Stripe formation in doped Hubbard ladders

We investigate the formation of stripes in $7r\ifmmode\times\else\texttimes\fi{}6$ Hubbard ladders with $4r$ holes doped away from half filling using the density-matrix renormalization group (DMRG) method. A parallelized code allows us to keep enough density-matrix eigenstates (up to $m=8000$) and to study sufficiently large systems (with up to $7r=21$ rungs) to extrapolate the stripe amplitude to the limits of vanishing DMRG truncation error and infinitely long ladders. Our work gives strong evidence that stripes exist in the ground state for strong coupling $(U=12t)$ but that the structures found in the hole density at weaker coupling $(U=3t)$ are an artifact of the DMRG approach.