Opening of a gap in an inhomogeneous external field
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We study the one-dimensional spin-1/2 antiferromagnetic Heisenberg model exposed to an external field, which is a superposition of a homogeneous field h3 and a small periodic field of strength h1. For the case of a transverse staggered field a gap opens, which scales with \(\), where \(\) is given by the critical exponent \(\) defined through the transverse structure factor of the model at h 1 =0. For the case of a longitudinal periodic field with wave vector \(\) and strength hq a plateau is found in the magnetization curve at M=1/4. The difference of the upper- and lower magnetic field scales with \(\), where \(\) is given by the critical exponent \(\) defined through the longitudinal structure factor of the model at h q =0.
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