Flat band electron energy dispersion

Flat band electron energy dispersion in superconducting cuprates... 

On the other hand, molecular crystals are characterized by the existence of strongly bound (Frenkel type) excitons, and it has been shown that the lower-energy part of the absorption spectrum (say, the first 2 eV) is completely dominated by these excitons [168], even to the extent that the absorption corresponding to electron-hole pair generation is completely hidden in the exciton spectrum [128] and is revealed only by such methods as modulated electrorefletance [169]. The only states in the exciton bands that are accessible by photon absorption are those at the center of the Brillouin zone, so the absorption is not a continuous band as for semiconductors, but a sharp line. The existence of this sharp line therefore does not mean that the exciton band is narrow (i.e., that its dispersion relation in the Brillouin zone is flat). On the contrary, since that dispersion is caused by dipolar interactions, exciton bandwidths can be several eV [168,170] the total bandwidth is four times the coupling term. This will be particularly... 

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