• Issue front cover thumbnail

      Volume 69, Issue 4

      October 2007,   pages  491-692b

    • Fifty years of Szigeti's dielectric theory – A review

      D B Sirdeshmukh L Sirdeshmukh K G Subhadra

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      During the period 1949–1961 Szigeti published four seminal papers on the dielectric behaviour of crystals. Szigeti's theory is applicable to isotropic and anisotropic, ionic and covalent crystals with different structures. Szigeti's theory connects dielectric, spectroscopic and elastic properties. An important outcome of Szigeti's theory is the concept of the effective ionic charge (𝑠). It is pointed out that 𝑠 correlates with a number of physical properties and is a measure of ionicity of the interatomic bond. Since Szigeti's work, several theoretical models have been proposed to account for the fact that $s < 1$. These models provide an insight into the complex polarization mechanisms in solids. This review summarizes Szigeti's work and the work that followed; the implications and applications of Szigeti's theory are discussed. Some new results are also included.

    • Charmless nonleptonic 𝐵 decays into scalar and pseudoscalar mesons

      B Mawlong

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      The charmless nonleptonic decay modes $B \rightarrow f_{0} K(\pi)$ involving a scalar and a pseudoscalar meson in the final state are studied. The scalar meson $f_{0}$ is considered as a $q\bar{q}$ state, as favored by some recent studies. Using the generalized factorization approach, the branching ratios and CP violation parameters are computed for these modes. The form factors are calculated using the results from relativistic light front quark model and the ISGW2 model. It is found that the direct CP violation parameters in these modes are small. However, the obtained branching ratios are not in agreement with the experimental data. Therefore, these modes may be considered as possible probes for new physics.

    • Deviation from tri-bimaximal mixings through flavour twisters in inverted and normal hierarchical neutrino mass models

      N Nimai Singh Monisa Rajkhowa Abhijit Borah

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      We explore a novel possibility for lowering the solar mixing angle ($\theta_{12}$) from tri-bimaximal mixings, without sacrificing the predictions of maximal atmospheric mixing angle ($\theta_{23} = 45^{\circ}$) and zero reactor angle ($\theta_{13} = 0^{\circ}$) in the inverted and normal hierarchical neutrino mass models having 2-3 symmetry. This can be done through the identification of a flavour twister term in the texture of neutrino mass matrix and the variation of such term leads to lowering of solar mixing angle. For the observed ranges of $\Delta m_{21}^{2}$ and $\Delta m_{23}^{2}$, we calculate the predictions on tan2 $\theta_{12} = 0.5, 0.45, 0.35$ for different input values of the parameters in the neutrino mass matrix. We also observe a possible transition from inverted hierarchical model having even CP parity (Type-IHA) to inverted hierarchical model having odd CP parity (Type-IHB) in the first two mass eigenvalues, when there is a change in input values of parameters in the same mass matrix. The present work differs from the conventional approaches for the deviations from tri-bimaximal mixing, where the 2-3 symmetry is broken, leading to $\theta_{23} \neq 45^{\circ}$ and $\theta_{13} \neq 0^{\circ}$.

    • Development of 2D particle-in-cell code to simulate high current, low energy beam in a beam transport system

      S C L Srivastava S V L S Rao P Singh

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      A code for 2D space-charge dominated beam dynamics study in beam transport lines is developed. The code is used for particle-in-cell (PIC) simulation of 𝑧-uniform beam in a channel containing solenoids and drift space. It can also simulate a transport line where quadrupoles are used for focusing the beam. Numerical techniques as well as the results of beam dynamics studies are presented in the paper.

    • Particularization of alpha contamination using CR-39 track detectors

      M F Zaki Y H El-Shaer

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      Solid-state nuclear track detectors have found wide use in various domains of science and technology, e.g. in environmental experiments. The measurement of alpha activity on sources in an environment, such as air is not easy because of short penetration range of alpha particles. Furthermore, measurement of alpha activity by most gas ionization detectors suffers from high background induced by the accompanying gamma radiation. Solid state nuclear track detectors (SSNTDs) have been used successfully as detecting devices and as a passive system to detect alpha contamination on different surfaces. This work presents the response of CR-39 (for two types) to alpha particles from two sources, 238Pu with energy 5 MeV and 241Am with energy 5.4 MeV. The methods of etching and counting are investigated, along with the achievable linearity, efficiency and reproducibility. The sensitivity to low activity and energy resolution are studied.

    • Perturbative effects on ultra-short soliton self-switching

      Amarendra K Sarma Ajit Kumar

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      A numerical study of ultra-short self-soliton switching along with the corresponding analysis of coupler parameters is carried out for a Kerr coupler with intermodal dispersion. The influence of perturbations like third-order dispersion, self-steepening and intrapulse Raman scattering, on switching characteristics is also studied.

    • Interionic pair potentials and partial structure factors of compound-forming quaternary NaSn liquid alloy: First principle approach

      Anil Thakur P K Ahluwalia

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      In this paper formulae for partial structure factors have been used to study partial structure factors of compound-forming quaternary liquid alloys by considering Hoshino's m-component hard-sphere mixture, which is based on Percus-Yevic equation of Hiroike. Formulae are applied to NaSn (Na, Sn, NaSn, Na3Sn) which is considered as a quaternary liquid mixture with the formation of two compounds simultaneously. We have compared the total structure factors for ternary and quaternary alloys with experimental total structure factors which are found to be in good agreement. This suggests that, for suitable stoichiometric composition, two compounds are formed simultaneously. The hard-sphere diameters needed have been calculated using Troullier and Martins ab-initio pseudopotentials.

    • Surface segregation of the metal impurity to the (100) surface of fcc metals

      Jian-Min Zhang Bo Wang Ke-Wei Xu

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      The surface segregation energies for a single metal impurity to the (100) surface of nine fcc metals (Cu, Ag, Au, Ni, Pd, Pt, Rh, Al and Ir) have been calculated using the MAEAM and molecular dynamics (MD) simulation. The results show that the effect of the surface is down to the fourth-layer and an oscillatory or monotonic damping $(|E_{1}| > |E_{2}| > |E_{3}| > |E_{4}|)$ phenomenon in segregation energy has been obtained. The absolute value of the segregation energy $E_{1}$ for a single impurity in the first atomic layer is much higher than that in the nether layers. Thus, whether the surface segregation will work or not is mainly determined by $E_{1}$ which is in good relation to the differences in surface energy between the impurity and host crystals $\Delta Q = Q_{\text{imp}} - Q_{\text{hos}}$. So we conclude that an impurity with lower surface energy will segregate to the surface of the host with higher surface energy.

    • Creation evidence of the second non-dispersive Zakharenko wave by helium atomic beams in superfluid helium-II at low temperatures

      A A Zakharenko

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      In this work, the experimental results of the creation of the second non-dispersive Zakharenko wave ($C_{ph} = C_{g} \neq 0$) in the negative roton branch (the so-called second sound) of the bulk elementary excitations (BEEs) energy spectra are introduced. Several BEE signals detected by a bolometer situated in the isotopically pure liquid helium-II at low temperatures $\sim 100$ mK are shown, which give evidence of negative roton creation in the liquid by helium atomic beams striking the liquid surface. The negative roton signals were clearly distinguished by the following ways: the negative roton signal created by helium atomic beams appeared earlier than the positive roton signal created by the beams, and presence of both positive and negative roton signals together. It is natural that the negative roton creation by the beams requires the 4He-atom energies $\sim 12$ K, while the positive roton creation by the atomic beams requires energies $\sim 35$ K. Therefore, successive increase in the heater power resulting in an increase in the 4He-atom energies gives solid evidence that the negative rotons are first created in the liquid by the helium atomic beams.

    • A variable electron beam and its irradiation effect on optical and electrical properties of CdS thin films

      M Singh Y K Vijay B K Sharma

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      A low energy electron accelerator has been constructed and tested. The electron beam can operate in low energy mode (100 eV to 10 keV) having a beam diameter of 8–10 mm. Thin films of CdS having thickness of 100 nm deposited on ITO-coated glass substrate by thermal evaporation method have been irradiated by electron beam in the above instrument. The $I–V$ characteristic is found to be nonlinear before electron irradiation and linear after electron irradiation. The TEP measurement confirms the n-type nature of the material. The TEP and $I–V$ measurements also confirm the modification of ITO/CdS interface with electron irradiation.

    • A general approach to bosonization

      Girish S Setulur V Meera

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      We summarize recent developments in the field of higher dimensional bosonization made by Setlur and collaborators and propose a general formula for the field operator in terms of currents and densities in one dimension using a new ingredient known as a `singular complex number'. Using this formalism, we compute the Green function of the homogeneous electron gas in one spatial dimension with short-range interaction leading to the Luttinger liquid and also with long-range interactions that lead to a Wigner crystal whose momentum distribution computed recently exhibits essential singularities. We generalize the formalism to finite temperature by combining with the author's hydrodynamic approach. The one-particle Green function of this system with essential singularities cannot be easily computed using the traditional approach to bosonization which involves the introduction of momentum cutoffs, hence the more general approach of the present formalism is proposed as a suitable alternative.

    • Density of states in an electrically biased quantum well

      A Khan S Sinha P Panchadhyayee

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      Density of states in a quantum well has been studied in the presence of an electric field applied perpendicular to the growth direction. We have shown that an extra quantization is introduced to the motion of the electron due to the discrete energy levels known as Wannier–Stark ladder states and the nature of density of electronic states changes from quasi two-dimensional to quasi one-dimensional.

    • Electron transport through SWNT/trans-PA/ SWNT structure (the role of solitons): A t-matrix technique

      S A Ketabi H Milani Moghaddam N Shahtamasebi

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      Using a tight-binding model and a transfer-matrix technique, we numerically investigate the effects of the coupling strength and the role of solitons on the electronic transmission through a system in which trans-polyacetylene (trans-PA) molecule is sandwiched between two semi-infinite single-walled carbon nanotubes (SWNT). We rely on Landauer formalism as the basis for studying the conductance properties of this system. Our calculations show that the solitons play an important role in the response of this system causing a large enhancement in the conductance. Also our results suggest that the conductance is sensitive to the CNT/molecule coupling strength.

    • Neutron response study using poly allyl diglycol carbonate

      Basma A El-Badry M F Zaki Tarek M Hegazy A Ahmed Morsy

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      The results of an experimental work aimed at improving the performance of the CR-39 nuclear track detector for neutron dosimetry applications are reported. A set of CR-39 plastic detectors was exposed to 252Cf neutron source, which has the emission rate of $0.68 \times 10^{8}$ s-1, and neutron dose equivalent rate 1 m apart from the source is equal to 3.8 mrem/h. The detection of fast neutrons performed with CR-39 detector foils, subsequent chemical etching and evaluation of the etched tracks by an automatic track counting system was studied. It is found that the track density increases with the increase of neutron dose and etching time. The track density in the detector is directly proportional to the neutron fluence producing the recoil tracks, provided the track density is in the countable range. This fact plays an important role in determining the equivalent dose in the field of neutron dosimetry. These results are compared with previous work. It is found that our results are in good agreement with their investigations.

    • Normal coordinate analysis and quantum chemical study of tris(𝑝-fluorophenyl)antimony di(𝑁-phenylglycinate) [(𝑝-FC6H4)3Sb(O2CCH2NHC6H5)2]

      Tanveer Hasan P K Singh K Singhal P Raj Neeraj Misra

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      A complete normal coordinate analysis was performed by two different methods: a classical Wilson's G-F matrix method and the semi-empirical molecular orbital PM3 method, for a five coordinate tris(𝑝-fluorophenyl)antimony di(𝑁 -phenylglycinate) [(𝑝-FC6H4)3Sb(O2CCH2NHC6H5)2], known to be an in vitro antitumour molecule.

    • Composition dependence of density of states in a-Se$_{100−x}$Sn$_{x}$ thin films

      N Sharma S P Singh S Kumar

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      The present paper reports the DC conductivity measurements at high electric fields in vacuum evaporated amorphous thin films of a-Se$_{100−x}$Sn$_{x}$ ($x = 0, 2, 4, 6$) glassy alloys. Current–voltage ($I–V$) characteristics have been measured at various fixed temperatures. In these samples, at low electric fields, ohmic behaviour is observed. However, at high electric fields ($E \sim 10^{4}$ V/cm), non-ohmic behaviour is observed. An analysis of the experimental data confirms the presence of space charge limited conduction (SCLC) in the glassy materials studied in the present case. From the fitting of the data to the theory of SCLC, the density of defect states (DOS) near Fermi level is calculated. Increase in DOS with increase in Sn concentration has been found which could be correlated with the electronegativity difference between the two elements used here in making the glassy alloys. The peculiar role of the element Sn as an impurity in the pure Se glassy alloy is also discussed.

    • Estimation of various scattering parameters and 2-DEG mobilities from electron mobility calculations in the three conduction bands 𝛤, L and X of gallium arsenide

      Sonal Singhal A K Saxena S Dasgupta

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      The electron drift mobility in 𝛤 conduction band of GaAs has been calculated before, but for the first time, we have made attempts to estimate the electron mobilities in higher energy L and X minima. We have also calculated the value of mobility of two-dimensional electron gas needed to predict hetero-structure device characteristics using GaAs. Best scattering parameters have been derived by close comparison between experimental and theoretical mobilities. Room temperature electron mobilities in 𝛤, L and X valleys are found to be nearly 9094, 945 and 247 cm2 /V-s respectively. For the above valleys, the electron masses, deformation potentials and polar phonon temperatures have been determined to be (0.067, 0.22, 0.39m 0 ), (8.5, 9.5, 6.5 eV), and (416, 382, 542 K) as best values, respectively. The 2-DEG electron mobility in 𝛤 minimum increases to $1.54 \times 10^{6}$ from $1.59 \times 10^{5}$ cm2 /V-s (for impurity concentration of 1014 cm-3) at 10 K. Similarly, the 2-DEG electron mobility values in L and X minima are estimated to be $2.28 \times 10^{5}$ and $1.44 \times 10^{5}$ cm2 /V-s at 10 K, which are about $\sim 4.5$ and $\sim 3.9$ times higher than normal value with impurity scattering present.

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