ONE-DIMENSIONAL FRUSTRATED HEISENBERG-ANTIFERROMAGNET OF SPIN 1 2 IN A STRONG MAGNETIC-FIELD/

The properties of the one-dimensional antiferromagnet of spin 1/2 in a strong magnetic field h with nearest- and next-nearest-neighbor inter actions are studied in terms of the equivalent Bose-gas problem. We fi nd that below the first critical field h(cu) a phase with a gap in the elementary excitation spectrum and corresponding plateau in the magne tization curve should exist due to the two-particle attraction in the vicinity of the commensurate wave number p0u = pi. In the interval bet ween the first and second critical fields h(cu) < h < h(cf), the syste m behaves as the one-dimensional Bose gas with particle repulsion and the magnetization has a typical square-root dependence on field.