Avadh Saxena (Los Alamos National Laboratory): Skyrmions, Merons and Monopoles: Topological Excitations in Chiral Magnets


Marți, 21 mai 2019, începând cu ora 12:00, în sala Hermann Oberth din Clădirea Centrală a UBB, va avea loc prelegerea cu tema

Skyrmions, Merons and Monopoles: Topological Excitations in Chiral Magnets

susținută de dl. Avadh Saxena, Los Alamos National Laboratory, SUA.


Stable topological excitations such as domain walls and vortices are ubiquitous in condensed matter as well as high energy physics and are responsible for many emergent henomena. In 2009 a new mesoscopic spin texture called skyrmion was discovered experimentally in certain conducting and insulating magnets. It is now believed to exist in Bose-Einstein condensates, 2D electron gases, superconductors, nematic liquid crystals among many other systems. This topological excitation was originally proposed by Tony Skyrme in 1958 in a nonlinear field theory of baryons. In the temperature-magnetic field phase diagram of chiral magnets, skyrmions form a triangular lattice in the low temperature and intermediate magnetic field region (in thin films). In metallic magnets, skyrmions can be driven by a spin polarized current while in insulating magnets by magnons. The threshold current density to depin skyrmions is 4 to 5 orders of magnitudes weaker than that for magnetic domain walls. The low depinning current makes skyrmions extremely promising for applications in spintronics. I will first attempt to summarize the experiments and present an overview on skyrmions. Then I will demonstrate how increasing the easy-plane anisotropy results in a transition from a triangular lattice of skyrmions to a square lattice and eventually to merons. The latter are essentially half-skyrmions, with half the topological charge and have been experimentally observed. Finally, I will show that under current driving skyrmion tubes can split or merge at certain points leading to the formation of magnetic monopoles and anti-monopoles connected by a Dirac string.