PHASE STRING EFFECT IN A DOPED ANTIFERROMAGNET

Based on the t-J model, it is shown that a hole moving on an antiferro magnetic spin background always induces a phase-string effect. Such a previously unnoticed phase string is revealed by explicitly tracking t he Marshall sign and can be rigorously shown to be nonrepairable at lo w energy. Its quantum interference can drastically modify the long-wav elength behavior of the doped hole, leading to a vanishing spectral we ight Z at the ground-state energy. Implication for finite doping is al so discussed.