On the experimental verification of the Landau-Pomeranchuk-Migdal effect

A theoretical explanation is given for the “unexpected” behavior recently observed in the radiation spectrum of ultrarelativistic electrons in a thin layer of matter in an experimental investigation of the Landau-Pomeranchuk-Migdal effect at SLAC [S. R. Klein et al., Preprint SLAC-6378, Stanford (1993); P. L. Anthony et al., Phys. Rev. Lett. 75, 1949 (1995)]. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 11, 837–840 (10 June 1996)     

The ideal polymer chain near planar hard wall beyond the Dirichlet boundary conditions

We present a new ab initio approach to describe the statistical behavior of long ideal polymer chains near a plane hard wall. Forbidding the solid half-space to the polymer explicitly (by the use of Mayer functions) without any other requirement, we derive and solve an exact integral equation for the partition function G _D(r,r′, N) of the ideal chain consisting of N bonds with the ends fixed at the points r and r′ . The expression for G(r,r′, s) is found to be the sum of the commonly accepted Dirichlet result G _D(r,r′, N) = G ₀(r,r′, N) - G ₀(r,r”, N) , where r” is the mirror image of r′ , and a correction. Even though the correction is small for long chains, it provides a non-zero value of the monomer density at the very wall for finite chains, which is consistent with the pressure balance through the depletion layer (so-called wall or contact theorem). A significant correction to the density profile (of magnitude 1/is obtained away from the wall within one coil radius. Implications of the presented approach for other polymer-colloid problems are discussed.     

Fine structure of splitting of the separatrix of a nonlinear resonance

This report is a continuation of an analysis, initiated elsewhere V.V. Vecheslavov and B. V. Chirikov, Zh. éksp. Teor. Fiz. 114, 1516 (1998) [JETP 86, 823 (1998)], of the effect of splitting of the separatrix of a nonlinear resonance for the model of standard mapping, based on results of direct measurements of the splitting angle α(K), where K is the system parameter. Measurements were made in the previously used wide range 0.1≳α≳10⁻²⁰⁸ (1⩾K⩾0.0004), but with significantly higher relative (better than 10⁵⁰) and average (∼10⁻⁵⁵) accuracy. This procedure made it possible to substantially refine the effects observed in Ref. 1 and construct qualitatively new empirical dependences providing reliable extrapolation of the data obtained for the angle and the invariant in the intermediate asymptotic limit K≲10⁻² beyond the limits of the investigated region. The results obtained by us can be useful for further development of the theory of separatrix splitting and formation of the stochastic layer of a nonlinear resonance. Zh. éksp. Teor. Fiz. 116, 336–346 (July 1999)     

Thermal boundary layer for non-Newtonian fluid

Summary Analytical and numerical solutions for the momentum and thermal boundary layer equations of a non-Newtonian power law fluid are presented. The flow is assumed to be under the influence of an external magnetic fieldB (x) applied perpendicular to the surface and an electric fieldE(x) perpendicular toB(x) and the direction of the longitudinal velocity in the boundary layer. For the power law fluid it is assumed that the shear stress is proportional to then-th power of the velocity gradient andn is called the flow index. The variations of the velocity fieldf′, the temperature field θ, the shear stress on the surfaceτ _W , the displacement thicknessδ ₁ and the momentum thicknessδ ₂ with the magnetic-field parameter γ, the flow indexn, the heat transfer indexS and the Prandtl number Pr are studied. It is found that, if the outer flow velocityU(x) (potential flow) is proportional to the arc lengthx raised to a powerm, then the similarity solution for the thermal boundary layer equation is possible only whenm=1/3, which represents flow past a wedge of included angle π/2. It is established that the temperature of the wedge increases with the increase of γ, Pr,S and the decrease ofn. In general the magnetic field can be used as a heater for the surface of the wedge.     

On the electron mobility of δ layers in the presence of diamagnetic “ejection” of size-quantization levels

The Hall mobility of electrons is investigated as a function of the population of size-quantization subbands in the two-dimensional electron gas of a δ-doped layer in GaAs with constant total electron density N _s =3.2×10¹² cm⁻² (three initially filled subbands) at T=4.2 K. The population of the subbands is varied by diamagnetic “ejection” of size-quantization levels (i.e., pushing them over the Fermi level) by a magnetic field oriented parallel to the plane of the δ-doped layer. The measurements are made in magnetic fields making small angles (5°) with the plane of the doping. The magnetic field component normal to the plane was used to measure the Hall mobility and density. It is found that the measured Hall mobility as a function of the ejecting magnetic field has a distinct maximum. This maximum is due to an increase in the electron mobility in the first subband (the ground subband is assigned the index 0) and electron redistribution between subbands with in increasing ejecting magnetic field parallel to the plane of the δ layer. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 11, 704–708 (10 December 1997)     

Evaluation of the rate constants of dissociative and ternary recombination reactions of argon ions on the basis of the results of ballistic experiments

AbstractExperimental data on the change in the electron density in the wake of a ballistic object traveling at velocities V _∞=3.4–4.9 km/s in argon at pressures p _∞=30–100 Torr are processed and analyzed. A reaction scheme is proposed which takes into account the recombination of charged particles, processes of ionic conversion, and the excited states of the atom. The solution of the equations of a nonequilibrium boundary layer for flow in the wake is used to formulate the inverse problem of determining the rate constants for dissociative recombination Ar ₂ ⁺ +e→Ar+Ar and ternary recombination Ar⁺+e+Ar→Ar+Ar. The “nearest-neighbor” approximation is used to obtain theoretically an expression for the ternary recombination coefficient as a function of temperature and pressure. Numerous solutions of inverse problems and a comparison with experiments demonstrates the validity of the expression obtained for the ternary recombination coefficient. It is shown that this expression is valid for moderate pressures and complements the Pitaevskii result for low pressures and the Langevin result for high pressures. Zh. Tekh. Fiz. 67, 12–18 (May 1997)     

Role of surface phenomena in the magnetoresistivity of polycrystalline manganites La1−x CaxMnO3

The dc electrical resistivity and magnetoresistivity of polycrystalline manganites La_1−x Ca_xMnO₃ (x=0–0.3) are investigated as functions of the temperature, magnetic field and electric field, along with the microwave surface resistance. The investigations show that the dc electrical resistivity and magnetoresistivity are governed by the surface properties of the intergranular boundaries. The dc electrical resistivity is observed to decrease substantially (tenfold) for a comparatively small electric field (E⋟100 V/cm). Estimates are obtained for the internal electrical resistivity of the granules, the thickness of the contact layer (which depends on the temperature and the magnetic field), and the height of the potential barrier between the interfaces separating the surface layer and inner layer of a granule. Fiz. Tverd. Tela (St. Petersburg) 40, 1881–1884 (October 1998)     

Nonadiabatic effects in the phonon spectra of superconductors

The nonadiabatic corrections to the self-energy part Σ_s(q, ω) of the phonon Green’s function are studied for various values of the phonon vectors q resulting from electron-phonon interactions. It is shown that the long-range electron-electron Coulomb interaction has no direct influence on these effects, aside from a possible renormalization of the corresponding constants. The electronic response functions and Σ_s(q, ω) are calculated for arbitrary vectors qand energy ω in the BCS approximation. The results obtained for q=0 agree with previously obtained results. It is shown that for large wave numbers q, vertex corrections are negligible and Σ_s(q, ω) possesses a logarithmic singularity at ω=2Δ, where Δ is the superconducting gap. It is also shown that in systems with nesting, Σ_s(Q, ω) (where Q is the nesting vector) possesses a square-root singularity at ω=2Δ, i.e., exactly of the same type as at q=0. The results are used to explain the recently published experimental data on phonon anomalies, observed in nickel borocarbides in the superconducting state, at large q. It is shown, specifically, that in these systems nesting must be taken into account in order to account for the emergence of a narrow additional line in the phonon spectral function S(q, ω)≈−π ⁻¹ Im D _s (q, ω), where D _s (q, ω) is the phonon Green’s function, at temperatures T<T _c . Zh. éksp. Teor. Fiz. 115, 1799–1817 (May 1999)     

Specific features in the temperature dependence of photoluminescence from CdTe/ZnTe size-quantized islands and ultrathin quantum wells

A study has been carried out of the temperature dependences of luminescence spectra on a large number of CdTe/ZnTe structures differing in average thickness, 〈L _z〉=0.25–4 monolayers (ML), and CdTe layer geometry (continuous, island type). The influence of geometric features in the structure of ultrathin layers on linewidth, the extent of lateral localization of excitons, their binding energy, and exciton-phonon coupling is discussed. It is shown that in island structures there is practically no lateral exciton migration. The exciton-phonon coupling constant in a submonolayer structure has been determined, Γ_ph=53 meV, and it is shown that in structures with larger average thicknesses Γ_ph is considerably smaller. Substantial lateral exciton migration was observed to occur in a quantum well with 〈L _z〉=4 ML, and interaction with acoustic phonons was found to play a noticeable part in transport processes. It has been established that the depth of the exciton level in a quantum well and structural features of an ultrathin layer significantly affect the temperature dependences of integrated photoluminescence intensity. Fiz. Tverd. Tela (St. Petersburg) 41, 717–724 (April 1999)     

Reciprocity relations for the effective electrical conductivity of randomly inhomogeneous media in the fractal regime

An exact relation for the realization-averaged effective conductivities in the fractal region is found for two-dimensional randomly inhomogeneous media. It has the form {σ _e (τ,L)~× {1/σ _e (−τ,L)~⁻¹=σ _e ² (τ=0, L≫ξ), where ξ is the correlation length (the self-averaging scale), L is the size of the system, τ=(p-p _c )/p _c , and p _c is the percolation threshold. For L≫ ξ, the system is self-averaged, and the relation transforms into the Dykhne reciprocity relation, A. M. Dykhne, Zh. éksp. Teor. Fiz. 59, 110 (1970) [Sov. Phys. JETP 32, 63 (1971)] σ _e (τ)σ _e (−τ])=σ _e ² (τ=0)= σ ₁ σ ₂. A similar relation is obtained for media with an exponentially broad distribution of local conductivities, as well as for individual realizations of some deterministic structures. Zh. éksp. Teor. Fiz. 113, 1484–1490 (April 1998)     

An approximate expression for the galvanomagnetic tensor

Using the iterative solution to the Boltzmann equation for electrons in d.c. electric and magnetic fields, an expression for the resistivity tensor can be obtained in the form of an infinite series. This series can be approximated by retaining only the first two terms. In the cases where relaxation times exist — in the sense that the collision term in the Boltzmann equation can be written asg(k)/τ(k), whereτ(k) is the relaxation time, andf (k) = f _E(ɛ k) + [∂f _E(εk)/∂ε]·g(k) the distribution function for electrons with wavevectork — this approximation is exact. For polyvalent metals in the one-OPW approximation, the complete galvanomagnetic tensor can be obtained using this approximation and the result differs from that obtained by using a time of relaxation given by an expression suggested byZiman. A calculation for a simple model Fermi surface, with screened Coulomb scattering, is carried out and the results compared with those of the relaxation time approximation.     

Structure of the free exciton luminescence band of heteroepitaxial ZnSe/GaAs layers

The exciton reflectance and photoluminescence spectra of epitaxial ZnSe/GaAs layers with a thickness of 2–4 μm are investigated in the temperature range 10–120 K. It is shown that one of the causes of the formation of the doublet structure of the A _n=1 photoluminescence band is interference of the exciton radiation at the boundaries of the near-surface dead layer. Fiz. Tverd. Tela (St. Petersburg) 40, 881–883 (May 1998)     

Rotation of the Fermi surface in the transition layer of bicrystals of an anisotropic metal

The Shubnikov-de Haas effect is investigated in bicrystals of p-type Bi_1−x Sb_x (0.063⩽x⩽0.088) alloys in static magnetic fields up to 15 T. The constant-energy surface of -holes in the transition layer of the bicrystals is found to be rotated ∼90° from its position in single domains (monoblocks). Fiz. Tverd. Tela (St. Petersburg) 39, 1713–1716 (October 1997)     

Concentration or modulation layering of perfect single crystals Bi2Sr2?xLaxCuO6+δ and Bi2?yPbySr2?xLaxCuO6

Precision structural studies of layering of perfect crystals Bi ₂ Sr _2−x La _x CuO _6+δ (BSLCO) ∼ 10 μm thick, grown by free growth within crystallized melt cavities, detect a macrolayer structure with each layer up to 0.1 μm thick. In the lanthanum concentration range x = 0.6−0.8, only modulation layering is observed. In the concentration range x = 0.3−0.5, either layers with two different lanthanum concentrations, but with the same modulated superlattice type, or layers with the same lanthanum concentration, but with two different modulated superlattice types are observed. At low lanthanum concentrations (0 < x < 0.26), layering into two or even three layer types with different lanthanum concentrations are almost always observed. Modulation suppression when lead is added to a mixture leads to the same layered structure of samples, but with appreciable variations in lattice parameters in the ab plane of individual layers and a and b axis rotation by several degrees with respect to each other in these layers. Thus, the superlattice in BSLCO single crystals stabilizes their composition in the ab plane, and inevitable variations in growth conditions lead to the layered structure of such crystals. Original Russian Text ? V.P. Martovitsky, A. Krapf, 2008, published in Kratkie Soobshcheniya po Fizike, 2008, Vol. 35, No. 3, pp. 29–38.     

A disclination-based approach to describing the structure of fullerenes

The possibility of using a disclination approach to describe the structure and properties of fullerenes is discussed. It is shown that the conversion of a planar carbon monolayer into a spherical macromolecule can be viewed as the result of introducing 12 disclinations with power π/3 into the original layer. Fiz. Tverd. Tela (St. Petersburg) 40, 1178–1180 (June 1998)     

Scattering of Rayleigh waves by a statistical inhomogeneity of the mass density

The problem of the scattering of a Rayleigh wave by a surface inhomogeneity of the mass density of an isotropic solid is solved in the Born approximation of perturbation theory. The inhomogeneity is statistical with a Gaussian correlation function in the plane parallel to the surface and is deterministic with an exponentially decaying dependence on the coordinate perpendicular to the surface. Expressions are derived for the displacement fields in the scattered longitudinal (P), transverse (SV and SH), and Rayleigh (R) waves at large distances from the inhomogeneity. The Rayleigh wave energy scattering coefficients are calculated as functions of the wavelength λ, the correlation length a of the inhomogeneity, the depth d of the defective layer, and the Poisson ratio of the medium, σ. The angular distribution of the scattered Rayleigh wave energy is determined. Asymptotic expressions are obtained for the scattering coefficient in various limiting cases with respect to the parameters a/λ and λ/d. The relation between the energies in the scattered P, SV, SH, and R waves is established. The resulting equations are used to calculate the scattering coefficients numerically over a wide range of variation of the parameters a/λ, λ/d, and σ; the results are presented in the form of graphs and a table. A physical pattern of the scattering process is constructed and used as a basis for interpreting the results of the study. Fiz. Tverd. Tela (St. Petersburg) 39, 267–274 (February 1997)     

Surface electron scattering in d-wave superconductors

A theoretical model of a rough surface in a d-wave superconductor is studied for the general case of arbitrary strength of electron scattering by an impurity layer covering the surface. Boundary conditions for quasiclassical Eilenberger equations are derived at the interface between the impurity layer and the d-wave superconductor. The model is applied to the self-consistent calculation of the surface density of states and the critical current in d-wave Josephson junctions. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 3, 242–246 (10 February 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Effect of vacancies and interstitials on the dynamic properties of the La2−x SrxCuO4 superconductor

The effect of vacancies and interstitials in the CuO₂ layer on the vibrational spectrum in the La_2−x Sr_xCuO₄ system has been calculated by molecular dynamics. It is shown that the excitation probability for local ∼0.4-eV high-frequency vibrations of nonphonon origin in the vicinity of Sr impurity atoms decreases if copper vacancies are introduced at a concentration x=0.17, which corresponds to the maximum superconducting transition temperature, this decrease being still more effective (by about ten times) if interstitial atoms are present. The appearance of interstitials makes a considerable region around them (five to six nearest neighbors) quasi-amorphous. A comparison with available experimental data is made. It is concluded that the behavior of the system under irradiation is accounted for primarily by interstitials, which bring about strong perturbation in the lattice (∼1 nm) up to making it completely amorphous. Fiz. Tverd. Tela (St. Petersburg) 40, 984–988 (June 1998)     

Large-scale amplification of a magnetic field by turbulent helicity fluctuations

In this paper the procedure of large-scale averaging of the magnetic-field diffusion equation with the α-term curlα(r,t)B(r,t) is used to show that a nonuniform distribution of the turbulent helicity fluctuations (more precisely, the fluctuations of the coefficient α) with a zero average value gives rise to large-scale amplification of the initial magnetic field. A detailed study is carried out of the dependence of the resulting large-scale α effect on the characteristics of the correlator 〈〈α(r, t)α(r″,t″)〉〉 in a rotating medium with a nonuniform distribution of the angular velocity ω=ω(ρ,z) (ρ is the distance for the rotation axis z). The effect of helicity fluctuations and the diffusion coefficient on the turbulent diffusion process is also investigated. Zh. éksp. Teor. Fiz. 116, 85–104 (July 1999)     

Hamiltonian dynamics of vortex lines in hydrodynamic-type systems

It is shown that the degeneracy of the noncanonical Poisson bracket operating on the space of solenoidal vector fields that arises due to the freezing-in of the curl of the velocity [E. A. Kuznetsov and A. V. Mikhailov, Phys. Lett. A 77, 37 (1980)] is lifted when the vorticity Ω is represented in terms of vortex lines. This representation makes it possible to integrate the equation of motion of the vorticity for a system with the Hamiltonian H=∫∣Ω∣d r. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 12, 1015–1020 (25 June 1998)