Exciton annihilation on dendrimeric trees

Exciton-exciton annihilation on Cayley tree like dendrimer molecules are investigated via Monte Carlo simulations. Annihilation reaction of the type A+A→0 is considered to calculate the exciton density decay for multiexciton diffusion on dendrimers. Exciton density decays as a power law with a continuously varying exponent in a linear potential. For the case of realistic nonlinear potential of phenylacetylene dendrimers (Phys. Rev. Lett. 77 (1998) 4656) the excitons accumulate around the free energy minimum and annihilate each other quickly.     

A four-neutrino mixing scheme for observed neutrino data

It has been observed that simultaneous explanation of the solar and atmospheric neutrino deficits and the reported evidence for oscillation from the Los Alamos Liquid Scintillator Detector (LSND) requires at least one extra neutrino species in addition to the three known ones. The extra neutrino must be sterile with respect to the known weak interactions. We present a new mass matrix for these four neutrinos in which the LSND effect and the atmospheric neutrino deficit are governed by only one parameter. We investigate the phenomenological implications of such a mass matrix ansatz and suggest possible ways to understand it in gauge theories.     

Ion Temperature Measurement in the Edge Region of SINP Tokamak by a Retarding Field Analyser

Based on the principle of retarding field analyser an ion energy analyser has been developed for the measurement of ion temperature in the edge plasma region of the SINP tokamak. This has been used to determine the dependence of the ion temperature on the safety factor q in the normal q region (q=2 to 4), plasma current, plasma electron temperature and also the Spitzer resistivity.     

An automated thermal relaxation calorimeter for operation at low temperature (0.5 K<T<10 K)

We describe an automated calorimeter for measurement of specific heat in the temperature range 10 K>T>0.5 K. It uses sample of moderate size (100–1000 mg), has a moderate precision and accuracy (2%–5%), is easy to operate and the measurements can be done quickly with He⁴ economy. The accuracy of this calorimeter was checked by measurement of specific heat of copper and that of aluminium near its superconducting transition temperature.     

Neutron-antineutron oscillations and SO (10) grand unification

Within the framework of the survival hypothesis for Higgs scalars we comprehensively examine the following question: could there be neutron-antineutron oscillations in SO(10) grand unified theories which would be detectable in the forthcoming experiments? In the process of answering this, we critically discuss and supplement the existing knowledge of the relevant patterns of SO(10) symmetry breakdown in relation to the said oscillations. However, our conclusions are negative with the oscillation period being 10²⁵ years or higher.     

On the energy of Yang-Mills fields in the presence of an external source

An expression is derived for the energy of a non-Abelian solution of the Yang-Mills equations in the presence of an external source, which permits an immediate comparison with the energy of the corresponding Coulomb-type solution, For any semisimple gauge group, a number of different field configurations are exhibited which always lead to solutions of lower energy than the Coulomb solution independent of the strength and symmetry properties of the source. The various implications of these results in view of the known instability of the Coulomb-type solution of a spherically symmetric source are discussed.     

A comparison of barrier type tunnel junction and point contact tunnel junction formed on the same highT c material

By making a combination of both point contact and barrier type tunnel junctions on a single sample of the highT _c 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/dV —V 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.     

Synthesis of multicyclic pyrrolidines via copper(I) catalyzed photobicyclization of ethyl N,N-diallyl carbamates

Ultraviolet irradiation of ethyl N,N-diallylcarbamates in the presence of copper(I) trifluoromethanesulfonate (CuOTf) catalyst results in photobicyclization to give bi or tricyclic pyrrolidines incorporating the N-carboethoxy-3-azabicyclo [3.2.0] heptane moiety.