NUMERICAL STUDY OF A MODEL FOR NENP - ONE-DIMENSIONAL S=1 ANTIFERROMAGNET IN A STAGGERED FIELD

In order to explain some mysteries in ESR experiments on NENP, we study the S = 1 antiferromagnetic chain with a transverse staggered magnetic field. The staggered field is caused by the crystal structure of NENP and makes the ESR absorption possible from the ground state to the lowest excited state with k = pi. In addition it removes the phase transition, which was predicted for the Heisenberg model without the staggered field. Numerical diagonalization study on this model qualitatively agrees with the NMR, ESR and heat capacity measurements. In particular, it explains well the field dependence of the energy gap, the intensity of ESR and the staggered moment. It is also found that the standard theory of ESR is valid above the ''critical field'' corresponding to a minimum of the energy gap, if the sublattice moments are suitably reinterpreted.