RESISTIVITY OF EUBA2(CU1-YZNY)3OX AS A FUNCTION OF TEMPERATURE, MAGNETIC-FIELD, PRESSURE AND ZN CONCENTRATION

Abstract We report electrical resistance and magnetoresistance measurements on EuBa2(Cu1-yZny)3Ox under pressures to 17 kbar. With Zn substitution, Tc is depressed rapidly (∼19 K/ at.% Zn) and there are qualitative changes in the temperature and field response of the resistance. Pressure drives the Zn-substituted samples toward undoped EuBa2Cu3Ox behavior. Analysis of the temperature dependent resistance for y=0.05 and P=0, together with Hall effect and thermopower data, suggests that the suppression of superconductivity may arise from weak three-dimensional localization promoted by Coulomb interactions.

Understanding the mechanism for high superconducting transition temperatures T c in the oxide compounds RBa2Cu30 x is among topics most actively pursued currently. The observation [1] of virtual independence of T c when R is a rare earth highlighted the importance of the Cu-O planes/chains for superconductivity. Recent studies of the oxygen content [2] and 3d element substitution for Cu [3] have reinforced this viewpoint. However, there is still no consensus on the actual processes leading to superconducting pair formation and condensation.
We have studied the influence of small amounts of Zn in EuBa2(CUl_yZny)3Ox, where y = 1, 3.5 and 5 at. %, through electrical resistivity and magnetoresistance under pressures P< 17 kbar and Hall effect and thermopower measurements using conventional ac and/or dc techniques. As shown in fig. 1 the superconducting transition temperature is depressed by Zn substitution at a rate dTJdy =-19 K/at.%, which is over five times larger than for Cr and Mn doping [4] and nearly three times larger than for Ni [5].
Because Zn has a full 3d shell, this depression clearly is not related to magnetic pair-breaking effects.
In fig. 2   pressures as a function of temperature. In contrast to R(T) for EuBa2Cu3Ox, which is linear in T from above T~ to room temperature, the resistance for x = 0.01 varies non-monotonically with decreasing temperature, reaching a maximum near 80K before dropping to zero at ~45 K. With increasing pressure, the region of linear resistance extends to lower temperatures, T c increases, AT~ (10-90%) sharpens, and at the highest pressures the maximum in R disappears.
The inset of fig. 2 shows that dTjdP is almost ten times larger for x = 0.01 than for x = 0 and for other RBa2C%O ~ samples previously studied [6]. on the magnetic and thermal history of the sample. Magnetoresistance curves obtained in a similar manner for P = 11.6 kbar resemble those in fig. 4 for H > 0.05 T, while for P = 0 the resistance changes by less than 0.5% under an 8T field at all temperatures T < 300 K, as is also the case for undoped EuBa2Cu30 x.
In an earlier study of 3d element substitutions for Cu in YBa2(Cu0.09A0.i)306+a, Xiao [3] noted, on the basis of a moderate resistance upturn for T > To, that the conduction electrons appeared to become weakly localized before superconductivity set in and that this was most pronounced for Zn. The inset of fig. 3 shows a plot of electrical conductance versus T 2/3. The linear variation, 1/R~ T 2/3, found for 4< T< 120 K, is consistent with the power law dependence expected [7] for weak three-dimensional localization. If the observed behavior is due to localization, it arises from more than simple disorder because 5% Cr or Mn substitutions, while causing a resistance upturn at low temperatures before the onset of superconductivity, do not have such a dramatic effect.
Both the thermopower and Hall coefficient, positive for y = 0, remain positive but change substantially, by factors of nine and three respectively, upon substituting 5 at.% Zn, indicative of strong modifications to the Fermi surface character. Previously we have argued [8] for the presence of strong Coulomb correlations in YBa2Cu30 x. The data of figs. 1-4 might suggest that Coulomb effects become increasingly important with Zn doping. Reasoning for a plausible argument is as follows: Because Zn has a full d-band, substitution of Zn reduces the d-densityof-states at Ev, believed to be important for superconductivity [9], and consequently inhibits conduction screening of Coulomb interactions. The enhanced Coulomb effects in turn promote localization at the expense of superconductivity. On the other hand, pressure favors superconduc-tivity as shown in fig. 2. These arguments are consistent with a large body of data, in addition to those presented here, on the Cu-O superconductors.
In summary, Zn substitution for Cu in EuBa2(Cu 1 yZny)30 x results in a strong depression of superconductivity that may arise from Coulomb-correlation assisted localization. The application of pressure tends to drive y > 0 samples towards behaviors found in undoped EuBa2Cu30 x.