3,265 research outputs found
The ratio FK/Fpi in QCD
We determine the ratio FK/Fpi in QCD with Nf=2+1 flavors of sea quarks, based
on a series of lattice calculations with three different lattice spacings,
large volumes and a simulated pion mass reaching down to about 190 MeV. We
obtain FK/Fpi=1.192 +/- 0.007(stat) +/- 0.006(syst). This result is then used
to give an updated value of the CKM matrix element |Vus|. The unitarity
relation for the first row of this matrix is found to be well observed.Comment: 15 pages, 4 figures, 2 table
Optimized Effective Potential Method in Current-Spin Density Functional Theory
Current-spin density functional theory (CSDFT) provides a framework to
describe interacting many-electron systems in a magnetic field which couples to
both spin- and orbital-degrees of freedom. Unlike in usual (spin-) density
functional theory, approximations to the exchange-correlation energy based on
the model of the uniform electron gas face problems in practical applications.
In this work, explicitly orbital-dependent functionals are used and a
generalization of the Optimized Effective Potential (OEP) method to the CSDFT
framework is presented. A simplifying approximation to the resulting integral
equations for the exchange-correlation potentials is suggested. A detailed
analysis of these equations is carried out for the case of open-shell atoms and
numerical results are given using the exact-exchange energy functional. For
zero external magnetic field, a small systematic lowering of the total energy
for current-carrying states is observed due to the inclusion of the current in
the Kohn-Sham scheme. For states without current, CSDFT results coincide with
those of spin density functional theory.Comment: 11 pages, 3 figure
Precision computation of the kaon bag parameter
Indirect CP violation in K \rightarrow {\pi}{\pi} decays plays a central role
in constraining the flavor structure of the Standard Model (SM) and in the
search for new physics. For many years the leading uncertainty in the SM
prediction of this phenomenon was the one associated with the nonperturbative
strong interaction dynamics in this process. Here we present a fully controlled
lattice QCD calculation of these effects, which are described by the neutral
kaon mixing parameter B_K . We use a two step HEX smeared clover-improved
Wilson action, with four lattice spacings from a\approx0.054 fm to
a\approx0.093 fm and pion masses at and even below the physical value.
Nonperturbative renormalization is performed in the RI-MOM scheme, where we
find that operator mixing induced by chiral symmetry breaking is very small.
Using fully nonperturbative continuum running, we obtain our main result
B_K^{RI}(3.5GeV)=0.531(6)_{stat}(2)_{sys}. A perturbative 2-loop conversion
yields B_K^{MSbar-NDR}(2GeV)=0.564(6)_{stat}(3)_{sys}(6)_{PT}, which is in good
agreement with current results from fits to experimental data.Comment: 10 pages, 7 figures. v2: Added one reference and one figure, replaced
2 figures for better readability and updated ensembles, conclusions
unchanged. Final, published versio
Exchange-correlation orbital functionals in current-density-functional theory: Application to a quantum dot in magnetic fields
The description of interacting many-electron systems in external magnetic
fields is considered in the framework of the optimized effective potential
method extended to current-spin-density functional theory. As a case study, a
two-dimensional quantum dot in external magnetic fields is investigated.
Excellent agreement with quantum Monte Carlo results is obtained when
self-interaction corrected correlation energies from the standard local
spin-density approximation are added to exact-exchange results. Full
self-consistency within the complete current-spin-density-functional framework
is found to be of minor importance.Comment: 5 pages, 2 figures, submitted to PR
The Effect of 45{\deg} Grain Boundaries and associated Fe particles on Jc and resistivity in Ba(Fe0.9Co0.1)2As2 Thin Films
The anisotropy of the critical current density Jc depends in general on both
the properties of the flux lines (such as line tension, coherence length and
penetration depth) and the properties of the defects (such as density, shape,
orientation etc.). Whereas the Jc anisotropy in microstructurally clean films
can be scaled to an effective magnetic field containing the Ginzburg-Landau
anisotropy term, it is in general not possible (or only in a limited field
range) for samples containing extended defects. Here, the Jc anisotropy of a
Co-doped BaFe2As2 sample with 45{\deg} [001] tilt grain boundaries (GBs), i.e.
grain boundaries created by 45{\deg} in-plane rotated grains, as well as
extended Fe particles is investigated. This microstructure leads to c-axis
correlated pinning, both due to the GBs and the Fe particles and manifests in a
c-axis peak in the Jc anisotropy at low magnetic fields and a deviation from
the anisotropic Ginzburg-Landau scaling at higher fields. Strong pinning at
ellipsoidal extended defects, i.e. the Fe particles, is discussed, and the full
Jc anisotropy is fitted successfully with the vortex path model. The results
are compared to a sample without GBs and Fe particles. 45{\deg} GBs seem to be
good pinning centers rather than detrimental to current flow.Comment: 8 pages, 7 figures, CEC-ICMC 2013 proceeding, accepted for
publication in Advances in Cryogenic Engineering (Materials
Electromagnetic corrections to light hadron masses
At the precision reached in current lattice QCD calculations, electromagnetic
effects are becoming numerically relevant. We will present preliminary results
for electromagnetic corrections to light hadron masses, based on simulations in
which a degree of freedom is superimposed on QCD
configurations from the BMW collaboration.Comment: 7 pages, 2 figures, The XXVIII International Symposium on Lattice
Field Theory, June 14-19,2010, Villasimius, Sardinia Ital
Scaling behaviour of the critical current in clean epitaxial Ba(Fe1-xCox)2As2 thin films
The angular-dependent critical current density, Jc(theta), and the upper
critical field, Hc2(theta), of epitaxial Ba(Fe1-xCox)2As2 thin films have been
investigated. No Jc(theta) peaks for H || c were observed regardless of
temperatures and magnetic fields. In contrast, Jc(theta) showed a broad maximum
at theta=90 degree, which arises from intrinsic pinning. All data except at
theta=90 degree can be scaled by the Blatter plot. Hc2(theta) near Tc follows
the anisotropic Ginzburg-Landau expression. The mass anisotropy increased from
1.5 to 2 with increasing temperature, which is an evidence for multi-band
superconductivity.Comment: Accepted in Physical Review B rapid communication
Highly anisotropic energy gap in superconducting Ba(FeCo)As from optical conductivity measurements
We have measured the complex dynamical conductivity, , of superconducting Ba(FeCo)As ( K) at terahertz frequencies and temperatures 2 - 30 K. In the frequency
dependence of below , we observe clear signatures of the
superconducting energy gap opening. The temperature dependence of
demonstrates a pronounced coherence peak at frequencies below 15 cm (1.8
meV). The temperature dependence of the penetration depth, calculated from
, shows power-law behavior at the lowest temperatures. Analysis of
the conductivity data with a two-gap model, gives the smaller isotropic s-wave
gap of meV, while the larger gap is highly anisotropic with
possible nodes and its rms amplitude is meV. Overall, our
results are consistent with a two-band superconductor with an gap
symmetry.Comment: 6 pages, 4 figures, discussion on pair-barking scattering and
possible lifting of the nodes is adde
Influenced of Fe buffer thickness on the crystalline quality and the transport properties of Fe/Ba(Fe1-xCox)2As2 bilayers
The implementation of an Fe buffer layer is a promising way to obtain
epitaxial growth of Co-doped BaFe2As2 (Ba-122). However, the crystalline
quality and the superconducting properties of Co-doped Ba-122 are influenced by
the Fe buffer layer thickness, dFe. The well-textured growth of the Fe/Ba-122
bilayer with dFe = 15 nm results in a high Jc of 0.45 MAcm at 12 K in
self-field, whereas a low Jc value of 61000 Acm is recorded for the
bilayer with dFe = 4 nm at the corresponding reduced temperature due to the
presence of grain boundaries
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