How the substitution of Zn for Cu destroys superconductivity in YBCO system

Atomistic simulation techniques were used to determine the Zn substitution sites in YBa 2 Cu 3 O 7 (123) and YBa 2 Cu 4 O 8 (124). It was found that Zn dopant is energetically favourable to substitute at the Cu(2) site. Zn substitution at both Cu(1) and Cu(2) sites is also possible, but Zn substitution at Cu(1) site seems unlikely. Our result implies that most Zn dopants go to the Cu(2) sites and a small percentage of Zn dopants goes to the Cu(1) sites, which is in agreement with the experimental results. We also found that the structural changes in Zn doped 124 and Zn doped 123 are opposite to that in 124 under pressure and in YBa 2 Cu 3 O 6.6 under pressure, respectively (note that T c is enhanced in the latter two cases). It is found that Zn subsitution for Cu(2) causes similar atomic displacements in YBa 2 Cu 3 O 7 and YBa 2 Cu 4 O 8 . We also found that Zn doping on the Cu(2) site causes of the Cu(1)O(1) bond length and an elongation of the Cu(2)O(1) bond length in these two materials. These structural changes may induce the charge carriers to transfer from CuO 2 plane to CuO chain resulting in T c decrease.

Structural changes in YBa2Cu3O7 and YBa2Cu4O8 by Zn doping

The partial molar enthalpy of incorporation of oxygen (Δ ox H ) and enthalpy of formation (Δ f H ) have been measured via high-temperature reaction calorimetry for YBa 2 Cu 3− x M x O y (where M=Ni, Zn) in the composition range 0.0 x ox H and Δ f H values were nearly independent of dopant for both Ni and Zn, with enthalpies of oxidation in the range −86 to −110 kJ/(mol 1 2 O 2 ) and enthalpies of formation in the range −110 to −118 kJ/mol. The independence of thermodynamic behavior from superconducting properties is consistent with an electronic mechanism for T c suppression in Zn-doped YBCO superconductors. Oxygen contents, lattice parameters, and T c measurements are also reported.

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Thermochemistry of YBa2Cu3−xMxOy (M=Ni, Zn)

The change of the electronic structure of YBa 2 Cu 3 O 7 caused by the local substitution of Li for Cu has been studied by the recursion method. The characteristics including the partial wave density of state, the types and intensities of bonds, and the valence on a site etc. are obviously affected by Li on the replacing site and its neighbours. In the case of the substitution of Li for the Cu(2) site, the two-dimensional character of the electronic structure in CuO 2 planes is locally destroyed, accompanied by the migration of electrons or holes, which causes the decrease of the hole concentration in the CuO 2 planes. The local substitution of Li for Y has also been discussed. This substitution can strengthen the two-dimensional character of the carriers in the CuO 2 planes. From our point of view, these changes may be responsible for the changes of T c in the Li-doped YBa 2 Cu 3 O 7 material.

Effect of the local substitution of Li for Cu on the electronic structure of YBa2Cu3O7

Polycrystalline samples of nominal composition Y 1− x Ca x Ba 2 Cu 3− x M x O 7− δ (M = Ni or Zn, 0.0⩽x⩽0.30) were investigated for their phase stability and superconducting properties. X-ray diffraction (XRD) patterns revealed that single-phase materials result only for x T c ( R = 0) value decreases from 92 K to 62 K and 41 K, respectively, for MNi and Zn, as the level of doping ( x ) increases from 0.0 to 0.15. However, for x ⩾0.20, the T c ( R =0) value remained nearly constant in each series. A comparative analysis of the superconducting behaviour of Y 1− x Ca x Ba 2 Cu 3− x M x O 7− δ , Y 1− x Ca x Ba 2 Cu 3 O 7− δ and YBa 2 Cu 3− x M x O 7− δ (M =Ni or Zn) showed that co-substitution of Ca does not counteract the adverse influence of Ni or Zn within the single-phase limit. However, it tends to reduce the detrimental effect of Ni and Zn on the T c values for x ⩾0.20, due to phase segragation. In addition, the effect of annealing co-doped samples in argon on T c was studied for x = 0.20. Upon co-doping, the samples were found to exhibit complex behaviour.

Superconducting behaviour of co-doped Y1−xCaxBa2Cu3−xMxO7−δ (M = Ni or Zn and 0.0 ⩽x⩽0.30)

The temperature dependence of the specific heat C(T) has been measured for YBa2(Cu1−xMx)3O7 (YBCO) with M = Zn and Ni and 1.2 λ LT = C(T) T at T = 1.5 K reaching values of 100–150 mJ⧸mol K2 for modest Zn and Ni doping. This γLT dependency for Zn doped YBCO has been associated with a localization of d-holes within the CuO2 planes caused by a disorder induced disruption of hybridization within the planes.

Light heavy-fermions in Zn and Ni doped 123 Cu-O superconductors

Single crystals and expitaxial thin films of Y 1−x Tb x Ba 2 Cu 3 O 7−δ F> ( x ⩽0.50) were prepared and char acterized by X-ray diffraction along with SEM and EDX. The films are single phase and highly c -axis aligned with Tb ions uniformly distributed. The temperature dependence of the electrical resistivity ϱ ( T ) and magnetic susceptibility ξ ( T ) (1.8 K⩽ T ⩽ 350 K) have been measured for these samples. The substitution of magnetic Tb for Y does not shift T c (92.5 K) and ϱ ( T ) is metallic-like (i.e., d ϱ d T>0 ) for T > T c , even for x as high as 0.5. The effective magnetic moment, ∼8.25μ B , estimated from the magnetic susceptibility, tends to suggest that the Tb is in a mixed-valence state. The results of this work underscore the complexity of possible mixed-valence rare earth elements in high- T c systems.

Superconductivity in Tb-doped YBa2Cu3O7−δ single crystals and thin films

The thermal conductivity K(T) of polycrystalline YBa2(Cu1−xNx)3O7 with N=Zn and Al (0 ⩽x⩽0.1) and Y1−yMyBa2Cu3O7 with M=Pr and Gd (0⩽y⩽1.0) has been measured for temperatures 1 .2⩽T⩽200 K. In the undoped samples, K(T) decreases slightly with decreasing temperature above Tc, with a strong enhancement of K(T) below Tc reaching a maximum at ∼50 K followed by a sharp decrease. Doping with Zn and Pr, which have been shown to dramatically disturb the superconductivity, rapidly depresses the maximum in K(T) even at concentrations that remain superconducting. This effect is not as dramatic for Al- and Gd-doped samples, which do not strongly depress the superconductivity. These data are discussed considering both phonon and electron thermal conduction mechanisms, and are compared with relevant theories, such as a modified BRT theory of phonon conduction within the superconducting state.

S.T. Ting

Papers

Light heavy-fermions in Zn and Ni doped 123 Cu-O superconductors

Superconductivity in Tb-doped YBa2Cu3O7−δ single crystals and thin films

Thermal conductivity of impurity-doped high-Tc superconductors