4,171 research outputs found
2dF-AAOmega spectroscopy of massive stars in the Magellanic Clouds: The north-eastern region of the Large Magellanic Cloud
We present spectral classifications from optical spectroscopy of 263 massive
stars in the north-eastern region of the Large Magellanic Cloud. The observed
two-degree field includes the massive 30 Doradus star-forming region, the
environs of SN1987A, and a number of star-forming complexes to the south of 30
Dor. These are the first classifications for the majority (203) of the stars
and include eleven double-lined spectroscopic binaries. The sample also
includes the first examples of early OC-type spectra (AAOmega 30 Dor 248 and
280), distinguished by the weakness of their nitrogen spectra and by C IV 4658
emission. We propose that these stars have relatively unprocessed CNO
abundances compared to morphologically normal O-type stars, indicative of an
earlier evolutionary phase. From analysis of observations obtained on two
consecutive nights, we present radial-velocity estimates for 233 stars, finding
one apparent single-lined binary and nine (>3sigma) outliers compared to the
systemic velocity; the latter objects could be runaway stars or large-amplitude
binary systems and further spectroscopy is required to investigate their
nature.Comment: Accepted by A&
Ultralong-range order in the Fermi-Hubbard model with long-range interactions
We use the dual boson approach to reveal the phase diagram of the
Fermi-Hubbard model with long-range dipole-dipole interactions. By using a
large-scale finite-temperature calculation on a square lattice
we demonstrate the existence of a novel phase, possessing an `ultralong-range'
order. The fingerprint of this phase -- the density correlation function --
features a non-trivial behavior on a scale of tens of the lattice sites. We
study the properties and the stability of the ultralong-range ordered phase,
and show that it is accessible in modern experiments with ultracold polar
molecules and magnetic atoms
Two-particle correlations and the metal-insulator transition: Iterated Perturbation Theory revisited
Recent advances in many-body physics have made it possible to study
correlated electron systems at the two-particle level. In Dynamical Mean-Field
theory, it has been shown that the metal-insulator phase diagram is closely
related to the eigenstructure of the susceptibility. So far, this situation has
been studied using accurate but numerically expensive solvers. Here, the
Iterated Perturbation Theory (IPT) approximation is used instead. Its
simplicity makes it possible to obtain analytical results for the two-particle
vertex and the DMFT Jacobian. The limited computational cost also enables a
detailed comparison of analytical expressions for the response functions to
results obtained using finite differences. At the same time, the approximate
nature of IPT precludes an interpretation of the metal-insulator transition in
terms of a Landau free energy functional.Comment: Revised versio
Thermodynamic consistency of the charge response in dynamical mean-field based approaches
We consider the thermodynamic consistency of the charge response function in
the (extended) Hubbard model. In DMFT, thermodynamic consistency is preserved.
We prove that the static, homogeneous DMFT susceptibility is consistent as long
as vertex corrections obtained from the two-particle impurity correlation
function are included. In presence of a nonlocal interaction, the problem may
be treated within extended DMFT (EDMFT), or its diagrammatic extension, the
dual boson approach. We show that here, maintaining thermodynamic consistency
requires knowledge of three- and four-particle impurity correlation functions,
which are typically neglected. Nevertheless, the dual boson approximation to
the response is remarkably close to consistency. This holds even when
two-particle vertex corrections are neglected. EDMFT is consistent only in the
strongly correlated regime and near half-filling, where the physics is
predominantly local.Comment: 11 pages (incl. appendix), 4 figure
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