Persistent current generation by small polarons in a spin-density wave   background
  Persistent current generation by small polarons embedded in a spin-density wave background in a two dimensional lattice is theoretically studied. When the embedded small polarons become cores of merons and antimerons (vortices in spin configuration with winding numbers +1 and -1, respectively), a spin Berry phase arises from the spiral spin texture of the merons and antimerons, and generates a spin current around the polarons. Electric current is, however, absent due to the exact cancellation of the up- and down-spin electron currents. By addition of an imaginary magnetic flux that negates the fictitious magnetic flux from the spin Berry phase, the state with merons and antimerons is stabilized and non-zero electric current appears; this imaginary flux becomes a real one when side-by-side bipolaronic cores of merons and antimerons are formed, where the real flux arises spontaneously from the wave function for the motion of the system as a whole; thus, the bipolarons serve as pumps for the persistent electric current. It is also shown that a Nernst signal stems from thermal diffusion of the bipolarons. Implications of the present results to anomalous properties of cuprate superconductors, including the enhanced Nernst signal, Drude peak, and Fermi arc are also discussed. The present result suggests the appearance of the phase variable or ``Nambu-Goldstone'' mode for the superconductivity without the breakdown of the global U(1) gauge invariance in the cuprates. 
Hiroyasu Koizumi