N Kumar
Articles written in Pramana – Journal of Physics
Volume 1 Issue 2 August 1973 pp 98-103 Solids
A three dimensional ferromagnetic Ising ‘fluid’ model
The three dimensional ferromagnetic spin-half Ising model with an arbitrary external magnetic field is considered in the spatial continuum limit and under a certain tempering condition to be imposed on the pair-wise spin-spin interaction. An expression for the partition function has been obtained for a tempered RKKY type interaction. The solution predicts the classical mean-field behaviour above a critical temperature below which the spontaneous magnetization jumps discontinuously from zero to the saturation value.
Volume 25 Issue 4 October 1985 pp 363-367 Quantum Statistics
Formulation of quantum first passage problem is attempted in terms of a restricted Feynman path integral that simulates an absorbing barrier as in the corresponding classical case. The positivity of the resulting probability density, however, remains to be demonstrated.
Volume 27 Issue 1-2 July 1986 pp 33-42 Electromagnetics, Statistical Mechanics And General Relativity
Quantum-ohmic resistance fluctuation in disordered conductors—An invariant imbedding approach
It is now well known that in the extreme quantum limit, dominated by the elastic impurity scattering and the concomitant quantum interference, the zero-temperature d.c. resistance of a strictly one-dimensional disordered system is non-additive and non-self-averaging. While these statistical fluctuations may persist in the case of a physically thin wire, they are implicitly and questionably ignored in higher dimensions. In this work, we have re-examined this question. Following an invariant imbedding formulation, we first derive a stochastic differential equation for the complex amplitude reflection coefficient and hence obtain a Fokker-Planck equation for the full probability distribution of resistance for a one-dimensional continuum with a gaussian white-noise random potential. We then employ the Migdal-Kadanoff type bond moving procedure and derive the
Volume 28 Issue 5 May 1987 pp 503-508
Low-temperature resistance fluctuation in disordered conductors
At low temperatures the electron elastic mean free path in a disordered conductor can become much smaller than the inelastic mean free path (or more precisely the Thouless length) which in turn may be comparable with, or even larger than the sample size. In this quantum regime, the electrical resistance is dominated by the coherence effects that eventually lead to the now well-known weak or strong localization. Yet another remarkable manifestation of the quantum coherence is that it makes the resistance non-additive in series and, more importantly, non-self averaging, thus replacing the classical Ohm’s law with a quantum Ohm’s law describing statistical fluctuations. In this paper, we report on some of our recent work on the statistics of these “Sinai” fluctuations of residual resistance for one and higher space dimensions (
Volume 41 Issue 3 September 1993 pp 285-289 Brief Report
Further studies on Ag/BPSCCO tapes using low purity materials
S R Shukla Y S Reddy N Kumar S K Sharma R G Sharma
Transmission electron microscopic (TEM) studies are reported on Ag-clad Bi_{1.7} Pb_{0.4}Sr_{1.8}Ca_{2}Cu_{3.5}O_{x} tapes prepared by using low purity (98–99%) commercial grade materials. The self-field
Volume 46 Issue 6 June 1996 pp 403-410
Optimal barrier subdivision for Kramers’ escape rate
Mulugeta Bekele G Ananthakrishna N Kumar
We examine the effect of subdividing the potential barrier along the reaction coordinate on Kramers’ escape rate for a model potential. Using the known supersymmetric potential approach, we show the existence of an optimal number of subdivisions that maximizes the rate.
Volume 56 Issue 6 June 2001 pp 767-778 Research Articles
Photon transport in thin disordered slabs
Venkatesh Gopal S Anantha Ramakrishna AK Sood N Kumar
We examine using Monte Carlo simulations, photon transport in optically ‘thin’ slabs whose thickness
Volume 67 Issue 1 July 2006 pp 101-112
Bose-Einstein condensation and superfluidity are well known to occur in the dilute gaseous as well as in the dense liquid state of matter having a fixed number of Bose particles. Very recently, experimental evidence has been obtained for the probable realization of BEC and superfluidity in ^{4}He in the solid state too, as revealed through its non-classical rotational moment of inertia — smaller than that for the solid. Such a solid that can also subtend a flow as superfluid — and hence a supersolid — is indeed a surprise of condensed matter physics. In this conversation, an order-parameter description for the supersolid state will be given in which the superfluid flow is decoupled from the crystalline density modulation which remains at rest in the laboratory frame, thus giving it a nonclassical inertia.
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