Optical quenching of the magnetoplastic effect in NaCl crystals

Observations indicate that illuminating NaCl crystals by ultraviolet light (λ=350 nm) suppresses the magnetoplastic effect. The processes induced by illumination take place in a subsystem of point defects and are related to a change in the state of magnetically sensitive dislocation pinning sites. Fiz. Tverd. Tela (St. Petersburg) 39, 1389–1391 (1996)     

Defect entropies and enthalpies in BaF<Subscript>2</Subscript>

Various experimental techniques have revealed that the predominant intrinsic point defects in BaF₂ are anion Frenkel defects. Their formation, enthalpy and entropy, as well as the corresponding parameters for the fluorine vacancy and fluorine interstitial motion have been determined. In addition, low temperature dielectric relaxation measurements in BaF₂doped with uranium leads to the parameters τ₀, E in the Arrhenius relation τ = τ₀ exp(E/k _B T) for the relaxation time τ. For the relaxation peak associated with a single tetravalent uranium, the migration entropy deduced from the pre-exponential factor τ₀ is smaller than the anion Frenkel defect formation entropy by almost two orders of magnitude. We show that, despite their great variation, the defect entropies and enthalpies are interconnected through a model based on anharmonic properties of the bulk material which have been recently studied by employing density-functional theory and density-functional perturbation theory.     

X-ray studies of magnetic phase transitions in the intermetallic compounds RMn2Ge2 (R=La, Sm, Gd, Nd, Tb, and Y)

The lattice parameters a and c of the tetragonal intermetallic compounds RMn₂Ge₂ (R=La, Sm, Gd, Nd, Tb, and Y) have been measured by x-ray diffraction in the temperature interval 10–800 K. Anomalies are observed in the temperature dependence of a and c due to phase transitions from the paramagnetic to the magnetically ordered state in the Mn subsystem, transitions between various magnetically ordered phases due to a change in the magnitude and sign of the Mn-Mn exchange interaction, and magnetic transitions caused by ordering of the rare-earth subsystem leading to a rearrangement of the magnetic structure of the Mn subsystem. It is concluded that, along with the lattice parameter a, the lattice parameter c also has an influence on the Mn-Mn exchange interaction. Fiz. Tverd. Tela (St. Petersburg) 41, 2053–2058 (November 1999)     

Properties of phase transitions in easy-axis tetragonal antiferromagnets

Properties of phase transitions in a magnetic field H parallel to the easy axis have been investigated, and it has been shown, in particular, that the nature of the transition of the magnetic subsystem from the antiferromagnetic phase to the angular phase depends on the “sign” of the Dzyaloshinskii interaction. The conditions for orientation of the antiferromagnetism vector l in the basis plane in fields larger than the threshold field have been determined. It is shown that the transition from the angular phase to the state where the resulting magnetic moment m is parallel to the easy axis takes place in the field corresponding to a spin-flip transition. From an analysis of the configuration of the magnetic subsystem for arbitrary orientation of the external magnetic field, it follows that the values of the critical angle (ψ _cr ) for which a first-order phase transition takes place satisfy the condition in the case in which the anisotropy constant f in the basis plane is of the order of the first anisotropy constant b. Usually so that the tricritical point in the phase diagram H _y ,H _z satisfies the condition H _y ∼H _z . Fiz. Tverd. Tela (St. Petersburg) 41, 2044–2046 (November 1999)     

Influence of antiferromagnetic ordering on the de Haas-van Alphen effect in a semimetal

The distinctive characteristics of the de Haas-van Alphen effect in semimetals with antiferromagnetic long-range order are investigated theoretically. It is shown that the transition of the subsystem of localized spins from the canted antiferromagnetic phase to the ferromagnetic phase is accompanied by an abrupt change in the “frequency” of the magnetization oscillations of band carriers M _∼. In the below-critical range of magnetic fields, M _∼ is not a function periodic in 1/H. Significantly, the additional contribution to the phase of the oscillatory factors is proportional to H ² and is determined entirely by quantum fluctuations in the antiferromagnetic subsystem. Fiz. Tverd. Tela (St. Petersburg) 39, 204–210 (February 1997)     

Calculation of the electron-ion interaction-potential components linear and quadratic in the ion displacements for YBa2Cu3O7

A simple method is proposed to calculate the component of electron interaction potential with the ionic subsystem of a crystal quadratic in ionic displacement. The electron-ion interaction potential is calculated for the high-frequency A _g modes in YBa₂Cu₃O₇. The important role of the Madelung potential, which results in violation of the selection rules valid in the rigid muffin-tin approximation, is pointed out. The quadratic component of the potential can greatly affect the calculated matrix elements which correspond to electronic transitions involving absorption or emission of zero wave-vector phonons. Fiz. Tverd. Tela (St. Petersburg) 39, 1323–1327 (August 1997)     

Point defects and ring defects in a nematic liquid crystal in a cylindrical capillary

The bidirectional escape into the third dimension of a linear disclination of strength m=1 (L ₊₁ ^p in a cylindrical capillary with normal boundary conditions is investigated. It is shown that in this case two types of defects arise in the capillary: point defects and ring defects, each of which can be of the radial or hyperbolic type. Exact solutions are obtained for the equation of equilibrium of the elastic field. The free energy of the point and ring defects is calculated approximately in a narrow, long capillary. New scenarios are proposed for the escape of the disclination L ₊₁ ^p . Zh. Tekh. Fiz. 69, 29–32 (July 1999)     

Strong coupling in the Kondo problem in the low-temperature region

The magnetic field dependence of the average spin of a localized electron coupled to conduction electrons with an antiferromangetic exchange interaction is found for the ground state. In the magnetic field range μH∼0.5T _c (T _c is the Kondo temperature) there is an inflection point, and in the strong magnetic field range μH≫T _c , the correction to the average spin is proportional to (T _c /μ H)². In zero magnetic field, the interaction with conduction electrons also leads to the splitting of doubly degenerate spin impurity states. Zh. éksp. Teor. Fiz. 115, 1263–1284 (April 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor     

Exchange interactions and spin dynamics in a [CuNd2(C4O4)4(H2O)16]·2H2O crystal

Crystalline [CuNd₂(C₄O₄)₄(H₂O)₁₆]·2H₂O constructed of complexes of trivalent neodymium and divalent copper, has been synthesized and studied by EPR. The square anion groups (C₄O₄) enter as bridge ligands, forming chains of neodymium ions interconnected by (C₄O₄)Cu(C₄O₄) fragments. It is found that the relaxation rate of the neodymium subsystem at room temperature significantly exceeds the exchange interaction rate between copper and neodymium ions. Under these conditions the magnetic properties of the crystal are determined by two magnetically nonequivalent chains of copper ions, which do not interact. The intrachain exchange interaction via hydrogen bonds is estimated to be ∼0.1 cm⁻¹. As one proceeds from the high-temperature (250<T<300 K) to the low-temperature region (T<40 K), a substantial change in the nature of the interaction is revealed. An unusual magnetic structure given in a crystal is observed at low temperatures, which is determined by the presence of two magnetically nonequivalent “ribbons,” formed by the interacting copper and neodymium ions: chains of copper ions are framed on two sides by chains of neodymium ions. The magnitude of the parameter of the exchange interaction between the copper and neodymium ions is estimated as J ^Cu-Nd⩾0.2 cm⁻¹. An exchange interaction between magnetically nonequivalent neodymium ions is not revealed in the EPR spectra. Fiz. Tverd. Tela (St. Petersburg) 39, 2057–2061 (November 1997)     

Formation and growth of an α-PbF2 phase during the plastic deformation of β-PbF2 crystals

The microstructure of strained β-PbF₂ crystals is investigated by optical and scanning electron microscopy. It is established that the α-PbF₂ previously observed by x-ray diffraction methods during the straining of cubic PbF₂ nucleates predominantly on structural defects like slip lines and bands. The kinetics of the growth of α-PbF₂ particles during the straining of β-PbF₂ crystals and during a postdeformation anneal is investigated. The results of the experiments point to the involvement of diffusion in the growth of α-PbF₂. Fiz. Tverd. Tela (St. Petersburg) 39, 640–646 (April 1997)     

Influence of samarium on optical absorption in thin films of the solid electrolyte RbAg4I5

The fundamental optical absorption of films of the solid electrolyte RbAg₄I₅ electrolyte films decreases (by approximately 25%) after vacuum evaporation of Sm films onto them, and a broad strong-absorption band with a maximum at 2.4 eV appears within the bad gap. The films bleach after 5–10 days in dry air. The observed phenomena are attributed to a high concentration (∼3×10²⁰ cm⁻³) of point defects, including F-centers, in nonstoichiometric RbAg₄I₅:Sm, and also to the oxidation of Sm. In colored films the ionic conductivity is σ⋍0.9σ ₀, and in bleached films it is close to the initial value σ ₀. Fiz. Tverd. Tela (St. Petersburg) 39, 1544–1547 (September 1997)     

Valence instability of uranium in U(Al1−x Gex)3

The electronic structure of U and Ge in the solid solutions U(Al_1−x Ge_x)₃ is investigated by measuring x-ray line shifts. It is shown that uranium has the mixed valence U³⁺ [Rn](5f ³)-U⁴⁺ [Rn](5f ²) over the entire composition range (0⩽x⩽1) and that the population of the uranium 5f shell increases by ∼0.28 5f electrons/U atom from UAl₃ (x=0) to UGe₃ (x=1). The electronic structure of Ge is close to the electronic structure of Ge metal over the entire composition range 0<x⩽1. No variation of the population of the Ge 4p shell is detected to within the experimental error (∼0.1 4p electrons/Ge atom) as the composition varies from x=0.2 to 1. It is established that the delocalization of a U 5f electron occurs as a result of its transition to the s or d band of the same uranium atom. Fiz. Tverd. Tela (St. Petersburg) 39, 1505–1508 (September 1997)     

Electronic structure of lead (II) fluoride and lead (II) chloride crystals

The method of complete neglect of differential overlap was used to calculate the electronic structure of α PbF₂, β PbF₂, and PbCl₂ crystals which form the basic matrices for the synthesis of new superionic conductors. It was shown that the electronic subsystem is fairly stable on transition from the high-symmetry β PbF₂ crystal to the low-symmetry α PbF₂. The electronic structure of the cotunnite crystal PbCl₂ was calculated for the first time. Fiz. Tverd. Tela (St. Petersburg) 40, 235–236 (February 1998)     

Excitation of intrinsic defects in ionic crystals by high-power optical and electron beams

The efficiency of excitation and recharging of intrinsic defects by hot charge carriers has been investigated in ionic crystals acted on by high-power optical and electron beams. The interaction cross sections of hot electrons and holes with intrinsic lattice sites and F _n -type defects (n=1,2) are shown to be commensurate. It is also shown that the potential of the intracrystalline field in the vicinity of F and F ₂ centers is nearly regular. Fiz. Tverd. Tela (St. Petersburg) 40, 1030–1035 (June 1998)     

Structure,elasticity and phase transitions in liquid crystals with deformations

ABSTRACT

A molecular-statistical theory describing the nematic liquid crystals (LCs) with spherical inclusions (or point defects) is proposed. At given size of inclusions and nematic order parameters at the surfaces of inclusions (zero in the case of point defects) and far from inclusions (where the nematic LC is almost uniform), the distribution of nematic order parameters in the bulk of LC with inclusions was found to be fully determined by the elastic constants of LC. We have found and explained the two-step heat-driven transformation from the nematic phase into the isotropic phase, with the intermediate phase in between. The nematic order parameters and the elastic constants are evaluated in the framework of a unified approach based on the features of pair interaction potentials of the individual LC molecules. It is shown that, in the case of K₃₃ < K₁₁, the point defects should destroy the conventional nematic phase.     

Asymptotic coefficients for atoms and ions

We derive convenient analytical formulas in the effective-range approximation for the asymptotic coefficient C _κ of the radial wave function at infinity and for the average radius of the system. A comparison with the results of numerical calculations (done by the Hartree-Fock method) for multi-electron atoms and ions reveals that this approximation has good accuracy for valence s-electrons in all atoms from hydrogen to uranium. We calculate the values of the scattering lengths and the effective ranges for electron-atom and electron-ion scattering. We also examine the quasiclassical approximation for C _κ. Finally, we discuss the logarithmic increase in the effective ranges of ns states as n→∞. Zh. éksp. Teor. Fiz. 115, 521–541 (February 1999)     

Thermally activated glide of edge superdislocations in a single-crystalline Ni<Subscript>3</Subscript>Ge alloy with point defects

Temperature dependence of the yield stress for Ni ₃ Ge single crystals is investigated. Positive temperature dependence of the critical yield stress in the high-temperature region (above 420 K) is attributed to a thermally activated accumulation of a high nonscrew-superdislocation density. The mechanism of interaction of point defects with superdislocations and its contribution to the temperature anomaly is examined. Correlation is found between the calculated contribution from point defects and the observed increase in the critical yield stress with the test temperature. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 83–88, April, 2007.     

Investigation of metamagnetic transitions in the itinerant d subsystem of the intermetallics RCo2 in superstrong magnetic fields up to 300 T

The time derivatives of the magnetization of intermetallic compounds RCo₂ (R = Y, Tm, Er, Ho, or Dy) with a metamagnetic subsystem of itinerant d electrons have been measured in pulsed magnetic fields up to 300 T generated by an explosive method. Peaks associated with abrupt demagnetization or magnetization of the d subsystem are found in the field dependences of dM/dH. The results obtained are compared with theoretical estimates and the values derived for the critical fields from measurements performed on substituted compounds. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 3, 188–192 (10 August 1996)