• Volume 36, Issue 6

      June 1991,   pages  581-656

    • Theoretical evaluation of the overall values of Booth type R-indices based on intensity variables

      S Parthasarathy N Elango K Sekar

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      Theoretical expressions for the overall values of three Booth type R-indices based on intensity variables are derived. The results are applicable to crystals of any space group containing any number and type of atoms at general positions in the asymmetric unit. The theoretical results were tested in the case of models of a few crystal structures.

    • Stereochemical studies of oligomers XXIX. Reinvestigation of the structure of N-m-tolyl phthalimide

      G Bocelli

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      C15H11NO2, Mr 237.3, monoclinic, space groupCc, a=8.539(2),b=19.865(4),c=7.599(2)Å,β=111.44(2)°,V=1199.8 Å3,Z=4,Dc=1.31 gcm−3,λ(CuKα)=1.5418 Å,µ=6.74cm−1,F(000)=496, room temperature. The structure was solved by direct methods with SHELX-86 and refined down to agreement valueR=0.046 for 1117 reflections above 2σ(I). The angle between the plane of the phthalimide group, which shows a little bent [1.2(2)°] between its two rings, and the tolyl group is 56.1(1)°. The packing of the molecules is stabilized by van der Waal’s forces only.

    • A phenomenological approach for the phonon dispersion curves of barium

      A E Arua L A Hussain

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      A phenomenological model which combines the axially symmetric ion-ion interaction with the electron-ion interaction scheme of Bhatia has been successfully used to calculate the phonon dispersion curves of bcc Ba. In particular, when the elastic constants determined by Mizukiet al are used as input data, the model successfully reproduces the anomalous feature of the dispersion curves measured by them. On the other hand, when the elastic constants determined by Buchenauet al are used, the model produces a cross over of the longitudinal and transverse branches in the [100] direction, a feature not established in their measurements. Thus the features of the dispersion curves in the [100] direction which is due to the hybridization of thes andd states are evident in the measured elastic constants and are consistent with the values of C12-C44.

    • Vibrational spectra of (NH4)3ZnCl5

      M Balasubramanian V Ramakrishnan S Rajendran

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      IR and Raman spectra of (NH4)3ZnCl5 have been recorded. The observed spectra have been analysed on the basis of the vibrations of ZnCl42− and NH4+ ions. The appearance of multiple Raman bands indicates the presence of two different types of ammonium ions. The effect of anisotropic crystalline field over the ZnCl4 and NH4 tetrahedra is also discussed. The assignment of internal modes has been verified by the potential energy distribution calculations.

    • Scaling theory of quantum resistance distributions in disordered systems

      A M Jayannavar

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      We have derived explicitly, the large scale distribution of quantum Ohmic resistance of a disordered one-dimensional conductor. We show that in the thermodynamic limit this distribution is characterized by two independent parameters for strong disorder, leading to a two-parameter scaling theory of localization. Only in the limit of weak disorder we recover single parameter scaling, consistent with existing theoretical treatments.

    • A comparison of barrier type tunnel junction and point contact tunnel junction formed on the same highTc material

      H Srikanth A K Raychaudhuri

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      By making a combination of both point contact and barrier type tunnel junctions on a single sample of the highTc superconductor BSCCO (2212) single crystal, we have shown that as the tunneling tip is slowly retracted from the surface a point contact junction gradually evolves from a N-S short to a high resistance tunnel junction. The scaled dynamic conductance (dI/dV) of this point contact tunnel junction becomes almost identical to that of a conventional barrier type tunnel junction and both show a linear dI/dVV curve. The observation implies that at high resistance a point contact junction behaves in the same way as a barrier type tunnel junction. We suggested that the almost linear tunneling conductance obtained in both the cases most likely arises due to an intrinsic characteristic of the surface of the crystal comprising of a mosaic of superconducting regions of the order of a few nanometers. We also conclude that the barrierless (N-S) point contact obtained by piercing the surface oxide layer of the crystal shows Andreev reflection which we suggest as the origin of the zero bias anomaly often observed in point contact junctions.

    • Electrical properties of simultaneously evaporated Sb-Te thin films

      P S Nikam R R Pawar

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      Thin films of Sb-Te of varying compositions have been deposited on glass substrates following the three temperature method. The dc conductivity (σ), Hall coefficient (RH) and thermoelectric power (α) of annealed samples have been measured in the temperature range 300–470°K. Films exhibit metallic as well as semiconducting characteristics withp-type conductivity. The properties are found to be dependent on composition and thickness of the film.

    • Intermolecular energy transfer in mixed laser dyes: photophysical properties of triplet states

      A D Mulla N N Math M I Savadatti

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      Triplet-singlet energy transfer in laser dyes have been studied in EPA at 77K using N2 laser as an excitation source. Phosphorescence of the donor (D) and the delayed fluorescence of the acceptor (A) and their lifetimes have been measured for coumarin 102 (D)-rhodamine B(A) and 9(10H)-acridone (D)-rhodamine 6G(A) dye systems as a function of acceptor concentration. These data yield energy transfer rate constants of ∼103 dm3 mol−1 s−1 for the donor acceptor combinations, consistent with the Forster mechanism. The phosphorescence quantum efficiency and other spectral parameters are also reported.

    • Laser optogalvanic and emission spectrum of N2 in the 5400–6150 Å region

      G Ullas S B Rai

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      Laser optogalvanic (LOG) and emission spectra of the N2 molecule have been recorded at different discharge voltages and gas pressures in the spectral region 5400–6150 Å. It is observed that in the optogalvanic spectrum bands belonging to several systems develop extensively. Under the present discharge conditions bands of the first positive system are predominant. The variation of the signals with pressure and voltage in the two cases is discussed.

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