Showing papers on "Effective mass (solid-state physics) published in 2004"

Tonks–Girardeau gas of ultracold atoms in an optical lattice

Abstract: Strongly correlated quantum systems are among the most intriguing and fundamental systems in physics. One such example is the Tonks-Girardeau gas, proposed about 40 years ago, but until now lacking experimental realization; in such a gas, the repulsive interactions between bosonic particles confined to one dimension dominate the physics of the system. In order to minimize their mutual repulsion, the bosons are prevented from occupying the same position in space. This mimics the Pauli exclusion principle for fermions, causing the bosonic particles to exhibit fermionic properties. However, such bosons do not exhibit completely ideal fermionic (or bosonic) quantum behaviour; for example, this is reflected in their characteristic momentum distribution. Here we report the preparation of a Tonks-Girardeau gas of ultracold rubidium atoms held in a two-dimensional optical lattice formed by two orthogonal standing waves. The addition of a third, shallower lattice potential along the long axis of the quantum gases allows us to enter the Tonks-Girardeau regime by increasing the atoms' effective mass and thereby enhancing the role of interactions. We make a theoretical prediction of the momentum distribution based on an approach in which trapped bosons acquire fermionic properties, finding that it agrees closely with the measured distribution.

Valence band effective-mass expressions in the sp 3 d 5 s * empirical tight-binding model applied to a Si and Ge parametrization

Abstract: Exact, analytic expressions for the valence band effective masses in the spin-orbit, ${\mathrm{sp}}^{3}{d}^{5}{s}^{*}$ empirical tight-binding model are derived. These expressions together with an automated fitting algorithm are used to produce improved parameter sets for Si and Ge at room temperature. Detailed examinations of the analytic effective-mass expressions reveal critical capabilities and limitations of this model in reproducing simultaneously certain gaps and effective masses. The [110] masses are shown to be completely determined by the [100] and [111] masses despite the introduction of $d$ orbitals into the basis.

New grids of stellar models including tidal-evolution constants up to carbon burning. I. From 0.8 to 125 M at Z = 0.02

Abstract: We present new stellar models based on updated physics (opacities, expanded nuclear network and mass loss rates). We compute stellar models suitable for the mean solar neighborhood, i.e. for Z = 0.02 and X = 0.70. The covered mass range is from 0.8 up to 125 M ○. and the models are followed until the exhaustion of carbon in the core, for the more massive ones. In addition, the effective temperatures of the more massive models are corrected for the effects of stellar winds, while models with lower effective temperatures are computed using a special treatment of the equation of state (CEFF). Convective core overshooting is assumed to be moderate and is modelled with α ov = 0.20. Besides the classical ingredients of stellar models, we also provide the internal structure constants needed to investigate apsidal motion and tidal evolution in close binaries. The latter constants are made public for the first time. According to the current theories of tidal evolution, the time scales for synchronization and circularization for cool stars depend - apart from the mass, radius and effective temperature - on the depth of the convective envelope x bf and on the radius of gyration β. For stars with higher effective temperatures, these dependencies are mainly incorporated in the tidal torque constant E 2 . All these parameters are steep functions of mass and time, and thus require a special numerical treatment. The new mass loss formalism produces more mass concentrated configurations than previously, especially for more massive and more evolved stellar models. As the present grid is designed mainly for the study of double-lined eclipsing binaries, the gravity-darkening exponents necessary to calculate the surface brightness distribution in rotationally and/or tidally distorted stars are computed following the method described recently by Claret (1998), and made available for each point of every evolutionary track.

The 6.1 Å family (InAs, GaSb, AlSb) and its heterostructures: a selective review

Abstract: The three semiconductors InAs, GaSb, and AlSb form an approximately lattice-matched set around 6.1 A , covering a wide range of energy gaps and other properties. Of particular interest are heterostructures combining InAs with one or both of the antimonides, and they are emphasized in this review. In addition to their use in conventional device types (FETs, RTDs, etc.), several heterostructure configurations with unique properties have been explored, especially InAs/AlSb quantum wells and InAs/GaSb superlattices. InAs/AlSb quantum wells are an ideal medium to study the low-temperature transport properties in InAs itself. With gate-induced electron sheet concentrations on the order 10 12 cm −2 , they exhibit a pronounced conductivity quantization. The very deep wells (1.35 eV ) provide excellent electron confinement, and also permit modulation doping up to at least 10 13 electrons cm −2 . Because of the very low effective mass in InAs, heavily doped wells are essentially metals, with Fermi energies around 200 meV , and Fermi velocities exceeding 10 8 cm s −1 . Contacted with superconducting electrodes, such structures can act as superconductive weak links. InAs/GaSb-related superlattices with their broken-gap lineup behave like semimetals at large lattice periods, but if the lattice period is shortened, increasing quantization effects cause a transition to a narrow-gap semiconductor, making such structures of interest for infrared detectors, often combined with the deliberate addition of strain.

Photoluminescence in quantum-confined SnO2 nanocrystals: Evidence of free exciton decay

Abstract: Nanocrystalline SnO2 quantum dots were synthesized at room temperature by hydrolysis reaction of SnCl2. The addition of tetrabutyl ammonium hydroxide and the use of hydrothermal treatment enabled one to obtain tin dioxide colloidal suspensions with mean particle radii ranging from 1.5 to 4.3 nm. The photoluminescent properties of the suspensions were studied. The particle size distribution was estimated by transmission electron microscopy. Assuming that the maximum intensity photon energy of the photoluminescence spectra is related to the band gap energy of the system, the size dependence of the band gap energies of the quantum-confined SnO2 particles was studied. This dependence was observed to agree very well with the weak confinement regime predicted by the effective mass model. This might be an indication that photoluminescence occurs as a result of a free exciton decay process.

Energy position of near-band-edge emission spectra of InN epitaxial layers with different doping levels

Abstract: We studied the shape and energy position of near-band-edge photoluminescence spectra of InN epitaxial layers with different doping levels. We found that the experimental spectra of InN layers with moderate doping level can be nicely interpreted in the frames of the "free-to-bound" recombination model in degenerate semiconductors. For carrier concentrations above n>5x10(18) cm(-3) the emission spectra can also be modeled satisfactorily, but a contribution due to a pushing up of nonequilibrium holes over the thermal delocalization level in the valence band tails should be considered in the model. The emission spectra of samples with low doping level were instead explained as a recombination from the bottom of the conduction band to a shallow acceptor assuming the same value of the acceptor binding energy estimated from the spectra of highly doped samples. Analyzing the shape and energy position of the free-electron recombination spectra we determined the carrier concentrations responsible for the emissions and found that the fundamental band gap energy of InN is E-g=692+/-2 meV for an effective mass at the conduction-band minimum m(n0)=0.042m(0).

Unification of the band anticrossing and cluster-state models of dilute nitride semiconductor alloys.

Abstract: We show that a quantitative description of the conduction band in Ga(In)NAs is obtained by combining the experimentally motivated band anticrossing model with detailed calculations of nitrogen cluster states. The unexpectedly large electron effective mass values observed in many GaNAs samples are due to hybridization between the conduction band edge E- and nitrogen cluster states close to the band edge. Similar effects explain the difficulty in observing the higher-lying E+ level at low N composition. We predict a decrease of effective mass with hydrostatic pressure in many GaNAs samples.

Spatial structure of an individual Mn acceptor in GaAs.

Abstract: The wave function of a hole bound to an individual Mn acceptor in GaAs is spatially mapped by scanning tunneling microscopy at room temperature and an anisotropic, crosslike shape is observed. The spatial structure is compared with that from an envelope-function, effective mass model and from a tight-binding model. This demonstrates that anisotropy arising from the cubic symmetry of the GaAs crystal produces the crosslike shape for the hole wave function. Thus the coupling between Mn dopants in GaMnAs mediated by such holes will be highly anisotropic.

Demonstration of a 1/4-cycle phase shift in the radiation-induced oscillatory magnetoresistance in GaAs/AlGaAs devices.

Abstract: We examine the phase and the period of the radiation-induced oscillatory magnetoresistance in GaAs/AlGaAs devices utilizing in situ magnetic field calibration by electron spin resonance of diphenyl-picryl-hydrazal. The results confirm a f-independent 1/4-cycle phase shift with respect to the hf=j variant Planck's over 2pi omega(c) condition for j>/=1, and they also suggest a small ( approximately 2%) reduction in the effective mass ratio, m(*)/m, with respect to the standard value for GaAs/AlGaAs devices.

Electronic Structure and Electrical Conductivity of Undoped LiFePO4

Abstract: The electronic structure of LiFePO 4 underpins transport properties important to its use as a lithium storage electrode. Here we have calculated the electronic structure of LiFePO 4 in the ordered olivine structure by a first-principles method to determine (i) the effective mass of carriers and (ii) the nature of the band structure. The electrical conductivity in high purity undoped LiFePO 4 has also been measured experimentally. Spin-polarized calculations show a large electron effective mass and a much smaller but highly anisotropic hole effective mass, suggesting that hole-doped compositions should have the greater electronic conductivity. More surprisingly, the calculations show that this polyanion compound is a half-metal with spin-sensitive band structure, like some other oxides being studied for spintronics applications. This previously unappreciated aspect of the LiFePO 4 electronic structure may play a role in determining transport properties including those relevant to electrochemical applications.

Effective mass of InN epilayers

Abstract: We report on the study of plasma edge absorption of InN epilayers with free electron concentration ranging from 3.5×1017to5×1019cm−3. Together with the previously reported data, the wide range variation of effective mass cannot be explained by Kane’s two band k∙p model alone. We show that the combination of Kane’s two band k∙p model, band renormalized effect due to electron–electron interaction, and electron–ionized impurity interaction can provide an excellent description. The effective mass of the free electron at the bottom of the conduction band was found to be m*=0.05m0, which is in good agreement with the very recent theoretical calculation.

Symmetry of boundary conditions of the Dirac equation for electrons in carbon nanotubes

Abstract: We consider the effective mass model of spinless electrons in single-wall carbon nanotubes that is equivalent to the Dirac equation for massless fermions. Within this framework we derive all possible energy independent hard wall boundary conditions that are applicable to metallic tubes. The boundary conditions are classified in terms of their symmetry properties and we demonstrate that the use of different boundary conditions will result in varying degrees of valley degeneracy breaking of the single-particle energy spectrum.

Generalized effective mass approach for cubic semiconductor n-MOSFETs on arbitrarily oriented wafers

Abstract: The general theory for quantum simulation of cubic semiconductor n-MOSFETs is presented within the effective mass equation approach. The full three-dimensional transport problem is described in terms of coupled transverse subband modes which arise due to quantum confinement along the body thickness direction. Couplings among the subbands are generated for two reasons: due to spatial variations of the confinement potential along the transport direction, and due to non-alignment of the device coordinate system with the principal axes of the constant energy conduction band ellipsoids. The problem simplifies considerably if the electrostatic potential is separable along transport and confinement directions, and further if the potential variations along the transport direction are slow enough to prevent dipolar coupling (Zener tunneling) between subbands. In this limit, the transport problem can be solved by employing two unitary operators to transform an arbitrarily oriented constant energy ellipsoid into a regular ellipsoid with principal axes along the transport, width and confinement directions of the device. The effective masses for several technologically important wafer orientations for silicon and germanium are calculated in this paper.

Zero-resistance states induced by electromagnetic-wave excitation in GaAs/AlGaAs Heterostructures

Abstract: We report the detection of novel zero-resistance states induced by electromagnetic wave excitation in ultra high mobility GaAs/AlGaAs heterostructure devices, at low magnetic fields, B , in the large filling factor limit. Vanishing resistance is observed in the vicinity of B =[4/(4 j +1)] B f , where B f =2πfm ∗ /e , where m ∗ is the effective mass, e is the charge, and f is the microwave frequency. The dependence of the effect is reported as a function of f , the temperature, and the power.

Universal tunneling behavior in technologically relevant P/N junction diodes

Abstract: Band-to-band tunneling was studied in ion-implanted P/N junction diodes with profiles representative of present and future silicon complementary metal–oxide–silicon (CMOS) field effect transistors. Measurements were done over a wide range of temperatures and implant parameters. Profile parameters were derived from analysis of capacitance versus voltage characteristics, and compared to secondary-ion mass spectroscopy analysis. When the tunneling current was plotted against the effective tunneling distance (tunneling distance corrected for band curvature) a quasi-universal exponential reduction of tunneling current versus, tunneling distance was found with an attenuation length of 0.38 nm, corresponding to a tunneling effective mass of 0.29 times the free electron mass (m0), and an extrapolated tunneling current at zero tunnel distance of 5.3×107 A/cm2 at 300 K. These results are directly applicable for predicting drain to substrate currents in CMOS transistors on bulk silicon, and body currents in CMOS transistors in silicon-on-insulator.

Further explorations of Skyrme-Hartree-Fock-Bogoliubov mass formulas. III: Role of particle-number projection

Abstract: Starting from HFB-6, we have constructed a new mass table, referred to as HFB-8, including all the 9200 nuclei lying between the two drip lines over the range of $Z$ and $N\ensuremath{\geqslant}8$ and $Z\ensuremath{\leqslant}120$. It differs from HFB-6 in that the wave function is projected on the exact particle number. Like HFB-6, the isoscalar effective mass ${M}_{s}^{*}$ is constrained to the value $0.80M$ and the pairing is density independent. The rms errors of the mass-data fit is $0.635\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$, i.e. better than almost all our previous HFB mass formulas. The extrapolations of this new mass formula out to the drip lines do not differ significantly from the previous HFB-6 mass formula.

Doping mechanism in aluminum doped zinc oxide films

Abstract: The doping mechanism in aluminum doped zinc oxide films has been interpreted by considering the relationship between Hall mobility and effective mass of electrons with carrier concentrations. Both degeneracy and the nonparabolic nature of the conduction band are taken into account for determining the charge state of the dopant. It is ascertained that aluminum liberates one free carrier in the zinc oxide lattice by substituting the zinc atom.

Influence of conduction-band nonparabolicity on electron confinement and effective mass in GaNxAs1−x∕GaAs quantum wells

Abstract: We derive an analytical model to describe the conduction-band states of GaNAs-based quantum well structures, including the band anticrossing effect between N resonant states and the conduction-band edge. The predictions of the model are compared to those obtained using a full ten-band k·p model based on the same set of parameters. Both methods are then tested by comparison with the experimentally determined ground- and excited-state interband transition energies of GaNxAs1−x quantum wells of different well widths and N composition x obtained at 300 K and under hydrostatic pressures up to 2.0 GPa . We show that the transition energies can be described by a consistent set of material parameters in all the samples studied, and present how the conduction to valence-band offset ratio varies strongly with x in GaNxAs1−x∕GaAs quantum well structures. We conclude that the model presented can be used to predict the transition energies and electron subband structure of any GaNxAs1−x∕GaAs quantum well with well width between 2 and 25 nm , and N composition x between 1 and 4% , although further work is still required to confirm the optimum choice for the variation of band offset ratio with composition.

Cluster dynamical mean field analysis of the mott transition.

Abstract: We investigate the Mott transition using a cluster extension of dynamical mean field theory (DMFT). In the absence of frustration we find no evidence for a finite temperature Mott transition. Instead, in a frustrated model, we observe signatures of a finite temperature Mott critical point in agreement with experimental studies of $\ensuremath{\kappa}$ organics and with single-site DMFT. As the Mott transition is approached, a clear momentum dependence of the electron lifetime develops on the Fermi surface with the formation of cold regions along the diagonal direction of the Brillouin zone. Furthermore, the variation of the effective mass is no longer equal to the inverse of the quasiparticle residue, as in DMFT, and is reduced approaching the Mott transition.

Confinement effects in quasi-stoichiometric CeO2 nanoparticles

Abstract: This paper deals with the analysis of structural and electronic effects of size in quasi-stoichiometric CeO2 nanoparticles prepared by a microemulsion method. The preparation method yields highly controlled materials in terms of particle size distribution and chemical oxidation state, with the presence of Ce(III) species only below an average particle size of ca. 8 nm. The rather low quantity of Ce reduced ions produces marked differences with confinement effects previously reported in the literature. A steady behavior of the fluorite lattice parameter is observed as a function of size in the 5–10 nm range. In this range, the bandgap displays a small decrease of ca. 0.1 eV, with significant differences from the behavior expected on the basis of the effective mass approximation. These structural and electronic properties are rationalized on the basis of the characterization of the materials.

Condensation, excitation, pairing, and superfluid density in high-$T_{c}$ superconductors: magnetic resonance mode as a roton analogue and a possible spin-mediated pairing

Abstract: To find out a primary determing factor of $T_{c}$ and a pairing mechanism in high-$T_{c}$ cuprates, we combine the muon spin relaxation results on $n_{s}/m^{*}$ (superconducting carrier density / effective mass), accumulated over the last 15 years, with the results from neutron and Raman scattering, STM, specific heat, Nernst effect and ARPES measurements. We identify the neutron magnetic resonance mode as an analogue of roton minimum in the superfluid $^{4}$He, and argue that $n_{s}/m^{*}$ and the resonance mode energy $\hbar\omega_{res}$ play a primary role in determining $T_{c}$ in the underdoped region. We propose a picture that roton-like excitations in the cuprates appear as a coupled mode, which has the resonance mode for spin and charge responses at different momentum transfers but the same energy transfers, as detected respectively, by the neutron S=1 mode and the Raman S=0 A1$_{g}$ mode. We shall call this as the ``hybrid spin/charge roton''. After discussing the role of dimensionality in condensation, we propose a generic phase diagram of the cuprates with spatial phase separation in the overdoped region as a special case of the BE-BCS crossover conjecture where the superconducting coupling is lost rapidly in the overdoped region. Using a microscopic model of charge motion resonating with antiferomagnetic spin fluctuations, we propose a possibility that the hybrid spin/charge roton and higher-energy spin fluctuations mediate the superconducting pairing. In this model, the resonance modes can be viewed as a meson-analogue and the ``dome'' shape of the phase diagram can be understood as a natural consequence of departure from the competing Mott insulator ground state via carrier doping.

Band dispersion relations of zinc-blende and wurtzite InN

Abstract: The fundamental band-gap energy of the III-V compound semiconductor InN has been corrected due to recent experiments on high-quality InN samples. The advances in the InN growth technique also open new possibilities to realize high-frequency field effect transistors at elevated temperatures due to the small effective mass in the InN conduction-band minimum and high optical phonon frequencies. For device simulations the correct band dispersion relations around the InN band gap are of great interest and motivate the reexamination of previous theoretical works on InN. We present the band structure on InN in the zinc-blende and wurtzite-type configuration calculated within the empirical pseudopotential method approach by exploiting the transferability of ionic model potential parameters. The modified potential parameters of In support the achieved reliability of the presented InN zinc-blende and wurtzite band structure and related properties, e.g., Luttinger and Luttinger-like $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ parameters.

Effective mass approximation for two extreme semiconductors: Band gap of PbS and CuBr nanoparticles

Abstract: An effective mass approximation (EMA) with finite-depth square-well potential is used to investigate the size-dependent band gap (BG) of PbS and CuBr nanoparticles embedded in different matrices. These two materials are interesting from the theoretical point of view as PbS is a low-BG material with smaller effective masses and larger dielectric constants, whereas CuBr is a wide-BG material with larger effective masses and smaller dielectric constants. Comparing the experimental BGs with our theoretical calculations, it is shown that EMA provides a better description of the experimental data, especially for CuBr, when the Coulomb interaction having the size-dependent dielectric constant is included in the calculation. Further, comparing the change in the BG of spherical nanoparticle, nanowire and thin film, it is predicted that the effective dimensionality of semiconductor nanoparticles can be increased by embedding them in another semiconducting matrix.

Condensation, excitation, pairing, and superfluid density in high-Tc superconductors: the magnetic resonance mode as a roton analogue and a possible spin-mediated pairing

Abstract: To find a primary factor determining T c and a pairing mechanism in high-T c cuprates, we combine the muon spin relaxation results on n s /m* (superconducting carrier density/effective mass), accumulated over the last 17 years, with the results from neutron and Raman scattering, scanning tunnelling microscopy, specific heat, Nernst effect, and angle-resolved photoemission spectroscopy measurements. We identify the neutron magnetic resonance mode as an analogue of the roton minimum in the superfluid 4 He, and argue that n s /m* and the resonance mode energy ∞ω res play a primary role in determining T c in the underdoped region. We propose a picture wherein roton-like excitations in the cuprates appear as a coupled mode, which has resonance modes for spin and charge responses at different momentum transfers but the same energy transfer, as detected respectively by means of the neutron S = I mode and the Raman S = 0 A 1 g mode. We shall call this the 'hybrid spin/charge roton'. After discussing the role of dimensionality in condensation, we propose a generic phase diagram for the cuprates with spatial phase separation in the overdoped region as a special case of the Bose-Einstein to Bardeen-Cooper-Schrieffer crossover conjecture where the superconducting coupling is lost rapidly in the overdoped region. Using a microscopic model of charge motion resonating with antiferromagnetic spin fluctuations, we propose the possibility that the hybrid spin/charge roton and higher-energy spin fluctuations mediate the superconducting pairing. In this model, the resonance modes can be viewed as a meson analogue and the 'dome' shape of the phase diagram can be understood as a natural consequence of departure from the competing Mott insulator ground state via carrier doping.

Molecular wires: guiding the super-exchange interactions between two electrodes

Abstract: After discussing experimental works on the measurement of the conductance of a metal–molecule–metal tunnel junction with many down to a few molecules and down to only one molecule in the junction, we propose a comprehensive way of understanding the electron transport phenomenon through such a junction. The dependence of the junction conductance on the length of the molecule(s) is explained starting from the quantum super-exchange electron transfer phenomenon up to the effective mass of the tunnelling electrons in the coherent limit. This super-exchange mechanism results from the electronic coupling between the two electrodes introduced by the molecule(s). The molecular wire guides this interaction better than the electronic coupling through vacuum between the two electrodes of the junction. Dephasing and thermal effects during the electron transfer events along the molecular wire are described using a density matrix formalism. The implication of our understanding of this through junction electronic transport is described starting from hybrid molecular electronics towards mono-molecular electronics.