Dissipation mechanism in a high-T c Bi 1:7 Pb 0:3 Sr 2 Ca 2 Cu 3 O x granular superconductor G.L. Bhalla * , Pratima, Amita Malik 1 , K.K. Singh Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India Received 27 September 2002; received in revised form 6 February 2003; accepted 7 February 2003 Abstract Magneto-resistance studies in a high-T c granular superconductor Bi 1:7 Pb 0:3 Sr 2 Ca 2 Cu 3 O x in an applied magnetic field rangingfrom0to0.6Tinthelowcurrentlimits(1mA)havebeencarriedouttounderstandthedissipationmechanism in high-T c superconducting materials. The Kosterlitz–Thouless (KT) as well as the Ambegaoker–Halperin (AH) widely employed models are invoked to interpret the results. The KT theory is found to account for dissipation in the tem- perature range T KT < T < T GL , i.e. between the Kosterlitz–Thouless temperature and the Ginzberg–Landau transition temperature. The dissipation around T c cannot be explained adequately. It is suggested that dissipation in the entire range of temperature can be accounted for on the basis of a modified Ambegaokar–Halperin theory incorporating two mechanisms, viz. vortex-dynamics and the fluctuations of the order parameter and the two mechanisms crossover at a transition temperature (T BP ). Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: High-T c superconductors; Magneto-resistance; Dissipation; Order parameter; Vortex-dynamics 1. Introduction The complex mechanism of dissipation in high- temperature superconducting materials is a matter of interest because of their technological potentials particularly, in polycrystalline materials rather than single crystals. An understanding of their dynamic properties in the presence of a magnetic field is essential, since the properties are effected detrimentally by the magnetic field. Several mod- els, superconducting glass [1], flux creep [2], flux flow [3], Kosterlitz–Thouless transition [4], fluctu- ations [5], Josephson coupling [6], Ambegaokar and Halperin [7], thermally activated flux flow [8], etc.havebeenputforwardtodescribethedynamic properties and the dissipation near transition temperature. Usually these models invoke the motion of fluxons under the influence of Lorentz force as the fundamental mechanism for the dis- sipation in the superconductors. The dissipation, in the absence of magnetic field, is also attributed to the motion of the flux lines, but their origin and dynamics are still an open question. Soulen et al. [9] have examined various models and concluded that the modified Ambegaokar–Halperin [7] model satisfactorily describes the dissipation behaviour * Corresponding author. Tel.: +91-11-27667725x1114; fax: +91-11-27667061. E-mail address: glbhalla@physics.du.ac.in (G.L. Bhalla). 1 SRI-Delhi, India. 0921-4534/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-4534(03)00805-0 Physica C 391 (2003) 17–24 www.elsevier.com/locate/physc