We analyze the gaps in the excitation spectrum of a Haldane chain with single-ion anisotropy in a staggered field. We show that the gap along the direction of the field increases at a faster rate than the others, while its spectral weight decreases, being transferred to a two-magnon continuum. In recent times the preparation of a new class of magnetic materials of the general formula (RxY1−x)2BaNiO5, where R is one of the magnetic rare-earth ions, has arisen a new theoretical interest for the study of integer-spin Haldane chains in a staggered magnetic field. The reference compound Y2BaNiO5  is one of the best examples of a magnetic material with a singlet ground state and a spin gap which has been interpreted in the framework of the Haldane conjecture , according to which integer-spin chains have disordered ground state separated by a gap from a degenerate triplet of excitations. In all these materials , the Ni ions are arranged along linear chains (the a-axis of thei
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