Two-carrier System Research Articles

Electronic transport of co-doped misfit-layered cobaltites

We have fabricated two series of co-doped misfit-layered cobaltites Ca3Co4O9+δ, Ca3−xYbxCo4−yAgyO9+δ with (x = 0.05, y = 0.05), (x = 0.05, y = 0.10), and (x = 0.05, y = 0.10), and Ca3−xEuxCo4−yAgyO9+δ with (x = 0.05, y = 0.05), (x = 0.05, y = 0.10), and (x = 0.05, y = 0.10) using conventional solid state reaction. The electrical resistivity and thermopower were measured between 300 and 700 K. For all the samples, the temperature dependence of electrical resistivity exhibits broad maximum, indicating disappearance of quasiparticle resonance. Unlike the level-off thermopower behavior between 200 and 300 K, which is often observed for the misfit-layered cobaltites, the thermopower of all the samples increases with increasing temperature up to 700 K, which could be associated with the strong temperature dependence of quasiparticle resonance. Considering that both the sublattices of CoO2 and Ca2CoO3 could make contribution to the electronic transport, it is plausible to explain the variation of the size and temperature dependence of electrical resistivity and thermopower in the framework of two-carrier system. Among the samples, Ca2.95Eu0.05Co3.95Ag0.05O9+δ exhibits the highest power factor of 3.36 μW cm−1K−2 at 700 K. This value represents an improvement of about 110 % compared to the undoped Ca3Co4O9+δ. Ca2.95Eu0.05Co3.95Ag0.05O9+δ had the highest dimensionless figure of merit of 0.037 at 300 K, representing an improvement of about 84 % compared to the undoped Ca3Co4O9+δ.

High temperature thermoelectric properties of co-doped Ca3−xAgxCo3.95Fe0.05O9+δ (0≤x≤0.3)

We have fabricated a series of co-doped misfit-layered cobaltites Ca3−xAgxCo3.95Fe0.05O9+δ with x=0.0, 0.1, 0.2 and 0.3 using conventional solid state reaction. The electrical resistivity and thermopower were measured between 300 and 700K. For all the samples, the temperature dependence of electrical resistivity exhibits broad maximum. The level-off thermopower behavior of misfit-layered cobaltites in the temperature range of 200 and 300K is often observed. Instead, the thermopower of Ca3−xAgxCo3.95Fe0.05O9+δ increases with increasing temperature up to 700K, which could be associated with the strong temperature dependence of quasiparticle resonance. Partial substitution of Ag+ for Ca2+ in the Ca2CoO3 sublattice results in simultaneous increase of conductivity and thermopower. Considering that both the sublattices of CoO2 and Ca2CoO3 could make contribution to the transport properties, it is therefore likely that the conduction behavior of the Ca2CoO3 sublattice is activated type in the framework of two-carrier system. Ca2.9Ag0.2Co3.95Fe0.05O9+δ exhibits the highest power factor and highest figure of merit (zT) among this series of samples.

General analysis of the modulated-photocurrent experiment including the contributions of holes and electrons.

In this paper we concentrate on an analysis of the modulated photocurrent (MPC) experiment applied to samples of amorphous semiconductors built in coplanar geometry. Taking into account both types of photocarriers, the basic equations describing the modulation of the occupation of the localized states are derived according to the statistics of Simmons and Taylor. Generalized expressions for the phase shift and the modulus of the modulated photocurrent are obtained without any restrictive assumptions and discussed. It is shown that, if one type of carrier is predominant, the modulated photocurrent gives the density as well as the capture cross section of the localized states interacting with these carriers. The precise conditions under which the two-carrier system is reduced to a single-carrier system are given. The main features of the method are illustrated by means of a simulation, where we study the influence of several parameters. We show that the dominant contribution to the modulated photocurrent comes from the carrier type which presents the higher value of \ensuremath{\mu}/(N\ensuremath{\sigma}), where \ensuremath{\mu} is the free-carrier mobility; \ensuremath{\sigma}, the capture cross section; and N, the density of trapping states for which the emission rate is equal to the angular frequency \ensuremath{\omega} at which the experiment is performed. Consequently, only the trapping states corresponding to this type of carrier can be probed. Our study underlines some possible experimental misuses of the MPC technique which could lead to erroneous results regarding the inferred density of states.

Interactions of external and internal H+ and Na+ with Na+/Na+ and Na+/H+ exchange of rabbit red cells: Evidence for a common pathway

We have studied the kinetic properties of rabbit red cell (RRBC) Na+/Na+ and Na+/H+ exchanges (EXC) in order to define whether or not both transport functions are conducted by the same molecule. The strategy has been to determine the interactions of Na+ and H+ at the internal (i) and external (o) sites for both exchanges modes. RRBC containing varying Nai and Hi were prepared by nystatin and DIDS treatment of acid-loaded cells. Na+/Na+ EXC was measured as Nao-stimulated Na+ efflux and Na+/H+ EXC as Nao-stimulated H+ efflux and delta pHo-stimulated Na+ influx into acid-loaded cells. The activation of Na+/Na+ EXC by Nao at pHi 7.4 did not follow simple hyperbolic kinetics. Testing of different kinetic models to obtain the best fit for the experimental data indicated the presence of high (Km 2.2 mM) and low affinity (Km 108 mM) sites for a single- or two-carrier system. The activation of Na+/H+ EXC by Nao (pHi 6.6, Nai less than 1 mM) also showed high (Km 11 mM) and low (Km 248 mM) affinity sites. External H+ competitively inhibited Na+/Na+ EXC at the low affinity Nao site (KH 52 nM) while internally H+ were competitive inhibitors (pK 6.7) at low Nai and allosteric activators (pK 7.0) at high Nai. Na+/H+ EXC was also inhibited by acid pHo and allosterically activated by Hi (pK 6.4). We also established the presence of a Nai regulatory site which activates Na+/H+ and Na+/Na+ EXC modifying the affinity for Nao of both pathways. At low Nai, Na+/Na+ EXC was inhibited by acid pHi and Na+/H+ stimulated but at high Nai, Na+/Na+ EXC was stimulated and Na+/H+ inhibited being the sum of both pathways kept constant. Both exchange modes were activated by two classes of Nao sites, cis-inhibited by external Ho, allosterically modified by the binding of H+ to a Hi regulatory site and regulated by Nai. These findings are consistent with Na+/Na+ EXC being a mode of operation of the Na+/H+ exchanger. Na+/H+ EXC was partially inhibited (80-100%) by dimethyl-amiloride (DMA) but basal or pHi-stimulated Na+/Na+ EXC (pHi 6.5, Nai 80 mM) was completely insensitive indicating that Na+/Na+ EXC is an amiloride-insensitive component of Na+/H+ EXC. However, Na+ and H+ efflux into Na-free media were stimulated by cell acidification and also partially (10 to 40%) inhibited by DMA; this also indicates that the Na+/H+ EXC might operate in reverse or uncoupled modes in the absence of Na+/Na+ EXC.(ABSTRACT TRUNCATED AT 400 WORDS)

Intestinal glucose transport using perfused rat jejunum in vivo: model analysis and derivation of corrected kinetic constants.

1. The transport model that best describes intestinal glucose transport in vivo remains unsettled. Three models have been proposed: (1) a single carrier, (2) a single carrier plus passive diffusion, and (3) a two-carrier system. The objectives of the current studies were to define the transport model that best fits experimental data and to devise methods to obtain the kinetic constants, corrected for diffusion barrier resistance, with this model. 2. Intestinal glucose uptake was measured during perfusion of rat jejunum in vivo over a wide range of perfusate concentrations and diffusion barrier resistance was determined under identical experimental conditions. The data were fitted to the transport equations that describe the three models with appropriate diffusion barrier corrections, and the kinetic constants were derived by non-linear regression techniques. The fit of each model to the data was assessed using six statistical tests, five of which favoured a model described by a single carrier and passive diffusion. 3. The main conclusions of these studies are: (1) kinetic constants uncorrected for diffusion barrier resistance are seriously in error; (2) values for the derived kinetic constants are strongly dependent on the transport model selected for the data analysis which underscores the need for rigorous model analysis; (3) corrected kinetic constants may be obtained by either non-linear regression or by a simpler graphical analysis once the correct transport model has been selected and diffusion barrier resistance determined; (4) only corrected kinetic constants should be used for inter-species comparisons or to study the effect of specific interventions on intestinal glucose transport.

A CMOS integrated circuit for the digitally-controlled analog processing of dual-channel TV sound

A single-supply 4- mu m silicon-gate CMOS IC for the processing of dual-channel TV sound transmitted with the West German two-carrier system is presented. Both transmission-mode identification and audio signal processing are performed. The processing functions are performed with continuous-time and sampled-data analog techniques and are digitally controlled by an external microprocessor through a serial bus, thus eliminating any need for analog controls or manual adjustments. The measured performance of the IC has demonstrated the effectiveness of the approach. Full integration of the required functions in a limited silicon area was achieved while retaining advantages resulting from the software programmability, in particular the elimination of any need for analog controls and manual adjustments, which result in improved factory productivity, better TV-set serviceability, and higher long-term stability and low temperature dependence of processing parameters.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Stereophonic and two-channel sound in terrestrial television broadcasting

A number of possible methods exist for the transmission and reception of the additional sound signal required for stereo on existing television services and some of them are also suitable for the transmission of independent sound signals, such as may be required for a bi-lingual service. The BBC has investigated several of these possibilities and these are described in the paper. The most recent tests are of a two-carrier system, similar to that now used in West Germany, but modified to suit UK System I television. The early trials of this system indicate that there may be difficulty in providing a signal that would be both compatible with existing receivers and able to provide a two-channel service over the existing coverage area.

Theory of electrical resistivity in an interacting two-carrier system: application to (SN)Xand (SNBry)X

Electrical resistivity of an interacting two-carrier system has been investigated by using the coupled Boltzmann equation. An exact formula for the resistivity due to carrier-carrier scattering is derived under a plausible angular dependence of the distribution function. This formula indicates that the interband carrier-carrier normal scattering acts so as to attenuate the relative motion of the two types of carriers, while the intraband carrier-carrier normal scattering acts only so as to renormalise the masses of two types of carriers. It is also shown that the centre-of-mass motion of the two types of carriers cannot be attenuated in the absence of carrier-carrier Umklapp scattering and other scattering mechanisms such as carrier-phonon scattering. Moreover, in order to consider both carrier-carrier and carrier-phonon scattering mechanisms simultaneously, an optimum formula for the resistivity is also given and applied to the case of (SN)x and (SNBry)x whose Fermi surface consist of highly anisotropic electron and hole pockets. The observed behaviour in the magnitude and the temperature dependence of the resistivity along the chain axis of these materials is satisfactorily explained by the calculated results.

Characteristics of synaptosomal tyrosine uptake in various brain regions: Effect of other amino acids

Tyrosine uptake by rat synaptosomes was maximal after 5–10 min of incubation and at 30°C; uptake was inhibited by dinitrophenol (10 −4 M) or ATP (10 −3 M) and increased by reducing sodium concentrations or increasing calcium or potassium. The best model for uptake is a two-carrier system, in which one carrier shows high-affinity uptake and the other may be diffusional. Both uptake mechanisms are more concentrated in catecholamine-rich brain areas, and are inhibited in vitro by other large, neutral amino acids. At physiologic amino acid brain concentrations, each system probably carries about half of the tyrosine into the nerve terminal.

Theory of cyclotron resonance in Si(100) inversion layers under [001] uniaxial stress

Abstract Employing the Landau Fermi liquid theory and evaluating the Landau parameters microscopically in the RPA, we have calculated the line shape of the cyclotron resonance in a two-carrier system which can be realized in Si(100) inversion layers by the application of the uniaxial stress in the [001] direction. Calculated results are compared with the observed line-shape measured recently by Abstreiter et al.

Theory of cyclotron resonance in Si(100) inversion layers under [001] uniaxial stress

Employing the Landau Fermi liquid theory and evaluating the Landau parameters microscopically in the RPA, we have calculated the line shape of the cyclotron resonance in a two-carrier system which can be realized in Si(100) inversion layers by the application of the uniaxial stress in the [001] direction. Calculated results are compared with the observed line-shape measured recently by Abstreiter et al.

Electron-electron scattering: Hall coefficient and magnetoresistance

We calculate the Hall coefficient and the magnetoresistance of an interacting two-carrier system. Our results, derived from two simple coupled kinetic equations, are identical to those obtained from the coupled Boltzmann equations. Specializing to a semimetal, we calculate ${R}_{H}$ and $\ensuremath{\rho}$ as a function of doping and pressure. We find that a strong electron-electron interaction has a large effect on these quantities.

Magnetoplasma wave propagation in the two-carrier system provided by p-type semiconductors: a study of the transport properties of p-type germanium using magnetoplasma waves

When the cyclotron frequency relaxation time product is smaller than unity, the propagation coefficient of magnetoplasma waves increases with increasing magnetic field. At higher magnetic fields, where the cyclotron frequency relaxation time product is larger than unity, the magnetoplasma wave propagation coefficient decreases with increasing magnetic field. This inversion of the magnetic field dependence of the propagation coefficient of these waves allows them to be used to study the transport properties of a plasma.

Anisotropy of the seebeck coefficients of bismuth telluride

Anisotropy in the Seebeck effect is found to arise in two separate theoretical situations. It will arise in a single-carrier system when there are two competing scattering mechanisms. One of these (lattice scattering, for example) must be isotropic. The anisotropy will arise in a two-carrier system when there is only a single scattering mechanism. In this case, the anisotropy arises because the conduction and valence band structures are not identical; hence the ratio of the mobility of holes to the mobility of electrons is different in the two principal crystallographic directions. The single-carrier, multiple-scattering theory of the anisotropy is confirmed experimentally on n-type iodine-doped Bi 2Te 3 at low temperatures (100–300°K). The multiple-carrier, single-scattering theory is confirmed on stoichiometric undoped Bi 2Te 3 at high temperatures (300–700°K), when mixed conduction has set in and lattice scattering predominates.