Optimization of high-temperature superconducting bilayer structures using a vortex dynamics simulation.

We argue that the current carrying properties of high-temperature superconducting thin films can be further improved, in particular under the mid-field range (B ≈ 0.1-2 T), via introduction of multilayer structures that compromise between good zero field critical current and vortex pinning performance. In this work we focus on a simple bilayer structure consisting of two adjacent layers of pure YBa2Cu3O6+x(YBCO) and BaZrO3(BZO) doped YBCO under magnetic field within the mid-field range oriented parallel to thec-axis of the YBCO unit cell. We have utilized a computational model to simulate the vortex dynamics limited critical current separately from the associated zero field current, which is addressed analytically. The obtained results have allowed us to estimate the optimal layer thicknesses as a function of magnetic field. Our idealized model suggests that the thickness of the doped layer should be substantially smaller than the undoped one, that is around 30% of the total thickness of the film. We have estimated that the current carrying capability of the optimized bilayer structure can be up to 50% higher when compared with corresponding single layer films. Possible deviations from the obtained results associated with the idealized model, most prominently the effect of natural defects, are comprehensively discussed. Our results provide the foundation for the future experimental realization of the proposed bilayer structures. The comparison between the presented results and experimental realization would enable further study of the underlying primitive vortex interactions.

Vortex dynamics simulation for pinning structure optimization in the applications of high-temperature superconductors.

We introduce a molecular dynamics based simulation model that enables the efficient optimization of complex pinning structures in unpresented wide magnetic field and angular ranges for high-temperature superconductor applications. The fully three-dimensional simulation allows the modeling of the critical current and the associated anisotropy in the presence of any kinds of defects despite their size and orientation. Most prominently, these include artificial defects such as nanorods along with intrinsic weak-links orab-plane oriented stacking faults, for example. In this work, we present and analyze the most fundamental results of the simulation model and compare them indirectly with a wide range of previous experimental and computational observations. With the provided validation for the proposed simulation model, we consider it to be an extremely useful tool in particular for pushing the limits of ampacity in the coated conductor industry.

Angle dependent molecular dynamics simulation of flux pinning in YBCO superconductors with artificial pinning sites.

A molecular dynamics (MD) simulation to simulate the vortices in superconductors with artificial pinning sites is presented. The simulation reproduces the correct anisotropic behavior in angular dependence of critical current. We also show that the shape of the [Formula: see text] curve depends on the size of the pinning sites and the change from p = 0.5 to [Formula: see text] is due to the breaking of the vortex lattice to individually acting vortices. The results beautifully correspond to experimental data. Furthermore, we found that the size and shape of the c-axis peak observed with columnar pinning sites in [Formula: see text] also depends on the size of the rods, larger pinning sites leading to wider peaks. The results obtained from the MD-simulation are similar to those of the much more computationally intensive Ginzburg-Landau simulations. Furthermore, the MD-simulations can provide insight to the vortex dynamics within the samples.

Topography and work function measurements of thin MgO(001) films on Ag(001) by nc-AFM and KPFM.

The surface topography and local surface work function of ultrathin MgO(001) films on Ag(001) have been studied by noncontact atomic force microscopy (nc-AFM) and Kelvin probe force microscopy (KPFM). First principles calculations have been used to explain the contrast formation of nc-AFM images. In agreement with literature, thin MgO films grow in islands with a quasi rectangular shape. Contrary to alkali halide films supported on metal surfaces, where the island heights can be correctly measured, small MgO islands are either imaged as depressions or elevations depending on the electrostatic potential of the tip apex. Correct island heights therefore cannot be given without knowing the precise contrast formation discussed in this paper. KPFM shows a silver work function which is reduced by the MgO islands. The values for the work function differences for one and two layer thin films are -1.1 and -1.4 eV, respectively, in good agreement with recent calculations and experiments.