Archiv 2017

Raum: Seminar Room I-II, EG.076-078

Hideo Aoki - Quantum phases induced by polarised light --- Floquet-topological states and Higgs modes

MPSD Seminar
Putting materials out of equilibrium can produce new quantum phenomena. When driven by a circularly-polarised laser, a massless Dirac system such as graphene can be converted into the Floquet topological insulator of photon-dressed electrons, which realises the quantum anomalous Hall effect. The phase diagram becomes increasingly intricate for lower laser frequencies where transitions between different Chern numbers take place. If we consider electron correlation, we can predict transitions between Floquet topological insulators and Mott insulator[1]. In another avenue a linearlypolarised light can induce the collective Higgs amplitude mode in superconductors, a condensed-matter analogue of the renowned particle. Then a resonantly strong third-harmonic generation arises, where the Higgs mode contribution is shown to dominate[2] unlike in the BCS approximation.[1] T. Mikami et al, Phys. Rev. B 93, 144307 (2016).[2] R. Matsunaga et al, Phys. Rev. B 96, 020505(R) (2017). [mehr]

Shinji Saito - Supercooled water: Fluctuation, glass transition, and vibrational entropy

MPSD ARD Seminar
Water is the most common liquid. Its properties are not, however, common. Since the anomalies of water become pronounced at low temperatures, especially below its melting temperature, it has been proposed that the anomalous properties are attributed to the presence of two liquid states corresponding to the two amorphous ices. Experimental studies of supercooled water are however very difficult. This is due to the fact that bulk water is easily transferred to a crystalline phase in the “no man’s land”, which is a temperature range bounded by the crystallization of supercooled water at ~235K and by that of highly viscous liquid water at ~150K. Therefore, various theoretical and computational studies have been conducted for understanding the properties in the no man’s land. We have performed extensive molecular dynamics simulations from normal liquid to deeply supercooled states to reveal the structural and dynamical instabilities in the no man’s land. The spatiotemporal fluctuations, dynamic transition, glass transition, and vibrational contribution to Kauzmann temperature of supercooled water will be discussed. [mehr]

Frederike Ahr - High energy multi-cycle terahertz generation

Spectroscopic mapping by STEM/EELS has proven to be a powerful technique for determining the structure, chemistry and bonding of interfaces, reconstructions, and defects. So far, most efforts in the physical sciences have focused on room temperature measurements where atomic resolution mapping of composition and bonding has been demonstrated [1-3]. For many materials, including those that exhibit electronic and structural phase transitions below room temperature and systems that involve liquid/solid interfaces, STEM/EELS measurements at low temperature are required. Operating close to liquid nitrogen temperature gives access to a range of emergent electronic states in solid materials and allows us to study processes at liquid/solid interfaces immobilized by rapid freezing [4,5]. [mehr]

Irene Dornmair - Advanced Beam Dynamics and Diagnostics Concepts for Laser-Plasma Accelerators


Yoni Toker: Watching Isolated Molecules Change Their Shape

CFEL Molecular Physics Seminar
Recently it has been shown that using two stages of ion mobility it is possible to measure isomerizations - structural changes in isolated molecules. Moreover, it is possible to measure the relative energies of different isomers and to deduce the barrier energy for each isomerization pathway. We have applied the technique to one of the most biologically important cases; that of the retinal protonated Schiff base (RPSB). RPSB is the chromophore which acts as a photon detector in every known form of animal vision. The primary step in vision is photoisomerization of the RPSB, which then activates the surrounding protein. We have studied the barrier energies for isomerizations of the RPSB and have shown them to be significantly lower than within the protein. We have also compared different derivatives of the chromophore and shown that slight changes to the structure of the chromophore, can dramatically effects its energy landscape. [mehr]
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