Exact energy eigenvalues for an attractive Coulomb potential with zero-radius hard core

Following a peratization procedure, the exact energy eigenvalues for an attractive Coulomb potential, with a zero-radius hard core, are obtained as roots of a certain combination of di-gamma functions. The physical significance of this entirely new energy spectrum is discussed.

Magnetotransport of Dirac fermions on the surface of a topological insulator

We study the properties of Dirac fermions on the surface of a topological insulator in the presence of crossed electric and magnetic fields. We provide an exact solution to this problem and demonstrate that, in contrast to their counterparts in graphene, these Dirac fermions allow relative tuning of the orbital and Zeeman effects of an applied magnetic field by a crossed electric field along the surface. We also elaborate and extend our earlier results on normal-metal-magnetic film-normal metal (NMN) and normal-metal-barrier-magnetic film (NBM) junctions of topological insulators [S. Mondal, D. Sen, K. Sengupta, and R. Shankar, Phys. Rev. Lett. 104, 046403 (2010)]. For NMN junctions, we show that for Dirac fermions with Fermi velocity vF, the transport can be controlled using the exchange field J of a ferromagnetic film over a region of width d. The conductance of such a junction changes from oscillatory to a monotonically decreasing function of d beyond a critical J which leads to the possible realization of magnetic switches using these junctions. For NBM junctions with a potential barrier of width d and potential V-0, we find that beyond a critical J, the criteria of conductance maxima changes from chi=eV(0)d/h upsilon(F)=n pi to chi=(n+1/2)pi for integer n. Finally, we compute the subgap tunneling conductance of a normal-metal-magnetic film-superconductor junctions on the surface of a topological insulator and show that the position of the peaks of the zero-bias tunneling conductance can be tuned using the magnetization of the ferromagnetic film. We point out that these phenomena have no analogs in either conventional two-dimensional materials or Dirac electrons in graphene and suggest experiments to test our theory.

Energy dispersive backscattering of electrons from surface resonances of a disordered medium and 1/f noise

Anderson localised states in the bulk of a disordered medium appear as sharp resonances near the surface. The resonant backscattering leads to an energy-dependent random time delay for an incident electron. We derive an analytic expression for the delay-time probability distribution at a given energy. This is shown to give a 1/f noise for the surface currents in general.

Surface modifications in single crystal surfaces of YBa2Cu3O7-delta upon high energy ion irradiation

Atomic force microscopy investigations on swift heavy ion (200 MeV An) irradiated surfaces of a high T-c single crystal YBa2Cu3O7-delta are presented. Results obtained revealed an ion-induced erosion/sputtering clearly confirming our earlier observation on grain boundary dominated thin films. Apart from sputtering, notable effects were seen with many defect structures like dikes/hillocks surrounded by craters, dikes, holes, pearl like structures and ripple formation of sub-micron undulations, all in one crystal. Results are discussed in the light of co-operative phenomena of material re-distribution mechanism related to mass transfer and crater formations.

A molecular dynamics study of cage-to-cage migration in sodium Y zeolite: role of surface-mediated diffusion

The details of cage-to-cage migration have been obtained from an analysis of the molecular dynamics trajectory of a probe adsorbate. It is observed that particles utilize the region within a radius of 2 angstrom from the window center but with diffusion taking place predominantly at 1.6 angstrom from the window center and a potential energy of nearly -12 kJ/mol. A barrier of about 0.5 kJ/mol is observed for surface-mediated diffusion. Surprisingly, for diffusion without surface mediation for a particle going from one cage center to another, there is an attractive well near the window instead of a barrier. At low adsorbate concentrations and room temperature, the predominant mode for cage-to-cage migration is surface-mediated diffusion. The analysis suggests that particles slide along the surface of the inner walls of the alpha-cages during migration from one cage to another.

Modelling vacuum arcs : from plasma initiation to surface interactions

A better understanding of vacuum arcs is desirable in many of today's 'big science' projects including linear colliders, fusion devices, and satellite systems. For the Compact Linear Collider (CLIC) design, radio-frequency (RF) breakdowns occurring in accelerating cavities influence efficiency optimisation and cost reduction issues. Studying vacuum arcs both theoretically as well as experimentally under well-defined and reproducible direct-current (DC) conditions is the first step towards exploring RF breakdowns. In this thesis, we have studied Cu DC vacuum arcs with a combination of experiments, a particle-in-cell (PIC) model of the arc plasma, and molecular dynamics (MD) simulations of the subsequent surface damaging mechanism. We have also developed the 2D Arc-PIC code and the physics model incorporated in it, especially for the purpose of modelling the plasma initiation in vacuum arcs. Assuming the presence of a field emitter at the cathode initially, we have identified the conditions for plasma formation and have studied the transitions from field emission stage to a fully developed arc. The 'footing' of the plasma is the cathode spot that supplies the arc continuously with particles; the high-density core of the plasma is located above this cathode spot. Our results have shown that once an arc plasma is initiated, and as long as energy is available, the arc is self-maintaining due to the plasma sheath that ensures enhanced field emission and sputtering. The plasma model can already give an estimate on how the time-to-breakdown changes with the neutral evaporation rate, which is yet to be determined by atomistic simulations. Due to the non-linearity of the problem, we have also performed a code-to-code comparison. The reproducibility of plasma behaviour and time-to-breakdown with independent codes increased confidence in the results presented here. Our MD simulations identified high-flux, high-energy ion bombardment as a possible mechanism forming the early-stage surface damage in vacuum arcs. In this mechanism, sputtering occurs mostly in clusters, as a consequence of overlapping heat spikes. Different-sized experimental and simulated craters were found to be self-similar with a crater depth-to-width ratio of about 0.23 (sim) - 0.26 (exp). Experiments, which we carried out to investigate the energy dependence of DC breakdown properties, point at an intrinsic connection between DC and RF scaling laws and suggest the possibility of accumulative effects influencing the field enhancement factor.

Crystal Structure Engineering by Fine-Tuning the Surface Energy: The Case of CdE (E = S/Se) Nanocrystals

We prove that CdS nanocrystals can be thermodynamically stabilized in both wurtzite and zinc-blende crystallographic phases at will, just by the proper choice of the capping ligand. As a striking demonstration of this, the largest CdS nanocrystals (similar to 15 nm diameter) ever formed with the zinc-blende structure have been synthesized at a high reaction temperature of 310 degrees C, in contrast to previous reports suggesting the formation of zinc-blende CdS only in the small size limit (< 4.5 nm) or at a lower reaction temperature (<= 240 degrees C). Theoretical analysis establishes that the binding energy of trioctylphosphine molecules on the (001) surface of zinc-blende CdS is significantly larger than that for any of the wurtzite planes. Consequently, trioctylphosphine as a capping agent stabilizes the zinc-blende phase via influencing the surface energy that plays an important role in the overall energetics of a nanocrystal. Besides achieving giant zinc-blende CdS nanocrystals, this new understanding allows us to prepare CdSe and CdSe/CdS core/shell nanocrystals in the zinc-blende structure.

An Efficient Robust Algorithm for the Surface-Potential Calculation of Independent DG MOSFET

Although the recently proposed single-implicit-equation-based input voltage equations (IVEs) for the independent double-gate (IDG) MOSFET promise faster computation time than the earlier proposed coupled-equations-based IVEs, it is not clear how those equations could be solved inside a circuit simulator as the conventional Newton-Raphson (NR)-based root finding method will not always converge due to the presence of discontinuity at the G-zero point (GZP) and nonremovable singularities in the trigonometric IVE. In this paper, we propose a unique algorithm to solve those IVEs, which combines the Ridders algorithm with the NR-based technique in order to provide assured convergence for any bias conditions. Studying the IDG MOSFET operation carefully, we apply an optimized initial guess to the NR component and a minimized solution space to the Ridders component in order to achieve rapid convergence, which is very important for circuit simulation. To reduce the computation budget further, we propose a new closed-form solution of the IVEs in the near vicinity of the GZP. The proposed algorithm is tested with different device parameters in the extended range of bias conditions and successfully implemented in a commercial circuit simulator through its Verilog-A interface.

Surface chemical characterisation of Paenibacillus polymyxa before and after adaptation to sulfide minerals

A heterotroph Paenibacillus polymyxa bacteria is adapted to pyrite, chalcopyrite, galena and sphalerite minerals by repeated subculturing the bacteria in the presence of the mineral until their growth characteristics became similar to the growth in the absence of mineral. The unadapted and adapted bacterial surface have been chemically characterised by zeta-potential, contact angle, adherence to hydrocarbons and FT-IR spectroscopic studies. The surface free energies of bacteria have been calculated by following the equation of state and surface tension component approaches. The aim of the present paper is to understand the changes in surface chemical properties of bacteria during adaptation to sulfide minerals and the projected consequences in bioflotation and bioflocculation processes. The mineral-adapted cells became more hydrophilic as compared to unadapted cells. There are no significant changes in the surface charge of bacteria before and after adaptation, and all the bacteria exhibit an iso-electric point below pH 2.5. The contact angles are observed to be more reliable for hydrophobicity assessment than the adherence to hydrocarbons. The Lifschitz–van der Waals/acid–base approach to calculate surface free energy is found to be relevant for mineral–bacteria interactions. The diffuse reflectance FT-IR absorbance bands for all the bacteria are the same illustrating similar surface chemical composition. However, the intensity of the bands for unadapted and adapted cells is significantly varied and this is due to different amounts of bacterial secretions underlying different growth conditions.

The effect of nature of raw coal on the adhesion of bacteria to coal surface

The surface properties of coal and solution pH play a major role in determining the adhesion of microorganisms. In this study, three Indian coal samples with different compositions have been used and the adhesion of the bacterium Bacillus polymyxa to these coals has been investigated. It was found that due to the high ash content of coal, the zeta-potential was negative over most of the pH range which is close to the values exhibited by pure quartz as well as B. polymyxa. Similarly, the surface free energy components of coal (derived from contact angle measurements) showed that the electron-donor component increased with ash content. Adhesion experiments revealed that maximum adhesion of the bacterium B. polymyxa occurred on to the coal samples around the point-of-zero-charge of the coal and the bacterium i.e. about pH 2. Further, adhesion was found to be dependent on the ash content and the surface free energy of the coals. (C) 2002 Published by Elsevier Science Ltd.

Potential and Electric Field Profiles for Transmission line Insulators

Transmission of bulk power at high voltages over very long distances has become very imperative. At present, throughout the globe, this task has been mostly performed by overhead transmission lines. The dual task of mechanically supporting and electrically isolating the live phase conductors from the support tower is performed by string insulators. Whether in clean condition or under polluted conditions, the electrical stress distribution along the insulators governs the possible flashover, which is quite detrimental to the system. However, a reliable data on stress distribution in commonly employed string insulators are rather scarce. Considering this, the present work has made an attempt to study accurately, the field distribution in 220 kV strings for six different types of porcelain/ceramic insulators (Normal and Antifog discs) used for high voltage transmission. The surface charge simulation method is employed for the required field computation. Voltage and electric stress distribution is deduced and compared across different types of discs. A comparison on normalised surface resistance, which is an indicator for the stress concentration under polluted condition, is also attempted.

Potential of biomass energy for electricity generation in sub-Saharan Africa

The paper explores the biomass based power generation potential of Africa. Access to electricity in sub-Saharan Africa (SSA) is about 26% and falls to less than 1% in the rural areas. On the basis of the agricultural and forest produce of this region, the residues generated after processing are estimated for all the countries. The paper also addresses the use of gasification technology - an efficient thermo-chemical process for distributed power generation - either to replace fossil fuel in an existing diesel engine based power generation system or to generate electricity using a gas engine. This approach enables the implementation of electrification programs in the rural sector and gives access to grid quality power. This study estimates power generation potential at about 5000 MW and 10,000 MW by using 30% of residues generated during agro processing and 10% of forest residues from the wood processing industry, respectively. A power generation potential of 15000 MW could generate 100 terawatt-hours (TWh), about 15% of current generation in SSA. The paper also summarizes some of the experience in using the biomass gasification technology for power generation in Africa and India. The paper also highlights the techno economics and key barriers to promotion of biomass energy in sub-Saharan Africa. (C) 2011 International Energy Initiative. Published by Elsevier Inc. All rights reserved.