Traditionally, in three dimensions, the only symmetries essential for superconductivity are time reversal ($\mathcal{T}$) and inversion ($\mathcal{I}$). Here, we examine superconductivity in two dimensions and find that $\mathcal{T}$ and $\mathcal{I}$ are not required, and having a combination of either symmetry with a mirror operation ($M_z$) on the basal plane is sufficient. By combining energetic and topological arguments, we classify superconducting states when $\mathcal{T}$ and $\mathcal{I}$ are not present, a situation encountered in several experimentally relevant systems, such as transition metal dichalcogenides or a two-dimensional Rashba system, when subject to an applied field, and in superconducting monolayer FeSe with Néel antiferromagnetic order. Energetic arguments suggest interesting superconducting states arise. For example, we find a unique pure intraband pairing state with Majorana chiral edge states in Néel-ordered FeSe. Employing topological arguments, we find when the only symmetry is the combination of $\mathcal{I}$ with $M_z$, the superconducting states are generically fully gapped and can have topologically protected chiral Majorana edge modes. In all other cases, there are no chiral Majorana edge states, but the superconducting bulk can have point nodes with associated topologically protected flatband Majorana edge modes. Our analysis provides guidance on the design and search for novel two-dimensional superconductors and superconducting heterostructures.

• Received 17 March 2018
• Revised 18 June 2018

DOI:https://doi.org/10.1103/PhysRevLett.121.157003