Properties Of Two-dimensional Electron Systems Research Articles

BinPo: An open-source code to compute the band structure of two-dimensional electron systems

We introduce BinPo, an open-source Python code to compute electronic properties of two-dimensional electron systems. A bulk tight binding Hamiltonian is constructed from relativistic density functional theory calculations represented in the basis of maximally localized Wannier functions. BinPo has a Schrödinger-Poisson solver, integrating an electric field-dependent relative permittivity to obtain self-consistently the confining electrostatic potential energy term in the derived tight binding slab system. The band structure, energy slices, and other properties, along with different projections and orientations can be computed. High resolution and publishable figures of the simulations can be generated. In BinPo, priority has been given to ease-of-use, efficiency, readability and modularity, therefore becoming suitable to produce reliable electronic structures simulations at low computational cost. Along with the code itself, we provide files from first-principles calculations for some materials, instructions of use, and detailed examples of its wide range of capabilities. The code was developed with a focus on the ABO3 perovskite structure-based systems, such as SrTiO3 and KTaO3, because of their increasing impact in the materials community. Some features, such as the projection onto orbital states, are restricted to calculations using the relevant t2g orbitals for this family of materials, yet it is possible to include more elements in the basis for the band structure determination of other systems. The use of a relativistic approach allows for the inspection of the role of spin-orbit coupling and the resulting Rashba effect on the systems. We detail the approaches used in the code, so that it can be further exploited and adapted to other problems, such as adding new materials and functionalities which can strength the initial code scopes. Program summaryProgram title: BinPoCPC Library link to program files:https://doi.org/10.17632/zk4xy7vfy4.1Developer's repository link:https://github.com/emanuelm33/BinPoCode Ocean capsule:https://codeocean.com/capsule/0987578Licensing provisions: GPLv3Programming language: PythonExternal routines/libraries: NumPy, SciPy, Matplotlib, ASE.Nature of problem: Compute the band structure and other electronic properties in two-dimensional electron systems.Solution method: Construction of a slab Hamiltonian from one built using a basis of maximally localized Wannier functions. Solve the Schrödinger-Poisson scheme to get the self-consistent potential energy along the slab. Use this potential to compute the band structure and additional properties by solving the Schrödinger equation.Additional comments including restrictions and unusual features: Other features included are the visualization of the self-consistent process and final solutions, band structure and energy slices with different projections. High resolution and customizable figures can be obtained in all cases.

Structural, elastic, and electronic properties of chemically functionalized boron phosphide monolayer.

Surface functionalization is one of the useful techniques for modulating the mechanical and electronic properties of two-dimensional systems. In the present study, we investigate the structural, elastic, and electronic properties of hexagonal boron phosphide monolayer functionalized by Br and Cl atoms using first-principles predictions. Once surface-functionalized with Br/Cl atoms, the planar structure of BP monolayer is transformed to the low-buckled lattice with the bucking constant of about 0.6 Å for all four configurations of functionalized boron phosphide, i.e., Cl–BP–Cl, Cl–BP–Br, Br–BP–Cl, and Br–BP–Br. The stability of functionalized BP monolayers is confirmed via their phonon spectra analysis and ab initio molecular dynamics simulations. Our calculations indicate that the functionalized BP monolayers possess a fully isotropic elastic characteristic with the perfect circular shape of the angle-dependent Young's modulus and Poisson's ratio due to the hexagonal symmetry. The Cl–BP–Cl is the most stiff with the Young's modulus C2D = 43.234 N m−1. All four configurations of the functionalized boron phosphide are direct semiconductors with a larger band gap than that of a pure BP monolayer. The outstanding stability, isotropic elastic properties, and moderate band gap make functionalized boron phosphide a very intriguing candidate for next-generation nanoelectromechanical devices.

High sensitivity variable-temperature infrared nanoscopy of conducting oxide interfaces

Probing the local transport properties of two-dimensional electron systems (2DES) confined at buried interfaces requires a non-invasive technique with a high spatial resolution operating in a broad temperature range. In this paper, we investigate the scattering-type scanning near field optical microscopy as a tool for studying the conducting LaAlO3/SrTiO3 interface from room temperature down to 6 K. We show that the near-field optical signal, in particular its phase component, is highly sensitive to the transport properties of the electron system present at the interface. Our modeling reveals that such sensitivity originates from the interaction of the AFM tip with coupled plasmon–phonon modes with a small penetration depth. The model allows us to quantitatively correlate changes in the optical signal with the variation of the 2DES transport properties induced by cooling and by electrostatic gating. To probe the spatial resolution of the technique, we image conducting nano-channels written in insulating heterostructures with a voltage-biased tip of an atomic force microscope.

Direct extraction of electron parameters from magnetoconductance analysis in mesoscopic ring array structures

We report an approach for examining electron properties using information about the shape and size of a nanostructure as a measurement reference. This approach quantifies the spin precession angles per unit length directly by considering the time-reversal interferences on chaotic return trajectories within mesoscopic ring arrays (MRAs). Experimentally, we fabricated MRAs using nanolithography in InGaAs quantum wells which had a gate-controllable spin-orbit interaction (SOI). As a result, we observed an Onsager symmetry related to relativistic magnetic fields, which provided us with indispensable information for the semiclassical billiard ball simulation. Our simulations, developed based on the real-space formalism of the weak localization/antilocalization effect including the degree of freedom for electronic spin, reproduced the experimental magnetoconductivity (MC) curves with high fidelity. The values of five distinct electron parameters (Fermi wavelength, spin precession angles per unit length for two different SOIs, impurity scattering length, and phase coherence length) were thereby extracted from a single MC curve. The methodology developed here is applicable to wide ranges of nanomaterials and devices, providing a diagnostic tool for exotic properties of two-dimensional electron systems.

Spin polarization and exchange-correlation effects in transport properties of two-dimensional electron systems in silicon

We show that the parallel magnetic field-induced increase in the critical electron density for the Anderson transition in a strongly interacting two-dimensional electron system is caused by the effects of exchange and correlations. If the transition occurs when electron spins are only partially polarized, additional increase in the magnetic field is necessary to achieve the full spin polarization in the insulating state due to the exchange effects.

Electron confinement induced by diluted hydrogen-like ad-atoms in graphene ribbons

We report the electronic properties of two-dimensional systems made of graphene nanoribbons, which are patterned with ad-atoms in two separated regions. Due to the extra electronic confinement induced by the presence of impurities, we find resonant levels, quasi-bound and impurity-induced localized states, which determine the transport properties of the system. Regardless of the ad-atom distribution in the system, we apply band-folding procedures to simple models and predict the energies and the spatial distribution of those impurity-induced states. We take into account two different scenarios: gapped graphene and the presence of randomly distributed ad-atoms in a low dilution regime. In both cases the defect-induced resonances are still detected. Our findings would encourage experimentalists to synthesize these systems and characterize their quasi-localized states by employing, for instance, scanning tunneling spectroscopy (STS). Additionally, the resonant transport features could be used in electronic applications and molecular sensing devices.

Antilocalization sensing of interactions between two-dimensional electrons and surface species

We apply antilocalization measurements to experimentally study the interactions and exchange between InAs surface accumulation electrons and local magnetic moments of the rare earth ions Sm3+, Gd3+, Ho3+, and Dy3+, of the transition metal ions Ni2+, Co2+, and Fe3+, and of Fe3O4 nanoparticles and Fe3+-phthalocyanine deposited on the surface. The influence of the deposited species on the surface electrons is observed through the changes in the spin–orbit scattering and magnetic spin-flip scattering rates, which carry information about magnetic interactions. Experiments indicate a temperature-dependent magnetic spin-flip scattering for Ho3+, Dy3+, Ni2+, and Co2+. Concerning the spin–orbit scattering rate, we observe an increase, except for the cases of Ni2+, Fe3+, Fe3O4 nanoparticles and Fe3+-phthalocyanine. We also observe an increase in SO scattering in another system where we study the interactions of Au nanoparticles and ferromagnetic Co0.6Fe0.4 nanopillars and an In0.53Ga0.47As quantum well. Experimental results are analyzed and compared to theoretical models. Our method provides a controlled way to probe the quantum properties of two-dimensional electron systems, either on the surface of InAs or in a quantum well.

Ferromagnetic Instability and Finite-Temperature Properties of Two-Dimensional Electron Systems with van Hove Singularities

We study a ferromagnetic tendency in the two-dimensional Hubbard model near van Hove filling by using a functional renormalization-group method. We compute temperature dependences of magnetic susceptibilities including incommensurate magnetism. The ferromagnetic tendency is found to occur in a dome-shaped region around van Hove filling with an asymmetric property: incommensurate magnetism is favored near the edge of the dome above van Hove filling whereas a first-order-like transition to the ferromagnetic ground state is expected below van Hove filling. The dome-shaped phase diagram is well captured in the Stoner theory by invoking a smaller Coulomb interaction. Triplet p-wave superconductivity tends to develop at low temperatures inside the dome and extends more than the ferromagnetic region above van Hove filling.

The interplay of Landau level broadening and temperature on two-dimensional electron systems

This work investigates the influence of low temperature and broadened Landau levels on the thermodynamic properties of two-dimensional electron systems. The interplay between these two physical parameters on the magnetic field dependence of the chemical potential, the specific heat and the magnetization is calculated. In the absence of a complete theory that explains the Landau level broadening, experimental and theoretical studies in literature perform different model calculations of this parameter. Here it is presented that different broadening parameters of Gaussian-shaped Landau levels cause width variations in their contributions to interlevel and intralevel excitations. Below a characteristic temperature, the interlevel excitations become negligible. Likewise, at this temperature range, the effect of the Landau level broadening vanishes.

Optical mapping of properties of two-dimensional electron system in magnetic fields

Real-space mappings of a Hall photovoltage are performed by local laser excitation using optical microscope setup with a spatial resolution of about 2 μ m . The obtained images of broad and narrow strips presumably reflect the diffusion of the optically created electrons in the bulk and the edge states. The image near a current contact is considered to reflect the potential gradient near the contact.

Electric control of spin polarization orientation in a magnetic–electric barrier structure

We investigate spin-dependent transport properties of two-dimensional electron systems modulated by both the stray field from a ferromagnetic metal (FM) stripe and the electrostatic potential (EP) provided by two normal metal Schottky stripes which sandwich the FM stripe. The EP consists of a single barrier and a single well. By switching the gate voltages applied to the Schottky stripes, the barrier and the well exchange their positions. Accordingly, the spatial region, where spin-dependent resonant tunneling happens, is transferred from one side of the FM stripe to the other, resulting in a rotation or a reversal of the spin polarization orientation.

Transport properties of two-dimensional electron systems on silicon (111) surfaces

We theoretically study transport properties of a two-dimensional electron system on a hydrogen-passivated Si(111) surface in the field-effect-transistor (FET) configuration. We calculate the density and temperature dependent mobility and resistivity for the recently fabricated Si(111)-vacuum FET by using a semiclassical Boltzmann theory including screened charged impurity scattering. We find reasonable agreement with the corresponding experimental transport properties, indicating that the screened disorder potential from random charged impurities is the main scattering mechanism. We also find that the theoretical results with the valley degeneracy $g_v=2$ give much better agreement with experiment than the $g_v=6$ situation indicating that the usual bulk six-valley degeneracy of Si is lifted in this system.

Goldstone Mode Induced by Skyrmions in Quantum Hall Systems

The properties of two-dimensional electron system exhibit rich physics in the presence of strong magnetic field. Electron-electron interactions add more interesting many-body phenomena to the systems, such as the frractional quantum Hall effect. From field theory method [1,2], a new type of topological excitation, Skyrmion, is predicted to occur around filling factor, 1/m, with odd integer m. The optically pumped nuclear magnetic resonance (OPNMR) measurements [3] support the existence of Skyrmions or spin texture excitations. Recently the present author has proposed that in quasi-2 + 1 dimensional quantum antiferromagnet the doped hole induces magnetic-disordered hedgehog-like soliton, which is composed of the doped hole and the cloud of SU(2) Yang-Mills fields with the spin disorder around the hole, by using the gauge-invariant effective Lagrangian with spontaneous symmetry breaking [4-7]. In this study, extending the theoretical formula, we introduce the gaugeinvariant effective Lagrangian density for the skyrmions around the filling factor = 1, and present the massless gauge field (Goldstone mode) induced by the Skyrmion. The massless gauge field introduces the long-range interaction, which will play an important role in the Skyrme crystal [8]. In addition, we suggest the presence of the skyrmion glass.

Collective effects in the transport properties of two-dimensional electron systems

The results of the experimental study of the nontraditional manifestations of collective effects in the transport properties of two-dimensional electron systems are reported.

Subsurface charge accumulation imaging of a quantum Hall liquid

The unusual properties of two-dimensional electron systems that give rise to the quantum Hall effect have prompted the development of new microscopic models for electrical conduction1,2,3,4,5,6. The bulk properties of the quantum Hall effect have also been studied experimentally using a variety of probes including transport7,8, photoluminescence9,10, magnetization11 and capacitance12,13 measurements. However, the fact that two-dimensional electron systems typically exist some distance (about 1,000 A) beneath the surface of the host semiconductor has presented an important obstacle to more direct measurements of microscopic electronic structure in the quantum Hall regime. Here we introduce a cryogenic scanning-probe technique—‘subsurface charge accumulation’ imaging—that permits very high resolution examination of systems of mobile electrons inside materials. We use this technique to image directly the nanometre-scale electronic structures that exist in the quantum Hall regime.

Wigner crystallization of two-dimensional electron systems on solid H2-films

We have investigated the transport properties of two-dimensional electron systems (2DES) on quench-condensed hydrogen films for various electron densities up to n e =1.5× 10 cm -2 . In the range of the plasma parameter Γ= / from 40 to 180 we have observed a behavior lnμ∞Γ of the electron mobility μ with a distinct transition from a smaller slope for Γ 125. This signature is interpreted as the Wigner crystallization of the 2DES

Novel strained InP/InAsxP1−x quantum-well modulation-doped heterostructures

We report the first investigation of transport properties of two-dimensional electron systems in strained InAsxP1−x quantum-well channels. The InP/InAsxP1−x modulation-doped heterostructures were grown by the low-pressure OMCVD technique. The dependence of the transport parameters on the arsenic composition (x) has been studied using Hall, Shubnikov de Haas, and cyclotron resonance measurements. Hall mobilities of 6100 and 52 700 cm2/V s, with carrier concentrations of 2.3×1012 and 1.5×1012 cm−2 at 300 and 77 K, respectively, were measured for x=0.6.

Transport properties of InxGa1−xAs/GaAs strained quantum well delta-doped heterostructures grown by molecular beam epitaxy

We have investigated the transport properties of two-dimensional electron systems in strained InxGa1−xAs channels confined in a potential well formed by delta doping and GaAs barriers. The dependence of the transport parameters on the indium composition has been studied using Hall, Shubnikov–de Haas, and cyclotron resonance measurements. Experimental measurements of the effective mass have been compared with theoretical data obtained from self-consistent calculations, which take account of effects due to biaxial strain and nonparabolicity on the band structure of InxGa1−xAs.

Electronic Properties of Two-Dimensional Systems

A tutorial review is given of some of the basic electronic properties of two-dimensional systems and their physical realisations.

Electronic properties of two-dimensional systems

The electronic properties of inversion and accumulation layers at semiconductor-insulator interfaces and of other systems that exhibit two-dimensional or quasi-two-dimensional behavior, such as electrons in semiconductor heterojunctions and superlattices and on liquid helium, are reviewed. Energy levels, transport properties, and optical properties are considered in some detail, especially for electrons at the (100) silicon-silicon dioxide interface. Other systems are discussed more briefly.