• Volume 42, Issue 3

      March 1994,   pages  167-283

    • Photoelasticity in quasicrystals

      K Rama Mohana Rao P Hemagiri Rao

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      The maximum number of non-vanishing and independent second order photoelastic coefficients required by the seven pentagonal and the two icosahedral point groups 5(C5),$$\bar 5$$(S10),$$\overline {10} $$(C5h),$$\overline {10} $$ m2(D5h), 52(D5), 5m(C5v),$$\bar 5$$ 2m(D5d); 235(I), 2/m$$\bar 3$$$$\bar 5$$(Ih)—that describe the quasicrystals symmetry groups in two and three dimensions—is obtained. The schemes of non-vanishing and independent coefficients have been calculated and listed. Finally the results of this group-theoretical study are briefly discussed.

    • Elastic anomalies in strontium titanate

      K S Viswanathan B Subramanyam

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      The anomalies of the second and third-order elastic constants have been considered for the phase transition of strontium titanate within the framework of Landau’s theory. All the anomalies of the second-order elastic constants have been obtained in a single formula using Kronecker delta functions and relations among them have been established. The real parts ofC*11 andC*44 decrease steeply across the transition temperature and thereafter flatly tend to their asymptotic values in the low temperature phase agreeing qualitatively with experimental observations. We have also derived expressions for the third-order elastic anomalies and discussed the temperature variation of the real part ofC*111. We have derived expressions for the attenuation of the longitudinal and transverse waves along certain simple symmetry directions and have shown that there is nearly good agreement with experimental observations.

    • Relation of coordination number to memory and threshold switching in chalcogenides

      Manohar Lal Navdeep Goyal Anil Vohra

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      The paper reports a structural study of some memory and threshold chalcogenides in terms of coordination numberC, defined byC=8−N, and is the average coordination number for covalently bonded materials. The average number of nearest neighbours surrounding a central atom, obtained for As-Ge-Te (memory) and Se-Ge-Te (threshold) systems have been used to estimate the cohesive energies, assuming simple additivity of bond energies. The bonding pattern so obtained, explains certain properties of these glasses.

    • Magnetic phase diagram of anisotropic layered superconductors via magnetization measurements forHc in Bi2212

      A K Grover F Iga Y Yamaguchi N Aoki K Ino N Goyal Y Nishihara

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      The Bi2Sr2CaCu2O8 system is viewed as an archetypal of superconductors modelled as Josephson coupled CuO2 bilayers. The isothermal and temperature dependent DC and AC magnetization measurements forHc in a single crystal of Bi2212 have been performed. Qualitative changes are observed to occur over a narrow range of temperature values before reaching the superconducting-normal transition. The observed behaviour can be ascribed to the rapid variation in the strength of the coupling between the superconducting CuO2 planes (i.e., bilayers in the case of Bi2212). Strongly coupled planes behave like a 3D superconductor, whereas weakly coupled planes have a two component response attributable to 2D planes and interplanar couplings. We believe that this paper is a plethora of new findings. Our observations imply that resistivity across the planes becomes zero earlier than that within the planes. A new line (designated asH2D(T)) above which the change in the electromagnetic response is dominated by quasi 2D-planes has been determined for the first time. This paper also contains the first observation of Differential Diamagnetic Effect (DDE) in the In-phase AC susceptibility data which signals the onset (atT2D(H)) of dominance of response from 2D-planes. In addition to a host of interesting thermomagnetic history effects which are a consequence of interplay between the diamagnetic responses from the two components, a comparison of irreversibility lines (of the 3D state) determined by different methods on the same specimen of a HTSC is also being presented for the first time. We have come across Paramagnetic Meissner Effect (PME), first recognized in ceramic samples of Bi2212, in the temperature region of dimensional crossover in our single crystal sample, whichinter-alia confirms our labelling of the two component behaviour. A schematic phase diagram summarizing the various transformations that can occur nearTc in the electromagnetic response of an anisotropic layered system has been drawn.

    • Self focusing of elliptic Gaussian laser beams: Saturable nonlinearity

      S Konar A Sengupta

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      An analysis of self focusing of an elliptic Gaussian laser beam, propagating through a medium with saturable nonlinearity has been presented. It has been established that stationary self trap propagation of the beam is not possible. Though stationary self trap propagation does not occur, one can define a virtual threshold power for self focusing. Above this threshold power value, but not far from it, the beam focuses; below this threshold value it defocuses. As a whole three different defocusing zones and two focusing zones have been identified. It is also shown that effective beam radius never reaches zero.

    • High resolution optogalvanic spectrum of N2-rotational structure of (11, 7) band in the first positive system

      P R Sasikumar S S Harilal V P N Nampoori C P G Vallabhan

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      High resolution optogalvanic spectrum of the (11, 7) band in the first positive system of nitrogen molecule has been recorded from 17179 to 17376 cm−1. Assignment of 432 rotational lines belonging to the 27 branches of this band has been carried out.

    • New band system of YbI molecule

      K N Uttam M M Joshi

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      Thermal emission spectrum of YbI molecule has been photographed for the first time in the spectral regionλλ 6100–6400 Å using Saha’s high temperature furnace at a reciprocal linear dispersion of 7.3 Å/mm. A total of 52 single-headed and violet degraded bands have been recorded and are classified into a single system. Vibrational analysis has been carried out and it has been suggested that system arises from the ground state with the vibrational constantω″e=153.0 cm−1.

    • CH local mode overtone excitations in benzyl chloride—A conformational study

      T M A Rasheed V P N Nampoori

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      The CH local mode overtone spectrum of benzyl chloride in the visible and NIR regions studied by laser induced thermal lens and conventional NIR absorption is presented. The analysis shows that the −CH2Cl group is symmetrically oriented with respect to the benzene ring, thus finalizing one of the two possible conformational models predicted by electron diffraction studies. The aryl CH bonds have a slightly larger force constant than that in benzene.

    • A calorimeter for operation in the temperature range 2–150 K

      I Das E V Sampathkumaran

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      The description of a calorimeter for measurement of heat-capacity in the temperature range, 2–150 K, by semi-adiabatic heat-pulse method is presented. The data collection is computerized and various parameters monitored through the software ensure high quality of the data. The performance of the calorimeter was verified with the well-known standard materials. Results on the alloys, PrCu2Si2 and GdCu2Si2, are also presented.

    • Losses in pendular suspensions due to centrifugal coupling

      Sangita N Pitre S V Dhurandhar D G Blair Ju Li

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      We present an analysis of the centrifugal coupling of a simple pendulum to a dissipative support. We show that such a coupling leads to an amplitude dependent quality factor. For amplitudes which could be present in laser interferometer gravitational wave detector suspensions, this mechanism could limit the quality factor of the test mass suspension significantly to 1010 and should be considered in the design of advanced LIGO type detectors.

    • High pressure measurement with simple piston gauge in static condition

      J K N Sharma Kamlesh K Jain H N P Poddar

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      This paper describes the characterization of the newly developed piston gauge pressure standard at the National Physical Laboratory (NPL), with particular reference to its fall rate, engagement length of the piston and the deceleration rate for the measurement of hydraulic hydrostatic pressure up to 60 MPa. The low pressure effective area of the gauge derived from its dimensional measurements when compared with the value obtained by its direct calibration against NPL transfer pressure standard agrees within 0.025%. The pressure gauge is quite stable, reproducible and has a sensitivity of 3 ppm. Though the theoretically calculated value of the pressure coefficient is low as compared to the experimentally observed one in its absolute terms, the pressure dependent effective area agrees within ± 0.025% over whole of the pressure range which is well within the uncertainty statement of the two independent techniques used.

    • The white line inL absorption spectra of rare earth oxides

      N V Moghe V S Pai G G Sahasrabudhe V B Sapre

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