Transition between chaotic and stochastic universality classes of kinetic roughening

The dynamics of nonequilibrium spatially extended systems are often dominated by fluctuations, e.g., due to deterministic chaos or intrinsic stochasticity. This reflects into generic scale invariant or kinetic roughening behavior that can be classified into universality classes defined by critical exponent values and by the probability distribution function (PDF) of field fluctuations. Suitable geometrical constraints are known to change secondary features of the PDF while keeping the values of the exponents unchanged, inducing universality subclasses. Working on the Kuramoto-Sivashinsky equation as a paradigm of spatiotemporal chaos, we show that the physical nature of the prevailing fluctuations (chaotic or stochastic) can also change the universality class while respecting the exponent values, as the PDF is substantially altered. This transition takes place at a nonzero value of the stochastic noise amplitude and may be suitable for experimental verification.This work has been supported by Ministerio de Economía y Competitividad, Agencia Estatal de Investigación, and Fondo Europeo de Desarrollo Regional (Spain and European Union) through Grant No. PGC2018-094763-B-I00. E.R.-F. also acknowledges financial support by Ministerio de Educación, Cultura y Deporte (Spain) through Formación del Profesorado Universitario Scholarship No. FPU16/06304

Non-KPZ fluctuations in the derivative of the Kardar-Parisi-Zhang equation or noisy Burgers equation

The Kardar-Parisi-Zhang (KPZ) equation is a paradigmatic model of nonequilibrium low-dimensional systems with spatiotemporal scale invariance, recently highlighting universal behavior in fluctuation statistics. Its space derivative, namely the noisy Burgers equation, has played a very important role in its study, predating the formulation of the KPZ equation proper, and being frequently held as an equivalent system. We show that, while differences in the scaling exponents for the two equations are indeed due to a mere space derivative, the field statistics behave in a remarkably different way: while the KPZ equation follows the Tracy-Widom distribution, its derivative displays Gaussian behavior, hence being in a different universality class. We reach this conclusion via direct numerical simulations of the equations, supported by a dynamic renormalization group study of field statistics.We acknowledge valuable comments by B. G. Barreales, M. Castro, J. Krug, P. Rodríguez-López, and J. J. Ruiz-Lorenzo. This work has been supported by Ministerio de Economía y Competitividad, Ministerio de Ciencia, Innovación y Universidades, Agencia Estatal de Investigación, and Fondo Europeo de Desarrollo Regional (Spain and European Union) through Grants No. FIS2015-66020-C2-1-P and No. PGC2018-094763-B-I00. E.R.-F. also acknowledges financial support from Ministerio de Educación, Cultura y Deporte (Spain) through Formación del Profesorado Universitario sco-larship No. FPU16/06304

Surface nanopatterning by ion beam irradiation: compositional effects

Surface nanopatterning induced by ion beam irradiation (IBI) has emerged as an effective nanostructuring technique since it induces patterns on large areas of a wide variety of materials, in short time, and at low cost. Nowadays, two main subfields can be distinguished within IBI nanopatterning depending on the irrelevant or relevant role played by the surface composition. In this review, we give an up-dated account of the progress reached when surface composition plays a relevant role, with a main focus on IBI surface patterning with simultaneous co-deposition of foreign atoms. In addition, we also review the advances in IBI of compound surfaces as well as IBI systems where the ion employed is not a noble gas species. In particular, for the IBI with concurrent metal co-deposition, we detail the chronological evolution of these studies because it helps us to clarify some contradictory early reports. We describe the main patterns obtained with this technique as a function of the foreign atom deposition pathway, also focusing in those systematic studies that have contributed to identify the main mechanisms leading to the surface pattern formation and development. Likewise, we explain the main theoretical models aimed at describing these nanopattern formation processes. Finally, we address two main special features of the patterns induced by this technique, namely, the enhanced pattern ordering and the possibility to produce both morphological and chemical patterns.This work was supported by Ministerio de Economía, Industria y Competitividad (MINECO, Spain), Agencia Estatal de Investigación (AEI, Spain), and Fondo Europeo de Desarrollo Regional (FEDER, EU) through Grant No. PGC2018-094763-B-I00, and by Comunidad de Madrid (Spain) under the Multiannual Agreement with UC3M in the line of Excellence of University Professors, No. EPUC3M23, in the context of the V Plan Regional de Investigación Científica e Innovación Tecnológica (PRICIT), as well as under the TRANSNANOAVANSENS program (S2018-NMT-4349)

Anomalous ballistic scaling in the tensionless or inviscid Kardar-Parisi-Zhang equation

The one-dimensional Kardar-Parisi-Zhang (KPZ) equation is becoming an overarching paradigm for the scaling of nonequilibrium, spatially extended, classical and quantum systems with strong correlations. Recent analytical solutions have uncovered a rich structure regarding its scaling exponents and fluctuation statistics. However, the zero surface tension or zero viscosity case eludes such analytical solutions and has remained ill-understood. Using numerical simulations, we elucidate a well-defined universality class for this case that differs from that of the viscous case, featuring intrinsically anomalous kinetic roughening (despite previous expectations for systems with local interactions and time-dependent noise) and ballistic dynamics. The latter may be relevant to recent quantum spin chain experiments which measure KPZ and ballistic relaxation under different conditions. We identify the ensuing set of scaling exponents in previous discrete interface growth models related with isotropic percolation, and show it to describe the fluctuations of additional continuum systems related with the noisy Korteweg-de Vries equation. Along this process, we additionally elucidate the universality class of the related inviscid stochastic Burgers equation.This work has been partially supported by Ministerio de Ciencia, Innovación y Universidades (Spain), Agencia Estatal de Investigación (AEI, Spain), and Fondo Europeo de Desarrollo Regional (FEDER, EU) through Grants No. PGC2018-094763-B-I00 and No. PID2019-106339GB-I00, and by Comunidad de Madrid (Spain) under the Multiannual Agreements with UC3M in the line of Excellence of University Professors (No. EPUC3M14 and No. EPUC3M23), in the context of the V Plan Regional de Investigación Científica e Innovación Tecnológica (PRICIT). E.R.-F. acknowledges financial support through Contract No. 2022/018 under the EPUC3M23 line

Nanopatterning of rotating highly oriented pyrolytic graphite (0001) surfaces by ion beam irradiation: Experiments and modeling

We produce ordered surface nanostructures on azimuthally rotating HOPG(0001) by irradiation with low-energy ion beams. Thus, small dots at low fluences evolve into cellular structures at large fluences. This transition is a consequence of the fast growth of dots and their subsequent concatenation to form walls along the grain boundaries, leading to cells. The walls compete and ripen with continued ion beam irradiation, resulting in both coarsening and roughening of the cell pattern. We reproduce the same morphological and scaling behavior using different ions, Kr or Ar, so the mechanisms involved in pattern formation and coarsening are robust. We compare the experimental findings with simulations of a nonlinear continuum model based on ion-driven viscous flow, which reproduces many qualitative properties seen in the experiments. Such a comparison underscores the need for a suitable consideration of both external noise and the grain structure of the target to fully account for the large-scale properties of the process.This work was supported by the Korean Research Foundation through Grants No. NRF- 2016R1D1A1B03930532 and No. 2019R1F1A1040955, by Ministerio de Economía, Industria y Competitividad (MINECO, Spain), Agencia Estatal de Investigación (AEI, Spain), and Fondo Europeo de Desarrollo Regional (FEDER, EU) through Grants No. PGC2018-094763-B-I00, No. PID2019-106339GB-I00, and No. PID2019-109320GB-I00, and by Comunidad de Madrid (Spain) under the Multiannual Agreement with UC3M in the line of Excellence of University Professors, No. EPUC3M23, in the context of the V Plan Regional de Investigación Científica e Innovación Tecnológica (PRICIT)

Spreading fronts of wetting liquid droplets: microscopic simulations and universal fluctuations

We have used kinetic Monte Carlo (kMC) simulations of a lattice gas to study front fluctuations in the spreading of a nonvolatile liquid droplet onto a solid substrate. Our results are consistent with a diffusive growth law for the radius of the precursor layer, R ∼ t δ , with δ ≈ 1 / 2 in all the conditions considered for temperature and substrate wettability, in good agreement with previous studies. The fluctuations of the front exhibit kinetic roughening properties with exponent values which depend on temperature T , but become T independent for sufficiently high T . Moreover, strong evidence of intrinsic anomalous scaling has been found, characterized by different values of the roughness exponent at short and large length scales. Although such a behavior differs from the scaling properties of the one-dimensional Kardar-Parisi-Zhang (KPZ) universality class, the front covariance and the probability distribution function of front fluctuations found in our kMC simulations do display KPZ behavior, agreeing with simulations of a continuum height equation proposed in this context. However, this equation does not feature intrinsic anomalous scaling, at variance with the discrete model.This work was partially supported by Ministerio de Economía, Industria y Competitividad (MINECO, Spain), Agencia Estatal de Investigación (AEI, Spain), and Fondo Europeo de Desarrollo Regional (FEDER, EU) through Grants No. PID2020-112936GB-I00 and No. PGC2018-094763-BI00, by the Junta de Extremadura (Spain) and Fondo Europeo de Desarrollo Regional (FEDER, EU) through Grants No. GRU18079 and No. IB20079, and by Comunidad de Madrid (Spain) under the Multiannual Agreement with UC3M in the line of Excellence of University Professors (EPUC3M23), in the context of the 5th Regional Programme of Research and Technological Innovation (PRICIT). J.M.M. was supported by Programa Propio de Investigación a la Investigación de la Universidad de Extremadura through Scolarship No. 1362. P.R.-L. was supported by "AYUDA PUENTE 2021, URJC." Our kMC simulations have been performed in the computing facilities of the Instituto de Computación Científica Avanzada de Extremadura (ICCAEx)

Modelos bidimensionales de vértices y simitrías bajo álgebras cuánticas

Tesis doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física Teórica. Fecha de lectura: 15-11-199

Towards ordered Si surface nanostructuring: Role of an intermittent ion beam irradiation approach

The dynamical characteristics of surface nanopatterning using low-energy ion beams remains a central theme within ion beam sputtering. Most previous studies have focused on nanostructure evolution by bombarding surfaces using a continuous ion beam. Here, we study the effect of sputtering from an intermittent ion beam on nanopatterning of a Si surface, using a 900 eV or (mostly) 500 eV Ar+ion beam at an incident angle of 67°, up to a total fluence of 10 × 1019 ions cm-2. Nanoripples are predominantly found on the irradiated surfaces, alongside a hierarchical triangular morphology at the lower energy condition. Ripple ordering is superior for intermediate values of the sputtering interval used in the intermittent sputtering approach. The area of the triangular structures also depends on the intermittent sputtering time intervals. At larger length scales than the ripple wavelength or the triangular structures, all surfaces display strong height fluctuations with a well-defined roughness exponent. Our results can be rationalized via known properties of the nonlinear regime of evolution for surfaces that become amorphous under irradiation and relax stress via ion-induced viscous flow, as borne out from numerical simulations of a continuum model previously proven to provide a significant description of the present class of experiments.This work has been partially supported by DST-SERB, Govt. of India through Grant No. SR/S2/CMP-0112/2012, Ministerio de Ciencia e Innovación (Spain), by Agencia Estatal de Investigación (AEI, Spain, 10.130 39/501100011033), and by European Regional Development Fund (ERDF, A way of making Europe) through Grants No. PGC2018-094763-B-I00, No. PID2019-109320GB-I00, and No. PID2021-123969NB-I00, and by CAM (Spain) under the Multiannual Agreements with UC3M in the line of Excellence of University Professors (Grant No. EPUC3M23), in the context of the V Plan Regional de Investigación Científica e Innovación Tecnológica (PRICIT)

Texturization of polycrystalline titanium surfaces after low-energy ion beam irradiation: impact on biocompatibility

In this study, the impact of low-energy (1 keV) Ar+ ion-beam irradiation on the morphology of polycrystalline Ti disks was investigated. Targets were prepared by cutting and mechanically polishing commercial rods. The surface topographies before and after irradiation were characterized by scanning electron microscopy (SEM) and mechanical profilometry. Irradiation was performed using a wide range of incident angles (αi) from normal to grazing geometries at a total dose of 1018 ions/cm2. SEM analysis of the irradiated Ti targets revealed clear texturing with various attainable surface morphologies depending on αi. The surface features varied from ripples within patches with fingerprint-like patterns (0 ≤ αi ≤ 60°) to oriented structures parallel to the direction of the ion beam, such as pillar/tip structures (65 ≤ αi ≤ 75°) and shallow ripples (αi ≈ 80°). This morphological selectivity could be attributed to the competitive diffusive and erosive regimes, where the lateral uniformity of the morphology was affected by the limited size of the crystal grains. Finally, the wettability and biocompatibility of the characteristic topographies were evaluated, and the results indicated improved performance of the ion-beam-textured surfaces compared to the untreated ones.This research was financially supported by Dirección General de Asuntos del Personal Académico (DGAPA) from Universidad Nacional Autónoma de México (UNAM) under grant PAPIIT IN-114120, by Ministerio de Ciencia, Innovación y Universidades (Spain), Agencia Estatal de Investigación (AEI, Spain), and Fondo Europeo de Desarrollo Regional (FEDER, EU) through grants PGC2018-094763-B-I00, PID2021-123969NB-I00, and by Comunidad de Madrid (Spain) under the Multiannual Agreement with UC3M for Excellence of University Professors (EPUC3M23), in the context of the “V Plan Regional de Investigación Científica e Innovación Tecnológica” (PRICIT). Additional funding was obtained from Consejo Nacional de Ciencia y Tecnología (CONACyT, Mexico) under “Estancias Posdoctorales en el Extranjero Vinculadas a la Consolidación de Grupos de Investigación y Fortalecimiento del Posgrado Nacional” with scholarship numbers 711146, and 740556.Publicad