Showing papers by "Tata Institute of Fundamental Research published in 1981"

Production spectrum of gamma rays in interstellar space through neutral pion decay

Abstract: A simple representation is obtained of the observed invariant cross section for the production of neutral pions in proton-proton collisions. Using this representation, the differential and integral production spectra of gamma rays in the galaxy are calculated from interactions of cosmic ray nuclei with interstellar gas. It is shown that the uncertainties in deducing interstellar proton spectrum by demodulating the observed spectrum have only a limited effect on the gamma ray spectrum. Also determined is the gamma ray production spectrum through bremsstrahlung process for a typical interstellar electron spectrum derived from the radio spectrum in the galaxy.

Strong itinerant magnetism in ternary boride CeRh3B2

Abstract: Reports a high-temperature itinerant ferromagnet CeRh3B2 which orders magnetically at 115K. The magnetic ordering seems to be intrinsic and presumably arises from the Rh d-band. Similar compounds of La, Pr and Nd do not show any magnetic ordering down to 77K.

Chewing and smoking habits in relation to precancer and oral cancer

Abstract: In a prospective epidemiologic house-to-house survey of a random sample in the district of Ernakulam in Kerala State, the annual incidence rate of leukoplakia per 1,000 adults was found to be 2.1 for males and 1.5 for females. The rate was highest in the mixed tobacco habits group and lowest (0) in the no habits group. During the same period, oral cancer developed only among the individuals, who had a history of a previously diagnosed oral lesion. Malignant transformation was significantly higher among the speckled leukoplakia cases. The rate of malignant transformation was also highest among leukoplakias associated with tobacco chewing habits. These results suggest that leukoplakias associated with different tobacco habits may have a different natural history.

Dyson-Schwinger equations approach to the large- N limit: Model systems and string representation of Yang-Mills theory

Abstract: Simple model systems like the $\mathrm{O}(N)$ $\ensuremath{\sigma}$ model, the Gross-Neveu model, and the random matrix model are solved at $N\ensuremath{\rightarrow}\ensuremath{\infty}$ using Dyson-Schwinger equations and the fact that the Hartree-Fock approximation is exact at $N\ensuremath{\rightarrow}\ensuremath{\infty}$. The complete string equations of the $U(\ensuremath{\infty})$ lattice gauge theory are presented. These must include both string rearrangement and splitting. Comparison is made with the "continuum" equations of Makeenko and Migdal which are structurally different. The difference is ascribed to inequivalent regularization procedures in the treatment of string splitting or rearrangement at intersections.

Geometric construction of cohomology for arithmetic groups I

Abstract: (1) a proof that every unitary representation with non-zero cohomology, see Betel [2], occurs in L ~ ( I ~ S O (n, 1)) for a suitable uniform F, thereby proving for the group SO (n, 1) a conjecture which seems to be widely believed, though the group SO (n, 1) represents the only case where it has been proved. In terms of differential geometry we show that the n dimensional manifolds of constant negative curvature constructed at the end of Borel [4] admit finite covers with all Betti numbers strictly between 0 and n not equal to zero--and hence arbitrarily large by Borel ~2];

A gustatory mutant of Drosophila defective in pyranose receptors

Abstract: Mutations in an X-linked gene, gust-A, block the responses of Drosophila melanogaster to a group of pyranose sugars. It is shown that the behavioural effects of this mutation are correlated with a loss of electrical responses in taste receptors. The mutation affects the chemoacceptors for pyranose sugars leaving the furanose acceptors intact.

Measurement of Richardson constant of GaAs Schottky barriers

Abstract: The Richardson constants for near-ideal AuGaAs and AlGaAs Schottky diodes have been determined from an analysis of forward current-voltage characteristics. Measurement of capacitance-voltage characteristics at different temperatures shows that the barrier heights of the diodes have very similar temperature dependence for both Au and Al contacts. On taking into account the temperature dependence of the barrier height, the corrected value of the Richardson constant for AuGaAs diode is found to be very close to the predicted theoretical value. The corresponding value for AlGaAs diode is about a factor of five smaller which is explained on the basis of a thin interfacial layer between Al and GaAs.

Dynamics of a two-dimensional order-disorder transition

Abstract: We present results of a Monte Carlo study of the time development of a two-dimensional order-disorder model binary alloy following a quench to low temperature from a disordered, high-temperature state. The behavior is qualitatively quite similar to that seen in a recent study of a three-dimensional system. The structure function exhibits a scaling of the form K/sup 2/(t)S(k,t) = G(k/K(t)) where the moment K(t) decreases with time approximately like t/sup -1/2/. If one interprets this moment as being inversely proportional to the domain size, the characteristic domain growth rate is proportional to t/sup -1/2/. Additional insight into this time evolution is obtained from studying the development of the short-range order, as well as from monitoring the growth of a compact ordered domain embedded in a region of opposite order. All these results are consistent with the picture of domain growth as proposed by Lifshitz and by Cahn and Allen.

Secondary production of antiprotons in cosmic radiation

Abstract: A reliable representation to the invariant cross-section for the production of antiprotons ( $$\bar P$$ ) in inclusive reactions has been obtained, which fits the data extremely well from threshold to ISR energies. Using this, the production spectrum of $$\bar P$$ by cosmic ray interaction with interstellar gas is calculated and is compared with other existing calculations. The equilibrium spectrum $$\bar P$$ in the Galaxy has been derived from about 100 MeV to a few hundred GeV in kinetic energy using Leaky Box Model for the propagation of cosmic rays, by taking into account all energy loss processes. It is found that $$\bar P$$ /P ratio calculated here is very much smaller than the observed ratio. In view of the fact that this excess of observed $$\bar P$$ /P ratio is much larger than the measured upper limits on the fraction of antimatter in the form of antinuclei, the excess $$\bar P$$ /P ratio has not been attributed to the existence of antimatter. Instead, it is shown that Closed Galaxy Model for the propagation of cosmic rays predicts more antiprotons than observed. Therefore, it is suggested that if cosmic rays contain about 50% new component of local origin, the resulting Closed Galaxy Model explains well the observations. The present calculations also predict too small a flux of $$\bar P$$ below a few hundred MeV to make this energy region ideally suited to look for antiprotons of primordial origin.

Effect of annealing on the Richardson constant of Al-GaAs Schottky diodes

Abstract: Effect of heat-treatment on near-ideal Al-GaAs Schottky barrier diodes has been studied. Forward current-voltage and capacitance-voltage characteristics are used to determine Richardson constant and barrier height. It is shown that due to annealing at 250°C, the Richardson constant reduces by a factor of 2–3. Also, the difference between barrier heights measured from I–V and C–V characteristics is found to increase after heat-treatment. The results are interpreted in terms of an interfacial AlxGa1-xAs layer which gets formed during annealing.

Cosmic ray antiprotons and modified closed galaxy model

Abstract: Observations suggest that a small flux of secondary antiprotons (p), created in the interaction of cosmic ray nuclei with interstellar gas, should also be present in the primary cosmic radiation. Calculations based on accelerator data for the production of p and existing models for the propagation of cosmic rays, predict too small a flux of p compared with the finite flux observed recently1. Although the observed excess could be attributed to the existence of primary cosmic ray antimatter, the experimental upper limits2,3 of the ratio of antinuclei to nuclei of charge |Z| ≥ 2 seem to contradict this. Here we use a ‘modified closed galaxy’ model to attribute this excess to secondary antiprotons. In this model, ∼50% of the observed cosmic ray nucleons are of recent origin and they propagate according to the ‘nested leaky box’ model, while the rest propagate according to ‘closed galaxy’ model. This model explains the observations on p and e+, and predicts more D and 3He than do the existing models.

55Mn NMR Study in Magnetically Ordered State of RMn2 (R=Rate Earth) Intermetallic Compounds

Abstract: NMR of 55 Mn has been observed in magnetically ordered state of cubic Laves phase compounds GdMn 2 , TbMn 2 , DyMn 2 and HoMn 2 and in hexagonal Lave phases ErMn 2 and TmMn 2 . Magnetization and neutron diffraction studies have shown non zero Mn moment in cubic compounds only. Accordingly, we find that hyperfine fields at Mn site are larger in cubic than in hexagonal compounds. The observed hyperfine field values are used to estimate Mn moments in these compounds.

Magnetic and structural studies on copper(II) dialkyldithiocarbamates

Abstract: The magnetic susceptibilities of a series of copper(II) dialkyldithiocarbamates have been measured over the range 4–290 K, the alkyl groups being methyl, ethyl, isopropyl, and n-butyl. Contrary to a previous report of strong ferromagnetic interaction in the diethyl derivative, we find no evidence of any significant exchange interaction in this compound. The dimethyl and di-isopropyl analogues show weak antiferromagnetic interactions. Only the di-n-butyl derivative in the dialkyl series shows evidence of strong magnetic exchange interaction but of an antiferromagnetic nature; this effect is peculiar to the phase recrystallized from chloroform–light petroleum (α), the phase obtained from chloroform–ethanol (β) showing no such interaction. To seek the origin of the exchange interaction in the α derivative, its crystal structure has been determined by single-crystal X-ray diffraction methods at 295 K and refined by least squares to a residual 0.032 for 1 976 ‘observed’ reflections. Crystals are triclinic, P, a= 15.29(1), b= 9.963(7), c= 9.243(7)A, α= 67.94(7), β= 82.92(7), γ= 71.55(7)°, and Z= 2. The copper environment is the usual pseudo-square-planar array of four sulphur atoms from two bidentate ligands (〈Cu–S〉, 2.31 A), but there is a long fifth interaction [Cu ⋯ S, 2.899(4)A] through the inversion centre leading to pseudo-dimer formation. Although similar to the diethyl analogue in this respect, differences are observed in regard to (a) the bridging geometry in the ‘dimer’ and (b) the proximity to the ‘dimer’ sulphur ligands of sulphur atoms from neighbouring dimers at ca. 3.8 A. The likely relative importance of these two features in determining the origin of the antiferromagnetic coupling is discussed. The structure of the β phase has also been determined, the final residual being 0.036 for 1 324 ‘observed’ reflections. Crystals are monoclinic, P21/n, a= 14.593(5), b= 7.840(2), c= 10.822(5)A, β= 101.55(3)°, and Z= 2. The molecules are located with the copper atoms on crystallographic centres of symmetry, and the only significant intermolecular interactions observed are S ⋯ H contacts. The CuS4 entity is planar with 〈Cu–S〉 2.30 A.

The minimal forbidden subgraphs for generalized line-graphs

Abstract: It is shown, by exhibiting the list, that there are exactly 31 nonisomorphic minimal forbidden subgraphs for generalized line graphs, that is, graphs which are representable by a subset of the root system Dn for some n. This solves a problem of Hoffman. Further, it is proved that the family of minimal forbidden subgraphs for graphs whose adjacency matrix has the least eigen-value at least-2, is finite.

Structure and function of propranolol: A β-adrenergic blocking drug

Abstract: In our earlier studies using quantum chemical methods we had proposed that propranolol has an extended structure. These results were confirmed using proton NMR. We have now carried out extensive magnetic resonance and model building studies to examine the interaction of this drug with model membranes. The effect of propranolol on organization of lipid bilayers has been studied using ESR spin labeling technique. Spin label Tempo and spin labeled stearic acid (5 SASL) have been used to monitor changes in the fluidity of model membranes. Presence of the drug is found to fluidize the lipids. In case of 0.2M dipalmitoyl phosphatidyl choline (DPPC), presence of drug (0.1M) is found to decrease the gel-liquid crystalline phase transition temperature by about 10°C. The order parameter measured from the spectrum of 5 SASL shows a 4% decrease on incorporation of the drug in membranes. 13 C spin lattice relaxation time (T 1 ) measurements have been carried out for different nuclear sites of the drug. The aromatic moiety shows a high degree of molecular rigidity when the drug is bound to the lipid bilayers. The oxypropanolamine group is however relatively flexible. It appears from these studies that the aromatic group binds strongly to the hydrophobic regions of the lipid bilayer, while the oxypropanolamine moiety remains relatively free and lies in the hydrophilic region. The 13 C chemical shifts indicate the involvement of the β-hydroxyl group in hydrogen bonding with the lipids. The NH 2 + group may be involved in electrostatic interactions with the negatively charged phosphate group of the lipid bilayers.