- Advanced electron microscopy
- Controlled increase of critical currents of GBs by overdoping
- EFM imaging of plastic vortex motion near GBs
- MO imaging of percolative current flow in HTS polycrystals
- Probing vortex dynamics and pinning on grain boundaries by measuring transport V-J characteristics and critical currents of HTS bicrystals
On nanoscales (0.1-10 nm), the high-resolution electron microscopy has revealed detailed atomic structure of GB dislocation cores and changes due to controlled overdoping of GBs. In turn, these data are being used to address theoretically the local electronic and charge states of dislocation cores, electron screening around GBs and their effect on current transport through nanoscale channels between the GB dislocation cores. These issues determine the behavior of vortices on GBs, which is of prime importance for the current-carrying capability of HTS conductors. By combined experimental and theoretical analysis, we have recently revealed the nature of vortices on low-angle GBs in thin-film YBCO. This gives the first clear understanding of in-field current-limiting mechanisms of GBs, enabling us to measure the intrinsic boundary critical current density Jb in nanoscale current channels between the grain boundary dislocations, a fundamental quantity that has not been accessible by other techniques. We have developed a theory of vortices driven along a grain boundary by dc and ac currents. The theory describes very well the observed field dependence of the flux flow resistance of low-angle YBCO bicrystals. This enabled us to prove the existence of mixed Abrikosov-Josephson vortices on low angle GBs and measure for the first time their core length, and the intrinsic value of Jb. This new method will be used for systematic analysis of the effect of local overdoping on current transport through grain boundaries.
On microscales (0.1-10 µm), electron microscopy is being used to study facet structure, strain fields and local nonstoichiometry around GBs. This multiscale structural disorder is very important for pinning of GB vortices, which is also strongly affected by their magnetic interaction with bulk vortices in the grains. This results in a rich composite behavior of GB vortices influenced by both GB and grain properties. For instance, plastic vortex flow channels along GB and transition from single to multiple vortex row flow along GBs in the presence of current. We probe dynamics and pinning of GB vortices by transport measurements of extended V-J characteristics and critical currents at different temperatures and magnetic fields. Recently, we have been able to considerably increase in-field critical currents of low angle GBs in thin film YBCO bicrystals by Ca overdoping.
On macroscales (10-10µm), we use magneto-optic imaging combined with current reconstruction algorithms and theoretical modeling of nonlinear current flow around defects to address key issues of current limiting mechanisms in HTS conductors. Our calculations of nonlinear current transport in HTS with macroscopic random inhomogeneities, and current flow around planar obstacles model different aspects of percolative current flow in polycrystalline HTS and indicate a very complex and rich behavior of global V-I characteristics. In particular, using the hodograph method, we were able to solve analytically highly nonlinear Maxwell’s equations, which describe current flow around planar defects and discovered extended domains of strongly enhanced electric field and dissipation near high-angle GBs. These "hot spots" can dominate the transport behavior and stability of current flow, even if GBs occupy a small fraction of the geometrical cross-section.
Exploring Science Issues
A number of unique features of MgB2 raise additional scientific challenges. Recent theoretical and experimental work suggests that MgB2 may exhibit two-gap superconductivity. We are therefore curious whether there are novel effects due to the coupling between the gaps, and whether this produces new properties when quantized field lines move along grain boundaries or when MgB2 is exposed to microwave radiation. The competition between thermal fluctuations and flux pinning is also being explored.