arXiv subjects

F. M. Peeters

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

At least 37 records · Page 2Linked to original sources

arXiv2023

Proton transport through nanoscale corrugations in two-dimensional crystals

Defect-free graphene is impermeable to all atoms and ions at ambient conditions. Experiments that can resolve gas flows of a few atoms per hour through micrometre-sized membranes found that monocrystalline graphene is completely impermeable to helium, the smallest of atoms. Such membranes were also shown to be impermeable to all ions, including the smallest one, lithium. On the other hand, graphene was reported to be highly permeable to protons, nuclei of hydrogen atoms. There is no consensus, however, either on the mechanism behind the unexpectedly high proton permeability or even on whether it requires defects in graphene's crystal lattice. Here using high resolution scanning electrochemical cell microscopy (SECCM), we show that, although proton permeation through mechanically-exfoliated monolayers of graphene and hexagonal boron nitride cannot be attributed to any structural defects, nanoscale non-flatness of 2D membranes greatly facilitates proton transport. The spatial distribution of proton currents visualized by SECCM reveals marked inhomogeneities that are strongly correlated with nanoscale wrinkles and other features where strain is accumulated. Our results highlight nanoscale morphology as an important parameter enabling proton transport through 2D crystals, mostly considered and modelled as flat, and suggest that strain and curvature can be used as additional degrees of freedom to control the proton permeability of 2D materials.

cond-mat.mes-hall↗

arXiv2023

Magnetism in twisted triangular bilayer graphene quantum dots

Using a tight-binding model along with the mean-field Hubbard method, we investigate the effect of twisting angle on the magnetic properties of twisted bilayer graphene (tBLG) quantum dots (QDs) with triangular shape and zigzag edges. We consider such QDs in two configurations: when their initial untwisted structure is a perfect AA- or AB-stacked BLG, referred to as AA- or AB-like dots. We find that AA-like dots exhibit an antiferromagnetic spin polarization for small twist angles, which transits to a ferromagnetic spin polarization beyond a critical twisting angle $θ_c$. Our analysis shows that $θ_c$ decreases as the dot size increases, obeying a criterion, according to which once the maximum energy difference between electron and hole edge states (in the single-particle picture) is less than $(U / γ_0)\, t_0$, the spin-polarized energy levels are aligned ferromagnetically [$U$ is the Hubbard parameter and $γ_0$ ($t_0$) the graphene intralayer (interlayer) hopping]. Unlike AA-like dots, AB-like dots exhibit finite magnetization for any twist angle. Furthermore, in the ferromagnetic polarization state, the ground net spin for both dot configurations agrees with prediction from Lieb's theorem.

cond-mat.mes-hall↗

arXiv2022

Enhanced thermoelectric properties in phosphorene nanorings

Using the tight-binding approach, we investigate the thermoelectric (TE) properties of rectangular phosphorene nanorings for both symmetrically and asymmetrically attaching to phosphorene nanoribbon leads. We design our phosphorene-based nanostructures to enhance the TE performance in the absence and the presence of perpendicular magnetic fields. Our results show that when zigzag phosphorene nanoribbons (ZPNRs) are coupled symmetrically to rectangular rings, a comparatively large band gap is induced in the electronic conductance due to the suppression of the contribution of edge states. This gives rise to a remarkable increase in the thermopower response compared to the case of pristine ZPNRs. More intriguingly, we found that though the maximum power factor in this system is about the same as the one for its ZPNR counterpart, the much smaller electronic thermal conductance of this phosphorene-based nanostructure can remarkably contribute to the improvement of the figure of merit. Also, we found that the symmetry/asymmetry of our designed nanostructures, the geometrical characteristics of the ring, and the magnetic flux are three important factors that control the thermoelectric properties of phosphorene quantum rings. Our numerical calculations show that by changing the magnetic flux through the nanoring, a drastic increase in the thermopower is observed near an antiresonance point. We demonstrate the tunability of the thermopower and the possibility to switch on and off the TE response of phosphorene nanorings with the magnetic flux. Moreover, for asymmetric connection configurations with armchair-edged leads, we found that though the thermopower is almost intact, a remarkable reduction of the electronic thermal conductance can lead to a notable improvement in the figure of merit. Our results suggest phosphorene nanorings as promising candidate nanostructures for TE applications.

cond-mat.mes-hall↗

arXiv2022

Photo-accelerated water dissociation across one-atom-thick electrodes

Recent experiments demonstrated that interfacial water dissociation (H2O = H+ + OH-) could be accelerated exponentially by an electric field applied to graphene electrodes, a phenomenon related to the Wien effect. Here we report an order-of-magnitude acceleration of the interfacial water dissociation reaction under visible-light illumination. This process is accompanied by spatial separation of protons and hydroxide ions across one-atom-thick graphene and enhanced by strong interfacial electric fields. The found photo-effect is attributed to the combination of graphene's perfect selectivity with respect to protons, which prevents proton-hydroxide recombination, and to proton transport acceleration by the Wien effect, which occurs in synchrony with the water dissociation reaction. Our findings provide fundamental insights into ion dynamics near atomically-thin proton-selective interfaces and suggest that strong interfacial fields can enhance and tune very fast ionic processes, which is of relevance for applications in photo-catalysis and designing reconfigurable materials.

physics.chem-ph↗

arXiv2022

Field-free superconducting diode in a magnetically nanostructured superconductor

A strong superconducting diode effect (SDE) is revealed in a thin superconducting film periodically nanostructured with magnetic dots. The SDE is caused by the current-activated dissipation mitigated by vortex-antivortex pairs (VAPs), which periodically nucleate under the dots, move and annihilate in the superconductor - eventually driving the system to the high-resistive state. Inversing the polarity of the applied current destimulates the nucleation of VAPs, the system remains superconducting up to far larger currents, leading to the pronounced diodic response. Our dissipative Ginzburg-Landau simulations detail the involved processes, and provide reliable geometric and parametric ranges for the experimental realization of such a non-volatile superconducting diode, which operates in absence of any applied magnetic field while being fluxonic by design.

cond-mat.supr-con↗

arXiv2022

Wien effect in interfacial water dissociation through proton-permeable graphene electrodes

Strong electric fields can accelerate molecular dissociation reactions. The phenomenon known as the Wien effect was previously observed using high-voltage electrolysis cells that produced fields of about 10^7 V m-1, sufficient to accelerate the dissociation of weakly bound molecules (e.g., organics and weak electrolytes). The observation of the Wien effect for the common case of water dissociation (H2O = H+ + OH-) has remained elusive. Here we study the dissociation of interfacial water adjacent to proton-permeable graphene electrodes and observe strong acceleration of the reaction in fields reaching above 10^8 V m-1. The use of graphene electrodes allow measuring the proton currents arising exclusively from the dissociation of interfacial water, while the electric field driving the reaction is monitored through the carrier density induced in graphene by the same field. The observed exponential increase in proton currents is in quantitative agreement with Onsager's theory. Our results also demonstrate that graphene electrodes can be valuable for the investigation of various interfacial phenomena involving proton transport.

physics.chem-ph↗

arXiv2022

Alternating superconducting and charge density wave monolayers within bulk 6R-TaS2

Van der Waals (vdW) heterostructures continue to attract intense interest as a route of designing materials with novel properties that cannot be found in naturally occurring materials. Unfortunately, this approach is currently limited to only a few layers that can be stacked on top of each other. Here we report a bulk material consisting of superconducting monolayers interlayered with monolayers displaying charge density waves (CDW). This bulk vdW heterostructure is created by phase transition of 1T-TaS2 to 6R at 800 °C in an inert atmosphere. Electron microscopy analysis directly shows the presence of alternating 1T and 1H monolayers within the resulting bulk 6R phase. Its superconducting transition (Tc) is found at 2.6 K, exceeding the Tc of the bulk 2H phase of TaS2. The superconducting temperature can be further increased to 3.6 K by exfoliating 6R-TaS2 and then restacking its layers. Using first-principles calculations, we argue that the coexistence of superconductivity and CDW within 6R-TaS2 stems from amalgamation of the properties of adjacent 1H and 1T monolayers, where the former dominates the superconducting state and the latter the CDW behavior.

cond-mat.supr-con↗

arXiv2022

Electronic Mach-Zehnder interference in a bipolar hybrid monolayer-bilayer graphene junction

Graphene matter in a strong magnetic field, realizing one-dimensional quantum Hall channels, provides a unique platform for studying electron interference. Here, using the Landauer-Büttiker formalism along with the tight-binding model, we investigate the quantum Hall (QH) effects in unipolar and bipolar monolayer-bilayer graphene (MLG-BLG) junctions. We find that a Hall bar made of an armchair MLG-BLG junction in the bipolar regime results in valley-polarized edge-channel interferences and can operate a fully tunable Mach-Zehnder (MZ) interferometer device. Investigation of the bar-width and magnetic-field dependence of the conductance oscillations shows that the MZ interference in such structures can be drastically affected by the type of (zigzag) edge termination of the second layer in the BLG region [composed of vertical dimer or non-dimer atoms]. Our findings reveal that both interfaces exhibit a double set of Aharonov-Bohm interferences, with the one between two oppositely valley-polarized edge channels dominating and causing a large-amplitude conductance oscillation ranging from 0 to $ 2 e^2 / h$. We explain and analyze our findings by analytically solving the Dirac-Weyl equation for a gated semi-infinite MLG-BLG junction.

cond-mat.mes-hall↗

arXiv2022

Anisotropic and tunable optical conductivity of a two-dimensional semi-Dirac system in the presence of elliptically polarized radiation

We investigate the effect of ellipticity ratio of the polarized radiation field on optoelectronic properties of a two-dimensional (2D) semi-Dirac (SD) system. The optical conductivity is calculated within the energy balance equation approach derived from the semiclassical Boltzmann equation. We find that there exists the anisotropic optical absorption induced via both the intra- and interband electronic transition channels in the perpendicular $xx$ and $yy$ directions. Furthermore, we examine the effects of the ellipticity ratio, the temperature, the carrier density, and the band-gap parameter on the optical conductivity of the 2D SD system placed in transverse and vertical directions, respectively. It is shown that the ellipticity ratio, temperature, carrier density, and band-gap parameter can play the important roles in tuning the strength, peak position, and shape of the optical conductivity spectrum. The results obtained from this study indicate that the 2D SD system can be a promising anisotropic and tunable optical and optoelectronic material for applications in innovative 2D optical and optoelectronic devices, which are active in the infrared and terahertz bandwidths.

cond-mat.mes-hall↗

arXiv2022

Isolated and hybrid bilayer graphene rings

Using the continuum model, we investigate the electronic properties of two types of bilayer graphene (BLG) quantum ring (QR) geometries: (i) an isolated BLG QR and (ii) a monolayer graphene (MLG) with a QR put on top of an infinite graphene sheet (hybrid BLG QR). Solving the Dirac-Weyl equation in the presence of a perpendicular magnetic field and applying the infinite-mass boundary condition at the ring boundaries, we obtain analytical results for the energy levels and corresponding wave spinors for both structures. In the case of isolated BLG QR, we observe a sizeable and magnetically tunable band gap which agrees with the tight-binding transport simulations. Our analytical results also show the intervalley symmetry $ E^K_e (m) = -E^{K'}_h(m) $ between the electron (e) and hole (h) states ($ m $ being the angular momentum quantum number) for the energy spectrum of the isolated BLG QR. The presence of interface boundary in a hybrid BLG QR modifies drastically the energy levels as compared to that of an isolated BLG QR. Its energy levels are tunable from MLG dot, to isolated BLG QR, and to MLG Landau energy levels as magnetic field is varied. Our predictions can be verified experimentally using different techniques such as by magnetotransport measurements.

cond-mat.mes-hall↗

arXiv2022

Strong anisotropic optical properties of 8-Pmmn borophene: a many-body perturbation study

Using first-principle many-body perturbation theory, we investigate the optical properties of 8- borophene at two levels of approximations; the GW method considering only the electron-electron interaction and the GW in combination with the Bethe-Salpeter equation including electron-hole coupling. The band structure exhibits anisotropic Dirac cones with semimetallic character. The optical absorption spectra are obtained for different light polarizations and we predict strong optical absorbance anisotropy. The absorption peaks undergo a global redshift when the electron-hole interaction is taken into account due to the formation of bound excitons which have an anisotropic excitonic wave function.

cond-mat.mes-hall↗

arXiv2021

Pivotal role of magnetic ordering and strain in lattice thermal conductivity of chromium-trihalide monolayers

Understanding the coupling between spin and phonons is critical for controlling the lattice thermal conductivity ($κ_l$) in magnetic materials, as we demonstrate here for CrX$_3$ (X = Br and I) monolayers. We show that these compounds exhibit large spin-phonon coupling (SPC), dominated by out-of-plane vibrations of Cr atoms, resulting in significantly different phonon dispersions in ferromagnetic (FM) and paramagnetic (PM) phases. Lattice thermal conductivity calculations provide additional evidence for strong SPC, where particularly large $κ_l$ is found for the FM phase. Most strikingly, PM and FM phases exhibit radically different behavior with tensile strain, where $κ_l$ increases with strain for the PM phase, and strongly decreases for the FM phase -- as we explain through analysis of phonon lifetimes and scattering rates. Taken all together, we uncover the very high significance of SPC on the phonon transport in CrX$_3$ monolayers, a result extendable to other 2D magnetic materials, that will be useful in further design of thermal spin devices.

cond-mat.mtrl-sci↗

arXiv2021

Exponentially selective molecular sieving through angstrom pores

Two-dimensional crystals with angstrom-scale pores are widely considered as candidates for a next generation of molecular separation technologies aiming to provide extreme, exponentially large selectivity combined with high flow rates. No such pores have been demonstrated experimentally. Here we study gas transport through individual graphene pores created by low intensity exposure to low kV electrons. Helium and hydrogen permeate easily through these pores whereas larger species such as xenon and methane are practically blocked. Permeating gases experience activation barriers that increase quadratically with molecules' kinetic diameter, and the effective diameter of the created pores is estimated as ~2 angstroms, about one missing carbon ring. Our work reveals stringent conditions for achieving the long sought-after exponential selectivity using porous two-dimensional membranes and suggests limits on their possible performance.

cond-mat.mes-hall↗

arXiv2021

Stability of adsorption of Mg and Na on sulfur-functionalized MXenes

Two-dimensional materials composed of transition metal carbides and nitrides (MXenes) are poised to revolutionize energy conversion and storage. In this work, we used density functional theory (DFT) to investigate adsorption of Mg and Na adatoms on five M$_{2}$CS$_{2}$ monolayers (where M= Mo, Nb, Ti, V, Zr) for battery applications. We assessed the stability of the adatom (i.e. Na and Mg)-monolayer systems by calculating adsorption and formation energies, as well as voltages as a function of surface coverage. For instance, we found that Mo$_{2}$CS$_{2}$ cannot support a full layer of Na nor even a single Mg atom. Na and Mg exhibit the strongest binding on Zr$_{2}$CS$_{2}$, followed by Ti$_{2}$CS$_{2}$, Nb$_{2}$CS$_{2}$ and V$_{2}$CS$_{2}$. Using the nudged elastic band method (NEB) we computed promising diffusion barriers for both dilute and nearly-full ion surface coverage cases. In the dilute ion adsorption case, a single Mg and Na atom on Ti$_{2}$CS$_{2}$ experience $\sim$0.47 eV and $\sim$0.10 eV diffusion barriers between the lowest energy sites, respectively. For a nearly full surface coverage, a Na ion moving on Ti$_{2}$CS$_{2}$ experiences a $\sim$0.33 eV energy barrier, implying a concentration dependent diffusion barrier. Our molecular dynamics results indicate that three (one) layers (layer) of Mg (Na) ion on both surfaces of Ti$_{2}$CS$_{2}$ remain stable at T=300 K. While, according to voltage calculations, Zr$_{2}$CS$_{2}$ can store Na up to three atomic layers, our MD simulations predict that the outermost layers detach from Zr$_{2}$CS$_{2}$ monolayer due to weak interaction between Na ions and the monolayer. This suggests that MD simulations are essential to confirming the stability of an ion-electrode system - an insight that is mostly absence in previous studies.

cond-mat.mtrl-sci↗

arXiv2021

Infrared to terahertz optical conductivity of $n$-type and $p$-type monolayer MoS$_2$ in the presence of Rashba spin-orbit coupling

We investigate the effect of Rashba spin-orbit coupling (SOC) on the optoelectronic properties of n- and p-type monolayer MoS2. The optical conductivity is calculated within the Kubo formalism. We find that the spin-flip transitions enabled by the Rashba SOC result in a wide absorption window in the optical spectrum. Furthermore, we evaluate the effects of the polarization direction of the radiation, temperature, carrier density, and the strength of the Rashba spin-orbit parameter on the optical conductivity. We find that the position, width, and shape of the absorption peak or absorption window can be tuned by varying these parameters. This study shows that monolayer MoS2 can be a promising tunable optical and optoelectronic material that is active in the infrared to terahertz spectral range.

cond-mat.mes-hall↗

arXiv2021

Terahertz magneto-optical properties of graphene hydrodynamic electron liquid

The discovery of the hydrodynamic electron liquid (HEL) in graphene [D. Bandurin \emph{et al.}, Science {\bf 351}, 1055 (2016) and J. Crossno \emph{et al.}, Science {\bf 351}, 1058 (2016)] has marked the birth of the solid-state HEL which can be probed near room temperature in a table-top setup. Here we examine the terahertz (THz) magneto-optical (MO) properties of a graphene HEL. Considering the case where the magnetic length $l_B=\sqrt{\hbar/eB}$ is comparable to the mean-free path $l_{ee}$ for electron-electron interaction in graphene, the MO conductivities are obtained by taking a momentum balance equation approach on the basis of the Boltzmann equation. We find that when $l_B\sim l_{ee}$, the viscous effect in a HEL can weaken significantly the THz MO effects such as cyclotron resonance and Faraday rotation. The upper hybrid and cyclotron resonance magnetoplasmon modes $ω_\pm$ are also obtained through the RPA dielectric function. The magnetoplasmons of graphene HEL at large wave-vector regime are affected by the viscous effect, and results in red-shifts of the magnetoplasmon frequencies. We predict that the viscosity in graphene HEL can affect strongly the magneto-optical and magnetoplasmonic properties, which can be verified experimentally.

cond-mat.mes-hall↗

arXiv2021

Tunable coupling of terahertz Dirac plasmons and phonons in transition metal dicalchogenide-based van der Waals heterostructures

Dirac plasmons in graphene hybridize with phonons of transition metal dichalcogenides (TMDs) when the materials are combined in so-called van der Waals heterostructures (vdWh), thus forming surface plasmon-phonon polaritons (SPPPs). The extend to which these modes are coupled depends on the TMD composition and structure, but also on the plasmons' properties. By performing realistic simulations that account for the contribution of each layer of the vdWh separately, we calculate how the strength of plasmon-phonon coupling depends on the number and composition of TMD layers, on the graphene Fermi energy and the specific phonon mode. From this, we present a semiclassical theory that is capable of capturing all relevant characteristics of the SPPPs. We find that it is possible to realize both strong and ultra-strong coupling regimes by tuning graphene's Fermi energy and changing TMD layer number.

cond-mat.mes-hall↗

arXiv2021

Zitterbewegung of moiré excitons in twisted MoS$_2$/WSe$_2$ hetero-bilayers

The moiré pattern observed in stacked non-commensurate crystal lattices, such as hetero-bilayers of transition metal dichalcogenides, produces a periodic modulation of their bandgap. Excitons subjected to this potential landscape exhibit a band structure that gives rise to a quasi-particle dubbed moiré exciton. In the case of MoS$_2$/WSe$_2$ hetero-bilayers, the moiré trapping potential has honeycomb symmetry and, consequently, the moiré exciton band structure is the same as that of a Dirac-Weyl fermion, whose mass can be further tuned down to zero with a perpendicularly applied field. Here we show that, analogously to other Dirac-like particles, moiré exciton exhibits a trembling motion, also known as zitterbewegung, whose long timescales are compatible with current experimental techniques for exciton dynamics. This promotes the study of the dynamics of moiré excitons in van der Waals heterostructures as an advantageous solid-state platform to probe zitterbewegung, broadly tunable by gating and inter-layer twist angle.

cond-mat.other↗