Depth-resolved core level spectroscopy of noncentrosymmetric solid BiPd

Understanding exotic solids is a difficult task as interactions are often hidden by the symmetry of the system. Here, we study the electronic properties of a noncentrosymmetric solid, BiPd, which is a rare material exhibiting both superconductivity and the topological phase of matter. Employing high-resolution photoemission spectroscopy with photon energies ranging from the hard x-ray to extreme-ultraviolet regime, we show that hard x-ray spectroscopy alone is not enough to reveal surface-bulk differences in the electronic structure. We derive the escape depths close to the extreme surface sensitivity and find that the photon energies used for high-resolution photoemission measurements fall in the surface sensitive regime. In addition, we discover a deviation of the branching ratio of Bi core level features derived from conventional quantum theories of the core hole final states. Such a breakdown of the atomic description of the core level spectroscopy can be attributed to the absence of a center of symmetry and spin-orbit interactions.