Supercriticality and Transmission Resonances in the Dirac equation

It is shown that a Dirac particle of mass $m$ and arbitrarily small momentum will tunnel without reflection through a potential barrier $V=U_c(x)$ of finite range provided that the potential well $V=-U_c(x)$ supports a bound state of energy $E=-m\mathrm{.}$ This is called a supercritical potential well.