Polaron dynamics in a two-dimensional Holstein-Peierls system.

A semiclassical model for studying charge transport in a two-dimensional molecular lattice is presented and applied to both a well ordered system and a system with disorder. The model includes both intra- and inter-molecular electron-lattice interactions and the focus of the studies is to describe the dynamics of a charge carrier in the system. In particular, we study the dynamics of the system in which the polaron solution is dynamically stable. It is found that the parameter space for which the polaron is moving through the system is quite restricted and that the polaron is immobile for large electron-phonon coupling and weak intermolecular electron interactions and dynamically unstable and disassociates into a delocalized electronic state decoupled from the lattice for small electron-phonon coupling and strong intermolecular electron interactions. Disorder further limits the parameter space in which the polaron is mobile.

Charge and energy dynamics in photo-excited poly(para-phenylenevinylene) systems.

We report results from simulations of charge and energy dynamics in poly(para-phenylenevinylene) (PPV) and PPV interacting with C60. The simulations were performed by solving the time-dependent Schrodinger equation and the lattice equation of motion simultaneously and nonadiabatically. The electronic system and the coupling of the electrons to the lattice were described by an extended three-dimensional version of the Su-Schrieffer-Heeger model, which also included an external electric field. Electron and lattice dynamics following electronic excitations at different energies have been simulated. The effect of additional lattice energy was also included in the simulations. Our results show that both exciton diffusion and transitions from high to lower lying excitations are stimulated by increasing the lattice energy. Also field induced charge separation occurs faster if the lattice energy is increased. This separation process is highly nonadiabatic and involves a significant rearrangement of the electron distribution. In the case of PPV coupled to C60, we observe a spontaneous charge separation. The separation time is in this case limited by the local concentration of C60 molecules close to the PPV chain.

Cross-linked nano-onions of carbon nitride in the solid phase: existence of a novel C(48)N(12) aza-fullerene.

We report a new fullerenelike material consisting of cross-linked nano-onions of C and N. Growth of the onion shells takes place atom by atom on a substrate surface and yields thin solid films during magnetron sputter deposition. Electron microscopy and energy loss spectroscopy show that the core shell contains up to 20 at. % N corresponding to C(48)N(12) aza-fullerene composition. Nanoindentation of this nanostructured material gives high resilience with hardness 7 GPa, Young's modulus 37 GPa, and complete elastic recovery after loading with 0.5 mN to a depth of 75 nm. Total energy calculations show the stability of C(60-2n)N(2n) aza-fullerenes and suggest the existence of a novel C(48)N(12) molecule.

Band resonant tunneling in DNA molecules.

The charge migration process in DNA is subject to a lively debate among researchers at the moment. We have performed calculations on poly(G)-poly(C) DNA which substantiate a recent report indicating bandlike conduction for this particular form of DNA. Our results show that both guanine and cytosine give rise to conducting channels along the DNA strands. The conductivity results from the overlap of the pi orbitals along the base stacks. We also demonstrate that the measured increase of the threshold voltage with temperature in poly(G)-poly(C) DNA is the result of electron localization due to the structural disorder following high temperature.

Polaron dynamics in a system of coupled conjugated polymer chains.

The motion of excitations such as polarons is believed to be of fundamental importance for the transport properties of conjugated polymers for the use in, e.g., polymer based LED's. We have investigated polaron dynamics in a system of coupled polymer chains in the presence of an external electric field. In particular, we focus on how a polaron migrates through the polymer lattice, i.e., the situation in which a polaron reaches a chain end and is scattered to the surrounding chains. We show that the outcome of this event strongly depends on the strength of the electric field, and we identify three different cases for the polaron migration.

Negative Poisson's ratios for extreme states of matter

Negative Poisson's ratios are predicted for body-centered-cubic phases that likely exist in white dwarf cores and neutron star outer crusts, as well as those found for vacuumlike ion crystals, plasma dust crystals, and colloidal crystals (including certain virus crystals). The existence of this counterintuitive property, which means that a material laterally expands when stretched, is experimentally demonstrated for very low density crystals of trapped ions. At very high densities, the large predicted negative and positive Poisson's ratios might be important for understanding the asteroseismology of neutron stars and white dwarfs and the effect of stellar stresses on nuclear reaction rates. Giant Poisson's ratios are both predicted and observed for highly strained coulombic photonic crystals, suggesting possible applications of large, tunable Poisson's ratios for photonic crystal devices.

Materials with negative compressibilities in one or more dimensions

Rare crystal phases that expand in one or more dimensions when hydrostatically compressed are identified and shown to have negative Poisson's ratios. Some of these crystals (i) decrease volume and expand in two dimensions when stretched in a particular direction and (ii) increase surface area when hydrostatically compressed. Possible mechanisms for achieving such negative linear and area compressibilities are described for single crystals and composites, and sensor applications are proposed. Materials with these properties may be used to fabricate porous solids that either expand in all directions when hydrostatically compressed with a penetrating fluid or behave as if they are incompressible.