Gerd Teniers

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 An=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 Tc–H phase boundary. The calculated phase boundaries are compared with the experimental Tc(H) curves measured for squares, triangles, disks, rectangles,...

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 Tcby suppressing the superconducting state in high magnetic fields. Here we present the normal state properties of YBa2Cu3Oxthin films in the underdoped regime and the normal state resistance of La2-xSrxCuO4thin films under epitaxial strain, measured below Tcby applying pulsed fields up to 60 T. We interpret these data in terms of the recently proposed 1D quantum transport model with the 1D paths corresponding to the charge stripes.

Nucleation of superconductivity in a mesoscopic triangle

Abstract 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.

Nucleation of superconductivity in regular polygons: superconducting vector potential gauge approach

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 An ¼ 0 for the normal component along the boundary line of an arbitrary regular polygon. As a result the corresponding linearised Ginzburg–Landau equation reduces to an eigenvalue problem in the basis set of functions obeying Neumann boundary condition. 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 and equilateral triangles where novel vortex patterns containing and antivortex in the centre are obtained on the Tc–H phase boundary. The stability of these solutions against small deviations from the phase boundary line deeper into the superconducting state is investigated and the conditions for the experimental observation of the novel vortex patterns are discussed. 2001 Elsevier Science B.V. All rights reserved.

Vortex dynamics in superconductors with a lattice of magnetic dots

We report on molecular dynamics simulations of vortex patterns in the presence of a periodic array of ferromagnetic dots with out-of-plane magnetization. Dots and vortices show the interesting behaviour of having attractive or repulsive interaction depending on the mutual orientation of the flux of the vortices and the magnetic moment of the dots. This microscopic property strongly influences the simulated magnetization loops and critical force curves. Different configurations of the magnetization direction of the dot array have been systematically studied. For all dots magnetized in one direction the experimentally observed asymmetric magnetization loops are clearly reproduced. We also investigate random distributions of up and down magnetized dots. These simulations show that new vortex phases can be stabilized by the magnetic dot array.

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 An ¼ 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. r 2003 Elsevier Science B.V. All rights reserved.

Normal state resistivity of underdoped YBa2Cu3O7-d films and La2-XSrXCuO4 ultra -thin films in fields up to 60 T

Abstract We present the normal-state resistance of La 2-x Sr x CuO 4 thin films under epitaxial strain, measured below T c by applying pulsed fields up to 60 T. We compare these data with earlier data on YBa 2 Cu 3 O x thin films in the underdoped regime. The data are analyzed in terms of the recently proposed 1D quantum transport model and charge-stripe models. The high field data have been used to identify the new regimes and dimensional crossovers caused by the formation of stripes and their interplay with disorder.

Stripe formation and disorder induced stripe fragmentation

Abstract We have analyzed the zero field and high field transport properties of several underdoped high-Tc systems. A convincing scaling behavior of the temperature dependence of the resistivity has been found for all of them, thus proving that the underlying scattering mechanism remains the same as we approach the metal-insulator transition from the metallic side. The scaling temperature Δ grows substantially with the decreasing hole concentration x. We associate Δ with the opening of the spin-gap in the spin ladders appearing due to the stripe formation. Three distinctly different temperature regimes have been identified: (i) high-temperature Heisenberg-like 2D regime; (ii) intermediate temperature 1D stripe regimes: and (iii) low-temperature disorder-induced stripe fragmentation and pinning with an enhanced inter-fragment and inter-stripe hole hopping, which recovers an effective 2D weak localization with a logarithmic behavior of resistivity with temperature. This third regime has been revealed by suppressing superconductivity in fields up to 60 T. Hall conductivity data are used to estimate the evolution of the stripe “order parameter” and mobility with temperature in the normal state.

COMPARISON OF THE TRANSPORT MECHANISM IN UNDERDOPED HIGH TEMPERATURE SUPERCONDUCTORS AND IN SPIN LADDERS

Abstract Recently, the normal state resistivity of high-temperature superconductors (HTSC) (in particular in La 2− x Sr x CuO 4 single crystals) has been studied extensively in the region below T c by suppressing the superconducting state in high magnetic fields [Y. Ando, G.S. Boebinger, A. Passnet, Phys. Rev. Lett. 75, 1995, pp. 4662–4665.]. In the present work, we report on the normal state resistance of underdoped La 2− x Sr x CuO 4 thin films under epitaxial strain [J.P. Locquet, J. Perret, J. Fompeyrine, E. Machler, J.W. Seo, G. Van Tendeloo, Nature 394, 1998, pp. 453–456.], measured far below T c by applying pulsed fields up to 60 T [F. Herlach, L. Van Bockstal, R. Bogaerts, I. Deckers, G. Heremans, L. Li, G. Pitsi, J. Vanacken, A. Van Esch, Physica B 201, 1994, p. 542.]. We will compare the transport measurements on these HTSCs with transport data reported for the Sr 2.5 Ca 11.5 Cu 24 O 41 spin ladder compound [T. Nagata, M. Uehara, J. Goto, N. Komiya, J. Akimitsu, N. Motoyama, H. Eisaki, S. Uchida, H. Takahashi, N. Nakanishe, N. Mori, Physica C 282–287, 1997, pp. 153–156.]. This comparison leads to an interpretation of the data in terms of the recently proposed 1D quantum transport model and the charge-stripe models [V.V. Moshchalkov, Solid State Commun. 86, 1993, p. 715; V.V. Moshchalkov, Cond-Mat/9802281; V.V. Moshchalkov, L. Trappeniers, J. Vanacken, Europhys. Lett. 46, 1999, pp. 75–81; J. Vanacken, L. Trappeniers, J. Perret, J.-P. Locquet, V.V. Moshchalkov, Y. Bruynseraede, unpublished.].