Spin fluctuations in a two-dimensional marginal Fermi liquid

Hidden itinerant-spin phase in heavily overdoped La(2-x)Sr(x)CuO4 superconductors revealed by dilute Fe doping: a combined neutron scattering and angle-resolved photoemission study

We demonstrated experimentally a direct way to probe a hidden propensity to the formation of a spin-density wave in a nonmagnetic metal with strong Fermi surface nesting. Substituting Fe for a tiny amount of Cu (1%) induced an incommensurate magnetic order below 20 K in heavily overdoped La(2-x)Sr(x)CuO(4). Elastic neutron scattering suggested that this order cannot be ascribed to the localized spins on Cu or doped Fe. Angle-resolved photoemission revealed a strong Fermi surface nesting inherent in the pristine La(2-x)Sr(x)CuO(4) that likely drives this order. Our finding presents the first example of the long-sought "itinerant-spin extreme" of cuprates, where the spins of itinerant doped holes define the magnetic ordering ground state; it complements the current picture of cuprate spin physics that highlights the predominant role of localized spins at lower dopings.

Vertex corrections and the Korringa ratio in strongly correlated electron materials

We show that the Korringa ratio, associated with nuclear magnetic resonance in metals, is unity if vertex corrections to the dynamic spin susceptibility are negligible, the hyperfine coupling is momentum independent, and there exists an energy scale below which the density of states is constant. In the absence of vertex corrections we also find a Korringa behaviour for T(1), the nuclear spin relaxation rate, i.e., [Formula: see text], and a temperature independent Knight shift. These results are independent of the form and magnitude of the self-energy (so far as is consistent with neglecting vertex corrections) and of the dimensionality of the system.

Persistence of high-frequency spin fluctuations in overdoped superconducting La2-xSrxCuO4 (x=0.22)

We report a detailed inelastic neutron scattering study of the collective magnetic excitations of overdoped superconducting La(1.78)Sr(0.22)CuO(4) for the energy range 0-160 meV. Our measurements show that overdoping suppresses the strong response present for optimally doped La(2-x)Sr(x)CuO(4) which is peaked near 50 meV. The remaining response is peaked at incommensurate wave vectors for all energies investigated. We observe a strong high-frequency magnetic response for E approximately >80 meV suggesting that significant antiferromagnetic exchange couplings persist well into the overdoped part of the cuprate phase diagram.

Imaging quasiparticle interference in Bi2Sr2CaCu2O8+delta

Scanning tunneling spectroscopy of the high-Tc superconductor Bi2Sr2CaCu2O8+delta reveals weak, incommensurate, spatial modulations in the tunneling conductance. Images of these energy-dependent modulations are Fourier analyzed to yield the dispersion of their wavevectors. Comparison of the dispersions with photoemission spectroscopy data indicates that quasiparticle interference, due to elastic scattering between characteristic regions of momentum-space, provides a consistent explanation for the conductance modulations, without appeal to another order parameter. These results refocus attention on quasiparticle scattering processes as potential explanations for other incommensurate phenomena in the cuprates. The momentum-resolved tunneling spectroscopy demonstrated here also provides a new technique with which to study quasiparticles in correlated materials.

Commensurate dynamic magnetic correlations in La2Cu0.9Li0.1O4

When sufficient numbers of holes are introduced into the two-dimensional CuO2 square lattice, dynamic magnetic correlations become incommensurate with underlying lattice in all previously investigated La(2-x)A(x)Cu(1-z)B(z)O(4+y) ( A = Sr or Nd, B = Zn) including high T(c) superconductors and insulators, and in bilayered superconducting YBa2Cu3O6.6 and Bi2Sr2CaCu2O8. Magnetic correlations also become incommensurate in structurally related La2NiO4 when doped with Sr or O. We report an exception to this so-far well-established experimental "rule" in La(2)Cu(1-z)Li(z)O4 in which magnetic correlations remain commensurate.