Excitonic and Quasiparticle Life Time Effects on Silicon Electron Energy Loss Spectrum from First Principles

The quasiparticle decays due to electron-electron interaction in silicon are studied by means of first-principles all-electron GW approximation. The spectral function as well as the dominant relaxation mechanisms giving rise to the finite life time of quasiparticles are analyzed. It is then shown that these life times and quasiparticle energies can be used to compute the complex dielectric function including many-body effects without resorting to empirical broadening to mimic the decay of excited states. This method is applied for the computation of the electron energy loss spectrum of silicon. The location and line shape of the plasmon peak are discussed in detail.Comment: 4 pages, 3 figures, submitted to PR

Huge excitonic effects in layered hexagonal boron nitride

The calculated quasiparticle band structure of bulk hexagonal boron nitride using the all-electron GW approximation shows that this compound is an indirect-band-gap semiconductor. The solution of the Bethe-Salpeter equation for the electron-hole two-particle Green function has been used to compute its optical spectra and the results are found in excellent agreement with available experimental data. A detailed analysis is made for the excitonic structures within the band gap and found that the excitons belong to the Frenkel class and are tightly confined within the layers. The calculated exciton binding energy is much larger than that obtained by Watanabe {\it et al} using a Wannier model to interpret their experimental results and assuming that h-BN is a direct-band-gap semiconductor.Comment: 4 pages, 3 figure

Pressure-Induced Simultaneous Metal-Insulator and Structural-Phase Transitions in LiH: a Quasiparticle Study

A pressure-induced simultaneous metal-insulator transition (MIT) and structural-phase transformation in lithium hydride with about 1% volume collapse has been predicted by means of the local density approximation (LDA) in conjunction with an all-electron GW approximation method. The LDA wrongly predicts that the MIT occurs before the structural phase transition. As a byproduct, it is shown that only the use of the generalized-gradient approximation together with the zero-point vibration produces an equilibrium lattice parameter, bulk modulus, and an equation of state that are in excellent agreement with experimental results.Comment: 7 pages, 4 figures, submitted to Europhysics Letter

Comment on "Origin of Giant Optical Nonlinearity in Charge-Transfer--Mott Insulators: A New Paradigm for Nonlinear Optics"

Comment on Phys. Rev. Lett. 86, 2086 (2001)Comment: 1 page, 1 eps figur

Elaboration of Inorganic Polymer for Removal of Organic Compound by Dynamic Column Test

The present research aims to synthesize inorganic polymer by geopolymerization reaction and to evaluate its ability to removal organic matter from water. The inorganic polymer was prepared by activation of metakaolin by a mixture of sodium hydroxide and sodium silicate. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) were employed to characterize the geo-adsorbent. The XRD and SEM analysis indicates that formation of new phase and aluminosilicate gel after the geopolymerization process. The adsorption of cationic dye into elaborated inorganic polymer matrix was investigated by a dynamic column test. To predict the breakthrough curves and determine the  main  fixed  bed  column  parameters,  three  kinetic  models;  Tomas,  Bohart-Adams  and Yoon-Nelson  models  are  applied  to  fit  the  experimental  data. The kinetic models of the adsorption in dynamic column confirmed that the prepared adsorbent can be used repeatedly for decolouration of water contaminated by textile dye.

Enhancing the Removal of Organic Pollutant-Methylene Blue- by a Morroccan Natural Clay

In this study, the removal of a cationic dye namely, methylene blue (MB) by natural clay (NC) from aqueous solution was investigated. The morphology, structure and surface state of NC were characterized using X-Ray Diffraction (XRD), X-Ray Fluorescence (XRF), Fourier Transform Infrared Spectroscopy (FTIR), Differential Scanning Calorimetry (DSC), Scanning Electron Microscopy (SEM)) and Optical Microscope, respectively. Affecting factors solution pH, dye concentration, contact time and temperature were investigated.  The maximum removal was obtained at basic pH. The experimental equilibrium data were examined using Langmuir, Freundlich, Temkin and Dubinin–Radushkevich. The equilibrium data for cationic dye adsorption was fitted to the Langmuir, and the maximum adsorption capacity of natural clay for MB was up to 25 mg.g−1. The adsorption kinetic dye was analyzed using pseudo-first order, pseudo-second order, Elovish and the intraparticle diffusion model.  The kinetic data of cationic dye could be better described by the pseudo second-order model. The thermodynamic parameters such as the change in free energy (ΔGo), enthalpy (ΔHo) and entropy (ΔSo) of adsorption were determined. These values show that the adsorption was exothermic and spontaneous

Structural relaxation effects on interface and transport properties of Fe/MgO(001) tunnel junctions

The interface structure of Fe/MgO(100) magnetic tunnel junctions predicted by density functional theory (DFT) depends significantly on the choice of exchange and correlation functional. Bader analysis reveals that structures obtained by relaxing the cell with the local spin-density approximation (LSDA) display a different charge transfer than those relaxed with the generalized gradient approximation (GGA). As a consequence, the electronic transport is found to be extremely sensitive to the interface structure. In particular, the conductance for the LSDA-relaxed geometry is about one order of magnitude smaller than that of the GGA-relaxed one. The high sensitivity of the electronic current to the details of the interface might explain the discrepancy between the experimental and calculated values of magnetoresistance.Comment: Submitted to PRL, 5 figure

Reconfigurable Hardened Latch and Flip-Flop for FPGAs

In this paper, we propose Joint Latch (JLatch) and Joint Flip-Flop (JFF), two novel reconfigurable structures which bring the reconfigurability of reliability to user latches and flip-flops (FFs) in reconfigurable devices such as FPGAs. Specifically, we implement two reconfigurable storage elements that exploit a trade-off between reliability and amount of available resources. In fault prone conditions, JLatch (or JFF) is configured in such a way that four pre-selected normal static latches (or FFs) are combined together at circuit level to form one hardened storage cell. Solution focuses on transient faults such as soft errors, where we show that critical charge is increased by at least three orders of magnitude (1000X) to practically bring immunity against any Single Event Upset (SEU). If four latches inside an FPGA logic block are far enough, it can effectively cope with Multiple Bit Upsets (MBUs) as well. Additionally, provided that special transistor sizing is applied (only necessary for some latch structures), JLatch and JFF take advantage of a novel self-correcting technique to correct any single fault immediately. Our solution provides reconfigurability of reliability with negligible performance and area overhead with only one (two) extra transistor(s) per latch (FF). The delay of this technique is less than the delay of conventional TMR (Triple Modular Redundancy) technique with a majority voter at output. © 2017 IEEE

NS-SRAM: Neighborhood Solidarity SRAM for Reliability Enhancement of SRAM Memories

Technology shift and voltage scaling increased the susceptibility of Static Random Access Memories (SRAMs) to errors dramatically. In this paper, we present NS-SRAM, for Neighborhood Solidarity SRAM, a new technique to enhance error resilience of SRAMs by exploiting the adjacent memory bit data. Bit cells of a memory line are paired together in circuit level to mutually increase the static noise margin and critical charge of a cell. Unlike existing techniques, NS-SRAM aims to enhance both Bit Error Rate (BER) and Soft Error rate (SER) at the same time. Due to auto-adaptive joiners, each of the adjacent cells' nodes is connected to its counterpart in the neighbor bit. NS-SRAM enhances read-stability by increasing critical Read Static Noise Margin (RSNM), thereby decreasing faults when circuit operates under voltage scaling. It also increases hold-stability and critical charge to mitigate soft-errors. By the proposed technique, reliability of SRAM based structures such as cache memories and register files can drastically be improved with comparable area overhead to existing hardening techniques. Moreover it does not require any extra-memory, does not impact the memory effective size, and has no negative impact on performance. © 2016 IEEE

Pressure Tuning of the Charge Density Wave in the Halogen-Bridged Transition-Metal (MX) Solid Pt2Br6(NH3)4Pt_2Br_6(NH_3)_4

We report the pressure dependence up to 95 kbar of Raman active stretching modes in the quasi-one-dimensional MX chain solid $Pt_2Br_6(NH_3)_4$. The data indicate that a predicted pressure-induced insulator-to-metal transition does not occur, but are consistent with the solid undergoing either a three-dimensional structural distortion, or a transition from a charge-density wave to another broken-symmetry ground state. We show that such a transition cacan be well-modeled within a Peierls-Hubbard Hamiltonian. 1993 PACS: 71.30.+h, 71.45.Lr, 75.30.Fv, 78.30.-j, 81.40.VwComment: 4 pages, ReVTeX 3.0, figures available from the authors on request (Gary Kanner, [email protected]), to be published in Phys Rev B Rapid Commun, REVISION: minor typos corrected, LA-UR-94-246