Dimer versus twist order in theJ1-J2model

Abstract

We diagonalize fully the 4×4 S=1/2 Heisenberg model with nearest-neighbor (${\mathit{J}}_1$) and second-neighbor (${\mathit{J}}_2$) exchanges for several values of ${\mathit{J}}_2$/${\mathit{J}}_1$. For ${\mathit{J}}_2$/${\mathit{J}}_1$≊1/2 the specific heat shows a peak near T=0.27${\mathit{J}}_1$, and the dimer order increases sharply around that temperature. In contrast, the twist order considered earlier by Dagotto and Moreo drops below that temperature, and its T=0 value is roughly the same as that at T=∞. This shows that while the system may develop long-range dimer order (and it certainly has appreciable short-range dimer order) for these values of ${\mathit{J}}_2$/${\mathit{J}}_1$, there is no indication of enhanced twist order.