Detailed understanding of the excitation-intensity dependent photoluminescence of ZnO materials: Role of defects

The photoluminescence (PL) of ZnO is shown to be dependent on the excitation intensity (EI) of the laser, and the substantial shift observed in the band to band transition is attributed to the heating effect. In order to understand this phenomenon in detail, we investigate the EI dependent PL of various ZnO samples systematically from liquid nitrogen (LN) to room temperature by varying the laser power. Some of the samples exhibit substantial red shift in the band to band transition with increasing EI even in LN environment, negligible effect is observed for others. Hence, our results strongly suggest that the EI dependent PL is not a characteristic of all ZnO samples. This indicates that laser-induced heating effect is not the dominant factor that governs the shifts in the PL spectra. Rather, the defect level excitation accounts for such observation. (C) 2014 AIP Publishing LLC.

Static shear strength and fatigue life of refill friction stir spot welded AA 6061-T6 sheets

This study was aimed at evaluating the static shear strength and fatigue properties of the newly developed refilled friction stir spot welded AA 6061-T6 joints. The keyhole, the process disadvantage of conventional friction stir spot welding, was refilled successfully, using an additional filler plate, with specially designed tools. Two different tool profiles, namely, convex and concave, were used for the refilling process. Sound and defect free joints were obtained by the refilling process. Joints refilled with convex tools showed better static shear strength than those with the concave ones. The variation of microhardness in different regions of the weld was analysed. Fatigue tests were conducted on the lap shear specimens at a stress ratio of R=0.1. The optical micrographs of the welds after fatigue failure in both the conventional and refilled processes were examined to study the fatigue crack propagation and failure modes.

Shock tunnel experiments on control of shock induced large separation bubble using boundary layer bleed

Shock-Boundary Layer Interaction (SBLI) often occurs in supersonic/hypersonic flow fields. Especially when accompanied by separation (termed strong interaction), the SBLI phenomena largely affect the performance of the systems where they occur, such as scramjet intakes, thus often demanding the control of the interaction. Experiments on the strong interaction between impinging shock wave and boundary layer on a flat plate at Mach 5.96 are carried out in IISc hypersonic shock tunnel HST-2. The experiments are performed at moderate flow total enthalpy of 1.3 MJ/kg and freestream Reynolds number of 4 million/m. The strong shock generated by a wedge (or shock generator) of large angle 30.96 degrees to the freestream is made to impinge on the flat plate at 95 mm (inviscid estimate) from the leading edge, due to which a large separation bubble of length (75 mm) comparable to the distance of shock impingement from the leading edge is generated. The experimental simulation of such large separation bubble with separation occurring close to the leading edge, and its control using boundary layer bleed (suction and tangential blowing) at the location of separation, are demonstrated within the short test time of the shock tunnel (similar to 600 mu s) from time resolved schlieren flow visualizations and surface pressure measurements. By means of suction - with mass flow rate one order less than the mass flow defect in boundary layer - a reduction in separation length by 13.33% was observed. By the injection of an array of (nearly) tangential jets in the direction of mainstream (from the bottom of the plate) at the location of separation - with momentum flow rate one order less than the boundary layer momentum flow defect - 20% reduction in separation length was observed, although the flow field was apparently unsteady. (C) 2014 Elsevier Masson SAS. All rights reserved.

Higher spin entanglement entropy from CFT

We consider free fermion and free boson CFTs in two dimensions, deformed by a chemical potential mu for the spin-three current. For the CFT on the infinite spatial line, we calculate the finite temperature entanglement entropy of a single interval perturbatively to second order in mu in each of the theories. We find that the result in each case is given by the same non-trivial function of temperature and interval length. Remarkably, we further obtain the same formula using a recent Wilson line proposal for the holographic entanglement entropy, in holomorphically factorized form, associated to the spin-three black hole in SL(3, R) x SL(3, R) Chern-Simons theory. Our result suggests that the order mu(2) correction to the entanglement entropy may be universal for W-algebra CFTs with spin-three chemical potential, and constitutes a check of the holographic entanglement entropy proposal for higher spin theories of gravity in AdS(3).

Fermi-Edge Transmission Resonance in Graphene Driven by a Single Coulomb Impurity

The interaction between the Fermi sea of conduction electrons and a nonadiabatic attractive impurity potential can lead to a power-law divergence in the tunneling probability of charge through the impurity. The resulting effect, known as the Fermi edge singularity (FES), constitutes one of the most fundamental many-body phenomena in quantum solid state physics. Here we report the first observation of FES for Dirac fermions in graphene driven by isolated Coulomb impurities in the conduction channel. In high-mobility graphene devices on hexagonal boron nitride substrates, the FES manifests in abrupt changes in conductance with a large magnitude approximate to e(2)/h at resonance, indicating total many-body screening of a local Coulomb impurity with fluctuating charge occupancy. Furthermore, we exploit the extreme sensitivity of graphene to individual Coulomb impurities and demonstrate a new defect-spectroscopy tool to investigate strongly correlated phases in graphene in the quantum Hall regime.

Unzipping Carbon Nanotubes at High Impact

The way nanostructures behave and mechanically respond to high impact collision is a topic of intrigue. For anisotropic nanostructures, such as carbon nanotubes, this response will be complicated based on the impact geometry. Here we report the result of hypervelocity impact of nanotubes against solid targets and show that impact produces a large number of defects in the nanotubes, as well as rapid atom evaporation, leading to their unzipping along the nanotube axis. Fully atomistic reactive molecular dynamics simulations are used to gain further insights of the pathways and deformation and fracture mechanisms of nanotubes under high energy mechanical impact. Carbon nanotubes have been unzipped into graphene nanoribbons before using chemical treatments but here the instability of nanotubes against formation, fracture, and unzipping is revealed purely through mechanical impact. defect

Sodium-ion battery cathodes Na2FeP2O7 and Na2MnP2O7: diffusion behaviour for high rate performance

Na-ion batteries are currently the focus of significant research activity due to the relative abundance of sodium and its consequent cost advantages. Recently, the pyrophosphate family of cathodes has attracted considerable attention, particularly Li2FeP2O7 related to its high operating voltage and enhanced safety properties; in addition the sodium-based pyrophosphates Na2FeP2O7 and Na2MnP2O7 are also generating interest. Herein, we present defect chemistry and ion migration results, determined via atomistic simulation techniques, for Na2MP2O7 (where M = Fe, Mn) as well as findings for Li2FeP2O7 for direct comparison. Within the pyrophosphate framework the most favourable intrinsic defect type is found to be the antisite defect, in which alkali-cations (Na/Li) and M ions exchange positions. Low activation energies are found for long-range diffusion in all crystallographic directions in Na2MP2O7 suggesting three-dimensional (3D) Na-ion diffusion. In contrast Li2FeP2O7 supports 2D Li-ion diffusion. The 2D or 3D nature of the alkali-ion migration pathways within these pyrophosphate materials means that antisite defects are much less likely to impede their transport properties, and hence important for high rate performance.

Deformation and strength of Ti-6Al-4V alloyed with B at cryogenic temperatures

Plastic deformation and strength of Ti-6Al-4V (Ti64) alloyed with minor additions of B at cryogenic temperatures were investigated through unnotched and notched tensile tests at 20 and 77 K Marked microstructural refinement that occurs with the trace addition of B to Ti64 was exploited for examining the role of microstructural length scales on the cryogenic plastic deformation. The tensile tests were complemented with detailed microstructural characterisation using transmission electron microscopy and electron back scattered diffraction imaging of the deformed specimens. Experimental results show that the addition of 0.30 wt% and above of B to Ti64 reduces ductility, and in turn enhances the notch sensitivity to the extent that those alloys become unsuitable for low temperature applications. However, the addition of similar to 0.10 wt% B is beneficial in enhancing the low temperature strength. An examination of the yield strength variation at various temperatures reveals that at 77 K, the colony size determines the yield strength of the alloy, just as it does at room temperature; implying dislocation-mediated plasticity continues to dominate up to 77 K At 20 K however, twinning dominates the flow response, with the activation of {11 (2) over bar1} and {5 (6) over bar1 (3) over bar} twinning in addition to {10 (1) over bar2} in the base alloy resulting in enhanced ductility of it as compared to either B-modified alloys at 20 K or the base alloy itself at 77 K The observation of a reasonable correlation between the lath aspect ratio, given by the colony-to-lath thickness ratios, and yield strength variation at 20 K suggests that coarse colony size in the base alloy allows for the activation of additional twinning mechanisms. (C) 2014 Elsevier B.V. All rights reserved.

Hot deformation behavior of Ni-Fe-Ga-based ferromagnetic shape memory alloy - A study using processing map

Ni-Fe-Ga-based alloys form a new class of ferromagnetic shape memory alloys (FSMAs) that show considerable formability because of the presence of a disordered fcc gamma-phase. The current study explores the deformation processing of this alloy using an off-stoichiometric Ni55Fe59Ga26 alloy that contains the ductile gamma-phase. The hot deformation behavior of this alloy has been characterized on the basis of its flow stress variation obtained by isothermal constant true strain rate compression tests in the 1123-1323 K temperature range and strain rate range of 10(-3)-10 s(-1) and using a combination of constitutive modeling and processing map. The dynamic recrystallization (DRX) regime for thermomechanical processing has been identified for this Heusler alloy on the basis of the processing maps and the deformed microstructures. This alloy also shows evidence of dynamic strain-aging (DSA) effect which has not been reported so far for any Heusler FSMAs. Similar effect is also noticed in a Ni-Mn-Ga-based Heusler alloy which is devoid of any gamma-phase. (C) 2014 Elsevier Ltd. All rights reserved.

Observation of Combined Effect of Temperature and Pressure on Cubic to Hexagonal Phase Transformation in ZnS at the Nanoscale

The temperature of allotropic phase transformation in ZnS (cubic to wurtzite) changes with pressure and particle size. In this paper we have explored the interrelation among these through a detailed study of ZnS powders obtained by a temperature-controlled high energy milling process. By employing the combined effect of temperature and pressure in an indigenously built cryomill, we have demonstrated a large-scale, low-temperature synthesis of wurtzite ZnS nanoparticles. The synthesized products have been characterized for their phase and microstructure by the use of X-ray diffraction and transmission electron microscopic techniques. Further, it has been demonstrated that the synthesized materials exhibit photoluminescence emissions in the UV-visible region with an unusual doublet pattern due to the presence of both cubic and hexagonal wurtzite domains in the same particles. By further fine-tuning the processing conditions, it may be possible to achieve controlled defect related photoluminescence emissions from the ZnS nanoparticles.

Hot deformation behaviour and microstructure control in AlCrCuNiFeCo high entropy alloy

An AlCrCuNiFeCo high entropy alloy (HEA), which has simple face centered cubic (FCC) and body centered cubic (BCC) solid solution phases as the microstructural constituents, was processed and its high temperature deformation behaviour was examined as a function of temperature (700-1030 degrees C) and strain rate (10(-3)-10(-1) s(-1)), so as to identify the optimum thermo-mechanical processing (TMP) conditions for hot working of this alloy. For this purpose, power dissipation efficiency and deformation instability maps utilizing that the dynamic materials model pioneered by Prasad and co-workers have been generated and examined. Various deformation mechanisms, which operate in different temperature-strain rate regimes, were identified with the aid of the maps and complementary microstructural analysis of the deformed specimens. Results indicate two distinct deformation domains within the range of experimental conditions examined, with the combination of 1000 degrees C/10(-3) s(-1) and 1030 degrees C/10(-2) s(-1) being the optimum for hot working. Flow instabilities associated with adiabatic shear banding, or localized plastic flow, and or cracking were found for 700-730 degrees C/10(-3)-10(-1) s(-1) and 750-860 degrees C/10(-1.4)-10(-1) s(-1) combinations. A constitutive equation that describes the flow stress of AlCrCuNiFeCo alloy as a function of strain rate and deformation temperature was also determined. (C) 2014 Elsevier Ltd. All rights reserved.

Tuning photoinduced terahertz conductivity in monolayer graphene: Optical-pump terahertz-probe spectroscopy

Optical-pump terahertz-probe differential transmission measurements of as-prepared single layer graphene (AG) (unintentionally hole dopedwith Fermi energy E-F at similar to -180 meV), nitrogen doping compensated graphene (NDG) with E-F similar to -10 meV, and thermally annealed doped graphene (TAG) are examined quantitatively to understand the opposite signs of photoinduced dynamic terahertz conductivity Delta sigma. It is negative for AG and TAG but positive for NDG. We show that the recently proposed mechanism of multiple generations of secondary hot carriers due to Coulomb interaction of photoexcited carriers with the existing carriers together with the intraband scattering can explain the change of photoinduced conductivity sign and its magnitude. We give a quantitative estimate of Delta sigma in terms of controlling parameters-the Fermi energy E-F and momentum relaxation time tau. Furthermore, the cooling of photoexcited carriers is analyzed using a supercollision model which involves a defect mediated collision of the hot carriers with the acoustic phonons, thus giving an estimate of the deformation potential.

Spinning strings and minimal surfaces in AdS(3) with mixed 3-form fluxes

Motivated by the recent proposal for the S-matrix in AdS(3) x S-3 with mixed three form fluxes, we study classical folded string spinning in AdS(3) with both Ramond and Neveu-Schwarz three form fluxes. We solve the equations of motion of these strings and obtain their dispersion relation to the leading order in the Neveu-Schwarz flux b. We show that dispersion relation for the spinning strings with large spin S acquires a term given by -root lambda/2 pi b(2) log(2) S in addition to the usual root lambda/pi log S term where root lambda is proportional to the square of the radius of AdS(3). Using SO(2, 2) transformations and re-parmetrizations we show that these spinning strings can be related to light like Wilson loops in AdS(3) with Neveu-Schwarz flux b. We observe that the logarithmic divergence in the area of the light like Wilson loop is also deformed by precisely the same coefficient of the b(2) log(2) S term in the dispersion relation of the spinning string. This result indicates that the coefficient of b(2) log(2) S has a property similar to the coefficient of the log S term, known as cusp-anomalous dimension, and can possibly be determined to all orders in the coupling lambda using the recent proposal for the S-matrix.

Impact of Stone-Wales and lattice vacancy defects on the electro-thermal transport of the free standing structure of metallic ZGNR

We report the effect of topological as well as lattice vacancy defects on the electro-thermal transport properties of the metallic zigzag graphene nano ribbons at their ballistic limit. We employ the density function theory-Non equilibrium green's function combination to calculate the transmission details. We then present an elaborated study considering the variation in the electrical current and the heat current transport with the change in temperature as well as the voltage gradient across the nano ribbons. The comparative analysis shows, that in the case of topological defects, such as the Stone-Wales defect, the electrical current transport is minimum. Besides, for the voltage gradient of 0.5 Volt and the temperature gradient of 300 K, the heat current transport reduces by similar to 62 % and similar to 50% for the cases of Stones-Wales defect and lattice vacancy defect respectively, compared to that of the perfect one.

TiO2/ZnO core/shell nano-heterostructure arrays as photo-electrodes with enhanced visible light photoelectrochemical performance

The present article reports a facile method for preparing the vertically-aligned 1D arrays of a new type of type II n-n TiO2/ZnO core/shell nano-heterostructures by growing the nano-shell of ZnO on the electrochemically fabricated TiO2 nanotubes core for visible light driven photoelectrochemical applications. The strong interfacial interaction at the type II heterojunction leads to an effective interfacial charge separation and charge transport. The presence of various defects such as surface states, interface states and other defects in the nano-heterostructure enable it for improved visible light photoelectrochemical performance. The presence of such defects has also been confirmed by the UV-vis absorption, cathodoluminescence, and crystallographic studies. The TiO2/ZnO core/shell nano-heterostructures exhibit strong green luminescence due to the defect transitions. The TiO2/ZnO core/shell nano-heterostructures photo-electrode show significant enhancement of visible light absorption and it provides a photocurrent density of 0.7 mA cm(-2) at 1 V vs. Ag/AgCl, which is almost 2.7 times that of the TiO2/ZnO core/shell nano-heterostructures under dark conditions. The electrochemical impedance spectroscopy results demonstrate that the substantially improved photoelectrochemical and photo-switching performance of the nano-heterostructures photo-anode is because of the enhancement of interfacial charge transfer and the increase in the charge carrier density caused by the incorporation of the ZnO nano-shell on TiO2 nanotube core.