  Two-dimensional (2D) materials with a Lieb lattice can host exotic electronic
band structures. Such a system does not exist in nature, and it is also
difficult to obtain in the laboratory due to its structural instability. Here,
we experimentally realized a 2D system composed of a tin overlayer on an
aluminum substrate by molecular beam epitaxy. The specific arrangement of Sn
atoms on the Al(100) surface, which benefits from favorable interface
interactions, forms a stabilized buckled Lieb lattice. Our theoretical
calculations indicate a partially broken nodal line loop protected by its
mirror reflection symmetry and a topologically nontrivial insulating state with
a spin-orbital coupling (SOC) effect in the band structure of this Lieb
lattice. The electronic structure of this system has also been experimentally
characterized by scanning tunnelling spectroscopy and angle-resolved
photoemmision spectroscopy. Our work provides an appealing method for
constructing 2D quantum materials based on the Lieb lattice.
