Band gap reduction and redshift of lattice vibrational spectra in Nb and Fe co-doped PLZT

Nb and Fe co-doped PLZT (Pb_0.97La_0.02(Zr_0.52Ti_0.48)_1?2x (Nb_0.5Fe_0.5)_2x O₃ for x = 0.00, 0.02, 0.04, 0.06 and 0.08) samples have been prepared using sol–gel method. X-ray diffraction (XRD) and Raman spectroscopy studies confirmed that the samples are single phase even for the highest tested doping of 8 mol% of Nb and Fe. Incorporation of Nb and Fe atoms into PLZT lattice has been confirmed by XRD study where a systematic peak shift has been observed with increasing dopant concentration. The lattice parameters are found to decrease gradually with increase in Nb and Fe contents. From Raman spectroscopic investigation, redshift of several modes has been observed. Rietveld refinement has been performed to correlate XRD results with the fitting of Raman spectra. A total of 14 distinguished modes have been identified by de-convolution of Raman spectra, and they are in good agreement with the theoretically calculated modes for PbTiO₃ and also with those reported on PZT and PLZT previously. The Burstein–Moss shift of absorption edge has been observed by diffuse reflectance spectroscopy experiment, and the analysis shows change in band gap from 3.21 eV (for x = 0.00) to 2.59 eV (for x = 0.08). The underlying mechanisms and the observed electronic behavior have been confirmed and analyzed by photoluminescence study which revealed several transitions and supported the effect of Nb and Fe co-doping as observed from XRD and Raman spectroscopy.     

Correction to: Enhanced critical current density of ex situ MgB2 via control of Mg assisted sintering

Journal of Materials Science: Materials in Electronics -     

Effect of Ag Addition on the Surface Topography and the Vibrational Dynamics of MgB<Subscript>2</Subscript>

We fabricated MgB₂ samples with Ag additions using in situ solid-state reaction via a single-step sintering to study the effect of Ag on the structural, vibration, and superconducting properties of MgB₂ samples. Ag addition to MgB₂ resulted in a significant improvement in J _c although no appreciable effect was observed in the lattice parameters and the superconducting transition temperature T _c. Dramatic increase in the grain size was observed with Ag addition and topographic measurements with atomic force microscopy revealed the formation of Ag–Mg nanoparticles 5–20 nm in size at 2 and 4 wt% Ag additions. The fact that these samples showed high J _c values suggests that the nanoparticles formed as a result of Ag addition are responsible for enhanced flux pinning. Raman spectroscopy measurements showed that Ag additions also increased disorder in the system and thereby affected the line width of the Raman active E _2g mode.     

Vortex interaction with large particles in bulk RE-123 compounds

Excellent electromagnetic properties of ternary RE-123 compounds at low fields rely on vortex trapping at relatively large particles of secondary phases. Mechanism of vortex interaction with large particles, both in the single-vortex and multi-vortex regimes is studied. It is shown why the pinning effeciency is high at low magnetic fields but rapidly drops with increasing field.     

Modeling of Physical Defects in PN Junction Based Graphene Devices

Graphene’s exceptional electro-mechanical properties make it a strong contender to replace silicon-based Complementary Metal-Oxide Semiconductor (CMOS) devices in the future. Among other novel material-based devices, graphene is pushing the research community to find new technological solutions that exploit its special characteristics. As it is a semimetal, the key challenge for graphene-based devices to be used in digital circuits is introducing band gap. Among the proposed approaches, electrostatic doping represents a key option. It allows the implementation of graphene pn junctions through which building a new class of reconfigurable logic gates is possible. This devices are analyzed in this work. Recent works presented a quantitative analysis of such gates in terms of area, delay and power consumptions, confirming their superiority w.r.t. CMOS technologies below the 22 nm. This paper explores another dimension, that is testability, and proposes a study of possible physical defects that might alter the functionality of the graphene logic gates. Two major kinds of manufacturing defects, which are possible in these gates, namely the S h o r t s between the device’s terminals and O p e n terminals, are considered. These faults have been injected into non faulty devices at the SPICE-level and the resulting behavior is mapped to appropriate fault model. Most of such models belong to the CMOS domain, but for some specific class of defects, new fault definitions are needed.     

Enhanced critical current density of ex situ MgB2 via control of Mg assisted sintering

Journal of Materials Science: Materials in Electronics - In this work,&nbsp;ex situ&nbsp;MgB2&nbsp;was mixed with 0.5&nbsp;mol of Mg and sintered. The sintering conditions were...     

Improvement of critical current density of MgB2 bulk superconductor processed by Spark Plasma Sintering

Spark Plasma Sintering (SPS) is a promising rapid consolidation technique that allows a better understanding and optimization of the sintering kinetics and therefore makes it possible to obtain MgB₂ bulk superconductor with tailored microstructures consisting of grains with either spherical or elongated morphology. In this contribution, the role of the precursor powders on the superconducting properties of MgB₂ is investigated. Three sets of bulk MgB₂ material were processed from: (i) a commercially available MgB₂ powder; (ii) a mixture of Mg metal and amorphous B using a single-step solid-state reaction process and (iii) a mixture of amorphous boron coated with carbon and Mg metal. The samples were prepared in the same SPS processing conditions. The microstructure of the samples was investigated by X-ray diffraction, SEM and TEM and correlated to their superconducting properties. The critical current density of the best sample at 20K was J_c = 500 kA/cm² in self-field, which is one of the highest critical current density reported for MgB₂ bulk superconductors.