Self-Adjoint Extensions of the Dirac Hamiltonian with a delta-Sphere   Interaction
  The purpose of this paper is to make an explicit construction of specific self-adjoint extensions of the Dirac Hamiltonian in the presence of a $\delta$-sphere interaction of finite radius. The exact resolvent kernel of the free Dirac operator is given. This specifies related results that have recently appeared in the literature. 
Gabriel Y.H. Avossevou (CIPMA, Univ. Abomey-Calavi, Rep. Benin), Jan   Govaerts (UCL, Louvain-la-Neuve, Belgium), M. Norbert...