Showing papers on "Electronic structure published in 2010"

Atomically thin MoS2: a new direct-gap semiconductor

Abstract: The electronic properties of ultrathin crystals of molybdenum disulfide consisting of N=1,2,…,6 S-Mo-S monolayers have been investigated by optical spectroscopy Through characterization by absorption, photoluminescence, and photoconductivity spectroscopy, we trace the effect of quantum confinement on the material's electronic structure With decreasing thickness, the indirect band gap, which lies below the direct gap in the bulk material, shifts upwards in energy by more than 06 eV This leads to a crossover to a direct-gap material in the limit of the single monolayer Unlike the bulk material, the MoS₂ monolayer emits light strongly The freestanding monolayer exhibits an increase in luminescence quantum efficiency by more than a factor of 10⁴ compared with the bulk material

Unique Electronic Structure Induced High Photoreactivity of Sulfur-Doped Graphitic C3N4

Abstract: Electronic structure intrinsically controls the light absorbance, redox potential, charge-carrier mobility, and consequently, photoreactivity of semiconductor photocatalysts. The conventional approach of modifying the electronic structure of a semiconductor photocatalyst for a wider absorption range by anion doping operates at the cost of reduced redox potentials and/or charge-carrier mobility, so that its photoreactivity is usually limited and some important reactions may not occur at all. Here, we report sulfur-doped graphitic C(3)N(4) (C(3)N(4-x)S(x)) with a unique electronic structure that displays an increased valence bandwidth in combination with an elevated conduction band minimum and a slightly reduced absorbance. The C(3)N(4-x)S(x) shows a photoreactivity of H(2) evolution 7.2 and 8.0 times higher than C(3)N(4) under lambda > 300 and 420 nm, respectively. More strikingly, the complete oxidation process of phenol under lambda > 400 nm can occur for sulfur-doped C(3)N(4), which is impossible for C(3)N(4) even under lambda > 300 nm. The homogeneous substitution of sulfur for lattice nitrogen and a concomitant quantum confinement effect are identified as the cause of this unique electronic structure and, consequently, the excellent photoreactivity of C(3)N(4-x)S(x). The results acquired may shed light on general doping strategies for designing potentially efficient photocatalysts.

Probing layer number and stacking order of few-layer graphene by Raman spectroscopy.

Abstract: Graphene is a two-dimensional material defined as a planar honeycomb lattice of close-packed carbon atoms, where the electrons exhibit a linear dispersion near Dirac K points and behave as massless Dirac fermions. However, the valence and conduction bands in an AB stacked graphene bilayer split into two parabolic branches near the K point originating from the interaction of p electrons, and the electrons are hence described by massive Dirac fermions. Moreover, a graphene bilayer is a tunable-gap semiconductor under electric-field biasing. With a further increase in the number of layers along with AB stacking, the electronic structure reveals stepwise variations that eventually approach that of the three-dimensional counterpart. Considering the close relation between the electronic properties and layer number of few-layer graphene (FLG), the ability to accurately determine the layer number and correlating this with the electronic structure is a prerequisite in understanding the evolution of the electronic properties from twoto threedimensional graphitic materials. In addition to graphene layers with AB stacking, FLG with arbitrary stacking (Figure 1) is considered to possess distinct properties arising from its different crystalline structure and p electron interactions. Experimentally, it has been observed that the electroand magnetotransport properties for folded graphene sheets are different to thoseofAB stackedbilayers. Furthermore,FLG grown on SiC, Ni, and Ru also have non-AB stacking order. Therefore, elucidating the detailed character-

Quantum computing with defects

Abstract: Identifying and designing physical systems for use as qubits, the basic units of quantum information, are critical steps in the development of a quantum computer. Among the possibilities in the solid state, a defect in diamond known as the nitrogen-vacancy (NV-1) center stands out for its robustness—its quantum state can be initialized, manipulated, and measured with high fidelity at room temperature. Here we describe how to systematically identify other deep center defects with similar quantum-mechanical properties. We present a list of physical criteria that these centers and their hosts should meet and explain how these requirements can be used in conjunction with electronic structure theory to intelligently sort through candidate defect systems. To illustrate these points in detail, we compare electronic structure calculations of the NV-1 center in diamond with those of several deep centers in 4H silicon carbide (SiC). We then discuss the proposed criteria for similar defects in other tetrahedrally coordinated semiconductors.

Electronic and optical properties of Cu2ZnSnS4 and Cu2ZnSnSe4

Abstract: The electronic structure as well as the optical response of kesterite and stannite structures of Cu2ZnSnS4 and Cu2ZnSnSe4 are analyzed by a relativistic full-potential linearized augmented plane wa ...

Intra-unit-cell electronic nematicity of the high- T c copper-oxide pseudogap states

Abstract: In the high-transition-temperature (high-T(c)) superconductors the pseudogap phase becomes predominant when the density of doped holes is reduced. Within this phase it has been unclear which electronic symmetries (if any) are broken, what the identity of any associated order parameter might be, and which microscopic electronic degrees of freedom are active. Here we report the determination of a quantitative order parameter representing intra-unit-cell nematicity: the breaking of rotational symmetry by the electronic structure within each CuO(2) unit cell. We analyse spectroscopic-imaging scanning tunnelling microscope images of the intra-unit-cell states in underdoped Bi(2)Sr(2)CaCu(2)O(8 +) (delta) and, using two independent evaluation techniques, find evidence for electronic nematicity of the states close to the pseudogap energy. Moreover, we demonstrate directly that these phenomena arise from electronic differences at the two oxygen sites within each unit cell. If the characteristics of the pseudogap seen here and by other techniques all have the same microscopic origin, this phase involves weak magnetic states at the O sites that break 90 degrees -rotational symmetry within every CuO(2) unit cell.

Injection and Trapping of Tunnel-Ionized Electrons into Laser-Produced Wakes

Abstract: A method, which utilizes the large difference in ionization potentials between successive ionization states of trace atoms, for injecting electrons into a laser-driven wakefield is presented. Here a mixture of helium and trace amounts of nitrogen gas was used. Electrons from the K shell of nitrogen were tunnel ionized near the peak of the laser pulse and were injected into and trapped by the wake created by electrons from majority helium atoms and the L shell of nitrogen. The spectrum of the accelerated electrons, the threshold intensity at which trapping occurs, the forward transmitted laser spectrum, and the beam divergence are all consistent with this injection process. The experimental measurements are supported by theory and 3D OSIRIS simulations.

Chirality and Electronic Structure of the Thiolate-Protected Au38 Nanocluster

Abstract: Structural, electronic, and optical properties of the thiolate-protected Au38(SR)24 cluster are studied by density-functional theory computations (R = CH3 and R = C6H13) and by powder X-ray crystallography (R = C12H25). A low-energy structure which can be written as Au23@(Au(SR)2)3(Au2(SR)3)6 having a bi-icosahedral core and a chiral arrangement of the protecting gold−thiolate Au x (SR) y units yields an excellent match between the computed (for R = C6H13) and measured (for R = C12H25) powder X-ray diffraction function. We interpret in detail the electronic structure of the Au23 core by using a particle-in-a-cylinder model. Although the alkane thiolate ligands are achiral, the chiral structure of the ligand layer yields strong circular dichroism (CD) in the excitations below 2.2 eV for Au38(SCH3)24. Our calculated CD spectrum is in quantitative agreement with the previously measured low-energy CD signal of glutathione-protected Au38(SG)24. Our study demonstrates a new mechanism for the strong chiral respo...

Probing Strain-Induced Electronic Structure Change in Graphene by Raman Spectroscopy

Abstract: Two-phonon Raman scattering in graphitic materials provides a distinctive approach to probing the material's electronic structure through the spectroscopy of phonons. Here we report studies of Raman scattering of the two-dimensional mode of single-layer graphene under uniaxial stress and which implicates two types of modification of the low-energy electronic structure of graphene: a deformation of the Dirac cone and its displacement away from the K point.

Nematic Electronic Structure in the “Parent” State of the Iron-Based Superconductor Ca(Fe1–xCox)2As2

Abstract: The mechanism of high-temperature superconductivity in the newly discovered iron-based superconductors is unresolved. We use spectroscopic imaging-scanning tunneling microscopy to study the electronic structure of a representative compound CaFe1.94Co0.06As2 in the "parent" state from which this superconductivity emerges. Static, unidirectional electronic nanostructures of dimension eight times the inter-iron-atom distance a(Fe-Fe) and aligned along the crystal a axis are observed. In contrast, the delocalized electronic states detectable by quasiparticle interference imaging are dispersive along the b axis only and are consistent with a nematic alpha2 band with an apparent band folding having wave vector q vector congruent with +/-2pi/8a(Fe-Fe) along the a axis. All these effects rotate through 90 degrees at orthorhombic twin boundaries, indicating that they are bulk properties. As none of these phenomena are expected merely due to crystal symmetry, underdoped ferropnictides may exhibit a more complex electronic nematic state than originally expected.

Predicted band structures of III-V semiconductors in the wurtzite phase

Abstract: While non-nitride III-V semiconductors typically have a zinc-blende structure, they may also form wurtzite crystals under pressure or when grown as nanowhiskers. This makes electronic structure calculation difficult since the band structures of wurtzite III-V semiconductors are poorly characterized. We have calculated the electronic band structure for nine III-V semiconductors in the wurtzite phase using transferable empirical pseudopotentials including spin-orbit coupling. We find that all the materials have direct gaps. Our results differ significantly from earlier ab initio calculations, and where experimental results are available (InP, InAs, and GaAs) our calculated band gaps are in good agreement. We tabulate energies, effective masses, and linear and cubic Dresselhaus zero-field spin-splitting coefficients for the zone-center states. The large zero-field spin-splitting coefficients we find may facilitate the development of spin-based devices.

Electronic structure calculations with the Tran-Blaha modified Becke-Johnson Density Functional

Abstract: We report a series of calculations testing the predictions of the Tran-Blaha functional for the electronic structure and magnetic properties of condensed systems. We find a general improvement in the properties of semiconducting and insulating systems, relative to calculations with standard generalized gradient approximations, although this is not always by the same mechanism as other approaches such as the quasiparticle GW method. In ZnO the valence bands are narrowed and the band gap is increased to a value in much better agreement with experiment. The Zn d states do not move to higher binding energy as they do in local-density approximation+U calculations. The functional is effective for systems with hydride anions, where correcting self-interaction errors in the 1s state is important. Similarly, it correctly opens semiconducting gaps in the alkaline-earth hexaborides. It correctly stabilizes an antiferromagnetic insulating ground state for the undoped cuprate parent CaCuO{sub 2}, but seriously degrades the agreement with experiment for ferromagnetic Gd relative to the standard local-spin-density approximation and generalized gradient approximations. This is due to positioning of the minority-spin 4f states at too low an energy. Conversely, the position of the La 4f conduction bands of La{sub 2}O{sub 3} is in reasonable accord with experimentmore » as it is with standard functionals. The functional narrows the Fe d bands of the parent compound LaFeAsO of the iron high-temperature superconductors while maintaining the high Fe spectral weight near the Fermi energy.« less

Electronic structure of few-layer graphene: experimental demonstration of strong dependence on stacking sequence.

Abstract: The electronic structure of few-layer graphene (FLG) samples with crystalline order was investigated experimentally by infrared absorption spectroscopy for photon energies ranging from 0.2-1 eV. Distinct optical conductivity spectra were observed for different samples having precisely the same number of layers. The different spectra arise from the existence of two stable polytypes of FLG, namely, Bernal (AB) stacking and rhombohedral (ABC) stacking. The observed absorption features, reflecting the underlying symmetry of the two polytypes and the nature of the associated van Hone singularities, were reproduced by explicit calculations within a tight-binding model. The findings demonstrate the pronounced effect of stacking order on the electronic structure of FLG.

Crystal Phase Quantum Dots

Abstract: In semiconducting nanowires, both zinc blende and wurtzite 1 crystal structures can coexist. 2-4 The band structure difference between the two structures can lead to charge confinement. 5 Here we fabricate and study single quantum dot devices 6 defined solely by crystal phase in a chemically homogeneous nanowire and observe single photon generation. More generally, our results show that this type of carrier confinement represents a novel degree of freedom in device design at the nanoscale.

Nanocrystal quantum dots, second edition

Abstract: "Soft" Chemical Synthesis and Manipulation of Semiconductor Nanocrystals, J.A. Hollingsworth and V.I. Klimov Electronic Structure in Semiconductor Nanocrystals: Optical Experiment, D.J. Norris Fine Structure and Polarization Properties of Band-Edge Excitons in Semiconductor Nanocrystals, A.L. Efros Intraband Spectroscopy and Dynamics of Colloidal Semiconductor Quantum Dots, P. Guyot-Sionnest, M. Shim, and C. Wang Multiexciton Phenomena in Semiconductor Nanocrystals, V.I. Klimov Optical Dynamics in Single Semiconductor Quantum Dots, K.T. Shimizu and M.G. Bawendi Electrical Properties of Semiconductor Nanocrystals, N.C. Greenham Optical and Tunneling Spectroscopy of Semiconductor Nanocrystal Quantum Dots, U. Banin and O. Millo Quantum Dots and Quantum Dot Arrays: Synthesis, Optical Properties, Photogenerated Carrier Dynamics, Multiple Exciton Generation, and Applications to Solar Photon Conversion, A. J. Nozik and O.I. Micic Potential and Limitations of Luminescent Quantum Dots in Biology, H. Mattoussi Colloidal Transition-Metal-Doped Quantum Dots, R. Beaulac, S.T. Ochsenbein, and D.R. Gamelin Index

Electronic structure and properties of transition metal compounds : introduction to the theory

Abstract: Preface. Foreword to the First Edition. Mathematical Symbols. Abbreviations. 1 Introduction: Subject and Methods. 1.1 Objectives. 1.2 Definitions of Chemical Bonding and Transition Metal Coordination System. 1.3 The Schrodinger Equation. Summary Notes. References. 2 Atomic States. 2.1 One-Electron States. 2.2 Multielectron States: Energy Terms. Summary Notes. Questions. Exercises and Problems. References. 3 Symmetry Ideas and Group-Theoretical Description. 3.1 Symmetry Transformations and Matrices. 3.2 Groups of Symmetry Transformations. 3.3 Representations of Groups and Matrices of Representations. 3.4 Classification of Molecular Terms and Vibrations, Selection Rules, and Wigner-Eckart Theorem. 3.5 Construction of Symmetrized Molecular Orbitals and Normal Vibrations. 3.6 The Notion of Double Groups. Summary Notes. Exercises. References. 4 Crystal Field Theory. 4.1 Introduction. 4.2 Splitting of the Energy Levels of One d Electron in Ligand Fields. 4.3 Several d Electrons. 4.4 f -Electron Term Splitting. 4.5 Crystal Field Parameters and Extrastabilization Energy. 4.6 Limits of Applicability of Crystal Field Theory. Summary Notes. Questions. Exercises and Problems. References. 5 Method of Molecular Orbitals and Related Approaches. 5.1 Basic Ideas of the MO LCAO Method. 5.2 Charge Distribution and Bonding in the MO LCAO Method and the Case of Weak Covalency. 5.3 Methods of Calculation of MO Energies and LCAO Coefficients. 5.4 Semiquantitative Approaches. 5.5 Semiempirical Methods. 5.6 Fragmentary Calculations, Molecular Mechanics, and Combined Quantum/Classical (QM/MM) Modeling. 5.7 General Comparison of Methods. Summary Notes. Exercises and Problems. References. 6 Electronic Structure and Chemical Bonding. 6.1 Classification of Chemical Bonds by Electronic Structure and Role of d and f Electrons in Coordination Bonding. 6.2 Qualitative Aspects and Electronic Configurations. 6.3 Ligand Bonding. 6.4 Energies, Geometries, and Charge Distributions. 6.5 Relativistic Effects. Summary Notes. Exercises and Problems. References. 7 Electronic Control of Molecular Shapes and Transformations via Vibronic Coupling. 7.1 Molecular Vibrations. 7.2 Vibronic Coupling. 7.3 The Jahn-Teller Effect. 7.4 Pseudo-Jahn-Teller Effect and the Two-Level Paradigm. Summary Notes. Exercises and Problems. References. 8 Electronic Structure Investigated by Physical Methods. 8.1 Band Shapes of Electronic Spectra. 8.2 d d, Charge Transfer, Infrared, and Raman Spectra. 8.3 X-Ray and Ultraviolet Photoelectron Spectra EXAFS. 8.4 Magnetic Properties. 8.5 Gamma-Resonance Spectroscopy. 8.6 Electron Charge and Spin Density Distribution in Diffraction Methods. Summary Notes. Exercises and Problems. References. 9 Stereochemistry and Crystal Chemistry. 9.1 Definitions. Semiclassical Approaches. 9.2 Vibronic Effects in Stereochemistry. 9.3 Mutual Influence of Ligands. 9.4 Crystal Stereochemistry. Summary Notes. Exercises and Problems. References. 10 Electron Transfer, Redox Properties, and Electron-Conformational Effects. 10.1 Electron Transfer and Charge Transfer by Coordination. 10.2 Electron Transfer in Mixed-Valence Compounds. 10.3 Electron-Conformational Effects in Biological Systems. Summary Notes. Exercises and Problems. References. 11 Reactivity and Catalytic Action. 11.1 Electronic Factors in Reactivity. 11.2 Electronic Control of Chemical Activation via Vibronic Coupling. 11.3 Direct Computation of Energy Barriers of Chemical Reactions. Summary Notes. Questions and Problems. References. Appendixes. Answers and Solutions. Subject Index. Formula Index.

Unfolding first-principles band structures.

Abstract: A general method is presented to unfold band structures of first-principles supercell calculations with proper spectral weight, allowing easier visualization of the electronic structure and the degree of broken translational symmetry. The resulting unfolded band structures contain additional rich information from the Kohn-Sham orbitals, and absorb the structure factor that makes them ideal for a direct comparison with angle resolved photoemission spectroscopy experiments. With negligible computational expense via the use of Wannier functions, this simple method has great practical value in the studies of a wide range of materials containing impurities, vacancies, lattice distortions, or spontaneous long-range orders.

Imaging the Fano lattice to ‘hidden order’ transition in URu 2 Si 2

Abstract: Within a Kondo lattice, the strong hybridization between electrons localized in real space (r-space) and those delocalized in momentum-space (k-space) generates exotic electronic states called ‘heavy fermions’. In URu2Si2 these effects begin at temperatures around 55 K but they are suddenly altered by an unidentified electronic phase transition at To = 17.5 K. Whether this is conventional ordering of the k-space states, or a change in the hybridization of the r-space states at each U atom, is unknown. Here we use spectroscopic imaging scanning tunnelling microscopy (SI-STM) to image the evolution of URu2Si2 electronic structure simultaneously in r-space and k-space. Above To, the ‘Fano lattice’ electronic structure predicted for Kondo screening of a magnetic lattice is revealed. Below To, a partial energy gap without any associated density-wave signatures emerges from this Fano lattice. Heavy-quasiparticle interference imaging within this gap reveals its cause as the rapid splitting below To of a light k-space band into two new heavy fermion bands. Thus, the URu2Si2 ‘hidden order’ state emerges directly from the Fano lattice electronic structure and exhibits characteristics, not of a conventional density wave, but of sudden alterations in both the hybridization at each U atom and the associated heavy fermion states. A long-standing mystery in condensed matter physics is that of the appearance of a 'hidden order' state in URu2Si2 at low temperature, an unexpected phase change that is accompanied by a sharp change in bulk properties of the material. The problem is related to the appearance of a 'heavy fermion' state (already at a higher temperature) where electron-like charge carriers propagate through the solid with an effective mass thousands of times larger than that of a free electron. Schmidt et al. have now used scanning tunnelling microscopy and spectroscopy to visualize the electronic structure of URu2Si2 with subatomic resolution. In the process, they observe the electronic structure associated with a magnetic 'Kondo' lattice, which was assumed to cause heavy fermion effects, but never observed directly. Further, the spectroscopic findings show how the hidden order state evolves with decreasing temperature from this lattice. A longstanding mystery in condensed-matter physics involves the appearance of a 'hidden order' state in URu2Si2 at low temperature — an unexpected phase change that is accompanied by a sharp change in the bulk properties of the material. The problem is related to the appearance of a 'heavy fermion' state. Here, scanning tunnelling microscopy and spectroscopy have been used to image the electronic structure of URu2Si2 at sub-atomic resolution, revealing how the hidden order state evolves with decreasing temperature.

Energy gap tuning in graphene on hexagonal boron nitride bilayer system

Abstract: We use a tight-binding approach and density functional theory calculations to study the band structure of graphene/hexagonal boron nitride bilayer system in the most stable configuration We show that an electric field applied in the direction perpendicular to the layers significantly modifies the electronic structure of the whole system, including shifts, anticrossing and other deformations of bands, which can allow to control a value of the energy gap It is shown that band structure of biased system may be tailored for specific requirements of nanoelectronics applications The carriers' mobilities are expected to be higher than in the bilayer graphene devices

Light non-metallic atom (B, N, O and F)-doped graphene: a first-principles study

Abstract: First-principles calculations are performed to study the geometry, electronic structure and magnetic properties of light non-metallic atom-doped graphene (B, N, O and F). The planar structure and the quasi-linear energy dispersion near the Dirac point remain through doping with B and N atoms, by which p-type doping and n-type doping graphene are respectively induced. A bandgap of about 0.5 eV is generated through O doping, and geometrically the O atom is also in the graphene plane. No magnetic moment is detected in B- , N- and O-doped graphene. For F doping, the F atom bonds with one of the carbon atoms close to the vacancy, with the other two carbon atoms undergoing a Jahn-Teller distortion. A weak polarized magnetic moment of 0.71 µ(B) is detected through F doping.

The evolution of electronic structure in few-layer graphene revealed by optical spectroscopy

Abstract: The massless Dirac spectrum of electrons in single-layer graphene has been thoroughly studied both theoretically and experimentally. Although a subject of considerable theoretical interest, experimental investigations of the richer electronic structure of few-layer graphene (FLG) have been limited. Here we examine FLG graphene crystals with Bernal stacking of layer thicknesses N = 1,2,3,...8 prepared using the mechanical exfoliation technique. For each layer thickness N, infrared conductivity measurements over the spectral range of 0.2-1.0 eV have been performed and reveal a distinctive band structure, with different conductivity peaks present below 0.5 eV and a relatively flat spectrum at higher photon energies. The principal transitions exhibit a systematic energy-scaling behavior with N. These observations are explained within a unified zone-folding scheme that generates the electronic states for all FLG materials from that of the bulk 3D graphite crystal through imposition of appropriate boundary conditions. Using the Kubo formula, we find that the complete infrared conductivity spectra for the different FLG crystals can be reproduced reasonably well within the framework a tight-binding model.

Time-dependent density functional theory calculations of the spectroscopy of core electrons

Abstract: Recent advances in X-ray sources have led to a renaissance in spectroscopic techniques in the X-ray region. These techniques that involve the excitation of core electrons can provide an atom specific probe of electronic structure and provide powerful analytical tools that are used in many fields of research. Theoretical calculations can often play an important role in the analysis and interpretation of experimental spectra. In this perspective, we review recent developments in quantum chemical calculations of X-ray absorption spectra, focusing on the use of time-dependent density functional theory to study core excitations. The practical application of these calculations is illustrated with examples drawn from surface science and bioinorganic chemistry, and the application of these methods to study X-ray emission spectroscopy is explored.

Vacancy ordering and electronic structure of gamma-Fe2O3 (maghemite): a theoretical investigation

Abstract: The crystal structure of the iron oxide gamma-Fe2O3 is usually reported in either the cubic system (space group P4332) with partial Fe vacancy disorder or in the tetragonal system (space group P41212) with full site ordering and c/a\approx 3. Using a supercell of the cubic structure, we obtain the spectrum of energies of all the ordered configurations which contribute to the partially disordered P4332 cubic structure. Our results show that the configuration with space group P41212 is indeed much more stable than the others, and that this stability arises from a favourable electrostatic contribution, as this configuration exhibits the maximum possible homogeneity in the distribution of iron cations and vacancies. Maghemite is therefore expected to be fully ordered in equilibrium, and deviations from this behaviour should be associated with metastable growth, extended anti-site defects and surface effects in the case of small nanoparticles. The confirmation of the ordered tetragonal structure allows us to investigate the electronic structure of the material using density functional theory (DFT) calculations. The inclusion of a Hubbard (DFT+U) correction allows the calculation of a band gap in good agreement with experiment. The value of the gap is dependent on the electron spin, which is the basis for the spin-filtering properties of maghemite.

High-resolution tunnelling spectroscopy of a graphene quartet

Abstract: The unique electronic structure of graphene, a material made of carbon sheets just one atom thick, makes it of interest both to materials scientists looking for possible technological applications and to the study of fundamental aspects of physics. Young Jae Song et al. have studied one aspect of graphene's uniqueness — the fourfold energy degeneracy that means that a single Landau level (a peak in the density of states produced by a magnetic field) consists of four separate quantum states. Using a high-resolution scanning tunnelling microscope that operates at a record low temperature of down to 10 millikelvin, they perform tunnelling spectroscopy measurements on epitaxial graphene. They obtain spectral fingerprints of the Landau levels, showing in fine detail how they evolve with magnetic fields and how they split (at high fields) into the four separate quantum states. The authors observe states with fractional Landau level filling factors of 7/2, 9/2 and 11/2, which are suggestive of new many-body states in graphene. In graphene, two particular sets of electrons exist that have a fourfold energy degeneracy. To study the corresponding four quantum states comprising a Landau level, these authors perform measurements on epitaxial graphene at 10 millikelvin. They take spectral 'fingerprints' of the Landau levels, showing in detail how they evolve with magnetic field and how they split into the four separate quantum states. They also observe states with Landau level filling factors of 7/2, 9/2 and 11/2. Electrons in a single sheet of graphene behave quite differently from those in traditional two-dimensional electron systems. Like massless relativistic particles, they have linear dispersion and chiral eigenstates. Furthermore, two sets of electrons centred at different points in reciprocal space (‘valleys’) have this dispersion, giving rise to valley degeneracy. The symmetry between valleys, together with spin symmetry, leads to a fourfold quartet degeneracy of the Landau levels, observed as peaks in the density of states produced by an applied magnetic field. Recent electron transport measurements have observed the lifting of the fourfold degeneracy in very large applied magnetic fields, separating the quartet into integer1,2,3,4 and, more recently, fractional5,6 levels. The exact nature of the broken-symmetry states that form within the Landau levels and lift these degeneracies is unclear at present and is a topic of intense theoretical debate7,8,9,10,11. Here we study the detailed features of the four quantum states that make up a degenerate graphene Landau level. We use high-resolution scanning tunnelling spectroscopy at temperatures as low as 10 mK in an applied magnetic field to study the top layer of multilayer epitaxial graphene. When the Fermi level lies inside the fourfold Landau manifold, significant electron correlation effects result in an enhanced valley splitting for even filling factors, and an enhanced electron spin splitting for odd filling factors. Most unexpectedly, we observe states with Landau level filling factors of 7/2, 9/2 and 11/2, suggestive of new many-body states in graphene.

Electronic band structure of zirconia and hafnia polymorphs from the GW perspective

Abstract: The electronic structure of crystalline ZrO2 and HfO2 in the cubic, tetragonal, and monoclinic phase has been investigated using many-body perturbation theory in the GW approach based on density-functional theory calculations in the local-density approximation LDA. ZrO2 and HfO2 are found to have very similar quasiparticle band structures. Small differences between them are already well described at the LDA level indicating that the filled f shell in HfO2 has no significant effect on the GW corrections. A comparison with direct and inverse photoemission data shows that the GW density of states agrees very well with experiment. A systematic investigation into the structural and morphological dependence of the electronic structure reveals that the internal displacement of the oxygen atoms in the tetragonal phase has a significant effect on the band gap.

Modeling electronic structure and transport properties of graphene with resonant scattering centers

Abstract: We present a detailed numerical study of the electronic properties of single-layer graphene with resonant (hydrogen) impurities and vacancies within a framework of noninteracting tight-binding model on a honeycomb lattice. The algorithms are based on the numerical solution of the time-dependent Schr\"odinger equation and applied to calculate the density of states, quasieigenstates, ac and dc conductivities of large samples containing millions of atoms. Our results give a consistent picture of evolution of electronic structure and transport properties of functionalized graphene in a broad range of concentration of impurities (from graphene to graphane), and show that the formation of impurity band is the main factor determining electrical and optical properties at intermediate impurity concentrations, together with a gap opening when approaching the graphane limit.

Effect of contamination on the electronic structure and hole-injection properties of MoO3/organic semiconductor interfaces

Abstract: The electronic structure and hole-injection properties of ambient contaminated molybdenum trioxide (MoO3) surfaces are studied by ultraviolet and inverse photoemission spectroscopy, and current-voltage measurements. Contamination reduces the work function (WF), electron affinity (EA) and ionization energy by about 1 eV with respect to the freshly evaporated film, to values of 5.7 eV, 5.5 eV, and 8.6 eV, respectively. However, the WF and EA remain sufficiently large that the hole-injection properties of MoO3 are not affected by contamination. The results are of particular importance in view of potential applications of transition metal oxides for low-cost manufacturing of devices in low-vacuum or nonvacuum environment.

Vacancy ordering and electronic structure of γ-Fe2O3 (maghemite): a theoretical investigation

Abstract: The crystal structure of the iron oxide γ-Fe₂O₃ is usually reported in either the cubic system (space group P4(3)32) with partial Fe vacancy disorder or in the tetragonal system (space group P4(1)2(1)2) with full site ordering and c/a≈3. Using a supercell of the cubic structure, we obtain the spectrum of energies of all the ordered configurations which contribute to the partially disordered P4(3)32 cubic structure. Our results show that the configuration with space group P4(1)2(1)2 is indeed much more stable than the others, and that this stability arises from a favourable electrostatic contribution, as this configuration exhibits the maximum possible homogeneity in the distribution of iron cations and vacancies. Maghemite is therefore expected to be fully ordered in equilibrium, and deviations from this behaviour should be associated with metastable growth, extended anti-site defects and surface effects in the case of small nanoparticles. The confirmation of the ordered tetragonal structure allows us to investigate the electronic structure of the material using density functional theory (DFT) calculations. The inclusion of a Hubbard (DFT + U) correction allows the calculation of a band gap in good agreement with experiment. The value of the gap is dependent on the electron spin, which is the basis for the spin-filtering properties of maghemite.