Paraconductivity study in ErBa₂Cu_3−xM_xO_7−δ (M = Zn, Fe) superconductors

We report here the paraconductivity of ErBa₂Cu_3−xM_xO_7−δ (M = Zn and Fe) superconductors. The logarithmic plots of excess conductivity δσ and reduced temperature ℂ reveal two different exponents corresponding to crossover temperature as a result of shifting the order parameter from 2 to 3. The first exponent in the normal field region is close to 1, in which the order parameter dimensionality (OPD) is 2. The second exponent in the critical field region is close to 0.5, in which the OPD is 3. The coherence length, interlayer coupling, interlayer separation and carrier concentration decrease with increasing doping content, and their values for Fe samples are different from those of Zn samples. While anisotropy is increased with increasing doping content, it is generally higher for a Zn sample than that for an Fe sample. We also estimate several physical parameters such as upper critical magnetic fields in the a—b plane and along the c axis (B_ab and B_c), and critical current density J at 0 K. Although B_ab and B_c are generally increased with doping content increasing, the value of B_ab is found to be twice more than that of B_c. A similar behavior is obtained for J (0 K) and its value is higher in the Fe sample than that in the Zn sample. These results are discussed in terms of oxygen deficiency, localization of carriers, and flux pinning, which are produced by doping