Showing papers on "Fermi energy published in 1993"

Carbon 1s near-edge-absorption fine structure in graphite.

Abstract: High-resolution p z - and p x,y -projected fine structure for the 1s carbon core level in graphite are compared with symmetry-projected calculations. A very sharp transition occurs about 7.6 eV above the Fermi energy in the p x,y results, agreeing closely with the nondispersive σ * band. An additional feature near 8.5 eV above the Fermi energy may be due to the Γ 2 - branch of the free-electron-like interlayer states

Quantum-chaotic scattering effects in semiconductor microstructures.

Abstract: We show that classical chaotic scattering has experimentally measurable consequences for the quantum conductance of semiconductor microstructures. These include the existence of conductance fluctuations-a sensitivity of the conductance to either Fermi energy or magnetic field-and weak-localization-a change in the average conductance upon applying a magnetic field. We develop a semiclassical theory and present numerical results for these two effects in which we model the microstructures by billiards attached to leads. We find that the difference between chaotic and regular classical scattering produces a qualitative difference in the fluctuation spectrum and weak-localization lineshape of chaotic and nonchaotic structures. While the semiclassical theory within the diagonal approximation accounts well for the weak-localization lineshape and for the spectrum of the fluctuations, we uncover a surprising failure of the semiclassical diagonal-approximation theory in describing the magnitude of these quantum transport effects.

Adsorption and scanning-tunneling-microscope imaging of benzene on graphite and MoS2.

Abstract: We report ab intio total energy and electronic structure calculations for benzene physisorbed on graphite and MoS 2 . Our results constitute the first detailed ab initio investigation of the states through which tunneling occurs in the scanning tunneling microscope (STM) through an insulating molecule. At low voltages the density of states near the Fermi energy some way from the surface is dominated by a weak admixture of molecular states into the substrate states; the resulting STM images reflect the details of the molecule-substrate interaction. At higher voltages, images of molecular states are obtained

Band structure effects of transport properties in icosahedral quasicrystals.

Abstract: Transport properties and optical conductivity are calculated for the crystalline approximant AlMn alloy. The number of electrons at the Fermi energy is very small and, based on the band structure, the anomalously small dc conductivity and temperature dependent thermoelectric power are explained. A model calculation for the two-dimensional Penrose lattice with random phason strain shows that it becomes more conductive than the perfect Penrose lattice. We propose the mechanism of the interband transition by phason randomness and inelastic scattering for the origin of the anomalous conductivity in real quasicrystals

Spectroscopy of a single adsorbed atom.

Abstract: We have used a low-temperature ultrahigh-vacuum scanning tunneling microscope to perform atomically localized spectroscopic measurements on single Fe atoms adsorbed onto the Pt(111) surface. Using a simple tunneling model, we are able to quantitatively deconvolute the tip and adatom local densities of states (LDOS) from the dI/dV spectra. We find that a resonance occurs in the adatom LDOS that is centered 0.5 eV above the Fermi energy. This feature has a width of approximately 0.6 eV, and occurs only when the tip is within angstroms (laterally) of the center of an Fe adatom

Resonant reflection and transmission in a conducting channel with a single impurity.

Abstract: We consider a narrow conducting channel in a two-dimensional electron gas with a single impurity in the bottleneck part of the channel. In contrast with previous works no specific assumptions concerning the shape of the channel and the impurity potential are made. It is shown that any attractive impurity potential generates sharp Breit-Wigner-type resonances-dips and peaks-in the graph of conductance versus Fermi energy. These correspond to resonant reflection and transmission due to quasibound states in the impurity potential. A simple formula for the conductance in terms of resonance energies and partial widths is found. The widths are directly related to the impurity and channel potentials

Magnetoconductivity of quantum wires with elastic and inelastic scattering.

Abstract: We use a Boltzmann equation to determine the magnetoconductivity of quantum wires. The presence of a confining potential in addtion to the magnetic field removes the degeneracy of the Landau levels and allows one to associate a group velocity with each single-particle state. The distribution function describing the occupation of these single-particle states satisfies a Boltzmann equation, which may be solved exactly in the case of impurity scattering. In the case where the electrons scatter against both phonons and impurities we solve numerically---and in certain limits analytically---the integral equation for the distribution function and determine the conductivity as a function of temperature and magnetic field. The magnetoconductivity exhibits a maximum at a temperature, which depends on the relative strength of the impurity and electron-phonon scattering and shows oscillations when the Fermi energy or the magnetic field is varied.

Van Hove singularities and the role of doping in the stabilization, synthesis and superconductivity of HgBa2Can−1CunO2n+2+δ

Abstract: The electronic structure and Fermi surface of the recently discovered HgBa2Can-1CunO2n+2+δ superconductors have been determined using the full potential linear muffin-tin orbital method and precise structural information determined with neutrons by Radaelli et al. Whereas for stoichiometric HgBa2CuO4 (Hg-1201) the only band crossing the Fermi energy derived from the CuO antibonding state is half-filled, an additional HgO band that crosses EF exists in the case of HgBa2CaCu2O6 (Hg-1212) and HgBa2Ca2CuO4O8 (Hg-1223). Thus, stoichiometric HgBa2CuO4 is expected to be a Mott insulator with dopants essential for forming the normal metallic state that leads to superconductivity at 95 K, in contrast to two other members of the Hg family that are expected to be “self-doped” to a metallic normal state. As in Hg-1201, the electronic structure is two-dimensional and is dominated by van Hove singularities (vHS's) to which EF is pinned by dopants whose calculated concentration is found to agree well with that determined by Radaelli et al. for Hg-1212 for their maximum Tc = 128 K sample. Finally, predicted doping levels for stabilizing a large volume of the high-Tc Hg-1223 phase - and hence its highest Tc - are made on the basis of pinning E F to the vHS.

Current and rate equation for resonant tunneling.

Abstract: We have used nonequilibrium quantum statistical mechanics to derive an exact expression for the current and a rate equation for a simple model of a resonant tunneling diode including scattering within the resonant state. The rate equation is identical to the classical rate equation for resonant tunneling provided that two conditions are met. First, the driving frequency must be slow compared with the Fermi energy of incoming electrons; the lifetime of the resonance does not set a scale. Second, the resonance must be narrow compared with the range of incoming energies and on a scale set by the variation in energy of the tunneling rates of the individual barriers. Our derivation shows that the rate equation holds both in the coherent limit, in which transport can be described by elementary wave mechanics alone, and in the limit where scattering within the resonant state makes a classical ``sequential'' picture more appropriate; all information about coherence is lost in the averaging required to deduce the rate equation. The current through the resonant state is independent of scattering provided that the rate equation holds. We also examine the current when the rate equation does not hold, and show how strongly inelastic processes enter the quantum kinetic description through scattering-out and scattering-in processes at different energies.

Image states and the proper work function for a single layer of Na and K on Cu(111), Co(0001), and Fe(110).

Abstract: High-resolution photoemission and two-photon photoemission spectroscopy have been used for a comparative study of the electronic structure of a single layer of Na and K on Cu(111), Co(0001), and Fe(110). All systems under investigation exhibit an occupied alkali-induced state near the Fermi energy as well as a series of unoccupied states converging toward the vacuum energy, which are identified as image-potential states. For Na (K) on any of the substrates we find the lowest image state to be located at an energy of 2.04\ifmmode\pm\else\textpm\fi{}0.03 eV (1.62\ifmmode\pm\else\textpm\fi{}0.03 eV) above the Fermi level. From this and from the convergence behavior of the image-state series we conclude that the work function for the perfect single alkali layer is independent of the substrates investigated, and is characteristic for the alkali metal. This is in contrast to results of conventional work-function measurements, but in accordance with the theoretical expectation.

Coulomb interactions and the integer quantum Hall effect: Screening and transport.

Abstract: We examine the influence of Coulomb interactions on the integer quantum Hall effect in high-mobility, wide spacer-layer heterostructures. In these devices, the potential due to disorder is expected to be smooth on the scale of the spacer-layer thickness, which can be much larger than the magnetic length. Screening of this potential is accompanied by large fluctuations in electron density and has dramatic consequences. In particular, the screened potential can be pinned to the Fermi energy in regions of the sample, and these regions can percolate over a range of Landau-level filling fractions. We present a theory for transport that includes screening within the Thomas-Fermi approximation

Electronic structure and Fermi surface of the HgBa2CuO4+δ superconductor: Apparent importance of the role of van Hove singularities on high Tc

Abstract: The electronic structure and Fermi surface of the recently discovered HgBa 2 CuO 4+δ superconductor with T c =95 K is calculated making use of the full-potential linear muffin-tin orbital (FLMTO) method. Similarly to the other high- T c cuprates, the main feature of the electronic structure of undoped HgBa 2 CuO 4 is a single free-electron-like-two-dimensional dpσ band crossing E F . As for the “infinite layered” compound, (Sr 1− x Ca x ) 1− y CuO 2 with T c =110 K, the Fermi surface has the shape of a rounded square, and a major van Hove saddle-point singularity (vHS) exists near E F . Drastic changes of the density of states and Fermi surface are found when the hole doping moves the Fermi energy precisely onto the vHs, which is now seen to have a strong influence on the superconducting properties of this compound. These striking results also call attention to and provide possible support for vHs based excitonic pairing mechanisms for high T c .

Coulomb gaps in a strong magnetic field

Abstract: We report on a study of interaction effects in the tunneling density of states of a disordered two-dimensional electron gas in the strong magnetic field limit where only the lowest Landau level is occupied. Interactions in the presence of disorder are accounted for by performing finite-size self-consistent Hartree-Fock calculations. We find evidence for the formation of a pseudogap with a tunneling density of states which vanishes at the Fermi energy.

Quantitative characterization of modulation-doped strained quantum wells through line-shape analysis of room-temperature photoluminescence spectra

Abstract: Room‐temperature photoluminescence (PL) has been used to characterize modulation‐doped AlGaAs/InGaAs/superlattice strained layer quantum wells. A phenomenological line‐shape model has been developed which can be fitted to PL spectra in order to obtain key parameters such as the subband energies, Fermi energy, and transition amplitudes. Quantum well sheet densities calculated from fits to the PL spectra (taken at both room temperature and 77 K) have been compared to sheet densities obtained from low‐temperature Hall measurements. It has also been shown how variations in quantum well composition, width, and symmetry can be characterized by shifts in values of the relevant fitting parameters.

Optical and electrical investigation of subband populations, mobilities and Fermi level pinning in delta-doped quantum wells

Abstract: Measurements are reported on the electrical and optical properties of a series of GaAs/Al0.33Ga0.67As quantum well structures in which a Si delta-doped plane has been placed at the centre of the well. By using a range of well widths for a given planar doping level, it has been possible to control the populations of the electron subbands, as evidenced by Hall, Shubnikov-de Haas, photoluminescence and photoluminescence excitation measurements. Combination of these techniques with the results of a self-consistent Poisson-Schrodinger model has enabled the bandgap renormalization to be determined as a function of electron density, and has also demonstrated that the Fermi energy is pinned at 190 meV above the Gamma conduction band minimum at the delta-doped plane. There is, however, no evidence for a Fermi-edge singularity in the optical spectra of these layers. The transport and the quantum mobilities of the individual subbands have been measured at low temperature, and were found to be in the ratio of approximately 2:1.

Single-electron capacitance spectroscopy of a few electron box

Abstract: This paper presents a technique which permits a quantitative spectroscopy of discrete quantum levels in structures containing as few as one electron. The ground state energy in a quantum dot can be measured for an arbitrary number of electrons and followed as a function of magnetic field. The method involves monitoring the capacitance signal resulting from the tunneling of single electrons. In a microscopic capacitor fabricated in GaAs we study the confined states of a single 1 μm disk to which electrons can tunnel from a nearby metallic layer. Charge transfer occurs only for bias voltages at which a quantum level is resonant with the Fermi energy of the metallic layer. This creates a sequence of distinct capacitance peaks whose bias positions can ve directly converted to an energy scale to determine the electronic spectrum of the confined structure. The evolution of the spectrum in magnetic field allows deduction of the nature of the bound states.

Magnetic splitting of image states at Fe(110).

Abstract: The exchange splitting of image-induced surface states on Fe(110) is calculated by spin-polarized near-surface embedding The splitting is 55 meV for the n=1 state and is primarily a result of coupling to the spin-polarized substrate potential The effect of the spin-polarized surface barrier is relatively small and of opposite sign to the substrate contribution This surprising result is a direct consequence of the negative spin density of surface electrons at the Fermi energy, illustrating the sensitivity of spin-split image states to surface magnetic properties

Electronic structure and Fermi surface topology of the infinite-layered superconductor Sr1−xCaxCuO2

Abstract: Superconductivity reported at 110 K, and recently at 150–170 K, in the infinite-layered (Sr x Ca 1− x )CuO 2 system is investigated by means of the full potential linear muffin-tin orbital (FLMTO) method. As in other high- T c cuprates, the electronic structure of the parent compounds, CaCuO 2 and SrCuO 2 , and of the separately calculated composition Sr 0.7 Ca 0.3 CuO 2 using the experimental lattice parameters, displays strong 2D features including a low density of states at E F (lower than in the other cuprates) and a simple 2D Fermi surface (rounded square) with strong nesting due to a single hybridized Cu d-O p band. As in La 2 CuO 4 , a major van Hove saddle-point singularity exists near E F . The drastic changes of the Fermi surface when Ca/Sr vacancies shift the Fermi energy to the van Hove singularity may have a strong influence on the superconducting properties of the compounds and indicate the need for Sr/Ca vacancies in inducing the high T c .

Electronic properties of quasicrystals

Abstract: We present and analyze briefly partial valence and conduction band states distributions of Al6Mn and both crystalline and quasicrystalline AlCuFe phases of close nominal composition. Low densities of states and a pseudo-gap are found at the Fermi energy. We discuss the formation of this pseudo-gap as due to the diffraction of electronic conduction states by Bragg planes and emphasize its consequences for the densities of d states of the transition element. We discuss also briefly the transport properties of some systems like AlCuFe which show the proximity of a metal-insulator transition.

Low temperature specific heat of VSi2, NbSi2, and TaSi2

Abstract: We present low temperature specific heat measurements of VSi2, NbSi2 and TaSi2. The three disilicides crystallize in the same hexagonal structure (C40, space group P6222). The measured values of the electronic Density of States at the Fermi Energy,D(EF), are in good agreement with theoretical band structure calculations. The obtained Debye temperatures vary asM −1/2 (M is the molar mass of the compound), showing that the interatomic forces are similar in the three disilicides. NbSi2 and TaSi2 are found to be superconductors at 0.130K and 0.353K respectively. The variation of the transition temperatures is discussed.

Photoinduced charge carriers in insulating cuprates: fermi glass insulator, metal-insulator transition and superconductivity

Abstract: The results of photoexcitation experiments in partially doped, insulating cuprates are reviewed, and the physical implications of these “photodoping” experiments are discussed. The existence of a Fermi-glass insulating state at intermediate doping levels is thoroughly explored. The localized electronic states near the Fermi energy (EF) are characterized through transport, steady-state photoconductivity (including spectral response) and time resolved transient photodoping experiments (sub-nanosecond through microsecond). The experimental results indicate an Anderson-type metal-insulator transition in the doped cuprates; i.e., the transition from metal to insulator is dominated by disorder-induced localization. The Fermi energy can be shifted across the mobility edge either by increasing the doping level or by transient photoexcitation at high pump intensities, thereby causing the metal-insulator transition. The high-Tc superconductors, therefore, can be characterized as disordered metals with the Fermi energy relatively close to the mobility edge (Ec). Although the photo-generated carriers are generated homogeneously, the data indicate electronic phase separation into metallic “droplets”. The temperature dependence of the photoinduced conductivity implies that these droplets become superconducting below the intrinsic transition temperature observed in the heavily doped metallic regime.

Electronic structure of HgBa2CuO4

Abstract: Local density approximation calculations of the electronic structure of HgBa 2 CuO 4 are reported along with a number of properties identifiable from it. For the stoichiometric material, the only band crossing the Fermi energy is the Cu-O derived antibonding state characteristic of high- T c cuprates; this band is half-filled. Thus, the stoichiometric material is expected to be a Mott insulator, as are the other undoped cuprates, and the excess oxygen reported in the as-synthesized material is no doubt essential for its superconductivity. Electric field gradients are calculated and compared with those in other cuprates. The position wave-function has maximum weight in the rather large holes in the Hg layer. However, unlike the majority of high- T c materials, there is substantial weight throughout the unit cell, including the Cu-O layer region.

Spectral function of holes in the Emery model.

Abstract: We calculate the single-particle excitation spectrum of holes in the Emery model thereby extending and improving previous calculations The system is considered at half filling ([ital n][sub [ital h]]=1, one hole per CuO[sub 2] unit) and for hole doping, where the on-site hole-hole repulsions are kept finite A paramagnetic form of the ground state is used For the determination of the retarded Green's functions of copper and oxygen holes, the projection technique is applied solving the resulting equations of motions self-consistently At half filling, the excitation spectrum exhibits a charge-transfer gap bounded by Zhang-Rice singlet states and the upper Hubbard band Upon hole doping the flat singlet band crosses the Fermi level giving rise to a large Fermi surface at a hole concentration of [ital n][sub [ital h]]=125 Moreover, spectral weight is shifted from the upper Hubbard band to the states near the Fermi energy The calculated spectral densities, the singlet dispersion for the doped system, and the transfer of spectral weight are in good quantitative agreement with exact diagonalization results for 2[times]2 CuO[sub 2] cluster

Ground state of granular metals.

Abstract: We argue that for granular metals a sizable fraction of the grains becomes charged because the energy fluctuation of the highest-occupied level of each grain, as predicted by random matrix theory, is larger than the charging energy. We have computed the ground state density of states and the degree of ionization of granular metals. The density of states shows a Coulomb gap around the Fermi energy, produced by the long-range part of the Coulomb interactions, which should dominate transport properties at low temperatures

Multichannel ballistic magnetotransport through quantum wires with double circular bends.

Abstract: A theoretical study of magnetotransport through two-dimensional quantum wires with double circular bends is made within the ballistic approximation. By means of the mode-matching method, the S matrix of the single bend with a hard-wall confining potential is calculated as a function of the magnetic field or the Fermi energy for the bending radius and the bending angle as parameters. The combination of S matrices is performed for bends in series. The technique is applied to systems consisting of double bends. The symmetry properties of the S matrix are clarified for the two-terminal systems with rotational symmetry or mirror-plane symmetry. The transmission matrix is evaluated in detail for the case of three propagating channels. The strong dependence of the interchannel scattering on the magnetic field is found. The structure of dips (antiresonances) appearing in the conductance is studied as a function of the distance between bends.

Two-dimensional hole gas and Fermi-edge singularity in Be δ-doped GaAs

Abstract: The subband structure of the quasi-two-dimensional hole gas (2DHG) formed at a single Be \ensuremath{\delta}-doped layer in GaAs has been studied by photoluminescence spectroscopy. To confine the photogenerated minority carriers, and thus to enhance the efficiency of radiative recombination from the 2DHG, the \ensuremath{\delta}-doping spike was placed in the center of an ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As double heterostructure. Recombination involving different hole subbands has been resolved which enabled us to analyze the subband occupation as a function of dopant concentration and sample temperature. In sample structures where the Fermi level is located close to unoccupied subbands, a pronounced Fermi-edge singularity (FES) is observed in the low-temperature (20 K) luminescence spectrum. The temporal evolution of the FES has been studied by time-resolved luminescence spectroscopy. The enhancement in emission intensity at the Fermi edge can be understood in terms of a transfer of excitonic oscillator strength from the unoccupied subbands to nearby occupied states at the Fermi energy. Self-consistent subband calculations have been performed to compute the hole confining potential and the subband energies for the present \ensuremath{\delta}-doped structures. The results of these calculations, which take into account the finite spread of the dopant atoms in accordance with secondary-ion-mass spectroscopic data, are in good agreement with the measured subband spacings. The assignment of light- and heavy-hole transitions is supported by luminescence measurements using circularly polarized light.

Room-temperature photoreflectance characterization of pseudomorphic GaAlAs/InGaAs/GaAs high electron mobility transistor structures including the two-dimensional electron gas density

Abstract: Using contactless photoreflectance at 300 K the authors have characterized three pseudomorphic GaAlAs/InGaAs/GaAs high electron mobility transistor structures. The spectra from the InGaAs modulation-doped quantum well (MDQW) channel can be accounted for on the basis of a step-like two-dimensional density of states (screened exciton) and a Fermi level filling factor. A detailed lineshape fit makes it possible to evaluate the Fermi energy, and hence the two-dimensional electron gas concentration. Furthermore, other important parameters of the system such as built-in electric fields, In composition and well width of the InGaAs MDQW channel can be evaluated.