Department of Physics
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Item (e,2e) Impact Ionization Processes for Surface Science(Springer, 2018-04) Choubisa, RakeshWe present a scattering theoretic approach to the calculation of the cross-section of (e,2e) impact spectroscopy where all the electrons involved are treated within the real space multiple scattering framework. This approach is particularly suited to the reflection geometry at low kinetic energies, with the ejection of a core-level electron. In this case, we expect (e,2e) spectroscopy can be turned into an extremely sensitive surface structure probe.Item Photoluminescence spectroscopy of many-body effects in heavily doped AlxGa1−xAs(APS, 2005-06) Sarkar, NiladriWe present an experimental investigation of heavy doping-induced many-body effects such as band gap narrowing and Fermi-edge singularity (FES) in AlxGa1−xAs using photoluminescence (PL) spectroscopy. The band-to-band transition energy shifts to lower energies and the FES feature in PL spectra grows with increasing electron concentration. We show that the FES feature is a nonmonotonic function of temperature and excitation intensity. Our data lead us to suggest that a small concentration of nonequilibrated holes is required to enhance the FES feature in the PL spectra.Item Opening of DNA chain due to force applied on different locations(APS, 2016-09) Singh, NavinWe consider a homogeneous DNA molecule and investigate the effect of random force applied on the unzipping profile of the molecule. How the critical force varies as a function of the chain length or number of base pairs is the objective of this study. In general, the ratio of the critical forces that is applied on the middle of the chain to that which is applied on one of the ends is two. Our study shows that this ratio depends on the length of the chain. This means that the force which is applied to a point can be experienced by a section of the chain. Beyond a length, the base pairs have no information about the applied force. In the case when the chain length is shorter than this length, this ratio may vary. Only in the case when the chain length exceeds a critical length, this ratio is found to be two. Based on the de Gennes formulation, we developed a method to calculate these forces at zero temperature. The exact results at zero temperature match numerical calculations.Item Optical spectroscopy of x-ray-selected young stars in the carina nebula(IOP, 2015-12) Vaidya, KausharWe present low-resolution optical spectra for 29 X-ray sources identified as either massive star candidates or low-mass pre-main-sequence (PMS) star candidates in the clusters Trumpler 16 and Trumpler 14 of the Carina Nebula. Spectra of two more objects (one with an X-ray counterpart, and one with no X-ray counterpart), not originally our targets, but found close (∼3'') to two of our targets, are presented as well. Twenty early-type stars, including an O8 star, seven B1–B2 stars, two B3 stars, a B5 star, and nine emission-line stars, are identified. Eleven T Tauri stars, including eight classical T Tauri stars (CTTSs) and three weak-lined T Tauri stars, are identified. The early-type stars in our sample are more reddened compared to the previously known OB stars of the region. The Chandra hardness ratios of our T Tauri stars are found to be consistent with the Chandra hardness ratios of T Tauri stars of the Orion Nebula Cluster. Most early-type stars are found to be nonvariable in X-ray emission, except the B2 star J104518.81–594217.9, the B3 star J104507.84–594134.0, and the Ae star J104424.76–594555.0, which are possible X-ray variables. J104452.20–594155.1, a CTTS, is among the brightest and the hardest X-ray sources in our sample, appears to be a variable, and shows a strong X-ray flare. The mean optical and near-infrared photometric variability in the V and Ks bands, of all sources, is found to be ∼0.04 and 0.05 mag, respectively. The T Tauri stars show significantly larger mean variation, ∼0.1 mag, in the Ks band. The addition of one O star and seven B1–B2 stars reported here contributes to an 11% increase of the known OB population in the observed field. The 11 T Tauri stars are the first ever confirmed low-mass PMS stars in the Carina Nebula region.Item A study of the effect of annealing temperatures and time periods on the hyperfine field distributions of Fe79B16Si5 metallic glass near and below the crystallization temperature was carried out using 57Fe Mössbauer spectroscopy. The effect of crystallization during annealing as a function of annealing time on the average hyperfine magnetic field, , and the relative change of the probability of hyperfine fields, P(H), with respect to that of the as-received sample are also discussed.(Springer, 1999-11) Bandyopadhyay, DebashisThis paper reports 57Fe Mössbauer spectroscopic studies of the polycrystalline samples of the substitutionally disordered, isostructural (fcc), ternary alloy system Fe80-xNixCr20 for x = 30, 26, 19 and 14 in the temperature range of 10–295 K. The data have been analyzed in terms of the magnetic phase transitions occurring in these alloys by examining the temperature dependence of the various Mössbauer parameters like line‐width, center shift, resonance area, distribution of hyperfine field, P(H), and the average hyperfine field 〈H〉. An estimate of the magnetic transition temperature T C is made for the alloys with x = 30 and 26 and these results are compared with those previously obtained by magnetic measurements and neutron diffraction experiments. The data for the second order Doppler shift have been analyzed to estimate the Debye temperature ΘD for alloys with x = 30 and 26.Item Purpose-led Publishing logo. Study of hyperfine-field distributions and the lattice dynamics of Fe50Ni30Cr20 alloy by using 57Fe Mössbauer spectroscopy(IOP, 1999) Bandyopadhyay, DebashisThis paper reports a 57Fe Mössbauer spectroscopic study of a polycrystalline sample of the substitutionally disordered, isostructural (fcc), ternary alloy Fe50Ni30Cr20 over the temperature range 10 to 295 K. The data have been analysed in terms of the magnetic phase transitions occurring in the alloy by examining the temperature dependence of the Mössbauer parameters. The Curie temperature has been determined by fitting the average hyperfine-field values obtained at different temperatures. In order to achieve a detailed understanding of the nature of the hyperfine-field distribution corresponding to the Mössbauer spectra of this alloy, the curves representing the probabilities of the hyperfine-field distributions versus the hyperfine field have been refitted by using two independent Gaussians. On the basis of these fits, the local magnetic ordering of this alloy at low temperature has been described. The data on the second-order Doppler shift have been analysed in order to obtain an estimate of the Debye temperature, ΘD, and a description of the lattice dynamics.Item Time resolved fluorescence spectroscopy of quinine sulphate, quinidine and 6-methoxyquinoline: pH dependence(Elsevier, 1992-03) Pant, Debi D.The excited state dynamics of quinine sulphate (QS), quinidine (Qd) and 6-methoxyquinoline (6MQ) has been studied as a function of pH in steady state and nanosecond time resolved fluorescence experiments. The solvent relaxation process is a dominant process for all the molecules studied, irrespective of pH. Moreover, 6MQ undergoes a proton transfer reaction in the excited state at pH 7 whereas QS and Qd do not exhibit excited state protonation.Item Time resolved fluorescence spectroscopy of quinine sulphate, quinidine and 6-methoxyquinoline: pH dependence(Elsevier, 1992-03) Pant, Debi D.The excited state dynamics of quinine sulphate (QS), quinidine (Qd) and 6-methoxyquinoline (6MQ) has been studied as a function of pH in steady state and nanosecond time resolved fluorescence experiments. The solvent relaxation process is a dominant process for all the molecules studied, irrespective of pH. Moreover, 6MQ undergoes a proton transfer reaction in the excited state at pH 7 whereas QS and Qd do not exhibit excited state protonation.Item Li{sub 2}SO{sub 4}−Li{sub 2}O−P{sub 2}O{sub 5} ionic glass dispersed with [Bmim] [PF{sub 6}] ionic liquid: Electrical transport and thermal stability investigations(Office of Scientific and Technical Information, 2014) Dalvi, AnshumanA fast ionic composite is prepared by dispersion of Ionic liquid [Bmim][PF{sub 6}] in Li{sub 2}SO{sub 4}−Li{sub 2}O−P{sub 2}O{sub 5} glass matrix by mixing and through grinding. Amorphous/glassy nature of the samples is confirmed by X-Ray diffraction (XRD). Surprisingly, the electrical conductivity of the samples is found to be increasing by ∼ 2 orders of magnitude and exhibits typical Arrhenius behavior with low activation energy. DC polarization and impedance spectroscopy measurements suggest that samples are essentially ionic in nature. The conductivity isotherms were also obtained at different temperatures (T < 100 °C) and found to be appreciably stable at least for ∼ 10 days.Item Synthesis of mixed ionic–electronic Li+–NASICON glass-ceramic nanocomposites for cathode applications(Springer, 2020-06) Dalvi, AnshumanA novel route for preparation of mixed ionic–electronic (MIE) glass-ceramics is proposed. Glass-ceramic nanocomposites are prepared by externally dispersing MIE glass viz. [Li2O]x-[V2O5-P2O5]100 − x (where x = 10 and 20 wt% with and the ratio of V2O5 and P2O5 has been kept to 9:1 and 3:1) in the NASICON-structured LiTi2(PO4)3 (LTP) matrix using mechanical milling-assisted synthesis route. After subjecting these glass-ceramic composites to suitable sintering temperature, the samples are structurally and electrically characterized using XRD, FESEM, and impedance spectroscopy. The maximum conductivity (100 °C) is found to be 5 × 10−5 Ω−1 cm−1 for an LTP-glass-ceramic composite, which is significantly higher than that of LTP. Impedance spectroscopy, electronic conductivity, and cyclic voltammetry scans strongly suggest simultaneous transport of ions and electrons. These composites exhibit thermal stability up to ~250 °C. These mixed conductors are found to be potential candidates as electrode/cathode materials in all-solid-state Li+ ion batteries.