Study of magnetoresistance and conductance of bicrystal grain boundary in La0.67Ba0.33MnO3 thin film

La_0.67Ba_0.33MnO₃ (LBMO) thin film is deposited on a 36.7°C SrTiO₃ bicrystal substrate using laser ablation technique. A microbridge is created across bicrystal grain boundary and its characteristics are compared with a microbridge on the LBMO film having no grain boundary. Presence of grain boundary exhibits substantial magnetoresistance ratio (MRR) in the low field and low temperature region. Bicrystal grain boundary contribution in MRR disappears at temperature T>175 K. At low temperature, I-V characteristic of the microbridge across bicrystal grain boundary is nonlinear. Analysis of temperature dependence of dynamic conductance-voltage characteristics of the bicrystal grain boundary indicates that at low temperatures (T<175 K) carrier transport across the grain boundary in LBMO film is dominated by inelastic tunneling via pairs of manganese atoms and tunneling through disordered oxides. At higher temperatures (T>175 K), magnetic scattering process is dominating. Decrease of bicrystal grain boundary contribution in magnetoresistance with the increase in temperature is due to enhanced spin-flip scattering process.     

A theory of rheed

Elastic Reflection High Energy Electron Diffraction (RHEED) by solid surfaces is studied theoretically. First, the problem of finding the electron reflection and transmission coefficients of a crystal slab is formally solved. Following this, it is shown how the formal solution may be used in a practical computation of the diffracted beam intensities. These two results are applied to a study of high energy (20 keV) electron diffraction by the Ag(001) surface. Rocking curves are computed to illustrate the dependence of the reflection coefficients on the glancing angle of the incident beam, the incident beam azimuth being in the [110] direction. The curves are shown to have several features in common with a typical set of LEED I-V plots: primary Bragg peaks, secondary Bragg peaks and resonance peaks are all present. The dependence of the reflection coefficients on the deviation of the incident beam azimuth from the [110] direction is also described. Additional computations are made to illustrate the sensitivity of the RHEED pattern to the details of the surface structure: the relative heights of the peaks in the rocking curves are shown to be quite sensitive to the spacing of the topmost atomic layers.     

Calculation of MEED intensities in the 5–10 keV electron energy range

A method for computing MEED intensities in the 5–10 keV electron energy range is described. The method is based on improving the computational efficiency of a RHEED program so that it can be used to handle the larger matrices involved in MEED calculations. As an example of its use rocking curves are computed for 5 keV electrons incident on the Al(110) surface in the 11?0 azimuth. Further numerical results are then presented to show that smaller scale calculations, in which only beams in the zeroth Laue zone are taken into account, can give a useful approximation to the exact rocking curves. Finally, the conditions under which these calculations are likely to be valid are discussed.     

Molar extinction coefficients of some carbohydrates in aqueous solutions

Molar extinction coefficients of some carbohydrates viz. l-arabinose (C₅H₁₀O₅), d-glucose (C₆H₁₂O₆), d-mannose (C₆H₁₂O₆), d-galactose (C₆H₁₂O₆), d(-) fructose (C₆H₁₂O₆) and maltose (C₁₂H₂₄O₁₂) in aqueous solutions have been determined at 81, 356, 511, 662, 1173 and 1332 keV by gamma ray transmission method in a narrow beam good geometry set-up. These coefficients have been found to depend upon the photon energy following a 4-parameter polynomial. These extinction coefficients for different sugars having the same molecular formula have same values varying within experimental uncertainty. Within concentration ranges studied, Beer-Lambert law is obeyed very well.     

Cavity ring-down spectroscopy of the torsional motions of 1,4-bis(phenylethynyl)benzene

The torsional motions of jet-cooled 1,4-bis(phenylethynyl)benzene (BPEB), a prototype molecular wire, were studied using cavity ring-down spectroscopy in the first UV absorption band (316-321 nm). The torsional spectrum of 1,4-bis(phenylethynyl)-2,3,5,6-tetradeuteriobenzene was also recorded in the gas phase. Both spectra were successfully simulated using simple cosine potentials to describe the torsional motions. The ground-state barrier to rotation was estimated to be 220-235 cm(-1), which is similar to that of diphenylacetylene (tolane). Complementary DFT calculations were found to overestimate the torsional barrier.     

Two-photon absorption and photoluminescence of europium based emissive probes for bioactive systems

Observation of two-photon excitation (760 nm) and emission of two responsive water soluble europium complexes is reported with cross-sections of up to 2 GM. Two-photon excitation spectra have also been measured, acquisition being achieved by the use of a cavity-dumped mode locked Ti-sapphire laser. Time-gated detection is used to differentiate the ligand fluorescence and metal centred emission in these europium complexes.     

Sensitization of europium(III) luminescence by benzophenone-containing ligands: regioisomers, rearrangements and chelate ring size, and their influence on quantum yields

A series of europium(III) complexes based on the macrocyclic azacarboxylate structure, DO3A, have been investigated, incorporating benzophenone appended at N10 of the macrocycle via linkers containing amide bonds (H3DO3A = 1,4,7,10-tetraazacyclododecane-1,4,7-tris-acetic acid). Complexes [EuL(1-3)] incorporate N10-CH2CONH-BP linkers (BP = benzophenone), which allow formation of a five-membered chelate ring containing the metal ion upon chelation of the amide oxygen; these three isomeric complexes differ from one another in the substitution position of the BP unit, namely para, meta, and ortho for L1, L2, and L3 respectively. The quantum yields of europium luminescence sensitized via the chromophore are found to be highly dependent upon the position of substitution, being 20 times smaller for the ortho compared to the para-substituted complex. A related para-substituted BP complex [EuL(4)], prepared by an unusual Michael reaction of the azamacrocycle with a BP-containing acrylamide, incorporates an additional methylene unit in the linker, namely N10-CH2CH2CONH-BP. Despite the longer linker, this complex equals the luminescence quantum yield achieved with [EuL(1)] (Phi(lum) = 0.097 and 0.095, respectively, in H2O at 298 K). Analysis of the pertinent kinetics reveals that the decreased energy transfer efficiency in this complex, arising from the longer donor-acceptor distance, is compensated by an increased radiative rate constant. Under basic conditions, the ortho-substituted complex [EuL(3)] undergoes an intramolecular rearrangement to generate an unprecedented complex [EuL(5)] incorporating a 4-phenyl-2-hydroxyquinoline unit directly bound to the ring nitrogen. Although this complex is a poor emitter, an analogous complex obtained from 2-amino-acetophenone, which generates 4-methyl-2-hydroxyquinoline during the corresponding rearrangement, is an order of magnitude more emissive while still benefiting from relatively long-wavelength absorption. The emission from this complex is pH sensitive, being dramatically quenched under mildly basic conditions.     

Ionic potential analysis of RHEED rocking curves from fluoride structures

Rocking curves of reflection high energy electron diffraction from MnF₂ and CaF₂(1 1 1) surfaces have been calculated with ionic scattering potentials. The potentials were derived from tabulated X-ray scattering factors and expressed in the Doyle-Turner representation. The Coulomb potential of the ions was introduced into the calculations of diffracted intensity using dynamical diffraction theory. Comparison of the calculated and measured rocking curves for CaF₂(1 1 1) surface confirmed that it is bulk terminated; the correction to volume average potential was found to be 1.1 eV (much less than when using atomic potentials). Analysis of the rocking curves from ultra-thin MnF₂ layers on CaF₂(1 1 1) indicated that MnF₂ inherits the cubic lattice of fluorides up to a thickness of three molecular layers.