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Register a new userNernst Effect in NdBa_2[Cu_{1-y}Ni_y]_3O_{7-delta}
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In NdBa_2[Cu_{1-y}Ni_y]_3O_{7-delta}, magnetic Ni-impurities suppress Tc but at the same time the pseudogap is strongly enhanced. This unique feature makes it an ideal system to study possible relations between the anomalous Nernst effect, superconductivity and the pseudogap. We present Nernst effect measurements on a series of optimally doped (O_7) and underdoped (O_{6.8}) samples with Ni contents ranging from y=0 to 0.12. In all samples an onset of the Nernst signal is found at T^ u > Tc. For the optimally doped samples T^ u and Tc decrease simultaneously with increasing Ni content. The underdoped samples show a different behavior, i.e. the onset of the Nernst signal is hardly affected by increasing the Ni content from y=0 to 0.03. Irrespective of the oxygen content, T^ u clearly does not track the enhanced pseudogap temperature T*.
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The intragrain pinning in high-$T_c$ superconductor compounds Y$_{1-x}$RE$_{x}$Ba$_{2}$Cu$_{3}$O$_{7-delta}$ with low concentration of RE (La, Ce, Pr) was investigated. Magnetic and transport measurements reveal that the pinning is maximal for the concentration of heterovalent RE such that the average distance between the impurity ions in the plane of rare-earth elements close to the diameter of Abrikosov vortices in YBCO.
I-V characteristics of polycrystalline Y(1-x)CaxBa2Cu3O(7-{delta}) samples (x=0.025 and 0.20) have been measured at different temperatures and magnetic fields in the range 0.1 T-6.9 T. The scaling behavior has been established for both samples at all magnetic fields. The dynamic exponent z displays some morphology dependence with higher value for small grain size sample Y0.8Ca0.2Ba2Cu3O(7-{delta}). The static exponent { u} has been determined from {rho} vs. T dependence at given magnetic field. The critical exponents are field independent with one only exception ({ u} - for Y0.975Ca0.025Ba2Cu3O(7-{delta}) sample). This is connected with the special interrelation between the vortex correlation length, {xi}, and intervortex spacing {alpha} ({xi} leq {alpha}) at all magnetic fields above Tg for this sample and its better pinning.
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