Showing papers on "Effective mass (solid-state physics) published in 1990"

Intrinsic concentration, effective densities of states, and effective mass in silicon

Abstract: An inconsistency between commonly used values of the silicon intrinsic carrier concentration, the effective densities of states in the conduction and valence bands, and the silicon band gap is resolved by critically assessing the relevant literature. As a result of this assessment, experimentally based values for the valence‐band ‘‘densities‐of‐states’’ effective mass are determined in the 300–500 K range and are shown to be in good agreement with recent theoretical calculations. At 300 K, experimentally based values of 3.1×1019 cm−3 for the valence‐band effective densities of states and 1.08×1010 cm−3 for the intrinsic carrier concentration are determined. Although in good agreement with theoretical calculations, these are significantly higher and lower, respectively, than commonly used values in the past. These results have important implications in the calculation of other silicon material and device parameters.

Twisted boundary conditions and effective mass in Heisenberg-Ising and Hubbard rings.

Abstract: We identify the boundary energy of a many-body system of fermions on a lattice under twisted boundary conditions as the inverse of the effective charge-carrying mass, or the stiffness, renormalizing nontrivially under interactions due to the absence of Galilean invariance. We point out that this quantity is a sensitive and direct probe of the metal-insulator transitions possible in these systems, i.e., the Mott-Hubbard transition or Density-wave formation. We calculate exactly the stiffness, or the effective mass, in the 1D Heisenberg-Ising ring and the 1D Hubbard model by using the ansatz of Bethe. For the Hubbard ring we also calculate a spin stiffness by extending the nested ansatz of Bethe-Yang to this case.

High-resolution angle-resolved photoemission study of the Fermi surface and the normal-state electronic structure of Bi2Sr2CaCu2O8

Abstract: High-resolution angle-resolved photoelectron spectroscopic measurements were made of the Fermi edge of a single crystal of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} at 90 K along several directions in the Brillouin zone. The resultant Fermi-level crossings are consistent with local-density band calculations, including a point calculated to be of Bi-O character. Additional measurements were made where bands crossed the Fermi level between 100 and 250 K, along with measurements on an adjacent Pt foil. The Fermi edges of both materials agree to within the noise. Below the Fermi level the spectra show correlation effects in the form of an increased effective mass, but the essence of the single-particle band structure is retained. The shape of the spectra can be explained by a lifetime-broadened photohole and secondary electrons. The effective inverse photohole lifetime is linear in energy.

Repulsive gravitational effects of global monopoles.

Abstract: A monopole formed as a consequence of the spontaneous breakdown of a global symmetry should have a mass that grows linearly with the distance off its core. It was recently shown by Barriola and Vilenkin that the gravitational effect of this configuration is equivalent to that of a deficit solid angle in the metric, plus that of a relatively tiny mass at the origin. Here we show that this small effective mass is negative. Global monopoles thus share with other topological defects, such as domain walls and global strings, a repulsive gravitational potential. We solve numerically the coupled equations for the metric and the scalar field, to precisely determine this repulsive potential and in order to analyze the solution when gravitational effects are already significant close to the monopole core. We study the motion of test particles in a monopole background, and discuss the possible implications of a negative effective mass.

Conduction electrons in GaAs: Five-level k⋅p theory and polaron effects

Abstract: Properties of conduction electrons in GaAs are described theoretically using a five-level k\ensuremath{\cdot}p model, which consistently accounts for inversion asymmetry of the material. The dispersion relation E(k) is computed and it is shown that the conduction band is both nonparabolic and nonspherical. The energy dependence of the electron effective mass, the energy-momentum relation in the forbidden gap, and the spin splitting of the band are calculated. Analytical expressions for the band-edge effective mass, the spin splitting, and the Land\'e factor ${g}^{\mathrm{*}}$ are presented, taking explicitly into account an interband matrix element of the spin-orbit interaction. A five-level P\ensuremath{\cdot}p theory for the conduction band in the presence of an external magnetic field is developed. Resonant and nonresonant effects due to polar electron--optic-phonon interaction are included in the theory. The spin g value of conduction electrons is calculated as a function of energy and magnetic field. Spin-doublet splitting of the cyclotron resonance and the cyclotron-resonance-mass anisotropy are described. A comparison of the theory with experimental data of various authors is used to determine important band parameters for GaAs. It is shown that away from the band edge the polaron effects in GaAs are comparable to the band-structure effects.

Operator ordering in effective-mass theory for heterostructures. I. Comparison with exact results for superlattices, quantum wells, and localized potentials.

Abstract: L'operateur d'energie cinetique est 1/2 mα pmβ pmα, pour une masse effective dependante de la position. Ici, 2 α + β = - 1. Lorsque la theorie de la masse effective est applicable, α = 0 et β = - 1

Image charges in semiconductor quantum wells: Effect on exciton binding energy

Abstract: Binding energies of excitons in a quantum-well structure are calculated including fully the effects of image charges, finite barriers, the z correlation of electrons and holes, and anisotropic hole masses. The influence of discontinuous masses and discontinuous dielectric constants across the interfaces is evaluated in detail: While the mass difference becomes important only when the excitonic wave function penetrates into the barrier, the image charges appreciably modify the Coulomb interaction and therefore influence the exciton binding energy even at well widths larger than the exciton Bohr radius. Results for technologically important, particular material systems are presented.

The spin-polaron theory of high-Tc superconductivity

Abstract: An outline is given of the model for some high-temperature superconductors which assumes that the carriers are holes in the (hybridized) oxygen 2p band and form ‘spin polarons’ with the moments on the copper atoms. A comparison is made with observations of spin polarons in Gd3-x v x S4 and with the properties of La1-x Sr x VO3 in relation to those of La2-x Sr x CuO4. It is assumed, following several authors, that in the superconductors the polarons form bipolarons, which are bosons, and a comparison is made with some other treatments of this hypothesis. It is proposed that in many such superconductors the boson, essentially a pair of these holes, moves in an impurity band, and that normally all the polarons (fermions) form bipolarons; the fermions repel each other on the same site (positive Hubbard U) but attract when on adjacent sites; the critical temperature T c is then that at which the Bose gas becomes non-degenerate. In such materials a non-degenerate gas of bosons would carry the current a...

Quantum‐confinement effects in CdTe‐glass composite thin films produced using rf magnetron sputtering

Abstract: Quantum‐confinement‐induced shifts in the fundamental absorption edge of isolated CdTe crystallites are reported in CdTe‐glass composite thin films produced using a sequential rf magnetron sputtering process employing two separate sputtering sources. Films ranging in thickness from 0.5 to 4.5 μm and containing as much as 30 vol % CdTe have been produced, illustrating the versatility of this technique over a more conventional melting approach. Post‐deposition heat treatments were used to produce average crystallite sizes in the range 46–158 A. An improved fit to theory at larger crystal sizes is found if a cylindrical crystal morphology is assumed. The effective mass of the confined specie, which governs the shift of the absorption edge with crystal size, is found to be 0.20m0 (spherical morphology) and 0.12m0 (cylindrical morphology), both of which are greater than the exciton‐reduced mass in bulk CdTe. The data suggests, therefore, that a non‐negligible Coulomb interaction may still exist in crystals eve...

Determination of the bulk band structure of Ag in Ag/Cu(111) quantum-well systems

Abstract: Ag is grown epitaxially on Cu(111) to form quantum wells. The resulting quantum-well states and resonances, observed with angle-resolved photoemission, exhibit shifts in energy for varying emission directions and for changing Ag-film thicknesses. The results are utilized in a determination of the Ag bulk sp-band dispersion relation near the L point in the Brillouin zone. Effective masses and the Fermi surface near the L point are deduced. The Fermi surface agrees well with that obtained earlier from de Haas--van Alphen measurements.

Influence of the confinement potential on the electron-hole-pair states in semiconductor microcrystallites.

Abstract: The energies of the two energetically lowest dipole-allowed electron-hole-pair states in semiconductor microcrystallites are computed variationally. Details of the quantum confinement conditions, such as the finite value of the quantum confinement potential and the different effective electron-hole masses inside and outside the crystallites, are considered explicitly. Significant deviations from the infinite-potential approximation are obtained.

Band nonparabolicities in lattice-mismatch-strained bulk semiconductor layers

Abstract: Analytic expressions for band nonparabolicities and effective masses are derived for lattice-mismatch-strained bulk semiconductor layers. We have investigated both a full 6\ifmmode\times\else\texttimes\fi{}6 valence-band Hamiltonian and an 8\ifmmode\times\else\texttimes\fi{}8 model. In the latter, the interactions between the singlet conduction band and the triplet valence bands were treated exactly, with the effects of higher bands calculated to order ${\mathit{k}}^{2}$. To our knowledge, the present work constitutes the first explicit calculation of strain and k\ensuremath{\cdot}p matrix elements using a basis consistent with the formalism of Luttinger and Kohn [Phys. Rev. 97, 869 (1955)] throughout. The present results should prove to be valuable in determining strained-layer heterostructure band alignments using excitation spectroscopy and in applications requiring highly accurate estimates of confinement energies in narrow quantum-well structures (such as those used in long-wavelength infrared detection).

Kinks in the Frenkel-Kontorova model with long-range interparticle interactions

Abstract: We study a nonlocal Frenkel-Kontorova model that describes a one-dimensional chain of atoms moving in a periodic external potential and repulsing one another according to a long-range law, e.g., the power law \ensuremath{\sim}${\mathit{x}}^{\mathrm{\ensuremath{-}}\mathit{n}}$. The investigation is carried out both numerically and analytically in approximations of a weak or strong bond between atoms. Static characteristics of kinks (topological solitons) such as the effective mass, shape, and amplitude of the Peierls potential, the interaction energy of kinks, and the creation energy of kink-antikink pairs are calculated for different (exponential and power with n=1 and 3) laws of the interparticle interaction and various concentrations of atoms, i.e., ratio between the external potential period and the average spacing of atoms in the chain. The anharmonicity of the interaction potential between atoms is shown to result in differences between kink and antikink parameters, which are proportional to the value of the anharmonicity that rises with increasing exponent n of the interaction potential as well as at a changeover from a complex to a simpler unit cell. It is noted that at a power law of the interparticle repulsion this law describes also the asymptotics of the kink shape as well as the interaction energy of the kinks. Because of this, the dependence of, e.g., the amplitude of the Peierls potential versus the atom concentration, is similar to the ``devil's staircase.'' The applicability of the extended Frenkel-Kontorova model for describing diffusion characteristics of a quasi-one-dimensional layer adsorbed on a crystal surface is discussed.

Polaron effects in one-dimensional lateral quantum wires and parabolic quantum dots.

Abstract: Calcul des effets de polarons pour des electrons confines dans des fils quantiques lateraux monodimensionnels et des points quantiques paraboliques, en prenant en compte a la fois l'interaction electron-phonon LO volumique et electron-phonon d'interface

Spectral function of a hole in a Hubbard antiferromagnet.

Abstract: The Green's function G(k,\ensuremath{\omega}) is calculated for a hole in a Hubbard antiferromagnet by a new nonperturbative, variational method for an infinite square lattice. The quasiparticle and excited-state wave functions show that the substantial structure in the ``incoherent'' spectrum is related to string states for large U. The strength of the quasiparticle pole and the anisotropic effective mass are obtained as a function of (U/t). The quasiparticle lifetime is infinite for most k.

Numerical studies of femtosecond carrier dynamics in GaAs.

Abstract: We present simulations of 2-eV femtosecond differential transmission experiments in GaAs. Electron and heavy-, light-, and split-off-hole dynamics are calculated by an ensemble Monte Carlo method. To account for valence-band nonparabolicity and anisotropy, a 30-band k\ensuremath{\cdot}p method is used to determine hole band structure, optical matrix elements, density of states, and Bloch overlap factors. Using the distribution functions obtained from the Monte Carlo simulations, we calculate the differential transmission and compare directly with experimental spectra. We show that the inclusion of both collisional broadening during photoexcitation and holes is essential to reproduce accurately the experimental results. We also discuss the effects of intervalley and carrier-carrier scattering in these measurements.

Relativistic band structure of Si, Ge, and GeSi: Inversion-asymmetry effects

Abstract: We present relativistic (including spin) linear muffin-tin orbitals (LMTO) calculations of the band structures of Si, Ge, and zinc-blende-like GeSi. The errors in excitation energies introduced by the use of the local-density approximation to the exchange-correlation potential are corrected with ``ad hoc'' potentials placed at the atomic sites. Effective masses, matrix elements of p, and Luttinger parameters are evaluated. Special emphasis is placed on the effects of inversion asymmetry in GeSi, such as ionicity and spin splittings. The former is very small, probably with Ge acting as the cation. The latter are appreciable and can be related to the asymmetry in the spin-orbit splittings of both constituent atoms. A detailed study of these spin splittings is made, also with the help of k\ensuremath{\cdot}p perturbation theory. The coefficients of terms linear and cubic in k around k=0 are obtained. They should be experimentally observable when high-quality samples become available and should help to understand similar splittings in (Ge${)}_{\mathit{n}}$/(Si${)}_{\mathit{m}}$ (n,m odd) superlattices.

Heavily doped GaAs:Se. I. Photoluminescence determination of the electron effective mass

Abstract: A systematic study of the photoluminescence (PL) of Se‐doped n‐type GaAs grown by metalorganic chemical vapor deposition is reported. A new method is presented to determine the electron effective mass of n+‐direct‐gap semiconductors from the PL spectrum. GaAs samples with electron densities from 1015 to 8×1018 cm−3 were investigated over the temperature range of 13 to 353 K. The PL spectra of n+‐GaAs are analyzed using a physical model which for the first time explains in a consistent manner both the energy of the peak and the full width at half‐maximum, and accounts for the electron density. An accurate fit of the PL spectra is obtained by invoking band‐to‐band transitions without k selection. The electron exchange and correlation interactions account for all the observed band shrinkage, which reaches 48 meV for n=8.0×1018 cm−3. No significant density of band‐tail states is observed. The Fermi energy is obtained directly from the PL fitting and is used with the measured Hall electron density n to determi...

Energy spectrum and charge-density-wave instability of a double quantum well in a magnetic field.

Abstract: Treating the electrons in the Hartree-Fock approximation, we study the wave-vector dependence of the spin-density excitations and the charge-density excitations of a double-quantum-well (DQW) system in a strong magnetic field with total \ensuremath{ u}=1. We conclude that as the distance between the wells increases, the DQW system undergoes a phase transition, probably to a charge-density-wave state. We discuss the possible effects of this phase transition on the quantum Hall effect.

Low-energy excitations in heavy fermion systems

Abstract: Band structure effects play an important role in determining the low-energy excitations in heavy fermion systems. The influence of coherence is evidenced by the recent de Haas-van Alphen experiments where well-defined Fermi surfaces for the heavy quasiparticles are observed and determined. Quasiparticles states in highly correlated electron systems can be described within the renormalized band method which merges realistic ab initio band calculations with phenomenological Fermi liquid considerations. In the present paper we compare calculated Fermi surfaces and quasiparticle bands of heavy fermion compounds with measured dHvA data. Emphasis is placed on the question: how do high effective masses and crystalline electric field (CEF) splitting influence the Fermi surface topology? Under which conditions can one expect conventional band theory to predict the correct Fermi surface?

Nonrelativistic zitterbewegung in two-band systems.

Abstract: The notion of zitterbewegung and the resulting formalism, originally proposed for relativistic quantum dynamics, is applied to describe the acceleration of a nonrelativistic electron moving in a crystal, due to the periodic force experienced. A general linear-combination-of-atomic-orbitals (LCAO) approach is developed for multiband systems, and the special case of two-band systems is studied in detail. It is shown that the zitterbewegung determines the minimum spatial extension of localized wave packets formed by a combination of Bloch functions belonging to a single band, in a manner that depends strongly on the relative parity of the orbitals entering the LCAO bands. In the case of two orbitals with opposite parity, one is able to extend the notion of effective mass to deep defect levels, and to estimate the width of the tails of localized states in glassy semiconductors, as a function of the separation between the mobility edges.

Intersubband infrared absorption in a GaAs/Al0.3Ga0.7As quantum well structure

Abstract: The linewidth, total integrated area, and peak position (ν0) of the intersubband transition (IT) in a GaAs/Al0.3Ga0.7As multiple quantum well, with doping in the barrier, are studied as a function of temperature using the infrared absorption technique. From the temperature dependence of the linewidth and the configuration coordinate model we find that the electrons in the GaAs well are weakly coupled to the GaAs normal optical phonon mode. The electron density (σ) in the quantum well is extracted from the total integrated area of the IT. From the temperature‐dependence of σ we conclude that the Fermi energy is also temperature dependent and that at 5 K it is about 36 meV above the ground state energy. We also find that ν0 increases as the temperature decreases. We calculated the absorption spectrum for the quantum well in a nonparabolic‐anisotropic envelope function approximation including temperature‐dependent effective masses, nonparabolicity, conduction‐band offsets, the Fermi level, and line shape bro...

Theoretical study on the electronic structure of Si-Ge copolymers

Abstract: The electronic structure of polysilane, polygermane, and their copolymers have been calculated by the first principle local density functional method. Polysilane and polygermane with trans-planar skeleton have direct band gaps of 3.89 and 3.31 eV, respectively. This direct-type band structure is conserved independently of the skeleton forms and the copolymerization. The ordered regular and/or block Si-Ge copolymerization introduces the zone-folding image in the copolymer band structures. Si{sub m}Ge{sub n} ordered copolymers have the potential to be the 1D superlattice high polymers. For Si-Ge copolymers having over five blocks, the band-edge electronic structure can be approximately estimated by using the effective mass theory, and a picture of a 1D-QW wire model can be imaged. Typical characteristics in the superlattice, the energy gaps, and optical transition profiles are theoretically discussed.

Almost-localized electrons in a magnetic field.

Abstract: Within a Gutzwiller type of approach to correlated electrons, the effective mass of quasiparticles composing an almost-localized Fermi liquid is spin dependent and varies strongly with magnetic field. The magnetization of such a system saturates in physically accessible fields. The results are used to explain the field dependence of both the effective mass and of the specific heat in the heavy-fermion systems at low temperature

Electron-phonon coupling in the rare-earth metals

Abstract: We have estimated the strength of the mass enhancement of the conduction electrons due to electron-phonon interaction in the rare metals Sc, Y, and La--Lu. The underlying self-consistent energy bands were obtained by means of the scalar relativistic linear-muffin-tin-orbital method, and the electron-phonon parameters were calculated within the Gaspari-Gyorffy formulation. For the heavier rare earths Gd--Tm spin polarization was included both in the band-structure calculations and in the treatment of the electron-phonon coupling to take into account the spin splitting of the conduction electrons induced by the 4f states. The calculated electron-phonon mass enhancement \ensuremath{\lambda} exhibits a pronounced variation through the series with a maximum value of 1.07 in Pr and a minimum of 0.3 in Ho. We analyze the experimental data from specific heat and de Haas--van Alphen measurements in light of the calculated electron-phonon contribution to the mass enhancement. Finally, we present for the superconducting elements Sc, Y, La, and Lu a comparison with the empirical electron-phonon coupling constants derived from the transition temperatures.