5,532 research outputs found
XPS characterization of silver electrodes and catalyst for oxygen reduction
The combined analysis of the silver GDE using an ex-situ surface sensitive technique (XPS) and in-situ electrochemical measurements (EIS, CV) show that the performance of the silver GDE is significantly influenced by the degree of degradation of the electrodes, e. g., the reduction of the active surface due to the decomposition of the PTFE. These findings indicate a different degree of decomposition of the PTFE on the on the GDE
Optical exciton Aharonov-Bohm effect, persistent current, and magnetization in semiconductor nanorings of type I and II
The optical exciton Aharonov-Bohm effect, i. e. an oscillatory component in
the energy of optically active (bright) states, is investigated in nanorings.
It is shown that a small effective electron mass, strong confinement of the
electron, and high barrier for the hole, achieved e. g. by an InAs nanoring
embedded in an AlGaSb quantum well, are favorable for observing the optical
exciton Aharonov-Bohm effect. The second derivative of the exciton energy with
respect to the magnetic field is utilized to extract Aharonov-Bohm oscillations
even for the lowest bright state unambiguously. A connection between the
theories for infinitesimal narrow and finite width rings is established.
Furthermore, the magnetization is compared to the persistent current, which
oscillates periodically with the magnetic field and confirms thus the
non-trivial (connected) topology of the wave function in the nanoring.Comment: 12 pages, 11 figure
Thermoelectric phenomena in a quantum dot asymmetrically coupled to external leads
We study thermoelectric phenomena in a system consisting of strongly
correlated quantum dot coupled to external leads in the Kondo regime. We
calculate linear and nonlinear electrical and thermal conductance and
thermopower of the quantum dot and discuss the role of asymmetry in the
couplings to external electrodes. In the linear regime electrical and thermal
conductances are modified, while thermopower remains unchanged. In the
nonlinear regime the Kondo resonance in differential conductance develops at
non-zero source-drain voltage, which has important consequences on
thermoelectric properties of the system and the thermopower starts to depend on
the asymmetry. We also discuss Wiedemann-Franz relation, thermoelectric figure
of merit and validity of the Mott formula for thermopower.Comment: 6 pages, 7 figure
Do Evaporating Black Holes Form Photospheres?
Several authors, most notably Heckler, have claimed that the observable
Hawking emission from a microscopic black hole is significantly modified by the
formation of a photosphere around the black hole due to QED or QCD interactions
between the emitted particles. In this paper we analyze these claims and
identify a number of physical and geometrical effects which invalidate these
scenarios. We point out two key problems. First, the interacting particles must
be causally connected to interact, and this condition is satisfied by only a
small fraction of the emitted particles close to the black hole. Second, a
scattered particle requires a distance ~ E/m_e^2 for completing each
bremsstrahlung interaction, with the consequence that it is improbable for
there to be more than one complete bremsstrahlung interaction per particle near
the black hole. These two effects have not been included in previous analyses.
We conclude that the emitted particles do not interact sufficiently to form a
QED photosphere. Similar arguments apply in the QCD case and prevent a QCD
photosphere (chromosphere) from developing when the black hole temperature is
much greater than Lambda_QCD, the threshold for QCD particle emission.
Additional QCD phenomenological arguments rule out the development of a
chromosphere around black hole temperatures of order Lambda_QCD. In all cases,
the observational signatures of a cosmic or Galactic halo background of
primordial black holes or an individual black hole remain essentially those of
the standard Hawking model, with little change to the detection probability. We
also consider the possibility, as proposed by Belyanin et al. and D. Cline et
al., that plasma interactions between the emitted particles form a photosphere,
and we conclude that this scenario too is not supported.Comment: version published in Phys Rev D 78, 064043; 25 pages, 3 figures;
includes discussion on extending our analysis to TeV-scale,
higher-dimensional black hole
Steps on current-voltage characteristics of a silicon quantum dot covered by natural oxide
Considering a double-barrier structure formed by a silicon quantum dot
covered by natural oxide with two metallic terminals, we derive simple
conditions for a step-like voltage-current curve. Due to standard chemical
properties, doping phosphorus atoms located in a certain domain of the dot form
geometrically parallel current channels. The height of the current step
typically equals to (1.2 pA)N, where N=0,1,2,3... is the number of doping atoms
inside the domain, and only negligibly depends on the actual position of the
dopants. The found conditions are feasible in experimentally available
structures.Comment: 4 pages, 3 figure
Effect of gas flow on electronic transport in a DNA-decorated carbon nanotube
We calculate the two-time current correlation function using the experimental
data of the current-time characteristics of the Gas-DNA-decorated carbon
nanotube field effect transistor. The pattern of the correlation function is a
measure of the sensitivity and selectivity of the sensors and suggest that
these gas flow sensors may also be used as DNA sequence detectors. The system
is modelled by a one-dimensional tight-binding Hamiltonian and we present
analytical calculations of quantum electronic transport for the system using
the time-dependent nonequilibrium Green's function formalism and the adiabatic
expansion. The zeroth and first order contributions to the current
and are calculated, where is the Landauer formula. The formula for the time-dependent current
is then used to compare the theoretical results with the experiment.Comment: 14 pages, 5 figures and 2 table
Bremsstrahlung and pair production processes at low energies, multi-differential cross section and polarization phenomena
Radiative electron-proton scattering is studied in peripheral kinematics,
where the scattered electron and photon move close to the direction of the
initial electron. Even in the case of unpolarized initial electron the photon
may have a definite polarization. The differential cross sections with
longitudinally or transversal polarized initial electron are calculated. The
same phenomena are considered for the production of an electron-positron pair
by the photon, where the final positron (electron) can be also polarized.
Differential distributions for the case of polarized initial photon are given.
Both cases of unscreened and completely screened atomic targets are considered.Comment: 15 pages, 6 figure
Efficient graphene-based photodetector with two cavities
We present an efficient graphene-based photodetector with two Fabri-P\'erot
cavities. It is shown that the absorption can reach almost 100% around a given
frequency, which is determined by the two-cavity lengths. It is also shown that
hysteresis in the absorbance is possible, with the transmittance amplitude of
the mirrors working as an external driving field. The role of non-linear
contributions to the optical susceptibility of graphene is discussed.Comment: 10 pages, 8 figures. published version: minor revisio
Spin-polarized tunneling currents through a ferromagnetic insulator between two metallic or superconducting leads
Using the Keldysh formalism the tunneling current through a hybrid structure
where a confined magnetic insulator (I) is sandwiched between two non-magnetic
leads is calculated. The leads can be either normal metals (M) or
superconductors (S). Each region is modelled as a single band in tight-binding
approximation in order to understand the formation of the tunneling current as
clearly as possible. The tunneling process itself is simulated by a
hybridization between the lead and insulator conduction bands. The insulator is
assumed to have localized moments which can interact with the tunneling
electrons. This is described by the Kondo Lattice Model (KLM) and treated
within an interpolating self-energy approach. For the superconductor the
mean-field BCS theory is used. The spin polarization of the current shows a
strong dependence both on the applied voltage and the properties of the
materials. Even for this idealized three band model there is a qualitative
agreement with experiment.Comment: 15 pages, 23 figures, accepted for publication in PR
Theory of the optical absorption of light carrying orbital angular momentum by semiconductors
We develop a free-carrier theory of the optical absorption of light carrying
orbital angular momentum (twisted light) by bulk semiconductors. We obtain the
optical transition matrix elements for Bessel-mode twisted light and use them
to calculate the wave function of photo-excited electrons to first-order in the
vector potential of the laser. The associated net electric currents of first
and second-order on the field are obtained. It is shown that the magnetic field
produced at the center of the beam for the mode is of the order of a
millitesla, and could therefore be detected experimentally using, for example,
the technique of time-resolved Faraday rotation.Comment: Submitted to Phys. Rev. Lett. (23 Jan 2008
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