223 research outputs found
Recursive Calculation of Effective Potential and Variational Resummation
We set up a method for a recursive calculation of the effective potential
which is applied to a cubic potential with imaginary coupling. The result is
resummed using variational perturbation theory (VPT), yielding an exponentially
fast convergence.Comment: Author Information under
http://www.physik.fu-berlin.de/~kleinert/institution.html Latest update of
paper (including all PS fonts) at
http://www.physik.fu-berlin.de/~kleinert/350
Recursive Graphical Solution of Closed Schwinger-Dyson Equations in phi^4-Theory -- Part1: Generation of Connected and One-Particle Irreducible Feynman Diagrams
Using functional derivatives with respect to the free correlation function we
derive a closed set of Schwinger-Dyson equations in phi^4-theory. Its
conversion to graphical recursion relations allows us to systematically
generate all connected and one-particle irreducible Feynman diagrams for the
two- and four-point function together with their weights.Comment: Author Information under http://www.physik.fu-berlin.de/~pelster
Large-D Expansion from Variational Perturbation Theory
We derive recursively the perturbation series for the ground-state energy of
the D-dimensional anharmonic oscillator and resum it using variational
perturbation theory (VPT). From the exponentially fast converging approximants,
we extract the coefficients of the large-D expansion to higher orders. The
calculation effort is much smaller than in the standard field-theoretic
approach based on the Hubbard-Stratonovich transformation.Comment: Author Information under http://hbar.wustl.edu/~sbrandt and
http://www.theo-phys.uni-essen.de/tp/ags/pelster_di
Beyond Mean-Field Low-Lying Excitations of Dipolar Bose Gases
We theoretically investigate various beyond mean-field effects on Bose gases
at zero temperature featuring the anisotropic and long-range dipole-dipole
interaction in addition to the isotropic and short-range contact interaction.
Within the realm of the Bogoliubov-de Gennes theory, we consider static
properties and low-lying excitations of both homogeneous and harmonically
trapped dipolar bosonic gases. For the homogeneous system, the condensate
depletion, the ground-state energy, the equation of state, and the speed of
sound are discussed in detail. Making use of the local density approximation,
we extend these results in order to study the properties of a dipolar Bose gas
in a harmonic trap and in the regime of large particle numbers. After deriving
the equations of motion for the general case of a triaxial trap, we analyze the
influence of quantum fluctuations on important properties of the gas, such as
the equilibrium configuration and the low-lying excitations in the case of a
cylinder-symmetric trap. In addition to the monopole and quadrupole oscillation
modes, we also discuss the radial quadrupole mode. We find that the latter
acquires a quantum correction exclusively due to the dipole-dipole interaction.
As a result, we identify the radial quadrupole as a reasonably accessible
source for the signature of dipolar many-body effects and stress the enhancing
character that dipolar interactions have for quantum fluctuations in the other
oscillation modes.Comment: Version published in PR
Fluctuation Pressure of a Stack of Membranes
We calculate the universal pressure constants of a stack of N membranes
between walls by strong-coupling theory. The results are in very good agreement
with values from Monte-Carlo simulations.Comment: Author Information under
http://www.physik.fu-berlin.de/~kleinert/institution.html Latest update of
paper also at http://www.physik.fu-berlin.de/~kleinert/31
Mean-field phase diagram of disordered bosons in a lattice at non-zero temperature
Bosons in a periodic lattice with on-site disorder at low but non-zero
temperature are considered within a mean-field theory. The criteria used for
the definition of the superfluid, Mott insulator and Bose glass are analysed.
Since the compressibility does never vanish at non-zero temperature, it can not
be used as a general criterium. We show that the phases are unambiguously
distinguished by the superfluid density and the density of states of the
low-energy exitations. The phase diagram of the system is calculated. It is
shown that even a tiny temperature leads to a significant shift of the boundary
between the Bose glass and superfluid
Order via Nonlinearity in Randomly Confined Bose Gases
A Hartree-Fock mean-field theory of a weakly interacting Bose-gas in a
quenched white noise disorder potential is presented. A direct continuous
transition from the normal gas to a localized Bose-glass phase is found which
has localized short-lived excitations with a gapless density of states and
vanishing superfluid density. The critical temperature of this transition is as
for an ideal gas undergoing Bose-Einstein condensation. Increasing the
particle-number density a first-order transition from the localized state to a
superfluid phase perturbed by disorder is found. At intermediate number
densities both phases can coexist.Comment: Author Information under
http://www.theo-phys.uni-essen.de/tp/ags/pelster_dir/. International Journal
of Bifurcation and Chaos (in press
Bose-Einstein Condensate in Weak 3d Isotropic Speckle Disorder
The effect of a weak three-dimensional (3d) isotropic laser speckle disorder
on various thermodynamic properties of a dilute Bose gas is considered at zero
temperature. First, we summarize the derivation of the autocorrelation function
of laser speckles in 1d and 2d following the seminal work of Goodman. The goal
of this discussion is to show that a Gaussian approximation of this function,
proposed in some recent papers, is inconsistent with the general background of
laser speckle theory. Then we propose a possible experimental realization for
an isotropic 3d laser speckle potential and derive its corresponding
autocorrelation function. Using a Fourier transform of that function, we
calculate both condensate depletion and sound velocity of a Bose-Einstein
condensate as disorder ensemble averages of such a weak laser speckle potential
within a perturbative solution of the Gross-Pitaevskii equation. By doing so,
we reproduce the expression of the normalfluid density obtained earlier within
the treatment of Landau. This physically transparent derivation shows that
condensate particles, which are scattered by disorder, form a gas of
quasiparticles which is responsible for the normalfluid component
Direct Transfer Patterning of Electrically Small Antennas onto Three‐Dimensionally Contoured Substrates
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/90030/1/1166_ftp.pd
Manure Management Practices and Policies in Sub-Saharan Africa: Implications on Manure Quality as a Fertilizer
Manure has been used as a fertilizer since ancient times and if well-managed it can be an asset, promoting sustainable agriculture, and increasing crop production, particularly for smallholder farmers in sub-Saharan Africa (SSA). However, most farmers in SSA do not apply recommended manure management practices, such as roofing animal housing, having a water-proof floor or covering manure during storage, causing large nutrient losses during manure storage, increasing greenhouse gas emissions, and reducing the quality of the manure as a fertilizer. This paper compares manure management practices in representative SSA countries, and summarizes government policies and socio-cultural practices that influence the adoption of good (recommended) manure management practices. Three steps were applied in this analysis: (i) review of manure management practices from various literature sources, (ii) interviews on manure management practices and policies with key stakeholders from 13 SSA countries, and (iii) surveys of manure management practices on small, medium, and large scale farms in Ethiopia and Malawi. The review confirms the potential of manure to improve crop yields and promote sustainable agriculture in SSA. Unfortunately, most SSA countries (a) do not explicitly mention manure management in their policies (b) have different ministries that share responsibilities on manure management, often leading to incoherent policies and abnegation of these responsibilities (c) take limited action to promote good practices or enforce legislation on manure management. Also, the field survey indicated that farmers lack knowledge on manure management. However, farmers are able to access agricultural extension services from both government and non-government agencies, although these extension services rarely included information on improved manure management practices. Extension services that encourage exchange and interaction between farmers were most successful in increasing adoption of good manure management practices, and are recommended. In addition, efforts to improve manure management in SSA should strengthen the enforcement of existing policies and provide an enabling environment for adoption of good manure management practices
- …