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arXiv:cond-mat/9309028 · Physical review. B, Condensed matter

Superconductivity near phase separation in models of correlated electrons

Elbio R. Dagotto, José A. Riera, Y.-C. Chen, Adriana Moreo, Alexander Nazarenko, Francisco Castilho Alcaraz, F. Ortolani

Published Feb 1, 1994• 129 citations
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Abstract

Numerical and analytical studies of several models of correlated electrons are discussed. Based on exact diagonalization and variational Monte Carlo techniques, we have found indications that the two-dimensional t-J model superconducts near phase separation in the regime of quarter-filling density, in agreement with previous results reported by Dagotto and Riera [Phys. Rev. Lett. 70, 682 (1993)]. At this density the dominant channel is ${\mathit{d}}_{\mathit{x}}^{2}$-${\mathit{y}}^{2}$, but a transiton to s-wave superconductivity is observed decreasing the electronic density. In addition, the one-band t-U-V model has also been studied using a mean-field approximation. An interesting region of ${\mathit{d}}_{\mathit{x}}^{2}$-${\mathit{y}}^{2}$ superconductivity near phase separation is observed in the phase diagram, and its implications for recent self-consistent studies of d-wave condensates in the context of the high-${\mathit{T}}_{\mathit{c}}$ cuprates are briefly discussed. Finally, the two-band Hubbard model on a chain is also analyzed. Superconducting correlations near phase separation exist in this model, as it occurs in the t-J model. Based on these nontrivial examples it is conjectured that electronic models tend to have superconducting phases in the vicinity of phase separation, and this regime of parameter space should be explored first when a new model for superconductivity is proposed. Reciprocally, if it is established that a model does not phase separate, even in an extended parameter space, then we believe that its chances of presenting a superconducting phase are considerably reduced.

DOI
10.1103/physrevb.49.3548

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The phase-separation instability occurring with increasing nearest-neighbor repulsion VVV in a two-band Hubbard model (CuO chain) is discussed. Quantum Monte Carlo simulations indicate that this transition is strongly discontinuous if the filling fraction 〈n〉〈n〉〈n〉 is close to 1 (half-filled lower band). Spin-density-wave fluctuations then dominate before phase separation. Superconducting fluctuations dominate in the neighborhood of phase separation only at considerably higher doping levels, where the transition to the phase-separated state is much smoother.