arXiv subjects

Hristina Popova

Publications and source records attributed to Hristina Popova.

9 recordsLinked to original sources

arXiv2026

Morphological Transition: From Meanders to Mound Structures

Mound formation on flat and miscut crystal surfaces exhibits distinct growth behaviors. While mound structures are the predominant feature on flat surfaces, miscut surfaces display a smooth transition from meandered patterns to three-dimensional mounds, depending on both internal and external conditions. We investigate this morphological evolution-from meander-like surface patterns to faceted pyramidal structures-using a Vicinal Cellular Automaton modeling framework. The transition is shown to be governed by the competition between the Ehrlich-Schwoebel barrier and adatom mobility on terraces. Under moderate barrier strengths and sufficiently high terrace diffusivity, the system demonstrates a reversible transition from mounded configurations to regular step meandered patterns. This reveals a complex interplay between kinetic barriers and mass transport. Our simulations cover a wide range of growth conditions, including variations in deposition flux, surface diffusion rates, temperature, and miscut angle. By applying the height-height correlation function, we calculate the correlation lengths along and across the steps and analyze their scaling behavior. These results offer insight into the continuum pathways that connect distinct classes of surface structures and provide a unified framework for describing pattern evolution across different crystal growth regimes.

cond-mat.mtrl-sci↗

arXiv2026

Complex surface patterning in homo- and heteroepitaxial contexts: (simultaneous) step bunching and step meandering

We confront a meso-scale continuum model, archetypical for the heteroepitaxial context, with an atomistic Vicinal Cellular Automaton (VicCA), built as a homoepitaxial counterpart, to show that in (2+1)D complex surface instabilities are fundamental growth phenomena rather than context-specific artifacts. Our approach is to first construct a Ginzburg-Landau-type model, designed to extend the previously (1+1)D Tersoff-type models in (2+1)D. We complement the continuum approach with a discrete one - the VicCA, in which we use a novel version of the potential landscape for the diffusing particles - a double-well potential located at each step edge. Notably, this framework also reproduces step bunching and step meandering - which are typically treated as incompatible in the theoretical paradigm, but coexist in real material systems. Thus we establish a cross-context correspondence at the level of obtained morphologies and morphology diagrams and, additionally, a multiscale perspective on the governing parameters, bridging the gap between the mesoscale and atomistic modeling.

cond-mat.mtrl-sci↗

arXiv2025

Step meandering: The balance between the potential well and the Ehrlich-Schwoebel barrier

This study presents a comprehensive and innovative exploration of how the surface potential energy landscape influences meander formation. Using the Vicinal Cellular Automaton model, which distinguishes surface diffusion from adatom incorporation into the crystal, the research delves into various factors affecting surface pattern dynamics. By isolating the diffusion process within a defined energy potential, the study provides a detailed analysis of how changes in the potential energy well and the barrier at the top of the step contribute to meander formation. Remarkably, the results reveal that the mere presence of a potential well at the step's bottom is sufficient to induce meandering. The role of the Ehrlich-Schwoebel barrier on formed meanders is further investigated, and a mechanism for meander formation is proposed to clarify this process. The derived relation successfully reflects the wavelength of the meandered patterns observed in the simulations, emphasizing its reliability. Overall, the results illustrate the crucial influence of the surface energy potential's shape in driving surface pattern formation.

cond-mat.mes-hall↗

arXiv2022

Predicted universality class of step bunching found on DC-heated Si(111) surfaces

Concerted experimental and numerical studies of step bunching on vicinal crystal surfaces resulting from step-down electromigration of partially charged adatoms, confirmed the theoretical prediction of scaling dependence of the minimal bunch distance $l_{\rm min}$ on the bunch size $N$: $l_{\rm min} \sim N^{-γ}$, with $γ= 2/3$. The value of the so called size-scaling exponent $γ$ was observed in experiments on vicinal surfaces of semiconducting, metallic, and dielectric materials. Careful theoretical investigations and numerical calculations predict a second value of $γ= 1/2$. However, this value is still not been reported from experiments. And we report here experimental observation of step bunching in the universality class relative to $γ= 1/2$. This is achieved by monitoring step flow during sublimation of Si(111)-vicinals heated by a direct step-down current at ~1200$^\circ$C. In the experiment we also measure other characteristic for the bunching quantities, such as the mean total number of steps in the bunch $N$ and the mean bunch width $W$. We then compare our findings with published experimental and numerical data to arrive at a theoretically consistent framework in terms of universality classes. The ultimate benefit of our study is not only to advance fundamental knowledge but also to provide further guidance for bottom-up synthesis of vicinal nanotemplates.

cond-mat.mes-hall↗

arXiv2020

Bunch width versus macrostep height: A quantitative study of the effects of step-step repulsion

Bunching of steps at the surface of growing crystals can be induced by both directions of the driving force: step up and step down. The processes happen in different adatom concentrations and differ in character. In this study we show how the overall picture of the bunching process depends on the strength of short range step-step repulsion. The repulsive interaction between steps, controlled by an additional parameter, is introduced into the recently studied atomistic scale model of vicinal crystal growth, based on cellular automata. It is shown that the repulsion modifies bunching process in a different way, depending on the direction of the destabilizing force. In particular, bunch profiles, stability diagrams and time-scaling dependences of various bunch properties are affected when the step-step repulsion increases. The repulsion between steps creates a competition between two characteristic sizes - bunch width and macrostep height, playing the role of the second length scale that describes the step bunching phenomenon. A new characteristic time scale dependent on the step-step repulsion parameter emerges as an effect of interplay between (01) faceted macrosteps and (11) faceted bunches. The bunch height being the major characteristic size of the bunches is not influenced dramatically by the repulsion.

cond-mat.mtrl-sci↗

arXiv2018

Unstable dynamics of model vicinal crystal surfaces: Initial and intermediate stages

We approach the old-standing problem of vicinal crystal surfaces destabilized by step-down and step step-up currents from a unified modelling viewpoint with focus on both the initial and the intermediate stages of the instability. We develop further our atomistic scale model of vicinal crystal growth (Gr) destabilized by SD drift of the adatoms in order to account for also the vicinal crystal sublimation (Sbl) and the SU drift of the adatoms as an alternative mode of destabilization. In order to study the emergence of the instability we use the number of steps in the bunch (bunch size) N as a measure and probe with small-size systems the models stability against step bunching (SB) on a dense grid of points in the parameter space formed by the diffusion rate/step transparency, surface miscut and drift direction, for each of the four possible cases - Gr+SD, Gr+SU, Sbl+SD, Sbl+SU. The obtained stability diagrams show where the system is initially most unstable and provide a ground to study there the intermediate stages of the developed instability quantifying the surface self-similarity by the time-scaling of N. For each of the four enumerated cases we show that it reaches the universal curve N=2sqrt(T/3), where T is the time, properly rescaled with the model parameters. We confirm the value of the numerical pre-factor with results from a parallel study of models based on systems of ordinary differential equations (ODE) for the step velocity.

cond-mat.mtrl-sci↗

arXiv2017

Step bunching with both directions of the current: Vicinal W(110) surfaces versus atomistic scale model

We report for the first time the observation of bunching of monoatomic steps on vicinal W(110) surfaces induced by step up or step down currents across the steps. Measurements reveal that the size scaling exponent γ, connecting the maximal slope of a bunch with its height, differs depending on the current direction. We provide a numerical perspective by using an atomistic scale model with a conserved surface flux to mimic experimental conditions, and also for the first time show that there is an interval of parameters in which the vicinal surface is unstable against step bunching for both directions of the adatom drift.

cond-mat.mtrl-sci↗

arXiv2015

Unstable vicinal crystal growth from cellular automata

In order to study the unstable step motion on vicinal crystal surfaces we devise vicinal Cellular Automata. Each cell from the colony has value equal to its height in the vicinal, initially the steps are regularly distributed. Another array keeps the adatoms, initially distributed randomly over the surface. The growth rule defines that each adatom at right nearest neighbor position to a (multi-) step attaches to it. The update of whole colony is performed at once and then time increases. This execution of the growth rule is followed by compensation of the consumed particles and by diffusional update(s) of the adatom population. Two principal sources of instability are employed: biased diffusion and infinite inverse Ehrlich-Schwoebel barrier (iiSE). Since these factors are not opposed by step-step repulsion the formation of multi-steps is observed but in general the step bunches preserve a finite width. We monitor the developing surface patterns and quantify the observations by scaling laws with focus on the eventual transition from diffusion-limited to kinetics-limited phenomenon. The time-scaling exponent of the bunch size N is 1/2 for the case of biased diffusion and 1/3 for the case of iiSE. Additional distinction is possible based on the time-scaling exponents of the sizes of multi-steps, these are 0.36-0.4 (for biased diffusion) and 1/4 (iiSE).

cond-mat.mtrl-sci↗

arXiv2008

Anomalous diffusion of a tethered membrane: A Monte Carlo investigation

Using a continuum bead-spring Monte Carlo model, we study the anomalous diffusion dynamics of a self-avoiding tethered membrane by means of extensive computer simulations. We focus on the subdiffusive stochastic motion of the membrane's central node in the regime of flat membranes at temperatures above the membrane folding transition. While at times, larger than the characteristic membrane relaxation time $τ_R$, the mean-square displacement of the center of mass of the sheet, $ $, as well as that of its central node, $ $, show the normal Rouse diffusive behavior with a diffusion coefficient $D_N$ scaling as $D_N \propto N^{-1}$ with respect to the number of segments $N$ in the membrane, for short times $t\le τ_R$ we observe a {\em multiscale dynamics} of the central node, $ \propto t^α$, where the anomalous diffusion exponent $α$ changes from $α\approx 0.86$ to $α\approx 0.27$, and then to $α\approx 0.5$, before diffusion turns eventually to normal. By means of simple scaling arguments we show that our main result, $α\approx 0.27$, can be related to particular mechanisms of membrane dynamics which involve different groups of segments in the membrane sheet. A comparative study involving also linear polymers demonstrates that the diffusion coefficient of self-avoiding tethered membranes, containing $N$ segments, is three times smaller than that of linear polymer chains with the same number of segments.

cond-mat.soft↗