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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. Ryazanov31Institut 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).

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