Bond and charge density waves in the isotropic interacting

 two-dimensional quarter-filled band and the insulating state proximate to

 organic superconductivity

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Abstract

We report two surprising results regarding the nature of the spatial broken symmetries in the two-dimensional (2D), quarter-filled band with strong electron-electron interactions. First, in direct contradiction to the predictions of one-electron theory, we find a coexisting ``bond-order and charge density wave'' (BCDW) insulating ground state in the 2D rectangular lattice for all anisotropies, including the isotropic limit. Second, we find that the BCDW further coexists with a spin-density wave (SDW) in the range of large anisotropy. Further, in contrast to the interacting half-filled band, in the interacting quarter-filled band there are two transitions: first, a similar singlet-to-AFM/SDW transition for large anisotropy and second, an AFM/SDW-to-singlet transition at smaller anisotropy. We discuss how these theoretical results apply to the insulating states that are proximate to the superconducting states of 2:1 cationic charge-transfer solids (CTS). An important consequence of this work is the suggestion that organic superconductivity is related to the proximate Coulomb-induced BCDW, with the SDW that coexists for large anisotropies being also a consequence of the BCDW, rather than the driver of superconductivity.

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Is Open Access

Correlated Electrons in High Temperature Superconductors

Elbio Dagotto (1993)

Theoretical ideas and experimental results concerning high temperature superconductors are reviewed. Special emphasis is given to calculations carried out with the help of computers applied to models of strongly correlated electrons proposed to describe the two dimensional \({\rm Cu O_2}\) planes. The review also includes results using several analytical techniques. The one and three band Hubbard models, and the \({\rm t-J}\) model are discussed, and their behavior compared against experiments when available. Among the conclusions of the review, we found that some experimentally observed unusual properties of the cuprates have a natural explanation through Hubbard-like models. In particular abnormal features like the mid-infrared band of the optical conductivity \(\sigma(\omega)\), the new states observed in the gap in photoemission experiments, the behavior of the spin correlations with doping, and the presence of phase separation in the copper oxide superconductors may be explained, at least in part, by these models. Finally, the existence of superconductivity in Hubbard-like models is analyzed. Some aspects of the recently proposed ideas to describe the cuprates as having a \(\dx2y2\) superconducting condensate at low temperatures are discussed. Numerical results favor this scenario over others....(continues).