Showing papers on "Electronic structure published in 2008"

Fine Structure Constant Defines Visual Transparency of Graphene

Abstract: There are few phenomena in condensed matter physics that are defined only by the fundamental constants and do not depend on material parameters. Examples are the resistivity quantum, h/e2 (h is Planck's constant and e the electron charge), that appears in a variety of transport experiments and the magnetic flux quantum, h/e, playing an important role in the physics of superconductivity. By and large, sophisticated facilities and special measurement conditions are required to observe any of these phenomena. We show that the opacity of suspended graphene is defined solely by the fine structure constant, a = e2/hc feminine 1/137 (where c is the speed of light), the parameter that describes coupling between light and relativistic electrons and that is traditionally associated with quantum electrodynamics rather than materials science. Despite being only one atom thick, graphene is found to absorb a significant (pa = 2.3%) fraction of incident white light, a consequence of graphene's unique electronic structure.

Unconventional Pairing Originating from the Disconnected Fermi Surfaces of Superconducting LaFeAsO 1-x F x

Abstract: For a newly discovered iron-based high ${T}_{c}$ superconductor ${\mathrm{LaFeAsO}}_{1\ensuremath{-}x}{\mathrm{F}}_{x}$, we have constructed a minimal model, where inclusion of all five Fe $d$ bands is found to be necessary. The random-phase approximation is applied to the model to investigate the origin of superconductivity. We conclude that the multiple spin-fluctuation modes arising from the nesting across the disconnected Fermi surfaces realize an extended $s$-wave pairing, while $d$-wave pairing can also be another candidate.

Novel Jeff=1/2 Mott state induced by relativistic spin-orbit coupling in Sr2IrO4.

Abstract: We investigated the electronic structure of 5d transition-metal oxide Sr2IrO4 using angle-resolved photoemission, optical conductivity, x-ray absorption measurements, and first-principles band calculations. The system was found to be well described by novel effective total angular momentum Jeff states, in which the relativistic spin-orbit coupling is fully taken into account under a large crystal field. Despite delocalized Ir 5d states, the Jeff states form such narrow bands that even a small correlation energy leads to the Jeff=1/2 Mott ground state with unique electronic and magnetic behaviors, suggesting a new class of Jeff quantum spin driven correlated-electron phenomena.

Hydrogen on graphene: Electronic structure, total energy, structural distortions and magnetism from first-principles calculations

Abstract: Density-functional calculations of electronic structure, total energy, structural distortions, and magnetism for hydrogenated single-layer, bilayer, and multilayer graphenes are performed. It is found that hydrogen-induced magnetism can survive only at very low concentrations of hydrogen (single-atom regime) whereas hydrogen pairs with optimized structure are usually nonmagnetic. Chemisorption energy as a function of hydrogen concentration is calculated, as well as energy barriers for hydrogen binding and release. The results confirm that graphene can be perspective material for hydrogen storage. Difference between hydrogenation of graphene, nanotubes, and bulk graphite is discussed.

Why multilayer graphene on 4H-SiC(0001[over ]) behaves like a single sheet of graphene.

Abstract: We show experimentally that multilayer graphene grown on the carbon terminated SiC(0001[over ]) surface contains rotational stacking faults related to the epitaxial condition at the graphene-SiC interface. Via first-principles calculation, we demonstrate that such faults produce an electronic structure indistinguishable from an isolated single graphene sheet in the vicinity of the Dirac point. This explains prior experimental results that showed single-layer electronic properties, even for epitaxial graphene films tens of layers thick.

Periodically Rippled Graphene: Growth and Spatially Resolved Electronic Structure

Abstract: We grow epitaxial graphene monolayers on Ru(0001) that cover uniformly the substrate over lateral distances larger than several microns. The weakly coupled graphene monolayer is periodically rippled and it shows charge inhomogeneities in the charge distribution. Real space measurements by scanning tunneling spectroscopy reveal the existence of electron pockets at the higher parts of the ripples, as predicted by a simple theoretical model. We also visualize the geometric and electronic structure of edges of graphene nanoislands.

Electronic structure methods for studying surface-enhanced Raman scattering.

Abstract: This critical review highlights recent advances in using electronic structure methods to study surface-enhanced Raman scattering Examples showing how electronic structure methods, in particular time-dependent density functional theory, can be used to gain microscopic insights into the enhancement mechanism are presented (150 references)

Interlayer Interaction and Electronic Screening in Multilayer Graphene

Abstract: Interlayer interaction and electronic screening in multilayer graphene Taisuke Ohta, 1, 2 Aaron Bostwick, 1 J. L. McChesney, 1, 3 Thomas Seyller, 4 Karsten Horn, 2 and Eli Rotenberg 1 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, USA Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany Montana State University, Bozeman, Montana, USA Institut f¨ r Physik der Kondensierten Materie, Universit¨ t Erlangen-N¨ rnberg, Erlangen, Germany u a u (Dated: March 30, 2007) The unusual transport properties of graphene are the direct consequence of a peculiar bandstruc- ture near the Dirac point. We determine the shape of the π bands and their characteristic splitting, and find the transition from two-dimensional to bulk character for 1 to 4 layers of graphene by angle-resolved photoemission. By detailed measurements of the π bands we derive the stacking order, layer-dependent electron potential, screening length and strength of interlayer interaction by comparison with tight binding calculations, yielding a comprehensive description of multilayer graphene’s electronic structure. PACS numbers: Much recent attention has been given to the electronic structure of multilayer films of graphene, the honeycomb carbon sheet which is the building block of graphite, car- bon nanotubes, C 60 , and other mesoscopic forms of car- bon [1]. Recent progress in synthesizing or isolating mul- tilayer graphene films [2–4] has provided access to their physical properties, and revealed many interesting trans- port phenomena, including an anomalous quantum Hall effect [5, 6], ballistic electron transport at room temper- ature [7], micron-scale coherence length [7, 8] and novel many-body couplings [9]. These effects originate from the effectively massless Dirac Fermion character of the carri- ers derived from graphene’s valence bands, which exhibit a linear dispersion degenerate near the so-called Dirac point energy, E D [10]. These unconventional properties of graphene offer a new route to room temperature, molecular-scale electron- ics capable of quantum computing [6, 7]. For example, a possible switching function in bilayer graphene has been suggested by reversibly lifting the band degeneracy at the Fermi level (E F ) upon application of an electric field [11, 12]. This effect is due to a unique sensitivity of the bandstructure to the charge distribution brought about by the interplay between strong interlayer hopping and weak interlayer screening, neither of which are currently well-understood [13, 14]. In order to evaluate the interlayer screening, stack- ing order and interlayer coupling, we have systemati- cally studied the evolution of the bandstructure of one to four layers of graphene using angle-resolved photoemis- sion spectroscopy (ARPES). We demonstrate experimen- tally that the interaction between layers and the stacking sequence affect the topology of the π bands, the former inducing an electronic transition from two-dimensional (2D) to 3D (bulk) character when going from one layer to multilayer graphene. The interlayer hopping integral and screening length are determined as a function of the num- ber of graphene layers by exploiting the sensitivity of π FIG. 1: (color online) Photoemission images revealing the bandstructure of (a) single and (b) bilayer graphene along high symmetry directions, Γ-K-M-Γ. The blue dashed lines are scaled DFT bandstructure of free standing films [16]. Inset in (a) shows the 2D Brillouin zone of graphene. states to the Coulomb potential, and the layer-dependent carrier concentration is estimated. The films were synthesized on n-type (nitrogen, 1 × 10 18 cm −3 ) 6H-SiC(0001) substrates (SiCrystal AG) that were etched in hydrogen at 1550 C. Annealing in a vac- uum first removes the resulting silicate adlayer and then causes the growth of the graphene layers between 1250 to 1400 C [15]. Beyond the first layer, the samples have a ± 0.5 monolayer thickness variation; the bandstructures of different thicknesses were extracted using the method of Ref. [11]. ARPES measurements were conducted at the Electronic Structure Factory endstation at beamline 7.01 of the Advanced Light Source, equipped with a Sci- enta R4000 electron energy analyzer. The samples were cooled to ∼ 30K by liquid He. The photon energy was 94 eV with the overall energy resolution of ∼30 meV for Fig. 1 and Fig. 2(a-d). The bandstructures of a single (Fig. 1 (a)) and a bi-

Structures of neutral Au7, Au19, and Au20 clusters in the gas phase.

Abstract: The catalytic properties of gold nanoparticles are determined by their electronic and geometric structures. We revealed the geometries of several small neutral gold clusters in the gas phase by using vibrational spectroscopy between 47 and 220 wavenumbers. A two-dimensional structure for neutral Au7 and a pyramidal structure for neutral Au20 can be unambiguously assigned. The reduction of the symmetry when a corner atom is cut from the tetrahedral Au20 cluster is directly reflected in the vibrational spectrum of Au19.

Edge-functionalized and substitutionally doped graphene nanoribbons: Electronic and spin properties

Abstract: Graphene nanoribbons are the counterpart of carbon nanotubes in graphene-based nanoelectronics. We investigate the electronic properties of chemically modified ribbons by means of density functional theory. We observe that chemical modifications of zigzag ribbons can break the spin degeneracy. This promotes the onset of a semiconducting-metal transition, or of a half-semiconducting state, with the two spin channels having a different band gap, or of a spin-polarized half-semiconducting state, where the spins in the valence and conduction bands are oppositely polarized. Edge functionalization of armchair ribbons gives electronic states a few eV away from the Fermi level and does not significantly affect their band gap. N and B produce different effects, depending on the position of the substitutional site. In particular, edge substitutions at low density do not significantly alter the band gap, while bulk substitution promotes the onset of semiconducting-metal transitions. Pyridinelike defects induce a semiconducting-metal transition.

Au-Fe3O4 Dumbbell Nanoparticles as Dual-Functional Probes

Abstract: Synthesis of dumbbell-shaped nanoparticles containing different functionalities has attracted much attention recently.[1] In such a dumbbell structure, one nanoparticle is linked to another, and electronic communication across the junction can drastically change the local electronic structure, leading to an additional dimension of control in catalytic, magnetic, and optical properties.[2] Moreover, the dumbbell structure offers two functional surfaces for the attachment of different kinds of molecules, making such species especially attractive as multifunctional probes for diagnostic and therapeutic applications.[3] Au-Fe3O4 nanoparticles represent one such multifunctional system. They contain both Au and Fe3O4 nanoparticles, which are known to be biocompatible and have been used extensively for optical and magnetic applications in biomedicine.[4] Compared with conventional single-component Au or Fe3O4 nanoparticles, the dumbbell-like Au-Fe3O4 systems have distinct advantages: 1) The structure contains both a magnetic (Fe3O4) and an optically active plasmonic (Au) unit and is suitable for simultaneous optical and magnetic detection. 2) The presence of Fe3O4 and Au surfaces facilitates the attachment of different chemical functionalities for target-specific imaging and delivery applications. 3) The size of either of the two nanoparticles can be controlled to optimize magnetic and optical properties, and the small particle is only capable of accommodating a few DNA strands, proteins, antibodies, or therapeutic molecules, thus facilitating kinetic studies in cell targeting and drug release. Herein, we report that dumbbell Au-Fe3O4 nanoparticles can be made biocompatible and used as magnetic and optical probes for cell imaging applications.

Excess electron states in reduced bulk anatase TiO2: Comparison of standard GGA, GGA+U, and hybrid DFT calculations

Abstract: The removal of lattice O atoms, as well as the addition of interstitial H atoms, in TiO(2) is known to cause the reduction in the material and the formation of "Ti(3+)" ions. By means of electronic structure calculations we have studied the nature of such oxygen vacancy and hydrogen impurity states in the bulk of the anatase polymorph of TiO(2). The spin polarized nature of these centers, the localized or delocalized character of the extra electrons, the presence of defect-induced states in the gap, and the polaronic distortion around the defect have been investigated with different theoretical methods: standard density functional theory (DFT) in the generalized-gradient approximation (GGA), GGA+U methods as a function of the U parameter, and two hybrid functionals with different admixtures of Hartree-Fock exchange. The results are found to be strongly dependent on the method used. Only GGA+U or hybrid functionals are able to reproduce the presence of states at about 1 eV below the conduction band, which are experimentally observed in reduced titania. The corresponding electronic states are localized on Ti 3d levels, but partly delocalized solutions are very close in energy. These findings show the limited predictive power of these theoretical methods to describe the electronic structure of reduced titania in the absence of accurate experimental data.

Slow electron cooling in colloidal quantum dots.

Abstract: Hot electrons in semiconductors lose their energy very quickly (within picoseconds) to lattice vibrations. Slowing this energy loss could prove useful for more efficient photovoltaic or infrared devices. With their well-separated electronic states, quantum dots should display slow relaxation, but other mechanisms have made it difficult to observe. We report slow intraband relaxation (>1 nanosecond) in colloidal quantum dots. The small cadmium selenide (CdSe) dots, with an intraband energy separation of approximately 0.25 electron volts, are capped by an epitaxial zinc selenide (ZnSe) shell. The shell is terminated by a CdSe passivating layer to remove electron traps and is covered by ligands of low infrared absorbance (alkane thiols) at the intraband energy. We found that relaxation is markedly slowed with increasing ZnSe shell thickness.

Correlated electronic structure of LaO1-xFxFeAs.

Abstract: We compute the electronic structure, momentum resolved spectral function and optical conductivity of the new superconductor LaO1-xFxFeAs within the combination of the density functional theory and dynamical mean field theory. We find that the compound in the normal state is a strongly correlated metal and the parent compound is a bad metal at the verge of the metal insulator transition. We argue that the superconductivity is not phonon mediated.

2,3-Disubstituted Thiophene-Based Organic Dyes for Solar Cells

Abstract: New organic dyes that contain variable lengths of conjugation, featuring oligothiophene and arylamines at the 2- and 3-position, have been synthesized. These compounds are characterized by photophysical, electrochemical, and theoretical computational methods. Nanocrystalline TiO2-based dye-sensitized solar cells were fabricated using these molecules as light-harvesting sensitizers. The overall efficiencies of the sensitized cells range from 4.11 to 6.15%, compared to a cis-di(thiocyanato)-bis(2,2′-bipyridyl)-4,4′-dicarboxylate ruthenium(II)-sensitized device (7.86%) fabricated and measured under similar conditions. The devices made from these compounds have higher open-circuit voltage (VOC) compared to oligothiophene congeners with arylamines at the 2-position only. The hydrophobic segment at the 3-position appears to help retarding the charge transfer from the conduction band of TiO2 to the electrolyte, I3−. Supplementary studies of the transient photovoltage and electrochemical impedance are in support ...

Transient Electronic Structure and Melting of a Charge Density Wave in TbTe3

Abstract: Obtaining insight into microscopic cooperative effects is a fascinating topic in condensed matter research because, through self-coordination and collectivity, they can lead to instabilities with macroscopic impacts like phase transitions. We used femtosecond time- and angle-resolved photoelectron spectroscopy (trARPES) to optically pump and probe TbTe3, an excellent model system with which to study these effects. We drove a transient charge density wave melting, excited collective vibrations in TbTe3, and observed them through their time-, frequency-, and momentum-dependent influence on the electronic structure. We were able to identify the role of the observed collective vibration in the transition and to document the transition in real time. The information that we demonstrate as being accessible with trARPES will greatly enhance the understanding of all materials exhibiting collective phenomena.

Electronic structure and doping in BaFe 2 As 2 and LiFeAs: Density functional calculations

Abstract: We report density functional calculations of the electronic structure and Fermi surface of the ${\text{BaFe}}_{2}{\text{As}}_{2}$ and LiFeAs phases including doping via the virtual-crystal approximation. The results show that contrary to a rigid-band picture, the density of states at the Fermi energy is only weakly doping dependent and that the main effect of doping is a change in the relative sizes of the electron and hole Fermi surfaces as required by Luttinger's theory. This is partly a consequence of a change in As height with doping, in particular a shift of As toward Fe as holes are introduced in the Fe plane, as might be expected from simple ionic considerations. The main effect of doping is therefore a reduction in the degree of nesting of the Fermi surface. This provides a framework for understanding the approximate electron-hole symmetry in the phase diagrams of the Fe-As based superconductors.

Cholesky Decomposition-Based Multiconfiguration Second-Order Perturbation Theory (CD-CASPT2): Application to the Spin-State Energetics of Co(III)(diiminato)(NPh).

Abstract: The electronic structure and low-lying electronic states of a Co-III(diiminato)(NPh) complex have been studied using mulficonfigurational wave function theory (CASSCF/CASPT2) The results have been compared to those obtained with density functional theory. The best agreement with ab initio results is obtained with a modified B3LYP functional containing a reduced amount (15%) of Hartree-Fock exchange. A relativistic basis set with 869 functions has been employed in the most extensive ab initio calculations, where a Cholesky decomposition technique was used to overcome problems arising from the large size of the two-electron integral matrix. It is shown that this approximation reproduces results obtained with the full integral set to a high accuracy, thus opening the possibility to use this approach to perform multiconfigurational wave-function-based quantum chemistry on much larger systems relative to what has been possible until now.

Divalent Carbon(0) Chemistry, Part 1: Parent Compounds

Abstract: Quantum-chemical calculations with DFT (BP86) and ab initio methods [MP2, SCS-MP2, CCSD(T)] have been carried out for the molecules C(PH(3))(2) (1), C(PMe(3))(2) (2), C(PPh(3))(2) (3), C(PPh(3))(CO) (4), C(CO)(2) (5), C(NHC(H))(2) (6), C(NHC(Me))(2) (7) (Me(2)N)(2)C=C=C(NMe(2))(2) (8), and NHC (9), where NHC=N-heterocyclic carbene and NHC(Me)=N-methyl-substituted NHC. The electronic structure in 1-9 was analyzed with charge- and energy-partitioning methods. The results show that the bonding situations in L(2)C compounds 1-8 can be interpreted in terms of donor-acceptor interactions between closed-shell ligands L and a carbon atom which has two lone-pair orbitals L-->C C((1)D) donor-acceptor bonds are roughly twice as strong as the respective L-->BH(3) bond.

Changes in the electronic structure and properties of graphene induced by molecular charge-transfer

Abstract: Interaction with electron donor and acceptor molecules such as aniline and nitrobenzene brings about marked changes in the Raman spectrum and the electronic structure of graphene, prepared by the exfoliation of graphitic oxide.

Dithiomethylether as a ligand in the hydrogenase h-cluster.

Abstract: An X-ray crystallographic refinement of the H-cluster of [FeFe]-hydrogenase from Clostridium pasteurianum has been carried out to close-to atomic resolution and is the highest resolution [FeFe]-hydrogenase presented to date. The 1.39 A, anisotropically refined [FeFe]-hydrogenase structure provides a basis for examining the outstanding issue of the composition of the unique nonprotein dithiolate ligand of the H-cluster. In addition to influencing the electronic structure of the H-cluster, the composition of the ligand has mechanistic implications due to the potential of the bridge-head γ-group participating in proton transfer during catalysis. In this work, sequential density functional theory optimizations of the dithiolate ligand embedded in a 3.5−3.9 A protein environment provide an unbiased approach to examining the most likely composition of the ligand. Structural, conformational, and energetic considerations indicate a preference for dithiomethylether as an H-cluster ligand and strongly disfavor the ...

Giant magneto-elastic coupling in multiferroic hexagonal manganites

Abstract: The motion of atoms in a solid always responds to cooling or heating in a way that is consistent with the symmetry of the given space group of the solid to which they belong1,2. When the atoms move, the electronic structure of the solid changes, leading to different physical properties. Therefore, the determination of where atoms are and what atoms do is a cornerstone of modern solid-state physics. However, experimental observations of atomic displacements measured as a function of temperature are very rare, because those displacements are, in almost all cases, exceedingly small3,4,5. Here we show, using a combination of diffraction techniques, that the hexagonal manganites RMnO3 (where R is a rare-earth element) undergo an isostructural transition with exceptionally large atomic displacements: two orders of magnitude larger than those seen in any other magnetic material, resulting in an unusually strong magneto-elastic coupling. We follow the exact atomic displacements of all the atoms in the unit cell as a function of temperature and find consistency with theoretical predictions based on group theories. We argue that this gigantic magneto-elastic coupling in RMnO3 holds the key to the recently observed magneto-electric phenomenon in this intriguing class of materials6.

How Cooper pairs vanish approaching the Mott insulator in Bi 2 Sr 2 CaCu 2 O 8+δ

Abstract: The antiferromagnetic ground state of copper oxide Mott insulators is achieved by localizing an electron at each copper atom in real space (r-space). Removing a small fraction of these electrons (hole doping) transforms this system into a superconducting fluid of delocalized Cooper pairs in momentum space (k-space). During this transformation, two distinctive classes of electronic excitations appear. At high energies, the mysterious 'pseudogap' excitations are found, whereas, at lower energies, Bogoliubov quasi-particles-the excitations resulting from the breaking of Cooper pairs-should exist. To explore this transformation, and to identify the two excitation types, we have imaged the electronic structure of Bi(2)Sr(2)CaCu(2)O(8+delta) in r-space and k-space simultaneously. We find that although the low-energy excitations are indeed Bogoliubov quasi-particles, they occupy only a restricted region of k-space that shrinks rapidly with diminishing hole density. Concomitantly, spectral weight is transferred to higher energy r-space states that lack the characteristics of excitations from delocalized Cooper pairs. Instead, these states break translational and rotational symmetries locally at the atomic scale in an energy-independent way. We demonstrate that these unusual r-space excitations are, in fact, the pseudogap states. Thus, as the Mott insulating state is approached by decreasing the hole density, the delocalized Cooper pairs vanish from k-space, to be replaced by locally translational- and rotational-symmetry-breaking pseudogap states in r-space.

Simultaneous measurements of electronic conduction and Raman response in molecular junctions.

Abstract: Electronic conduction through single molecules is affected by the molecular electronic structure as well as by other information that is extremely difficult to assess, such as bonding geometry and chemical environment. The lack of an independent diagnostic technique has long hampered single-molecule conductance studies. We report simultaneous measurement of the conductance and the Raman spectra of nanoscale junctions used for single-molecule electronic experiments. Blinking and spectral diffusion in the Raman response of both p-mercaptoaniline and a fluorinated oligophenylyne ethynylene correlate in time with changes in the electronic conductance. Finite difference time domain calculations confirm that these correlations do not result from the conductance modifying the Raman enhancement. Therefore, these observations strongly imply that multimodal sensing of individual molecules is possible in these mass-producible nanostructures.

Electronic structure calculations of liquid-solid interfaces: Combination of density functional theory and modified Poisson-Boltzmann theory

Abstract: A robust and efficient computational method for electronic structure calculations of liquid-solid interfaces is presented. The theory employs the density functional theory and a modified Poisson-Boltzmann theory, combining them through a smooth dielectric model function. The free energy, including electrostatic and nonelectrostatic interactions between solutes and the solvation medium, is formulated, and its first derivatives with atomic positions are presented. This methodology is applied to two different topics; one is the potential of zero charge (PZC) of Pt(111), and the other is a poisoning of active sites for the oxygen-reduction reaction (ORR) by interfacial water molecules on Pt(111). The results of the first topic show that induced charge redistributions caused by the adsorption of water molecules form a surface dipole moment that dominates the experimentally observed negative shift in the PZC when platinum is immersed in an aqueous electrolyte. The results of the second topic show the possibility of a decrease in the surface coverage of the first reaction precursor to the ORR due to site blocking by the adsorbed water molecules.

Charge self-regulation upon changing the oxidation state of transition metals in insulators.

Abstract: Transition-metal atoms embedded in an ionic or semiconducting crystal can exist in various oxidation states that have distinct signatures in X-ray photoemission spectroscopy and 'ionic radii' which vary with the oxidation state of the atom. These oxidation states are often tacitly associated with a physical ionization of the transition-metal atoms--that is, a literal transfer of charge to or from the atoms. Physical models have been founded on this charge-transfer paradigm, but first-principles quantum mechanical calculations show only negligible changes in the local transition-metal charge as the oxidation state is altered. Here we explain this peculiar tendency of transition-metal atoms to maintain a constant local charge under external perturbations in terms of an inherent, homeostasis-like negative feedback. We show that signatures of oxidation states and multivalence--such as X-ray photoemission core-level shifts, ionic radii and variations in local magnetization--that have often been interpreted as literal charge transfer are instead a consequence of the negative-feedback charge regulation.

Two-Dimensional Electron Gases at Oxide Interfaces

Abstract: Two-dimensional electron gases (2DEGs) based on conventional semiconductors such as Si or GaAs have played a pivotal role in fundamental science and technology. The high mobilities achieved in 2DEGs enabled the discovery of the integer and fractional quantum Hall effects and are exploited in high-electron-mobility transistors. Recent work has shown that 2DEGs can also exist at oxide interfaces. These electron gases typically result from reconstruction of the complex electronic structure of the oxides, so that the electronic behavior of the interfaces can differ from the behavior of the bulk. Reports on magnetism and superconductivity in oxide 2DEGs illustrate their capability to encompass phenomena not shown by interfaces in conventional semiconductors. This article reviews the status and prospects of oxide 2DEGs.

n-Type behavior of graphene supported on Si/SiO(2) substrates.

Abstract: Results are presented from an experimental and theoretical study of the electronic properties of back-gated graphene field effect transistors (FETs) on Si/SiO2 substrates. The excess charge on the graphene was observed by sweeping the gate voltage to determine the charge neutrality point in the graphene. Devices exposed to laboratory environment for several days were always found to be initially p-type. After ∼20 h at 200 °C in ∼5 × 10−7 Torr vacuum, the FET slowly evolved to n-type behavior with a final excess electron density on the graphene of ∼4 × 1012 e/cm2. This value is in excellent agreement with our theoretical calculations on SiO2, where we have used molecular dynamics to build the SiO2 structure and then density functional theory to compute the electronic structure. The essential theoretical result is that the SiO2 has a significant surface state density just below the conduction band edge that donates electrons to the graphene to balance the chemical potential at the interface. An electrostati...