arXiv subjects

Bart Partoens

Publications and source records attributed to Bart Partoens.

At least 37 records · Page 2Linked to original sources

arXiv2016

Extended homologous series of Sn-O layered systems: a first-principles study

Apart from the most studied tin-oxide compounds, SnO and SnO2, intermediate states have been claimed to exist for more than a hundred years. In addition to the known homologous series (Seko et al., Phys. Rev. Lett. 100, 045702 (2008)), we here predict the existence of several new compounds with an O concentration between 50 % (SnO) and 67 % (SnO2). All these intermediate compounds are constructed from removing one or more (101) oxygen layers of SnO2. Since the van der Waals (vdW) interaction is known to be important for the Sn-Sn interlayer distances, we use a vdW-corrected functional, and compare these results with results obtained with PBE and hybrid functionals. We present the electronic properties of the intermediate structures and we observe a decrease of the band gap when (i) the O concentration increases and (ii) more SnO-like units are present for a given concentration. The contribution of the different atoms to the valence and conduction band is also investigated.

cond-mat.mtrl-sci↗

arXiv2016

First-principles study of the optoelectronic properties and photovoltaic absorber layer efficiency of Cu-based chalcogenides

Cu-based chalcogenides are promising materials for thin-film solar cells with more than 20% measured cell efficiency. Using first-principles calculations based on density functional theory, the optoelectronic properties of a group of Cu-based chalcogenides Cu$_2$-II-IV-VI$_4$ is studied. They are then screened with the aim of identifying potential absorber materials for photovoltaic applications. The spectroscopic limited maximum efficiency (SLME) introduced by Yu and Zunger is used as a metric for the screening. After constructing the current-voltage curve, the maximum spectroscopy dependent power conversion efficiency is calculated from the maximum power output. The role of the nature of the band gap, direct or indirect, and also of the absorptivity of the studied materials on the maximum theoretical power conversion efficiency is studied. Our results show that Cu$_2$-II-GeSe$_4$ with II=Cd and Hg, and Cu$_2$-II-SnS$_4$ with II=Cd and Zn have a higher theoretical efficiency compared to the materials currently used as absorber layer.

cond-mat.mtrl-sci↗

arXiv2016

Correlation and current anomalies in helical quantum dots

We theoretically investigate the ground-state properties of a quantum dot defined on the surface of a strong three-dimensional time-reversal invariant topological insulator. Confinement is realized by ferromagnetic barriers and Coulomb interaction is treated numerically for up to seven electrons in the dot. Experimentally relevant intermediate interaction strengths are considered. The topological nature of the dot has interesting consequences: i) spin polarization increases and the ground state exhibits quantum phase transitions at specific angular momenta as a function of interaction strength ii) the onset of Wigner correlations takes place mainly in one spin channel, iii) the ground state is characterized by a persistent current which changes sign as a function of the radius of the dot.

cond-mat.mes-hall↗

arXiv2016

Easily doped p-type, low hole effective mass, transparent oxides

Fulfillment of the promise of transparent electronics has been hindered until now largely by the lack of semiconductors that can be doped p-type in a stable way, and that at the same time present high hole mobility and are highly transparent in the visible spectrum. Here, a high-throughput study based on first-principles methods reveals four oxides, namely X2SeO2, with X = La, Pr, Nd, and Gd, which are unique in that they exhibit excellent characteristics for transparent electronic device applications-i.e., a direct band gap larger than 3.1 eV, an average hole effective mass below the electron rest mass, and good p-type dopability. Furthermore, for La2SeO2 it is explicitly shown that Na impurities substituting La are shallow acceptors in moderate to strong anion-rich growth conditions, with low formation energy, and that they will not be compensated by anion vacancies VO or VSe.

cond-mat.mtrl-sci↗

arXiv2015

Inelastic electron-vortex-beam scattering

Recent theoretical and experimental developments in the field of electron vortex beam physics have raised questions on what exactly this novelty in the field of electron microscopy (and other fields, such as particle physics) really provides. An important part in the answer to those questions lies in scattering theory. The present investigation explores various aspects of inelastic quantum scattering theory for cylindrically symmetric beams with orbital angular momentum. The model system of Coulomb scattering on a hydrogen atom provides the setting to address various open questions: How is momentum transferred? Do vortex beams selectively excite atoms, and how can one employ vortex beams to detect magnetic transitions? The analytical approach presented here provides answers to these questions. OAM transfer is possible, but not through selective excitation; rather, by pre- and post-selection one can filter out the relevant contributions to a specific signal.

quant-ph↗

arXiv2014

First-principles study of carbon impurities in CuIn$_{1-x}$Ga$_x$Se$_{2}$, present in nonvacuum synthesis methods

A first-principles study of the structural and electronic properies of carbon impurities in CuIn$_{1-x}$Ga$_x$Se$_{2}$ is presented. Carbon is present in organic molecules in the precursor solutions used in nonvacuum growth methods, making more efficient use of material, time and energy than traditional vacuum methods. The formation energies of several carbon impurities are calculated using the hybrid HSE06 functional. C$_{\mathrm{Cu}}$ acts as a shallow donor, C$_{\mathrm{In}}$ and interstitial C yield deep donor levels in CuInSe$_{2}$, while in CuGaSe$_{2}$ C$_{\mathrm{Ga}}$ and interstitial C act as deep amphoteric defects. So, if present, these defects reduce the majority carrier (hole) concentration by compensating the acceptor levels and become trap states for the photogenerated minority carriers (electrons). However, the formation energies of the calculated carbon impurities are high, even under C-rich growth conditions. Therefore, these impurities are not likely to form and will probably be expelled to the intergranular region and out of the absorber layer.

cond-mat.mes-hall↗

arXiv2014

Native point defects in CuIn$_{1-x}$Ga$_x$Se$_{2}$: hybrid density functional calculations predict origin of p- and n-type conductivity

We have performed a first-principles study of the p- and n-type conductivity in CuIn$_{1-x}$Ga$_x$Se$_{2}$ due to native point defects, based on the HSE06 hybrid functional. Band alignment shows that the band gap becomes larger with $x$ due to the increasing conduction band minimum, rendering it hard to establish n-type conductivity in CuGaSe$_{2}$. From the defect formation energies, we find that In/Ga$_{\mathrm{Cu}}$ is a shallow donor, while V$_{\mathrm{Cu}}$, V$_{\mathrm{In}/\mathrm{Ga}}$ and Cu$_{\mathrm{In}/\mathrm{Ga}}$ act as shallow acceptors. Using total charge neutrality of ionized defects and intrinsic charge carriers to determine the Fermi level, we show that under In-rich growth conditions In$_{\mathrm{Cu}}$ causes strongly n-type conductivity in CuInSe$_{2}$. Under In-poor growth conditions the conductivity type in CuInSe$_{2}$ alters to p-type and compensation of the acceptors by In$_{\mathrm{Cu}}$ reduces, as observed in photoluminescence experiments. In CuGaSe$_{2}$, the native acceptors pin the Fermi level far away from the conduction band minimum, thus inhibiting n-type conductivity. On the other hand, CuGaSe$_{2}$ shows strong p-type conductivity under a wide range of Ga-poor growth conditions. Maximal p-type conductivity in CuIn$_{1-x}$Ga$_x$Se$_{2}$ is reached under In/Ga-poor growth conditions, in agreement with charge concentration measurements on samples with In/Ga-poor stoichiometry, and is primarily due to the dominant acceptor Cu$_{\mathrm{In}/\mathrm{Ga}}$.

cond-mat.mes-hall↗

arXiv2014

Rutherford scattering of electron vortices

By considering a cylindrically symmetric generalization of a plane wave, the first Born approximation of screened Coulomb scattering unfolds two new dimensions in the scattering problem: transverse momentum and orbital angular momentum of the incoming beam. In this paper, the elastic Coulomb scattering amplitude is calculated analytically for incoming Bessel beams. This reveals novel features occurring for wide angle scattering when the incoming beam is correctly prepared. The result successfully generalizes the well known Rutherford formula, incorporating transverse and orbital angular momentum into the formalism.

quant-ph↗

arXiv2013

The 30-band k.p modeling of electron and hole states in silicon quantum wells

We modeled the electron and hole states in Si/SiO2 quantum wells within a basis of standing waves using the 30-band k.p theory. The hard-wall confinement potential is assumed, and the influence of the peculiar band structure of bulk silicon on the quantum-well sub-bands is explored. Numerous spurious solutions in the conduction-band and valence-band energy spectra are found and are identified to be of two types: (1) spurious states which have large contributions of the bulk solutions with large wave-vectors (the high-k spurious solutions) and (2) states which originate mainly from the spurious valley outside the Brillouin zone (the extra-valley spurious solutions). An algorithm to remove all those nonphysical solutions from the electron and hole energy spectra is proposed. Furthermore, slow and oscillatory convergence of the hole energy levels with the number of basis functions is found and is explained by the peculiar band mixing and the confinement in the considered quantum well. We discovered that assuming the hard-wall potential leads to numerical instability of the hole states computation. Nonetheless, allowing the envelope functions to exponentially decay in a barrier of finite height is found to improve the accuracy of the computed hole states.

cond-mat.mes-hall↗

arXiv2013

Stone-Wales Defects in Silicene: Formation, Stability and Reactivity of Defect Sites

During the synthesis of ultra-thin materials with hexagonal lattice structure Stone-Wales (SW) type of defects are quite likely to be formed and the existence of such topological defects in the graphene-like structures results in dramatical changes of their electronic and mechanical properties. Here we investigate the formation and reactivity of such SW defects in silicene. We report the energy barrier for the formation of SW defects in freestanding (~2.4 eV) and Ag(111)-supported (~2.8 eV) silicene and found it to be significantly lower than in graphene (~9.2 eV). Moreover, the buckled nature of silicene provides a large energy barrier for the healing of the SW defect and therefore defective silicene is stable even at high temperatures. Silicene with SW defects is semiconducting with a direct bandgap of 0.02 eV and this value depends on the concentration of defects. Furthermore, nitrogen substitution in SW defected silicene shows that the defect lattice sites are the least preferable substitution locations for the N atoms. Our findings show the easy formation of SW defects in silicene and also provide a guideline for bandgap engineering in silicene-based materials through such defects.

cond-mat.mtrl-sci↗

arXiv2013

Spin effects in electron vortex states

The recent experimental realization of electron vortex beams opens up a wide research domain previously unexplored. The present paper explores the relativistic properties of these electron vortex beams, and quantifies deviations from scalar wave theory. It is common in electron optics to use the Schrödinger equation neglecting spin. The present paper investigates the role of spin and the total angular momentum Jz and how it pertains to the vortex states. As an application, we also investigate if it is possible to use holographic reconstruction to create novel total angular momentum eigenstates in a Transmission Electron Microscope. It is demonstrated that relativistic spin coupling effects disappear in the paraxial limit, and spin effects in holographically created electron vortex beams can only be exploited by using specialized magnetic apertures.

quant-ph↗

arXiv2013

Exploration of stable and unstable vortex patterns in a superconductor under a magnetic disc

The stable and unstable solutions of a square 2D extreme type-II superconductor are studied in the field of a magnetic disc. We use a preconditioned Newton-Krylov solver to find the solutions and use numerical continuation to track the solutions as the field strength varies. For a disc with a small radius, we have identified generic scenarios through which the state loses and regains its stability.

cond-mat.supr-con↗

arXiv2013

Dirac Kirchhoff diffraction theory

Kirchhoff's scalar diffraction theory is applied throughout photon and electron optics. It is based on the stationary electromagnetic or Schrödinger wave equation, and is useful in describing interference phenomena for both light and matter waves. Here, Kirchhoff's diffraction theory is derived from the relativistic Dirac equation, thus reformulated to work on Dirac spinors. The parallels with the "classic" scalar theory are highlighted, and a basic interpretation of the result obtained for the Fraunhofer diffraction limit is given. The goal of this paper is to emphasize the similarity between the two equations on the most fundamental level.

quant-ph↗

arXiv2013

Adsorption and absorption of Boron, Nitrogen, Aluminium and Phosphorus on Silicene: stability, electronic and phonon properties

Ab initio calculations within the density-functional theory formalism are performed to investigate the chemical functionalization of a graphene-like monolayer of silicon - silicene - with B, N, Al or P atoms. The structural, electronic, magnetic and vibrational properties are reported. The most preferable adsorption sites are found to be valley, bridge, valley and hill site for B, N, Al and P adatoms, respectively. All the relaxed systems with adsorbed/substituted atoms exhibit metallic behaviour with strongly bonded B, N, Al, and P atoms accompanied by an appreciable electron transfer from silicene to the B, N and P adatom/substituent. The Al atoms exhibit opposite charge transfer, with n-type doping of silicene and weaker bonding. The adatoms/substituents induce characteristic branches in the phonon spectrum of silicene, which can be probed by Raman measurements. Using molecular dynamics we found that the systems under study are stable up to at least T = 500 K. Our results demonstrate that silicene has a very reactive and functionalizable surface.

cond-mat.mtrl-sci↗

arXiv2013

Gapless interface states at the junction between two topological insulators

We consider a junction between two topological insulators, and calculate the properties of the interface states with an effective low energy Hamiltonian for topological insulators with a single cone on the surface. This system bears a close resemblance to bilayer graphene, as both result from the hybridization of Dirac cones. We find gapless interface states not only when the helicity direction of the topological surface states are oppositely oriented, but they can also exist if they are equally oriented. Furthermore, we find that the existence of the interface states can be understood from the closing of the bulk gap when the helicity changes orientation. Recently, superluminal tachyonic excitations were also claimed to exist at the interface between topological insulators. However, here we show that these interface states do not exist.

cond-mat.mes-hall↗

arXiv2013

First-principles investigation of bilayer fluorographene

\textit{Ab initio} calculations within the density functional theory formalism are performed to investigate the stability and electronic properties of fluorinated bilayer graphene (bilayer fluorographene). A comparison is made to previously investigated graphane, bilayer graphane, and fluorographene. Bilayer fluorographene is found to be a much more stable material than bilayer graphane. Its electronic band structure is similar to that of monolayer fluorographene, but its electronic band gap is significantly larger (about 1 eV). We also calculate the effective masses around the $Γ$-point for fluorographene and bilayer fluorographene and find that they are isotropic, in contrast to earlier reports. Furthermore, it is found that bilayer fluorographene is almost as strong as graphene, as its 2D Young's modulus is approximately 300 $\mathrm{N} \mathrm{m}^{-1}$.

cond-mat.mtrl-sci↗

arXiv2013

Adsorption of titanium and titanium dioxide on graphene: n and p-type doping

Ab initio calculations within the density-functional theory formalism are performed to investigate the ground state, electric charge doping, and electronic properties of titanium and titanium dioxide monolayers adsorbed on a graphene surface. A new ground state structrure of Ti monolayer adsorbed on graphene is reported which is shown to be stable up to T = 500 K. Effects due to lower and higher Ti adatoms coverage are studied. We find that the adsorbed Ti provides a strong n-type doping which supports recent experimental observations. On the other hand, TiO_2 can induce both p- and n-type doping in the carbon monolayer depending on whether oxygen or titanium atoms are closest to the substrate. We identify the structures which are responsible for the experimentally observed autocompensation mechanism that leads to the reversion of adsorbate effects after oxidation of the adsorbed Ti.

cond-mat.mtrl-sci↗

arXiv2012

Efficient determination of the energy landscape of nonlinear Schrödinger-type equations

We describe a systematic approach for the efficient numerical solution of nonlinear Schrödinger-type partial differential equations of the form $(K +V + g|ψ|^2)ψ=0$, with an energy operator $K$, a scalar potential $V$, and a scalar parameter $g$. Instrumental to the approach are developments in numerical linear and nonlinear algebra, specifically numerical parameter continuation. We demonstrate how a continuous sequence of solutions can be obtained regardless of their stability, so that finally the spectrum of stable and unstable solutions in the specified parameter range is fully revealed. The method is demonstrated for the GL equation in a three-dimensional superconducting domain with an inhomogeneous magnetic field, a numerically demanding problem known to have an involved solution landscape.

physics.comp-ph↗