Non conventional screening of the Coulomb interaction in low dimensional and finite size system

We study the screening of the Coulomb interaction in non polar systems by polarizable atoms. We show that in low dimensions and small finite size systems this screening deviates strongly from that conventionally assumed. In fact in one dimension the short range interaction is strongly screened and the long range interaction is anti-screened thereby strongly reducing the gradient of the Coulomb interaction and therefore the correlation effects. We argue that this effect explains the success of mean field single particle theories for large molecules.Comment: 4 pages, 5 figure

Electronic Correlations in Oligo-acene and -thiophene Organic Molecular Crystals

From first principles calculations we determine the Coulomb interaction between two holes on oligo-acene and -thiophene molecules in a crystal, as a function of the oligomer length. The relaxation of the molecular geometry in the presence of holes is found to be small. In contrast, the electronic polarization of the molecules that surround the charged oligomer, reduces the bare Coulomb repulsion between the holes by approximately a factor of two. In all cases the effective hole-hole repulsion is much larger than the calculated valence bandwidth, which implies that at high doping levels the properties of these organic semiconductors are determined by electron-electron correlations.Comment: 5 pages, 3 figure

Charged excitons in doped extended Hubbard model systems

We show that the charge transfer excitons in a Hubbard model system including nearest neighbor Coulomb interactions effectively attain some charge in doped systems and become visible in photoelectron and inverse photoelectron spectroscopies. This shows that the description of a doped system by an extended Hubbard model differs substantially from that of a simple Hubbard model. Longer range Coulomb interactions cause satellites in the one electron removal and addition spectra and the appearance of spectral weight if the gap of doped systems at energies corresponding to the excitons of the undoped systems. The spectral weight of the satellites is proportional to the doping times the coordination number and therefore is strongly dependent on the dimension.Comment: 10 pages revtex, 5 figures ps figures adde

Inter-site Coulomb interaction and Heisenberg exchange

Based on exact diagonalization results for small clusters we discuss the effect of inter-site Coulomb repulsion in Mott-Hubbard or charge transfer insulators. Whereas the exchange constant J for direct exchange is substantially enhanced by inter-site Coulomb interaction, that for superexchange is suppressed. The enhancement of J in the single-band models holds up to the critical value for the charge density wave (CDW) instability, thus opening the way for large values of J. Single-band Hubbard models with sufficiently strong inter-site repulsion to be near a CDW instability thus may provide `physical' realizations of t-J like models with the `unphysical' parameter ratio J/t=1.Comment: Revtex file, 4 PRB pages, with 5 embedded ps-files. To appear in PRB, rapid communications. Hardcopies of figures or the entire manuscript may also be obtained by e-mail request to: [email protected]

New light on magnetic excitations: indirect resonant inelastic X-ray scattering on magnons

Recent experiments show that indirect resonant inelastic X-ray scattering (RIXS) is a new probe of spin dynamics. Here I derive the cross-section for magnetic RIXS and determine the momentum dependent four-spin correlation function that it measures. These results show that this technique offers information on spin dynamics that is complementary to e.g. neutron scattering. The RIXS spectrum of Heisenberg antiferromagnets is calculated. It turns out that only scattering processes that involve at least two magnons are allowed. Other selection rules imply that the scattering intensity vanishes for specific transferred momenta ${\bf q}$, in particular for ${\bf q}=0$. The calculated spectra agree very well with the experimental data.Comment: 4 pages, 3 figure