Paraconductivity study in ErBa_2Cu_(3-x)M_xO_(7-δ)(M=Zn,Fe) superconductors

We report here the paraconductivity of ErBa_2Cu_(3-x)M_xO_(7-δ)(M=Zn, Fe) superconductors.The logarithmic plots of excess conductivityσand reduced temperatureCreveal 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 to1,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(Baband Bc),and critical current density J at 0 K.Although Baband Bcare generally increased with doping content increasing,the value of Babis found to be twice more than that of Bc.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.