Electric field gradients in PrBa2Cu3O7: suppression of (super)conductivity by mis-substitution

Full text: Since the early years of discovery the high-temperature superconductivity, the most prepared PrBa2Cu3O7 (Pr-123) samples have shown non(super)conducting behavior in contrast to RBa2Cu3O7 (R=Y or a rare earth element) samples (R-123). However, (super)conductivity in some Pr-123 samples has recently been reported by some groups. Some people argue that some mis-substitutions (mainly Pr on Ba site (PrBa)) suppress the intrinsic (super)conductivity in pure Pr-123. In spite of it, the intrinsic (super)conductivity of Pr-123 has not been accepted, yet. In the present work, the electric field gradient (EFG) and asymmetry parameter at all oxygen sites of the PrBa2Cu3O7 system were calculated using the full-potential (linearized)-augmented-plane-wave plus local orbitals (FP-(L)APW+lo) method. The plane-wave cutoff was chosen RminKmax=7.5 and the wave function in the atomic region was expanded up to l=10. The Brillouin-zone sampling in the self-consistent calculations was performed using 16 special k-points in the irreducible Brillouin zone (IBZ). Calculations with 72 k points yielded almost the same EFG results. To describe the exchange and correlation interactions the LSDA+U was employed for Pr(4f) and Cu(3d) orbitals. We have shown that there is a direct relation between the number of superconducting holes in O 2 (O 3) site and the EFG at this site in R-123 systems. The amount of calculated EFG at O 2 site of Pr-123 is close to the EFG at O 3 site. However, the experimental EFG at O 2 site are noticeably less than O 3 site in the non(super)conducting Pr-123 samples. It was shown that PrBa mis-substitution comprehensively reduces the superconducting holes in both O 2 and O 3 sites. Our calculations argue that PrBa mis-substitution, which forms the migration of O 4 atoms from their sites to the O 5 sites, can be responsible for the suppression of superconductivity in Pr-123 samples. (authors)