Eur. Phys. J. B 7, 225-231 (1999)
https://doi.org/10.1007/s100510050608

Opening of a gap in an inhomogeneous external field

Physics Department, University of Wuppertal, 42097 Wuppertal, Germany

Received: 4 June 1998
Accepted: 21 August 1998
Published online: 15 January 1999

Abstract

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 . For the case of a longitudinal periodic field with wave vector and strength h_q 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 .