Calendar of Physics Talks Vienna

Ultrafast carrier dynamics in ferromagnetic thin films and molecular layers
Speaker:Martin Weinelt (Max-Born-Institut und Freie Universität Berlin/D)
Abstract:Understanding carrier dynamics is of importance for many applications in solid state and molecular physics. In this talk I will address a few problems related to femtomagnetism and molecular electronics. Recent experiments demonstrate that significant demagnetization of thin films of itinerant ferromagnets can be achieved within a few hundred femtoseconds upon optical excitation. Within this timescale, the excited electronic system and the underlying lattice are not in equilibrium and it seems that the transient hot electron population is responsible for the change of the magnetization. It remains controversial to date, which microscopic processes are fast enough to provoke femtomagnetism. To approach these problems we combined time-, angle- and energy-resolved two-photon photoemission with spin-resolved electron detection and investigated ultrathin iron and cobalt films on Cu(100). Electrons only undergo a spin flip in the presence of spin-orbit coupling significantly enhanced at hybridization points in the band structure. We have identified these so-called spin hot-spots by linear magnetic dichroism in time-resolved photoemission. Excellent agreement with ab initio fully relativistic calculations of the cobalt fcc band-structure allows us to precisely determine spin-orbit hybridization points close to the Fermi level. For iron we show that magnon emission significantly contributes to the lifetime of optically excited carriers and thus allows for transfer of angular momentum of hot electrons on the femtosecond timescale. In contrast for the Heisenberg ferromagnet Gadolinium equilibration of the excited state involves more than one timescale, because the optical excitation occurs in the valence band but the magnetic moment is dominated by localized 4f electrons. Thus, beside electronic, phononic, and magnetic excitations, which affect the Gd valence band, equilibration includes the 4f magnetic moment. Demagnetization and equilibration is therefore established on a much slower picosecond time scale involving spin-lattice interaction. Molecular motion induced by optical excitation provides a new prospect for the functionalization of surfaces on the nanoscale. In this context azobenzene-based photo-switches are widely explored prototype systems. However, coupling between the chromophore and the surface leads to low quantum efficiency and renders application of molecular switches at surfaces difficult. We use alkane-thiols to decouple the azobenzene switch from gold surfaces and to form well-ordered self-assembled monolayers. While the azobenzene entity is sufficiently decoupled from the metal substrate by the alkane chain, lateral interactions between neighboring molecules within the molecular layer strongly influence properties and functionality of the optical switch. Charge transfer among the chromophores occurs on the femtosecond timescale and therefore quenches the optical excitation. Introducing defects into the molecular layer by X-ray beam damage activates the photochromic molecular switch and we can follow the trans to cis isomerization.
Date: Mon, 19.05.2008
Time: 14:30
Location:Technische Universität Wien, Getreidemarkt 9, 1060 Wien, MB und Chemie-Inst., Vortmann HS, GM 3
Contact:Mag. Ch. Toupal-Pinter

Time-resolved reconstruction of photonic Schrödinger cats in a cavity: A movie of decoherence
Speaker:Serge HAROCHE (Ecole Normale Supérieure and Collège de France, Paris)
Abstract:The development of super-high finesse mirrors which bounce microwave photons back and forth for times up to a few tenths of a second has opened a new domain in quantum optics. It is now possible to "look" at stored light without destroying it, to prepare by quantum non-demolition (QND) measurements well-defined photons number states and to observe the quantum jumps between them. One can also generate and study mesoscopic non-classical field states containing several photons, which survive long enough to be measured and fully characterized. We will describe how a single atom passing between mirrors trapping a coherent field can prepare a superposition of photonic states with opposite phases. This state is called a "Schrödinger cat" by reference to the feline that the founding father of quantum physics imagined to be suspended in a superposition of life and death. This highly non-classical state is represented in phase space by a quasi-probability distribution, called Wigner function, which exhibits Gaussian peaks corresponding to its classical components (the "dead" and "alive" parts of the cat) along with interference fringes describing its quantum coherence. By performing QND measurements of the photon number parity, we have reconstructed these Wigner function and recorded the progressive disappearance of their interference features. Movies of the decoherence process are realized in this way, which directly illustrate how a mesoscopic system evolves from quantum to classical.
Date: Mon, 19.05.2008
Time: 17:00
Location:TU Wien Freihaus, Hörsaal 5, Turm A, grüner Bereich, 2. Stock, Wiedner Hauptstr. 8-10, 1040 Wien
Contact:Prof. J. Schmiedmayer

Effect of a locally repulsive interaction on s--wave superconductors
Speaker:Jean Bernard Bru (Univ. Wien) (Fakultät für Physik)
Abstract:im Rahmen des Seminars für Mathematische Physik
Date: Tue, 20.05.2008
Time: 14:15
Duration: 60 min
Location:Großer Seminarraum, Boltzmanngasse 5, 5. Stock
Contact:J. Yngvason