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

摘要

We present a Quantum Monte Carlo study for Heisenberg spin-1/2 two leg laddersystems doped with non magnetic impurities. The simulations are applied to thedoped spin ladder compound SrCu2O3 doped with Zn, where a large broadening ofthe 65Cu NMR lines has been observed in experiment at low temperatures but farabove the Neel temperature. We find that interladder-couplings with a sizeablecoupling in stacking direction are required to describe the line broadening,which can not be explained by considering a single ladder only. Around a singleimpurity, spin correlations cause an exponentially decaying antiferromagneticlocal magnetization in a magnetic field. We develop an effective model for thelocal magnetization of systems with many randomly distributed impurities, withfew parameters which can be extracted out of QMC calculations with a singleimpurity. The broadening arises from a drag effect, where the magnetizationaround an impurity works as an effective field for spins on the neighboringladders, causing a non-exponentially decaying magnetization cloud around theimpurity. 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 quantitativeagreement with experiment. We also develop a simple model for the effectiveinteraction of two impurity spins.