Two Hole Dynamics in Spin Ladders

We present an analytic theory for the energy spectrum of a two leg spin ladder doped with two holes. Starting from a pseudo fermion bond boson representation of the corresponding t 1,2 J 1,2 Hamiltonian we apply a diagrammtic approach adapted to the limit of strong rung coupling, which includes both, the coupling of holes to the spin background as well as the two hole interactions. The two hole spectrum is calculated and the formation of bound states is discussed. Additionally the evolution of the spin gap of the ladder upon doping is analyzed. A comparison with existing exact diagonalization data is presented and good agreement is found.