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Comment: 6 pages, 3 figures; longer version of the paper, to appear in PRB
Comment: 5 pages and 5 figures, Accepted in Physical Review Letters
The Coulomb interaction between two holes on oligo-thiophene molecules is studied systematically as a function of the oligomer length using first principles density function calculations. The effect of molecular geometry relaxation upon this interaction is found to be small. In contrast, electronic polarization of the molecules that surround the charged oligomer in the crystal lattice reduces the bare Coulomb repulsion between the holes by approximately a factor of 2. In all cases, the effective hole–hole repulsion is much larger than the valence band width, which means that at high doping levels, strong correlation effects should become important.
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 electronic polarization of the molecules that surround the charged oligomer reduces the bare Coulomb repulsion between the holes by approximately a factor of 2. The effects of relaxing the molecular geometry in the presence of holes is found to be significantly smaller. In all cases the effective hole-hole repulsion is much larger than the valence bandwidth, which implies that at high doping levels the properties of these organic semiconductors are determined by electron-electron correlations.
Comment: Submitted version in compliance with editorial policies - final version to appear in Nature Physics
Comment: This manuscript will be published on the Proceedings of the 2010 Nobel Symposium on Graphene and Quantum Matter
We have investigated the contact resistance of rubrene single-crystal field-effect transistors (FETs) with Nickel electrodes by performing scaling experiments on devices with channel length ranging from 200 nm up to 300 $\mu$m. We find that the contact resistance can be as low as 100 $\Omega$cm with narrowly spread fluctuations. For comparison, we have also performed scaling experiments on similar Gold-contacted devices, and found that the reproducibility of FETs with Nickel electrodes is largely superior. These results indicate that Nickel is a very promising electrode material for the reproducible fabrication of low resistance contacts in organic FETs.
We investigate electronic transport through pentacene thin films intercalated with potassium. From temperature-dependent conductivity measurements we find that potassium-intercalated pentacene shows metallic behavior in a broad range of potassium concentrations. Surprisingly, the conductivity exhibits a re-entrance into an insulating state when the potassium concentration is increased past one atom per molecule. We analyze our observations theoretically by means of electronic structure calculations, and we conclude that the phenomenon originates from a Mott metal-insulator transition, driven by electron-electron interactions.
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