2,259 research outputs found
The RKKY interactions and the Mott Transition
A two-site cluster generalization of the Hubbard model in large dimensions is
examined in order to study the role of short-range spin correlations near the
metal-insulator transition (MIT). The model is mapped to a two-impurity
Kondo-Anderson model in a self-consistently determined bath, making it possible
to directly address the competition between the Kondo effect and RKKY
interactions in a lattice context. Our results indicate that the RKKY
interactions lead to qualitative modifications of the MIT scenario even in the
absence of long range antiferromagnetic ordering.Comment: 10 pages, 10 figures; to appear in Phys. Rev. B (1999
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
Charge and spin density wave ordering transitions in strongly correlated metals
We study the quantum transition from a strongly correlated metal, with heavy
fermionic quasiparticles, to a metal with commensurate charge or spin density
wave order. To this end, we introduce and numerically analyze a large
dimensionality model of Ising spins in a transverse field, coupled to two
species of fermions; the analysis borrows heavily from recent progress in the
solution of the Hubbard model in large dimensions. At low energies, the Ising
order parameter fluctuations are characterized by the critical exponent , while above an energy scale, , there is a crossover to criticality. We show that is of the order of the width of the
heavy quasiparticle band, and can be made arbitrarily small for a correlated
metal close to a Mott-Hubbard insulator. Therefore, such a correlated metal has
a significant intermediate energy range of behavior, a single
particle spectrum with a narrow quasiparticle band, and well-formed analogs of
the lower and upper Hubbard bands; we suggest that these features are
intimately related in general.Comment: 14 pages, REVTEX 3.0, 2 postscript figure
Transfer of Spectral Weight in Spectroscopies of Correlated Electron Systems
We study the transfer of spectral weight in the photoemission and optical
spectra of strongly correlated electron systems. Within the LISA, that becomes
exact in the limit of large lattice coordination, we consider and compare two
models of correlated electrons, the Hubbard model and the periodic Anderson
model. The results are discussed in regard of recent experiments. In the
Hubbard model, we predict an anomalous enhancement optical spectral weight as a
function of temperature in the correlated metallic state which is in
qualitative agreement with optical measurements in . We argue that
anomalies observed in the spectroscopy of the metal are connected to the
proximity to a crossover region in the phase diagram of the model. In the
insulating phase, we obtain an excellent agreement with the experimental data
and present a detailed discussion on the role of magnetic frustration by
studying the resolved single particle spectra. The results for the periodic
Anderson model are discussed in connection to recent experimental data of the
Kondo insulators and . The model can successfully explain
the different energy scales that are associated to the thermal filling of the
optical gap, which we also relate to corresponding changes in the density of
states. The temperature dependence of the optical sum rule is obtained and its
relevance for the interpretation of the experimental data discussed. Finally,
we argue that the large scattering rate measured in Kondo insulators cannot be
described by the periodic Anderson model.Comment: 19 pages + 29 figures. Submitted to PR
Finite temperature numerical renormalization group study of the Mott-transition
Wilson's numerical renormalization group (NRG) method for the calculation of
dynamic properties of impurity models is generalized to investigate the
effective impurity model of the dynamical mean field theory at finite
temperatures. We calculate the spectral function and self-energy for the
Hubbard model on a Bethe lattice with infinite coordination number directly on
the real frequency axis and investigate the phase diagram for the Mott-Hubbard
metal-insulator transition. While for T<T_c approx 0.02W (W: bandwidth) we find
hysteresis with first-order transitions both at U_c1 (defining the insulator to
metal transition) and at U_c2 (defining the metal to insulator transition), at
T>T_c there is a smooth crossover from metallic-like to insulating-like
solutions.Comment: 10 pages, 9 eps-figure
Absence of hysteresis at the Mott-Hubbard metal-insulator transition in infinite dimensions
The nature of the Mott-Hubbard metal-insulator transition in the
infinite-dimensional Hubbard model is investigated by Quantum Monte Carlo
simulations down to temperature T=W/140 (W=bandwidth). Calculating with
significantly higher precision than in previous work, we show that the
hysteresis below T_{IPT}\simeq 0.022W, reported in earlier studies, disappears.
Hence the transition is found to be continuous rather than discontinuous down
to at least T=0.325T_{IPT}. We also study the changes in the density of states
across the transition, which illustrate that the Fermi liquid breaks down
before the gap opens.Comment: 4 pages, 4 eps-figures using epsf.st
Single-Pion Production in pp Collisions at 0.95 GeV/c (II)
The single-pion production reactions , and
were measured at a beam momentum of 0.95 GeV/c (
400 MeV) using the short version of the COSY-TOF spectrometer. The central
calorimeter provided particle identification, energy determination and neutron
detection in addition to time-of-flight and angle measurements from other
detector parts. Thus all pion production channels were recorded with 1-4
overconstraints. Main emphasis is put on the presentation and discussion of the
channel, since the results on the other channels have already been
published previously. The total and differential cross sections obtained are
compared to theoretical calculations. In contrast to the channel we
find in the channel a strong influence of the excitation
already at this energy close to threshold. In particular we find a dependence in the pion angular distribution, typical for a
pure s-channel excitation and identical to that observed in the
channel. Since the latter is understood by a s-channel resonance in
the partial wave, we discuss an analogous scenario for the
channel
Transport properties of strongly correlated metals:a dynamical mean-field approach
The temperature dependence of the transport properties of the metallic phase
of a frustrated Hubbard model on the hypercubic lattice at half-filling are
calculated. Dynamical mean-field theory, which maps the Hubbard model onto a
single impurity Anderson model that is solved self-consistently, and becomes
exact in the limit of large dimensionality, is used. As the temperature
increases there is a smooth crossover from coherent Fermi liquid excitations at
low temperatures to incoherent excitations at high temperatures. This crossover
leads to a non-monotonic temperature dependence for the resistance,
thermopower, and Hall coefficient, unlike in conventional metals. The
resistance smoothly increases from a quadratic temperature dependence at low
temperatures to large values which can exceed the Mott-Ioffe-Regel value, hbar
a/e^2 (where "a" is a lattice constant) associated with mean-free paths less
than a lattice constant. Further signatures of the thermal destruction of
quasiparticle excitations are a peak in the thermopower and the absence of a
Drude peak in the optical conductivity. The results presented here are relevant
to a wide range of strongly correlated metals, including transition metal
oxides, strontium ruthenates, and organic metals.Comment: 19 pages, 9 eps figure
Density distributions, magnetic field structures and fragmentation in high-mass star formation
Methods: Observing the large pc-scale Stokes I mm dust continuum emission
with the IRAM 30m telescope and the intermediate-scale (<0.1pc) polarized submm
dust emission with the Submillimeter Array toward a sample of 20 high-mass
star-forming regions allows us to quantify the dependence of the fragmentation
behaviour of these regions depending on the density and magnetic field
structures.
Results: We infer density distributions n~r^{-p} of the regions with typical
power-law slopes p around ~1.5. There is no obvious correlation between the
power-law slopes of the density structures on larger clump scales (~1pc) and
the number of fragments on smaller core scales (<0.1pc). Comparing the
large-scale single-dish density profiles to those derived earlier from
interferometric observations at smaller spatial scales, we find that the
smaller-scale power-law slopes are steeper, typically around ~2.0. The
flattening toward larger scales is consistent with the star-forming regions
being embedded in larger cloud structures that do not decrease in density away
from a particular core. Regarding the magnetic field, for several regions it
appears aligned with filamentary structures leading toward the densest central
cores. Furthermore, we find different polarization structures with some regions
exhibiting central polarization holes whereas other regions show polarized
emission also toward the central peak positions. Nevertheless, the polarized
intensities are inversely related to the Stokes I intensities. We estimate
magnetic field strengths between ~0.2 and ~4.5mG, and we find no clear
correlation between magnetic field strength and the fragmentation level of the
regions. Comparison of the turbulent to magnetic energies shows that they are
of roughly equal importance in this sample. The mass-to-flux ratios range
between ~2 and ~7, consistent with collapsing star-forming regions.Comment: Accepted for Astronomy & Astrophysics, 14 pages, 14 figures plus
appendices, also download option at
https://www2.mpia-hd.mpg.de/homes/beuther/papers.htm
- …