The emission spectrum of <Emphasis Type="Italic">p</Emphasis>-AlGaAs/GaAsP/<Emphasis Type="Italic">n</Emphasis>-AlGaAs diodes under uniaxial compression

New experimental data are presented on the effects of uniaxial compression of up to 4 kbar along the [110] and [1$ \bar 1 $ \bar 1 0] crystallographic directions on the spectra of electroluminescence and the current-voltage characteristics of diodes based on n-Al _x Ga_{1 − x} As/GaAs _y P_{1 − y} /p-Al _x Ga_{1 − x} As (y = 0.84) heterostructures that were designed for injection lasers. With increasing pressure, the spectra show a shift to shorter wavelengths, reaching 25 meV at 3 kbar; the intensity increases 2–3 times as well. Numerical calculations were carried out on the band structure of the investigated heterostructures under compression along the [110] axis, which indicate the increase in the effective band gap in the quantum well (QW) GaAs _y P_{1 − y} , with a pressure coefficient of about 8.5 meV/kbar and reduction of the barrier height at the boundaries of the QW. The calculations predict the possibility that light and heavy holes crossover at pressures above 4.5–5 kbar. The increase in the effective band gap completely describes the experimental data on the shift of the electroluminescence spectra. The mixing of light and heavy holes when approaching the band crosspoint is the probable cause of an increase in the intensity of radiation under uniaxial compression.     

Spin-dependent electron dynamics and recombination in GaAs<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>N<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> alloys at room temperature

Conduction-electron spin polarization dynamics achieved by pulsed optical pumping at room temperature in GaAs_1−x N _x alloys with a small nitrogen content (x = 2.1, 2.7, and 3.4%) is studied both experimentally and theoretically. It is found that the photoluminescence circular polarization determined by the mean spin of free electrons reaches 40–45% and this giant value persists within 2 ns. Simultaneously, the total free-electron spin decays rapidly with the characteristic time ≈ 150 ps. The results are explained by spin-dependent capture of free conduction electrons on deep paramagnetic centers resulting in the dynamical polarization of bound electrons. A nonlinear theory of spin dynamics in the coupled system of spin-polarized free and localized carriers has been developed which describes the experimental dependencies, in particular, the electron spin quantum beats observed in a transverse magnetic field. The text was submitted by the authors in English.     

Temperature and electric field dependences of the mobility of electrons in vertical transport in GaAs/Ga<Subscript>1−<Emphasis Type="Italic">y</Emphasis></Subscript>Al<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>As barrier structures containing quantum wells

The mobility of electrons in vertical transport in GaAs/Ga_1−y Al _y As barrier structures was investigated using geometric magnetoresistance measurements in the dark. The samples studied had Ga_1−y Al _y As (0 ≤ y ≤ 0:26) linearly graded barriers between the n⁺-GaAs contacts and the Ga_0:74Al_0:26As central barrier, which contain N _w (=0, 2, 4, 7 and 10) n-doped GaAs quantum wells. The mobility was determined as functions of (i) temperature (80–290 K) at low applied voltage (0.01–0.1 V) and (ii) applied voltage (0.005–1.6 V) at selected temperatures in the range 3.5–290 K. The experimental results for the temperature dependence of low-field mobility suggest that space-charge scattering is dominant in the samples with N _w =0 and 2, whereas ionized impurity scattering is dominant in the samples with N _w =4, 7 and 10. The effect of polar optical phonon scattering on the mobility becomes significant in all barrier structures at temperatures above about 200 K. The difference between the measured mobility and the calculated total mobility in the samples with N _w =4, 7 and 10, observed above 200 K, is attributed to the reflection of electrons from well-barrier interfaces in the quantum wells and interface roughness scattering. The rapid decrease of mobility with applied voltage at high voltages is explained by intervalley scattering of hot electrons.      

Band structure and dominating electron-scattering mechanisms in Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis>−<Emphasis Type="Italic">y</Emphasis></Subscript>Mn<Subscript>x</Subscript>Fe<Subscript>y</Subscript>Se

Magnetic and kinetic properties as well as transmission and absorption spectra of Hg _1−x−y Mn _x Fe _ySe (0.09 ≤ x ≤ 0.099 and 0.001 ≤ y ≤ 0.01) crystals are investigated at H ≈ 0.5–6 kOe in the temperature range T = 77–300 K. The band parameters are determined on the basis of experimental data. It is found that in the crystals under study at T ≈ 300 K, electron scattering by polar optical phonons dominates, direct optical band-to-band transitions occur, and replacement of a part of Mn atoms by Fe for x + y = 0.1 results in an increase in E_g ^op with Fe content. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 35–39, March, 2007.     

Optical properties of the ternary compound Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Se:Co

We present the results of experimental studies of the optical properties of cobalt-doped Cd _x H_1−x Se (x = 0.18) single crystals with cobalt ion concentrations of NCo = 5·10¹⁸, 5·10¹⁹, and 1·10²⁰ cm⁻³ at T = 90 K and 300 K. The composition (x = 0.18) of the Cd _x Hg_1−x Se solid solution was selected so that the hypothetical resonance level is found on the bottom of the conduction band. We show that the cobalt ions in the mercury selenide can form a resonance donor level only for cobalt concentrations N_Co < 5·10¹⁸ cm⁻³. For N_Co ∼ 5·10¹⁸ cm⁻³, the cobalt ions substitute for mercury atoms, forming a solid solution and leading to an increase in the bandgap width and a change in the physical properties. The solubility of cobalt in the HgSe lattice can be greater than 5%–10%. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 1, pp. 73–77, January–February, 2007.     

Universal behavior of CePd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Rh<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> ferromagnet at the quantum critical point

The heavy-fermion metal CePd_1−x Rh _x can be tuned from ferromagnetism at x = 0 to the nonmagnetic state at some critical concentration x _c . The non-Fermi liquid behavior (NFL) at x ≃ x _c is recognized by the power-law dependence of the specific heat C(T) given by the electronic contribution susceptibility X(T) and volume expansion coefficient α(T) at low temperatures: C/T ∝ X(T) ∝ α(T)/T∝ 1/ √T. We also demonstrate that the behavior of the normalized effective mass M _N ^* observed in CePd_1−x Rh _x at x ≃ 0.8 agrees with that of M _N ^* observed in paramagnetic CeRu₂Si₂ and conclude that these alloys exhibit the universal NFL thermodynamic behavior at their quantum critical points. We show that the NFL behavior of CePd_1−x Rh _x can be accounted for within the frameworks of the quasiparticle picture and fermion condensation quantum phase transition, while this alloy exhibits a universal thermodynamic NFL behavior that is independent of the characteristic features of the given alloy such as its lattice structure, magnetic ground state, dimension, etc. The text was submitted by the authors in English.     

GaSe<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript><Emphasis Type="Italic">S</Emphasis><Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> solid solutions

The most probable parameters of the pure, doped, and mixed GaSe_1−xS_x (x ≤ 0.4) crystals (solid solutions) are presented together with the results of investigations of their crystal structure and defects, optical and mechanical properties, and damage thresholds. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 35–40, June, 2007.     

Two-Photon Absorption in Nanoheterostructures with <Emphasis Type="Italic">D</Emphasis><Superscript>(−)</Superscript>-Centers

A two-photon impurity absorption coefficient of the “quantum dot — D⁽⁻⁾-center” complexes synthesized in a transparent dielectric matrix is calculated within the model of zero-radius potential. The evolution of spectral dependence of the absorption coefficient of the nanoheterostructure based on semiconductor CdS _x Se _1−x glasses is studied versus the average quantum-dot radius. It is shown that the contribution of the two-photon impurity absorption to the exited two-photon luminescence is fairly significant at a reasonable quantum-dot concentration. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 46–50, July, 2005.     

Temperature Dependence of the Transmission Spectra of Silicate Glasses Containing CdS<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Semiconducting Nanocrystals

The transmission spectra of silicate glasses containing CdS_{1 − x} Se _x semiconducting nanocrystals of various sizes are investigated in the temperature range 20–300°C. It is shown that for explaining optical properties of these materials, not only the nanocrystal sizes, but also the structural changes occurring in nanocrystals during their formation must be taken into account.     

Trap elimination and reduction of size dispersion due to aging in CdS<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> quantum dots

Quantum Dots of CdS _x Se_1−x embedded in borosilicate glass matrix have been grown using Double-Step annealing method. Optical characterization of the quantum dots has been done through the combinative analysis of optical absorption and photoluminescence spectroscopy at room temperature. Decreasing trend of photoluminescence intensity with aging has been observed and is attributed to trap elimination. The changes in particle size, size distribution, number of quantum dots, volume fraction, trap related phenomenon and Gibbs free energy of quantum dots, has been explained on the basis of the diffusion-controlled growth process, which continues with passage of time. For a typical case, it was found that after 24 months of aging, the average radii increased from 3.05 to 3.12 nm with the increase in number of quantum dots by 190% and the size-dispersion decreased from 10.8% to 9.9%. For this sample, the initial size range of the quantum dots was 2.85 to 3.18 nm. After that no significant change was found in these parameters for the next 12 months. This shows that the system attains almost a stable nature after 24 months of aging. It was also observed that the size-dispersion in quantum dots reduces with the increase in annealing duration, but at the cost of quantum confinement effect. Therefore, a trade off optimization has to be done between the size-dispersion and the quantum confinement.     

Superconductivity at 36 K in gadolinium-arsenide oxides GdO<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>F<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>FeAs

In this paper we report the fabrication and superconducting properties of GdO_1−x F _x FeAs. It was found that when x is equal to 0.17, GdO_0.83F_0.17FeAs is a superconductor with the onset transition temperature T _c^on ≈ 36.6 K. Resistivity anomaly near 130 K was observed for all samples up to x = 0.17, and such a phenomenon is similar to that of LaO_1−x F _x FeAs. Hall coefficient indicates that GdO_1−x F _x FeAs is conducted by electron-like charge carriers. Recommended by Prof. Nie Yuxin, Executive Editor of Science in China Series G-Physics, Mechanics & Astronomy Supported by the National Natural Science Foundation of China (Grant Nos. 10221002/A0402 and 10774170/A0402), the National Basic Research Program of China (973 Program) (Grant Nos. 2006CB601000, 2006CB921107 and 2006CB921802), and the Chinese Academy of Sciences (ITSNEM)     

Dual properties of orthogonal polynomials of discrete variables associated with the quantum algebra <Emphasis Type="Italic">U</Emphasis><Subscript><Emphasis Type="Italic">q</Emphasis></Subscript>(<Emphasis Type="Italic">su</Emphasis>(2))

We show that for every set of discrete polynomials y _n (x(s)) on the lattice x(s), defined on a finite interval (a, b), it is possible to construct two sets of dual polynomials z _k (ξ(t)) of degrees k = s-a and k = b-s-1. Here we do this for the classical and alternative Hahn and Racah polynomials as well as for their q-analogs. Also we establish the connection between classical and alternative families. This allows us to obtain new expressions for the Clerbsch-Gordan and Racah coefficients of the quantum algebra U _q (su(2)) in terms of various Hahn and Racah q-polynomials. Dedicated to the memory of our teacher and friend Arnold F. Nikiforov (18.11.1930–27.12.2005).     

Composition dependence of density of states in a-Se<Subscript>100−<Emphasis Type="Italic">x</Emphasis></Subscript>Sn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> thin films

The present paper reports the DC conductivity measurements at high electric fields in vacuum evaporated amorphous thin films of a-Se_100−x Sn _x (x = 0, 2, 4, 6) glassy alloys. Current-voltage (I–V) characteristics have been measured at various fixed temperatures. In these samples, at low electric fields, ohmic behaviour is observed. However, at high electric fields (E ∼ 10⁴ V/cm), non-ohmic behaviour is observed. An analysis of the experimental data confirms the presence of space charge limited conduction (SCLC) in 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. Increase in DOS with increase in Sn concentration has been found which could be correlated with the electronegativity difference between the two elements used here in making the glassy alloys. The peculiar role of the element Sn as an impurity in the pure Se glassy alloy is also discussed.      

Re-evaluation of the LHC potential for the measurement of <Emphasis Type="Italic">m</Emphasis><Subscript><Emphasis Type="Italic">W</Emphasis></Subscript>

We present a study of the LHC sensitivity to the W boson mass based on simulation studies. We find that both experimental and phenomenological sources of systematic uncertainties can be strongly constrained with Z measurements: the lineshape, dσ _Z /dm, is robustly predicted, and its analysis provides an accurate measurement of the detector resolution and absolute scale, while the differential cross-section analysis, d² σ _Z /dydp _T , absorbs the strong interaction uncertainties. A sensitivity δ m _W ∼7 MeV for each decay channel (W→e ν, W→μ ν), and for an integrated luminosity of 10 fb⁻¹, appears as a reasonable goal.     

Modifying the properties of lead titanate in PbTi<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3</Subscript> solid solutions

Spontaneous deformation in the PbTi_{1 − x} Mn _x O₃ system at room temperature is found to decrease as x increases, indicating a drop in the temperature of the ferroelectric phase transition.     

The determination of <Emphasis Type="Italic">α</Emphasis><Subscript><Emphasis Type="Italic">S</Emphasis></Subscript> from <Emphasis Type="Italic">τ</Emphasis> decays revisited

We revisit the determination of α _S (m _τ ²) using a fit to inclusive τ hadronic spectral moments in light of (1) the recent calculation of the fourth-order perturbative coefficient K ₄ in the expansion of the Adler function, (2) new precision measurements from BABAR of e⁺e⁻ annihilation cross sections, which decrease the uncertainty in the separation of vector and axial-vector spectral functions, and (3) improved results from BABAR and Belle on τ branching fractions involving kaons. We estimate that the fourth-order perturbative prediction reduces the theoretical uncertainty, introduced by the truncation of the series, by 20% with respect to earlier determinations. We discuss to some detail the perturbative prediction of two different methods: fixed-order perturbation theory (FOPT) and contour-improved perturbative theory (CIPT). The corresponding theoretical uncertainties are studied at the τ and Z mass scales. The CIPT method is found to be more stable with respect to the missing higher order contributions and to renormalization scale variations. It is also shown that FOPT suffers from convergence problems along the complex integration contour. Nonperturbative contributions extracted from the most inclusive fit are small, in agreement with earlier determinations. Systematic effects from quark-hadron duality violation are estimated with simple models and found to be within the quoted systematic errors. The fit based on CIPT gives α _S (m _τ ²)=0.344±0.005±0.007, where the first error is experimental and the second theoretical. After evolution to M _Z we obtain α _S (M _Z ²)=0.1212±0.0005±0.0008±0.0005, where the errors are respectively experimental, theoretical and due to the evolution. The result is in agreement with the corresponding N³LO value derived from essentially the Z width in the global electroweak fit. The α _S (M _Z ²) determination from τ decays is the most precise one to date.     

Transmission spectra of films of Mn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se solid solutions

Films of Mn_1−x Fe _x Se (x = 0–0.45) solid solutions were flash-sputtered. We measured the transmission spectra of the films in the wavelength range 200–1000 nm at room temperature. From these spectra, we calculated the absorption coefficients and determined the fundamental absorption edge, the position of which is shifted from 2.65 eV in MnSe to 2.30 eV in solid solutions of compositions x = 0.20–0.45. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 1, pp. 136–138, January–February, 2007.     

Thermoelectric figure-of-merit in <Emphasis Type="Italic">p</Emphasis>-type bismuth- and antimony-chalcogenide-based solid solutions above room temperature

The specific features of the variation in the thermoelectric figure-of merit Z for p-type bismuth- and antimony-chalcogenide-based solid solutions p-(Bi,Sb)₂(Te,Se)₃ have been analyzed with allowance made for the data amassed in the investigation of thermoelectric and galvanomagnetic properties. It has been shown that, in the samples with optimum carrier concentrations, the increase in Z in the multicomponent p-Bi_{2 − x} Sb _x Te_{3 − y} Se _y composition (x = 1.3, y = 0.06) in the temperature range 300–370 K is mediated by the high carrier mobility and the low lattice thermal conductivity. The higher effective mass of the density of states and the larger slope of the temperature dependence of the mobility as compared to the other compositions bring about an increase in Z in the p-Bi_{2 − x} Sb _x Te₃ solid solution for x = 1.6 in the temperature range 370–550 K. The increase in the figure-of merit reached in the compositions under study stems also from the increasing contraction of constant-energy ellipsoids along the binary and bisector directions and from the change in the angle θ between the principal axes of the ellipsoids and the crystallographic axes.     

<Emphasis Type="Italic">Λ</Emphasis>N scattering length from the reaction <Emphasis Type="Italic">γ</Emphasis>d→K<Superscript>+</Superscript><Emphasis Type="Italic">Λ</Emphasis>n

The prospects of utilizing the strangeness production reaction γd→K ⁺ Λn for the determination of the Λn low-energy scattering parameters are investigated. The spin observables that need to be measured in order to isolate the Λn singlet (¹ S ₀) and triplet (³ S ₁) states are identified. Possible kinematical regions where the extraction of the Λn scattering lengths might be feasible are discussed.     

Static and dynamic properties of KCN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Cl<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>

An extended three-body force shell model (ETSM) has been applied to investigate the static and dynamic properties of KCN _x Cl_1−x for the compositionx = 0.56 and 1.0 at 300 K. The phonon dispersion curves computed by us are compared with the single crystal neutron diffraction data. The unusual features of these curves are the upward curvature seen in some of the acoustic branches. This is a result ofK-dependent softening of the phonon due to translation-rotation coupling. The transverse acoustic branch is more soft near the zone centre.