Signatures of Deconfined Quantum Criticality in the 2D J-Q-h Model

Abstract

Deconfined quantum criticality is a type of quantum critical behavior characterized by the presence of exotic fractionalized excitations. The transition between the Neel antiferromagnet and the valence-bond solid in the J-Q model is believed to be an example of such a deconfined quantum critical point where the excitations are spinons—bosons carrying spin-1/2. Using a magnetic field, one can induce a finite ground state density of magnetic excitations, which can then form a Bose–Einstein condensate (BEC) at low temperature. Previous work has predicted that a BEC of spinons would have an anomalous temperature dependence due to the presence of a gapless quadratic mode, providing a way to distinguish deconfined spinons from conventional (nonfractional) magnons (Scammell and Sushkov, Phys Rev Lett 114:055702, 2015). We study this possibility using quantum Monte Carlo methods. We do not find the predicted anomalous temperature dependence in the spinon BEC. At higher temperatures, the spinons are in a gaseous rather than BEC phase; this spinon gas should also have a gapless quadratic mode. In this deconfined spinon gas we do detect the expected anomalous temperature dependence, providing direct evidence for the existence of deconfined spinons. The introduction of a field also admits a phase transition of the Berezinskii–Kosterlitz–Thouless (BKT) type, which separates the spinon BEC and spinon gas phase. We estimate TBKT(h) and also show that this transition produces a non-monotonic temperature dependence in the magnetization.