LAUSR

We classify interacting topological insulators and superconductors with order-two crystal symmetries (reflection and twofold rotation), focusing on the case where interactions reduce the noninteracting classification. We find that the free-fermion $\mathbb{Z}_2$ classifications are stable against quartic contact interactions, whereas the $\mathbb{Z}$ classifications reduce to $\mathbb{Z}_N$, where $N$ depends on the symmetry class and the dimension $d$. These results are derived using a quantum nonlinear $\sigma$ model (QNLSM) that describes the effects of the quartic interactions on the boundary modes of the crystalline topological phases. We use Clifford algebra extensions to derive the target spaces of these QNLSMs in a unified way. The reduction pattern of the free-fermion classification then follows from the presence or absence of topological terms in the QNLSMs, which is determined by the homotopy group of the target spaces. We show that this derivation can be performed using either a complex fermion or a real Majorana representation of the crystalline topological phases and demonstrate that these two representations give consistent results. To illustrate the breakdown of the noninteracting classification we present examples of crystalline topological insulators and superconductors in dimensions one, two, and three, whose surfaces modes are unstable against interactions. For the three-dimensional example, we show that the reduction pattern obtained by the QNLSM method agrees with the one inferred from the stability analysis of the boundary modes using bosonization.