arXiv subjects

P. Süle

Publications and source records attributed to P. Süle.

At least 19 recordsLinked to original sources

arXiv2017

Alloy formation at the bottom and top Fe interfaces of Ti/Fe/Ti trilayers

The chemical mixing at the Fe-on-Ti and Ti-on-Fe, i.e. the bottom and top interfaces of Fe in atomically deposited layers, were studied experimentally and by molecular dynamics simulations of the layer growth. The basic structure and chemical composition of the layers were explored in cross-sections using transmission electron microscopy. The calculations show a concentration distribution along the layer growth direction which is atomically sharp at the Ti-on-Fe interface for the (001) and (110) crystallographic orientations of the Fe layer, while it varies over a few atomic layers for Fe(111) substrate and at the Fe-on-Ti interface for all studied substrate orientations. Conversion electron Mössbauer spectroscopy and X-ray reflectometry measurements indicate an even larger asymmetry of the bottom and top Fe interface in Ti/Fe/Ti trilayers grown over Si(111) substrate by vacuum evaporation.

cond-mat.mes-hall↗

arXiv2015

Time-lapsed graphene moiré superlattice on Cu(111)

The detailed study of the graphene (gr) moiré superlattices emerging due to the mismatch between the substrate's and gr-overlayer crystal lattices is inevitable because of its high technological relevance. However, little is known about the dynamics of moiré superstructures on gr. Here, we report the first classical molecular dynamics simulation (CMD) of the moiré superlattice of graphene on Cu(111) using a new parameterized Tersoff-potential for the graphene/Cu(111) interface fitted in this paper to nonlocal van der Waals density functional theory (DFT) calculations. The interfacial force field with time-lapsed CMD provides superlattices in good quantitative agreement with the available experimental results. The long range coincidence supercells of $2 \times 2$ and $3 \times 3$ with nonequivalent moiré hills have also been identified and analyzed. The moiré superlattice exhibits a pattern which is dynamical rather than statically pinned to the support and can be observed mostly via time lapsing. The instantaneous snapshots of the periodic moiré pattern already at low temperature are weakly disordered lacking the apparent sharpness of the time averaged pattern and scanning tunneling microscopy images. This suggests the existence of competing orders between a static (1st order) and a dynamical (2nd order) moiré superstructures.The revealed random height fluctuations may limit the important electronic properties of supported graphene such as the mobility of charge carriers.

cond-mat.mtrl-sci↗

arXiv2014

Rotation misorientated graphene moire superlattices on Cu(111): classical molecular dynamics simulations and scanning tunneling microscopy studies

Graphene on copper is a system of high technological relevance, as Cu is one of the most widely used substrates for the CVD growth of graphene. However, very little is known about the details of their interaction. One approach to gain such information is studying the superlattices emerging due to the mismatch of the two crystal lattices. However, graphene on copper is a low-corrugated system making both their experimental and theoretical study highly challenging. Here, we report the observation of a new rotational Moire superlattice of CVD graphene on Cu (111), characterized by a periodicity of $1.5 \pm 0.05$ nm and corrugation of $0.15 \pm 0.05$ $\hboxÅ$ , as measured by Scanning Tunneling Microscopy. To understand the observed superlattice we have developed a newly parameterized Tersoff-potential for the graphene/Cu (111) interface fitted to nonlocal van der Waals density functional theory (DFT) calculations. The interfacial force field with time-lapsed CMD provides superlattices in good quantitative agreement with the experimental results, for a misorientation angle of $10.4 \pm 0.5,^{\circ}$ without any further parameter adjustment. Furthermore, the CMD simulations predict the existence of two non-equivalent high-symmetry directions of the Moiré pattern that could also be identified in the experimental STM images.

cond-mat.mtrl-sci↗

arXiv2014

The DFT and molecular dynamics multiscale study of the corrugation of graphene on Ru(0001): the unexpected stability of the moire-buckled structure

Results from first principles density functional theory (DFT) calculations and classical molecular dynamics (CMD) simulations are presented on moire-corrugation of graphene (gr). We find that the moire-corrugated graphene could be surprisingly stable against the perfectly flat gr-sheet as pointed out by CMD simulations and DFT calculations. We also show that using the cost-effective CMD approach one can simulate graphene on e.g. Ru(0001) with a correct binding registry and reasonable corrugation and adhesion energy. A new force field has been parameterized for the interface using an angular-dependent Abell-Tersoff potential. The newly parameterized Abell-Tersoff interface potential provides correct moire superstructures in accordance with scanning tunnelling microscopy images and with DFT results. Based on ab initio DFT calculations, we also find that the CMD moire superstructure can be used as a preoptimized structure for DFT calculations and for further geometry optimization. The nearly flat gr (the corrugation $ξ\approx 0.2$ $\hboxÅ$) on Ru(0001) is slightly energetically unfavorable vs. the moire-corrugated gr-system ($ξ\approx 2.0$ $\hboxÅ$) as revealed by van der Waals DFT structural relaxation.

cond-mat.mtrl-sci↗

arXiv2014

Anomalous nanoscale diffusion in Pt/Ti: superdiffusive intermixing

Probing the anomalous nanoscale intermixing using molecular dynamics (MD) simulations in Pt/Ti bilayer we characterize the superdiffusive nature of interfacial atomic transport. In particular, the low-energy ($0.5$ keV) ion-sputtering induced transient enhanced intermixing has been studied by MD simulations. {\em Ab initio} density functional calculations have been used to check and reparametrize the employed heteronuclear interatomic potential. We find a robust intermixing in Pt/Ti driven by nanoscale mass-anisotropy . The sum of the square of atomic displacements $\langle R^2 \rangle$ asymptotically scales nonlinearly ($\langle R^2 \rangle \propto t^2$), where $t$ is the time of ion-sputtering, respectively which is the fingerprint of superdiffusive features. This anomalous behavior explains the high diffusity tail in the concentration profile obtained by Auger electron spectroscopy depth profiling (AES-DP) analysis in Pt/Ti bilayer (reported in ref.: P. Süle, et al., J. Appl. Phys., 101, 043502 (2007)). In Ti/Pt bilayer a linear time scaling of $\langle R^2 \rangle \propto t$ has been found at the Ti/Pt interface indicating the suppression of superdiffusive features. These findings are inconsistent with the standard ion-mixing models. Instead a simple accelerative effect of the downward fluxes of energetic particles on the unidirectional fluxes of preferential intermixing of Pt atoms seems to explain the enhancement of interfacial broadening in Pt/Ti. Contrary to this in Ti/Pt the fluxes of recoils are in counterflow with intermixing Pt atoms and hence slows down the nanoscale mass-effect driven ballistic preferential mobility of Pt atoms.

cond-mat.mtrl-sci↗

arXiv2014

Fe impurity induced ion-nanopatterning: atomistic simulations using a new force field for FeSi

The ion-bombardment induced nanopatterning of Si(001) has been simulated by atomistic simulations with and without Fe impurity. The surface contamination has been simulated by using a new force field developed for FeSi. This is a fitted bond order potential (BOP) given for Si and for Fe by Albe et al. This BOP formula has been optimized simultaneously for FeSi, Si and for Fe. Using this new force field we are able to follow the ion-beam assisted deposition for Si in the presence of Fe contamination in the surface region. As an overall result, we get an unexpectedly rich variety of nanopatterns formed by the reorganization of the crater rims of the individual ion impacts. The previously thought simple atomistically roughened surfaces show unprecedented landscapes and topography with nanoscale features. The characteristic size of the units of the pattern is in the range of a few nms. Typical of the occurred pattern is the network of interconnected elongated adatom islands. We also see the self-organization of this pattern upon ion-bombardments. At $50^{\circ}$ impact angle we get a nanoporous surface (sponge-like) both for Fe-contaminated and Fe-free simulations. At $70^{\circ}$ of impact (grazing angle of incidence) the pattern resembles to that of elongated atomic chains (adatom islands) along the ion impact direction. This latter pattern could be understood as a prepattern state towards rippling. At lower angles ($30^{\circ}$) nanoholes rule the landsdcape. The obtained pattern corresponds to low fluence experiments which are used to consider as simple roughening without showing any sing of patterning.

cond-mat.mtrl-sci↗

arXiv2014

Bond order potential for FeSi

A new parameter set has been derived for FeSi using the Albe-Erhart-type bond order potential (BOP) and the PONTIFIX code for fitting the parameters on a large training set of various polymorphs. Ab initio calculations are also carried out to study the relative stability of various polymorphs and to use the obtained data in the training set. The original BOP formalism was unable to account for the correct energetic relationship between the B20 ($ε$-FeSi) and B2 (CsCl) phases and notoriusly slightly favors incorrectly the B2 polymorph. In order to correct this improper behavior the BOP potential has been extended by a Columbic term (BOP+C) in order to account for the partial ionic character of FeSi. Using this potential we are able to account for the correct phase order between the most stable B20 and B2 (CsCl) polymorphs when the net atomic charges are properly set. Although this brings in a new somewhat uncertain parameter (the net charges) one can adjust properly the BOP+C potential for specific problems. To demonstrate this we study under high pressure the B2 phase which becomes more stable vs. B20 as it is found experimentally and expected to be taken place in the Earth mantle. The obtained BOP has also been tested for the metallic and semiconducting disilicides ($α$-FeSi$_2$ and $β$-FeSi$_2$) and for the Si/$β$-FeSi$_2$ heterostructure. The obtained BOP, as many other BOP, overestimates the melting point ($T_m$) of the B20 phase by $\sim 1000$ K if the parameters in the radial part of the potential were obtained according to the Pauling relation (regular way).

cond-mat.mtrl-sci↗

arXiv2013

The classical molecular dynamics simulation of graphene on Ru(0001) using a fitted Tersoff interface potential

The accurate molecular dynamics simulation of weakly bound adhesive complexes, such as supported graphene, is challenging due to the lack of an adequate interface potential. Instead of the widely used Lennard-Jones potential for weak and long-range interactions we use a newly parameterized Tersoff-potential for graphene/Ru(0001) system. The new interfacial force field provides adequate moir$\acute{e}$ superstructures in accordance with scanning tunnelling microscopy images and with DFT results. In particular, the corrugation of $ξ\approx 1.0 \pm 0.2$ $\hboxÅ$ is found which is somewhat smaller than found by DFT approaches ($ξ\approx 1.2$ $\hboxÅ$) and is close to STM measurements ($ξ\approx 0.8 \pm 0.3$ $\hboxÅ$).The new potential could open the way towards large scale simulations of supported graphene with adequate moir$\acute{e}$ supercells in many fields of graphene research. Moreover, the new interface potential might provide a new strategy in general for getting accurate interaction potentials for weakly bound adhesion in large scale systems in which atomic dynamics is inaccessible yet by accurate DFT calculations.

cond-mat.mtrl-sci↗

arXiv2008

Lévy-flight intermixing: anomalous nanoscale diffusion in Pt/Ti

Probing the anomalous nanoscale intermixing using molecular dynamics (MD) simulations in Pt/Ti bilayer we reveal the superdiffusive nature of interfacial atomic transport. It is shown that the Pt atoms undergo anomalous atomic transport across the anisotropic interface of Pt/Ti with suprisingly high rates which can be characterized as Lévy flights. Lévy flight is not a unique phenomenon in nature, however, no such events have been reported yet for bulk interdiffusion. In particular, the low-energy (0.5 keV) ion-sputtering induced transient enhanced intermixing has been studied by MD simulations. Ab initio density functional calculations have been used to check and reparametrize the employed heteronuclear interatomic potential. The Lévy-intermixing behavior explains the high diffusity tail in the concentration profile obtained by Auger electron spectroscopy depth profiling (AES-DP) analysis in Pt/Ti bilayer (reported in ref.: P. Süle, {\em et al.}, J. Appl. Phys., {\bf 101}, 043502 (2007)).

cond-mat.mtrl-sci↗

arXiv2008

Self-organized transient facilitated atomic transport in Pt/Al(111)

During the course of atomic transport in a host material, impurity atoms need to surmount an energy barrier driven by thermodynamic bias or at ultra-low temperatures by quantum tunneling. In the present article we demonstrate using atomistic simulations that at ultra-low temperature transient inter-layer atomic transport is also possible without tunneling when the Pt/Al(111) impurity/host system self-organizes itself spontaneously into an intermixed configuration. No such extremely fast athermal concerted process has been reported before at ultra low temperatures. The outlined novel transient atomic exchange mechanism could be of general validity. We find that the source of ultra-low temperature heavy particle barrier crossing is intrinsic and no external bias is necessary for atomic intermixing and surface alloying in Pt/Al although the dynamic barrier height is few eV. The mechanism is driven by the local thermalization of the Al(111) surface in a self-organized manner arranged spontaneously by the system without any external stimulus. The core of the short lived thermalized region reaches the local temperature of $\sim 1000$ K (including few tens of Al atoms) while the average temperature of the simulation cell is $\sim 3$ K. The transient facilitated intermixing process also takes place with repulsive impurity-host interaction potential leading to negative atomic mobility hence the atomic injection is largely independent of the strength of the impurity-surface interaction. We predict that similar exotic behaviour is possible in other materials as well.

cond-mat.mtrl-sci↗

arXiv2008

Anisotropy driven ultrafast nanocluster burrowing

We explore the occurrence of low-energy and low-temperature transient cluster burrowing leading to intact cluster inclusions. In particular, the anomalously fast (ballistic) Pt nanocluster implantation into Al and Ti substrates has been found by molecular dynamics simulations using a tight-binding many-body potential with the 1-5 eV/atom low impact energy. Similar behavior has also been found for many other cluster/substrate couples such as Cu/Al and Ni/Ti, Co/Ti, etc. In particular, in Ni/Ti at already $\sim 0.5$ eV/atom impact energy burrowing takes place. At this few eV/atom low impact energy regime instead of the expected stopping at the surface we find the propagation of the cluster through a thin Al slab as thick as $\sim 50$ $\hboxÅ$ with a nearly constant speed ($\propto 1$ eV/atom). Hence the cluster moves far beyond the range of the impact energy which suggests that the mechanism of cluster burrowing can not be explained simply by collisional cascade effects. In the couples with reversed succession (e.g. Ti/Pt, Al/Pt) no burrowing has been found, the clusters remain on the surface (the asymmetry of burrowing). We argue that cluster penetration occurs at few eV/atom impact energy when the cluster/substrate interaction is size-mismatched and mass anisotropic atomically.

cond-mat.mtrl-sci↗

arXiv2006

Fingerprint of super-interdiffusion: anomalous intermixing in Pt/Ti

The ion-sputtering induced transient enhanced intermixing has been studied by molecular dynamics (MD) simulations in Pt/Ti and its anomalous nature has been explained as a superdiffusive transient enhanced interdiffusion. We find ballistic mixing and a robust mass effect in Pt/Ti. The sum of the square of atomic displacements ($ $) asymptotically grows nonlinearily and scales as $N^2$ and $\sim t^2$, where $N$ and $t$ are the ion-number fluence and the time of ion-sputtering, respectively. This anomalous behavior explains the high diffusity tail in the concentration profile obtained by Auger electron spectroscopy depth profiling (AES-DP) analysis in Pt/Ti/Si substrate (Pt/Ti) multilayer. In Ta/Ti/Pt/Si multilayer we find a linear time scaling of $ \propto t$ at the Ti/Pt interface indicating the suppression of superdiffusive features. We propose a qualitative explanation based on the accelerative effect of nonlinear forces provbided by the anharmonic host lattice.

cond-mat.mtrl-sci↗

arXiv2006

Asymmetric intermixing in Pt/Ti

The ion-sputtering induced intermixing is studied by Monte-Carlo TRIM, molecular dynamics (MD) simulations, and Auger electron spectroscopy depth profiling (AES-DP) analysis in Pt/Ti/Si substrate (Pt/Ti) and Ta/Ti/Pt/Si substrate (Ti/Pt) multilayers. Experimental evidence is found for the asymmetry of intermixing in Pt/Ti, and in Ti/Pt. In Ti/Pt we get a much weaker interdiffusion than in Pt/Ti. The unexpected enhancement of the interdiffusion of the Pt atoms into the Ti substrate has also been demonstrated by simulations. We are able to capture the essential features of intermixing using TRIM and MD simulations for ion-beam sputtering and get reasonable values for interface broadening which can be compared with the experimental measurements. However, the origin of the asymmetry remains poorly understood yet.

cond-mat.mtrl-sci↗

arXiv2006

Intermixing in Cu/Co: molecular dynamics simulations and Auger electron spectroscopy depth profiling

The ion-bombardment induced evolution of intermixing is studied by molecular dynamics simulations and by Auger electron spectroscopy depth profiling analysis (AESD) in Cu/Co multilayer. It has been shown that from AESD we can derive the low-energy mixing rate and which can be compared with the simulated values obtained by molecular dynamics (MD) simulations. The overall agreement is reasonably good hence MD can hopefuly be used to estimate the rate of intermixing in various interface systems.

cond-mat.mtrl-sci↗

arXiv2005

Simulated ion-sputtering and Auger electron spectroscopy depth profiling study of intermixing in Cu/Co

The ion-bombardment induced evolution of intermixing is studied by molecular dynamics simulations and by Auger electron spectroscopy depth profiling analysis (AESD) in Cu/Co multilayer. It has been shown that from AESD we can derive the low-energy mixing rate and which can be compared with the simulated values obtained by molecular dynamics (MD) simulations.The overall agreement is reasonably good hence MD can hopefully be used to estimate the rate of intermixing in various interface systems.

cond-mat.mtrl-sci↗

arXiv2005

Interface-anisotropy induced asymmetry of intermixing in bilayers

The ion-sputtering induced intermixing is studied by molecular dynamics (MD) simulations and by Auger electron spectroscopy depth profiling (AES-DP) analysis in Pt/Ti/Si substrate (Pt/Ti) and Ta/Ti/Pt/Si substrate (Ti/Pt) multilayers. Experimental evidence is found for the asymmetry of intermixing in Pt/Ti and in Ti/Pt. An unexpected enhancement of the injection of the heavy Pt atoms into the Ti substrate is observed both by AES-DP and by MD simulations. In Ti/Pt we get a much weaker interdiffusion than in Pt/Ti. The asymmetry is explained by the backscattering of hyperthermal particles at the mass-anisotropic interface and which is reproduced by computer atomistic simulations. The AES-DP measurements support our earlier predictions (P. Süle, M. Menyhárd, Phys. Rev., {\bf B71}, 113413 (2005)) obtained for mass-anisotropic bilayers.

cond-mat.mtrl-sci↗

arXiv2004

Strong mass effect on ion beam mixing in metal bilayers

Molecular dynamics simulations have been used to study the mechanism of ion beam mixing in metal bilayers. We are able to explain the ion induced low-temperature phase stability and melting behavior of bilayers using only a simple ballistic picture up to 10 keV ion energies. The atomic mass ratio of the overlayer and the substrate constituents seems to be a key quantity in understanding atomic mixing. The critical bilayer mass ratio of $δ< 0.33$ is required for the occurrence of a thermal spike (local melting) with a lifetime of $τ> 0.3$ ps at low-energy ion irradiation (1 keV) due to a ballistic mechanism. The existing experimental data follow the same trend as the simulated values.

cond-mat.mtrl-sci↗

arXiv2004

What is the real driving force of ion beam mixing?

Molecular dynamics simulations have been used to study the driving force of ion irradiation induced interfacial mixing in metal bilayers in which the relative mass of the constituents is considerable. We find no apparent effect of chemical forces, such as heat of mixing or cohesive energy up to 8 keV ion energy, although a considerable number of liquid and high energy particles (hot atoms) persist up to even 20 ps during the thermal spike. This result is in direct conflict with the widely accepted theory of thermal spike mixing (chemical interdiffusion model). The supersaturation of vacancies also occurs and which induces a thermally activated intermixing of the lighter constituent of the bilayer. The delay and the decoupling of the intermixing of the light constituent is explained as a backscattering effect at the interface: the interface acts as a diffusional barrier for high energy particles due to the large difference in atomic masses. The heavier atoms are predominantly ejected to the overlayer at the beginning of the thermal spike while the light atoms are injected to the bulk at the beginning of the cooling period (in Ti/Pt) or during the thermal spike with some time delay (Al/Pt).

cond-mat.mtrl-sci↗