Quantum Monte Carlo Simulations for Stacked Spin Ladder Systems Containing Low Concentrations of Non-Magnetic Impurities; Application to the Low Temperature Broadening of NMR-Spectra in SrCu2O3

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.

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