 Continuum p-wave Majorana interface mode

For the long-wavelength Bogoliubov--de Gennes Hamiltonian $H=-\mu (x){\tau }_z-i\hslash \Delta {\tau }_x{\partial }_x$, a sign change from $\mu >0$ in a one-dimensional p-wave superconductor to $\mu <0$ in the vacuum binds a zero mode of width $\xi =\hslash \Delta /|\mu |$. Neglecting the quadratic kinetic term is valid when $|\mu |\ll m{\Delta }^2$ up to an inessential numerical factor.