Study of Ageing and Fluctuation Effects in MgB 2 Superconductor
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1099-II04-04
Study of Ageing and Fluctuation Effects in MgB2 Superconductor Suchitra Rajput, and Sujeet Chaudhary Department of Physics, Indian Institute of Technology Delhi, New Delhi, 110016, India ABSTRACT For device applications of a superconducting material, its stability to ambient conditions is an issue of critical concern. The study of the ageing effects of the recently discovered intermetallic compound MgB2 is vital for knowing the degree of usefulness of this material. We report here the study of ageing and fluctuation conductivity behavior in bulk MgB2 pellets and stainless-steel sheathed, in-situ synthesized MgB2 tapes. A possible degradation mechanism is proposed based on ac-magnetic susceptibility and XRD investigations of MgB2 samples exposed to ambient over prolonged durations of time. In addition, the fluctuation induced conductivity behavior is also analyzed based on the observed resistivity (ρ)-vs-T measurements in MgB2 tape samples. This study indicated a three dimensional character of the MgB2 with reasonably good estimates of coherence the length. The scaling of fluctuation induced conductivity enhancement is also investigated from the ρ(H) vs T data obtained for different applied field strengths. These findings support the three-dimensional nature of thermodynamic fluctuations operative in MgB2 in contrast to that prevalent in several other high-temperature superconductors (HTSCs). INTRODUCTION In the case of HTSCs, there exist several reports on the ageing and excess conductivity arising due to thermodynamics fluctuations in the order parameter. The study of the ageing effects of the newly discovered MgB2 superconductor is vital to understanding its usefulness and, so far, there are few reports on the effect of ambient atmosphere and water exposure on MgB2 samples [1,2]. In this paper, we report a detailed study on the correlation of degradation of superconducting properties and/or phase dissociation of MgB2 samples on exposure to ambient environment for a wide span of time, so as to find out the stability of the samples and the possible degradation mechanism in bulk MgB2 samples. It may be recalled here that few reports exist on the fluctuation study of doped [3] and undoped [4, 5] MgB2 thin films and single crystal MgB2 [6]. The second part of this study pertains to this fluctuation-induced conductivity on insitu fabricated stainless steel sheathed MgB2 tapes via their R vs. T measurements. The dimensionality of the conduction of the carriers and the coherence length is estimated from the various fits to the observed fluctuation enhanced conductivity data for MgB2 tapes. In addition, scaling of fluctuation induced conductivity enhancement data corresponding to the different applied magnetic fields has also been carried out to find the nature of the thermodynamic fluctuations operative in the system. It may be emphasized that such studies are relatively easy in the case of HTSCs due to large broadening of the superconducting transition near TC due to higher thermal energy/TC. EXPERIMENTAL D
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