482 research outputs found
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
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
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.
Intrinsic optical conductivity of modified-Dirac fermion systems
We analytically calculate the intrinsic longitudinal and transverse optical
conductivities of electronic systems which govern by a modified-Dirac fermion
model Hamiltonian for materials beyond graphene such as monolayer MoS and
ultrathin film of the topological insulator. We analyze the effect of a
topological term in the Hamiltonian on the optical conductivity and
transmittance. We show that the optical response enhances in the non-trivial
phase of the ultrathin film of the topological insulator and the optical Hall
conductivity changes sign at transition from trivial to non-trivial phases
which has significant consequences on a circular polarization and optical
absorption of the system.Comment: 14 pages, 9 figures. To appear in Phys. Rev.
Indirect exchange interaction between magnetic adatoms in a monolayer MoS2
We study the Ruderman-Kittle-Kasuya-Yosida (RKKY) interaction in a monolayer
MoS. We show that the rotation of the itinerant electron spin due to the
spin-orbit coupling causes a twisted interaction between two magnetic adatoms
which consists of different RKKY coupling terms, the Heisenberg,
Dzyaloshinsky-Moriya and Ising interactions. We find that the interaction terms
are very sensitive to the Fermi energy values and change dramatically from
doped to undoped systems. A finite doping causes that all parts of the
interaction oscillate with the distance of two magnetic impurities, and the
interaction behaves like for the long distance between two localized
spins. We explore a beating pattern of oscillations of the RKKY interaction
which occurs for the doped system.Comment: Published version. To appear in Phys. Rev.
Andreev reflection in monolayer MoS2
Andreev reflection in a monolayer molybdenum disulfide superconducting-normal
(S/N) hybrid junction is investigated. We find, by using a modified-Dirac
Hamiltonian and the scattering formalism, that the perfect Andreev reflection
happens at normal incidence with -doped S and N regions. The probability of
the Andreev reflection and the resulting Andreev conductance, in this system,
are demonstrated to be large in comparison with corresponding gapped graphene
structure. We further investigate the effect of a topological term ( in
the Hamiltonian and show that it results in an enhancement of the Andreev
conductance with -doped S and N regions, while in the corresponding
structure with -doped S region it is strongly reducible in comparison. This
effect can be explained in terms of the dependence of the Andreev reflection
probability on the sign of and the chemical potential in the
superconducting region.Comment: 8 pages, 6 figures, accepted for publication in Phys. Rev.
Edge modes in zigzag and armchair ribbons of monolayer MoS
We explore the electronic structure, orbital character and topological aspect
of a monolayer MoS nanoribbon using tight-binding (TB) and low-energy
() models. We obtain a mid-gap edge mode in the zigzag
ribbon of monolayer MoS, which can be traced back to the topological
properties of the bulk band structure. Monolayer MoS can be considered as a
valley Hall insulator. The boundary conditions at armchair edges mix the
valleys on the edges, and a gap is induced in the edge modes. The spin-orbit
coupling in the valence band reduces the hybridization of the bulk states.Comment: 28 pages, 13 figures, Published:J. Phys.: Condens. Matter 28 (2016)
49500
Effective lattice Hamiltonian for monolayer MoS2 : Tailoring electronic structure with perpendicular electric and magnetic fields
We propose an effective lattice Hamiltonian for monolayer MoS in order to
describe the low-energy band structure and investigate the effect of
perpendicular electric and magnetic fields on its electronic structure. We
derive a tight-binding model based on the hybridization of the orbitals of
molybdenum and orbitals of sulfur atoms and then introduce a modified
two-band continuum model of monolayer MoS by exploiting the
quasi-degenerate partitioning method. Our theory proves that the low-energy
excitations of the system are no longer massive Dirac fermions. It reveals a
difference between electron and hole masses and provides trigonal warping
effects. Furthermore, we predict a valley degeneracy breaking effect in the
Landau levels. Besides, we also show that applying a gate voltage perpendicular
to the monolayer modifies the electronic structure including the band gap and
effective masses.Comment: 7 pages, 3 figures, To appear in PR
Charge compressibility and quantum magnetic phase transition in MoS
We investigate the ground-state properties of monolayer MoS incorporating
the Coulomb interaction together with a short-range intervalley interaction
between charged particles between two valleys within the Hartree-Fock
approximation. We consider four variables as independent parameters, namely
homogeneous charge (electron or hole) density, averaged dielectric constant,
spin degree of freedom and finally the Hubbard repulsion coefficient which
originates mostly from orbits of Mo atoms. We find the electronic charge
compressibility within the mean-field approximation and show that non-monotonic
behavior of the compressibility as a function of carrier density which is
rather different from those of the two-dimensional electron gas. We also
explore a paramagnetic-to-ferromagnetic quantum phase transition for the wide
range of the electron density in the parameter space.Comment: 9 pages, 4 figure
Theory of strain in single-layer transition metal dichalcogenides
Strain engineering has emerged as a powerful tool to modify the optical and
electronic properties of two-dimensional crystals. Here we perform a systematic
study of strained semiconducting transition metal dichalcogenides. The effect
of strain is considered within a full Slater-Koster tight-binding model, which
provides us with the band structure in the whole Brillouin zone. From this, we
derive an effective low-energy model valid around the K point of the BZ, which
includes terms up to second order in momentum and strain. For a generic profile
of strain, we show that the solutions for this model can be expressed in terms
of the harmonic oscillator and double quantum well models, for the valence and
conduction bands respectively. We further study the shift of the position of
the electron and hole band edges due to uniform strain. Finally, we discuss the
importance of spin-strain coupling in these 2D semiconducting materials
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