Hole depletion and localization due to disorder in insulating PrBa2Cu3O7-d: a Compton scattering study

The (mostly) insulating behaviour of PrBa2Cu3O7-d is still unexplained and even more interesting since the occasional appearance of superconductivity in this material. Since YBa2Cu3O7-d 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, the wavefunction coherence length in PrBa2Cu3O7-d being strongly suppressed. We conclude that Pr on Ba-site substitution disorder is responsible for the metal-insulator transition. Preliminary efforts at growth with a method to prevent disorder yield 90K superconducting PrBa2Cu3O7-d crystallites.Comment: 4 pages, 3 figures, revised version submitted to PR

Credit Protection and Lending Relationships

We examine the impact CDS protection on lending relationships and efficiency. CDS insulate lenders against losses from forcing borrowers into default and liquidation. This improves the credibility of foreclosure threats, which can have positive implications for borrower incentives and credit availability ex ante. However, lenders may also abuse their enhanced bargaining power vis-a-vis borrowers and extract additional surplus in debt renegotiations. If this hold up threat becomes severe, borrowers will be reluctant to agree to debt maturity designs or control right transfers that would have been optimal in the absence of CDS protection. The introduction of CDS markets may then ultimately tighten credit constraints and be detrimental to welfare

High-energy Cu spin excitations in PrBa₂Cu₃O_6+x

This paper describes high-energy neutron inelastic scattering measurements of propagating magnetic excitations in PrBa2Cu3O6+x (x approx 0.2 and 0.93). The measurements probe the acoustic and optic modes of the antiferromagnetically-ordered copper--oxygen bilayers in the energy range 50-150 meV. The observed magnon dispersion can be described satisfactorily in this energy range by a spin wave model including intra- and inter-layer nearest-neighbour exchange constants J_parallel and J_perp. We find J_parallel = 127 +/- 10 meV and J_perp = 5.5 +/- 0.9 meV. The value of J_parallel is virtually the same as that found in YBa2Cu3O6.2, but J_perp is a factor of two smaller. To within experimental error the values of J_parallel and J_perp for PrBa2Cu3O6+x do not vary with oxygen doping.Comment: 6 pages, 5 figures. To be published in Phys. Rev.