Regensburg 2002 – scientific programme
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DY: Dynamik und Statistische Physik
DY 20: Neuronale Netze / Spinmodelle
DY 20.4: Talk
Tuesday, March 12, 2002, 10:30–10:45, H2
Bond-disordered spin systems: Theory and application to doped high-Tc compounds — •Frank Krüger and Stefan Scheidl — Institut für Theoretische Physik, Universität zu Köln, Zülpicher Str. 77, D-50937 Köln, Germany
We examine the stability of magnetic order in a classical Heisenberg model with quenched random exchange couplings. This system represents the spin degrees of freedom in high-Tc compounds with immobile dopants. Starting from a replica representation of the nonlinear σ-model, we perform a renormalization-group analysis. The importance of cumulants of the disorder distribution to arbitrarily high orders necessitates a functional renormalization scheme. From the renormalization flow equations we determine the magnetic correlation length numerically as a function of the impurity concentration and of temperature. From our analysis follows that two-dimensional layers can be magnetically ordered for arbitrarily strong but sufficiently diluted defects. We further consider the dimensional crossover in a stack of weakly coupled layers. The resulting phase diagram is compared with experimental data for La2−xSrxCuO4.