Superconductivity and Spin Fluctuations in the Electron-Doped Infinitely-Layered High Tc Superconductor Sr0.9_0.9La0.1_0.1CuO2_2 (Tc=42K)

This paper describes the first 63-Cu NMR study of an electron-doped infinitely-layered high Tc superconductor Sr$_0.9$La$_0.1$CuO$_2$ (Tc=42K). The spin dynamics in the normal state above Tc exhibits qualitatively the same behavior as some hole-doped materials with significantly enhanced spin fluctuations. Below Tc, we observed no signature of a Hebel-Slichter coherence peak, suggesting an unconventional nature of the symmetry of the superconducting order parameter.Comment: Invited Paper to SNS-95 Conference (Spectroscopies on Novel Superconductors 1995 at Stanford). Also presented at Aspen Winter Conference on Superconductivity and Grenoble M^2S-HTSC in 199

Charge and spin density wave ordering transitions in strongly correlated metals

We study the quantum transition from a strongly correlated metal, with heavy fermionic quasiparticles, to a metal with commensurate charge or spin density wave order. To this end, we introduce and numerically analyze a large dimensionality model of Ising spins in a transverse field, coupled to two species of fermions; the analysis borrows heavily from recent progress in the solution of the Hubbard model in large dimensions. At low energies, the Ising order parameter fluctuations are characterized by the critical exponent $z \nu = 1$, while above an energy scale, $\Gamma$, there is a crossover to $z\nu = 1/2$ criticality. We show that $\Gamma$ is of the order of the width of the heavy quasiparticle band, and can be made arbitrarily small for a correlated metal close to a Mott-Hubbard insulator. Therefore, such a correlated metal has a significant intermediate energy range of $z\nu=1/2$ behavior, a single particle spectrum with a narrow quasiparticle band, and well-formed analogs of the lower and upper Hubbard bands; we suggest that these features are intimately related in general.Comment: 14 pages, REVTEX 3.0, 2 postscript figure

Crossover and scaling in a nearly antiferromagnetic Fermi liquid in two dimensions

We consider two-dimensional Fermi liquids in the vicinity of a quantum transition to a phase with commensurate, antiferromagnetic long-range order. Depending upon the Fermi surface topology, mean-field spin-density-wave theory predicts two different types of such transitions, with mean-field dynamic critical exponents $z=1$ (when the Fermi surface does not cross the magnetic zone boundary, type $A$) and $z=2$ (when the Fermi surface crosses the magnetic zone boundary, type $B$). The type $A$ system only displays $z=1$ behavior at all energies and its scaling properties are similar (though not identical) to those of an insulating Heisenberg antiferromagnet. Under suitable conditions precisely stated in this paper, the type $B$ system displays a crossover from relaxational behavior at low energies to type $A$ behavior at high energies. A scaling hypothesis is proposed to describe this crossover: we postulate a universal scaling function which determines the entire, temperature-, wavevector-, and frequency-dependent, dynamic, staggered spin susceptibility in terms of 4 measurable, $T=0$, parameters (determining the distance, energy, and order parameter scales, plus one crossover parameter). The scaling function contains the full scaling behavior in all regimes for both type $A$ and $B$ systems. The crossover behavior of the uniform susceptibility and the specific heat is somewhat more complicated and is also discussed. Explicit computation of the crossover functions is carried out in a large $N$ expansion on a mean-field model. Some new results for the critical properties on the ordered side of the transition are also obtained in a spin-density wave formalism. The possible relevance of our results to the doped cuprate compounds is briefly discussed.Comment: 20 pages, REVTeX, 6 figures (uuencoded compressed PostScript file for figures is appended

A Fermi liquid model for the overdoped and optimally doped cuprate superconductors: scattering rate, susceptibility, spin resonance peak and superconducting transition

This paper treats a number of issues of the cuprates, ranging from the spin resonance peak and the linear one-particle scattering rate to the superconducting transition, in the frame of a Fermi liquid model. Recent ARPES expts. by Valla et al., Science vol. 285, 2110 (1999), and e-print cond-mat/0003407, directly support the linearity of the one-particle scattering rate everywhere in the Brillouin zone we obtained here. We show that the origin of this linearity is the strong linear in energy term of the imaginary part of the carrier susceptibility. This result yields directly a linear in temperature resistivity and linear in 1/energy optical conductivity. We show that the low energy dependence of the susceptibility can have a purely fermionic origin. We introduce an antiferromagnetic ansatz for the susceptibility of the carriers.Inter alia, this ansatz may explain the appearance of the spin resonance peak (observed in neutron scattering) in the normal state of the cuprates. Further, we obtain particularly high transition temperatures $T_c$ from our Eliashberg scheme by using this ansatz: we have a $d_{x^2-y^2}$ gap with $T_c > 120 ^o$K for nearest neighbour hopping $t=250meV$.Comment: This version corrects a few misprints of the printed versio

Impact of Vitamin D Supplementation on Arterial Vasomotion, Stiffness and Endothelial Biomarkers in Chronic Kidney Disease Patients

Background: Cardiovascular events are frequent and vascular endothelial function is abnormal in patients with chronic kidney disease (CKD). We demonstrated endothelial dysfunction with vitamin D deficiency in CKD patients; however the impact of cholecalciferol supplementation on vascular stiffness and vasomotor function, endothelial and bone biomarkers in CKD patients with low 25-hydroxy vitamin D [25(OH)D] is unknown, which this study investigated. Methods: We assessed non-diabetic patients with CKD stage 3/4, age 17–80 years and serum 25(OH)D ,75 nmol/L. Brachial artery Flow Mediated Dilation (FMD), Pulse Wave Velocity (PWV), Augmentation Index (AI) and circulating blood biomarkers were evaluated at baseline and at 16 weeks. Oral 300,000 units cholecalciferol was administered at baseline and 8-weeks. Results: Clinical characteristics of 26 patients were: age 50614 (mean61SD) years, eGFR 41611 ml/min/1.73 m2, males 73%, dyslipidaemia 36%, smokers 23% and hypertensives 87%. At 16-week serum 25(OH)D and calcium increased (43616 to 84629 nmol/L, p,0.001 and 2.3760.09 to 2.4260.09 mmol/L; p = 0.004, respectively) and parathyroid hormone decreased (10.868.6 to 7.464.4; p = 0.001). FMD improved from 3.163.3% to 6.163.7%, p = 0.001. Endothelial biomarker concentrations decreased: E-Selectin from 566662123 to 525662058 pg/mL; p = 0.032, ICAM-1, 3.4560.01 to 3.1061.04 ng/mL; p = 0.038 and VCAM-1, 54633 to 42633 ng/mL; p = 0.006. eGFR, BP, PWV, AI, hsCRP, von Willebrand factor and Fibroblast Growth Factor-23, remained unchanged. Conclusion: This study demonstrates for the first time improvement of endothelial vasomotor and secretory functions with vitamin D in CKD patients without significant adverse effects on arterial stiffness, serum calcium or FGF-23. Trial Registration: ClinicalTrials.gov NCT0200571

Effect of Vitamin D Supplementation on Markers of Vascular Function: A Systematic Review and Individual Participant Meta- Analysis

Background-—Low 25-hydroxyvitamin D levels are associated with an increased risk of cardiovascular events, but the effect of vitamin D supplementation on markers of vascular function associated with major adverse cardiovascular events is unclear. Methods and Results-—We conducted a systematic review and individual participant meta-analysis to examine the effect of vitamin D supplementation on flow-mediated dilatation of the brachial artery, pulse wave velocity, augmentation index, central blood pressure, microvascular function, and reactive hyperemia index. MEDLINE, CINAHL, EMBASE, Cochrane Central Register of Controlled Trials, and http://www.ClinicalTrials.gov were searched until the end of 2016 without language restrictions. Placebo-controlled randomized trials of at least4 weeks duration were included. Individual participant data were sought from investigators on included trials. Trial-level metaanalysis was performed using random-effects models; individual participant meta-analyses used a 2-stage analytic strategy, examining effects in prespecified subgroups. 31trials (2751 participants) were included; 29 trials (2641participants) contributed data to trial-level meta-analysis, and24trials (2051 participants) contributed to individual-participant analyses. VitaminD3daily dose equivalents ranged from 900 to 5000 IU; duration was 4 weeks to12 months. Trial-level meta-analysis showed no significant effect of supplementation on macrovascularmeasures(flow-mediateddilatation,0.37%[95%confidenceinterval, 0.23to0.97]; carotid-femoralpulsewavevelocity, 0.00 m/s [95% confidence interval, 0.36 to 0.37]); similar results were obtained from individual participant data. Microvascular function showed a modest improvement in trial-level data only. No consistent benefit was observed in subgroup analyses or between different vitamin D analogues. Conclusions-—Vitamin D supplementation had no significant effect on most markers of vascular function in this analysis

Quantum Monte Carlo Study of Weakly Coupled Spin Ladders

We report a quantum Monte Carlo study of the thermodynamic properties of arrays of spin ladders with various widths ($n$), coupled via a weak inter-ladder exchange coupling $\alpha J$, where $J$ is the intra-ladder coupling both along and between the chains. This coupled ladder system serves as a simplified model for the magnetism of presumed ordered spin and charge stripes in the two-dimensional CuO$_2$ planes of hole-doped copper oxides. Our results for $n=3$ with weak inter-ladder coupling $\alpha=0.05$, estimated from the $t-t'-t''-J$ model, show good agreement with the ordering temperature of the recently observed spin density wave condensation in La$_2$CuO$_{4+y}$. We show that there exists a quantum critical point at $\alpha_c \simeq 0.07$ for $n=4$, and determine the phase diagram. Our data at this quantum critical point agree quantitatively with the universal scaling predicted by the quantum nonlinear $\sigma$ model. We also report results on random mixtures of $n=2$ and $n=3$ ladders, which correspond to the doping region near but above 1/8. Our study on the magnetic static structure factor reveals a saturation of the incommensurability of the spin correlations around 1/8, while the incommensurability of the charge stripes grows linearly with hole concentration. The implications of this result for the interpretation of neutron scattering experiments on the dynamic spin fluctuations in La$_{2-x}$Sr$_x$CuO$_4$ are discussed.Comment: 12 pages, LaTeX, 11 eps figure

Spin-Charge Separation in the t−Jt-J Model: Magnetic and Transport Anomalies

A real spin-charge separation scheme is found based on a saddle-point state of the $t-J$ model. In the one-dimensional (1D) case, such a saddle-point reproduces the correct asymptotic correlations at the strong-coupling fixed-point of the model. In the two-dimensional (2D) case, the transverse gauge field confining spinon and holon is shown to be gapped at {\em finite doping} so that a spin-charge deconfinement is obtained for its first time in 2D. The gap in the gauge fluctuation disappears at half-filling limit, where a long-range antiferromagnetic order is recovered at zero temperature and spinons become confined. The most interesting features of spin dynamics and transport are exhibited at finite doping where exotic {\em residual} couplings between spin and charge degrees of freedom lead to systematic anomalies with regard to a Fermi-liquid system. In spin dynamics, a commensurate antiferromagnetic fluctuation with a small, doping-dependent energy scale is found, which is characterized in momentum space by a Gaussian peak at ($\pi/a$, $\pi/a$) with a doping-dependent width ($\propto \sqrt{\delta}$, $\delta$ is the doping concentration). This commensurate magnetic fluctuation contributes a non-Korringa behavior for the NMR spin-lattice relaxation rate. There also exits a characteristic temperature scale below which a pseudogap behavior appears in the spin dynamics. Furthermore, an incommensurate magnetic fluctuation is also obtained at a {\em finite} energy regime. In transport, a strong short-range phase interference leads to an effective holon Lagrangian which can give rise to a series of interesting phenomena including linear-$T$ resistivity and $T^2$ Hall-angle. We discuss the striking similarities of these theoretical features with those found in the high-$T_c$ cuprates and give aComment: 70 pages, RevTex, hard copies of 7 figures available upon request; minor revisions in the text and references have been made; To be published in July 1 issue of Phys. Rev. B52, (1995

Competing charge transfer pathways at the photosystem II-electrode interface.

The integration of the water-oxidation enzyme photosystem II (PSII) into electrodes allows the electrons extracted from water oxidation to be harnessed for enzyme characterization and to drive novel endergonic reactions. However, PSII continues to underperform in integrated photoelectrochemical systems despite extensive optimization efforts. Here we carried out protein-film photoelectrochemistry using spinach and Thermosynechococcus elongatus PSII, and we identified a competing charge transfer pathway at the enzyme-electrode interface that short-circuits the known water-oxidation pathway. This undesirable pathway occurs as a result of photo-induced O2 reduction occurring at the chlorophyll pigments and is promoted by the embedment of PSII in an electron-conducting fullerene matrix, a common strategy for enzyme immobilization. Anaerobicity helps to recover the PSII photoresponse and unmasks the onset potentials relating to the QA/QB charge transfer process. These findings impart a fuller understanding of the charge transfer pathways within PSII and at photosystem-electrode interfaces, which will lead to more rational design of pigment-containing photoelectrodes in general.This work was supported by the U.K. Engineering and Physical Sciences Research Council (EP/H00338X/2 to E. Reisner), the U.K. Biology and Biotechnological Sciences Research Council (BB/K010220/1 to E. Reisner), a Marie Curie International Incoming Fellowship (PIIF-GA-2012-328085 RPSII to J.J.Z.). N.P. was supported by the Winton Fund for the Physics of Sustainability. E. Romero. and R.v.G. were supported by the VU University Amsterdam, the Laserlab-Europe Consortium, the TOP grant (700.58.305) from the Foundation of Chemical Sciences part of NWO, the Advanced Investigator grant (267333, PHOTPROT) from the European Research Council, and the EU FP7 project PAPETS (GA 323901). R.v.G. gratefully acknowledges his `Academy Professor' grant from the Royal Netherlands Academy of Arts and Sciences (KNAW). We would also like to thank Miss Katharina Brinkert and Prof A. William Rutherford for a sample of T. elongatus PSII, and H. v. Roon for preparation of the spinach PSII samples

Effects of vitamin D supplementation on endothelial function:a systematic review and meta-analysis of randomised clinical trials

Background: In addition to regulating calcium homoeostasis and bone health, vitamin D influences vascular and metabolic processes including endothelial function (EF) and insulin signalling. This systematic review and meta-analysis of randomised clinical trials (RCTs) were conducted to investigate the effect of vitamin D supplementation on EF and to examine whether the effect size was modified by health status, study duration, dose, route of vitamin D administration, vitamin D status (baseline and post-intervention), body mass index (BMI), age and type of vitamin D.  Methods: We searched the Medline, Embase, Cochrane Library and Scopus databases from inception until March 2015 for studies meeting the following criteria: (1) RCT with adult participants, (2) vitamin D administration alone, (3) studies that quantified EF using commonly applied methods including ultrasound, plethysmography, applanation tonometry and laser Doppler.  Results: Sixteen articles reporting data for 1177 participants were included. Study duration ranged from 4 to 52 weeks. The effect of vitamin D on EF was not significant (SMD: 0.08, 95 % CI −0.06, 0.22, p = 0.28). Subgroup analysis showed a significant improvement of EF in diabetic subjects (SMD: 0.31, 95 % CI 0.05, 0.57, p = 0.02). A non-significant trend was found for diastolic blood pressure (β = 0.02; p = 0.07) and BMI (β = 0.05; p = 0.06).  Conclusions: Vitamin D supplementation did not improve EF. The significant effect of vitamin D in diabetics and a tendency for an association with BMI may indicate a role of excess adiposity and insulin resistance in modulating the effects of vitamin D on vascular function. This remains to be tested in future studies