Article
TwoDimensional Geometry of Spin Excitations in the High Temperature Superconductor YBa2Cu3O6+x
MaxPlanckInstitut für Festkörperforschung, 70569 Stuttgart, Germany.
Nature (Impact Factor: 41.46). 09/2004; 430(7000):6504. DOI: 10.1038/nature02774 Source: PubMed
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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 twodimensional Hubbard model. Singleparticle quantities, such as the spectral function, the quasiparticle weight and the entropy, display a crossover between two distinct ground states: a Fermi liquid at low filling and a nonFermi liquid with a pseudogap at high filling. Both states are found to cross over to a marginal Fermiliquid state at higher temperatures. For finite nextnearestneighbour hopping t', we find a classical critical point at temperature T(c). This classical critical point is found to be associated with a phaseseparation transition between a compressible Mott gas and an incompressible Mott liquid corresponding to the Fermi liquid and the pseudogap 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 pseudogap region, and that the marginal Fermiliquid behaviour in its vicinity is the analogue of the supercritical region in the liquidgas transition.Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 04/2011; 369(1941):167086. DOI:10.1098/rsta.2010.0228 · 2.15 Impact Factor 
 "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 coaligned on three Alplates with a mosaicity of < 1.2 • . "
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ABSTRACT: The pseudogap is one of the most pervasive phenomena of hightemperature superconductors1. It is attributed either to incoherent Cooper pairing setting in above the superconducting transition temperature, Tc, or to a hidden order parameter competing with superconductivity. Here, we use inelastic neutron scattering from underdoped YBa2Cu3O6.6 to show that the dispersion relations of spin excitations in the superconducting and pseudogap states are qualitatively different. Specifically, the extensively studied 'hour glass' shape of the magnetic dispersions in the superconducting state is no longer discernible in the pseudogap state and we observe an unusual 'vertical' dispersion with pronounced inplane anisotropy. The differences between superconducting and pseudogap states are thus more profound than generally believed, suggesting a competition between these two states. Whereas the highenergy excitations are common to both states and obey the symmetry of the copper oxide square lattice, the lowenergy excitations in the pseudogap state may be indicative of collective fluctuations towards a state with broken orientational symmetry predicted in theoretical work.Nature Physics 07/2008; 3(11). DOI:10.1038/nphys720 · 20.15 Impact Factor 
Article: Crystal growth and normal state transport of electron doped high temperature superconductors
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ABSTRACT: In der vorliegenden Arbeit werden verbesserte Wachstumsbedingungen und Temperparameter sowie der Einfluss von Sauerstoffstörstellen auf die strukturellen und elektronischen Eigenschaften von Re<sub>2x</sub>Ce<sub>x</sub>CuO<sub>4</sub> (Re = La, Pr, Nd, Sm) Einkristallen diskutiert. Für die Transportmessungen stand ein hochreiner Nd<sub>2x</sub>Ce<sub>x</sub>CuO<sub>4</sub> Probensatz zur Verfügung, der einen großen Bereich des elektrondotierten Phasendiagramms der Kuprat Hochtemperatursupraleiter abdeckt. Die sich ergebenden ungewöhnlichen Transportdaten im optimalen und überdotierten Bereich können im Rahmen konventioneller BoltzmannTransporttheorie erklärt werden, indem ein ZweiBand Modell mit anisotropen Streuraten angewandt wird.