Self-injection length in La0.7Ca0.3MnO3-YBa2Cu3O7-δ ferromagnet-superconductor multilayer thin films

We have carried out extensive studies on the self-injection problem in barrierless heterojunctions between La_0.7Ca_0.3MnO₃ (LCMO) and YBa₂Cu₃O_7-δ (YBCO) thin films. The heterojunctions were formed in situ by sequentially growing LCMO and YBCO films on 〈100〉 LaAlO₃ (LAO) substrate using a pulsed laser deposition (PLD) system. YBCO micro-bridges with 64 μm width were patterned both on the LAO (control) and LCMO side of the substrate. Critical current, I _c, was measured at 77 K on both the control side as well as the LCMO side for different YBCO film thickness. It was observed that while the control side showed a J _c of ∼ 2 × 10⁶ A/cm², the LCMO side showed about half the value for the same thickness (1800 ?). The difference in J _c indicates that a certain thickness of YBCO has become ‘effectively’ normal due to self-injection. From the measurement of J _c at two different thicknesses (1800 ? and 1500 ?) of YBCO films both on the LAO as well as the LCMO side, the value of self-injection length (at 77 K) was estimated to be ∼ 900 ?. To the authors’ best knowledge, this is the first time that self-injection length has been quantified. A control experiment carried out with LaNiO₃ deposited by PLD on YBCO did not show any evidence of self-injection.     

Characterization of titanium dioxide nanoparticles using molecular dynamics simulations

Molecular dynamics simulations of titanium dioxide nanoparticles in the three commonly occurring phases (anatase, brookite, and rutile) are reported. The structural properties inferred by simulated X-ray diffraction patterns of the nanoparticles were investigated. The titanium-oxygen bond length as a function of size, phase, and temperature was determined and was found to be dependent on the coordination environment of the titanium and independent of phase and size. The equilibrium Ti-O bond length is 1.86 A for a four-coordinated titanium ion, 1.92 A for a five-coordinated titanium ion, and 1.94 A for an octahedral titanium ion. Smaller nanoparticles are characterized by a higher fraction of titanium ions that are four and five coordinated, due to the larger surface area-to-volume ratios. The surface energies for anatase, rutile, and brookite particles were reported. The surface energy of the nanoparticle increases and approaches a constant value as the particle gets bigger. The surface energies of small rutile particles are higher than that for anatase particles of a similar size, consistent with anatase being the more stable phase of nanocrystalline titanium dioxide.     

Energy dependence of multiplicity in proton-nucleus collisions and models of multiparticle production

This is a continuation of our earlier investigation (Gurtuet al 1974Phys. Lett. 50 B 391) on multiparticle production in proton-nucleus collisions based on an exposure of emulsion stack to 200 GeV/c beam at the NAL. It is found that the ratioR _em = 〈n _s〉/〈n _ch〉, where 〈n _ch〉 is the charged particle multiplicity in pp-collisions, increases slowly from about 1 at 10 GeV/c to 1·6 at 68 GeV/c and attains a constant value of 1·71 ± 0·04 in the region 200 to 8000 GeV/c. Furthermore,R _em = 1·71 implies an effectiveA-dependence ofR _A =A ^0.18,i.e., a very weak dependence. Predictions ofR _em on various models are discussed and compared with the emulsion data. Data seem to favour models of hadron-nucleon collisions in which production of particles takes place through adouble step mechanism,e.g., diffractive excitation, hydrodynamical and energy flux cascade as opposed to models which envisage instantaneous production.     

Magnetoelectric effect in cobalt ferrite-barium titanate composites and their electrical properties

CoFe₂O₄-BaTiO₃ composites were prepared using conventional ceramic double sintering process with various compositions. Presence of two phases in the composites was confirmed using X-ray diffraction. The dc resistivity and thermoemf as a function of temperature in the temperature range 300 K to 600 K were measured. Variation of dielectric constant (ɛ′) with frequency in the range 100 Hz to 1 MHz and also with temperature at a fixed frequency of 1 kHz was studied. The ac conductivity was derived from dielectric constant (ɛ′) and loss tangent (tan δ). The nature of conduction is discussed on the basis of small polaron hopping model. The static value of magnetoelectric conversion factor has been studied as a function of magnetic field.     

Pi-interaction tuning of the active site properties of metalloproteins

The influence of pi-interactions with a His ligand have been investigated in a family of copper-containing redox metalloproteins. The Met16Phe and Met16Trp pseudoazurin, and Leu12Phe spinach and Leu14Phe Phormidium laminosum plastocyanin variants possess active-site pi-contacts between the introduced residue and His81 and His87/92 respectively. The striking overlap of the side chain of Phe16 in the Met16Phe variant and that of Met16 in wild type pseudoazurin identifies that this position provides an important second coordination sphere interaction in both cases. His-ligand protonation and dissociation from Cu(I) occurs in the wild type proteins resulting in diminished redox activity, providing a [H(+)]-driven switch for regulating electron transfer. The introduced pi-interaction has opposing effects on the pKa for the His ligand in pseudoazurin and plastocyanin due to subtle differences in the pi-contact, stabilizing the coordinated form of pseudoazurin whereas in plastocyanin protonation and dissociation is favored. Replacement of Pro36, a residue that has been suggested to facilitate structural changes upon His ligand protonation, with a Gly, has little effect on the pKa of His87 in spinach plastocyanin. The mutations at Met16 have a significant influence on the reduction potential of pseudoazurin. Electron self-exchange is enhanced, whereas association with the physiological partner, nitrite reductase, is only affected by the Met16Phe mutation, but kcat is halved in both the Met16Phe and Met16Trp variants. Protonation of the His ligand is the feature most affected by the introduction of a pi-interaction.     

Analysis and simulation of the structure of nanoparticles that undergo a surface-driven structural transformation

A room temperature solid-state structural transformation was observed in 3 nm ZnS nanoparticles in methanol following the addition of water (Zhang et al., Nature 424, 1025, 2003). Experimental wide angle x-ray scattering (WAXS), x-ray absorption near edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) spectroscopy measurements show a large increase in crystallinity associated with water addition, in agreement with molecular dynamics (MD) predictions. Here we perform first-shell EXAFS and pair distribution function analysis and whole-nanoparticle calculations of WAXS, EXAFS and XANES to compare structural data with the MD predictions. The predicted WAXS patterns give excellent agreement with data, while the predicted EXAFS and XANES spectra give poor agreement. Relative to WAXS, XANES and EXAFS spectra contain additional structural information related to the distribution of disorder. The discrepancy between the x-ray diffraction and x-ray absorption results indicates that structural disorder is partitioned between interior and surface regions more strongly than predicted in the MD simulations.     

The importance of the long type 1 copper-binding loop of nitrite reductase for structure and function

The long 15-residue type 1 copper-binding loop of nitrite reductase has been replaced with that from the cupredoxin amicyanin (7 residues). This sizable loop contraction does not have a significant effect on the spectroscopy, and therefore, the structures of both the type 1 and type 2 Cu(II) sites. The crystal structure of this variant with Zn(II) at both the type 1 and type 2 sites has been determined. The coordination geometry of the type 2 site is almost identical to that found in the wild-type protein. However, the structure of the type 1 centre changes significantly upon metal substitution, which is an unusual feature for this class of site. The positions of most of the coordinating residues are altered of which the largest difference was observed for the coordinating His residue in the centre of the mutated loop. This ligand moves away from the active site, which results in a more open metal centre with a coordinating water molecule. Flexibility has been introduced into this region of the protein. The 200 mV increase in the reduction potential of the type 1 copper site indicates that structural changes upon reduction must stabilise the cuprous form. The resulting unfavourable driving force for electron transfer between the two copper sites, and an increased reorganisation energy for the type 1 centre, contribute to the loop variant having very little nitrite reductase activity. The extended type 1 copper-binding loop of this enzyme makes a number of interactions that are important for maintaining quaternary structure.     

Working group report: Heavy-ion physics and quark-gluon plasma

This is the report of Heavy Ion Physics and Quark-Gluon Plasma at WHEPP-09 which was part of Working Group-4. Discussion and work on some aspects of quark-gluon plasma believed to have created in heavy-ion collisions and in early Universe are reported.