1,042 research outputs found
Laser-assisted spin-polarized transport in graphene tunnel junctions
Keldysh nonequilibrium Green's function method is utilized to study
theoretically the spin polarized transport through a graphene spin valve
irradiated by a monochromatic laser field. It is found that the bias dependence
of the differential conductance exhibits successive peaks corresponding to the
resonant tunneling through the photon-assisted sidebands. The multi photon
processes originate from the combined effects of the radiation field and the
graphene tunneling properties, and are shown to be substantially suppressed in
a graphene spin valve which results in a decrease of the differential
conductance for a high bias voltage. We also discussed the appearance of a
dynamical gap around zero bias due to the radiation field. The gap width can be
tuned by changing the radiation electric field strength and the frequency. This
leads to a shift of the resonant peaks in the differential conductance. We also
demonstrate numerically the dependencies of the radiation and spin valve
effects on the parameters of the external fields and those of the electrodes.
We find that the combined effects of the radiation field, the graphene, and the
spin valve properties bring about an oscillatory behavior in the tunnel
magnetoresistance (TMR), and this oscillatory amplitude can be changed by
scanning the radiation field strength and/or the frequency.Comment: 31 pages, 5 figures, corrected version to the paper in J. Phys.:
Condens. Matter 24 (2012) 26600
Single or multi-flavor Kondo effect in graphene
Based on the tight-binding formalism, we investigate the Anderson and the
Kondo model for an adaom magnetic impurity above graphene. Different impurity
positions are analyzed. Employing a partial wave representation we study the
nature of the coupling between the impurity and the conducting electrons. The
components from the two Dirac points are mixed while interacting with the
impurity. Two configurations are considered explicitly: the adatom is above one
atom (ADA), the other case is the adatom above the center the honeycomb (ADC).
For ADA the impurity is coupled with one flavor for both A and B sublattice and
both Dirac points. For ADC the impurity couples with multi-flavor states for a
spinor state of the impurity. We show, explicitly for a 3d magnetic atom,
, (,), and (,) couple
respectively with the , , and representations (reps) of group in ADC case. The basses for
these reps of graphene are also derived explicitly. For ADA we calculate the
Kondo temperature.Comment: 11 pages, 1 fures, 2 tables, accepted by EP
Dependence of electronic and optical properties on a high-frequency field for carbon nanotubes
We study theoretically the electronic structure, transport and optical
properties for a zigzag single-wall carbon nanotube connected to two normal
conductor leads under the irradiation of an external electromagnetic field at
low temperatures, with particular emphasis on the features of high-frequency
response. Using the standard nonequilibrium Green's function techniques, we
examine the time-averaged density of states, the conductivity, the dielectric
function and the electron energy loss spectra for the system with photon
polarization parallel with the tunneling current direction, respectively.
Through some numerical examples, it is shown that the density of states is
strongly dependent on the incident electron energy, the strength and frequency
of the applied field. For higher electron energies in comparison with
lead-nanotube coupling energy, the system conductance decreases with increasing
the field strength and increases with increasing the field frequency
respectively, and shows some oscillation structures. Moreover, the optical
functions for the system have also a rich structure with the variation of field
frequency. It may demonstrate that this transport dependence on the external
field parameters can be used to give the energy spectra information of carbon
nanotubes and to detect the high-frequency microwave irradiation.Comment: 6 Revtex pages, 4 figures. to be appeared in JA
Synthesis and Characterization of Crystalline Silicon Carbide Nanoribbons
In this paper, a simple method to synthesize silicon carbide (SiC) nanoribbons is presented. Silicon powder and carbon black powder placed in a horizontal tube furnace were exposed to temperatures ranging from 1,250 to 1,500°C for 5–12 h in an argon atmosphere at atmospheric pressure. The resulting SiC nanoribbons were tens to hundreds of microns in length, a few microns in width and tens of nanometers in thickness. The nanoribbons were characterized with electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, Raman spectroscopy and X-ray photoelectron spectroscopy, and were found to be hexagonal wurtzite–type SiC (2H-SiC) with a growth direction of . The influence of the synthesis conditions such as the reaction temperature, reaction duration and chamber pressure on the growth of the SiC nanomaterial was investigated. A vapor–solid reaction dominated nanoribbon growth mechanism was discussed
Magneto-transport in impurity-doped few-layer graphene spin valve
Using Keldysh nonequilibrium Green's function method we study the
spin-dependent transport through impurity-doped few layer graphene sandwiched
between two magnetic leads with an arbitrary mutual orientations of the
magnetizations. We find for parallel electrodes magnetizations that the
differential conductance possesses two resonant peaks as the applied bias
increases. These peaks are traced back to a buildup of a magnetic moment on the
impurity due to the electrodes spin polarization. For a large mutual angle of
the electrodes magnetization directions, the two resonant peaks approach each
others and merge into a single peak for antiparallel orientation of the
electrodes magnetizations. We point out that the tunneling magnetoresistance
(TMR) may change sign for relatively small changes in the values of the
polarization parameters. Furthermore, we inspect the behaviour of the
differential conductance and TMR upon varying the temperature.Comment: 8 pages, 7 figures, accepted by Phys. Rev.
Magnetotransport through graphene spin valves
We present a theoretical study on the spin-dependent transport through a spin
valve consisting of graphene sandwiched between two magnetic leads with an
arbitrary orientation of the lead magnetization. No gate voltage is applied.
Using Keldysh's nonequilibrium Green's function method we show that, in absence
of external magnetic fields, the current-voltage curves are nonlinear. Around
zero bias the differential conductance versus bias voltage possesses a strong
dip. The zero-bias anomaly in the tunnel magnetoresistance (TMR) is affected
strongly by the leads spin polarization. Depending on the value of the bias
voltage TMR exhibits a behavior ranging from an insulating to a metallic-type.
In presence of a static external magnetic field the differential conductance
and TMR as a function of the bias voltage and the strength of the magnetic
field show periodic oscillations due to Landau-level crossings. We also inspect
the effects of the temperature and the polarization degrees on the differential
conductance and TMR
A possible role of the ATP-sensitive potassium ion channel in determining the duration of spike-bursts in mouse pancreatic β-cells
AbstractThe pancreatic β-cell displays an electrical activity consisting of spike bursts and silent phases at glucose concentrations of about 10 mM. The mechanism of initial depolarization induced by glucose is well defined. However, the mechanism inducing the silent phase has not been fully elucidated. In the present study, the possibility of involvement of ATP-sensitive K+ channels in repolarization was examined using the patch-clamp technique in the cell-attached recording configuration. Ouabain (0.1 mM), an inhibitor of Na+/K+-ATPase, caused a complete suppression of ATP-sensitive K+ channel activity followed by typical biphasic current deflections, which were due to action potentials. The channel activity was also inhibited by removal of K+ from a perifusion solution. Furthermore, the activity of ATP-sensitive K+ channels was markedly inhibited either by replacement of external NaCl with LiCl or by addition of amiloride (0.2 mM), a blocker of Na+/H+ antiport. Addition of L-type Ca2+ channel blockers such as Nifedipine or Mn2+ induced the complete suppression of K+ channel activity. These findings strongly suggest that a fall in ATP consumption results in sustained depolarization, and that the repolarizations interposed between spike-bursts under normal ionic conditions are due to the periodical fall of ATP concentration brought about by periodical acceleration of ATP consumption at Na+/K+-pumps. It is concluded that the elevation of intracellular Na+ concentration as a consequence of accelerated Na+/Ca2+-countertransport during the period of spike-burst enhances ATP consumption, leading to a fall in ATP concentration which is responsible for termination of spike-burst and initiation of repolarization
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