  We present a Quantum Monte Carlo study for Heisenberg spin-1/2 two leg ladder
systems doped with non magnetic impurities. The simulations are applied to the
doped spin ladder compound SrCu2O3 doped with Zn, where a large broadening of
the 65Cu NMR lines has been observed in experiment at low temperatures but far
above the Neel temperature. We find that interladder-couplings with a sizeable
coupling in stacking direction are required to describe the line broadening,
which can not be explained by considering a single ladder only. Around a single
impurity, spin correlations cause an exponentially decaying antiferromagnetic
local magnetization in a magnetic field. We develop an effective model for the
local magnetization of systems with many randomly distributed impurities, with
few parameters which can be extracted out of QMC calculations with a single
impurity. The broadening arises from a drag effect, where the magnetization
around an impurity works as an effective field for spins on the neighboring
ladders, causing a non-exponentially decaying magnetization cloud around the
impurity. Our results show that even for impurity concentrations as small as x
= 0.001 and x = 0.0025, the broadening effect is large, in good quantitative
agreement with experiment. We also develop a simple model for the effective
interaction of two impurity spins.
