Comparison of the Transport Mechanism in Underdoped High Temperature Superconductors and in Spin Ladders

Recently, the normal state resistivity of high temperature superconductors (in particular in La2-xSrxCuO4 single crystals) has been studied extensively in the region below Tc by suppressing the superconducting state in high magnetic fields. In the present work we report on the normal state resistance of underdoped La2-xSrxCuO4 thin films under epitaxial strain, measured far below Tc by applying pulsed fields up to 60 T. We will compare the transport measurements on these high temperature superconductors with transport data reported for the Sr2.5Ca11.5Cu24O41 spin ladder compound. This comparison leads to an interpretation of the data in terms of the recently proposed 1D quantum transport model and the charge-stripe models.Comment: 5 pages, PDF fil

Vortex-antivortex patterns in mesoscopic superconductors

We have studied the nucleation of superconductivity in mesoscopic structures of different shape (triangle, square and rectangle). This was made possible by using an analytical gauge transformation for the vector potential A which gives A(n) = 0 for the normal component along the boundary line of the rectangle. As a consequence the superconductor-vacuum boundary condition reduces to the Neumann boundary condition. By solving the linearized Ginzburg-Landau equation with this boundary condition we have determined the field-temperature superconducting phase boundary and the corresponding vortex patterns. The comparison of these patterns for different structures demonstrates that the critical parameters of a superconductor can be manipulated and fine-tuned through nanostructuring. (C) 2003 Elsevier Science B.V. All rights reserved.status: publishe

Pseudo Jahn-Teller mechanism for symmetry-breaking phase transitions in vortex molecules

The phase transitions from a symmetric superconducting order parameter to a different or broken symmetry phase are investigated for thin mesoscopic superconductors well below the nucleation temperature. By using an effective two-state model the mechanism of these transitions has been revealed and four distinct types of transitions have been found. The symmetry-breaking phase transition has the same structure as the pseudo Jahn-Teller instability of high symmetry nuclear configurations in molecules. This analogy provides an interesting connection between real and vortex molecules. The existence of broken-symmetry phases is predicted to be strongly dependent on the size of the samples.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 A which gives 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 = infinity). 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.status: publishe

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 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.status: publishe

Nucleation of superconductivity in a mesoscopic triangle

The nucleation of superconductivity in a mesoscopic equilateral triangle is studied experimentally by transport measurements and theoretically by using the linearized Ginzburg-Landau equation. The voltage and current leads, intrinsic to transport measurements. give rise to a broadening of the resistive transition R(T). However, by choosing the right resistance criterion to determine the experimental superconducting/normal phase boundary T-c(H) and by reducing the coupling between the triangle and the contacts, achieved by changing the shape of the contacts, this influence can be minimized. From the theoretical study, we found that the trigonal symmetry of the sample has a profound effect on the superconducting state in the presence of a magnetic field H leading, in particular. to the formation of antivortices in symmetry-consistent states. A good agreement between the theory and the experimental T-c(H) data is observed. (C) 2001 Elsevier Science B.V. All rights reserved.status: publishe