### Full-text

Dapeng P. Chen, Jul 06, 2015 Available from:- [Show abstract] [Hide abstract]

**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.Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 04/2011; 369(1941):1670-86. DOI:10.1098/rsta.2010.0228 · 2.86 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**The pseudogap is one of the most pervasive phenomena of high-temperature 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 in-plane anisotropy. The differences between superconducting and pseudogap states are thus more profound than generally believed, suggesting a competition between these two states. Whereas the high-energy excitations are common to both states and obey the symmetry of the copper oxide square lattice, the low-energy 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; DOI:10.1038/nphys720 · 20.60 Impact Factor -
##### Article: Crystal growth and normal state transport of electron doped high temperature superconductors

[Show abstract] [Hide abstract]

**ABSTRACT:**In der vorliegenden Arbeit werden verbesserte Wachstumsbedingungen und Temperparameter sowie der Einfluss von Sauerstoffstörstellen auf die strukturellen und elektronischen Eigenschaften von Re<sub>2-x</sub>Ce<sub>x</sub>CuO<sub>4</sub> (Re = La, Pr, Nd, Sm) Einkristallen diskutiert. Für die Transportmessungen stand ein hochreiner Nd<sub>2-x</sub>Ce<sub>x</sub>CuO<sub>4</sub> Probensatz zur Verfügung, der einen großen Bereich des elektron-dotierten Phasendiagramms der Kuprat Hochtemperatursupraleiter abdeckt. Die sich ergebenden ungewöhnlichen Transportdaten im optimalen und überdotierten Bereich können im Rahmen konventioneller Boltzmann-Transporttheorie erklärt werden, indem ein Zwei-Band Modell mit anisotropen Streuraten angewandt wird.