• Volume 1, Issue 1

      July 1973,   pages  1-60

    • Editorial

      S Ramaseshan

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    • Atmospheric gamma rays in the energy region 40–1000 GeV

      K C Anand R R Daniel S A Stephens

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      A large stack of lead-emulsion sandwich detector assembly was flown over Hyderabad, India. High energy gamma rays at the float altitude were unambiguously identified from the cascades they induced, and their energies reliably determined by improved methods. From an analysis of 163 gamma rays of energy ≳ 30 GeV, it is found that the differential energy spectrum is represented by the power lawJr(E)= 129·4E−2·62±0·12 photons m−2 sr−1sec−1 GeV−1 at an effective atmospheric depth of 14·3 g cm−2; this is the first reliable balloon measurement of atmospheric gamma rays in the energy range 40–1000 GeV. After correcting for the gamma rays radiated by the primary cosmic ray electrons, the production spectrum of gamma rays, resulting from the collisions of cosmic ray nuclei with air nuclei, at the top of the atmosphere isPr(E, 0)=8·2 × 10−4 E2.60±0.09 photons g−1sr−1sec−1 GeV−1. The atmospheric propagation of the electromagnetic component due to the cascade process is also derived from the gamma ray production spectrum.

    • Magnetic and electric birefringence in the isotropic phase of nematic liquid crystals

      N V Madhusudana S Chandrasekhar

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      A theory is developed of the anomalous magnetic and electric birefringence in the isotropic phase of nematic liquid crystals taking into account orientational correlations between neighbouring molecules. Use is made of a modification of Bethe’s method due to Krieger and James, and the properties of the system are derived in terms of a single parameter, viz., the two-particle interaction constant. The expressions for the magnetic and electric birefringence are similar in form to those given by the phenomenological model of de Gennes. Theoretical curves forp-azoxyanisole reproduce the trends in the observed data. A calculation of the nematic-isotropic transition point confirms that this treatment is an improvement over the mean field approximation in describing pre-transition phenomena in the isotropic phase.

    • High pressure behaviour of liquid caesium

      T G Ramesh

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      This paper deals with the anomalous behaviour of liquid caesium at high pressures. A model for the phenomenon of electron collapse in the liquid phase, based on the anomalous density variation of liquid caesium with pressure has been proposed. The process of 6s→5d electron collapse is pictured as the formation of a virtual bound state and the tunnelling process accounts for the 6s⇌5d dynamic conversion. The same model together with the Friedel sum rule has been used to explain the resistivity variation of liquid caesium with pressure. The resistivity minimum observed in most of the liquid alkali metals in the low pressure region has been explained. The agreement with the experimental curve is good in the low pressure region whereas a large discrepancy exists at higher pressures. This may be due to the breakdown of the Ziman’s resistivity formula under conditions of resonance scattering.

    • Plasmon-magnon interaction in magnetic semiconductors

      G Baskaran K P Sinha

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      In appreciably doped semiconductors (e.g. EuO, CdCr2S4, etc.) plasmon and magnon energies are comparable. Therefore, there will be resonant interaction between these modes of excitations. On the basis of a new microscopic theory formulated for plasmon-magnon interaction, the effect of this interaction on the energies and lifetimes of plasmons and magnons has been calculated using the double-time Green’s function. The energy shifts are very small and the lifetimes of plasmons,τp, and magnons,τm, are of the order of 10−2 and 10−3 sec respectively.

    • Off-diagonal long-range order and currents in weakly coupled superconductors

      S Rangarajan Sudhanshu S Jha

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      Within the framework of quasi equilibrium approximation, Josephson’s expression for current in a weakly coupled superconducting system is derived without the use of any specific microscopic model. It is based only on the existence of the off-diagonal long-range order in the two-particle reduced density matrix. Allowing for deviations from the quasi equilibrium approximation, generalised Josephson equations are obtained which include ohmic terms. The effect of relaxation and thermal fluctuations is examined in detail to emphasise the physical origin of various terms in the expression for current.

    • Pressure effects on the semiconductor-semimetal transition in Ti2O3

      B Viswanathan S Usha Devi C N R Rao

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      Hydrostatic pressure has negligible effect on the resistivity anomaly and thecH/aH ratio of Ti2O3. The results are consistent with the band-crossing mechanism wherein the aT and eT bands cross as thecH/aH ratio increases.

    • Local magnetic ordering of Fe impurities in Pd2MnSn

      K R P M Rao P K Iyengar

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      Mössbauer effect of Fe57 embedded as very dilute substitutional impurities in Pd2MnSn was studied. The impurities are seen to replace the three elements in the alloy. Although the Curie temperature of the alloy is 189K, well below the room temperature, the Mössbauer spectrum recorded at room temperature consisted of two distinct 6-finger magnetic hyperfine spectra and a single unsplit line. One of the 6-finger patterns which corresponds to an internal magnetic field ofHint=−375 kOe is inferred to arise due to local magnetic coupling of the localized magnetic moments of Fe impurities at the Pd sites with those of the 4 Mn first nearest neighbours of the Fe impurities. The other 6-finger pattern which corresponds to an internal magnetic field ofHint=−335 kOe is inferred to arise due to the local magnetic coupling of the localized magnetic moments of the Fe impurities at the Sn sites with those of the 6 Mn second nearest neighboours of the Fe impurities. The difference in the internal magnetic fields observed at the Pd and Sn sites in the alloy could be understood qualitatively, on the basis of RKKY theory, as arising due to the different conduction electron polarization contributions to the net internal magnetic field at the Fe impurity sites. The results of the measurements suggest that the localized magnetic moments of Fe57 impurities at Pd and Sn sites are antiferromagnetically coupled with the moments of their neighbouring Mn atoms.

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