Measurement of the atomic orbital composition of the near-fermi-level electronic states in the lanthanum monopnictides LaBi, LaSb, and LaAs

Recent debates in the literature over the relationship between topology and Extreme Magnetoresistance (XMR) have drawn attention to the Lanthanum Monopnictide family of binary compounds. Angle resolved photoemission spectroscopy (ARPES) is used to measure the electronic structure of the XMR topological semimetal candidates LaBi, LaSb, and LaAs. The orbital content of the near-E F states in LaBi and LaSb are extracted using varying photon polarizations and both dominant d and p bands are observed near X. The measured bulk bands are shifted in energy when compared to the results of Density Functional Calculations. This disagreement is minor in LaBi, but large in LaSb and LaAs. The measured bulk band structure of LaBi shows a clear band inversion and puts LaBi in the υ = 1 class of Topological Insulators (or semimetals), as predicted by calculations and consistent with the measured Dirac-like surface states. LaSb is on the verge of a band inversion with a less-clear case for any distinctly topological surface states and in disagreement with calculations. Lastly, these same bands in LaAs are clearly non-inverted implying its topological triviality and demonstrating a topological phase transition in the Lanthanum monopnictides. Using a wide range of photon energies the true bulk states are cleanly disentangled from the various types of surface states which are present. These surface states exist due to surface projections of bulk states in LaSb and for topological reasons in LaBi. Lanthanum monopnictides are drawing interest because they are topological semimetal candidates and they exhibit extreme magnetoresistance, that is, they transition from being good metals to having high resistance when a magnetic field is applied. Extreme magnetoresistance is very interesting in view of applications, but the mechanism behind it is not understood yet. Daniel Dessau at the University of Colorado and collaborators performed angle-resolved photoemission spectroscopy measurements and density functional theory calculations to investigate the electronic structure of lanthanum monopnictides and its connection with extreme magnetoresistance. LaBi shows band inversion and is thus a topological material, whereas the case is less clear for LaSb, which is close to band inversion. LaAs is topologically trivial. The results suggest that the connection between topology and extreme magnetoresistance might be less clear than previously thought.