High-temperature superconductivity and long-range order in strongly correlated electronic systems

Abstract

A selective review of the question of how repulsive electron correlations might give rise to off-diagonal long-range order (ODLRO) in high-temperature superconductors is presented. The article makes detailed explanations of the relevance to superconductivity of reduced electronic density matrices and how these can be used to understand whether ODLRO might arise from Coulombic repulsions in strongly correlated electronic systems. Time-reversed electron pairs on alternant Cuprate and the iron-based pnictide and chalcogenide lattices may have a weak long-range attractive tail and much stronger short-range repulsive Coulomb interaction. The long-range attractive tail may find its origin in one of the many suggested proposals for high-Tc superconductivity and thus has an uncertain origin. A phenomenological Hamiltonian is invoked whose model parameters are obtained by fitting to experimental data. A detailed summary is given of the arguments that such interacting electrons can cooperate to produce a superconducting state in which time-reversed pairs of electrons effectively avoid the repulsive hard-core of the Coulomb interaction but reside on average in the attractive well of the long-range potential. Thus, the pairing of electrons itself provides an enhanced screening mechanism. The alternant lattice structure is the key to achieving robust high-temperature superconductivity with dx2-y2 or sign alternating s-wave or s± condensate symmetries in cuprates and iron-based compounds. Some attention is also given to the question first raised by Leggett as to where the Coulombic energy is saved in the superconducting transition in cuprates. A mean-field-type model in which the condensate density serves as an order parameter is discussed. Many of the observed trends in the thermal properties of cuprate superconductors are reproduced giving strong support for the proposed model for high-temperature superconductivity in such strongly correlated electronic systems. © 2015 Wiley Periodicals, Inc.