Spin ordering and dynamics in percolating quantum magnets: a model system for superconductors and magnetic semiconductors

Joel E. Moore

University of California, Berkeley

Percolation models with quantum degrees of freedom have been used to model both lattice-scale randomness in dopant distribution and larger-scale randomness that emerges from phase competition in complex materials. This talk reviews recent theoretical and experimental work on 2D percolating magnets and examines how physical properties near the 2D percolation threshold become determined by the fractal structure at criticality. Theoretical methods that have been applied to this problem include quantum Monte Carlo, spin-wave expansion, and real-space renormalization group. Comparisons are made to experimental data of Greven et al. on (Zn,Mg) diluted La2CuO4: the primary experimental consequences include magnetic correlation lengths at finite temperature that are extremely short compared to the undiluted case. The final portion of the talk discusses experimental methods for spin dynamics in complex materials, concentrating on the recent observation of spin drag in semiconductor quantum wells by Orenstein et al.