UNCORRECTED PROOF Effect of grain boundaries on paraconductivity of YBa 2 Cu 3 O x D.K. Aswal a, * , Ajay Singh a , Shashwati Sen a , Manmeet Kaur a , C.S. Viswandham b , G.L. Goswami b , S.K. Gupta a a Technical Physics and Prototype Engineering Division, Bhabha Atomic Research Center, Mumbai 400 085, India b Atomic Fuels Division, Bhabha Atomic Research Center, Mumbai 400 085, India Received 25 June 2001; revised 29 November 2001; accepted 30 November 2001 Abstract To investigate the effect of grain boundaries on paraconductivity of YBa 2 Cu 3 O x , melt-textured and c-axis oriented thin ®lms with controlled grain boundaries superconducting transition width, DT, varying between 0.54 and 2.85 K) were prepared, and dc-conductivity has been measured as a function of temperature. In the logarithmic plots of excess-conductivity Ds ) and reduced temperature e ), starting from low values of e , we have observed three different regions namely critical region, mean ®eld region and short wave ¯uctuation region. A correlation is observed between the range of critical region and DT, which is found to increase with DT. While for DT values smaller than 2.5 K only static critical region is observed, for higher DTs both static and dynamic critical regions are observed. In the mean ®eld region a crossover from 3D to 2D was observed for all the samples. At e values larger than 0.24, the excess-conductivity decreased sharply as e 23 , which suggested the existence of the short wave ¯uctuations. q 2002 Published by Elsevier Science Ltd. Keywords: A. Oxides; B. Epitaxial growth; D. Electrical conductivity; D. Superconductivity 1. Introduction In high temperature superconductors HTS), due to their high transition temperature T c ), short coherence length j 0 ) and high anisotropy j c p j ab ), the effects of thermo- dynamic ¯uctuations are prominently well above the mean ®eld transition temperature T mf c : Effects of thermal ¯uc- tuations have been observed in various properties of HTS, including speci®c heat [1,2], magnetic susceptibility [3], magnetoresistance [4,5], Hall effect [6,7] and electrical conductivity [6,8±19]. In electrical conductivity measure- ments, as the sample temperature is lowered from room temperature towards T c , the ¯uctuating Cooper pairs begin to be created spontaneously from a temperature around 2T c . As the temperature approaches T c , the number of Cooper pairs increases rapidly at the expense of the normal electron density. As a result, the net resistance of the sample decreases, which is known as ¯uctuation-induced excess- conductivity or paraconductivity. The excess-conductivity in the Ginzburg±Landau GL) region [21] or mean ®eld region i.e. 1.01T c to 1.1T c ) has been measured in a wide variety of polycrystalline [6,8,9], thin-®lm [5,10±15] and single crystal [16±20] samples of YBCO and the results are explained using Aslamazov and Larkin AL) [22] or Lawrence and Doniach LD) [23] theories. According to LD theory [23], the excess-conductivity in GL region depends upon the dimensionality of the ¯uctuation and a dimensional crossover from 2D to 3D may occur due to interlayer coupling, which has experimentally been observed in YBCO samples [9]. The GL theory, however, breaks down both at temperatures very close to T c [24] and very far away from T c [25], i.e. on both sides of the mean ®eld region, and not many experimental studies are reported on these two regions. The small coherence length, on the other hand, implies that inhomogeneities such as structural twin boundaries, stacking faults, grain boundaries, etc.) and chemical imperfections oxygen de®ciency, etc.) will in¯uence both normal state and superconducting properties. For example, thicker grain boundaries lead to increased room temperature resistivity, broadened superconducting transition, reduced critical current density, etc. [26,27]. It is expected that the grain boundaries would also affect the ¯uctuation-induced excess-conductivity. Recently, Cukauskas et al. [28] have Journal of Physics and Chemistry of Solids 00 2002) 000±000 PCS2607 0022-3697/02/$ - see front matter q 2002 Published by Elsevier Science Ltd. PII: S0022-369701)00266-9 www.elsevier.com/locate/jpcs * Corresponding author. Tel.: 191-22-559-0451; fax: 191-22- 550-5151. E-mail address: dkaswal@apsara.barc.ernet.in D.K. Aswal). ARTICLE IN PRESS