Promotionsvortrag Physik: „Effective models for interacting spin waves in two-dimensional quantum antiferromag-nets“ (Hörsaal HE, Physikum, Staudtstr. 5, Erlangen)

Ankündigung des Promotionsvortrags von: Herrn Matthias Walther

Deriving effective models is an important approach to describe interacting many-body systems that also gives insights about the relevant quasi particles and their interactions.

In my thesis, I focus on magnetically ordered phases in spin-1/2 antiferromagnets in two dimensions on a square lattice.

Technically, this is achieved by continuous similarity transformations (CSTs) based on the non-Hermitian Dyson-Maleev representation of elementary, collective excitations, called magnons.

The CST flow equations are truncated in momentum space by the scaling dimension.

For the paradigmatic antiferromagnetic spin-1/2 Heisenberg model on the square lattice, this method marked the first demonstration of an effective model based on the magnon picture that captures the entire single-particle dispersion and spectral densities.

It agrees excellently with experimental and numerical data.

We extend the usage of the CST to gapped phases and continuous phase transitions.

As a prototypical gapped phase, we study the antiferromagnetic easy-axis XXZ-model. We find a quantitative description of the closing of the single-particle gap and determine the excitation energies of two-magnon bound states and their decay into the two-magnon continuum.

Next, we determine the quantum phase transition out of the Néel-ordered and columnar phases in the frustrated J_1-J_2 model on the square lattice.

We extract quantum-critical points that are in good agreement with the literature.

Further, we estimate the associated critical exponents α and β which turns out to be a challenging task for the CST approach.

(Vortrag auf Englisch)

Dem Vortrag schließt sich eine Diskussion von 15 Minuten an. Vortrag und Diskussion sind öffentlich. Diesen Verfahrensteilen folgt ein nicht öffentliches Rigorosum von 45 Minuten.