Ensemble v-representable ab-initio density functional calculation of energy and spin in atoms: atest of exchange-correlation approximations

The total energies and the spin states for atoms and their first ions with Z = 1-86 are calculated within the the local spin-density approximation (LSDA) and the generalized-gradient approximation (GGA) to the exchange-correlation (xc) energy in density-functional theory. Atoms and ions for which the ground-state density is not pure-state v-representable, are treated as ensemble v- representable with fractional occupations of the Kohn-Sham system. A newly developed algorithm which searches over ensemble v-representable densities [E. Kraisler et al., Phys. Rev. A 80, 032115 (2009)] is employed in calculations. It is found that for many atoms the ionization energies obtained with the GGA are only modestly improved with respect to experimental data, as compared to the LSDA. However, even in those groups of atoms where the improvement is systematic, there remains a non-negligible difference with respect to the experiment. The ab-initio electronic configuration in the Kohn-Sham reference system does not always equal the configuration obtained from the spectroscopic term within the independent-electron approximation. It was shown that use of the latter configuration can prevent the energy-minimization process from converging to the global minimum, e.g. in lanthanides. The spin values calculated ab-initio fit the experiment for most atoms and are almost unaffected by the choice of the xc-functional. Among the systems with incorrectly obtained spin there exist some cases (e.g. V, Pt) for which the result is found to be stable with respect to small variations in the xc-approximation. These findings suggest a necessity for a significant modification of the exchange-correlation functional, probably of a non-local nature, to accurately describe such systems. PACS numbers: 31.15.

Preliminary study on low-density polystyrene microplastics bead removal from drinking water by coagulation-flocculation and sedimentation

Microplastics (MPs), sized ~150 μm, have been found in tap water at levels of ~5 particles/L, suggesting that water treatment plants are not effectively removing MPs. Therefore, there is an urgent need to evaluate their fate in drinking water treatment processes. Coagulation-flocculation and sedimentation are applied in water treatment to primarily decrease turbidity, and MPs contribute to water turbidity. This study focuses on the removal of polystyrene (PS) beads of 100 μm with density 1.04–1.06 g/cm3. The low-density PS beads pose a removal challenge because they have similar density to the media. The effects of initial water pH and stirring speed on MPs removal by coagulation-flocculation and sedimentation were studied. The most effective conditions found for removing the PS beads from water, that led to removal rates up to 98.9 ± 0.94%, were 3.4 mg Al/L of coagulant, pH 5, flocculation time of 7 min and sedimentation time of 30 min. For the first time, floc breakage and regrowth following the addition of Al, has shown to favour the removal of the PS beads. Based on this research, coagulation-flocculation can play a very important role in removing MPs during drinking water treatment

Narrow structure in the coherent population trapping resonances in rubidium and Rayleigh scattering

The measurement of the coherent-population-trapping (CPT) resonances in uncoated Rb vacuum cells has shown that the shape of the resonances is different in different cells. In some cells the resonance has a complex shape - a narrow Lorentzian structure, which is not power broadened, superimposed on the power broadened CPT resonance. The results of the performed investigations on the fluorescence angular distribution are in agreement with the assumption that the narrow structure is a result of atom interaction with Rayleigh scattering light. The results are interesting for indication of the vacuum cleanness of the cells and building of magnetooptical sensors

Detailed studies of non-linear magneto-optical resonances at D1 excitation of Rb-85 and Rb-87 for partially resolved hyperfine F-levels

Experimental signals of non-linear magneto-optical resonances at D1 excitation of natural rubidium in a vapor cell have been obtained and described with experimental accuracy by a detailed theoretical model based on the optical Bloch equations. The D1 transition of rubidium is a challenging system to analyze theoretically because it contains transitions that are only partially resolved under Doppler broadening. The theoretical model took into account all nearby transitions, the coherence properties of the exciting laser radiation, and the mixing of magnetic sublevels in an external magnetic field and also included averaging over the Doppler profile. Great care was taken to obtain accurate experimental signals and avoid systematic errors. The experimental signals were reproduced very well at each hyperfine transition and over a wide range of laser power densities, beam diameters, and laser detunings from the exact transition frequency. The bright resonance expected at the F_g=1 --> F_e=2 transition of Rb-87 has been observed. A bright resonance was observed at the F_g=2 --> F_e=3 transition of Rb-85, but displaced from the exact position of the transition due to the influence of the nearby F_g=2 --> F_e=2 transition, which is a dark resonance whose contrast is almost two orders of magnitude larger than the contrast of the bright resonance at the F_g=2 --> F_e=3 transition. Even in this very delicate situation, the theoretical model described in detail the experimental signals at different laser detunings.Comment: 11 pages, 9 figure

Efficient nonlinear room-temperature spin injection from ferromagnets into semiconductors through a modified Schottky barrier

We suggest a consistent microscopic theory of spin injection from a ferromagnet (FM) into a semiconductor (S). It describes tunneling and emission of electrons through modified FM-S Schottky barrier with an ultrathin heavily doped interfacial S layer . We calculate nonlinear spin-selective properties of such a reverse-biased FM-S junction, its nonlinear I-V characteristic, current saturation, and spin accumulation in S. We show that the spin polarization of current, spin density, and penetration length increase with the total current until saturation. We find conditions for most efficient spin injection, which are opposite to the results of previous works, since the present theory suggests using a lightly doped resistive semiconductor. It is shown that the maximal spin polarizations of current and electrons (spin accumulation) can approach 100% at room temperatures and low current density in a nondegenerate high-resistance semiconductor.Comment: 7 pages, 2 figures; provides detailed comparison with earlier works on spin injectio

Optical properties and electronic structure of β′−NiAl

The optical constants and their temperature derivatives have been determined for β′−NiAl from absorption and thermoreflectance measurements in the energy range of 0.2-4.4 eV. The results are interpreted using the self-consistent energy bands of Moruzzi, Williams, and Gelatt. By comparing a calculated joint density of states with ε2, the imaginary part of the dielectric function, good overall agreement is found between theory and experiment. In contrast to earlier analyses, it is found that the 2.5-eV peak in ε2 is primarily due to direct interband transitions terminating near the Fermi surface. This new interpretation of the 2.5-eV feature is discussed in relation to previously reported concentration effects and the rigid-band model

Scattering polarization of hydrogen lines from electric-induced atomic alignment

We consider a gas of hydrogen atoms illuminated by a broadband, unpolarized radiation with zero anisotropy. In the absence of external fields, the atomic J-levels are thus isotropically populated. While this condition persists in the presence of a magnetic field, we show instead that electric fields can induce the alignment of those levels. We also show that this electric alignment cannot occur in a two-term model of hydrogen (e.g., if only the Ly-alpha transition is excited), or if the level populations are distributed according to Boltzmann's law.Comment: 10 pages, 4 figures. Accepted by J.Phys.B: At.Mol.Opt.Phy

Scattering polarization of hydrogen lines in the presence of turbulent electric fields

We study the broadband polarization of hydrogen lines produced by scattering of radiation, in the presence of isotropic electric fields. In this paper, we focus on two distinct problems: a) the possibility of detecting the presence of turbulent electric fields by polarimetric methods, and b) the influence of such fields on the polarization due to a macroscopic, deterministic magnetic field. We found that isotropic electric fields decrease the degree of linear polarization in the scattered radiation, with respect to the zero-field case. On the other hand, a distribution of isotropic electric fields superimposed onto a deterministic magnetic field can generate a significant increase of the degree of magnetic-induced, net circular polarization. This phenomenon has important implications for the diagnostics of magnetic fields in plasmas using hydrogen lines, because of the ubiquitous presence of the Holtsmark, microscopic electric field from neighbouring ions. In particular, previous solar magnetographic studies of the Balmer lines of hydrogen may need to be revised because they neglected the effect of turbulent electric fields on the polarization signals. In this work, we give explicit results for the Lyman-alpha and Balmer-alpha lines.Comment: 15 pages, 6 figure

Four-terminal resistance of an interacting quantum wire with weakly invasive contacts

We analyze the behavior of the four-terminal resistance, relative to the two-terminal resistance of an interacting quantum wire with an impurity, taking into account the invasiveness of the voltage probes. We consider a one-dimensional Luttinger model of spinless fermions for the wire. We treat the coupling to the voltage probes perturbatively, within the framework of non-equilibrium Green function techniques. Our investigation unveils the combined effect of impurities, electron-electron interactions and invasiveness of the probes on the possible occurrence of negative resistance.Comment: 10 pages, 7 figure