
Calendar of Physics Talks Vienna
Search for new physics with CMS  Joint Seminar HEPHY / SMI 
Speaker:  Oliver Buchmueller (Imperial College London) 
Date:  Tue, 11.10.2011 
Time:  11:00 
Location:  Institut für Hochenergiephysik der ÖAW, Nikolsdorfer Gasse 18, 1050 Wien, Bibliothek, 1. Stock 
Contact:  Christian Fabjan (HEPHY) / Eberhard Widmann (SMI) 
Finite family symmetries in the lepton sector 
Speaker:  Patrick Otto Ludl (Uni Wien) 
Abstract:  The fact that at least two of the three known active neutrinos have nonvanishing masses is the most important evidence for physics beyond the standard model of particle physics. In the same way as the introduction of quark mass terms leads to quark mixing, the introduction of neutrino mass terms leads to lepton mixing in charged current interactions. While in the quarksector the mixing matrix is close to the unit matrix, in the lepton sector this is not the case. In fact in the lepton sector two of the three mixing angles are quite large. An interesting possibility to "explain" these large values for the mixing angles is to impose certain symmetries on the actions of the leptonic and scalar sector, the most popular symmetry groups being finite groups. After an introduction to the physics of lepton mixing we will take a look on the mathematics of finite groups. Finally we will study some simple models using finite groups in order to reproduce some of the features required to accommodate the experimental results. 
Date:  Tue, 11.10.2011 
Time:  12:30 
Duration:  60 min 
Location:  TU Wien (Wiedner Hauptstr. 810), yellow area, 10th floor, seminar room E136 
Contact:  Sabine Ertl 
Bosonen in Zufallspotentialen 
Speaker:  Jakob Yngvason 
Abstract:  im Rahmen des Seminars für Mathematische Physik 
Date:  Tue, 11.10.2011 
Time:  14:15 
Duration:  60 min 
Location:  Fakultät für Physik, Erwin SchrödingerHörsaal, Boltzmanngasse 5, 5. Stock 
Contact:  J. Yngvason 
Theoretical Study of Charge and Spin Ordering at the Reconstructed Magnetite (001) Surface AntiPhase Domain Boundary 
Speaker:  Thomas Manz (Theoretical Study of Charge and Spin Ordering at the Reconstructed Magnetite (001) Surface AntiPhase Domain Boundary) 
Abstract:  Magnetite is a mixed oxidation state material that exhibits charge and spin ordering transitions. For example, bulk magnetite is a halfmetal above the Verwey transition temperature of ~120K and an insulator below this temperature [1]. Published calculations show a subsurface chargeordering transition is associated with the magnetite 001 surface reconstruction [2,3]. Recent scanning tunneling microscopy (STM) experiments show the reconstructed magnetite 001 surface can contain surface antiphase domain boundaries [4].
In this talk, I will discuss planewave density functional theory calculations performed to model the surface reconstruction antiphase domain boundary. Common generalized gradient approximations (GGA) of the electronelectron exchangecorrelation energy are known to overly delocalize electric charge. A semiempirical onsite parameter, Ueff, was used to correct this error. The rational behind chosen Ueff values for different atoms in the system will be discussed. Net atomic charges and atomic spin moments were computed using two different atomic population analysis methods: (a) Bader’s atomsinmolecules method and (b) the recently developed Density Derived Electrostatic and Chemical (DDEC) method [57]. The reconstruction antiphase domain boundary was found to be caused by chargeordering of the Fe atoms in the first subsurface octahedral layer. Specifically, we find that for the reconstructed surface without antiphase domain boundary the Fe atoms in the first subsurface octahedral layer are arranged in rows with alternating pairs in the +2 and +3 oxidataion states; that is, the iron atoms along a subsurface octahedral row have the repeating sequence ..FeIIFeIIFeIIIFeIII... If two different surface domains grow together during synthesis, the charge ordering may change phase at the boundary leading to the observed surface reconstruction antiphase domain boundary. DFT+U calculations clearly show this interruption of charge ordering at the phase boundary. Finally, simulated STM images will be discussed that show the accumulation of electrons on Fe atoms in the surface octahedral layer. This accumulation of charge explains certain features of the experimental STM images. DFT+U calculations also show a phase change in the displacement of surface atoms at the antiphase domain boundary. The magnitude of surface atom displacements is smaller for computations than for experiments, but the two are in qualitative agreement.
References:
[1] Verwey, E. J. W. Nature 1939, 144, 327328.
[2] Lodziana, Z. Phys. Rev. Lett. 2007, 99, 206402.
[3] Pentcheva, R.; Wendler, F.; Meyerheim, H. L.; Moritz, W.; Jedrecy, N.; Scheffler, M. Phys. Rev.Lett. 2005, 94, 126101.
[4] Gareth Parkinson and Ulrike Diebold, to be published.
[5] Bader, R. F. W.; Macdougall, P. J.; Lau, C. D. H. J. Am. Chem. Soc. 1984, 106, 15941605.
[6] Manz, T. A.; Sholl, D. S. J. Chem. Theory Comput. 2010, 6, 24552468.
[7] Manz, T. A.; Sholl, D. S. J. Chem. Theory Comput. 2011, in press, http://pubs.acs.org/doi/abs/10.1021/ct200539n.

Date:  Tue, 11.10.2011 
Time:  16:00 
Location:  Technische Universität Wien, Institut für Angewandte Physik, Seminarraum 134A, Turm B (gelbe Leitfarbe), 5. OG, 1040 Wien, Wiedner Hauptstraße 810 
Contact:  Univ.Prof. Dr. Ulrike Diebold 
The Stability of the Riemannian Positive Mass Theorem and the Intrinsic Flat Distance 
Speaker:  Christina Sormani (New York) 
Abstract:  im Rahmen des Literaturseminars 
Date:  Thu, 13.10.2011 
Time:  14:15 
Duration:  60 min 
Location:  Arbeitsgruppe: Gravitation, Währinger Strasse 17, Seminarraum A, 2. Stock 
Contact:  R. Beig 
