Inverted Band Structure Research Articles

Nonmetallization and band inversion in beryllium dicarbide at high pressure.

Carbides have attracted much attention owing to their interesting physical and chemical properties. Here, we systematically investigated global energetically stable structures of BeC2 in the pressure range of 0–100 GPa using a first-principles structural search. A transition from the ambient-pressure α-phase to the high-pressure β-phase was theoretically predicted. Chemical bonding analysis revealed that the predicted phase transition is associated with the transformation from sp2 to sp3 C-C hybridization. The electrical conductivity of the high-pressure phase changed from a metal (α-phase) to a narrow bandgap semiconductor (β-phase), and the β-phase had an inverted band structure with positive pressure dependence. Interestingly, the β-phase was a topological insulator with the metallic surface states protected by the time-reversal symmetry of the crystal. The results indicate that pressure modulates the electronic band structure of BeC2, which is an important finding for fundamental physics and for a wide range of potential applications in electronic devices.

Pressure-induced topological phases of KNa2Bi.

We report an ab initio study of the effect of hydrostatic pressure and uniaxial strain on electronic properties of KNa2Bi, a cubic bialkali bismuthide. It is found that this zero-gap semimetal with an inverted band structure at the Brillouin zone center can be driven into various topological phases under proper external pressure. We show that upon hydrostatic compression KNa2Bi turns into a trivial semiconductor with a conical Dirac-type dispersion of electronic bands at the point of the topological transition while the breaking of cubic symmetry by applying a uniaxial strain converts the compound into a topological insulator or into a three-dimensional Dirac semimetal with nontrivial surface Fermi arcs depending on the sign of strain. The calculated phonon dispersions show that KNa2Bi is dynamically stable both in the cubic structure (at any considered pressures) and in the tetragonal phase (under uniaxial strain).

Quantum Oscillation in Narrow-Gap Topological Insulators.

The canonical understanding of quantum oscillation in metals is challenged by the observation of the de Haas-van Alphen effect in an insulator, SmB_{6} [Tan etal, Science 349, 287 (2015)]. Based on a two-band model with inverted band structure, we show that the periodically narrowing hybridization gap in magnetic fields can induce the oscillation of low-energy density of states in the bulk, which is observable provided that the activation energy is small and comparable to the Landau level spacing. Its temperature dependence strongly deviates from the Lifshitz-Kosevich theory. The nontrivial band topology manifests itself as a nonzero Berry phase in the oscillation pattern, which crosses over to a trivial Berry phase by increasing the temperature or the magnetic field. Further predictions to experiments are also proposed.

Far infrared edge photoresponse and persistent edge transport in an inverted InAs/GaSb heterostructure

Direct current (DC) transport and far infrared photoresponse were studied an InAs/GaSb double quantum well with an inverted band structure. The DC transport depends systematically upon the DC bias configuration and operating temperature. Surprisingly, it reveals robust edge conduction despite prevalent bulk transport in our device of macroscopic size. Under 180 GHz far infrared illumination at oblique incidence, we measured a strong photovoltaic response. We conclude that quantum spin Hall edge transport produces the observed transverse photovoltages. Overall, our experimental results support a hypothesis that the photoresponse arises from direct coupling of the incident radiation field to edge states.

Scaling of local and nonlocal resistances in a 2D topological insulator based on HgTe quantum well

We report the observation and systematic investigation of a local and nonlocal transport in HgTe quantum wells with inverted band structure corresponding to the two-dimensional topological insulator phase. We examine the probe spacing and probe configuration dependencies of the resistance near the charge neutrality point, where the transport is dominated by edge states. We provide details on the model, which takes into account the edge and bulk contribution to the total current and reproduces our experimental results.

Terahertz detection of magnetic field-driven topological phase transition in HgTe-based transistors

We report on terahertz photoconductivity under magnetic field up to 16 T of field effect transistor based on HgTe quantum well (QW) with an inverted band structure. We observe pronounced cyclotron resonance and Shubnikov-de Haas-like oscillations, indicating a high mobility electron gas in the transistor channel. We discover that nonlinearity of the transistor channel allows for observation of characteristic features in photoconductivity at critical magnetic field corresponding to the phase transition between topological quantum spin Hall and trivial quantum Hall states in HgTe QW. Our results pave the way towards terahertz topological field effect transistors.

Terahertz studies of 2D and 3D topological transitions

We report terahertz measurements on bulk HgCdTe crystals at the semiconductor- to-semimetal topological transition and InAs/GaSb inverted band structure quantum well. We show that physical parameters of these narrow gap systems driven in specific topological phases can be efficiently extracted by means of terahertz photoconductivity studies.

Terahertz excitations in HgTe-based field effect transistors

We report on Terahertz detection by inverted band structure HgTe-based Field Effect Transistor. Photoconductivity measurements allow for the observation of cyclotron resonance and Shubnikov-de-Haas-like oscillations. However, an unexpected peak was observed at the critical magnetic field value for which zero mode Landau Levels are crossing. Therefore, this specific feature of TeraHertz photoconductivity spectra can tentatively attributed to this magnetic field driven topological phase transition.

Shot noise of the edge transport in the inverted band HgTe quantum wells

We investigate the current noise in HgTe-based quantum wells with an inverted band structure in the regime of disordered edge transport. Consistent with previous experiments, the edge resistance strongly exceeds $h/e^2$ and weakly depends on the temperature. The shot noise is well below the Poissonian value and characterized by the Fano factor with gate voltage and sample to sample variations in the range $0.1<F<0.3$. Given the fact that our devices are shorter than the most pessimistic estimate of the ballistic dephasing length, these observations exclude the possibility of one-dimensional helical edge transport. Instead, we suggest that a disordered multi-mode conduction is responsible for the edge transport in our experiment.

Single- and Few-Electron States in Deformed Topological Insulator Quantum Dots**Supported by the National Natural Science Foundation of China under Grant No 11434010, and the National Basic Research Program of China under Grant No 2011CB922204.

We theoretically investigate the single- and few-electron states in deformed HgTe quantum dots (QDs) with an inverted band structure using the full configuration interaction method. For the circular and deformed QD, it is found that the energy of edge states is robust against the shape from the circular QD in various elliptic ones. For the few electron states, electrons will firstly fill the edge states localized at the short axis, then the states localized at the long axis of the QD before filling the bulk states. The filling of the edge states can be controlled by tuning the dot size or the deformation of the geometry of the HgTe QD, respectively.

Anticrossing of Landau levels in HgTe/CdHgTe (013) quantum wells with an inverted band structure

The simultaneous splitting of lines of an interband transition and cyclotron resonance in the conduction band has been detected in the absorption spectra of HgTe/CdHgTe quantum wells with an inverted band structure in quantizing magnetic fields. It has been shown that it is due to the absence of an inversion center in the crystal, which results in the interaction between the lower Landau level of the conduction band and the upper Landau level of the valence band.

Topological states inα−Snand HgTe quantum wells: A comparison ofab initioresults

Both $\ensuremath{\alpha}\text{-Sn}$ and HgTe are expected to have similar topological properties because of their inverted band structure and zero-gap character. We investigate how the different crystal symmetries and the bonding to barrier materials act to the quantum phase transition versus the thickness of the corresponding quantum well (QW) structures. They are simulated by ${(\text{SnSn})}_{N}{(\text{CdTe})}_{M}$ and ${(\text{HgTe})}_{N}{(\text{CdTe})}_{M}(110)$ superlattices. Their electronic structures and eigenstates are studied by means of first-principles calculations using the modified Becke-Johnson exchange-correlation functional and spin-orbit interaction. Significant differences are observed for the two QW materials. A topological transition between trivial insulator and quantum spin Hall phase together with the formation of topologically protected edge states are observed in the case of HgTe QWs, while these features are considerably modified for $\ensuremath{\alpha}\text{-Sn}$. The different behaviors are discussed in the light of the different symmetry, spin-orbit interaction, and interface bonding. For a better understanding of the influence of the interface electrostatics also results for ${(\text{HgTe})}_{N}{(\text{InSb})}_{M}(110)$ systems are discussed.

Strong terahertz absorption in long-period InAs/GaSb type-II superlattices with inverted band structures

We present a theoretical investigation on the terahertz (THz) absorption by long-period InAs/GaSb type-II superlattices (SLs) with inverted band structures. It is found that in such SLs the band inversion causes a significant electron–hole hybridization and a strong THz absorption can be induced by this hybridization. The THz absorption coefficient is even larger than mid-infrared absorption coefficient for short-period InAs/GaSb SLs. Moreover, we find that the strong THz absorption can be further improved and optimized by the proper choice of InAs/GaSb layer widths. The interesting absorption features are well manifested in hybridization gaps and optical transition matrix elements. This study is pertinent to the potential application of long-period inverted InAs/GaSb type-II SLs as high-efficiency THz photodetectors.

Aharonov Bohm effect in 2D topological insulator

We present magnetotransport measurements in HgTe quantum well with inverted band structure, which expected to be a two-dimensional topological insulator having the bulk gap with helical gapless states at the edge. The negative magnetoresistance is observed in the local and nonlocal resistance configuration followed by the periodic oscillations damping with magnetic field. We attribute such behaviour to Aharonov–Bohm effect due to magnetic flux through the charge carrier puddles coupled to the helical edge states. The characteristic size of these puddles is about 100nm.

Half-Heusler topological insulators

Abstract

Singular robust room-temperature spin response from topological Dirac fermions.

Topological insulators are a class of solids in which the non-trivial inverted bulk band structure gives rise to metallic surface states that are robust against impurity scattering. In three-dimensional (3D) topological insulators, however, the surface Dirac fermions intermix with the conducting bulk, thereby complicating access to the low-energy (Dirac point) charge transport or magnetic response. Here we use differential magnetometry to probe spin rotation in the 3D topological material family (Bi2Se3, Bi2Te3 and Sb2Te3). We report a paramagnetic singularity in the magnetic susceptibility at low magnetic fields that persists up to room temperature, and which we demonstrate to arise from the surfaces of the samples. The singularity is universal to the entire family, largely independent of the bulk carrier density, and consistent with the existence of electronic states near the spin-degenerate Dirac point of the 2D helical metal. The exceptional thermal stability of the signal points to an intrinsic surface cooling process, probably of thermoelectric origin, and establishes a sustainable platform for the singular field-tunable Dirac spin response.

Topological field-effect quantum transistors in HgTe nanoribbons

We propose practical designs to realize topological field-effect quantum transistors in an HgTe nanoribbon with an inverted band structure. Our theoretical calculations show that, as a strip-shape top gate is placed on the HgTe nanoribbon and with an increasing gate voltage, two new conductance channels develop in the HgTe nanoribbon and are localized to the lattice sites neighboring the boundaries of the gate, leading to an additional quantization of the conductance of 2e2/h. The quantum states in the new channels are not only robust against a short-range Anderson disorder, but can also couple with the intrinsic helical edge states in the boundaries of the HgTe nanoribbon to open a gap in the energy spectrum, indicating their topological characteristics. More importantly, the newly developed conductance channels can be turned on or off easily by adjusting the gate voltage. The proposal of controllable topological edge states produced by the gate voltage opens a new route for future topological field-effect quantum transistors in nanoelectronics and spintronics.

One-dimensional weak antilocalization due to the berry phase in HgTe wires.

We study the weak antilocalization (WAL) effect in the magnetoresistance of narrow HgTe wires fabricated in quantum wells with normal and inverted band ordering. Measurements at different gate voltages indicate that the WAL is only weakly affected by Rashba spin-orbit splitting and persists when the Rashba splitting is about zero. The WAL amplitude in wires with normal band ordering is an order of magnitude smaller than for wires with an inverted band structure. These observations are attributed to the Dirac-like dispersion of the energy bands in HgTe quantum wells. From the magnetic-field and temperature dependencies we extract the dephasing lengths and band Berry phases. The weaker WAL for samples with a normal band structure can be explained by a nonuniversal Berry phase which always exceeds π, the characteristic value for gapless Dirac fermions.

Strained Hgte 3D Topological Insulator

First proposed as a new class of matter depicted as a bulk insulator with conducting surfaces, topological insulators have recently stimulated an intensive research activity due to their unique electronic and spin properties. In particular, surface conduction is expected to occur from mass-less Dirac fermions with spin orientation locked to momentum.A general requirement for turning a conventional insulator into its topological counterpart is the realization of an inverted band structure. Such inversion can be achieved experimentally using confinement or band-offset effects in hetero-structures, but may also be intrinsically present in the band structure of low band-gap materials with very large spin-orbit interaction (SOI).Since the SOI scales with Z4, the family of intrinsic topological insulator material thus shrinks to very heavy elements. In particular, the most widely investigated Bi1-xSbx and Bi2Se3/Bi2Te3 compounds have shown, from angle resolved photo-emission spectroscopy, to exhibit such surfaces states in the form of Dirac fermions, but usually suffer from parasitic bulk conduction which has so far prevented the demonstration of convincing surface transport experiments.In this picture, HgTe appears to be a good candidate for topological insulator structures, and we will show that the epitaxial growth of intentionally strained HgTe can provide “insulator-like” band structure material of very high quality. In particular, we will present extensive material characterization of the bulk crystal quality of HgTe and the surfaces and interfaces of HgTe/HgCdTe/CdTe structures designed for electronic transport experiments.Samples processed into top-gated Hall-bar structures are investigated using variable magnetic field, low-temperature, electronic transport set-up. Strong experimental signatures of surface transport of Dirac particles will be given for different epitaxial configurations. Finally, perspective work as well as potential applicative developments will be discussed.

Electron relaxation and mobility in the inverted band quantum well CdTe/Hg1-xCdxTe/CdTe

Electron relaxation processes at nitrogen temperatures in CdTe/Hg1- xCdxTe/CdTe quantum well (QW) with an inverted band structure is modelled. In this structure, scattering by longitudinal optical phonons, charged impurities, acoustic phonons and interfaces were taken into account. It was found that for undoped and lightly doped QWs (concentration of background n-type charged impurities in the well is 10 14 - 10 16 cm -3 or less), for x close to the band inversion value 0.16, the electron mobility grows considerably when the QW width decreases. This mobility is higher for samples with smaller concentrations of charged impurities.