• N Kumar

      Articles written in Pramana – Journal of Physics

    • A three dimensional ferromagnetic Ising ‘fluid’ model

      N Kumar K B Athreya

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      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.

    • Quantum first-passage problem

      N Kumar

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      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.

    • Quantum-ohmic resistance fluctuation in disordered conductors—An invariant imbedding approach

      N Kumar

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      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 thed-dimensional generalization of the above probability distribution, or rather the associated cumulant function—‘the free energy’. Ford=3, our analysis shows that the dispersion dominates the mobility edge phenomena in that (i) a one-parameterβ-function depending on the mean conductance only does not exist, (ii) one has a line of fixed-points in the space of the first two cumulants of conductance, (iii) an approximate treatment gives a diffusion-correction involving the second cumulant. It is, however, not clear whether the fluctuations can render the transition at the mobility edge ‘first-order’. We also report some analytical results for the case of the one-dimensional system in the presence of a finite electric field. We find a cross-over from the exponential to the power-law length dependence of resistance as the field increases from zero. Also, the distribution of resistance saturates asymptotically to a Poissonian form. Most of our analytical results are supported by the recent numerical simulation work reported by some authors.

    • Low-temperature resistance fluctuation in disordered conductors

      N Kumar

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      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 (d). In particular we show that the physics at the mobility edge may be dominated by these fluctuations. We also show that an external electric field tends to harness these fluctuations. Some observational consequences such as 1/f-noise at low temperatures are discussed. Our approach is based on invariant imbedding extended by us for this purpose.

    • Further studies on Ag/BPSCCO tapes using low purity materials

      S R Shukla Y S Reddy N Kumar S K Sharma R G Sharma

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      Transmission electron microscopic (TEM) studies are reported on Ag-clad Bi1.7 Pb0.4Sr1.8Ca2Cu3.5Ox tapes prepared by using low purity (98–99%) commercial grade materials. The self-fieldJc values of these tapes viz. 6.14 × 103 A.cm−2 at 77 K and 1.4 × 105 A.cm−2 at 4.2 K, reported in an earlier publication, were significantly higher than the correspondingJc values in tapes prepared with high purity (99.99%) materials. The TEM pictures on the low purity core material of the tapes reveal the presence of stacking faults and the intergrowth of the 2212 and 2223 phases which could be acting as flux pinning sites and responsible for enhancedJc values. These defects can perhaps be traced back to the presence of 60 ppm iron in the low purity CuO as revealed by atomic absorption analysis reported earlier.

    • Optimal barrier subdivision for Kramers’ escape rate

      Mulugeta Bekele G Ananthakrishna N Kumar

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      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.

    • Photon transport in thin disordered slabs

      Venkatesh Gopal S Anantha Ramakrishna AK Sood N Kumar

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      We examine using Monte Carlo simulations, photon transport in optically ‘thin’ slabs whose thickness L is only a few times the transport mean free path l*, with particles of different scattering anisotropies. The confined geometry causes an auto-selection of only photons with looping paths to remain within the slab. The results of the Monte Carlo simulations are borne out by our analytical treatment that incorporates directional persistence by the use of the Ornstein-Uhlenbeck process, which interpolates between the short time ballistic and long time diffusive regimes.

    • Bose-Einstein condensation: Where many become one and, therefore, there is plenty of room at the bottom

      N Kumar

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      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 4He 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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