International Journal of Modern Physics B

PHASE SEPARATION AND SUPERCONDUCTIVITY IN THE U=∞ LIMIT OF THE EXTENDED MULTIBAND HUBBARD MODEL

M. Grilli, Roberto Raimondi, Claudio Castellani, C. Di Castro, Gabriel Kotliar

Published Jan 1, 1991• 34 citations

Abstract

The three band Hubbard model with nearest neighbour repulsion is studied in the U=∞ limit using the large N expansion technique to order [Formula: see text]. A charge transfer (CT) instability is found like in weak coupling theory. However at small doping a major role is played by the Brinkman-Rice point. The CT instability is always associated with a diverging compressibility leading to a phase separation. The evaluation to order [Formula: see text] of the effective scattering amplitude in the Cooper channel shows the presence of superconducting instabilities in the s and d wave channel near the phase separation.

DOI: 10.1142/s0217979291000195

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International Journal of Modern Physics B

PHASE SEPARATION AND SUPERCONDUCTIVITY IN THE U=∞ LIMIT OF THE EXTENDED MULTIBAND HUBBARD MODEL

M. Grilli, Roberto Raimondi, Claudio Castellani, C. Di Castro, Gabriel Kotliar

Published Jan 1, 1991• 34 citations

Publisher

Abstract

DOI: 10.1142/s0217979291000195

Related papers

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{\mathit{F}}_{0}^{\mathit{s}}

>-1 is violated, due to a coupling to the charge-transfer exciton. We also study the frequency dependence of the effective interaction, mediated by the exchange of collective modes, between the quasiparticles.

We analyze the dynamics of the charge degrees of freedom in the extended Hubbard model for the

{\mathrm{CuO}}_{2}

planes in copper oxides in the strong-coupling limit. We analyze the behavior of the collective modes near the charge-transfer instability (CTI). The CTI is driven by an overdamped zero-sound mode when the Landau stability criterion

{\mathit{F}}_{0}^{\mathit{s}}

>-1 is violated due to the charge-transfer mode-mediated attraction. The divergence of the compressibility at the CTI requires a Maxwell construction, which determines a region of phase separation. Near the phase-separation boundary, at intermediate doping, the singlet Cooper coupling is attractive both in the s- and d-wave channels. In the strong-coupling limit the excitonic energy ${\mathrm{\ensuremath{\omega}}}_{\mathrm{exc}}$ is large and the energy scale for pairing is the Fermi energy itself.

We study the Cu-O valence instability (VI) and the related phase separation (PS) driven by Cu-O nearest-neighbor repulsion

U_{pd}

, using an effective extended one-band Hubbard model (

H_{eff}

) obtained from the extended three-bandHubbard model, through an appropriate low-energy reduction.

H_{eff}

is solved by exact diagonalization of a square cluster with 10 unit cells and also within a slave-boson mean-field theory. Its parameters depend on doping for

U_{pd}\neq 0

or on-site O repulsion

U_p\neq 0

. The results using both techniques coincide in that there is neither VI nor PS for doping levels

x 0.6

and increases with increasing

U_{pd}

. The PS also increases with increasing on-site Cu repulsion

U_d

The doped Hubbard model is a simple model for high-

{T}_{c}

cuprate superconductors, while its ground state remains a challenge. Here, by performing state-of-the-art variational Monte Carlo calculations for the strong-coupling Hubbard model, we find evidence that the

d

-wave superconducting phase emerges always near the phase separation region and the superconducting order has one-to-one correspondence with the enhancement of charge compressibility. The order as well as the phase separation are vulnerable to realistic intersite Coulomb interaction, while the superexchange interaction enhances both. An appropriate combination of these two widens the stable superconducting phase.