Observation of Dirac surface states in the hexagonal PtBi2, a possible origin of the linear magnetoresistance

The nonmagnetic compounds showing extremely large magnetoresistance are attracting a great deal of research interests due to their potential applications in the field of spintronics. PtBi$_2$ is one of such interesting compounds showing large linear magnetoresistance (MR) in its both the hexagonal and pyrite crystal structure. We use angle-resolved photoelectron spectroscopy (ARPES) and density functional theory (DFT) calculations to understand the mechanism of liner MR observed in the hexagonal PtBi$_2$. Our results uncover for the first time linear dispersive surface Dirac states at the $\bar{\Gamma}$-point, crossing Fermi level with node at a binding energy of $\approx$ 900 meV, in addition to the previously reported Dirac states at the $\bar{M}$-point in the same compound. We further notice from our dichroic measurements that these surface states show an asymmetric spectral intensity when measured with left and right circularly polarized light, hinting at a substantial spin polarization of the bands. Following these observations, we suggest that the linear dispersive Dirac states at the $\bar{\Gamma}$ and $\bar{M}$-points are likely to play a crucial role for the linear field dependent magnetoresistance recorded in this compound.