Session U03: Degenerate Fermi Gases

Spin-Energy Correlation in a Weakly-Interacting; Degenerate Fermi gas

Active manipulation of spin and spin current can be used to transport information with low dissipation and for creating quantum-entangled states. We study the formation of spin-energy correlations in a weakly interacting Fermi gas of $^6$Li contained in an optical trap with a spin-dependent potential. A tailored radio-frequency pulse creates an initial coherent superposition of two spin states with a controllable spin-energy spiral. We observe the subsequent evolution of the spin-up and spin-down density profiles over 1000 ms, demonstrating a long coherence time. For both quantum-degenerate and thermal clouds we obtain excellent quantitative agreement with a new collision-less mean-field model of spin-energy correlation. Further, we precisely measure the magnetic field at which the s-wave scattering length vanishes (where spin segregation ceases) for both 1-2 and 2-3 hyperfine state mixtures, providing new constraints on the molecular potentials.   

A strongly interacting Fermi gas of ytterbium-173 in low dimensions

Fermionic ytterbium as an alkaline-earth-like atom features a metastable excited state. The so-called clock state opens up the possibility of exploring interacting two-orbital many-body systems. The strong interorbital spin-exchange interaction of ytterbium-173 leads to a novel type of Feshbach resonance due to the orbital and nuclear spin degree of freedom. This recently observed orbital Feshbach resonance allows to tune the interaction strength in an external magnetic field. In our experiment, a strongly interacting two-orbital Fermi gas of ytterbium-173 is prepared in quasi-2D using a state-independent optical lattice. In addition to the magnetic field, the properties of the interacting system are modified by strong confinement. We probe the effect of reduced dimensions onto the strongly interacting two-orbital Fermi gas with high-resolution spectroscopy on the clock transition.   

Collisional properties of ultracold potassium in the vicinity of a d-wave Feshbach resonance

The massive impact of ultracold atoms on modern physics resides in the great versatility they offer to explore new quantum phenomena. Most notably, Feshbach resonances represent a formidable tool in order to tune interactions between atoms. Originally conceived in the context of nuclear physics by Herman Feshbach, it has, since then, found numerous other applications in several fields, including atomic (and molecular) physics and condensed matter. In particular, high partial waves Feshbach resonances (l\textgreater 1) have been predicted to give access to complex order parameters and quantum phase transition such as p-wave superfluid and d-wave high-Tc superconductors. Here, we report on our progress on studying the temperature dependent collisional properties of a Fermi gas of $^{\mathrm{40}}$K in the vicinity of a new d-wave Feshbach resonance. To this end, we first characterize the nature of the two body losses before analyzing the dynamical evolution of the spin composition while reaching equilibrium.   

Effective field theory approach to the p-wave spin-triplet Fermi superfluid

The low-energy physics of a fully gapped Fermi superfluid is governed by Nambu-Goldstone bosons resulting from its spontaneous symmetry breaking. Here we construct an effective field theory of the Balian-Werthamer state, which is one of the p-wave spin-triplet superfluid state. The effective field theory is constructed up to the next-to-leading order in a derivative expansion, so as to be consistent with all available symmetries in curved space, which are the $\textrm{U}(1)_{\textrm{phase}}\times \textrm{SU}(2)_{\textrm{spin}}\times \textrm{SO}(3)_{\textrm{orbital}}$ gauge invariance and the nonrelativistic general coordinate invariance. The obtained low-energy effective field theory is capable of predicting gyromagnetic responses of the Balian-Werthamer state, such as a magnetization generated by a rotation and an orbital angular momentum generated by a magnetic field, in a model-independent and nonperturbative way. We furthermore show that the stress tensor exhibits a dissipationless Hall viscosity with coefficients uniquely fixed by the orbital angular momentum.   

Sound in a Unitary Fermi Gas

Low temperature thermodynamics and transport properties of systems ranging from solids to quantum gases are determined by phonons (sound). We measure important properties of phonons in a homogeneous unitary Fermi gas, including its dispersion relation and quantum limited attenuation. We present our measurements of the sound diffusion coefficient, a quantity which fully characterizes sound attenuation, and is closely related to viscosity and thermal conductivity of the Fermi gas.   

Collective excitations of a resonantly interacting Fermi gas across the superfluid transition

We explore the temperature dependence of the elementary excitations across the superfluid phase transition in a homogeneous unitarity Fermi gas using low-momentum Bragg spectroscopy. At long wavelength, Bragg scattering probes collective excitations of the gas which are closely linked to the superfluid order parameter. In the experiments, we shine two tightly focussed laser beams into the central, nearly homogeneous, volume of an optically trapped cloud of lithium-6 atoms with equal spin population and measure the density-density response. The dominant feature in the measured Bragg spectra is a peak corresponding to the phonon mode, which dramatically changes in both amplitude and width across the superfluid to normal fluid transition. We can use this to study dynamic properties such as damping and the evolution of the speed of sound. The latter allows also us to link the density response to the thermodynamics of the system via the pressure equation of state.   

Interaction Effects in Normal 2D Fermion Gases

$\newline$We present results on the effect of short-range, attractive interactions on the properties of balanced 2D Fermi gases in the non-superfluid (normal) phase. Our approach combines the renormalization group (RG) with perturbation theory, yielding observables such as the equation of state and compressibility. We find good agreement with recent experiments that measured the equation of state in trapped gases in the balanced regime [1,2], showing that these results are consistent with logarithmic corrections in the equation of state. We also discuss predictions for our theory in the imbalanced regime. $\newline$ [1] K. Fenech et al, Phys. Rev. Lett. $\textbf{116}$, 045302 (2016) $\newline$ [2] I. Boettcher et al, Phys. Rev. Lett. $\textbf{116}$, 045303 (2016)   

Observation of the Higgs mode in a strongly interacting fermionic superfluid

Higgs and Goldstone modes are possible collective modes of an order parameter upon spontaneously breaking a continuous symmetry. Whereas the low-energy Goldstone (phase)mode is always stable, additional symmetries are required to prevent the Higgs (amplitude)mode from rapidly decaying into low-energy excitations. In the realm of condensed--matter physics, particle--hole symmetry can play the role of Lorentz invariance in high energy physics and a Higgs mode has been observed in weakly-interacting superconductors. However, whether the Higgs mode is also stable for strongly-correlated superconductors in which particle--hole symmetry is not precisely fulfilled or whether this mode becomes overdamped has been subject of numerous discussions. Experimental evidence is still lacking, in particular owing to the difficulty to excite the Higgs mode directly. Here, we observe the Higgs mode in a strongly-interacting superfluid Fermi gas. By inducing a periodic modulation of the amplitude of the superconducting order parameter, we observe an excitation resonance at frequency 2$\Delta $/h. For strong coupling, the peak width broadens and eventually the mode disappears when the Cooper pairs turn into tightly bound dimers signalling the eventual instability of the Higgs mode.   

Higgs mode in a superfluid Fermi gas in the BCS-BEC crossover

In quantum many-body systems with spontaneous breaking of a continuous symmetry, Higgs modes emerge as collective amplitude oscillations of order parameters. Recently, Higgs modes have been observed in superconductors and in Bose gases in optical lattices. However, it has yet to be observed in Fermi gases. We use the time-dependent Bogoliubov–de Gennes equations to investigate Higgs amplitude oscillations of the superfluid order parameter in a trapped Fermi gas induced by a sudden changes of the s-wave scattering length. In particular, we investigate the Higgs mode with different values of the initial scattering length and discuss how the frequency and damping of the Higgs mode changes in the BCS-BEC crossover.