Phase Separation and Pairing Fluctuations in Oxide Materials

Abstract

The microscopic mechanism of charge instabilities and the formation of inhomogeneous states in systems with strong electron correlations is investigated. We demonstrate that within a strong coupling expansion the single-band Hubbard model shows an instability towards phase separation and extend the approach also for an analysis of phase separation in the Hubbard-Kanamori hamiltonian as a prototypical multiband model. We study the pairing fluctuations on top of an inhomogeneous stripe state where superconducting correlations in the extended s-wave and d-wave channels correspond to (anti)bound states in the two-particle spectra. Whereas extended s-wave fluctuations are relevant on the scale of the local interaction parameter U, we find that d-wave fluctuations are pronounced in the energy range of the active subband which crosses the Fermi level. As a result, low energy spin and charge fluctuations can transfer the d-wave correlations from the bound states to the low energy quasiparticle bands. Our investigations therefore help to understand the coexistence of stripe correlations and d-wave superconductivity in cuprates.

Absence of superconductivity in the two-dimensional Hubbard model

“…The momentum dependence of the vertex V + q (j ′ , j) is shown in figure 4 for two values of temperature. The dependence has the pronounced minimum at the momentum of the antiferromagnetic ordering q = Q = (π, π), which is connected with the spin vertex V s in (12). A less pronounced local minimum at q = (0, 0) is caused by the charge vertex V c .…”

“…Indeed, many properties of cuprate perovskites are correctly described in the model. The most prominent of them are antiferromagnetism and its doping dependence, charge instability, and pseudogap formation (see, e.g., [3,4,5,6,7,8,9,10,11,12]). As for the ability to describe superconductivity, results are more diverseboth affirmative and negative answers were obtained depending on used methods (see, e.g., [4,5,13,14,15,16,17,18]).…”

“…In the present work, the ladder diagrams are constructed from second-order cumulants of electron operators and renormalized hopping lines containing electron Green's functions. These functions are self-consistently calculated using the same approach as in our earlier works [9,10,11,12,28]. We consider both the t-U model, with the non-zero hopping constant t between nearest-neighbor sites only, and the t-t ′ -t ′′ -U model, with additional non-zero hopping constants between second and third neighbors.…”

“…We consider both the t-U model, with the non-zero hopping constant t between nearest-neighbor sites only, and the t-t ′ -t ′′ -U model, with additional non-zero hopping constants between second and third neighbors. The Hubbard model exhibits phase separation when the chemical potential µ is near zero [11,12]. For the considered regime of strong correlations with the on-site Coulomb repulsion U = 8t, this instability occurs near electron concentrations n = 0.8 in the t-U model and near n = 0.87 in the t-t ′ -t ′′ -U model with t ′ = −0.3t, t ′′ = 0.2t, and the temperature T ≈ 0.1t.…”

Absence of superconductivity in the repulsive Hubbard model on a square lattice in the regime of strong coupling

“…The most prominent of them are antiferromagnetism and its doping dependence, charge instability, and pseudogap formation (see, e.g. [3][4][5][6][7][8][9][10][11][12]). As for the ability to describe superconductivity, results are more diverse-both affirmative and negative answers were obtained depending on used methods (see, e.g.…”