Magnetic correlations on the full chains of Ortho-II YBa2_2Cu3_3O6.5_{6.5}

We propose that the NMR line shape on the chain Cu in the stoichiometric high-$T_c$ superconductor Ortho-II YBa$_2$Cu$_3$O$_{6.5}$ is determined by the magnetization induced on Cu near O vacancies, due to strong magnetic correlations in the chains. An unrestricted Hartree-Fock calculation of a coupled chain-plane Hubbard model with nearest-neighbor d-wave pairing interaction shows that the broadening of NMR lines is consistent with disorder-induced magnetization at low temperatures. In addition, we give a possible explanation of the anomalous bimodal line shape observed at high temperatures in terms of nonuniform Cu valence in the chains. The proximity between chains and CuO plane induces anisotropic magnetization on the planar Cu, and broadens the plane NMR lines in accordance with that of the chain lines, in agreement with experiment. We discuss implications of the model for other experiments on underdoped YBCO.Comment: 8 pages, 8 figures, submitted to PR

Spin excitations in layered antiferromagnetic metals and superconductors

The proximity of antiferromagnetic order in high-temperature superconducting materials is considered a possible clue to the electronic excitations which form superconducting pairs. Here we study the transverse and longitudinal spin excitation spectrum in a one-band model in the pure spin density wave (SDW) state and in the coexistence state of SDW and the superconductivity. We start from a Stoner insulator and study the evolution of the spectrum with doping, including distinct situations with only hole pockets, with only electron pockets and with pockets of both types. In addition to the usual spin-wave modes, in the partially gapped cases we find significant weight of low-energy particle-hole excitations. We discuss the implications of our findings for neutron scattering experiments and for theories of Cooper-pairing in the metallic SDW state.Comment: (14 pages, 6 figures

Two impurities in a d-wave superconductor:local density of states

We study the problem of two local potential scatterers in a d-wave superconductor, and show how quasiparticle bound state wave functions interfere. Each single-impurity electron and hole resonance energy is in general split in the presence of a second impurity into two, corresponding to one even parity and one odd parity state. We calculate the local density of states (LDOS), and argue that scanning tunneling microscopy (STM) measurements should be capable of extracting information about the Green's function in the pure system by a systematic study of 2-impurity configurations. In some configurations highly localized, long-lived states are predicted. We discuss the effects of realistic band structures, and how 2-impurity STM measurements could help distinguish between current explanations of LDOS impurity spectra in the BSCCO-2212 system.Comment: 16 pages,21 figure,New Version to be Published on P.R.

Quantum interference in nested d-wave superconductors: a real-space perspective

We study the local density of states around potential scatterers in d-wave superconductors, and show that quantum interference between impurity states is not negligible for experimentally relevant impurity concentrations. The two impurity model is used as a paradigm to understand these effects analytically and in interpreting numerical solutions of the Bogoliubov-de Gennes equations on fully disordered systems. We focus primarily on the globally particle-hole symmetric model which has been the subject of considerable controversy, and give evidence that a zero-energy delta function exists in the DOS. The anomalous spectral weight at zero energy is seen to arise from resonant impurity states belonging to a particular sublattice, exactly as in the 2-impurity version of this model. We discuss the implications of these findings for realistic models of the cuprates.Comment: 12 pages, 10 figs, submitted to Phys. Rev.