Calendar of Physics Talks Vienna

Ab initio Low-Dimensional Physics Opened Up by Constrained RPA for Energy and Space: Applications to LaFeAsO and kappa-(BEDT-TTF)_2
Speaker:Kazuma Nakamura (University of Tokyo)
Abstract:Studies on outstanding electron correlation effects such as non-Fermi liquid behavior and unconventional superconductivity discovered in systems with low-dimensional anisotropy have continuously been at a front of condensed matter physics. Analyses for 1D or 2D simplified models have played a primary role in understanding essence of correlation effects, but to a large extent, the studies rely on ad hoc adjustable parameters as in the Hubbard models. We develop a new ab initio downfolding scheme for deriving effective low-energy models with low spatial dimensions [1]. The scheme is based on constrained random- phase-approximations by imposing constraints not only in ``energy'' but also in ``space''. We show real applications for 2D-layered superconductors of LaFeAsO and $\kappa $-(BEDT-TTF)$_{2}$X. The derived interactions in the effective models become short ranged essentially within up to next-nearest neighbors and thus justify multiband 2D Hubbard models as effective models for these materials from first principles. [1] K. Nakamura, Y. Yoshimoto, Y. Nohara, and M. Imada, J. Phys. Soc. Jpn. 79, 123708 (2010).
Date: Mon, 25.01.2010
Time: 09:00
Duration: 60 min
Location:Seminar room 138C (TU Freihaus 9. Stock, gelb) Wiedenerhauptstr. 8-12
Contact:Karsten Held

Iron‐based superconductors: what can we learn from DFT?
Speaker:Lilia Boeri, PhD (head of MPI junior research group) (Max-Planck Institute Stuttgart )
Abstract:The discovery of superconductivity with a critical temperature Tc of 26 K in fluorine‐doped LaOFeAs in early 2008 has initiated a so‐called "iron age" in the field of superconductivity. In a few months, several new iron‐based superconductors, with Tc's as high as 55 K, have been found. Their crystal structures show a common motive, characterized by a square Fe lattice, surrounded by distorted tetrahedra of pnictogen or chalchogen atoms. Besides superconductivity, iron‐based superconductors show a spin density wave (SDW) transition accompanied by a lattice distortion. This points to a possible unconventional origin of superconductivity, in analogy with the high‐Tc cuprates, although there is no general consensus on the pairing mechanism. In this talk, I will first review and discuss the main experimental findings concerning iron‐based superconductors. I will then use Density Functional theory to describe the electronic and vibrational properties of LaOFeAs, which is a prototype compound for iron pnictides. Using linear response calculations, I will show that the standard Migdal‐Eliashberg theory fails to account for the observed critical temperature.[1] Using an ab‐initio effective tight‐binding Hamiltonian, based on Fe d and As p Wannier orbitals, derived from NMTO downfolding, I will then analyze in detail the origin of the complicated band structure of iron pnictides, and use this tight‐binding model to discuss the origin of magnetism in Fe-based superconductors and its itinerant nature[2], which is probably at the basis of the observed failure of LDA in the description of their magnetic properties.[3]. Finally, I will present some recent results about the interplay between magnetic and lattice degrees of freedom in these systems [4]. [1] L. Boeri, O. V. Dolgov, and A. A. Golubov, Phys. Rev. Lett. 101, 026403 (2008) and Physica C, 469, 628, 2009. [2] O.K. Andersen and L. Boeri, in preparation. [3] I. I. Mazin, M. D. Johannes, L. Boeri, K. Koepernik, and D. J. Singh, Phys. Rev. B 78, 085104 (2008). [4] L. Boeri, M. Calandra, I.I. Mazin, O.V. Dolgov and F. Mauri, in preparation.
Date: Mon, 25.01.2010
Time: 16:00
Duration: 45 min
Location:Seminar room 138C (TU Freihaus 9. Stock, gelb)
Contact:Prof. Dr. Karsten Held

Fundamental constants in physics and their time dependence
Speaker:Harald Fritzsch (Physik-Department, Ludwig-Maximilians-Universität München)
Abstract:In the Standard Model of Particle Physics there are 28 fundamental constants. In the experiments these constants can be measured, but theoretically they are not understood. I will discuss these constants, which are mostly mass parameters. Astrophysical measurements indicate that the finestructure constant depends on time. Grand unification implies a time variation of the QCD scale. Thus the masses of the atomic nuclei and the magnetic moments of the nuclei will depend on time. I proposed an experiment, which is currently done by Prof. Haensch in Munich and his group. The first results indicate a time dependence of the QCD scale. I will discuss the theoretical implications.
Date: Mon, 25.01.2010
Time: 17:00
Location:TU Wien Freihaus, Hörsaal 5 (2. Stock, grüner Bereich), Wiedner Hauptstr. 8-10, 1040 Wien
Contact:J. Schmiedmayer

Vienna Theory Lunch Club - Loop Calculations in MSSM
Speaker:Wolfgang Frisch (HEPHY)
Abstract:Overview Lunch Seminar
Date: Tue, 26.01.2010
Time: 12:30
Duration: 60 min
Location:University of Vienna, Boltzmanngasse 5, 5th Floor, Large Seminar Room
Contact:Theo Adaktylos, Maximilian Attems, David Burke, Marcus Huber

Examples and explicit construction of Hadamard states
Speaker:Claudio Dappiaggi (ESI) (Fakultät für Physik)
Abstract:im Rahmen des Seminars für Mathematische Physik
Date: Tue, 26.01.2010
Time: 14:15
Duration: 60 min
Location:Erwin Schrödinger-Hörsaal, Boltzmanngasse 5, 5. Stock
Contact:J. Yngvason

Neue Möglichkeiten der funktionellen Bildgebung retinaler Gefäße
Speaker:Prof. Dr. Leopold Schmetterer (Zentrum für Biomedizinische Physik und Technik, Universitätsklinik für Klinische Pharmakologie, Medizinische Universität Wien)
Abstract:Eine Vielzahl okulärer Erkrankungen wie Glaukom, senile Makuladegeneration, diabetische Retinopathie und Gefäßverschlüsse sind mit Durchblutungsstörungen des Auges assoziiert. Obwohl klinisch sehr wichtig, stellt die Messung der okulären Durchblutung dennoch ein schwieriges Problem dar. In den letzten Jahren wurden mehrere Verfahren entwickelt um Einsicht in die okuläre Zirkulation zu erhalten. Hier ist einerseits die retinale Gefäßanalyse zu nennen. Dabei werden retinale Gefäße mittels einer Funduskamera auf eine CCD Kamera abgebildet und danach retinale Gefäßdurchmesser bestimmt. Besonders interessant ist, dass durch Stimulation mittels Flackerlicht auch Einblick in die neurovaskuläre Kopplung der Retina gewonnen werden kann. Auch zur Messung der retinalen Blutflussgeschwindgkeit stehen neue Möglichkeiten zur Verfügung. Hier ist vor allem die optische Doppler-Tomographie zu nennen. Diese basiert auf einer Kombination der optischen Kohärenztomographie mit den Prinzipien der Doppler-Velozimetrie. Mittels bi-direktionaler Verfahren ist dabei auch eine Messung der absoluten Blutflussgeschwindgkeit möglich. Diese neuen Verfahren ermöglichen auch im klinischen Alltag eine Untersuchung des retinalen Gefäßstatus von Patienten mit okulären Erkrankungen.
Date: Tue, 26.01.2010
Time: 16:00
Location:Technische Universität Wien, Institut für Angewandte Physik, Seminarraum 134A, Turm B (gelbe Leitfarbe), 5. OG, 1040 Wien, Wiedner Hauptstraße 8-10
Contact:Ao.Univ.Prof. Dr. Martin Gröschl

The covariant *-product
Speaker:Dimitri Vassilevich (ABC Federal U.)
Abstract:It is well known that for a given Poisson structure one has infinitely many star products related through the Kontsevich gauge transformations. These gauge transformations have an infinite functional dimension (i.e., correspond to an infinite number of degrees of freedom per point of the base manifold). I show that on a symplectic manifold this freedom may be almost completely eliminated if one extends the star product to all tensor fields in a covariant way and impose some natural conditions on the tensor algebra. The remaining ambiguity either correspond to constant renormalizations to the symplectic structure, or to maps between classically equivalent field theory actions. I also discuss how one can introduce the Riemannian metric in this approach and the consequences of our results for noncommutative gravity theories.
Date: Thu, 28.01.2010
Time: 10:15
Duration: 60 min
Location:Schroedinger Seminar Room, University of Vienna
Contact:Daniel Grumiller

From Classical Statistical Physics to Quantum Mechanics: Dimers and Crystal Melting
Speaker:Helmuth Hüffel (Univ. Wien) (Fakultät für Physik)
Abstract:im Rahmen der gemeinsam veranstalteten Seminare "Komplexe Stochastische Systeme"(Univ.Wien) u."Analyse Komplexer Systeme" (Med.Univ.Wien)
Date: Fri, 29.01.2010
Time: 14:30
Duration: 90 min
Location:Medizinische Univ., Bibliothek, Bauteil 88 (Zugang über Spitalgasse 23) 3. Stock
Contact:H. Hüffel