Elemental noncentrosymmetric superconductors

Abstract: Superconductivity has been discovered in many materials including elements, cuprates and nickelates. Noncentrosymmetric superconductors get attention due to their physical properties results from spatial inversion symmetry breaking. Here we propose the existence of elemental noncentrosymmetric superconductors which needs to break spatial inversion symmetry in single element phases realized by occupations of more than one Wyckoff positions. Elemental noncentrosymmetric superconductors allow us to study the admixture of spin-singlet and spin-triplet pairing states in elements with simple crystal structures and chemical conditions. We find that four-layer La thin films coming from bulk {\alpha}-La with a double hexagonal close-packed structure are elemental noncentrosymmetric superconductors since the dimension reduction breaks the spatial inversion symmetry. In the normal state, four-layer La is a polar rare-earth metal with a spontaneous polarization having a polar layer group like ferroelectrics. Hence, to be more precise, four-layer La can also be called the elemental polar superconductor in the superconductivity state. Based on this method of breaking inversion symmetry, we discovery polar rare-earth metals, polar transition metals, polar alkali metals and polar alkali earth metals. Other candidate materials are bulk polar materials formed with group 15 elements, Bi thin films with polar stacking, polar double layer plumbene and Al thin films. The question of building the periodic table of elemental noncentrosymmetric superconductors is raised.