349 research outputs found
Plasmons in dimensionally mismatched Coulomb coupled graphene systems
We calculate the plasmon dispersion relation for Coulomb coupled metallic
armchair graphene nanoribbons and doped monolayer graphene. The crossing of the
plasmon curves, which occurs for uncoupled 1D and 2D systems, is split by the
interlayer Coulomb coupling into a lower and an upper plasmon branch. The upper
branch exhibits a highly unusual behavior with endpoints at finite .
Accordingly, the structure factor shows either a single or a double peak
behavior, depending on the plasmon wavelength. The new plasmon structure is
relevant to recent experiments, its properties can be controlled by varying the
system parameters, and be used in plasmonic applications.Comment: 5 pages, 3 figures; in press in Phys. Rev. Let
Electron polarization function and plasmons in metallic armchair graphene nanoribbons
We calculate the polarization function of Dirac fermions in metallic armchair
graphene nanoribbons for an arbitrary temperature and doping. We find that at
finite temperatures due to the phase space redistribution among inter-band and
intra-band electronic transitions in the conduction and valence bands, the full
polarization function becomes independent of the temperature and the position
of the chemical potential. As a result, for a given width of nanoribbons there
exists a single plasmon mode, with the energy dispersion determined by the
graphene's fine structure constant. In Coulomb-coupled nanoribbons, this
plasmon splits into the basic in-phase and out-of-phase plasmon modes, with the
splitting energy determined additionally by the inter-ribbon spacing.Comment: 7 pages, 4 figures; in press in Phys. Rev.
Plasma Wave Instabilities in Non-Equilibrium Graphene
We study two-stream instabilities in a non-equilibrium system in which a
stream of electrons is injected into doped graphene. As with equivalent
non-equilibrium parabolic band systems, we find that the graphene systems can
support unstable charge-density waves whose amplitudes grow with time. We
determine the range of wavevector that are unstable, and their
growth rates. We find no instability for waves with wavevectors parallel or
perpendicular to the direction of the injected carriers. We find that, within
the small wavevector approximation, the angle between and the
direction of the injected electrons that maximizes the growth rate increases
with increasing . We compare the range and strength of the
instability in graphene to that of two and three dimensional parabolic band
systems.Comment: 21 pages, 7 figure
Numerical studies of tunneling in a nonharmonic time-dependent potential
Azbel' has recently carried out a WKB-analysis of the effects of a
nonharmonic time-dependent perturbation embedded in an opaque potential
barrier. He suggests the existence of three different transmission regimes:
direct tunneling, activation assisted tunneling, and elevator resonant
activation. We address the same problem with a numerical technique, and find
qualitative agreement with Azbel's picture.Comment: LaTeX document, 15 pages. 4 figures (Fig. 2 comes in 7 pages) in
postscript appended to the LaTeX documen
Atomic carbon chains as spin-transmitters: an \textit{Ab initio} transport study
An atomic carbon chain joining two graphene flakes was recently realized in a
ground-breaking experiment by Jin {\it et al.}, Phys. Rev. Lett. {\bf 102},
205501 (2009). We present {\it ab initio} results for the electron transport
properties of such chains and demonstrate complete spin-polarization of the
transmission in large energy ranges. The effect is due to the spin-polarized
zig-zag edge terminating each graphene flake causing a spin-splitting of the
graphene bands, and the chain states. Transmission occurs when the
graphene -states resonate with similar states in the strongly hybridized
edges and chain. This effect should in general hold for any -conjugated
molecules bridging the zig-zag edges of graphene electrodes. The polarization
of the transmission can be controlled by chemically or mechanically modifying
the molecule, or by applying an electrical gate
Surface decorated silicon nanowires: a route to high-ZT thermoelectrics
Based on atomistic calculations of electron and phonon transport, we propose
to use surface decorated Silicon nanowires (SiNWs) for thermoelectric
applications. Two examples of surface decorations are studied to illustrate the
underlying deas: Nanotrees and alkyl functionalized SiNWs. For both systems we
find, (i) that the phonon conductance is significantly reduced compared to the
electronic conductance leading to high thermoelectric figure of merit, ,
and (ii) for ultra-thin wires surface decoration leads to significantly better
performance than surface disorder.Comment: Accepted for PR
Current oscillations in a metallic ring threaded by a time-dependent magnetic flux
We study a mesoscopic metallic ring threaded by a magnetic flux which varies
linearly in time PhiM(t)=Phi t with a formalism based in Baym-Kadanoff-Keldysh
non-equilibrium Green functions. We propose a method to calculate the Green
functions in real space and we consider an experimental setup to investigate
the dynamics of the ring by recourse to a transport experiment. This consists
in a single lead connecting the ring to a particle reservoir. We show that
different dynamical regimes are attained depending on the ratio hbar Phi/Phi0
W, being Phi0=h c/e and W, the bandwidth of the ring. For moderate lengths of
the ring, a stationary regime is achieved for hbar Phi/Phi0 >W. In the opposite
case with hbar Phi/Phi0 < W, the effect of Bloch oscillations driven by the
induced electric field manifests itself in the transport properties of the
system. In particular, we show that in this time-dependent regime a tunneling
current oscillating in time with a period tau=2piPhi0/Phi can be measured in
the lead. We also analyze the resistive effect introduced by inelastic
scattering due to the coupling to the external reservoir.Comment: 17 pages, 13 figure
Analysis of optical properties of strained semiconductor quantum dots for electromagnetically induced transparency
Using multiband k*p theory we study the size and geometry dependence on the
slow light properties of conical semiconductor quantum dots. We find the V-type
scheme for electromagnetically induced transparency (EIT) to be most favorable,
and identify an optimal height and size for efficient EIT operation. In case of
the ladder scheme, the existence of additional dipole allowed intraband
transitions along with an almost equidistant energy level spacing adds
additional decay pathways, which significantly impairs the EIT effect. We
further study the influence of strain and band mixing comparing four different
k*p band structure models. In addition to the separation of the heavy and light
holes due to the biaxial strain component, we observe a general reduction in
the transition strengths due to energy crossings in the valence bands caused by
strain and band mixing effects. We furthermore find a non-trivial quantum dot
size dependence of the dipole moments directly related to the biaxial strain
component. Due to the separation of the heavy and light holes the optical
transition strengths between the lower conduction and upper most valence-band
states computed using one-band model and eight-band model show general
qualitative agreement, with exceptions relevant for EIT operation.Comment: 9 pages, 12 figure
Transient Charging and Discharging of Spin-polarized Electrons in a Quantum Dot
We study spin-polarized transient transport in a quantum dot coupled to two
ferromagnetic leads subjected to a rectangular bias voltage pulse.
Time-dependent spin-resolved currents, occupations, spin accumulation, and
tunneling magnetoresistance (TMR) are calculated using both nonequilibrium
Green function and master equation techniques. Both parallel and antiparallel
leads' magnetization alignments are analyzed. Our main findings are: a
dynamical spin accumulation that changes sign in time, a short-lived pulse of
spin polarized current in the emitter lead (but not in the collector lead), and
a dynamical TMR that develops negative values in the transient regime. We also
observe that the intra-dot Coulomb interaction can enhance even further the
negative values of the TMR.Comment: 7 pages, 6 figures. Typos corrections corresponding to the published
versio
Nanoplasmonics beyond Ohm's law
In tiny metallic nanostructures, quantum confinement and nonlocal response
change the collective plasmonic behavior with important consequences for e.g.
field-enhancement and extinction cross sections. We report on our most recent
developments of a real-space formulation of an equation-of-motion that goes
beyond the common local-response approximation and use of Ohm's law as the
central constitutive equation. The electron gas is treated within a
semi-classical hydrodynamic model with the emergence of a new intrinsic length
scale. We briefly review the new governing wave equations and give examples of
applying the nonlocal framework to calculation of extinction cross sections and
field enhancement in isolated particles, dimers, and corrugated surfaces.Comment: Invited paper for TaCoNa-Photonics 2012 (www.tacona-photonics.org),
to appear in AIP Conf. Pro
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