Excited states and the thermodynamics of a fully frustrated quantum spin chain

A one-dimensional fully frustrated spin system called a {Δ} chain is considered and its excited states and the specific heat are investigated. As in the Heisenberg antiferromagnet on the kagome$iaa--- lattice, the classical ground state of the system has infinite continuous degeneracies. Numerical studies of finite systems show that the low-lying excited states have an almost dispersionless spectrum near a small energy gap. As a result the specific heat exhibits a two-peak structure, one of which arises from these low-lying excited states.