Polarization Of Charge Carriers Research Articles

Spin polarization of charge carriers and Andreev reflection in (LaCa)MnO/superconductor point contacts

Spin polarization of charge carriers in La0.65Ca0.35MnO3 (LCMO) is studied using point-contact Andreev spectroscopy. Pb and MgB2 are used to make superconducting electrodes. In all cases, the transport spin polarization obtained from the conductivity of LCMO/superconductor point contacts does not exceed 80–85%. Different models of the current flow through the superconductor-ferromagnetic metal contact and possible reasons for noncomplete spin polarization of a current in manganites are explored. The level of spin polarization observed in Sharvin contacts (contact area ∼104 A2) is most naturally explained in terms of a model that suggests separation of the crystal into nanosized magnetic phases, only one of which is a ferromagnetic metal with full spin polarization of charge carriers.

Above-room-temperature ferromagnetism in half-metallic Heusler compounds NiCrP, NiCrSe, NiCrTe, and NiVAs: A first-principles study

We study the interatomic exchange interactions and Curie temperatures in half-metallic semi-Heusler compounds NiCrZ (Z=P, Se, Te) and NiVAs. The study is performed within the framework of density functional theory. The calculation of exchange parameters is based on the frozen-magnon approach. It is shown that the exchange interactions in NiCrZ vary strongly depending on the Z constituent. The Curie temperature is calculated within mean field and random phase approximations. The difference between two estimated values is related to the properties of the exchange interactions. The predicted Curie temperatures of all four systems are considerably above room temperature. The relation between half-metallicity and the value of the Curie temperature is discussed. The combination of a high spin polarization of charge carriers and a high Curie temperature makes these Heusler alloys interesting candidates for spintronics applications.

Tunneling of spin-polarized charge carriers in La0.8Ag0.1MnO3+δ film with variant structure: Magnetotransport and magnetooptical data

Epitaxial La0.8Ag0.1MnO3+δ film with variant structure on ZrO2(Y2O3) substrate was grown for the first time, and the optical, magnetooptical, and transport properties of this film were studied in order to elucidate special features in the mechanism of conductivity and magnetoresistance (MR) in this film. An original method has been developed for separating the MR contributions related to the colossal MR near TC and the tunneling MR component. This method is based on the comparative analysis of data on the MR and the IR magnetotransmission of the film. It is established that the tunneling MR component is related to the large-angle boundaries of structural domains and its temperature dependence is described by the function \(\Delta \rho /\rho \sim (a + b/\sqrt T )\). The degree of spin polarization of charge carriers in the La0.8Ag0.1MnO3+δ film with variant structure amounts to P ∼ 0.5.

Effect of Cr doping in charge ordered Y1−Ca MnO3(x∼0.5)

Abstract The magnetic investigation shows that the AFM charge-ordered Y, Ca manganites are converted on Cr doping into a glassy magnetic state with minor FM inclusions. According to thermopower data, the Cr induced FM regions are intrinsically non-conducting for samples with large octahedral tilts (x⩽0.45) and become metallic for doped Y0.5Ca0.5MnO3. The role of an orbital polarization of charge carriers (O’ phase) is discussed.

Energetic trap distributions in organic semiconductors

The electronic trap distributions of the vapor deposited electron transport material Alq 3 [tris(8-(hydroxyquinoline) aluminum] and hole transport material 1-NaphDATA [4,4′,4″-tris( N-(1-naphthyl)- N-phenylamino)-triphenylamine] have been analyzed by the fractional TSC method (thermally stimulated current) and the thermally stimulated luminescence (TL) technique. The obtained trap distributions can be described by a Gaussian distribution in case of Alq 3 and two discrete trap levels in case of 1-NaphDATA. In the case of Alq 3, a distinction between the trapped charge carrier polarity was possible. A correlation between the I– V curves of electron- and hole-only devices and the respective trap distributions suggests that the trap distribution is responsible for the shape of the observed current–voltage characteristics. It will be shown that the observed trap states cannot be explained by intrinsic tail states of regular HOMO and LUMO levels.

Electronic structure and magnetic, electrical and optical properties of ferromagnetic Heusler alloys

Measurements of the electrical resistivity and the Hall effect of the Heusler alloys NiMnSb, PtMnSb, CoMnSb, AuMnSb, CuMnSb and PtMnSn are reported. The anomalous Hall effect is analyzed in terms of the contributions from skew scattering and side jump. The spin polarization of the charge carriers is proportional to the magnetization. This is an indication that the electronic structure as a function of the temperature is best described by a local band model, with complete spin polarization of charge carriers parallel to the local magnetization at all temperatures. The spin disorder resistivity of NiMnSb is calculated with the local band model. The magneto-optical Kerr effect of PtMnSb is enhanced by the plasma resonance of the charge carriers.

The dielectic response of K x Al x Ti8?x O16 and K x Mg x/2 Ti8?x/2 O16

Materials of the hollandite structure with the general formulae Kx Alx Ti8−x O16 and Kx Mgx/2 Ti8−x/2 O16 have been synthesized in the composition range 1.6⩽x⩽2.0 and their dielectric properties have been measured in the temperature range 77 to 800 K and the frequency range 10−3 to 106 Hz. The observed response shows a whole range of features characteristic for both charge carrier and dipolar polarization processes and these are seen as being associated with the one-dimensional transport in channels in the hollandite structure. At low temperatures the dominant response is the “universal” dielectric relation in which the loss follows the law x″(ω) ∢ ωn−1, with the exponent n<1 and equal specifically to approximately 0.7. This is followed at 120 to 180 K by a distinct loss peak superimposed on the above law, and finally at higher temperatures by a region of strong dispersion which is associated with strongly interacting many-body processes between charged carriers restricted by defects to move in limited regions of the channels.

Dielectric properties of aluminum oxide films

This paper presents the results of an investigation on the dielectric properties and discusses the observed ac and dc electrical characteristics of aluminum oxide thin films. ac properties are deduced from the capacitance measurements at intermediate and high frequencies. Measurements indicate the existence of two relaxation mechanisms. Hopping polarization of charge carriers is responsible for the high-frequency dissipation mechanism. This polarization is influenced by an additional mechanism at low frequencies. dc current-voltage characteristics are obtained from the steady-state region of the current transients for various step voltages. They exhibit two distinct types of conduction mechanisms depending on the applied voltage: (a) Ohmic conductivity at low voltages and (b) space-charge-limited current (SCLC) at high voltages. Specimens are polarized in the upper limit of the Ohmic region. According to the additional effect of the polarization field in a formed sample with respect to applied field, two alternative types of current transients are posible, which are called ’’same’’ and ’’reverse’’ polarity transients. Relaxation frequencies, obtained by means of the current transient data using Hamon’s equation, depend on the applied electrical field. The polarization field also moderates the relaxation frequencies.

Electrical properties and structural transitions in the mitochondrion

Charge carrier generation and transport in the mitochondrial lipoprotein system has been investigated by electrical conductivity, low frequency dielectric relaxation, and thermoelectric power. A parallel study was conducted on morphological/structural changes by DTA and NMR. The results obtained confirm the need to consider concurrently free charge carrier processes and polarization phenomena. All techniques show a "transition" at the same temperature. The steady state conductivity is correlated with main chain segmental reorientations of the phospholipid moiety below the transition and with an interfacial polarization process above it. The Seebeck coefficient provides a useful new aid to characterizing the charge carriers, confirming that they are electronic. The terminal cytochrome oxidase component was investigated separately but it largely reflected ionic impurities characteristic of the isolation process, so that the results were of no intrinsic value.

On the Interaction of Electrons and Holes in a Molecular Crystal

AbstractA self‐consistent polarization field (SCPF) method is used in the framework of a classical electrostatic model to compute single charge carrier polarization energies and electron‐hole interaction energies in naphthalene and anthracene crystals. The crystal polarization energy Pe‐h of an electron‐hole pair localized on specific sites and the energy of the respective charge transfer (CT) states ECT have been calculated for nearest‐neighbour states in naphthalene and five specific sites in anthracene. The estimated polarization energy of a single charge carrier is 1.27 eV for naphthalene, 0.07 eV below the mean reported experimental value, and 1.52 eV for anthracene, 0.06 eV below the mean experimental value. The energy of the nearest‐neighbour CT state is estimated to be 4.29 eV for naphthalene, 0.09 eV below the reported experimental value, and 3.19 eV for anthracene, 0.26 eV below the reported experimental value. The energy of the first conductivity level for electrons E'C is estimated to be 3.96 eV for anthracene, 0.06 eV above the reported threshold for intrinsic photoconductivity. The validity and perspectives of the SCPF method are discussed.

On the physical nature of traps in molecular crystals

The possible physical nature of charge carrier traps of quasi-continuous distribution in molecular crystals is discussed. It is shown that the formation of quasi-continuous trap distribution may be connected with the statistical dispersion of the charge carrier polarization energy caused by fluctuational structural irregularities of the lattice. The dispersion of the polarization energy is evaluated and compared with experimental data of trap distribution in anthracene single crystals. Some suggestions are made about the formation of discrete traps and surface states caused by charge carrier polarization effects in molecular crystals. [Russian Text Ignored]