= 4 c^2$ could not be seen with this approach due to the decomposition to the spherical harmonics.

It took then 25 years until Dirac operators with singular interactions supported on more general surfaces in $\mathbb{R}^3$ were studied. In a series of papers [@AMV14; @AMV15; @AMV16] Arrizabalaga, Mas, and Vega showed the self-adjointness and derived several basic properties of $A_{\eta, \tau}$, in particular for the special case of purely electrostatic interactions, i.e. for $\tau = 0$. Moreover, for purely electrostatic and purely scalar interactions it was shown in [@MP18] that $A_{\eta, \tau}$ can be regarded as a limit of Dirac operators with squeezed potentials. Inspired by the approach in [@AMV14] the authors of the present paper applied the abstract concept of quasi boundary triples and Weyl functions from extension theory of symmetric operators to Dirac operators with singular interactions and provided in the recent paper [@BEHL17] a deeper analysis of the spectral properties of $A_{\eta, 0}$ for purely electrostatic potentials. We should note that in all of the above mentioned papers the case $\eta^2-\tau^2=4c^2$ was excluded and it turns out that in this [*critical case*]{} the operator $A_{\eta, \tau}$ has different properties as in the [*noncritical case*]{} $\eta^2 - \tau^2 