2016/03/21 by Bimla Danu, Gautam Nambiar, R. Ganesh · 1 citation
Physics and Astronomy · #cond-mat.str-el #cond-mat.stat-mech
paper · pdf · doi:10.1103/physrevb.94.094438
published as Phys. Rev. B 94, 094438 (2016) · 10 pages
arxiv created 2016/03/21 · arxiv updated 2016/10/05
The square lattice antiferromagnet with frustrating next nearest neighbour coupling continues to generate tremendous interest, with an elusive quantum disordered phase in the vicinity of J2 = J1/2. At this precise value of frustration, the classical model has a very large degeneracy which makes the problem difficult to handle. We show that introducing a ferromagnetic J3 coupling partially lifts this degeneracy. It gives rise to a four-site magnetic unit cell with the constraint that the spins on every square must add to zero. This leads to a two-parameter family of ground states and an emergent vector order parameter. We reinterpret this family of ground states as coexistence states of three spirals. Using spin wave analysis, we show that thermal and quantum fluctuations break this degeneracy differently. Thermal fluctuations break it down to a threefold degeneracy with a Néel phase and two stripe phases. This threefold symmetry is restored via a Z3 thermal transition, as we demonstrate using classical Monte Carlo simulations. On the other hand, quantum fluctuations select the Néel state. In the extreme quantum limit of spin-1/2, we use exact diagonalization to demonstrate Néel ordering beyond a critical J3 coupling. For weak J3, a variational approach suggests an s-wave plaquette-RVB state. Away from the J2 = J1/2 line, we show that quantum fluctuations favour Néel ordering strongly enough to stabilize it within the classical stripe region. Our results shed light on the origin of the quantum disordered phase in the J1-J2 model.