Materials Science and Engineering B56 (1998) 77 – 85
Possible influence of band bending on the normal state properties
of grain boundaries in high-T
c
superconductors
J. Mannhart *, H. Hilgenkamp
Experimentalphysik VI, Center for Electronic Correlations and Magnetism, Augsburg Uniersity, D-86391 Augsburg, Germany
Abstract
The normal state transport properties of grain boundaries in high-T
c
superconductors, of central importance for a large number
of electronic applications, still lack a thorough explanation. In this contribution we consider the implications of grain boundaries
on the electronic band structures of high-T
c
cuprates. It is shown that the normal state properties observed may be caused by
depletion layers formed at the interfaces. © 1998 Published by Elsevier Science S.A. All rights reserved.
Keywords: High temperature superconductivity; Josephson junctions; Grain boundaries
1. Introduction
Grain boundaries in high-T
c
superconductors have
been used with outstanding success for experiments
elucidating the fundamental physics of high-T
c
super-
conductivity [1 – 3], and have been exploited for appli-
cations in electronic devices such as superconducting
quantum interference devices or rapid single flux
quantum based logic circuits [4,5]. Furthermore, be-
cause of their current-limiting properties, their opti-
mization plays a critical role for the fabrication of
wires and tapes from high-T
c
materials [6].
Experiments elucidating the physics of grain
boundaries in high-T
c
superconductors have been car-
ried out by many groups for a large number of high-
T
c
compounds. It was found that in nominally all
high-T
c
materials grain boundaries display a charac-
teristic and almost universal set of properties
1
. Of
these we mention in particular:
1. the existence of a misorientation-dependent upper
limit to the grain boundary critical current density
J
c
[7,8],
2. the SNS-(superconductor-normal metal-supercon-
ductor) Josephson junction-type current versus
voltage-characteristics of [001] tilt boundaries with
a misorientation exceeding 10° [8,9],
3. the typical specific normal state interface resistivity
R
n
A 10
-7
–10
-9
cm
2
[8]. Here R
n
is the nor-
mal state resistance of the grain boundary and A
denotes the grain boundary area. This resistivity is
large compared to the normal-state interface resis-
tivity of grain boundaries in conventional super-
conductors like Al, which is determined by the
Sharvin resistance ranging at 10
-11
cm
2
[10].
Although these properties are striking and crucial
for many applications of high-T
c
superconductors, the
electronic behavior of the grain boundaries in these
materials is still poorly understood [11]. To elucidate
their behavior, in this contribution we pursue an ear-
lier suggestion [12] and investigate the relevance of
bending of the band structure of the cuprates in the
vicinity of the grain boundaries and of d
x2 -y2
-wave
superconductivity for grain boundary transport.
2. Mechanisms suggested to explain the transport
properties of grain boundaries in high-T
c
superconductors
To explain the intriguing behavior of the grain
boundaries in high-T
c
superconductors, various mech-
* Corresponding author. Tel.: +49 821 3650; fax: +49 821 3652.
1
It is noted that due to difficulties associated with the sample
preparation no systematic studies have been published on the trans-
port properties of superconducting Nd
1.85
Ce
0.15
CuO
4
-grain
boundaries.
0921-5107/98/$ - see front matter © 1998 Published by Elsevier Science S.A. All rights reserved.
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