Topological insulator
[5] Since this results from a global property of the topological insulator's band structure, local (symmetry-preserving) perturbations cannot damage this surface state.
At a given energy the only other available electronic states have different spin, so "U"-turn scattering is strongly suppressed and conduction on the surface is highly metallic.
Despite their origin in quantum mechanical systems, analogues of topological insulators can also be found in classical media.
Existence of interface Dirac states in HgTe/CdTe was experimentally verified by Laurens W. Molenkamp's group in 2D topological insulators in 2007.
topological invariant was constructed and the importance of the time reversal symmetry was clarified in the work by Kane and Mele.
[23] 2D Topological insulators were first realized in system containing HgTe quantum wells sandwiched between cadmium telluride in 2007.
Using angle-resolved photoemission spectroscopy, and many other measurements, it was observed that Bi1 − xSbx alloy exhibits an odd surface state (SS) crossing between any pair of Kramers points and the bulk features massive Dirac fermions.
[27] This prediction is of particular interest due to the observation of charge quantum Hall fractionalization in 2D graphene[28] and pure bismuth.
[36][37] In some of these materials, the Fermi level actually falls in either the conduction or valence bands due to naturally-occurring defects, and must be pushed into the bulk gap by doping or gating.
[31] Fully bulk-insulating or intrinsic 3D topological insulator states exist in Bi-based materials as demonstrated in surface transport measurements.
[40] In a new Bi based chalcogenide (Bi1.1Sb0.9Te2S) with slightly Sn - doping, exhibits an intrinsic semiconductor behavior with Fermi energy and Dirac point lie in the bulk gap and the surface states were probed by the charge transport experiments.
Discrete time quantum walks (DTQW) have been proposed for making Floquet topological insulators (FTI).
An atomic lattice empowered by distance selective Rydberg interaction could simulate different classes of FTI over a couple of hundred sites and steps in 1, 2 or 3 dimensions.
[62] In addition, topological insulator materials have also found practical applications in advanced magnetoelectronic and optoelectronic devices.
The growth of thin film topological insulators is governed by weak van der Waals interactions.
MBE has an advantage over other methods due to the fact that the synthesis is performed in high vacuum hence resulting in less contamination.
Additionally, lattice defect is reduced due to the ability to influence the growth rate and the ratio of species of source materials present at the substrate interface.
[74] Due to the weak van der Waals bonding, which relaxes the lattice-matching condition, TI can be grown on a wide variety of substrates[75] such as Si(111),[76][77] Al2O3, GaAs(111),[78] InP(111), CdS(0001) and Y3Fe5O12 .
The physical vapor deposition (PVD) technique does not suffer from the disadvantages of the exfoliation method and, at the same time, it is much simpler and cheaper than the fully controlled growth by molecular-beam epitaxy.
The PVD method enables a reproducible synthesis of single crystals of various layered quasi-two-dimensional materials including topological insulators (i.e., Bi2Se3, Bi2Te3).
[79] The resulted single crystals have a well-defined crystallographic orientation; their composition, thickness, size, and the surface density on the desired substrate can be controlled.
The thickness control is particularly important for 3D TIs in which the trivial (bulky) electronic channels usually dominate the transport properties and mask the response of the topological (surface) modes.
By reducing the thickness, one lowers the contribution of trivial bulk channels into the total conduction, thus forcing the topological modes to carry the electric current.
[41] The choice of chalcogenides is related to the van der Waals relaxation of the lattice matching strength which restricts the number of materials and substrates.
The van der Waals interaction in TIs exhibit important features due to low surface energy.
Generally, regardless of the substrate used, the resulting films have a textured surface that is characterized by pyramidal single-crystal domains with quintuple-layer steps.
[73] Due to the weak van der Waals bonding, graphene is one of the preferred substrates for TI growth despite the large lattice mismatch.
The first step of topological insulators identification takes place right after synthesis, meaning without breaking the vacuum and moving the sample to an atmosphere.
[74] Further measurements includes structural and chemical probes such as X-ray diffraction and energy-dispersive spectroscopy but depending on the sample quality, the lack of sensitivity could remain.
[7] Specifically, the number of connected components of the space indicates how many different "islands" of insulators exist amongst the metallic states.
