Yang-Lee circle theorem for an antiferromagnetic Heisenberg ladder

The Yang-Lee zeros of an antiferromagnetic Heisenberg ladder model are dete rmined. It is found that if J(4) less than or equal to 0 Yang-Lee zeros are located on the unit circle and on the negative real axis in the complex ac tivity plane. In particular, if J(4) less than or equal to 0 and 2J(2) grea ter than or equal to J(4), Yang-Lee zeros are located on the unit circle an d the Yang-Lee circle theorem is valid. If J(4) > 0, Yang-Lee zeros are loc ated on some complicated curves.