Scanning Hall probe microscopy has been used to make a microscopic study of flux structures and dynamics in yttrium barium copper oxide thin-film disks containing a regular 10-m-period square array of 2.5-m-sized holes ͑antidots͒. Images obtained after field cooling the sample to 77 K in very low fields reveal that the holes can trap two flux quanta at this temperature. Scans obtained after zero-field cooling ͑ZFC͒ to 77 K and a subsequent applied field cycle clearly display preferential flux channeling along chains of antidots in the direction of maximum induction gradient. Remarkably, upon reversal of field sweep direction, we observe flux "streaming" out of the holes towards the sample edges with almost uniform density flux "stripes" bridging the holes in the exit direction. We estimate that the antidots can preferentially trap about 15 flux quanta in these ZFC experiments. Classical electrodynamics simulations of our samples appear to be in good qualitative agreement with our results, indicating that many of the observed phenomena may be geometrical effects that depend primarily on the shape and topology of the sample, and potential applications are discussed.

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We present a novel method, based on vortex imaging by low-temperature scanning electron microscopy (LTSEM), to directly image the sheet-current distribution in YBa2Cu3O7 dc SQUID washers. We show that the LTSEM vortex signals are simply related to the scalar stream function describing the vortex-free circulating sheet-current distribution J . Unlike previous inversion methods that infer the current distribution from the measured magnetic field, our method uses pinned vortices as local detectors for J . Our experimental results are in very good agreement with numerical calculations of J .PACS numbers: 68.37. Hk, 74.25.Op, 85.25.Dq Spatially resolved techniques can provide important insight into current flow, arrangement of vortices, flux pinning, and noise in superconductors and their mutual interactions. So far there has been only one method of imaging the current distribution in superconductors: The magnetic field distribution on top of a superconducting thin film is measured, e.g. by magneto-optics, from which the current distribution can then be calculated by inverting the Biot-Savart law [1].In this paper we present a novel method to directly image the sheet-current distribution in a YBa 2 Cu 3 O 7 thin film. We use low-temperature scanning electron microscopy (LTSEM) [2,3,4,5] to image vortices in dc SQUID washers [6,7]. Most techniques for vortex imaging, such as Lorentz microscopy [8], scanning SQUID microscopy [9,10], scanning Hall microscopy [11] or magneto-optics[12] rely on the detection of the stray magnetic field produced in close proximity to a vortex. In contrast, vortex imaging by LTSEM is different from those techniques, as it is based on the electron-beaminduced apparent displacement of a vortex, pinned at position r in the (x, y)-plane of a SQUID washer, which is detected as a change of stray magnetic flux Φ(r) coupled to the SQUID. Hence, the contrast of the LTSEM vortex signals directly senses ∇Φ(r). Recently, Clem and Brandt [13] have shown that Φ(r) is proportional to the scalar stream function G(r) that describes the circulating sheet-current density J(r) flowing in the vortex-free case at position r in the SQUID washer. In this paper we show that this relationship allows us to use the vortices as local detectors for J(r): At each position a vortex has been imaged, we can directly determine J(r) without complicated calculations.In our experiments, we investigated several dc SQUID washers [see Fig. 1(a)] fabricated from epitaxially grown d=80 nm thick c-axis oriented YBa 2 Cu 3 O 7 (YBCO) thin * Electronic address: koelle@uni-tuebingen.de films. We will present an analysis of LTSEM data obtained from one representative device with washer size 120 µm × 305 µm, with a 100 µm long and 4 µm wide slit. The 1 µm wide Josephson junctions are formed by a 24 • symmetric grain boundary in the underlying SrTiO 3 substrate. For imaging by LTSEM, the YBCO SQUIDs are mounted on a magnetically shielded, liquid nitrogen cooled cryostage of an SEM [14] and read out by a standard flux-locked loop (FLL) w...

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Vortex loops are generated by the inhomogeneous stray field of a magnetic dipole on top of a current-carrying mesoscopic superconductor. Cutting and recombination processes unfold under the applied drive, resulting in periodic voltage oscillations across the sample. We show that a direct and detectable consequence of the cutting and recombination of these vortex loops in the present setup is the onset of vortices at surfaces where they were absent prior to the application of the external current. The nonlinear dynamics of vortex loops is studied within the time-dependent Ginzburg-Landau theory to describe the profound three-dimensional features of their time evolution.