Non-Technical Abstract Superconductors, which transport electric current without loss, also expel a magnetic field. However, above some critical field magnetic flux enters a superconductor through the formation of an array of swirling currents of superconducting electrons, called vortices. The movement of these vortices can destroy the superconducting property. High field magnets avoid this problem due to microscopic defects that pin the vortices. Rather than having the vortices pinned randomly by defects, the approach of this research team is to pin them in a controlled way by creating patterns of nanoscopic holes within a thin film to which they attach. By introducing external electric and magnetic fields it should then be possible to move the vortices between the pinning sites in a controlled way. Such manipulations can, potentially, be used to either store data, perform calculations, or both, all within the same assembly of holes. Since vortices are inherently stable, being destroyed only at the edges of the film or on encountering an anti-vortex, the “information bits†they carry are secure. Furthermore, they can have sub micron dimensions and may respond on sub nanosecond time scales potentially facilitating high-speed, high density data processing. But vortices are also quantum mechanical in nature and this suggests the possibility they may be manipulated in a quantum mechanically coherent way by exploiting both quantum tunneling and superposition. Therefore, this project could lead to the development of devices for quantum information technology. The proposed project is in line with national goals to better understand and better exploit quantum phenomena. In addition, it will equip students with the expertise needed to join the work force of future industrial efforts to realize and exploit the principles and techniques established for quantum information technology.
This project aims to explore various quasi-static and dynamic properties of thin film vortices pinned on patterned holes in thin superconducting films. An external magnetic field perpendicular to the film will control the average flux density within the film, with special emphasis on strengths corresponding to an integer occupations. One class of experiments involves a search for resonant responses of vortices pinned on isolated holes, since displacement of the vortex center generates a restoring force. This project will in addition study the use of applied d.c. (or pulsed) currents, and in combination with oscillatory fields, to move vortices between neighboring pairs of holes, down a one-dimensional line of holes, or across a two dimensional array of holes, which should result in r.f./microwave emissions, the frequency of which is controlled by the hole spacing and the local current density. Application of an oscillatory current at the hopping frequency should produce Shapiro-like steps in the d.c transport characteristics. In the presence of quantum-tunneling between the neighboring potential wells, a single vortex pinned on the pair could distribute itself between the wells in a symmetric or antisymmetric manner from which two-state super-positions might be formed, which could be searched for spectroscopically. The manipulation of such vortices can be used as flux-flow oscillators, computer bits, or data storage elements. The phenomena to be examined, if realized, could, potentially, lead to new classes of super-conducting devices, all based on the manipulation of hole-pinned vortices. These include: i) various flux flow oscillators, a multi-bit flux line memory element, and iii) a possible qubit. Graduates of this program will be equipped to enter various industrial environments.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
