Neutron diffraction investigation of the H-T phase diagram above the longitudinal incommensurate phase of BaCo2V2O8

The quasi-one-dimensional antiferromagnetic Ising-like compound BaCo2V2O8 has been shown to be describable by the Tomonaga-Luttinger liquid theory in its gapless phase induced by a magnetic field applied along the Ising axis. Above 3.9 T, this leads to an exotic field-induced low-temperature magnetic order, made of a longitudinal incommensurate spin-density wave, stabilized by weak interchain interactions. By single-crystal neutron diffraction we explore the destabilization of this phase at a higher magnetic field. We evidence a transition at around 8.5 T towards a more conventional magnetic structure with antiferromagnetic components in the plane perpendicular to the magnetic field. The phase diagram boundaries and the nature of this second field-induced phase are discussed with respect to previous results obtained by means of nuclear magnetic resonance and electron spin resonance, and in the framework of the simple model based on the Tomonaga-Luttinger liquid theory, which obviously has to be refined in this complex system.Comment: 7 pages, 5 figure

Nonmagnetic insulator state in Na1CoO2 and phase separation of Na vacancies

Crystallographic, magnetic, and NMR properties of a NaxCoO2 single crystal with x≃1 are presented. We identify the stoichiometric Na1CoO2 phase, which is shown to be a nonmagnetic insulator, as expected for homogeneous planes of Co3+ ions with S=0. In addition, we present evidence that, because of slight average Na deficiency, chemical and electronic phase separation leads to a segregation of Na vacancies into the well-defined, magnetic, Na0.8CoO2 phase. The importance of phase separation is discussed in the context of magnetic order for x≃0.8 and the occurrence of a metal-insulator transition for x→1

Cu(2) nuclear resonance evidence for an original magnetic phase in aged 60K-superconductors RBa2Cu3O6+x (R=Tm,Y)

It is widely believed that the long-range antiferromagnetic order in the RBa2Cu3O6+x compounds (R=Y and rare earths except of Ce, Pr, Tb) is totally suppressed for the oxygen index x>0.4 (AFM insulator-metal transition). We present the results of the copper NQR/NMR studies of aged RBa2Cu3O6+x (R=Tm,Y) samples showing that a magnetic order can still be present at oxygen contents x up to at least 0.7 and at temperatures as high as 77K.Comment: 7 pages, 6 figures. Submitted to Phys.Rev.

Transport Coefficients of the Anderson Model via the Numerical Renormalization Group

The transport coefficients of the Anderson model are calculated by extending Wilson's NRG method to finite temperature Green's functions. Accurate results for the frequency and temperature dependence of the single--particle spectral densities and transport time $\tau(\omega,T)$ are obtained and used to extract the temperature dependence of the transport coefficients in the strong correlation limit. The low temperature anomalies in the resistivity, $\rho(T)$, thermopower, $S(T)$, thermal conductivity $\kappa(T)$ and Hall coefficient, $R_{H}(T)$, are discussed. All quantities exhibit the expected Fermi liquid behaviour at low temperature with power law dependecies on $T/T_{K}$ in very good agreement with analytic results based on Fermi liquid theory. Scattering of conduction electrons in higher, $l>0$, angular momentum channels is also considered and an expression is derived for the corresponding transport time and used to discuss the influence of non--resonant scattering on the transport properties.Comment: 45 pages, RevTeX, 28 figures, available on reques

Superconducting Fluctuations and the Pseudogap in the Slightly-overdoped High-Tc Superconductor TlSr2CaCu2O6.8: High Magnetic Field NMR Studies

From measurements of the ^{63}Cu Knight shift (K) and the nuclear spin-lattice relaxation rate (1/T_{1}) under magnetic fields from zero up to 28 T in the slightly overdoped superconductor TlSr_{2}CaCu_{2}O_{6.8} (T_{c}=68 K), we find that the pseudogap behavior, {\em i.e.}, the reductions of 1/T_{1}T and K above T_{c} from the values expected from the normal state at high T, is strongly field dependent and follows a scaling relation. We show that this scaling is consistent with the effects of the Cooper pair density fluctuations. The present finding contrasts sharply with the pseudogap property reported previously in the underdoped regime where no field effect was seen up to 23.2 T. The implications are discussed.Comment: 10 pages, 4 GIF figures, to be published in Phys. Rev. Let

Fine structure in the off-resonance conductance of small Coulomb blockade systems

We show how a fine, multiple-peak structure can arise in the off-resonance, zero-bias conductance of Coulomb blockade systems. In order to understand how this effect comes about one must abandon the orthodox, mean-field understanding of the Coulomb blockade phenomenon and consider quantum fluctuations in the occupation of the single-particle electronic levels. We illustrate such an effect with a spinless Anderson-like model for multi-level systems and an equation-of-motion method for calculating Green's functions that combines two simple decoupling schemes.Comment: 5 pages, 3 figures, postscript file also available at http://www.pa.uky.edu/~palacios/papers/eom.ps One figure added. Discussion of results extende

Phase diagrams in nonlocal PNJL models constrained by Lattice QCD results

Based on lattice QCD-adjusted SU(2) nonlocal Polyakov--Nambu--Jona-Lasinio (PNJL) models, we investigate how the location of the critical endpoint in the QCD phase diagram depends on the strenght of the vector meson coupling, as well as the Polyakov-loop (PL) potential and the form factors of the covariant model. The latter are constrained by lattice QCD data for the quark propagator. The strength of the vector coupling is adjusted such as to reproduce the slope of the pseudocritical temperature for the chiral phase transition at low chemical potential extracted recently from lattice QCD simulations. Our study supports the existence of a critical endpoint in the QCD phase diagram albeit the constraint for the vector coupling shifts its location to lower temperatures and higher baryochemical potentials than in the case without it.Comment: 23 pages, 10 figures. Version accepted in Phys. Part. Nucl. Lett. (to appear), references adde

Multiband model of high Tc superconductors

We propose an extension to other high T_{c } compounds of a model introduced earlier for YBCO. In the ''self-doped'' compounds we assume that the doping part (namely the BiO, HgO, TlO planes in BSCCO, HBCCO, TBCCO respectively) is metallic, which leads to a multiband model. This assumption is supported by band structure calculations. Taking a repulsive pairing interaction between these doping bands and the CuO_{2} bands leads to opposite signs for the order parameter on these bands and to nodes whenever the Fermi surfaces of these bands cross. We show that in BSCCO the low temperature dependence of the penetration depth is reasonably accounted for. In this case the nodes are not located near the 45^{o} direction, which makes the experimental determination of the node locations an important test for our model. The situation in HBCCO and TBCCO is rather analogous to BSCCO. We consider the indications given by NMR and find that they rather favor a metallic character for the doping bands. Finally we discuss the cases of NCCO and LSCO which are not ''self-doped'' and where our model does not give nodes.Comment: 11 pages, revtex, 1 figure

Planar Cu and O hole densities in high-Tc cuprates determined with NMR

The electric hyperfine interaction observable in atomic spectroscopy for O and Cu ions in various configurations is used to analyze the quadrupole splitting of O and Cu nuclear magnetic resonance (NMR) in La2-xSrxCuO4 and YBa2Cu3O6+y and to determine the hole densities at both sites as a function of doping. It is found that in La2-xSrxCuO4 all doped holes (x) reside in the Cu-O plane but almost exclusively at O. For YBa2Cu3O6+y and y<0.6 doped holes are found at planar Cu as well as O. For y>0.6 further doping increases the hole content only for planar O. The phase diagram based on NMR data is presented. Further implications from the Cu A and B site in La2-xSrxCuO4 and the two planar O sites in YBa2Cu3O6+y and consequences for the phase diagram are discussed.Comment: 18 pages, 1 figure, 2 tables, 2 appendice