Disorder Driven Suppression of Topological Flat Bands on a Cuprate Side Surface

Theory predicts that the chiral symmetry of singlet d-wave superconductors endows nodal quasiparticles with a nontrivial winding number, resulting in zero-energy flat-band topological surface states on most side surfaces. Such states, expected to show distinctive momentum dependence and potentially drive new superconducting or magnetic phases, have long been inferred from zero-bias tunnelling peaks in cuprates, but no momentum-resolved measurement of any cuprate side surface has previously been reported.

Here we overcome this long-standing experimental barrier by using focused ion-beam milling to cleave flat (110) side surfaces of the high‐temperature superconductor La$_{1-x}$Sr$_{x}$CuO$_{4}$ (LSCO) with $x = 0.22$ and probe them with angle-resolved photoelectron spectroscopy, directly revealing their momentum-resolved electronic structure. Surprisingly, however, we do not observe a clear signature of the expected flat band, despite relatively low surface disorder measured by atomic force microscopy. Tight-binding calculations indicate that bulk disorder—a common feature of cuprate superconductors—with a magnitude comparable to the superconducting gap can strongly suppress these surface states.

Our work provides the first momentum-resolved view of a cuprate side surface and shows that topological surface states in unconventional superconductors are more susceptible to disorder than may naively be expected. Cleaner samples may therefore enable the observation of topological flat-band dispersions in the cuprates.