Influence of quantum resonance-percolation trajectories in a disordered <Emphasis Type="Italic">I</Emphasis> layer on the critical current of a Josephson <Emphasis Type="Italic">S-I-S</Emphasis> junction

Formulae for critical current and the magnitude of its mesoscopic structural fluctuations in the tunneling resonance energy range of an S-I-S (S = superconductor, I = insulator) tunnel junction with a weak structural disorder (low impurity density) in an I layer at temperature T = 0 are obtained in the form of sums describing the quantum resonant-percolation trajectories [1] that randomly form in a disordered I layer and the connecting opposite S banks of a contact.     

压力下应变异质结中施主杂质态的Stark效应

对应变GaN/Al_xGa_1-xN异质结系统,考虑理想界面突变势垒,引入简化相干势近似,采用变分法讨论了流体静压力下外界电场对束缚于界面附近的浅杂质态结合能的影响.对GaN为衬底的闪锌矿应变异质结,分别计算了(001)和(111)取向时杂质态的结合能随压力、杂质位置、电场强度以及组分的变化关系.结果表明,杂质态结合能随流体静压力呈近线性变化.电场对杂质态的Stark效应则随杂质位置不同而呈现谱线蓝、红移动.此外,还讨论了在不同压力情况下,Al组分对杂质结合能的影响.当杂质处于GaN材料中且距界面较远时,Al组分的增加使电子的二维特性增强,从而使结合能增大,且压力加剧增幅的增加;当杂质处于Al_xGa_1-xN材料中,Al组分的增加削弱了杂质与电子间的库仑相互作用,故而结合能降低. 关键词： xGa_1-xN异质结')" href="#">GaN/Al_xGa_1-xN异质结 杂质态 压力 Stark效应     

Friedel oscillation about a Friedel-Anderson impurity

The Friedel oscillations in the vicinity of a Friedel-Anderson (FA) impurity are investigated numerically. For an FA impurity in the local moment limit the normalized amplitude A(ξ) is S-shaped, approximately zero at short distances, approaching two at large distances and crossing the value one at the characteristic length ξ _1/2. Surprisingly, the Friedel oscillations of a simple non-interacting Friedel impurity with a narrow resonance at the Fermi level show a very similar behavior of their amplitude A(ξ). A comparison correlates the resonance width and the Kondo energy of the FA impurity with the characteristic length ξ _1/2 of the Friedel oscillations.     

Influence of quantum resonance-percolation trajectories in a disordered I layer on the critical current of a Josephson S-I-S junction

Formulas for a critical current and the magnitude of its mesoscopic structural fluctuations are obtained in the form of sums over quantum resonance-percolation trajectories [1] randomly formed in a disordered I layer and connecting opposite S banks of the junction at T = 0 in the energy region of tunnel S-I-S resonances (S denotes the superconductor, I, insulator) for a tunnel junction with weak structural disorders (low impurity concentrations) in the I layer.     

The volt-ampere characteristics of thin-film metal-InSb-metal structures

The temperature dependences of VA-characteristics for thin-film metal-InSb-metal (Al-InSb-Al, Au-InSb-Au) structures are analyzed within the framework of the theory of the barrier mechanism of electrical conductivity. Qualitative agreement is achieved between the experimental curves and theoretical relations. It is shown that the VA-characteristics may be described by the lawI=I _sexp (U). As follows from the measurements, the temperature dependenceI _s=f(1/T) is mainly given by the temperature dependence of the carrier concentration. It is possible to determine the height of the energy barrier in the region of impurity conduction from the dependence logI _s=f(1/T).     

Surface segregation of the metal impurity to the (100) surface of fcc metals

The surface segregation energies for a single metal impurity to the (100) surface of nine fcc metals (Cu, Ag, Au, Ni, Pd, Pt, Rh, Al and Ir) have been calculated using the MAEAM and molecular dynamics (MD) simulation. The results show that the effect of the surface is down to the fourth-layer and an oscillatory or monotonic damping (|E ₁| > |E ₂| > |E ₃| > |E ₄|) phenomenon in segregation energy has been obtained. The absolute value of the segregation energy E ₁ for a single impurity in the first atomic layer is much higher than that in the nether layers. Thus, whether the surface segregation will work or not is mainly determined by E ₁ which is in good relation to the differences in surface energy between the impurity and host crystals ΔQ = Q _imp − Q _hos. So we conclude that an impurity with lower surface energy will segregate to the surface of the host with higher surface energy.      

The effect of UV irradiation on temperature dependence of photoluminescence and photoacoustic response in porous silicon

The influence of UV irradiation on temperature dependence of the integral intensity of luminescence I _lum(T) in porous silicon was studied. It was found that, if luminescence decays with temperature nonmonotonically, the peak of I _lum(T) shifts toward higher temperatures as the exciting radiation density increases. Under UV irradiation, the function I _lum(T) becomes monotonically decreasing. If the function I _lum(T) is initially monotonically decreasing, UV preirradiation changes the emission spectrum and accelerates temperature quenching of the red-orange emission band. The variation of the amplitude of the pulsed photoacoustic response with UV irradiation dose was studied. The dependence found is explained by the removal of foreign inclusions from the developed surface of porous silicon. An energy level diagram that makes it possible to explain the behavior of I _lum(T) is suggested. It is noted that the shape of the function I _lum(T) can be used as a test whereby the contributions from dissimilar oscillators to the red-orange emission band are estimated.     

Impurity-induced coherent current oscillations in one-dimensional conductors

The electronic transport through a single impurity in a repulsive Luttinger liquid (LL) has been theoretically studied. It has been found that the direct current Ī above the threshold voltage related to the strength of the impurity potential is accompanied by coherent oscillations with frequency f= Ī/e. There is an analogy with the Josephson junctions: the well-known regime of the power-law I−V curves in the LL corresponds to the damping of the Josephson current below the critical one, while the oscillatory regime in the LL can be compared with the Josephson oscillations above the critical current. The text was submitted by the authors in English.     

CdTe:Li晶体中ALi°受主与电子及激子的相互作用

用两种不同方法实现了在CdTe晶体中Li杂质的低温扩散。其中后一种无Te预扩散的方法是首次报导的。为研究在CdTe:Li中ALi°受主与电子和激子的相互作用,分析了各种发光机制中的动力学过程,观测了(e,ALi°)及(ALi°,X)发光的激发光强度效应。(ALi°、X)束缚激子可以热离解成一个自由电子和一个自由空穴而留下一个中性受主ALi°,其束缚能为Eb=4meV。在不同强度的光激发下,研究了(e,ALi°)辐射复合中电子—声子耦合。当杂质中心不变时,平均声子数N～0.1基本上保持不变,它是杂质中心波函数半径的一个量度。若假定杂质中心上电荷分布为Gaussian形,计算出表征电荷在杂质中心局域程度的常数α～160Å。     

Stochastic fluctuations of persistent currents in mesoscopic rings

We investigate systematically the effect of the electronic Coulomb interaction on the stochastic fluctuations (from sample to sample) in the persistent currents I(Φ) of mesoscopic rings threaded by a magnetic flux Φ. In contrast to our recent publication [Europhys. Lett. 18 (1992) 457], we avoid separation of impurity and interaction scattering events and we present here a comprehensive view. By this alternative procedure, we are able to confirm our earlier result, namely that 〈I²〉^1/2 ～ ev_F/L, which is a much larger quantity than the average current 〈I〉 ～ ev_Fl/L².     

Noise in the Helical Edge Channel Anisotropically Coupled to a Local Spin

We calculate the frequency-dependent shot noise in the edge states of a two-dimensional topological insulator coupled to a magnetic impurity with spin S = 1/2 of arbitrary anisotropy. If the anisotropy is absent, the noise is purely thermal at low frequencies, but tends to the Poisson noise of the full current I at high frequencies. If the interaction only flips the impurity spin but conserves those of electrons, the noise at high voltages eV ≫ T is frequency-independent. Both the noise and the backscattering current I_bs saturate at voltageindependent values. Finally, if the Hamiltonian contains all types of non-spin-conserving scattering, the noise at high voltages becomes frequency-dependent again. At low frequencies, its ratio to 2eI_bs is larger than 1 and may reach 2 in the limit I_bs→0. At high frequencies, it tends to 1.

     

Pumping current of a Luttinger liquid with finite length

We study transport properties in a Tomonaga-Luttinger liquid in the presence of two time-dependent point like weak impurities, taking into account finite-length effects. By employing analytical methods and performing a perturbation theory, we compute the backscattering pumping current (I _bs ) in different regimes which can be established in relation to the oscillatory frequency of the impurities and to the frequency related to the length and the renormalized velocity (by the electron-electron interactions) of the charge density modes. We investigate the role played by the spatial position of the impurity potentials. We also show how the previous infinite length results for I _bs are modified by the finite size of the system.     

Intense laser effects on donor impurity in a cylindrical single and vertically coupled quantum dots under combined effects of hydrostatic pressure and applied electric field

Using the effective mass and parabolic band approximations and a variational procedure we have calculated the combined effects of intense laser radiation, hydrostatic pressure, and applied electric field on shallow-donor impurity confined in cylindrical-shaped single and double GaAs-Ga_1−xAl_xAs QD. Several impurity positions and inputs of the heterostructure dimensions, hydrostatic pressure, and applied electric field have been considered. The laser effects have been introduced by a perturbative scheme in which the Coulomb and the barrier potentials are modified to obtain dressed potentials. Our findings suggest that (1) for on-center impurities in single QD the binding energy is a decreasing function of the dressing parameter and for small dot dimensions of the structures (lengths and radius) the binding energy is more sensitive to the dressing parameter, (2) the binding energy is an increasing/decreasing function of the hydrostatic pressure/applied electric field, (3) the effects of the intense laser field and applied electric field on the binding energy are dominant over the hydrostatic pressure effects, (4) in vertically coupled QD the binding energy for donor impurity located in the barrier region is smaller than for impurities in the well regions and can be strongly modified by the laser radiation, and finally (5) in asymmetrical double QD heterostructures the binding energy as a function of the impurity positions follows a similar behavior to the observed for the amplitude of probability of the noncorrelated electron wave function.     

Pilot Study: Understanding the Effects of Voicing Intervention on HFCC Therapy in People with Cystic Fibrosis

The efficacy of High-Frequency Chest Compression (HFCC) airway clearance therapy is linked to the induced-peak expiratory airflow pulse (IPEF) at the patient's mouth. The authors' goal was to determine the conditions that yield the highest IPEF using HFCC running at 6 Hz in conjunction with voicing intervention. A pilot experimental study was conducted in a laboratory setting. Six adults with moderate to mild cystic fibrosis (CF) and 10 healthy adults participated. When the component characteristics of voicing were disregarded in data analysis of four conditions, voicing only intervention (V₁I₀), HFCC only intervention (V₀I₁), voicing intervention and HFCC intervention combinations (V₁I₁) and nonintervention (V₀I₀), V₀I₁ had significantly higher (P < 0.0001) IPEF. Data analyses of 64 separate voicing component characteristics, frequency (×4), amplitude (×4), and rhythm (×2) of voicing intervention, in addition to absence and presence of HFCC intervention (V₁I₀ and V₁I₁), were examined. One condition in V₁I₀ had significantly higher (P < 0.000001) IPEF than other conditions in V₁I₀ and V₁I₁ in both experimental and control groups. Based on these findings, V₁I₁ may yield higher IPEF than V₀I₁. One condition of amplitude component of voicing and one condition of rhythm component of voicing had significantly higher (P < 0.0001) IPEF than other conditions of amplitude and rhythm components in both CF and control subjects. Analysis of this combined condition of V₁I₁ showed that this specific condition of V₁I₁ had significantly higher (P < 0.000001) IPEF than any other conditions in V₁I₁ and V₀I₁.     

Einfluß der Kristallorientierung auf die Plasmonenintensität in Ag und Si

In energy loss measurements of electrons on monocrystals of silver and silicon in transmission the intensity of the volume plasma loss (Ag 3.8 eV, Si 16.6 eV) is found to be dependent on the crystal orientation. By tilting the crystal relatively to the incident beam (in general some degrees) the intensities of the elastic peak (I _el), the volume plasma peak (I _v) and in Ag additionally that of the surface loss (I _s) at 3.1 eV (damped by a carboncoating) show different variations.I _s in Ag andI _v in thick (>150 nm) Si is as expected proportional toI _el, whereasI _v in Ag and thin Si varies much stronger thanI _el, so that the quotientI _v/I_el is not a constant. Presumably an explanation of this phenomenon lies in the dynamic theory of electron diffraction including inelastic processes.     

Genetic Algorithms for Design of Discrete Phase-only Reconfigurable Array Antennas with Fixed Dynamic Range Ratio

In this paper, we present a new method based on real-coded Genetic Algorithm (GA) with elitist model for optimal design of a reconfigurable symmetrical dual-beam uniformly spaced linear isotropic antenna array with phase-only control of quantized phase shifters. The problem is to find a common amplitude distribution that will generate a pencil beam with zero phases and a flat-top beam with discrete phases of a six-bit discrete phase shifter, without or with pre-fixing the value of dynamic range ratio (|I _max/I _min|) of excitation current amplitude distribution equal to or less than five.     

How does contrasting dependence of impurity-atom diffusivity on the density of host disordered medium arise?

We report results of molecular dynamics investigations into neutral impurity diffusing within an amorphous solid as a function of the size of the diffusant and density of the host amorphous matrix. We find that self diffusivity exhibits an anomalous maximum as a function of the size of the impurity species. An analysis of properties of the impurity atom with maximum diffusivity shows that it is associated with lower mean square force, reduced backscattering of velocity autocorrelation function, near-exponential decay of the intermediate scattering function (as compared to stretched-exponential decay for other sizes of the impurity species) and lower activation energy. These results demonstrate the existence of size-dependent diffusivity maximum in disordered solids. Further, we show that the diffusivity maximum is observed at lower impurity diameters with increase in density. This is explained in terms of the Levitation parameter and the void structure of the amorphous solid. We demonstrate that these results imply contrasting dependence of self diffusivity (D) on the density of the amorphous matrix, ρ. D increases with ρ for small sizes of the impurity but shows an increase followed by a decrease for intermediate sizes of the impurity atom. For large sizes of the impurity atom, D decreases with increase in ρ. These contrasting dependence arises naturally from the existence of Levitation Effect.      

Shift of the Fermi level during cointercalation of copper and iron into TiSe<Subscript>2</Subscript>

The influence of copper intercalation into the TiSe₂ compound on the Fermi energy has been investigated using two independent methods. The first method is based on the analysis of the amplitude of the deformation (induced by impurity intercalation and providing the formation of polarons) as a function of the binding energy of the band of polaron states. For this purpose, the Fe _x Cu _y TiSe₂ system has been synthesized for the first time by cointercalation of copper and iron into the TiSe₂ lattice and studied. The second method consists in measuring the electromotive force of an open-circuit electrochemical cell with respect to a metal reference electrode. Both methods lead to consistent results in the range of moderate copper contents. However, considerable discrepancies have been observed in the copper concentration range that corresponds to the beginning of filling of Cu/Ti hybrid states. These discrepancies are explained by the softening of the lattice due to an increase in the density of states at the Fermi level.     

Line vortices in a three-dimensional ordered Josephson medium

Two equilibrium configurations of a line vortex in a three-dimensional ordered Josephson medium are considered: (i) the vortex core is at the center of a cell and (ii) the vortex core is on a contact. Infinite systems of equations describing these configurations are derived. In going to a finite system, the currents far away from the center are neglected. A new technique for solving the finite system of equations is suggested. It does not require smallness of phase discontinuities at all vortex cells and, therefore, can be applied for any values of pinning parameter I down to zero. The structures and energies of both equilibrium states for isolated line vortices are calculated for any I from the range considered. For I >0.3, a vortex can be thought of as fitting a square of 5×5 cells. For lower I, the vortex energy can be expressed as a sum of the energies of the small discrete core and the quasi-continuous outside. The core energy is comparable to the energy of the outside and is a major contributor to the vortex energy when I is not too small. For any I, the energy of the vortex centered on the contact is higher than the energy of the configuration centered at the center of the cell.     

Visible photoluminescence in porous GaAs capped by GaAs

A typical porous structure with pores diameters ranging from 10 to 50 nm has been obtained by electrochemical etching of (1 0 0) heavily doped p-type GaAs substrate in HF solution. Room temperature photoluminescence (PL) investigations of the porous GaAs (π-GaAs) reveal the presence of two PL bands, I₁ and I₂, located at 1.403 and 1.877 eV, respectively. After GaAs capping, the I₁ and I₂ PL bands exhibit opposite shift trends. However, the emission efficiency of these two bands is not strongly modified. Low temperature PL of capped porous GaAs versus injection levels shows that the I₁ PL band exhibits a red shift while the I₂ PL band exhibits a blue shift with increasing injection levels. The I₂ PL band intensity temperature dependence shows an anomalous behaviour and its energy location shows a blue shift as temperature increases. The observed PL bands act independently and are attributed to electron – hole recombination in porous GaAs and to the well-known quantum confinement effects in GaAs nanocrystallites. The I₂ PL band excitation power and temperature dependencies were explained by the filling effect of GaAs nanocrystallites energy states.