Speaker: Professor Ward Plummer (Louisiana State University)

Time: June 15, 2016 16:00

Place: 理科楼三楼报告厅C302

Abstract：Complexity in transition-metal compounds originates for the close and nonlinear coupling of spin, charge, orbital, and lattice degrees of freedom. Creating a surface disturbs the balance between these degrees of freedom, and in many cases results in phased Hidden in the bulk. Several examples will be presented to illustrate this phenomenon. 1) Dynamic antiphase spin domain walls have been proposed for the parent compound of the Fe based super-conductor BaFe2As2. The surface (√2x√2)R45o phase displays a coupled structural-magnetic antiphase domain walls with a reduced symmetry compared to the bulk. 2) The (1x2) surface phase Ca(Fe1-xCox)2stabilizes Antiferromagnetic Ordering leading to the coexistence of superconductivity and antiferromagnetic ordering.  3) The final example is for Mn doped double-layerRuthenate Sr3(Ru1-xMnx)2O7, where the surface phase diagram is almost opposite from the bulk due to surface induced tilt of the octahedral in the parent compound.  The surface of Sr3(Ru0.84Mn0.16)2O7 is metallic while the bulk is insulating, because the surface tilt has been removed.

 Bio: Professor Plummer is a world-renowned scholar in surfaces, with a BA in Math and Physics from Lewis and Clark College, Portland, Oregon, and a Ph. D in Physics from Cornell University. He is a member of the National Academy of Science, a fellow of American Academy of Arts and Sciences (AAAS) and a recipient ofDavission and Germer Prize of American Physical Society. He has authored ~400 papers, and has mentored for over 110 young scholars as graduate students or postdoctoral fellows.