Correlation of tunneling spectra with surface nano-morphology and doping in thin YBa2Cu3O7-delta films

Tunneling spectra measured on thin epitaxial YBa2Cu3O7-delta films are found to exhibit strong spatial variations, showing U and V-shaped gaps as well as zero bias conductance peaks typical of a d-wave superconductor. A full correspondence is found between the tunneling spectra and the surface morphology down to a level of a unit-cell step. Splitting of the zero bias conductance peak is seen in optimally-doped and overdoped films, but not in the underdoped ones, suggesting that there is no transition to a state of broken time reversal symmetry in the underdoped regimeComment: accepted to ep

Resistivity scaling and critical dynamics of fully frustrated Josephson-junction arrays with on-site dissipation

We study the scaling behavior and critical dynamics of the resistive transition in Josephson-junction arrays, at f=1/2 flux quantum per plaquette, by numerical simulation of an on-site dissipation model for the dynamics. The results are compared with recent simulations using the resistively-shunted-junction model. For both models, we find that the resistivity scaling and critical dynamics of the phases are well described by the same critical temperature as for the chiral (vortex-lattice) transition, with a power-law divergent correlation length. The behavior is consistent with the single transition scenario, where phase and chiral variables order at the same temperature, but with different dynamic exponents z for phase coherence and chiral order.Comment: 17 pages, 13 figures, to appear in Phys. Rev.

Direct observation of nanometer-scale pinning sites in (Nd0.33_{0.33},Eu0.20_{0.20},Gd0.47_{0.47})Ba2_2Cu3_3O7−δ_{7-\delta} single crystals

We report on the observation of self-organized stripe-like structures on the as-grown surface and in the bulk of (Nd,Eu,Gd)Ba$_2$Cu$_3$O$_y$ single crystals. The periodicity of the stripes on the surface lies between 500 and 800 nm. These are possibly the growth steps of the crystal. Transmission electron microscopy investigations revealed stripes of periodicity in the range of 20 to 40 nm in the bulk. From electron back scattered diffraction investigations, no crystallographic misorientation due to the nanostripes has been found. Scanning tunneling spectroscopic experiments revealed nonsuperconducting regions, running along twin directions, which presumably constitute strong pinning sites.Comment: 4 pages, 5 Figure

Magnetic domain tuning and the emergence of bubble domains in the bilayer manganite La 2−2x Sr 1+2x Mn 2 O 7 (x=0.32)

We report a magnetic force microscopy study of the magnetic domain evolution in the layered manganite La2-2x Sr1+2x Mn2O7 (with x = 0.32). This strongly correlated electron compound is known to exhibit a wide range of magnetic phases, including a recently uncovered biskyrmion phase. We observe a continuous transition from dendritic to stripelike domains, followed by the formation of magnetic bubbles due to a field-and temperaturedependent competition between in-plane and out-of-plane spin alignments. The magnetic bubble phase appears at comparable field and temperature ranges as the biskyrmion phase, suggesting a close relation between both phases. Based on our real-space images we construct a temperature-field phase diagram for this composition.open115Ysciescopu

Using Single loxP Sites to Enhance Homologous Recombination: ts Mutants in Sec1 of Dictyostelium discoideum

Dictyostelium discoideum amoebae are haploid and, as they share many features with animal cells, should be an ideal creature for studying basic processes such as cell locomotion. Isolation of mutants in this amoeba has largely been limited to non-essential genes: nsfA-the gene for NEM-sensitive factor-remains the only essential gene for which conditional (ts) mutants exist. These ts mutants were generated by gene replacement using a library of mutagenised nsfA containing a selectable marker: transformants were then screened for temperature sensitivity. The success of this approach depended on the high level of homologous recombination prevailing at this locus: approximately 95% of selected clones were homologous recombinants. This is unusually high for Dictyostelium: homologous recombination at other loci is usually much less, usually between 0-30%, making the isolation of ts mutants much more tedious.In trying to make ts mutants in sec1A, homologous recombination was found to be only approximately 25%. A new approach, involving single loxP sites, was investigated. LoxP sites are 34 bp sequences recognised by Cre recombinase and between which this enzyme catalyses recombination. A Dictyostelium line containing a single loxP site adjacent to the 3' end of the sec1A gene was engineered. A sec1A replacement DNA also containing a single loxP site in a homologous position was then introduced into this cell line. In the presence of CRE recombinase, homologous recombination increased to approximately 80% at this locus, presumably largely driven by intermolecular recombination between the two single loxP sites.A route to increase the rate of homologous recombination at a specific locus, sec1A, is described which enabled the isolation of 30 ts mutants in sec1A. One of these, sec1Ats1,has been studied and found to cease moving at the restrictive temperature. The approach described here may be valuable for enhancing homologous recombination at specified loci and thus for introducing mutations into specific genes in Dictyostelium and other creatures

Flux-Induced Vortex in Mesoscopic Superconducting Loops

We predict the existence of a quantum vortex for an unusual situation. We study the order parameter in doubly connected superconducting samples embedded in a uniform magnetic field. For samples with perfect cylindrical symmetry, the order parameter has been known for long and no vortices are present in the linear regime. However, if the sample is not symmetric, there exist ranges of the field for which the order parameter vanishes along a line, parallel to the field. In many respects, the behavior of this line is qualitatively different from that of the vortices encountered in type II superconductivity. For samples with mirror symmetry, this flux-induced vortex appears at the thin side for small fluxes and at the opposite side for large fluxes. We propose direct and indirect experimental methods which could test our predictions.Comment: 6 pages, Latex, 4 figs., uses RevTex, extended to situations far from cylindrical symmetr