Articles written in Pramana – Journal of Physics

    • ESR study of exchange coupled pairs in copper diethyldithiocarbamate

      R Kumari Cowsik R Srinivasan

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      ESR investigations on exchange coupled pairs of Cu ions in single crystals of Cu(dtc)2, isomorphously diluted with the corresponding diamagnetic zinc salt, are reported. The spin Hamiltonian parameters for the coupled species (S=1) are:g=2.1025,g+=2.031,A=75.1×10−4 cm−1,B=14.8×10−4,D=276.0×10−4 cm−1 andE=46.7×10−4 cm−1. While theg andA tensors show tetragonal symmetry, the zeor-field splitting tensor is rhombic and has principal axes different from those of theg andA tensors. Intensity measurements made down to 4.2 K indicate that the exchange is ferromagnetic with |FFF| ∼ 10 cm−1. Direct dipole-dipole interaction appears to be the major contribution to the zero-field splitting. A calculation on the distributed point dipole model shows that dipolar interaction is considerably modified by the high covalency of the Cu-S bond and accounts for the rhombic nature of the tensor. The possible exchange mechanisms in Cu(dtc)2—direct exchange and superexchange through the bridging sulphurs—are discussed.

    • Equation of state and temperature variation of the bulk modulus of the alkaline earth fluorides

      R Srinivasan V Ramachandran

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      The isotherms for the alkaline earth fluorides (CaF2, SrF2 and BaF2) have been computed using the expression for the total free energy of a crystal in the quasiharmonic approximation. The theoretical points for SrF2 and BaF2 have been compared with the points derived from Bridgman’s experimental relation.

      The temperature variation of the isothermal bulk modulus of the alkaline earth fluorides has been worked out on the basis of Axe’s shell model. The theoretica points are compared with the points obtained from the temperature variation of the elastic constants data. It is found that the vibrational contribution to the temperature variation of the elastic constants in these crystals is significant.

    • Studies in molecular structure, symmetry and conformation—Part XIV. Crystal and molecular structure of L-isoleucine hydrochloride monohydrate form II

      K I Varughese R Srinivasan

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      The crystal structure of a new form of L-isoleucine hydrochloride monohydrate C6H13O2NHClH2O (termed form II) has been determined using three-dimensional photographic data. This differs conformationally from the hydrochloride derivative (termed form I, Trommel and Bijvoet 1954) reported earlier. The crystal belongs to the orthorhombic space group P212121 with cell dimensions,a=5.87±0.01,b=24.77±0.02 andc=6.85±0.01 Å and four molecules per cell,ρobs=1.240 g/cm3,ρcal=1.238 g/cm3,µ for CuKa=32.6 cm−1.

    • Determination of imaginary components of anomalous scattering of some elements from experimental data

      S S Rajan R Srinivasan

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      Using the Bijvoet difference data reported in the literature for four compounds,viz./it, zinc sulphide, L-α-glyceryl phosphoryl ethanolamine monohydrate, 1-β-arabinofurinosyl cytosine hydrochloride and lithium aluminium oxide, the anomalous dispersion components Δf″ of the stronger anomalous scatterer in each case is calculated accepting the theoretical values for the lighter anomalous scatterers. For MoKα radiation the values turn out to be Δf″zn=1.470 (85), and for CuKα Δf″P=0.430 (25) Δf″C1=0.715 (18) and Δf″A1=0.264 (21).

    • Site symmetry and internal strains

      R Srinivasan G Lakshmi

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      The determination of the internal strains on the coupling parameter approach becomes very involved particularly when the number of atoms per unit cell is very large. It is shown in this paper that a knowledge of the site symmetry of the atoms helps one in determining the number of non-vanishing internal strain coefficients easily. The internal strain coefficients of two symmetry connected atoms can also be related. Examples are shown to illustrate these ideas.

    • ESR study of exchange coupled pairs in copper diethyldithiocarbamate—explanation of the low temperature hyperfine anomaly

      CV Manjunath Kumari Santosh R Srinivasan

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      A study of the hyperfine interaction in the ESR of Cu-Cu pairs in single crystals of copper diethyldithiocarbamate as a function of temperature has shown distinct differences in the hyperfine structure in the two fine structure transitions at 20 K, the spectrum not having the same hyperfine intensity pattern in the low field fine structure transition in contrast to that of the high field transition. The details of the structure of both the fine structure transitions in the 20 K spectrum have now been explained by recognizing the fact that the mixing of the nuclear spin states caused by the anisotropic hyperfine interaction affects the electron spin states | + 1 > and | −> differently. This has incidentally led to a determination of the sign ofD confirming the earlier model. The anomalous hyperfine structure is found to become symmetric at 77 K and 300 K. It is proposed that the reason for this lies in the dynamics of spin-lattice interaction which limits the lifetime of the spin states in each of the electronic levels | − 1 >, | 0 > and | + 1 > The estimate of spin-lattice relaxation time agrees with those indicated from other studies. The model proposed here for the hyperfine interaction of pairs in the electronic triplet state is of general validity.

    • Molecular packing for planar molecules

      Y T Thathachari R Kamalam R Srinivasan

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      The crystal packing of five planar molecules is considered in this paper. Each unit cell contains two non-equivalent molecules whose planes are inclined to each other. It is shown that the angle of inclination between the planes is completely determined by a simple geometrical criterion. A simple sequential arrangement of the four molecules defining the elementary parallelopiped of which the entire crystal is built leads to various configurations from which the one which has the least interaction energy can be picked out. Using a crude Lennard-Jones potential for the non-bonded interaction and a hard sphere model for the atoms, one can compute the crystal structure from the minimum energy criterion and this is found to be in fair agreement with the observed structure. This simple sequential packing with some modifications can provide an useful model for calculating the radial distribution function in amorphous solids involving planar groups.

    • Temperature variation of the Debye-Waller factors of Ba++ and F ions in BaF2 powder by x-ray diffraction

      R Srinivasan K S Girirajan

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      The temperature variation of the Debye-Waller factors of Ba++ and F ions in BaF2 powder has been studied using x-ray powder diffraction over the temperature range 77°–298°K. A continuous flow cryostat has been specially fabricated for this purpose for the YPC 50 NM powder diffractometer available in the department. The Debye-Waller factors of Ba++ and F between room temperature and 879°K have been measured using single crystal neutron diffraction by Cooperet al. Theoretical lattice dynamics shell model calculations using a 7-parameter model in a quasiharmonic approximation have been done over a temperature range 77° to 879°K. The theoretical values have been compared with the present x-ray measurements and the single crystal neutron diffraction values and the results are discussed.

    • The crystal fields inα-alums—A high pressure magnetic resonance study

      Shantanu Sinha R Srinivasan

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      The origin of the crystal field and its variation with temperature in severalα-alums have been studied bynmr of27Al andepr of Cr3+ at high hydrostatic pressures and low temperatures. The results lead to an explanation of the anomalous temperature variation of the axial crystal field at the trivalent ion site. The mechanism of the phase transition in ammonium alum is also explained. A correlation between the axial crystal field as determined bynmr (e2qQ/h) and that determined byepr (D) has been obtained.

    • 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.

    • Enhancement of thermopower in the highTc superconductor YBa2Cu3O7 and related compounds

      R Srinivasan V Sankaranarayanan N P Raju S Natarajan U V Varadaraju G V Subba Rao

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      The absolute thermopower of single phase YBa2Cu3O7 and Y0.8Er0.2Ba2Cu3O7 has been measured in the range 250 K to the superconducting transition temperature. It is found that these compounds show a large enhancement of thermopower in the range 150 K down toTc. This enhancement shows a steep exponential drop as the temperature increases from the transition temperature. The temperature variation of the enhancement is too steep to be accounted for by electron-phonon or electron-local structural excitation mechanisms.

    • Specific heat of YBa2Cu3O7−x from 4.2 to 60 K

      N Sankar V Sankaranarayanan R Srinivasan G Rangarajan G V Subba Rao

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      The specific heat of superconducting oxide compound, YBa2Cu3O7 −x, is studied using a quasi-adiabatic calorimeter from 4.2 to 60 K. The analysis of the specific heat data below 15 K gives a value of 17 mJ/mole K2 for the electronic heat capacity coefficient. The value ofθD(0) is determined to be 397±8 K. The variation ofθD with temperature was calculated in the temperature range 4.2 to 60 K.

    • Thermal conductivity of YBa2Cu3O7 from 7 K to 260 K

      T R Ravindran V Sankaranarayanan R Srinivasan

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      Thermal conductivity of YBa2Cu3O7 has been measured on samples of a few mm thickness. AboveTc thermal conductivity is found to decrease with increase in temperature, pointing towards a contribution to thermal resistivity from three-phonon Umklapp processes. BelowTc thermal conductivity increases rapidly before reaching a maximum at about 50 K and then falls towards zero at lower temperatures. The experimental set up is described and results discussed.

    • Critical current density of a sample of melt grown Y1.2Ba1.8Cu2.4Ox

      V Ganesan R Srinivasan R Gopalan T Rajasekaran

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      Melt grown samples of Y1.2Ba1.8Cu2.4Ox have been prepared and studied for their current carrying capacity. The composition was chosen to include Y2BaCuO5 (211) particles in the YBa2Cu3Ox (123) phase. The critical current density (Jc) of these samples was studied as a function of magnetic field using magnetization technique. The micrographic investigation shows well aligned grains in this material. The magnetic hysteresis measurements were done using a MPMS SQUID magnetometer up to the fields of 5.5 T. TheJc was estimated from the remanent magnetization using Bean model. Isothermal magnetization hysteresis loops at low fields reveal the presence of only one kind of hysteresis loops (corresponding to intragrain magnetizations). This is a valid proof that the weak links are greatly eliminated in these samples prepared by MG process. TheJc behaviour as a function of magnetic field has two components, a rapidly decaying exponential function of field and the other component that predominates at higher fields. This could be explained if we assume that the sample contains two phases of superconductors, one having a lowHc2 becoming normal at fairly medium fields of the order of a few kilogauss will act as pinning centres for the other phase having higherHc2 and hence higherJc at high fields.

    • Even harmonics of AC susceptibility in some sintered pellets of high temperature superconductors

      A Das A Bajpai A Banerjee R Srinivasan

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      The real (χ′n) and imaginary (χ″n) parts of even harmonic susceptibility (n⩽6) are measured as a function of external DC field (Hdc) in the field increasing (H↑) and decreasing (H↑) cycle. Hysteresis is observed betweenH↑ andH↓ cycles. In theH↓ cycle, at a field,Hcomp=4·2G, bothχ′ andχ″ of all the even harmonics vanish indicating a true cancellation of fields in the intergrain region (Heff≈0) caused by the balance betweenHdc and the remanent magnetization of the grains. The position of the extrema and the zero of the various harmonics undergo a shift proportional to the remanent magnetization of the grains at that particular field.

    • One-dimensional nuclear design analyses of the SST-2


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      Steady State Tokamak-2 (SST-2) will be an intermediate fusion machine before Indian DEMOnstration power reactor (DEMO) development to realise the reactor technologies. It is designed for fusion gain Q = 5 andfusion power in the range of 100–300 MW. Nuclear design analyses of SST-2 machine have been carried out to support the conceptual design work. Analyses have been carried out for two breeding blanket concepts: Indian lead–lithium ceramic breeder (LLCB) and helium-cooled ceramic breeder (HCCB). The analyses assess the tritium production and radiation shielding capability of the machine referring to the engineering design parameters. In this study, one-dimensional radiation transport calculations have been performed to assess the SST-2 nuclear responses for 1 full power year (FPY) operation. Nuclear responses such as tritium breeding ratio (TBR), various radiation loads to toroidal field (TF) coil have been calculated to obtain the radial build-up of SST-2 capable of breeding tritium and satisfying the shielding requirements. The assessment has been made using the ANISEN code andFENDL 2.1 cross-section library. It is observed that the TBRs with LLCB and HCCB blankets are 0.85 and 0.94, respectively. Shielding calculations confirm that the radial build is sufficient to protect the superconducting TF coils for 1 FPY.

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