Charge-order transition in the extended Hubbard model on a two-leg ladder

We investigate the charge-order transition at zero temperature in a two-leg Hubbard ladder with additional nearest-neighbor Coulomb repulsion V using the density matrix renormalization group technique. We consider electron de nsities between quarter and half filling. For quarter filling and U = 8t, w e find evidence for a continuous phase transition between a homogeneous sta te at small V and a broken-symmetry state with "checkerboard" [wave vector Q = (pi, pi)] charge order at large V. This transition to a checkerboard ch arge-ordered state remains present at all larger fillings, but becomes disc ontinuous at sufficiently large filling. We discuss the influence of U/t on the transition and estimate the position of the tricritical points. [S0163 -1829(99)52236-4].