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April 2018

Wed., 25.

Seminar in theoretical physics

Exploring the electronic properties of 2D layered materials by using a new spin and angle resolved photoemission experiment

Prof. Geoffroy KREMER, Institut Jean Lamour, Nancy, France

Time: 14:00 - 15:00h

Location: UNI-Perolles, Physics Department, building 8 auditorium 2.73, Chemin du Musee 3, 1700 Fribourg, Switzerland

Since the beginning of 2015, the Institut Jean Lamour in Nancy hosts a new and unique instrument: the TUBE. Fully under ultra-high vacuum (UHV), this exceptional device allows the connexion of twenty experiments of deposition and analysis. One of these is a new angle and spin resolved photoemission spectroscopy setup. First, I will show the relevance of photoemission, and in particular of this new experiment, for the characterization of the electronic structure of materials through different examples, going from benchmark results of the literature to recent developments of our group. One of the main thematic of the TUBE is the synthesis and analysis of two dimensional (2D) artificial materials. 2D materials like graphene, transition metal dichalcogenides and ultrathin oxides have revealed unique properties very different from bulk. As an example, new quasi-2D phases of atomically thin silica films have been discovered recently [1,2]. Reduced dimensionality is expected to ease structural phase transitions, a desirable characteristic in view of faster and less energy costly applications. Although the structural properties of ultrathin silica phases have been characterized, experimental electronic properties of this material are yet to be explored, especially the band structure. In this context, I will present recent results we obtained concerning the electronic properties of an half-lamella of silica epitaxially grown on Ru(0001) [3], also called silicatene. The atomic structure has been studied by XPS, LEED and STM. It is presented in figure(a). These data bring to light a (2x2) reconstruction ordered as an honeycomb like structure and characteristic chemical bondings for silicatene [1-5]. The band structure has been studied by ARPES. It shows dispersive states with the expected periodicity, in good agreement with our theoretical calculations: see figure(b). I will demonstrate that this study bring a new contribution to the comprehension of the structural and electronic properties of the system, in particular concerning the interactions of the half-lamella of silicatene with Ru(0001) substrate.

Wed., 25.

Colloquium Physics

Fundamentals of Pulsed Laser Deposition and Applications for Renewable Energy Applications

Prof. Dr. Thomas Lippert - Paul Scherrer Institut, Villigen

Time: 17:15h

Location: UNI-Perolles, Physics Departement, building 8, auditoire 0.51, Chemin du Musee 3, 1700 Fribourg Switzerland


Thin films of organic, polymeric, biological, and inorganic thin films are utilized in many applications, e.g. catalysis, microelectronics, sensors, food industry, tools, optics, decorative coatings, and renewable energy applications, etc.. The preparation of these thin films can be achieved with a variety of tools, ranging e.g. from chemical to physical vapor deposition methods (PVD). One PVD method which is used extensively in research, especially for oxides, but recently also in industry in pulsed laser deposition (PLD). PLD can be divided into 3 steps, i.e. laser ablation and plasma formation, plasma expansion, and film growth. Different processes associated to these steps can have a pronounced influence on the film growth and film composition, i.e. properties. It is noteworthy, that it is often assumed, without further analysis, that the films will have automatically the same composition as the target, which is not really the case.

I will show, that all steps and the associated parameter, such as background gas type and pressure, target composition, and substrate type and temperature have a pronounced effect on the PLD process, and therefore on the film composition and properties. The sum of all these effects suggests, that in many cases it can be difficult and time-consuming to find conditions to achieve the desired film composition (properties).

The application of thin films as model systems for renewable energy applications will be shown for two topics, i.e. photocatalysis and ion conduction. Pulsed reactive crossed beam laser ablation (PRCLA), a modification of PLD, is used to obtain oxynitride films. Oxynitrides have gained a lot of attention over the last decade due to their photocatalytic properties using visible light. We utilize photoelectrocatalytic measurements (PEC) to study the oxynitride thin films, mainly LaTiOxNy, where we could show that the crystalline orientation has a pronounced influence on the activity. For ion conductors, we are looking mainly at the influence of strain on oxygen and proton conductivity. We could show, that tensile strain will improve conductivity but not by orders of magnitude as reported elsewhere.



May 2018

Thu., 03.

Seminar FriMAT

Nitrogen-vacancy centers in diamonds for applications in biophysics

Dominika Lyzwa - Cambridge, US Research Laboratory of Electronics

Time: 14:00h

Location: UNI-Perolles, Physics Departement, building 8, auditoire 0.58.5, Chemin du Musee 3, 1700 Fribourg Switzerland



Thu., 17.

Seminar in theoretical physics

Interaction range effects and universality in the BCS-BEC crossover of spin-orbit coupled Fermi gases

Davide Giambastiani, University of Pisa

Time: 14:00 - 15:00h

Location: UNI-Perolles, Physics Department, building 8, auditorium 2.73, Chemin du Musee 3, 1700 Fribourg, Switzerland

We explore the evolution of a cold quantum gas of interacting fermions crossing from a Bardeen-Cooper-Schrieffer (BCS) superfluidity to a Bose-Einstein condensation (BEC) of molecular bosons in the presence of a tunable-range interaction among the fermions and of an artificial magnetic field, which can be used to simulate a pseudo spin-orbit coupling (SOC). We find that the crossover is affected by the competition between the longer-range interaction tending to correlate the system over large distances and favoring the BCS-like state, and the SOC term tending instead to favor the BEC-like state with formation of tightly bound molecules with small size. We then calculate the critical temperature, chemical potential and condensate fraction in different regimes for the SOC strength and the strength and range of the atomic interactions, finding that universal behavior sets in and persists for weak spin-orbit interaction, when the crossover is described in terms of the correlation length. We then argue that for larger values of the SOC universality can be restored by taking into account the existence of SOC-driven bosonic molecules, on the BCS side. Our results can be relevant in view of current experiments with cold atoms in optical cavities, where tunable-range effective atomic interactions can be engineered.

Wed., 25.04.2018 - Tue., 22.05.2018

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