Complex-temperature Phase Diagrams of 1d Spin Models with Next-nearest-neighbor Couplings

We study the dependence of complex-temperature phase diagrams on details of the Hamiltonian, focusing on the effect of non-nearest-neighbor spin-spin couplings. For this purpose, we consider a simple exactly solvable model, the 1D Ising model with nearest-neighbor (NN) and next-to-nearest-neighbor (NNN) couplings. We work out the exact phase diagrams for various values of Jnnn/Jnn and compare these with the case of pure nearest-neighbor (NN) couplings. We also give some similar results for the 1D Potts model with NN and NNN couplings. PACS nos. 05.50.+q, 05.70.Fh, 64.60.-i, 64.60.Cn ∗email: [email protected] ∗∗email: [email protected]