Nonexistence in Thomas-Fermi-Dirac-von Weizsäcker Theory with Small Nuclear Charges

We study the ionization problem in the Thomas-Fermi-Dirac-von Weizsäcker theory for atoms and molecules. We prove the nonexistence of minimizers for the energy functional when the number of electrons is large and the total nuclear charge is small. This nonexistence result also applies to external potentials decaying faster than the Coulomb potential. In the case of arbitrary nuclear charges, we obtain the nonexistence of stable minimizers and radial minimizers.     

Analysis of the Thomas-Fermi-von Weizsäcker equation for an infinite atom without electron repulsion

The equation $$\left\{ { - \Delta + |\psi (x)|^{2p - 2} - |x|^{ - 1} } \right\}\psi (x) = 0$$ in three dimensions is investigated. Uniqueness and other properties of the positive solution are proved for 3/2<p<2. There are two physical interpretations of this equation forp=5/3: (i) As the TFW equation for an infinite atomwithout electron repulsion; (ii) The positive solution, ψ, suitably scaled, is asymptotically equal to the solution of the TFW equation for an atom or moleculewith electron repulsion in the regime where the nuclear charges are large andx is close to one of the nuclei.     

Statistical errors in Weizscker-Skyrme mass model

The statistical uncertainties of 13 model parameters in the Weizscker-Skyrme(WS*) mass model are investigated for the first time with an efficient approach,and the propagated errors in the predicted masses are estimated.The discrepancies between the predicted masses and the experimental data,including the new data in AME 2016,are almost all smaller than the model errors.For neutron-rich heavy nuclei,the model errors increase considerably,and go up to a few MeV when the nucleus approaches the neutron drip line.The most sensitive model parameter which causes the largest statistical error is analyzed for all bound nuclei.We find that the two coefficients of symmetry energy term significantly influence the mass predictions of extremely neutron-rich nuclei,and the deformation energy coefficients play a key role for well-deformed nuclei around the β-stability line.     

Developments in geothermal energy in Mexico—part nineteen. Corrosion in Mexican geothermal wells

The Instituto de Investigaciones Eléctricas and the Comisión Federal de Electricidad have initiated a collaborative study to define the specifications of steels for use in geothermal well construction in Mexico. Tests have been designed to characterize and control identifiable factors affecting corrosion. The study includes three main areas of activity: (a) studies of cases of material failure from several Mexican fields were made; (b) studies of general, localized and stress corrosion of sample coupons exposed to geothermal fluid were made in wellhead pressure chambers; (c) laboratory tests are being carried out under controlled hydrodynamic conditions.     

Self-consistent inhomogeneity of quantum wells in II–VI semiconductors

An X-ray diffraction method that uses a slightly diverging (3′) beam and maximally attainable diffraction angles ? _B (as large as 77°) was developed to study quantum wells (QWs) with widths of 5–8 nm separated by wide (100–220 nm) barrier layers. The advantage of this method compared to the use of a parallel beam is an increase by two orders of magnitude in the intensity of the beam incident on the sample and an increase in the probability of diffraction for all QWs as a unified single crystal. It is found that the growth on GaAs substrates misoriented by 10° from the (001) plane in the [111]_II direction brings about monoclinization of crystal lattices of the QW layers and barrier layers in opposite directions. Inhomogeneity of composition over the thickness of each well is observed. In the case of growth of a ZnSe/ZnMgSSe structure in which the layers have a crystal-lattice period close to the lattice period of the GaAs substrate, the QWs are inhomogeneously doped with elements from the composition of the barrier layers. The inhomogeneity of QW composition observed in the growth of mismatched layers in ZnCdSe/ZnSSe and ZnCdS/ZnSSe structures is caused by the fact that mismatch between the lattice parameters of QWs and barriers stimulates the growth of self-consistent compositions; this occurs due to a decrease in the Cd concentration in the Zn_1?x Cd _x Se QW in the initial stages of growth compared to the Cd concentration in the flow of gases and an increase in the Zn concentration in the Cd_1?x Zn _x S QW at small values of x up to the concentration matching GaAs (x = 0.4). The mismatch stresses are partially relaxed via dislocations with the (111)_II glide planes, as a result of which is observed the combination of rotation of the crystal planes of the layers and QW around the [1\(\overline 1 \)0] axis and almost cylindrical bending of the entire sample around the perpendicular [110] axis. Mismatch between lattice parameters of the ZnMgSSe barrier layers and the substrate brings about decomposition of these layers into two phases; this decomposition is caused by thermodynamic instability of the alloy.     

The breakdown of Navier—Stokes hydrodynamics in small periodic systems

The results of direct particle simulations of 2-dimensional, convective flow are compared with truncated solutions of the Navier—Stokes equation, and show that the Navier—Stokes predictions are quite accurate throughout the transient development of the flow, if the correct value of the viscosity is used. The viscosity determines the steady values of the flow velocities, and the Navier—Stokes predictions are accurate for systems with multiple k vector forcing. At higher values of the fields, there is a limiting value of the forcing for which the character of the response matches the character of the forcing. Beyond this point there is a breakdown in hydrodynamic behaviour where the particle nature of the fluid dominates. Both multiphase solid and fluid regions are observed, plus cavitation, depending upon the forcing and the system geometry.     

Localized excitonic states in ZnS–ZnSe single quantum wells

We present low temperature photoluminescence spectra taken from an 11Å ZnSe quantum well in ZnS barriers. The samples are grown by the technique of photo-assisted vapour phase epitaxy (PAVPE) and the spectra show evidence for interface disorder. The observed dependences of the excitonic luminescence on excitation power and temperature are interpreted by a model involving excitonic localization below an exciton mobility edge. This mobility edge is measured for these samples to be 6 meV below the free exciton energy in the ideal quantum well.     

Dynamical current–current correlation in two-dimensional parabolic Dirac systems

We theoretically investigate the current–current correlation of the two-dimensional (2D) parabolic Dirac system in hexogonal lattice. The analytical expressions of the random phase approximation (RPA) susceptibility, Ruderman–Kittel–Kasuya–Yosida (RKKY) Hamiltonian, and the diamagnetic orbital susceptibility in noninteracting case base on the density–density or current–current correlation function are derived and quantitatively analyzed. In noninteracting case, the dynamical polarization within RPA, and spin transverse susceptibility as well as the RKKY interaction (when close to the half-filling) are related to the current–current response in the 2D parabolic Dirac systems. Both the cases of anisotropic dispersion and isotropic dispersion are discussed.     

The symmetry of donor–bound electron wavefunctions in quantum wells

In order to investigate the symmetry (i.e. sphericity) of donor–bound electron wavefunctions in quantum wells, we have invoked a two-parameter trial wavefunction. One parameter is the Bohr radius λ, whilst the other is the eccentricity parameter ζ. The latter incorporates the effect of the quantum well (QW) on the carrier motion in the growth (i.e. the z) direction. Working within the envelope function approximation it is shown that the donor wavefunction has the form of a prolate spheroid. However, calculations of the ratio λ/ζ shows that it is the value of λ which determines the essential symmetry of the wavefunction.     

Stabilised bright solitons in Bose-Einstein condensates in an expulsive parabolic and complex potential

The exact solitonic solutions of the one-dimensional nonlinear Schr?dinger equation, which describes the dynamics of bright soliton in Bose—Einstein condensates with the time-dependent interaction in an expulsive parabolic and complex potential, are obtained by Darboux transformation. The results show that one can compress a bright soliton into an assumed peak of matter wave density by adusting the experimental parameter of the ratio of axial oscillation to radial oscillation or feeding parameter. Especially,when parameters satisfy the relation λ=2γ, the soliton is stable with time evolution without changing its shape and amplitude.     

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.     

Nonlinear optical properties in n-type quadruple δ-doped GaAs quantum wells

The effects of the interlayer distance on the nonlinear optical properties of n-type quadruple δ-doped GaAs quantum well were theoretically investigated. Particularly, the absorption coefficient and the relative refraction index change were determined. In the effective mass approach and within the framework of the Thomas-Fermi theory, the Schrödinger equation was resolved. Thereby, the subband energy levels and their respective wave functions were calculated. The variations in the nonlinear optical properties were determined by using the density matrix solutions. The achieved results demonstrate that the interlayer distance causes optical red-shift on nonlinear optical properties. Therefore, it can be deduced that the suitably chosen interlayer distance can be used to tune optical properties within the infrared spectrum region in optoelectronic devices such as far-infrared photo-detectors, high-speed electronic-optical modulators, and infrared lasers.     

Effects of an anisotropic parabolic potential and Coulomb’s impurity potential on the energy characteristics of asymmetrical semi-exponential CsI quantum wells

Because of its unique optoelectronic properties,people have studied the characteristics of polarons in various quantum well(QW)models.Among them,the asymmetrical semiexponential QW(ASEQW)is a new model for studying the structure of QWs in recent years.It is of great significance to study the influences of the impurity and anisotropic parabolic confinement potential(APCP)on the crystal’s properties,because some of the impurities,usually regarded as Coulomb’s impurity potential(CIP),will exist in the crystal more or less,and the APCP has flexible adjustment parameters.However,the energy characteristics of the ASEQW under the combined actions of impurities and APCP have not been studied,which is the motivation of this paper.Using the linear combination operation and Lee-Low-Pines unitary transformation methods,we investigate the vibrational frequency and the ground state energy of the strong coupling polaron in an ASEQW with the influences of the CIP at the origin of coordinates and APCP,and make a comparison between our results and previous literature’s.Our numerical results about the energy properties in the ASEQW influenced by the CIP and APCP may have important significances for experimental design and device preparation.     

Higher-order effects on self-similar parabolic pulse in the microstructured fibre amplifier

By considering higher-order effects, the properties of self-similar parabolic pulses propagating in the microstructured fibre amplifier with a normal group-velocity dispersion have been investigated. The numerical results indicate that the higher-order effects can badly distort self-similar parabolic pulse shape and optical spectrum, and at the same time the peak shift and oscillation appear, while the pulse still reveals highly linear chirp but grows into asymmetry. The influence of different higher-order effects on self-similar parabolic pulse propagation has been analysed. It shows that the self-steepening plays a more important role. We can manipulate the geometrical parameters of the microstructured fibre amplifier to gain a suitable dispersion and nonlinearity coefficient which will keep high-quality self-similar parabolic pulse propagation. These results are significant for the further study of self-similar parabolic pulse propagation.