Regensburg 2004 – wissenschaftliches Programm

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TT: Tiefe Temperaturen

TT 18: Supraleitung: Tunneln, Josephson-Kontakte, SQUIDs

TT 18.3: Vortrag

Mittwoch, 10. März 2004, 15:00–15:15, H20

Imaging of Josephson Current Distribution in YBa₂Cu₃O₇-Nb Zigzag Junctions — •D. Dönitz¹, E. Goldobin¹, R. Straub¹, R. Kleiner¹, D. Koelle¹, H.J.H. Smilde², Ariando², H. Hilgenkamp², and H. Rogalla² — ¹Universität Tübingen, Physikalisches Institut – Experimentalphysik II, Auf der Morgenstelle 14, D-72076 Tübingen, Germany — ²Low Temperature Division, Faculty of Science and Technolgoy and MESA+ Research Institute, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands

We used low temperature scanning electron microscopy (LTSEM) to image the supercurrent distribution in zigzag YBa₂Cu₃O₇ (YBCO)–Nb ramp edge Josephson junctions. These junctions consist of several artificial facets of length a ≤ λ_J and alternating orientation along the in-plane crystallographic axes of the YBCO film. At zero applied magnetic field we clearly observed the reversal of the direction of the Josephson current flowing through neighboring facets. This is a direct visualization of the opposite sign of the critical current in neighboring facets, i.e. they can be considered as 0 and π junctions. This constitutes direct imaging of the sign change in the phase of the d_x²−y² order parameter symmetry in YBCO. We performed numerical simulations of the sine–Gordon model which showed that for our particular geometry the phase profile along the zigzag junctions corresponds to a compressed chain of antiferromagnetically ordered semifluxons. At the matching field, for which the critical current is maximum, the simulations predict a supercurrent distribution which has the same sign and an arc–like profile in all facets. This prediction is directly confirmed by LTSEM.