Universal Metallic Surface States in Electrides

Robust metallic surface states (MSS) of topological insulators (TIs) against imperfections and perturbations are important in broad applications such as chemical catalysis and quantum computing. Unfortunately, they suffer from the narrow band gap that can be accessed. Searching for MSS with a large bulk band gap beyond conventional TIs becomes a quest. In this work, inspired by the adiabatic connection principle in real space, we identify that all electrides, a new class of emerging materials, must host robust and universal MSS that resist any disturbances, in spite of the fact that some of them could be classified as trivial in standard topology theory. This counterintuitive property is traced to the specific charge localization-delocalization change intrinsic to the electride when approaching the crystalline surface or interface, which is a kind of interstice-centered to atom-centered transition in the real-space topology of the charge density distribution, and is sharply different from the band inversion in the standard topology theory. The new mechanism circumvents the obstacle that limits the band gap of TI. Robust and universal MSS in an electride conventionally determined as trivial but with a colossal band gap beyond 6.13 eV are demonstrated. This gap size is about 6-fold larger than the highest record of known “wide-gap” TIs, thus opening up new avenues to universal MSS with gigantic bulk gap.