1,125 research outputs found
Impact Excitation by Hot Carriers in Carbon Nanotubes
We investigate theoretically the efficiency of intra-molecular hot carrier
induced impact ionization and excitation processes in carbon nanotubes. The
electron confinement and reduced screening lead to drastically enhanced
excitation efficiencies over those in bulk materials. Strong excitonic coupling
favors neutral excitations over ionization, while the impact mechanism
populates a different set of states than that produced by photoexcitation. The
excitation rate is strongly affected by optical phonon excitation and a simple
scaling of the rate with the field strength and optical phonon temperature is
obtained.Comment: 5 pages 4 figure
Anisotropic exciton Stark shift in black phosphorus
We calculate the excitonic spectrum of few-layer black phosphorus by direct
diagonalization of the effective mass Hamiltonian in the presence of an applied
in-plane electric field. The strong attractive interaction between electrons
and holes in this system allows one to investigate the Stark effect up to very
high ionizing fields, including also the excited states. Our results show that
the band anisotropy in black phosphorus becomes evident in the direction
dependent field induced polarizability of the exciton
Disorder induced local density of states oscillations on narrow Ag(111) terraces
The local density of states of Ag(111) has been probed in detail on
disordered terraces of varying width by dI/dV-mapping with a scanning tunneling
microscope at low temperatures. Apparent shifts of the bottom of the
surface-state band edge from terrace induced confinement are observed.
Disordered terraces show interesting contrast reversals in the dI/dV maps as a
function of tip-sample voltage polarity with details that depend on the average
width of the terrace and the particular edge profile. In contrast to perfect
terraces with straight edges, standing wave patterns are observed parallel to
the step edges, i.e. in the non-confined direction. Scattering calculations
based on the Ag(111) surface states reproduce these spatial oscillations and
all the qualitative features of the standing wave patterns, including the
polarity-dependent contrast reversals.Comment: 19 pages, 12 figure
Operation of Graphene Transistors at GHz Frequencies
Top-gated graphene transistors operating at high frequencies (GHz) have been
fabricated and their characteristics analyzed. The measured intrinsic current
gain shows an ideal 1/f frequency dependence, indicating an FET-like behavior
for graphene transistors. The cutoff frequency fT is found to be proportional
to the dc transconductance gm of the device. The peak fT increases with a
reduced gate length, and fT as high as 26 GHz is measured for a graphene
transistor with a gate length of 150 nm. The work represents a significant step
towards the realization of graphene-based electronics for high-frequency
applications
ОПАЛЬНИЙ ГОЛОВА ОБЛВИКОНКОМУ Г.Г. ДЕМЕНТЬЄВ
У статті висвітлюється життя та діяльність в органах міського самоврядування першого секретаря Дніпропетровського обкому КП(б)У та голови виконкому обласної ради депутатів трудящих, учасника Великої Вітчизняної війни 1941-1945 рр Г.Г.ДементьєваThis article introduces G.G.Dementyevs life and activities in local governmental organs. G.G.Dementyev was the first Secretari of Dniepropetrovsk Regional Executive Committee of the Communist Party ( bolchevyks) of Ukraine, participant in the Great Patriotic War ( from 1941-1945 )
How does the substrate affect the Raman and excited state spectra of a carbon nanotube?
We study the optical properties of a single, semiconducting single-walled
carbon nanotube (CNT) that is partially suspended across a trench and partially
supported by a SiO2-substrate. By tuning the laser excitation energy across the
E33 excitonic resonance of the suspended CNT segment, the scattering
intensities of the principal Raman transitions, the radial breathing mode
(RBM), the G-mode and the D-mode show strong resonance enhancement of up to
three orders of magnitude. In the supported part of the CNT, despite a loss of
Raman scattering intensity of up to two orders of magnitude, we recover the E33
excitonic resonance suffering a substrate-induced red shift of 50 meV. The peak
intensity ratio between G-band and D-band is highly sensitive to the presence
of the substrate and varies by one order of magnitude, demonstrating the much
higher defect density in the supported CNT segments. By comparing the E33
resonance spectra measured by Raman excitation spectroscopy and
photoluminescence (PL) excitation spectroscopy in the suspended CNT segment, we
observe that the peak energy in the PL excitation spectrum is red-shifted by 40
meV. This shift is associated with the energy difference between the localized
exciton dominating the PL excitation spectrum and the free exciton giving rise
to the Raman excitation spectrum. High-resolution Raman spectra reveal
substrate-induced symmetry breaking, as evidenced by the appearance of
additional peaks in the strongly broadened Raman G band. Laser-induced line
shifts of RBM and G band measured on the suspended CNT segment are both linear
as a function of the laser excitation power. Stokes/anti-Stokes measurements,
however, reveal an increase of the G phonon population while the RBM phonon
population is rather independent of the laser excitation power.Comment: Revised manuscript, 20 pages, 8 figure
Electron-phonon effects and transport in carbon nanotubes
We calculate the electron-phonon scattering and binding in semiconducting
carbon nanotubes, within a tight binding model. The mobility is derived using a
multi-band Boltzmann treatment. At high fields, the dominant scattering is
inter-band scattering by LO phonons corresponding to the corners K of the
graphene Brillouin zone. The drift velocity saturates at approximately half the
graphene Fermi velocity. The calculated mobility as a function of temperature,
electric field, and nanotube chirality are well reproduced by a simple
interpolation formula. Polaronic binding give a band-gap renormalization of ~70
meV, an order of magnitude larger than expected. Coherence lengths can be quite
long but are strongly energy dependent.Comment: 5 pages and 4 figure
Transmission through a biased graphene bilayer barrier
We study the electronic transmission through a graphene bilayer in the
presence of an applied bias between layers. We consider different geometries
involving interfaces between both a monolayer and a bilayer and between two
bilayers. The applied bias opens a sizable gap in the spectrum inside the
bilayer barrier region, thus leading to large changes in the transmission
probability and electronic conductance that are controlled by the applied bias.Comment: 10 pages, 8 figures, extended versio
Spatial distribution of local currents of massless Dirac fermions in quantum transport through graphene nanoribbons
We employ the formalism of bond currents, expressed in terms of the
nonequilibrium Green functions, to image the charge flow between two sites of
the honeycomb lattice of graphene ribbons of few nanometers width. In sharp
contrast to nonrelativistic electrons, current density profiles of quantum
transport at energies close to the Dirac point in clean zigzag graphene
nanoribbons (ZGNR) differs markedly from the profiles of charge density peaked
at the edges due to zero-energy localized edge states. For transport through
the lowest propagating mode induced by these edge states, edge vacancies do not
affect current density peaked in the center of ZGNR. The long-range potential
of a single impurity acts to reduce local current around it while concurrently
increasing the current density along the zigzag edge, so that ZGNR conductance
remains perfect .Comment: 5 pages, 5 figure
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