1,676 research outputs found
New Solutions of the T-Matrix Theory of the Attractive Hubbard Model
This short paper summarizes a calculational method for obtaining the
dynamical properties of many-body theories formulated in terms of
(unrenormalized) bare propagators (and more generally, in terms of meromorphic
functions, or convolutions over meromorphic functions) to a very high accuracy.
We demonstrate the method by applying it to a T-matrix theory of the attractive
Hubbard model in two dimensions. We expand the pair propagator using a partial
fraction decomposition, and then solve for the residues and pole locations of
such a decomposition using a computer algebra system to an arbitrarily high
accuracy (we used MapleV and obtained these quantities to a relative error of
10^(-80)). Thus, this method allows us to bypass all inaccuracies associated
with the traditional analytical continuation problem. Our results for the
density of states make clear the pronounced development of a pseudogap as the
temperature is lowered in this formulation of the attractive Hubbard model.Comment: 2 pages, 2 figure
Demonstration of a robust pseudogap in a three-dimensional correlated electronic system
We outline a partial-fractions decomposition method for determining the
one-particle spectral function and single-particle density of states of a
correlated electronic system on a finite lattice in the non self-consistent
T-matrix approximation to arbitrary numerical accuracy, and demonstrate the
application of these ideas to the attractive Hubbard model. We then demonstrate
the effectiveness of a finite-size scaling ansatz which allows for the
extraction of quantities of interest in the thermodynamic limit from this
method. In this approximation, in one or two dimensions, for any finite lattice
or in the thermodynamic limit, a pseudogap is present and its energy diverges
as Tc is approached from above; this is an unphysical manifestation of using an
approximation that predicts a spurious phase transition in one or two
dimensions. However, in three dimensions one expects the transition predicted
by this approximation to represent a true continuous phase transition, and in
the thermodynamic limit any pseudogap predicted by this formulation will remain
finite. We have applied our method to the attractive Hubbard model on a
three-dimensional simple cubic lattice, and find that for intermediate coupling
a prominent pseudogap is found in the single-particle density of states, and
this gap persists over a large temperature range. In addition, we also show
that for weak coupling a pseudogap is also present. The pseudogap energy at the
transition temperature is almost a factor of three larger than the T=0 BCS gap
for intermediate coupling, whereas for weak coupling the pseudogap and BCS gap
energies are essentially equal.Comment: 28 pages, 9 figure
Dynamical properties of the single--hole -- model on a 32--site square lattice
We present results of an exact diagonalization calculation of the spectral
function for a single hole described by the -- model
propagating on a 32--site square cluster. The minimum energy state is found at
a crystal momentum , consistent with
theory, and our measured dispersion relation agrees well with that determined
using the self--consistent Born approximation. In contrast to smaller cluster
studies, our spectra show no evidence of string resonances. We also make a
qualitative comparison of the variation of the spectral weight in various
regions of the first Brillouin zone with recent ARPES data.Comment: 10 pages, 5 postscript figures include
Unifying the Phase Diagrams of the Magnetic and Transport Properties of La_(2-x)Sr_xCuO_4, 0 < x < 0.05
An extensive experimental and theoretical effort has led to a largely
complete mapping of the magnetic phase diagram of La_(2-x)Sr_xCuO_4, and a
microscopic model of the spin textures produced in the x < 0.05 regime has been
shown to be in agreement with this phase diagram. Here we use this same model
to derive a theory of the impurity-dominated, low temperature transport. Then,
we present an analysis of previously published data for two samples: x = 0.002
data from Chen et. al., and x = 0.04 data from Keimer et. al. We show that the
transport mechanisms in the two systems are the same, even though they are on
opposite sides of the observed insulator-to-metal transition. Our model of
impurity effects on the impurity band conduction, variable-range hopping
conduction, and coulomb gap conduction, is similar to that used to describe
doped semiconductors. However, for La_(2-x)Sr_xCuO_4 we find that in addition
to impurity-generated disorder effects, strong correlations are important and
must be treated on a equal level with disorder. On the basis of this work we
propose a phase diagram that is consistent with available magnetic and
transport experiments, and which connects the undoped parent compound with the
lowest x value for which La_(2-x)Sr_xCuO_4 is found to be superconducting, x
about 0.06.Comment: 7 pages revtex with one .ps figur
Development of a coaxial plasma gun for space propulsion final report
Current sheet accelerators and pulsed plasma thrustors for spacecraft propulsio
Thermal Analyzer for Planetary Soil (TAPS): an in Situ Instrument for Mineral and Volatile-element Measurements
Thermal Analyzer for Planetary Soil (TAPS) offers a specific implementation for the generic thermal analyzer/evolved-gas analyzer (TA/EGA) function included in the Mars Environmental Survey (MESUR) strawman payload; applications to asteroids and comets are also possible. The baseline TAPS is a single-sample differential scanning calorimeter (DSC), backed by a capacitive-polymer humidity sensor, with an integrated sampling mechanism. After placement on a planetary surface, TAPS acquires 10-50 mg of soil or sediment and heats the sample from ambient temperature to 1000-1300 K. During heating, DSC data are taken for the solid and evolved gases are swept past the water sensor. Through ground based data analysis, multicomponent DSC data are deconvolved and correlated with the water release profile to quantitatively determine the types and relative proportions of volatile-bearing minerals such as clays and other hydrates, carbonates, and nitrates. The rapid-response humidity sensors also achieve quantitative analysis of total water. After conclusion of soil-analysis operations, the humidity sensors become available for meteorology. The baseline design fits within a circular-cylindrical volume less than 1000 cm(sup 3), occupies 1.2 kg mass, and consumes about 2 Whr of power per analysis. Enhanced designs would acquire and analyze multiple samples and employ additional microchemical sensors for analysis of CO2, SO2, NO(x), and other gaseous species. Atmospheric pumps are also being considered as alternatives to pressurized purge gas
Observation of the cluster spin-glass phase in La_{2-x}Sr_{x}CuO_{4} by anelastic spectroscopy
An increase of the acoustic absorption is found in La_{2-x}Sr_{x}CuO_{4} (x =
0.019, 0.03 and 0.06) close to the temperatures at which freezing of the spin
fluctuations in antiferromagnetic-correlated clusters is expected to occur. The
acoustic absorption is attributed to changes of the sizes of the quasi-frozen
clusters induced by the vibration stress through magnetoelastic coupling.Comment: LaTeX, 2 PostScript figures, submitted to Phys. Rev.
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