Energetics and many-particle mechanisms of two-dimensional cluster diffusion on Cu(100) surfaces

We study the energetics and stability of small Cu clusters on Cu(100) surfaces using molecular statics combined with systematic saddle-point search methods. We find several previously overlooked concerted many-particle processes that play an important role in cluster energetics. In particular, for smaller clusters there is an internal atom row shear mechanism that in some cases determines the rate-limiting step for center-of-mass motion. Our results suggest specific reaction paths for experimentally observed cluster diffusion events.Peer reviewe

Finding transition paths and rate coefficients through accelerated Langevin dynamics

We present a technique to resolve the rare event problem for a Langevin equation describing a system with thermally activated transitions. A transition event within a given time interval (0,tf) can be described by a transition path that has an activation part during (0,tM) and a deactivation part during (tM,tf)(0<tM<tf). The activation path is governed by a Langevin equation with negative friction while the deactivation path by the standard Langevin equation with positive friction. Each transition path carries a given statistical weight from which rate constants and related physical quantities can be obtained as averages over all possible paths. We demonstrate how this technique can be used to calculate activation rates of a particle in a two dimensional potential for a wide range of temperatures where standard molecular dynamics techniques are inefficient.Peer reviewe

Searching for transition paths in multidimensional space with a fixed repulsive bias potential

An efficient method for searching for transition paths in a multidimensional configuration space is proposed. It is based on using a fixed, locally repulsive bias potential, which forces the system to move from a given initial state to a different final state. This simple method is very effective in determining nearby configurations and possible transition paths for many-particle systems. Once the approximate transition paths are known, the corresponding activation energies can be computed using, e.g., the nudged elastic band method. The usefulness of the present method is demonstrated for both classical and quantum-mechanical systems.Peer reviewe

Status of the Fatigue Studies on the CLIC Accelerating Structures

The need for high accelerating gradients for the future multi-TeV e+e- Compact Linear Collider (CLIC) imposes considerable constraints on the materials of the accelerating structures. The surfaces exposed to high pulsed RF (Radio Frequency) currents are subject to cyclic thermal stresses which are expected to induce surface break up by fatigue. Since no fatigue data exists in the literature up to very large numbers of cycles and for the particular stress pattern present in RF cavities, a comprehensive study of copper alloys in this parameter range has been initiated. Fatigue data for selected copper alloys in different states are presente

Interface dynamics and kinetic roughening in fractals

We consider the dynamics and kinetic roughening of single-valued interfaces in two-dimensional fractal media. Assuming that the local height difference distribution function of the fronts obeys Levý statistics with a well-defined power-law decay exponent, we derive analytic expressions for the local scaling exponents. We also show that the kinetic roughening of the interfaces displays anomalous scaling and multiscaling in the relevant correlation functions. For invasion percolation models, the exponents can be obtained from the fractal geometry of percolation clusters. Our predictions are in excellent agreement with numerical simulations.Peer reviewe

Kokemuksia haastavista ja kuormittavista tilanteista varhaiskasvattajien työssä

Opinnäytetyön lisäosan tarkoituksena oli selvittää, millaisia haastavia ja kuormittavia tilanteita varhaiskasvattajat voivat kohdata työssään lasten, huoltajien, työyhteisön ja muiden tilanteiden parissa. Tutkimustehtävä keskittyi erityisesti siihen, millä tavalla haasteisiin voidaan vaikuttaa ja millaisiin tilanteisiin varhaiskasvattajat kokevat itse pystyvänsä vaikuttamaan ja millaisiin eivät. Tutkielmassa käsiteltiin myös varhaiskasvattajien kokemia työn voimavaroja, jotka osaltaan tukevat varhaiskasvattajien työssä jaksamista. Tutkielmassa vertailtiin varhaiskasvattajien ja muskariopettajien kokemuksien samankaltaisuuksia, koska tämä tutkielma täydensi aikaisempaa musiikkipedagogin (AMK) opinnäytetyötäni (Mäkipelto 2014). Teoreettinen viitekehys muodostui tässä tutkielmassa työhyvinvointiin liittyvästä kirjallisuudesta ja aikaisemmasta tutkimuksesta, joissa tarkasteltiin työkäytäntöjen ja työn voimavara- ja kuormitustekijöiden vaikutuksia työhyvinvointiin. Tutkielma toteutettiin kvalitatiivisena eli laadullisena tutkimuksena ja tutkimusmenetelmänä käytettiin puolistrukturoitua teemahaastattelua. Teemahaastattelu analysoitiin teemoittelun ja aineistolähtöisen sisällönanalyysin avulla. Yhteistyökumppanina oli Norlandia päiväkodit. Haastatteluun osallistui neljä varhaiskasvattajaa, joiden kokemukset eivät liittyneet ainoastaan nykyiseen työpaikkaan Norlandian päiväkodissa, vaan ne koskivat myös aikaisempaa työhistoriaa. Tutkimustulokset osoittivat, että haastavat ja kuormittavat kokemukset liittyivät erityisesti lapsiryhmän hallintaan, huoltajien kohtaamiseen, työyhteisön ristiriitoihin, työn ulkopuoliseen elämään ja työnkuvaan liittymättömiin tilanteisiin kuten siivoamiseen. Ongelmanratkaisukeinoiksi nimettiin lasten parissa tunteiden käsittely ja rauhallinen vuorovaikutus. Varhaiskasvattajat kokivat, että huoltajien parissa auttoi kunnioittava suhtautuminen ja säännöllinen tiedottaminen. Työyhteisössä korostui yhteisten sääntöjen, kommunikoinnin ja toimivan johtajuuden merkitys. Muissa tilanteissa painotettiin työn ulkopuolisesta elämästä huolehtimista. Varhaiskasvattajat kokivat, että lasten perheoloihin ja resurssien puutteeseen he eivät pystyneet vaikuttamaan. Työn voimavaroina painottui lasten riemu ja kehitys, toimiva yhteistyö huoltajien ja työyhteisön parissa sekä oman työn kehittäminen, arvostaminen ja rajaaminen. Edeltävissä tuloksissa oli paljon yhteneväisyyksiä aikaisempien tutkimustulosten kanssa. Varhaiskasvattajat ja muskariopettajat olivat kokeneet yhtäläisiä haastavia tilanteita erityisesti lapsiryhmän hallintaan, huoltajien kohtaamiseen ja työyhteisössä heikkoon perehdytykseen ja sääntöjen epäselvyyteen liittyvissä tilanteissa. Ongelmanratkaisukeinoina korostui molemmissa ammattiryhmissä lasten parissa keskusteleminen, yhteisten sääntöjen, periaatteiden ja toimintatapojen vahvistaminen. Varhaiskasvattajat ja muskariopettajat kokivat lasten riemun ja työkavereiden tuen voimavaraksi. Johtopäätöksistä ilmeni, että varhaiskasvattajien ja muskariopettajien työssä korostui vuorovaikutuksen merkitys. Mikäli vuorovaikutus oli heikkoa, se aiheutti enemmän kuormitusta. Jos vuorovaikutus toimi, yhteisöllisyys lisääntyi ja toimi voimavarana. Jatkossa voitaisiin tutkia enemmän erityisesti työn ulkopuolisen elämän yhteyttä varhaiskasvattajien hyvinvointiin ja moniammatillisten varhaiskasvattajien erilaisia kokemuksia. Niiden merkitys korostui tässä tutkielmassa ja aikaisemmissa tutkimuksissa.The purpose of the supplementary thesis was to find out what kind of challenging and stressful situations early childhood educators may face in their work with children, guardians, the work community and other situations. The aim was to find ways to influence the challenges and find out which situations early childhood educators feel themselves capable of influencing and which not. The thesis focused also on the strengthening factors experienced by early childhood educators, which contribute to the well-being of early childhood educators. Moreover, the thesis compares similarities between the experiences of early childhood educators and music playschool teachers as this thesis complements my earlier bachelor’s thesis in early childhood music education (Mäkipelto 2014). The theoretical framework of this thesis consists of occupational well-being literature and earlier studies, which examined the effects of work practices and work strength and stress factors on occupational well-being. The thesis was carried out as qualitative research and a semi-structured thematic interview was used as the research method. The thematic interview was analyzed with theme and content analysis. The cooperation partner was Norlandia Päiväkodit. Four early childhood educators participated in the interview. Their experiences were not only related to the current job at the Norlandia kindergarten, but also on past work history. The results demonstrated that challenging and stressful experiences were especially related to group management with children, the interaction with guardians, conflicts in the work community, life outside the workplace, and work assignment issues not included in the job description such as cleaning. Problem solving included emotional management and peaceful interaction with children. Early childhood educators felt that respect and regular information helped with guardians. In the work community the importance of common rules, communication and effective leadership were emphasized. The life outside of work was emphasized in other situations. Early childhood educators felt that they were unable to influence children's family conditions and lack of resources. The work strength-factors consisted of the joy and development of children, effective cooperation with guardians and the work community, as well as the development, appreciation and delimitation of their own work. These results had several similarities with previous research results. Early childhood educators and music playschool teachers had experienced similar challenging situations, especially in the group management with children, the interactions with guardians, and in the work community in situations involving weak orientation and unclear rules. Both professional groups solved problems by discussing with children and enforcing common rules and principles. Both professional groups felt that the joy of children and support from colleagues strengthened their work resources. The study concluded that interaction is important for both early childhood educators and music playschool teachers. If interaction was weak, it caused more stress. If it worked, communality increased and it served as a strength factor. The occupational wellbeing of early childhood educators could be studied further by focusing on life-factors outside work and a multi-professional perspective. The importance of these perspectives was emphasized in this thesis and former studies

Comment on Surface diffusion near the points corresponding to continuous phase transitions

It is well known that unlike static equilibrium properties, kinetic quantities in Monte Carlo simulations are very sensitive to the details of the algorithm used for the microscopic transition rates. This is particularly true near the critical region where fluctuations are pronounced. We demonstrate that when diffusion of oxygen adatoms near the order–disorder transition of a lattice-gas model of the O/W(110) model system is studied, the transition rates must be chosen carefully. In particular, we show that the choice by Uebing and Zhdanov [J. Chem. Phys. 109, 3197 (1998)] is inappropriate for the study of critical effects in diffusion.Peer reviewe

Langevin Dynamics Simulations of Polymer Translocation through Nanopores

We investigate the dynamics of polymer translocation through a nanopore using two-dimensional Langevin dynamics simulations. In the absence of an external driving force, we consider a polymer which is initially placed in the middle of the pore and study the escape time τe required for the polymer to completely exit the pore on either side. The distribution of the escape times is wide and has a long tail. We find that τe scales with the chain length N as τe∼N1+2ν, where ν is the Flory exponent. For driven translocation, we concentrate on the influence of the friction coefficient ξ, the driving force E, and the length of the chain N on the translocation time τ, which is defined as the time duration between the first monomer entering the pore and the last monomer leaving the pore. For strong driving forces, the distribution of translocation times is symmetric and narrow without a long tail and τ∼E−1. The influence of ξ depends on the ratio between the driving and frictional forces. For intermediate ξ, we find a crossover scaling for τ with N from τ∼N2ν for relatively short chains to τ∼N1+ν for longer chains. However, for higher ξ, only τ∼N1+ν is observed even for short chains, and there is no crossover behavior. This result can be explained by the fact that increasing ξ increases the Rouse relaxation time of the chain, in which case even relatively short chains have no time to relax during translocation. Our results are in good agreement with previous simulations based on the fluctuating bond lattice model of polymers at intermediate friction values, but reveal additional features of dependency on friction.Peer reviewe

Heat flux and information backflow in cold environments

© 2016 American Physical Society. We examine non-Markovian effects in an open quantum system from the point of view of information flow. To this end, we consider the spin-boson model with a cold reservoir, accounting for the exact time-dependent correlations between the system and the bath to study the exchange of information and heat. We use an information-theoretic measure of the relevant memory effects and demonstrate that the information backflow from the reservoir to the system does not necessarily correlate with the backflow of heat. We also examine the influence of temperature and coupling strength on the loss and gain of information between the system and the bath. Finally, we discuss how additional driving changes the backflow of information, giving rise to potential applications in reservoir engineering

Deterministic Walks in Quenched Random Environments of Chaotic Maps

This paper concerns the propagation of particles through a quenched random medium. In the one- and two-dimensional models considered, the local dynamics is given by expanding circle maps and hyperbolic toral automorphisms, respectively. The particle motion in both models is chaotic and found to fluctuate about a linear drift. In the proper scaling limit, the cumulative distribution function of the fluctuations converges to a Gaussian one with system dependent variance while the density function shows no convergence to any function. We have verified our analytical results using extreme precision numerical computations.Comment: 18 pages, 9 figure