Topological Lifshitz transition in Weyl semimetal NbP decorated with heavy elements

Studies of the Fermi surface modification after in situ covering NbP semimetal with heavy elements Pb and Nb ultrathin layers were performed by means of angle-resolved photoemission spectroscopy (ARPES). First, the electronic structure was investigated for pristine single crystals with two possible terminations (P and Nb) of the (0 0 1) surface. The nature of the electronic states of these two cleaving planes is different: the P-terminated surface shows spoon and bow tie-shaped surface states, whereas these shapes are not present in the Nb-terminated surface. ARPES studies show that even one monolayer (ML) of Pb causes topological Lifshitz transition (TLT) in P-terminated NbP where the surface Fermi arcs (SFAs) teleport to another pair of Weyl points connecting two adjacent Brillouin zones. Depositing one ML of Pb modifies the Fermi surface along with a shift in the Fermi energy. On the other hand, the deposition of approximately 0.8 ML of Nb modifies the electronic structure of P-terminated NbP, pushing the system on the verge of TLT but not yet fully transformed. Regardless of the dramatic surface evolution, SFAs remain connected to topologically protected Weyl points. Additionally, we studied the Nb-terminated NbP covered with 1.9 ML of Pb with only altered trivial surface states caused by an ordinary Lifshitz transition.