SCOPF analysis of YBa 2 Cu 3 O 7  d þ xCr 2 O 3 superconductor composite M. Sahoo n , D. Behera Department of physics, National institute of technology, Rourkela-769008, India article info Article history: Received 21 September 2012 Received in revised form 7 February 2013 Accepted 12 February 2013 Available online 18 February 2013 Keywords: A. HTSC C. X-ray diffraction D. Electrical conductivity D. Microstructure D. Intragranular abstract A series of YBa 2 Cu 3 O 7d þxCr 2 O 3 (x ¼0.0, 2.3, 4.4, and 6.1 wt%) samples were prepared using solid state reaction method. Microstructural analysis reveal the reduced grain size with the incorporation of magnetic Cr 2 O 3 particles in the YBCO matrix. XRD graphs show the unchanged orthorhombic structure and improved oxygen ordering in the composite samples. With the increase of Cr 2 O 3 wt%, it is found that the superconducting transition temperatures determined from standard four-probe method decreases gradually. Excess conductivity fluctuation analysis using Aslamazov–Larkin model fitting reveals transition of two dominant regions (2D and 3D) above T c . 2D to 3D crossover temperature i.e. Lawerence–Doniach temperature that demarcates dimensional nature of fluctuation inside the grains is influenced by Cr 2 O 3 incorporation in YBCO matrix. The decrease in Lawerence–Doniach temperature in the mean field region has been observed as a consequent dominance of 3D region with increase in wt% of Cr 2 O 3 in the composite. The decrease in zero-resistance critical temperature and increase in transition width signifies the degradation of intergrain weak links. & 2013 Elsevier Ltd. All rights reserved. 1. Introduction The Type-II superconductors are marked by short coherence length, high transition temperature and anisotropy property. They are also recognized for their ability to carry large transport currents in the presence of a magnetic field by strong pinning force. Even after several decades of research on high-T c super- conductors, these materials are still interesting for many researchers. Recent interest in these materials is to combine them with ferromagnetic, ferroelectric, etc materials for new insights and device applications [1–3]. The high temperature supercon- ductor (HTSC) YBa 2 Cu 3 O 7 d (YBCO) is one of the materials being considered as potential candidates for applications such as microwave devices, power transmission tape, and delay lines. This is the most useful material for conductor applications because of its high irreversibility field. Superconductivity in YBCO triple perovskite structures resides in the Cu–O planes. It has crystallographically two distinct Cu sites i.e. Cu(1) site in CuO chains and Cu(2) in CuO 2 planes. It is known that CuO 2 layers play an important role in transferring of the charge carriers, whereas CuO chains are non-superconducting and acts as ‘charge reser- voir’. From the structural point of view, substitution at the Cu(1) site can cause an increase in the oxygen content in the basal plane, inducing orthorhombic to tetragonal (O–T) transition with increasing dopant concentration. If the substitution takes place on Cu(2) site, there is no observed structural O–T transition, the structure remains orthorhombic even for higher dopant concentrations and oxygen order in the CuO chains remains intact [4]. The oxygen in the basal plane acts as a charge reservoir introducing holes into CuO 2 plane copper atoms. The theoretical developments have indicated that the antiferromagnetic spin fluctuations in HTSC favors d-wave pairing. But due to disorder in the structure of the orthorhombic HTSC, distortion of the pairing interaction takes places, giving rise to s-wave pairing component in the original d-wave pairing. Many experiments have shown that, the CuO 2 sheet in the YBCO structure is a superconducting layer (S) where as the CuO chain layer is a non- superconductor (N) [5]. The hopping interaction between these S–N layers leads to the suppression of T c [6,7]. The divalent cations substitute preferentially at the Cu(2) site where as trivalent cations substitute preferentially at the Cu(1) site. The substitution of Cu by non-magnetic ions such as Zn, Ca and Ga or magnetic ions (Cr, Co, Fe) in the high-temperature superconductor is a useful tool for probing superconductivity parameters [8]. Excess conductivity studies in the paracoherence region of HTSC are a subject of interest for the past few years. Superconducting transition is a second-order phase transition characterized by an order parameter which is zero above the transition temperature T c and grows to a finite value below T c . The effect of fluctuations of this order parameter is a tool to under- stand the nature of the phase transition near T c . The resistive transition of sintered samples of the HTSC shows a characteristic two stage behavior. When the temperature is decreased, one first Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jpcs Journal of Physics and Chemistry of Solids 0022-3697/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jpcs.2013.02.011 n Corresponding author. Tel.: þ9166 124 627 24, fax: þ916 612 462 999. E-mail addresses: sahoomousumi0@gmail.com, hellomousumi05@yahoo.co.in (M. Sahoo). Journal of Physics and Chemistry of Solids 74 (2013) 950–956