第38回 原研・兵県大合同コロキウム

日時: 平成20年2月22日(金) 16:00〜
Date and time: 22nd Feb. (Fri.) 16:00〜
場所: SPring-8, 萌光館
Place: SPring-8, "HOUKOUKAN" seminar room
Title: What is the role of strong hole-lattice interaction in the hole-doped cuprate superconductivity?
講演者: 小泉裕康 氏 (筑波大学数理物質科学)
Speaker: Prof. Hiroyasu Koizumi (Institute of materials science, University of Tsukuba)
Abstract:

We first present our molecular orbital cluster calculation results that indicate the presence of strong interaction between doped holes and the underlying lattice in the cuprate. It is also indicated that the naive Zhang-Rice singlet picture for the local electronic state needs to be revised: the doped hole is mainly concentrated in a single bridging oxygen, and the bridging oxygen p orbital and two nearby copper dx2-y2 orbitals form a valence bond type bonding; this state is stabilized by local lattice distortions and the doped hole becomes a small polaron.

Next, we consider the current generation mechanism compatible with the above small polaron formation; actually, we present a new current generation mechanism that is effective even if all the doped holes become self-trapped small polarons. The current generation here reverses a long-standing belief that at half-filing the effective Hamiltonian for the U >> t Hubbard model is the Heisenberg model, thus, possible low energy excitations are those include spin degrees of freedom only. The new current generation mechanism yields the Fermi arc seen in the ARPES simulation; it also suggests that the phase of the superconductivity order parameter in underdoped cuprates arises without Cooper pair formation; i.e., the London current formula with effective charge 2e is realized without the Cooper pair formation.

The electric current here is generated by a fictitious magnetic field induced by spin vortices: because the presence of spin vortices causes a sign ambiguity of wave functions for electron hoping motion, a phase factor that ensures the single-valuedness of the wave function appears; and this phase factor gives rise to a vector potential for the fictitious magnetic field. The strong hole-lattice interaction stabilizes spin vortices by pinning each center at a polaron occupied site; thus, it will also stabilize the spin vortex induced current. Based on the new current generation mechanism, possibility of an artificial superconducting nano wire is presented. It is made in the Cu-O plane, and consists of pining centers of nonmagnetic atoms arranged in two parallel lines; here the nonmagnetic atoms play the role of lattice small polarons; and the region between the two arrays of vortex centers is a superconducting wire where a thermodynamically stable current flows.