Phase Transitions

Yan Qin, Sheng-Chang Li, Ke Li +1 more

We propose a theoretical model to study the quantum phase transition in a double-cavity magnonic system. We find that the system exhibits a second-order phase transition from a parity-symmetric phase to a parity-symmetry-broken phase or a first-order phase transition from a parity-symmetric phase to a bistable phase when the driving strength of one cavity is above a critical value. We obtain the phase diagram, the critical point, and the corresponding critical exponent to characterize the phase transition. We can identify different phase transitions by the different behaviors of the mean magnon number and correlation fluctuation in the vicinity of the critical point. In particular, we show that the phase transition in one cavity can be precisely and efficiently controlled by adjusting the parameters of the other cavity, which suggests that we can easily observe the phase transition at low driving strength in experiment. The effects of additional microwave pulses on the dynamical behavior of the phase-transition observable and the experimental feasibility of our theoretical scheme are discussed as well.

Pedro D Sacramento

Wolfgang Nolting

Lokman Tsui, Yen-Ta Huang, Hong‐Chen Jiang +1 more

The study of continuous phase transitions triggered by spontaneous symmetry breaking has brought revolutionary ideas to physics. Recently, through the discovery of symmetry protected topological phases, it is realized that continuous quantum phase transition can also occur between states with the same symmetry but different topology. Here we study a specific class of such phase transitions in 1 + 1 dimensions – the phase transition between bosonic topological phases protected by Z n × Z n . We find in all cases the critical point possesses two gap opening relevant operators: one leads to a Landau-forbidden symmetry breaking phase transition and the other to the topological phase transition. We also obtained a constraint on the central charge for general phase transitions between symmetry protected bosonic topological phases in 1 + 1 D .

L. Bányai

Tsui, Lokman

The study of continuous phase transitions triggered by spontaneous symmetry breaking has brought revolutionary ideas to physics. Recently, through the discovery of symmetry protected topological phases, it is realized that continuous quantum phase transition can also occur between states with the same symmetry but different topology. The subject of this dissertation is the phase transition between these symmetry protected topological states (SPTs). There are two main parts in this dissertation.In the first part we consider spatial dimension $d$ and symmetry group $G$ so that the cohomology group, $H^{d+1}(G,U(1))$, contains at least one $Z_{2n}$ or $Z$ factor. We show that the phase transition between the trivial SPT and the root states that generate the $ Z_{2n} $ or $Z$ groups can be induced on the boundary of a d+1 dimensional $G\\times Z_2^T$-symmetric SPT by a $Z_2^T$ symmetry breaking field. Moreover we show these boundary phase transitions can be ``transplanted'' to d dimensions and realized in lattice models as a function of a tuning parameter. The price one pays is for the critical value of the tuning parameter there is an extra non-local (duality-like) symmetry. In the case where the phase transition is continuous, our theory predicts the presence of unusual (sometimes fractionalized) excitations corresponding to delocalized boundary excitations of the non-trivial SPT on one side of the transition. This theory also predicts other phase transition scenarios including first order transition and transition via an intermediate symmetry breaking phase. In the second part, we study the phase transition between bosonic topological phases protected by $Z_n\\times Z_n$ in 1+1 dimensions. We find a direct transition occurs when $ n=2,3,4 $ and in all cases the critical point possesses two gap opening relevant operators: one leads to a Landau-forbidden symmetry breaking phase transition and the other to the topological phase transition. We also obtained a constraint($ c\\geq1 $) on the central charge for general phase transitions between symmetry protected bosonic topological phases in 1+1D.