LaAs: a Key Material to Understanding the Origin of Extreme Magnetoresistance*

The first growth and complete study of the LaAs single crystal are presented. LaAs belongs to the lanthanum monopnictide family, where the two heavy members, LaBi and LaSb, have recently attracted much attention due to their non-trivial topology and extreme magnetoresistance (XMR). The origin of XMR is unclear because both of the possible explanations- non-trivial topology and electron-hole compensation- have been shown to exist in LaBi and LaSb. Therefore, it is impossible to recognize which mechanism is responsible for XMR. By studying LaAs crystals, we observed a lack of Dirac cones from ARPES, and ruled out the role of topology in LaAs. From transport measurements, we established the fermiology of LaAs, confirmed it has the same degree of compensation as LaSb and LaBi, and observed all the features of XMR remain in LaAs while the magnitude was reduced by 10²~10³. As a result, we concluded that electron-hole compensation is the true origin of XMR and the mobility mismatch constrains the magnitude of XMR. We also pointed out the difficulty in capturing the band structure using DFT in semimetals with very small Fermi surfaces such as LaAs.