Topological insulators are a class of materials with unique electrical properties that arise from their topological band structure. Unlike conventional insulators, which have an energy gap between the conduction and valence bands, topological insulators have a time-reversal symmetry protected Dirac cone in the surface states. This results in a conducting surface state that is immune to disorder and backscattering, making them ideal for use in electronic devices.
Topological insulators were first predicted theoretically in the 1980s by Tsuyoshi Kimura and others, but it was not until 2007 that they were experimentally observed in HgTe/CdTe quantum wells by Molenkamp et al. Since then, numerous other topological insulator materials have been discovered, including Bi_2Se_3, Bi_2Te_3, Sb_2Te_3, InAs/GaSb quantum wells, etc. The unique electrical properties of these materials have led to many potential applications in electronics and photonics. For example, topological insulators can be used as novel electron sources for next-generation semiconductor devices or as efficient light emitters for LEDs and lasers. Additionally, because of their immunity to backscattering from disorder, topological insulators could be used to create ultra-low power dissipation electronic devices or even new types of computer memory.
The study of topological insulators is an active area of research with many open questions yet to be answered. For example, it is not yet known what kind of material is necessary to create a three-dimensional (3D) topological insulator; all known 3D materials are either metals or semimetals. Additionally, there are many interesting phenomena that occur at the interface between different topological phases which are not fully understood yet. With continued research, it is hoped that scientists will be able to unlock all the secrets of these fascinating materials and harness their full potential for use in real-world applications