EXCITED-STATES AND THE THERMODYNAMICS OF A FULLY FRUSTRATED QUANTUM SPIN CHAIN

A one-dimensional fully frustrated spin system called a DELTA chain is considered and its excited states and the specific heat are investigated. As in the Heisenberg antiferromagnet on the kagome 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.