Higgs Mode and Critical Temperature in the BCS-BEC Crossover

Abstract

For this thesis, analogue quantum simulations of the BCS-BEC crossover have been performed. To this end, an ultracold gas of fermionic ⁶Li atoms was prepared in an optical dipole trap. The gas is in a balanced mixture of two hyperfine states which constitute the two spin states necessary for the implementation of the crossover. The strength of the attractive interaction between these states is controlled by an external magnetic field with the help of a Feshbach resonance.<br /> New techniques to detect superfluidity, based on machine learning, were developed. These give a clear indication of the phase transition for arbitrary interaction strengths. By reconstructing the density distribution of the inhomogeneous gas, a phase diagram of the homogeneous BCS-BEC crossover could be established. It confirmed, for the first time, an increase of critical temperature from the unitary point towards the BEC limit.<br /> With a newly developed coil, the interaction strength of the gas could be quenched to excite the Higgs mode of the superfluid. It is observed as a fast, interaction strength-dependent oscillation in the condensed fraction of the gas and constitutes the first observation of the Higgs mode in time domain in the whole BCS-BEC crossover. The oscillations exhibit a strong damping, which a model based on the local-density approximation attributes to dephasing caused by the inhomogeneity of the gas. Towards the BEC limit of the crossover, a stronger damping than expected from dephasing is observed indicating an additional instability of the Higgs mode. This has been predicted theoretically in this regime, where the system cedes to be particle-hole symmetric.<br /> As a complementary method to the interaction quench, some first results of experiments in which the interaction strength is periodically modulated are presented. Here, a resonance in the response of the superfluid, matching some predictions for the Higgs mode, is observed.