Berlin 2005 – scientific programme
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TT: Tiefe Temperaturen
TT 10: Superconductivity - Tunneling, Josephson Junctions, SQUIDs
TT 10.6: Talk
Saturday, March 5, 2005, 09:45–10:00, TU H104
SIFS junctions for Semifluxon Qubits — •M. Weides1, H. Kohlstedt1, R. Waser1, E. Goldobin2, D. Koelle2, R. Kleiner2, and V. Ryazanov3 — 1Institut für Festkörperforschung,Forschungszentrum Jülich — 2Physikalisches Institut –Experimentalphysik II, Universität Tübingen — 3Institute for Solid State Physics,Chernogolovka, Russia
There are several approaches to realize qubits using π or 0-π
Josephson junctions (JJs). Such JJs can be fabricated using a ferromagnetic
barrier of thickness dF between two superconductors, i.e. SFS or SIFS
structures. Due to damped spatial oscillations of the order parameter in
the F-layer, dF determines the phase coupling (0 or π) and the
critical current density jc of such JJs.
We fabricated SIFS multilayers Nb/Al2O3/Cu40Ni60/Nb by
magnetron sputtering. Similar JJs without an F-layer (SIS JJs) show
jc∼2.5 kA/cm2, low sub gap current and a large
superconducting gap. In SIFS JJs the first side maximum of jc(dF)
corresponds to the π ground state and is clearly seen in our samples.
For applications such as digital logic and qubits, a figure of merit is
Vc=IcRn in the π state, which for our SIFSs is about 10 µV
— the highest value obtained so far for SIFS JJs.
The appearance of a degenerated semifluxon state at the boundary between 0-
π parts1 is interesting both for fundamental studies and
applications. In the quantum limit, semifluxons could be used as quiet
qubits or as a playground for testing quantum mechanics. We propose a way
of designing 0-π SIFS junctions for the investigation of semifluxons.
1 E. Goldobin et al., Phys. Rev. B 66, 100508 (2002).