Catégorie d'événements: Seminars

A seminar given by

Dr. Niels F.W. Ligterink

SNSF Ambizione Fellow, Space Research & Planetary Sciences division, Physics Institute, University of Bern

titled:

Frozen starships: Studies of comet 67P and the interstellar object ‘Oumuamua

Abstract: Comets are the frozen remnants of planet formation. Their icy composition reflects the physical conditions to which these objects were exposed, such as temperature and radiation. During this talk I present several new studies that help us understand the nature of comet 67P/Churyumov-Gerasimenko and the interstellar object 1I/’Oumuamua.

In the laboratory we simulate the desorption processes of noble gases and nitrogen (N2) from comet-like ice to unravel the ice structure and “warm” formation mechanism of 67P. For ‘Oumuamua I show that accelerating this object by releasing “invisible” H2 molecules requires a highly unusual composition in combination with a much older age than currently thought.

A seminar given by

Dr. David LUNNEY

CNRS researcher at the IJCLab (CNRS/IN2P3), Orsay (France)

titled:

The rise or fall of antihydrogen: the GBAR* experiment at CERN

A seminar given by

Dr. Marie LANDEIRO DOS REIS

Enseignante-Chercheuse à La Rochelle Université, LaSIE (France)

titled:

Interaction between point-defects and dislocation

Abstract: The plastic properties of crystalline materials are not solely determined by the presence of defects within the crystal structure, but also by the interactions between these defects. In metals, the interactions between point defects and dislocations have been extensively studied and are known to play a crucial role in various mechanical behaviors. For instance, at low or moderate temperatures, point defects significantly influence strain hardening or softening and strain ageing. At higher temperatures, the absorption or emission of point defects along the dislocation lines enables dislocation climb and thus impacts creep. Understanding these interactions is essential for predicting and controlling the mechanical behavior of crystalline materials. The aim of this presentation is to introduce methods for investigating the interaction between dislocations and point defects at different scales. We will explore these methods in the context of several crystalline materials, including aluminum, magnesium oxide, and iron, and examine the interaction for edge and screw dislocations with various point defects such as vacancies and hydrogen.

Dislocations are extended defects with a complex core structure that varies depending on the material. Hence, to fully capture the intricacies of the core and its local interactions with point defects, atomic-scale simulations are necessary. In particular, Density Functional Theory (DFT) simulations are a crucial step in capturing the electronic effects that arise when point defects are located within the dislocation core [1]. Point defects may be trapped by the core, which can hinder dislocation motion influenced by the Peierls barrier which depends on the presence of point defects. They may also experience accelerated diffusion through pipe diffusion mechanism. Alternatively, point defects can be absorbed by the core and participate in dislocation climb, which can affect the material’s creep behavior [2].  In order to accurately simulate the effect of defect concentration on dislocation glide or dislocation climb, larger-scale simulations in time and space are thus necessary. In these cases, an empirical potential approach using molecular dynamic or kinetic monte carlo simulations can be used to describe the interactions between point defects and dislocations, both at short and long ranges.

However, these simulations are extremely computationally expensive and cumbersome. When the distance between a point defect and a dislocation is large enough, their interaction can be modeled within the framework of elasticity theory [3], which is less computationally demanding than atomic-scale simulations. Studies have shown that a good agreement between theory and simulation results can be achieved using an approach based on elasticity theory and the dipole elastic approach [3, 4]. This approach enables us to describe not only the interaction between point defects and the dislocation’s far elastic field but also the diffusion of point defects [3, 5].

[1] Allera, Ribeiro, Perez and Rodney, Phys. Rev. Materials 6 (2022) 013608
[2] Landeiro, Proville, Marinica and Sauzay, Phys. Rev. Materials 4 (2020) 103603
[3] Clouet, Varvenne, Jourdan, Comput. Mater. Sci. 147 (2018) 49-63
[4] Landeiro, Carrez, and Cordier, Phys. Rev. Materials 5 (2021) 063602
[5] Landeiro, Giret, Carrez and Cordier, Comput. Mater. Sci. 211 (2022) 111490

*Corresponding author: mlandeirATuniv-lr.fr (M. Landeiro Dos Reis)

A seminar given by

Dr. Abdou DIAW

Researcher at the Oak Ridge National Laboratory (United States of America)

titled:

Advancing Iterative Synthetic Diagnosis Workflow for Light Impurities on WEST

Abstract: Despite limited poloidal diagnostic coverage in fusion devices, understanding impurity distributions across the poloidal extent is crucial for interpreting experimental results. Our developed iterative synthetic diagnosis workflow provides valuable insights into impurity sources and transport in the main Scrape Off Layer (SOL) plasma. The workflow utilizes uses hydrogenic plasma from a multifluid MHD code as fixed background, allowing an impurity transport code to determine poloidal charge state abundances. These abundances are further processed using the collisional radiative code ColRadPy, which converts them into spectral line intensities. To account for 3D effects such as sight lines and reflections off in-vessel components, we employ Raysect.

By comparing experimental measurements with synthetic results generated by the workflow, we iteratively adjust free parameters in the impurity transport code to achieve consistency between the synthetic and experimental outcomes. Our focus lies on oxygen, assumed to be the primary source of tungsten (W) sputtering, and includes experimental and SOLEDGE SOL power scans conducted on the WEST platform. The constructed synthetic workflow demonstrates good agreement with measured O II emission, offering valuable insights for future measurements of higher charge states.

We gratefully acknowledge the support of the U.S. Department of Energy (DOE) under Grant Numbers DE-SC0014664 and DE-AC05-00OR22725.

A seminar given by

Dr. Antonio TEJEDA

Researcher at the Laboratoire de Physique des Solides (Université Paris-Saclay/CNRS)

titled:

Many-body effects in phase transitions in Sn/Ge(111) as a function of the temperature

Abstract: We present here an investigation on the phase transitions of 0.33 ML of Sn on Ge(111) at low temperature. We have identified a (3 × 3) phase, characterized by a charge ordering settled by electronic correlation, which appears between the known metallic- (3 × 3) and the root3 insulating phase at very low temperature, The vertical distortion characteristic of the (3 × 3) phase is lost across the phase transition,. We identify the atomistic mechanism behind the stabilization of this charge ordered insulating phase and interpret these findings on the basis of theoretical calculations. Moreover, our experimental and theoretical results show a giant electron-phonon interaction at the (3×3) phase between 150 and 120 K. The electron-phonon interaction in α−Sn/Ge(111)−(3×3) is unusually large, since we find that theoretically λ, the electron mass enhancement for the half-filled band, is λ=1.3. This result is in good agreement with the experimental value obtained from high-resolution angle-resolved photoemission spectroscopy measurements, which yield λ=1.45±0.1. The giant electron-phonon interaction can be considered at least partially responsible for the different phases that this system shows at very low temperature.

Figure (a) Second derivative of the ARPES data along the ΓM3×3 direction. (b) Left: Fermi level region from (a). Right: Momentum distribution curves corresponding to the red lines shown in the left panel. (c) Experimental points and fit to the bare (black line) and the renormalized (dashed purple line) bands. The kink associated to electron phonon coupling is visible.

References:

  ¹⁾ R. Cortés et al. Phys. Rev. Lett. 96, 126103 (2006).

  ²⁾ R. Cortés et al. Phys. Rev. B 88, 125113 (2013).

  ³⁾ M.N. Nair et al. Phys. Rev. B 107, 045303 (2023).

Bio: Antonio TEJEDA’s research focuses on the control of electronic properties by structural modifications. His favorite objects are surfaces and other two-dimensional systems where physical properties can be significantly affected by defects, phase transitions or quantum confinement in nanostructures.

A seminar given by

Dr. Ghassan ANTAR

Professor at the American University of Beirut (Lebanon)

titled:

The Suppression of large-scale turbulence in the Tokamak Scrape-off Layer

Abstract: Reducing and controlling turbulence at the edge of magnetic fusion devices have been the paradigm of fundamental and applied research for several decades in magnetic fusion. We show that when using radio-frequency waves to heat or drive the current in the plasma, turbulence is reduced and large scales are suppressed. This effect is now confirmed on many tokamaks and is shown to affect the whole scrape-off layer, which indicates that the wave affects the source of the instability. The mechanism by which this interaction takes place is shown to be the excitation by ‘sound waves’ that couple to the turbulent fluctuations.

Bio: Ghassan Antar is professor at the American University of Beirut and leader of the Laboratory for Plasma and Fluid Dynamics. Find more information here

A seminar given by

Dr. María E. DAVILA

Professor at Instituto de Ciencia de Materiales de Madrid – ICMM – CSIC (Spain)

titled:

Reducing the dimensionality: Silicon nanoribbons

Abstract: In recent years the discovery of different forms of nanostructures in many  materials (nanospheres, nanotubes, nanowires, as well as their derivatives) has raised much scientific and technological excitement. As the most important electronic materials, silicon in its nanoscale forms, has stimulated major interest because of the peculiar emerging physical properties, such as light emission, field emission, and quantum confinement effects. In my seminar, I will especially focus on silicon nanoribbons, a new allotrope uniquely made of Si pentagonal buiding blocks, whose physical properties could be explored  in details.

Bio: María E Dávila research focuses on “The synthesis and characterization of low-dimensional materials with special emphasis on semiconductors”. Her interests include “Determining the structural and electronic structure of those materials”. She has expertise in “The use of synchrotron radiation techniques to explore the physics and chemistry of low-dimensional materials”. She completed her PhD in Condensed Matter Physics at University Auronoma of Madrid in 1996, followed by a Post-doctoral fellowship at University of Uppsala and KTH in Sweden.