Department of Physics

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The authors provide two routes to increase the photocurrent of organic photodetectors that are based on electron-donor and electron-acceptor materials. During device fabrication, they introduce carbon nanotubes, which act as channels for electron transport. The channels in effect provide an approach to address the problem of low carrier mobility in the conjugated organics. The authors also make use of the (electrically) bistable nature of the acceptor material, namely, rose bengal in a favorable way. During device characterization, they switch its conductivity to decrease device resistance and yield higher photocurrent. Both the routes enhance photocurrent in these donor-acceptor-type photodetectors. © 2007 American Institute of Physics.

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Applied Physics Letters

Organic photodetectors with electrically bistable electron acceptors and nanotubes

The stress patterns relevant for the assessment of the post production service behaviour of plain carbon hot strip mill plates were evaluated on the surface and at various depths. The hole drilling strain gage (HDSG) is a standard, well proven, semi-destructive technique and is employed for the purpose. However, this technique requires skill and is slow in operation. The magnetic Barkhausen technique (MBE) is non destructive and is still in an experimental stage, but has tremendous potential and has been tried here in steel plant environment. The objective is to establish whether MBE technique could be used as an alternative to HDSG technique. The calibration for MBE results was done under controlled compressive and tensile loading. The stress profiles were determined at six locations along rolling and transverse directions using these two techniques. The rolling stresses using HDSG method were found to be tensile near the surface, increase with depth and saturate at 0.5 to 1 mm depths. The transverse stresses were low tensile or compressive near the surface and saturate to low tensile values at similar depths. The stresses (averaged over a certain depth) by MBE are tensile in rolling direction and compressive in transverse direction. Correlation plots between residual stresses by HDSG method up to different depths and average stress by MBE show that that a good correlation exists at 0.35 mm depth. Beyond this depth, correlation is not good. It is true that for the material of the sheet and the frequency employed, the MBE results are not relevant beyond this depth. Also, the equivalent uniform stress as measured by HDSG method would deviate more and more from the actual stress as we go deeper. © Springer Science+Business Media, LLC 2007.

Journal of Nondestructive Evaluation

Evaluation and correlation of residual stress measurement in steel

The paper addresses the structural, crystallization, soft magnetic and Curie temperature behaviour of Co36Fe36Si4B20Nb4 alloy. The material, prepared in the form of ribbons by melt-spinning technique, was amorphous in the as-cast state. Differential scanning calorimetry (DSC) showed two stages of crystallization whereas thermal variation of electrical resistivity (TER) carried out to a higher range of temperature indicated three stages of crystallization. The first crystallization stage, which occurred at 845 K and 825 K in DSC and TER, respectively, was due to the formation of nanophase (CoFe)2Si as evidenced by X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The formation of these nanoparticles reduced the magnetocrystalline anisotropy, thereby revealing good soft magnetic properties in the samples annealed between 825 K and 875 K with coercivity less than 49.9 A m-1 (627 mOe) and susceptibility 0.72 103. In this optimum nanocrystalline state, the material also exhibited a high Curie temperature above 1100 K, opening the scope of the present nanocrystalline alloy for high temperature applications.

Philosophical Magazine

A potential Co36Fe36Si4B 20Nb4 nanocrystalline alloy for high temperature soft magnetic applications

The experimental work reported here is a validation of Langmuir probe measurements against emission spectroscopic and discharge resistivity measurements of atmospheric glow discharge plasma parameters. The probe measurements of plasma density are close to that obtained by spectroscopy and discharge resistivity measurement. However, it is found that the electron temperature measurement by the probe is higher than that obtained by the spectroscopic technique. Variation of plasma parameters with operating voltage and frequency, which are estimated by the three techniques, shows a similar trend. © 2007 IEEE.

IEEE Transactions on Plasma Science

Characterization of atmospheric pressure glow discharge in helium using langmuir probe, emission spectroscopy, and discharge resistivity

The authors demonstrate thin-film formation of capped-CdSe nanoparticles via layer-by-layer electrostatic assembly. The assembly of two types of nanoparticles in sequence-with anionic and cationic capping agents, respectively-results in thin films of CdSe nanoparticles. Devices based on such thin films demonstrate electrical bistability. The bistability, which is reversible in nature, is due to charge confinement in the nanoparticles and has an associated memory phenomenon. The devices based on the CdSe nanoparticles exhibit high on/off ratio and demonstrate read-only and random-access memory applications. © 2007 American Institute of Physics.

Layer-by-layer assembly of capped CdSe nanoparticles: Electrical bistability and memory phenomenon

Marginal glass forming Al-based alloys have attracted considerable attention due to the occurrence of a primary crystallization reaction that yields a microstructure consisting of uniformly dispersed Al-nanocrystals embedded in an amorphous matrix. Rapidly solidified Al-alloys ribbons containing 8 at.% Ni and 4-6 at.% La were prepared by single roller melt-spinning technique. As-melt-spun and annealed ribbons were investigated by means of differential scanning calorimetry (DSC), X-ray diffractometry (XRD) and hardness measurements. XRD studies revealed that ribbons are fully amorphous in the as-melt-spun state. DSC studies showed that crystallization proceeds in two or three stages that are sensitively influenced by La content. Crystallizations kinetics were analysed by Kissinger and Johnson-Mehl-Avrami approaches. Microhardness of all the ribbons was examined at different temperatures and correlated with the corresponding structural evolution.

Materials and Manufacturing Processes

Kinetic aspects of the nanocrystallization and evolution of microhardness in Al92-xNi8Lax amorphous alloys

A first-principles study of the effects of spin-orbit coupling (SOI) on the structural, electronic and optical properties of 16 bismuth nanowires, Bi n with n = 7-18, has been performed. The density functional theory (DFT) in the local density approximation (LDA) has been used. The inclusion of the SOI significantly alters the electronic and optical properties of the wires. The stable structures for the Bin wires with n = 7-18 form two groups: non-helical and helical configurations. In addition to the most stable non-helical 5-Bi pentagonal, 6-Bi hexagonal and 6-Bi triple-zigzag wire configurations found in a previous report, the present study adds to this list three more non-helical structures, namely the non-helical 7-Bi hexagonal, 8-Bi heptagonal and 11-Bi pentagonal cross-sectional wire configurations. The present result is in sharp contrast to the conclusions of our previous studies of Pb- and Tl-nanowires, where it was observed that, in general, a structure possessing a high coordination number value has large binding energy and, therefore, the helical wire structures are the most stable ones. All of the wires are metallic in the LDA. The number of channels in the nanowires is large which will lead to high quantum ballistic conduction. The consideration of the many body effects such as the GW approximation (GWA) may destroy the metallicity predicted here in the Bin wire configurations for n≤7. However, for the wire configurations having n≥8, we find that even in the GWA, one may not observe the opening of the energy gaps because of the violation of the electron counting principle. The optical absorption calculated with SOI is much stronger compared to the one obtained after neglecting the SOI. For the wires containing a large number of atoms in the unit cell, the optical absorption is multi-peaked, strong and extended over the whole energy region from infrared to the ultraviolet electromagnetic radiation including the visible region. These nanowires may thus be used as a source of white radiation. © IOP Publishing Ltd.

Nanotechnology

Effect of spin-orbit interaction on the electronic and optical properties of ultrathin bismuth nanowires - A density functional approach

We consider the effect of new physics on the branching ratio of B s → l+l-γ where l = e, μ. If the new physics is of the form scalar/pseudoscalar, then it makes no contribution to Bs → l+l-γ, unlike in the case of Bs → l+l-, where it can potentially make a very large contribution. If the new physics is in the form of vector/axial-vector operators, then the present data on B → (K, K*)l+l- does not allow a large enhancement for B(Bs → l+l-γ). If the new physics is in the form of tensor/pseudotensor operators, then the data on B → (K, K*)l+l- gives no useful constraint but the data on B → K*γ does. Here again, a large enhancement of B(Bs → l+l-γ), much beyond the Standard Model expectation, is not possible. Hence, we conclude that the present data on b → s transitions allow a large boost in B(Bs → l +l-) but not in B(Bs → l+l -γ). © World Scientific Publishing Company.

Modern Physics Letters A

New physics upper bound on the branching ratio of Bs → l+l-γ

We consider the most general new physics effective Lagrangian for b → s l+ l-. We derive the upper limit on the branching ratio for the processes Bs → l+ l- where l = e, μ, subject to the current experimental bounds on related processes, B → (K, K*) l+ l-. If the new physics interactions are of vector/axial-vector form, the present measured rates for B → (K, K*) l+ l- constrain B (Bs → l+ l-) to be of the same order of magnitude as their respective Standard Model (SM) predictions. On the other hand, if the new physics interactions are of scalar/pseudoscalar form, B → (K, K*) l+ l- rates do not impose any useful constraint on B (Bs → l+ l-) and the branching ratios of these decays can be as large as present experimental upper bounds. If future experiments measure B (Bs → l+ l-) to be ≥ 10-8 then the new physics giving rise to these decays has to be of the scalar/pseudoscalar form. We also consider the effect of new physics on B (Bs → l+ l- γ) subject to the present experimental constraints on B → (K, K*) l+ l- and B → K* γ. New physics in form scalar/pseudoscalar, which makes a very large contribution to Bs → l+ l-, makes no contribution at all to Bs → l+ l- γ due to angular momentum conservation. New Physics in the form of vector/axial-vector operators is constrained by the data on B → (K, K*) l+ l- and new physics in the form of tensor/pseudo-tensor is constrained by the data on B → K* γ. In both cases, enhancement of B (Bs → l+ l- γ) much beyond the SM expectation is impossible. In conclusion, present data on B → (K, K*) transitions allow for large B (Bs → l+ l-) but do not allow B (Bs → l+ l- γ) to be much larger than its SM expectation. © 2007 Elsevier B.V. All rights reserved.

Nuclear Physics B - Proceedings Supplements

New physics upper bound on the branching ratio of Bs → l+ l- and Bs → l+ l- γ

We study some discrete symmetries of unbiased (Hadamard) and biased quantum walk on a line, which are shown to hold even when the quantum walker is subjected to environmental effects. The noise models considered in order to account for these effects are the phase flip, bit flip, and generalized amplitude damping channels. The numerical solutions are obtained by evolving the density matrix, but the persistence of the symmetries in the presence of noise is proved using the quantum trajectories approach. We also briefly extend these studies to quantum walk on a cycle. These investigations can be relevant to the implementation of quantum walks in various known physical systems. We discuss the implementation in the case of NMR quantum information processor and ultracold atoms. © 2007 The American Physical Society.