Dc and rf transport in growth boundary networks

of band gap - or Mott insulators in the

normal or superconducting state

J. Halbritter,

Forschungszentrum Karlsruhe, IHM, Postfach 3640

76021 Karlsruhe, Germany

 

ABSTRACT

 

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 GBs 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.