Observation of a flat and extended surface state in a topological semimetal

A topological flatband, also known as drumhead states, is an ideal platform to drive new exotic topological quantum phases. Using angle-resolved photoemission spectroscopy experiments, we reveal the emergence of a highly localized possible drumhead surface state in a topological semimetal BaAl4 and provide its full energy and momentum space topology. We find that the observed surface state is highly localized in momentum, inside a square-shaped bulk Dirac nodal loop, and in energy, leading to a flat band and a peak in the density of state. These results establish this class of materials as a possible experimental realization of drumhead surface states and provide an important reference for future studies of fundamental physics of topological quantum phase transition.