Direct numerical evidence of deconfined spinons using magnetic field effects

Adam Iaizzi^1*, Harley D Scammell², Oleg P Sushkov², Anders W Sandvik³

¹Physics, National Taiwan University, Taipei, Taiwan
²Physics, University of New South Wales, Sydney, New South Wales, Australia
³Physics, Boston University, Boston, MA, USA

* Presenter:Adam Iaizzi, email:iaizzi@bu.edu

We study a 2D S=1/2 quantum antiferromagnet (the J-Q model) in the presence of an external magnetic field using quantum Monte Carlo methods. The J-Q model combines the standard Heisenberg exchange (J) with a four-spin interaction (Q) that drives a quantum phase transition from the O(3) Néel state to the valence-bond solid (a nonmagnetic state breaking Z₄ lattice symmetry). This transition is believed to be an example of deconfined quantum criticality, where the critical point is described by exotic fractionalized excitations called spinons (S=1/2 bosons) [1]. We present direct evidence for the presence of these fractionalized excitations. Using a magnetic field we induce a finite ground-state density of magnetic excitations at the critical point and measure energy as a function of field and temperature. Expanding on previous work [2], we include an extra U(1) gauge field in our analysis, resulting in new predictions for the behavior of both a BEC and gas of deconfined spinons. We compare these predictions to numerical results. At low temperatures, we find behavior consistent with a Bose-Einstein condensate of deconfined spinons. At higher temperatures we find an anomalous temperature dependence that can only be explained by a gas of deconfined spinons.
[1] H Shao, W Guo & AW Sandvik, Science 352, 213 (2016)
[2] HD Scammell & OP Sushkov, Phys. Rev. Lett. 114 055702 (2015)

Keywords: quantum phase transitions, spinons, deconfined quantum criticality