≲ 30 Hz.     

ESR study of exchange coupled pairs in copper diethyldithiocarbamate

ESR investigations on exchange coupled pairs of Cu ions in single crystals of Cu(dtc)₂, isomorphously diluted with the corresponding diamagnetic zinc salt, are reported. The spin Hamiltonian parameters for the coupled species (S=1) are:g _‖=2.1025,g ₊=2.031,A=75.1×10⁻⁴ cm⁻¹,B=14.8×10⁻⁴,D=276.0×10⁻⁴ cm⁻¹ andE=46.7×10⁻⁴ cm⁻¹. While theg andA tensors show tetragonal symmetry, the zeor-field splitting tensor is rhombic and has principal axes different from those of theg andA tensors. Intensity measurements made down to 4.2 K indicate that the exchange is ferromagnetic with |FFF| ∼ 10 cm⁻¹. Direct dipole-dipole interaction appears to be the major contribution to the zero-field splitting. A calculation on the distributed point dipole model shows that dipolar interaction is considerably modified by the high covalency of the Cu-S bond and accounts for the rhombic nature of the tensor. The possible exchange mechanisms in Cu(dtc)₂—direct exchange and superexchange through the bridging sulphurs—are discussed.     

Photoluminescence,thermoluminescence and electron spin resonance studies on Eu<Superscript>3+</Superscript> doped Ca<Subscript>3</Subscript>Al<Subscript>2</Subscript>O<Subscript>6</Subscript> red phosphors

Tricalcium aluminate doped with Eu³⁺ was prepared at furnace temperatures as low as 500°C by using the convenient combustion route and examined using powder X-ray diffraction, scanning electron microscope and photoluminescence techniques. A room-temperature photoluminescence study showed that the phosphors can be efficiently excited by UV/Visible region, emitting a red light with a peak wavelength of 616 nm corresponding to the ⁵D₀–⁷F₂ transition of Eu³⁺ ions. The phosphor exhibits three thermoluminescence (TL) peaks at 195°C, 325°C and 390°C. Electron Spin Resonance (ESR) studies were carried out to study the defect centres induced in the phosphor by gamma irradiation and also to identify the defect centres responsible for the TL process. Room-temperature ESR spectrum of irradiated phosphor appears to be a superposition of three distinct centres. One of the centres (centre I) with principal g-value 2.0130 is identified as O⁻ ion while centre II with an axially symmetric principal values g _∥=2.0030 and g _⊥=2.0072 is assigned to an F⁺ centre (singly ionized oxygen vacancy). O⁻ ion (hole centre) correlates with the TL peak at 195°C and the F⁺ centre (electron centre), which acts as a recombination centre, is also correlated to the 195°C TL peak. F⁺ centre further appears to be related to the high temperature peak at 390°C. Centre III is also assigned to an F⁺ centre and seems to be the recombination centre for the TL peak at 325°C.     

Dissociation equilibrium of dinitrogen tetroxide in organic solvents: An electron paramagnetic resonance measurement

The dissociation equilibrium of N₂O₄−NO₂ has been measured in hexane, carbon tetrachloride and chloroform at different temperatures. The equilibrium constants at 298.15 K (25°C),K _m (molality basis), are 3.5·10⁻⁵ in hexane, 5.9·10⁻⁶ in carbon tetrachloride and 5.3·10⁻⁶ in chloroform. The EPR technique has been used to quantify the NO₂ radical. These data are compared with gas-phase and solution data of previous reports. The applicability of Hildebrand and Scatchard theory of solutions is also discussed and some thermodynamic properties are deduced, such as Henry’s N₂O₄ and NO₂ constants in different solvents.     

Effect of interlayer single-particle hoppings on the superconducting transition temperature

It is well known that the superconducting transition temperature of high-T _c cuprates depends on the number of CuO₂ planes in the unit cell. The multilayer structure implies the possibility of interlayer hopping. Under the assumption that the interlayer hopping can be specified by the parameter t _⊥(k) = t _⊥(cos(k _x ) − cos(k _y ))², the quasiparticle excitation spectrum for the bilayer cuprate in the superconducting state has been determined in the framework of the t − t′ − t″ − t _⊥ − J* model using the generalized mean-field approximation. It turns out that the interlayer hoppings does not create any additional mechanism of the Cooper paring and does not lead to an increase in T _c . The splitting of the upper Hubbard quasiparticle band attributed to the interlayer hoppings is manifested as two peaks in the doping dependence of the superconducting transition temperature at temperatures below the maximum T _c value for a single-layer cuprate. It has been found that antiferromagnetic interlayer correlations suppress the interlayer splitting. This probably leads to the common doping dependence of T _c for both single-layer and bilayer cuprates.     

Static charged spheres with anisotropic pressure in general relativity

We report a generalization of our earlier formalism [Pramana, 54, 663 (1998)] to obtain exact solutions of Einstein-Maxwell’s equations for static spheres filled with a charged fluid having anisotropic pressure and of null conductivity. Defining new variables: w=(4π/3)(ρ+ε)r ², u=4πξr ², v _r=4πp _r r ², v _⊥=4πp _⊥ r ²[ρ, ξ(=−(1/2)F ₁₄ F ¹⁴), p _r, p _⊥ being respectively the energy densities of matter and electrostatic fields, radial and transverse fluid pressures whereas ε denotes the eigenvalue of the conformal Weyl tensor and interpreted as the energy density of the free gravitational field], we have recast Einstein’s field equations into a form easy to integrate. Since the system is underdetermined we make the following assumptions to solve the field equations (i) u=v _r=(a ²/2κ)r ⁿ⁺², v _⊥=k ₁ v _r, w=k ₂ v _r; a ², n(>0), k ₁, k ₂ being constants with κ=((k ₁+2)/3+k ₂) and (ii) w+u=(b ²/2)r ⁿ⁺², u=v _r, v _⊥−v _r=k, with b and k as constants. In both cases the field equations are integrated completely. The first solution is regular in the metric as well as physical variables for all values of n>0. Even though the second solution contains terms like k/r ² since Q(0)=0 it is argued that the pressure anisotropy, caused by the electric flux near the centre, can be made to vanish reducing it to the generalized Cooperstock-de la Cruz solution given in [14]. The interior solutions are shown to match with the exterior Reissner-Nordstrom solution over a fixed boundary. Dedicated to Prof. F A E Pirani.     

Critical point phenomena in the electrical resistivity of binary liquid systems: CS2 + CH3CN and CS2 + CH3NO2

Electrical resistance measurements are reported on the binary liquid mixtures CS₂ + CH₃CN and CS₂ + CH₃NO₂ with special reference to the critical region. Impurity conduction seems to be the dominant mechanism for charge transport. For the liquid mixture filled at the critical composition, the resistance of the system aboveT _c follows the relationR=R _c−A(T−T _c) ^b withb=0·6±0·1. BelowT _c the conductivities of the two phases obey a relation σ₂−σ₁=B(T _c−T)^β with β=0·34±0·02, the exponent of the transport coefficient being the same as the exponent of the order parameter, an equilibrium property.     

Electron paramagnetic resonance of NH 3 + radical in potassium sulphate

Room temperature EPR spectra of (NH₄)₂SO₄ doped K₂SO₄ monocrystals irradiated with x-rays show the presence of NH₃ ⁺ radicals. The EPR parameters areg ‖=2.0037 andg ⊥ = 2.0068;¹⁴NA _XX=13.75;A _YY=24.5;A _ZZ=25.5 gauss;¹HA _XX=A _YY=22 andA _ZZ=25 gauss. From the¹⁴N and¹H coupling constants it has been inferred that at room temperature the planar NH₃ ⁺ radical undergoes rotation about theC ₃ axis which corroborates with the equivalence of the protons, but the radical itself is in an asymmetric crystal field environment. The 77K spectra indicate a considerable reduction in the motion of the radical with the free motion almost completely stopped. Part of Ph.D. work of the second author     

High-Frequency ESR Measurements of <Emphasis Type="Italic">S</Emphasis> = 1/2 1-D Heisenberg Antiferromagnetic Zig-Zag Chain (VO)(SO<Subscript>4</Subscript>)(2,2-bpy)

High-frequency electron spin resonance (ESR) measurements of S = 1/2 one-dimensional Heisenberg antiferromagnetic zig-zag chain substance (VO)(SO₄)(2,2-bpy) were performed in the temperature region from 1.8 to 265 K. ESR measurements at 265 K show typical powder pattern spectra of the V⁴⁺ ion and g _|| = 1.925 ± 0.001 and g _⊥ = 1.976 ± 0.001 were obtained. Although the magnetic susceptibility shows a maximum at 3.1 K suggesting the existence of a short-range order, no g-shift and line width broadening were observed down to 1.8 K. We suggest that these temperature-independent features of ESR can be attributed to the result of frustration in (VO)(SO₄)(2,2-bpy).