Collective defect formation in the high-temperature superconductor YBa2Cu3O7 under the influence of adsorbed water molecules

The mechanisms for defect formation stimulated by the adsorption of water molecules in the surface of YBa₂Cu₃O₇ ceramic are studied, together with the types of defects and their distributions. It is found that a water layer physically bound to the surface reduces the rates of annihilation and capture of positrons, changes the amount of barium and copper on the surface by a factor of two, and inhibits diffusive jumps of nickel atoms. A layer of adsorbed water excites subthreshold formation of 10²¹ cm⁻³ interstitial Ba and Cu1 atoms and transitions of oxygen from O1 to O5, and vice versa in the volume of crystallites, and the migration of defects and accumulation of Ba atoms in the surface layer, which block diffusive jumps of Ni within the volume of the crystals. These effects are related to the excitation of collective, low-frequency weakly damped motion of heavy holes in the crystal volume when defects are formed on the surface by physically adsorbed H₂O molecules, which is accompanied by Coulomb repulsion of cations from intermediate layers into interstitials and the migration of defects in the field of the collective excitations. Zh. éksp. Teor. Fiz. 116, 586–603 (August 1999)     

A Selective Spectrofluorometric Determination of Micromolar Level of Cyanide in Water Using Naphthoquinone Imidazole Boronic-Based Sensors and a Surfactant Cationic CTAB Micellar System

We developed a new spectrofluorometric method for qualitative and quantitative determination of cyanide in water using the incorporation of naphthoquinone imidazole boronic-based sensors (m -NQB and p -NQB) and a cationic surfactant, certyltrimethyl ammonium bromide (CTAB). This micellar system exhibited great selectivity for cyanide detection with an assistance of the cationic surface of micelle. The interaction of boronic acid of the sensor toward cyanide in CTAB micellar media gave a quantitative measure of cyanide concentration in the micromolar level. Under the optimal condition, fluorescence intensity at 460 nm of m -NQB and p -NQB provided two sets of linear ranges, 0.5–15 μM and 20–40 μM and the limit of cyanide detection of 1.4 μM. Hence, both sensors in CTAB aqueous micellar system offered a considerably promising cyanide detection with 1000–fold enhancement of the detection limit compared to those studied in DMSO: H₂O. The proposed sensors could also be used to determine cyanide in water with good analytical characteristics.     

Growth of crystallites consisting of C60 molecules on heated (100)Mo

The initial stages of growth of films of C₆₀ molecules on a (100)Mo surface are studied by high-resolution Auger spectroscopy and atomic force microscopy under ultrahigh vacuum conditions. It is shown that at T<750 K, after a specific coating with molecular density 1.5×310¹⁴ cm⁻², which the authors term a high-temperature (HT) monolayer, has formed on the surface, crystallites in the form of “towers” with flat tops grow on the surface. The fraction of the area occupied by towers depends strongly on the substrate temperature and the flux density of the molecules on the surface but is virtually independent of the exposure time. At temperatures _T>760 K the C₆₀ molecules making up the crystallites desorb, while the molecules making up the HT monolayer decompose, and the carbon released is dissolved in the interior of the substrate. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 12, 969–972 (25 June 1998)     

Photoexcitation of rare-gas neon and argon clusters doped with <Emphasis Type="Bold">H</Emphasis><Subscript><Emphasis Type="Bold">2</Emphasis></Subscript><Emphasis Type="Bold">O</Emphasis>

We have studied the fluorescence of electronically excited OH^*, H^* and H₂O^+* dissociation fragments after VUV excitation ( h ν≥11.6 eV) of rare-gas clusters (Rg = Ne, Ar) doped with H₂O molecules. In contrast to a free molecule, where Balmer H-series dominate the UV-visible spectra, only the OH ^* ( A ² Σ ⁺ ↦ X ² Π) emission band is observed in neon clusters. No emission of excited water ions has been observed. We find that while higher excitation energies (Ne vs. Ar) induce higher vibrational excitation of the OH^* ( A ) fragment, the rotational temperature is lower. This effect is attributed to the difference in the geometric position of the H₂O molecule on the surface or inside the Rg-cluster. The rotational relaxation in neon clusters is rapid while the vibrational relaxation is slow because of the coupling with the low energy matrix phonons. Received 7 March 2002 / Received in final form 27 May 2002 Published online 19 July 2002     

A comparison of the effectiveness of GaAs surface passivation with sodium and ammonium sulfide solutions

To explore the possibility of improving the effectiveness of chemical and electronic passivation, a study has been made of the properties of GaAs surface treated with solutions of inorganic sulfides [Na₂S and (NH₄)₂S] in various amphiprotic solvents (water, alcohols). X-ray photoelectron and photoluminescence spectroscopy shows that the efficiency of both chemical and of electronic passivation of GaAs surface increases with decreasing dielectric permittivity of the solvent. The degree of this increase reached with solutions of the sulfide of a strong base (Na₂S) is larger than that of a weak-base sulfide [(NH₄)₂S]. Fiz. Tverd. Tela (St. Petersburg) 39, 63–66 (January 1997)     

Superconductivity of SrTiO

Superconducting SrTiO _{3 - δ} was obtained by annealing single crystalline SrTiO₃ samples in ultra high vacuum. An analysis of the V ( I ) characteristics revealed very small critical currents I _c which can be traced back to an unavoidable doping inhomogeneity. R ( T ) curves were measured for a range of magnetic fields B at I ≪ I _c , thereby probing only the sample regions with the highest doping level. The resulting curves B _c2 ( T ) show upward curvature, both at small and strong doping. These results are discussed in the context of bipolaronic and conventional superconductivity with Fermi surface anisotropy. We conclude that the special superconducting properties of SrTiO _{3 - δ} can be related to its Fermi surface and compare this finding with properties of the recently discovered superconductor MgB₂. Received 4 December 2002 / Received in final form 10 March 2003 Published online 23 May 2003 RID="a" ID="a"e-mail: jourdan@uni-mainz.de     

Condensed water in superfluid He-II

It was found that when ⁴He gas containing water vapor as an impurity condenses on the surface of superfluid He-II cooled to ∼ 1.4 K, semitransparent clouds (icebergs) form in the volume of a glass cell filled with He-II below the He-II surface. The form of the icebergs extracted from the superfluid liquid remains virtually unchanged on heating up to ∼ 1.8 K. In the temperature range 1.8–2.2 K the thermometers register sharp temperature jumps, which are accompanied by jumps in the gas pressure in the cell and a repeated decrease, by more than two orders of magnitude, in the total volume of the condensate, i.e., the water content in the volume of an iceberg does not exceed 10²⁰H₂O molecules per 1 cm³. It can be inferred that porous icebergs, permeated with superfluid liquid and containing cores consisting of small clusters surrounded by a layer of solidified helium, form in the volume of He-II as the gas mixture condenses. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 11, 744–748 (10 December 1999)     

Luminescence detection of multiphoton ionization-fragmentation of the molecular CrO 4 2− anions adsorbed on the surface of dispersed SiO2

It is shown that luminescence detection of multiphoton ionization-fragmentation of the molecular CrO ₄ ²⁻ anions adsorbed on the surface of dispersed SiO₂ is possible under excitation with the fundamental frequency of a Nd:YAG pulsed laser (λ=1.064 μm). The structure and the process of formation of the adsorbed complexes under thermal activation of the surface and the nature of luminescence transitions in CrO ₄ ²⁻ anions are studied in detail. It is shown that luminescence is excited as a result of the recombination of photoelectrons and ionized chromate ions. Multiphoton ionization of the ions occurs under three-photon resonance conditions. The resonance level is an antibonding state of the adsorption complex formed with the participation of an oxygen vacancy on the SiO₂ surface. The dynamics of the multiphoton luminescence excitation process includes autoionization (stimulated by intercomplex electronic excitation) in superexcited states, fragmentation of chromate anions, and annealing of surface oxygen vacancies. The rate equations for three-photon-resonance multiphoton ionization are studied. The cross sections for two-and one-photon transitions on the nonresonance steps of multiphoton absorption are obtained. It is concluded that the nonlinear polarizability of the donor-acceptor adsorption bond in “chromate anion-oxygen vacancy” complexes is very important. Zh. éksp. Teor. Fiz. 111, 1748–1774 (May 1997)     

Giant waves in weakly crossing sea states

The formation of rogue waves in sea states with two close spectral maxima near the wave vectors k ₀ ± Δk/2 in the Fourier plane is studied through numerical simulations using a completely nonlinear model for long-crested surface waves [24]. Depending on the angle θ between the vectors k ₀ and Δk, which specifies a typical orientation of the interference stripes in the physical plane, the emerging extreme waves have a different spatial structure. If θ ≲ arctan(1/√2), then typical giant waves are relatively long fragments of essentially two-dimensional ridges separated by wide valleys and composed of alternating oblique crests and troughs. For nearly perpendicular vectors k ₀ and Δk, the interference minima develop into coherent structures similar to the dark solitons of the defocusing nonlinear Schroedinger equation and a two-dimensional killer wave looks much like a one-dimensional giant wave bounded in the transverse direction by two such dark solitons.     

Efficiency of Pd impregnated sol-gel derived γ-alumina porous spheres as catalyst

Porous γ-alumina spheres having surface area as large as 300 m ² /gm are obtained by using sol-gel process. γ-alumina supported Pd catalyst is synthesized by soaking it in PdCl ₂ solution followed by drying and heating at 300 ^° C/1 hr in hydrogen atmosphere. Specific Surface Area of Pd coated alumina remained unaltered. The scanning electron microscopy revealed uniform coating of Pd metal on the surface. The equivalent spherical diameter (ESD) of the coated Pd metal particles is estimated to be 50. The catalytic efficiency is evaluated by H ₂ -O ₂ combination reaction. The performance of this supported catalyst is found to be much superior in comparison with Pd catalyst realized on commercially available alumina having surface area of 80 m ² /gm. The improved efficiency of the catalyst could be co-related to enhanced surface of nano-structured Pd available for catalytic action. The added advantage of spherical shape of the support material appeared in uniform and homogeneous loading of the catalyst bed. The results highlight the important role played by support material in these type of catalysts. Received 24 November 2000     

Polymorphism of biperiodic stripe domain structures

Magnetic-field-induced polymorphism of biperiodic stripe domain structures of films with a positive anisotropy constant and easy axis parallel to the normal to the surface is studied. A symmetry classification of the observed types of domain structures is performed, and the phase diagram of the films in the (H _⊥, H _∥) plane, where H _⊥ and H _∥ are, respectively, the components of the magnetic field vector perpendicular and parallel to the normal to the surface, is determined. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 9, 701–706 (10 May 1998)     

Nonequilibrium phase transition in quenched samples of Fe0.1TiTe2

The influence of the heat treatment of Fe_0.1TiTe₂ samples on the appearance of thermally induced “levitation” is investigated. It is postulated that this phenomenon is most probably caused by undamped fluctuations of the electric charge on the surface of particles of the material. Fiz. Tverd. Tela (St. Petersburg) 41, 680–683 (April 1999)     

Effect of a bounding surface on the rotational viscosity of liquid crystals

The effect of a surface, bounding a nematic-liquid phase, on the rotational viscosity γ ₁ is investigated on the basis of a statistical approach employing direct correlation functions. Specific calculations are performed for a model system consisting of ellipsoidal molecules interacting via a Gay-Berne potential near the bounding surface. Fiz. Tverd. Tela (St. Petersburg) 40, 1945–1949 (October 1998)     

Sputtering of AlxGa1−x As semiconductor targets by Ar+ ions with energies of 2–14 keV

Investigations of the sputtering of Al_xGa_1−x As semiconductor solid solutions by Ar⁺ ions with energies of 2–14 keV are performed. The dependence of the sputtering yield on the energy and angle of incidence of the ions are determined and the character of the surface relief formed during the sputtering is investigated. A comparison with theory shows that the best agreement between theory and experiment is achieved when the Haff-Switkowski formula is used together with Yudin’s stopping cross section. It is shown that the surface binding energies obtained differ from the atomization energies by an amount approximately equal to the amorphization energy. Zh. Tekh. Fiz. 67, 113–117 (June 1997)     

Dependence of the intrinsic line width of surface states on the wave vector: The Cu(111) and Ag(111) surfaces

The dependence of the intrinsic line width Γ of electron and hole states due to inelastic scattering on the wave vector k _∥ in the occupied surface state and the first image potential state on the Cu(111) and Ag(111) surfaces has been calculated using the GW approximation, which simulates the self-energy of the quasiparticles by the product of the Greens’s function and the dynamically screened Coulomb potential. Different contributions to the relaxation of electron and hole excitations have been analyzed. It has been demonstrated that, for both surfaces, the main channel of relaxation of holes in the occupied surface states is intraband scattering and that, for electrons in the image potential states, the interband transitions play a decisive role. A sharp decrease in the intrinsic line width of the hole state with an increase in k _∥ is caused by a decrease in the number of final states, whereas an increase in Γ of the image potential state is predominantly determined by an increase of its overlap with bulk states.     

Pseudogap and symmetry of superconducting order parameter in cuprates

The phase diagram, nature of the normal state pseudogap, type of the Fermi surface, and behavior of the superconducting gap in various cuprates are discussed in terms of a correlated state with valence bonds. The variational correlated state, which is a band analogue of the Anderson (RVB) states, is constructed using local unitary transformations. Formation of valence bonds causes attraction between holes in the d-channel and corresponding superconductivity compatible with antiferromagnetic spin order. Our calculations indicate that there is a fairly wide range of doping with antiferromagnetic order in isolated CuO₂ planes. The shape of the Fermi surface and phase transition curve are sensitive to the value and sign of the hopping interaction t′ between diagonal neighboring sites. In underdoped samples, the dielectrization of various sections of the Fermi boundary, depending on the sign of t′, gives rise to a pseudogap detected in photoemission spectra for various quasimomentum directions. In particular, in bismuth-and yttrium-based ceramics (t′>0), the transition from the normal state of overdoped samples to the pseudogap state of underdoped samples corresponds to the onset of dielectrization on the Brillouin zone boundary near k=(0,π) and transition from “large” to “small” Fermi surfaces. The hypothesis about s-wave superconductivity of La-and Nd-based ceramics has been revised: a situation is predicted when, notwithstanding the d-wave symmetry of the superconducting order parameter, the excitation energy on the Fermi surface does not vanish at all points of the phase space owing to the dielectrization of the Fermi boundary at k _x=± k _y. The model with orthorhombic distortions and two peaks on the curve of T _c versus doping is discussed in connection with experimental data for the yttrium-based ceramic. Zh. éksp. Teor. Fiz. 115, 649–674 (February 1999)     

Origin of “Hot Spots” in the pseudogap regime of Nd1.85Ce0.15CuO4: An LDA + DMFT + Σk study

The material-specific electronic band structure of the electron-doped high- T _c cuprate Nd_1.85Ce_0.15CuO₄ (NCCO) is calculated in the pseudogap regime using the recently developed generalized LDA + DMFT + Σ _k scheme. The LDA/DFT (density-functional theory within local density approximation) provides model parameters (hopping integral values and local Coulomb interaction strength) for the one-band Hubbard model, which is solved by the DMFT (dynamical mean-field theory). To take pseudogap fluctuations into account, the LDA + DMFT is supplied with an “external” k-dependent self-energy Σ _k that describes interaction of correlated conducting electrons with nonlocal Heisenberg-like antiferromagnetic (AFM) spin fluctuations responsible for the pseudogap formation. Within this LDA + DMFT + Σ _k approach, we demonstrate the formation of pronounced hot spots on the Fermi surface (FS) map in NCCO, opposite to our recent calculations for Bi₂Sr₂CaCu₂O_{8 − δ} (Bi2212), which have produced a rather extended region of the FS “destruction.” There are several physical reasons for this fact: (i) the hot spots in NCCO are located closer to the Brillouin zone center; (ii) the correlation length ξ of AFM fluctuations is longer for NCCO; (iii) the pseudogap potential Δ is stronger than in Bi2212. Comparison of our theoretical data with recent bulk-sensitive high-energy angle-resolved photoemission (ARPES) data for NCCO provides good semiquantitative agreement. Based on that comparison, an alternative explanation of the van Hove singularity at −0.3 eV is proposed. Optical conductivity for both Bi2212 and NCCO is also calculated within the LDA + DMFT + Δ _k scheme and is compared with experimental results, demonstrating satisfactory agreement. The text was submitted by the authors in English.     

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)     

Anomalous energy dependence of the electron relaxation time in bismuth

The relaxation time of highly nonequilibrium electrons whose excitation energy |Δε| is of the order of the Fermi energy ε _F is measured. The measurements are made on bismuth samples with the aid of transverse electron focusing with micron intercontact separations. The dependence of the electron collision frequency on the excitation energy is found to be close to linear. The possibility of observing the topological restructuring of a closed isoenergetic surface into an open surface in Bi with an increase in the electron energy is discussed. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 12, 835–840 (25 December 1996)     

Accurate method for determining tilt bias angles in thin films of nematic liquid crystals

We have developed a new method for measuring tilt bias angles in spatially uniform and nonuniform thin films of nematic liquid crystals. The method employs modulation ellipsometry, based on the use of an exponentially decaying light wave to probe the boundary layer. Oscillations of the director of the liquid crystal, which are induced by the flexoelectric torque, are excited with an external periodic field. A periodic variation of the ellipticity of the light wave reflected from the interface is detected at both the first and second harmonics of the exciting electric field. When these two Fourier components of the electrooptic response are known, it is possible to calculate both the tilt bias angle θ₀ of the director and the dynamic deviation δ₀ of the tilt bias angle. The angles θ₀ and δ₀ measured by this method on the surface of an electrode (ITO) and on the surface of a ferroelectric film (a copolymer of vinylidene fluoride and trifluoroethylene), oriented in a corona discharge, were equal to θ ₀=5.1°, δθ=0.5° and θ₀=89°, δθ=0.06°, respectively. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 6, 387–392 (25 March 1998)