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Physics Colloquium

Speaker: Avadh Saxena Location: LD 010

Skyrmions, Merons and Monopoles: Topological Excitations in Chiral Magnets

Avadh Saxena

Abstract: 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  phenomena. 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.   


Refreshments will be served in LD 154B from 3:00-3:30pm.