Concentration Of Conduction Electrons Research Articles

The Structure of the Indirect Absorption Edge of CdO

AbstractThe optical absorption in CdO single crystals is reported for photon energies just below the absorption edge (≈ 2.4 eV) at several temperatures between 80 and 300 °K. The investigation of the shape of the intrinsic absorption indicates the existence of an indirect gap. The influence of the higher concentration of conduction electrons on the indirect absorption edge of CdO has been studied. Extrapolation for n = 0 leads to the value Eig = (1.09 ± 0.04) eV.

Magnetic Studies of Clb-Compounds CuMnSb, PdMnSb and Cu1-x (Ni or Pd)x MnSb

Magnetic properties of manganese compounds with C1 b -type crystal structure are studied. CuMnSb (lattice parameter a =6.09 5 A) is antiferromagnetic with a Neel temperature of 55°K and an asymptotic Curie temperature of -160°K. The paramagnetic Bohr magneton number per Mn is 5.4 7 . PdMnSb ( a =6.24 6 A) is ferromagnetic with a Curie temperature of 500°K and the magnetic moment per Mn is 3.9 5 µ B . Magnetic studies are also carried out for the compounds in which Ni or Pd are substituted for Cu in CuMnSb, resulting in a conclusion that the concentration of conduction electrons is very important in determining the magnetic interaction. Qualitative interpretation of experimental results is given based on the theory of resonance scattering by Friedel, Caroli and Blandin.

Nuclear Magnetic Resonance in Uranium Hydride and Deuteride

$\ensuremath{\beta}$-uranium-hydride and $\ensuremath{\beta}$-uranium-deuteride are ferromagnetic with Curie temperatures of 182 and 178\ifmmode^\circ\else\textdegree\fi{}K, respectively. Nuclear-magnetic-resonance studies of the proton in $\ensuremath{\beta}$-U${\mathrm{H}}_{3}$, and of the deuteron in $\ensuremath{\beta}$-U${\mathrm{D}}_{3}$, were made in the paramagnetic state of these compounds. The measured hydrogen Knight shifts ($K$) in $\ensuremath{\beta}$-U${\mathrm{H}}_{3}$ and $\ensuremath{\beta}$-U${\mathrm{D}}_{3}$ are given by $K=(0.40\ifmmode\pm\else\textpm\fi{}0.03){\ensuremath{\chi}}_{M}$, where ${\ensuremath{\chi}}_{M}$ is the molar susceptibility. The second moments are determined as ${M}_{2}=(24\ifmmode\pm\else\textpm\fi{}2)+(45\ifmmode\pm\else\textpm\fi{}3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}{h}^{2}$ ${\mathrm{Oe}}^{2}$ in $\ensuremath{\beta}$-U${\mathrm{H}}_{3}$, and ${M}_{2}=(56.4\ifmmode\pm\else\textpm\fi{}1.5)+(48.5\ifmmode\pm\else\textpm\fi{}3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}{h}^{2}$ ${\mathrm{Oe}}^{2}$ in $\ensuremath{\beta}$-U${\mathrm{D}}_{3}$, where $h=\frac{{H}_{0}}{(T\ensuremath{-}\ensuremath{\theta})}$, $\ensuremath{\theta}$ being the appropriate paramagnetic Curie temperature. Analysis of the linewidth, line shape, and second moment showed that there are three line-broadening mechanisms (four in the deuteride): internuclear dipolar broadening, the (classical) dipolar field of the $5f$ electrons on the uranium ions, and an effect due to the use of powder samples (in the deuteride, a quadrupolar interaction is also present, with $|\frac{{e}^{2}\mathrm{qQ}}{h}|=14.5\ifmmode\pm\else\textpm\fi{}0.5$ kHz). The line shapes, due to the simultaneous presence of several broadening mechanisms, are discussed in some detail. The $5f$ electrons are shown to be localized and do not form a band. The ferromagnetic interaction was analyzed using a simple Ruderman-Kittel-Kasuya-Yosida model, from which it is deduced that the coupling constant $\ensuremath{\Gamma}\ensuremath{\sim}20$ eV ${\mathrm{\AA{}}}^{3}$, $\frac{{m}^{*}}{{m}_{e}}\ensuremath{\sim}3$, and the concentration of conduction electrons is about 2.5 per uranium ion. The Knight shift and quadrupole shielding were calculated, assuming the presence of a high-density interacting electron gas. At temperatures above 370\ifmmode^\circ\else\textdegree\fi{}K, line narrowing was observed, from which the activation energy for hydrogen diffusion was obtained as $E=8.4\ifmmode\pm\else\textpm\fi{}0.9$ kcal/mole for the hydride and 8.9\ifmmode\pm\else\textpm\fi{}0.9 kcal/mole for the deuteride. It was concluded, on the basis of the pre-exponential term ${\ensuremath{\tau}}_{0}$ of the correlation time ${\ensuremath{\tau}}_{c}$, that diffusion takes place by a vacancy mechanism, and that the higher values of ${\ensuremath{\tau}}_{0}$ and ${\ensuremath{\tau}}_{c}$ for the deuterium are mainly due to a difference in the relaxation mechanism. The valency of the uranium ions present has not been definitely established; they are either ${\mathrm{U}}^{3+}$ or ${\mathrm{U}}^{4+}$, probably the latter.

Spin Scattering and Magnetic Ordering in EuB6

Magnetic and transport measurements in ferromagnetic EuB6 show (a) evidence for a Fisher-Langer-type resistance anomaly near Tc, (b) that EuB6 may not be a simple ferromagnet, and (c) that the magnetic coupling might be influenced by a temperature-dependent conduction electron concentration.

A spin echo study of the 59Co hyperfine field in GdCo2 and several related series of substitutional compounds

The hyperfine fields at the 59Co nucleus have been observed in GdCo2 (GdY)Co2, (GdDy)Co2 and Gd(CoNi)2 using a spin echo technique. The field strength in the basic compound GdCo2 was found to be 607 kOe. The results can be interpreted in terms of contributions to the hyperfine field arising from the transition metal sublattice and from the rare earth sublattice. The former appears to be proportional to the magnetic moment associated with the transition metal ion while the rare earth contribution is taken to arise predominantly from conduction electron polarization. With yttrium substitution this second term is found to be directly proportional to the gadolinium concentration. In the nickel substituted compounds the hyperfine field due to the rare earth sublattice is observed to decrease. This is believed to be the result of a change in conduction electron concentration.

H−Impurity States and Nuclear Magnetic Relaxation

<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="normal">H</mml:mi></mml:mrow><mml:mrow><mml:mo>−</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math>Impurity States and Nuclear Magnetic Relaxation

Electric Conductivity and Hall Effect in Evaporated Cadmium Selenide Films

AbstractConductivity and Hall effect were investigated on evaporated CdSe films either non‐treated or annealed at 400 and 700°C. The measurements are performed in the temperature range from −150 to 400°C. In the lower temperature range the mobility of electrons follows an exponential temperature dependence of the form μeff = μ0 exp (– ΔE/kT) typical for polycrystalline films, which is not only determined by scattering mechanisms alone but also by the numbers and properties of potential barriers at the crystallite boundaries. These properties of polycrystalline film structure also explain the dependence of conductivity and mobility on film thickness found in non‐treated CdSe films. In the high temperature range conductivity and electron concentration strongly increase, indicating a self‐activated conductivity mechanism. The conduction electron concentration is determined here by thermal intrinsic disorder of lattice atoms.

S−dModel with Nonspherical Fermi Surface

A study is made of the effects of a nonspherical and multiply connected Fermi surface on the properties of the Ruderman-Kittel-Kasuya-Yosida $s\ensuremath{-}d$ interaction between localized spins and the conduction electrons. At a critical value of the conduction-electron concentration ${\overline{N}}_{e}$, the Fermi surface touches a number of Brillouin-zone faces, giving rise to a number of necks as in copper and nickel. It is shown that, in this region of electron concentration, the paramagnetic Curie temperature $\ensuremath{\theta}$ exhibits a kink associated with a discontinuity in the exchange-stiffness parameter $D\ensuremath{\propto}\ensuremath{\Sigma}{\mathbf{R}}^{2}J(\mathbf{R})$, where $J(\mathbf{R})$ is the indirect-exchange parameter. These effects are expected to be more pronounced in materials with a large number of necks in the multiply connected Fermi surface. We choose the simplest possible model consistent with the crystal symmetry and the Bragg condition at the Brillouin-zone faces, i.e., a tight-binding approximation for the conduction electrons; but a number of results may be shown to be model-independent. We also neglect interband transitions, but these are shown not to affect the long-range component of the conduction-electron polarization. The long-range effects are shown to be model-independent because they are related only to the level-density function $\ensuremath{\rho}(\ensuremath{\epsilon})$ and its derivatives at the Fermi level, whereas the short-range effects are shown to be sensitive to the details of the band structure. The paramagnetic temperature $\ensuremath{\theta}$ can be separated into a model-independent term related to $\ensuremath{\rho}({\ensuremath{\epsilon}}_{F})$ and a model-dependent term related to short-range effects. In the region corresponding to the spherical Fermi surface, $\ensuremath{\theta}$ is nearly proportional to ${{\overline{N}}_{e}}^{\frac{1}{3}}$, and the model-dependent contribution to $\ensuremath{\theta}$ is relatively small, at least for the simple cubic structure. It is also shown that the exchange moments $〈{\mathbf{R}}^{2}〉\ensuremath{\propto}{\ensuremath{\Sigma}}_{\mathbf{R}} {\mathbf{R}}^{2}J(\mathbf{R})$ and $〈{\mathbf{R}}^{4}〉\ensuremath{\propto}{\ensuremath{\Sigma}}_{\mathbf{R}} {\mathbf{R}}^{4}J(\mathbf{R})$ are model-independent and related to the level-density function $\ensuremath{\rho}(\ensuremath{\epsilon})$ and its first and second derivatives at the Fermi level. In the neighborhood of the critical concentration, relatively large positive values of $\frac{〈{\mathbf{R}}^{4}〉}{({a}^{2}〈{\mathbf{R}}^{2}〉)}$ can be obtained for the simple cubic case, in contrast to the negative values of $\frac{〈{\mathbf{R}}^{4}〉}{〈{\mathbf{R}}^{2}〉}$ obtained by Kasuya in the limit ${k}_{F}a\ensuremath{\rightarrow}0$.

Electrical and Optical Properties of Sputtered In2O3 films. I. Electrical Properties and Intrinsic Absorption

AbstractThin films of n‐conducting In2O3 are prepared by reactive sputtering. The concentration of conduction electrons is changed by a heat treatment between ne = 1017 and 1020 cm−3 (T = 300°K). Concentration and temperature dependence of the electron mobility indicate an intense scattering at charged imperfection centres which are due to a considerable intrinsic lattice disorder. The intrinsic absorption is also influenced by this disorder.

Time dependent conductivity in titanium oxide

An instability of electrical conductivity of nonstoichiometric ceramic and crystalline titanium oxide has been observed. The conductivity was found to decrease slowly for as much as several hundred hours at constant measuring temperatures lying between 0 and 100° following quenching from temperatures between 200 and 400°. Experimental results are reported which indicate that the conductivity decrease is associated with a decrease in the bulk conduction electron concentration and that the rate of change of conduction electrons may be represented by a second order kinetic equation.

EFFECTS OF CHEMISORBED OXYGEN ON THE ELECTRICAL PROPERTIES OF CHEMICALLY SPRAYED CdS THIN FILMS

The Hall mobility μ and conduction electron concentration n of semiconducting, spray-deposited CdS thin films are both reduced by chemisorbed oxygen, the former by a larger factor than the latter. The temperature dependence of n is not affected by chemisorption whereas that of μ is changed from practically no dependence on temperature to a thermally activated dependence. This change is attributed to additional scattering introduced by the chemisorbed ions.

The degenerate semiconductor thin films. I—The Fermi energy

A formula is derived for the electronic Fermi energy of the strongly degenerate mono-crystalline n-type semiconductor thin films. If a set of these films, having equal concentration of conduction electrons, is borne in mind, then the dependence of the Fermi energy (related to the conduction band bottom) upon the film thickness is shown to have a tendency to oscillate, provided that no Tamm levels are present in the forbidden gap. The n-type InSb films are discussed in connection with the topic of this paper.

Magnetic Studies of Gadolinium Compounds with the CsCl Structure

The dominant interaction in rare-earth compounds with the CsCl structure is the Ruderman-Kittel-Yosida exchange due to polarization of the conduction electrons. In the rare-earth systems RA (where A=Ag, Cu, Zn, Cd, In), it was found that RAg and RCu are antiferromagnetic; RZn and RCd are ferromagnetic; RIn is antiferromagnetic. This change in magnetic properties can be attributed to the variation of conduction-electron concentration. We have prepared the ternary solid solutions: Gd(ZnxAg1−x), Gd(ZnxIn1−x), Gd(ZnxCu1−x), and (Gd1.2Cd)x(GdAg)1−x [0≦x≦1] to study the effect of changes in electron concentration on the magnetic properties of these materials. The Curie temperature of GdZn and Gd1.2Cd are about 280°K; the Néel temperatures of GdAg, GdCu, and GdIn are reported to be 145°, 41°, and 28°K. The change of the paramagnetic Curie temperatures in the Gd(ZnxAg1−x) and Gd(ZnxIn1−x) systems is in qualitative agreement with the Gd(AgxIn1−x) system as reported by Sekizawa et al. and is consistent with theoretical expectations, assuming that Ag, Zn, and In give one, two, and three conduction electrons. In the composition region 0.4≦x≦1, both systems are ferromagnetic and the Curie temperature decreases as x decreases. At x=0.2 the compounds are probably antiferromagnetic. The total magnetic moment also diminishes as x decreases, in a field of 4 kOe. We expect the total moment to remain constant at 7 μB/Gd for the ferromagnetic compounds at higher fields. The Gd(ZnxCu1−x) and (Gd1.2Cd)x(GdAg)1−x systems show the same trend.

Effect of Stress on the Electrical Properties ofn-Type Gallium Arsenide

The electrical resistivity and Hall coefficient of undoped $n$-type GaAs samples with room-temperature carrier concentrations between 1.9\ifmmode\times\else\texttimes\fi{}${10}^{14}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ and 3.7\ifmmode\times\else\texttimes\fi{}${10}^{16}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ have been measured as a function of uniaxial compression (up to 2\ifmmode\times\else\texttimes\fi{}${10}^{8}$ dyn/${\mathrm{cm}}^{2}$) at various temperatures between 77 and 298\ifmmode^\circ\else\textdegree\fi{}K and as a function of hydrostatic pressure (up to 6\ifmmode\times\else\texttimes\fi{}${10}^{9}$ dyn/${\mathrm{cm}}^{2}$) between 195 and 298\ifmmode^\circ\else\textdegree\fi{}K. Some measurements were also made on two vanadium-doped samples. The results indicate that conduction takes place in a single band at k=0 and that the concentration $n$ and mobility $\ensuremath{\mu}$ of the carriers in this band decrease with increasing compressional stress, the rate of decrease of $n$ being much greater than that of $\ensuremath{\mu}$ in some cases. Between 195 and 298\ifmmode^\circ\else\textdegree\fi{}K the conduction-electron concentration is explained quantitatively by the presence of non-shallow donors having a pressure-dependent ionization energy ${E}_{I}\ensuremath{\approx}[0.17+{10}^{\ensuremath{-}11}{P}_{\mathrm{dyn}/{\mathrm{cm}}^{2}}]$ eV, as well as of shallow donors and acceptors. At lower temperatures the stress dependence of $n$ cannot be explained using the above nonshallow level but seems to imply the presence of a less deep, nonshallow level. The dependence of the mobility on pressure is accounted for in most cases by the variation of the electron effective mass with pressure. Impurity-level concentrations deduced from the electrical measurements, the nature of the nonshallow donors, and the role of vanadium impurity are discussed with the aid of mass-spectrographic analyses. Carbon, nitrogen, and oxygen seem to be likely sources of the 0.17-eV donor levels.

Oxygen Vacancies and Electrical Conduction in Metal Oxides

Oxygen vacancies and their effects on electrical conduction in some metal oxides were considered through an exact solution of the equilibrium relations between oxygen partial pressure in the ambient gas and concentrations of oxygen vacancies and conduction electrons in the oxide. The mass-action law was assumed but no state of ionization of oxygen vacancies was neglected. Concentrations of electrons in conduction states and total oxygen vacancies were considered. The results, which possess considerable complexity not contained in the usual limiting-case solutions, were compared with pertinent experiments.

Investigations of Photoconductivity in a Reaction Kinetic Model Involving an Indirect Generation Process

AbstractAn investigation is made of a reaction kinetic model in which the generation rate of conduction electrons is assumed to be proportional to the number of occupied traps in the appropriate energy‐range. Solutions of the steady‐state and transient concentrations of conduction electrons are obtained. The influence of various trap distributions (discrete level, homogeneous, exponential and Gaussian) is investigated in detail. The quantum efficiency increases with increasing electron concentration and results in a superlinearity and an S‐shaped growth of photocurrent. The change of the quantum efficiency with the electron concentration is virtually determined by the decay or half width, respectively, of the trap distribution.

Space charge injection into impurity semiconductors

An n-type semiconductor placed between two metal electrodes is considered. The ohmic contacts formed facilitate the injection of electrons into the semiconductor. A non-linear differential equation is obtained which relates the concentration of conduction electrons to the distance through the crystal. This equation is integrated exactly for the zero current case. For the current carrying case accurate numerical solutions are reported. The results are discussed in the light of apparent nonohmic conductivity and theories of space charge limited currents.

Low temperature electron trapping lifetimes and extrinsic photoconductivity in n-type silicon doped with shallow impurities

The trapping lifetimes of conduction electrons photoexcited from shallow impurities have been measured in the liquid helium temperature range for samples of n-type silicon whose donor and compensating acceptor concentrations vary from 10 13 to 10 16 impurities/cm 3. A steady state method was employed, in which the conduction electron lifetime t L is equal to n e τ i / N o D . n e is the conduction electron concentration which is determined from low temperature photo-Hall measurements, and N O D τ i is the electron generation rate. The neutral donor concentration No is measured by the Hall effect and T i , the lifetime against photoionization of a neutral donor, is obtained directly by an electron spin resonance technique. The trapping lifetimes t L are generally independent of temperature between 4-2°K and 1–2°K, and are inversely proportional to the compensating acceptor concentration. This latter was determined using a recently developed combined infrared radiation and spin resonance method. The trapping cross-section result for phosphorus donors is about 5 × 10 −12 cm 2 at 3°K. This is about an order of magnitude larger than the value obtained from the giant trap theory of L ax. Also, the concentration independence of the cross-section in the region of temperature independent t L . is not easily accounted for. The mobility of electrons photoexcited with 2 micron radiation considerably exceeds the mobility of electrons photoexcited with 8–25 micron radiation. This suggests the possibility of excitation to another band or minimum by the 2 micron radiation, as is also suggested by the free carrier absorption peak near 2 microns found in n-type silicon by S pitzer and F an.

Effective Masses of Electrons in Indium Arsenide and Indium Antimonide

Measurements of the electrical conductivity and Hall effect of $n$-type indium arsenide and indium antimonide specimens have been made between 1.5\ifmmode^\circ\else\textdegree\fi{}K and 300\ifmmode^\circ\else\textdegree\fi{}K. The dilute metallic character of these substances at low temperatures permits the Fermi level at absolute zero to be obtained from the electrical conductivity versus temperature data. From each Fermi level and the respective conduction electron concentration determined from the Hall effect, we obtained the following effective electron masses: InAs, $\frac{{m}^{*}}{{m}_{0}}=0.020\ifmmode\pm\else\textpm\fi{}0.005$; and InSb, $\frac{{m}^{*}}{{m}_{0}}=0.016\ifmmode\pm\else\textpm\fi{}0.007$.

Lattice Thermal Conductivity of Some Copper Alloys

The thermal and electrical conductivities of three copper-zinc alloys annealed at high temperatures, and of two copper-gold alloys, were measured over a wide range of low temperatures, and their lattice component of thermal conductivity was deduced in the range 2?90 �K. It appears that the high lattice thermal resistance at liquid helium temperatures previously found in copper-zinc alloys is a function of solute content rather than of concentration of conduction electrons and that this resistance can be reduced by high-temperature annealing.