813 research outputs found
Double Exchange model for nanoscopic clusters
We solve the double exchange model on nanoscopic clusters exactly, and
specifically consider a six-site benzene-like nanocluster. This simple model is
an ideal testbed for studying magnetism in nanoclusters and for validating
approximations such as the dynamical mean field theory (DMFT). Non-local
correlations arise between neighboring localized spins due to the Hund's rule
coupling, favoring a short-range magnetic order of ferro- or antiferromagnetic
type. For a geometry with more neighboring sites or a sufficiently strong
hybridization between leads and the nanocluster, these non-local correlations
are less relevant, and DMFT can be applied reliably.Comment: 9 pages, 9 figures, 1 tabl
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
First principles characterization of reversible martensitic transformations
Reversible martensitic transformations (MTs) are the origin of many
fascinating phenomena, including the famous shape memory effect. In this work,
we present a fully ab initio procedure to characterize MTs in alloys and to
assess their reversibility. Specifically, we employ ab initio molecular
dynamics data to parametrize a Landau expansion for the free energy of the MT.
This analytical expansion makes it possible to determine the stability of the
high- and low-temperature phases, to obtain the Ehrenfest order of the MT, and
to quantify its free energy barrier and latent heat. We apply our model to the
high-temperature shape memory alloy Ti-Ta, for which we observe remarkably
small values for the metastability region (the interval of temperatures in
which the high-and low-temperature phases are metastable) and for the barrier:
these small values are necessary conditions for the reversibility of MTs and
distinguish shape memory alloys from other materials
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