5,450 research outputs found
NaCoO in the x -> 0 Regime: Coupling of Structure and Correlation effects
The study of the strength of correlations in NaCoO is extended to the
x=0 end of the phase diagram where Mott insulating behavior has been widely
anticipated. Inclusion of correlation as modeled by the LDA+U approach leads to
a Mott transition in the subband if U is no less than U=2.5 eV. Thus
U smaller than U is required to model the metallic, nonmagnetic CoO
compound reported by Tarascon and coworkers. The orbital-selective Mott
transition of the state, which is essentially degenerate with the
states, occurs because of the slightly wider bandwidth of the
bands. The metal-insulator transition is found to be strongly coupled to the
Co-O bond length, due to associated changes in the bandwidth, but the
largest effects occur only at a reduced oxygen height that lies below the
equilibrium position.Comment: 8 pages with 9 embedded figure
An Analysis of the Design-Build Delivery Method in Air Force Military Construction
The design-build method for construction project delivery continues to grow in both the private and public sector. Several government agencies have observed, through experience with design-build, positive results which give “anecdotal” credibility to design-build methods. The objective of this study is to compare the performance of the design-build delivery method with traditional design-bid-build approaches for Air Force (AF) military construction (MILCON). Data related to 835 (278 design-build, 557 traditional) MILCON projects were gathered from the Automated Civil Engineer System – Project Management Module (ACES-PM) for Fiscal Years 1996-2006. The design-build method had better performance for six of eight metrics with highly significant results for cost growth and number of modifications per million dollars. The traditional method experienced a highly significant advantage for the metrics of construction timeline and total project time. The historical analysis revealed that design-build MILCON has improved significantly for cost growth, modifications per million dollars, construction timeline, and total project time. The traditional method also improved for the cost growth and modifications per million dollars metrics. Finally, the facility type analysis revealed that the design-build method was best suited for seven of the nine facility types. This study provides empirical evidence of where the design-build delivery method provides an advantage to the traditional method for AF MILCON execution
Vibrational Modes in LiBC: Theory Compared with Experiment
The search for other superconductors in the MgB2 class currently is focussed
on Li{1-x}BC, which when hole-doped (concentration x) should be a metal with
the potential to be a better superconductor than MgB2. Here we present the
calculated phonon spectrum of the parent semiconductor LiBC. The calculated
Raman-active modes are in excellent agreement with a recent observation, and
comparison of calculated IR-active modes with a recent report provides a
prediction of the LO--TO splitting for these four modes, which is small for the
B-C bond stretching mode at ~1200 cm^{-1}, but large for clearly resolved modes
at 540 cm^{-1} and 620 cm^{-1}.Comment: 4 pages, two embedded figures. Physica B (in press
Enhancement of the upper critical field in codoped iron-arsenic high-temperature superconductors
We present the first study of codoped iron-arsenide superconductors of the
122 family (Sr/Ba)_(1-x)K_xFe_(2-y)Co_yAs_2 with the purpose to increase the
upper critical field H_c2 compared to single doped (Sr/Ba)Fe_2As_2 materials.
H_c2 was investigated by measuring the magnetoresistance in high pulsed
magnetic fields up to 64 T. We find, that H_c2 extrapolated to T = 0 is indeed
enhanced significantly to ~ 90 T for polycrystalline samples of
Ba_0.55K_0.45Fe_1.95Co_0.05As_2 compared to ~75 T for Ba_0.55K_0.45Fe_2As_2 and
BaFe_1.8Co_0.2As_2 single crystals. Codoping thus is a promising way for the
systematic optimization of iron-arsenic based superconductors for
magnetic-field and high-current applications.Comment: 7 pages, 5 figures, submitted to Journal of Applied Physic
Prior Cardiac Surgery is Independently Associated with Decreased Survival following Infant Tracheostomy
INTRODUCTION: Previous reports have demonstrated that prior cardiac surgery is independently associated with in-hospital mortality after infant tracheostomy. We aimed to determine whether these infants would continue to be at increased risk for death following hospital discharge.
METHODS: A retrospective review was performed on subjects < 2 y of age who recovered from tracheostomy in the pediatric ICU at our institution between January 2007 and December 2011, with follow-up to December 2013. Survival to 1 y following tracheostomy was the primary outcome variable for the study. Multivariate Cox regression analysis was then performed to determine independent risk factors for death after infant tracheostomy.
RESULTS: Forty-two subjects met inclusion criteria, 18 of whom had undergone prior cardiac surgery. Twenty-six subjects (62%) were alive at 1 y post-tracheostomy. Age at tracheostomy, concomitant genetic abnormalities or prematurity, and ventilator dependence at discharge were not statistically different between survivors and those who died. Subjects who died, however, were more likely to have had cardiac surgery prior to tracheostomy (11 [69%] vs 7 [27%], P = .008) and had longer hospital stay (median 3.4 months [interquartile range: 2.6–4.6] vs 2.2 months [interquartile range: 1.1–3.5], P = .045). Multivariate Cox regression analysis revealed only prior cardiac surgery to be independently associated with decreased survival after tracheostomy (hazard ratio 4.7, 95% CI 1.3–16.4, P = .02).
CONCLUSIONS: Prior cardiac surgery is independently associated with decreased survival within 1 y following tracheostomy. Clinicians and families of infants with prior cardiac surgery in whom tracheostomy after cardiac surgery is deemed necessary should consider this risk when planning long-term care
Microscopic magnetic modeling for the =1/2 alternating chain compounds NaCuSbO and NaCuTeO
The spin-1/2 alternating Heisenberg chain system NaCuSbO features
two relevant exchange couplings: within the structural CuO
dimers and between the dimers. Motivated by the controversially
discussed nature of , we perform extensive density-functional-theory
(DFT) calculations, including DFT+ and hybrid functionals. Fits to the
experimental magnetic susceptibility using high-temperature series expansions
and quantum Monte Carlo simulations yield the optimal parameters =
217 K and = 174 K with the alternation ratio 1.25. For the closely related system
NaCuTeO, DFT yields substantially enhanced , but weaker
. The comparative analysis renders the buckling of the chains as the
key parameter altering the magnetic coupling regime. Numerical simulation of
the dispersion relations of the alternating chain model clarify why both
antiferromagnetic and ferrromagnetic can reproduce the experimental
magnetic susceptibility data.Comment: published version: 11 pages, 8 figures, 5 tables + Supplemental
materia
Crystal water induced switching of magnetically active orbitals in CuCl2
The dehydration of CuCl2*2(H2O) to CuCl2 leads to a dramatic change in
magnetic behavior and ground state. Combining density functional electronic
structure and model calculations with thermodynamical measurements we reveal
the microscopic origin of this unexpected incident -- a crystal water driven
switching of the magnetically active orbitals. This switching results in a
fundamental change of the coupling regime from a three-dimensional
antiferromagnet to a quasi one-dimensional behavior. CuCl2 can be well
described as a frustrated J1-J2 Heisenberg chain with ferromagnetic exchange J1
and J2/J1 ~ -1.5 for which a helical ground state is predicted.Comment: 6 pages, 5 figures, 1 table (PRB, accepted
Electronic Structure of ZnCNi3
According to a recent report by Park et al, ZnCNi3 is isostructural and
isovalent to the superconducting (Tc = 8 K) anti-perovskite, MgCNi3, but shows
no indication of a superconducting transition down to 2K. A comparison of
calculated electronic structures shows that the main features of MgCNi3,
particularly the van Hove singularity near the Fermi energy, are preserved in
ZnCNi3. Thus the reported lack of superconductivity in ZnCNi3 is not
explainable in terms of Tc being driven to a very low value by a small Fermi
level density of states. We propose that the lack of superconductivity, the
small value of the linear specific heat coefficient, gamma, and the discrepancy
between theoretical and experimental lattice constants can all be explained if
the material is assumed to be a C-deficient alpha-ZnCNi3 similar to the
analogous non-superconducting phase of MgCNi3
Free serum cortisol during the postoperative acute phase response determined by equilibrium dialysis liquid chromatography-tandem mass spectrometry
In severely ill patients low concentrations of the corticosteroid binding globulin are typically found; the aim of this study was to quantify directly free bioactive cortisol concentrations in the sera of postoperative cardiosurgical patients. Serum samples of 12 consecutive patients undergoing aortocoronary bypass surgery taken preoperatively and on the postoperative days 1 to 4 were analyzed. Total serum cortisol was quantified using liquid chromatographytandem mass spectrometry with an online sample extraction system and trideuterated cortisol as the internal standard, and free serum cortisol was measured after overnight equilibrium dialysis. Whereas on the first postoperative day, the median total serum cortisol concentration was approximately twofold increased compared to preoperative samples (preoperatively, 245 nmol/l (interquartile range (IQR) 203293 nmol/l); first postoperative day, 512 nmol/l (IQR 410611 nmol/l)), median dialyzable free cortisol concentration was almost sevenfold increased (preoperatively, 14.2 nmol/l (IQR 10.920.7 nmol/l); first postoperative day, 98.3 nmol/l (IQR 81.3134 nmol/l)). On the fourth postoperative day, median free cortisol was still significantly increased compared to baseline sampling (p < 0.05), whereas median total cortisol was not. A median of 5.7% (IQR 5.47.0%) of total cortisol was found as free cortisol on the preoperative day, 21.2% (IQR 18.9 23.5%) on the first postoperative day and 10.5% (IQR 9.814.0%) on the fourth postoperative day. It is concluded that during the postoperative period the freeto bound ratio of cortisol is highly variable and that during the acute phase response direct quantification of free bioactive cortisol concentrations seems to be biologically more appropriate than the measurement of total cortisol concentrations
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