 Chiral Dirac interface state

For $H=c(p_x{\sigma }_x+p_y{\sigma }_y)+V(y){\sigma }_z$, $c>0$, and a mass changing from negative to positive as $y$ increases, $e^{i{k}_{x}x}\exp [-{\int }_0^{y}V(s)\mathrm{d}s/(\hslash c)](1,-1{)}^T$ has energy $-\hslash c{k}_x$. The derivative and mass terms cancel. Normalizable confinement additionally requires the envelope to be square-integrable, as for nonzero asymptotic masses. Reversing the wall reverses the propagating direction.