Determination Of Band Structure Research Articles

A floating spherical gaussian orbital model for polymers: I. general formalism and computational procedure

The application of the floating spherical Gaussian orbital (FSGO) method to the determination of band structures of regular polymers is described. Related computational problems are discussed.

Spectral element method for band structures of two-dimensional anisotropic photonic crystals

A spectral element method (SEM) is proposed for the accurate calculation of band structures of two-dimensional anisotropic photonic crystals. It uses Gauss-Lobatto-Legendre polynomials as the basis functions in the finite-element framework with curvilinear quadrilateral elements. Coordination mapping is introduced to make the curved quadrilateral elements conformal with the problem geometry. Mixed order basis functions are used in the vector SEM for full vector calculation. The numerical convergence speed of the method is investigated with both square and triangular lattices, and with isotropic and in-plane anisotropic media. It is shown that this method has spectral accuracy, i.e., the numerical error decreases exponentially with the order of basis functions. With only four points per wavelength, the SEM can achieve a numerical error smaller than 0.1%. The full vector calculation method can suppress all spurious modes with nonzero eigenvalues, thus making it easy to filter out real modes. It is thus demonstrated that the SEM is an efficient alternative method for accurate determination of band structures of two-dimensional photonic crystals.

Direct determination of photonic band structure for waveguiding modes in two-dimensional photonic crystals

We directly determine the experimental photonic band dispersion structure of waveguiding modes under the light line in a two-dimensional photonic crystal (2D PhC) waveguide by using angle-resolved attenuated total reflection spectroscopy. Resonance coupling between the external evanescent wave from total reflection within the prism and the waveguiding modes in the 2D PhC provides clear information on individual band components by resolving the angle (i.e., wave vector k) and photon energy. The experimentally determined photonic band structure, which is essential for understanding the novel light propagation properties of PhC systems with many degrees of freedom, agrees well with the band structure predicted by theory. Furthermore, we demonstrate the accuracy and suitability of this method by analyzing field distribution and eigen-photon-energy calculations for a model structure identical to the experimental arrangement of the prism and sample structure.

A comparative study of the electronic band structures of hydrogen-terminated silicon chains by density functional theory with and without GW correction

A comparative study of the electronic band structures of hydrogen-terminated silicon atomic chains with 〈 110 〉 and 〈 112 〉 orientations was conducted using density functional theory (DFT) both with and without GW correction. A direct band gap was revealed in the 〈 110 〉 chain and an indirect band gap was revealed in the 〈 112 〉 chain. The same order of the energy levels at specific k points was observed with both levels of theory. The GW correction to the energy difference of the conduction band minima at points Γ and X was within 0.16 eV and does not change the order of energy levels. This study supports the validity of DFT for the qualitative determination of electronic band structures of low-dimensional hydrogen-terminated silicon systems.

Determination of the electronic band structure for a graded modulation-doped AlGaN∕AlN∕GaN superlattice

The electronic band structure of a modulation-doped AlGaN∕AlN∕GaN superlattice structure where the AlGaN layer is compositionally graded has been experimentally determined by electron holography. It is shown that all periods in the superlattice have a similar two-dimensional-electron-gas distribution, indicating no degradation in the quality of the heterostructures during growth. High-resolution potential energy profiles show that the nominally linear grading of the AlGaN barrier layers results in a parabolic profile in Al composition. Knowledge of the nature of energy barriers for electron transfer between channels is important in the optimization of the perpendicular conductivity of AlGaN∕GaN superlattice structures.

A phenomenological approach of joint density of states for the determination of bandstructure in the case of a semi-metal studied by FT-STS

The authors show that an accurate determination of the band structure can be achieved by usingFourier transform scanning tunnelling microscopy (FT-STM) techniques in the case of a semi-metallicErSi2 layer grown on a Si(111) substrate. This material provides an ideally confined2D electron and hole gas that is reflected in a complex standing wavepattern at 77 K. The quasi-particles exist over a wide energy range from−800 to+300 meV without mixing with silicon bulk excitations. The Fourier transform ofdI/dV maps have been successfully interpreted using the concept of the joint density ofstates (JDOS), which will be properly introduced. We present here an intuitiveinterpretation of the quasiparticle interference process based on a geometric constructionwhich also allows us to clearly demonstrate that hole–hole and hole–electronquantum interferences dominate over electron–electron quantum interference.

Determination of band structure in one-dimensional complex photonic crystals: A bandedge approach

We present a numerical stable method to easily compute bandgaps of one-dimensional complex basis photonic crystals using novel bandedge equations rather than the traditional dispersion equation. The bandedge equations are derived by the concept of scattering matrix and concisely expressed by the transport coefficients with operations of multiplication and addition only. It is not required to calculate the global transfer matrix or the cosine of Bloch phase to avoid numerical instability. Moreover, we present closed-form expressions to calculate the global scattering matrix without using recursive computation. Finally, numerical examples show that use of the derived bandedge equation has better numerical stability than using the traditional dispersion equation for the band structure analysis.

Recovering metallicity inA4C60: The case of monomericLi4C60

The restoration of metallicity in the high-temperature, cubic phase of Li4C60 represents a remarkable feature for a member of the A4C60 family (A = alkali metal), invariably found to be insulators. Structural and resonance technique investigations on Li4C60 at T > 600 K, show that its fcc structure is associated with a complete (4e) charge transfer to C60 and a sparsely populated Fermi level. These findings not only emphasize the crucial role played by lattice symmetry in fulleride transport properties, but also re-dimension the role of Jahn-Teller effects in band structure determination. Moreover, they suggest the present system as a potential precursor to a new class of superconducting fullerides.

All-Electron Exact Exchange Treatment of Semiconductors: Effect of Core-Valence Interaction on Band-Gap andd-Band Position

We present the first all-electron full-potential exact exchange (EXX) Kohn-Sham density functional calculations on a range of semiconductors and insulators (Ge, GaAs, CdS, Si, ZnS, C, BN, Ne, Ar, Kr, and Xe). We remove one of the main computational obstacles of such calculations by the use of a highly efficient basis for inversion of the response function. We find that the band gaps are not as close to experiment as those obtained from previous pseudopotential EXX calculations. The locations of d bands, determined using the full-potential EXX method, are in excellent agreement with experiment, irrespective of whether these are core, semicore, or valence states. We conclude that the inclusion of the core-valence interaction is necessary for accurate determination of EXX Kohn-Sham band structures and that EXX alone is not a complete answer to the band-gap problem in semiconductors.

Band-structure determination for finite 3-D photonic crystals

The partial band structure from a finite photonic crystal is determined using a model based on light diffraction and the transfer-matrix formalism. The predictions from such a model are compared to an experimental measurement of the bands in the LU direction of a face centered cubic colloidal crystal. Then, both the theoretical predictions and the experimental measurements are compared with the usual band-structure calculation based on a plane-wave expansion with perfectly periodic boundary conditions. As in measurements performed in the past, discrepancies between the predictions of this later model and the experimentally determined bands are observed. On the contrary, using the model presented based on light propagation through a finite crystal, where no periodicity is imposed in the direction perpendicular to any of the set of planes considered to determine a specific branch of the band structure, we found a very good agreement between the experimentally determined and the predicted bandwidths.

Photoelectron spectroscopy—An overview

We give an overview about the development of photoelectron spectroscopy (PES) from first historic observations of the photoelectric effect to state-of-the-art experiments. We present selected examples for the application of PES for chemical analysis of solids (ESCA), the determination of the valence band structure by angle resolved photoelectron spectroscopy (ARUPS), and the investigation of many-body effects by high-resolution PES. Furthermore, we give a brief introduction to the possibilities of spacially resolved PES and PES with high photon energies.

Photoemission spectroscopy—from early days to recent applications

In this review we describe the development of photoemission spectroscopy (PES) from the first historic observations of the photoelectric effect by Hertz and Hallwachs to state-of-the-art experiments. We present several examples for the application of PES for chemical analysis of solids (ESCA), the determination of the valence band structure by angle-resolved photoemission (ARUPS), and the investigation of many-body effects, in particular by high-resolution PES on the meV-scale. Furthermore, we give a brief overview about the wide spectrum of experimental methods based on PES.

Calculations of electronic and optical properties in p-doped AlGaN/GaN superlattices and quantum wells

We present the band structure and photoluminescence spectra calculations of h-AlGaN/GaN multiple quantum wells and superlattices. The calculations are performed within a self-consistent approach to the k → · p → theory by means of the resolution of a full eight-band Kane Hamiltonian together with Poisson equation for the carriers charge density. Exchange-correlation effects are included within the local density approximation. These are found to play a important role in the correct determination of band structures and potential profiles. In our calculations, electric fields, strain effects and split-off hole band are taken into account. Theoretical PL spectra of the systems are shown. Similar calculations to c-AlGaN/GaN MQWs and SLs were done for comparison.

Spin-polarized low energy electron microscopy of ferromagnetic thin films*

After a brief summary of the fundamentals, the possibilities and limitations of spin-polarized low energy electron microscopy and its application is illustrated with examples from recent work. These range from fundamental problems such as the determination of the exchange-split excited state band structure to problems of more practical interest such as the evolution of interlayer coupling with film thickness.

Magnetic-field-dependent band gaps in two-dimensional photonic crystals

This paper concerns the first band-structure determination of two-dimensional periodic semiconductor-dielectric photonic crystals in the presence of an applied magnetic field. We introduce an augmented-block-matrix method that allows to transform the determination of photonic band structure for H polarization into a standard eigenvalue problem. The complete polarization-dependent band gaps are reported for a filling fraction $f=10%,$ for both square and hexagonal arrangements.

Photoemission spectral lineshapes from metal overlayers

Photoelectron spectral peaks derived from bulk band-to-band transitions contain information on band structure and quasiparticle lifetime. However, the lineshapes can be significantly distorted by surface photoemission and final-state broadening. The latter is often a dominant source of peak width and may be strongly dependent on the experimental geometry. The use of a thin film sample provides an alternate method of resolving k and removes the dependence on the matrix element and final states from the measurement. Many thin film systems display quantum-well peaks in their photoemission spectra, but in the typical case, the line widths are dominated by interfacial and surface roughness. It has been demonstrated that films that are atomically uniform over the area used for measurement can be grown. In such films broadening due to roughness is eliminated, and the observed peak widths are directly related to the quasiparticle lifetime. The film behaves like an optical interferometer. Imperfection of this interferometer, such as scattering or leakage at the boundaries, introduces additional broadening, but this contribution can be accounted for using a Fabry–Pérot analysis. Quantum-well peaks derived from the sp band can be resolved in films of Ag on Fe(100) to thicknesses in excess of 100 ML. Application of the Fabry–Pérot model to data covering a large thickness range yields an accurate band-structure determination, quasiparticle lifetimes, as well as properties of the well including the interfacial reflectivity and phase shift. Through a study of the temperature dependence of the spectra, the contribution to the lifetime width due to phonon scattering can be determined. This results in a value for the electron–phonon mass-enhancement parameter λ . The observation of quantum well states is not limited to sp-derived states: the Ag d bands also provide discrete states, although the range of film thicknesses over which they are observable is more limited than for the sp-derived states.

Photoemission spectroscopy in solids

AbstractThe photoelectric effect was discovered and explained in terms of quantum physics about a hundred years ago. Today, photoemission spectroscopy using vacuum ultraviolet radiation and x‐rays is an important tool for studying the electronic properties and atomic structure of solids. This paper reviews the development of an understanding of the photoemission process and the basic applications of photoemission spectroscopy in solids, including band structure determination, studies of quasiparticle properties, and core level spectroscopy.

Photoemission spectroscopy in solids

The photoelectric effect was discovered and explained in terms of quantum physics about a hundred years ago. Today, photoemission spectroscopy using vacuum ultraviolet radiation and x-rays is an important tool for studying the electronic properties and atomic structure of solids. This paper reviews the development of an understanding of the photoemission process and the basic applications of photoemission spectroscopy in solids, including band structure determination, studies of quasiparticle properties, and core level spectroscopy.

Experimental determination of the valence band structure of the InxGa1-x-As(001) surface

InxGaj1-xAs layers were grown on GaAs(00l) substrates by molecular beam epitaxy. The structure and quality of the surface and the layer thickness were monitored in situ by dynamical reflection high-energy electron diffraction (RHEED oscillations). Angle-resolved photoemission spectroscopy studies using He-I radiation were performed on the epitaxial layers. The energy-distribution curves of the photoelectrons were determined along the high symmetric directions in the surface Brillouin zone. The experimental band structure of InxGa1-xAs was determined with the help of the experimental and theoretical band structures of GaAs and In As.

Quantum well photoemission from atomically uniform Ag films: determination of electronic band structure and quasi-particle lifetime in Ag(100)

Photoemission from atomically uniform layers of Ag on a Fe(100) substrate is employed for the determination of the bulk electronic band structure of Ag. The high degree of control of the layer thickness allows the interferometric measurement of the electron momentum normal to the sample surface. This leads to an unprecedented precision in the electronic band structure determined from the experiment. The high quality of the layers also allows the discussion of the line shape of the quantum well peaks for the first time and gives access to the intrinsic and extrinsic broadening parameters of the line shape.