Two-dimensional Dirac operators with singular interactions supported on closed curves

This paper is devoted to the study of the two-dimensional Dirac operator with an arbitrary combination of an electrostatic and a Lorentz scalar δ-interaction of constant strengths supported on a closed curve. For any combination of the coupling constants a rigorous description of the self-adjoint realization of the operators is given and the spectral properties are described. For a non-zero mass and a critical combination of coupling constants the operator appears to have an additional point in the essential spectrum, which is related to a loss of regularity in the operator domain, and the position of this point is expressed in terms of the coupling constants.