Paramagnetic adsorbates on graphene: a charge transfer analysis

We introduce a modified version of the Hirshfeld charge analysis method and demonstrate its accurateness by calculating the charge transfer between the paramagnetic molecule NO2 and graphene. The charge transfer between paramagnetic molecules and a graphene layer as calculated with ab initio methods can crucially depend on the size of the supercell used in the calculation. This has important consequences for adsorption studies involving paramagnetic molecules such as NO2 physisorbed on graphene or on carbon nanotubes.Comment: 4 pages, 4 figures, submitted to Applied Physics Letter

Graphene: a perfect nanoballoon

We have performed a first-principles density functional theory investigation of the penetration of helium atoms through a graphene monolayer with defects. The relaxation of the graphene layer caused by the incoming helium atoms does not have a strong influence on the height of the energy barriers for penetration. For defective graphene layers, the penetration barriers decrease exponentially with the size of the defects but they are still sufficiently high that very large defects are needed to make the graphene sheet permeable for small atoms and molecules. This makes graphene a very promising material for the construction of nanocages and nanomembranes.Comment: 4 pages, 4 figures, submitted to Applied Physics Letter

Polaron effects in electron channels on a helium film

Using the Feynman path-integral formalism we study the polaron effects in quantum wires above a liquid helium film. The electron interacts with two-dimensional (2D) surface phonons, i.e. ripplons, and is confined in one dimension (1D) by an harmonic potential. The obtained results are valid for arbitrary temperature ($T$), electron-phonon coupling strength ($\alpha$), and lateral confinement ($\omega_{0}$). Analytical and numerical results are obtained for limiting cases of $T$, $\alpha$, and $\omega_{0}$. We found the surprising result that reducing the electron motion from 2D to quasi-1D makes the self-trapping transition more continuous.Comment: 6 pages, 7 figures, submitted to Phys. Rev.

Adsorption of H2O, NH3, CO, NO2, and NO on graphene: A first-principles study

Motivated by the recent realization of graphene sensors to detect individual gas molecules, we investigate the adsorption of H2O, NH3, CO, NO2, and NO on a graphene substrate using first-principles calculations. The optimal adsorption position and orientation of these molecules on the graphene surface is determined and the adsorption energies are calculated. Molecular doping, i.e. charge transfer between the molecules and the graphene surface, is discussed in light of the density of states and the molecular orbitals of the adsorbates. The efficiency of doping of the different molecules is determined and the influence of their magnetic moment is discussed.Comment: 6 pages, 6 figure

First-principles investigation of graphene fluoride and graphane

Different stoichiometric configurations of graphane and graphene fluoride are investigated within density functional theory. Their structural and electronic properties are compared, and we indicate the similarities and differences among the various configurations. Large differences between graphane and graphene fluoride are found that are caused by the presence of charges on the fluorine atoms. A new configuration that is more stable than the boat configuration is predicted for graphene fluoride. We also perform GW calculations for the electronic band gap of both graphene derivatives. These band gaps and also the calculated Young's moduli are at variance with available experimental data. This might indicate that the experimental samples contain a large number of defects or are only partially covered with H or F.Comment: 6 pages, 3 figures, submitted to PR

Reply to the comment by D. Kreimer and E. Mielke

We respond to the comment by Kreimer et. al. about the torsional contribution to the chiral anomaly in curved spacetimes. We discuss their claims and refute its main conclusion.Comment: 9 pages, revte

Dynamics of Vortex Shells in Mesoscopic Superconducting Corbino Disks

In mesoscopic superconducting disks vortices form shell structures as recently observed in Nb disks. We study the dynamics of such vortices, driven by an external current I_0, in a Corbino setup. At very low I_0, the system exhibits rigid body rotation while at some critical current I_c,i vortex shells rotate separately with angular velocities omega_i. This critical current I_c,i has a remarkable non-monotonous dependence on the applied magnetic field which is due to a dynamically-induced structural transition with a rearrangement of vortices over the shells similar to the Coster-Kronig transition in hollow atoms. Thermally-activated externally-driven flux motion in a disk with pinning centers explains experimentally observed omega_i as a function of I_0 and T and the dynamically-induced melting transition.Comment: 5 pages, 5 figure