52 research outputs found
Excitons and trions in monolayer transition metal dichalcogenides: A comparative study between the multiband model and the quadratic single-band model
The electronic and structural properties of excitons and trions in monolayer
transition metal dichalcogenides are investigated using both a multiband and a
single-band model. In the multiband model we construct the excitonic
Hamiltonian in the product base of the single-particle states at the conduction
and valence band edges. We decouple the corresponding energy eigenvalue
equation and solve the resulting differential equation self-consistently, using
the finite element method (FEM), to determine the energy eigenvalues and the
wave functions. As a comparison, we also consider the simple single-band model
which is often used in numerical studies. We solve the energy eigenvalue
equation using the FEM as well as with the stochastic variational method (SVM)
in which a variational wave function is expanded in a basis of a large number
of correlated Gaussians. We find good agreement between the results of both
methods, as well as with other theoretical works for excitons, and we also
compare with available experimental data. For trions the agreement between both
methods is not as good due to our neglect of angular correlations when using
the FEM. Finally, when comparing the two models, we see that the presence of
the valence bands in the mutiband model leads to differences with the
single-band model when (interband) interactions are strong.Comment: 14 pages, 11 figures, 3 table
Wigner crystallization in transition metal dichalcogenides: A new approach to correlation energy
We introduce a new approach for the correlation energy of one- and two-valley
two-dimensional electron gas (2DEG) systems. Our approach is based on a random
phase approximation at high densities and a classical approach at low
densities, with interpolation between the two limits. This approach gives
excellent agreement with available Quantum Monte Carlo (QMC) calculations. We
employ the two-valley 2DEG model to describe the electron correlations in
monolayer transition metal dichalcogenides (TMDs). The zero-temperature
transition from a Fermi liquid to a quantum Wigner crystal phase in monolayer
TMDs is obtained using density-functional theory within the local-density
approximation. Consistent with QMC, we find that electrons crystallize at
in one-valley 2DEG. For two-valleys, we predict Wigner
crystallization at , indicating that valley degeneracy has little
effect on the critical , in contrast to an earlier claim.Comment: 5 pages, 3 figure
Enhancement of electron-hole superfluidity in double few-layer graphene
We propose two coupled electron-hole sheets of few-layer graphene as a new
nanostructure to observe superfluidity at enhanced densities and enhanced
transition temperatures. For ABC stacked few-layer graphene we show that the
strongly correlated electron-hole pairing regime is readily accessible
experimentally using current technologies. We find for double trilayer and
quadlayer graphene sheets spatially separated by a nano-thick hexagonal
boron-nitride insulating barrier, that the transition temperature for
electron-hole superfluidity can approach temperatures of 40 K.Comment: 17 pages, 5 figure
Electron-electron interactions in bilayer graphene quantum dots
A parabolic quantum dot (QD) as realized by biasing nanostructured gates on
bilayer graphene is investigated in the presence of electron-electron
interaction. The energy spectrum and the phase diagram reveal unexpected
transitions as function of a magnetic field. For example, in contrast to
semiconductor QDs, we find a novel valley transition rather than only the usual
singlet-triplet transition in the ground state of the interacting system. The
origin of these new features can be traced to the valley degree of freedom in
bilayer graphene. These transitions have important consequences for cyclotron
resonance experiments.Comment: 5 pages, 5 figures, to appear in Phys. Rev.
Multiband Mechanism for the Sign Reversal of Coulomb Drag Observed in Double Bilayer Graphene Heterostructures
Coupled 2D sheets of electrons and holes are predicted to support novel
quantum phases. Two experiments of Coulomb drag in electron-hole (e-h) double
bilayer graphene (DBLG) have reported an unexplained and puzzling sign reversal
of the drag signal. However, we show that this effect is due to the multiband
character of DBLG. Our multiband Fermi liquid theory produces excellent
agreement and captures the key features of the experimental drag resistance for
all temperatures. This demonstrates the importance of multiband effects in
DBLG: they have a strong effect not only on superfluidity, but also on the
drag.Comment: 5 pages, 3 figure
Quantum transport across van der Waals domain walls in bilayer graphene
Bilayer graphene can exhibit deformations such that the two graphene sheets
are locally detached from each other resulting in a structure consisting of
domains with different inter-layer coupling. Here we investigate how the
presence of these domains affect the transport properties of bilayer graphene.
We derive analytical expressions for the transmission probability, and the
corresponding conductance, across walls separating different inter-layer
coupling domain. We find that the transmission can exhibit a valley-dependent
layer asymmetry and that the domain walls have a considerable effect on the
chiral tunnelling properties of the charge carriers. We show that transport
measurements allow one to obtain the strength with which the two layers are
coupled. We performed numerical calculations for systems with two domain walls
and find that the availability of multiple transport channels in bilayer
graphene modifies significantly the conductance dependence on inter-layer
potential asymmetry.Comment: 20 pages, 24 Figure
Snake states in graphene quantum dots in the presence of a p-n junction
We investigate the magnetic interface states of graphene quantum dots that
contain p-n junctions. Within a tight-binding approach, we consider rectangular
quantum dots in the presence of a perpendicular magnetic field containing p-n,
as well as p-n-p and n-p-n junctions. The results show the interplay between
the edge states associated with the zigzag terminations of the sample and the
snake states that arise at the p-n junction, due to the overlap between
electron and hole states at the potential interface. Remarkable localized
states are found at the crossing of the p-n junction with the zigzag edge
having a dumb-bell shaped electron distribution. The results are presented as
function of the junction parameters and the applied magnetic flux.Comment: 13 pages, 23 figures, to be appeared in Phys. Rev.
Strain-induced topological phase transition in phosphorene and phosphorene nanoribbons
Using the tight-binding (TB) approximation with inclusion of the spin-orbit
interaction, we predict a topological phase transition in the electronic band
structure of phosphorene in the presence of axial strains. We derive a
low-energy TB Hamiltonian that includes the spin-orbit interaction for bulk
phosphorene. Applying a compressive biaxial in-plane strain and perpendicular
tensile strain in ranges where the structure is still stable leads to a
topological phase transition. We also examine the influence of strain on zigzag
phosphorene nanoribbons (zPNRs) and the formation of the corresponding
protected edge states when the system is in the topological phase. For zPNRs up
to a width of 100 nm the energy gap is at least three orders of magnitude
larger than the thermal energy at room temperature.Comment: 10 pages, 6 figure
Simplified model for the energy levels of quantum rings in single layer and bilayer graphene
Within a minimal model, we present analytical expressions for the eigenstates
and eigenvalues of carriers confined in quantum rings in monolayer and bilayer
graphene. The calculations were performed in the context of the continuum
model, by solving the Dirac equation for a zero width ring geometry, i.e. by
freezing out the carrier radial motion. We include the effect of an external
magnetic field and show the appearance of Aharonov-Bohm oscillations and of a
non-zero gap in the spectrum. Our minimal model gives insight in the energy
spectrum of graphene-based quantum rings and models different aspects of finite
width rings.Comment: To appear in Phys. Rev.
Optimisation of quantum Monte Carlo wave function: steepest descent method
We have employed the steepest descent method to optimise the variational
ground state quantum Monte Carlo wave function for He, Li, Be, B and C atoms.
We have used both the direct energy minimisation and the variance minimisation
approaches. Our calculations show that in spite of receiving insufficient
attention, the steepest descent method can successfully minimise the wave
function. All the derivatives of the trial wave function respect to spatial
coordinates and variational parameters have been computed analytically. Our
ground state energies are in a very good agreement with those obtained with
diffusion quantum Monte Carlo method (DMC) and the exact results.Comment: 13 pages, 3 eps figure
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