171 research outputs found
Detecting and Characterizing Small Dense Bipartite-like Subgraphs by the Bipartiteness Ratio Measure
We study the problem of finding and characterizing subgraphs with small
\textit{bipartiteness ratio}. We give a bicriteria approximation algorithm
\verb|SwpDB| such that if there exists a subset of volume at most and
bipartiteness ratio , then for any , it finds a set
of volume at most and bipartiteness ratio at most
. By combining a truncation operation, we give a local
algorithm \verb|LocDB|, which has asymptotically the same approximation
guarantee as the algorithm \verb|SwpDB| on both the volume and bipartiteness
ratio of the output set, and runs in time
, independent of the size of the
graph. Finally, we give a spectral characterization of the small dense
bipartite-like subgraphs by using the th \textit{largest} eigenvalue of the
Laplacian of the graph.Comment: 17 pages; ISAAC 201
Squeezing based on nondegenerate frequency doubling internal to a realistic laser
We investigate theoretically the quantum fluctuations of the fundamental
field in the output of a nondegenerate second harmonic generation process
occuring inside a laser cavity. Due to the nondegenerate character of the
nonlinear medium, a field orthogonal to the laser field is for some operating
conditions indepedent of the fluctuations produced by the laser medium. We show
that this fact may lead to perfect squeezing for a certain polarization mode of
the fundamental field. The experimental feasibility of the system is also
discussed.Comment: 6 pages, 5 figure
Thermodynamic gauge-theory cascade
It is proposed that the cooling of a thermalized SU() gauge theory can be
formulated in terms of a cascade involving three effective theories with
successively reduced (and spontaneously broken) gauge symmetries, SU()
U(1) Z. The approach is based on the assumption that away
from a phase transition the bulk of the quantum interaction inherent to the
system is implicitly encoded in the (incomplete) classical dynamics of a
collective part made of low-energy condensed degrees of freedom. The properties
of (some of the) statistically fluctuating fields are determined by these
condensate(s). This leads to a quasi-particle description at tree-level. It
appears that radiative corrections, which are sizable at large gauge coupling,
do not change the tree-level picture qualitatively. The thermodynamic
self-consistency of the quasi-particle approach implies nonperturbative
evolution equations for the associated masses. The temperature dependence of
these masses, in turn, determine the evolution of the gauge coupling(s). The
hot gauge system approaches the behavior of an ideal gas of massless gluons at
asymptotically large temperature. A negative equation of state is possible at a
stage where the system is about to settle into the phase of the (spontaneously
broken) Z symmetry.Comment: 25 pages, 6 figures, 1 reference added, minor corrections in text,
errors in Sec. 3.2 corrected, PRD versio
Violation of the Wiedemann-Franz Law in a Large-N Solution of the t-J Model
We show that the Wiedemann-Franz law, which holds for Landau Fermi liquids,
breaks down in a large-n treatment of the t-J model. The calculated ratio of
the in-plane thermal and electrical conductivities agrees quantitatively with
experiments on the normal state of the electron-doped Pr_{2-x}Ce_xCuO_4 (x =
0.15) cuprate superconductor. The violation of the Wiedemann-Franz law in the
uniform phase contrasts with other properties of the phase that are Fermi
liquid like.Comment: 4 pages, 2 figures. Typos corrected, one added reference, revised
discussion of experiment on 214 cuprate material (x = 0.06
Net ecosystem exchange from two formerly afforested peatlands undergoing restoration in the Flow Country of northern Scotland
Northern peatlands are important in the global carbon (C) cycle as they help regulate local, regional and global C budgets through high atmospheric carbon dioxide (CO2) uptake and low net CO2 losses to the atmosphere. Since the 1900s (but particularly the 1950s) land-use change has affected many peatland areas, driven in part by attempts to improve their commercial value. During this period, many peatlands in the UK were drained and planted with non-native conifer plantations. Efforts are now underway to restore the ecosystem functioning of these peatlands to those characteristic of pristine peatlands, in particular C flux dynamics. A lack of ecosystem level measurements means that the timescales of restoration and the degree to which they are successful remains poorly determined. In this research, we present the first year-round study of net ecosystem CO2 exchange (NEE) from peatlands undergoing restoration from forestry. Annual NEE was measured from two sites between March 2014 and June 2015, where restoration commenced 10 years and 16 years prior to the start of this study, and the results were then compared to existing measurements from a near-pristine peatland. Existing NEE data (expressed as CO2-C) from the near-pristine peatland indicated a C sink of 114 g m-2 yr-1, and our estimates suggest that the older restored site (16 years) was also a NEE sink (71 g m-2 yr-1). In contrast, the younger site (10 years) was a NEE source (80 g m-2 yr-1). We critically assess the confidence of these measurements and also present these data in relation to other northern hemisphere peatlands to better understand the timeframe in which a peatland site can turn from a C source to a C sink after restoration
Magnetic Order in YBaCuO Superconductors
Polarized and unpolarized neutron diffraction has been used to search for
magnetic order in YBaCuO superconductors. Most of the
measurements were made on a high quality crystal of YBaCuO. It
is shown that this crystal has highly ordered ortho-II chain order, and a sharp
superconducting transition. Inelastic scattering measurements display a very
clean spin-gap and pseudogap with any intensity at 10 meV being 50 times
smaller than the resonance intensity. The crystal shows a complicated magnetic
order that appears to have three components. A magnetic phase is found at high
temperatures that seems to stem from an impurity with a moment that is in the
- plane, but disordered on the crystal lattice. A second ordering occurs
near the pseudogap temperature that has a shorter correlation length than the
high temperature phase and a moment direction that is at least partly along the
c-axis of the crystal. Its moment direction, temperature dependence, and Bragg
intensities suggest that it may stem from orbital ordering of the -density
wave (DDW) type. An additional intensity increase occurs below the
superconducting transition. The magnetic intensity in these phases does not
change noticeably in a 7 Tesla magnetic field aligned approximately along the
c-axis. Searches for magnetic order in YBaCuO show no signal
while a small magnetic intensity is found in YBaCuO that is
consistent with c-axis directed magnetic order. The results are contrasted with
other recent neutron measurements.Comment: 11 pages with 10 figure
DDW Order and its Role in the Phase Diagram of Extended Hubbard Models
We show in a mean-field calculation that phase diagrams remarkably similar to
those recently proposed for the cuprates arise in simple microscopic models of
interacting electrons near half-filling. The models are extended Hubbard models
with nearest neighbor interaction and correlated hopping. The underdoped region
of the phase diagram features density-wave (DDW) order. In a
certain regime of temperature and doping, DDW order coexists with
antiferromagnetic (AF) order. For larger doping, it coexists with
superconductivity (DSC). While phase diagrams of this form
are robust, they are not inevitable. For other reasonable values of the
coupling constants, drastically different phase diagrams are obtained. We
comment on implications for the cuprates.Comment: 7 pages, 3 figure
A radium assay technique using hydrous titanium oxide adsorbent for the Sudbury Neutrino Observatory
As photodisintegration of deuterons mimics the disintegration of deuterons by
neutrinos, the accurate measurement of the radioactivity from thorium and
uranium decay chains in the heavy water in the Sudbury Neutrino Observatory
(SNO) is essential for the determination of the total solar neutrino flux. A
radium assay technique of the required sensitivity is described that uses
hydrous titanium oxide adsorbent on a filtration membrane together with a
beta-alpha delayed coincidence counting system. For a 200 tonne assay the
detection limit for 232Th is a concentration of 3 x 10^(-16) g Th/g water and
for 238U of 3 x 10^(-16) g U/g water. Results of assays of both the heavy and
light water carried out during the first two years of data collection of SNO
are presented.Comment: 12 pages, 4 figure
Radiofrequency ablation versus hepatic resection for hepatocellular carcinoma within the Milan criteria – A comparative study
AbstractBackgroundTo compare the results of radiofrequency ablation (RFA) with hepatic resection in the treatment of hepatocellular carcinoma (HCC) within the Milan criteria.MethodsA nonrandomized comparative study was performed with 111 consecutive patients who underwent laparoscopic RFA (n = 31) or curative hepatic resection (n = 80) for HCC within Milan criteria.ResultsProcedure related complications were less often and severe after RFA than resection (3.2% vs. 25%). There was no significant difference in hospital mortality (0% vs. 3.8%). Hospital stay was significantly shorter in the RFA group than in the resection group (mean, 3.8 vs. 6.8 days). The 1-, 3-, and 5-year disease-free survival rates for the RFA group and the resection group were 76%, 40%, 40% and 76%, 60%, 60%, respectively. Disease-free survival was significantly lower in the RFA group than in the resection group. The corresponding 1-, 3-, and 5-year overall survival rates for the RFA group and the resection group were 100%, 92%, 84%, and 92%, 75%, 71%, respectively. The overall survival for RFA and resection were not significantly different.ConclusionsOur result showed comparable overall survival between RFA and surgery, although RFA was associated with a significantly higher tumor recurrence rate. RFA had the advantages over surgical resection in being less invasive and having lower morbidity
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