= Chiral Dirac interface state
{c}
{title2=$E=-\hbar ck_x$}
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, $e^{ik_xx}\exp[-\int_0^y V(s)\,\mathrm ds/(\hbar c)](1,-1)^T$ has energy $-\hbar ck_x$. 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.
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