Termination-dependent topological surface states in nodal-loop semimetal HfP2

Symmetry plays a major role in all disciplines of physics. Within the field of topological materials there is a great interest in understanding how the mechanics of crystalline and internal symmetries protect crossings between the conduction and valence bands. Additionally, exploring this direction can lead to a deeper understanding on the topological properties of crystals hosting a variety of symmetries. We report the experimental observation of topological surface states in the nodal-loop semimetal ${\mathrm{HfP}}_{2}$ using angle-resolved photoemission spectroscopy which is supported by our first-principles calculations. Our paper shows termination-dependent surface states in this compound. Our experimental data reveal surface states linked to three unique nodal loops confirmed by theoretical calculation to be topologically nontrivial. This paper demonstrates that transition-metal dipnictides provide a good platform to study nontrivial topological states protected by nonsymmorphic symmetry.