Mirror-selective quasiparticle interference in bilayer nickelate superconductor

The recent discovery of high-temperature superconductivity in bilayer nickelates has attracted widespread interest, yet the electronic structure and pairing symmetry essential for superconductivity remain debated. Here, inspired by recent STM experiments on thin films, we investigate the quasiparticle interference (QPI) of bilayer nickelates in both normal and superconducting states. We demonstrate that the mirror symmetry of the bilayer structure induces mirror-selective quasiparticle scattering through selection rules governed by the mirror properties of impurities and the mirror eigenvalues of electronic wavefunctions. In the normal state, this selectivity enables differentiation of distinct Fermiologies, as QPI patterns differ markedly between scenarios with and without the \({d}_{{z}^{2}}\) -bonding Fermi surface (FS). In the superconducting state, it allows separate detection of gap sign changes within and between different FSs. Crucially, neglecting the mirror-symmetry selection rules causes the QPI of an s ± -wave state to mimic that of a conventional s -wave state, leading to misidentification of the pairing symmetry. Combined with field-dependent and phase-reference QPI measurements, this approach enables robust determination of pairing symmetry even in the presence of FS-dependent gaps and gap anisotropy. Our findings establish mirror-selective QPI as a powerful tool for identifying Fermiology and pairing symmetry in bilayer nickelate superconductors.