Session K08: Degenerate Fermi Gases

Rotating uniform 3D Fermi gases

Peculiar response to rotation is one of the hallmarks of quantum fluids. Ultracold-atom experiments in harmonic traps have demonstrated several such manifestations, for instance the existence of critical frequency for vortex nucleation, and the observation of ordered vortex lattices. The advent of optical boxes has opened the prospect of studying rotation and vortices in homogeneous systems with sharp boundaries. Indeed, crucial new qualitative features such as the absence of surface modes are expected to significantly alter rotation and vortex-nucleation dynamics. Here, we present protocols to rotate three-dimensional box trapped uniform Fermi gases and the response of the system to slow rotation. We also discuss the measurement of the superfluid fraction of the system in the slow rotation limit, the analog of the Andronikashvili rotating bucket experiment, and the observation of the quench of the gas's moment of inertia. Finally, we explore the feasibility of nucleating individual vortex filaments in a homogeneous 3D gas.   

Spin transport at non-equilibrium interfaces in the unitary Fermi gas

Quantum gases of fermionic atoms provide a model system for studying the dynamics of strongly correlated fermions. A multi-region box trap allows exploration of transport and equilibration by enabling samples prepared in different thermodynamic states to be brought into contact. In particular, a highly spin-polarized normal-phase region and a weakly spin-polarized superfluid region provide a non-equilibrium normal-superfluid interface. The two regions can evolve towards equilibrium by exchange of particles. We implement a phenomenological mean-field model and calculate the spin and mass currents across the interface between normal and superfluid regions. We find that transport into an unpolarized superfluid from a polarized normal fluid exhibits a threshold in the normal fluid polarization, analogous to the current-voltage curve of a normal-superconductor junction. However, the threshold nearly vanishes when the superfluid reaches critical polarization. We describe progress towards experimental realization of this system using a digital micromirror device (DMD) to create programmable barriers between regions of the trap.   

Fermi gas hydrodynamics in perfect box potentials

We create a perfect optical box potential for a ⁶ Li Fermi gas. The box walls are six sheets of repulsive light, which are created by two Digital Micromirror Devices (DMD). We use a third DMD to generate a customized weakly repulsive potential inside the box, to cancel the attractive potential arising from the bias magnetic field. The net result is to give an almost perfect uniform density profile. We will report on our latest hydrodynamic transport experiments in a perfect box potential.   

Weakly-Interacting Fermi Gases as Chains of Interacting Spins

In a weakly-interacting (a_s < ~15 Bohr) Fermi gas, the energies of individual atoms are conserved on the order of minutes. As pairs of atoms forward-scatter, the spin states are mutually affected, producing long-range spin coherence and spin waves. This system may be described as a spin chain in energy space, with site-to-site couplings that are controlled either magnetically or optically. We discuss our progress in developing new methods to characterize this system.   

Searching for Breakdowns of Classical Spin Rotation Models in Weakly-Interacting Fermi Gases

Weakly-interacting Fermi gases have been modeled as classical spin chains in energy space. This spin vector evolution model is able to predict the spin-density profiles successfully for short (<1s) spin evolution after coherent excitation. However, fitting this model to out-of-time order correlation measurements requires a scattering length that is 2.5 times larger than the measured value, and the spin-density amplitude diverges from the model at longer evolution times. We will report our recent measurements attempting to understand these discrepancies.   

Density fluctuation dynamics in SU(N)-symmetric fermions

Spin multiplicity N affects thermodynamics in a SU(N)-symmetric Fermi gas, in which intriguing properties have been extensively studied theoretically and experimentally. However, there is still a lack of study about thermodynamics in imbalanced SU(N) fermions, especially when a system undergoes non-equilibrium dynamics. In this talk, we will introduce our studies about the imbalanced SU(N) degenerate Fermi gas of tunable spin configuration through density fluctuation measurements. This approach enables monitoring non-equilibrium dynamics of imbalanced SU(N) fermions after the sudden quench of spin configuration. Our findings may extend our understanding of SU(N)-symmetric Fermi liquid and its spin diffusion.   

Three-component Fermi gases: stability and energy dynamics

Uniform ultracold quantum gases with three (pseudo-)spin components are a promising platform for exploring novel physics, such as exotic “spin-1” superfludity and analogues of QCD. But much remains to be understood about the stability of these systems. Building on our recent observation of anomalous losses in three-component mixtures of ⁶Li [1], we expand our measurements to additional interaction regimes, and we present energy-resolved loss measurements. Seeking to shed light on the microscopic mechanism behind these anomalous losses, we explore the scaling of the loss rate with the average energy of the system.

[1] G. Schumacher, et al, arXiv:2301.02237   

Higgs oscillations in a unitary Fermi superfluid

Ultracold atomic gases with tunable interactions provide a versatile setting to explore quantum many-body systems out of equilibrium. Here, we study the dynamics in a two-component strongly interacting Fermi gas following a quench of the inter-atomic interactions. Small quenches within the superfluid phase excite Higgs oscillations of the superfluid order parameter. We observe these directly using Bragg spectroscopy which measures the imaginary part of the density-density response function. Our measurements at unitarity show a strong temperature dependence, and the oscillations decay according to a power law with a damping exponent approximately midway between the well-known Bardeen-Cooper-Schrieffer (BCS) and Bose-Einstein Condensate (BEC) limits. As the interactions are tuned through the BCS-BEC crossover regime, we observe a monotonic change of the oscillation frequency reflecting the evolution of the pair-breaking energy, as well as the damping exponent. Our data show good qualitative agreement with the time-dependent BCS theory.   

Emergence of Fermi's Golden Rule in the Spectroscopy of a Strongly Correlated Fermi Gas

Connecting experimental observables to many-body quantities often relies on linear-response theory via Fermi's Golden Rule. However, its range of applicability is not always straightforward, especially when probing a strongly interacting system. Here, we explore the radio-frequency spectroscopic response of a homogeneous, balanced spin-½ Fermi gas of ⁶Li, where we drive one of the two components into a third unoccupied level and measure the transfer fraction rate. We observe three distinct regimes. First, at early stages, the transfer fraction scales quadratically with pulse duration. At intermediate times, the 'Fermi’s Golden Rule' regime emerges. For long enough pulse duration, we observe the saturation of the transfer fraction, beyond linear response. This clarifies the validity of linear-response theory in many-body spectroscopy.   

Measuring two-body correlations via photoexcitation of 40K2 in an optical lattice

The properties of many-body systems with short-range interactions can be characterized by the strength of two-body correlations expressed via the contact. In atomic gases, these correlations may be probed using photoexcitation-induced trap loss of interacting atomic pairs. Here, we use a 3D optical lattice to create isolated two-body systems of fermionic ⁴⁰K and tune their interaction strength via a magnetic s-wave Feshbach resonance. In contrast to a bulk gas, the correlations of this precisely two-body system can be exactly calculated across all interaction strengths. We measure photoexcitation spectra and induced trap loss rates of these isolated Feshbach molecules and their dependence on magnetic field and lattice confinement. The measured photoexcitation loss rates are compared to those calculated using the expected short-range correlations, as well as to those of a bulk gas. These measurements of isolated two-body systems provide insight into the correlations and universal contact relations in the many-body regime.