Influence of oxygen orbitals and boundary conditions on the pairing behavior in the Emery model for doped ladders

We investigate the Emery model on several ladder-like lattices including two legs of copper <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mi>d</a:mi></a:math> orbitals and various numbers of oxygen <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mi>p</b:mi></b:math> orbitals. Pair binding energy, pair spatial structure, density distribution, and pairing correlation functions are calculated using the density-matrix renormalization group (DMRG). We show that a Luther-Emery phase with enhanced pairing correlations can be found for hole doping as well as for electron doping with realistic model parameters. Ladder properties depend sensitively on model parameters, the oxygen <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mi>p</c:mi></c:math> orbitals taken into account, and boundary conditions. The pair binding energy is a more reliable quantity than correlation functions for ascertaining the occurrence of pairing in ladders. Overall, our results for two-leg Emery ladders support the possibility of superconductivity in the hole-doped 2D model. The issue is rather to determine which of the various ladder structures and model parameters are appropriate to approximate the two-dimensional cuprates.