+++ONLINE COLLOQUIUM+++ DOUBLE FEATURE Dr. Thomas Chalopin and Dr. Rafał Ołdziejewski

Dr. Thomas Chalopin talks about "Realising the Symmetry-Protected Haldane Phase in Fermi-Hubbard Ladders"

The Haldane antiferromagnetic spin-1 chain constitutes a paradigmatic model of a quantum system which holds a symmetry protected topological phase. Such a system features a ground state with gapped bulk excitations and a non-local order parameter. In the case of open boundary conditions, the ground state is furthermore four-fold degenerate due to the presence of topologically protected edge states.

In this talk, I will present our experimental realization of the Haldane phase using Fermi-Hubbard ladders in an ultracold quantum gas microscope [1]. Site-resolved potential shaping allows us to create tailored spin-1/2 geometries which permit the exploration of such a topological chain and its comparison to a topologically trivial configuration. We use spin- and density-resolved measurements to probe edge and bulk properties of the system, revealing a clear distinction between the trivial and topological cases. The measurement of the non-local string order parameter, in particular, allows to directly capture the underlying protecting symmetry of the topological phase. We furthermore investigate the robustness of the topological phase upon the onset of density fluctuations by tuning the Hubbard interaction.
[1] P. Sompet et. al., arXiv:210310421 (2021).

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Dr. Rafał Ołdziejewski talks about "Excitonic Tonks-Girardeau and charge-density wave phases in monolayer semiconductors"

Excitons in two-dimensional semiconductors provide a novel platform for fundamental studies of many-body interactions. In particular, dipolar interactions between spatially indirect excitons may give rise to strongly correlated phases of matter that so far have been out of reach of experiments. We will discuss that excitonic few-body systems in atomically thin transition-metal dichalcogenides undergo a crossover from a Tonks-Girardeau to a charge-density-wave regime. In our approach, we take into account realistic system parameters and predict the effective exciton-exciton interaction potential. We find that the pair correlation function contains key signatures of the many-body crossover already at small exciton numbers and show that photoluminescence spectra provide readily accessible experimental fingerprints of these strongly correlated quantum many-body states.