477 research outputs found
Transmission of a Symmetric Light Pulse through a Wide QW
The reflection, transmission and absorption of a symmetric electromagnetic
pulse, which carrying frequency is close to the frequency of an interband
transition in a QW (QW), are obtained. The energy levels of a QW are assumed
discrete, one exited level is taken into account. The case of a wide QW is
considered when a length of the pulse wave, appropriate to the carrying
frequency, is comparable to the QW's width. In figures the time dependencies of
the dimensionless reflection, absorption are transmission are represented. It
is shown, that the spatial dispersion and a distinction in refraction indexes
influence stronger reflection.Comment: 8 pages,8 figures with caption
Electrical excitation of shock and soliton-like waves in two-dimensional electron channels
We study electrical excitation of nonlinear plasma waves in heterostructures
with two-dimensional electron channels and with split gates, and the
propagation of these waves using hydrodynamic equations for electron transport
coupled with two-dimensional Poisson equation for self-consistent electric
potential. The term related to electron collisions with impurities and phonons
as well as the term associated with viscosity are included into the
hydrodynamic equations. We demonstrate the formation of shock and soliton-like
waves as a result of the evolution of strongly nonuniform initial electron
density distribution. It is shown that the shock wave front and the shape of
soliton-like pulses pronouncedly depend on the coefficient of viscosity, the
thickness of the gate layer and the nonuniformity of the donor distribution
along the channel. The electron collisions result in damping of the shock and
soliton-like waves, while they do not markedly affect the thickness of the
shock wave front.Comment: 9 pages, 11 figure
Bifurcations and chaos in semiconductor superlattices with a tilted magnetic field
We study the effects of dissipation on electron transport in a semiconductor
superlattice with an applied bias voltage and a magnetic field that is tilted
relative to the superlattice axis.In previous work, we showed that although the
applied fields are stationary,they act like a THz plane wave, which strongly
couples the Bloch and cyclotron motion of electrons within the lowest miniband.
As a consequence,the electrons exhibit a unique type of Hamiltonian chaos,
which creates an intricate mesh of conduction channels (a stochastic web) in
phase space, leading to a large resonant increase in the current flow at
critical values of the applied voltage. This phase-space patterning provides a
sensitive mechanism for controlling electrical resistance. In this paper, we
investigate the effects of dissipation on the electron dynamics by modifying
the semiclassical equations of motion to include a linear damping term. We
demonstrate that even in the presence of dissipation,deterministic chaos plays
an important role in the electron transport process. We identify mechanisms for
the onset of chaos and explore the associated sequence of bifurcations in the
electron trajectories. When the Bloch and cyclotron frequencies are
commensurate, complex multistability phenomena occur in the system. In
particular, for fixed values of the control parameters several distinct stable
regimes can coexist, each corresponding to different initial conditions. We
show that this multistability has clear, experimentally-observable, signatures
in the electron transport characteristics.Comment: 14 pages 11 figure
Terahertz imaging and spectroscopy of large-area single-layer graphene
We demonstrate terahertz (THz) imaging and spectroscopy of a 15x15-mm^2
single-layer graphene film on Si using broadband THz pulses. The THz images
clearly map out the THz carrier dynamics of the graphene-on-Si sample, allowing
us to measure sheet conductivity with sub-mm resolution without fabricating
electrodes. The THz carrier dynamics are dominated by intraband transitions and
the THz-induced electron motion is characterized by a flat spectral response. A
theoretical analysis based on the Fresnel coefficients for a metallic thin film
shows that the local sheet conductivity varies across the sample from {\sigma}s
= 1.7x10^-3 to 2.4x10^-3 {\Omega}^-1 (sheet resistance, {\rho}s = 420 - 590
{\Omega}/sq).Comment: 6 pages, 5 figure
Gain in quantum cascade lasers and superlattices: A quantum transport theory
Gain in current-driven semiconductor heterostructure devices is calculated
within the theory of nonequilibrium Green functions. In order to treat the
nonequilibrium distribution self-consistently the full two-time structure of
the theory is employed without relying on any sort of Kadanoff-Baym Ansatz. The
results are independent of the choice of the electromagnetic field if the
variation of the self-energy is taken into account. Excellent quantitative
agreement is obtained with the experimental gain spectrum of a quantum cascade
laser. Calculations for semiconductor superlattices show that the simple 2-time
miniband transport model gives reliable results for large miniband widths at
room temperatureComment: 8 Pages, 4 Figures directly included, to appear in Physical Review
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Generation of multicycle terahertz pulses via optical rectification in periodically inverted GaAs structures
The authors demonstrate the generation of multicycle narrow-bandwidth terahertz pulses in periodically inverted GaAs structures using optical rectification of 2 μm, 100 fs pump pulses. Three different types of orientation-inverted samples are employed: optically contacted multilayer, orientation-patterned, and diffusion-bonded GaAs. The terahertz pulses are characterized by two-color (pump at 2 μm and probe at 0.8 μm) terahertz time-domain spectroscopy and terahertz Michelson interferometry
Two-dimensional electron gas in a linearly varying magnetic field: Quantization of the electron and current density
Published versio
THERMOELECTRIC PROPERTIES OF HOT-PRESSED p-TYPE Mg2Si0.3Sn0.7 SOLID SOLUTION
It is shown that thermoelectric energy conversion which gives the possibility for utilizing a low potential heat is one of the ways for adoption of energy-saving technologies; and semiconductor materials with p-type and n-type conductivities having high thermoelectric figure of merit are necessary for operation of thermoelectric generators. The paper deals with possibility of usage of the p-Mg2Si0.3Sn0.7 solid solution (with a nanostructured modification) as a couple for the well studied thermoelectric material based on n-Mg2Si-Mg2Sn. A technological scheme for fabrication of heavily doped Mg2Si0.3Sn0.7 solid solution of p-type by hot pressing from nanopowder is developed. The given technology has made it possible to reduce duration of a homogeneous material fabrication and has improved its physical and chemical properties. The samples were made by three ways: direct fusion for polycrystals fabrication; hot pressing from microparticles; nanostructuring, i.e. hot pressing from nanoparticles. By X-ray diffraction it is shown that sizes of structural elements in the fabricated samples are about 40 nm. The probe technique is used for measurement of electric conductivity and Seebeck coefficient. The stationary absolute method is used for measurement of thermal conductivity. Thermoelectric figure of merit is defined by measured values of kinetic coefficients in the temperatures range of 77 – 800 K. It was demonstrated, that electric conductivity, Seebeck coefficient and the power factor do not depend practically on a way of solid solution preparation. Thermal conductivity of samples pressed from nanoparticles has appeared to be higher, than of samples, obtained by direct fusion; i.e. in this case nanostructuring has not led to increase of thermoelectric figure of merit. The conclusion is drawn, that polycrystalline semiconductor Mg2Si0.3Sn0.7 can be used as a p-branch for a thermoelectric generator though nanostructuring has not led to the figure of merit growth. The assumption is made, that thermoelectric figure of merit improvement can be expected at the further reduction of the nanograins size
Theory of Transmission through disordered superlattices
We derive a theory for transmission through disordered finite superlattices
in which the interface roughness scattering is treated by disorder averaging.
This procedure permits efficient calculation of the transmission thr ough
samples with large cross-sections. These calculations can be performed
utilizing either the Keldysh or the Landauer-B\"uttiker transmission
formalisms, both of which yield identical equations. For energies close to the
lowest miniband, we demonstrate the accuracy of the computationally efficient
Wannier-function approximation. Our calculations indicate that the transmission
is strongly affected by interface roughness and that information about scale
and size of the imperfections can be obtained from transmission data.Comment: 12 pages, 6 Figures included into the text. Final version with minor
changes. Accepted by Physical Review
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