Especially in condensed matter physics, an energy gap is often known more abstractly as a spectral gap, a term which need not be specific to electrons or solids.
The superconducting energy gap is a key aspect in the theoretical description of superconductivity and thus features prominently in BCS theory.
, it starts to open upon entering the superconducting state at
Pseudogaps are experimentally observed in a variety of material classes; a prominent example are the cuprate high-temperature superconductors.
[4] If the density of states vanishes over an extended energy range, then this is called a hard gap.
If instead the density of states exactly vanishes only for a single energy value (while being suppressed, but not vanishing for nearby energy values), then this is called a soft gap.
A prototypical example of a soft gap is the Coulomb gap that exists in localized electron states with Coulomb interaction.