Catégorie d'événements: Seminars

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.

A seminar given by

Dr. Petr GRIGOREV

Research Fellow at Centre Interdisciplinaire de Nanoscience de Marseille (France)

titled:

Hybrid ab initio-machine learning simulations of extended defects

Abstract: In the context of plasma facing materials, interaction of radiation induced defects, plasma components and their clusters with material microstructure is important both in terms of material degradation as well as transport and retention of plasma components. Within this presentation I will describe how hybrid ab initio/machine learning methods can be used to study unfeasibly large systems with ab initio accuracy. I will show successful application of the method to study interaction of dislocations with hydrogen, helium and vacancy clusters in tungsten. I will also discuss future applications to fracture and grain boundary segregation and how this QM/ML data can be used to test and improve modern machine learning interatomic potentials.

Bio: Petr Grigorev is research Fellow working in the field of computational material science. He obtained his master degree in Physics from Peter the Great St. Petersburg Polytechnic University in 2012. The same year he enrolled in a Ph.D. program shared between Ghent University and Complutense University of Madrid within the framework of Erasmus Mundus FUSION-DC. He defended his Ph.D. in April 2017 and, shortly after that, joined Warwick Centre for Predictive Modelling as a Research Fellow. In December 2020 he started as a postdoc at the Départment Théorie et Simulation Numérique of Centre Interdisciplinaire de Nanoscience de Marseille (CINaM). Find more info here.

A seminar given by

Dr. Mykola IALOVEGA

Research Associate at the University of Wisconsin–Madison (USA) and in the Plasma Edge Physics with 3D Boundaries group

titled:

Development of Additive Manufacturing Methods of Plasma-Facing Components for Novel Magnetic Fusion Confinement Devices

Abstract: This talk will give an overview on the development of cold spray coatings for functional surfaces of plasma-facing components, specifically, for the novel magnetic confinement fusion devices which are now under construction in Wisconsin, US. Wisconsin HTS Axisymmetric Mirror Experiment (WHAM) is a result of a public-private partnership between the UW-Madison, MIT and Commonwealth Fusion Systems (CFS) to build and operate a compact, high-field mirror device and demonstrate a pathway to a commercial axisymmetric tandem mirror for production of grid—scale electrical power, hydrogen and more. Currently in its final construction phase to achivev pirst plasma, WHAM will utilize REBCO high temperature supercondicting coils (17 T mirrors) with the bore radius of 5.5 cm to confine hot and dense plasmas created using high frequency ECH. RF heating scenario and NBI injecttion will be used for creating fast sloshing ions. Quasi-stationary plasmas (plasma duration >> ion slowing down and characteristic confinement times) will be created with electron temperatures of 1 keV, average ion energies of 20 keV and densities that approach the plasma pressure limit. One of the WHAM’s technilogy missions is to demonstrate advanced particle hadling techniques by utilizing non-evaporatable Tantalum (Ta) gettering interface for effective neutral pressure reduction in the plasma edge which will limit loses associated with charge-exchange and improve the overal plasma performance. This objective is being achieved by developing the solid-state, high-velocity powder-particle-impact-based process—known as cold spray—to create Tantalum coatings on Stainless Steel (SS) substrates. Several cold spray parameters have been tested to achive dense and well-adhered coatings with minimal porosity level. Recent plasma exposure and subsequent annealing experiments have demonstrated the ability of the coatings to withsdand the impact of a high particle fluence (3e25 D/m2) and high surface temperature while maintaing the integrity, surface and bulk morphology of the coatings. Thermal desorption experiments performed in the collaboration with the PIIM laboratory (Aix-Marseille University) revealed the high Hydrogen storage capacity of the coatings (more than two orders of magnitude compared to sintered Tungsten exposed to the same D implantation conditions). Finally, tests of a large-scale Ta cold-sprayed PFC featuring a heating functionality for reguating Hydrogen adsorption and release is underway at the UW-Madison. 
In addition, this research provides an expertise on the additive manufacturing options for functional first-walls and divertor components for stellarators and tokamaks. For example, the Helically Symmetric eXperiment (HSX) device at the UW-Madison—a small-scale stellarator—is planned to be upgraded with a significant heating power (up to 800 kW of injected power and yielding plasma densities up to 1e19 m^-3) and a devertor for testing the non-resonant divertor concept in detached plasma conditions. It is proposed to 3D print parts the HSX and utilize the cold spray technology for adding functionality to the PFCs of the device.

Bio:  BSc and MSc, Physics, V.N. Karazin Kharkiv National University, Ukraine, 2015; MSc, Nuclear Engineering, Stuttgart University (Germany) and Ghent University (Belgium), 2017; PhD, Plasma Physics and Materials Science, CNRS, Aix-Marseille University and CEA Cadarache (France), 2021. Find more information here.

A seminar given by

Dr. Hyeon K. Park

professor at the Departement of Physics of UNIST (South Korea) and director of UNIST Fusion Plasma Laboratory

titled:

History, Science and Perspective of Fusion Energy Development

Abstract: Long history of fusion plasma research is briefly reviewed and relevant lessons will be emphasized. Following the lessons, common factor of a variety of the claimed improved confinement regimes in the magnetic fusion plasma (e.g. L-, H-, VH-, Super-H-, I-, Supershot-, High_beta_P-, RS-, ERS-, FIRE-mode, etc.) is suggested. In addition, study of MHD instabilities by a new 2-D visualization tool (Electron Cyclotron Emission Imaging system) demonstrated the physics of MHDs that we understand and what we do not understand. Based on accumulated empirical knowledge of the toroidal fusion plasmas such as scaling laws for the confinement time and triple products for the fusion power, a comprehensive and effective path for the ignition is suggested along with the perspective of ITER and size limit of a possible compact magnetic confinement device for ignition will be discussed.

Bio: Hyeon K. Park is professor at the Department of Physics at UNIST (Ulsan National Institute of Science & Technology) and director of the UNIST Fusion plasma Laboratory. Find more information here.

A seminar given by

Dr. Junji Yuhara,

professor at the School of Engineering, Nagoya University (Japan)

titled:

Fabrication and characterization of two-dimensional materials on solid surface

Abstract: In my research activities, I have been studying the following topics using variety of techniques such as LEED, AES, STM, LEIS, RBS, PES, and ARUPS; (a) binary metal adsorbates on Si(111) surface, (b) two dimensional alloy on metal surfaces, (c) intermetallic compound and metal oxide quasicrystals, (d) oxide surface and metal-oxide thin films on metal surfaces, (e) single crystal stainless steel surface, and (f) group 14 post-graphene materials. I will review some of these topics in this seminar.

• It has been studied for a decade that binary metal adsorbates on Si(111) surface induce a unique superstructure that is different from single metal adsorbate. In the case of lead and tin, they form a variety of superstructures, like √7×√3, √3×√3, and 2√7×3 structures, although lead and tin are immiscible in bulk [1]. For the immiscible binary adsorbates of silver and copper on Si(111), Cu atoms behave a dissolution and segregation depending on the surface [2].
• To fabricate two-dimensional (2D) alloy, the substrate has to be static and inert against adsorbates. The 2D alloy is an overlayer monolayer film and should be incommensurate to the substrate. I succeeded in fabricating Pb-Sn 2D ordered 2D alloy and Pb-Bi solid solution 2D alloy for the first time using Rh(111) surface [3].
• I demonstrated to identify the Al surface segregation on an Al-Co-Ni and Al-Co-Cu decagonal quasicrystal surfaces and determine the atomic arrangement [4]. Recently, oxide quasicrystal on Pt(111) has been prepared and structural properties are examined [5].
• The synthesis and characterization of post-graphene materials have been intensively studied with the aim of utilizing novel 2D properties. Most studies adopted molecular beam epitaxy as a synthesis method of 2D materials grown on clean crystalline surfaces. In my study, I will focus on the epitaxial growth of germanene, stanene, and plumbene on surface alloy and alloy surface by segregation and deposition methods[6].

References

[1] J. Yuhara et al, Surf. Sci., 482/485, 1374 (2001); J. Yuhara et al, Mat. Sci. and Eng. 96, 145 (2002); J. Yuhara et al, Nucl. Instr. and Meth. B 199, 422 (2003).

[2] J. Yuhara et al, Surf. Sci. 326, 133 (1995); J. Yuhara et al, Appl. Surf. Sci. 104/105, 163 (1996).

[3] J. Yuhara et al, Phys. Rev. B 67, 195407 (2003); J. Yuhara et al, J. of Appl. Phys. 110, 074314 (2011).

[4] J. Yuhara et al, Phys. Rev. B 70, 24203 (2004); R. Zenkyu et al, Phys. Rev. B 86, 115422 (2012).

[5] J. Yuhara et al, Phys. Rev. Mater. 4, 103402 (2020); Li et al, Appl. Surf. Sci. 561, 150099 (2021).

[6] J. Yuhara et al, ACS Nano 12, 11632 (2018); J. Yuhara et al, 2D Materials 5, 025002 (2018); J. Yuhara et al, Adv. Mater. 31, 1901017 (2019); J. Yuhara and G. Le Lay, Jpn. J. Appl. Phys. 59, SN0801 (2020); T. Ogikubo et al, Adv. Mater. Inter. 7, 1902132 (2020); W. Pang et al, Appl. Surf. Sci. 517, 146224 (2020); J. Yuhara et al, Appl. Surf. Sci. 550, 149236 (2021); J. Yuhara et al, Phys. Rev. Mater. 5, 053403 (2021); J. Yuhara et al, 2D Mater. 8, 045039 (2021); S. Mizuno et al, Appl. Phys. Express 14, 125501 (2021).

Bio: Junji Yuhara is currently associate professor at Department of Energy Science and Engineering at the Nagoya University. In 1995, he obtained a PhD in engineering at the Nagoya University, performing his research at Department of Crystalline Materials Science. He was visiting researcher in the USA, Austria and Singapore. His research focuses in the synthesis and characterization of 2D materials.

A seminar given by

Dr. Anne Lafosse, professor at the Paris-Saclay University, ISMO laboratory, Saclay (France)

titled:

Quantifying the unavoidable contribution of electron – supported molecular film interactions – Nanolithography and astrophysics

Abstract: What do chemical enrichment of the interstellar medium and nanolithography processes have in common? Electron-induced processes within molecular films deposited, respectively, on interstellar dust grains and on substrates to be functionalized. The interaction of high-energy radiations (X-ray, ions, electrons) with a condensed medium leads to the formation of low-energy (<20 eV) secondary electron bursts. These electrons created within the irradiated medium contribute efficiently to its chemical modification. In all application contexts (astrochemistry, radiation damage, nanolithography), one of the important issues is to quantify the efficiency of the induced processes, in terms of yields, but also of effective cross sections and required doses.
The approach proposed by the “electron-solids” group is to study directly the effects of electron irradiation on interfaces deposited on substrates by combining: (i) quantitative mass analysis of neutrals desorbing during irradiation (ESD) and (ii) analysis of the deposits before and after irradiation by temperature programmed désorption (TPD) and vibrational spectroscopy HREELS (High Resolution Electron Energy Loss Spectroscopy).
Two systems will be discussed and the derived quantitative markers presented:
– the decomposition under electron irradiation of a film of a halogenated unsaturated compound potentially of interest in the development of EUV lithographic resists,
– the non-thermal desorption from molecular ice of methanol CH3OH, with an interpretation linked to XESD (X-ray induced electron-stimulated desorption) processes.

Bio : Anne Lafosse is full professor at the University of Paris-Saclay. She performs her research in the laboratory ISMO (Institut des Sciences Moléculaire d’Orsay). She is the team leader of the Surface chemistry & slow electrons team.