Yang-Lee Circle Theorem for an Antiferromagnetic Heisenberg Ladder

The Yang-Lee zeros of an antiferromagnetic Heisenberg ladder model are determined. It is found that if J₄ ≤ 0 Yang-Lee zeros are located on the unit circle and on the negative real axis in the complex activity plane. In particular, if J₄ ≤ 0 and 2J₂ ≥ J₄, Yang-Lee zeros are located on the unit circle and Yang-Lee circle theorem is valid. If J₄ > 0, Yang Lee zeros are locted on some complicated curves.