Synthesis,structural, and spectroscopic (FT-IR,NMR, and UV) Characterization of 1-(Cyclohexylmethyl)-2-(pyridin-2-yl)-1<Emphasis Type="Italic">H</Emphasis>-benzo[<Emphasis Type="Italic">d</Emphasis>]imidazole by experimental techniques and quantum chemical calculations

The title compound (II), 1-(cyclohexylmethyl)-2-(pyridin-2-yl)-1H-benzo[d]imidazole (C₁₉H₂₁N₃), was synthesized via N-alkylation of 2-(pyridin-2-yl)-1H-benzo[d]imidazole (I). Both compounds I and II were characterized by IR, NMR and UV-vis spectroscopy. Solid-state structure of compound II was determined by single-crystal X-ray diffraction technique. Furthermore, quantum chemical calculations employing density functional theory (DFT/B3LYP) method with the 6–311++G(d, p) basis set were performed for the theoretical characterization of the molecular and spectroscopic features of the compounds. Using the TD-DFT method, electronic absorption spectra of the compounds have been predicted at same level. When the obtained results were compared with the experimental findings, it is seen that theoretical results support the experimental data and a good agreement exists between them.     

V<Subscript>52</Subscript>O<Subscript>64</Subscript> tetragonal superstructure of cubic vanadium monoxide with vacancies in the metal sublattice

The atom-vacancy ordering of cubic vanadium monoxide VO_1.29, which has basis cubic structure B1 and structural vacancies in the metal sublattice, has been studied using the x-ray diffraction method. It has been shown that the formation of the tetragonal (space group I4₁/amd) ordered phase V₅₂O₆₄ of cubic vanadium monoxide VO_y proceeds as a first-order phase transition through the disorder-order channel including 22 nonequivalent superstructure vectors of four stars {k ₁₀}, {k ₄}, {k ₃}, and {k ₂}. The distribution function of the vanadium atoms in the V₅₂O₆₄ tetragonal superstructure has been calculated.     

Magnetic nesting and coexistence of ferromagnetism and superconductivity

If the condition ε_σ(p)=ε_?σ(?p+nI/v_F) for magnetic nesting is fulfilled for the electron dispersion law with spin σ along a certain preferential direction n, ferromagnetism and the inhomogeneous superconducting state can coexist up to a very high magnetization I. This fact was used to explain the coexistence of ferromagnetism and superconductivity for layered cuprates of the RuSr₂GdCu₂O₈ type, which possess a finite, though rather high, critical magnetization, because the conditions for magnetic nesting are fulfilled only approximately.     

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.     

Penetration of a magnetic field into the YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−δ</Subscript> high-temperature superconductor: Magnetoresistance in weak magnetic fields

The penetration of a magnetic field into superconducting grains and weak links of YBa₂Cu₃O_7?δ ceramic high-temperature superconductors is investigated using measurements of the transverse and longitudinal magnetoresistances at T=77.3 K and 0≤H≤～500 Oe as a function of the transport current in the range ～0.01≤I/I _c ≤～0.99. The effects associated with the complete penetration of Josephson vortices into weak links of the high-temperature superconductor in magnetic fields H_c2J, the onset of penetration of Abrikosov vortices into superconducting grains in magnetic fields H_c1A, and the first-order transition from the Bragg glass phase to the vortex glass phase in fields H_BG-VG are revealed and interpreted. The I-H phase diagrams YBa₂Cu₃O_7?δ high-temperature superconductors are constructed for I ⊥ H and I ‖ H.     

Some exact solutions of the local induction equation for the motion of a vortex in a Bose–Einstein condensate with a Gaussian density profile

The dynamics of a vortex filament in a Bose–Einstein condensate whose equilibrium density in the reference frame rotating at the angular velocity Ω is Gaussian with the quadratic form r·D?r has been considered. It has been shown that the equation of motion of the filament in the local-induction approximation permits a class of exact solutions in the form R(β, t) = βM(t) + N(t) of a straight vortex, where β is the longitudinal parameter and is the time. The vortex slips over the surface of an ellipsoid, which follows from the conservation laws N · D?N=C ₁ and M · D?N=C ₀=0. The equation of the evolution of the tangential vector M(t) appears to be closed and has integrals of motion M ·D?M=C ₂ and (|M| ? M· G?Ω) = C, with the matrix G? = 2(I?TrD? ? D?)^?1. Crossing of the respective isosurfaces specifies trajectories in the phase space.     

Galvanomagnetic properties of atomically disordered Sr<Subscript>2</Subscript>RuO<Subscript>4</Subscript> single crystals

The effect of neutron-bombardment-induced atomic disorder on the galvanomagnetic properties of Sr₂RuO₄ single crystals has been experimentally studied in a broad range of temperatures (1.7–380 K) and magnetic fields (up to 13.6 T). The disorder leads to the appearance of negative temperature coefficients for both the in-plane electric resistivity (ρ_a) and that along the c axis (ρ_c), as well as the negative magnetoresistance Δρ, which is strongly anisotropic to the magnetic field orientation (H ∥ a and H ∥ c), with the easy magnetization direction along the c axis and a weak dependence on the probing current direction in the low-temperature region. The experimental ρ_a(T) and ρ_c(T) curves obtained for the initial and radiation-disordered samples can be described within the framework of a theoretical model with two conductivity channels. The first channel corresponds to the charge carriers with increased effective masses (～10m _e , where m _e is the electron mass) and predominantly electron-electron scattering, which leads to the quadratic temperature dependences of ρ_a and ρ_c. The second channel corresponds to the charge carriers with lower effective masses exhibiting magnetic scattering at low temperatures, which leads to the temperature dependence of the ρ_{a, c}(T) ∝ 1/T type.     

Optical 4<Emphasis Type="Italic">f</Emphasis>-4<Emphasis Type="Italic">f</Emphasis> transitions in multiferroic HoMnO<Subscript>3</Subscript>

In the absorption spectra of the hexagonal single-crystal manganite HoMnO₃ in the paramagnetic ferroelectric state, lines near 1.1 and 2.0 μm were observed associated with the transitions ⁵ I ₈ → ⁵ I ₆ and ⁵ I ₈ → ⁵ I ₇, respectively, within the 4f ¹⁰ configuration of the Ho³⁺ ion. At T = 80 K, to the ⁵ I ₈ → ⁵ I ₇ transition corresponds one band at 1.9 μm for both polarizations E ∥ c and E ⊥ c. As the temperature increases from 80 to 293 K, a low-energy band with a peak at 2.04 μm for E ⊥ c and a peak at 2.07 μm for E ∥ c arises associated with transitions from an excited Stark level of the ground ⁵ I ₈ multiplet to the Stark levels of the ⁵ I ₇ multiplet and with an increase in the population of the initial Stark level, the energy of which is ～100 K.     

Influence of the ground state of the Pr<Superscript>3+</Superscript> ion on magnetic and magnetoelectric properties of the PrFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> multiferroic

The magnetic, magnetoelectric, and magnetoelastic properties of a PrFe₃(BO₃)₄ single crystal and the phase transitions induced in this crystal by the magnetic field are studied both experimentally and theoretically. Unlike the previously investigated ferroborates, this material is characterized by a singlet ground state of the rare-earth ion. It is found that, below T _N = 32 K, the magnetic structure of the crystal in the absence of the magnetic field is uniaxial (l ∥ c), while, in a strong magnetic field H ∥ c (H _cr ～ 43 kOe at T = 4.2 K), a Fe³⁺ spin reorientation to the basal plane takes place. The reorientation is accompanied by anomalies in magnetization, magnetostriction, and electric polarization. The threshold field values determined in the temperature interval 2–32 K are used to plot an H-T phase diagram. The contribution of the Pr³⁺ ion ground state to the parameters under study is revealed, and the influence of the praseodymium ion on the magnetic and magnetoelectric properties of praseodymium ferroborate is analyzed.     

Liquid-crystalline dendrimer Cu(II) complexes and Cu(0) nanoclusters based on the Cu(II) complexes: An electron paramagnetic resonance investigation

New nanostructured materials, namely, the liquid-crystalline copper(II) complexes that contain poly(propylene imine) dendrimer ligands of the first (ligand 1) and second (ligand 2) generations and which have a columnar mesophase and different copper contents (x = Cu/L), are investigated by EPR spectroscopy. The influence of water molecules and nitrate counterions on the magnetic properties of complex 2 (x = 7.3) is studied. It is demonstrated that water molecules can extract some of the copper ions from dendrimer complexes and form hexaaqua copper complexes with free ions. The dimer spectra of fully hydrated complex 2 (x = 7.3) are observed at temperatures T < 10 K. For this complex, the structure is identified and the distance between the copper ions is determined. It is shown that the nitrate counterion plays the role of a bridge between the hexaaqua copper(II) complex and the dendrimer copper(II) complex. The temperature-induced valence tautomerism attended by electron transport is revealed for the first time in blue dendrimer complexes 1 (x = 1.9) with a dimer structure. The activation energy for electron transport is estimated to be 0.35 meV. The coordination of the copper ion site (NO₄) and the structural arrangement of green complexes 1 (x = 1.9) in the columnar mesophase are determined. Complexes of this type form linear chains in which nitrate counterions serve as bridges between copper centers. It is revealed that green complexes 1 (x = 1.9) dissolved in isotropic inert solvents can be oriented in the magnetic field (B ₀ = 8000 G). The degree of orientation of these complexes is rather high (S _z = 0.76) and close to that of systems with a complete ordering (S _z = 1) in the magnetic field. Copper(0) nanoclusters prepared by reduction of complex 2 (x = 7.3) in two reducing agents (NaBH₄, N₂H₄ · H₂O) are examined. A model is proposed for a possible location of Cu(0) nanoclusters in a dendrimer matrix.     

Field transformation in a multimode optical waveguide with random irregularities of the film surface

A self-consistent mathematical model for the transformation of the average intensity of the mode spectrum I(z) of a waveguide field in a multimode planar optical waveguide with a step profile and rough surface is developed. This model is based on the matrix model for multiple scattering of modes in an optical waveguide. The elements of the intermode scattering matrix are found, which describe the process of mutual transfer of the energy of modes along a waveguide and their transformation into radiation modes. The transformation of the I(z) modes in waveguides with large-and small-scale inhomogeneities is investigated. It is shown that the largest qualitative differences in the noted dependences manifest themselves only in the initial portions of the optical waveguide. The length z of these portions is much smaller than the characteristic scale length L _k at which the fundamental energy of the kth mode excited in the optical waveguide is renewed. The effect of self-filtration of the mode spectrum I(z) is described, as a result of which a stable (normalized), independent of distance z, distribution I* is formed. It is established that irregularities of the optical waveguide boundaries exert a depolarizing effect on a guided light beam. The specific features of the normalization of the radiative dissipation of a group of modes I_i(z) in an optical waveguide are investigated. It is ascertained that, in the case of small-scale irregularities, the attenuation coefficient is described by a nonlinear monotonic dependence α(z), which asymptotically converges to the value α*, characteristic of the normalized field I*. When the optical-waveguide film has large irregularities, the dependence α(z) is characterized by a pronounced maximum due to the formation of alternative channels of radiative dissipation of the energy of waveguide modes.     

Magnetic structure of Er<Subscript>5</Subscript>Ge<Subscript>3</Subscript> at 4.2 K

A symmetry analysis of the possible magnetic structures of Er₅Ge₃ in the ground state is performed using the results of measurements of elastic magnetic neutron scattering at 4.2 K. It is shown that the minimum discrepancy factor R _m ≈9.5% corresponds to a modulated collinear magnetic structure in which the magnetic moments of erbium atoms are oriented along the a ₃ axis of the unit cell of the crystal structure and induce an antiferromagnetic longitudinal spin wave (AFLSW). The magnetic structure is characterized by the wave vector k=2π(0, 0, μ /a ₃) (where μ≈0.293) and the modulation period λ≈3.413a ₃. The magnetic ordering temperature T _N ≈38 K is determined from the temperature dependence of the intensity of magnetic reflections. __________ Translated from Fizika Tverdogo Tela, Vol. 45, No. 9, 2003, pp. 1653–1659. Original Russian Text Copyright ? 2003 by Vokhmyanin, Dorofeev.     

Field Confinement Energy at a Nonlinear Interface between Nonlinear Defocusing Media

The dependences of the resistance of the layered quasi-one-dimensional semiconductor TiS₃ on the direction and magnitude of the magnetic field B have been measured. The anisotropy and angular dependences of the magnetoresistance indicate the two-dimensional character of the conductivity at T < 100 K. Below T₀ ≈ 50 K, the magnetoresistance for the directions of the field in the plane of the layers (ab plane) increases sharply, whereas the transverse magnetoresistance (B ∥ c) becomes negative. The results confirm the possibility of an electron phase transition to a collective state at T₀. The negative magnetoresistance (at B ∥ c) below T₀ is explained by the magnetic-field-induced suppression of two-dimensional weak localization. The positive magnetoresistance (at B ∥ ab) is explained by the effect of the magnetic field on the spectrum of electronic states.     

The disorder-order transition in cubic vanadium monoxide with vacancies in the metal sublattice

The disorder-order transition in cubic vanadium monoxide VO_y (y = 1.29, 1.30) possessing a B1 type structure and containing vacancies only in the metal sublattice has been studied by X-ray diffraction and symmetry analyses. It is established that the formation of a tetragonal (space group I4₁/amd) ordered V₅₂O₆₄ phase in cubic vanadium monoxide VO_y proceeds in the form of the first-order phase transition via a channel involving 22 nonequivalent superstructural vectors of four stars ({k ₁₀}, {k ₄}, {k ₃}, and {k ₂}). The distribution function of V atoms in the tetragonal V₅₂O₆₄ superstructure is calculated, and it is found that the real ordered V_51.6O₆₄ phase exhibits significant atomic displacements. The boundaries of the domain of existence of the V₅₂O₆₄ phase at 54–60% O are determined in the phase diagram of the V-O system.     

Role of critical fluctuations in the formation of a skyrmion lattice in MnSi

The region in the H–T phase diagram near the critical temperature (T _c ) of the cubic helicoidal MnSi magnet is comprehensively studied by small-angle neutron diffraction. Magnetic field H is applied along the [111] axis. The experimental geometry is chosen to simultaneously observe the following three different magnetic states of the system: (a) critical fluctuations of a spin spiral with randomly orientated wavevector k _f , (b) conical structure with k _c ∥ H, and (c) hexagonal skyrmion lattice with k_sk ⊥ H. Both states (conical structure, and skyrmion lattice) are shown to exist above critical temperature T _c = 29 K against the background of the critical fluctuations of a spin spiral. The conical lattice is present up to the temperatures where fluctuation correlation length ξ becomes comparable with pitch of spiral d _s . The skyrmion lattice is localized near T _c and is related to the fluctuations of a spiral with correlation length ξ ≈ 2d _s , and the propagation vector is normal to the field (k_sk ⊥ H). These spiral fluctuations are assumed to be the defects that stabilize the skyrmion lattice and promote its formation.     

Substituent Dependent Optical Properties of <Emphasis Type="Italic">p</Emphasis>-phenyl Substituted ethenyl-<Emphasis Type="Italic">E</Emphasis>-thiophenes

Various electron donor and acceptor substituted (NO₂, CN, Cl, H, OCH₃, NH₂) p-phenyl ethenyl-E- thiophenes (1–6) were synthesized and substituent dependent optical properties (dipole moment, transition dipole moment, oscillator strength, optical band gap, hyperpolarizability) were studied using Solvatochromism and Density functional theory. It is shown that thiophene acts as a weak electron donor in presence of an electron withdrawing p-phenyl substituent (NO₂, CN, Cl), whereas thiophene acts as a weak electron acceptor in presence of an electron donating p-phenyl substituent (OCH₃, NH₂). In comparison to ethenyl thiophene 4, the HOMO-LUMO energy band gap is decreased upon increasing the electron donating or electron withdrawing capacity of p-phenyl substituent. From the excited state dipole moment calculation, it is shown that the excited state is highly dipolar for nitro and amino compounds 1 and 6, whereas compounds 2–5 show a non-polar excited state. As compared to the ethenyl thiophene 4, the first hyperpolarizability (β) increases upon substitution either with a strong electron withdrawing or strong electron donating p-phenyl substituent. A large β value is found for p-nitro phenyl ethenyl-E-thiophene and p-amino phenyl ethenyl-E- thiophene. Overall, these studies provide useful information in understanding the optical properties of phenyl and heterocyclic based ethenyl systems.     

Critical current in S-FNF-S Josephson structures with the noncollinear magnetization vectors of ferromagnetic films

The critical current I _C of S-FNF-S Josephson junctions, which are ferromagnet (F)-normal metal (N)-ferromagnet multilayer structures whose ends are in contact with the superconducting (S) electrodes, has been calculated. It has been shown that both the magnitude and sign of I _C depends significantly on the misorientation angle α of the magnetization vectors M _{1, 2} of the ferromagnetic films and the distance L between the superconducting electrodes. The effect of the triplet superconducting component ～〈ψ↑ψ↑〉 ～〈ψ↓ψ↓〉 appearing in the structure on I _C(α) has been analyzed. It has been proven that a new type of the π junction exists, appearing due to the superposition of two contributions to I _C that decrease monotonically with L and are damped at lengths about the coherence length of the normal metal. It has been shown that the effective control over the magnitude and sign of I _C of the structure is achieved at a small deflection of the vectors M _{1, 2} from the antiferromagnetic (M ₁ ↑↓ M ₂) configuration.     

Strong short-range magnetic order in a frustrated FCC lattice and its possible role in the iron structural transformation

The magnetic properties of a frustrated Heisenberg antiferromagnet with the fcc lattice and exchange interaction between the nearest (J ₁) and next-to-nearest (J ₂) neighbors are studied in this work. For the collinear phase with the wave vector Q= (π,π,π), the equations of the self-consistent spin-wave theory are obtained and solved numerically for the sublattice magnetization and the averaged short-range order parameter. The dependence of the Néel temperatureT _N on the ratio J ₂/J ₁ is found. It is shown that, in the case of a sufficiently strong frustration, strong short-range magnetic order persists over a wide temperature range above T _N. The possible application of this result to the mechanism of structural phase transition from α-Fe to γ-Fe is considered.     

The dynamic structure of <Emphasis Type="Italic">cis</Emphasis> and <Emphasis Type="Italic">trans</Emphasis> conformers of substituted phosphinous acids

The potential energy surfaces of bis-(trifluoromethyl)-phosphinous (III), diemthylphosphinous (IV), and bis-(pentafluorophenyl)-phosphinous (V) acids, the geometric parameters of the cis and trans conformers of these acids, and their P-O and O-H stretching vibration frequencies were calculated by the density functional theory method (PBE, 4Z). The potential energy surface sections corresponding to internal rotations about the P-O bonds were constructed, and dynamic problems for such a motion of the proton in a cyclic potential were solved by the Ritz method using a basis set of 100 trigonometric functions. According to calculations, the energy differences between the cis and trans conformers of acids III–V were of 4–7 kJ/mol, and the heights of potential barriers separating these conformers were of from 11 to 16 kJ/mol. In acids III and V, the cis, and, in IV, the trans conformer was stabler. At 298 K, only the ground vibrational states were populated for both rotamers of IV and V and the cis conformer of III. The special features of the potential of III were such that, at 298 K, the first excited vibrational level of its trans conformer was also noticeably populated.     

Electrical resistivity and magnetotransport in La<Subscript>0.67</Subscript>Ba<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> films asymmetrically biaxially compressed by an NdGaO<Subscript>3</Subscript>(001) substrate

The La_0.67Ba_0.33MnO₃(40 nm) films are quasi-coherently grown on an NdGaO₃(001) substrate with an orthorhombic unit cell distortion of ～1.4%. The biaxial compressive stresses generated during nucleation and growth lead to a decrease in the unit cell volume of the grown layers. This, in turn, results in a decrease (by ～35 K) in the temperature of the maximum in the dependence of the electrical resistivity ρ of the layers on the temperature. For T < 150 K, the electrical resistivity ρ of the films increases in proportion to ρ₂ T ^4.5 and the coefficient ρ₂ decreases almost linearly with increasing magnetic field H. The negative magnetoresistance (≈?0.17 for μ₀ H = 1 T) reaches a maximum at temperatures close to room temperature. The response of the electrical resistivity ρ of the La_0.67Ba_0.33MnO₃(40 nm) films to the magnetic field depends on the crystallographic direction of the film orientation and the angle between H and I (where I is the electric current through the film).