FIRST-ORDER TRANSITION IN THE FRUSTRATED ANTIFERROMAGNETIC HEISENBERGS=1 QUANTUM SPIN CHAIN

We study a frustrated Heisenberg S = 1 quantum spin chain with next-ne arest neighbor (NNN) coupling alpha using a variational ansatz and the density matrix renormalization group. We find as quantum remnants of the phase transition in the classical chain a disorder point at alpha( D) = 0.284(1) and a Lifshitz point at alpha(L) = 0.3725(25). Our main finding is a first-order transition from an Affleck-Kennedy-Lieb-Tasak i (AKLT) phase to a NNN generalization of the AKLT phase at alpha(T) = 0.7443(6). At the transition, string order jumps discontinuously by a pproximate to 0.085 to 0; correlation length and gap are finite.