Behavior of Dirac Fermions in a non-centrosymmetric superconductor, BiPd

Topological materials having Dirac fermions exhibiting superconductivity is believed to be the candidate material to realize exotic physics as well as advanced technology. We studied the electronic structure of a complex topological material BiPd, which has non-centrosymmetric crystal structure and exhibit superconductivity below 3.7 K. Employing high resolution photoemission spectroscopy in a wide photon energy range (ultraviolet to hard x-ray), we show that the surface electronic structure of this material is confined only within the top few layers and there is an enhancement in orbital moment contribution leading to a deviation from atomic description of the core level spectroscopy. Our high-resolution ARPES study at varied conditions helped to identify the location of the Dirac node accurately and show the two dimensional behavior of the Dirac states. These states have significantly high binding energy and hence may not be responsible for the superconductivity in this system. The dispersion of the Dirac bands show unusual anisotropy - isotropic close to the Dirac point and becomes anisotropic at higher momenta. We have constructed a model Hamiltonian considering higher-order spin-orbit terms and demonstrate that this model provides an excellent description of the observed anisotropy.