Nuclear Magnetic Resonance and High-Temperature Superconductivity in YTTRIUM(1-X)PRASEODYMIUM(X)BARIUM Cuprate

Copper nuclear magnetic resonance (NMR) has been performed on praseodymium-doped YBa_2Cu _3O_7, to investigate the nature of the depression of the superconducting transition temperature T_{c} with Pr concentration in this series and to provide insight into the microscopic and magnetic properties of high T_{c } planar cuprate materials. Praseodymium is unique among rare-earth dopants in suppressing superconductivity in the high T _{c} cuprate YBa _2Cu_3O_7 while maintaining the orthorhombic structure of the host. Temperature dependence of the Knight shift K and the nuclear relaxation rate 1/T_1 has been observed in Y-rich Y_{1-x}Pr _ xBa_2Cu _3O_7. Its striking resemblance to the behavior found in oxygen deficient YBa_2Cu_3O _{7-y} provides evidence that T_{c} depression in the former is primarily due to the removal of hole carriers as a consequence of Pr being close to tetravalent state. This hole-filling mechanism is consistent with the observed antiferromagnetic ordering of Cu spins on the plane sites in both PrBa_2Cu_3O_7 and YBa_2Cu_3O_6 since the absence of the doped-holes on the Cu -O_2 planes enhances spin correlations among Cu local moments. On the other hand, extensive analysis of the spin susceptibility has shown a tendency towards trivalency of Pr in specimens with low x values, suggesting that in the dilute limit pair-breaking due to conduction band-4f hybridization is also involved in the depression of T_{c}. The mixed-valent nature of Pr must therefore be considered for any adequate explanation of the suppression of superconductivity in Y_{1-x}Pr_ xBa_2Cu_3 O_7. The phenomenological model of antiferromagnetic Fermi liquid (AFL) proposed by Millis, Monien and Pines has been used to explain the behavior of the planar ^{63}Cu relaxation rate and to extract information on the strength of correlations among the local spins. Analysis has shown that the unusual behavior of the relaxation rate is a consequence of a competition between the temperature dependence of the correlation length and spin susceptibility. Magnetic correlations have been found to increase with higher Pr doping, consistent with a decrease of mobile hole concentration. The intimate relation between spin fluctuations and hole-carrier density in these materials offers the suggestion that magnetism may be important in high temperature superconductivity.