Enhancement of the d -wave pairing correlations by charge and spin ordering in the spin-one-half Falicov-Kimball model with Hund and Hubbard coupling

Abstract

Projector Quantum-Monte-Carlo Method is used to examine effects of the spin-independent U f d as well as spin-dependent J z Coulomb interaction between the localized f and itinerant d electrons on the stability of various types of charge/spin ordering and superconducting correlations in the spin-one-half Falicov-Kimball model with Hund and Hubbard coupling. The model is studied for a wide range of f and d-electron concentrations and it is found that the interband interactions U f d and J z stabilize three basic types of charge/spin ordering, and namely, (i) the axial striped phases, (ii) the regular n-molecular phases and (iii) the phase separated states. It is shown that the d-wave pairing correlations are enhanced within the axial striped and phase separated states, but not in the regular phases. Moreover, it was found that the antiferromagnetic spin arrangement within the chains further enhances the d-wave paring correlations, while the ferromagnetic one has a fully opposite effect.

Influence of Spin Ordering on Superconducting Correlations in the Spin-One-Half Falicov-Kimball Model with Hund and Hubbard Coupling

“…Numerical solutions obtained within so called restricted set phase diagram method [8,9] as wel as our well controlled gradient method [10,11] showed that this model is able to describe various types of charge and spin orderings observed experimentally in strongly correlated systems, including the diagonal and axial charge stripes with the antiferromagnetic or ferromagnetic arrangement of spins within the lines. Moreover, using the exact diagonalization calculations [11] on small clusters (L = 16) and the Projector Quantum-Monte-Carlo Method [7] on larger clusters (L ≤ 64), we have found that in the strong coupling U f d limit (U f d ≥ 4) the ground states of the model (1) found for U dd = 0 persist as ground states also for nonzero U dd , up to relatively large values (U c dd ∼ 3). This fact allows us to avoid the exhaustive numerical calculations on the full model Hamiltonian (1) and represent its ground states directly by a set of ground states of numerically much simpler single particle Hamiltonian (2), at least in the strong coupling U f d limit and U dd < U c dd .…”

“…The reasons for such a selection of N f values are following. The previous numerical results[11] obtained for the case N f = L/2 showed that the ground states of the model (2) in this case are mainly the segregated or axial striped charge phases.However, according to our very recent results[7] these configuration types enhance the d-wave pairing correlations in the d x 2 −y 2 channel of the full model Hamiltonian (1) and thus they are ideal candidates for the examination of effects of spin ordering on this charge induced superconducting state. In addition, it was found[11] that for both charge phases, the segregated one as well as the axial striped one, there are many different spin arrangements that minimize the ground state energy of the model at different d-electron fillings N d .…”

“…However, it should be noted that the potential V i is phenomenological and as such has no direct microscopic origin that corresponds to a degree of freedom in the actual materials. Contrary to this approach, we have presented very recently [7] an alternative model of coexistence of the charge/spin stripe order and superconductivity in the strongly correlated systems. Our approach is based on a generalized spin-one-half Falicov-Kimball model that besides the spin-independent U f d as well as spin-dependent J z Coulomb interaction between the localized f and itinerant d electrons takes into account the Hubbard interaction between d (f ) electrons of opposite spins.…”