4,111 research outputs found
Switching and Rectification of a Single Light-sensitive Diarylethene Molecule Sandwiched between Graphene Nanoribbons
The 'open' and 'closed' isomers of the diarylethene molecule that can be
converted between each other upon photo-excitation are found to have
drastically different current-voltage characteristics when sandwiched between
two graphene nanoribbons (GNRs). More importantly, when one GNR is metallic and
another one is semiconducting, strong rectification behavior of the 'closed'
diarylethene isomer with the rectification ratio >10^3 is observed. The
surprisingly high rectification ratio originates from the band gap of GNR and
the bias-dependent variation of the lowest unoccupied molecular orbital (LUMO)
of the diarylethene molecule, the combination of which completely shuts off the
current at positive biases. Results presented in this paper may form the basis
for a new class of molecular electronic devices.Comment: The Journal of Chemical Physics 135 (2011
Shear Viscosity of Uniform Fermi Gases with Population Imbalance
The shear viscosity plays an important role in studies of transport phenomena
in ultracold Fermi gases and serves as a diagnostic of various microscopic
theories. Due to the complicated phase structures of population-imbalanced
Fermi gases, past works mainly focus on unpolarized Fermi gases. Here we
investigate the shear viscosity of homogeneous, population-imbalanced Fermi
gases with tunable attractive interactions at finite temperatures by using a
pairing fluctuation theory for thermodynamical quantities and a gauge-invariant
linear response theory for transport coefficients. In the unitary and BEC
regimes, the shear viscosity increases with the polarization because the excess
majority fermions cause gapless excitations acting like a normal fluid. In the
weak BEC regime the excess fermions also suppress the noncondensed pairs at low
polarization, and we found a minimum in the ratio of shear viscosity and
relaxation time. To help constrain the relaxation time from linear response
theory, we derive an exact relation connecting some thermodynamic quantities
and transport coefficients at the mean-field level for unitary Fermi
superfluids with population imbalance. An approximate relation beyond
mean-field theory is proposed and only exhibits mild deviations from numerical
results.Comment: 11 pages, 4 figure
Dielectric properties and lattice dynamics of alpha-PbO2-type TiO2: The role of soft phonon modes in pressure-induced phase transition to baddeleyite-type TiO2
Dielectric tensor and lattice dynamics of alpha-PbO2-type TiO2 have been
investigated using the density functional perturbation theory, with a focus on
responses of the vibrational frequencies to pressure. The calculated Raman
spectra under different pressures are in good agreement with available
experimental results and the symmetry assignments of the Raman peaks of
alpha-PbO2-type TiO2 are given for the first time. In addition, we identified
two anomalously IR-active soft phonon modes, B1u and B3u, respectively, around
200 cm-1 which have not been observed in high pressure experiments. Comparison
of the phonon dispersions at 0 and 10 GPa reveals that softening of phonon
modes also occurs for the zone-boundary modes. The B1u and B3u modes play an
important role in transformation from the alpha-PbO2-type phase to baddeleyite
phase. The significant relaxations of the oxygen atoms from the Ti4 plane in
the Ti2O2Ti2 complex of the baddeleyite phase are directly correlated to the
oxygen displacements along the directions given by the eigenvectors of the soft
B1u and B3u modes in the alpha-PbO2-type phase.Comment: 8 pages, 9 figure
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