1,185 research outputs found
Hydrodynamic model for electron-hole plasma in graphene
We propose a hydrodynamic model describing steady-state and dynamic electron
and hole transport properties of graphene structures which accounts for the
features of the electron and hole spectra. It is intended for electron-hole
plasma in graphene characterized by high rate of intercarrier scattering
compared to external scattering (on phonons and impurities), i.e., for
intrinsic or optically pumped (bipolar plasma), and gated graphene (virtually
monopolar plasma). We demonstrate that the effect of strong interaction of
electrons and holes on their transport can be treated as a viscous friction
between the electron and hole components. We apply the developed model for the
calculations of the graphene dc conductivity, in particular, the effect of
mutual drag of electrons and holes is described. The spectra and damping of
collective excitations in graphene in the bipolar and monopolar limits are
found. It is shown that at high gate voltages and, hence, at high electron and
low hole densities (or vice-versa), the excitations are associated with the
self-consistent electric field and the hydrodynamic pressure (plasma waves). In
intrinsic and optically pumped graphene, the waves constitute quasineutral
perturbations of the electron and hole densities (electron-hole sound waves)
with the velocity being dependent only on the fundamental graphene constants.Comment: 11 pages, 6 figure
Effect of plasma resonances on dynamic characteristics of double graphene-layer optical modulator
We analyze the dynamic operation of an optical modulator based on double
graphene-layer(GL) structure utilizing the variation of the GL absorption due
to the electrically controlled Pauli blocking effect. The developed device
model yields the dependences of the modulation depth on the control voltage and
the modulation frequency. The excitation of plasma oscillations in double-GL
structure can result in the resonant increase of the modulation depth, when the
modulation frequency approaches the plasma frequency, which corresponds to the
terahertz frequency for the typical parameter values.Comment: 8 pages, 4 figure
Flux Penetration in Superconducting Strip with Edge-Indentation
The flux penetration near a semicircular indentation at the edge of a thin
superconducting strip placed in a transverse magnetic field is investigated.
The flux front distortion due to the indentation is calculated numerically by
solving the Maxwell equations with a highly nonlinear law. We find that
the excess penetration, , can be significantly ( 50%) larger than
the indentation radius , in contrast to a bulk supercondutor in the
critical state where . It is also shown that the flux creep tends
to smoothen the flux front, i.e. reduce . The results are in very good
agreement with magneto-optical studies of flux penetration into an
YBaCuO film having an edge defect.Comment: 5 pages, 7 figure
Mechanism for flux guidance by micrometric antidot arrays in superconducting films
A study of magnetic flux penetration in a superconducting film patterned with
arrays of micron sized antidots (microholes) is reported. Magneto-optical
imaging (MOI) of a YBCO film shaped as a long strip with perpendicular antidot
arrays revealed both strong guidance of flux, and at the same time large
perturbations of the overall flux penetration and flow of current. These
results are compared with a numerical flux creep simulation of a thin
superconductor with the same antidot pattern. To perform calculations on such a
complex geometry, an efficient numerical scheme for handling the boundary
conditions of the antidots and the nonlocal electrodynamics was developed. The
simulations reproduce essentially all features of the MOI results. In addition,
the numerical results give insight into all other key quantities, e.g., the
electrical field, which becomes extremely large in the narrow channels
connecting the antidots.Comment: 8 pages, 7 figure
Interaction between superconducting vortices and Bloch wall in ferrite garnet film
Interaction between a Bloch wall in a ferrite-garnet film and a vortex in a
superconductor is analyzed in the London approximation. Equilibrium
distribution of vortices formed around the Bloch wall is calculated. The
results agree quantitatively with magneto-optical experiment where an in-plane
magnetized ferrite-garnet film placed on top of NbSe2 superconductor allows
observation of individual vortices. In particular, our model can reproduce a
counter-intuitive attraction observed between vortices and a Bloch wall having
the opposite polarity. It is explained by magnetic charges appearing due to
discontinuity of the in-plane magnetization across the wall.Comment: 4 pages, 5 figure
Reentrant stability of superconducting films and the vanishing of dendritic flux instability
We propose a mechanism responsible for the abrupt vanishing of the dendritic flux instability found in many superconducting films when an increasing magnetic field is applied. The onset of flux avalanches and the subsequent reentrance of stability in NbN films were investigated using magneto-optical imaging, and the threshold fields were measured as functions of critical current density jc. The results are explained with excellent quantitative agreement by a thermomagnetic model published recently [D. V. Denisov et al., Phys. Rev. B 73, 014512 (2006)], showing that the reentrant stability is a direct consequence of a monotonously decreasing jc versus fiel
Light-Controlled Polarization of MM-Waves with Photo-Excited Gratings in a Resonant Semiconductor Slab
We investigated photoconductive gratings in the resonant semiconductor layers as light-controlled polarizers for the millimeter (MM) waves. We compared the effects of strip-like, wire-like, and fin-like gratings excited by the red light and the IR radiation in Silicon wafers, respectively. The fin-like gratings are shown to be the preferred structures that can operate at the limited light intensity. The light-sensitive shift of maxima of transmitted power and polarizing efficiency towards the lower frequency band is observed. The effect makes photoconductive gratings and similar patterns potentially suitable for the design of light-controlled frequency-tuning and frequency-modulating components of resonant quasi-optical devices
Very strong intrinsic supercurrent carrying ability and vortex avalanches in (Ba,K)Fe2As2 superconducting single crystals
We report that single crystals of (Ba,K)Fe2As2 with Tc = 32 K have a pinning
potential, U0, as high as 10^4 K, with U0 showing very little field
depend-ence. In addition, the (Ba,K)Fe2As2 single crystals become isotropic at
low temperatures and high magnetic fields, resulting in a very rigid vortex
lattice, even in fields very close to Hc2. The rigid vortices in the two
dimensional (Ba,K)Fe2As2 distinguish this compound from 2D high Tc cuprate
superconductors with 2D vortices, and make it being capable of cearrying very
high critical current.Flux jumping due to high Jc was also observed in large
samples at low temperatures.Comment: 4 pages, 7 figures. submitte
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