Thema: 
Spintronics: from magnetic memories to black holes 
Datum: 
16.07.18 
Uhrzeit: 
16:15 
Ort: 
H6 
Vortragender: 

Utrecht University 

Inhalt: 
Over the past two decades spintronics has been concerned with utilizing the electron spin rather than its charge. Most of the envisioned applications, predominantly in the context of magnetic memories, make use of the interaction between spinpolarized currents and magnetization dynamics in ferromagnets. A great deal of research has been devoted to understanding, characterizing and controlling these interactions. In this talk I will review these developments. I will in particular discuss the example of magnetic domain walls, and how their motion can be controlled by spin currents. An envision application is the socalled magnetic racetrack memory that I will discuss briefly. Building further on this understanding, I will also describe how the interaction between spin currents and magnetization dynamics can be used to implement blackhole horizons for spin waves. The latter are wavelike oscillations in the magnetization direction, also referred to as magnons. Finally, I will discuss how these blackholelike horizons can be used to amplify spin waves and make a magnon laser. The latter idea may be important in the field of magnonics, that seeks to replace the electron with spin waves/magnons in dataprocessing applications. 
Ansprechpartner: 
Thema: 
Upper and lower Lipschitz bounds for the perturbation of edges of the essential spectrum 
Datum: 
01.06.18 
Uhrzeit: 
16:15 
Ort: 
V3204 
Vortragender: 

TU Dortmund 

Inhalt: 
Let $A$ be a selfadjoint operator, $B$ a bounded symmetric operator and $A+t B$ a perturbation. I will present upper and lower Lipschitz bounds on the function of $t$ which locally describes the movement of edges of the essential spectrum. Analogous bounds apply also for eigenvalues within gaps of the essential spectrum. The bounds hold for an optimal range of values of the coupling constant $t$. This is result is applied to Schroedinger operators on unbounded domains which are perturbed by a nonnegative potential which is mostly equal to zero. Unique continuation estimates nevertheless ensure quantitative bounds on the lifting of spectral edges due to this semidefinite potential. This allows to perform spectral engineering in certain situations. The talks is based on the preprint https://arxiv.org/abs/1804.07816 
Ansprechpartner: 
Thema: 
Perturbative construction of a stringlocalized Dirac field in a Hilbert space representation of QED  A programme and some results. 
Datum: 
13.07.18 
Uhrzeit: 
14:15 
Ort: 
D5153 
Vortragender: 
Jens Mund 
UFJF, Juiz de Fora, Brazil 

Inhalt: 
The construction of charged physical states in QED [Morchio and Strocchi 1983/2003; Steinmann 1984] has been a difficult task due to the infrared problems related to Gauss' law, which imply that the charge cannot be localized in finite regions and that the electron is an infraparticle, i.e., it does not correspond to a discrete eigenvalue of the mass operator. I propose a new strategy for a straightforward perturbative construction of the interacting Dirac field acting in a Hilbert space, which describes the electron as an infraparticle. It is not point but ''stringlocalized'', i.e., localized on halfrays extending to spacelike infinity. The construction works in a new framework which relates the free (GuptaBleuler) vector potential acting in a Krein space with its stringlocalized version acting in the physical (positive semidefinite) subspace. I construct the stringlocalized scalar field chi which implements the gauge transformation between the two potentials, and consider the free stringlocalized ''dressed Dirac field'' $\exp(i e \chi) \psi$ , where $\psi$ is the free Dirac field and $e$ is the electron charge. The adiabatic limit of its interacting version in the EpsteinGlaser perturbative scheme is my candidate for the Dirac field. My conjecture that it exists and satisfies the mentioned properties has the following basis. Firstly, it has been verified at lower orders that this field leaves the physical subspace invariant before the adiabatic limit. Therefore the weak adiabatic limit (whose existence was shown by Blanchard and Seneor) satisfies positivity by a recent result of Duch. Secondly, the field $\chi$ has the same infrared structure as the free scalar field in 2 dimensions, whose exponentiation leads to an infraparticle representation [Schroer 1963]. I expect that the same holds true for our $\chi$ in the adiabatic limit. This would mean that our free ''dressed Dirac field'' already describes the free (but dressed) electron as an infraparticle. Then there is no obstruction from general reasons to the conjecture that the Hilbert space of the free dressed Dirac field is the GNS space of the adiabatic limit. 
Ansprechpartner: 
Thema: 
Physikalische Erlebnispädagogik für Kinder 
Datum: 
28.06.18 
Uhrzeit: 
14:15 
Ort: 
D5153 
Vortragender: 
MariaBernadette Riedl 
Universität Bielefeld 

Inhalt: 

Ansprechpartner: 
Thema: 
Symmetry Transition from GUE to chGUE protecting Chirality 
Datum: 
12.07.18 
Uhrzeit: 
16:15 
Ort: 
D5153 
Vortragender: 
Mario Kieburg 
Bielefeld University 

Inhalt: 
Symmetry transitions of systems have been always of particular interest in physics. There are only few real systems, that are pure and ideal yielding the desired results predicted by simplified, analytically feasible models. This is also the case for the spectral statistics of linear operators corresponding to such realistic systems, which are usually described by random matrices. Especially the global symmetries can be wellcaptured by random matrices, since the local spectral statistics on the level of the mean level spacing is extremly sensitive to these symmetries. Therefore, the question arises what the statistics would look like when a symmetry transition takes place to compare these results efficiently with physical measurements. Exactly this has been the goal of my joint work with Takuya Kanazawa when we studied an interpolation between the Gaussian unitary ensemble (GUE) and the chiral Gaussian unitary ensemble (chGUE) while protecting the chirality of the matrix. This transition is motivated by several QCD applications. Particularly the protection of the chirality leads to surprising effects. I am going to report on these results which comprise finite matrix size as well as the limit of large matrix dimensions. 
Ansprechpartner: 
Thema: 
Symmetry Transition from GUE to chGUE protecting Chirality 
Datum: 
12.07.18 
Uhrzeit: 
16:15 
Ort: 
D5153 
Vortragender: 
Mario Kieburg 
Bielefeld University 

Inhalt: 
Symmetry transitions of systems have been always of particular interest in physics. There are only few real systems, that are pure and ideal yielding the desired results predicted by simplified, analytically feasible models. This is also the case for the spectral statistics of linear operators corresponding to such realistic systems, which are usually described by random matrices. Especially the global symmetries can be wellcaptured by random matrices, since the local spectral statistics on the level of the mean level spacing is extremly sensitive to these symmetries. Therefore, the question arises what the statistics would look like when a symmetry transition takes place to compare these results efficiently with physical measurements. Exactly this has been the goal of my joint work with Takuya Kanazawa when we studied an interpolation between the Gaussian unitary ensemble (GUE) and the chiral Gaussian unitary ensemble (chGUE) while protecting the chirality of the matrix. This transition is motivated by several QCD applications. Particularly the protection of the chirality leads to surprising effects. I am going to report on these results which comprise finite matrix size as well as the limit of large matrix dimensions. 
Ansprechpartner: 