Sergey A. Fedoseev

Rectifying differences in transport, dynamic, and quasi-equilibrium measurements of critical current density

The dependence of the critical current density (Jc) on electric field criteria (Ecr) is studied for high-quality YBCO (YBa2Cu3O7) thin films over the entire applied magnetic field (Ba) range. The quantitative model describing the Jc(Ba) dependence is compared and explained for the critical current densities obtained by different measurement techniques. Transport current and quasi-equilibrium magnetization measurement data can successfully be fitted by the model with appropriate electric field criteria. The dependence of the irreversibility field on the Ecr criterion can be obtained within the model. At the same time, the dynamic magnetization measurements of the Jc(Ba) curves strongly depend on instrumentally defined parameters, introducing inconsistencies in the experimental results. Therefore, the model calculations are able to explain the Jc(Ba) curves only if the instrumental vibrations affecting vortex behaviour are taken into account. However, the nature of the observed dependence on the vibration o...

Large, Controllable Spikes of Magnetoresistance in La2/3Ca1/3MnO3/SrTiO3 Superlattices

We have investigated superlattices consisting of up to 30 epitaxial nanomultilayers (3-7 nm thick) of ferromagnetic La(2/3)Ca(1/3)MnO(3) (LCMO) and insulating SrTiO(3) (STO) hybrids. The superlattices demonstrate dramatic shifts of Curie temperature, indicating the possibility of its tunability. The metal-insulator transition (MIT) has been observed around 140 K. Below the MIT temperature, the superlattices have shown sharp drops of resistivity, facilitating the largest and sharpest magnetoresistance peaks (>2000%) ever observed in LCMO films and superlattices at low temperatures. The observed experimental results can be explained in the frame of the phase separation model in manganites with well-organized structures. The results of magnetic and transport measurements of such hybrid structures are discussed, indicating a magnetodielectric effect in STO interlayers. The magnetic and transport properties of the superlattices are shown to be technology-dependent, experiencing dimensional transitions, which enables the creation of structures with prescribed magnetoresistance characteristics for a broad range of applications.

Analysis of low-field isotropic vortex glass containing vortex groups in YBa2Cu3O7−x thin films visualized by scanning SQUID microscopy

The glass-like vortex distribution in pulsed laser deposited YBa2Cu3O7 − x thin films is observed by scanning superconducting quantum interference device microscopy and analysed for ordering after cooling in magnetic fields significantly smaller than the Earths field. Autocorrelation calculations on this distribution show a weak short-range positional order, while Delaunay triangulation shows a near-complete lack of orientational order. The distribution of these vortices is finally characterised as an isotropic vortex glass. Abnormally closely spaced groups of vortices, which are statistically unlikely to occur, are observed above a threshold magnetic field. The origin of these groups is discussed, but will require further investigation.

Observation of Transient Overcritical Currents in YBCO Thin Films using High-Speed Magneto-Optical Imaging and Dynamic Current Mapping

The dynamics of transient current distributions in superconducting YBa2Cu3O7−δ thin films were investigated during and immediately following an external field ramp, using high-speed (real-time) Magneto-Optical Imaging and calculation of dynamic current profiles. A number of qualitatively unique and previously unobserved features are seen in this novel analysis of the evolution of supercurrent during penetration. As magnetic field ramps up from zero, the dynamic current profile is characterized by strong peaks, the magnitude of which exceed the conventional critical current density (as determined from static current profiles). These peaks develop close to the sample edges, initially resembling screening currents but quickly growing in intensity as the external field increases. A discontinuity in field and current behaviour is newly observed, indicating a novel transition from increasing peak current toward relaxation behaviour. After this transition, the current peaks move toward the centre of the sample while reducing in intensity as magnetic vortices penetrate inward. This motion slows exponentially with time, with the current distribution in the long-time limit reducing to the expected Kim-model profile.

Quantitative model for tunable microstructure in magnetic FePt thin films by pulsed laser deposition

Pulsed laser deposition (PLD) is employed to fabricate FePt L10 thin films from elemental targets. Dramatic structure variations are obtained by varying the laser frequency while keeping the thickness of the films constant. A new theoretical model based on the mean field approach is proposed, which quantitatively describes the structural changes obtained experimentally. The experiment and the model exhibit the opposite growth development to the trend reported for the modulated flux in the literature. The new model considers different growth rates in the lateral and transverse directions due to different responses of the normal and tangential film surfaces being deposited to the incident flux and the migration kinetics of adatoms and clusters. The quantitative results obtained confirm that the migration kinetics and self-assembly can easily be controlled by the PLD frequency which is consistent with the experiments. Magnetic properties of the films are shown to be extremely sensitive to the structure variations allowing practical tunability.

Vibration effect on magnetization and critical current density of superconductors

In this work the effect of vibrations on critical current density (J c ) of superconductors has been studied. The vibrations are shown to affect J c of all types of superconductors during their measurements, employing a vibrating sample magnetometer (VSM). Increasing vibration frequency (f) and/or amplitude (A) leads to progressive reduction of J c as a function of magnetic field (B a ). The effect of vibrations is substantially stronger in thin films. It leads to development of unexpected kinks on curves. Analysis of magnetization loops and relaxation of magnetization in YBCO films revealed that the vibration effect can be treated as the effective reduction of pinning potential. The asymmetry of the vibration effect in ascending and descending B a is observed, indicating differences in free energy of the corresponding vortex structures. Thermal effects induced by vibrations with large f and A are shown to have rather insignificant influence, while the vibrational vortex dynamics exhibit a strong impact. The irreversibility field () is shown to be instrumentally defined, and its value depends on VSM settings. In addition, the practical importance of for J c modeling is demonstrated.

Effect of Substrate and Buffer Layer Materials on Properties of Thin

High-temperature superconducting thin films (YBa₂Cu₃ O_{7 - x}) are emerging in superconducting single photon detector (SSPD) research as a novel replacement for conventional and semiconductor detectors. The major hindrance for this is the degradation of the superconducting properties of YBa₂Cu₃O_{7 - x} (YBCO) thin film with reduction of its lateral and longitudinal dimensions (i.e., film thickness and width of the stripe). Furthermore, the surface of the film should be smooth to enable fabrication of the SSPD device. In order to improve the quality of YBCO thin films, we exploited various buffer layers (i.e., SrTiO₃, CeO₂, and PrBa₂Cu₃O₇) with thickness of 30 ± 5 nm. We have also investigated the properties of (65 ± 5-nm-thick) YBCO films grown simultaneously on different substrates (i.e., SrTiO₃ , LaAlO₃, MgO, and yttrium stabilized zirconia). For some substrate/buffer material combinations, the surface morphology of the YBCO film has been effectively improved. Also, there was only a small or no degradation of their critical temperature values. These structures give a precursor for further development of fabrication technology for YBCO-based SSPD devices.

\hbox{YBa}_{2}\hbox{Cu}_{3}\hbox{O}_{7 - {\rm x}}

The variation of topological defect density and hexatic order parameter were measured over a range of micro-Tesla fields in a 2-D superconducting vortex glass. This was achieved through scanning SQUID microscopy of the vortex distribution in YBa2Cu3O7-δ thin films under field-cooled conditions. It was discovered that, although the defect density decreased for increasing magnetic fields, giving the impression of a more lattice-like vortex distribution, the hexatic order parameter also decreased, showing that the distribution was less orientationally ordered.

Films

We are developing two-layered Yttrium Barium Copper Oxide (YBCO) thin film structures for energy efficient data links for superconducting electronics and present the results of their property measurements. High temperature superconductors (HTS) are advantageous for the implementation of energy-efficient cables interconnecting low temperature superconductor-based circuits and other cryogenic electronics circuits at higher temperature stages. The advantages of the HTS cables come from their low loss and low dispersion properties, allowing ballistic transfer of low power signals with very high bandwidth, low heat conduction and negligible inter-line crosstalk. The microstrip line cable geometry for typical materials is a two-layered film, in which the two superconducting layers are separated by an insulation layer with a minimized permittivity. We have made a proof of concept design of two YBCO films grown by pulsed laser deposition and then assembled into a sandwich with uniform insulating interlayer of tens of micrometers thick. We report on results obtained from such systems assembled in different ways. Structural and electromagnetic properties have been examined on individual films and on the corresponding sandwich composite.