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January 2018

Thu., 11.

Seminar

Ultrafast dynamics of bands and bonds during the photo-induced phase transition in In/Si(111)

Christopher Nicholson (Fritz-Haber Institute, Berlin)

Time: 10:15h

Location: Room 0.58.5 - Physics Department - University of Fribourg - Ch. du Musee 3 - 1700 Fribourg Switzerland

 

Abstract:

In the Born-Oppenheimer picture, nuclear dynamics evolve on a free energy surface determined by the transient occupation of electronic states. Access to the non-equilibrium distribution of excited electronic states therefore allows a determination of the forces that govern the trajectory along the reaction co-ordinate during a photo-induced ultrafast structural transition. Such a description including the complete electron dynamics goes far beyond that of a “molecular movie” [1]. To address these ideas we investigate a model phase transition system – indium nanowires at the silicon(111) surface – which undergoes an order-order structural transition accompanied by an electronic insulator-to-metal transition [2]. Utilizing femtosecond time and angle resolved photoemission spectroscopy with a novel 500 kHz XUV laser source, we obtain direct access to the transient k-resolved electronic structure during the photo-induced phase transition (PIPT) in In/Si(111). By observing the dynamically changing band structure a detailed reaction pathway is resolved, including temporal separation of the insulator-to-metal (200 fs) and structural (700 fs) phase transitions; the latter in extremely good agreement with recent time-resolved electron diffraction measurements [3]. The reaction pathway is reproduced by ab initio molecular dynamics simulations, which reveal the crucial role played by localized photo-holes in shaping the potential energy landscape of the structural transition. This furthermore allows us to extend the description of ultrafast PIPTs to real space, and chart the ultrafast formation of chemical bonds during the phase transition.

 

1.        Dwyer, J. R. et al. Femtosecond electron diffraction: ‘Making the molecular movie’. Philos. Trans. A. 364, 741–78 (2006)

2.        Yeom, H. et al. Instability and Charge Density Wave of Metallic Quantum Chains on a Silicon Surface. Phys. Rev. Lett. 82, 4898–4901 (1999)

3.        Frigge, T. et al. Optically excited structural transition in atomic wires on surfaces at the quantum limit. Nature 544, 207 (2017)

 

 

Fri., 12.

Conference

Complex networks: from data through models to knowledge (Titularisation procedure)

Dr. Matus Medo - Physics department - University of Fribourg

Time: 17:15h

Location: Petit Auditoire de Physique 0.51, Rez - Perolles 8, Ch. du Musee 3, Fribourg

 

Since almost 20 years, complex networks are used to represent a wide range of real systems. In this talk, we will highlight the importance of models of complex systems. Albeit inevitably partial, these models help us understand the analysed systems and design better techniques to measure them. To illustrate our discussion, we will use information networks that are based on the interconnected information that nowadays surrounds us: citations among patents or scientific papers, and links between users and items in e-commerce systems such as Amazon.com, for example. We will address questions such as how to rank the nodes in a network, and how to find densely connected clusters in a network.

 

Mon., 15.

Candidate review confererence, Professorship succession

Surgical manipulation of collective modes at the nano-scale

Prof. Dr. Fabrizio Carbone, Laboratory for ultrafast Microscopy and Electron Scattering, Institute of Physics, EPFL, CH

Time: 10:00 - 11:00h

Location: Auditoire de Physique 2.73, 2eme etage, Perolles 8, Ch. du Musee 3, Fribourg

 

Inherent properties such as low-dimensionality, strong electron-electron correlations or topological protection are the fundamental ingredients of materials displaying novel emergent electronic, optical, and magnetic properties. Microscopically, such properties are often ruled by ordered textures of spins and charges and their concerted motions. Key to understand and manipulate these states of matter is the ability to act on specific degrees of freedom externally while being able to monitor at the atomic level in space and time the consequences. In this seminar, I will show that collective electronic modes in nanostructures (surface plasmon polaritons) can be imaged and controlled in an ultrafast Transmission Electron Microscope down to the nanometer and attosecond space and time scales. The implications and possibilities opened by such an ability will be also discussed, with a particular focus on the light-induced control of magnetic textures of interest for novel data storage applications.

 

Wed., 17.

Seminar in theoretical physics

Spinon confinement and field-induced transition in the quasi-1D spin system BaCo2V2O8

Shintaro Takayoshi, University of Geneva

Time: 11:00 - 12:00h

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

Calculations of dynamical correlation with high precision in one-dimensional strongly correlated electron systems become possible thanks to the advance of numerical simulation techniques. We study the dynamical susceptibility of quasi-one-dimensional antiferromagnets with Ising anisotropy BaCo2V2O8 using numerical simulations. Our numerical results can be directly compared with experimental measurements of inelastic neutron scattering and electron spin resonance. We treat the effects of interchain interaction by a mean field theory, which introduces an effective staggered field in the system. This effective field causes the confinement of spinons, an elementary excitation in Ising-Heisenberg antiferromagnets and excitation spectra are discretized. We also discuss the effect of externally applied magnetic field. The magnetic field along the anisotropy axis provokes the Zeeman splitting of transverse excitations. The magnetic field perpendicular to the anisotropy axis, on the other hand, gives rise to a quantum phase transition, which is described through a dual-field double sine-Gordon model in terms of a bosonized effective field theory.

Fri., 22.12.2017 - Thu., 18.01.2018

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