Effect of 3d metal ion (Fe, Ni, Zn) substitution in REBa2(Cu3−xMx)O7 (RE=Sm, Dy)

Tight-binding potentials for transition metals and alloys.

Abstract: The parameters of many-body potentials for fcc and hcp transition metals, based on the second-moment approximation of a tight-binding Hamiltonian, have been systematically evaluated. The potential scheme, cast in analytical form, allows us to reproduce correctly the thermal behavior of transition metals making use of a small set of adjustable parameters. The large cutoff, which extends the range of the interactions up to the fifth-neighbor distance, ensures good quantitative agreement with the experimental data up to temperatures close to the melting point. The ability of the potentials to describe real systems has been checked by calculating point-defect properties, lattice dynamics, and finite-temperature behavior, and by comparing the results with other potential schemes. Application of this scheme to bcc transition metals has proved unsuccessful. Examples of derivation of many-body potentials for a few transition-metal alloys with cubic structure are also reported.

Influence of Ni, Fe, and Zn substitution on the superconducting and antiferromagnetic state of YBa2Cu3O7- delta.

Abstract: We report magnetic and electric measurements on $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ with up to 15 at.% substitution of Ni, Fe, and Zn for Cu in the insulating state ($\ensuremath{\delta}\ensuremath{\simeq}1$) and the metallic state ($\ensuremath{\delta}\ensuremath{\simeq}0.1$). The concentration dependence of the Pauli susceptibility and the electrical conductivity indicates that the shift of the Fermi energy towards the mobility edge with increasing degree of substitution is one important mechanism for the suppression of the superconducting transition temperature. In the insulating antiferromagnetic state the Fe and Ni spins are very weakly coupled to the Cu spins. The Ni spins remain paramagnetic down to the lowest measuring temperature of 3 K, the Fe spins order at 5 K, independent of the Fe concentration. The N\'eel temperature depends weakly on the Fe and Zn concentration but decreases strongly with the Ni concentration. This different behavior reflects the different site preference for the substitution in the insulating state, namely Cu(2) for Ni and Cu(1) for Fe and Zn.

Influence of Cu-site substitution on the structure and superconducting properties of the NdBa2Cu3-xMxO7+ delta (M=Fe,Co) and NdBa2Cu3-xMxO7- delta (M=Ni,Zn) systems.

Abstract: The effect of substitution at the Cu site by the transition elements Fe, Co, Ni, and Zn on the structure and superconducting properties of ${\mathrm{NdBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ has been investigated. Compared to the ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3\mathrm{\ensuremath{-}}\mathit{x}}$${\mathit{M}}_{\mathit{x}}$${\mathrm{O}}_{7+\mathrm{\ensuremath{\delta}}}$ (M=Fe, Co, Ni, Zn) system, considerably high solid solubility and drastic ${\mathit{T}}_{\mathit{c}}$ suppression is achieved in the Nd 1:2:3 system. The maximum solid solubilities achieved for the dopants are x=1.0 for M=Fe and Co and x=0.8 for M=Ni and Zn. An orthorhombic-to-tetragonal (O\ensuremath{\rightarrow}T) transition is observed at x=0.08 for M=Fe, Co similar to the case of the M-doped Y 1:2:3 system. However, a second T\ensuremath{\rightarrow}O transition is observed in Nd 1:2:3 at x=0.5 for M=Co and x=0.8 for M=Fe. For the Ni- and Zn-doped phases, an O\ensuremath{\rightarrow}T transition is observed at x=0.4 for M=Ni and x=0.5 for M=Zn. The doped phases are superconducting up to x=0.2 (M=Fe, Co, Ni) and x=0.08 (M=Zn). The initial rate of ${\mathit{T}}_{\mathit{c}}$ suppression (up to 3.33 at. %) is \ensuremath{\sim}14 K/at. % for M=Fe, Co, and Ni and \ensuremath{\sim}28 K/at. % for M=Zn and is much higher than that reported for the Y 1:2:3 system. The Fe- and Zn-doped phases, heated in flowing ${\mathrm{N}}_{2}$ gas at 800 \ifmmode^\circ\else\textdegree\fi{}C followed by reoxygenation at 450 \ifmmode^\circ\else\textdegree\fi{}C show higher ${\mathit{T}}_{\mathit{c}}$ and sharper superconducting transition. Hole doping by Ca substitution (y) at the Nd site in the Zn-doped (x=0.1) semimetallic-semiconducting Nd 1:2:3 phase induces metallicity and superconductivity for y\ensuremath{\ge}0.1 and a ${\mathit{T}}_{\mathit{c}}$ of 32 K is obtained for y=0.2.

Ion size effect on Tc in RBa2Cu3−χGaχO7−y systems (R=Er, Y, Dy, Eu and Nd)

Abstract: The effect of Ga substitution for Cu in RBa2Cu3−χGaχO7−y (R=Er, Y, Dy, Eu and Nd) has been investigated for nominal composition χ=0, 0.05, 0.1, 0.15, 0.2 and 0.3. The superconducting transition temperature Tc falls with increasing Ga concentration, and at a constant Ga concentration, Tc decreases with increasing radius of the R ions. We suggest that the decrease of density of states N(EF) or the localization of carriers due to Ga substitution are the possible mechanisms of superconductivity suppression. The superconducting-nonsuperconducting transition accompanied by a transition into the semiconducting phase in these systems is interpreted in terms of the Mott transition.