Condensed Matter Seminar: Few to Many: Observing the emergence of a quantum phase transition

Luca Bayha and Marvin Holten (Heidelberg University)
Collective excitations and phase transitions are striking manifestations of many-body physics.

Luca Bayha and Marvin Holten (Heidelberg University)
Online Seminar
Mon 19. October 2020, 11:30 am (MEZ)

Abstract:

Collective excitations and phase transitions are striking manifestations of many-body physics. Understanding how collective behaviour emerges when assembling a system from individual particles is an ongoing challenge. Here, we present an observation of the few-body precursor of a quantum phase transition from a normal to a superfluid phase. The measurement relies on the deterministic preparation of the ground state of a few ultracold fermions in a two-dimensional harmonic oscillator. Its level structure gives rise to stable closed-shell configurations at certain magic numbers like 2, 6 or 12 atoms.
We perform spectroscopy on this system and find a pair excitation that shows a non-monotonous excitation energy when increasing the attraction from zero to being larger than the shell spacing. This mode can be identified as the few-body precursor of a Higgs mode associated to a normal to superfluid quantum phase transition in the thermodynamic limit.
In a second set of experiments we investigate the momentum distribution of the closed-shell configurations of a spin polarized Fermi gas with single particle resolution. We observe strong higher-order correlations in the density. These so-called Pauli crystals arise purely from the anti-symmetry of the wave function of the indistinguishable particles.
In the future, our newly established methods will allow us to explore questions like the emergence of Cooper pairing and Goldstone modes when scaling up the system size.