Deconfined quantum criticality in the two-dimensional antiferromagnetic Heisenberg model with next-nearest-neighbor Ising exchange

Abstract

We have considered the $S=1∕2$ antiferromagnetic Heisenberg model in two dimensions, with an additional Ising next-nearest-neighbor interaction. Antiferromagnetic next-nearest-neighbor interactions will lead to frustration, and the system responds with flipping the spins down in the $xy$ plane. For large next-nearest-neighbor coupling the system will order in a striped phase along the $z$ axis, this phase is reached through a first-order transition. We have considered two generalizations of this model, one with random next-nearest-neighbor interactions, and one with an enlarged unit cell, where only half of the atoms have next-nearest-neighbor interactions. In both cases the transition is softened to a second-order transition separating two ordered states. In the latter case we have estimated the quantum critical exponent $\ensuremath{\beta}\ensuremath{\approx}0.25$. These two cases then represent candidate examples of deconfined quantum criticality.