The role of radiative damping in shaping of the cyclotron resonance line in a two-dimensional electron system

It is shown that the broadening of a cyclotron resonance line in a two-dimensional (2D) electron system relative to the inverse scattering time of electrons is connected with the radiative damping accompanying the cyclotron motion of electrons. Using the radiative damping concept, a simple and physically informative formula describing the cyclotron resonance curve shape in a 2D electron system was obtained.     

The effect of interference in the substrate on the electromagnetic wave polarization transformation under cyclotron resonance conditions in a two-dimensional electron system

The effect of interference in the substrate on the polarization transformation of an electromagnetic wave in the presence of cyclotron resonance in a two-dimensional (2D) electron system is studied theoretically. It is predicted that the interference can dramatically enhance the transformation if the 2D electron system is situated on the rear face of a quarter-wave substrate plate. As a result, the reflected wave exhibits a virtually complete polarization transformation.     

3D reconstruction and characterization of polycrystalline microstructures using a FIB–SEM system

A novel methodology is described in this paper which is a step towards three-dimensional representation of grain structures for microstructure characterization and processing microstructural data for subsequent computational analysis. It facilitates evaluation of stereological parameters of grain structures from a series of two-dimensional (2D) electron backscatter diffraction (EBSD) maps. Crystallographic orientation maps of consecutive serial sections of a micron-size specimen are collected in an automated manner using a dual-beam focused ion beam–scanning electron microscope (FIB–SEM) outfitted with an EBSD system. Analysis of the serial-sectioning data is accomplished using a special purpose software program called “Micro-Imager”. Micro-Imager is able to output characterization parameters such as the distribution of grain size, number of neighboring grains, and grain orientation and misorientation for every 2D section. Some of these data can be compared with results from stereological exercises. Stacking the 2D statistical information obtained from the analysis of the serial-sectioning data provides a means to quantify the variability of grain structure in 3D.     

Bioinspired detection of light using a porphyrin-sensitized single-wall nanotube field effect transistor

Single-wall carbon nanotube (SWNT) field effect transistors (FETs), functionalized noncovalently with a zinc porphyrin derivative, were used to directly detect a photoinduced electron transfer (PET) within a donor/acceptor (D/A) system. We report here that the SWNTs act as the electron donor and the porphyrin molecules as the electron acceptor. The magnitude of the PET was measured to be a function of both the wavelength and intensity of applied light, with a maximum value of 0.37 electrons per porphyrin for light at 420 nm and 100 W/m2. A complete understanding of the photophysics of this D/A system is necessary, as it may form the basis for applications in artificial photosynthesis and alternative energy sources such as solar cells.     

Proximity Coupling of Graphene to a Submonolayer 2D Magnet

Imprinting magnetism into graphene may lead to unconventional electron states and enable the design of spin logic devices with low power consumption. The ongoing active development of 2D magnets suggests their coupling with graphene to induce spin-dependent properties via proximity effects. In particular, the recent discovery of submonolayer 2D magnets on surfaces of industrial semiconductors provides an opportunity to magnetize graphene coupled with silicon. Here, synthesis and characterization of large-area graphene/Eu/Si(001) heterostructures combining graphene with a submonolayer magnetic superstructure of Eu on silicon are reported. Eu intercalation at the interface of the graphene/Si(001) system results in a Eu superstructure different from those formed on pristine Si in terms of symmetry. The resulting system graphene/Eu/Si(001) exhibits 2D magnetism with the transition temperature controlled by low magnetic fields. Negative magnetoresistance and the anomalous Hall effect in the graphene layer provide evidence for spin polarization of the carriers. Most importantly, the graphene/Eu/Si system seeds a class of graphene heterostructures based on submonolayer magnets aiming at applications in graphene spintronics.     

First dc Measurements of Electrons on Liquid Helium: the Helium-FET

We present the first dc-measurements on a 2-dimensional (2D) electron system floating above a liquid ⁴ He-film which covers a structured metal surface. With our arrangement of a source-, gate-, and drain-electrode a 2-dimensional charge transport is realized in analogy to a field-effect-transistor. The electrons which are moving along the x-direction due to different dc potentials are directly measured. This dc current, of the order of pA, is strongly dependent on the applied split-gate voltage. So the electrons were laterally confined to a narrow channel between the two gate electrodes. The effective width of the channel is reduced by the gate potential, so that a quasi-1D configuration can be realized. The measured electron current through the split-gate is analyzed and discussed on grounds of reduced dimensionality and 1D electron transport behaviour.     

Viscoelastic effects in a two-dimensional classical electron liquid

The shear viscosity of a classical two-dimensional (2D) electron liquid is estimated by adapting the theory of Kirkwood, Buff, and Green for three dimensions to two dimensions. It is found to be large enough so that shear modes, if not overdamped by other scattering mechanisms, should be able to propagate through the electron liquid above a minimum temperature-dependent frequency, which is a small fraction of the highest frequency in the corresponding 2D electron solid.     

Indirect evidence for the existence of viscoelastic shear modes in a two-dimensional classical electron liquid

It has recently been predicted that viscoelastic shear modes should propagate in a two-dimensional (2D) classical electron liquid above a minimum temperature dependent frequency. A 2D electron layer held above a liquid helium substrate is considered in this paper. The contribution of scattering between viscoelastic shear modes of the electron fluid and ripplons on the liquid helium surface to the mobility of electrons is calculated. This contribution explains the differences observed between the existing experimental data and single electron-ripplon scattering calculations quite well. This may be taken as indirect evidence for the existence of viscoelastic shear modes in a 2D electron liquid.     

Temperature Dependence of Polaronic Correction to the Ground State Energy in a GaAs Parabolic Quantum Dot

Within the framework of second-order Rayleigh-Schrodinger perturbation theory (RSPT), the polaronic corrections to the ground state (GS) energy of an electron in both two-dimensional (2D) and three-dimensional (3D) parabolic quantum dots (QDs) are presented at finite temperature. We apply our calculations to GaAs. It is found that the polaronic corrections to the GS energy of an electron in both 2D and 3D QDs increase with temperature increasing and size of the QD decreasing. Furthermore, this trend is much more pronounced with dimensionality decreasing.     

Direct identification of monolayer rhenium diselenide by an individual diffraction pattern

In the current extensive studies of layered two-dimensional (2D) materials,compared to hexagonal structures such as graphene,hBN,and MoS2,lowsymmetry 2D materials have shown great potential for applications in anisotropic devices.Rhenium diselenide (ReSe2) possesses the bulk space group P(1) and belongs to the triclinic crystal system with a deformed cadmium-iodide-type structure.Here,we propose an electron diffraction-based method to distinguish the monolayer ReSe2 membrane from multilayer ReSe2 and its two different vertical orientations.Our method is also applicable to other low-symmetry crystal systems,including both triclinic and monoclinic lattices,as long as their third unit-cell basis vectors are not perpendicular to the basal plane.Our experimental results are well explained by kinematical electron diffraction theory and the corresponding simulations.Generalization of our method to other 2D materials,such as graphene,is also discussed.     

Local strain in a 3D nano-crystal revealed by 2D coherent X-ray diffraction imaging

The coherent X-ray diffraction from an isolated strained nano-crystal is given by the Fourier transform of a complex-valued electron density where the modulus and phase are linked to the physical electron density and the displacement field, respectively. The possibility to reconstruct a complex-valued object from a coherent diffraction pattern is demonstrated using iterative algorithms. In the case of a 2D intensity slice, the reconstructed 2D object is strongly dependent on the distribution function of the displacement field values along the direction perpendicular to the observation plane. It is shown that valuable 3D information can, however, still be extracted. This work is of particular interest as soon as the complete 3D measurement is not accessible.     

Magnetic impurity in a luttinger liquid: A view from conformal field theory

Exact results are reported for the low-temperature thermodynamics of a spin-1/2 magnetic impurity coupled to a 1D interacting electron system. The use of conformal field theory techniques reveals that there are only two types of critical behaviors consistent with the symmetries of the problem: either a local Fermi liquid, or else a theory identical to that recently proposed by Furusaki and Nagaosa. We also show that forward electron scattering on the impurity produces the same critical behavior as the two-channel Kondo effect for noninteracting electrons.This work is supported by NSF grant DMR-9205125 (P. F.), and a grant from the Swedish Natural Science Research Council (H. J.).     

Two-dimensional SnS nanosheets fabricated by a novel hydrothermal method

Two-dimensional (2D) Tin (II) sulfide (SnS) nanosheets were successfully synthesized by a novel thioglycolic acid (TGA) assisted hydrothermal method. X-ray diffraction characterization reveals that the product is well-crystallized SnS with orthorhombic structure. Transmission electron microscopy observation shows that the SnS crystals display 2D sheet-like nanomorphology. Further structure characterization by selected area electron diffraction identifies that the SnS nanosheets are single crystalline in nature. Furthermore, the mechanism and critical factors for the TGA-assisted hydrothermal synthesis of the SnS nanosheets have been preliminarily discussed.     

Nanopillar arrays on semiconductor membranes as electron emission amplifiers

A new transmission-type electron multiplier was fabricated from silicon-on-insulator (SOI) material by integrating an array of one-dimensional (1D) silicon nanopillars onto a two-dimensional (2D) silicon membrane. Primary electrons are injected into the nanopillar-membrane (NPM) system from the flat surface of the membrane, while electron emission from the nanopillars is probed by an anode. The secondary electron yield (SEY) from the nanopillars in the current device is found to be about 1.8 times that of the plain silicon membrane. This gain in electron number is slightly enhanced by the electric field applied from the anode. Further optimization of the dimensions of the NPM and an application of field emission promise an even higher gain for detector applications and allow for probing of electronic/mechanical excitations in an NPM system stimulated by incident particles or radiation.     

Direct imaging of a two-dimensional silica glass on graphene

Large-area graphene substrates provide a promising lab bench for synthesizing, manipulating, and characterizing low-dimensional materials, opening the door to high-resolution analyses of novel structures, such as two-dimensional (2D) glasses, that cannot be exfoliated and may not occur naturally. Here, we report the accidental discovery of a 2D silica glass supported on graphene. The 2D nature of this material enables the first atomic resolution transmission electron microscopy of a glass, producing images that strikingly resemble Zachariasen's original 1932 cartoon models of 2D continuous random network glasses. Atomic-resolution electron spectroscopy identifies the glass as SiO(2) formed from a bilayer of (SiO(4))(2-) tetrahedra and without detectable covalent bonding to the graphene. From these images, we directly obtain ring statistics and pair distribution functions that span short-, medium-, and long-range order. Ab initio calculations indicate that van der Waals interactions with graphene energetically stabilizes the 2D structure with respect to bulk SiO(2). These results demonstrate a new class of 2D glasses that can be applied in layered graphene devices and studied at the atomic scale.     

Possible Magnetic Background of the Metal-Insulator Transition in Two-Dimensional Correlated Electron System

Whether the apparent metal-insulator transition in two-dimensional (2D) correlated electron system is a true quantum phase transition or is a crossover phenomena—this question is in the core of ongoing debates. I present here a novel scenario of this phenomenon, based on experimental finding of the two-phase state in the correlated 2D system. The transport features in the suggested picture are the finite temperature phenomena and a consequence of the magnetic phase transition; the latter manifests in the sign change of the spin magnetization-per-electron. Physically, the magnetic transition means changing the tendency of the two-phase system to either paramagnetic Fermi liquid state (high density), or to the disordered ferromagnet (low density).     

Mobility and Localization of Carriers in a Quasi-One-Dimensional Electron System over Liquid Helium

The properties of a quasi-one-dimensional (Q1D) electron system on liquid helium are considered and the experiments on the carrier transport are presented. It has been shown that the electron mobility changes from high values close to that for bulk helium to very small values as a function of the Q1D channel width. Such a behaviour of is connected to a gradual transition from the regime of the quasi-free movement of the carriers in a Q1D system to the conditions of strong localization of the electrons. In the last case depends on temperature exponentially with the activation energy of a few degrees. It is predicted that in Q1D channels in regime of the localization two optical plasmon branches can exist. One of them (a high frequency branch) is connected with the oscillations of electrons in potential wells and another one has lower frequency and is due to the oscillations of dimples, which are created by localized electrons on the liquid helium surface, in the potential wells. Possible new experiments on study of magnetotransport, plasma oscillations and phase diagram of ordered-disordered states in a Q1D electron system of a finite length over liquid helium are proposed and discussed.     

Air-Stable Monolayer Cu2Se Exhibits a Purely Thermal Structural Phase Transition

Materials possessing structural phase transformations exhibit a rich set of physical and chemical properties that can be used for a variety of applications. In 2D materials, structural transformations have so far been induced by strain, lasers, electron injection, electron/ion beams, thermal loss of stoichiometry, and chemical treatments or by a combination of such approaches and annealing. However, stoichiometry-preserving, purely thermal, reversible phase transitions, which are fundamental in physics and can be easily induced, have not been observed. Here, the fabrication of monolayer Cu₂Se, a new 2D material is reported, demonstrating the existence of a purely thermal structural phase transition. Scanning tunneling microscopy, scanning transmission electron microscopy, and density functional theory (DFT) identify two structural phases at 78 and 300 K. DFT calculations trace the phase-transition mechanism via the existence/absence of imaginary (unstable) phonon modes at low and high temperatures. In situ, variable-temperature low-energy electron diffraction patterns demonstrate that the phase transition occurs across the whole sample at ≈147 K. Angle-resolved photoemission spectra and DFT calculations show that a degeneracy at the Γ point of the energy bands of the high-temperature phase is lifted in the low-temperature phase. This work opens up possibilities for studying such phase transitions in 2D materials.     

Is high-T c superconductor an undegenerated doped semiconductor?

The model of electron spectrum of HTS is suggested. It is supposed that the main feature of the spectrum is the presence of a narrow local pair level placed near the top of the filled two-dimensional (2D) electron band. Holes in this band are the result of thermal activation of electrons from the band to the pair level. The temperature dependences of resistivity and Hall constant of HTS in this model are in agreement with experiment on YBCO. The possibility of a first-order phase transition in such a system is considered.     

Neutron and electron diffraction of the FeTi  D(H) - γ - phase

Neutron diffraction data are presented on the γ-phase of the FeTiD system at the temperatures T = 300 K, 100 K and 4.2 K. The structure is monoclinic, space group P 2/m, with four special D sites in octahedral coordination between Fe atoms as nearest neighbours. The temperature dependence of the structure parameters and atomic distances is discussed. Comparative models of the α-, β-, and γ-phases are presented. Additional electron diffraction results on FeTi hydride show that pronounced isotopic differences can be excluded in the FeTiD(H) system.