Insulating PrBa2Cu3O7−δ and superconducting YBa2Cu3O7−δ: charge transfer modified by disorder

The (mostly) insulating behaviour of PrBa2Cu3O7−δ is still unexplained and even more interesting since the occasional appearance of superconductivity in this material. Since YBa2Cu3O7−δ is nominally iso-structural and always superconducting, we have measured the electron momentum density in these materials. We find that they differ in a striking way whereas LDA based calculations predict similar behaviour for both. We conclude that some Pr on Ba-site substitution disorder exists and is responsible for the observed effects. Preliminary efforts at growth with a method to prevent disorder directly yield 90K superconducting PrBa2Cu3O7−δ crystallites. PACS numbers: 74.72Jt, 74.72Bk, 74.62Dh, 78.70Ck Typeset using REVTEX 1 The peculiar case of PrBa2Cu3O7−δ stands out in the RBa2Cu3O7−δ family (R for rare earth or Y) because it is the only easily fabricated member which until recently was consistently insulating in the normal state and non-superconducting (for a review see [1,2]). Superconducting samples have since been fabricated [3,4], some of which show metallic behaviour in the normal state, akin to that seen in YBa2Cu3O7−δ. The non-superconducting behaviour has been attributed to various causes: (1) +4 valence for Pr leading to hole filling in the Cu-O layers, (2) hybridization between the Pr 4f and O 2p orbitals [5], leading to hole localization in these layers, (3) a chain-based model for superconductivity and magnetic pair-breaking induced by Pr on the Ba site [3]. The first possibility has been invalidated on the grounds that Pr on the rare-earth site is essentially trivalent [6,7] while the last is unlikely if only because it fails to explain why non-superconducting PrBa2Cu3O7−δ should be insulating in the normal state. The Fehrenbacher-Rice model of hole-localization [5] remains a possible explanation, and to get further insight into this matter we have measured the electron momentum density in both YBa2Cu3O7−δ and PrBa2Cu3O7−δ using Compton scattering of high energy X-rays. This is the first high statistics and high resolution measurement in these materials, which are difficult candidates for such experiments given the problems related to fluorescence background from heavy atoms and low count-rates due to absorption. These problems have been overcome thanks to high energy synchrotron radiation, and dedicated instrumentation at ID15B at the ESRF [8,9]. Our measurements show that the experimental momentum density is globally similar to that given by our LDA band-structure calculations for YBa2Cu3O7−δ. However in PrBa2Cu3O7−δ, a striking difference is seen between measurement and calculation, a difference that cannot be explained only by allowing for Pr-O hybridization in the calculation. Furthermore, a calculation which supposes an isotropic, spherical contribution of the Pr and Ba-O planes to the total momentum density is closer to the measured spectrum. This would be the case if there were no long-range order due to Pr on Ba site defects (labelled PrBa hence-forward) in these planes. These facts are interpreted in the light of other experiments [6–10,14–17,22–27,30–35,39] in the Y1−xPrxBa2Cu3O7−δ system. 2 The experiments were performed on crystals grown by the flux method in the non reactive crucible material BaZrO3, known to give uncontaminated, high-purity samples [10]. The YBa2Cu3O7−δ crystals were annealed under 1 bar of oxygen at 530 C for 170 hours yielding a Tc of 91.3 K and an oxygen content of 6.89 [12]. The PrBa2Cu3O7−δ crystals were annealed under 1 bar of oxygen at 500C for 200 hours yielding an oxygen content of 6.92 [13], and were insulating and non-superconducting. Both exhibited uniform twins in the a-b plane, as is usually observed after such a treatment. These twinned crystals were used for the experiment. Compton scattering, or inelastic scattering with very high momentum and energy transfer is a probe of the ground state, one-electron properties of the system [11]. The measured one dimensional quantity J(p z ) or the Compton profile,is a projection of the three dimensional electronic momentum density ρ(p) onto the scattering vector (parallel to p z ):