London equation studies of thin-film superconductors with a triangular antidot lattice

We report on a study of vortex pinning in nanoscale antidot defect arrays in the context of the London Theory. Using a wire network model, we discretize the array with a fine mesh, thereby providing a detailed treatment of pinning phenomena. The use of a fine grid has enabled us to examine both circular and elongated defects, patterned in the form of a rhombus. The latter display pinning characteristics superior to circular defects constructed with the similar area. We calculate pinning potentials for defects containing zero and single quanta, and we obtain a pinning phase diagram for the second matching field, $H = 2 \Phi_{o}$.Comment: 10 pages and 14 figure

Flux Confinement in Mesoscopic Superconductors

We report on flux confinement effects in superconducting submicron line, loop and dot structures. The main idea of our study was to vary the boundary conditions for confinement of the superconducting condensate by taking samples of different topology and, through that, modifying the lowest Landau level E_{LLL}(H). Since the critical temperature versus applied magnetic field T_{c}(H) is, in fact, E_{LLL}(H) measured in temperature units, it is varied as well when the sample topology is changed. We demonstrate that in all studied submicron structures the shape of the T_{c}(H) phase boundary is determined by the confinement topology in a unique way.Comment: 10 pages, 5 EPS figures, uses LaTeX's sup.sty, contribution to a special issue of "Superlattices and Microstructures

Flux Jumps Driven by a Pulsed Magnetic Field

The understanding of flux jumps in the high temperature superconductors is of importance since the occurrence of these jumps may limit the perspectives of the practical use of these materials. In this work we present the experimental study of the role of heavy ion irradiation in stabilizing the HTSC against flux jumps by comparing un-irradiated and 7.5 10^10 Kr-ion/cm2 irradiated (YxTm1-x)Ba2Cu3O7 single crystals. Using pulsed field magnetization measurements, we have applied a broad range of field sweep rates from 0.1T/s up to 1800 T/s to investigate the behavior of the flux jumps. The observed flux jumps, which may be attributed to thermal instabilities, are incomplete and have different amplitudes. The flux jumps strongly depend on the magnetic field, on the magneto-thermal history of the sample, on the magnetic field sweep rate, on the critical current density jc, on the temperature and on the thermal contact with the bath in which the sample is immersed.Comment: 5 pages, PDF-fil

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

Crossover from the 2D Heisenberg to the 1D Quantum Spin Ladder Regime in Underdoped High Tc Cuprates

The enigmatic scaling behaviour of the normal state properties of the high Tc cuprates has been explained by assuming that a crossover from the two- dimensional Heisenberg (2D-H) to the one-dimensional spin ladder (1D-SL) regime takes place at temperature T=T*. For T<T* stripe formation results in the quantum 1D transport with the characteristic inelastic length L_phi being fully controlled by the magnetic correlation length xi_m of the even-chain SL, whereas for T> T* the 2D quantum transport is realized with L_\phi governed by the 2D-H correlations L_phi=xi_m=exp(J/T)$. Therefore, the pseudogap found in underdoped (p<p_opt) high Tc's is the spin-gap Delta(p) in even-chain 1D-SL.Comment: 5 pages, 5 EPS figures, submitted to PR

'Giant' normal state magnetoresistances of Bi2Sr2CaCu2O8+.

Magnetoresistance (MR) of Bi-2212 single crystals with T$_{c}$ $\approx 87-92 K$ is studied in pulsed magnetic fields up to 50T along the c-axis in a wide temperature range. The negative out-of-plane and the positive in-plane MRs are measured in the normal state. Both MRs have similar magnitudes, exceeding any orbital contribution by two orders in magnitude. These are explained as a result of the magnetic pair-breaking of preformed pairs. Resistive upper critical fields H$_{c2}$(T) determined from the in-- and out-of-plane MRs are about the same. They show non-BCS temperature dependences compatible with the Bose-Einstein condensation field of preformed charged bosons

Giant vortex state in perforated aluminum microsquares

We investigate the nucleation of superconductivity in a uniform perpendicular magnetic field H in aluminum microsquares containing a few (2 and 4) submicron holes (antidots). The normal/superconducting phase boundary T_c(H) of these structures shows a quite different behavior in low and high fields. In the low magnetic field regime fluxoid quantization around each antidot leads to oscillations in T_c(H), expected from the specific sample geometry, and reminiscent of the network behavior. In high magnetic fields, the T_c(H) boundaries of the perforated and a reference non-perforated microsquare reveal cusps at the same values of Phi/Phi_0 (where Phi is the applied flux threading the total square area and Phi_0 is the superconducting flux quantum), while the background on T_c(H) becomes quasi-linear, indicating that a giant vortex state is established. The influence of the actual geometries on T_c(H) is analyzed in the framework of the linearized Ginzburg-Landau theory.Comment: 14 pages, 6 PS figures, RevTex, accepted for publication in Phys. Rev.

Anisotropic vortex pinning in superconductors with a square array of rectangular submicron holes

We investigate vortex pinning in thin superconducting films with a square array of rectangular submicron holes ("antidots"). Two types of antidots are considered: antidots fully perforating the superconducting film, and "blind antidots", holes that perforate the film only up to a certain depth. In both systems, we observe a distinct anisotropy in the pinning properties, reflected in the critical current Ic, depending on the direction of the applied electrical current: parallel to the long side of the antidots or perpendicular to it. Although the mechanism responsible for the effect is very different in the two systems, they both show a higher critical current and a sharper IV-transition when the current is applied along the long side of the rectangular antidots

Weak ferromagnetism in La-doped BiFeO3 multiferroics thin films

Bi1xLaxFeO3 thin films (x ¼ 0.0, 0.3, 0.5) were grown on glass substrates by thermal physical vapor deposition. The monoclinically distorted crystal structure of the films was revealed by x-ray diffraction at room temperature. Field and temperature (up to 1000 K) dependences of magnetization were studied. Saturation of the room temperature magnetic hysteresis loop has been observed at magnetic field above 0.15 T, demonstrating the weak ferromagnetic nature of the thin films. Our magnetic force microscopy results show clearly the presence of magnetic domains in BFO thin films. These structural and magnetic properties suggest the absence of magnetic spiral spin structure in monoclinically distorted BFO-based thin films

Magnetic properties and electronic structure of Bi0.75Sm0.25FeO3 multiferroic

Room temperature multiferroic Bi0.75Sm0.25FeO3 samples were synthesized using a solid-state reaction method under high-pressure conditions (∼4 GPA). The calculations of band structure, electronic and spin densities distribution of Bi0.75Sm0.25FeO3 multiferroic were performed using the local spin-density approximation plus Hubbard U (LSDA+U) method in the framework of density functional theory. The calculations took into account a collinear antiferromagnetic ordering of the Fe and Sm magnetic moments. Temperature and field dependencies of the specific magnetization were investigated and correlated to the electronic structure of the multiferroic sample