Quantum Spin Excitations through the metal-to-insulator crossover in YBa2Cu3O6+yY Ba_2 Cu_3 O_{6+y}

We use inelastic neutron scattering to study the temperature dependence of the spin excitations of a detwinned superconducting YBa$_2$Cu$_3$O$_{6.45}$ ($T_c=48$ K). In contrast to earlier work on YBa$_2$Cu$_3$O$_{6.5}$ ($T_c=58$ K), where the prominent features in the magnetic spectra consist of a sharp collective magnetic excitation termed ``resonance'' and a large ($\hbar\omega\approx 15$ meV) superconducting spin gap, we find that the spin excitations in YBa$_2$Cu$_3$O$_{6.45}$ are gapless and have a much broader resonance. Our detailed mapping of magnetic scattering along the $a^\ast$/$b^\ast$-axis directions at different energies reveals that spin excitations are unisotropic and consistent with the ``hourglass''-like dispersion along the $a^\ast$-axis direction near the resonance, but they are isotropic at lower energies. Since a fundamental change in the low-temperature normal state of YBa$_2$Cu$_3$O$_{6+y}$ when superconductivity is suppressed takes place at $y\sim0.5$ with a metal-to-insulator crossover (MIC), where the ground state transforms from a metallic to an insulating-like phase, our results suggest a clear connection between the large change in spin excitations and the MIC. The resonance therefore is a fundamental feature of metallic ground state superconductors and a consequence of high-$T_c$ superconductivity.Comment: 9 pages, 7 figures, accepted by Phys. Rev.

Electron-Spin Excitation Coupling in an Electron Doped Copper Oxide Superconductor

High-temperature (high-Tc) superconductivity in the copper oxides arises from electron or hole doping of their antiferromagnetic (AF) insulating parent compounds. The evolution of the AF phase with doping and its spatial coexistence with superconductivity are governed by the nature of charge and spin correlations and provide clues to the mechanism of high-Tc superconductivity. Here we use a combined neutron scattering and scanning tunneling spectroscopy (STS) to study the Tc evolution of electron-doped superconducting Pr0.88LaCe0.12CuO4-delta obtained through the oxygen annealing process. We find that spin excitations detected by neutron scattering have two distinct modes that evolve with Tc in a remarkably similar fashion to the electron tunneling modes in STS. These results demonstrate that antiferromagnetism and superconductivity compete locally and coexist spatially on nanometer length scales, and the dominant electron-boson coupling at low energies originates from the electron-spin excitations.Comment: 30 pages, 12 figures, supplementary information include

Resonance in the electron-doped high-Tc superconductor Pr0.88LaCe0.12CuO(4-delta)

In conventional superconductors, the interaction that pairs the electrons to form the superconducting state is mediated by lattice vibrations (phonons). In high-transition temperature (high-Tc) copper oxides, it is generally believed that magnetic excitations play a fundamental role in the superconducting mechanism because superconductivity occurs when mobile 'electrons' or 'holes' are doped into the antiferromagnetic parent compounds. Indeed, a sharp magnetic excitation termed "resonance" has been observed by neutron scattering in a number of hole-doped materials. The resonance is intimately related to superconductivity, and its interaction with charged quasi-particles observed by photoemission, optical conductivity, and tunneling suggests that it plays a similar role as phonons in conventional superconductors. However, the relevance of the resonance to high-Tc superconductivity has been in doubt because so far it has been found only in hole-doped materials. Here we report the discovery of the resonance in electron-doped superconducting Pr0.88LaCe0.12CuO(4-delta) (Tc = 24 K). We find that the resonance energy (Er) is proportional to Tc via Er = 5.8kBTc (kB is the Boltzmann's constant) for all high-Tc superconductors irrespective of electron- or hole-doping (Fig. 1e). Our results demonstrate that the resonance is a fundamental property of the superconducting copper oxides and therefore must play an essential role in the mechanism of superconductivity.Comment: PDF file with 4 Figure

Microscopic annealing process and its impact on superconductivity in T'-structure electron-doped copper oxides

High-transition-temperature superconductivity arises in copper oxides when holes or electrons are doped into the CuO2 planes of their insulating parent compounds. While hole-doping quickly induces metallic behavior and superconductivity in many cuprates, electron-doping alone is insufficient in materials such as R2CuO4 (R is Nd, Pr, La, Ce, etc.), where it is necessary to anneal an as-grown sample in a low-oxygen environment to remove a tiny amount of oxygen in order to induce superconductivity. Here we show that the microscopic process of oxygen reduction repairs Cu deficiencies in the as-grown materials and creates oxygen vacancies in the stoichiometric CuO2 planes, effectively reducing disorder and providing itinerant carriers for superconductivity. The resolution of this long-standing materials issue suggests that the fundamental mechanism for superconductivity is the same for electron- and hole-doped copper oxides.Comment: 23 pages, 3 figures, accepted for publication in Nature Material

A comprehensive evaluation of food fortification with folic acid for the primary prevention of neural tube defects

BACKGROUND: Periconceptional use of vitamin supplements containing folic acid reduces the risk of a neural tube defect (NTD). In November 1998, food fortification with folic acid was mandated in Canada, as a public health strategy to increase the folic acid intake of all women of childbearing age. We undertook a comprehensive population based study in Newfoundland to assess the benefits and possible adverse effects of this intervention. METHODS: This study was carried out in women aged 19–44 years and in seniors from November 1997 to March 1998, and from November 2000 to March 2001. The evaluation was comprised of four components: I) Determination of rates of NTDs; II) Dietary assessment; III) Blood analysis; IV) Assessment of knowledge and use of folic acid supplements. RESULTS: The annual rates of NTDs in Newfoundland varied greatly between 1976 and 1997, with a mean rate of 3.40 per 1,000 births. There was no significant change in the average rates between 1991–93 and 1994–97 (relative risk [RR] 1.01, 95% confidence interval [CI] 0.76–1.34). The rates of NTDs fell by 78% (95% CI 65%–86%) after the implementation of folic acid fortification, from an average of 4.36 per 1,000 births during 1991–1997 to 0.96 per 1,000 births during 1998–2001 (RR 0.22, 95% CI 0.14–0.35). The average dietary intake of folic acid due to fortification was 70 μg/day in women aged 19–44 years and 74 μg/day in seniors. There were significant increases in serum and RBC folate levels for women and seniors after mandatory fortification. Among seniors, there were no significant changes in indices typical of vitamin B(12 )deficiencies, and no evidence of improved folate status masking haematological manifestations of vitamin B(12 )deficiency. The proportion of women aged 19–44 years taking a vitamin supplement containing folic acid increased from 17% to 28%. CONCLUSIONS: Based on these findings, mandatory food fortification in Canada should continue at the current levels. Public education regarding folic acid supplement use by women of childbearing age should also continue

Genetic Dissection of the Canq1 Locus Governing Variation in Extent of the Collateral Circulation

<div><h3>Background</h3><p>Native (pre-existing) collaterals are arteriole-to-arteriole anastomoses that interconnect adjacent arterial trees and serve as endogenous bypass vessels that limit tissue injury in ischemic stroke, myocardial infarction, coronary and peripheral artery disease. Their extent (number and diameter) varies widely among mouse strains and healthy humans. We previously identified a major quantitative trait locus on chromosome 7 (<em>Canq1</em>, LOD = 29) responsible for 37% of the heritable variation in collateral extent between C57BL/6 and BALB/c mice. We sought to identify candidate genes in <em>Canq1</em> responsible for collateral variation in the cerebral pial circulation, a tissue whose strain-dependent variation is shared by similar variation in other tissues.</p> <h3>Methods and Findings</h3><p>Collateral extent was intermediate in a recombinant inbred line that splits <em>Canq1</em> between the C57BL/6 and BALB/c strains. Phenotyping and SNP-mapping of an expanded panel of twenty-one informative inbred strains narrowed the <em>Canq1</em> locus, and genome-wide linkage analysis of a SWRxSJL-F2 cross confirmed its haplotype structure. Collateral extent, infarct volume after cerebral artery occlusion, bleeding time, and re-bleeding time did not differ in knockout mice for two vascular-related genes located in <em>Canq1</em>, <em>IL4ra</em> and <em>Itgal</em>. Transcript abundance of 6 out of 116 genes within the 95% confidence interval of <em>Canq1</em> were differentially expressed >2-fold (p-value<0.05÷150) in the cortical <em>pia mater</em> from C57BL/6 and BALB/c embryos at E14.5, E16.5 and E18.5 time-points that span the period of collateral formation.</p> <h3>Conclusions</h3><p>These findings refine the <em>Canq1</em> locus and identify several genes as high-priority candidates important in specifying native collateral formation and its wide variation.</p> </div

Deciphering the influence of TOD on metro ridership: An integrated approach of extended node-place model and interpretable machine learning with planning implications

Many global high-density cities have embraced transit-oriented development (TOD) strategies around metro stations in a strong push toward promoting transit trips. However, the general TOD principles must be adapted to a variety of local features. Thus, conducting a baseline study that deciphers the locality-specific differences and similarities in the influence of TOD on metro ridership is of paramount importance. Using the case of Shanghai in China, this paper demonstrated a novel approach to unveiling the locality-specific influence of TOD on metro ridership that integrated the node-functionality-place model with interpretable machine learning. By exploring a wide range of explanatory variables, we discovered that the relative importance of TOD structural factors (e.g., node, place and functionality) and neighborhood sociodemographics as well as their interactions presented great spatial and temporal heterogeneity for boarding and alighting on weekdays and weekends. Among the TOD structural factors, functionality was of the highest importance, and a greater contribution was observed within the central districts during peak hours. Regarding the interactions among the TOD structural factors, the interaction between node and functionality presented the highest relative importance, followed by that between place between functionality and that between node and place. Additionally, neighborhood sociodemographics also accounted for a noticeable contribution, especially in the outskirts. Based on the locality-specific estimations, the affinity propagation clustering algorithm was further used to cluster the TODs into difference groups as nuanced representations of the TOD-metro ridership relationships. The findings refreshed the knowledge base concerning the spatiotemporal heterogeneity in the nonlinear influence of TOD on metro ridership and provided new insights into spatial planning. The proposed approach showed high computational feasibility with strong theoretical underpinnings and thus offered broad potential applications into other high-density cities worldwide

Transit-oriented development (TOD) typologies around metro station areas in urban China: A comparative analysis of five typical megacities for planning implications

Transit-oriented development (TOD), which is regarded as an efficient planning strategy for urban sustainability, has surged in use across the globe in the recent past. While the lessons learned from case studies of individual cities can provide valuable references, they also result in a gap between existing theoretical principles and actual planning practices. The comparative analysis of TOD typologies among cities affords unique strengths for addressing such a challenge. By extending the classic ‘node–place’ model with a third dimension, this paper first constructs a ‘node–functionality–place’ model in the form of a magic cube as the theoretical basis for classifying TOD typologies. Then, the model is applied to five typical Chinese megacities, namely, Beijing, Shanghai, Shenzhen, Wuhan and Hangzhou. After establishing an indicator system, the analytic hierarchy process (AHP) is used to determine the TOD degree, and a two-step cluster is employed to classify TOD typologies. The results show that the TOD degree, although it varies with cities, presents a similar spatial pattern, with a general tendency to decline from the central core to the outskirts. In total, six TOD types are distinguished and present notable variations within and across the five megacities. The identified TOD typologies thus equip urban planners and policymakers with a useful tool for designing more targeted strategies. The discoveries made in the comparative context differ from those made in the individual context in prior studies. This paper is thus believed to make a new contribution to the existing TOD literature

Unraveling the relative contribution of TOD structural factors to metro ridership: A novel localized modeling approach with implications on spatial planning

TOD (transit-oriented development) has gradually earned the reputation as a promising spatial planning strategy to encourage public transit usage. Many cities across the world, especially those in the global south, have established TOD projects around metro stations. From this standpoint, the paramount question of how TOD is conductive to metro ridership is at the heart of scholarly discourse. Theoretically, the territorial organization of a TOD comprises three structural factors –node (metro station), place (surrounding land use) and their feedback. An accurate judgement of the TOD-metro ridership relationship would not be achieved if we neglected any of the three structural factors forming the TOD architecture. However, the three TOD structural factors have not frequently been considered simultaneously in prior literature. A gap remains regarding the relative contribution of TOD structural factors to metro ridership across time and space. This paper aims to address this unresolved issue using a case study of the Hangzhou metropolitan area in China. As the dependent variable, metro ridership is measured using one week of smart card records, with the three TOD structural factors as explanatory variables and sociodemographic factors as control variables, described by a set of indicators. A novel localized modeling approach using variance decomposition of geographically temporally weighted regression is demonstrated to quantify the spatially and temporally varying relative contribution of TOD structural factors. The results show that both similarities and discrepancies were identified compared to earlier studies. Most importantly, new knowledge was gained, particularly that ‘metro station’ factors contribute the most to metro ridership, followed by the feedback factor, ‘surrounding land use’ factors and sociodemographic factors both on workdays and nonworkdays. Furthermore, our analysis highlights that the relative contribution of TOD structural factors presents noticeable spatial heterogeneities across metro station areas both on workdays and nonworkdays but exhibits only obvious temporal heterogeneity on nonworkdays. Finally, TOD clusters are generated based on the relative contribution of TOD structural factors with implications for spatial planning. This study is believed to open the door for framing locally representative strategies of TOD to stimulate the use of public transit

A facile method to prepare ultra-wideband anti-reflective borosilicate glass with subwavelength structure surface

Anti-reflective (AR) films have been widely investigated due to their various applications in optoelectronic devices. Among all the investigation, the manufacturing method is the most popular research field which directly determines the performance of anti-reflective films. Herein, Na2HPO4 and AlCl3 mixed neutral solution were used to etch the surface of K9 glass matrix to obtain the AR structure. The morphology of SEM images indicated that subwavelength structure (SWS) was formed on the glass surface via precisely controlling the solution concentration, treatment time and temperature. Owing to the porous structure, the refractive index mutation of the interface between air and glassss can be effectively weakened. Therefore, the integral transmittance, covering the whole visible light (VIS) to near-infrared (NIR) wavelength (300-2500 nm), of glass matrix was increased remarkably to 95.77%, much higher than 87.72% of blank glass, achieving the effect of ultra-wideband anti-reflection. Particularly, the transmittance at 520 nm and 1990 nm can even reach to 99.14% and 97.86%, respectively. According to the composite analysis and reaction process, Al3+ ions act as both the regulator of the solution pH and part of the new-formed glass skeleton, resulting in the more uniform and regular SWS on the glass surface. Moreover, in virtue of -OH in silanol (Si-OH) on the expanded glass surface attracting more H2O molecules or additional silylation modification on glass surface repulsing H2O molecules, the surface characteristics of AR glass with SWS films can be easily turned between hydrophilic and hydrophobic in order to meet the requirements of various application environments. These ultra-wideband AR glasses with controllable hydrophilic and hydrophobic properties are expected to be applied in many optoelectronic fields