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 ($K_x$-picene), recently characterized as a superconductor with $T_c = 18K$. The inclusion of exchange interactions by means of hybrid functionals reproduces the correct gap for the undoped compound and predicts an antiferromagnetic state for $x=3$, 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 $\rm K_2CuPc$ the additional electrons that are introduced by potassium are fully transferred to the $e_g$ 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 $e_g$ 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 $\rm K_2CuPc$ 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, $\rm K_{2.75}{CuPc}$, 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 TMTTF2_2-PF6_6 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$_2$-PF$_6$ 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$_2$-PF$_6$ 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 RMn$_2$O$_5$, 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 HoMn$_2$O$_5$ 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