An Electronic Model for CoO2CoO_2 layer based systems: Chiral RVB metal and Superconductivity

Takada et al. have reported superconductivity in layered Na__x CoO_2.yH_2O ($T_c \approx5 K$) and more recently Wen et al. in $A_xCoO_{2+\delta}$ ($A = Na,K$)(\tc$\approx~31 K$). We model a reference neutral \cob layer as an orbitally non-degenerate spin-\half antiferromagnetic Mott insulator on a triangular lattice and Na__x CoO_2.yH_2O and $A_xCoO_{2+\delta}$ as electron doped Mott insulators described by a t-J model. It is suggested that at optimal doping chiral spin fluctuations enhanced by the dopant dynamics leads to a d-wave superconducting state. A chiral RVB metal, a PT violating state with condensed RVB gauge fields, with a possible weak ferromagnetism and low temperature p-wave superconductivity are also suggested at higher dopings.Comment: 4 pages of LaTex file, 6 figures in eps files. Typos and minor corrections mad

RVB gauge theory and the Topological degeneracy in the Honeycomb Kitaev model

We relate the Z$_2$ gauge theory formalism of the Kitaev model to the SU(2) gauge theory of the resonating valence bond (RVB) physics. Further, we reformulate a known Jordan-Wigner transformation of Kitaev model on a torus in a general way that shows that it can be thought of as a Z$_2$ gauge fixing procedure. The conserved quantities simplify in terms of the gauge invariant Jordan-Wigner fermions, enabling us to construct exact eigen states and calculate physical quantities. We calculate the fermionic spectrum for flux free sector for different gauge field configurations and show that the ground state is four-fold degenerate on a torus in thermodynamic limit. Further on a torus we construct four mutually anti-commuting operators which enable us to prove that all eigenstates of this model are four fold degenerate in thermodynamic limit.Comment: 12 pages, 3 figures. Added affiliation and a new section, 'Acknowledgements'.Typos correcte

Factors associated with prolonged length of stay following cardiac surgery in a major referral hospital in Oman: a retrospective observational study

Two objectives were set for this study. The first was to identify factors influencing prolonged postoperative length of stay (LOS) following cardiac surgery. The second was to devise a predictive model for prolonged LOS in the cardiac intensive care unit (CICU) based on preoperative factors available at admission and to compare it against two existing cardiac stratification systems.Observational retrospective study.A tertiary hospital in Oman.All adult patients who underwent cardiac surgery at a major referral hospital in Oman between 2009 and 2013.30.5% of the patients had prolonged LOS (≥11 days) after surgery, while 17% experienced prolonged ICU LOS (≥5 days). Factors that were identified to prolong CICU LOS were non-elective surgery, current congestive heart failure (CHF), renal failure, combined coronary artery bypass graft (CABG) and valve surgery, and other non-isolated valve or CABG surgery. Patients were divided into three groups based on their scores. The probabilities of prolonged CICU LOS were 11%, 26% and 28% for group 1, 2 and 3, respectively. The predictive model had an area under the curve of 0.75. Factors associated with prolonged overall postoperative LOS included the body mass index, the type of surgery, cardiopulmonary bypass machine use, packed red blood cells use, non-elective surgery and number of complications. The latter was the most important determinant of postoperative LOS.Patient management can be tailored for individual patient based on their treatments and personal attributes to optimise resource allocation. Moreover, a simple predictive score system to enable identification of patients at risk of prolonged CICU stay can be developed using data that are routinely collected by most hospitals

Weak phase separation and the pseudogap in the electron-doped cuprates

We study the quantum transition from an antiferromagnet to a superconductor in a model for electron- and hole-doped cuprates by means of a variational cluster perturbation theory approach. In both cases, our results suggest a tendency towards phase separation between a mixed antiferromagnetic-superconducting phase at low doping and a pure superconducting phase at larger doping. However, in the electron-doped case the energy scale for phase separation is an order of magnitude smaller than for hole doping. We argue that this can explain the different pseudogap and superconducting transition scales in hole- and electron-doped materials.Comment: Final version, accepted for publication in Europhysics Letter

Atmospheric depositional fluxes of \u3csup\u3e7\u3c/sup\u3eBe and \u3csup\u3e210\u3c/sup\u3ePb at Galveston and College Station, Texas

The bulk depositional fluxes of 210Pb and 7Be were measured at a coastal (Galveston) and an inland (College Station) station for about 3 years, between 1989 and 1991. The annual depositional fluxes of 7Be and 210Pb at Galveston during this period varied by a factor of about 2.5, between 8.9 and 23.2 disintegrations per minute (dpm) cm−2 yr−1, with a mean of 14.7 dpm cm−2 yr−1 for 7Be, and 0.67 and 1.71 dpm cm−2 yr−1, with a mean of 1.03 dpm cm−2 yr−1 for 210Pb, respectively. The precipitation-normalized 7Be flux increases with increasing amount of precipitation. There is no systematic and consistent seasonal trend in the depositional fluxes for 7Be or for 210Pb. The volume-weighted 210Pb concentrations, when normalized to the amount of precipitation, seem to be constant over the time period of this study. Four to six heavy rain events (\u3e 5 cm) in a single day account for 20–30% of the annual deposition of 7Be and 210Pb. Such events account, however, for only about 4–6% of the total number of rainy days in a year. The dry depositional fluxes of these nuclides appear to be a significant fraction of the bulk depositional flux only during the months when there is very little rain. The fraction of dry to total depositional flux of 210Pb appears to be higher than that of 7Be. The strong positive correlation between 7Be and 210Pb depositional fluxes indicates that the flux of both nuclides is controlled by scavenging processes by local precipitation. This correlation also indicates that a major portion of the air masses that brings precipitation to Galveston and College Station is of continental origin. Our data therefore suggest that 7Be and 210Pb cannot be used as independent atmospheric tracers in our coastal station. This observation is consistent with those observed at many other continental and coastal stations

The Principle of Valence Bond Amplitude Maximization in Cuprates: How it breeds Superconductivity, Spin and Charge Orders

A simple microscopic principle of `Valence bond (nearest neighbor singlet) amplitude maximization '(VBAM) is shown to be present in undoped and optimally doped cuprates and unify the very different orderings such as antiferromagnetism in the Mott insulator and the robust superconductivity accompanied by an enhanced charge and stripe correlations in the optimally doped cuprates. VBAM is nearly synonymous with the energy minimization principle. It is implicit in the RVB theory and thereby makes the predictions of RVB mean field theory of superconductivity qualitatively correct.Comment: 4 pages, RevTe

Spin-Charge Separation in Two Dimensions - A Numerical Study

The question of spin-charge separation in two-dimensional lattices has been addressed by numerical simulations of the motion of one hole in a half-filled band. The calculations have been performed on finite clusters with Hubbard and t-J models. By comparing the time evolution of spin and charge polarisation currents in one and two dimensions, evidence in favor of spin-charge separation in two dimensions is presented. In contrast with this, spin-charge separation is absent in a highly doped, metallic, system.Comment: RevTeX 3.0, 10 Pages, 6 PostScript Figures (on request

Scavenging, cycling and removal fluxes of 210Po and 210Pb at the Bermuda time-series study site

Author Posting. © The Author(s), 2012. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part II: Topical Studies in Oceanography 93 (2013):108–118, doi:10.1016/j.dsr2.2013.01.005.Quantifying relative affinities of Po and Pb in different populations of marine particulate matter is of great importance in utilizing 210Po as a tracer for carbon cycling. We collected and analyzed water samples for the concentrations of dissolved and total 210Po and 30 210Pb from the upper 600 m of the water column at Bermuda Time-series Study site (September 1999 to September 2000) to investigate their seasonality of concentrations and their activity ratio (210Po/210Pb activity ratio, AR). Sinking particles collected in sediment traps at depths of 500 m, 1500 m, and 3200 m from the Oceanic Flux Program (OFP) time-series sediment traps were analyzed over a period of 12 months (May 1999 to May 2000). The objective was to compare the deficiencies of 210Po with respect to 210Pb in the water column to that measured in the sediment traps and to assess the relative affinities of Po and Pb with different particle pools. Inventories of 210Po in the upper 500 m water column varied by a factor of 2, indicating seasonal variations of particulate flux dominated the removal of 210Po. The 210Po/210Pb ARs in the dissolved phase were generally less than the secular equilibrium value (1.0) in the upper 600 m, while were generally greater than 1.0 in the particulate phase, indicating higher removal rates of 210Po relative to 210Pb by particulate matter. The measured fluxes of 210Po and 210Pb in the 500 m, 1500 m, and 3200 m traps increased with depth, while the 210Po/210Pb ARs decreased with depth except from May-August 1999. From the measured fluxes of 210Po and 210Pb at these three traps and the concentrations of 210Po and 210Pb in the water column, this region appears to be a sink for 210Pb which is likely brought-in by lateral advection.GHH’s sabbatical leave was supported by Korea Ocean Research and Development Institute (renamed as Korea Institute of Ocean Science and Technology) (PG47900 and PE98742). The Oceanic Flux Program has been supported since inception by the NSF Chemical Oceanography Program, most recently by grants OCE-0325627/0509602, OCE-0623505 and OCE-0927098. Partial support in writing this manuscript was supported by OCE-0961351 (MB)

g-on Mean Field Theory of the t-J Model

Implication of our recent proposal [J. Phys. Soc. Jpn. 65 (1996) 687] to treat large-amplitude gauge-field fluctuations around the slave-boson mean-field theory for the t-J model has been explored in detail. By attaching gauge flux to spinons and holons and then treating them as free g-on's which respect the time-reversal symmetry, the optimum exclusion (g) and exchange (\a) statistics have been determined in the plane of doping rate and temperature. Two different relations between \a and g have been investigated, namely g=|\a| (Case1) and g=|\a|(2-|\a|) (Case2). The results indicate that slave fermion is favored at low doping while slave boson at high doping. For two dimension, in Case1 intermediate statistics are found in between, while in Case2 no intermediate statistics are found. The consequences of varying the dimensionality and strength of J have been studied also. The latter has no qualitative effect for both cases, while the former has a profound effect in Case1.Comment: 18 pages, 11 figures two of them are figure 8; submitted to Phys. Rev. B; notes and citations are added, as seen in page 17; E-mails: [email protected], [email protected]

Possible Pairing Symmetry of Superconductor Na_xCoO_2yH_2O

To discuss a possibility that the superconductivities in Na_xCoO_2yH_2O are induced by the electron correlation, we investigate the possible pairing symmetry based on the single-band Hubbard model whose dispersion of the bare energy band is obtained by using FLAPW-LDA band structure calculation of Na_xCoO_2yH_2O. The superconducting transition temperature is estimated by solving the Eliashberg equation. In this equation, both normal and anomalous self-energies are calculated up to the third-order terms with respect to the Coulomb repulsion. In the case of spin-singlet pairing, the candidate of pairing symmetry (the maximum eigen value \lambda_max^SS of Eliashberg's equation) belongs to d-wave(E_2 representation of D_6 group). In the case of spin-triplet pairing, the candidate of pairing symmetry (the maximum eigen value \lambda_max^ST of Eliashberg's equation) belongs to f_{y(y^{2}-3x^{2})}-wave (B_1 representation of D_6 group). It is found that \lambda_max^SS\simeq\lambda_max^ST and the transition temperatures of unconventional pairing state are estimated to be low compared with observed temperature within our simple model.Comment: 10 pages, 5 figure