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Abstract:
We propose the concept of the spin superconductor (SSC), a counterpart to the charge superconductor. We carry out theoretical study to show the existence of a spin superconductor in a ferromagnetic graphene, in which the spin-polarized electron-hole excitons play the roles of the `Cooper' pairs. We present a BCS-type theory and the Laudau-Ginzburg theory for the SSC. With the ``London-type equations" of the super-spin-current density, we show the existence of an electric ``Meissner effect" against a spatial varying electric field. We further study a SSC/normal conductor/SSC junction and predict a spin-current Josephson effect.
By viewing the exciton in a bilayer system as an electric dipole, we provide a general theory for the electric dipole superconductivity, and derive the London-type and Ginzburg-Landau-type equations for the electric dipole superconductors. By using these equations, we discover the Meissner-type effect and the electric dipole current Josephson effect. These effects can provide direct evidence for the formation of the exciton superfluid state in bilayer systems and pave new ways to drive an electric dipole current.
About the Speaker:
Prof. Xie obtained his B.Sc. in physics from University of Science and Technology of China in 1982 and Ph.D. from University of Maryland in 1988. He became a faculty member in Department of Physics at Oklahoma State University in USA since 1991 and was named Regents Professor in 2004. He worked as a Chief Scientist and Director of Laboratory of Condensed Matter Theory and Computation at the Institute of Physics, Chinese Academy of Sciences in 2005-2010. Prof. Xin-Cheng Xie joined Peking University in 2010 as a Chair Professor and the Founding Director of International Center for Quantum Materials. In 2011 he was appointed as Dean of School of Physics at Peking University, and a Co-Director of National “2011 project” Collaborative Innovation Center of Quantum Matter.
Prof. Xie’s main research focuses include quantum Hall effect, quantum transport, topological matter and strongly correlated electron systems.
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