Disorder-enhanced delocalization and local-moment quenching in a disordered antiferromagnet

The interplay of disorder and spin-fluctuation effects in a disordered antiferromagnet is studied. In the weak-disorder regime (W<U), while the energy gap decreases rapidly with disorder, the sublattice magnetization, including quantum corrections, is found to remain essentially unchanged in the strong correlation limit. Magnon energies and Néel temperature are enhanced by disorder in this limit. A single paradigm of disorderenhanced delocalization qualitatively accounts for all these weak disorder effects. Vertex corrections and magnon damping, which appear only at order (W/U), are also studied. With increasing disorder a crossover is found at W;U , characterized by a rapid decrease in sublattice magnetization due to quenching of local moments, and formation of spin vacancies. The latter suggests a spin-dilution behavior that is indeed observed in softened magnon modes, lowering of Néel temperature, and enhanced transverse spin fluctuations. @S0163-1829~98!00137-4#