Dc and rf transport in growth boundary networks
of band gap - or Mott – insulators in the
normal or superconducting state
J. Halbritter,
76021
Karlsruhe, Germany
Island/grain boundaries (GB) occur naturally in film growth or in
sintering. The hindrance of electric transport by boundary resistances Rbn(Wcm2) in distances aJ (≲ 10 mm) is easy to measure by normal
conducting transport in such GB networks. The resistivity r(T) = Rbn/aJ + p ri (T) is fitted to observations with
percolation factors p > 1 by current diverting boundaries with Rbn ≳ aJri(T), where ri(T) is due to the grain interior
(IG), whereas Rbn/aJ describe the effects of growth boundaries and p of
networks. In superconducting transport GB may act as Josephson junctions (JJ)
with jcJ (A/cm2) as short junction critical current density acting as maximal
possible pinning potential for Josephson fluxons (JF). For superconducting
networks a simple separation in IG and GB is not possible. But low Ic values, p
> 1 and large Rbn values are indications for GB networks. GB in, e.g., YBCO,
MgB2 and NbX contain insulating oxides, in YBCO due to an intrinsic transition
to a Mott insulator but in MgB2 and NbX due to chemical reactions with oxygen.
Aside of the central insulator of thickness dGB acting as JJ, interface
deterioration up to depths dGB* >> dGB occur due to nucleated growth or
chemical reactions enforced by strain, which may pin JF or Abrikosov fluxons
(AF). Analysis of Ic(T,B,q,w) as function of temperature, field
B, angle q and
frequency f, below and above 0.1GHz, give crucial information about GB’s and
their dGB* and dGB layers about flux flow or pinning of JF or AF related to WSL
and IG. The combination of normal and superconducting analysis, e.g. of r(T), of Ic(T,B,q,w), of jcJ and of jcJRbn, is of
crucial importance for dc, ac and rf engineering applications and for the
understanding of the related material science and the transport, e.g.,
especially for doped Mott insulators the easy transition to a Mott insulator is
the reason for the enhanced pinning in the dGB* and dGB layers enforcing
Ic(T,B,q <qc,w) for epitaxial films and for
bicrystal junctions.