6,663 research outputs found
High Mass Star Formation. II. The Mass Function of Submillimeter Clumps in M17
We have mapped an approximately 5.5 by 5.5 pc portion of the M17 massive
star-forming region in both 850 and 450 micron dust continuum emission using
the Submillimeter Common-User Bolometer Array (SCUBA) on the James Clerk
Maxwell Telescope (JCMT). The maps reveal more than 100 dusty clumps with
deconvolved linear sizes of 0.05--0.2 pc and masses of 0.8--120 solar masses,
most of which are not associated with known mid-infrared point sources. Fitting
the clump mass function with a double power law gives a mean power law exponent
of alpha_high = -2.4 +/- 0.3 for the high-mass power law, consistent with the
exponent of the Salpeter stellar mass function. We show that a lognormal clump
mass distribution with a peak at about 4 solar masses produces as good a fit to
the clump mass function as does a double power law. This 4 solar mass peak mass
is well above the peak masses of both the stellar initial mass function and the
mass function of clumps in low-mass star-forming regions. Despite the
difference in intrinsic mass scale, the shape of the M17 clump mass function
appears to be consistent with the shape of the core mass function in low-mass
star-forming regions. Thus, we suggest that the clump mass function in
high-mass star-forming regions may be a scaled-up version of that in low-mass
regions, instead of its extension to higher masses.Comment: 33 pages, 6 figures, 3 tables. Accepted for publication in the
Astrophysical Journa
One-Flavour Hybrid Monte Carlo with Wilson Fermions
The Wilson fermion determinant can be written as product of the determinants
of two hermitian positive definite matrices. This formulation allows to
simulate non-degenerate quark flavors by means of the hybrid Monte Carlo
algorithm. A major numerical difficulty is the occurrence of nested inversions.
We construct a Uzawa iteration scheme which treats the nested system within one
iterative process.Comment: 11 pages, to appear in proceedings of the workshop "Numerical
Challenges in Lattice QCD", Springer Verla
CDW, Superconductivity and Anomalous Metallic Behavior in 2D Transition Metal Dichalcogenides
We propose a theory for quasi-two-dimensional transition metal
dichalcogenides that provides a unified microscopic picture of the charge
density wave (CDW) and superconducting phases. We show, based on the
electron-phonon coupling and Fermi surface topology, that a CDW order parameter
with six-fold symmetry and nodes (f-wave) gives a consistent description of the
available experimental data. The elementary excitations in the CDW phase are
Dirac electrons. The superconducting state has its origin on the attractive
interaction mediated by phonons. The theory predicts strong deviations from
Fermi liquid theory in the CDW phase.Comment: 4 pages, 3 figure
Functional renormalization group approach to zero-dimensional interacting systems
We apply the functional renormalization group method to the calculation of
dynamical properties of zero-dimensional interacting quantum systems. As case
studies we discuss the anharmonic oscillator and the single impurity Anderson
model. We truncate the hierarchy of flow equations such that the results are at
least correct up to second order perturbation theory in the coupling. For the
anharmonic oscillator energies and spectra obtained within two different
functional renormalization group schemes are compared to numerically exact
results, perturbation theory, and the mean field approximation. Even at large
coupling the results obtained using the functional renormalization group agree
quite well with the numerical exact solution. The better of the two schemes is
used to calculate spectra of the single impurity Anderson model, which then are
compared to the results of perturbation theory and the numerical
renormalization group. For small to intermediate couplings the functional
renormalization group gives results which are close to the ones obtained using
the very accurate numerical renormalization group method. In particulare the
low-energy scale (Kondo temperature) extracted from the functional
renormalization group results shows the expected behavior.Comment: 22 pages, 8 figures include
Nonperturbative renormalization in a scalar model within Light-Front Dynamics
Within the covariant formulation of Light-Front Dynamics, in a scalar model
with the interaction Hamiltonian , we calculate
nonperturbatively the renormalized state vector of a scalar "nucleon" in a
truncated Fock space containing the , and sectors. The
model gives a simple example of non-perturbative renormalization which is
carried out numerically. Though the mass renormalization diverges
logarithmically with the cutoff , the Fock components of the "physical"
nucleon are stable when .Comment: 22 pages, 5 figure
The Role of Zero-Modes in the Canonical Quantization of Heavy-Fermion QED in Light-Cone Coordinates
Four-dimensional heavy-fermion QED is studied in light-cone coordinates with
(anti-)periodic field boundary conditions. We carry out a consistent light-cone
canonical quantization of this model using the Dirac algorithm for a system
with first- and second-class constraints. To examine the role of the zero
modes, we consider the quantization procedure in {the }zero-mode {and the
non-zero-mode} sectors separately. In both sectors we obtain the physical
variables and their canonical commutation relations. The physical Hamiltonian
is constructed via a step-by-step exclusion of the unphysical degrees of
freedom. An example using this Hamiltonian in which the zero modes play a role
is the verification of the correct Coulomb potential between two heavy
fermions.Comment: 22 pages, CWRUTH-93-5 (Latex
A gentle introduction to the functional renormalization group: the Kondo effect in quantum dots
The functional renormalization group provides an efficient description of the
interplay and competition of correlations on different energy scales in
interacting Fermi systems. An exact hierarchy of flow equations yields the
gradual evolution from a microscopic model Hamiltonian to the effective action
as a function of a continuously decreasing energy cutoff. Practical
implementations rely on suitable truncations of the hierarchy, which capture
nonuniversal properties at higher energy scales in addition to the universal
low-energy asymptotics. As a specific example we study transport properties
through a single-level quantum dot coupled to Fermi liquid leads. In
particular, we focus on the temperature T=0 gate voltage dependence of the
linear conductance. A comparison with exact results shows that the functional
renormalization group approach captures the broad resonance plateau as well as
the emergence of the Kondo scale. It can be easily extended to more complex
setups of quantum dots.Comment: contribution to Les Houches proceedings 2006, Springer styl
The low-energy scale of the periodic Anderson model
Wilson's Numerical Renormalization Group method is used to study the
paramagnetic ground state of the periodic Anderson model within the dynamical
mean-field approach. For the particle-hole symmetric model, which is a Kondo
insulator, we find that the lattice Kondo scale T_0 is strongly enhanced over
the impurity scale T_K; T_0/T_K ~ exp(1/3I), where I is the Schrieffer-Wolff
exchange coupling. In the metallic regime, where the conduction band filling is
reduced from one, we find characteristic signatures of Nozi\`eres exhaustion
scenario, including a strongly reduced lattice Kondo scale, a significant
suppression of the states available to screen the f-electron moment, and a
Kondo resonance with a strongly enhanced height. However, in contrast to the
quantitative predictions of Nozi\`eres, we find that the T_0 ~ T_K with a
coefficient which depends strongly on conduction band filling.Comment: 11 pages, 9 figures, submitted to Phys. Rev.
Zero temperature metal-insulator transition in the infinite-dimensional Hubbard model
The zero temperature transition from a paramagnetic metal to a paramagnetic
insulator is investigated in the Dynamical Mean Field Theory for the Hubbard
model. The self-energy of the effective impurity Anderson model (on which the
Hubbard model is mapped) is calculated using Wilson's Numerical Renormalization
Group method. Results for quasiparticle weight, spectral function and
self-energy are discussed for Bethe and hypercubic lattice. In both cases, the
metal-insulator transition is found to occur via the vanishing of a
quasiparticle resonance which appears to be isolated from the Hubbard bands.Comment: 4 pages, 3 eps-figures include
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