497 research outputs found
Electronic Correlation effects in superconducting picene from ab-initio calculations
We show, by means of ab-initio calculations, that electron-electron
correlations play an important role in potassium-doped picene (-picene),
recently characterized as a superconductor with . The inclusion of
exchange interactions by means of hybrid functionals reproduces the correct gap
for the undoped compound and predicts an antiferromagnetic state for ,
where superconductivity has been observed. The latter finding is compatible
with a sizable value of the correlation strength, in agreement with simple
estimates. Our results highlight the similarity between potassium-doped picene
and alkali-doped fulleride superconductors.Comment: 5 pages, 3 figure
FDI in Business Services has general TFP effects : evidence from Italy
This paper studies the effect of FDI in business services on Total Factor Productivity of Italian manufacturing firms, over the period 2003-2008. More precisely, the paper tests the impact of forward inter industry linkages at local level. Our results, robust to different specifications, show that foreign capital infl
ows improve the performance of domestic manufacturing firms. This relationship is particularly strong in the case of high tech sectors, such as mechanics and machinery. Traditional sectors, on the other hand, seem to be less sensitive to the availability of foreign business services in the same location.
Electronic structure and correlations in pristine and potassium doped Cu-Phthalocyanine molecular crystals
We investigate the changes in the electronic structure of copper
phthalocyanine (CuPc) crystals that is caused by intercalation with potassium.
This is done by means of {\it ab initio} LSDA and LSDA+U calculations of the
electronic structure of these molecular crystals. Pristine CuPc is found to be
an insulator with local magnetic moments and a Pc-derived valence band with a
width of 0.32 eV. In the intercalated compound the additional
electrons that are introduced by potassium are fully transferred to the
states of the Pc-ring. A molecular low spin state results, preserving, however,
the local magnetic moment on the copper ions. The degeneracy of the
levels is split by a crystal field that quenches the orbital degeneracy and
gives rise to a band splitting of 110 meV. Molecular electronic Coulomb
interactions enhance this splitting in to a charge gap of 1.4 eV.
The bandwidth of the conduction band is 0.56 eV, which is surprisingly large
for a molecular solid. This is line with the experimentally observation that
the system with additional potassium doping, , is a metal
as the unusually large bandwidth combined with the substantial carrier
concentration acts against localization and polaron formation, while strongly
promoting the delocalization of the charge carriers.Comment: 5 pages, 7 figures embedde
Magnetism and Charge ordering in TMTTF-PF organic crystals
Using a combination of Density Functional Theory, mean-field analysis and
exact diagonalization calculations we reveal the emergence of a dimerized
charge ordered state in TMTTF-PF organic crystal. The interplay between
charge and spin order leads to a rich phase diagram. Coexistence of charge
ordering with a structural dimerization results in a ferroelectric phase, which
has been observed experimentally. The tendency to the dimerization is
magnetically driven revealing TMTTF-PF as a multiferroic material
Electronic correlations decimate the ferroelectric polarization of multiferroic HoMn2O5
We show that electronic correlations decimate the intrinsic ferroelectric
polarization of the recently discovered class of multiferroic manganites
RMnO, where R is a rare earth element. Such is manifest from {\it ab
initio} bandstructure computations that account for the strong local Coulomb
interactions between the manganese 3d electrons --the root of magnetism in
these materials. When including these the computed electronic, magnetic and
lattice structure of multiferroic HoMnO results in an amplitude and
direction of polarization that is in accordance with experiment. The
microscopic mechanism behind the decimation is a near cancellation of the ionic
polarization induced by ferroelectric lattice displacements and the electronic
one caused by valence charge redistributions.Comment: 4 pages, 4 figure
Evidence for multiferroicity in TTF-CA organic molecular crystals
We show by means of ab-initio calculations that the organic molecular crystal
TTF-CA is multiferroic: it has an instability to develop spontaneously both
ferroelectric and magnetic ordering. Ferroelectricity is driven by a Peierls
transition of the TTF-CA in its ionic state. Subsequent antiferromagnetic
ordering strongly enhances the opposing electronic contribution to the
polarization: it is so large that it switches the direction of the total
ferroelectric moment. Within an extended Hubbard model we capture the essence
of the electronic interactions in TTF-CA, confirm the presence of a
multiferroic groundstate and clarify how this state develops microscopically.Comment: 4 pages, 4 figure
Multiferroicity in rare-earth nickelates RNiO3
We show that charge ordered rare-earth nickelates of the type RNiO3 (R= Ho,
Lu, Pr and Nd) are multiferroic with very large magnetically induced
ferroelectric (FE) polarizations. This we determine from first principles
electronic structure calculations. The emerging FE polarization is directly
tied to the long-standing puzzle of which kind of magnetic ordering is present
in this class of materials: its direction and size indicate the type of
ground-state spin configuration that is realized. Vice versa, the small energy
differences between the different magnetic orderings suggest that a chosen
magnetic ordering can be stabilized by cooling the system in presence of an
electric field.Comment: 4 pages, 4 figure
Substrate-induced bandgap in graphene on hexagonal boron nitride
We determine the electronic structure of a graphene sheet on top of a
lattice-matched hexagonal boron nitride (h-BN) substrate using ab initio
density functional calculations. The most stable configuration has one carbon
atom on top of a boron atom, the other centered above a BN ring. The resulting
inequivalence of the two carbon sites leads to the opening of a gap of 53 meV
at the Dirac points of graphene and to finite masses for the Dirac fermions.
Alternative orientations of the graphene sheet on the BN substrate generate
similar band gaps and masses. The band gap induced by the BN surface can
greatly improve room temperature pinch-off characteristics of graphene-based
field effect transistors.Comment: 5 pages, 4 figures, Phys. Rev. B, in pres
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