O(N) vector model with twisted boundary conditions

Prompted by a recent article of Chakravarty, we reexamine theO(N) vector model with twisted boundary conditions ind dimensions in the various frameworks of the =d–2 expansion, the =4–d expansion, and the large-N expansion. These continuum models describe the physics below the critical temperatureT _c and nearT _c of a latticeO(N) spin model. We determine the effect of the twisting on finite-size scaling functions, for various geometries.On leave from G. Nadjakov Institute of Solid State Physics, 1784 Sofia, Bulgaria.     

Linear and Nonlinear Optical Properties Of Racemic (±)2-(α-Methylbenzylamino)-5-Nitropyridine Single Crystals

Linear and nonlinear optical properties of racemic (±)2-(α-methylbenzylamino)-5-nitropyridine ((±)MBANP) single crystals have been comprehensively investigated and compared with those of the enantiomorph (–)2-(α-methylbenzylamino)-5-nitropyridine ((–)MBANP) crystals. (±)MBANP crystal exhibits very high chemical and physical stability, but relatively small nonlinear optical coefficients (d₃₁ = 6.8 pm/V, d₃₂ = 4.7 pm/V, d₃₃ = 0.84 pm/V). A comparison between the nonlinear optical coefficients of (±)MBANP and (–)MBANP demonstrates the validity of the oriented-gas model in molecular crystals that neglects all the contributions from intermolecular interaction.     

Preparation and optical constants of the nano-crystal and polymer composite Bi4Ti3O12/PMMA thin films

Bismuth titanate, Bi₄Ti₃O₁₂ (BTO), is a typical ferroelectric material with useful properties for optical memory, piezoelectric and electro-optic devices. Its nano-crystals were compounded by the chemical solution decomposition technique. Its structure and size were analyzed by X-ray diffraction and transmissive electron microscopy. The composite thin film of BTO nano-crystals and high transparency polymethylmethacrylate (PMMA) polymer was prepared by spin coating. The transmitted spectrum of BTO/PMMA composite thin film in 300–1500 nm was measured. The film thickness d and the optical constants of the film, such as the refractive index n, the absorption coefficient α, and the extinction coefficient κ were obtained using the data from the transmitted spectrum.     

Apparent conflicting indications on the conformation of dimethylether–argon from the rotational spectra of the d6 and C species

The rotational spectra of dimethylether–d₆–Ar and dimethylether–¹³C–Ar have been investigated by molecular beam Fourier transform microwave spectroscopy. The obtained rotational constants, when compared to those of the normal species, give contrasting indications on the conformation of the complex. We believe the indications given by the dimethylether–Ar/dimethylether–¹³C–Ar pair to be the correct ones. They suggest the Ar atom to lie in the σ_ν symmetry plane of dimethylether perpendicular to the C–O–C plane, shifted towards the oxygen atom. This conformation is in agreement with several kinds of theoretical calculations. Probably isotopic substitution effects make the data from dimethylether–d₆–Ar not directly compatible with those of the other species.     

Electronic structure and spectrum of Cr3+ in LiCaAlF6

The electronic structure and spectrum of Cr³⁺ in LiCaAlF₆ are investigated by using the discrete variatitional-local density functional (DV-LDF) method with embedded cluster model. The clusters (CrF₆)^3– withC ₃,D _3d andO _h point group symmetries embedded in the crystal are treated. The one-electron energy levels, densities of states, orbital populations, spin polarization splittings and energies of some terms are calculated. The results show that the relaxation of F^– ions around the Cr³⁺ impurity is inevitable, and that theD _3d andO _h (CrF₆)^3– clusters, with an extended bond-lengthR(Cr–F) chosen to be equal to 1.88 Å can represent this relaxation in a much better way. All the ligand-field transition energies, which are obtained from the transition-state energy and the Griffith parameters, as yielded by a restricted one-electron DV-LDF calculation, compare well with the experimental ones.     

Dispersive optical constants of a-Se100−xSbx films

Thin films of amorphous Se_100−xSb_x (x=5,10 and 20 at%) system are deposited on a silicon substrate at room temperature (300 K) by thermal evaporation technique. The optical constant such as refractive index (n) has been determined by a method based on the envelope curves of the optical transmission spectrum at normal incidence by a Swanpoel method. The oscillator energy (E_o), dispersion energy (E_d) and other parameters have been determined by the Wemple–DiDomenico method. The absorption coefficient (α) has been determined from the reflectivity and transmitivity spectrum in the range 300–2500 nm. The optical-absorption data indicate that the absorption mechanism is a non-direct transition. We found that the optical band gap, E_g^opt, decreases from 1.66±0.01 to 1.35±0.01 eV with increase Sb content.     

Effect of temperature on the band structure of MnxHg1−xSe

The band structure of the solid solutions Mn_xHg_1–xSe is constructed on the basis of the band parameters obtained from the electrophysical and optical investigations of these solid solutions. It is shown that the band structure of Mn_xHg_1–xSe and the dynamics of the bands as a function of the composition and temperature are analogous to those for Cd_xHg_1–xTe. It is shown that the variation of the nonparabolicity of the band spectrum of Mn_xHg_1–xSe as a function of the temperature greatly affects the temperature coefficient of the change of the width of the optical forbidden band (d_gop/dT).Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 40–44, April, 1991.     

Ion aging effects on the dissociative-attachment instability in CO2 lasers

Onset of the dissociative-attachment instability requires that the rate coefficient for electron detachment (k _d) from negative ions be below a critical value. The predominant negative and positive ions in a CO₂N₂He gas-discharge plasma are known to change with time. As secondary by-products form and the predominant negative-ion species changes from CO ₃ ^– to NO ₂ ^– , a decrease ink _d occurs destabilizing the discharge. Since NO ₂ ^– and NO ₃ ^– are largely unreactive with respect to associative detachment,k _d depends in a sensitive fashion on the concentration of certain minority negative ions (O^–, O ₂ ^– ) and neutrals (CO, O, N). The sufficient conditions for the dissociative-attachment instability are much less sensitive to changes in the electron-ion and ion-ion recombination rate coefficients resulting from the ion aging process.     

Volume and grain boundary diffusion of implanted 113Sn in aluminium

Volume and grain boundary diffusion of ¹¹³Sn in aluminium was investigated with the radiotracer method. The implantation technique was used for tracer deposition to avoid problems of tracer hold-up caused by the oxide layer always present on aluminium. The diffusion penetration was chosen large enough to permit serial sectioning of samples with the aid of a microtome.The temperature dependence of the volume diffusivity was determined as D(T)=4.54×10^–5×exp[–(114.5±1.2)kJmol^–1/RT] m ² s ^–1. This confirms previous measurements from our group which already showed that Sn is the fastest foreign metal diffusor so far investigated in aluminium.Grain boundary diffusion of ¹¹³Sn in Al polycrystals was measured in the type-B kinetic regime. The grain boundary diffusion product P=sD _gb (s=segregation factor, =grain boundary width, D _gb=grain boundary diffusivity) was found to be strongly affected by the impurity content of aluminium. For Al polycrystals of 99.9992% nominal purity we obtained P _5N(T)=1.08×10^–8exp [–(96.9±7.5) kJ mol^–1/RT] m³ s^–1 and for less pure Al polycrystals of 99.99% nominal purity P _4N(T)=3.0×10^–10 exp [–(90.1±4.2) kJ mol^–1/RT] m³ s^–1 was determined. The grain boundary diffusion product in the purer material is more than one order of magnitude higher than in the less pure material. Very likely this is an effect of co-segregation of non-diffusant impurities into the grain boundaries.     

The ESR spectrum of oxygen vacancies in sodium tellurite glasses

A theoretical analysis is presented for the shape of the ESR spectrum of the radiation-induced centers of oxygen vacancy type in sodium tellurite glasses. It is found that a qualitative agreement between the theoretical and observed curves can be obtained only if one assumes a completely anisotropic g factor and an inhomogeneous structure for the paramagnetic centers, whose parameters are found as g_x = 1.930; g_y = 1.971; g_z = 2.046; x ² >=4–10^–4-2.5·10^–3; y ² > = 4.9·10^–8-1·10^–6; z ² > = 2.2·10^–4-9·10^–4; d ² > = 36–100 G². It is concluded that the glass composition affects the center nonuniformity.Translated from Izvestiya VUZ, Fizika, No. 3, pp. 63–68, March, 1973.     

Amorphous to crystalline transition in Dy-Fe thin films

The crystallization of vacuum-deposited amorphous Dy-Fe thin films was studied by transmission electron microscopy and electron diffraction. The effect of thickness, deposition rate and substrate temperature on the crystallization process have been investigated. The results show that the crystallization thicknessd _c decreases with increasing deposition rate and substrate temperature. The number density of Dy-Fe islands were found to be almost constant at (4–5)×10¹¹ cm^–2 in the thickness range 20 Å<d <50 Å. The number density decreases with increase ind _c .     

Effect of deep dislocation levels in silicon on the properties of p-n junctions

We present the results of studies on the influence of deep levels, due to dislocations in electronic-grade silicon, on the lifetime of minority carriers and on the current-voltage and capacitance-voltage characteristics of p-n junctions. The parameters of the deep levels were determined by means of dynamic spectroscopy. The carrier lifetime in the high-resistance region of the p-n junction correlates well with the dislocation density and varies from 10^–7 sec to 3 · 10^–8 sec when the dislocation density N_d varies from 10⁷ cm^–2 to 5 · 10³ cm^–2. The voltage across the p-n junction at a high level of injection varies 1.6 to 6.2 v as a function of N_d. The ionization energy of deep levels associated with dislocation in silicon is 0.44 and 0.57 eV, measured from the bottom of the conduction band.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 81–84, January, 1988.     

The effect of surface roughness on the resistivity increase of thin metal films during gas adsorption

The resistivity increase of very thin metal films during gas adsorption is stronger than predicted by the commond ^–1 law. This effect is contributed to surface roughness. A quantitative theory is presented which leads to an additionald ^–3 term. The theory is checked using literature data obtained for the Ag/O₂ system.     

Manifestation of nano-sized effects in photoinduced nonlinear optics of CdI2–Cu layered single crystals

The infrared (IR) induced second harmonic generation in the thin CdI₂–Cu crystalline layers was discovered. With decreasing thickness of the CdI₂–Cu crystals up to several nanometers value of second-order nonlinear optical susceptibility d₁₄ (at wavelength λ=2.76 μm) for the output IR-induced SHG significantly increases. For the Cu content about 0.4% the output SHG signal achieves its maximum at the sample thickness below 2 nm. It is important that for the samples with larger film thickness the corresponding changes have substantially different properties. Limitation of the d₁₄ at the larger concentration is probably caused by formation of the Cu clusters limiting the enhancement of the hyperpolarizability for particular cluster as well as total nonlinear dielectric susceptibility.     

A Cud-d excitation model for the pairing in the high-T c cuprates

A new model for the pairing mechanism in the ceramic superconductors is presented. Like the magnetic models, we assume the limit of large correlation energies for the Cud electrons. We postulate that the pairing of the Op conduction holes occurs viad–d orbital excitations within thee _g manifold of thed hole of Cu⁺⁺, which is split because of tetragonal or lower symmetry at the Cu sites. This valence conserving charge degree of freedom has been ignored in the magnetic pairing models. Thed–d excitation model may provide a simple qualitative understanding of many experimental results.     

Analyticity properties and a convergent expansion for the inverse correlation length of the low-temperatured-dimensional Ising model

We show that the inverse correlation lengthm(z) of the truncated spin-spin correlation function of theZ ^d Ising model with + or — boundary conditions admits the representationm(z) = –(4d–4)ln z(1–_d1) + r(z) for smallz=e ^–, i.e., large inverse temperatures is ad-dependent analytic function atz = 0, already known in closed form ford = 1 and 2; ford = 3 b_n can be computed explicitly from a finite number of the Z^d limits of z = 0 Taylor series coefficients of the finite lattice correlation function at a finite number of points ofZ ^d.     

Study of the magnetic and electronic properties of the Fe4N with pressure

In the present work, we report ab-initio studies of the magnetic property variations with pressure of both iron sites in the structure of Fe₄N, using full-potential linearized augmented plane wave method and the Perdew–Burke–Ernzerhof functional and generalized gradient approximation to describe the exchange-correlation potential are reported. The results show that the magnetic moment of FeI is almost constant while the magnetic moment of FeII presents a discontinuity when the lattice parameter is varied. This is reflected in the compression of the spin up and down energy bands to different concentration points. The variation in the FeII magnetization arises mainly from changes in the d_xy, d_xz, d_yz and d_x2−y2 orbitals.     

The Ising Model on a Quenched Ensemble of c=−5 Gravity Graphs

We study with Monte Carlo methods an ensemble of c=–5 gravity graphs, generated by coupling a conformal field theory with central charge c=–5 to two-dimensional quantum gravity. We measure the fractal properties of the ensemble, such as the string susceptibility exponent _s and the intrinsic fractal dimension d _H. We find _s=–1.5(1) and d _H=3.36(4), in reasonable agreement with theoretical predictions. In addition, we study the critical behavior of an Ising model on a quenched ensemble of the c=–5 graphs and show that it agrees, within numerical accuracy, with theoretical predictions for the critical behavior of an Ising model coupled dynamically to two-dimensional quantum gravity, with a total central charge of the matter sector c=–5.     

Paraconductivity for a d-wave superconductor in short-wavelength fluctuation regime

A theoretical study of the fluctuation conductivity above T_c (paraconductivity) is reported for a d-wave superconductor with resonant scattering impurities. A d-wave system is modeled by tight-binding electrons in the two-dimensional squared lattice, and the impurity scattering is treated in the T-matrix approximation in a unitary limit. In calculating the Aslamazov–Larkin (AL) and the Maki–Thompson (MT) terms, we also consider effects of a short-wavelength cutoff in the fluctuation spectrum. The d-wave character in the AL and MT terms manifests itself to renormalization effects on the fluctuation amplitude and reduced temperature, whereas an anomalous-MT term is absent. The present calculations can describe fairly well experiments on the paraconductivity in zinc-doped cuprate superconductors provided that effects of a total-energy cutoff are taken into account.