We study the zero-temperature phase diagram of the frustrated square-latticeS=1/2 antiferromagnet in an external magnetic field numerically with theLanczos algorithm. For strong frustration, we find disordered phases at high(and low) magnetic fields. Between these two disordered phases we find aplateau in the magnetization curve at half of the saturation magnetizationwhich corresponds to a state with up-up-up-down (uuud) spin order. This andother considerations [cond-mat/0003343] suggest an unusual ordering scenario:There are an ordered phase with a spin gap (the plateau) and disorderedmagnetically gapless phases above and below. The transition to saturation is studied in further detail and problematicconclusions in earlier investigations of this region are pointed out.
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