SP-STM: Skyrmions, Magnetic Textures, and Magnetic Topological Insulators
Magnetic skyrmions are topologically protected, particle-like excitations that appear in chiral magnetic systems. Due to their stability and the low-power required to move them through materials, they’re a candidate for use in magnetic memory and spin based logic. MnGe is a low symmetry material that allows the formation of magnetic skyrmions. Because of its strong spin-orbit coupling, skyrmions form a 3D lattice instead of a 2D sheet. With Spin polarized (SP) STM we can directly observe winding of the spins that give rise to these skyrmions, and how the presence of the surface itself affects the properties that make them useful for devices.
The general theme of this project is to explore the ways in which the local environment influences the properties of individual defects in semiconductors. In prior studies focused on GaAs, we have demonstrated that proximity to the surface and to other defects allows tuning of the carrier binding energy, ionization state and can induce shallow/deep transitions. We have also developed methods to better quantify and control the local electrostatic potential at semiconductor surfaces by atomic manipulation of charged species. We are now working to extend these methods to a broader range of impurities and semiconductor materials.
Defect-mediated surface chemistry
Semiconductor photocatalysis has long been studied for the promise of renewable energy storage into energy-dense fuels such as hydrogen and saturated hydrocarbons. For example, use of sunlight to convert atmospheric CO2 into transportation fuels like methanol addresses several sustainability challenges at once. Toward this end, we are studying surface chemistry relevant for CO2 photocatalysis on semiconducting oxides such as Cu2O and CuO. These oxides have shown unusual selectivity for methanol production in the bulk, but an atomic scale understanding of the process has yet to be developed.
The discovery of graphene has increased interest in 2D materials beyond the surface science community. Our early work in this area was focused on the properties of one layer thick Cu2N grown on Cu substrates. Despite being only one layer thick, we found Cu2N acts as an insulator with a sizeable band gap of ~ 6 eV, that could be tuned by varying the lateral area in islands comprising as few as 12 atoms. We are now extending this work to other 2D materials including graphene, boron nitride, and a variety of transition metal dichalcogenides (TMDs).
Electron and nuclear spins are promising degrees of freedom for quantum computation and quantum sensing applications. Spins in metal-ion coordination complexes have the added advantage that their local environment is tunable via the chemical design of their molecular ligands. We have demonstrated that single molecule magnets such as vanadyl phthalocyanine (VOPc) can be studied on both metal and insulator-on-conductor surfaces using STM. We are now extending this work by implementing RF transmission to the tunneling junction, enabling the detection and coherent control of electron spin transitions with atomic precision. This work is a collaboration with Prof. Ezekiel Johnston-Halperin’s group also in OSU’s physics department, as well as the groups of Danna Freedman and Michael Flatté at MIT and University of Iowa respectively.
We are focusing on two approaches for combining optical excitation with STM. In one project, we are studying light/surface interactions by combining strong-field laser excitation with atomic resolution imaging. Our goal is to systematically monitor the evolution of laser-induced structures on the surface, starting from individual atomic defects to micron-scale periodic patterns. This is a collaborative project with Prof. Enam Chowdhury‘s group in OSU’s Department of Materials Science and Engineering. In a second project, we are developing methods for combining STM with soft-xray excitation, with an aim to perform element-specific imaging with atomic spatial resolution and sub-femtosecond time resolution. This project is taking place as part of the NSF NeXUS user facility construction, ongoing at OSU.