arXiv subjects

F. M. Peeters

Publications and source records attributed to F. M. Peeters.

At least 91 records · Page 5Linked to original sources

arXiv2018

Comment on "Impurity spectra of graphene under electric and magnetic fields"

In a recent publication [Phys. Rev. B $\textbf{89}$, 155403 (2014)], the authors investigated the spectrum of a Coulomb impurity in graphene in the presence of magnetic and electric fields using the coupled series expansion approach. In the first part of their publication they investigated how Coulomb impurity states collapse in the presence of a perpendicular magnetic field. We argue that the obtained spectrum does not give information about the atomic collapse and that their interpretation of the spectrum regarding atomic collapse is not correct. We also argue that the obtained results are only valid up to the dimensionless charge $\mid α\mid=0.5$ and in order to obtain correct results for $α>0.5$ a proper regularisation of the Coulomb interaction is required. Here we present the correct numerical results for the spectrum for arbitrary values of $α$.

cond-mat.mes-hall↗

arXiv2018

Ballistic electron channels including weakly protected topological states in delaminated bilayer graphene

We show that delaminations in bilayer graphene (BLG) with electrostatically induced interlayer asymmetry can provide one with ballistic channels for electrons with energies inside the electrostatically induced BLG gap. These channels are formed by a combination of valley-polarised evanescent states propagating along the delamination edges (which persist in the presence of a strong magnetic field) and standing waves bouncing between them inside the delaminated region (in a strong magnetic field, these transform into Landau levels in the monolayers). For inverted stacking between BLGs on the left and right of the delamination (AB-2ML-BA or BA-2ML-AB), the lowest energy ballistic channels are gapless, have linear dispersion and appear to be weakly topologically protected. When BLG stacking order on both sides of the delamination is the same (AB-2ML-AB or BA-2ML-BA), the lowest energy ballistic channels are gapped, with gap $\varepsilon_g$ scaling as $\varepsilon_g\propto W^{-1}$ with delamination width and as $\varepsilon_g\proptoδ^{-1}$ with the on-layer energy difference within the delamination. Depending on their width, delaminations may also support several `higher energy' waveguide modes. Our results are based on both an analytical study of the wavematching of Dirac states and tight binding model calculations, and we analyse in detail the dependence of the delamination spectrum on electrostatic conditions in the structure, such as the vertical displacement field.

cond-mat.mes-hall↗

arXiv2018

DC conductivity of twisted bilayer graphene: Angle-dependent transport properties and effects of disorder

The in-plane DC conductivity of twisted bilayer graphene (TBLG) is calculated using an expansion of the real-space Kubo-Bastin conductivity in terms of Chebyshev polynomials. We investigate within a tight-binding (TB) approach the transport properties as a function of rotation angle, applied perpendicular electric field and vacancy disorder. We find that for high-angle twists, the two layers are effectively decoupled, and the minimum conductivity at the Dirac point corresponds to double the value observed in monolayer graphene. This remains valid even in the presence of vacancies, hinting that chiral symmetry is still preserved. On the contrary, for low twist angles, the conductivity at the Dirac point depends on the twist angle and is not protected in the presence of disorder. Furthermore, for low angles and in the presence of an applied electric field, we find that the chiral boundary states emerging between AB and BA regions contribute to the DC conductivity, despite the appearance of strongly localized states in the AA regions. The results agree with recent conductivity experiments on twisted bilayer graphene.

cond-mat.mes-hall↗

arXiv2017

Electrostatics of electron-hole interactions in van der Waals heterostructures

The role of dielectric screening of electron-hole interaction in van der Waals heterostructures is theoretically investigated. A comparison between models available in the literature for describing these interactions is made and the limitations of these approaches are discussed. A simple numerical solution of Poissons equation for a stack of dielectric slabs based on a transfer matrix method is developed, enabling the calculation of the electron-hole interaction potential at very low computational cost and with reasonable accuracy. Using different potential models, direct and indirect exciton binding energies in these systems are calculated within Wannier-Mott theory, and a comparison of theoretical results with recent experiments on excitons in two-dimensional materials is discussed.

cond-mat.mtrl-sci↗

arXiv2017

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.

cond-mat.mes-hall↗

arXiv2017

Electronic properties of bilayer phosphorene quantum dots in the presence of perpendicular electric and magnetic fields

Using the tight-binding approach, we investigate the electronic properties of bilayer phosphorene (BLP) quantum dots (QDs) in the presence of perpendicular electric and magnetic fields. Since BLP consists of two coupled phosphorene layers, it is of interest to examine the layer-dependent electronic properties of BLP QDs, such as the electronic distributions over the two layers and the so-produced layer-polarization features, and to see how these properties are affected by the magnetic field and the bias potential. We find that in the absence of a bias potential only edge states are layer-polarized while the bulk states are not, and the layer-polarization degree (LPD) of the unbiased edge states increases with increasing magnetic field. However, in the presence of a bias potential both the edge and bulk states are layer-polarized, and the LPD of the bulk (edge) states depends strongly (weakly) on the interplay of the bias potential and the interlayer coupling. At high magnetic fields, applying a bias potential renders the bulk electrons in a BLP QD to be mainly distributed over the top or bottom layer, resulting in layer-polarized bulk Landau levels (LLs). In the presence of a large bias potential that can drive a semiconductor-to-semimetal transition in BLP, these bulk LLs exhibit different magnetic-field dependences, i.e., the zeroth LLs exhibit a linear-like dependence on the magnetic field while the other LLs exhibit a square-root-like dependence.

cond-mat.mes-hall↗

arXiv2017

Multi-component plasmons in monolayer MoS$_2$ with circularly polarized optical pumping

By making use of circularly polarized light and electrostatic gating, monolayer molybdenum disulfide (ML-MoS$_2$) can form a platform supporting multiple types of charge carriers. They can be discriminated by their spin, valley index or whether they're electrons or holes. We investigate the collective properties of those charge carriers and are able to identify new distinct plasmon modes. We analyze the corresponding dispersion relation, lifetime and oscillator strength, and calculate the phase relation between the oscillations in the different components of the plasmon modes. All platforms in ML-MoS$_2$ support a long-wavelength $\sqrt{q}$ plasmon branch at zero Kelvin. In addition to this, for an $n$-component system, $n-1$ distinct plasmon modes appear as acoustic modes with linear dispersion in the long-wavelength limit. These modes correspond to out-of-phase oscillations in the different Fermion liquids and have, although being damped, a relatively long lifetime. Additionally, we also find new distinct modes at large wave vector that are stronger damped by intra-band processes.

cond-mat.mes-hall↗

arXiv2017

Graphene membrane as a pressure gauge

Straining graphene results in the appearance of a pseudo-magnetic field which alters its local electronic properties. Applying a pressure difference between the two sides of the membrane causes it to bend/bulge resulting in a resistance change. We find that the resistance changes linearly with pressure for bubbles of small radius while the response becomes non-linear for bubbles that stretch almost to the edges of the sample. This is explained as due to the strong interference of propagating electronic modes inside the bubble. Our calculations show that high gauge factors can be obtained in this way which makes graphene a good candidate for pressure sensing.

cond-mat.mes-hall↗

arXiv2017

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.

cond-mat.mes-hall↗

arXiv2017

A tight-binding investigation of biaxial strain induced topological phase transition in GeCH$_3$

We propose a tight-binding (TB) model, that includes spin-orbit coupling (SOC), to describe the electronic properties of methyl-substituted germanane (GeCH$_3$). This model gives an electronic spectrum in agreement with first principle results close to the Fermi level. Using the $\mathbb{Z}_2$ formalism, we show that a topological phase transition from a normal insulator (NI) to a quantum spin Hall (QSH) phase occurs at 11.6\% biaxial tensile strain. The sensitivity of the electronic properties of this system on strain, in particular its transition to the topological insulating phase, makes it very attractive for applications in strain sensors and other microelectronic applications.

cond-mat.mtrl-sci↗

arXiv2017

Double quantum dots defined in bilayer graphene

Artificial molecular states of double quantum dots defined in bilayer graphene are studied with the atomistic tight-binding and its low-energy continuum approximation. We indicate that the extended electron wave functions have opposite parities on each of the sublattices at both graphene layers and that the ground-state wave function components change from bonding to antibonding with the interdot distance. In the weak coupling limit -- the most relevant for the quantum dots defined electrostatically -- the signatures of the interdot coupling include -- for the two-electron ground state -- formation of states with symmetric or antisymmetric spatial wave functions split by the exchange energy. In the high energy part of the spectrum the states with both electrons in the same dot are found with the splitting of energy levels corresponding to simultaneous tunneling of the electron pair from one dot to the other.

cond-mat.mes-hall↗

arXiv2017

Multicomponent electron-hole superfluidity and the BCS-BEC crossover in double bilayer graphene

Superfluidity in coupled electron-hole sheets of bilayer graphene is predicted here to be multicomponent because of the conduction and valence bands. We investigate the superfluid crossover properties as functions of the tunable carrier densities and the tunable energy band gap $E_g$. For small band gaps there is a significant boost in the two superfluid gaps, but the interaction driven excitations from the valence to the conduction band can weaken the superfluidity, even blocking the system from entering the BEC regime at low densities. At a given larger density, a band gap $E_g\sim 40$-$60$ meV can carry the system into the strong-pairing multiband BCS-BEC crossover regime, the optimal range for realization of high-$T_c$ superfluidity.

cond-mat.supr-con↗

arXiv2017

Strong anisotropic optical conductivity in two dimensional puckered structures: The role of Rashba effect

We calculate the optical conductivity of an anisotropic two-dimensional system with Rashba spin-flip excitation within the Kubo formalism. We show that the anisotropic Rashba effect caused by an external field changes significantly the magnitude of the spin splitting. Furthermore, we obtain an analytical expression for the longitudinal optical conductivity associated with inter-band transitions as a function of the frequency for an arbitrary polarization angle. We find that the diagonal components of the optical conductivity tensor are direction-dependent and the spectrum of optical absorption is strongly anisotropic with an absorption window. The height and width of this absorption window are very sensitive to the system anisotropy. While the height of absorption peak increases with increasing effective mass anisotropy ratio, the peak intensity is larger when the light polarization is along the armchair direction. Moreover, the absorption peak width becomes broader as the density of state mass or Rashba interaction is enhanced. These features can be used to determine parameters relevant for spintronics through the optical absorption spectrum.

cond-mat.mes-hall↗

arXiv2017

Piezoelectricity in two-dimensional materials: a comparative study between lattice dynamics and ab-initio calculations

Elastic constant C_{11} and piezoelectric stress constant e_{1,11} of two-dimensional (2D) dielectric materials comprising h-BN, 2H MoS2 and other transition metal dichalcogenides (TMDCs) and -dioxides (TMDOs) are calculated using lattice dynamical theory. The results are compared with corresponding quantities obtained by ab-initio calculations. We identify the difference between clamped-ion and relaxed-ion contributions with the dependence on inner strains which are due to the relative displacements of the ions in the unit cell. Lattice dynamics allows to express the inner strains contributions in terms of microscopic quantities such as effective ionic charges and optoacoustical couplings, which allows us to clarify differences in the piezoelectric behavior between h- BN versus MoS2. Trends in the different microscopic quantities as functions of atomic composition are discussed.

cond-mat.mtrl-sci↗

arXiv2017

Quantum transport in graphene Hall bars: Effects of side gates

Quantum electron transport in side-gated graphene Hall bars is investigated in the presence of quantizing external magnetic fields. The asymmetric potential of four side-gates distorts the otherwise flat bands of the relativistic Landau levels, and creates new propagating states in the Landau spectrum (i.e. snake states). The existence of these new states leads to an interesting modification of the bend and Hall resistances, with new quantizing plateaus appearing in close proximity of the Landau levels. The electron guiding in this system can be understood by studying the current density profiles of the incoming and outgoing modes. From the fact that guided electrons fully transmit without any backscattering (similarly to edge states), we are able to analytically predict the values of quantized resistances, and they match the resistance data we obtain with our numerical (tight-binding) method. These insights in the electron guiding will be useful in predicting the resistances for other side-gate configurations, and possibly in other system geometries, as long as there is no backscattering of the guided states.

cond-mat.mes-hall↗

arXiv2017

Scanning gate microscopy of magnetic focusing in graphene devices: quantum vs. classical simulation

We compare classical versus quantum electron transport in recently investigated magnetic focusing devices [S. Bhandari et al., Nano Lett. 16, 1690 (2016)] exposed to the perturbing potential of a scanning gate microscope (SGM). Using the Landauer-Büttiker formalism for a multi-terminal device, we calculate resistance maps that are obtained as the SGM tip is scanned over the sample. There are three unique regimes in which the scanning tip can operate (focusing, repelling, and mixed regime) which are investigated. Tip interacts mostly with electrons with cyclotron trajectories passing directly underneath it, leaving a trail of modified current density behind it. Other (indirect) trajectories become relevant when the tip is placed near the edges of the sample, and current is scattered between the tip and the edge. We also discuss possible explanations for spatial asymmetry of experimentally measured resistance maps, and connect it with specific configurations of the measuring probes.

cond-mat.mes-hall↗

arXiv2016

Strained graphene Hall bar

The effects of strain, induced by a Gaussian bump, on the magnetic field dependent transport properties of a graphene Hall bar are investigated. The numerical simulations are performed using both classical and quantum mechanical transport theory and we found that both approaches exhibit similar characteristic features. The effects of the Gaussian bump are manifested by a decrease of the bend resistance, $R_B$, around zero-magnetic field and the occurrence of side-peaks in $R_B$. These features are explained as a consequence of bump-assisted scattering of electrons towards different terminals of the Hall bar. Using these features we are able to give an estimate of the size of the bump. Additional oscillations in $R_B$ are found in the quantum description that are due to the population/depopulation of Landau levels. The bump has a minor influence on the Hall resistance even for very high values of the pseudo-magnetic field. When the bump is placed outside the center of the Hall bar valley polarized electrons can be collected in the leads.

cond-mat.mes-hall↗

arXiv2016

Electric- and magnetic-field dependence of the electronic and optical properties of phosphorene quantum dots

Recently, black phosphorus quantum dots were fabricated experimentally. Motivated by these experiments, we theoretically investigate the electronic and optical properties of rectangular phosphorene quantum dots (RPQDs) in the presence of an in-plane electric field and a perpendicular magnetic field. The energy spectra and wave functions of RPQDs are obtained numerically using the tight-binding (TB) approach. We find edge states within the band gap of the RPQD which are well separated from the bulk states. In an undoped RPQD and for in-plane polarized light, due to the presence of well-defined edge states, we find three types of optical transitions which are between the bulk states, between the edge and bulk states, and between the edge states. The electric and magnetic fields influence the bulk-to-bulk, edge-to-bulk, and edge-to-edge transitions differently due to the different responses of bulk and edge states to these fields.

cond-mat.mes-hall↗