• Volume 35, Issue 2

      August 1990,   pages  1-191

    • A definition for time in quantum cosmology

      T Padmanabhan

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      A definition for the intrinsic time co-ordinate is proposed, using the phase of the wave function of the universe. This definition generalizes the notion of time co-ordinate which arises in the semiclassical cosmology. It also leads to acceptable results for the evaluation of expectation values of physical variables.

    • A new formalism for the statistical dynamics of planar spin systems

      Kamlesh Kumari Deepak Kumar

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      The relaxational dynamics of a classical planar Heisenberg spin system is studied using the Fokker-Planck equation. A new approach is introduced in which we attempt to directly calculate the eigenvalues of the Fokker-Planck operator. In this connection a number space representation is introduced, which enables us to visualize the eigenvalue structure of the Fokker-Planck operator. The mean field approximation is derived and a systematic method to improve the mean field approximation is presented.

    • Force constants from intensity analysis of molecules CH3Cl and CD3Cl

      V Dayanand K Ravindranath N Rajeswara Rao

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      The secular equation (GF - Eλ)L = 0 contains more force constants than can be calculated from the equations formulated using the frequencies. For a 3 × 3 matrix, there are 6 force constants but only 3 frequencies. Attempts were made by others to estimate all the 6 constants to satisfy the frequencies and Coriolis constants and rotation distortion constants. However, many attempts are not made in these estimations to satisfy the intensities. A full complement of equations is derived to evaluate all the force constants combining the intensity equationsI =L’A withLL’ =G and evaluated the force constants ofA1 species of CH3Cl and CD3Cl. A simple analysis of a 2 × 2 matrix shows thatF12/F22=G12−1/G22−1 as reported earlier.

    • Spectroscopic study of rotational energy distribution of BH (A1Π) and electronic excitation temperature determination

      N N Math M I Savadatti

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      Emission spectra of BH(A1Π-X1Σ+) system were recorded and studied using a low pressure (3.0 torr) arc in flowing hydrogen and argon + hydrogen mixture. The rotational distributions in theA1Π state determined from the intensities of rotational lines for the 0–0 band of theA-X system conforms to a Maxwellian distribution with effective rotational temperature of 1000 ± 50°K. Intensities of Balmer lines of hydrogen were measured and used to determine electronic excitation temperature which was found to be around 2000°K.

    • A ‘tree’ for generation and identification of the colour symmetry point groups

      K Rama Mohana Rao

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      A flow chart (inverted ‘tree’) for generating and identifying the 58 magnetic and 18 polychromatic point groups using a classification for the 32 generating crystallographic point groups is suggested. The idea of colour generator is explored for generating the colour symmetry point groups. The advantages in presenting the identification of colour groups through a tree are discussed.

    • Liquid-like defects in superionic fluorites

      Mohua Makur Sujata Ghosh

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      A model based on the presence of two-level systems identified with defect clusters is used to study different properties of superionic lead fluoride and calcium fluoride. The high degeneracy of the higher level indicates a liquid-like arrangement of anions within the cluster.

    • Effects of lattice dispersion and elastic anisotropy on the thermal properties of fcc metals

      Meenakshi Mohapatra S Tolpadi

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      New parameters which separately take into account the effects of elastic anisotropy and phonon dispersion on the lattice specific heat in the case of fcc metals have been evaluated. A new graded mesh method which uses 162-direction approximation in (1/16) part of the Brillouin zone has been developed to evaluate the two new parameters.

    • Pressure-tuned resonance Raman scattering and photoluminescence studies on MBE grown bulk GaAs at theE0 gap

      A Jayaraman G A Kourouklis R People S K Sputz L Pfeiffer

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      Hydrostatic pressure has been used to tune in resonance Raman scattering (RRS) in bulk GaAs. Using a diamond anvil cell, both the photoluminescence peak (PL) and the 2 LO and LO-phonon Raman scattered intensities have been monitored, to establish RRS conditions. When theE0 gap of GaAs matchesħωS orħωL, the 2 LO and LO-phonon intensity, respectively, exhibit resonance Raman scattering maxima, at pressures determined byħωL. With 647.1 nm radiation (ħωL = 1.916 eV), a sharp and narrow resonance peak at 3.75 GPa is observed for the 2 LO-phonon. At this pressure the 2 LO-phonon goes through its maximum intensity, and falls right on top of the PL peak, revealing thatħωS(2 LO) =E0. This is the condition for “outgoing” resonance. Experiments with other excitation energies (ħωL) show, that the 2 LO resonance peak-pressure moves to higher pressure with increasingħωL, and the shift follows precisely theE0 gap. Thus, the 2 LO RRS is an excellent probe to follow theE0 gap, far beyond the Γ-X cross-over point. A brief discussion of the theoretical expression for resonance Raman cross section is given, and from this the possibility of a double resonance condition for the observed 2 LO resonance is suggested. The LO-phonon resonance occurs at a pressure whenħωLE0, but the pressure-induced transparency of the GaAs masks the true resonance profile.

    • Pressure induced amorphization of AlPO4

      Hema Sankaran Surinder M Sharma S K Sikka R Chidambaram

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      AlPO4 has been compressed to pressures of 16 GPa in a diamond anvil cell and its X-ray diffraction pattern studied by the energy-dispersive technique. The compound is observed to become amorphous at ∼ 12 GPa. This explains the loss of Raman spectrum of AlPO4 reported by Jayaraman and coworkers (1987).

    • Ultrasonic investigation in KTaO3

      S C Deorani U C Naithani B S Semwal

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      The anomalous ultrasonic attenuationαc of longitudinal waves propagating along (100) direction in KTaO3 has been analyzed above the phase transition temperature in the frequency range 150–300 MHz in the paraelectric phase. The attenuation of longitudinal ultrasonic waves in KTaO3 is primarily due to a strong interaction with thermally-excited phonons in the soft mode. Frequency and temperature variations of attenuation are discussed.

    • A note on grating based soft X-ray monochromators

      V C Sahni

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      A few facts related to the aberrations for toroidal and plane gratings are presented. Some design aspects of the plane grating-based monochromator devised by Petersen (1982) are discussed. We also point out that for a cylindrical grating astigmatic correction can be reduced and suggest that following Petersen’s scheme some improvements in the monochromator performance could be achieved with such a grating.

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