• Volume 47, Issue 2

      August 1996,   pages  99-182

    • The origins of phase stability in ordered and disordered Ni-Pt alloys

      Prabhakar P Singh

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      Using the atomic-sphere approximation formulation of the Korringa-Kohn-Rostoker coherent potential approximation (KKR CPA) method, we have studied the effects of relativity on the electronic structure of ordered and substitutionally disordered Ni-Pt alloys. The inclusion of mass-velocity and Darwin terms are found to be essential for describing the experimentally observed ground-state properties. For the stability of disordered Ni-Pt alloys we find that, in addition to relativity, the minimization of charge-transfer effects are important. We also find that the treatment of ordering tendencies based on the band energy term alone is not sufficiently accurate for alloys with charge-transfer effects. Further analysis, in terms of basis functions, densities of states and non-spherically averaged charge densities, indicate the importance ofs- andd-electrons of Pt for the stability of both ordered and disordered Ni-Pt alloys.

    • Analytic solution of the BCS gap equation inD dimensions (D=1, 2, 3), at finite temperatures

      Arun Bhardwaj Satish K Muthu

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      The Bardeen-Cooper-Schrieffer (BCS) gap equation is solved analytically in one, two and three dimensions, for temperatures close to zero andTc. We work in the weak coupling limit, but allow the interaction widthνħωm/EF to lie in the interval (0, ∞) Here,ħωm is the maximum energy of a force-mediating boson, andEF denotes the Fermi energy. We obtain expressions forTc and ΔC, the jump in the electronic specific heat acrossT=Tc, in the limitsν≪1 (the usual phonon pairing) andν>1 (non-phononic pairing). This enables us to see howTc scales with the mediating boson cut off. Our results predict a larger jump in the specific heat for the caseν>1, compared toν≪1. We also briefly touch upon the role of a van Hove singularity in the density of states.

    • Internal irradiation effects in239Pu doped K2Ca2(SO4)3): EPR, TSL and PAS investigations

      T K Seshagiri V Natarajan A R Dhobale M D Sastry

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      The electron/hole trapped centres created during internal irradiation in239Pu doped K2Ca2(SO4)3 were investigated using electron paramagnetic resonance (EPR), thermally stimulated luminescence (TSL) and photoacoustic spectroscopic studies (PAS). These techniques were used to identify the defects and characterize the thermally induced relaxation processes. TSL studies of self (α)/γ-irradiated239Pu doped K2Ca2(SO4)3 revealed two glow peaks around 400 and 433K. Plutonium introduced as Pu4+ was partly reduced to Pu3+ due to self irradiation. This was ascertained from PAS studies. EPR studies carried out on these samples showed the formation of radical ions SO4, SO3, O3, etc. The thermal destruction of SO4 ion was found to be associated with the prominent glow peak around 433K. Pu3+ was found to act as luminescent centre for the observed TSL glow. The trap depth for the glow peak at 433K has been determined from TSL and EPR data.

    • Temperature dependent photoluminescence studies of ZnSe:I single crystals grown by chemical vapor transport

      Suruchi Anand Prabhat Verma S C Abbi K P Jain M J Tafreshi K Balakrishnan R Dhansekaran

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      The single crystal ZnSe:I sample was grown by the chemical vapor transport (CVT) method using iodine as the transporting agent. The iodine incorporates itself effectively as a donor in the lattice. The sample shows a 〈111〉 optical quality surface and has an absorption edge at 2.55 eV due to a deep impurity band nearly 0.15eV below the conduction band. The photoluminescence emission spectra of this crystal have been measured for its temperature dependence as well as for excitation energy dependence. The photoluminescence is in accordance with a donor-acceptor complex formation involving iodine activated donors and self-activated acceptors. The configuration coordinate model has been used to explain the temperature dependent changes in the peak position and the bandwidth of the emission band. The decrease in luminescence efficiency with increasing temperature is explained by using a simple model for thermal quenching. The activation energy at low temperature range (T<200K) is different from that at high temperature range (200K<T<300K).

    • Raman study of phase transition in ferroelectric Ba0.95Ca0.05TiO3

      Rekha Rao A P Roy B A Dasannacharya S Balakumar R Ilangovan P Ramasamy

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      Raman scattering investigation of phase transition in the ferroelectric Ba0.95Ca0.05TiO3 is reported. The results suggest onset of significant dynamic disorder at 105°C. This corroborates findings of recent structural study regarding large positional disorder associated with Ti and O1 atoms well below the tetragonal to cubic transition temperature (∼150°C).

    • Pressure-induced structural and electronic transition in KTb(MoO4)2 through Raman and optical studies

      A Jayaraman S K Sharma S Y Wang S R Shieh L C Ming S-W Cheong

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      Raman and optical absorption studies under pressure have been conducted on KTb(MoO4)2 up to 35.5 GPa. A phase transformation occurs at 2.7 GPa when the crystal is pressurized at ambient temperature in a hydrostatic pressure medium. The sample changes to a deep yellow color at the transition and visibly contracts in theα-axis direction. The color shifts to red on further pressure increase. The Raman spectral features and the X-ray powder pattern change abruptly at the transition indicating a structural change. The pressure-induced transition appears to be a property of the layer-type alkali rare earth dimolybdates. However, the color change at the transition in KTb(MoO4)2 is rather unusual and is attributed to a valence change in Tb initiated by the structural transition and consequent intervalence charge transfer between Tb and Mo.In situ high pressure X-ray diffraction data suggest that phase II could be orthorhombic with a unit cell having 3 to 4% smaller volume than that of phase I.

    • Measurement of optical constants of thin polymer films using spectrally resolved white light interferometry

      V Nirmal Kumar Y Chandrasekhar D Narayana Rao

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      Using the spectrally resolved white light interferometry we present our experimental results on the measurement of the optical constants of thin polymer films coated on a transparent substrate. As an extension to our previous work (J. Opt. Soc. Am.B12, 1559 (1995)) on thick glass plates, we have shown here that this technique can be effectively applied to very thin polymer films also. We have improved the accuracy of our results by using the Sellmeier dispersion formula for fitting the data. From the width and position of the zero-order fringe and the frequency of modulations in the white light spectrum, the refractive indexn(λ) and thicknesst of the thin polymer films are calculated. To study the accuracies involved in the technique, PVA, PMMA and PS films of varied thicknesses are coated on glass plates and the measured values are compared with ellipsometer studies.

    • Magnetic studies in mesoscopic length scale using polarized neutron spectrometer at Dhruva reactor, Trombay

      Sk Mohammad Yusuf L Madhav Rao

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      The details of construction and principle of polarized neutron spectrometer at Dhruva reactor, Trombay, for neutron depolarization studies have been described. The feasibility in carrying out neutron depolarization studies in order to know the nature of magnetic ordering in various types of magnetic systems on mesoscopic length scale has been shown.

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