Series expansions for variable electron density

Full text: We have developed a series expansion method for calculating the zero-temperature properties of lattice electron models for variable electron density, i.e. for finite doping away from the half-filled case. This is done by introducing particle fluctuation terms in the Hamiltonian, within perturbation theory. The method is demonstrated by application to the 2-chain t - J ladder. This model has been extensively studied [1-3], in connection with materials such as SrCu2O3, which has a spin-gap. There is evidence that some of these systems become superconducting upon doping. Our previous series calculations were restricted to half-filling and to 1 and 2-hole excitations at half-filling. Our new method allows us to explore the interesting region of finite doping. Results will be presented for the ground state energy, chemical potential, and spin excitations. Comparisons with previous work will be given