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What are excitons?
Coulomb interaction between electrons and holes can give rise to new excitations in
matter- these are called excitons.
Generally electrons and holes are created in the conduction and valence band of a solid
due to optical absorptions. Therefore, when we talk about excitons -there is always a
corresponding inter-band transition associated with it.
Excitons are seen mainly in semiconductors and insulators. Metals generally donot show excitonic effects due to large screening. But
there are exceptions.“hydrogenic system”