Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session H35: Surfaces; Morphology, Growth, and Phases |
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Sponsoring Units: DCMP Chair: Ted Einstein, University of Maryland Room: 298 |
Tuesday, March 14, 2017 2:30PM - 2:42PM |
H35.00001: Understanding structural modulations in ultrathin Fe films on Cu(001): A systematic approach based on the bulk transition path Xie Zhang, Tilmann Hickel, Jutta Rogal, Joerg Neugebauer Despite the application of various experimental and theoretical techniques, a proper structure model for the ultrathin Fe films on Cu(001) is missing. This is due to the challenging task to search for possible metastable modulated structures in the high-dimensional configuration space. In this work, we have developed a theoretical approach based on \textit{ab initio} computations to significantly reduce the dimensionality of the configuration space by transferring the insights obtained from our recently identified fcc-bcc transformation path in the bulk with the orientation relationship observed in the ultrathin Fe film experiments. Using this approach, we are able to interpret the complex lattice modulations and surface reconstructions in terms of simple structural motifs, and explain the structural and magnetic features of the experimentally observed distinct regimes in the films. These insights improve our understanding of the fundamental interplay between magnetism and structure in Fe-based materials. [Preview Abstract] |
Tuesday, March 14, 2017 2:42PM - 2:54PM |
H35.00002: Surface diffusion of a carbon-adatom on Au(110) surfaces E. Kim, A. Safavi-Naini, D.A. Hite, K.S. McKay, D.P. Pappas, P.F. Weck, H.R. Sadeghpour We have investigated the surface diffusion of carbon-adatom on gold surfaces using density functional theory and detailed scanning probe microscopy. The decoherence of trapped-ion quantum gates due to heating of their motional modes is a fundamental science and engineering problem. In an effort to understand heating at the trap-electrode surfaces, we investigate the possible source of noise by focusing on the diffusion of carbon-containing adsorbates onto the Au(110) surface. In this study, we show how the diffusive motion of carbon adatom on gold surface significantly affects the energy landscape and adatom dipole moment variation. A simple model for the diffusion noise, which varies quadratically with the variation of the dipole moment, qualitatively reproduces the measured noise spectrum, and the estimate of the noise spectral density is in accord with measured values. [Preview Abstract] |
Tuesday, March 14, 2017 2:54PM - 3:06PM |
H35.00003: Multi-atom induced step instability in Fe/Cu(100) growth Yunsic Shim, Jacques Amar We examine the step instability observed in Fe deposition on a Cu(100) vicinal substrate [1] in which it was found that a very small Fe coverage ($0.035$ ML) leads to a dramatic change in the step morphology from relatively straight [100] steps to a mixture of [100] and [110] step-edges. While this has been previously attributed to stress-induced step erosion which arises as a result of Fe embedding in Cu(100), our temperature-accelerated dynamics simulations indicate that instead it is due to a variety of complex low-barrier multiatom interlayer diffusion (MID) processes at step edges which are enhanced by strain effects as well as the existence of strong Fe-Fe and Fe-Cu interactions. Combined with the relatively high energy barrier for MID at [110] steps as well as the moderate mobility of Fe monomers on Cu(100) these processes lead to [110] step formation. [Preview Abstract] |
Tuesday, March 14, 2017 3:06PM - 3:18PM |
H35.00004: Reversible submonolayer island nucleation and growth with anomalous diffusion Ehsan Sabbar, Jacques Amar Motivated by recent experiments with anomalous diffusion, a general rate-equation (RE) theory of submonolayer island nucleation and growth has been developed [J.G. Amar and M. Semaan, Phys.Rev. E 93, 062805 (2016)] which takes into account the critical island-size $i$, island fractal dimension $d_f$, substrate dimension $d$, and diffusion exponent $\mu$, and good agreement with simulations was found for the case of irreversible growth ($i = 1$) with $d=2$. Here we present the results of simulations of reversible island growth ($i=d=2$) which were carried out for both the case of subdiffusion ($\mu< 1$) and superdiffusion ($1 <\mu \le 2$). In the case of superdiffusion, excellent agreement is obtained with the generalized RE theory for the exponents which describe the scaling of the island and monomer densities with deposition rate. In addition, the exponents do not depend on whether or not monomers remain superdiffusive or are thermalized after detaching from a dimer. In contrast, while there is good agreement with the RE theory for point-islands in the case of subdiffusion, for ramified islands ($d_f\simeq2$) the exponents are significantly higher than predicted. Some possible explanations for this discrepancy are discussed. [Preview Abstract] |
Tuesday, March 14, 2017 3:18PM - 3:30PM |
H35.00005: On Scaling Exponents for Roughening and Coarsening in Unstable Epitaxial Mound Growth: A Parametric Study Joshua Schneider, Christian Ratsch, Dionisios Margetis, Frederic Gibou We describe scaling behavior of growing crystal surfaces in the presence of Ehrlich-Schwoebel step-edge barriers and two surface transport mechanisms: Downward funneling (DF) and transient mobility (TM). DF and TM are mechanisms that enable freshly deposited atoms to descend to lower layers on the surface, leading to a downhill mass current. This downhill current opposes the effect of step-edge barriers, which tend to suppress interlayer transport. Using kinetic Monte Carlo simulations of atomistic motion and level set simulations of island dynamics, we find that scaling exponents for surface roughening and mound coarsening depend on the strength of both step-edge barriers and surface transport mechanisms during unstable growth. Specifically, as downward transport becomes stronger, roughening exponents decrease while coarsening exponents increase, measured after a fixed amount of deposition. Dependence of scaling behavior on Ehrlich-Schwoebel barriers, detachment rate of atoms from step edges, and edge diffusion is also discussed. The goal of this work is to provide a more thorough understanding of the interplay between several competing effects which influence mound evolution. [Preview Abstract] |
Tuesday, March 14, 2017 3:30PM - 3:42PM |
H35.00006: Temperature and coverage-dependent height distribution of Ag nano-islands on Si(111)7x7 P. F. Miceli, Yiyao Chen, M. W. Gramlich, S. T. Hayden The height distribution of Ag(111) nano-islands on Si(111)7x7 exhibits an unusual minimum height that is imposed by the quantum confinement of the conduction electrons. Here we report an investigation of this system by in situ x-ray reflectivity. For different coverages as well as different growth and annealing temperatures, the island height distribution was observed to exhibit a variance that is less than the mean by a constant amount. These results suggest Poisson-like statistics except that there is a constraint for the minimum island height. Motivated by this observation, a modified Poisson statistics model is presented and shown to provide a good description of the experimentally measured island height distributions. The dependence on deposition and annealing temperature is discussed in terms of mobility that leads to islands that are taller than the minimum height. [Preview Abstract] |
Tuesday, March 14, 2017 3:42PM - 3:54PM |
H35.00007: Ab initio thermodynamics and kinetics for coalescence on nanoislands and nanopits on metal(100) surfaces Jim Evans, Yong Han, Conrad Stoldt, Patricia Thiel Coalescence or sintering of nanoscale features on metal(100) surfaces is mediated by periphery or edge diffusion. These processes are highly sensitive to the multiple diffusion barriers for various local edge environments. We provide an optimal strategy to determine both thermodynamics and kinetics for these systems at the ab initio level. The former requires assessing conventional interactions between adatoms at adsorption sites. The latter requires assessing unconventional interactions between the hopping atom at a bridge site transition state and other nearby atoms. KMC simulation reveals that this formulation recovers observed sintering times for Ag nanoislands on Ag(100), including a novel size dependence. The formulation also applies for nanopits where there are additional challenges to capture kinetics. See J. Phys. Chem. C 120 (2016) 21617. [Preview Abstract] |
Tuesday, March 14, 2017 3:54PM - 4:06PM |
H35.00008: Complex Pattern Formation from Current-Driven Dynamics of Single-Layer Epitaxial Islands on Crystalline Conducting Substrates Ashish Kumar, Dwaipayan Dasgupta, Dimitrios Maroudas We report a systematic study of complex pattern formation resulting from the driven dynamics of single-layer homoepitaxial islands on face-centered cubic (FCC) crystalline conducting substrate surfaces under the action of an externally applied electric field. The analysis is based on an experimentally validated nonlinear model of mass transport via island edge atomic diffusion, which also accounts for edge diffusional anisotropy. We analyze the morphological stability and simulate the field-driven evolution of rounded islands for an electric field oriented along the fast diffusion direction. For larger than critical island sizes on \{110\} and \{100\} FCC substrates, we show that multiple necking instabilities generate complex island patterns, including void-containing islands, mediated by sequences of breakup and coalescence events and distributed symmetrically with respect to the electric field direction. We analyze the dependence of the formed patterns on the original island size and on the duration of application of the external field. Starting from a single large rounded island, we characterize the evolution of the number of daughter islands and their average size and uniformity. The analysis reveals that the pattern formation kinetics follows a universal scaling relation. [Preview Abstract] |
Tuesday, March 14, 2017 4:06PM - 4:18PM |
H35.00009: Specular reflections from fluctuating membranes and interfaces Amir Azadi, David R. Nelson We describe the interplay between optical specular reflections and material properties of the fluctuating membranes and interfaces in thermodynamic equilibrium. We focus on the statistical mechanics of two distinct elastic models, fluid membranes with a tension, and capillary-gravity interfaces in two and three dimensions. Our analysis exploits a hydrodynamic model of the statistical mechanics, in which the statistics of specular points is fully characterized by the correlation length, thickness and lateral dimension of the membrane or interface. The correlation lengths of the membranes and capillary interfaces are controlled respectively by the ratio of the surface tension and the bending rigidity or gravity-capillary length scale. By combination of a simple scaling analysis and numerical simulations, we find a simple universal scaling law for the specular density, $n_{spec}$ in $d$ dimensions described by $n_{spec}\propto\ell^{-(d-1)}$, for arbitrary thin fluctuating interfaces, $\ell\rightarrow0$. The specular density diverges for fluctuating interfaces in the limit of vanishing thickness and surprisingly shows no dependance on the correlation length. In contrast, the number of specular reflections from highly bendable fluid membranes grow by decreasing the correlation length. [Preview Abstract] |
Tuesday, March 14, 2017 4:18PM - 4:30PM |
H35.00010: Using Curved Crystals to Study Terrace-Width Distributions. Theodore L. Einstein Recent experiments on curved crystals of noble and late transition metals (Ortega and Juurlink groups) have renewed interest in terrace width distributions (TWD) for vicinal surfaces. Thus, it is timely to discuss refinements of TWD analysis that are absent from the standard reviews. Rather than by Gaussians, TWDs are better described by the generalized Wigner surmise, with a power-law rise and a Gaussian decay, thereby including effects evident for weak step repulsion: skewness and peak shifts down from the mean spacing. Curved crystals allow analysis of several mean spacings with the same substrate, so that one can check the scaling with the mean width. This is important since such scaling confirms well-established theory. Failure to scale also can provide significant insights. Complicating factors can include step touching (local double-height steps), oscillatory step interactions mediated by metallic (but not topological) surface states, short-range corrections to the inverse-square step repulsion, and accounting for the offset between adjacent layers of almost all surfaces. We discuss how to deal with these issues. For in-plane misoriented steps there are formulas to describe the stiffness but not yet the strength of the elastic interstep repulsion. [Preview Abstract] |
Tuesday, March 14, 2017 4:30PM - 4:42PM |
H35.00011: Theory of multiple quantum dot formation in strained-layer heteroepitaxy Dimitrios Maroudas, Lin Du We develop a morphological stability theory that explains the experimentally observed formation of multiple quantum dots (QDs) in strained-layer heteroepitaxy. Using a fully nonlinear model of surface morphological evolution that accounts for a substrate wetting potential as well as surface diffusional anisotropy, we demonstrate the formation of multiple QD patterns in self-consistent dynamical simulations of the evolution of the epitaxial film surface perturbed from its planar state. The simulation predictions are supported by weakly nonlinear analysis of the epitaxial film surface morphological stability. We find that, in addition to the Stranski-Krastanow instability, long-wavelength perturbations from the planar film surface morphology can trigger a nonlinear instability, resulting in the splitting of a single QD into multiple QDs of smaller sizes, and predict the critical wavelength of the film surface perturbation for the onset of the nonlinear tip-splitting instability. The theory provides a fundamental interpretation for the observations of “QD pairs” or “double QDs” and other multiple QDs reported in experimental studies of epitaxial growth of semiconductor strained layers on patterned and unpatterned substrates. [Preview Abstract] |
Tuesday, March 14, 2017 4:42PM - 4:54PM |
H35.00012: Formation and Control of Highly Crumpled Surfaces on Viscous Polymer Substrate. Jung Gun Bae, Won Bo Lee Folds, highly deformed structures have received extensive attention for their nonlinear responses due to a large strain on soft matters. Furthermore, there is still lack of understanding about folding due to biaxial compressive stress. To investigate this elusive phenomena, we exploit residual stress which is large enough to compress thin film coated on viscous polymer substrate and observe simultaneous occurrence of folding network. Manipulating the thickness and modulus of the substrate, so that there is a difference in the morphology of folding. To demonstrate the dependence on the substrate effect using scaling analysis and there is a good agreement with experimental results. By applying this point, self-generated ladder and flower-like graphoepitaxial structures originated from the manipulation of viscous substrate are designed. [Preview Abstract] |
Tuesday, March 14, 2017 4:54PM - 5:06PM |
H35.00013: Surface phases of the transition metal dichalcogenide IrTe$_{2}$ Chen Chen, Yifan Yang, Jisun Kim, Guixin Cao, Rongying Jin, E. W. Plummer Transition metal dichalcogenides have received great attention because of their fertile properties. The bulk of IrTe$_{2}$ exhibits first-order structural transitions from the expected trigonal structure at room temperature to unusual quasi-one-dimensional striped structure at low temperature. We have investigated, using low energy electron diffraction (LEED) and scanning tunneling microscopy (STM), the surface structural properties of IrTe$_{2}$. We observe complex striped lattice modulations as a function of temperature via cooling and warming processes, including 5x1 and 8x1 phases seen in the bulk. The ground state at the surface is 6x1 phase, not seen in the bulk, and the surface transition temperatures are distinct from the bulk. The broken symmetry at the surface creates a quite different phase diagram, with the coexistence of several periodicities resembling a devil's staircase phenomena. [Preview Abstract] |
Tuesday, March 14, 2017 5:06PM - 5:18PM |
H35.00014: Surface Segregation and Stability of PdRuM (M$=$Rh, Ir, Ni, Ag) Ternary Metal Alloy Surfaces Through First Principles-based Studies Susan Aspera, Ryan Arevalo, Hiroshi Nakanishi, Hideaki Kasai Owing to the wide variety of possible combinations and promising applications, interest in the studies on the properties and behavior of transition metal alloys have been shown from the numerous studies to date. Recent advancement in nanoparticle science shows that binary transition metal nano-alloy particle shows remarkable properties different from that of its bulk form. At this point, the possibility of unravelling more interesting properties may arise from the combination of ternary transition-metal alloys. In this study, we used density functional theory (DFT)-based calculation and analysis to determine surface segregation and stability of ternary transition metal alloys. To sample, we used the ternary PdRuM (M$=$ Rh, Ir, Ni, Ag). Our results show that surface effects and atom-atom interaction in the binary system have large factors in the formation of ternary metal nano-particle alloy. And depending on the type of atom combination, tendency for surface segregation has drastic effects. One of the distinct properties of nano-particles are its larger surface area as compared with its bulk form. As such, this could be translated to a dominating factor for stability and behavior of nano-particle alloys. [Preview Abstract] |
Tuesday, March 14, 2017 5:18PM - 5:30PM |
H35.00015: Annealing dependence of ferroelectric domain patterns in h-\textit{RE}MnO$_{\mathrm{3}}$ Jae Sung Shin, Nara Lee, Young Jai Choi, Seung Chul Chae Topological vortices with complex ferroelectric domains and domain walls exist in hexagonal rare-earth manganites(h-\textit{RE}MnO$_{\mathrm{3}})$. When h-\textit{RE}MnO$_{\mathrm{3}}$ crystals are grown below the ferroelectric-trimerization transition temperature (T$_{\mathrm{c}})$, they exhibit stripe domains. However, when a h-\textit{RE}MnO$_{\mathrm{3}}$ crystal with stripe domains is heated above and cooled down across T$_{\mathrm{c}}$, vortex domains emerge in the crystal. The networks of vortices are found to be in two different types : type-I domains with roughly equal fractions of upward and downward polarization domains and type-II domains with one dominant polarization. In this presentation, we report the post annealing effect on the ferroelectric topology under varying the ambient condition with N$_{\mathrm{2}}$ gas. We observed two different types (type-I and type-II) of vortex domains using piezoresponse force microscopy and compared the topology change with Monte Carlo simulation with varying internal electric field. We observed domains of YMnO$_{\mathrm{3}}$ start to increase upward polarization domain area fraction from 10{\%} to 65{\%} through the post annealing processes. We analyzed the correlation between internal electric field and off-stoichiometry condition. [Preview Abstract] |
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