1,268 research outputs found
Optical conductivity in cluster dynamical mean field theory: formalism and application to high temperature superconductors
The optical conductivity of the one-band Hubbard model is calculated using
the 'Dynamical Cluster Approximation' implementation of dynamical mean field
theory for parameters appropriate to high temperature copper-oxide
superconductors. The calculation includes vertex corrections and the result
demonstrates their importance. At densities of one electron per site, an
insulating state is found with gap value and above-gap absorption consistent
with measurements. As carriers are added the above gap conductivity rapidly
weakens and a three component structure emerges, with a low frequency 'Drude'
peak, a mid-infrared absorption, and a remnant of the insulating gap. The
mid-infrared feature obtained at intermediate dopings is shown to arise from a
pseudogap structure in the density of states. On further doping the
conductivity evolves to the Drude peak plus weakly frequency dependent tail
structure expected for less strongly correlated metals.Comment: 5 pages, 3 figure
A comparative DMFT study of the eg-orbital Hubbard model in thin films
Heterostructures of transition-metal oxides emerged as a new route to
engineer electronic systems with desired functionalities. Motivated by these
developments, we study a two-orbital Hubbard model in a thin-film geometry
confined along the cubic [001] direction using the dynamical mean-field theory.
We contrast the results of two approximate impurity solvers (exact
diagonalization and one-crossing approximation) to the results of the
numerically exact continuous-time quantum Monte Carlo solver. Consistent with
earlier studies, we find that the one-crossing approximation performs well in
the insulating regime, while the advantage of the exact-diagonalization based
solver is more pronounced in the metallic regime. We then investigate various
aspects of strongly correlated eg-orbital systems in thin film geometries. In
particular, we show how the interfacial orbital polarization dies off quickly a
few layers from the interface and how the film thickness affects the location
of the interaction-driven Mott transition. In addition, we explore the changes
in the electronic structure with varying carrier concentration and identify
large variations of the orbital polarization in the strongly correlated regime.Comment: 11 pages, 11 figure
IUE observations of Seyfert galaxies
The L alpha/H beta ratio and line profiles for several galaxies are presented. The continuous energy distribution of NGC 4151 and MKN 509 are presented from the X-ray region to the infrared
[TiII] and [NiII] emission from the strontium filament of eta Carinae
We study the nature of the [TiII] and [NiII] emission from the so-called
strontium filament found in the ejecta of eta Carinae. To this purpose we
employ multilevel models of the TiII and NiII systems which are used to
investigate the physical condition of the filament and the excitation
mechanisms of the observed lines. For the TiII ion, for which no atomic data
was previously available, we carry out ab initio calculations of radiative
transition rates and electron impact excitation rate coefficients. It is found
that the observed spectrum is consistent with the lines being excited in a
mostly neutral region with an electron density of the order of cm
and a temperature around 6000 K. In analyzing three observations with different
slit orientations recorded between March~2000 and November~2001 we find line
ratios that change among various observations, in a way consistent with changes
of up to an order of magnitude in the strength of the continuum radiation
field. These changes result from different samplings of the extended filament,
due to the different slit orientations used for each observation, and yield
clues on the spatial extent and optical depth of the filament. The observed
emission indicates a large Ti/Ni abundance ratio relative to solar abundances.
It is suggested that the observed high Ti/Ni ratio in gas is caused by dust-gas
fractionation processes and does not reflect the absolute Ti/Ni ratio in the
ejecta of \etacar. We study the condensation chemistry of Ti, Ni and Fe within
the filament and suggest that the observed gas phase overabundance of TiComment: 14 paginas, 12 figure
Numerically Exact Long Time Behavior of Nonequilibrium Quantum Impurity Models
A Monte Carlo sampling of diagrammatic corrections to the non-crossing
approximation is shown to provide numerically exact estimates of the long-time
dynamics and steady state properties of nonequilibrium quantum impurity models.
This `bold' expansion converges uniformly in time and significantly ameliorates
the sign problem that has heretofore limited the power of real-time Monte Carlo
approaches to strongly interacting real-time quantum problems. The new approach
enables the study of previously intractable problems ranging from generic long
time nonequilibrium transport characteristics in systems with large onsite
repulsion to the direct description of spectral functions on the real frequency
axis in Dynamical Mean Field Theory
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