BulkLücke

BulkLücke is a term used primarily in German-language solid-state physics to denote the energy gap present in the bulk electronic structure of a crystal. It is conceptually equivalent to the English term bulk band gap, but the usage emphasizes the bulk interior rather than surface or edge states. In this sense, a material with a finite BulkLücke is an insulator or wide-gap semiconductor in its bulk, while the presence of gapless surface or edge modes can occur in certain topological phases.

In electronic band theory, electrons occupy allowed Bloch states; the BulkLücke refers to the energy interval between the top of the valence band and the bottom of the conduction band where no bulk states exist. The size of the gap determines electrical conductivity, optical absorption onset, and intrinsic carrier concentration at finite temperature.

In topological insulators, a nonzero bulk gap coexists with conducting surface states protected by topology; the BulkLücke must be large enough to inhibit bulk conduction and isolate surface channels. The concept highlights the separation between bulk insulating behavior and surface electronics that can arise in certain materials.

Measurement and modeling: BulkLücke is inferred from optical spectroscopy, transport experiments, angle-resolved photoemission (ARPES) or scanning tunneling spectroscopy. It is described by band-structure calculations, including density functional theory; real materials may host impurity or defect states that partially fill the gap, affecting observed properties.

Usage and context: The term BulkLücke is used mainly in German texts and reviews; English-language literature more often uses “bulk band gap.” See also band gap, bulk material, topological insulator, ARPES, and STM.