ELFT HÍRADÓ

[Janos Asboth <asboth.janos AT ttk.bme.hu>]

Meghívó

BME Elméleti Fizika Szeminárium,

okt. 13. péntek 10h15,

1111 Budapest, Budafoki út 8., BME F III. magasföldszint 01.,
Elméleti Fizika Tanszék szemináriumi szoba

Frank György (BME Elm. Fiz. tanszék):
Singularity theory of Weyl-point creation and annihilation

Weyl points (WP) are robust spectral degeneracies, which can not be split
by small perturbations, as they are protected by their non-zero topological
charge. For larger perturbations, WPs can disappear via pairwise
annihilation, where two oppositely charged WPs merge, and the resulting
neutral degeneracy disappears. The neutral degeneracy is unstable, meaning
that it requires the fine-tuning of the perturbation. Fine-tuning of more
than one parameter can lead to more exotic WP mergers. In this work [1], we
reveal and analyze a fundamental connection of the WP mergers and
singularity theory: phase boundary points of Weyl phase diagrams, i.e.,
control parameter values where Weyl point mergers happen, can be classified
according to singularity classes of maps between manifolds of equal
dimension. We demonstrate this connection on a Weyl--Josephson circuit
where the merger of 4 WPs draw a swallowtail singularity, and in a random
BdG Hamiltonian which reveal a rich pattern of fold lines and cusp points.
Our results predict universal geometrical features of Weyl phase diagrams,
and generalize naturally to creation and annihilation of Weyl points in
electronic (phononic, magnonic, photonic, etc) band-structure models, where
Weyl phase transitions can be triggered by control parameters such as
mechanical strain.

[1]: Gy. Frank, G. Pintér, and A. Pályi: "Singularity theory of Weyl-point
creation and annihilation",  arXiv:2309.05506
<https://arxiv.org/abs/2309.05506> (2023)

Minden érdeklődőt szeretettel várunk.
Asbóth János,
 szemináriumi koordinátor