3,979 research outputs found
From infinite to two dimensions through the functional renormalization group
We present a novel scheme for an unbiased and non-perturbative treatment of
strongly correlated fermions. The proposed approach combines two of the most
successful many-body methods, i.e., the dynamical mean field theory (DMFT) and
the functional renormalization group (fRG). Physically, this allows for a
systematic inclusion of non-local correlations via the flow equations of the
fRG, after the local correlations are taken into account non-perturbatively by
the DMFT. To demonstrate the feasibility of the approach, we present numerical
results for the two-dimensional Hubbard model at half-filling.Comment: 5 pages, 4 figure
High-temperature optical spectral weight and Fermi liquid renormalization in Bi-based cuprates
The optical conductivity and the spectral weight W(T) of two superconducting
cuprates at optimum doping, Bi2Sr2-xLaxCuO6 and Bi2Sr2CaCu2O8, have been first
measured up to 500 K. Above 300 K, W(T) deviates from the usual T2 behavior in
both compounds, even though the zero-frequency extrapolation of the optical
conductivity remains larger than the Ioffe-Regel limit. The deviation is
surprisingly well described by the T4 term of the Sommerfeld expansion, but its
coefficients are enhanced by strong correlation. This renormalization is due to
strong correlation, as shown by the good agreement with dynamical mean field
calculations.Comment: 5 pages, 3 figures, Physical Review Letters in pres
Local Electronic Correlation at the Two-Particle Level
Electronic correlated systems are often well described by dynamical mean
field theory (DMFT). While DMFT studies have mainly focused hitherto on
one-particle properties, valuable information is also enclosed into local
two-particle Green's functions and vertices. They represent the main ingredient
to compute momentum-dependent response functions at the DMFT level and to treat
non-local spatial correlations at all length scales by means of diagrammatic
extensions of DMFT. The aim of this paper is to present a DMFT analysis of the
local reducible and irreducible two-particle vertex functions for the Hubbard
model in the context of an unified diagrammatic formalism. An interpretation of
the observed frequency structures is also given in terms of perturbation
theory, of the comparison with the atomic limit, and of the mapping onto the
attractive Hubbard model.Comment: 29 pages, 26 Figures. Accepted for publication in Phys. Rev.
Stochastic optimization of a cold atom experiment using a genetic algorithm
We employ an evolutionary algorithm to automatically optimize different
stages of a cold atom experiment without human intervention. This approach
closes the loop between computer based experimental control systems and
automatic real time analysis and can be applied to a wide range of experimental
situations. The genetic algorithm quickly and reliably converges to the most
performing parameter set independent of the starting population. Especially in
many-dimensional or connected parameter spaces the automatic optimization
outperforms a manual search.Comment: 4 pages, 3 figure
A single atom detector integrated on an atom chip: fabrication, characterization and application
We describe a robust and reliable fluorescence detector for single atoms that
is fully integrated into an atom chip. The detector allows spectrally and
spatially selective detection of atoms, reaching a single atom detection
efficiency of 66%. It consists of a tapered lensed single-mode fiber for
precise delivery of excitation light and a multi-mode fiber to collect the
fluorescence. The fibers are mounted in lithographically defined holding
structures on the atom chip. Neutral 87Rb atoms propagating freely in a
magnetic guide are detected and the noise of their fluorescence emission is
analyzed. The variance of the photon distribution allows to determine the
number of detected photons / atom and from there the atom detection efficiency.
The second order intensity correlation function of the fluorescence shows
near-perfect photon anti-bunching and signs of damped Rabi-oscillations. With
simple improvements one can boost the detection efficiency to > 95%.Comment: 24 pages, 11 figure
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Measurement of masses in the [Formula: see text] system by kinematic endpoints in pp collisions at [Formula: see text].
A simultaneous measurement of the top-quark, W-boson, and neutrino masses is reported for [Formula: see text] events selected in the dilepton final state from a data sample corresponding to an integrated luminosity of 5.0 fb-1 collected by the CMS experiment in pp collisions at [Formula: see text]. The analysis is based on endpoint determinations in kinematic distributions. When the neutrino and W-boson masses are constrained to their world-average values, a top-quark mass value of [Formula: see text] is obtained. When such constraints are not used, the three particle masses are obtained in a simultaneous fit. In this unconstrained mode the study serves as a test of mass determination methods that may be used in beyond standard model physics scenarios where several masses in a decay chain may be unknown and undetected particles lead to underconstrained kinematics
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