= Dimerized tight-binding chain
{title2=$E_\pm(k)=\pm2\sqrt{t^2\cos^2(ka)+\delta^2\sin^2(ka)}$}
For spin-degenerate <Electrons> at atomic spacing $a$, alternate the hopping amplitudes $-(t+\delta)$ and $-(t-\delta)$, with $t>0$ and $|\delta|<t$. The doubled <primitive unit cell> gives an off-diagonal <Bloch Hamiltonian> entry $-(t+\delta)-(t-\delta)e^{-2ika}$. Its two <eigenvalues> are the displayed <energy bands>, for $|k|\leq\pi/(2a)$. The <band gap> at the edge of the <Brillouin zone> is $4|\delta|$. At half filling there are two <Electrons> per doubled <primitive unit cell>, so the lower band is full and the upper empty. This explicit model realizes the period-doubling form of a <Peierls distortion>.
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