Domain-Direct Band Gaps: Classification and Material Realization

TL;DR

This paper introduces the concept of domain-direct band gaps, where conduction and valence band extrema form extended manifolds, demonstrated in twisted diamond with unique optical and electronic properties.

Contribution

It defines and explores the novel concept of domain-direct band gaps, showing their realization in twisted diamond through first-principles calculations and analyzing their anisotropic properties.

Findings

Nearly flat 2D manifolds of CBM and VBM in twisted diamond

Pronounced optical absorption peak at the band gap

Strong anisotropic carrier dynamics with suppressed in-plane velocities

Abstract

The conventional classification of direct band-gap semiconductors relies on point-like extrema in momentum space. Here, we introduce the concept of domain-direct band gaps, where the conduction-band minimum (CBM) and valence-band maximum (VBM) form extended manifolds in the Brillouin zone. We demonstrate this concept through the material realization of an extreme two-dimensional-two-dimensional (2D-2D) domain-direct band gap in twisted diamond. First-principles calculations show that both the CBM and VBM exhibit nearly flat 2D manifolds in the kx-ky plane with minimal energy variation (a few meV), yielding a direct band gap of 3.264 eV. In contrast, strong dispersion along the out-of-plane kz direction induces anisotropic carrier dynamics, with strongly suppressed in-plane Fermi velocities (down to about 10$^1$-10$^3$ m/s in certain directions) and much larger out-of-plane velocities (about 10$^6$ m/s). The nearly flat CBM and VBM manifolds enhance the joint density of states, leading to a pronounced optical absorption peak at the band gap onset. This new type of domain-direct gap, coupled with strong directional anisotropy, opens up opportunities for anisotropic optoelectronic applications. Our results establish domain-direct band gaps as a new class of semiconductors, demonstrating their feasibility in real materials.