Effect of Zn doping on charge transport in YBa2Cu3O7-y

Effect of Ni and Zn substitution on the physical and electrical properties of NdBaSrCu3O7−δ high temperature superconductor

Polycrystalline NdBaSrCu[Formula: see text]MxO[Formula: see text] samples ([Formula: see text] or Zn for [Formula: see text]) were prepared via solid-state reaction and examined by X-ray diffraction (XRD), Scanning electron microscopy (SEM), AC susceptibility and electrical resistance (dc) measurements. This study is focused on the effect of magnetic and non-magnetic particles substitution on the superconductivity of this tetragonal NdBaSrCu3O[Formula: see text] sample. There is no significant dependence of doping concentration for the changes of a-, b-lattice parameter but c-lattice parameter increased with both Ni and Zn substituted samples contrary to the ionic size considerations of the dopants. Results showed decrease of the critical temperature, [Formula: see text] and critical current density [Formula: see text] with increase of both doping contents. [Formula: see text] is the peak temperature of the imaginary part of the complex AC susceptibility, [Formula: see text]. Decrease of [Formula: see text] is related to the disruption of spin correlation of the CuO2 planes. Both doping only affect the [Formula: see text] without changing the transition width. The results showed that both Ni and Zn substitutions alter the quantity of spin-singlet pairs but have no effect on the size of the spin gap in the ladder. However, Zn doping more clearly lowered the [Formula: see text] than Ni doping. It is proposed that Zn, a non-magnetic dopant, caused a greater induction of unpaired charge carriers.

Pseudogap formation due to charge-transfer transition and Kondo effect

We investigate the doping evolution of the electronic state of the three-bandt-J-Umodel considering the normal state of the hole-doped high-Tcsuperconducting cuprate. In our model, when some number of holes are doped into the undoped state, thedelectron exhibits the charge-transfer (CT)-type Mott-Hubbard transition along with a chemical potential jump. A reduced CT gap is formed from thepband and the coherent component of thedband, and it shrinks due to charge fluctuations as more holes are doped as in the pseudogap (PG) phenomenon. This trend is reinforced as thed-pband hybridization is increased, and a Fermi liquid state is retrieved as in the Kondo effect. These suggest that the PG in the hole-doped cuprate emerges due to the CT transition and the Kondo effect.

Persistence of the superconducting condensate far above the critical temperature of YBa2(Cu,Zn)3O(y) revealed by c-axis optical conductivity measurements for several Zn concentrations and carrier doping levels

The superconductivity precursor phenomena in high temperature cuprate superconductors is studied by direct measurements of the superconducting condensate with the use of the c-axis optical conductivity of YBa2(Cu1-xZnx)3Oy for several doping levels (p) as well as for several Zn concentrations. Both the real and imaginary parts of the optical conductivity clearly show that the superconducting carriers persist up to the high temperatures Tp that is higher than the critical temperature Tc but lower than the pseudogap temperature T*. Tp increases with reducing doping level like T*, but decreases with Zn substitution unlike T*.

T(c) suppression in co-doped striped cuprates

We propose a model that explains the reduction of T(c) due to the pinning of stripes by planar impurity co-doping in cuprates. A geometrical argument about the planar fraction of carriers affected by stripe pinning leads to a linear T(c) suppression as a function of impurity concentration z. The critical value z(c) for the vanishing of superconductivity is shown to scale like T(2)(c) in the incompressible stripe regime and becomes universal in the compressible regime. Our theory agrees very well with the experimental data in single- and bilayer cuprates co-doped with Zn, Li, Co, etc.

63Cu NMR evidence for enhanced antiferromagnetic correlations around Zn impurities in YBa2Cu3O6.7

Doping the high- T(c) superconductor YBa2Cu3O6.7 with 1.5% of nonmagnetic Zn impurities in CuO2 planes is shown to produce a considerable broadening of 63Cu NMR spectra, as well as an increase of low-energy magnetic fluctuations detected in 63Cu spin-lattice relaxation measurements. A model-independent analysis demonstrates that these effects are due to the development of staggered magnetic moments on many Cu sites around each Zn and that the Zn-induced moment in the bulk susceptibility might be explained by this staggered magnetization. Several implications of these enhanced antiferromagnetic correlations are discussed.