Anomalous breaking of scale invariance in a two-dimensional Fermi gas

TL;DR

This paper investigates the quantum anomaly in a two-dimensional Fermi gas, demonstrating how scale invariance is broken by interactions, leading to measurable shifts in the breathing mode frequency that depend on temperature and interactions.

Contribution

It provides experimental measurements of the anomalous frequency shift in a strongly interacting 2D Fermi gas and compares results with theoretical predictions at zero temperature.

Findings

Significant frequency shifts observed due to quantum anomaly.

Shifts depend strongly on interactions and temperature.

Qualitative agreement with zero-temperature theory.

Abstract

The frequency of the breathing mode of a classical two dimensional Fermi gas in a harmonic confinement is fixed by the scale invariance of the Hamiltonian. Scale invariance is broken on the quantum mechanical level by introducing the two dimensional scattering length as a regulator. This is an example of a quantum anomaly in the field of ultracold atoms and leads to a shift of the frequency of the collective breathing mode of the cloud. In this work, we study this anomalous frequency shift for a two component Fermi gas in the strongly interacting regime. We measure significant shifts away from the scale invariant result that depend strongly on both interactions and temperature. We find qualitative agreement with theoretical calculations at zero temperature.