An insulator has a band structure in which occupied and empty bands are separated by an energy gap. The existence of an energy gap, however, does not guarantee that a material is a simple insulator.

This band structure typically consists of a conduction ... of antibonding orbitals (antisymmetric wave functions). In an insulator the Fermi energy lies in the bandgap between the minimum of ...

"A flat band in a band structure is a very special type of frequency ... into real-world applications such as sound and vibration insulation, energy harvesting for sensors or medical devices ...