12,699 research outputs found
Characteristic extraction tool for gravitational waveforms
We develop and calibrate a characteristic waveform extraction tool whose major improvements and corrections of prior versions allow satisfaction of the accuracy standards required for advanced LIGO data analysis. The extraction tool uses a characteristic evolution code to propagate numerical data on an inner worldtube supplied by a 3+1 Cauchy evolution to obtain the gravitational waveform at null infinity. With the new extraction tool, high accuracy and convergence of the numerical error can be demonstrated for an inspiral and merger of mass M binary black holes even for an extraction worldtube radius as small as R=20M. The tool provides a means for unambiguous comparison between waveforms generated by evolution codes based upon different formulations of the Einstein equations and based upon different numerical approximations
Determining the conformal window: SU(2) gauge theory with N_f = 4, 6 and 10 fermion flavours
We study the evolution of the coupling in SU(2) gauge field theory with
, 6 and 10 fundamental fermion flavours on the lattice. These values are
chosen close to the expected edges of the conformal window, where the theory
possesses an infrared fixed point. We use improved Wilson-clover action, and
measure the coupling in the Schr\"odinger functional scheme. At four flavours
we observe that the couping grows towards the infrared, implying QCD-like
behaviour, whereas at ten flavours the results are compatible with a Banks-Zaks
type infrared fixed point. The six flavour case remains inconclusive: the
evolution of the coupling is seen to become slower at the infrared, but the
accuracy of the results falls short from fully resolving the fate of the
coupling. We also measure the mass anomalous dimension for the case.Comment: 22 pages, 12 figures. Proof readin
Static forces in d=2+1 SU(N) gauge theories
Using a three-level algorithm we perform a high-precision lattice computation
of the static force up to 1fm in the 2+1 dimensional SU(5) gauge theory.
Discretization errors and the continuum limit are discussed in detail. By
comparison with existing SU(2) and SU(3) data it is found that \sigma
r_0^2=1.65-\pi/24 holds at an accuracy of 1% for all N>=2, where r_0 is the
Sommer reference scale. The effective central charge c_{eff}(r) is obtained and
an intermediate distance r_s is defined via the property c_{eff}(r_s)=\pi/24.
It separates in a natural way the short-distance regime governed by
perturbation theory from the long-distance regime described by an effective
string theory. The ratio r_s/r_0 decreases significantly from SU(2) to SU(3) to
SU(5), where r_s < r_0. We give a preliminary estimate of its value in the
large-N limit. The static force in the smallest representation of N-ality 2,
which tends to the k=2 string tension as r->oo, is also computed up to 0.7fm.
The deviation from Casimir scaling is positive and grows from 0.1% to 1% in
that range.Comment: 25 pages, 8 figures, 11 table
Absolute physical calibration in the infrared
We determine an absolute calibration for the Multiband Imaging Photometer for Spitzer 24 μm band and recommend adjustments to the published calibrations for Two Micron All Sky Survey (2MASS), Infrared Array Camera (IRAC), and IRAS photometry to put them on the same scale. We show that consistent results are obtained by basing the calibration on either an average A0V star spectral energy distribution (SED), or by using the absolutely calibrated SED of the Sun in comparison with solar-type stellar photometry (the solar analog method). After the rejection of a small number of stars with anomalous SEDs (or bad measurements), upper limits of ~1.5% root mean square (rms) are placed on the intrinsic infrared (IR) SED variations in both A-dwarf and solar-type stars. These types of stars are therefore suitable as general-purpose standard stars in the IR. We provide absolutely calibrated SEDs for a standard zero magnitude A star and for the Sun to allow extending this work to any other IR photometric system. They allow the recommended calibration to be applied from 1 to 25 μm with an accuracy of ~2%, and with even higher accuracy at specific wavelengths such as 2.2, 10.6, and 24 μm, near which there are direct measurements. However, we confirm earlier indications that Vega does not behave as a typical A0V star between the visible and the IR, making it problematic as the defining star for photometric systems. The integration of measurements of the Sun with those of solar-type stars also provides an accurate estimate of the solar SED from 1 through 30 μm, which we show agrees with theoretical models
Mass anomalous dimension in SU(2) with two adjoint fermions
We study SU(2) lattice gauge theory with two flavours of Dirac fermions in
the adjoint representation. We measure the running of the coupling in the
Schroedinger Functional (SF) scheme and find it is consistent with existing
results. We discuss how systematic errors affect the evidence for an infrared
fixed point (IRFP). We present the first measurement of the running of the mass
in the SF scheme. The anomalous dimension of the chiral condensate, which is
relevant for phenomenological applications, can be easily extracted from the
running of the mass, under the assumption that the theory has an IRFP. At the
current level of accuracy, we can estimate 0.05 < gamma < 0.56 at the IRFP.Comment: 30 pages, 11 figure
Converged GW quasiparticle energies for transition metal oxide perovskites
The ab initio calculation of quasiparticle (QP) energies is a technically and
computationally challenging problem. In condensed matter physics the most
widely used approach to determine QP energies is the GW approximation. Although
the GW method has been widely applied to many typical semiconductors and
insulators, its application to more complex compounds such as transition metal
oxide perovskites has been comparatively rare, and its proper use is not well
established from a technical point of view. In this work, we have applied the
single-shot G0W0 method to a representative set of transition metal oxide
perovskites including 3d (SrTiO3, LaScO3, SrMnO3, LaTiO3, LaVO3, LaCrO3,
LaMnO3, and LaFeO3), 4d (SrZrO3, SrTcO3, and Ca2RuO4) and 5d (SrHfO3, KTaO3 and
NaOsO3) compounds with different electronic configurations, magnetic orderings,
structural characteristics and bandgaps ranging from 0.1 to 6.1 eV. We discuss
the proper procedure to obtain well converged QP energies and accurate bandgaps
within single-shot G0W0 by comparing the conventional approach based on an
incremental variation of a specific set of parameters (number of bands, energy
cutoff for the plane-wave expansion and number of k-points and the basis-set
extrapolation scheme [Phys. Rev. B 90, 075125 (2014)]. In addition, we have
inspected the difference between the adoption of norm-conserving and ultrasoft
potentials in GW calculations. A minimal statistical analysis indicates that
the correlation of the GW data with the DFT gap is more robust than the
correlation with the experimental gaps; moreover we identify the static
dielectric constant as alternative useful parameter for the approximation of GW
gap in high-throughput automatic procedures. Finally, we compute the QP band
structure and spectra within the random phase approximation and compare the
results with available experimental data.Comment: Physical Review Materials, accepte
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