Why have no manifestations of the excitonic mechanism been detected in Ginzburg sandwiches?

Using spatially inhomogeneous éliashberg equations in the local-interaction limit, an exact solution of the problem of the superconducting transition temperature in a Ginzburg sandwich (a superconducting film coated with a dielectric layer containing Bose-type excitations, i.e., excitons) in the first order in a/L (where a is the interatomic distance and L is the film thickness) has been obtained. The result has been found to be independent of the exciton frequency. The excitonic mechanism appears only in second order in a/L since both components of the Cooper pair should enter a layer of thickness ∼a in order to interact through the exchange of excitons. Numerical estimates indicate that manifestations of the excitonic mechanism are practically undetectable in systems with L≫a. Calculations for the model with a narrow-gap and a wide-gap dielectric have been performed and compared to experimental data. Zh. éksp. Teor. Fiz. 111, 717–729 (February 1997)     

Magnetic and structural properties of Y2Fe15·3Si1·7 alloy under high pressure

Abstract

The pressure dependence of the crystal structure of a powder sample of Y₂Fe_15·3Si_1·7 was studied in the pressure range from 0 to 2 GPa, using the D-2 diffractometer with hydrostatic liquid pressure cell at the IVV-2M reactor (Ekaterinburg) and DN-12 time-of-flight diffractometer with a sapphire anvil cell at the IBR-2 pulsed reactor (Dubna). Also the pressure dependence of the Curie temperature T_c (P) of the alloy was studied. The correlation between the decreases of both T_c and Fe-Fe interatomic distances in “dumbbell” site 4f under pressure was obtained.     

An interatomic pontential for fullerences from their vibrational spectrum

Summary The vibrational analysis ofsp ²-bonded carbon clusters with different nearest-neighbour interatomic distances (2 in C₆₀, 8 in C₇₀), performed in the framework of the bond-charge model, leads to the determination of an exponential form for the short-range interatomic potential which is inclusive of charge transfer effects. The potential, besides leading to excellent agreement with the existing spectroscopic data andab initio Car-Parrinello calculations, ensures a good transferability of the model to other clusters and possibly an empirical basis for molecular dynamics simulations. In honour of Prof. Fausto Fumi on the occasion of his retirement from teaching.     

Synchrotron radiation interference in front of the silicon absorption edge for silicon-on-insulator structures

The synchrotron radiation (SR) interference phenomenon has been for the first time observed in a strained silicon nanolayer deposited on a dielectric SiO₂ layer (∼150 nm) on Si (100) single crystalline substrates (silicon-on-insulator (SOI) structures). Strong oscillations of spectra intensity depending on photon energy have been detected in the energy range preceding the elementary silicon Si L _2,3 absorption edge (≤100 eV) at grazing angles of SR smaller than 21° in the X-ray photoeffect quantum yield structure. The phase of the spectra oscillation structure is reversed for small variations of grazing angle in the 4°–21° range. The silicon nanolayer thickness (∼180 nm) has been estimated in the three-layer, Si nanolayer-SiO₂-Si substrate structure with the use of neighbor maxima positions of ultrasoft X-ray radiation interference in XANES (X-ray absorption near edge structure) spectra. A decrease in the crystal lattice parameter of a strained silicon layer along the normal to substrate has been determined by X-ray diffraction. An increase in the Si-Si interatomic distances in the strained silicon nanolayer lattice of SOI structure has been found using ultrasoft X-ray emission spectroscopy data.     

Study on Phase Formation and Evolution of High Impact Polystyrene with Poly(Cis‐Butadiene) Rubber Blends

Phase formation and evolution of high‐impact polystyrene with poly(cis‐butadiene) rubber blends was studied. The characteristic length, L, was defined to describe the size of particles, and the graph‐estimation method was introduced to determine the width of the distribution of L. Based on the method, the distribution of L proved to be a log‐normal distribution and the distribution width of L was calculated. The phase structure was also discussed in the wave‐number space. The correlation distance, a _c , was defined and computed, applying light‐scattering theory to power spectrum images obtained by 2‐dimensional Fourier transformation (2DFT). The change of a _c was in accord with that of L, which meant 2DFT was valid to study the phase structure. A fractal dimension, D _c , was introduced to describe the uniformity of the spatial distribution. The result showed that D _c was an effective parameter to study the distribution of particles of the dispersed phase.     

Local structure around In atoms in coherently grown m‐plane InGaN film

The local structure around In atoms in an m‐plane In_0.06Ga_0.94N film coherently grown on a freestanding m‐plane GaN substrate was investigated by polarization‐dependent X‐ray absorption fine‐structure. A step‐by‐step fitting procedure was proposed for the m‐plane wurtzite structure. The interatomic distance for the first nearest neighbour In—N atomic pairs was almost isotropic. For the second nearest In—Ga pairs, the interatomic distances along the m‐ and a‐axes were longer and shorter, respectively, than that in strain‐free virtual crystals as expected for the m‐plane compressive strain. In contrast, the In—Ga interatomic distance in the c‐direction was elongated in spite of the compressive strain, which was explained in terms of the anisotropic atomic structure on the m‐plane. The local strain in the m‐plane film was more relaxed than that in coherently grown c‐plane single quantum wells. A few In atoms were atomically localized in all directions, and thus localized excitonic emission is expected as in the case of c‐plane InGaN.     

Crystal Structures of Tetrakis‐(4‐chlorophenylthio)‐butatriene and Tetrakis‐(tert‐butylthio)‐butatriene

Tetrakis‐(4‐chlorophenylthio)‐butatriene (3a) and tetrakis‐(tert‐butylthio)‐butatriene (3b) were synthesized, and their crystal structures were determined. The compound 3a is monoclinic, space group P2₁/c, a=6.9785(8), b=8.6803(9), c=22.884(2) Å, β=93.887(6)^o, V=1383.0(3) ų, Z=2. The compound 3b is monoclinic, space group P2₁/n, a=11.0615(6), b=10.8507(4), c=11.2717(6) Å, β =116.427(2)^o, V=1211.5(1) ų, Z=4. The title compounds 3a and 3b reside on an inversion center so that only half of the molecule is crystallographically unique. Both compounds are not planar. The crystal structures of 3a and 3b have cumulated double bonds. The C7–C8–C8ⁱ and C5–C6–C6ⁱ angles that show the linearity in both structures, respectively, are 176.4(3)° in 3a and 175.6(2)° in 3b.     

Low-temperature structural phase transition in a monoclinic KDy(WO4)2 crystal

The low-temperature thermal and magnetic-resonance properties of a monoclinic KDy(WO₄)₂ single crystal are investigated. It is established that a structural phase transition takes place at T _c=6.38 K. The field dependence of the critical temperature is determined for a magnetic field oriented along the crystallographic a and c axes. The initial part of the H-T phase diagram is plotted for H∥a. The prominent features of the structural phase transition are typical of a second-order Jahn-Teller transition, which is not accompanied by any change in the symmetry of the crystal lattice in the low-temperature phase. The behavior of C(T) in a magnetic field shows that the transition goes to an antiferrodistortion phase. An anomalous increase in the relaxation time (by almost an order of magnitude) following a thermal pulse is observed at T>T _c(H), owing to the structural instability of the lattice. A theoretical model is proposed for the structural phase transition in a magnetic field, and the magnetic-field dependence of T _c is investigated for various directions of the field. Fiz. Tverd. Tela (St. Petersburg) 40, 750–758 (April 1998)     

Nuclear magnetic shielding and diamagnetic susceptibility of interacting hydrogen atoms

A quantum-mechanical study is made of the changes of the nuclear magnetic screening constant σ and the diamagnetic susceptibility X of two interacting hydrogen atoms due to van der Waals and overlap interatomic forces (effects of electron spin being neglected). At large distances the calculations show that van der Waals forces decrease the nuclear screening but increase the diamagnetic susceptibility (in magnitude). As the internuclear distance is reduced the first effect of overlap forces is to increase the screening in the repulsive (electronic triplet) state but this is followed by a further reduction. Attractive overlap forces (as in the ground state of H₂) ultimately lead to an increase in screening.     

Accurately solving the electronic Schrödinger equation of atoms and molecules using explicitly correlated (r 12-)MR-CI IV. The helium dimer (He 2)

The helium dimer interaction potential is computed using the recently proposed (explicitly correlated) r ₁₂-MR-ACPF (averaged coupled-pair functional) method and a [11s8p6d5f4g] basis set. With an MR-ACPF ansatz that contains 121 references we obtain interaction energies that are close to full CI. In a smaller reference space containing 9 functions, however, even by successively adding [3h] and [2i] functions to the basis set mentioned above, the basis set limit could not be reached. While convergence to the basis set limit is slow, it nevertheless is monotonic and therefore allows for extrapolation to the limit. We obtain basis set corrections at R = 4 a ₀ and 5.6 a ₀ which we further extend to all distances and which we apply to the potential energy curve mentioned above. From our calculations, we conclude that a very recent potential which has been calculated using the SAPT (symmetry adapted perturbation theory) method, and which previously was assumed to be the most accurate available, is insufficiently repulsive at short distances. We correct our extrapolated potential for retardation and finally calculate the expectation value of the interatomic distance (?R?) and dissociation energy (D ₀) by solving the Schrödinger equation of the vibrating ⁴He₂. Our results (?R? = 41 ± 13 Å and D ₀ = 2.2 ± 1.0 mK) are in acceptable agreement with very recent calculations in the literature, but they disagree with a recent experiment.     

Thermo-and photostimulated depolarization in self-compensated CdF2:Ga and CdF2:In crystals

A study is reported of the conductivity of CdF₂ semiconductor crystals doped by indium and gallium donor impurities and residing in a semi-isolated state. The latter results from self-compensation of the impurities, in the course of which one half of them creates two-electron DX centers, and the second is ionized. Photo-and thermostimulated depolarization of these crystals has been studied. It was shown that the observed polarization/depolarization phenomena have a nonlocal nature and are due to the charges present in these crystals changing their positions. These changes may be formally considered as charge displacement to macroscopic distances considerably in excess of the interatomic ones. The mechanisms responsible for these phenomena are discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 1575–1581 (September 1999)     

Phonons in red HgI2

A critical analysis of existing experimental and theoretical information on the lattice dynamics of the layered crystal α-HgI₂ is performed. It is shown that a satisfactory understanding of the character of the interatomic interaction and characteristic features of the phonon spectrum of this material can be obtained in a simple phenomenological model with eight parameters, assuming that E _g/B_1g degeneracy of the 29 cm⁻¹ Raman line is of a two-phonon nature. Fiz. Tverd. Tela (St. Petersburg) 40, 537–541 (March 1998)     

EXAFS and XRD studies of an amorphous Co57Ti43 alloy produced by mechanical alloying

We have investigated the local atomic order of an amorphous Co₅₇Ti₄₃ alloy produced by mechanical alloying by means of x-ray diffraction and EXAFS analyses on Co and Ti K-edges. Average coordination numbers and average interatomic distances between first neighbors where found from EXAFS and compared with those determined using an additive hard sphere (AHS) model associated with an deconvolution, and also with data from bcc- Co₂Ti compound. EXAFS results obtained indicated a shortening in the Co-Ti and Ti-Ti average interatomic distances when compared to those found by using the AHS-RDF method and an increase in the Co-Co and Ti-Ti average interatomic distances and a shortening in the Co-Ti one when compared to the interatomic distances found in the bcc- Co₂Ti compound. In spite of these differences, average coordination numbers obtained from EXAFS and AHS-RDF are similar to each other and also to those found in bcc- Co₂Ti.     

Spin relaxation of triplet pyrene in a crystal and in a glass

We have investigated the EPR spectral lines of the photo-excited triplet state of pyrene-d-10 both in a fluorene single crystal and an ethanol glass matrix. In the crystal we have measured the width, shape and saturation parameters, λ′, a and B _{1/2 ^u} , of the ΔM = 1 lines from both sites in the cleavage plane, Y-Z, at 193 K and along the principal directions, X, Y, Z, between 143 K and 300 K. In the glass the same parameters have been measured for the six resonances of the absorption derivative at 77 K. We have used a general deconvolution procedure to extract the unresolved inhomogeneous and homogeneous broadenings, ΔB _G and ΔB _L ≡ 1/γT ₂, and the spin-lattice relaxation rate, T ₁ ^-1, from λ′, a and B _{1/2 ^u} for the different types of resonances. The use of Fourier-series expansions permits optimal utilization of the data and resolution of the terms of different symmetries for these quantities. The application of the method of moments, using assumed spin densities, permits the demonstration that the main contribution to the constant term of ΔB _G, but only a small part of its anisotropy, originates from the intramolecular hyperfine interactions. As T ₂ and T ₁ are of the same order of magnitude, we have assumed the same mechanism, i.e. modulation of the fine-structure tensor D, for both relaxation effects. A first-order semi-classical treatment leads to partial agreement between the main features of the calculated and observed orientation dependences if one assumes strongly anisotropic librations coupled to large modulations of D. This is consistent with the maximal libration amplitudes estimated from the differences between the D tensors measured in the crystal and in the glass. Variations with temperature are attributed to a competition between the increase of modulation amplitudes and decrease of correlation time with heating.     

On electrostatic models of a metal-insulator phase transition in crystalline semiconductors with hydrogen-like impurities

Two well-known models for calculating the critical density N _C of a metal-insulator transition, as a function of the Bohr radius a _H of an isolated impurity as temperature T→0 K, are refined by making allowance for the screening of ions by electrons hopping along impurities. In one model, the transition at N _C1 is explained by the appearance of delocalized electrons as the impurity band is displaced into the allowed-energy band as a result of a decrease in the electron (hole) affinity of ionized impurities. In the other model the transition is explained by an unbounded increase in the static permittivity of the crystal as the density of impurity atoms increases to N _C2. The obtained approximations N _C1 ^1/3 a _H≈0.24 and N _C2 ^1/3 a _H≈0.20 for the degree of compensation K=0.01 describe existing experimental data for 1<a _H<10 nm. Fiz. Tverd. Tela (St. Petersburg) 40, 147–151 (January 1998)     

Isothermal compression of the L10-type alloy CuAu

The pressure dependence of the lattice parameters of the typical LI₀-type alloy CuAu has been measured up to 23 GPa using an X-ray diamond-anvil press with sodium chloride as an internal pressure marker. The axial ratio c/a increases with increasing pressure, indicating a larger resistance of the interatomic distances between unlike neighbors against compression compared with those between like neighbors. This increase is contrary to what is expected from the atomic-size-ratio consideration and suggests the significance of the conduction electron energy in determining the tetragonality of the Ll₀-type lattice.     

Collisions of alkali K and Rb atoms in ground state: Modification of interaction potentials

The interaction of alkali K and Rb atoms that reside in the ground state is considered in the range of collision energies E = 10⁻⁴ to 10⁻² au. The singlet (X ¹Σ⁺) and triplet (a ³Σ⁺) interaction potentials available in the literature are analyzed and modified. For the KRb dimer in the range of interatomic distances 15–21a ₀, we chose analytical representations of the singlet and triplet potentials that more accurately describe the interaction of alkali Rb and K atoms in the ground state. Complex cross sections of the spin exchange are calculated for the first time that permit one to calculate the processes of polarization transfer and relaxation times, as well as shifts in the magnetic resonance frequencies caused by K-Rb spin exchange collisions.     

Cutoff of long-wave phonons in a nanocrystal due to a nonuniform strain field

This paper considers the effect of extended monopole and dipole strain fields on the low-frequency boundary of the phonon spectrum in a crystal of finite dimensions. The boundary shift depends on the dynamical volume of the nonuniform strain region, which is determined by the parameters of the crystal and the sources of stress. An increase in the volume of the deformed region leads to a decrease in the undistorted part of the crystal, where a phonon with the largest wavelength can be produced. A monopole strain field is more efficient in cutting off long-wave phonons than a dipole strain field, and can “soften” the phonon spectrum. If a source generates stresses of the order of those on an interatomic scale, these effects can be the strongest and most diverse in crystals or phase precipitates with dimensions of less than 10⁻²⁶ cm. Zh. éksp. Teor. Fiz. 111, 1845–1857 (May 1997)     

Metamagnetism and magnetostriction of the Ising antiferromagnet DyCrO3

The magnetostriction of the Ising antiferromagnet DyCrO₃ during metamagnetic transitions induced by a magnetic field applied along the a and b axes of the crystal is investigated experimentally and theoretically. The magnetostriction measurements performed in the temperature range 1.5–4.2 K in magnetic fields up to 3 T clearly reveal the two-step character of the transition from the original antiferromagnetic structure to the ferromagnetic structure with the intermediate formation of a low-symmetry structure (when H is parallel to the a axis). The unusual behavior of the magnetostriction discovered during the magnetizing process provides information on the nature of magnetostriction in rare-earth Ising metamagnets. Fiz. Tverd. Tela (St. Petersburg) 39, 668–670 (April 1997)     

Crystal structure of stereocomplex of poly(L-lactide) and poly(D-lactide)

An equimolar mixture of poly(L-lactide) and poly(D-lactide) was crystallized into a stereocomplex whose crystal system is triclinic (P1) with cell dimensions: a = 0.916 nm, b = 0.916 nra, c (chain axis) = 0.870 nm, α = 109.2°, β - 109.2°, and γ = 109.8°. In the unit cell, a poly(L-lactide) segment and a poly (D-lactide) segment are contained as a pair and packed laterally in parallel fashion. The L- and D-poly(lactides) in the complex take a 3₁ helical conformation, which is a little extended from a 10₃ helix in the homopolymer crystal with the α-form. Homopolymers are also able to take the 3₁ helical conformation and form the β-form crystal. The 3₁ helix in the homopolymer crystal is less stable than the 10₃ one, and hence the β-form is easily transformed to the α-form by annealing.