Conventional Two-dimensional Electron Gas Research Articles

Moiré-Pattern Modulated Electronic Structures of GaSe/HOPG Heterostructure.

Conventional two-dimensional electron gas (2DEG) typically occurs at the interface of semiconductor heterostructures and noble metal surfaces, but it is scarcely observed in individual 2D semiconductors. In this study, few-layer gallium selenide (GaSe) grown on highly ordered pyrolytic graphite (HOPG) is demonstrated using scanning tunneling microscopy and spectroscopy (STM/STS), revealing that the coexistence of quantum well states (QWS) and 2DEG. The QWS are located in the valence bands and exhibit a peak feature, with the number of quantum wells being equal to the number of atomic layers. Meanwhile, the 2DEG is located in the conduction bands and exhibits a standing-wave feature. Additionally, monolayer GaSe/HOPG heterostructures with different stacking angles (0°, 33°, 8°) form distinct moiré patterns that arise from lattice mismatch and angular rotation between adjacent atomic layers in 2D materials, which effectively modulate the electron effective mass, charge redistribution, and band gap of GaSe. Overall, this work reveals a paradigm of band engineering based on layer numbers and moiré patterns that can modulate the electronic properties of 2D materials.

Formation of Artificial Fermi Surfaces with a Triangular Superlattice on a Conventional Two-Dimensional Electron Gas.

Imposing an external periodic electrostatic potential to the electrons confined in a quantum well makes it possible to engineer synthetic two-dimensional band structures, with electronic properties different from those in the host semiconductor. Here we report the fabrication and study of a tunable triangular artificial lattice on a GaAs/AlGaAs heterostructure where it is possible to transform from the original GaAs band structure and a circular Fermi surface to a new band structure with multiple artificial Fermi surfaces simply by altering a gate bias. For weak electrostatic modulation magnetotransport measurements reveal multiple quantum oscillations and commensurability oscillations due to the electron scattering from the artificial lattice. Increasing the strength of the modulation reveals new commensurability oscillations of the electrons from the artificial Fermi surface scattering from the triangular artificial lattice. These results show that low disorder gate-tunable lateral superlattices can be used to form artificial two-dimensional crystals with designer electronic properties.

Probing many-body interactions in the cyclotron resonance of h -BN/bilayer graphene/ h -BN

Unlike a conventional two-dimensional electron gas system, which has parabolic band structure, the nonparabolic band dispersion of mono- to few-layer graphene violates Kohn's theorem. Thus, Landau levels (LLs) in graphene are sensitive to many-body interactions. This modifies the LL spacing, depending on the location of the Fermi energy (${E}_{\mathrm{F}}$). Such effects have been extensively studied in $h$-BN/monolayer graphene/$h$-BN through observation of inter-LL optical transitions known as cyclotron resonances (CRs). However, thus far, the influence of many-body interactions on the CR of bilayer graphene (BLG) has been rarely studied, even though BLG also possesses nonparabolic band dispersion. Here, we investigate CR in the $h$-BN/BLG/$h$-BN structure via magneto-photothermoelectric measurements under infrared laser irradiation. This method enables sensitive detection of cyclotron resonances while tuning ${E}_{\mathrm{F}}$ of BLG. The CR magnetic field value shifted significantly when ${E}_{\mathrm{F}}$ of BLG approached the charge-neutrality point (the Dirac point, DP). We attribute this to a change in the Fermi velocity of BLG near the DP, which occurs as a result of many-body interactions.

Carrier concentration independent plasmons in biased twisted bilayer graphene

We calculate the density-density response function of biased twisted bilayer graphene (BTBG) and study its plasmon dispersion within the random phase approximation (RPA). At long wavelengths ($q\ensuremath{\rightarrow}0$), plasmon dispersion shows local classical behavior $\ensuremath{\omega}={\ensuremath{\omega}}_{0}\sqrt{q}$. Unlike the situation in conventional two-dimensional electron gas (2DEG), where the density dependence of the plasmon energy is of the form ${\ensuremath{\omega}}_{0}\ensuremath{\propto}\sqrt{n}$ ($n$ is the carrier concentration), the plasmon energy ${\ensuremath{\omega}}_{0}$ is independent of the carrier concentration ($n$) in biased twisted bilayer graphene. Furthermore, the plasmon energy (${\ensuremath{\omega}}_{0}$) is also independent of the Fermi energy ($\ensuremath{\mu}$) which is decided by the carrier concentration ($n$).

Dynamical polarization function and plasmons in monolayer XSe ( X=In,Ga )

By using the single-band and multiband effective Hamiltonian for monolayer $X\mathrm{Se}$ ($X=\mathrm{In},\mathrm{Ga}$), we calculate the density-density response function of monolayer $X\mathrm{Se}$ ($X=\mathrm{In},\mathrm{Ga}$) and study its plasmon dispersion within the random-phase approximation. At long wavelengths ($q\ensuremath{\rightarrow}0$), plasmon dispersion shows the local classical behavior $\ensuremath{\omega}={\ensuremath{\omega}}_{0}\sqrt{q}$. Although, monolayer $X\mathrm{Se}$ ($X=\mathrm{In},\mathrm{Ga}$) has a nonparabolic ``Mexican hat'' topmost valence band dispersion which is so different from the parabolic band structure in two-dimensional electron gas, the corresponding density dependence of the plasmon energy is still of the form ${\ensuremath{\omega}}_{0}\ensuremath{\propto}\sqrt{n}$ ($n$ is the carrier concentration) as in a conventional two-dimensional electron gas (2DEGs). However, the Fermi energy ($|\ensuremath{\mu}|$) dependence of the plasmon energy is of the form ${\ensuremath{\omega}}_{0}\ensuremath{\propto}{|\ensuremath{\mu}|}^{1/4}$, which is different from the conventional 2DEGs (${\ensuremath{\omega}}_{0}\ensuremath{\propto}\sqrt{|\ensuremath{\mu}|}$).

Landau quantization and highly mobile fermions in an insulator.

In strongly correlated materials, quasiparticle excitations can carry fractional quantum numbers. An intriguing possibility is the formation of fractionalized, charge-neutral fermions-for example, spinons1 and fermionic excitons2,3-that result in neutral Fermi surfaces and Landau quantization4,5 in an insulator. Although previous experiments in quantum spin liquids1, topological Kondo insulators6-8 and quantum Hall systems3,9 have hinted at charge-neutral Fermi surfaces, evidence for their existence remains inconclusive. Here we report experimental observation of Landau quantization in a two-dimensional insulator, monolayer tungsten ditelluride (WTe2), a large-gap topological insulator10-13. Using a detection scheme that avoids edge contributions, we find large quantum oscillations in the material's magnetoresistance, with an onset field as small as about 0.5tesla. Despite the huge resistance, the oscillation profile, which exhibits many periods, mimics the Shubnikov-de Haas oscillations in metals. At ultralow temperatures, the observed oscillations evolve into discrete peaks near 1.6tesla, above which the Landau quantized regime is fully developed. Such a low onset field of quantization is comparable to the behaviour of high-mobility conventional two-dimensional electron gases. Our experiments call for further investigation of the unusual ground state of the WTe2 monolayer, including the influence of device components and the possible existence of mobile fermions and charge-neutral Fermi surfaces inside its insulating gap.

Photon drag of superconducting fluctuations in two-dimensional systems

The theory of photon drag of superconducting fluctuations in the two-dimensional electron gas is developed. It is shown that the frequency dependence of the induced current is qualitatively similar to the case of photon drag of conventional two-dimensional degenerate electron gas. With the decreasing temperature the magnitude of the effect increases dramatically and the current of superconducting fluctuations carries an additional power of reduced temperature in comparison with the Aslamazov-Larkin contribution. The magnitude of the developed effect is expected sufficient to be visible against the conventional photocurrent background.

Dynamical density response and collective modes of topological-insulator ultrathin films

We analytically calculate the intra- and inter-surface dynamical density-density linear responses of ultra-thin topological insulator films with finite tunneling between their top and bottom surfaces in both metallic and insulating regimes. Employing the random phase approximation we investigate the dispersions of in-phase and out-of-phase collective density modes of this system in the metallic regime. We find that in contrast to the bilayers of the conventional two-dimensional electron gas, where finite tunneling gaps out the out-of-phase mode, in topological insulator thin films, this mode remains linear at long wavelengths. Depending on different system parameters, the velocity of out-of-phase mode can be tuned to be larger or substantially smaller than the Fermi velocity of electrons on the isolated surfaces of the topological insulator. Finite tunneling generally reduces the energy of collective modes, making them more confined in space.

Polymer-encapsulated molecular doped epigraphene for quantum resistance metrology

One of the aspirations of quantum metrology is to deliver primary standards directly to end-users thereby significantly shortening the traceability chains and enabling more accurate products. Epitaxial graphene grown on silicon carbide (epigraphene) is known to be a viable candidate for a primary realisation of a quantum Hall resistance standard, surpassing conventional semiconductor two-dimensional electron gases, such as those based on GaAs, in terms of performance at higher temperatures and lower magnetic fields. The bottleneck in the realisation of a turn-key quantum resistance standard requiring minimum user intervention has so far been the need to fine-tune the carrier density in this material to fit the constraints imposed by a simple cryo-magnetic system. Previously demonstrated methods, such as via photo-chemistry or corona discharge, require application prior to every cool-down as well as specialist knowledge and equipment. To this end we perform metrological evaluation of epigraphene with carrier density tuned by a recently reported permanent molecular doping technique. Measurements at two National Metrology Institutes confirm accurate resistance quantisation below 5 nΩ Ω−1. Furthermore, samples show no significant drift in carrier concentration and performance on multiple thermal cycles over three years. This development paves the way for dissemination of primary resistance standards based on epigraphene.

Spin-charge conversion in InSe bilayers

We find that the bilayer InSe possesses an intrinsic Rashba spin-orbit coupling and hence a spin-charge conversion effect due to the breaking of mirror symmetry. The interplay between the Rashba spin-orbit coupling and the nonparabolic Mexican hat dispersion leads to an interesting intraband Lifshitz transition and strongly amplifies the spin-charge conductivity, making it larger than that in conventional two-dimensional electron gas formed at oxide interfaces. This highlights bilayer InSe as a strong candidate for next-generation spintronic devices.

Phase transition and anomalous scaling in the quantum Hall transport of topological-insulator Sn−Bi1.1Sb0.9Te2S devices

The scaling physics of quantum Hall transport in optimized topological insulators with a plateau precision of ~1/1000 e2/h is considered. Two exponential scaling regimes are observed in temperature-dependent transport dissipation, one of which accords with thermal activation behavior with a gap of 2.8 meV (> 20 K), the other being attributed to variable range hopping (1-20 K). Magnetic field-driven plateau-to-plateau transition gives scaling relations of (dR$_{xy}$/dB)$^{max}$ \propto T$^{-\kappa}$ and \DeltaB$^{-1}$ \propto T$^{-\kappa}$ with a consistent exponent of \kappa ~ 0.2, which is half the universal value for a conventional two-dimensional electron gas. This is evidence of percolation assisted by quantum tunneling, and reveals the dominance of electron-electron interaction of the topological surface states.

Emergent Josephson current of N=1 chiral topological superconductor in quantum anomalous Hall insulator/superconductor heterostructures

Recently, a quantum anomalous Hall insulator (QAHI)/superconductor heterostructure has been realized and shows half-quantized conductance plateaus in two-terminal conductance measurements [Q. L. He \textit{et al.}, Science {\bf357}, 294 (2017)]. The half-quantized conductance plateaus are considered as a solid evidence of chiral Majorana edge modes. However, there is a strong debate over the origin of the half-quantized conductance plateaus. In this work, we propose a Josephson junction based on the QAHI/superconductor heterostructure to identify the existence of chiral Majorana edge modes. We find that the critical Josephson current dramatically increases to a peak value when a half-quantized conductance plateau $\sigma_{12}=e^2/2h$ is showing up for the $N=1$ chiral topological superconductor phase with a single chiral Majorana mode. Furthermore, we show that the critical Josephson current of the $N=1$ chiral topological superconductor exhibits an $h/e$-period oscillation and is robust to disorder, in contrast to the behaviors of conventional two-dimensional electron gas systems. We also estimate experimentally relevant parameters and believe that the supercurrent can be observed in experiments.

Magnetothermoelectric transport properties in phosphorene

We numerically study the electrical and thermoelectric transport properties in phosphorene in the presence of both a magnetic field and disorder. The quantized Hall conductivity is similar to that of a conventional two-dimensional electron gas, but the positions of all the Hall plateaus shift to the left due to the spectral asymmetry, in agreement with the experimental observations. The thermoelectric conductivity and Nernst signal exhibit remarkable anisotropy, and the thermopower is nearly isotropic. When a bias voltage is applied between top and bottom layers of phosphorene, both thermopower and Nernst signal are enhanced and their peak values become large.

Twisted Fermi surface of a thin-film Weyl semimetal

The Fermi surface of a conventional two-dimensional electron gas is equivalent to a circle, up to smooth deformations that preserve the orientation of the equi-energy contour. Here we show that a Weyl semimetal confined to a thin film with an in-plane magnetization and broken spatial inversion symmetry can have a topologically distinct Fermi surface that is twisted into a figure-8—opposite orientations are coupled at a crossing which is protected up to an exponentially small gap. The twisted spectral response to a perpendicular magnetic field B is distinct from that of a deformed Fermi circle, because the two lobes of a figure-8 cyclotron orbit give opposite contributions to the Aharonov–Bohm phase. The magnetic edge channels come in two counterpropagating types, a wide channel of width and a narrow channel of width (with the magnetic length and β the momentum separation of the Weyl points). Only one of the two is transmitted into a metallic contact, providing unique magnetotransport signatures.

Electrical and optical transport properties of single layer WSe2

The electronic properties of single layer WSe2 are distinct from the famous graphene due to strong spin orbit coupling, a huge band gap and an anisotropic lifting of the degeneracy of the valley degree of freedom under Zeeman field. In this work, band structure of the monolayer WSe2 is evaluated in the presence of spin and valley Zeeman fields to study the electrical and optical transport properties. Using Kubo formalism, an explicit expression for the electrical Hall conductivity is examined at finite temperatures. The electrical longitudinal conductivity is also evaluated. Further, the longitudinal and Hall optical conductivities are analyzed. It is observed that the contributions of the spin-up and spin-down states to the power absorption spectrum depend on the valley index. The numerical results exhibit absorption peaks as a function of photon energy, ℏω, in the range ∼1.5−2 eV. Also, the optical response lies in the visible frequency range in contrast to the conventional two-dimensional electron gas or graphene where the response is limited to terahertz regime. This ability to isolate carriers in spin-valley coupled structures may make WSe2 a promising candidate for future spintronics, valleytronics and optical devices.

Coupling between graphene and intersubband collective excitations in quantum wells

Recently, strong light-matter coupling between the electromagnetic modes in plasmonic metasurfaces with quantum-engineering electronic intersubband transitions in quantum wells has been demonstrated experimentally (Benz et al., [14], Lee et al., [15]). These novel materials combining different two-dimensional electronic systems offer new opportunities for tunable optical devices and fundamental studies of collective excitations driven by interlayer Coulomb interactions. In this work, our aim is to study the plasmon spectra of a hybrid structure consisting of conventional two-dimensional electron gas (2DEG) in a semiconductor quantum well and a graphene sheet with an interlayer separation of a. This electronic bilayer structure is immersed in a nonhomgeneous dielectric background of the system. We use a simple model in which the graphene surface plasmons and both; the intrasubband and intersubband collective electron excitations in the quantum well are coupled via screened Coulomb interaction. Here we calculate the dispersion of these relativistic/nonrelativistic new plasmon modes taking into account the thickness of the quantum well providing analytical expressions in the long-wavelength limit.

Anisotropic transport of normal metal-barrier-normal metal junctions in monolayer phosphorene

We study transport properties of a phosphorene monolayer in the presence of single and multiple potential barriers of height U0 and width d, using both continuum and microscopic lattice models, and show that the nature of electron transport along its armchair edge (x direction) is qualitatively different from its counterpart in both conventional two-dimensional electron gas with Schrödinger-like quasiparticles and graphene or surfaces of topological insulators hosting massless Dirac quasiparticles. We show that the transport, mediated by massive Dirac electrons, allows one to achieve collimated quasiparticle motion along x and thus makes monolayer phosphorene an ideal experimental platform for studying Klein paradox in the context of gapped Dirac materials. We study the dependence of the tunneling conductance as a function of d and U0, and demonstrate that for a given applied voltage V its behavior changes from oscillatory to decaying function of d for a range of U0 with finite non-zero upper and lower bounds, and provide analytical expression for these bounds within which G decays with d. We contrast such behavior of G with that of massless Dirac electrons in graphene and also with that along the zigzag edge (y direction) in phosphorene where the quasiparticles obey an effective Schrödinger equation at low energy. We also study transport through multiple barriers along x and demonstrate that these properties hold for transport through multiple barriers as well. Finally, we suggest concrete experiments which may verify our theoretical predictions.

Corrigendum: Artificial Gauge Field and Topological Phase in a Conventional Two-dimensional Electron Gas with Antidot Lattices.

Scientific Reports 5: Article number: 15266; published online: 16 October 2015; updated: 07 July 2016 The Authors neglected to cite previous studies related to the existence of intersubband spin-orbit coupling and the use of intersubband spin-orbit coupling to create topological insulator in double quantum wells with antidote lattices.

Charge Puddles in Graphene near the Dirac Point.

The charge carrier density in graphene on a dielectric substrate such as SiO_{2} displays inhomogeneities, the so-called charge puddles. Because of the linear dispersion relation in monolayer graphene, the puddles are predicted to grow near charge neutrality, a markedly distinct property from conventional two-dimensional electron gases. By performing scanning tunneling microscopy and spectroscopy on a mesoscopic graphene device, we directly observe the puddles' growth, both in spatial extent and in amplitude, as the Fermi level approaches the Dirac point. Self-consistent screening theory provides a unified description of both the macroscopic transport properties and the microscopically observed charge disorder.

Quantum magnetotransport properties of aMoS2monolayer

We study transport properties of a ${\text{MoS}}_{2}$ monolayer in the presence of a perpendicular magnetic field $B$. We derive and discuss its band structure and take into account spin and valley Zeeman effects. Compared to a conventional two-dimensional electron gas, these effects lead to new quantum Hall plateaus and new peaks in the longitudinal resistivity as functions of the magnetic field. The field $B$ leads to a significant enhancement of the spin splitting in the conduction band, to a beating of the Shubnikov--de Haas (SdH) oscillations in the low-field regime, and to their splitting in the high-field regime. The Zeeman fields suppress significantly the beating of the SdH oscillations in the low-field regime and strongly enhance their splitting at high fields. The spin and valley polarizations show a similar beating pattern at low fields and are clearly separated at high fields in which they attain a value higher than $90%$.