Measuring Excitations of a Strongly Interacting Superfluid Fermi Gas

Abstract

This thesis investigates experimentally the superfluid phase of strongly interacting ultracold Fermi gases. The diluteness of the considered gases of neutral atoms ensures that their interaction can be described by a single interaction parameter leading to universal behavior, which can be relevant for entirely different physical systems. The interactions are tunable by a magnetic offset field, which enables the exploration of the so called “BEC-BCS” crossover. This crossover is a smooth connection from a weakly attractive superfluid, which has similarities to a superconductor and is described by Bardeen-Cooper-Schrieffer (BCS) theory, across a strongly interacting regime with remarkably stable superfluidity, to a weakly repulsive Bose-Einstein condensate (BEC) of molecules.<br /> The critical temperature of superfluid systems is a crucial property and in the case of high temperature superconductivity, a better understanding of the critical temperature could have a practical impact. Nevertheless, previous measurements of the critical temperature of the superfluid phase transition across the BEC-BCS crossover show large deviations from theoretical predictions, especially towards the BEC side of the strongly interacting regime. The deviations can be attributed to difficulties in measuring the temperature of strongly interacting gases. This problem is addressed here by employing a more careful thermometry, based on the low density regions of the trapped sample, taking interaction effects into account. Additionally, by measuring the density of the gas at the trap center, the dependence of the critical temperature on compression effects is examined, which allows a comparison between the critical temperature of a trapped and a homogeneous gas.<br /> The superfluid state is typically detected by observing a bimodal density distribution in time-of-flight, caused by a macroscopic occupation of low momentum paired states, which can be distinguished from a thermal background. However, on the BCS side the pairs break up during time-of-flight, which is usually mitigated with a rapid ramp of the magnetic field towards the BEC side to project the pairs onto more robust molecules. Here, analysis methods based on machine learning are described, which do not require the rapid ramp but can detect superfluidity directly from time-of-flight images.<br /> Recently, the excitations of Fermi gases in the BEC-BCS crossover have been explored experimentally in quite some detail, except for the Higgs amplitude mode, which is difficult to excite and to detect. It directly couples to the interaction, but in order to excite the Higgs amplitude mode the interaction has to be changed on a timescale similar to the Fermi time of typically a few microseconds, which poses technical challenges. Here, the construction of a new fast magnetic coil is described, which allows the manipulation of the interaction on a timescale faster than the Fermi time and enables the implementation of two different theoretically proposed excitation schemes of the Higgs amplitude mode.<br /> The first method is an interaction quench, which leads to subsequent oscillation of the order parameter. Indeed, an experimental signature of these oscillations is found, but the low signal-to-noise ratio requires a careful analysis.<br /> The second method, interaction modulation, provides an excitation scheme with a narrow frequency resolution and reveals a resonance at the Higgs frequency. The mode can be distinguished from the background consisting of the pair breaking continuum and the dissociation of Feshbach molecules on the BEC side.