= Hybridization gap
A <hybridization gap> is a separation in <electronic band structure> caused by mixing orbital-derived states. For $E_a=Ak^2$, $E_b=E_0>0$, $A>0$ and constant coupling $\alpha>0$, the lower eigenvalue is everywhere below $E_0$ and tends to it as $k\to\infty$. The upper eigenvalue has minimum $(E_0+\sqrt{E_0^2+4\alpha^2})/2$. The unbounded model therefore has an indirect gap of width $(\sqrt{E_0^2+4\alpha^2}-E_0)/2$, approximately $\alpha^2/E_0$ at weak coupling. This differs from the direct separation $2\alpha$ at the bare crossing. A finite <Brillouin zone> bounds the lower band farther below $E_0$.
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