440 research outputs found
Regularization of Diagrammatic Series with Zero Convergence Radius
The divergence of perturbative expansions for the vast majority of
macroscopic systems, which follows from Dyson's collapse argument, prevents
Feynman's diagrammatic technique from being directly used for controllable
studies of strongly interacting systems. We show how the problem of divergence
can be solved by replacing the original model with a convergent sequence of
successive approximations which have a convergent perturbative series. As a
prototypical model, we consider the zero-dimensional
theory.Comment: 4 pages, 3 figure
Dynamical mean field solution of the Bose-Hubbard model
We present the effective action and self-consistency equations for the
bosonic dynamical mean field (B-DMFT) approximation to the bosonic Hubbard
model and show that it provides remarkably accurate phase diagrams and
correlation functions. To solve the bosonic dynamical mean field equations we
use a continuous-time Monte Carlo method for bosonic impurity models based on a
diagrammatic expansion in the hybridization and condensate coupling. This
method is readily generalized to bosonic mixtures, spinful bosons, and
Bose-Fermi mixtures.Comment: 10 pages, 3 figures. includes supplementary materia
Competition between pairing and ferromagnetic instabilities in ultracold Fermi gases near Feshbach resonances
We study the quench dynamics of a two-component ultracold Fermi gas from the
weak into the strong interaction regime, where the short time dynamics are
governed by the exponential growth rate of unstable collective modes. We obtain
an effective interaction that takes into account both Pauli blocking and the
energy dependence of the scattering amplitude near a Feshbach resonance. Using
this interaction we analyze the competing instabilities towards Stoner
ferromagnetism and pairing.Comment: 4+epsilon pages, 4 figure
Bosons Confined in Optical Lattices: the Numerical Renormalization Group revisited
A Bose-Hubbard model, describing bosons in a harmonic trap with a
superimposed optical lattice, is studied using a fast and accurate variational
technique (MF+NRG): the Gutzwiller mean-field (MF) ansatz is combined with a
Numerical Renormalization Group (NRG) procedure in order to improve on both.
Results are presented for one, two and three dimensions, with particular
attention to the experimentally accessible momentum distribution and possible
satellite peaks in this distribution. In one dimension, a comparison is made
with exact results obtained using Stochastich Series Expansion.Comment: 10 pages, 15 figure
Optimal Monte Carlo Updating
Based on Peskun's theorem it is shown that optimal transition matrices in
Markov chain Monte Carlo should have zero diagonal elements except for the
diagonal element corresponding to the largest weight. We will compare the
statistical efficiency of this sampler to existing algorithms, such as
heat-bath updating and the Metropolis algorithm. We provide numerical results
for the Potts model as an application in classical physics. As an application
in quantum physics we consider the spin 3/2 XY model and the Bose-Hubbard model
which have been simulated by the directed loop algorithm in the stochastic
series expansion framework.Comment: 6 pages, 5 figures, replaced with published versio
Identification of an iron–hepcidin complex
Following its identification as a liver-expressed antimicrobial peptide, the hepcidin peptide was later shown to be a key player in iron homoeostasis. It is now proposed to be the 'iron hormone' which, by interacting with the iron transporter ferroportin, prevents further iron import into the circulatory system. This conclusion was reached using the corresponding synthetic peptide, emphasizing the functional importance of the mature 25-mer peptide, but omitting the possible functionality of its maturation. From urine-purified native hepcidin, we recently demonstrated that a proportion of the purified hepcidin had formed iron-hepcidin complexes. This interaction was investigated further by computer modelling and, based on the sequence similarity of hepcidin with metallothionein, a three-dimensional model of hepcidin, containing one atom of iron, was constructed. To characterize these complexes further, the interaction with iron was analysed using different spectroscopic methods. Monoferric hepcidin was identified by MS, as were possibly other complexes containing two and three atoms of iron respectively, although these were present only in minor amounts. UV/visible absorbance and CD studies identified the iron-binding events which were facilitated at a physiological pH. EPR spectroscopy identified the ferric state of the bound metal, and indicated that the iron-hepcidin complex shares some similarities with the rubredoxin iron-sulfur complex, suggesting the presence of Fe(3+) in a tetrahedral sulfur co-ordination. The potential roles of iron binding for hepcidin are discussed, and we propose either a regulatory function in the maturation of pro-hepcidin into active hepcidin or as the necessary link in the interaction between hepcidin and ferroportin
Silent universes with a cosmological constant
We study non-degenerate (Petrov type I) silent universes in the presence of a
non-vanishing cosmological constant L. In contrast to the L=0 case, for which
the orthogonally spatially homogeneous Bianchi type I metrics most likely are
the only admissible metrics, solutions are shown to exist when L is positive.
The general solution is presented for the case where one of the eigenvalues of
the expansion tensor is 0.Comment: 11 pages; several typos corrected which were still present in CGQ
version; minor change
Exact quantum dynamics of bosons with finite-range time-dependent interactions of harmonic type
The exactly solvable quantum many-particle model with harmonic one- and
two-particle interaction terms is extended to include time-dependency. We show
that when the external trap potential and finite-range interparticle
interaction have a time-dependency the exact solutions of the corresponding
time-dependent many-boson Schr\"odinger equation are still available. We use
these exact solutions to benchmark the recently developed multiconfigurational
time-dependent Hartree method for bosons (MCTDHB) [Phys. Rev. Lett. {\bf 99},
030402 (2007), Phys. Rev. A {\bf 77}, 033613 (2008)]. In particular, we
benchmark the MCTDHB method for: (i) the ground state; (ii) the breathing
many-body dynamics activated by a quench scenario where the interparticle
interaction strength is suddenly turned on to a finite value; (iii) the
non-equilibrium dynamic for driven scenarios where both the trap- and
interparticle-interaction potentials are {\it time-dependent}. Excellent
convergence of the ground state and dynamics is demonstrated. The great
relevance of the self-consistency and time-adaptivity, which are the intrinsic
features of the MCTDHB method, is demonstrated by contrasting the MCTDHB
predictions and those obtained within the standard full configuration
interaction method spanning the Fock space of the same size, but utilizing as
one-particle basis set the fixed-shape eigenstates of the one-particle
potential. Connections of the model's results to ultra-cold Bose-Einstein
condensed systems are addressed.Comment: 31 pages, 5 figure
Dynamical mean-field theory for bosons
We discuss the recently developed bosonic dynamical mean-field (B-DMFT)
framework, which maps a bosonic lattice model onto the selfconsistent solution
of a bosonic impurity model with coupling to a reservoir of normal and
condensed bosons. The effective impurity action is derived in several ways: (i)
as an approximation to the kinetic energy functional of the lattice problem,
(ii) using a cavity approach, and (iii) by using an effective medium approach
based on adding a one-loop correction to the selfconsistently defined
condensate. To solve the impurity problem, we use a continuous-time Monte Carlo
algorithm based on a sampling of a perturbation expansion in the hybridization
functions and the condensate wave function. As applications of the formalism we
present finite temperature B-DMFT phase diagrams for the bosonic Hubbard model
on a 3d cubic and 2d square lattice, the condensate order parameter as a
function of chemical potential, critical exponents for the condensate, the
approach to the weakly interacting Bose gas regime for weak repulsions, and the
kinetic energy as a function of temperature.Comment: 26 pages, 19 figure
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