$E = 0.5,\ 1$ and $1.5$ are plotted respectively. Panels (c) and (d) show the EECs and the in-plane spin accumulation directions at the two values of $\lambda$ indicated by different colors in panel (b). The inset of panel (d) shows a zoomed in view of the EECs near the intersection of the two Fermi ‘circles’ showing that the inter surface coupling leads to a breaking away of the lens shaped region where the two circles overlap into separate EEC curves.[]{data-label="glambdaComb2"}](lambdaComb2.eps)

We now turn on the inter-surface coupling. Fig. \[glambdaComb2\] shows the dispersion relations and equal energy contours (EECs) at two values of $\lambda < |\vec{M}|$. At these small (relative to $|\vec{M}|$) values of $\lambda$ the two Dirac cones corresponding to the surface states localized at the upper and lower surfaces of the thin film are still distinctly evident. At low values of energy where the two cones do not overlap (panel (b) ), the EECs consist of two almost circular curves that correspond to the cross sections of the two Dirac cones. As the energy increases and the two almost-circular cross sections begin to almost touch each other, the inter-surface coupling pushes the EECs outwards in $k$-space so that 