goal is to understand "real" grain boundaries (GBs) of
high Tc superconductors in all their multi-scale complexity.
This requires a forefront, mix of sample design and fabrication,
film growth, superconducting property characterization, nanoscale
microstructure and electronic structure determination, methods to
modify GB properties and theory that takes full account of the complex
materials science of these materials. Different techniques are being
used to address key aspects of current transport through GBs:
Advanced electron microscopy.
vortex dynamics and pinning on grain boundaries by measuring
transport V-J characteristics and critical currents of HTS bicrystals.
increase of critical currents of GBs by overdoping.
imaging of plastic vortex motion near GBs.
imaging of percolative current flow in HTS polycrystals.
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
microscales (0.1-10 µm), the 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.
macroscales (10-100µm), 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 Maxwells 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.
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