Verlag des Forschungszentrums Jülich
JUEL-3526 Charge transport at interfaces of HTS ramp-type junctions with PrBa2Cu3O7-[delta]
barrier The Josephson junction is the most important basic device in active superconductor
electronics. Ramp-type junctions are used in high-temperature-superconductor (HTS) devices
because the fabrication process of these junctions is compatible with a complex multilayer
technology and provides a high design flexibility. Most HTS-applications require a high
normal resistance of the junctions. Therefore PrBa2Cu3O7-[delta]
(PBCO) can be used as the barrier material with a high resistivity. In this work, the
charge transport at the interfaces of the barrier to the superconducting electrodes was
investigated. To compare the influence of the degradation during the ion-beam milling
process of the ramp, a second fabrication process was used for an in-situ preparation of
the junctions in the same deposition system. The junctions with a PBCO barrier fabricated
by these two techniques showed comparable current-voltage characteristics and ICRN-products.
This indicates a neglectable suppression of the junction properties due to the ion-beam
milling process of the interface. The ICRN-products were in the
range of 5 – 18mV at 4.2K. The junctions showed an additional conductance G[alpha]U4/3
in the current-voltage characteristics at higher voltages and a temperature dependent
normal conductance GN[alpha]T4/3. Therefore the quasiparticle
current can be well described by resonant tunneling via one and two localized states in
the barrier. This was already observed by other groups [Sat95][Ver96b]. The transport of
the supercurrent by resonant tunneling via localized states is only possible by neglecting
the Coulomb repulsion between the two electrons of the Cooper pairs at the localized
state. But for barrier thicknesses of several 10nm the contribution of resonant tunneling
cannot be neglected. The current-voltage characteristics showed an excess-current which
cannot be described by the RCSJ-Model. This deviation can result from Andreev reflections
at the interfaces. By measuring the differential conductance of the junctions, a zero-bias
conductance peak could be observed. In a magnetic field the amplitude of the peak is
suppressed and the width is broadened. This can be related to surface bound states, which
are due to the d-wave symmetry of the superconducting order parameter. These states split
in a magnetic field due to the interacting with surface currents. The splitting saturates
at a magnetic field of about 1.5T and the linear dependency for larger fields cannot be
extrapolated to zero splitting at zero field. This behavior was recently explained by
Fogelström et al. with spontaneous surface currents generated by a sub-dominant order
parameter which is stabilized by the surface [Fog97]. The conductance measurements clearly
showed a subharmonic gap structure. These structure can be explained within a two gap
model including multiple Andreev reflections at the interfaces. The values of the two gaps
are in the range of [Delta]N=13-17meV and [Delta]S=21-26meV which is
in good agreement with the results of Polturak et al. [Pol93]. The origin of the second
smaller gap is not clearly identified. One possibility is a suppression of the order
parameter close to the interface. The two gaps at the interface seem to be a more
intrinsic property and are not caused by damaging during processing because the same gap
values are observed at junctions fabricated with different processes. Another possibility
is a second gap related to the crystallographic anisotropy of the YBCO superconductor and
a resulting anisotropy of the order parameter along the crystallographic axes. In
conclusion, the important role of Andreev reflections at the interfaces has been shown in
the measurements of the stationary and the dynamic properties. A two gap model and
multiple Andreev reflections can describe the subharmonic gap structure which was
observed. Assuming a supercurrent transport by resonant tunneling via localized states,
the linear temperature dependency of the critical current can be explained within the two
gap model.
Engelhardt, Achim
Ladungstransport an Grenzflächen von HTSL-Rampenkontakten mit PrBa2Cu3O7-delta-Barriere
125 S., 1998
Neuerscheinungen
Schriften des Forschungszentrums Jülich
Ihre Ansprechperson
Heike Lexis
+49 2461 61-5367
zb-publikation@fz-juelich.de