Department of Physics
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Item Randomness of random networks: A random matrix analysis(IOP, 2009-09) Bandyopadhyay, DebashisWe analyze complex networks under the random matrix theory framework. Particularly, we show that Δ3 statistics, which gives information about the long-range correlations among eigenvalues, provides a measure of randomness in networks. As networks deviate from the regular structure, Δ3 follows the random matrix prediction of logarithmic behavior (i.e., ) for longer scale.Item Magnetic phase transitions in Fe{sub 80-x}Ni{sub x}Cr{sub 20} (14 {<=} x {<=} 30) alloy studied by using {sup 57}Fe Moessbauer spectroscopy(ETDE Web, 1999-11) Bandyopadhyay, DebashisThis paper reports {sup 57}Fe Moessbauer spectroscopic studies of the polycrystalline samples of the substitutionally disordered, isostructural (fcc), ternary alloy system Fe{sub 80-x}Ni{sub x}Cr{sub 20} 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 Moessbauer parameters like line-width, center shift, resonance area, distribution of hyperfine field, P(H), and the average hyperfine field . An estimate of the magnetic transition temperature T{sub 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 {theta}D for alloys with x = 30 and 26.Item Study of hyperfine field distributions in the alloy Fe{sub 54}Ni{sub 26}Cr{sub 20} by using {sup 57}Fe Moessbauer spectroscopic technique(ETDE Web, 2000) Bandyopadhyay, DebashisThis paper reports the study of hyperfine field distributions of polycrystalline Fe{sub 54}Ni{sub 26}Cr{sub 20} sample of the substitutionally disordered, isostructural (fcc), ternary alloy by using {sup 57}Fe Moessbauer spectroscopic technique. Magnetic phase transitions occurring in the alloy have been studied by examining the temperature dependence of average hyperfine field. To understand the local magnetic order in this alloy at low temperature, probability of hyperfine field distribution curves have been re-fitted by using three independent Gaussians having different positions and widths. From this fits, local magnetic order of this alloy at low temperature has been described which supports the existence of nonequilibrium ferromagnetic state below Curie temperature. (orig.)Item The C-Hf-Ti System (Carbon-Hafnium-Titanium)(JOURNAL OF PHASE EQUILIBRIA, 2000) Bandyopadhyay, DebashisThe assessed phase diagram of the Ti-C system shown in[1990Oka] conducted a detailed assessment of the C-Hf Fig. 1 is taken from [1998Oka].[1995Alb],[1996Jon], and system, where only one intermediate phase exists. The [1996Sei] reported other assessments of this system, which assessed phase diagram shown in Fig. 3 is taken from consists of two terminal solids α-Ti and β-Ti and a refractory[1990Oka]. Three terminal solids, C (graphite), α-Hf, and monocarbide TiC. The other phases present are liquid and β-Hf, are present in this system.[Massalski1] has reported graphite (C). Two eutectic and one peritectoid reaction appear the temperature of β-Hf⇔ α-Hf allotropic transformation as in this system at 1646, 2776, and 920 C, respectively. There 1743 C. There are several conflicting reports in the literature seems to be a tendency of carbon ordering at compositions([1954Cot],[1961Por],[1962Kat], and [1965Rud]) regarding below stoichiometry; as a result, the Ti2C phase does not the eutectic reaction temperature between C and Hf and its appear in the binary phase diagram. The crystal structure corresponding composition. The crystal structure data of the data shown Table 1 are taken from [Massalski2]. C-Hf system shown in Table 3 are taken from [1990Oka].Item The C-Ti-Zr system (carbon-titanium-zirconium) (vol 22, pg 64, 1998)(JOURNAL OF PHASE EQUILIBRIA, 2001-04) Bandyopadhyay, DebashisItem Shell Magnetism in Chromium Doped Germanium Superatom(American Scientific Publishers, 2014-05) Bandyopadhyay, DebashisIt has been demonstrated that in a specific composition of a particular size nano-clusters can exhibit atom like behaviors in the periodic table and hence can be chosen as superatoms. According to the consequence of shell models some of the magic clusters can mimic chemical properties of elemental atoms. Following these, we have studied the structural, electronic and magnetic properties for Cr doped germanium nano-clusters using density functional theory based first-principles electronic structure methods. Clusters, which have enhanced chemical and thermo-dynamical stability, are classified by studying of their HOMO-LUMO gap, second order energy difference and adiabatic spin excitation energies. Cr doped Ge10 nanoclusters is found as the most stable cluster among all in the whole series studied. The present study illustrates that first and second ionization potential (IP) of such superatom are 7.41 eV and 10.93 eV, respectively, which are similar to that of Ca and Sr atom group which have IP around of 6.11 eV and 11.87 eV.Item Purification of thermo and acid tolerant extracellular phytase from a new soil isolate of Amycolatopsis vancoresmycina S-12(Elsevier, 2017-07) Bandyopadhyay, DebashisAmycolatopsis vancoresmycina S-12, a member of soil Actinomycetes, was explored regarding its extracellular phytase. The enzyme was recovered from the culture broth by ammonium sulfate fractionation and purified to near homogeneity following dialysis, DEAE Sepharose and Sphadex G-100 column. The enzyme was purified 5.51 fold with a final yield of 14.9%. It was characterized by SDS PAGE, Zymogram, Native PAGE, 2D PAGE as monomeric protein of 53 kDa. and pI 5.4. Peptide mass fingerprinting of MALDI-TOF MS result indicates its phytase nature. Optimum enzyme activity was at 37 °C and at pH 5.4. Optimum stability was at 60 °C and pH 8.0. The enzyme activity is stimulated in presence of Mg2+. The plant growth promotion experiment shows a significant effect, with an increase of 10% root and 25% of shoot length, resulting in a 2.2 fold increase in biomass.Item Magnetic behavior in (1≤n≤12) clusters: A density functional investigation(AIP, 2014-04) Bandyopadhyay, DebashisWith a goal to produce magnetic moment in Doped clusters which will be useful for practical applications, we have considered the structure and magnetic properties of Pure Germanium clusters and substitutionally doped it with Cr dimer to produce clusters. As the first step of calculation, geometrical optimizations of the nanoclusters have been done. These optimized geometries have been used in calculate the average binding energy per atom (BE), HOMO-LUMO gap and hence the relative stability of the clusters. These parameters have been demonstrated as structural and electronic properties of the clusters. Gap between highest occupied molecular orbital and lowest unoccupied molecular orbital indicate cluster to be a potential motif for generating magnetic cluster assembled materials. Based on these values a comparative study on different sized clusters has been done in order to understand the origin of structures, electronic and magnetic properties of nanoclusters.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 A study of the effect of annealing on the hyperfine field distributions in Fe79B16Si5 metallic glass(Elsevier, 1999-12) Bandyopadhyay, DebashisA 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.