The defining feature of gap materials is the presence of a bulk energy gap, which separates filled valence bands from empty conduction bands, while allowing conductive states to exist on their surfaces or edges. This phenomenon arises from strong spin-orbit coupling and time-reversal symmetry, leading to topologically protected surface states that are robust against disorder and defects. The most well-known example is bismuth selenide (Bi₂Se₃), which was the first three-dimensional topological insulator discovered in 2008.
Gap materials can be categorized into different types based on their dimensionality and the nature of their conducting states. Two-dimensional topological insulators, such as graphene with certain perturbations, exhibit conducting edge states, while three-dimensional topological insulators feature metallic surface states. Weyl semimetals and Dirac semimetals are additional classes where bulk conduction bands touch at discrete points (Weyl nodes) or lines (Dirac lines), respectively, leading to exotic electronic and transport properties.
Research into gap materials focuses on both fundamental studies of their electronic structure and practical applications. Potential uses include the development of quantum spin Hall effect devices, topological quantum computing components, and highly efficient thermoelectric materials. Challenges in this field involve synthesizing high-quality samples, understanding their behavior under external perturbations, and integrating them into functional devices.
The study of gap materials continues to evolve with advancements in experimental techniques, such as angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM), as well as theoretical modeling. Collaborations between physicists, chemists, and engineers drive innovation in this interdisciplinary field, promising breakthroughs in next-generation electronic technologies.