Over a large range of magnetic field and temperature, many superconductors contain regions which are threaded by localized magnetic flux lines. The quantum-mechanical wavefunction which characterizes the superconductivity winds its phase around one of these flux lines and vanishes at the center. These singularity points are appropriately called vortices, and exhibit a rich variety of phenomena, as they can interact with transport currents, temperature gradients, sample surfaces and defects, and other vortices. An understanding of the vortex potential landscape may allow for the control of vortex dynamics at the mesoscopic scale. Thus by microfabricating various sample edge shapes, surface step patterns, and bulk defects, it may be possible to influence the distribution and dynamics of vortices in superconductors. Eventually, the tailoring of this potential landscape may make possible electronic devices based on the control of vortex dynamics, as well as explorations of fundamental physical processes, such as ratchet dynamics.
Vortex dynamics can be studied by many methods, including transport measurements and direct magnetic imaging. In graduate school at the University of Illinois, BLTP worked in both areas and the images presented below were part of this work, supported by the National Science Foundation under Grants DMR91-20000 and DMR97-05695.
| Magnetic image, obtained with a Scanning SQUID Microscope (SSM), of a field-cooled a-MoGe film. Vortices form an amorphous, but evenly-spaced, pattern. The film edge is visible at the right side. Image is 508 microns on a side. |
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One type of fabricated pinning structure is a surface trench, where the film thickness is etched down in a particular region. Because of the increased vortex line energy in the thicker regions, vortices in such a trench are generally confined by the surface steps. By applying appropriate currents, it is possible to produce guided vortex motion through these trenches.
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Schematic showing vortices in thick and thin regions in the vicinity of a surface step. The SSM image on the right is a field-cooled film of a-MoGe with an octagonal trench etched into the surface. Vortices tend to orient parallel to the trenches due to interactions with the surface steps. |
| SSM images of vortices in a-MoGe strip with surface step running along length; thin edge towards bottom of images. Left: static vortex pattern with no transport current. Middle: transport current exerts Lorentz force towards top of image, pushing vortices into step. Right: opposite transport current exerting Lorentz force downwards; vortices move down step unimpeded. |    |
In addition to confining vortices in superconducting thin films with surface trenches, it is also possible to control the dynamics of vortices in arrays of Josephson junctions. In this case, each vortex corresponds to a winding of the phases of the junctions going around a particular closed path, or plaquette. By controlling the geometry and the distribution of the junction critical currents, it is possible to define the potential energy landscape which determines the dynamics of these vortices.
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SSM image of vortices in a square array of Josephson junctions. Applied magnetic field was near f = 1/2, corresponding to vortices in 1/2 of the plaquettes. Ground state configuration at f = 1/2 is a checkerboard arrangement of vortices. |
In our lab at Syracuse, we plan to fabricate various types of vortex pinning potentials both with surface trenches and Josephson junction arrays for investigations including:
Vortex dynamics in ratchet potentials -- systems with asymmetric potentials have been shown to exhibit directed motion when driven by fluctuations and have been proposed as the mechanism for biological motors. To study the dynamics of these ratchets, it is helpful to devise artificial model systems where the parameters can be varied for comparison with theoretical predictions. Vortices in a superconductor with patterned surface trenches and in Josephson junction arrays with a periodic asymmetric pinning potential provide an ideal system for studying ratchet behavior. An oscillatory Lorentz force, produced by varying the current driven through the device, can be rectified by the ratchet effect to produce a net displacement of the vortices. Studying the vortex response to the variation of several parameters including the drive strength and frequency, the pinning potential shape, and the temperature could reveal a rich variety of effects and could lead to a better understanding of ratchet dynamics in general. [A recent review of the ratchet effect and Brownian motors can be found in cond-mat/0410033.]
Vortex tunneling -- at dilution refrigerator temperatures, it may be possible to observe the tunneling of vortices through certain pinning potential structures.
| Possible arrangement for vortex ratchet with asymmetric surface pinning potential. Oscillatory Lorentz force, generated by ac bias current, drives vortices back and forth across the width of the strip, through the asymmetric potential. This basic scheme was initially proposed a few years ago by Wambaugh, et al. [Phys. Rev. Lett. 83, 5106 (1999)]. |  |