492 research outputs found
The Hubbard Model at Infinite Dimensions: Thermodynamic and Transport Properties
We present results on thermodynamic quantities, resistivity and optical
conductivity for the Hubbard model on a simple hypercubic lattice in infinite
dimensions. Our results for the paramagnetic phase display the features
expected from an intuitive analysis of the one-particle spectra and
substantiate the similarity of the physics of the Hubbard model to those of
heavy fermion systems. The calculations were performed using an approximate
solution to the single-impurity Anderson model, which is the key quantity
entering the solution of the Hubbard model in this limit. To establish the
quality of this approximation we compare its results, together with those
obtained from two other widely used methods, to essentially exact quantum Monte
Carlo results.Comment: 29 pages, 16 figure
Zeros of the Partition Function and Pseudospinodals in Long-Range Ising Models
The relation between the zeros of the partition function and spinodal
critical points in Ising models with long-range interactions is investigated.
We find the spinodal is associated with the zeros of the partition function in
four-dimensional complex temperature/magnetic field space. The zeros approach
the real temperature/magnetic field plane as the range of interaction
increases.Comment: 20 pages, 9 figures, accepted to PR
Calculations of the Knight Shift Anomalies in Heavy Electron Materials
We have studied the Knight shift and magnetic susceptibility
of heavy electron materials, modeled by the infinite U Anderson model
with the NCA method. A systematic study of and for
different Kondo temperatures (which depends on the hybridization width
) shows a low temperature anomaly (nonlinear relation between and
) which increases as the Kondo temperature and distance
increase. We carried out an incoherent lattice sum by adding the of
a few hundred shells of rare earth atoms around a nucleus and compare the
numerically calculated results with the experimental results. For CeSn_3, which
is a concentrated heavy electron material, both the ^{119}Sn NMR Knight shift
and positive muon Knight shift are studied. Also, lattice coherence effects by
conduction electron scattering at every rare earth site are included using the
average-T matrix approximation. Also NMR Knight shifts for YbCuAl and the
proposed quadrupolar Kondo alloy Y_{0.8}U_{0.2}Pd_{3} are studied.Comment: 31 pages of RevTex, 22 Postscript figures, submmitted to PRB, some
figures are delete
Numerical Renormalization Group Study of Pseudo-Fermion and Slave-Boson Spectral Functions in the Single Impurity Anderson Model
We use the numerical renormalization group to calculate the auxiliary
spectral functions of the Anderson impurity model. The slave--boson
and pseudo--fermion spectral functions diverge at the threshold with exponents
and given in terms of the conduction electron phase
shifts by the X--ray photoemission and the X--ray absorption exponents
respectively. The exact exponents obtained here depend on the impurity
occupation number, in contrast to the NCA results. Vertex corrections in the
convolution formulae for physical Green's functions are singular at the
threshold and may not be neglected in the Fermi liquid regime.Comment: 12 pages, RevTeX 3.0, 2 PS figures appende
Theory of One-Channel vs. Multi-Channel Kondo Effects for Ce Impurities
We introduce a model for Ce impurities in cubic metals which exhibits
competition between the Fermi-liquid fixed point of the single channel Kondo
model and the non-Fermi-liquid fixed point of the two- and three-channel Kondo
models. Using the non-crossing approximation and scaling theory, we find: (i) A
possible three-channel Kondo effect between the one- and two-channel regimes in
parameter space. (ii) The sign of the thermopower is a fixed point diagnostic.
(iii) Our results will likely survive the introduction of additional and
conduction states. We apply this model to interpret the non-Fermi liquid alloy
LaCeCuSi.Comment: 13 pages, Revtex, To appear in Phys. Rev. Let
Clusters and Fluctuations at Mean-Field Critical Points and Spinodals
We show that the structure of the fluctuations close to spinodals and
mean-field critical points is qualitatively different than the structure close
to non-mean-field critical points. This difference has important implications
for many areas including the formation of glasses in supercooled liquids. In
particular, the divergence of the measured static structure function in
near-mean-field systems close to the glass transition is suppressed relative to
the mean-field prediction in systems for which a spatial symmetry is broken.Comment: 5 pages, 1 figur
Phase Diagram of the Two-Channel Kondo Lattice
The phase diagram of the two-channel Kondo lattice model is examined with a
Quantum Monte Carlo simulation in the limit of infinite dimensions.
Commensurate (and incommensurate) antiferromagnetic and superconducting states
are found. The antiferromagnetic transition is very weak and continuous;
whereas the superconducting transition is discontinuous to an odd-frequency
channel-singlet and spin-singlet pairing state.Comment: 5 pages, LaTeX and 4 PS figures (see also cond-mat/9609146 and
cond-mat/9605109
Interannual variation patterns of total ozone and lower stratospheric temperature in observations and model simulations
We report results from a multiple linear regression
analysis of long-term total ozone observations (1979 to
2000, by TOMS/SBUV), of temperature reanalyses (1958
to 2000, NCEP), and of two chemistry-climate model simulations
(1960 to 1999, by ECHAM4.L39(DLR)/CHEM
(=E39/C), and MAECHAM4-CHEM). The model runs are
transient experiments, where observed sea surface temperatures,
increasing source gas concentrations (CO2, CFCs,
CH4, N2O, NOx), 11-year solar cycle, volcanic aerosols
and the quasi-biennial oscillation (QBO) are all accounted
for. MAECHAM4-CHEM covers the atmosphere from the
surface up to 0.01 hPa ( 80 km). For a proper representation
of middle atmosphere (MA) dynamics, it includes
a parametrization for momentum deposition by dissipating
gravity wave spectra. E39/C, on the other hand, has its top
layer centered at 10 hPa ( 30 km). It is targeted on processes
near the tropopause, and has more levels in this region.
Despite some problems, both models generally reproduce
the observed amplitudes and much of the observed lowlatitude
patterns of the various modes of interannual variability
in total ozone and lower stratospheric temperature. In
most aspects MAECHAM4-CHEM performs slightly better
than E39/C. MAECHAM4-CHEM overestimates the longterm
decline of total ozone, whereas E39/C underestimates
the decline over Antarctica and at northern mid-latitudes.
The true long-term decline in winter and spring above the
Correspondence to: W. Steinbrecht
([email protected])
Arctic may be underestimated by a lack of TOMS/SBUV
observations in winter, particularly in the cold 1990s. Main
contributions to the observed interannual variations of total
ozone and lower stratospheric temperature at 50 hPa come
from a linear trend (up to −10 DU/decade at high northern
latitudes, up to −40 DU/decade at high southern latitudes,
and around −0.7 K/decade over much of the globe), from
the intensity of the polar vortices (more than 40 DU, or 8 K
peak to peak), the QBO (up to 20 DU, or 2 K peak to peak),
and from tropospheric weather (up to 20 DU, or 2 K peak
to peak). Smaller variations are related to the 11-year solar
cycle (generally less than 15 DU, or 1 K), or to ENSO (up
to 10 DU, or 1 K). These observed variations are replicated
well in the simulations. Volcanic eruptions have resulted in
sporadic changes (up to −30 DU, or +3 K). At low latitudes,
patterns are zonally symmetric. At higher latitudes, however,
strong, zonally non-symmetric signals are found close
to the Aleutian Islands or south of Australia. Such asymmetric
features appear in the model runs as well, but often
at different longitudes than in the observations. The results
point to a key role of the zonally asymmetric Aleutian (or
Australian) stratospheric anti-cyclones for interannual variations
at high-latitudes, and for coupling between polar vortex
strength, QBO, 11-year solar cycle and ENSO
Heavy-Fermions in LiV2O4: Kondo-Compensation vs. Spin-Liquid Behavior?
7Li NMR measurements were performed in the metallic spinel LiV2O4. The
temperature dependencies of the line width, the Knight shift and the
spin-lattice relaxation rate were investigated in the temperature range 30 mK <
T < 280 K. For temperatures T < 1 K we observe a spin-lattice relaxation rate
which slows down exponentially. The NMR results can be explained by a
spin-liquid behavior and the opening of a spin gap of the order 0.6 K
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