Probing Inter- and Intrachain Exciton Coupling in Isolated Poly(3-hexylthiophene) Nanofibers: Effect of Solvation and Regioregularity.

We report wavelength and time-resolved photoluminescence studies of isolated extended (1-10 µm length) poly(3-hexylthiophene) (P3HT) nanofibers (xNFs) cast on glass from suspension. The PL spectra of xNFs show multiple vibronic replicas that appear to be associated with the existence of both H- and J-type aggregates. The PL spectra of xNFs made from regioregular (rr)- (93%) and highly regioregular (hrr)-P3HT (98%) both show similarities in PL spectra suggestive of common chain packing features, as well as subtle differences that can be attributed to higher long-range order in the hrr-xNFs. Specifically, PL spectral measurements on isolated xNFs made from highly regioregular (>98%) P3HT showed a red-shifted electronic origin (≈30 meV) and increased 0-0/0-1 PL intensity ratio for the J-type species, suggestive of enhanced structural coherence length and intrachain order.

Higher-energy composite fermion levels in the fractional quantum Hall effect.

Even though composite fermions in the fractional quantum Hall liquid are well established, it is not yet known up to what energies they remain intact. We probe the high-energy spectrum of the 1/3 liquid directly by resonant inelastic light scattering, and report the observation of a large number of new collective modes. Supported by our theoretical calculations, we associate these with transitions across two or more composite fermions levels. The formation of quasiparticle levels up to high energies is direct evidence for the robustness of topological order in the fractional quantum Hall effect.

Note: Interference technique for minimally invasive, subnanometer, microsecond measurements of displacements.

We present a novel high-resolution technique for single-molecule experiments, viz., differential traveling wave tracking. This is an interference-based scattering technique where we use gold nanoparticles for high scattering intensities and incorporate differential measurements along one in-plane direction to subtract mechanical noise and drift of the system. In addition, out-of-plane distances are measured via scattered light intensity in a total internal reflectance illumination field. In plane, we demonstrate a rms noise level of only 0.10 nm at 10 kHz and less than 0.5 nm at 600 kHz.

Spin texture and magnetoroton excitations at nu=1/3.

Neutral spin texture (ST) excitations at nu=1/3 are directly observed for the first time by resonant inelastic light scattering. They are determined to involve two simultaneous spin flips. At low magnetic fields, the ST energy is below that of the magnetoroton minimum. With increasing in-plane magnetic field these mode energies cross at a critical ratio of the Zeeman and Coulomb energies of eta(c)=0.020+/-0.001. Surprisingly, the intensity of the ST mode grows with temperature in the range in which the magnetoroton modes collapse. The temperature dependence is interpreted in terms of a competition between coexisting phases supporting different excitations. We consider the role of the ST excitations in activated transport at nu=1/3.

Velocity modulation of microtubules in electric fields.

We show that the speed of microtubules gliding over a kinesin-coated surface can be controlled over a wide range of values by the application of an electric field. The speed can be increased by up to a factor of 5 compared to the speed at zero field when assisting forces are applied and slowed down to zero velocity for opposing fields. Sideways applied fields also induce significant motion. The kinesin surface density impacts the rate of velocity change, whereas the ATP concentration does not seem to play a major role, provided that it is nonzero. A simple grab-and-release model is presented that explains the velocity change with applied electric fields.

Composite-fermion spin excitations as nu approaches 1/2: interactions in the Fermi sea.

Measurements of low-lying spin excitations by inelastic light scattering unveil a delicate balance between spin reversal and Fermi energies in the Fermi sea of composite fermions that emerges in the limit of nu --> 1/2. The interplays between these two fundamental quasiparticle interactions are uncovered in lowest spin-flip excitations in which the spin orientation and Landau level index of composite fermions change simultaneously. A collapse of the spin-flip excitation gap as nu --> 1/2 is linked to vanishing quasiparticle energy level spacings and loss of full spin polarization.

Splitting of long-wavelength modes of the fractional quantum hall liquid at nu = 1/3.

Resonant inelastic light scattering experiments at nu = 1/3 reveal a novel splitting of the long-wavelength modes in the low energy spectrum of quasi-particle excitations in the charge degree of freedom. We find a single peak at small wave vectors that splits into two distinct modes at larger wave vectors. The evidence of well-defined dispersive behavior at small wave vectors indicates a coherence of the quantum fluid in the micron length scale. We evaluate interpretations of long-wavelength modes of the electron liquid.

Evidence of Landau levels and interactions in low-lying excitations of composite fermions at 1/3

Excitation modes in the range 2/5>or=nu>or=1/3 of the fractional quantum Hall regime are observed by resonant inelastic light scattering. Spectra of spin-reversed excitations suggest a structure of lowest spin-split Landau levels of composite fermions that is similar to that of electrons. Spin-flip energies determined from spectra reveal significant composite fermion interactions. The filling factor dependence of mode energies displays an abrupt change in the middle of the range when there is partial population of a composite fermion level.