D. Abraimov

Observation of Breathers in Josephson Ladders

We report on the observation of spatially localized excitations in a ladder of small Josephson junctions. The excitations are whirling states which persist under a spatially homogeneous force due to the bias current. These states of the ladder are visualized using a low temperature scanning laser microscopy. We also compute breather solutions with high accuracy in corresponding model equations. The stability analysis of these solutions is used to interpret the measured patterns in the I-V characteristics.

Low-temperature scanning laser microscopy of individual filaments extracted from (Bi, Pb)2Sr2Ca2Cu3O10+x tapes

The method of low-temperature scanning laser microscopy is applied to visualize the resistive state in individual superconducting filaments extracted from (Bi, Pb)2Sr2Ca2Cu3O10+x/Ag tapes. This technique is capable of imaging the distributions of the critical currents over a sample. Using the nonbolometric response, a spatial resolution of about 1 μm is demonstrated for 10-μm-thick filaments. Some of the resistively visualized grain boundaries between crystallites show Josephson behavior.

Scanning laser imaging of dissipation in YBa2Cu3O7−δ-coated conductors

We investigate dc-current flow in high-jc YBa2Cu3O7−δ-coated conductors by low-temperature laser scanning microscopy (LTLSM) and correlate the LTLSM response to magneto-optical imaging (MOI) and grain boundary (GB) misorientation. Because the voltage response measured by LTLSM is associated with the local electric field, while MOI shows the local magnetic field, the combination of these two techniques unambiguously shows that the dominant sources of dissipation and easy flux flow occur at and near GBs. By correlating LTLSM images to grain misorientation maps determined by electron backscatter diffraction (EBSD), we can directly observe the overloading of current paths through low-angle GBs neighboring higher-angle GBs.

Spatially resolved measurements of critical parameters in superconducting filaments by laser scanning technique

We present an experimental study of local transport properties of superconducting filaments by means of low temperature scanning laser microscopy (LTSLM). A novel theoretical model that treats the laser-beam-induced voltage response is proposed and applied to analyze the measured data. The method of LTSLM allows to visualize the local inhomogeneities in individual filaments extracted from (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10+x//Ag tapes. Using the proposed theoretical analysis we extract the spatial distributions of the critical currents and critical temperatures over the sample. Tile data obtained by LTLSM show good agreement with standard non-local transport measurements. Using autocorrelation analysis of the voltage response, the characteristic length of the critical current variation is retrieved for filaments that belong to different parts of the tape.

Imaging local sources of intermodulation in superconducting microwave devices

This work presents new experimental results on low-temperature (LT) characterization of local RF properties of passive superconducting (SC) microwave devices using a novel laser scanning microscope (LSM). In this technique, a modulated laser beam is focused onto and scanned over the surface of a resonant SC device to probe the spatial distribution of RF current. The highly localized photo-induced change of the kinetic inductance of the SC device produces both a shift of the resonant frequency f/sub 0/ and change of the quality factor Q. An image of these changes is recorded as the laser spot is scanned over the device. We present the first measurements of spatially resolved intermodulation response in a high temperature superconducting (HTS) co-planar waveguide resonator, opening up a new window into the local origins of nonlinearity in the HTS materials.

Imaging local dissipation and magnetic field in YBCO films with artificial defects

We used Low Temperature Laser Scanning Microscopy (LTLSM) to investigate dc current flow in high-J/sub c/ YBCO thin films with artificially prepared defects and correlated the LTLSM response to Magneto Optical Images (MOI). Artificial defects model current blockages such as cracks, high angle grain boundaries or voids. Because the LTLSM voltage response is associated with the local electric field, while MOI shows the local perpendicular magnetic field, the combination of techniques gives complementary local information about the effect of current-limiting defects in superconductors.

Study of grain boundary transparency in(Yb1−xCax)Ba2Cu3Obicrystal thin films over a wide temperature, field, and field orientation range

The residual low angle grain boundary (GB) network is still the most important current-limiting mechanism operating in bi-axially textured rare earth barium copper oxide (REBCO) coated conductors. While Ca-doping is well established to improve super-current flow across low angle GBs in weak fields at high temperatures, Ca-doping also depresses Tc, making it so far impractical for high temperature applications of REBCO coated conductors. On the other hand, high field magnet applications of REBCO require low temperatures. Here we systematically evaluate the effectiveness of Ca-doping in improving the GB transparency, r

Imaging sub-millimeter waves in planar cryoelectronic circuits by scanning laser microscopy

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buffer Layer Growth, the Thickness Dependence of Jc in Coated Conductors, Local Identification of Current Limiting Mechanisms and Participation in the Wire Development Group

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Observation of important current-limiting defects in a recent high pinning force MOCVD IBAD-MgO coated conductor

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