Band Structure Of Topological Insulators Research Articles

The relationship between the plasma edge and topological properties of Sb2Te3 under pressure

Topological insulators have garnered considerable attention in recent years due to their unique electronic properties. However, pressure-induced topological materials often located in the middle of the two diamonds and cannot be detected by Angle resolved electron spectra. Such topological materials only stay in theoretical calculated. Since diamond has a high optical transmittance, its optical properties have been extensively studied to probe the band structure of topological insulators under pressure. In this paper, we determine the high-pressure infrared (IR) properties by measuring the transmittivity and reflectivity of Sb2Te3 up to 23.2 GPa. The plasma edge shows jumping at 4.2 GPa and 8.5 GPa, where is caused by the change of charge carrier features and a new phase appears at 8.5 GPa. Furthermore, by analyzing the position of plasma edge under pressure, we find that the absence of plasma edge at high pressure corresponds to the disappearance of the Sb2Te3 topological structure. Therefore, utilizing the plasma edge to investigate the modification of band structure in materials under high pressures is highly effective.

Surface coupling in Bi2Se3 ultrathin films by screened Coulomb interaction

Single-particle band theory has been very successful in describing the band structure of topological insulators. However, with decreasing thickness of topological insulator thin films, single-particle band theory is insufficient to explain their band structures and transport properties due to the existence of top and bottom surface-state coupling. Here, we reconstruct this coupling with an equivalently screened Coulomb interaction in Bi2Se3 ultrathin films. The thickness-dependent position of the Dirac point and the magnitude of the mass gap are discussed in terms of the Hartree approximation and the self-consistent gap equation. We find that for thicknesses below 6 quintuple layers, the magnitude of the mass gap is in good agreement with the experimental results. Our work provides a more accurate means of describing and predicting the behaviour of quasi-particles in ultrathin topological insulator films and stacked topological systems.

The theoretical development and prospect of two-dimensional topological insulators

Topological insulators can be described as rediscovery of band theory, where numerous secrets on edges and surfaces are unearthed. During the process of rediscovery, the concept of topology into novel theoretical models of topological insulators played a crucial role. This paper aims to give an introduction to the theoretical development of two-dimensional topological insulators in a nutshell, so it is indispensable and efficient to relate topological invariant with the band structure of topological insulators and use specific and vital theoretical models to reveal novel phenomena in those materials. The main focus of the paper is placed on historic landmarks in the theoretical development of two-dimensional topological insulators where fundamental concepts and models have paved the way for further enrichment of family of topological materials and experimental realizations.

Revealing hidden spin-momentum locking in a high-temperature cuprate superconductor.

Cuprate superconductors have long been thought of as having strong electronic correlations but negligible spin-orbit coupling. Using spin- and angle-resolved photoemission spectroscopy, we discovered that one of the most studied cuprate superconductors, Bi2212, has a nontrivial spin texture with a spin-momentum locking that circles the Brillouin zone center and a spin-layer locking that allows states of opposite spin to be localized in different parts of the unit cell. Our findings pose challenges for the vast majority of models of cuprates, such as the Hubbard model and its variants, where spin-orbit interaction has been mostly neglected, and open the intriguing question of how the high-temperature superconducting state emerges in the presence of this nontrivial spin texture.

Revealing spin-orbit coupling in a cuprate

Superconductivity Strong coupling between the spin and orbital degrees of freedom is crucial in generating the exotic band structure of topological insulators. The combination of spin-orbit coupling with electronic correlations could lead to exotic effects; however, these two types of interactions are rarely found to be strong in the same material. Gotlieb et al. used spin- and angle-resolved photoemission spectroscopy to map out the spin texture in the cuprate Bi2212. Surprisingly, they found signatures of spin-momentum locking, not unlike that seen in topological insulators. Thus, in addition to strong electronic correlations, this cuprate also has considerable spin-orbit coupling. Science , this issue p. [1271][1] [1]: /lookup/doi/10.1126/science.aao0980

Spontaneous breaking of time-reversal symmetry in topological insulators

The system of spinless fermions on a hexagonal lattice is studied. We have considered tight-binding model with the hopping integrals between the nearest-neighbor and next-nearest-neighbor lattice sites, that depend on the direction of the link. The links are divided on three types depending on the direction, the hopping integrals are defined by different phases along the links. The energy of the system depends on the phase differences, the solutions for the phases, that correspond to the minimums of the energy, lead to a topological insulator state with the nontrivial Chern numbers. We have analyzed distinct topological states and phase transitions, the behavior of the chiral gapless edge modes, have defined the Chern numbers. The band structure of topological insulator (TI) is calculated, the ground-state phase diagram in the parameter space is obtained. We propose a novel mechanism of realization of TI, when the TI state is result of spontaneous breaking of time-reversal symmetry due to nontrivial stable solutions for the phases that determine the hopping integrals along the links and show that the Haldane model [1] can be implemented in real compounds of the condensed matter physics.

Temperature Effects in the Band Structure of Topological Insulators.

We study the effects of temperature on the band structure of the Bi_{2}Se_{3} family of topological insulators using first-principles methods. Increasing temperature drives these materials towards the normal state, with similar contributions from thermal expansion and from electron-phonon coupling. The band gap changes with temperature reach 0.3eV at 600K, of similar size to the changes caused by electron correlation. Our results suggest that temperature-induced topological phase transitions should be observable near critical points of other external parameters.

Band structure of topological insulators from noise measurements in tunnel junctions

The unique properties of spin-polarized surface or edge states in topological insulators (TIs) make these quantum coherent systems interesting from the point of view of both fundamental physics and their implementation in low power spintronic devices. Here we present such a study in TIs, through tunneling and noise spectroscopy utilizing TI/Al2O3/Co tunnel junctions with bottom TI electrodes of either Bi2Te3 or Bi2Se3. We demonstrate that features related to the band structure of the TI materials show up in the tunneling conductance and even more clearly through low frequency noise measurements. The bias dependence of 1/f noise reveals peaks at specific energies corresponding to band structure features of the TI. TI tunnel junctions could thus simplify the study of the properties of such quantum coherent systems that can further lead to the manipulation of their spin-polarized properties for technological purposes.

On the influence of tetrahedral covalent-hybridization on electronic band structure of topological insulators from first principles

Based on first-principles calculations, we investigate the influence of tetrahedral covalent-hybridization between main-group and transition-metal atoms on the topological band structures of binary HgTe and ternary half-Heusler compounds, respectively. Results show that, for the binary HgTe, when its zinc-blend structure is artificially changed to rock-salt one, the tetrahedral covalent-hybridization will be removed and correspondingly the topologically insulating band character lost. While for the ternary half-Heusler system, the strength of covalent-hybridization can be tuned by varying both chemical compositions and atomic arrangements, and the competition between tetrahedral and octahedral covalent-hybridization has been discussed in details. As a result, we found that a proper strength of tetrahedral covalent-hybridization is probably in favor to realizing the topologically insulating state with band inversion occurring at the Γ point of the Brillouin zone.

Observations of a metal-insulator transition and strong surface states in Bi2-x SbxSe3 thin films.

High-quality thin films of the topological insulator Bi2-xSbxSe3 are grown by molecular beam epitaxy. A metal-insulator transition along with strong surface states - revealed by Shubnikov-de Haas oscillations - is observed as the Sb concentration is increased. This system represents a widely tunable platform for achieving high surface conduction, suppressing the bulk influence, and manipulating the band structure of topological insulators.

Tuning the Dirac point position in Bi(2)Se(3)(0001) via surface carbon doping.

Angular resolved photoemission spectroscopy in combination with abinitio calculations show that trace amounts of carbon doping of the Bi_{2}Se_{3} surface allows the controlled shift of the Dirac point within the bulk band gap. In contrast to expectation, no Rashba-split two-dimensional electron gas states appear. This unique electronic modification is related to surface structural modification characterized by an expansion of the top Se-Bi spacing of ≈11% as evidenced by surface x-ray diffraction. Our results provide new ways to tune the surface band structure of topological insulators.

Band Structure Engineering in Topological Insulator Based Heterostructures

The ability to engineer an electronic band structure of topological insulators would allow the production of topological materials with tailor-made properties. Using ab initio calculations, we show a promising way to control the conducting surface state in topological insulator based heterostructures representing an insulator ultrathin films on the topological insulator substrates. Because of a specific relation between work functions and band gaps of the topological insulator substrate and the insulator ultrathin film overlayer, a sizable shift of the Dirac point occurs resulting in a significant increase in the number of the topological surface state charge carriers as compared to that of the substrate itself. Such an effect can also be realized by applying the external electric field that allows a gradual tuning of the topological surface state. A simultaneous use of both approaches makes it possible to obtain a topological insulator based heterostructure with a highly tunable topological surface state.

Optical Properties and Electronic Band Structure of Topological Insulators (on A5 2B6 3 Compound Based)

We have performed a first principles study of structural, electronic, and optical properties of rhombohedral Sb2Te3 and Bi2Te3 compounds using the density functional theory within the local density approximation. The lattice parameters, bulk modulus, and its pressure derivatives of these compounds have been obtained. The linear photon-energy dependent dielectric functions and some optical properties such as the energy-loss function, the effective number of valance electrons and the effective optical dielectric constant are calculated and presented in the study.

Impurity scattering in the bulk of topological insulators

We study in this paper time-reversal $\ensuremath{\delta}$-impurity scattering effects in the bulk of topological insulators (TI) in two and three dimensions. Specifically we consider how impurity scattering strength is affected by the bulk band structure of topological insulators. An interesting band inversion effect associated with the change of the system from ordinary to topological insulator is pointed out. Experimental consequences of our findings are discussed.

Band structure engineering in (Bi1−xSbx)2Te3 ternary topological insulators

Topological insulators (TIs) are quantum materials with insulating bulk and topologically protected metallic surfaces with Dirac-like band structure. The most challenging problem faced by current investigations of these materials is to establish the existence of significant bulk conduction. Here we show how the band structure of topological insulators can be engineered by molecular beam epitaxy growth of (Bi(1-x)Sb(x))(2)Te(3) ternary compounds. The topological surface states are shown to exist over the entire composition range of (Bi(1-x)Sb(x))(2)Te(3), indicating the robustness of bulk Z(2) topology. Most remarkably, the band engineering leads to ideal TIs with truly insulating bulk and tunable surface states across the Dirac point that behaves like one-quarter of graphene. This work demonstrates a new route to achieving intrinsic quantum transport of the topological surface states and designing conceptually new topologically insulating devices based on well-established semiconductor technology.