Real-time dynamics in spin-1/2 chains with adaptive time-dependent DMRG

We investigate the influence of different interaction strengths and dimerizations on the magnetization transport in antiferromagnetic spin 1/2 XXZ chains. We focus on the real-time evolution of the inhomogeneous initial state vertical bar up arrow...up arrow down arrow...down arrow > in using the adaptive time-dependent density-matrix renormalization group (adaptive t-DMRG). Time scales accessible to us are of the order of 100 units of time measured in (h) over bar /J for almost negligible error in the observables. We find ballistic magnetization transport for small (SSz)-S-z interaction and arbitrary dimerization, but almost no transport for stronger (SSz)-S-z interaction, with a sharp crossover at J(z)=1. Additionally, we perform a detailed analysis of the error made by the adaptive time-dependent DMRG using the fact that the evolution in the XX model is known exactly. We find that the error at small times is dominated by the error made by the Trotter decomposition, whereas for longer times the DMRG truncation error becomes the most important, with a very sharp crossover at some "runaway" time. Overall, errors are extremely small before the "runaway" time.