92 research outputs found
Interpretation of surface diffusion data with Langevin simulations: a quantitative assessment
No full textDiffusion studies of adsorbates moving on a surface are often analyzed using 2D Langevin simulations. These simulations are computationally cheap and offer valuable insight into the dynamics, however, they simplify the complex interactions between the substrate and adsorbate atoms, neglecting correlations in the motion of the two species. The effect of this simplification on the accuracy of observables extracted using Langevin simulations was previously unquantified. Here we report a numerical study aimed at assessing the validity of this approach. We compared experimentally accessible observables which were calculated using a Langevin simulation with those obtained from explicit molecular dynamics simulations. Our results show that within the range of parameters we explored Langevin simulations provide a good alternative for calculating the diffusion procress, i.e. the effect of correlations is too small to be observed within the numerical accuracy of this study and most likely would not have a significant effect on the interpretation of experimental data. Our comparison of the two numerical approaches also demonstrates the effect temperature dependent friction has on the calculated observables, illustrating the importance of accounting for such a temperature dependence when interpreting experimental data
Crucial electronic contributions to measures of surface diffusion by He atom scattering
No full textWe examine theoretically recent He atom scattering measurements of Na/Cu(001), identifying a nontrivial relation between the dynamics of the electron distribution and that of the Na ions. The magnitude of the calculated variations in the charge density, and their dependence on the local density of adsorbates, account for the correlated three-dimensional (3D) motion experimentally observed. The results of this study further highlight the sensitivity of helium scattering to the electron distribution of the sample and point out the role of electronic contributions in high-resolution measures of surface dynamics
Surface dynamics and friction of K/Cu(001) characterized by helium-3 spin-echo and density functional theory
No full textHelium-3 spin-echo measurements of K/Cu(001) are presented, the diffusional surface dynamics of the system at low coverages and on picosecond time scales. Langevin molecular-dynamics simulations are used, together with a potential-energy surface derived from density functional theory calculations, to provide further understanding of the experimental data. An anisotropic potential with a corrugation of 35-64 meV and a friction parameter of 1/4.5 ps-1 are found to give a good fit to the data for the lower coverages with the adsorbate interactions modeled with a dipole-dipole repulsion. Additionally, at the highest coverage, θ=0.084, a component of motion is observed perpendicular to the surface, analogous to that recently found for Na/Cu(001)
Linewidths in bound state resonances for helium scattering from Si(111)-(1x1)H
No full textHelium-3 spin-echo measurements of resonant scattering from the Si(111)–(1 × 1)H surface, in the energy range 4–14 meV, are presented. The measurements have high energy resolution yet they reveal bound state resonance features with uniformly broad linewidths. We show that exact quantum mechanical calculations of the elastic scattering, using the existing potential for the helium/Si(111)–(1 × 1)H interaction, cannot reproduce the linewidths seen in the experiment. Further calculations rule out inelastic and other mechanisms that might give rise to losses from the elastic scattering channels. We show that corrugation in the attractive part of the atom–surface potential is the most likely origin of the experimental lineshapes
Observation of Uncorrelated Microscopic Motion in a Strongly Interacting Adsorbate System
No full textMisure di scattering di atomi di elio con la tecnica spin-echo permettono di individuare moti incorrelati delle molecole di CO adsorbite su platino (111). Questi risultati mostrano che le interazioni fra le molecole sono deboli
Quantum and Classical Image Charges at Metal Surfaces
No full textPoint charges at a metal surface are balanced by image charges canceling their electrostatic potential below the surface. This well-known phenomenon is at the basis of important observations also at the nanometer length scale, where a quantum description proves essential, but may escape common first-principle theoretical approaches.
By reviewing two different examples, I will discuss cases where standard description by Density Functional Theory with the independent-particle Kohn-Sham formalism (KS-DFT) can / cannot grasp the true findings. (i) The potential for an electron at a jellium surface in common uses of KS-DFT dramatically misses the correct decay at large distances, which is due to coupling with dynamical fluctuations in the surface charge density, and is restored by higher level of theory explicitly including electron-electron many-body interactions [1-3]. (ii) An alkali atom adsorbed on a metal typically charges positively, resulting in a strong dipole whose electric field below the surface is balanced by image dipole. The occurrence of the image dipole affects the interaction between the adsorbates, the charge transfer and magnitude of the dipole itself, aspects that we show to be described by KS-DFT calculations [4-9] well validated by the quantitative agreement to a variety of experimental findings [10,6,8].
References:
[1] A.G. Eguiluz, M. Heinrichsmeier, A. Fleszar, and W. Hanke, Phys. Rev. Lett. 68, 1359 (1992), DOI: http://dx.doi.org/10.1103/PhysRevLett.68.1359
[2] G. Fratesi, G.P. Brivio, P. Rinke, and R.W. Godby, Phys. Rev. B 68, 195404 (2003), DOI: http://dx.doi.org/10.1103/PhysRevB.68.195404
[3] G. Fratesi, G.P. Brivio, and L.G. Molinari, Phys. Rev. B 69, 245113 (2004), DOI: http://dx.doi.org/10.1103/PhysRevB.69.245113
[4] G. Fratesi, G. Alexandrowicz, M.I. Trioni, G.P. Brivio, and W. Allison, Phys. Rev. B 77, 235444 (2008), DOI: http://dx.doi.org/10.1103/PhysRevB.77.235444
[5] G. Fratesi, Phys. Rev. B 80, 045422 (2009), DOI: http://dx.doi.org/10.1103/PhysRevB.80.045422
[6] H. Hedgeland, P.R. Kole, H.R. Davies, A.P. Jardine, G. Alexandrowicz, W. Allison, J. Ellis, G. Fratesi, and G.P. Brivio, Phys. Rev. B 80, 125426 (2009), DOI: http://dx.doi.org/10.1103/PhysRevB.80.125426
[7] G. Fratesi, A. Pace, and G.P. Brivio, J. Phys.-Condens. Matter 22, 304005 (2010), DOI: http://dx.doi.org/10.1088/0953-8984/22/30/304005
[8] C. Huang, G. Fratesi, D.A. MacLaren, W. Luo, G.P. Brivio, and W. Allison, Phys. Rev. B 82, 081413(R) (2010), DOI: http://dx.doi.org/10.1103/PhysRevB.82.081413
[9] G. Fratesi, Phys. Rev. B 84, 155424 (2011), DOI: http://dx.doi.org/10.1103/PhysRevB.84.155424
[10] G. Alexandrowicz, A. P. Jardine, H. Hedgeland, W. Allison, and J. Ellis, Phys. Rev. Lett. 97, 156103 (2006), DOI: http://dx.doi.org/10.1103/PhysRevLett.97.15610
Nowe szaty króla – uwagi nad książką Krzysztofa Burczaka
No full textThe Emperor’s New Clothes – Remarks on Krzysztof Burczak’s Book
The review essay of the book on quinque compilationes antiquae by Krzysztof Burczak offers a critique of the scope and method applied to the research on the sources of medieval canon law within this monograph. The two objectives or hypotheses of the book are too broad and were not properly justified. The research was based on limited and outdated methods. The approach to the sources of quinque compilationes did not take into account the complicated history of the transmission of canon law texts. The author overlooked the relevance of the science of canon law (e.g., glosses) and application of compilations in courts which were crucial for defending his claims. Unfortunately, the book proves that the state and quality of the history of universal canon law in Poland resembles the emperor’s new clothes.
* Praca powstała w wyniku realizacji projektu badawczego o nr 2020/36/C/HS5/00365 finansowanego ze środków Narodowego Centrum Nauki
A goalkeeper engaged in penalties: The best action time
No full textIt is very important for an athlete and, in particular, for a soccer goalkeeper engaged in penalty, to choose the right time of motor action. Goalkeepers usually try to make a safe moving their legs before the kicker touches the ball. In a preliminary experiment, we obtained data showing that the performance of goalkeepers improves when the temporal gap between their action and the begin of the kick is near to zero. by using acoustic stimuli, twenty goalkeepers were tested in order to study the possibility to induce them to start the safe at the right time. The experimental variable was the temporal gap between the acoustic stimulus and the begin of the penalty kick (anticipation: none, 300ms, and 600ms from the beginning of the kick). Results confirm the improvement of the performance of the goalkeepers when acoustic stimulus induced them to move only when the kicker touched the ball and not before: The number of saves almost doubled. The method used in our experiments, and the results obtained, underline the possibility to implement specific training sessions for goalkeepers based on individual strategy aimed to determine the best timing for each athlete
G. F. de Martens on Asian Treaty Practice (1964)
No full textThis chapter examines the works of Georg Friedrich von [de] Martens in relation to Asian treaty practices. These include his principal treatise, the Précis du Droit des Gens Modernes de l’Europe fondé sur les traités et l’usage (1789); Cours Diplomatique ou Tableau des Relations extérieures des Puissances de l’Europe tant entre elles qu’avec d’autres états dans les diverses parties du globe; and Recueil des Traités. The interrelationship between these three works has been largely ignored by historians of the law of nations and deserves careful attention. While Martens does not draw any specific conclusions from the Asian treaty material which he had collected and included in his works, the material suggests compelling reasons for conceiving the existence of a fairly wide meeting ground on which European and Asian countries were not only able to engage in trade but also to exchange cultural values and legal ideas.</p
On Bruno Schulz’s Illustrations of the Story “From the Court of a Blind Goddess”
No full textThe cause of this investigation were two unknown drawings by Bruno Schulz (ink, two 4x7 cm, two 6x7 cm, each with the artist’s signature) found in the newspaper Chwila, a Polish language Zionist daily published in Lviv in 1919-1939. Chwila paid much attention to Schulz: 43 articles, reviews, and notes on his life and work were published throughout the paper’s history. Few texts published in the Chwila were illustrated, and book illustrations were also rare in the artistic career of the writer: next to illustrating his own books, he designed covers and illustrations only for Juliusz Witt and Witold Gombrowicz. Searching for information about the author of an illustrated story “Z dworu ślepej bogini,” Oskar Alexandrowicz (1885-1939?), a resident of Drogobych, painter, art critic, and lawyer, added to the knowledge about the circle of Schulz’s friends. For instance, an interesting character was Oskar’s brother, Roman Alexandrowicz (1882-1940), a well-known lawyer and collector of Schulz’s works which he used to show to the public in his apartment on the corner of Akademicki Square and Fredry Street in Lviv. The literary critic Ostap Ortwin (1876-1942), who was a regular patron of a famous café in the same building, most likely supported Schulz’s participation in the Spring Salon of 1922. Just opposite, on 7 Fredry Street, there was a studio of Kazimierz Sichulski, painter and professor of the Lviv State Industrial School, and Wanda Diamand’s photo studio “Światłocień.” The author also succeeded in establishing the identity and biographical data of another Alexandrowicz, Marek (1890-?), manager of one of the biggest oil companies in Europe, “Gazy Ziemne,” who was a close friend of Izydor Schulz
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