Berlin 2005 – wissenschaftliches Programm

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TT: Tiefe Temperaturen

TT 10: Superconductivity - Tunneling, Josephson Junctions, SQUIDs

TT 10.16: Vortrag

Samstag, 5. März 2005, 12:30–12:45, TU H104

Sheet current density distribution in a SQUID washer probed by vortices — •Dietmar Doenitz¹, Matthias Ruoff¹, Rainer Straub¹, Ernst Helmut Brandt², John R. Clem³, Reinhold Kleiner¹ und Dieter Koelle¹ — ¹Universität Tübingen, Physikalisches Institut - Experimentalphysik II, Auf der Morgenstelle 14, D-72076 Tübingen — ²Max-Planck-Institut für Metallforschung, D-70506 Stuttgart — ³Ames Laboratory - DOE and Department of Physics and Astronomy, Iowa State University, Ames Iowa 50011, USA

We use Low Temperature Scanning Electron Microscopy (LTSEM) to image vortices in YBCO washer SQUIDs. The imaging is based on the electron-beam-induced local displacement of vortices, which is detected as a flux change Δ Φ in the SQUID. The function Φ(r^→) describing the amount of flux a single vortex couples into the SQUID hole is directly measured by the SQUID. The scalar stream function G(r^→)=1/Φ₀· Φ(r^→) determines contrast C (∝ |∇^→G|) and direction d (|| ∇^→G) of the vortex signals. When there is a ring current I flowing around the SQUID loop, the vortex-free sheet current density distribution is J^→=I· (ẑ × ∇^→G), leading to |J^→ (r^→)| ∝ C and Ĵ ⊥ d. This allows to extract full information on the sheet current density distribution at every position a vortex has been imaged, thus using the vortices as local detectors for J^→. Our experimental results from vortex imaging are in very good agreement with numerical calculations of J^→(r^→).