Seminar | February 9 | 4-5 p.m. | 348 Hearst Memorial Mining Building
Dr. Padraic Shafer, Advanced Light Source, LBNL
Chirality determines many important properties in nature, ranging from the electroweak interaction in particle physics to the reactions of enzymes that are fundamental for life. In condensed matter physics, chiral structures determine the functional behavior of magnetic and other correlated-electron materials. Underlying these functionalities are a wealth of intriguing nanoscale ordering phenomena such as charge, orbital, and spin superstructures, as well as periodic arrays of domains and domains walls or vortices. This talk focuses on the chiral nanostructures in two distinct material systems: Chiral magnetic domain walls (DWs) in Co/Pt/Cu multilayers; and the emergent chirality of electric polarization vortices in PbTiO3/SrTiO3 superlattices. Our experimental measurements of DW chirality in ultrathin films are important for both the fundamental understanding of DWs and in research on topologically protected nanomagnetic structures . Additionally, they provide critical information for the development of DW-based spintronic devices, where DW chirality has been suggested to greatly suppress the critical current density for driving DW motion. Recently we have discovered chiral topologies of electric polarization that are reminiscent of rotational spin topologies. These nanometer-scale arrays of counterrotating vortex pairs can be created by employing the competition between charge, orbital, and lattice degrees of freedom in superlattices of alternating PbTiO3 and SrTiO3 layers . These observations have implications for the creation of new states of matter (such as dipolar skyrmions, hedgehog states) and associated phenomena in ferroic materials, such as electrically controllable chirality. For both studies we employ the unique capabilities of resonant soft x-ray diffraction (RSXD) as a tool for investigating electronic and magnetic nanostructures. RSXD uses x-ray wavelengths of ~1‒3 nm that are well matched to the periodicity of the nanostructures. By selecting wavelengths that correspond to resonant electronic transitions we gain sensitivity to probe the magnetic or electric polarization orientations. In particular, x-ray circular dichroism (XCD)the difference in diffraction intensity when circularly polarized x-rays of opposite helicity are usedis central to identifying chiral polarization textures that emerge in magnetic domain walls and in vortex superlattices composed of non-chiral constituents.