2,537 research outputs found
Theory of the Metal-Paramagnetic Mott-Jahn-Teller Insulator Transition in A_4C_{60}
We study the unconventional insulating state in A_4C_{60} with a variety of
approaches, including density functional calculations and dynamical mean-field
theory. While the former predicts a metallic state, in disagreement with
experiment, the latter yields a (paramagnetic) Mott-Jahn-Teller insulator. In
that state, conduction between molecules is blocked by on-site Coulomb
repulsion, magnetism is suppressed by intra-molecular Jahn-Teller effect, and
important excitations (such as optical and spin gap) should be essentially
intra-molecular. Experimental gaps of 0.5 eV and 0.1 eV respectively compare
well with molecular ion values, in agreement with this picture.Comment: 4 pages, 2 postscript figure
Isotope effects in the Hubbard-Holstein model within dynamical mean-field theory
We study the isotope effects arising from the coupling of correlated
electrons with dispersionless phonons by considering the Hubbard-Holstein model
at half-filling within the dynamical mean-field theory. In particular we
calculate the isotope effects on the quasi-particle spectral weight , the
renormalized phonon frequency, and the static charge and spin susceptibilities.
In the weakly correlated regime , where is the Hubbard
repulsion and is the bare electron half-bandwidth, the physical properties
are qualitatively similar to those characterizing the Holstein model in the
absence of Coulomb repulsion, where the bipolaronic binding takes place at
large electron-phonon coupling, and it reflects in divergent isotope responses.
On the contrary in the strongly correlated regime , where the
bipolaronic metal-insulator transition becomes of first order, the isotope
effects are bounded, suggesting that the first order transition is likely
driven by an electronic mechanism, rather then by a lattice instability. These
results point out how the isotope responses are extremely sensitive to phase
boundaries and they may be used to characterize the competition between the
electron-phonon coupling and the Hubbard repulsion.Comment: 10 pages, 8 figures. The paper has been already accepted on Phys.
Rev.
Pairing and polarization in systems with retarded interactions
In a system where a boson (e.g, a phonon) of finite frequency is
coupled to electrons, two phenomena occur as the coupling is increased:
electron pairing and polarization of the boson field. Within a path integral
formalism and a Dynamical Mean-Field approach, we introduce {\it ad hoc}
distribution function which allow us to pinpoint the two effects. When
is smaller than the bandwidth , pairing and polarization occur
for fairly similar couplings for all considered temperatures. When , the two phenomena tend to coincide only for , but are no
longer tied for low temperatures so that a state of paired particles without
finite polarization is stabilized.Comment: 4 pages, 2 figure
Temperature dependence of the optical spectral weight in the cuprates: Role of electron correlations
We compare calculations based on the Dynamical Mean-Field Theory of the
Hubbard model with the infrared spectral weight of
LaSrCuO and other cuprates. Without using fitting parameters we
show that most of the anomalies found in with respect to normal
metals, including the existence of two different energy scales for the doping-
and the -dependence of , can be ascribed to strong correlation
effects.Comment: 4 pages, 3 figures. Minor corrections, corrected some typos and added
reference
Pressure induced magnetic phase separation in LaCaMnO manganite
The pressure dependence of the Curie temperature T in
LaCaMnO was determined by neutron diffraction up to 8
GPa, and compared with the metallization temperature T \cite{irprl}.
The behavior of the two temperatures appears similar over the whole pressure
range suggesting a key role of magnetic double exchange also in the pressure
regime where the superexchange interaction is dominant. Coexistence of
antiferromagnetic and ferromagnetic peaks at high pressure and low temperature
indicates a phase separated regime which is well reproduced with a dynamical
mean-field calculation for a simplified model. A new P-T phase diagram has been
proposed on the basis of the whole set of experimental data.Comment: 5 pages, 4 figure
An assessment of the effect of supersonic aircraft operations on the stratospheric ozone content
An assessment of the potential effect on stratospheric ozone of an advanced supersonic transport operations is presented. This assessment, which was undertaken because of NASA's desire for an up-to-date evaluation to guide programs for the development of supersonic technology and improved aircraft engine designs, uses the most recent chemical reaction rate data. From the results of the present assessment it would appear that realistic fleet sizes should not cause concern with regard to the depletion of the total ozone overburden. For example, the NOx emission of one type designed to cruise at 20 km altitude will cause the ozone overburden to increase by 0.03% to 0.12%, depending upon which vertical transport is used. These ozone changes can be compared with the predictions of a 1.74% ozone decrease (for 100 Large SST's flying at 20 km) made in 1974 by the FAA's Climatic Impact Assessment Program
Off-equilibrium confined dynamics in a glassy system with level-crossing states
We study analytically the dynamics of a generalized p-spin model, starting
with a thermalized initial condition. The model presents birth and death of
states, hence the dynamics (even starting at equilibrium) may go out of
equilibrium when the temperature is varied. We give a full description of this
constrained out of equilibrium behavior and we clarify the connection to the
thermodynamics by computing (sub-dominant) TAP states, constrained to the
starting equilibrium configuration.Comment: 10 pages, 3 figures; longer version with appendi
Fiber suspension investigation in a backward-facing step by PIV
A dilute suspension (volume fraction 0.05%) of rod-like particles in a turbulent backward-facing step flow at Reynolds number ReH=14900, is investigated by means of Particle Image Velocimetry. Two-way interactions between fluid and dispersed phase are analyzed by exploiting the high spatial resolution of the acquisitions. Mutual interactions between phases can be investigated by considering flow turbulence modulations and phenomena related to preferential concentration and orientation of fibers. Slight turbulence enhancement is reported in the laden flow and concentration data show a moderate tendency of fibers to accumulate at the channel centreline. Orientation data display a strong preferential orientation of fibers. Local fiber orientation is correlated to the direction of maximum shear showing a high level of correlation also in the flow regions featuring strong gradients
A multi-blob representation of semi-dilute polymer solutions
A coarse-grained multi-blob description of polymer solutions is presented,
based on soft, transferable effective interactions between bonded and
non-bonded blobs. The number of blobs is chosen such that the blob density does
not exceed their overlap threshold, allowing polymer concentrations to be
explored deep into the semi-dilute regime. This quantitative multi-blob
description is shown to preserve known scaling laws of polymer solutions and
provides accurate estimates of amplitudes, while leading to orders of magnitude
increase of simulation efficiency and allowing analytic calculations of
structural and thermodynamic properties.Comment: 4 pages, 4 figure
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