Vortices are prevalent in nature all the way from the atmosphere, charge and uncharged plasmas, magnetic and superconducting materials. An important issue in this field is the way to anchor these vortices, the so called "pinning" because physical properties are modified in fundamental ways if vortices are pinned. A particularly interesting physical situations arises in superconductors where the magnetic field penetrates the material by the formation of arrays of superconducting vortices. These vortices can be pinned by artificially prepared pinning arrays which can be produced using novel lithographic techniques. This project is dedicated to the study of fundamental issue which arise when superconducting vortices interact with nanostructured arrays. Issues such as the effect of the array geometry, materials, shape of the pinning sites will be studied.In addition to its basic research interest, these studies may lead to schemes for reducing spontaneous noise and enhancing the superconducting properties of the material.
The study of vortex pinning in superconductors is an interesting basic research area, with implications for the fabrication of high critical current tapes and low noise superconducting devices. The interaction between artificially prepared pinning arrays and the superconducting vortex lattice leads to quantum matching phenomena which manifest as enhanced critical currents and decreased resistance at particular fields. This research project, will be dedicated to (a) finding the type of pinning structure which individually provides strong pinning of vortices, and (b) introducing appropriate "defects" within the periodic array of pins, such as stripes and/or random fluctuations on the pinning site positions. (a) will increase the domain wall energy for the periodic point pins and increase the magnetic field width for the periodic stripe pins, thereby reducing thermal fluctuations, while (b) can change the universality class of the commensurate phase, thereby suppressing thermal fluctuations in fundamental ways. The issues concerning (a) are mostly material-specific, involving the microscopic mechanism of individual vortex-pin interaction. The issues concerning (b) are statistical in nature, having to do with the collective interaction of the vortex lattice with the array of pins. Both of these issues will be investigate systematically, and through extensive experimental collaborations and interactions with theoretical groups active in the field.
Superconducting vortex lattices are model systems in which magnetic nano structure arrays are used to pin superconducting vortices in a well defined and controllable way (picture shows measurement geometry). This has a broad impact with relevance to other areas of science including Biology, Electronics, Chemistry, Quantum Information, Storage, and Irreversibility. It can also be applied to different types of devices such as magnetic field controlled rectification, reprogrammable logic, and radio frequency sensor (Pictures include superconducting cables and electronics). This award allowed us to research several novel effects: A hybrid Superconductor-Nanomagnet ratchet (picture shows a Superconducting-Ferromagnetic Hybrid Ratchet – i.e. Reversible AC current produces DC voltage) is an example of an electrical device which helps understanding how fundamentally reversible equations can give raise to irreversible responses. Generally the ratchet effect in electrical devices is controlled by some property, which is fixed and allows flow of charge only in one direction. In the present devices, we can turn the ratchet effect on and off depending on past magnetic history of the device. This provides a ratchet which can be tuned at will. Our studies using controlled disorder in the artificial lattice have shown that depending on the type of disorder, we can effectively pin the vortices for various magnetic fields. This pinning can help reduce noise in a variety of superconducting devices including SQUID magnetometers, superconducting electronics and superconducting magnets. In addition, we have manufactured structures with magnetic pinning sites in the Corbino geometry in which injected current flows outward from a point in the center of the sample. Since the current density i.e. force decreases with distance from the center there is a shearing force on the vortex lattice as different vortex rings are forced to rotate at different velocities. The artificial pinning centers cause this shearing effect to produce a bi-modal transition which occurs at specific magnetic fields. This has potential applications to switching devices in superconductors (figure shows switching the bi-modality in the resistance with small changes of magnetic fields).
