Study of the peak effect phenomenon in single crystals of 2H-NbSe2

The weakly pinned single crystals of the hexagonal 2H-NbSe₂ compound have emerged as prototypes for determining and characterizing the phase boundaries of the possible order-disorder transformations in the vortex matter. We present here a status report based on the ac and dc magnetization measurements of the peak effect phenomenon in three crystals of 2H-NbSe₂, in which the critical current densities vary over two orders of magnitude. We sketch the generic vortex phase diagram of a weakly pinned superconductor, which also utilizes theoretical proposals. We also establish the connection between the metastability effects and pinning.     

Pinacol Formation and Reduction of Aromatic Carbonyls with Magnesium–Methanol at Ambient Temperature

A simple and inexpensive procedure for the pinacol formation of aromatic aldehydes and reduction of aromatic ketones to the corresponding alcohols with magnesium in dry methanol at ambient temperature is reported. The pinacol formation and reduction are proposed to be proceeding by SET from magnesium.     

Novel nonperipheral octa-3-hydroxypropylthio substituted metallo-phthalocyanines: synthesis,characterization, and investigation of their electrochemical,photochemical and computational properties

The current study describes the synthesis, electrochemical, computational, and photochemical properties of octa (3-hydroxypropylthio) substituted cobalt (II) ( 4 ), copper (II) ( 5 ), nickel (II) ( 6 ) and zinc(II) ( 7 ) phthalocyanine derivatives. These novel compounds were characterized by elemental analysis,¹H,¹³C NMR, FT-IR, UV-Vis, and MS. The redox behaviors of these metallo-phthalocyanines were investigated by the cyclic voltammetric method. The optimized molecular structure and gauge-including atomic orbital (GIAO)¹H and¹³C NMR chemical shift values of these phthalocyanines in the ground state had been calculated by using B3LYP/6–31G(d,p) basis set. The outcomes of the optimized molecular structure were given and compared with the experimental NMR values. The photochemical properties including photodegradation and singlet oxygen generation of zinc(II) phthalocyanine were studied in DMSO solution for the determination of its photosensitizer behaviors.     

Synthesis and characterization of LaB6 thin films on tungsten, rhenium, silicon and other substrates and their investigations as?field emitters

The paper deals with the comparative study of nanocrystalline Lanthanum hexaboride (LaB₆) thin films grown on various substrates by Pulsed laser deposition and Arc plasma method. Field emission studies were carried out on LaB₆ films deposited on various substrates show metallic behavior of the emitters. The high value of field enhancement factors, indicating that the electron emission from LaB₆ nanoscale protrusions deposited on emitter surface. The post field emission surface morphology of the emitters showed no significant erosion of the films during continuous operation. The observed behavior indicates that it is linked with the growth of LaB₆ films on substrate crystal structure. The LaB₆ nanocrystallites/nanowires films were synthesized using arc plasma method shows good emission current stability. The LaB₆ micro/nanocrystallites were also obtained by picosecond laser irradiation which gives high enhancement β factor, and good emission current stability along with high current density. The results reveal that nanocrystalline LaB₆ films, exhibit high resistance to ion bombardment and excellent structural stability and are more promising emitters for practical applications in field emission based new generation devices.     

Study of magnetoresistance and conductance of bicrystal grain boundary in La0.67Ba0.33MnO3 thin film

La_0.67Ba_0.33MnO₃ (LBMO) thin film is deposited on a 36.7°C SrTiO₃ bicrystal substrate using laser ablation technique. A microbridge is created across bicrystal grain boundary and its characteristics are compared with a microbridge on the LBMO film having no grain boundary. Presence of grain boundary exhibits substantial magnetoresistance ratio (MRR) in the low field and low temperature region. Bicrystal grain boundary contribution in MRR disappears at temperature T>175 K. At low temperature, I-V characteristic of the microbridge across bicrystal grain boundary is nonlinear. Analysis of temperature dependence of dynamic conductance-voltage characteristics of the bicrystal grain boundary indicates that at low temperatures (T<175 K) carrier transport across the grain boundary in LBMO film is dominated by inelastic tunneling via pairs of manganese atoms and tunneling through disordered oxides. At higher temperatures (T>175 K), magnetic scattering process is dominating. Decrease of bicrystal grain boundary contribution in magnetoresistance with the increase in temperature is due to enhanced spin-flip scattering process.     

Development of a novel high speed (electron-mobility) epi-n-ZnO thin films by L-MBE for III–V opto-electronic devices

Intrinsic epitaxial zinc oxide (epi-ZnO) thin films were grown by laser-molecular beam epitaxy (L-MBE), i.e., pulsed laser deposition (PLD) technique using Johnson Matthey “specpure”-grade ZnO pellets. The effects of substrate temperatures on ZnO thin film growth, electrical conductivity (σ), mobility (μ) and carrier concentration (n) were studied. As well as the feasibility of developing high quality conducting oxide thin films was also studied simultaneously. The highest conductivity was found for optimized epi-ZnO thin films is σ=0.06×10³ ohm⁻¹ cm⁻¹ (n-type) (which is almost at the edge of semiconductivity range), carrier density n=0.316×10¹⁹ cm⁻³ and mobility μ=98 cm²/V s. The electrical studies further confirmed the semiconductor characteristics of epi-n-ZnO thin films. The relationship between the optical and electrical properties were also graphically enumerated. The electrical parameter values for the films were calculated, graphically enumerated and tabulated. As a novelty point of view, we have concluded that without doping and annealing, we have obtained optimum electrical conductivity with high optical transparency (95%) for as deposited ZnO thin films using PLD. Also, this is the first time that we have applied PLD made ZnO thin films to iso-, hetero-semiconductor–insulator–semiconductor (SIS) type solar cells as transparent conducting oxide (TCO) window layer. We hope that surely these data be helpful either as a scientific or technical basis in the semiconductor processing.     

Well-posedness of transonic characteristic discontinuities in two-dimensional steady compressible Euler flows

In our previous work, we have established the existence of transonic characteristic discontinuities separating supersonic flows from a static gas in two-dimensional steady compressible Euler flows under a perturbation with small total variation of the incoming supersonic flow over a solid right wedge. It is a free boundary problem in Eulerian coordinates and, across the free boundary (characteristic discontinuity), the Euler equations are of elliptic–hyperbolic composite-mixed type. In this paper, we further prove that such a transonic characteristic discontinuity solution is unique and L ¹–stable with respect to the small perturbation of the incoming supersonic flow in Lagrangian coordinates.     

Direct observation of slow morphological transformations and wetting behavior of pulsed laser deposited sub-monolayer gold on (0 0 0 1) sapphire in atmosphere

Using high-resolution atomic force microscope we observed in ambient atmosphere the slow morphological transitions of the incipient adlayer of gold grown on (0 0 0 1) sapphire substrate by pulsed laser deposition. The equivalent average uniform thickness of the gold deposition was about 0.55 Å, which is about one-fourth of its monolayer. A dynamic simulation revealed that about 10% of the gold was implanted into the substrate up to the depth of about 3.3 nm and the top monolayer of the sapphire surface was almost completely depleted of oxygen atoms due to the preferential sputtering by the plume particles. The gold adlayer transformed into a labile phase which enhanced the surface roughness and had a preferred orientation of a wavy structure during 24 h of the deposition. The auto-correlation function of this wavy structure in labile metastable phase revealed two-fold symmetry and provided a preferential size of about 4 nm (peak to peak) with a mean separation of 8 nm. At the end of about 6 days this phase was found to completely transform into an apparently de-wetted phase of beads with average in-plane diameter of ∼20 nm and height of ∼7 nm having large size distribution. Each bead was seen to have coating of a concentric corona layer, which might be that of the condensed moisture or other gaseous species from atmosphere because subjecting these samples to vacuum removed this layer. These observations shed light on the dynamics of the pulsed laser deposited metastable gold adlayer in the incipient stage of its growth on sapphire and their wetting or de-wetting mechanisms in ambient atmosphere.     

Charge transport properties of MDMO PPV thin films cast in different solvents

Charge transport properties in thin films of Poly(2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylenevinylene) (MDMO PPV) cast using either chloroform (CF), toluene (TOL), or chlorobenzene (CB) as solvent were investigated. Hole mobility (μ) in these thin films measured using time‐of‐flight transient photoconductivity showed an increasing trend with respect to the solvent used in the same order, that is, μ_CF (2.4 × 10^?7 cm²/Vs) < μ_TOL (6.9 × 10^?7 cm²/Vs) < μ_CB (2.3 × 10^?6 cm²/Vs). Observed variations in mobilities were attributed to different morphologies of MDMO PPV chains in thin films cast using the aforesaid solvents. Nature of the interchain interactions and aggregate formation were obtained using photoluminescence (PL), Raman spectroscopy, and AFM studies. Ratio of PL peak intensities of 0–0 and 0–1 transitions, which is a direct measure of interchain interaction, was the highest in CB and lowest in CF. Variation in the relative intensities of out‐of‐plane wagging of vinylene group (～963 cm^?1 mode) in Raman spectra suggested different extent of coiling of polymer chains in these thin films. From these observations, it was elicited that aggregate size and interchain interactions are highest in CB and least in CF. AFM‐based topographic images of thin films further supported these variations in the size of aggregates. Variation in the aggregate sizes and interchain interactions explained the corresponding variation in the mobility. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1431–1439