3,342 research outputs found
Polaron Crossover and Bipolaronic Metal-Insulator Transition in the Holstein model at half-filling
The evolution of the properties of a finite density electronic system as the
electron-phonon coupling is increased are investigated in the
Holstein model using the Dynamical Mean-Field Theory (DMFT).
We compare the spinless fermion case, in which only isolated polarons can be
formed, with the spinful model in which the polarons can bind and form
bipolarons. In the latter case, the bipolaronic binding occurs through a
metal-insulator transition. In the adiabatic regime in which the phonon energy
is small with respect to the electron hopping we compare numerically exact DMFT
results with an analytical scheme inspired by the Born-Oppenheimer procedure.
Within the latter approach,a truncation of the phononic Hilbert space leads to
a mapping of the original model onto an Anderson spin-fermion model. In the
anti-adiabatic regime (where the phonon energy exceeds the electronic scales)
the standard treatment based on Lang-Firsov canonical transformation allows to
map the original model on to an attractive Hubbard model in the spinful case.
The separate analysis of the two regimes supports the numerical evidence that
polaron formation is not necessarily associated to a metal-insulator
transition, which is instead due to pairing between the carriers. At the
polaron crossover the Born-Oppenheimer approximation is shown to break down due
to the entanglement of the electron-phonon state.Comment: 19 pages, 15 figure
Electronic correlations stabilize the antiferromagnetic Mott state in CsC
CsC in the A15 structure is an antiferromagnet at ambient pressure
in contrast with other superconducting trivalent fullerides. Superconductivity
is recovered under pressure and reaches the highest critical temperature of the
family. Comparing density-functional calculations with generalized gradient
approximation to the hybrid functional HSE, which includes a suitable component
of exchange, we establish that the antiferromagnetic state of CsC is
not due to a Slater mechanism, and it is stabilized by electron correlation.
HSE also reproduces the pressure-driven metalization. Our findings corroborate
previous analyses suggesting that the properties of this compound can be
understood as the result of the interplay between electron correlations and
Jahn-Teller electron-phonon interaction.Comment: 4 pages, 3 figure
Phonon softening and dispersion in the 1D Holstein model of spinless fermions
We investigate the effect of electron-phonon interaction on the phononic
properties in the one-dimensional half-filled Holstein model of spinless
fermions. By means of determinantal Quantum Monte Carlo simulation we show that
the behavior of the phonon dynamics gives a clear signal of the transition to a
charge-ordered phase, and the phase diagram obtained in this way is in
excellent agreement with previous DMRG results. By analyzing the phonon
propagator we extract the renormalized phonon frequency, and study how it first
softens as the transition is approached and then subsequently hardens in the
charge-ordered phase. We then show how anharmonic features develop in the
phonon propagator, and how the interaction induces a sizable dispersion of the
dressed phonon in the non-adiabatic regime.Comment: 7 pages, 6 figure
Electron-phonon Interaction close to a Mott transition
The effect of Holstein electron-phonon interaction on a Hubbard model close
to a Mott-Hubbard transition at half-filling is investigated by means of
Dynamical Mean-Field Theory. We observe a reduction of the effective mass that
we interpret in terms of a reduced effective repulsion. When the repulsion is
rescaled to take into account this effect, the quasiparticle low-energy
features are unaffected by the electron-phonon interaction. Phonon features are
only observed within the high-energy Hubbard bands. The lack of electron-phonon
fingerprints in the quasiparticle physics can be explained interpreting the
quasiparticle motion in terms of rare fast processes.Comment: 4 pages, 3 color figures. Slightly revised text and references. Kondo
effect result added in Fig. 2 for comparison with DMFT dat
Electron-phonon interaction and antiferromagnetic correlations
We study effects of the Coulomb repulsion on the electron-phonon interaction
(EPI) in a model of cuprates at zero and finite doping. We find that
antiferromagnetic correlations strongly enhance EPI effects on the electron
Green's function with respect to the paramagnetic correlated system, but the
net effect of the Coulomb interaction is a moderate suppression of the EPI.
Doping leads to additional suppression, due to reduced antiferromagnetic
correlations. In contrast, the Coulomb interaction strongly suppresses EPI
effects on phonons, but the suppression weakens with doping.Comment: 4 pages and 5 figure
Cluster Dynamical Mean-Field Theory of the density-driven Mott transition in the one-dimensional Hubbard model
The one-dimensional Hubbard model is investigated by means of two different
cluster schemes suited to introduce short-range spatial correlations beyond the
single-site Dynamical Mean-Field Theory, namely the Cluster-Dynamical
Mean-Field Theory and its periodized version. It is shown that both cluster
schemes are able to describe with extreme accuracy the evolution of the density
as a function of the chemical potential from the Mott insulator to the metallic
state. Using exact diagonalization to solve the cluster impurity model, we
discuss the role of the truncation of the Hilbert space of the bath, and
propose an algorithm that gives higher weights to the low frequency
hybridization matrix elements and improves the speed of the convergence of the
algorithm.Comment: 6 pages, 4 figures, minor corrections in v
Neutrino propagation in a medium with a magnetic field
We study the properties of neutrinos propagating in an isotropic magnetized medium in the two physical approximations of degenerate Fermi gas and classical plasma. The dispersion relation shows that, for peculiar configurations of the magnetic field, neutrinos can propagate freely as in vacuum, also for very large density; this result can be very important in the study of supernova evolution. For mixed neutrinos, the presence of a magnetic field can alter significatively MSW oscillations, and for particular configurations of the field the resonance condition no longer occurs. Furthermore, on the contrary to that happens in non-magnetized media, spatial dispersion arises and neutrino trajectory can be in principle deviated; however a simple estimate shows that this deviation is not detectable
Robust Energy Management for Green and Survivable IP Networks
Despite the growing necessity to make Internet greener, it is worth pointing
out that energy-aware strategies to minimize network energy consumption must
not undermine the normal network operation. In particular, two very important
issues that may limit the application of green networking techniques concern,
respectively, network survivability, i.e. the network capability to react to
device failures, and robustness to traffic variations. We propose novel
modelling techniques to minimize the daily energy consumption of IP networks,
while explicitly guaranteeing, in addition to typical QoS requirements, both
network survivability and robustness to traffic variations. The impact of such
limitations on final network consumption is exhaustively investigated. Daily
traffic variations are modelled by dividing a single day into multiple time
intervals (multi-period problem), and network consumption is reduced by putting
to sleep idle line cards and chassis. To preserve network resiliency we
consider two different protection schemes, i.e. dedicated and shared
protection, according to which a backup path is assigned to each demand and a
certain amount of spare capacity has to be available on each link. Robustness
to traffic variations is provided by means of a specific modelling framework
that allows to tune the conservatism degree of the solutions and to take into
account load variations of different magnitude. Furthermore, we impose some
inter-period constraints necessary to guarantee network stability and preserve
the device lifetime. Both exact and heuristic methods are proposed.
Experimentations carried out with realistic networks operated with flow-based
routing protocols (i.e. MPLS) show that significant savings, up to 30%, can be
achieved also when both survivability and robustness are fully guaranteed
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