Competing Bond Alternations in Spin-1/2 QuantumXYChain

Abstract

We propose an exactly solvable model which has numerous low-lying excited states and the dispersionless zero energy mode, by introducing the competing bond alternations into the spin-1/2 quantum X Y chain. We clarify the mechanism of the double peak in the temperature dependence of the specific heat and the residual entropy as follows; (1) Numerous low-lying excited states generate the double peak of the specific heat. (2) As an exception, only at the most competing points the double peak in the specific heat disappeares but the macroscopic residual entropy arises in the ground state. (3) If the competition is not so strong, the specific heat has a single peak.