Photoluminescence Measurement of Er,O-Codoped GaAs Under a Pulsed Magnetic Field up to 60 T

We have performed photoluminescence (PL) measurements on magnetic semiconductor GaAs:Er,O under pulsed magnetic fields up to 60 T at 4.2 K. We have succeeded in observing the Zeeman effect of the ⁴ I _13/2→⁴ I _15/2 PL transition of the Er³⁺ in the high magnetic field region. The effective g-factor of the dominant PL transition is estimated to be g _eff=1.54±0.04. This value is in reasonable agreement with the theoretical value based on the crystal field theory.     

Analysis of temperature dependent electrical characteristics of Au/n-GaAs/GaAs structures in a wide temperature range

The temperature dependence of some electrical parameters of Au/n-GaAs/GaAs structures obtained from the forward bias current-voltage (I-V) and reverse bias capacitance-voltage (C-V) characteristics is studied in the temperature range of 79-400 K. The forward bias current I is found to be proportional to I_o(T)exp(AV), where A is the slope of Ln(I)-V curves and almost independent of the voltage and temperature, and I_o(T) is relatively a weak function of temperature. The semi-logarithmic Ln(I)-V characteristics based on the thermionic emission (TE) mechanism showed a decrease in the ideality factor (n) and an increase in the zero-bias barrier height (Φ_Bo) with increasing temperature. These behaviors don't obey the pure thermionic emission (TE) theory. However, the barrier height Φ_B(C-V) determined from the C⁻²-V plot at high frequency decreased linearly with the temperature. Analysis of the data indicated that the predominant current conduction mechanism of our sample was a trap-assisted multistep tunneling rather than the other mechanisms.     

Circularly polarized electroluminescence of quantum-size InGaAs/GaAs heterostructures with ferromagnetic metal-GaAs Schottky contacts

Electroluminescence (EL) of InGaAs/GaAs heterostructures with quantum wells and ferromagnetic metal (Co, Ni)/GaAs Schottky contacts has been studied in magnetic fields up to 10 T at a temperature of 1.5 K. The EL line corresponding to the recombination of electrons with injected holes exhibits splitting into components corresponding to the Landau levels in the applied magnetic field and shows circular polarization that significantly exceeds the level typical of such structures with nonmagnetic (Au/GaAs) contacts. The degree of circular polarization (P _EL) exhibits a nonmonotonic dependence on the applied magnetic field and is correlated with the filling of Landau levels. The maximum degree of circular polarization reached in the heterostructures studied is P _EL = 40%.     

Magnetovoltage Measurements and Hysteresis Effects in Polycrystalline Superconducting Y1Ba2Cu3O7−x /Ag in Weak Magnetic Fields

The magnetovoltage measurements (V–H curves) with different sweeping rates (dH/dt) of the external magnetic field in Ag-added polycrystalline YBa₂Cu₃O_7?x sample (YBCO/Ag) were investigated. The measurements of V–H curves were carried out as functions of the transport current (I) and temperature (T). Upon cycling H, all V–H curves measured for different values of I exhibit a clockwise hysteresis effects in forward region. The hysteresis effects in the V–H curve were interpreted in terms of two-level magnetic system, which considers the superposition of the external magnetic field and the local magnetic fields in the intergrain boundaries induced by magnetic dipole moment of neighbor superconducting grains. The analysis of magnetovoltage data showed that the flux trapping in the junction network has a negligible effect on the evolution of the V–H curves and the irreversibilities arise mainly from the flux trapping inside the grains. It is shown that the width of V–H curves shows a universal scaling behavior with respect to the applied current below the critical temperature T _c . The comparison of V–H curves of the YBCO/Ag sample with those of YBCO shows that adding Ag to the superconducting structure weakens the pinning properties of Josephson medium and provokes the instabilities in measured dissipation. The presence of Ag in the superconducting matrix causes marked decrease in hysteresis effects and makes the V–H curves dH/dt dependent. At high values of dH/dt, the instabilities and plateau regions in V–H curves increase significantly as compared to those of observed in YBCO. In addition, the interrelation between the evolution of V–H curves obtained for different values of I and the critical current I _c was demonstrated experimentally.     

Power Law Dependence of I?CV Characteristics of Bi-2212 Whiskers at Low Magnetic Fields

In this study, BiSrCaCuO superconducting whiskers were fabricated by using a glass-ceramics process. The T _c value of the whiskers fabricated was found to be 90.2?K. I?CV characteristics of the whiskers were studied in the temperature range of 10?C70?K under low magnetic fields. The transport critical current density of whiskers was calculated between 19 and 0.63??10⁴ A/cm² at 10?C60?K. I?CV data were fitted using the Levenberg?CMarquardt technique. Results were discussed and so was their dependence on magnetic and electrical properties of the superconductors.     

Irreversibility Effects and Low Field Magnetovoltage Measurements in Superconducting MgB2 Near the Critical Temperature T c

We have investigated the hysteresis effects in magnetovoltage measurements (V–H curves) in a polycrystalline superconducting MgB₂ near critical temperature T _c . The measurements were carried out as functions of temperature (T) and transport current (I), and sweep rates of the external magnetic field (dH/dt) and perpendicular and parallel orientations of external magnetic field (H) with respect to I, i.e., the V _⊥–H curves for $\vec{H} \bot\vec{I}$ and V _∥–H curves for $\vec{H}\parallel \vec{I}$ . The hysteresis effects related to both forward and reverse regions of V–H curves were interpreted within two level magnetic system mainly flux trapping inside the grains. A series of expansion of V(H) in terms of H was approximated to describe macroscopically the experimental V–H curves. The analysis of zero field I–V curves obtained by standard and reverse procedures gives an evidence of nearly absence of macroscopic surface pinning. The instabilities in V–H curves were interpreted in terms of defective flow of flux lines and also local fluctuations in magnetic flux structure. Further, the anisotropic effects in MgB₂ sample were explored by measuring the angular dependence of dissipation at the fixed values of T, H and I by varying the angle (θ) between H and I, i.e., V–θ curves. The analysis of V–θ curves reveals that the measured dissipation does not exhibit any agreement with the Lorentz force driven free flux motion, V(θ)～sin²(θ). This suggests that the anisotropic effects are screened by the random distribution of the grains in different sizes.     

Space charge limited conduction in bulk Se100 − xSbx chalcogenide glasses

The present paper reports the measurements on space charge limited conduction in bulk glassy Se_100 − xSb_x (x = 2, 4, 6, 8). I–V characteristics have been measured at various fixed temperatures. These characteristics show that, at low electric fields, an ohmic behaviour is observed. However, at high electric fields (E ∼10⁴ V/cm), the current becomes superohmic.At high fields (10⁴ V/cm), current could be fitted to the theory of space charge limited conduction in case of uniform distribution of localized states in the mobility gap of these materials. Using the theory of SCLC for the uniform distribution of the traps, the density of localized states near Fermi level is also calculated. Density of localized states initially increases with Sb concentration till 6 at.% of Sb and then decreases with further increase of Sb. This decrease is explained in terms of average coordination number.     

Non-ohmic behavior in a-Se80−xTe20Cdx thin films

The present paper reports the d.c. conductivity measurements at high electric fields in vacuum evaporated thin films of amorphous Se_80−xTe₂₀Cd_x (x = 0, 5, 10, 15) systems. Current–voltage (I–V) characteristics have been measured at various fixed temperatures. In all the samples, at low electric fields ohmic behavior is observed. However, at high electric fields (E ∼ 10⁴ V/cm), non-ohmic behavior is observed. An analysis of the experimental data confirms the presence of space charge limited conduction (SCLC) in all the glassy materials studied in the present case. From the fitting of the data to the theory of SCLC, the density of defect states (DOS) near Fermi level is calculated. It is found that the incorporation of Cd like a third element as an impurity could not change the density of defect states significantly in pure binary Se–Te glassy system.     

Effect of some metallic impurities on the density of localized states in a-Se80Te20 thin films

The present paper reports the d.c. conductivity measurements at high electric fields in vacuum-evaporated thin films of amorphous Se₈₀Te₂₀, Se₇₅Te₂₀Ge₅ and Se₇₅Te₂₀Sb₅ systems. Current-voltage (I-V) characteristics have been measured at various fixed temperatures. In all the samples, at low electric fields ohmic behavior is observed. However, at high electric fields (E∼10⁴ V/cm), non-ohmic behavior is observed. An analysis of the experimental data confirms the presence of space charge limited conduction (SCLC) in all the glassy materials studied in the present case. From the fitting of the data to the theory of SCLC, the density of defect states (DOS) near Fermi level is calculated. The role of the aforesaid impurities in a-Se₈₀Te₂₀ is found to be entirely different. In case of Sb, an increase in DOS is observed. However, a decrease is observed in case of Ge. The change in DOS on impurity incorporation is explained in terms of the change in structure of these glasses.     

Carrier transport mechanisms and photovoltaic characteristics of p-H2Pc/n-Si heterojunctions

Heterojunction cells of p-H₂Pc/n-Si were fabricated by vacuum deposition of p-H₂Pc thin films onto n-Si single crystals. Measurements of the current-voltage (I-V) and the capacitance—voltage (C-V) characteristics have been evaluated to identify the mechanisms of barrier formation and, consequently, current flow. The forward current involves tunneling and could be explained by a multi-step tunneling recombination model due to a high density of interface defects. The C-V characteristics indicate an abrupt heterojunction model. The devices exhibit strong photovoltaic characteristics with an open-circuit voltage of 0.34 V, a short-circuit current density of 17.5 mA/cm² and a power conversion efficiency of 1.5%. These parameters have been estimated at room temperature and under constant illumination of 150 mW/cm².     

New lead vanadium phosphate with langbeinite-type structure: Pb1.5V2(PO4)3

The new lead vanadium phosphate Pb_1.5V₂(PO₄)₃ was synthesized by solid state reaction and characterized by X-ray powder diffraction, electron microscopy, and magnetic susceptibility measurements. The crystal structure of Pb_1.5V₂(PO₄)₃ (a = 9.78182(8) Å, S.G. P2₁3, Z = 4) was determined from X-ray powder diffraction data and belongs to the langbeinite-type structures. It is formed by corner-linked V³⁺O₆ octahedra and tetrahedral phosphate groups resulting in a three-dimensional framework. The lead atoms are situated in the structure interstices and only partially occupy their positions. An electron microscopy study confirmed the structure solution. Magnetic susceptibility measurements revealed Curie-Weiss (CW) behavior for Pb_1.5V₂(PO₄)₃ at high temperature whereas at around 14 K an abrupt increase on the susceptibility was observed.     

Flux Dynamics and Time Dependent Effects in Superconducting MgB2

We have investigated the time dependent flux motion in a polycrystalline superconducting MgB₂ sample by means of current-voltage (I?CV curves) and transport relaxation (V?Ct curves) measurements. The transport measurements were carried out as functions of temperature (T), transport current (I), and applied magnetic field (H). The time effects in the I?CV curves were investigated with the help of the sweep rate of transport current (dI/dt). The I?CV curves exhibit nearly reversible behavior for both slow and high current sweep rates (dI/dt) upon cycling the transport current. It was observed that the evolution of the I?CV curves is nearly independent of dI/dt. The small instabilities, voltage jump, and drops appearing at low dissipation levels were interpreted as a kind of plastic flow of the vortices evolving in the form of stripe or cluster in different sizes and local fluctuations in the superconducting order parameter. It was shown that the experimental I?CV curves are in good agreement with a power law behavior, V(I)??I ⁿ . The pinning potential U ₀ extracted from the fitting procedure is found to be approximately independent of dI/dt. In addition to the standard procedure, a reverse procedure was employed to study how the transport current (or associated vortices) penetrates from the surface into the sample. It was found that there is no marked difference between the I?CV curves obtained in the standard and reverse procedures. These results suggest that, in polycrystalline MgB₂, the weak links between grains (standard procedure) and surface weak links (reverse procedure) do not have a considerable effect on the evolution of the I?CV curves and also on the other transport measurements. In order to understand better the flux dynamics, the experimental results for polycrystalline MgB₂ sample were compared to previous similar studies on superconducting Y₁Ba₂Cu₃O_7?x (YBCO) and Bi_1.7Pb_0.3Sr₂Ca₂Cu₃O _x (BSCCO) polycrystalline samples.     

Towards All Electrical Spin Injection and Detection in GaAs in a Lateral Geometry

Herein we discuss our approach to realizing all electrical spin injection and detection in GaAs. We propose a lateral geometry, with two ferromagnetic electrodes crossing an n-doped GaAs channel. AlO _x tunnel barriers are to be used in order to overcome the impedance mismatch and different widths of the two electrodes ensure different coercive fields. We present a detailed theoretical analysis of the expected magnetoresistance. Differences in behavior between lateral and vertical devices, the influence of the applied bias (electric field), and opportunities offered by different measurement geometries were explored. The MBE grown wafer consisted of 100 nm Al_0.3Ga_0.7As, acting as confinement layer, 100 nm n-doped (4 × l0¹⁷ cm⁻³) GaAs, 3 nm n⁺⁺ GaAs (10²¹ cm⁻³), to suppress Schottky barrier formation, and 1.5 nm Al. The Al was oxidized naturally in order to obtain tunnel barriers. By making use of in-situ shadow masks, a 0.1 mm wide channel is defined by covering the rest of the sample by insulating SiO₂, followed by deposition of Ta bonding pads. Finally, 500 and 1000 nm wide CoFe electrodes crossing the GaAs channel are obtained by e-beam lithography and sputtering. We show that the I–V characteristics of the CoFe/AlO _x /GaAs interface are consistent with tunneling as the main injection mechanism. However, the resistance-area (5 × 10⁹ Ω μm²) of our barriers is too high compared to the GaAs conductance (50 Ω square resistance) leading to a strong suppression of magnetoresistance. Further experiments are in progress toward optimizing barrier and channel impedance matching.     

A dislocation theory of plasticity

Based on the observation that plastic behavior is a macroscopic manifestation of the motion and interaction of dislocations, a plasticity theory is proposed that represents a connecting link between solid state physics and the conventional plasticity and viscoplasticity theories. The dislocation picture is described-for a slip system a—by means of A^a_I (immobile dislocation densities), A^a_M(mobile dislocation densities), and the dislocation speeds V^a. Suitable constitutive equations are introduced for V^a and the time rate of change ofA^a_I,A^a_M.A two-dimensional extension of a rigid-viscoplastic body is analyzed to illustrate the nature of the proposed theory.     

The Influence of Lithium Halides on the Superconducting Properties of YB2Cu3O7−x

The effects of addition to YBa₂Cu₃O_7?x of lithium halides (YBa₂Cu₃O_7?x (LiF) _y , and YBa₂Cu₃O_7?x (LiCl) _y ) on the structural, electric, magnetic, and transport properties are analyzed. Both structural and superconducting properties depend weakly on the lithium content up to y= 0.10. The critical temperature keeps on a value well above 91 K for a small amount of lithium halide (reaching 93.48K. for y= 0.02 in YBa₂Cu₃O_7?x (LiF) _y and 91.30 K in YBa₂Cu₃O_7?x (LiCl) _y ), but for higher concentration of Li it rapidly decreases (81.68K for y= 0.20). The same behavior is exhibited in the lower intragranular critical field. The intragranular critical current density depends on the magnetic field as a power law:j _c∝B ^?α, with a lithium-concentration-dependent α. The voltage–current characteristics follow a law typical for granular superconductors, V∝(I?I _c(B,T)) ^n(B,T). The dependence of the intergranular critical current, I _c, and of the exponent, n, on temperature, magnetic field, and concentration is analyzed.     

A simple modification of the Hodgkin and Huxley equations explains type 3 excitability in squid giant axons

The Hodgkin and Huxley (HH) model predicts sustained repetitive firing of nerve action potentials for a suprathreshold depolarizing current pulse for as long as the pulse is applied (type 2 excitability). Squid giant axons, the preparation for which the model was intended, fire only once at the beginning of the pulse (type 3 behaviour). This discrepancy between the theory and experiments can be removed by modifying a single parameter in the HH equations for the K⁺ current as determined from the analysis in this paper. K⁺ currents in general have been described by I_K=g_K(V−E_K), where g_K is the membrane''s K⁺ current conductance and E_K is the K⁺ Nernst potential. However, I_K has a nonlinear dependence on (V−E_K) well described by the Goldman–Hodgkin–Katz equation that determines the voltage dependence of g_K. This experimental finding is the basis for the modification in the HH equations describing type 3 behaviour. Our analysis may have broad significance given the use of I_K=g_K(V−E_K) to describe K⁺ currents in a wide variety of biological preparations.     

Iodine Vacancy Redistribution in Organic–Inorganic Halide Perovskite Films and Resistive Switching Effects

Organic–inorganic halide perovskite (OHP) materials, for example, CH₃NH₃PbI₃ (MAPbI₃), have attracted significant interest for applications such as solar cells, photodectors, light‐emitting diodes, and lasers. Previous studies have shown that charged defects can migrate in perovskites under an electric field and/or light illumination, potentially preventing these devices from practical applications. Understanding and control of the defect generation and movement will not only lead to more stable devices but also new device concepts. Here, it is shown that the formation/annihilation of iodine vacancies (V_I's) in MAPbI₃ films, driven by electric fields and light illumination, can induce pronounced resistive switching effects. Due to a low diffusion energy barrier (≈0.17 eV), the V_I's can readily drift under an electric field, and spontaneously diffuse with a concentration gradient. It is shown that the V_I diffusion process can be suppressed by controlling the affinity of the contact electrode material to I^? ions, or by light illumination. An electrical‐write and optical‐erase memory element is further demonstrated by coupling ion migration with electric fields and light illumination. These results provide guidance toward improved stability and performance of perovskite‐based optoelectronic systems, and can lead to the development of solid‐state devices that couple ionics, electronics, and optics.     

Étude en impulsion de l'injection et de la conduction dans les structures Si-SiO2-Al

We have studied the injection mechanisms at the Si-SiO₂ interface and the conduction in Si-SiO₂-Al devices. The use of a pulsed voltage makes it possible to apply fields of the order of 10⁷ V/cm to the thin oxide layer. The experimental I(V) characteristics prove the existence of the following mechanisms:-at weak fields (ε<3.5 x 10⁶ V/cm), Schottky injection of electrons at the temperature of the thermal lattice vibrations;-for 3.5 x 10⁶ V/cm<ε<8 x 10⁶ V/cm, injection of hot electrons (T_m? 6500°K) produced by an avalanche effect;-for ε>8 x 10⁶ V/cm, tunnel injection of electrons of an effective mass m? 0.4m₀.     

Effect of alloy composition on structural, optical and morphological properties and electrical characteristics of GaxIn1−xP/GaAs structure

The structural, optical and morphological properties of Ga-rich Ga_xIn_1?xP layers with various gallium compositions grown on epi-ready semi-insulating (100)-oriented GaAs substrates by using Molecular Beam Epitaxy technique are presented in this study. The Ga_xIn_1?xP/GaAs structures (S1, S2 and S3) have been evaluated by means of high resolution X-ray diffraction, photoluminescence (PL) and atomic force microscopy measurements at room temperature. Experimental forward and reverse bias current–voltage (I–V) characteristics of structure S3 was investigated at room temperature due to its better characteristics when compared to the other two samples. The main electrical parameters such as ideality factor (n), barrier height (Φ _b ) and series resistance (R _s ) were extracted from forward bias I–V characteristics and Cheung’s function. In addition, Hall measurements were carried out as a function of temperature (30–300 K) and at a magnetic field of 0.4 T were presented for structure S3.     

dc conduction properties of TlSbSe2 crystals

TlSbSe₂ samples used for electrical measurements were cleaved from larger crystals grown by using the Bridgman-Stockbarger method. Electrical properties of TlSbSe₂ have been carried out as a function of temperature (203-258 K) at different dc electric fields. The dc studies revealed the non-ohmic type of conduction (log I versus log V plot). The field lowering coefficientβ is evaluated from log I versus E^1/2 plot. It is found that the dominant conduction mechanism in these samples is Poole-Frenkel type. The activation energies are also calculated at different voltages. It is seen that the activation energy decreases with increase of applied voltage.