Layers In Unit Cell Research Articles

Atomic Layer and Unit-Cell Layer Growth of (Ca,Sr)CuO2 and Bi2Sr2Can-1CunO2n+4 Thin Films by Laser Molecular Bean Epitaxy

ABSTRACTThin films of (Ca,Sr)CuO2 and Bi2Sr2Can-1CunO2n+4 are formed by laser molecular beam epitaxy with in-situ reflection high energy electron diffraction observation. The diffraction pattern shows that these materials are formed with layer-by-layer growth. The change of the diffraction intensity as well as the analysis of the total diffraction pattern makes It possible to control the grown of the atomic layer or the unit-cell layer.

Lower critical field, remanent magnetization and irreversibility line of ceramic (Bi,Pb)-2223-HTSC

A detailed magnetic study of the (Bi, Pb)-2223 superconductors was carried out over the temperature range 4–120 K. At temperatures below 25 K the lower critical field H c1 was drastically enhanced. This enhancement can probably be explained either by a theory based on an intrinsic proximity effect in the normal layers of the Bi-2223 unit cell or by a blocking of the flux penetration because of a surface barrier effect. An analysis of the anomalies in the irreversibility line has been undertaken in the framework of the proximity effect allowing us to obtain also plausible values for ξ 0, λ 0 and the Fermi velocities in the superconducting and normal layers. The temperature dependence of the large remanent magnetization at 4 K with a drastic decrease above 25 K offered evidence for a strong depinning of the vortex lattice around 25 K. Below this temperature the active pinning centers have a rather small pinning energy. Above 25 K a smaller group of pinning centers with larger activation energy gives rise to a small and weakly temperature-dependent remanent magnetization. A scaling behavior of the magnetization curves was obeyed in the temperature range between 40 and 60 K in good agreement with the predictions of an extended Bean model.

Superconducting superlattices: Verification of two-dimensional nature in high T c Bi2Sr2(Ca1−xYx)Cu2O8 superconductors

An approach to evaluate the two-dimensional nature in a high Tc superconductor is examined on the superconducting superlattice prepared by stacking of superconductor and nonsuperconductor layers. The Bi2Sr2(Ca1−xYx)1Cu2O8-based superlattices have been formed by stacking two different unit cell layers containing different yttrium concentrations. One unit layer contains lower yttrium concentration (x=0.15) to be a superconductor, and the other contains higher concentration (x=0.5) to be a semiconductor, and these layers are periodically stacked with appropriate ratios. These superlattices exhibit the same resistance-temperature curve as the original superconducting films with x=0.15. This behavior can be explained by the strong two-dimensional character in this Bi2Sr2(Ca1−xYx)Cu2O8 high Tc superconductor.

Interlayer coupling effect in high-Tc superconductors probed by YBa2Cu3O7-x/PrBa2Cu3O7-x superlattices.

The effect of coupling between layers of the high-${\mathit{T}}_{\mathit{c}}$ superconductors was studied by varying both the ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$ and ${\mathrm{PrBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$ layer thicknesses in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$/${\mathrm{PrBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$ superlattices. Superconductivity was found even when the ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$ layers had nominal thicknesses as small as 12 \AA{} (one unit-cell thickness) but coupling between unit-cell layers is needed to achieve a ${\mathit{T}}_{\mathit{c}}$ of 90 K. The decrease of ${\mathit{T}}_{\mathit{c}}$ in isolated ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$ layers with increasing sheet resistance is on the same scale of about \ensuremath{\Elzxh}/${\mathit{e}}^{2}$ as observed for ordinary superconductors.

Critical temperature in high-Tc layered superconductors

Abstract The generalized BCS pairing thoery for layered superconductors is used to investigate the dependence of the transition temperature T c on the number N of interacting conducting Cu-O layers in any unit cell. The coupling constants, which describe intralayer and interlayer interactions in these layers where the Cooper pairing is assumed to take place, are explicity related to the microscopic polarization functions of all layers in the unit cell, including the insulating layers. With reasonable interlayer couplings it is possible to understand the experimental values of T c in many of the high- T c compounds discovered recently. One can indeed increase T c as a function of N , but we find T c (∞) to be less than 150K in polytypes of the [-(CuO 2 ) N -] class, as well as in the 1:2:3 class polytypes [-(CuO 2 )-(CuO-CuO 2 ) n -], N =2+1. This is based on the approximation involving only up to the nearest neighbour conducting layer couplings and the use of experimental results for T c in various thallium, bismuth and 1:2:3 compounds. A much higher T c (∞) is possible only of the single-layer T c (1) is high (>90 K) and interlayer couplings are significant.

Electronic structure of the superconducting compounds TlBa 2CaCu 2O 7 and TlBa 2Ca 2Cu 3O 9

The electronic structure of the two simplest superconducting compounds belonging to the new homologous series TlBa 2 Ca n−1 Cu n O 2( n+1)+1 ( n=2,3) is calculated using the tight-binding method. Total and partial electronic densities of states as well as the valences of individual atoms are obtained. The two-dimensional nature of the electronic spectrum is found to become more prominent with the increase in the number of CuO 2 layers in the primitive unit cell. TlBa 2CaCu 2O 7 and TlBaCa 2Cu 3O 9 are shown to have hole pockets which disappear after replacement of bivalent Ca with trivalent Y.

High‐Gradient Magnetic Concentration of Chlorite and Hydroxy‐Interlayered Minerals in Soil Clays

AbstractIdentification and characterization of chlorite and hydroxy‐interlayered layer‐silicate minerals (HIM) common to many soils of the Atlantic Coastal Plain would benefit if the minerals could be concentrated and isolated by a suitable technique that would not alter them chemically or mineralogically. High‐gradient magnetic separation (HGMS) meets these requirements and was used in this study as a pretreatment in the analysis of soil clays. Deferrated coarse clay (2–0.2 µm) of soil material from several Ultisols on the Coastal Plain of North Carolina was subjected to HGMS using magnetic fields ranging from 0.04 to 0.89 T. Resulting fractions were analyzed by x‐ray diffraction, differential thermal analysis, Mössbauer spectroscopy, and total chemical analysis. Iron‐substituted Ti oxides, including rutile, ilmenite, anatase, and pseudorutile were found in the fractions separated at low magnetic field; chlorite was found at higher field strengths; HIM was separated at medium to high fields; kaolin and quartz plus HIM were found in the tailings. Mössbauer spectroscopy showed the chlorite to contain Fe2+ indicating that it is inherited primary chlorite. The chlorite is trioctahedral and constitutes <10 g kg−1 of the coarse clay; the HIM is dioctahedral and appears to make up >650 g kg−1 of the coarse clay. Structural formula of the HIM calculated from total chemical analysis of Casaturated clay and assuming a negative charge of 49 per unit cell layer is (Ca0.07K0.26Na0.15)(Si7.38Al0.62) (Al2.87Fe0.63Mg0.50)O20(OH)4Al1.86(OH)5(H2O)0.58, suggesting a phengitic muscovite or celadonite precursor. The cation exchange capacity (CEC) based on Ca content in the total analysis is about 15 cmolc kg−1 of HIM. The HGMS was effective in simplifying the mineralogical composition of materials studied, based on Fe content of the components.

Distortion induced bi-polaron model for oxide superconductors

Abstract The removal of an electron from the hybridised Cu O d p bond in cuprate superconductors may lead to a local structural distortion surrounding the vacant site as well as a local spin cloud polarisation. Under certain favourable conditions, two such distortion-induced polarons may become further bound to form a bi-polaron boson pair which has a lower energy than that of the isolated polarons. The relevance of layered structure and high Tc superconductivity is discussed. The introduction of car-riers can be accomplished by cation substitution, or by increasing oxygen content, or by self-doping between CuO2 layers in crystals having multi-copper layer unit cell (e.g. Bi2Sr2Ca2Cu3O10). This model may also be adopted, without its spin contribution, to the system of barium bismuthate superconductors.

The determination of monolayer structure by infrared spectroscopy: CO 2 on NaCl(100)

Infrared spectra of a 12C 16O 2 monolayer on NaCl(100) are reported for the first time. The polarization and coverage dependence of spectroscopic signals indicate that there is only one kind of adsorption site, in which the molecular axis is tilted 68° from the surface normal. The adsorbate layer grows in the form of constant-density islands. Plane group symmetry analysis reveals a unique structure: there are two tilted molecules per layer unit cell, arranged in herringbone fashion. This structure is in quantitative agreement with the photometric observations. Splitting of the molecular v 3 band is interpreted by a vibration-vibration coupling mechanism.

Generalized Transfer‐Matrix Description of the Optical Properties of Spatially Dispersive Periodic Metal Multilayers

AbstractA transfer matrix formalism is used to treat in a general way the optical response of different kind of metal superlattices. The dispersion relation of infinite and semi‐infinite superlattices and the reflectivity of semiinfinite superlattices are calculated. Spatial dispersion is included via additional boundary conditions. Each unit cell of the superlattices can be built up by an arbitrary number of layers with local and/or nonlocal dielectric functions. In the case of a spatially dispersive double layer unit cell the explicit dispersion relation is derived. The limits when one of the layers is local as well as the complete local case are shown to be included.

Surface green's function for a rumpled crystal surface

The computer program allows to calculate the Green's function of a three-dimensional system with two-dimensional translation symmetry. Examples for its application are the crystal surface and the interface between two crystals. The crystal can be composed from rumpled atomic layers, that is, the atoms in the layer unit cell may have small differences in heights. The layer-by-layer KKR scheme is used. An overlayer with a coincidence structure of a substrate layer may be involved; the unit cell of any layer may be composed of up to four different atoms. The Green's function is evaluated in a spherical wave expansion, with basis functions centred at the atoms. It directly allows to calculate the surface electron charge density and the local and projected density of states.

An ionic diffusion mechanism of chromite reduction

The occurrence of the various phases that form during the solid-state carbothermic reduction of chromite is explained by the use of a point-defect model. In this model each chromite particle is considered to be comprised of concentric layers of spinel unit cells (Fe2+and Mg2+ occupy tetrahedral sites, Cr3+, Al3+, and Fe3+ occupy octahedral sites). The phases that form are a result of the interchange of cations between unit cells within the particle effected by the presence of carbon monoxide at the surface of the particle. Four stages of reduction are identified: (1) the formation of a slightly iron-enriched core comprising distorted spinel unit cells surrounded by a region of normal spinel unit cells, effected by the reduction of Fe3+ to Fe2+; (2) the formation of Cr-Al sesquioxide and Mg-Cr-Al spinel phases effected by the metallization of Fe2+ ions and subsequent production of Cr2+ ions; (3) reduction of Fe2+ interstitials in the spinel core; and (4) metallization of chromium ionsvia the Cr2+ intermediate. The non-appearance of the sesquioxide phase at high temperatures is explained by considering the effect of temperature on the magnitudes of the diffusion coefficients of Cr2+, Mg2+, and interstitial Fe2+ ions.

Neutron diffraction from small numbers of Langmuir-Blodgett monolayers of manganese stearate

We report the observation of neutron diffraction from as few as three monolayers of manganese stearate on a plate of about 4-${\mathrm{cm}}^{2}$ area. These room-temperature (paramagnetic state) measurements could be adequately calculated on a laminar model of the crystal structure. Similar calculations, extended to a hypothetical magnetically ordered structure, indicate that the interlayer magnetic structure would be observable already with three layers of magnetic unit cells.

Structure and staging of intercalated Ag xTaS 2 and Ag xTiS 2

The crystal structure of the intercalation compounds Ag xTaS 2 ( 0≤ x≤ 2 3 ) and Ag xTiS 2 (0≤x≤0.42) are reported. For both intercalated dichalcogenides only stages n=2 and 1 are observed and there is no evidence for higher staging. In stage 1 Ag 2 3 TaS 2 , the silver ions reside in tetrahedral sites and the 2H b−TaS 2 structure changes to that of MoS 2. For stage 2 Ag 1 3 TaS 2 , we observe a six layer unit cell with the silver ions again in tetrahedral sites. In stage 1 Ag 0.42TiS 2 and stage 2 Ag 0.2TiS 2, the silver ions are in octahedral sites and there is no change of the host lattice stacking. The samples were prepared both thermally and electrolytically and the same lattice parameters were obtained in both cases.

X-ray diffraction study of a one-dimensional GaAs–AlAs superlattice

Low- and high-angle X-ray diffraction patterns have been obtained from one-dimensional superlattice crystals prepared artificially by alternately depositing predetermined thicknesses of GaAs and AlAs, on the (001) face of a GaAs single-crystal by molecular-beam epitaxy. The positions and intensities of several superlattice reflections obtained along the 00l, 11l and 02l reciprocal lattice rows have been recorded. The structure of the superlattice can be approximated by a model which incorporates elastic strains in the unit cell due to the lattice mismatch between GaAs and AlAs. The number of Ga and Al layers in the superlattice unit cell can be accurately determined from the low-angle scattering data while the relative intensities of the high-angle superlattice reflections are a sensitive measure of the elastic strain present in the lattice. It is shown that the elastic strain agrees with the value computed theoretically on the assumption that the strain is not relieved by dislocations at the GaAs–AlAs interfaces.

Incongruent dissolution and surface complexes of hydroxyapatite

Calcium/phosphorus ratios in solution were determined when samples of synthetic hydroxyapatite (HA) were subject to acid (HCl) dissolution (pH 4.5) in 0.1 M KCl at various solid-to-solution ratios. At a ratio of 500g in 1 liter, the Ca/P approached 2.0, but when the HA solid was washed in electrolyte and resuspended at approximately the same solid: solution ratio, dissolution at pH 4.5 was congruent (Ca/P = 1.67). Incongruent dissolution was associated with the initial contact between solid and solution. At ratios of less than 8g HA/liter the initial dissolution appeared congruent because the absolute amount of excess calcium or deficit of phosphorus was less than the experimental accuracy of estimation of the elements. At the highest solid: solution ratio used, all the calcium and phosphorus involved in the production of Ca/P greater than 1.67 could be attributed to variations in the composition of the surface layer of unit cells. However, the underlying units are of normal composition. The changes in this surface layer are thought to occur at the end of the preparation of the HA, and may be associated with the acetone washing before drying of the crystals.

Variations on the particle packing theme of Graton and Fraser

One of the foundations of the study of particle packing is the work by Graton and Fraser on simple, regular sphere packings. They described four particular packings, and it can be shown that there are in fact nine simple basic packings of that type. Starting from a simple cubic packing, certain movements of the unit cell layers can be described which lead to all nine packings. The Voronoi polyhedra associated with packings are simple in all cases except one, in which the number of faces of the Voronoi polyhedron exceeds the coordination number. This situation arises when R 2 < √2R 1 where R 1 is the distance from a reference sphere to the nearest spheres and R 2 is the distance to the next nearest.

Crystal Size Effects on the Exciton Absorption Spectrum of WSe2

The thickness dependence of the optical absorption spectrum of thin single crystals of W${\mathrm{Se}}_{2}$ (500 \AA{} to a unit-cell layer) was studied in the wavelength region 8000-5000 \AA{} at 77\ifmmode^\circ\else\textdegree\fi{}K. A significant shift (0.15 eV) to higher energies in the position of the exciton absorption peaks was observed with decreasing thickness. Such a shift is shown to follow a $\frac{1}{{t}^{2}}$ dependence on the thickness ($t$) down to 40 \AA{}. This fact suggests the presence of a quantum size effect. A deviation from the $\frac{1}{{t}^{2}}$ law has been observed for crystals less than 40 \AA{} thick.

Generation of α-SiC stacking sequences

α-SiC crystal structures are polymorphs usually consisting of 2-, 3- and 4-layer blocks arranged in stacking sequences in order to yield a specific number of layers in a hexagonal unit cell. A computer-generated tabulation of some combinations of these blocks into significant sequences representing possible crystal structures is available from the authors.

Crystal structure and morphology of the alloys for cast permanent magnets

The structure of the Ticonal-type alloys was investigated using a transmission electron microscope technique. Two-dimensional square macrolattice was observed in the section normal to the magnetic field which was applied at heat treatment. The translation vectors of that lattice coincide with the \langle100\rangle directions of the crystal lattice. There were equiaxed regions of various contrasts at the corners and center of the unit cell and flat intermediate layers on the sides. Thus the two-phase model of the structure is only an approximation. The prolonged tempering at high temperature (800°C, 20 hours) of specimens after they are subjected to the thermal magnetic treatment gives rise to the development of the complex modulated structure.