Two-Dimensional Geometry of Spin Excitations in the High Temperature Superconductor YBa2Cu3O6+x

Max-Planck-Institut für Festkörperforschung, 70569 Stuttgart, Germany.
Nature (Impact Factor: 41.46). 09/2004; 430(7000):650-4. DOI: 10.1038/nature02774
Source: PubMed


The fundamental building block of the copper oxide superconductors is a Cu4O4 square plaquette. The plaquettes in most of these materials are slightly distorted to form a rectangular lattice, for which an influential theory predicts that high-transition-temperature (high-T(c)) superconductivity is nucleated in 'stripes' aligned along one of the axes. This theory received strong support from experiments that indicated a one-dimensional character for the magnetic excitations in the high-T(c) material YBa2Cu3O6.6 (ref. 4). Here we report neutron scattering data on 'untwinned' YBa2Cu3O6+x crystals, in which the orientation of the rectangular lattice is maintained throughout the entire volume. Contrary to the earlier claim, we demonstrate that the geometry of the magnetic fluctuations is two-dimensional. Rigid stripe arrays therefore appear to be ruled out over a wide range of doping levels in YBa2Cu3O6+x, but the data may be consistent with liquid-crystalline stripe order. The debate about stripes has therefore been reopened.

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    • "Experiments suggest that cuprate superconductors are susceptible to charge inhomogeneities , such as stripes or checkerboard modulations (Hinkov et al., 2004). These inhomogeneous charge patterns have stimulated intense theoretical and experimental research. "
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    ABSTRACT: Transport measurements on the cuprates suggest the presence of a quantum critical point (QCP) hiding underneath the superconducting dome near optimal hole doping. We provide numerical evidence in support of this scenario via a dynamical cluster quantum Monte Carlo study of the extended two-dimensional Hubbard model. Single-particle quantities, such as the spectral function, the quasi-particle weight and the entropy, display a crossover between two distinct ground states: a Fermi liquid at low filling and a non-Fermi liquid with a pseudo-gap at high filling. Both states are found to cross over to a marginal Fermi-liquid state at higher temperatures. For finite next-nearest-neighbour hopping t', we find a classical critical point at temperature T(c). This classical critical point is found to be associated with a phase-separation transition between a compressible Mott gas and an incompressible Mott liquid corresponding to the Fermi liquid and the pseudo-gap state, respectively. Since the critical temperature T(c) extrapolates to zero as t' vanishes, we conclude that a QCP connects the Fermi liquid to the pseudo-gap region, and that the marginal Fermi-liquid behaviour in its vicinity is the analogue of the supercritical region in the liquid-gas transition.
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    • "The experiments were performed on an array of 180 individually detwinned YBa 2 Cu 3 O 6.6 crystal with superconducting transition temperatures (midpoint) of T c ≈ 61 K and width ∆T c ≈ 2 K, determined for each crystal by magnetometry [13]. They were co-aligned on three Al-plates with a mosaicity of < 1.2 • . "
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