Finite Carrier Research Articles

Computability of simple games: A characterization and application to the core

The class of algorithmically computable simple games (i) includes the class of games that have finite carriers and (ii) is included in the class of games that have finite winning coalitions. This paper characterizes computable games, strengthens the earlier result that computable games violate anonymity, and gives examples showing that the above inclusions are strict. It also extends Nakamura’s theorem about the nonemptyness of the core and shows that computable games have a finite Nakamura number, implying that the number of alternatives that the players can deal with rationally is restricted.

Determining the photoelectric parameters of an organic photoconductor by the photoelectromotive-force technique

We report on a theoretical model to describe the non-steady-state photoelectromotive-force (photo-EMF) effect in organic photoconductors. Unlike the conventional theory of the photo-EMF effect developed for crystalline materials, this model accounts for the field dependence of the charge generation quantum efficiency and charge carrier mobility. To verify our findings a detailed experimental study of the charge carrier generation and transport processes in a organic photorefractive composite was performed using the photo-EMF effect and ac photocurrent measurements. The investigated composite was based on a conjugated triphenyldiamine based polymer (TPD-PPV) sensitized with a highly soluble fullerene derivative (PCBM). Our results show that at zero and low dc field the dependence of the photo-EMF signal on frequency, grating period and external electric field is well described by the standard model originally developed for an inorganic monopolar photoconductor with finite charge carrier lifetime. In this regime the photo-EMF effect was used to determine zero- and low-field photoelectric material parameters including the low-field hole mobility $({\ensuremath{\mu}}_{0,h}=1.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}∕\mathrm{V}\phantom{\rule{0.2em}{0ex}}\mathrm{s})$, the effective charge carrier lifetime $(\ensuremath{\tau}=45\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s})$, the diffusion length $({L}_{D}=114\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$, and the primary charge carrier generation efficiency $(\ensuremath{\Phi}=5.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}%)$. In addition, the validity of the Einstein relation $(D∕\ensuremath{\mu}=25\phantom{\rule{0.3em}{0ex}}\mathrm{meV})$ was verified for the low-field regime. At high electric fields the signal behavior deviates significantly from the trend predicted by the standard model for inorganic photoconductors. This behavior which is mainly attributed to the strong field dependence of the charge generation rate is well described by our model.

Curie temperature of Kondo lattice films with finite itinerant charge carrier density

We present a model study of ferromagnetic films consisting of free Bloch electrons coupled to localized moments (Kondo lattice films). By mapping the local interaction onto an effective Heisenberg Hamiltonian, we obtain temperature and carrier density dependent exchange integrals mediating the interaction between local moments via the conduction electrons. The nonperturbative approach recovers analytically the weak-coupling Ruderman-Kittel-Kasuya-Yosida interaction and yields convincing numerical results in the strong-coupling (double exchange) regime. The Curie temperature is calculated for various coupling strengths, band fillings, and numbers of layers. The results are compared with total-energy calculations. We discuss the influence of charge transfer between film layers and of anisotropy on the Curie temperature. The model we investigate is considered relevant to the understanding of the basic magnetic properties of manganites, diluted magnetic semiconductors, and rare-earth substances such as gadolinium.

Effect of the compositional distribution on the photovoltaic power conversion of SiGe solar cells

Theoretical investigation on the photovoltaic power conversion of SiGe solar cells was carried out focusing on the impact of the compositional distribution. The absorption coefficient and the intrinsic carrier concentration were assumed to be macroscopically uniform parameters controlled by the compositional distribution. Under an assumption with finite minority carrier diffusion length, solar cell based on SiGe was revealed to have power conversion superior to that based on Si in a limited compositional window on the Si-rich side. In the window, the increase in the photocurrent density overcompensates the decrease in the voltage, and controls the overall conversion efficiency. The width of the window was greatly affected by the compositional distribution in SiGe. A comparison was made with experiments to support the existence of such a compositional window.

Optical response of many-polaron systems

Exact results for the density of states and the AC conductivity of the spinless Holstein model at finite carrier density are obtained combining Lanczos and kernel polynomial methods.

Mobility in Semiconducting Carbon Nanotubes at Finite Carrier Density

Carbon nanotube field-effect transistors operate over a wide range of electron or hole density, controlled by the gate voltage. Here we calculate the mobility in semiconducting nanotubes as a function of carrier density and electric field, for different tube diameters and temperatures. The low-field mobility is a nonmonotonic function of carrier density and varies by as much as a factor of 4 at room temperature. At low density, with increasing field the drift velocity reaches a maximum and then exhibits negative differential mobility, due to the non-parabolicity of the band structure. At a critical density, rho(c) approximately 0.35-0.5 electrons/nm, the drift velocity saturates at around one-third of the Fermi velocity. Above rho(c), the velocity increases with field strength with no apparent saturation.

Simulation and optimisation of terahertz emission from InGaAs and InP photoconductive switches

We simulate the terahertz emission from laterally biased InGaAs and InP using a three-dimensional carrier dynamics model in order to optimise the semiconductor material. Incident pump-pulse parameters of current Ti:Sapphire and Er:fibre lasers are chosen, and the simulation models the semiconductor's bandstructure using parabolic Γ, L and X valleys, and heavy holes. The emitted terahertz radiation is propagated within the semiconductor and into free space using a model based on the Drude–Lorentz dielectric function. As the InGaAs alloy approaches InAs an increase in the emitted power is observed, and this is attributed to a greater electron mobility. Additionally, low-temperature grown and ion-implanted InGaAs are modelled using a finite carrier trapping time. At sub-picosecond trapping times the terahertz bandwidth is found to increase significantly at the cost of a reduced emission power.

Quasi-Two-Dimensional Diluted Magnetic Semiconductor Systems

We develop a theory for two-dimensional diluted magnetic semiconductor systems (e.g., Ga(1-x)Mn(x)As layers) where the itinerant carriers mediating the ferromagnetic interaction between the impurity local moments, as well as the local moments themselves, are confined in a two-dimensional layer. The theory includes exact spatial disorder effects associated with the random local moment positions within a disordered RKKY lattice field theory description. We predict the ferromagnetic transition temperature (T(c)) as well as the nature of the spontaneous magnetization. The theory includes disorder and finite carrier mean-free path effects as well as the important correction arising from the finite temperature RKKY interaction, finding a strong density dependence of T(c) in contrast to the simple virtual crystal approximation.

Effect of electron–phonon energy exchange on thermal wave propagation in semiconductors considering carrier diffusion and recombination

AbstractThe electron, hole, and phonon temperatures are calculated in semiconductors by taking into account the finite carrier diffusion and nonradiative recombination time in the sample. We assume that the energy of the modulated excitation radiation is greater than the energy gap and absorbed at the surface of the semiconductor, therefore, a source of heat and carrier generation at the surface of the sample are time dependent and must be considered in the photothermal theory. Under this situation, the coupled one‐dimensional heat transport (for each quasiparticle system) and carrier diffusion equations in the strong hole–phonon energy interaction approximation and charge quasineutrality condition are solved. Since the nonequilibrium carrier concentration depends sensitively on the electron and phonon fluctuation temperature, the heat power density generated in the sample due to the recombination of the electron–hole pair will be greatly influenced by the inhomogeneous quasiparticle temperature distributions. This latter effect comes through the recombination rate of carriers and it will be considered in the photothermal signal. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Phenomenological models of dielectric functions and screened Coulomb potential

The evolution of static dielectric constants and polarization with doping are analyzed and discussed using established experimental results. The variation of screened Coulomb potential with doping is derived theoretically to justify the said evolutions. The latter justification arises due to the influence of ionization energy ( E I ) on the static dielectric functions. The bare Coulomb potential and the Thomas–Fermi screened potential with finite carrier density were recovered when E I → ∞ and E I → E F 0 , respectively. Basically, these phenomenological models are associated with the ionization energy based Fermi–Dirac statistics (iFDS).

Nonanonymity and sensitivity of computable simple games

This paper investigates algorithmic computability of simple games (voting games). It shows that (i) games with a finite carrier are computable, (ii) computable games have both finite winning coalitions and cofinite losing coalitions, and (iii) computable games violate any conceivable notion of anonymity, including finite anonymity and measure-based anonymity. The paper argues that computable games are excluded from the intuitive class of “nice” infinite games, employing the notion of “insensitivity”—equal treatment of any two coalitions that differ only on a finite set.

Metal-insulator transition inCa1−xLixPd3O4

Metal-insulator transition in ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{Li}}_{x}{\mathrm{Pd}}_{3}{\mathrm{O}}_{4}$ has been studied through charge transport measurements. The resistivity, the Seebeck coefficient, and the Hall coefficient are consistently explained in terms of a simple one-band picture, where a hole with a moderately enhanced mass is itinerant three dimensionally. Contrary to the theoretical prediction [Phys. Rev. B 62, 13 426 (2000)], ${\mathrm{CaPd}}_{3}{\mathrm{O}}_{4}$ is unlikely to be an excitonic insulator, and holds a finite carrier concentration down to 4.2 K. Thus the metal-insulator transition in this system is basically driven by localization effects.

Electron and phonon thermal waves in semiconductors: the effect of carrier diffusion and recombination on the photoacoustic signal

The electron and phonon temperature distributions are calculated in bipolar semiconductors by taking into account generation and heating of carriers on the surface due to the incident modulated laser beam as well as the finite carrier diffusion and recombination in the solid. We solve the coupled one-dimensional heat and carrier diffusion equations in the linear approximation using appropriate boundary conditions according to the photoacoustic experimental conditions. In addition, special emphasis is laid on the dependence of the electron system heat flux on the electron and phonon temperature. We also consider the influence of both temperatures on the recombination of the electron–hole pairs. The dependence of the amplitude of the electron and phonon temperatures on the modulation frequency are analyzed for different values of characteristic parameters of the problem: electron–phonon energy exchange and carrier thermal generation rate.

Photoacoustic effect in semiconductors: Influence of field temperature on carrier diffusion and recombination

A quantitative derivation is presented for the heat transport in bipolar semiconductors taking into account the generation and heating of carriers on the surface of the sample due to an incident modulated laser beam, finite carrier diffusion, and recombination in the solid. The temperature distribution is calculated using appropriate boundary conditions according to the photoacoustic experimental conditions. In addition, special emphasis is made on the heat power density generated in the sample due to the recombination of the electron–hole pair and the effect of the inhomogeneous temperature distribution on the thermal generation rate of carriers in the photoacoustic signal. Finally, the effect of two temperatures associated with the quasiparticle (electrons and phonons) systems is considered in the finite electron–phonon energy interaction in the solid.

Performance analysis on semiconductor laser amplifier loop mirrors

Operating characteristics of the semiconductor laser amplifier loop mirror (SLALOM) in optical time-division multiplexing (OTDM) applications are investigated in theory. The injection current of the semiconductor laser amplifier (SLA) should be adjusted to realize a flat and high switching window. The channel crosstalk induced by the finite carrier lifetime and the signal timing jitter should be balanced. The optimal switching window width should be slightly wider than half of the OTDM signal bit interval. The control pulse should be narrower than one-third of the OTDM bit interval in order not to deteriorate the bit error rate (BER) performance. Further discussion indicates that the nonlinear gain compression in the SLA becomes a crucial limit for the SLA-based all-optical switches to be used in the terahertz applications.

Loudspeaker linearization technique for parametric array in air

The use of a parametric array in air as a directional audio loudspeaker has been reported in previous literature. The generation of highly directional audible sound is made possible by the self-demodulation of a finite amplitude ultrasonic carrier which is amplitude-modulated with the audio signal. This self-demodulation process, which is a result of the airs nonlinear response, has undesirable side effects on the audible sound as it is subject to nonlinear distortion. The most widely accepted preprocessing technique for reducing this distortion is based on Bertays far field solution and involves using a square root operation which requires the use of an ultrasonic transducer with infinite bandwidth. We propose a novel approach to model the airs nonlinear response a priori using a nonlinear system identification process commonly used in conventional loudspeaker linearization. Inverse modeling of the identified nonlinear system is then performed online and used to predistort the audio signal before it is modulated onto the ultrasonic carrier. A modified Volterra filter structure, known as Multi-memory Decomposition (MMD) and first proposed by Frank et al., is used for the whole linearization process because of its simplicity and efficiency.

Theory of Trion Spectra in Semiconductor Nanostructures

A consistent density-matrix approach for the absorption spectra of semiconductor nanostructures at finite carrier densities is presented that describes both bound and scattering three-particle states (trions). It automatically includes the two-particle bound states (excitons). In optical transitions, the initial electron or hole momentum is transferred to the final trion state, which — due to recoil energy — gives rise to low-energy tails at the trion and exciton line. A high-energy tail at the exciton is due to the exciton–electron scattering states (trion continuum). The incomplete transfer of exciton oscillator strength to the main trion line and the trion continuum is exemplified by a quantum wire calculation.

The effect of carrier diffusion and recombination in semiconductors on the photoacoustic signal

A quantitative derivation is presented for the heat transport in bipolar semiconductors, taking into account generation and heating of carriers on the surface due to an incident modulated laser beam on the surface and finite carrier diffusion and recombination in the solid. The temperature distribution as function of the position and time in the semiconductor is calculated using appropriate boundary conditions according to the photoacoustic experimental conditions. In addition, special emphasis is made on the heat power density generated in the sample due to the recombination of the electron–hole pair and the effect of the inhomogeneous temperature distribution on the thermal generation rate of carriers in the photoacoustic signal.

Kinetic theory of shot noise in nondegenerate diffusive conductors

We investigate current fluctuations in nondegenerate semiconductors, on length scales intermediate between the elastic and inelastic mean free paths. We present an exact solution of the nonlinear kinetic equations in the regime of space-charge limited conduction, without resorting to the drift approximation of previous work. By including the effects of a finite voltage and carrier density in the contact region, a quantitative agreement is obtained with Monte Carlo simulations by GonzAilez et al., for a model of an energy-independent elastic scattering rate. The shot-noise power P is suppressed below the Poisson value PPoisson=2eAa (at mean current Aa) by the Coulomb repulsion of the carriers. The exact suppression factor is close to 1/3 in a three-dimensional system, in agreement with the simulations and with the drift approximation. Including an energy dependence of the scattering rate has a small effect on the suppression factor for the case of short-range scattering by uncharged impurities or quasielastic scattering by acoustic phonons. Long-range scattering by charged impurities remains an open problem.

Modelling of lightly doped collector of a bipolar transistor operating in quasi-saturation region

A physical model taking into consideration the effects of finite collector minority carrier lifetime and minority carrier blocking property of the low-high (n−n+) junction is proposed for predicting the bipolar transistor dc characteristics covering saturation and quasi-saturation operations. Solution for the electron and hole carrier profiles, recombination, drift and diffusion current densities, electric field distributions and voltage drops as a function of the collector current density is presented. The low-level injection model neglecting collector minority carrier recombination current within the injection region is also present. The validity of the complete model is verified by comparing the results obtained by the present analysis with experimental results available in the literature.