397 research outputs found
A gauge invariant and string independent fermion correlator in the Schwinger model
We introduce a gauge invariant and string independent two-point fermion
correlator which is analyzed in the context of the Schwinger model (QED_2). We
also derive an effective infrared worldline action for this correlator, thus
enabling the computation of its infrared behavior. Finally, we briefly discuss
possible perspectives for the string independent correlator in the QED_3
effective models for the normal state of HTc superconductors.Comment: 14 pages, LaTe
Single Superfield Representation for Mixed Retarded and Advanced Correlators in Disordered Systems
We propose a new single superfield representation for mixed retarded and
advanced correlators for noninteracting disordered systems. The method is
tested in the simpler context of Random Matrix theory, by comparing with well
known universal behavior for level spacing correlations. Our method is general
and could be especially interesting to study localization properties encoded in
the mixed correlators of Quantum Hall systems.Comment: 13 pages including two figures, RevTex4. Improved version. Figures
changed. To appear in Journal of Physics
Electron transport through a mesoscopic metal-CDW-metal junction
In this work we study the transport properties of a finite Peierls-Fr\"ohlich
dielectric with a charge density wave of the commensurate type. We show that at
low temperatures this problem can be mapped onto a problem of fractional charge
transport through a finite-length correlated dielectric, recently studied by
Ponomarenko and Nagaosa [Phys. Rev. Lett {\bf 81}, 2304 (1998)]. The
temperature dependence of conductance of the charge density wave junction is
presented for a wide range of temperatures.Comment: Latex, Revtex 3.0, 7 pages, 2 EPS figures (uses epfs
Overstating the evidence - double counting in meta-analysis and related problems
Background: The problem of missing studies in meta-analysis has received much attention. Less attention has been paid to the more serious problem of double counting of evidence.
Methods: Various problems in overstating the precision of results from meta-analyses are described and illustrated with examples, including papers from leading medical journals. These problems include, but are not limited to, simple double-counting of the same studies, double counting of some aspects of the studies, inappropriate imputation of results, and assigning spurious precision to individual studies.
Results: Some suggestions are made as to how the quality and reliability of meta-analysis can be improved. It is proposed that the key to quality in meta-analysis lies in the results being transparent and checkable.
Conclusions: Existing quality check lists for meta-analysis do little to encourage an appropriate attitude to combining evidence and to statistical analysis. Journals and other relevant organisations should encourage authors to make data available and make methods explicit. They should also act promptly to withdraw meta-analyses when mistakes are found
Pattern revivals from fractional Gouy phases in structured light
We investigate pattern revivals in specially designed optical structures that
combine different transverse modes. In general, the resulting pattern is not
preserved under free propagation and gets transformed due to non synchronized
Gouy phases. However, it is possible to build structures in which the Gouy
phases synchronize at specific fractional values, thus recovering the initial
pattern at the corresponding longitudinal positions. This effect is illustrated
with a radially structured light spot in which the beam energy can be addressed
to different positions without the need of intermediate optical components,
what can be useful for optical communications and optical tweezing with
structured beams
Non-perturbative behavior of the quantum phase transition to a nematic Fermi fluid
We discuss shape (Pomeranchuk) instabilities of the Fermi surface of a
two-dimensional Fermi system using bosonization. We consider in detail the
quantum critical behavior of the transition of a two dimensional Fermi fluid to
a nematic state which breaks spontaneously the rotational invariance of the
Fermi liquid. We show that higher dimensional bosonization reproduces the
quantum critical behavior expected from the Hertz-Millis analysis, and verify
that this theory has dynamic critical exponent . Going beyond this
framework, we study the behavior of the fermion degrees of freedom directly,
and show that at quantum criticality as well as in the the quantum nematic
phase (except along a set of measure zero of symmetry-dictated directions) the
quasi-particles of the normal Fermi liquid are generally wiped out. Instead,
they exhibit short ranged spatial correlations that decay faster than any
power-law, with the law and we
verify explicitely the vanishing of the fermion residue utilizing this
expression. In contrast, the fermion auto-correlation function has the behavior
. In this regime we also find that, at
low frequency, the single-particle fermion density-of-states behaves as
, where is larger
than the free Fermi value, N(0), and is a constant. These results confirm
the non-Fermi liquid nature of both the quantum critical theory and of the
nematic phase.Comment: 20 pages, 2 figures, 1 table; new version with minor changes; new
subsection 3C2 added with an explicit calculation of the quasiparticle
residue at the nematic transition; minor typos corrected, new references;
general beautification of the text and figure
Perioperative adjuvant corticosteroids for post-operative analgesia in knee arthroplasty – A meta-analysis of 1396 knees
Background and purpose — Immediate postoperative pain management offered in knee arthroplasty is suboptimal in up to onethird of patients resulting in high opiate consumption and delayed discharge. In this meta-analysis we investigate the analgesic effect and safety of perioperative adjuvant corticosteroids in knee arthroplasty. Methods — Databases Medline, Embase, and Central were searched for randomized studies comparing the analgesic effect of adjuvant perioperative corticosteroids in knee arthroplasty. Our primary outcome was pain score at 24 hours postoperatively. Secondary outcomes included pain at 12, 48, and 72 hours, opiate consumption, postoperative nausea and vomiting, infection, and discharge time. Systemic (intravenous) and local (intra-articular) corticosteroids were analyzed separately. Results — 14 randomized controlled trials (1,396 knees) were included. Mean corticosteroid dosages were predominantly 50–75mg oral prednisolone equivalents for both systemic and local routes. Systemic corticosteroids demonstrated statistically signifi cant and clinically modest reductions in pain at 12 hours by –1.1 points (95%CI –2.2 to 0.02), 24 hours by –1.3 points (CI –2.3 to –0.26) and 48 hours by –0.4 points (CI –0.67 to –0.04). Local corticosteroids did not reduce pain. Opiate consumption, postoperative nausea and vomiting, infection, or time till discharge were similar between groups. Interpretation — Corticosteroids modestly reduce pain postoperatively at 12 and 24 hours when used systemically without any increase in associated risks for dosages between 50 and 75 mg oral prednisolone equivalents
Mobility and Maternal Position during Childbirth in Tanzania: An Exploratory Study at Four Government Hospitals.
Emerging research evidence suggests a potential benefit in being upright in the first stage of labour and a systematic review of trials suggests both benefits and harmful effects associated with being upright in the second stage of labour. Implementing evidence-based obstetric care in African countries with scarce resources is particularly challenging, and requires an understanding of the cumulative nature of science and commitment to applying the most up to date evidence to clinical decisions. In this study, we documented current practice rates, explored the barriers and opportunities to implementing these procedures from the provider perspective, and documented women's preferences and satisfaction with care. This was an exploratory study using quantitative and qualitative methods. Practice rates were determined by exit interviews with a consecutive sample of postnatal women. Provider views were explored using semi-structured interviews (with doctors and traditional birth attendants) and focus group discussions (with midwives). The study was conducted at four government hospitals, two in Dar es Salaam and two in the neighbouring Coast region, Tanzania. MAIN OUTCOME MEASURES: Practice rates for mobility during labour and delivery position; women's experiences, preferences and views about the care provided; and provider views of current practice and barriers and opportunities to evidence-based obstetric practice. Across all study sites more women were mobile at home (15.0%) than in the labour ward (2.9%), but movement was quite restricted at home before women were admitted to labour ward (51.6% chose to rest with little movement). Supine position for delivery was used routinely at all four hospitals; this was consistent with women's preferred choice of position, although very few women are aware of other positions. Qualitative findings suggest obstetricians and midwives favoured confining to bed during the first stage of labour, and supine position for delivery. The barriers to change appear to be complicated and require providers to want to change, and women to be informed of alternative positions during the first stage of labour and delivery. We believe that highlighting the gap between actual practice and current evidence provides a platform for dialogue with providers to evaluate the threats and opportunities for changing practice
SUPPORT Tools for evidence-informed health Policymaking (STP) 10: Taking equity into consideration when assessing the findings of a systematic review
This article is part of a series written for people responsible for making decisions about health policies and programmes and for those who support these decision makers
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