Publications

In quantum field theory, Weyl fermions are relativistic particles that travel at the speed of light and strictly obey the celebrated Lorentz symmetry. Their low-energy condensed matter analogs are Weyl semimetals, which are conductors whose electronic excitations mimic the Weyl fermion equation of motion. Although the traditional (type I) emergent Weyl fermions observed in TaAs still approximately respect Lorentz symmetry, recently, the so-called type II Weyl semimetal has been proposed, where the emergent Weyl quasiparticles break the Lorentz symmetry so strongly that they cannot be smoothly connected to Lorentz symmetric Weyl particles. Despite some evidence of nontrivial surface states, the direct observation of the type II bulk Weyl fermions remains elusive. We present the direct observation of the type II Weyl fermions in crystalline solid lanthanum aluminum germanide (LaAlGe) based on our photoemission data alone, without reliance on band structure calculations. Moreover, our systematic data agree with the theoretical calculations, providing further support on our experimental results.

This paper presents structural, morphological and preliminary ultrasonic characterizations of the -Cyclodextrin/hydroxyapatite (CD-HAp) composites synthesized by an adapted co-precipitation method. The structural and morphological properties were evaluated by Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDX). The specific surface area, pore size and pore volume were determined using the methods of Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH), respectively. The novelty of our study consists in preliminary ultrasonic measurements conducted on CD-HAp composite, uniformly dispersed in distilled water. The benefit of this non-destructive method was to facilitate and simplify the characterization techniques of nanoparticles. Our experiments proved that the efficiency of lead ion removal by CD-HAp composites depended on the initial concentration of lead. The maximum adsorption capacity of the solid phase, for Pb2+ indicated a higher rate of removal by the CD-HAp_2. These adsorption results bring valuable insight into the beneficial contribution of our compounds, for the removal of heavy metal ions from aqueous solutions. Furthermore, in the present study, was evaluated the toxic effect of lead ions adsorbed by hydroxyapatite from contaminated water on HeLa cells.

Compatibility of screen-printed piezoelectric 0.65Pb(Mg1/3Nb2/3) O-3-0.35PbTiO(3) thick films with metalized low-temperature co-fired (LTCC) ceramic substrates is examined. Such substrates are interesting for micro-electro mechanical systems, for example for piezoelectric sensors and actuators, where functional layers are usually Pb-based piezoelectrics. In this study the special attention is given to the influence of the Au, Ag and Ag/Pd electrode materials coated over the LTCC on the functional properties of the films. The best phase purity, dielectric and piezoelectric properties were obtained in the films on gilded substrates. No secondary phases were observed at the film/Au interface by scanning electron microscope. The piezoelectric coefficient d(33) of the films on gilded substrates is equal to 120 pC/N.

Silver-doped hydroxyapatite (AgHAp) was prepared by co-precipitation method at room temperature. The obtained AgHAp was added in different amounts of collagen gel (AgHApC1 and AgHApC2). Afterward, the gel was lyophilized and the final AgHApC1 and AgHApC2 composite was achieved. The purity, crystallinity, and the phase composition of the AgHAp, AgHApC1, and AgHApC2 samples were evaluated by X-ray diffraction (XRD). The planes corresponding to the 2 values for hydroxyapatite were found in the three samples analyzed in agreement with the crystalline hydroxyapatite. In the Fourier transform infrared (FT-IR) spectra of AgHApC1 and AgHApC2 samples the peak characteristic to the presence of (2) phosphate mode at 472/cm was found. The peaks resulting from the (4) vibration of the P-O mode, (1) symmetric P-O stretching vibration and (3) P-O stretching vibration of PO43- were also evidenced in all the samples. The formation of agglomerated particles with uniform particle size was evidenced by scanning electron microscope (SEM). The uniform distribution of the constituent elements was evidenced by mapping analysis. Furthermore, the strong antibacterial activity of AgHAp, AgHApC1, and AgHApC2 samples against Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aureus) bacterial strains was shown. The inhibition zone increased drastically with the increase of silver concentration. POLYM. ENG. SCI., 57:537-545, 2017. (c) 2017 Society of Plastics Engineers

The substantial increase in the power conversion efficiency of hybrid perovskite solar cells, to date reaching more than 20% in the laboratory, has strongly motivated research on this class of organic-inorganic materials and related devices, particularly based on CH3NH3PbI3-xXx/TiO2 heterostructures (X = Cl,Br). Taking under consideration that a ferroelectric substrate may act as an efficient electron transporter, positively influencing charge collection across the interface and allowing the tuning of the halide perovskite (HP) - ferroelectric junction, we performed extensive density functional theory calculations on CH3NH3PbI3-xClx layers deposited on tetragonal PbTiO3 (PTO) (001) surfaces, to study their structural and electronic properties. The main findings of this study are as follows. (i) A ferroelectric polarization pointing from the PTO/HP interface to the PTO is favorable for the photogenerated electrons transfer across the interface and their transport to the collecting electrode. (ii) The PTO internal electric field leads to a position dependent energy levels diagram. (iii) The HP gap may be tuned by chlorine concentration at the interface, as well as the by the surface terminations of PbTiO3 and hybrid perovskite layers. (iv) The presence of the PTO ferroelectric surface is likely to have just a slight orientational effect on the (CH3NH3)(+) dipoles.

Based on simulations using the plane-wave expansion method, we show that the position of the first bandgap for TM polarized electromagnetic radiation in a high-filling ratio two-dimensional square photonic crystal is practically insensitive to the shape of dielectric rods with the same cross-sectional area. The width of this bandgap depends on the rod shape, but the difference between photonic crystals with circular and regular polygonal rod shapes with a number of edges higher than six can be neglected. For higher-order bandgaps the difference is larger. These results suggest that the fabrication tolerances related to the shape of dielectric rods in a PBG are not as strict as previously thought. (C) 2017 Elsevier B.V. All rights reserved.

In: ZnO (IZO) thin films were deposited on flexible plastic substrates by pulsed laser deposition (PLD) method. The obtained layers present adequate optical and electrical properties competitive with those based on indium tin oxide (ITO). The figure of merit (9 X 10(-3) Omega(-1)) calculated for IZO layers demonstrates that high quality coatings can be prepared by this deposition technique. A thermal annealing (150 degrees C for 1 h) or an oxygen plasma etching (6 mbar for 10 min.) were applied to the IZO layers to evaluate the influence of these treatments on the properties of the transparent coatings. Using vacuum evaporation, organic heterostructures based on cooper phthalocyanine (CuPc) and 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) were deposited on the untreated and treated IZO layers. The optical and electrical properties of the heterostructures were investigated by UVVis, FTIR and current-voltage (I-V) measurements. For the heterostructure fabricated on IZO treated in oxygen plasma, an improvement in the current value with at least one order of magnitude was evidenced in the I-V characteristics recorded in dark conditions. Also, an increase in the current value for the heterostructure deposited on untreated IZO layer can be achieved by adding an organic layer such as tris-8-hydroxyquinoline aluminium (Alq(3)).

We report on isochamp magneto-resistivity and ac susceptibility curves obtained in a high-quality single crystal of the isovalent optimally doped pnictide BaFe2(As0.68P0.32)(2) with superconducting temperature T-c = 27.8 K for H||c-axis. Plots of the logarithmic derivative of the resistivity curves allowed the identification of a vortex-glass (VG) phase and to obtain the values of the critical glass temperature T-g, the temperature T* marking the transition to the liquid phase and of the critical exponent s. The presence of the VG phase is confirmed by detailed measurements of the third harmonic signal of the ac magnetic susceptibility. The modified VG model was successfully applied to the data allowing the obtention of the temperature independent VG activation energy U-b. The activation energy U-0 obtained from the Arrhenius plots in the flux-flow region are compared with U-b and with U-0 obtained from flux-creep measurements on a M(H) isothermal in the same sample. A phase diagram of the studied sample is constructed showing the T-g glass line, the T* line representing a transition (melting) to the liquid phase, the mean field temperature T-c(H) line and the H-p line obtained from the peaks in isothermal critical current, J(c)(H) curves, which are explained in terms of a softening of the vortex lattice. The glass line was fitted by a theory presented in the literature which considers the effect of disorder.

Comparative studies between doped conducting polymers and electrochemical deposited organometallic compounds reveals the interplay between crystalline-amorphous phases with significant contributions to the internal quantum efficiency in the OLED devices. The coexistence of the amorphous and crystalline phase in the electrodeposited film is revealed by the minor micro-crystal products which are present in the amorphous phase in thin films, while the many micro-crystals are randomly distributed in the thick films. Concerning the doped conducting polymers, the level of doping induces crystalline effects as a result of the - stacking between molecules, due to the Forester energy transfer processes in which the transfer rate is increased with decreasing of the distances between neighboring molecules. The crystallization processes change the emission properties of the active layers both for the luminance level and all over color, ranging from yellow to red in the case of IrQ(ppy)(2) compounds.

Significant differences are reported for the IR spectra of poly[(2,5-bisoctyloy)-1, 4-phenylenevinylene] (BO-PPV) recorded onto the KBr, Ag and Au supports. In this work, a decrease in the absorbance of the IR spectra of the BO-PPV films deposited onto Au and Ag supports as increasing the macromolecular compound film thickness is reported. A preferential orientation of the BO-PPV molecules and its composite with single-walled carbon nanotubes (SWNTs) versus metallic supports of Ag and Au is assessed using the IR absorption spectroscopy under s and p polarized light. An anti-Stokes photoluminescence (ASPL) is reported to characterize the BO-PPV macromolecular compound. The BO-PPV ASPL spectra intensity dependent both of the thickness of the macromolecular compound layer deposited onto the metallic supports and the SWNTs weight in the BO-PPV/SWNTs composite mass. (C) 2017 Elsevier B.V. All rights reserved.