Higher-order topological superconductors in ${\mathcal P}$-, ${\mathcal T}$-odd quadrupolar Dirac materials

Abstract: The presence or absence of certain symmetries in the normal state (NS) also determines the symmetry of the Cooper pairs. Here we show that parity (${\mathcal P}$) and time-reversal (${\mathcal T}$) odd Dirac insulators (trivial or topological) or metals, sustain a local or intra-unit cell pairing that supports corner (in $d=2$) or hinge (in $d=3$) modes of Majorana fermions and stands as a higher-order topological superconductor (HOTSC), when the NS additionally breaks discrete four-fold ($C_4$) symmetry. Although these outcomes does not rely on the existence of a Fermi surface, around it (when the system is doped) the HOTSC takes the form of a mixed parity, ${\mathcal T}$-odd (due to the lack of ${\mathcal P}$ and ${\mathcal T}$ in the NS, respectively) $p+id$ pairing, where the $p$($d$)-wave component stems from the Dirac nature of quasiparticles (lack of $C_4$ symmetry) in the NS. Thus, when strained, magnetically ordered Dirac materials, such as doped magnetic topological insulators (MnBi$_2$Te$_4$), can harbor HOTSCs, while the absence of an external strain should be conducive for the axionic $p+is$ pairing.