Visualization of Unconventional Rashba Bands and Vortex\nZero Mode in the Topological Superconductor Candidate AuSn<sub>4</sub>

The\nnoble metal alloy AuSn<sub>4</sub> has recently been identified\nas an intrinsic surface topological superconductor, promisingly hosting\nthe Majorana zero mode (MZM) for topological quantum computing. However,\nthe atomic visualization of its nontrivial surface states and MZM\nremains elusive. Here, we report the direct observation of unconventional\nsurface states and vortex zero mode in AuSn<sub>4</sub> by scanning\ntunneling microscopy/spectroscopy. Unlike the trivial metallic bulk\nstates of Sn-terminated surfaces, the Au-terminated surfaces exhibit\npronounced surface states near the Fermi level, arising from unconventional\nRashba bands characterized by shared helical spin textures. In the\nsuperconducting state, the Sn-terminated surfaces exhibit conventional\nCaroli–de Gennes–Matricon bound states, while the Au-terminated\nsurfaces display sharp zero-energy core states resembling MZMs in\na nonquantum-limit condition. This distinction may result from the\ndominant contribution of unconventional Rashba bands on the Au-terminated\nsurface. Our results provide a new platform for studying termination-dependent\ntopological surface states and MZM in noble-metal-based superconductors.