Reduction in relaxation time due to ionized impurities in GaAs/AlGaAs quantum well structures

Time-resolved photoluminescence measurements in δ -doped GaAs/AlGaAs on the quantum well structures are performed to study effects of ionized impurities relaxation process of photoexcited carriers. It is theoretically shown that a thin quantum well with a δ -doping layer inserted in the barrier layer of double quantum wells enhances the impurity scattering rate significantly. Photoluminescence decay time in the δ -doped samples is found to decrease compared with the undoped samples.     

Deposition and characterization of YBCO films with BZO inclusions on single crystal substrate

YBa₂Cu₃O_7?δ thin films with BaZrO₃ (BZO) inclusions have been deposited on SrTiO₃ substrates in order to study the effect of nanoparticles addition into YBCO matrix. Samples with 7% (mol) BZO content were obtained by PLD varying the deposition conditions, in order to evaluate their effect on the films transport properties. The change in deposition parameters, especially of the deposition temperature, has been discovered to be efficient for a reduction or complete recovery of the critical temperature decrease produced by BZO addition. The effect of the deposition temperature on in-field films transport properties as well as on the presence of c-axis correlated defects typically ascribable to nanoparticles addition, can be recognised in an improvement in J_C retention in applied magnetic field and, for higher temperatures, the appearance of correlated pinning contribution, as confirmed by pinning force density measurements.     

Nonperturbative physics from interpolating actions

We study the expansion in an artificial parameter δ which interpolates between a solvable theory at δ = 0 and the desired theory at δ = 1. The interpolating actions are form δS+ (1−δ)S₀; and augmented by an optimization procedure which introduces nonperturbative features into our results. This procedure relies on the freedom in choosing the best S₀ without affecting the convergent results at δ=1. Our linear interpolation is similar in spirit but differs in detail from the novel δ expansion that was recently formulated for scalar theories where the parameter 2(1+δ) was the power of the field in the interaction lagrangian. Here we use interpolating actions for the first time in fermionic and gauge theories.     

Phase-function method for elasticα-α scattering

The phase-function method is adapted to deal with scattering on Coulomb plus nonlocal separable potentials. The Coulomb effect is included rigorously and a closed form expression is obtained for the scattering phase shiftδ _l (k). Based on the expression forδ _l (k) we present results forα-α elastic scattering phases which compare very well with more detailed calculations.     

Bound-to-extended state absorption in quantum wells confined byδ-like barriers

Within the framework of a simple envelope function and effective mass approximation, by including the spatial variation of effective mass and nonparabolicity effects, we have investigated the energy spectrum and intersubband optical absorption in a quantum well with additional thin and higher (δ -like) cladding barriers on either side of the well. The dependence of the absorption coefficient on the structure parameters, doping level, photon energy and temperature has been investigated. The absorption coefficient and spectrum strongly depend on the cladding barrier tunnel transparency. The peak absorption wavelength is shifted towards the high energies as the barrier transparency decreases. The temperature shift of the absorption peak is very small. The results are compared with experiments of Schneider et al. taking into account the broadening induced by well width fluctuations.     

Piston error characteristics of the self-phase-controlled stimulated Brillouin scattering phase conjugate mirrors

It was reported by Kong et al. that the self-phase-controlled stimulated Brillouin scattering phase conjugate mirror (SBS-PCM) is useful for the beam combination laser amplifier system, which generally introduces a phase delay (piston error) between combined beams along separate optical paths. In this work, we have investigated the piston error characteristics of two schemes for self-phase-controlled SBS-PCMs, a collinear scheme and a triangular scheme. Experimental results show that the piston errors of the reflected beams are 2kδ and kδ for the former and the latter scheme, respectively, where k is the wave number and δ is the optical path difference (OPD) variation introduced into the pass.     

Control of carrier concentration in Bi-2212

We realized highly underdoped Bi-2212 crystals by using a Bi substitution into a Sr site (x), together with the excess oxygen (δ) control. The samples with x = 0–0.3 were grown by a traveling solvent floating zone method. For each x, δ was varied using the precision annealing method [1]. Under several assumptions, the effective Bi valence was evaluated to be +2.2, which is much lower than the formal valence +3. This indicates that the Bi substitution accompanies additional uptake in excess oxygen. The x dependent δ-doping level p relation and the observed shrink in the c-axis length are consistent with this assumption. Based on the decomposition phase diagram obtained in this study, T_c of 22 K was realized by careful annealing treatments. The samples showed a superconducting volume fraction of over 50%.     

Transformation from δ-FeOOH to hematite in the presence of trace Fe(II)

δ-FeOOH, a poorly crystalline iron oxyhydroxide, can transform to the more stable iron oxyhydroxide or oxide. In the present work, the transformation from δ-FeOOH to goethite and hematite in the presence of trace Fe(II) has been investigated. The results show that Fe(II) can catalyze this transformation of δ-FeOOH. Based on experimental results and literature data, it is confirmed that two transformation mechanisms exist in the current system. One is the catalytic dissolution of δ-FeOOH, which leads to the formation of both hematite and goethite. The other is the catalytic solid-state transformation from δ-FeOOH to hematite. Which mechanism predominates depends on the temperature, pH, heating rate, etc. The results reveal that high temperatures (in the range from room temperature (RT) to 100 °C) favor the solid-state transformation as well as the formation of hematite. Given the structural relationships observed between δ-FeOOH and hematite, it is possible that the solid-state transformation from δ-FeOOH to hematite can exist.     

Extraordinary fast oxide ion conductivity in La1.61GeO5−δ thin film consisting of nano-size grain

Thin films of La_1.61GeO_5−δ, a new oxide ionic conductor, were fabricated on dense polycrystalline Al₂O₃ substrates by a pulsed laser deposition (PLD) method and the effect of the film thickness on the oxide ionic conductivity was investigated on the nanoscale. The deposition parameters were optimized to obtain La_1.61GeO_5−δ thin films with stoichiometric composition. Annealing was found necessary to get crystalline La_1.61GeO_5−δ thin films. It was also found that the annealed La_1.61GeO_5−δ film exhibited extraordinarily high oxide ionic conductivity. Due to the nano-size effects, the oxide ion conductivity of La_1.61GeO_5−δ thin films increased with the decreasing thickness as compared to that in bulk La_1.61GeO_5−δ. In particular, the improvement in conductivity of the film at low temperature was significant .The electrical conductivity of the La_1.61GeO_5−δ film with a thickness of 373 nm is as high as 0.05 S cm^− 1 (log(σ/S cm^− 1) = − 1.3) at 573 K.     

Density of states and Friedel sum rule in one- and quasi-one-dimensional wires

We analyze the relation between the density of states obtained from the energy derivative of the Friedel phase and that obtained from the Green's function of one- and quasi-one-dimensional wires with a double δ-potential. In the case of repulsive δ-potentials (in both one- and quasi-one-dimension), we show that the local Friedel sum rule is valid when a correction term is included. Various properties of the one-dimensional local density of states are also discussed. In the case of attractive δ-potentials in a quasi-one-dimensional wire, it is well known that the transmission probability may exhibit a Fano resonance (due to a zero-pole pair). In this case, we show that the local Friedel sum rule is valid provided that the tail of the quasibound state is taken into account by the integrated local density of states. In addition, we show that the density of states in a Fano resonance always has a Lorentz shape with peak position at the resonance energy regardless of the (Fano) asymmetry parameter.     

Semi-empirical analysis of magnetic moments of single-particle (or-hole) states around the 208Pb core

Magnetic moments of single-particle (or-hole) states around the ²⁰⁸Pb core were fitted by three state-independent parameters δg_l^(p), δg_l⁽ⁿ⁾ and α, where α is a parameter that is connected closely with δg_S and g_p. The analysis was performed under the assumptions that (i) the state-dependence of radial integral I((nl)²(n'l')²) involved in δg_s and g_p is calculated by the harmonic-oscillator potential, and (ii) δg_s is taken as being equal to ?4g_p, as expected from the δ-force type core polarization. An excellent fit was obtained when δg_l^(p) = 0.10(2), δg_l⁽ⁿ⁾ = ?0.05(1) and α = 1.2(1), which show that the main contributor to δg_s is the M1 core polarization and δg_l⁽ⁿ⁾ is certainly smaller than 0. B(M1)'s for allowed transitions calculated with the use of the same parameters as above are smaller than the observed B(M1)'s whereas α = 1.01(1) explains well the observed B(M1)'s.     

Role of growth temperature and oxygen partial pressure on the structural and electrical properties of pulsed laser deposited La1−xSrxnO3−δ thin films

The paper presents the fabrication and characterization of La_0.65Sr_0.35MnO_3−δ (LSMO) polycrystalline thin films deposited directly on Si (1 0 0) substrates using pulsed laser deposition technique. Various deposition parameters like substrate temperature and oxygen partial pressure have been varied systematically to obtain stoichiometric, crack-free films with smooth surface morphology having nearly monodisperse grain size distribution. The substrate temperature variation from 600 to 800 °C had profound effects on the microstructure and topography of the deposited film, with optimum result being obtained at 700 °C. The variation of partial pressure of oxygen controls the deposition kinetics as well as the stoichiometry of the film in terms of oxygen vacancy, which influences the magnetic and electrical transport properties of the manganate films. The microstructure and crystallinity of the deposited films have been studied using X-ray diffraction, scanning electron microscopy and atomic force microscopy. A correlation between the oxygen stoichiometry and micro-structural and transport properties of the deposited films has been obtained.     

Geometric conservation law and applications to high-order finite difference schemes with stationary grids

The geometric conservation law (GCL) includes the volume conservation law (VCL) and the surface conservation law (SCL). Though the VCL is widely discussed for time-depending grids, in the cases of stationary grids the SCL also works as a very important role for high-order accurate numerical simulations. The SCL is usually not satisfied on discretized grid meshes because of discretization errors, and the violation of the SCL can lead to numerical instabilities especially when high-order schemes are applied. In order to fulfill the SCL in high-order finite difference schemes, a conservative metric method (CMM) is presented. This method is achieved by computing grid metric derivatives through a conservative form with the same scheme applied for fluxes. The CMM is proven to be a sufficient condition for the SCL, and can ensure the SCL for interior schemes as well as boundary and near boundary schemes. Though the first-level difference operators δ₃ have no effects on the SCL, no extra errors can be introduced as δ₃ = δ₂. The generally used high-order finite difference schemes are categorized as central schemes (CS) and upwind schemes (UPW) based on the difference operator δ₁ which are used to solve the governing equations. The CMM can be applied to CS and is difficult to be satisfied by UPW. Thus, it is critical to select the difference operator δ₁ to reduce the SCL-related errors. Numerical tests based on WCNS-E-5 show that the SCL plays a very important role in ensuring free-stream conservation, suppressing numerical oscillations, and enhancing the robustness of the high-order scheme in complex grids.     

The phase shifts leading to the broadening and shift of spectral lines

The classical theory of collisional broadening and shift parameters (β, δ) of an isolated spectral line was used to obtain simple analytical formulas for calculating both β and δ. These formulas were obtained on the assumption that the short range interaction is effective only in the broadening while the long range is effective in the shift of the spectral line. These parameters β and δ depend on the limiting phase shifts responsible for broadening η_b and shift ηδ. It was found that the values of η_b and η_δ are not equal to each other as was proposed by Weisskopf η_b=η_δ=1. The maximum and average values of η_b (η_b max, η_b av) and η_δ (η_δ max, η_δ av) were obtained by numerical evaluation, using different inverse power potentials. By introducing these parameters into the approximated formulas for β and δ using Van der Waals and Lennard-Jones potential, it was found that the results of calculations for (β and δ) with different atomic transitions perturbed by different inert gases are in close agreement with earlier results. Those results, obtained earlier, were based on the Lindholm-Foley theory especially with the average values of η_b [η_b av=0.6057] and the maximum values of η_δ [η_δ max=1.57625]. The impact parameters ρ_b and ρ_δ leading to the broadening and shift of the spectral line were also obtained for different interactions. It was found that the end parameter for the broadening ρ_b is not equal to the starting parameter for the shift ρ_δ.     

Current transport property in GdBCO coated conductor with artificial pinning centers in a wide range of temperature,magnetic field up to 27 T,and field angle

We have investigated current transport property in Gd₁Ba₂Cu₃O_7−δ coated conductor with artificial pinning centers in a wide range of temperature, magnetic field, B up to 27 T, and field angle. Due to the additional c-axis correlated pins, critical current density, J_c in B//c was enhanced and the improvement was observed in wide range of B. On the other hand, around B⊥c below 65 K, the angular dependence of n-value showed a valley-like behavior, although the J_c was increasing. In addition, the temperature dependence of the pinning force density defined as J_c × B was not scaled on an expected master curve. These results indicate the pinning in B⊥c is governed by different mechanism below 65 K and high magnetic field.     

Memory-function conductivity formula and transport coefficients in underdoped cuprates

The two-band memory-function conductivity formula is derived from the quantum kinetic equation in the pseudogap state of underdoped cuprates. The conduction electrons are described by using the adiabatic version of the nested Fermi liquid model, and the effects of Mott correlations are taken into account phenomenologically. The linear dependence of the low-temperature effective number of conduction electrons on the doping level δ (for not too large δ) is found to be in agreement with experimental observation. The momentum distribution function turns out to play an important role in describing temperature effects. The closing of the antiferromagnetic pseudogap at temperatures of the order of room temperature is shown to be a direct consequence of a relatively large width of the quasiparticle peak in this distribution function. The coupling of conduction electrons to external magnetic fields is included in the two-band transport equations in the usual semiclassical way. It is shown that the low-temperature Hall number is proportional to δ as well (again for not too large δ) and that it exhibits singular behaviour when the Fermi surface changes from the hole-like shape into the electron-like shape.     

Study of In0.49Ga0.51P/GaAs/In0.49Ga0.51P doubleδ-doped heterojunction bipolar transistor

A lattice-matched In_0.49Ga_0.51P/GaAs/In_0.49Ga_0.51P doubleδ -doped heterojunction bipolar transistor, prepared by low-pressure metal organic chemical vapor deposition (LP-MOCVD), is fabricated successfully and reported. Due to the insertion of δ -doped sheets and setback layers both at base–emitter (B–E) and base–collector (B–C) heterojunctions, the potential spikes are suppressed significantly. In addition, the electron blocking effect is removed and a dramatic improvement of current gain is obtained. A modified Ebers–Moll model is employed to study and analyse the device performances. The experimental results show that the common-emitter current gain over 210 at the collector current of 35 mA and an offset voltageΔV_CE smaller than 50 mV are obtained. Also, a lower knee-shaped voltage of 1.4 V at the collector current of 40 mA is observed. These results indicate that the device studied is a good candidate for high-speed and high-power circuit applications.     

LiAlδNi1−δO2 solid solutions as cathodic materials for rechargeable lithium batteries

Layered LiAl_δNi_1−δO₂ solid solutions were synthesised via a solid-state reaction at 750°C under an oxygen stream. Single phase LiAl_δNi_1−δO₂ compounds were obtained. The structural integrity of the electrode could be preserved via an inert Al³⁺ dopant effect to prevent the overcharge of the electrode, which is beneficial for long cycle life of the cell. The LiAl_δNi_1−δO₂ electrode delivered approximately 150–160 mAh/g discharge capacity between 3 and 4.3 V, which is similar to LiNiO₂. A.c. impedance spectroscopy was employed to characterise the kinetic parameters of LiAl_δNi_1−δO₂ electrodes in lithium cells combined with the galvanostatic intermittent titration technique (GITT). It was found that the Al³⁺ dopant effect could decrease the charge-transfer resistance (R_CT) and increase the Li-ion diffusion coefficient.     

A two-particle exchange interaction model

The magnetisation reversal of two interacting particles was investigated within a simple model describing exchange coupling of magnetically uniaxial single-domain particles. Depending on the interaction strength W, the reversal may be cooperative or non-cooperative. A non-collinear reversal mode is obtained even for two particles with parallel easy axes. The model yields different phenomena as observed in spring magnets such as recoil hysteresis in the second quadrant of the field-magnetisation-plane, caused by exchange bias, as well as the mentioned reversal-rotation mode. The Wohlfarth’s remanence analysis performed on aggregations of such pairs of interacting particles shows that the deviation δM(H_m) usually being considered as a hallmark of magnetic interaction vanishes for all maximum applied fields H_m not only at W=0, but also for sufficiently large values of W. Furthermore, this so-called δM-plot depends on whether the sample is ac-field or thermally demagnetised.     

The Infinite Square Well with a Point Interaction: A Discussion on the Different Parameterizations

The construction of Dirac delta type potentials has been achieved with the use of the theory of self adjoint extensions of non-self adjoint formally Hermitian (symmetric) operators. The application of this formalism to investigate the possible self adjoint extensions of the one dimensional kinematic operator $K=-\frac{d^{2}}{dx^{2}}$ on the infinite square well potential is quite illustrative and has been given elsewhere. This requires the definition and use of four independent real parameters, which relate the boundary values of the wave functions at the walls. By means of a different approach, that fixes matching conditions at the origin for the wave functions, it is possible to define a perturbation of the type aδ(x)+bδ′(x), thus depending on two parameters, on the infinite square well. The objective of this paper is to investigate whether these two approaches are compatible in the sense that perturbations like aδ(x)+bδ′(x) can be fixed and determined using the first approach.