- 2250 Elings Hall
Title: New multicritical point in the frustrated spin-1 chain
Spin-1 antiferromagnetic spin chain has a venerable history and has been thought to be well understood. Here we show that inclusion of both next nearest neighbor (alpha) and biquadratic (beta) interactions results in a rich phase diagram with a multicritical point that has not been observed before. We study the problem using a combination of the density matrix renormalization group (DMRG), an analytic variational matrix product state wavefunction, and conformal field theory. For negative beta < beta*, we establish the existence of a spontaneously dimerized phase, separated from the Haldane phase by the critical line alpha_c(beta) of second-order phase transitions. In the opposite regime, beta > beta*, the transition from the Haldane phase becomes first-order into the next nearest neighbor (NNN) AKLT phase. Based on field-theoretical arguments and DMRG calculations, we find that these two regimes are separated by a multicritical point (beta*, alpha*) of a different universality class, described by the level-4 SU(2) Wess??"Zumino??"Witten critical theory. Furthermore, we find that the dimerized and NNN-AKLT phases are separated from each other by a line of first-order phase transitions that terminates at the multicritical point. We establish that transitions out of Haldane phase into dimer or NNN-AKLT phase are topological in nature and occur either with or without closing of the bulk gap, respectively. We also study short-range incommensurate-to-commensurate transitions in the resulting phase diagram.