Professor Anders W. Sandvik
Physics Department
Boston University


"Possible deconfined quantum-critical point in a 2D Heisenberg model with four-spin couplings"

Phase transitions driven by quantum fluctuations in 2D antiferromagnets have formed a central topic in condensed matter physics since the discovery of high-Tc superconductivity in layered cuprates. While spin-liquid and valence-bond-solid (VBS) states have been the subjects of numerous theoretical studies, large-scale numerical simulations have been lacking because of Monte Carlo sign problems affecting the frustrated spin models in which these ground states and phase transitions should exist. Here I will show that a transition from the Neel state to a VBS state also occurs in a 2D Heisenberg model including a particular type of four-spin interaction which is not frustrated in the standard sense, and therefore can be studied on large lattices using Monte Carlo methods. There is compelling evidence that the Neel-VBS transition in this model is the first example of the recently proposed, but still controversial, "deconfined" quantum critical point: The transition appears to be continuous with dynamic exponent z=1, the critical correlation function exponent is anomalously large, and an emergent U(1) symmetry can be seen explicitly in the VBS order-parameter fluctuations. The model thus offers opportunities for detailed unbiased studies of aspects of quantum-criticality falling outside the standard Landau-Ginzburg-Wilson framework.