Validated Modeling Tools for Spin-filter Cathode Designs

Spin-polarized electrons are required by several projects relevant to the U.S. Department of Energy mission. More- over, many next generation applications require generation of spin-polarized electron beams with high bunch charge (up to 50 nC) and/or high average current (50 mA). Currently, GaAs-related photocathodes, based on different designs, are used to emit electron beams with high polarization. However, it is still a major challenge to achieve the required high bunch charge and/or average current from GaAs photocathodes. In order to reduce the risk for the successful delivery and operation of next generation systems, development of new cathodes for emission of high bunch charge polarized electrons is of interest. Recent experiments with semiconductor/insulator/ferromagnetic metal heterostructures have demonstrated the potential to deliver very high-current spin-polarized electron beams. The optimal material configurations of these cathodes are still to be determined. Detailed, three-dimensional simulations are expected to provide an efficient way to explore the properties of new spin-filter cathodes and help guide their design. However, there are no available codes that provide these capabilities. We propose to address this problem by developing software to model transport of spin-polarized electrons through heterostructure materials, including spin-dependent electron-electron scattering in ferromagnetic metallic layers, and electron emission for efficient investigation and design of spin-filter cathodes. The Phase I portion of this project will specifically address the issues of developing a combined effort of simulation and validation. We plan to develop software to model the transport of electrons, with both charge and spin degrees of freedom included, through spin-filter cathode designs. Our approach also includes models of spin-dependent emission. We also plan to team with researchers at Los Alamos National Laboratory (LANL) to perform specific experiments to validate these models. The models we plan to implement will be of direct benefit to DOE scientists at Brookhaven National Lab and Jefferson National Lab who are working to design and develop high-average- current polarized electron sources for advanced electron-ion collider, electron cooling, and nuclear and accelerator physics applications. We expect that results from realistic simulations with a 3D PIC code will provide valuable feedback to them and will ultimately help them to produce novel polarized electron sources that meet, or exceed, the requirements for use in next generation DOE electron-ion collider facilities. In the commercial sector, the new multi-material (semiconductor, insulator, metal) modeling capabilities will be most relevant for design of semiconductor devices that depend on electron spin properties, such as spin-based electronics (spintronics) for magnetic storage media, semiconductor lasers, and magnetic tunnel transistors.