557 research outputs found
Strain and band-mixing effects on the excitonic Aharonov-Bohm effect in In(Ga)As/GaAs ringlike quantum dots
Neutral excitons in strained axially symmetric In(Ga)As/GaAs quantum dots
with ringlike shape are investigated. Similar to experimental self-assembled
quantum rings, the analyzed quantum dots have volcano-like shapes. The
continuum mechanical model is employed to determine the strain distribution,
and the single-band envelope function approach is adopted to compute the
electron states. The hole states are determined by the axially symmetric
multiband Luttinger-Kohn Hamiltonian, and the exciton states are obtained from
an exact diagonalization. We found that the presence of the inner layer
covering the ring opening enhances the excitonic Aharonov-Bohm (AB)
oscillations. The reason is that the hole becomes mainly localized in the inner
part of the quantum dot due to strain, whereas the electron resides mainly
inside the ring-shaped rim. Interestingly, larger AB oscillations are found in
the analyzed quantum dot than in a fully opened quantum ring of the same width.
Comparison with the unstrained ring-like quantum dot shows that the amplitude
of the excitonic Aharonov-Bohm oscillations are almost doubled in the presence
of strain. The computed oscillations of the exciton energy levels are
comparable in magnitude to the oscillations measured in recent experiments.Comment: 16 pages, 9 figures, accepted for publication in Physical Review
Adsorption and absorption of Boron, Nitrogen, Aluminium and Phosphorus on Silicene: stability, electronic and phonon properties
Ab initio calculations within the density-functional theory formalism are
performed to investigate the chemical functionalization of a graphene-like
monolayer of silicon - silicene - with B, N, Al or P atoms. The structural,
electronic, magnetic and vibrational properties are reported. The most
preferable adsorption sites are found to be valley, bridge, valley and hill
site for B, N, Al and P adatoms, respectively. All the relaxed systems with
adsorbed/substituted atoms exhibit metallic behaviour with strongly bonded B,
N, Al, and P atoms accompanied by an appreciable electron transfer from
silicene to the B, N and P adatom/substituent. The Al atoms exhibit opposite
charge transfer, with n-type doping of silicene and weaker bonding. The
adatoms/substituents induce characteristic branches in the phonon spectrum of
silicene, which can be probed by Raman measurements. Using molecular dynamics
we found that the systems under study are stable up to at least T = 500 K. Our
results demonstrate that silicene has a very reactive and functionalizable
surface.Comment: 9 pages, 5 figure
Orbital magnetic moments in insulating Dirac systems: Impact on magnetotransport in graphene van der Waals heterostructures
In honeycomb Dirac systems with broken inversion symmetry, orbital magnetic
moments coupled to the valley degree of freedom arise due to the topology of
the band structure, leading to valley-selective optical dichroism. On the other
hand, in Dirac systems with prominent spin-orbit coupling, similar orbital
magnetic moments emerge as well. These moments are coupled to spin, but
otherwise have the same functional form as the moments stemming from spatial
inversion breaking. After reviewing the basic properties of these moments,
which are relevant for a whole set of newly discovered materials, such as
silicene and germanene, we study the particular impact that these moments have
on graphene nanoengineered barriers with artificially enhanced spin-orbit
coupling. We examine transmission properties of such barriers in the presence
of a magnetic field. The orbital moments are found to manifest in transport
characteristics through spin-dependent transmission and conductance, making
them directly accessible in experiments. Moreover, the Zeeman-type effects
appear without explicitly incorporating the Zeeman term in the models, i.e., by
using minimal coupling and Peierls substitution in continuum and the
tight-binding methods, respectively. We find that a quasiclassical view is able
to explain all the observed phenomena
Spin-valley filtering in strained graphene structures with artificially induced carrier mass and spin-orbit coupling
The interplay of massive electrons with spin-orbit coupling in bulk graphene
results in a spin-valley dependent gap. Thus, a barrier with such properties
can act as a filter, transmitting only opposite spins from opposite valleys. In
this Letter we show that strain induced pseudomagnetic field in such a barrier
will enforce opposite cyclotron trajectories for the filtered valleys, leading
to their spatial separation. Since spin is coupled to the valley in the
filtered states, this also leads to spin separation, demonstrating a
spin-valley filtering effect. The filtering behavior is found to be
controllable by electrical gating as well as by strain
Electron pairing: from metastable electron pair to bipolaron
Starting from the shell structure in atoms and the significant correlation
within electron pairs, we distinguish the exchange-correlation effects between
two electrons of opposite spins occupying the same orbital from the average
correlation among many electrons in a crystal. In the periodic potential of the
crystal with lattice constant larger than the effective Bohr radius of the
valence electrons, these correlated electron pairs can form a metastable energy
band above the corresponding single-electron band separated by an energy gap.
In order to determine if these metastable electron pairs can be stabilized, we
calculate the many-electron exchange-correlation renormalization and the
polaron correction to the two-band system with single electrons and electron
pairs. We find that the electron-phonon interaction is essential to
counterbalance the Coulomb repulsion and to stabilize the electron pairs. The
interplay of the electron-electron and electron-phonon interactions, manifested
in the exchange-correlation energies, polaron effects, and screening, is
responsible for the formation of electron pairs (bipolarons) that are located
on the Fermi surface of the single-electron band.Comment: 17 pages, 6 figures, Journal of Physics Communications 201
Tuning of the electronic and optical properties of single layer black phosphorus by strain
Using first principles calculations we showed that the electronic and optical
properties of single layer black phosphorus (BP) depend strongly on the applied
strain. Due to the strong anisotropic atomic structure of BP, its electronic
conductivity and optical response are sensitive to the magnitude and the
orientation of the applied strain. We found that the inclusion of many body
effects is essential for the correct description of the electronic properties
of monolayer BP; for example while the electronic gap of strainless BP is found
to be 0.90 eV by using semilocal functionals, it becomes 2.31 eV when many-body
effects are taken into account within the G0W0 scheme. Applied tensile strain
was shown to significantly enhances electron transport along zigzag direction
of BP. Furthermore, biaxial strain is able to tune the optical band gap of
monolayer BP from 0.38 eV (at -8% strain) to 2.07 eV (at 5.5%). The exciton
binding energy is also sensitive to the magnitude of the applied strain. It is
found to be 0.40 eV for compressive biaxial strain of -8%, and it becomes 0.83
eV for tensile strain of 4%. Our calculations demonstrate that the optical
response of BP can be significantly tuned using strain engineering which
appears as a promising way to design novel photovoltaic devices that capture a
broad range of solar spectrum
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