163 research outputs found
Theory for entanglement of electrons dressed with circularly polarized light in Graphene and three-dimensional topological insulators
We have formulated a theory for investigating the conditions which are
required to achieve entangled states of electrons on graphene and
three-dimensional (3D) topological insulators (TIs). We consider the quantum
entanglement of spins by calculating the exchange energy. A gap is opened up at
the Fermi level between the valence and conduction bands in the absence of
doping when graphene as well as 3D TIs are irradiated with circularly-polarized
light. This energy band gap is dependent on the intensity and frequency of the
applied electromagnetic field. The electron-photon coupling also gives rise to
a unique energy dispersion of the dressed states which is different from either
graphene or the conventional two-dimensional electron gas (2DEG). In our
calculations, we obtained the dynamical polarization function for imaginary
frequencies which is then employed to determine the exchange energy. The
polarization function is obtained with the use of both the energy eigenstates
and the overlap of pseudo-spin wave functions. We have concluded that while
doping has a significant influence on the exchange energy and consequently on
the entanglement, the gap of the energy dispersions affects the exchange
slightly, which could be used as a good technique to tune and control
entanglement for quantum information purposes
Enhanced response of current-driven coupled quantum wells
We have investigated the conditions necessary to achieve stronger
Cherenkov-like instability of plasma waves leading to emission in the terahertz
(THz) regime for semiconductor quantum wells (QWs). The surface response
function is calculated for a bilayer two-dimensional electron gas (2DEG) system
in the presence of a periodic spatial modulation of the equilibrium electron
density. The 2DEG layers are coupled to surface plasmons arising from
excitations of free carriers in the bulk region between the layers. A current
is passed through one of the layers and is characterized by a drift velocity
for the driven electric charge. By means of a surface response function
formalism, the plasmon dispersion equation is obtained as a function of angular
frequency, the in-plane wave vector and reciprocal lattice vector of the
density modulation. The dispersion equation,is solved numerically in the
complex frequency plane for real wave vector. It is ascertained that the
imaginary part of the angular frequency is enhanced with decreasing period of
modulation, and with increasing the doping density of the free carriers in the
bulk medium for fixed period of the spatial modulation
Comparison of inelastic and quasi-elastic scattering effects on nonlinear electron transport in quantum wires
When impurity and phonon scattering coexist, the Boltzmann equation has been
solved accurately for nonlinear electron transport in a quantum wire. Based on
the calculated non-equilibrium distribution of electrons in momentum space, the
scattering effects on both the non-differential (for a fixed dc field) and
differential (for a fixed temperature) mobilities of electrons as functions of
temperature and dc field were demonstrated. The non-differential mobility of
electrons is switched from a linearly increasing function of temperature to a
parabolic-like temperature dependence as the quantum wire is tuned from an
impurity-dominated system to a phonon-dominated one [see T. Fang, {\em et al.},
\prb {\bf 78}, 205403 (2008)]. In addition, a maximum has been obtained in the
dc-field dependence of the differential mobility of electrons. The low-field
differential mobility is dominated by the impurity scattering, whereas the
high-field differential mobility is limited by the phonon scattering [see M.
Hauser, {\em et al.}, Semicond. Sci. Technol. {\bf 9}, 951 (1994)]. Once a
quantum wire is dominated by quasi-elastic scattering, the peak of the
momentum-space distribution function becomes sharpened and both tails of the
equilibrium electron distribution centered at the Fermi edges are raised by the
dc field after a redistribution of the electrons is fulfilled in a symmetric
way in the low-field regime. If a quantum wire is dominated by inelastic
scattering, on the other hand, the peak of the momentum-space distribution
function is unchanged while both shoulders centered at the Fermi edges shift
leftward correspondingly with increasing dc field through an asymmetric
redistribution of the electrons even in low-field regime [see C. Wirner, {\em
et al.}, \prl {\bf 70}, 2609 (1993)]
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