296 research outputs found
Spectral and polarization dependencies of luminescence by hot carriers in graphene
The luminescence caused by the interband transitions of hot carriers in
graphene is considered theoretically. The dependencies of emission in mid- and
near-IR spectral regions versus energy and concentration of hot carriers are
analyzed; they are determined both by an applied electric field and a gate
voltage. The polarization dependency is determined by the angle between the
propagation direction and the normal to the graphene sheet. The characteristics
of radiation from large-scale-area samples of epitaxial graphene and from
microstructures of exfoliated graphene are considered. The averaged over angles
efficiency of emission is also presented.Comment: 6 pages, 5 figure
Least action principle for envelope functions in abrupt heterostructures
We apply the envelope function approach to abrupt heterostructures starting
with the least action principle for the microscopic wave function. The
interface is treated nonperturbatively, and our approach is applicable to
mismatched heterostructure. We obtain the interface connection rules for the
multiband envelope function and the short-range interface terms which consist
of two physically distinct contributions. The first one depends only on the
structure of the interface, and the second one is completely determined by the
bulk parameters. We discover new structure inversion asymmetry terms and new
magnetic energy terms important in spintronic applications.Comment: 4 pages, 1 figur
Transient magnetoconductivity of photoexcited electrons
Transient magnetotransport of two-dimensional electrons with
partially-inverted distribution excited by an ultrashort optical pulse is
studied theoretically. The time-dependent photoconductivity is calculated for
GaAs-based quantum wells by taking into account the relaxation of electron
distribution caused by non-elastic electron-phonon interaction and the
retardation of the response due to momentum relaxation and due to a finite
capacitance of the sample. We predict large-amplitude transient oscillations of
the current density and Hall field (Hall oscillations) with frequencies
corresponding to magnetoplasmon range, which are initiated by the instability
owing to the absolute negative conductivity effect.Comment: 21 pages, 6 fig
Thermal-radiation-induced nonequilibrium carriers in an intrinsic graphene
We examine an intrinsic graphene connected to the phonon thermostat at
temperature T under irradiation of thermal photons with temperature T_r, other
than T. The distribution of nonequilibrium electron-hole pairs was obtained for
the cases of low and high concentration of carriers. For the case when the
interparticle scattering is unessential, the distribution function is
determined by the interplay of intraband relaxation of energy due to acoustic
phonons and interband radiative transitions caused by the thermal radiation.
When the Coulomb scattering dominates, then the quasi-equilibrium distribution
with effective temperature and non-equilibrium concentration, determined
through balance equations, is realized. Due to the effect of thermal radiation
with temperature concentration and conductivity of carriers in
graphene modify essentially. It is demonstrated, that at the negative
interband absorption, caused by the inversion of carriers distribution, can
occur, i.e. graphene can be unstable under thermal irradiation.Comment: 5 pages, 4 figure
Frequency dependence of induced spin polarization and spin current in quantum wells
Dynamic response of two-dimensional electron systems with spin-orbit
interaction is studied theoretically on the basis of quantum kinetic equation,
taking into account elastic scattering of electrons. The spin polarization and
spin current induced by the applied electric field are calculated for the whole
class of electron systems described by p-linear spin-orbit Hamiltonians. The
absence of nonequilibrium intrinsic static spin currents is confirmed for these
systems with arbitrary (nonparabolic) electron energy spectrum. Relations
between the spin polarization, spin current, and electric current are
established. The general results are applied to the quantum wells grown in
[001] and [110] crystallographic directions, with both Rashba and Dresselhaus
types of spin-orbit coupling. It is shown that the existence of the fixed
(momentum-independent) precession axes in [001]-grown wells with equal Rashba
and Dresselhaus spin velocities or in symmetric [110]-grown wells leads to
vanishing spin polarizability at arbitrary frequency of the applied electric
field. This property is explained by the absence of Dyakonov-Perel-Kachorovskii
spin relaxation for the spins polarized along these precession axes. As a
result, a considerable frequency dispersion of spin polarization at very low
frequency in the vicinity of the fixed precession axes is predicted. Possible
effects of extrinsic spin-orbit coupling on the obtained results are discussed.Comment: 14 pages, 6 figures; published with minor corrections in Phys. Rev.
Electronic states in heterostructures formed by ultranarrow layers
Low-energy electronic states in heterosrtuctures formed by ultranarrow layer
(single or several monolayers thickness) are studied theoretically. The host
material is described within the effective mass approximation and effect of
ultranarrow layers is taken into account within the framework of the transfer
matrix approach. Using the current conservation requirement and the inversion
symmetry of ultranarrow layer, the transfer matrix is written through two
phenomenological parameters. The binding energy of localized state, the
reflection (transmission) coefficient for the single ultranarrow layer case,
and the energy spectrum of superlattice are determined by these parameters.
Spectral dependency of absorption in superlattice due to photoexcitation of
electrons from localized states into minibands is strongly dependent on the
ultranarrow layers characteristics. Such a dependency can be used for
verification of the transfer matrix parameters.Comment: 7 pages, 7 figure
Superlattice formed by quantum-dot sheets: density of states and IR absorption
Low-energy continuous states of electron in heterosrtucture with periodically
placed quantum-dot sheets are studied theoretically. The Green's function of
electron is governed by the Dyson equation with the self-energy function which
is determined the boundary conditions at quantum-dot sheets with weak damping
in low-energy region. The parameters of superlattice formed by quantum-dot
sheets are determined using of the short-range model of quantum dot. The
density of states and spectral dependencies of the anisotropic absorption
coefficient under mid-IR transitions from doped quantum dots into miniband
states of superlattice strongly depend on dot concentration and on period of
sheets. These dependencies can be used for characterization of the multi-layer
structure and they determine parameters of different optoelectronic devices
exploiting vertical transport of carriers through quantum-dot sheets.Comment: 7 pages and 5 figure
Transient quantum evolution of 2D electrons under photoexcitation of a deep center
We have considered the ballistic propagation of the 2D electron Wigner
distribution, which is excited by an ultrashort optical pulse from a
short-range impurity into the first quantized subband of a selectively-doped
heterostructure with high mobility. Transient ionization of a deep local state
into a continuum conduction c-band state is described. Since the quantum nature
of the photoexcitation, the Wigner distribution over 2D plane appears to be an
alternating-sign function. Due to a negative contribution to the Wigner
function, the mean values (concentration, energy, and flow) demonstrate an
oscillating transient evolution in contrast to the diffusive classical regime
of propagation.Comment: 8 pages, 6 figures, pape
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