arXiv subjects

F. M. Peeters

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

At least 73 records · Page 4Linked to original sources

arXiv2018

Strain field in graphene induced by nanopillar mesh

The mechanical and electronic properties of a graphene membrane placed on top of a superlattice of nanopillars are investigated. We use molecular dynamics (MD) simulations to access the deformation fields and the tight-binding approaches to calculate the electronic properties. The system of interest consists of a triangular lattice of nanopillars with a period of a=750 nm over which the graphene layer is deposited. Ripples form in the graphene layer that span across the unit cell, connecting neighbouring pillars, in agreement with recent experiments. We find that the resulting pseudo-magnetic field (PMF) varies strongly across the unit cell. We investigate the dependence of PMF on unit cell boundary conditions, height of the pillars, and the strength of the van der Waals interaction between graphene and the substrate. We find direct correspondence with typical experiments on pillars, showing intrinsic "slack" in the graphene membrane. PMF values are confirmed by the local density of states calculations performed at different positions of the unit cell showing pseudo-Landau levels at varying spacings. Our findings regarding the relaxed membrane configuration and the induced strains are transferable to other flexible 2D membranes.

cond-mat.mes-hall↗

arXiv2018

Electrical dipole on gapped graphene: Bound states and atomic collapse

We investigate the energy spectrum, wave functions, and local density of states of an electrical dipole placed on a sheet of gapped graphene as function of the charge strength Zα for different sizes of the dipole and for different regularization parameters. The dipole is modeled as consisting of a positive and negative charge. Bound states are found within the gap region with some energy levels that anticross and others that cross as function of the impurity strength Zα. The anticrossings are more pronounced and move to higher charges Zα when the length of the dipole decreases. These energy levels turn into atomic collapse states when they enter the positive (or negative) energy continuum. A smooth transition from the single-impurity behavior to the dipole one is observed: The states diving towards the continuum in the single-impurity case are gradually replaced by a series of anticrossings that represent a continuation of the diving states in the single-impurity case. By studying the local density of states at the edge of the dipole we show how the series of anticrossings persist in the positive and negative continuum.

cond-mat.mes-hall↗

arXiv2018

Rich many-body phase diagram of electrons and holes in doped monolayer transition metal dichalcogenides

We use a variational technique to study the many-body phase diagram of electrons and holes in $n$-doped and $p$-doped monolayer transition metal dichalcogenides (TMDs). We find a total of four different phases. $i$) A fully spin polarized and valley polarized ferromagnetic state. $ii$) A state with no global spin polarization but with spin polarization in each valley separately, i.e. spin-valley locking. $iii$) A state with spin polarization in one of the valleys and little to no spin polarization in the other valley. $iv$) A paramagnetic state with no valley polarization. These phases are separated by first-order phase transitions and are determined by the particle density and the dielectric constant of the substrate. We find that in the presence of a perpendicular magnetic field the four different phases persist. In the case of $n$-doped MoS$_2$, a fifth phase, which is completely valley polarized but not spin polarized, appears for magnetic fields larger than 7 T and for magnetic fields larger than 23 T completely replaces the second phase.

cond-mat.str-el↗

arXiv2018

Topological phase transition in GeSnH$_2$ induced by biaxial tensile strain: A tight-binding study

An effective tight-binding (TB) Hamiltonian for monolayer GeSnH$_2$ is proposed which has an inversion-asymmetric honeycomb structure. The low-energy band structure of our TB model agrees very well with previous {\it ab initio} calculations under biaxial tensile strain. We predict a phase transition upon 7.5\% biaxial tensile strain in agreement with DFT calculations. Upon 8.5\% strain the system exhibits a band gap of 134 meV, suitable for room temperature applications. The topological nature of the phase transition is confirmed by: 1)the calculation of the $\mathbb{Z}_2$ topological invariant, and 2)quantum transport calculations of disordered GeSnH$_2$ nanoribbons which allows us to determine the universality class of the conductance fluctuations.

cond-mat.mes-hall↗

arXiv2018

Interlayer excitons in transition metal dichalcogenide heterostructures

Starting from the single-particle Dirac Hamiltonian for charge carriers in monolayer transition metal dichalcogenides (TMDs), we construct a four-band Hamiltonian describing interlayer excitons consisting of an electron in one TMD layer and a hole in the other TMD layer. An expression for the electron-hole interaction potential is derived, taking into account the effect of the dielectric environment above, below, and between the two TMD layers as well as polarization effects in the transition metal layer and in the chalcogen layers of the TMD layers. We calculate the interlayer exciton binding energy and average in-plane interparticle distance for different TMD heterostructures. The effect of different dielectric environments on the exciton binding energy is investigated and a remarkable dependence on the dielectric constant of the barrier between the two layers is found, resulting from competing effects as a function of the in-plane and out-of-plane dielectric constants of the barrier. The polarization effects in the chalcogen layers, which in general reduce the exciton binding energy, can lead to an increase in binding energy in the presence of strong substrate effects by screening the substrate. The excitonic absorbance spectrum is calculated and we show that the interlayer exciton peak depends linearly on a perpendicular electric field, which agrees with recent experimental results.

cond-mat.mes-hall↗

arXiv2018

Fast water flow through graphene nanocapillaries: a continuum model approach involving the microscopic structure of confined water

Water inside a nanocapillary becomes ordered, resulting in unconventional behavior. A profound enhancement of water flow inside nanometer thin capillaries made of graphene has been observed [B. Radha et.al., Nature (London) 538, 222 (2016)]. Here we explain this enhancement as due to the large density and the extraordinary viscosity of water inside the graphene nanocapillaries. Using the Hagen-Poiseuille theory with slippage-boundary condition and incorporating disjoining pressure term in combination with results from molecular dynamics (MD) simulations, we present an analytical theory that elucidates the origin of the enhancement of water flow inside hydrophobic nanocapillaries. Our work reveals a distinctive dependence of water flow in a nanocapillary on the structural properties of nanoconfined water in agreement with experiment, which opens a new avenue in nanofluidics.

cond-mat.soft↗

arXiv2018

Confined states in graphene quantum blisters

Bilayer graphene samples may exhibit regions where the two layers are locally delaminated forming a so-called quantum blister in the graphene sheet. Electron and hole states can be confined in this graphene quantum blisters (GQB) by applying a global electrostatic bias. We scrutinize the electronic properties of these confined states under the variation of interlayer bias, coupling, and blister's size. The spectra display strong anti-crossings due to the coupling of the confined states on upper and lower layers inside the blister. These spectra are layer localized where the respective confined states reside on either layer or equally distributed. For finite angular momentum, this layer localization can be at the edge of the blister and corresponds to degenerate modes of opposite momenta. Furthermore, the energy levels in GQB exhibit electron-hole symmetry that is sensitive to the electrostatic bias. Finally, we demonstrate that confinement in GQB persists even in the presence of a variation in the inter-layer coupling.

cond-mat.mes-hall↗

arXiv2018

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.

cond-mat.mes-hall↗

arXiv2018

Graphene quantum blisters: a tunable system to confine charge carriers

Due to Klein tunneling, electrostatic confinement of electrons in graphene is not possible. This hinders the use of graphene for quantum dot applications. Only through quasi-bound states with finite lifetime has one achieved to confine charge carriers. Here we propose that bilayer graphene with a local region of decoupled graphene layers is able to generate bound states under the application of an electrostatic gate. The discrete energy levels in such a quantum blister correspond to localized electron and hole states in the top and bottom layers. We find that this layer localization and the energy spectrum itself are tunable by a global electrostatic gate and that the latter also coincides with the electronic modes in a graphene disk. Curiously, states with energy close to the continuum exist primarily in the classically forbidden region outside the domain defining the blister. The results are robust against variations in size and shape of the blister which shows that it is a versatile system to achieve tunable electrostatic confinement in graphene.

cond-mat.mes-hall↗

arXiv2018

Veselago focusing of anisotropic massless Dirac fermions

Massless Dirac fermions (MDFs) emerge as quasiparticles in various novel materials such as graphene and topological insulators, and they exhibit several intriguing properties, of which Veselago focusing is an outstanding example with a lot of possible applications. However, up to now Veselago focusing merely occurred in p-n junction devices based on the isotropic MDF, which lacks the tunability needed for realistic applications. Here, motivated by the emergence of novel Dirac materials, we investigate the propagation behaviors of anisotropic MDFs in such a p-n junction structure. By projecting the Hamiltonian of the anisotropic MDF to that of the isotropic MDF and deriving an exact analytical expression for the propagator, precise Veselago focusing is demonstrated without the need for mirror symmetry of the electron source and its focusing image. We show a tunable focusing position that can be used in a device to probe masked atom-scale defects. This study provides an innovative concept to realize Veselago focusing relevant for potential applications, and it paves the way for the design of novel electron optics devices by exploiting the anisotropic MDF.

cond-mat.mes-hall↗

arXiv2018

Electrically controlled water permeation through graphene oxide membranes

Developing 'smart' membranes that allow precise and reversible control of molecular permeation using external stimuli would be of intense interest for many areas of science: from physics and chemistry to life-sciences. In particular, electrical control of water permeation through membranes is a long-sought objective and is of crucial importance for healthcare and related areas. Currently, such adjustable membranes are limited to the modulation of wetting of the membranes and controlled ion transport, but not the controlled mass flow of water. Despite intensive theoretical work yielding conflicting results, the experimental realisation of electrically controlled water permeation has not yet been achieved. Here we report electrically controlled water permeation through micrometre-thick graphene oxide (GO) membranes. By controllable electric breakdown, conductive filaments are created in the GO membrane. The electric field concentrated around such current carrying filaments leads to controllable ionisation of water molecules in graphene capillaries, allowing precise control of water permeation: from ultrafast permeation to complete blocking. Our work opens up an avenue for developing smart membrane technologies and can revolutionize the field of artificial biological systems, tissue engineering and filtration.

cond-mat.mtrl-sci↗

arXiv2018

Excitons, trions, and biexcitons in transition metal dichalcogenides: magnetic field dependence

The influence of a perpendicular magnetic field on the binding energy and structural properties of excitons, trions, and biexcitons in monolayers of semiconducting transition metal dichalcogenides (TMDs) is investigated. The stochastic variational method (SVM) with a correlated Gaussian basis is used to calculate the different properties of these few-particle systems. In addition, we present a simplified variational approach which supports the SVM results for excitons as a function of magnetic field. The exciton diamagnetic shift is compared with recent experimental results and we extend this concept to trions and biexcitons. The effect of a local potential fluctuation, which we model by a circular potential well, on the binding energy of trions and biexcitons is investigated and found to significantly increase the binding of those excitonic complexes.

cond-mat.mes-hall↗

arXiv2018

High temperature electron-hole superfluidity with strong anisotropic gaps in double phosphorene monolayers

Excitonic superfluidity in double phosphorene monolayers is investigated using the BCS mean-field equations. Highly anisotropic superfluidity is predicted where we found that the maximum superfluid gap is in the BEC regime along the armchair direction and in the BCS-BEC crossover regime along the zigzag direction. We estimate the highest Kosterlitz-Thouless transition temperature with maximum value up to $\sim 90$ K with onset carrier densities as high as $4 \times 10^{12}$ cm$^{-2}$. This transition temperature is significantly larger than what is found in double electron-hole few-layers of graphene. Our results can guide experimental research towards the realization of anisotropic condensate states in electron-hole phosphorene monolayers.

cond-mat.mes-hall↗

arXiv2018

Tuning the electronic properties of gated multilayer phosphorene: A self-consistent tight-binding study

By taking account of the electric-field-induced charge screening, a self-consistent calculation within the framework of the tight-binding approach is employed to obtain the electronic band structure of gated multilayer phosphorene and the charge densities on the different phosphorene layers. We find charge density and screening anomalies in single-gated multilayer phosphorene and electron-hole bilayers in dual-gated multilayer phosphorene. Due to the unique puckered lattice structure, both intralayer and interlayer charge screenings are important in gated multilayer phosphorene. We find that the electric-field tuning of the band structure of multilayer phosphorene is distinctively different in the presence and absence of charge screening. For instance, it is shown that the unscreened band gap of multilayer phosphorene decreases dramatically with increasing electric-field strength. However, in the presence of charge screening, the magnitude of this band-gap decrease is significantly reduced and the reduction depends strongly on the number of phosphorene layers. Our theoretical results of the band-gap tuning are in good agreement with recent experiments.

cond-mat.mes-hall↗

arXiv2018

Phase Transition and Field Effect Topological Quantum Transistor made of monolayer MoS2

We study topological phase transitions and topological quantum field effect transistor in monolayer Molybdenum Disulfide (MoS2) using a two-band Hamiltonian model. Without considering the quadratic (q^2) diagonal term in the Hamiltonian, we show that the phase diagram includes quantum anomalous Hall (QAH), quantum spin Hall (QSH), and spin quantum anomalous Hall effect (SQAH) regions such that the topological Kirchhoff law is satisfied in the plane. By considering the q^2 diagonal term and including one valley, it is shown that MoS2 has a non-trivial topology, and the valley Chern number is non-zero for each spin. We show that the wave function is (is not) localized at the edges when the q^2 diagonal term is added (deleted) to (from) the spin-valley Dirac mass equation. We calculate the quantum conductance of zigzag MoS2 nanoribbons by using the nonequilibrium Green function method and show how this device works as a field effect topological quantum transistor.

cond-mat.mes-hall↗

arXiv2018

Magnetic field dependence of electronic properties of MoS$_2$ quantum dots with different edges

Using the tight-binding approach, we investigate the energy spectrum of square, triangular and hexagonal MoS$_2$ quantum dots (QDs) in the presence of a perpendicular magnetic field. Novel edge states emerge in MoS$_2$ QDs, which are distributed over the whole edge which we call ring states. The ring states are robust in the presence of spin-orbit coupling (SOC). The corresponding energy levels of the ring states oscillate as function of the perpendicular magnetic field which are related to Aharonov-Bohm oscillations. Oscillations in the magnetic field dependence of the energy levels and the peaks in the magneto-optical spectrum emerge (disappear) as the ring states are formed (collapsed). The period and the amplitude of the oscillation decreases with the size of the MoS$_2$ QDs.

cond-mat.mes-hall↗

arXiv2018

Strong Valley Zeeman Effect of Dark Excitons in Monolayer Transition Metal Dichalcogenides in a Tilted Magnetic Field

The dependence of the excitonic photoluminescence (PL) spectrum of monolayer transition metal dichalcogenides (TMDs) on the tilt angle of an applied magnetic field is studied. Starting from a four-band Hamiltonian we construct a theory which quantitatively reproduces the available experimental PL spectra for perpendicular and in-plane magnetic fields. In the presence of a tilted magnetic field, we demonstrate that the dark exciton PL peaks brighten due to the in-plane component of the magnetic field and split for light with different circular polarization as a consequence of the perpendicular component of the magnetic field. This splitting is more than twice as large as the splitting of the bright exciton peaks in tungsten-based TMDs. We propose an experimental setup that will allow to access the predicted splitting of the dark exciton peaks in the PL spectrum.

cond-mat.mes-hall↗

arXiv2018

Transport of hydrogen isotopes through interlayer spacing in van der Waals crystals

Atoms start behaving as waves rather than classical particles if confined in spaces commensurate with their de Broglie wavelength. At room temperature this length is only about one angstrom even for the lightest atom, hydrogen. This restricts quantum-confinement phenomena for atomic species to the realm of very low temperatures. Here we show that van der Waals gaps between atomic planes of layered crystals provide angstrom-size channels that make quantum confinement of protons apparent even at room temperature. Our transport measurements show that thermal protons experience a notably higher barrier than deuterons when entering van der Waals gaps in hexagonal boron nitride and molybdenum disulfide. This is attributed to the difference in de Broglie wavelength of the isotopes. Once inside the crystals, transport of both isotopes can be described by classical diffusion, albeit with unexpectedly fast rates, comparable to that of protons in water. The demonstrated angstrom-size channels can be exploited for further studies of atomistic quantum confinement and, if the technology can be scaled up, for sieving hydrogen isotopes.

cond-mat.mes-hall↗