Magnetic Field Effect in the 2D J-Q Model: Metamagnetism and Evidence for Deconfined Spinons*

We present a quantum Monte Carlo study of a 2D S=1/2 quantum magnet, the J-Q model, which combines an antiferromagnetic Heisenberg exchange J and a competing four-spin interaction Q, mimicking the behavior of frustrated systems in a sign-problem-free manner. We subject this model to an external magnetic field, building on our previous work in 1D [1]. We show metamagnetism (magnetization jumps to saturation) above a critical coupling ratio (Q/J)_min. At a different coupling ratio (Q/J)_c, the Néel order is destroyed and replaced with a long-range-ordered valence-bond solid. This transition is a deconfined quantum critical point described by spinons: fractionalized S=1/2 bosons [2]. At (Q/J)_c the specific heat of a field-induced Bose-Einstein condensate of spinons is expected to exhibit anomalous linear temperature dependence [3]. Our quantum Monte Carlo simulations demonstrate that this prediction holds above a lower temperature bound set by a field-induced BKT transition.
[1] A. Iaizzi, et al., Phys. Rev. B 95, 174436 (2017)
[2] H. Shao, et al., Science 352, 213 (2016)
[3] H. D. Scammell & O. P. Sushkov, Phys. Rev. Lett. 114 055702 (2015)