Bosonic analog of a topological Dirac semimetal: Effective theory, neighboring phases, and wire construction

We construct a bosonic analog of a two-dimensional topological Dirac semimetal (DSM). The low-energy description of the most basic 2D DSM model consists of two Dirac cones at positions $\pm {\mathbf{k}}_0$ in momentum space. The local stability of the Dirac cones is guaranteed by a composite symmetry $Z_2^{\mathcal{TI}}$, where $\mathcal{T}$ is time reversal and $\mathcal{I}$ is inversion. This model also exhibits interesting time-reversal and inversion symmetry breaking electromagnetic responses. In this work we construct a bosonic version by replacing each Dirac cone with a copy of the $O\left(4\right)$ nonlinear sigma model (NLSM) with topological theta term and theta angle $\theta =\pm \pi$. One copy of this NLSM also describes the gapless surface termination of the 3D bosonic topological insulator (BTI). We compute the time-reversal and inversion symmetry breaking electromagnetic responses for our model and show that they are twice the value one gets in the DSM case matching what one might expect from, for example, a bosonic Chern insulator. We also investigate the stability of the BSM model and find that the composite $Z_2^{\mathcal{TI}}$ symmetry again plays an important role. Along the way we clarify many aspects of the surface theory of the BTI including the electromagnetic response, the charges and statistics of vortex excitations, and the stability to symmetry-allowed perturbations. We briefly comment on the relation between the various descriptions of the $O\left(4\right)$ NLSM with $\theta =\pi$ used in this paper (a dual vortex description and a description in terms of four massless fermions) and the recently proposed dual description of the BTI surface in terms of $2+1$-dimensional quantum electrodynamics with two flavors of fermion $\left(N=2 {\text{QED}}_3\right)$. In a set of four Appendixes we review some of the tools used in the paper and also derive some of the more technical results.