Intra-Unit-Cell Singlet Pairing Mediated by Altermagnetic Fluctuations

We investigate the superconducting instabilities induced by altermagnetic fluctuations. Because of the nontrivial sublattice structure of the altermagnetic order, shorter-range and longer-range fluctuations favor qualitatively different types of pairing states. Specifically, while the latter stabilize a standard spin-triplet p-wave state, just like ferromagnetic fluctuations, the former leads to intra-unit-cell pairing, in which the Cooper pairs are formed by electrons from different sublattices. The symmetry of the intra-unit-cell gap function can be not only p-wave, but also spin-singlet s-wave and d-wave, depending on the shape of the Fermi surface. We also show that coexistence with altermagnetic order promotes intrinsic nontrivial topology, such as protected Bogoliubov Fermi surfaces and higher-order topological superconductivity. Our Letter establishes the key role played by sublattice degrees of freedom in altermagnetic-fluctuation mediated interactions.