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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).
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| 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 |
| 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 |
| 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.
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| 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 |
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