• Volume 23, Issue 3

      September 1984,   pages  279-432

    • Raman effect in the x-ray region

      Ashok Pimpale Chintamani Mande

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      This paper presents a brief review of x-ray Raman scattering and some of our calculations on Raman scattered line shapes from light elements. We summarise the history of the Raman process in the x-ray region and present a detailed theory of the Raman scattering from an atomic many-electron system. Actual calculations of the Raman cross-section using this theory in single-particle approximation are given. The process of internal resonance Raman scattering is also discussed in the same formulation. The Raman cross-section is compared with the cross-sections of other x-ray scattering processes.

    • The electron-electron interaction in doubly-excited states of atoms

      ARP Rau

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      Doubly-excited states of the isoelectronic sequence of H, in which both electrons are in high principal quantum numbers, are examined on the basis of alternative pictures of the correlations between the two electrons. Restricting ourselves to the lowest1S states with both electrons in principal quantum numbern, we parametrise the electron-electron interaction on the basis of these pictures and compare the resulting simple expressions with more elaborate numerical calculations. This provides further understanding of the nature of correlations in such states.

    • 3He-rich solar flares

      S Ramadurai M N Vahia S Biswas K Sakurai

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      A new subgroup of3He rich solar flares is found on reanalysing the global data.3He/H ratio as a function of maximum proton flux at an energy of about 10 MeV shows a break-up of the data into two groups. The first group follows the anticorrelation of3He/H ratio with the proton flux, as expected in the plasma process acceleration models. But the second group has a constant3He/H ratio as a function of maximum proton flux. This is not in conformity with the plasma process models. But this is expected in models where the nuclear spallation reactions are responsible for the production of3He. It is also found that the same break-up into two distinct groups follows if one plots the location of the flares in the solar disc. The first group is more or less confined to the west limb of the Sun, whereas the second group is more widely spread out across the solar disk.

    • High resolution measurement of bragg cut-off in beryllium

      C L Thaper P S Goyal B A Dasannacharya

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      Bragg cut-off for$$(10\bar 10)$$ plane of polycrystalline beryllium of various lengths of 300 and 116 K has been measured with an energy resolution of 5 μeV. The natural width of the cut-off is 12.5±1.5 μeV, independent of temperature and length of beryllium and also of physical characteristics and certain metallurgical treatments of the powder. Such blocks of beryllium would be suitable for designing a ΔT-window spectrometer with resolution ⩾20 μeV. Bragg cut-offs corresponding to (0002) and$$(10\bar 11)$$ planes of beryllium have been separated for the first time. These can also be used for producing additional energy windows in a ΔT-window spectrometer, thus increasing its efficiency.

    • Absorption spectrum of the HCCO radical

      S L N G Krishnamachari R Venkatasubramanian

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      In the flash-photolysis of oxazole and isoxazole, new transient absorption bands are observed in the region 3080–3670 A. Vibrational analysis of these bands showed that they can be arranged into two electronic systems with their origin bands at 3333.60 A and 3367.01 A. Based on the experimental conditions under which the bands are produced and on the analysis of their vibrational and gross-rotational structure, these bands are assigned to a new carrier, the HCCO free radical.

    • Cumulant functions in optical coherence theory

      CL Mehta

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      Cumulant functions are introduced to describe the statistical state of a radiation field. These functions are simply related to the optical coherence functions but have some interesting features. It is shown that if the cumulant functions of all orders greater than some numberN0 vanish then they also vanish for all orders greater than 2. Thermal field is the only field having this property. This property holds whether the field is described by a classical stochastic process or by a quantum density operator. Further the particular operator ordering used in defining these cumulant functions for the quantized field affects only the second order cumulant function. To describe the statistical state of a vector field such as partially polarized or unpolarized radiation, one would need to introduce cumulant tensors.

    • Soliton and boundary condition induced fractional fermion number

      S M Roy Virendra Singh

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      We show that for a fermion in a bounded background potential in a finite box, eigenvalues of the total charge are independent of whether the potential is solitonic and depend only on the boundary condition: half-odd integral or integral for charge conjugation (C) invariant boundary conditions and an arbitrary fraction forC non-invariant boundary conditions. Fractional fermion numbers for infinite space Jackiw-Rebbi and Goldstone-Wilczek Hamiltonians are reproduced in finite space by using boundary conditions different from the periodic ones of Rajaraman and Bell.

    • Origin of fine structures in solar radio bursts

      G S Lakhina B Buti

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      The radiation, resulting from the nonlinear interaction of whistler solitons, which act as localized antennae, with the upperhybrid waves in the coronal loop, is shown to give rise to fine structures in solar radio bursts. All the observed features of microwave spikes in radio flares,e.g. their frequency, polarization and short duration can be explained by the presence of about 106 solitons occupying a volume of ∼ 108 m3, provided this interaction takes place at low altitudes. However, if this interaction takes place close to the top of the coronal loop, it gives rise to the isolated tadpole ‘eyes’ features in the dynamic spectra. About 109 solitons are needed to account for the observed flux of these ‘eyes’.

    • Potential double layers in double plasma device

      A N Sekar Y C Saxena

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      Results of the investigation on the formation of double layers in double plasma device are presented. By appropriate modifications in the biasing conditions, we have been able to obtain both weak (eΔφ<10KTe) and strong double layers (eΔφ>10kTe) in the device. Unlike previous experiments, we have not been limited to potential jumps equal to ionisation potential of the neutral gas. A detailed investigation has been carried out to find out why earlier experiments in similar devices were limited to only weak double layers.

      We have also investigated the phenomenon of the so-called psuedo-double layers and have shown that they are potential jumps over the thickness of the order of Debye length and precede plasma expanding with velocity many times the ion-acoustic velocity. They do not represent metastable states of the plasma as suggested by earlier investigators.

    • A geometric generalization of classical mechanics and quantization

      R K Varma

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      A geometrization of classical mechanics is presented which may be considered as a realization of the Hertz picture of mechanics. The trajectories in thef-dimensional configuration spaceVf of a classical mechanical system are obtained as the projections onVf of the geodesics in an (f + 1) dimensional Riemannian spaceVf + 1, with an appropriate metric, if the additional (f + 1)th coordinate, taken to be an angle, is assumed to be “cyclic”.

      When the additional (angular) coordinate is not cyclic we obtain what may be regarded as a generalization of classical mechanics in a geometrized form. This defines new motions in the neighbourhood of the classical motions. It has been shown that, when the angular coordinate is “quasi-cyclic”, these new motions can be used to describe events in the quantum domain with appropriate periodicity conditions on the geodesics inVf + 1.

    • Black hole thermodynamics from a possible model for internal structure

      P Goswami K P Sinha

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      Treating a black hole as a relativistic gas of microblack holes (planckions) which have fermionic character, expressions for some thermodynamic quantities are obtained. These have the same structure as those obtained by Hawking by other considerations.

    • Effect of correlation and spin-polarisation on the band structure of metallic dysprosium

      P K Mukhopadhyay C K Majumdar K C Das

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      Energy bands of dysprosium have been calculated considering the effects of correlation and spin-polarisation. The exchange and correlation contributions to the spherically symmetric crystalline potential have been taken in the forms suggested by (i) Kohn and Sham and (ii) Overhauser. The exchange and correlation potential of von Barth and Hedin has been used to study the effect of spin-polarisation on the band structures. The resulting bandwidth, density of states, magnetic moment and spin-splitting have been computed and compared with experimental results. Some discrepancies remain; theoretical calculations done so far agree reasonably among themselves, so accurate experimental data are probably called for.

    • Mechanism of phase transformations in ZnS

      M T Sebastian P Krishna

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      X-ray diffraction of the 2H-3C transformation in ZnS crystals has been studied to determine the mechanism of the phase transformation. Single crystals of 2H ZnS were annealed in vacuum at different temperatures to induce the phase transformation and then quenched to arrest it at different intermediate stages. The transformation is found to occur by the non-random nucleation of stacking faults in the 2H structure which produce characteristic diffuse steaks along reciprocal lattice rows parallel toc* for whichH−K ≠ 0 (mod 3). All the crystals finally transform to a disordered twinned 3C structure. A study of the broadening of the x-ray diffraction maxima reveals that the stacking faults involved in the transformation are basal plane deformation faults. Initially these nucleate at random producing a random distribution of cubic nuclei within the 2H structure. As the transformation proceeds these 3C nuclei grow into thick 3C regions by a preferential nucleation of the faults at 2-layer separations. Since the 3C nuclei can have twin orientations the resulting 3C structure invariably contains a random distribution of twin faults. This is confirmed by comparing the experimentally observed intensity profile of the 10.L reflections as recorded on a single crystal diffractometer, with those calculated theoretically for a randomly twinned cubic structure.

    • Pressure and volume dependence of thelo-to phonons in InAs

      A Jayaraman V Swaminathan B Batlogg

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      The pressure dependence of thelo-to phonons in InAs has been investigated by Raman scattering using the diamond anvil cell. Indium arsenide transforms, presumably to the rock-salt structure at 70±1 kbar. The mode Grüneisen parameters for thelo-to phonons are γlo=0.99±0.03, γto=1.2±0.03 respectively. The effective charge,e*T, for InAs decreases slightly with pressure and this trend is in accordance with the behaviour of other III–V zinc blende structured semiconductors: The structural phase transition is discussed in the light of theoretical calculations for phase stability of III–V compounds, as well as recent high pressure x-ray diffraction studies.

    • Ga0.47 In0.53 As—The material for high-speed devices

      Biswa Ranjan Nag

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      Electron transport properties of Ga0.47In0.53As are reviewed. The available physical constants of the material and results on electron mobility in bulk materials, 2deg systems and under hot-electron conditions are presented. Applications of the material in the construction offet’s and photo-conductive detectors are briefly discussed.

    • Electron transport in Chevrel phase superconductors, Cu1.8Mo6S8−ySey, 0⩽y⩽8 and Cu1.8Mo6S8−yTey, 0⩽y⩽4—Fit to Cote-Meisel theory

      V Sankaranarayanan G Rangarajan R Srinivasan

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      Normal state electrical resistivity of the Chevrel phase compounds of the type Cu1.8Mo6S8−ySey, 0⩽y⩽8 and Cu1.8Mo6S8−yTey, 0⩽y⩽4 is analysed on the basis of the generalized diffraction model which incorporates a postulate on electron-phonon interaction,viz phonons with wavelength exceeding the electron mean-free path are ineffective electron scatterers. Fit obtained by this model was found to be superior to other models based on the interbands-d scattering of electrons.

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