Phase separation and electron pairing in repulsive Hubbard clusters

Exact thermal studies of small (4-site, 5-site and 8-site) Hubbard clusters with local electron repulsion yield intriguing insight into phase separation, charge-spin separation, pseudogaps, condensation, in particular, pairing fluctuations away from half filling (near optimal doping). These exact calculations, carried out in canonical (i.e. for fixed electron number N) and grand canonical (i.e. fixed chemical potential $\mu$) ensembles, monitoring variations in temperature T and magnetic field h, show rich phase diagrams in a T-$\mu$ space consisting of pairing fluctuations and signatures of condensation. These electron pairing instabilities are seen when the onsite Coulomb interaction U is smaller than a critical value U$_c$(T) and they point to a possible electron pairing mechanism. The specific heat, magnetization, charge pairing and spin pairing provide strong support for the existence of competing (paired and unpaired) phases near optimal doping in these clusters as observed in recent experiments in doped La$_{2-x}$Sr$_x$CuO$_{4+y}$ high T$_c$ superconductors.Comment: 5 pages, 5 figure

Spin dynamics near the critical doping in weakly-superconducting underdoped YBa2Cu3O6.35 (Tc=18K)

Using neutron scattering we have determined the magnetic structure and fluctuations in the YBa2Cu3O6.35 superconductor (Tc=18 K). The long-range ordered collinear spins of the insulating antiferromagnet are replaced by a commensurate central mode arising from slow, isotropically polarized, short-range spin correlations. The inelastic spectrum up to 30 meV is broad in wave vector and commensurate. In contrast to the the resonance peak of higher Tc superconductors, the spins exhibit a single overdamped spectrum whose rate of relaxation decreases on cooling and saturates at 5 meV below 50 K. As the relaxation rate saturates the quasi-static spin correlations grow and become resolution limited in energy. The spin susceptibility at high temperatures illustrates that the dominant energy scale is set by the temperature. At low temperatures, the scale length is geometric and not linked by velocity to dynamic widths. There is no observable suppression of the spin fluctuations or central mode upon the onset of superconductivity. The spins respond not to coherent charge pairs but to hole doping allowing coexistence of glassy short range spin order with superconductivity. Since the physics of the weakly superconducting system YBCO6.35 must connect continuously with that in more strongly superconducting YBCO6.5, we find that neither incommensurate stripe-like spin modulations nor a well-defined neutron spin resonance are essential for the onset with doping of pairing in a high temperature cuprate superconductor.Comment: 22 pages, 19 figures, accepted for publication in Phys. Rev.

Central mode and spin confinement near the boundary of the superconducting phase in YBa2Cu3O6.353 (Tc=18 K)

We have mapped the neutron scattering spin spectrum at low-energies in YBa2Cu3O6.353 (Tc=18 K) where the doping ~0.06 is near the critical value (pc=0.055) for superconductivity. No coexistence with long range ordered antiferromagnetism is found. The spins fluctuate on two energy scales, one a damped spin response with a ~2 meV relaxation rate and the other a central mode with a relaxation rate that slows to less than 0.08 meV below Tc. The spectrum mirrors that of a soft mode driving a central mode. Extremely short correlation lengths, 42+-5 Angstrom in-plane and 8+-2 Angstrom along the c direction, and isotropic spin orientations for the central mode indicate that the correlations are subcritical with respect to any second order transition to Neel order. The dynamics follows a model where damped spin fluctuations are coupled to the slow fluctuations of regions with correlations shortened by the hole doping.Comment: 5 pages 4 figures. One figure revised and some text revision. Accepted PRB Rapids February 14, 200

Field- and pressure-induced magnetic quantum phase transitions in TlCuCl_3

Thallium copper chloride is a quantum spin liquid of S = 1/2 Cu^2+ dimers. Interdimer superexchange interactions give a three-dimensional magnon dispersion and a spin gap significantly smaller than the dimer coupling. This gap is closed by an applied hydrostatic pressure of approximately 2kbar or by a magnetic field of 5.6T, offering a unique opportunity to explore the both types of quantum phase transition and their associated critical phenomena. We use a bond-operator formulation to obtain a continuous description of all disordered and ordered phases, and thus of the transitions separating these. Both pressure- and field-induced transitions may be considered as the Bose-Einstein condensation of triplet magnon excitations, and the respective phases of staggered magnetic order as linear combinations of dimer singlet and triplet modes. We focus on the evolution with applied pressure and field of the magnetic excitations in each phase, and in particular on the gapless (Goldstone) modes in the ordered regimes which correspond to phase fluctuations of the ordered moment. The bond-operator description yields a good account of the magnetization curves and of magnon dispersion relations observed by inelastic neutron scattering under applied fields, and a variety of experimental predictions for pressure-dependent measurements.Comment: 20 pages, 17 figure

Risk factor-based screening compared to universal screening for gestational diabetes mellitus in marginalized Burman and Karen populations on the Thailand-Myanmar border: an observational cohort

Background: Gestational diabetes mellitus (GDM) contributes significantly to maternal and neonatal morbidity, but data from marginalized populations remains scarce. This study aims to compare risk-factor-based screening to universal testing for GDM among migrants along the Thailand-Myanmar border. Methods: From the prospective cohort (September 2016, February 2019), 374 healthy pregnant women completed a 75g oral glucose tolerance test (OGTT) at 24-32 weeks gestation. Fasting, one hour and two hour cut-offs were based on Hyperglycaemia and Adverse Pregnancy Outcomes (HAPO trial) criteria and cases were treated. The sensitivity and specificity of risk-factor-based screening criteria was calculated using OGTT as the gold standard. Risk factors included at least one positive finding among 10 criteria, e.g., obesity (body mass index (BMI) >/=27.5kg/m (2)), 1 (st) degree relative with diabetes etc. Adverse maternal and neonatal outcomes were compared by GDM status, and risk factors for GDM were explored. Results: GDM prevalence was 13.4% (50/374) (95% CI: 10.3-17.2). Risk-factors alone correctly identified 74.0% (37/50) OGTT positive cases: sensitivity 74.0% (59.7-85.4) and specificity 27.8% (3.0-33.0). Burman women accounted for 29.1% of the cohort population, but 38.0% of GDM cases. Percentiles for birthweight (p=0.004), head circumference (p=0.005), and weight-length ratio (p=0.010) were higher in newborns of GDM mothers compared with non-GDM, yet 21.7% (75/346) of newborns in the cohort were small-for-gestational age. In Burman women, overweight/obese BMI was associated with a significantly increased adjusted odds ratio 5.03 (95% CI: 1.43-17.64) for GDM compared to normal weight, whereas underweight and overweight/obese in Karen women were both associated with similarly elevated adjusted odds, approximately 2.4-fold (non-significant) for GDM. GDM diagnosis by OGTT was highest prior to peak rainfall. Conclusions: Risk-factor-based screening was not sufficiently sensitive or specific to be useful to diagnose GDM in this setting among a cohort of low-risk pregnant women. A two-step universal screening program has thus been implemented

Neutron scattering search for static magnetism in oxygen ordered YBa2Cu3O6.5

We present elastic and inelastic neutron scattering results on highly oxygen ordered YBa2Cu3O6.5 ortho-II. We find no evidence for the presence of ordered magnetic moments to a sensitivity of 0.003 Bohr magnetons, an order of magnitude smaller than has been suggested in theories of orbital or d-density-wave (DDW) currents. The absence of sharp elastic peaks, shows that the d-density-wave phase is not present, at least for the superconductor with the doping of 6.5 and the ordered ortho-II structure. We cannot exclude the possibility that a broad peak may exist with extremely short-range DDW correlations. For less ordered or more doped crystals it is possible that disorder may lead to static magnetism. We have also searched for the large normal state spin gap that is predicted to exist in an ordered DDW phase. Instead of a gap we find that the Q-correlated spin susceptibility persists to the lowest energies studied, 6 meV. Our results are compatible with the coexistence of superconductivity with orbital currents, but only if they are dynamic, and exclude a sharp phase transition to an ordered d-density-wave phase.Comment: 6 pages 4 figures RevTex Submitted to Phys Rev B January 23, 200

Ecological Modeling of Aedes aegypti (L.) Pupal Production in Rural Kamphaeng Phet, Thailand

Background - Aedes aegypti (L.) is the primary vector of dengue, the most important arboviral infection globally. Until an effective vaccine is licensed and rigorously administered, Ae. aegypti control remains the principal tool in preventing and curtailing dengue transmission. Accurate predictions of vector populations are required to assess control methods and develop effective population reduction strategies. Ae. aegypti develops primarily in artificial water holding containers. Release recapture studies indicate that most adult Ae. aegypti do not disperse over long distances. We expect, therefore, that containers in an area of high development site density are more likely to be oviposition sites and to be more frequently used as oviposition sites than containers that are relatively isolated from other development sites. After accounting for individual container characteristics, containers more frequently used as oviposition sites are likely to produce adult mosquitoes consistently and at a higher rate. To this point, most studies of Ae. aegypti populations ignore the spatial density of larval development sites. Methodology - Pupal surveys were carried out from 2004 to 2007 in rural Kamphaeng Phet, Thailand. In total, 84,840 samples of water holding containers were used to estimate model parameters. Regression modeling was used to assess the effect of larval development site density, access to piped water, and seasonal variation on container productivity. A varying-coefficients model was employed to account for the large differences in productivity between container types. A two-part modeling structure, called a hurdle model, accounts for the large number of zeroes and overdispersion present in pupal population counts. Findings - The number of suitable larval development sites and their density in the environment were the primary determinants of the distribution and abundance of Ae. aegypti pupae. The productivity of most container types increased significantly as habitat density increased. An ecological approach, accounting for development site density, is appropriate for predicting Ae. aegypti population levels and developing efficient vector control program

Nuclear Magnetic Relaxation in the Haldane-Gap Antiferromagnet Ni(C_2_H_8_N_2_)_2_NO_2_(ClO_4_)

A new theory is proposed to interpret nuclear spin-lattice relaxation-time (T_1_) measurements on the spin-1 quasi-one-dimensional Heisenberg antiferromagnet Ni(C_2_H_8_N_2_)_2_NO_2_(ClO_4_) (NENP). While Sagi and Affleck pioneeringly discussed this subject in terms of field-theoretical languages, there is no theoretical attempt yet to explicitly simulate the novel observations of 1/T_1_ reported by Fujiwara et al.. By means of modified spin waves, we solve the minimum of 1/T_1_ as a function of an applied field, pending for the past decade.Comment: to be published in J. Phys. Soc. Jpn. 73, No. 4 (2004

Spin-Wave Description of Haldane-gap antiferromagnets

Modifying the conventional antiferromagnetic spin-wave theory which is plagued by the difficulty of the zero-field sublattice magnetizations diverging in one dimension, we describe magnetic properties of Haldane-gap antiferromagnets. The modified spin waves, constituting a grand canonical bosonic ensemble so as to recover the sublattice symmetry, not only depict well the ground-state correlations but also give useful information on the finite-temperature properties.Comment: to be published in J. Phys. Soc. Jpn. Vol. 72, No. 4 (2003