Intra-unit-cell singlet pairing mediated by altermagnetic fluctuations

Abstract

We investigate the superconducting instabilities induced by altermagnetic fluctuations. Because of the non-trivial 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 non-trivial topology, such as protected Bogoliubov Fermi surfaces and higher-order topological superconductivity. Our work establishes the key role played by sublattice degrees of freedom in altermagnetic-fluctuation mediated interactions.

Figures (3)

• Figure 1: (a) Minimal AM model on the Lieb lattice antonenko2024mirrorchernbandsweyl. The unit cell has two atomic positions ($A$ and $B$) related by a $90^\circ$ rotation, where up spins (red) and down spins (blue) form a spatially staggered configuration. (b) Schematic phase diagram of a putative AM QCP with tuning parameter $r$. Fluctuations are large when $r \gtrsim J$. (c) Phase diagram of the leading pairing instabilities in the chemical potential ($\mu$) - $r$ plane. The corresponding pairing configurations are shown in Fig. \ref{['fig:phase']}, and representative Fermi surfaces for each $\mu$ range are shown in the insets where the color gives the sublattice-projected spectral weight. The parameters used were $t=1$, $t_a'=-0.3$, and $t'_b=0.2$.
• Figure 2: Real space configuration of the distinct pairing channels shown in Fig. \ref{['fig:summary']}(c). The primes denote intra-unit-cell pairings whose Cooper pairs are formed by electrons from different sublattices.
• Figure 3: (a) BdG spectrum of the $d'$-state at the BZ boundary, exhibiting Dirac nodes (without AM order, left) and Bogoliubov Fermi surfaces (with AM order, right). (b) Eigenvalues (inset) and the $n=0$ wavefunction obtained by diagonalizing the BdG Hamiltonian of the $p_B$-phase in the presence of AM order $N=0.3$, showcasing the Majorana corner modes. Here, $n$ labels the eigenvalues of the finite-size system.