369 research outputs found
Valley Zeeman effect and spin-valley polarized conductance in monolayer MoS in a perpendicular magnetic field
We study the effect of a perpendicular magnetic field on the electronic
structure and charge transport of a monolayer MoS nanoribbon at zero
temperature. We particularly explore the induced valley Zeeman effect through
the coupling between the magnetic field, , and the orbital magnetic moment.
We show that the effective two-band Hamiltonian provides a mismatch between the
valley Zeeman coupling in the conduction and valence bands due to the effective
mass asymmetry and it is proportional to similar to the diamagnetic shift
of exciton binding energies. However, the dominant term which evolves with
linearly, originates from the multi-orbital and multi-band structures of the
system. Besides, we investigate the transport properties of the system by
calculating the spin-valley resolved conductance and show that, in a low-hole
doped case, the transport channels at the edge are chiral for one of the spin
components. This leads to a localization of the non-chiral spin component in
the presence of disorder and thus provides a spin-valley polarized transport
induced by disorder.Comment: 12 pages, 7 figures, new references are adde
Phosphorene as a nanoelectromechanical material
Based on density functional simulations combined with the Landauer transport
theory, the mechanical strain impacts on the chemical bonds of phosphorene and
their effects on the electronic properties are studied. Moreover, the effect of
the tensile strain along the zigzag direction on the charge transport
properties of a two-terminal phosphorene device is evaluated. Enhancement of
the intraplanar interactions, in particular between the next-nearest neighbors
in strained phosphorene, is found to be essential in the band-structure
evolution. The charge transport analyzing shows that phosphorene has a strong
piezoconductance sensitivity, which makes this material highly desirable for
high-pressure nanoelectromechanical applications. The piezoconductance gauge
factor increases by strain from 46 in 5% tension to 220 in 12% tension, which
is comparable to state-of-the-art silicon strain sensors. The transmission
pathways monitor the current flowing in terms of the chemical bonds and
hopping, however, the transport mostly arises from the charge transferring
through the chemical bonds. The strong anisotropy in the transport properties
along zigzag and armchair directions is observed
Electronic ground state properties of strained graphene
We consider the effect of the Coulomb interaction in strained graphene using
tight-binding approximation together with the Hartree-Fock interactions. The
many-body energy dispersion relation, anisotropic Fermi velocity
renormalization and charge compressibility in the presence of uniaxial strain
are calculated. We show that the quasiparticle quantities are sensitive to
homogenous strain and indeed, to its sign. The charge compressibility is
enhanced by stretching and suppressed by compressing a graphene sheet. We find
a reduction of Fermi velocity renormalization along the direction of graphene
deformation, in good agreement with the recent experimental observation.Comment: 19 pages, 6 figures. To appear in Phys. Rev.
A Model of f(R) Gravity as an Alternative for Dark Matter in Spiral Galaxies
In this paper we study consistent solutions of spherically symmetric space in
metric f(R) gravity theory. Here we inversely obtain a generic action from
metric solutions that describe flat rotation curves in spiral galaxies without
dark matter. Then we show that obtained solutions are in conformity with
Tully-Fisher relation and modified Newtonian dynamics, which are two strong
constraints in justification of flat rotation curves in spiral galaxies.Comment: 4 pages, no figure, PD
Charge transport in doped zigzag phosphorene nanoribbons
The effects of lattice distortion and chemical disorder on charge transport
properties of two-terminal zigzag phosphorene nanoribbons (zPNRs), which shows
resonant tunneling behavior under an electrical applied bias, are studied. Our
comprehensive study is based on an {\it ab~initio} quantum transport
calculations on the basis of the Landauer theory. We use nitrogen and silicon
substitutional dopant atoms, and employ different physical quantities such as
curve, voltage drop behavior, transmission spectrum, transmission
pathway, and atomic current, to explore the transport mechanism of zPNR devices
under a bias voltage. The calculated transmission pathways show the transition
from a ballistic transport regime to a diffusive and in some particular cases
to localized transport regimes. Current flowing via the chemical bonds and
hopping are monitored, however, the conductance originates mainly from a charge
traveling through the chemical bonds in the vicinity of the zigzag edges. Our
results show that, in the doped systems, the device conductance decreases and
negative differential resistance characteristic becomes weak or eliminates.
Besides, the conductance in a pure zPNR system is almost independent of the
ribbon width
Electronic structure and layer-resolved transmission of bilayer graphene nanoribbon in the presence of vertical fields
Electronic properties of bilayer graphene are distinct from both the
conventional two dimensional electron gas and monolayer graphene due to its
particular chiral properties and excitation charge carrier dispersions. We
study the effect of strain on the electronic structure, the edge-states and
charge transport of bilayer graphene nanoribbon at zero-temperature. We
demonstrate a valley polarized quantum Hall effect in biased bilayer graphene
when the system is subjected to a perpendicular magnetic field. In this system
a topological phase transition from a quantum valley Hall to a valley polarized
quantum Hall phase can occur by tuning the interplanar strain. Furthermore, we
study the layer-resolved transport properties by calculating the layer
polarized quantity by using the recursive Green's function technique and show
that the resulting layer polarized value confirms the obtained phases. These
predictions can be verified by experiments and our results demonstrate the
possibility for exploiting strained bilayer graphene in the presence of
external fields for electronics and valleytronics devices.Comment: 10 pages, 9 figures, typos are correcte
Specular Andreev reflection in thin films of topological insulators
We theoretically reveal the possibility of specular Andreev reflection in a
thin film topological insulator normal-superconductor (N/S) junction in the
presence of a gate electric field. The probability of specular Andreev
reflection increases with the electric field, and electron-hole conversion with
unit efficiency happens in a wide experimentally accessible range of the
electric field. We show that perfect specular Andreev reflection can occur for
all angles of incidence with a particular excitation energy value. In addition,
we find that the thermal conductance of the structure displays exponential
dependence on the temperature. Our results reveal the potential of the proposed
topological insulator thin-film-based N/S structure for the realization of
intraband specular Andreev reflection.Comment: 10 pages, 9 figures. Some typos are correcte
Excitons and optical spectra of phosphorene nanoribbons
On the basis of many-body {\it ab-initio} calculations, using single-shot
GW method and Bethe-Salpeter equation, we study phosphorene nanoribbons
(PNRs) in the two typical zigzag and armchair directions. The electronic
structure, optical absorption, electron-hole (exciton) binding energy, exciton
exchange splitting, and exciton wave functions are calculated for different
size of PNRs. The typically strong splitting between singlet and triplet
excitonic states make PNRs favorable systems for application in optoelectronic.
Quantum confinement occurs in both kinds of PNRs, and it is stronger in the
zPNRs, as behave like quasi-zero-dimensional systems. Scaling laws are
investigated for the size-dependent behaviors of PNRs. The first bright
excitonic state in PNRs is explored in detail.Comment: 9 pages, 11 figure
A Deep Dive into f(R) Gravity Theory
In this paper we have derived the behavior of deceleration parameter with
respect to redshift in context of f(R) gravity in vacuum using Taylor expansion
of derivative of action. Here we have obtained that the two first terms in
Taylor expansion may describe the late time acceleration which is appeared by
SNeIa without need of dark energy and dark matter. Also we have derived that
any other terms higher than z in Taylor expansion may describe main
inflationary epoch in the early Universe. We have shown that f(R) gravity may
cover all the dynamical history of the Universe from the beginning to the late
time accelerating phase transition.Comment: 10 pages, 6 figures, Eq. (4) is corrected, comments welcom
Vacuum Solution of a Linear Red-Shift Based Correction in Gravity
In this paper we have considered a red-shift based linear correction in
derivative of action in the context of vacuum gravity. Here we have
found out that the linear correction may describe the late time acceleration
which is appeared by SNeIa with no need of dark energy. Also we have calculated
the asymptotic action for the desired correction. The value of all solutions
may reduce to de' Sitter universe in the absence of correction term.Comment: Revised version, 18 pages, 7 figure
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