1,203 research outputs found
Quantum Phase Transitions and the Extended Coupled Cluster Method
We discuss the application of an extended version of the coupled cluster
method to systems exhibiting a quantum phase transition. We use the lattice
O(4) non-linear sigma model in (1+1)- and (3+1)-dimensions as an example. We
show how simple predictions get modified, leading to the absence of a phase
transition in (1+1) dimensions, and strong indications for a phase transition
in (3+1) dimensions
Analysis of No-Difference Findings in Evaluation Research
Conclusions of no difference are becoming increasingly important in evaluation research. We delineate three major uses of no-difference findings and analyze their meanings. (1) No-differ ence findings in randomized experiments can be interpreted as support for conclusions of the absence of a meaningful treatment effect, but only if the proper analytic methods are used. (2) Statistically based conclusions in quasi-experiments do not allow causal statements about the treatment impact but do provide a metric to judge the size of the resulting difference. (3) Using no-difference findings to conclude equivalence on control variables is inefficient and potentially misleading. The final section of the article presents alternative methods by which conclusions of no difference may be supported when applicable. These methods include the use of arbitrarily high alpha levels, interval estimation, and power analysis.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/67182/2/10.1177_0193841X8901300604.pd
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Supplemental Release Limits for the Directed Reuse of Steel in Road Barriers and Lead in Shielding Products by the Department of Energy
The DOE National Center of Excellence for Metals Recycle (NMR) proposes to define and implement a complex-wide directed reuse strategy for surplus radiologically impacted lead (Pb) and steel as part of the U.S. Department of Energy's commitment to the safe and cost-effective recycle or reuse of excess materials and equipment across the DOE complex. NMR will, under this proposal, act on behalf of the DOE Office of Environmental Management, Office of Technical Program Integration (specifically EM-22), as the Department's clearinghouse for DOE surplus lead, steel and products created from these materials by developing and maintaining a cost-effective commercially-based contaminated lead and steel recycle program. It is NMR's intention, through this directed reuse strategy, to mitigate the adverse environmental and economic consequences of managing surplus lead and steel as a waste within the complex. This approach promotes the safe and cost-effective reuse of scrap metals in support of the Department's goals of resource utilization, energy conservation, pollution prevention and waste minimization. This report discusses recommendations for supplemental radiological release limits for the directed reuse of contaminated lead and steel by the DOE within the nuclear industry. The limits were originally selected from the American National Standards Institute and Health Physics Society standard N13.12 titled ''Surface and Volume Radioactivity Standards for Clearance'' (Health Physics Society, 1999) but were subsequently modified as a result of application-specific issues. Both the health and measurement implications from the adoption and use of the limits for directed reuse scenarios are discussed within this report
Statistical Mechanics of Nonuniform Magnetization Reversal
The magnetization reversal rate via thermal creation of soliton pairs in
quasi-1D ferromagnetic systems is calculated. Such a model describes e.g. the
time dependent coercivity of elongated particles as used in magnetic recording
media. The energy barrier that has to be overcome by thermal fluctuations
corresponds to a soliton-antisoliton pair whose size depends on the external
field. In contrast to other models of first order phase transitions such as the
phi^4 model, an analytical expression for this energy barrier is found for all
values of the external field. The magnetization reversal rate is calculated
using a functional Fokker-Planck description of the stochastic magnetization
dynamics. Analytical results are obtained in the limits of small fields and
fields close to the anisotropy field. In the former case the hard-axis
anisotropy becomes effectively strong and the magnetization reversal rate is
shown to reduce to the nucleation rate of soliton-antisoliton pairs in the
overdamped double sine-Gordon model. The present theory therefore includes the
nucleation rate of soliton-antisoliton pairs in the double sine-Gordon chain as
a special case. These results demonstrate that for elongated particles, the
experimentally observed coercivity is significantly lower than the value
predicted by the standard theories of N\'eel and Brown.Comment: 21 pages RevTex 3.0 (twocolumn), 6 figures available on request, to
appear in Phys Rev B, Dec (1994
The self-consistent bounce: an improved nucleation rate
We generalize the standard computation of homogeneous nucleation theory at
zero temperature to a scenario in which the bubble shape is determined
self-consistently with its quantum fluctuations. Studying two scalar models in
1+1 dimensions, we find the self-consistent bounce by employing a two-particle
irreducible (2PI) effective action in imaginary time at the level of the
Hartree approximation. We thus obtain an effective single bounce action which
determines the rate exponent. We use collective coordinates to account for the
translational invariance and the growth instability of the bubble and finally
present a new nucleation rate prefactor. We compare the results with those
obtained using the standard 1-loop approximation and show that the
self-consistent rate can differ by several orders of magnitude.Comment: 28 pages, revtex, 7 eps figure
Thermodynamic properties of the periodic Anderson model:X-boson treatment
We study the specific dependence of the periodic Anderson Model (PAM) in the
limit of employing the X-boson treatment in two fifferent regimes of
the PAM: the heavy fermion Kondo (HF-K) and the heavy fermion local magnetic
regime (HF-LMM). We obtain a multiple peak structure for the specific heat in
agreement with experimental results as well as the increase of the electronic
effective mass at low temperatures associated with the HF-K regime. The entropy
per site at low T tends to zero in the HF-K regime, corresponding to a singlet
ground state, and it tends to in the HF-LMM, corresponding to a
doublet ground state at each site. The linear coefficient
of the specific heat qualitatively agrees with the experimental results
obtained for differents materials in the two regimes considered here.Comment: 9 pages, 14 figure
Thermal Decay of the Cosmological Constant into Black Holes
We show that the cosmological constant may be reduced by thermal production
of membranes by the cosmological horizon, analogous to a particle ``going over
the top of the potential barrier", rather than tunneling through it. The
membranes are endowed with charge associated with the gauge invariance of an
antisymmetric gauge potential. In this new process, the membrane collapses into
a black hole, thus the net effect is to produce black holes out of the vacuum
energy associated with the cosmological constant. We study here the
corresponding Euclidean configurations ("thermalons"), and calculate the
probability for the process in the leading semiclassical approximation.Comment: 14 pages, 6 figures. Minor correction
One-loop corrections to the metastable vacuum decay
We evaluate the one-loop prefactor in the false vacuum decay rate in a theory
of a self interacting scalar field in 3+1 dimensions. We use a numerical
method, established some time ago, which is based on a well-known theorem on
functional determinants. The proper handling of zero modes and of
renormalization is discussed. The numerical results in particular show that
quantum corrections become smaller away from the thin-wall case. In the
thin-wall limit the numerical results are found to join into those obtained by
a gradient expansion.Comment: 31 pages, 7 figure
Combination therapy with an ACE inhibitor and an angiotensin receptor blocker for diabetic nephropathy - a meta-analysis
WSTĘP. Inhibitory konwertazy angiotensyny (ACEI)
i blokery receptora angiotensyny (ARB) zapobiegają
progresji nefropatii cukrzycowej (DN). Wyniki badań
sugerują, że połączenie układu renina-angiotensyna-aldosteron (RAAS) i czynników hamujących działa
addytywnie w procesie leczenia DN. Ponieważ badania
te obejmowały niewielkie grupy chorych, autorzy niniejszej pracy przeprowadzili metaanalizę
prób dotyczących leczenia skojarzonego DN.
METODY. Badania do metaanalizy wybrano na podstawie
baz danych MEDLINE, EMBASE, CINAHL i Cochrane.
Włączono wszystkie próby dotyczące skojarzonego
leczenia za pomocą ACEI i ARB. Głównym
punktem końcowym było dobowe wydalanie białka
z moczem, a dodatkowe punkty końcowe obejmowały: wartości ciśnienia tętniczego, stężenia potasu
we krwi i współczynnika przesączania kłębuszkowego
(GFR).
WYNIKI. W 10 włączonych do analizy badaniach
156 chorych otrzymało ACEI i ARB, a 159 jedynie ACEI. Większość badań trwało 8-12 tygodni. U osób
leczonych ACEI i ARB uzyskano zmniejszenie proteinurii
(p = 0,01), co wiązało się ze znaczną statystyczną heterogenicznością (p = 0,005). Terapia ACEI
i ARB była związana ze zmniejszeniem GFR [3,87 ml/min
(7,32-0,42); p = 0,03] i tendencją do wzrostu stężenia
kreatyniny w surowicy (6,86 umol/l 95% CI -0,76-13,73; p = 0,09). Stężenie potasu zwiększyło się
o 0,2 (0,08-0,32) mmol/l (p < 0,01) u chorych leczonych
ACEI i ARB. Skurczowe i rozkurczowe ciśnienie
krwi obniżyło się odpowiednio o 5,2 mm Hg (2,1-8,4) (p < 0,01) i 5,3 mm Hg (2,2-8,4) (p < 0,01).
WNIOSKI. Wyniki metaanalizy sugerują, że łączne stosowanie
ACEI + ARB w większym stopniu zmniejsza
24-godzinne wydalanie białka z moczem niż przyjmowanie
jedynie ACEI. Korzystne efekty terapii skojarzonej
są wynikiem niewielkiego wpływu leków na
GFR, stężenie kreatyniny i potasu w surowicy oraz
ciśnienie tętnicze. Rezultaty te należy interpretować
ostrożnie, ponieważ większość analizowanych badań charakteryzowała się krótkim czasem obserwacji,
a w kilku długoterminowych próbach (12 miesięcy) nie wykazano korzystnego wpływu leczenia.AIMS. Angiotensin-converting enzyme inhibitors
(ACEIs) and angiotensin receptor blockers (ARBs)
prevent the progression of diabetic nephropathy (DN).
Studies suggest that combination renin-angiotensin-aldosterone system (RAAS)-inhibiting therapy provides
additive benefit in DN. However, these studies
are small in size. We performed a meta-analysis of
studies investigating combination therapy for DN.
METHODS. Studies were identified through a search
of MEDLINE, EMBASE, CINAHL and the Cochrane
Database. All trials involving combined ACEI and ARB
for slowing progression of DN were included. The
primary end point was 24-
Blood pressure, serum potassium and glomerular
filtration rate (GFR) were secondary end points.
RESULTS. In the 10 included trials, 156 patients received
ACEI + ARB and 159 received ACEI only. Most
studies were 8–12 weeks in duration. Proteinuria was
reduced with ACEI + ARB (p = 0.01). This was associated with significant statistical heterogeneity (p = 0.005). ACEI + ARB was associated with a reduction
in GFR [3.87 ml/min (7.32-0.42); p = 0.03] and
a trend towards an increase in serum creatinine (6.86
umol/l 95% CI: -0.76-13.73; p = 0.09). Potassium was
increased by 0.2 (0.08-0.32) mmol/l (p < 0.01) with
ACEI + ARB. Systolic and diastolic blood pressure were
reduced by 5.2 (2.1-8.4) mm Hg (p < 0.01) and 5.3
(2.2-8.4) mm Hg (p < 0.01), respectively.
CONCLUSIONS. This meta-analysis suggests that ACEI +
+ ARB reduces 24-h proteinuria to a greater extent
than ACEI alone. This benefit is associated with small
effects on GFR, serum creatinine, potassium and blood
pressure. These results should be interpreted cautiously
as most of the included studies were of short
duration and the few long-term studies (12 months)
have not demonstrated benefi
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