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 ${V=U}_c\left(x\right)$ of finite range provided that the potential well ${V=-U}_c\left(x\right)$ supports a bound state of energy $E=-m$. This is called a supercritical potential well.