# Heidelberg 2015 – wissenschaftliches Programm

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# A: Fachverband Atomphysik

## A 4: Ultra-cold atoms, ions and BEC I (with Q)

### A 4.4: Vortrag

### Montag, 23. März 2015, 15:15–15:30, C/HSW

**Goldstone mode in the quench dynamics of an ultracold BCS Fermi gas** — •Peter Kettmann^{1}, Simon Hannibal^{1}, Mihail Croitoru^{2}, Alexei Vagov^{3}, Vollrath Martin Axt^{3}, and Tilmann Kuhn^{1} — ^{1}Institute of Solid State Theory, University of Münster — ^{2}Condensed Matter Theory, University of Antwerp — ^{3}Theoretical Physics III, University of Bayreuth

Ultracold Fermi gases are a convenient testbed for complex interacting Fermi systems like, e.g., superconductors. They are on the one hand easily accessible in experiment. On the other hand they can form not only a BEC but a BCS phase as well. A study of this BCS phase and the crossover to the BEC is expected to give insight into other fields like high temperature superconductivity.

We investigate the BCS phase of an ultracold Fermi gas. In particular we calculate the nonequilibrium dynamics of a confined ^{6}Li gas after a quantum quench, i.e., a sudden change of the BCS coupling strength induced by the abrupt change of an external magnetic field. We find that the excitation leads to a vibration of the cloud with the spectrum containing one dominant low frequency and several higher frequencies. We show that the low frequency corresponds to the Goldstone mode of the order parameter while the higher frequencies result from the amplitude oscillation of the gap, i.e., the Higgs mode.

We study the Goldstone mode over a wide range of parameters. We find that the size-dependent superfluid resonances [1] have a strong impact on the frequency of the Goldstone mode and on its dependence on the size of the cloud. [1] Shanenko et al., PRA 86, 033612 (2012)