Domain-wall mobility in irradiated triglycine sulfate

Results are presented of an investigation of the temperature dependences of the domain wall mobility μ as a function of the irradiation dose between 20 and 180 kR. It is shown that the values of μ depend nonuniquely on the amplitude of the switching field and the radiation dose for switching current pulses of different polarity. Fiz. Tverd. Tela (St. Petersburg) 40, 1321–1323 (July 1998)     

Quantum spin liquid in an antiferromagnet with four-spin interactions

A quantum Monte Carlo study of the anisotropic antiferromagnet with four-spin interaction (K) and with spin S=1/2 on a square lattice is reported. Temperature dependences of the capacity, susceptibility, spin correlation functions, and correlation length, and field dependences of magnetization are calculated. Phase diagrams of the ground-state antiferromagnet and gap (K>0) and gapless (K<0) quantum spin liquids are determined in the exchange-anisotropy-four-spin-coupling-constant (K), magnetic-field (H)-temperature (T), and temperature-(T)-K planes for an exchange anisotropy of 0.1 J. Fiz. Tverd. Tela (St. Petersburg) 39, 1404–1409 (August 1997)     

Evolution of the fractal surface of amorphous lead zirconate-titanate films during crystallization

The recrystallization kinetics of amorphous lead zirconate-titanate films prepared by sol-gel technology are investigated experimentally using elastic scattering of light. Sequences of elastic dependences of the scattered light intensity are recorded directly during thermal annealing. The evolution of the morphology of the film surface during annealing is described in terms of the variation of their fractal dimensionalities D _s. The experimental dependences D _s(t) are compared with the results of a computer simulation of the phase transition kinetics in a thin plate (film). Fiz. Tverd. Tela (St. Petersburg) 41, 306–309 (February 1999)     

Characteristic features of the extrinsic electric resistance in ferromagnets with low carrier density

Switching from simple semiconductors to more complicated chemical compositions, we encounter mainly nonstoichiometric or undoped compounds. Combined with other characteristic features of d(f) compounds, this can lead, together with the ordinary scattering by spin disorder in magnetic semiconductors, to an unusual impurity contribution to the total scattering of carriers even in intrinsic semiconductors. A unique scheme for calculating the energy structure of the conduction-band bottom of a ferromagnetic semiconductor and the temperature and field dependences of the impurity contribution to the resistivity is proposed on the basis of a model Hamiltonian. The computed magnetoresistance ratio is negative and has a maximum near T _c . A qualitative comparison is made between the results and the experimental temperature dependences of the Hall mobility and magnetoresistance ratio in the ternary semiconductor n-HgCr₂Se₄, which is nonstoichiometric with respect to the chalcogen. To identify previously unobserved temperature oscillations of the resistance, a careful analysis is made of the low-temperature part of the resistance using the relations obtained. Fiz. Tverd. Tela (St. Petersburg) 41, 68–76 (January 1999)     

Magneto-optical phenomena in MnAs/CaF2/Si(111) epitaxial films in a transverse magnetic field

Field and angular dependences of the rotation of the plane of polarization in a transverse magnetic field H⊥k under normal reflection of light (λ=633 nm) have been studied in MnAs ferromagnetic epitaxial films grown by MBE on CaF₂/Si(111) substrates. The angle of rotation of the plane of polarization a is shown to be determined by contributions even and odd in the magnetization M. The odd contribution is associated with the deviation of the easy plane of magnetic anisotropy from the film plane, which originates from misorientation of the Si surface from the (111) plane, or from a presence of small regions of ( )-oriented MnAs. The even contribution is due to the optical anisotropy of films connected with quadratic-in-M terms in the dielectric permittivity tensor ɛ _ij of manganese arsenide. A method based on measuring the angular dependences of a in a rotating magnetic field is proposed to separate these contributions. Fiz. Tverd. Tela (St. Petersburg) 41, 110–115 (January 1999)     

Enhancement of the backscattered intensity from fractal aggregates

Abstract

Comparative measurements were conducted for the backscattered intensities of light from uniform random and fractal aggregated media. Different features are found for the backscattered intensity peak shapes. A crossover between the θ^1?D and θ^?2 dependences of the backscattered intensity occurs in the case of fractal aggregated medium, where D indicates the fractal dimension.     

Magnetic phase transitions in Nd1 ? xDyxFe3(BO3)4 ferroborates

The magnetic properties of ferroborate single crystals with substituted compositions Nd_{1 − x} Dy _x Fe₃(BO₃)₄ (x = 0.15, 0.25) with competing exchange Nd-Fe and Dy-Fe interactions are investigated. For each composition, we observed a spontaneous spin-reorientation transition from the easy-axis to the easy-plane state and step anomalies on the magnetization curves for the spin-flop transition induced by a magnetic field B | c. The measured parameters and effects are interpreted using a unified theoretical approach based on the molecular field approximation and on calculations performed in the crystal-field model for the rare-earth ion. The experimental temperature dependences of the initial magnetic susceptibility from T = 2 K to T = 300 K, anomalies on the magnetization curves for B | c in fields up to 1.8 T, and their evolution with temperature, as well as temperature and field dependences of magnetization in fields up to 9 T are described. In the interpretation of experimental data, the crystal-field parameters in trigonal symmetry for the rare-earth subsystem are determined, as well as the parameters of Nd-Fe and Dy-Fe exchange interactions.     

Dependence of the superconducting transition parameters on the solid-solution composition and Te excess in Sn1−z PbzTe:In

A low-temperature (0.4–4.2 K) measurement of the temperature dependences of the resistivity of two series of samples, SnTe_1+y and Sn_0.8Pb_0.2Te_1+y solid solution, doped with 5 at.% In, is reported. The parameters of the superconducting transition, namely, the critical temperatures T _c and the second critical magnetic field H _c2, and their dependences on tellurium excess (0⩽y⩽0.06) have been determined. The observed variation of the critical parameters with increasing tellurium excess in the samples is associated with a change in the filling by holes of the indium-impurity resonance states. Fiz. Tverd. Tela (St. Petersburg) 41, 2132–2134 (December 1999)     

EPR spectrum of donors in 6H SiC in a broad temperature range

An EPR study of donors in 6H SiC crystals with an uncompensated donor concentration (N _D−N _A) of 2×10¹⁸ to 1×10¹⁶ cm⁻³ performed in the temperature range 4.2 to 160 K at frequencies of 9 and 140 GHz showed that 6H n-SiC samples have two donor states in the gap. One of them originates from nitrogen occupying three inequivalent lattice sites with ionization energies of 150 and 80 meV, and the second is connected with a structural defect lying deeper in the gap than nitrogen. The temperature dependences of donor EPR line intensities have been found to deviate from the Curie law. The observed EPR line-intensity peaks of donors are produced in a temperature-driven successive redistribution of donor electrons between the donor levels. The temperature dependences of EPR line intensities obtained from samples with low donor concentrations were used to determine the valley-orbit splitting of nitrogen in cubic sites. Fiz. Tverd. Tela (St. Petersburg) 40, 1824–1828 (October 1998)     

Behavior of sound velocities in the compounds La1−x SrxMnO3 near magnetic and structural phase transitions

The temperature dependences of the transverse V _t and longitudinal V _l sound velocities in single crystals of the perovskites La_1−x Sr_xMnO₃ (x=0.1, 0.175, 0.2, 0.25) in the temperature interval T=70–350 K are investigated by the resonance method. Anomalies — small minima and kinks in the temperature dependences V _l (T) and V _t (T) — are observed at the Curie points. A strong jumplike increase (by up to 30%) in both the longitudinal and transverse sound velocities, which attests to sizable hardening of the acoustic branches of the phonon spectrum, is observed near the temperatures of the structural transitions between the rhombohedral and orthorhombic phases (for x=0.175, 0.2, 0.25) and at the phase transition to the polaron-ordered state (for x=0.1). Pis’ma Zh. éksp. Teor. Fiz. 68, No. 2, 141–146 (25 July 1998)     

On the applicability of the resonance tunneling model for describing conductivity anisotropy in TmBCO single crystals

The temperature dependences of the anisotropy of the resistance in two single crystals of the system TmBa₂Cu₃O_x with oxygen indices x=6.71 and 6.47 are analyzed from the standpoint of the applicability of Abrikosov’s theory [Physica C 253, 53 (1996)]. It is shown that the agreement between theory and experiment improves substantially if in approximating the data, besides resonance tunneling, allowance is made for the contribution of the ordinary tunneling conductivity. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 11, 772–777 (10 December 1996)     

Calculation of the structure properties and P–T phase diagram of hafnium

Abstract

The full-potential linear muffin-tin orbitals (FP-LMTO) method is used to calculate the total energy and equilibrium lattice properties for the observed phases of Hf. The temperature and pressure dependences of the Gibbs energy are found for these structures within the Debye model. A quantitative agreement with the experimental points of the P-T phase diagram is obtained.     

Magnetic susceptibility of multilayered carbon nanotubes

A theoretical study of the magnetic susceptibility of multilayered carbon nanotubes in fields both parallel, H _∥, and perpendicular, H _⊥, to the tube axis has been carried out disregarding electron-electron interaction. The temperature dependences of the magnetic susceptibility obtained exhibit a nontrivial form for H _∥, which is related to the quasi-one-dimensionality of such a system as the nanotube. The dependences of the magnetic susceptibility on chemical potential χ(μ) have also been derived. At low temperatures, χ(μ) has sharp peaks in fields H _∥, which is connected with the presence of singularities in the density of states of nanotubes. The effect of interlayer coupling on magnetic susceptibility of small-radius tubes has been investigated numerically. Fiz. Tverd. Tela (St. Petersburg) 40, 1950–1954 (October 1998)     

Effect of hole-hole scattering on the conductivity of the two-component 2D hole gas in GaAs/(AlGa)As heterostructures

The temperature dependences of the zero-magnetic-field resistivity ρ and magnetoresistance of the 2D hole gas in GaAs/(AlGa)As heterostructures are investigated in the temperature interval 0.4–4.2 K. As the temperature T is increased, (i) the resistivity ρ grows with a decreasing derivative dρ/dT, and (ii) the positive magnetoresistance diminishes from about 40% at T=0.4 K to about 1% at T=4.2 K. The results are explained in terms of a temperature-dependent mutual scattering of the holes, accompanied by momentum transfer between two different spin-split subbands. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 2, 101–106 (25 January 1998) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Formation of defects in gallium phosphide grown in the presence of oxygen

The influence of oxygen introduced in the gaseous phase on the formation of defects in GaP epitaxial layers is investigated by deep-level transient spectroscopy. The extremal dependences of the concentrations of charge carriers and electron traps with energy E _c−0.24 eV on the oxygen flux are discussed. Zh. Tekh. Fiz. 67, 52–55 (September 1997)     

Classical trajectory calculations for state-resolved Penning ionisation reactions of polycyclic aromatic hydrocarbon C10H8 in collision with He*(23S)

ABSTRACT

The reaction dynamics of Penning ionisation of a polycyclic aromatic hydrocarbon (PAH), naphthalene C₁₀H₈, in collision with the metastable He*(2³S) atom is studied by classical trajectory calculations using an approximate interaction potential energy surface between He* and the molecule, which is constructed based on ab initio calculations for the isovalent Li?+?C₁₀H₈ system. The ionisation width (rate) around the molecular surface are obtained from overlap integrals of the He 1s orbital and the molecular orbital. The calculated collision energy dependences of partial Penning ionisation cross sections (CEDPICS) in the range 50–500?meV at 300?K have reproduced the experimental results semi-quantitatively. The opacity functions, which represent the reaction probability with respect to the impact parameter b, are discussed in connection with collision energy, interaction with He* and the exterior electron density of molecular orbitals. They indicate that the collisional ionisations of C₁₀H₈ can be classified into three types: π electron ionisations with negative collision energy dependences which are predominantly determined by attractive interaction with He*; σ orbitals ionisations of the hardcore type; σ orbital ionisations which reflect interaction potentials around CH bonds. The critical impact parameters b_c become larger with increasing collision energy due to the centrifugal barrier.     

Excess space charge in strontium titanate

An analysis is made of mechanisms of the formation and redistribution of space charge which influence the dielectric hysteresis in SrTiO₃. The excess space charge density in SrTiO₃ at 4.2 K is estimated by comparing calculated and experimental dependences of the initial capacitance and the dielectric hysteresis parameter ΔC/C ₀ as a function of the crystal thickness. Fiz. Tverd. Tela (St. Petersburg) 40, 245–247 (February 1998)     

Effective dielectric and piezoelectric constants of thin polycrystalline ferroelectric films

The averaged dielectric, piezoelectric, and elastic constants of thin polycrystalline barium titanate and lead titanate films are calculated within a modified effective-medium approximation, which takes fully into account piezoelectric interactions between crystallites. Films with c-or a-type crystal texture resulting from mechanical interaction with the substrate are considered when the film becomes ferroelectric under cooling of the heterostructure. The dependences of the effective material constants of textured films on the residual macroscopic polarization of a film are described. An analysis is made of the effect of two-dimensional clamping of a film on a thick substrate on measurements of dielectric and piezoelectric constants. Fiz. Tverd. Tela (St. Petersburg) 40, 2206–2212 (December 1998)     

Inelastic resonance tunneling in S-Sm-S tunnel structures

Inelastic resonance tunneling through junctions with an amorphous interlayer and superconducting electrodes is studied. The form of the current-voltage characteristic I(V) at low temperature and the temperature dependence of the conductance G(0) at low bias are calculated and are found to be much different from the analogous dependences of structures with normal electrodes. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 2, 159–163 (25 January 1997)     

Angular dependence of the spin-wave resonance spectra in multilayer films

The angular dependence of the spin-wave resonance spectra is investigated in multilayer magnetic films in which the spin pinning mechanism changes from dynamical to dissipative. A fundamental difference is observed in the character of the angular dependences of the spectra for three-layer films as opposed to two-layer films. In particular, the number of peaks in the spectrum of three-layer films is observed to decrease and then increase twice as the angle between the magnetic field and the normal to the film is varied by π/2. Zh. Tekh. Fiz. 69, 97–101 (November 1999)