Anomalous multicritical phenomena and frustration induced by synthetic magnetic fields

We study the anomalous multicriticality and superradiant phase transition (SPT) of a one-dimensional Dicke lattice under broken time-reversal symmetry induced by synthetic magnetic fields [1]. We find that the universality class of SPT depends on the total magnetic flux, which divides the phase diagram into a standard SPT characterized by mean-field exponents and an anomalous SPT where non-mean-field exponents appear on both sides of the transition. In addition, correlations and fluctuations are bounded in the anomalous normal phase while divergent in the anomalous superradiant phase, leading to discontinuity despite it being a second-order phase transition. We attribute this non-mean-field behavior in the anomalous normal phase to the asymmetric dispersion relation due to time-reversal symmetry breaking. In the broken symmetry phase, we show that the complex nearest-neighbor hopping energy leads to long-range interactions in the mean-field, the competition of which induces varying degrees of frustration [2] in order parameters analogously to the J1-J2 Ising model. Finally, we find the resulting multicritical points can have two distinct critical exponents on both sides of the critical point. Our work demonstrates the interplay between broken time-reversal symmetry and frustration in bosonic lattice systems can lead to anomalous critical phenomena that cannot be found in fermionic or time-reversal symmetric quantum optical systems.

[1] Jinchen Zhao and Myung-Joong Hwang, arXiv:2208.02268.

[2] Jinchen Zhao and Myung-Joong Hwang, Phys. Rev. Lett. 128, 163601 (2022)