Volume 44, Issue 4
April 1995, pages 1-391
pp 1- April 1995 Rapid Communication
Giant magnetoresistance in bulk samples of LaMnO_{3} with varying Mn^{4+} content
R Mahendiran R Mahesh A K Raychaudhuri C N R Rao
Magnetoresistance (MR) in bulk samples of LaMnO_{3} has been investigated by varying the Mn^{4+} content from 10 to 33 per cent by chemical means, without aliovalent doping. With the increase in Mn^{4+} content, the structure of LaMnO_{3} changes first from orthorhombic to rhombohedral and then to cubic and the material becomes increasingly ferromagnetic, exhibiting a resistivity maximum akin to an insulator-metal transition atT_{Peak}, just below the ferromagneticT_{c}. The magnitude of MR is highest in the cubic sample (with 33% Mn^{4+}) around theT_{Peak}, and negligible in the non-magnetic orthorhombic sample (12% Mn^{4+}).
pp 281-293 April 1995
Patterned chaos in a system of three coupled identical unidirectional ring lasers
Three coupled identical unidirectional B-type ring lasers are considered and the dynamics for varying coupling parameter investigated. For given system parameters like pumping, cavity damping etc. the system is characterized by three distinct types of steady states. The linear stability of these steady states is investigated in detail which are unstable beyond a critical value of the coupling parameter. For a certain range of the coupling parameter chaotic behaviour of the system is demonstrated. The dynamical variables are mapped on to a complex variable reflecting the symmetry of the system. The plot of the real vs. the imaginary part of this complex variable exhibits patterns with threefold symmetry. The largest Lyapunov exponent as well as the power spectra corresponding to the chaotic attractors are also calculated.
pp 295-302 April 1995
Invariants of chaotic Hamiltonian systems
The invariants of chaotic bounded Hamiltonian systems and their relation to the solutions of the first variational equations of the equations of motion are studied. We show that these invariants are characterized by the fact that they either lose the property of differentiability as functions on phase space or that a certain formal power series defined in terms of the derivatives of the invariants has zero radius of convergence. For a specific example, we show that the former possibility appears to apply.
pp 303-316 April 1995
On singularity-free spacetimes
Naresh Dadhich L K Patel R Tikekar
We consider here the metric for the singularity-free family of fluid models. The metric is unique for cylindrically symmetric space-time with metric potentials being separable functions of radial and time coordinates in the comoving coordinates. It turns out that fluid models separate out into two classes, withρ ≠µp in general butρ = 3p in particular andp =ρ. It is shown that in both the cases radial heat flow can be incorporated without disturbing the singularity-free character of the spacetime. The geodesics of the singularity-free metric are studied and the geodesic completeness is established. Several previously known solutions are derived as particular cases.
pp 317-322 April 1995
Exact solutions of Einstein and Einstein-scalar equations in 2 + 1 dimensions
A nonstatic and circularly symmetric exact solution of the Einstein equations (with a cosmological constant Λ and null fluid) in 2 + 1 dimensions is given. This is a nonstatic generalization of the uncharged spinless BTZ metric. For Λ = 0, the spacetime is though not flat, the Kretschmann invariant vanishes. The energy, momentum, and power output for this metric are obtained. Further a static and circularly symmetric exact solution of the Einsteinmassless scalar equations is given, which has a curvature singularity atr = 0 and the scalar field diverges atr = 0 as well as at infinity.
pp 323-332 April 1995
Charge screening in a two potential theory of electromagnetism
By surrounding an Abelian Dyon with an axion-like domain wall we calculate the screening effects generated by the domain wall on the dyon degrees of freedom.
pp 333-345 April 1995
Dual nature of Ricci scalar and creation of spinless particles
Manifestation of Ricci scalar like a matter field as well as a geometrical field, at high energy, has been noted earlier [9]. Here, its interaction with another scalar field is considered in four-dimensional curved space-time. This interaction leads to the production of a large number of pairs of spinless particle-antiparticle due to expansion of the early universe in the vacuum state (provided by temperature dependent Coleman-Weinberg like potential for Ricci field), where spontaneous symmetry breaking takes place.
pp 347-356 April 1995
Two-loop Majorana neutrino mass and magnetic moment in a gauge model
Two-loop contributions to Majorana mass and transition magnetic moment in a gauge model not in conflict with decaying neutrino dark matter (DDM) hypothesis have been studied. Another variant of an earlier model [J Dhar and S Dev,Pramana — J. Phys.39 541 (1992)] consistent with the DDM hypothesis is proposed and is shown to lead to large enough neutrino magnetic moment and consistent with the phenomenological constraints on neutrino mass.
pp 357-374 April 1995
S-wave baryons in an equally mixed scalar-vector square root potential model of independent quarks
A relativistic model of independent quarks based on Dirac equation with an equally mixed scalar-vector square root confining potential is used to compute the quark core contributions to the static properties like magnetic moments, charge radii and axial vector coupling constant ratios of the baryon octet. The results obtained with appropriate corrections due to centre-of-mass motion agree fairly well with experimental values. The model is also extended to the study of magnetic moments of the quark core of baryons in the charmed andb-flavoured sectors and the overall predictions so obtained compare well with other model predictions.
pp 375-391 April 1995
Subrata Sarangi Jitendra C Parikh
A simple dynamic procedure, based on the deformed Hartree-Fock solution of a nucleus, is presented to construct the IBM operators in microscopic basis. The parameters of these operators are evaluated by establishing a Marumori mapping from the truncated shell model space onto the boson space. The transitions from spherical to axial-rotor shape observed in the low-lying levels ofeven^{96–108}Mo and^{146–154}Sm isotopes are reproduced qualitatively by applying this procedure with a fixed set of fermion input parameters to each chain. Variation of a few parameters in fermion space leads to quantitative agreement.
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