Normal State Resistivity of Underdoped YBa2Cu3Ox Thin Films and La2-xSrxCuO4 Ultra-Thin Films under Epitaxial Strain

The normal state resistivity of high temperature superconductors can be probed in the region below Tc by suppressing the superconducting state in high magnetic fields. Here we present the normal state properties of YBa2Cu3Ox thin films in the underdoped regime and the normal state resistance of La2-xSrxCuO4 thin films under epitaxial strain, measured below Tc by applying pulsed fields up to 60 T. A universal rho(T) behaviour is reported. We interpret these data in terms of the recently proposed 1D quantum transport model with the 1D paths corresponding to the charge stripes.Comment: 5 pages, PDF and PS, including figures, presented at MOS99 and accepted for publication in J. of Low Temp. Phy

Vector potential gauge for superconducting regular polygons

An approach to the Ginzburg-Landau problem of superconducting polygons is developed, based on the exact fulfillment of superconducting boundary conditions along the boundary of the sample. To this end an analytical gauge transformation for the vector potential $\mathbf{A}$ is found which gives $A_{n}=0$ for the normal component along the boundary line of an arbitrary regular polygon. The use of the new gauge reduces the Ginzburg-Landau problem of superconducting polygons in external magnetic fields to an eigenvalue problem in a basis set of functions obeying Neumann boundary conditions. The advantages of this approach, especially for low magnetic fields, are illustrated and novel vortex patterns are obtained which can be probed experimentally

Vortex states inside the superconducting phase of a thin microtriangle

By combining the non-linear Ginzburg-Landau equations with a gauge transformation of the vector potential that accounts for the superconducting/vacuum boundary condition, the superconducting phase of a thin microtriangle under a perpendicular magnetic field is investigated. We determine the symmetry-breaking and symmetry-switching transitions that the nucleated order parameter may undergo when decreasing the temperature well below the phase boundary. It is shown that symmetry consistent vortex-antivortex patterns are stable in a broad range of temperatures and magnetic fields. The geometry of the sample also induces crossovers between vortex states unexpected for other regular polygons.status: publishe

Nucleation of superconductivity in a mesoscopic rectangle

We have studied the nucleation of superconductivity in a mesoscopic rectangle. We used an analytical gauge transformation for the vector potential $\bm{A}$ which gives $\bm{A}_{n}=0$ for the normal component along the boundary of the rectangle. Consequently, the linearized Ginzburg-Landau equation is reduced to an eigenvalue problem in the basis set of functions obeying the Neumann boundary condition. Through the application of this technique we are able to accurately determine the field-temperature superconducting phase boundary together with the corresponding vortex patterns. A range of aspect ratios for the rectangle has been investigated and compared with a superconducting square (aspect ratio = 1) and with a superconducting line (aspect ratio = ∞). This also allows us to determine the stability of the vortex patterns with an anti-vortex in the centre, which have been predicted for a superconducting square, with respect to the deformation of the square

Vortex patterns and nucleation of superconductivity in mesoscopic rectangles and in hybrid superconductor/ferromagnet structures

Nucleation of superconductivity and vortex patterns have been studied in mesoscopic samples providing the crossover between square and rectangular geometries. The measured nucleation line, $T_c(H)$, has been analyzed in the framework of the linearized Ginzburg-Landau theory. A very good agreement has been found between the theoretical $T_c(H)$ boundary and the experimental data for rectangles with different aspect ratios. The superconductor/ferromagnet hybrids, such as magnetic Co/Pd dot in a superconducting loop and the dot on top of a superconducting disk have also been investigated. Pronounced effects of the dot on the $T_c(H)$ boundary have been found, including strong asymmetry with respect to the field polarity

Vortex states inside the superconducting phase of a thin microtriangle

By combining the non-linear Ginzburg-Landau equations with a gauge transformation of the vector potential that accounts for the superconducting/vacuum boundary condition, the superconducting phase of a thin microtriangle under a perpendicular magnetic field is investigated. We determine the symmetry-breaking and symmetry-switching transitions that the nucleated order parameter may undergo when decreasing the temperature well below the phase boundary. It is shown that symmetry consistent vortex-antivortex patterns are stable in a broad range of temperatures and magnetic fields. The geometry of the sample also induces crossovers between vortex states unexpected for other regular polygons

Ginzburg-Landau description of confinement and quantization effects in mesoscopic superconductors

An approach to the Ginzburg-Landau problem for superconducting regular polygons is developed making use of an analytical gauge transformation for the vector potential A which gives A(n)=0 for the normal component along the boundary line of different symmetric polygons. As a result the corresponding linearized Ginzburg-Landau equation reduces to an eigenvalue problem in the basis set of functions obeying Neumann boundary condition. Such basis sets are found analytically for several symmetric structures. The proposed approach allows for accurate calculations of the order parameter distributions at low calculational cost (small basis sets) for moderate applied magnetic fields. This is illustrated by considering the nucleation of superconductivity in squares, equilateral triangles and rectangles, where vortex patterns containing antivortices are obtained on the T-c-H phase boundary. The calculated phase boundaries are compared with the experimental T-c(H) curves measured for squares, triangles, disks, rectangles, and loops. The stability of the symmetry consistent solutions against small deviations from the phase boundary line deep into the superconducting state is investigated by considering the full Ginzburg-Landau functional. It is shown that below the nucleation temperature symmetry-switching or symmetry-breaking phase transitions can take place. The symmetry-breaking phase transition has the same structure as the pseudo-Jahn-Teller instability of high symmetry nuclear configurations in molecules. The existence of these transitions is predicted to be strongly dependent on the size of the samples. (c) 2005 American Institute of Physics.status: publishe