Past exam of the physics A course of the University of Cambridge / 2021 / Paper 2 / Section C / C8 / c / Solution

Set $G=G_1$, $\kappa =\Delta k$, $U=|V_1|>0$ and $E_B={\hslash }^2{G}^2/(8m)$. The two kinetic energies satisfyConsequently the nearly-free electron dispersion near a one-dimensional band gap becomesThe Taylor expansion $\sqrt{U^2+s^2}=U+s^2/(2U)+O(s^4/U^3)$ givesFor the paper's energy convention, set $V_0=0$. There is no linear or cubic term because each branch is an even function of displacement from the crossing in this two-wave approximation. Each coefficient multiplying $(\Delta k{)}^2$ has units of energy times length squared, as required.

The expansion requiresa more restrictive condition than merely $|\Delta k|\ll G$ when the band gap is small. If $U=0$, the square root is proportional to $|\Delta k|$ and this analytic expansion of the energy-ordered branches is unavailable.