Session C4: Ultracold Fermi Gases

Engineered p-wave collisions in ultracold Fermionic systems

This experiment demonstrates the p-wave and higher odd partial waves describing interactions in a spin polarized Fermi gas: critical for engineering utlracold Fermi gases to support Majorana fermions [1]. Here, we describe an experiment which combines known experimental techniques for modifying interactions between atoms in a unique way to artificially engineer p-wave collisions in ultracold Fermionic systems. Using s-wave Feshbach resonances to tune interactions in degenerate Fermi gases have been very fruitful in studying many-body quantum physics; however, p-Wave Feshbach resonances are limited in their usefulness by large inelastic loss rates [2,3]. In combination with an s-Wave Feshbach resonance, we modify the fermionic interaction by laser-dressing. We already demonstrated that this technique introduces d- and g-wave contributions to the s-wave scattering in degenerate Bose gases [4].\\[4pt] [1] C. Zhang, S. Tewari, R. M. Lutchyn, S. Das Sarma, \textit{Phys. Rev. Lett.} \textbf{101}, 160401 (2008). \newline [2] C. Chin, R. Grimm, P. Julienne, E. Tiesinga, \textit{Rev. Mod. Phys.} \textbf{82}, 1225 (2010). \newline [3] C. A. Regal, C. Ticknor, J. L. Bohn, D. S. Jin, \textit{Phys. Rev. Lett.} \textbf{90}, 053201 (2003). \newline [4] R. A. Williams, et al. \textit{Science} \textbf{335}, 314 (2012).   

Optical Control of Collisional Interactions in $^{6}$Li using Dark Molecular States

We are developing ``dark-state'' two-optical field methods to control interactions in $^{6}$Li. Although external magnetic fields are typically used to tune the interaction strength in fermionic atoms near a Feshbach resonance, optical tuning methods can provide rapid temporal control and high-resolution spatial control thus enabling the study of non-equilibrium strongly interacting Fermi gases. However, optical tuning suffers from heating due to spontaneous scattering, which can be suppressed by a second optical field. We will report on the measurement of loss spectrum as a function of magnetic field and laser detuning near Feshbach resonances in $^{6}$Li and our progress on two-optical field loss suppression.   

Non-interacting Fermi gas in a magnetic quadrupole trap

A non-interacting gas of spin polarised 6Li Fermi gas in a magnetic quadrupole trap which is not in thermal equilibrium can nevertheless show thermal signatures in some cases. This puzzling behaviour can be seen by measuring the doubly integrated momentum distribution along a particular axis. This distribution can be extremely close to a Gaussian from which we can extract a temperature. However, we show, using molecular dynamics simulations that the temperature thus measured is generally different along different axes. We provide a general explanation of this phenomenon based on ergodicity and check it with further simulations.   

Anomalous minimum in the shear viscosity of a Fermi gas

We measure the static shear viscosity in a two-component Fermi gas near a broad collisional (Feshbach) resonance as a function of the interaction strength and energy. We implement new and more precise methods in comparison to our previous measurement of shear viscosity by utilizing an elliptical trap with a 1:2.7:33 aspect ratio and imaging the expanding atom cloud from two orthogonal directions. We investigate the dependence of the shear viscosity $\eta $ as a function of the interaction strength 1/(k$_{\mathrm{F}}$ a), where a is the s-wave scattering length and k$_{\mathrm{F}}$ is the Fermi wave vector for an ideal gas at the trap center. We find that near resonance at constant energy, $\eta $ has both a quadratic and linear dependence on 1/(k$_{\mathrm{F}}$ a). Further, we find the linear contribution diminishes as a function of energy while the quadratic dependence remains constant. At low energy just above the critical temperature we find the minimum in shear viscosity is less than the resonant value and is significantly shifted toward the BEC side of resonance to 1/(k$_{\mathrm{F}}$ a) $\approx $ 0.2. http://arxiv.org/abs/1311.2049   

The Scattering and Coherence of a Fermi Polaron

We probe the coherence of a strongly interacting $^{40}$K impurity in a Fermi sea of ${^6}$Li atoms using time-resolved Ramsey spectroscopy. The measured variation of the coherence with the interaction strength and temperature is well-explained by the low-energy scattering of the impurity in the Fermi liquid picture. For very strong interactions, we observe additional dynamics arising from the dressing of the impurity by its environment.   

Observation of scale invariance and conformal symmetry breaking in expanding Fermi gases

We precisely test scale invariance and examine local thermal equilibrium in the hydrodynamic expansion of a Fermi gas of atoms as a function of interaction strength. After release from an anisotropic optical trap, we observe that a resonantly interacting gas obeys scale-invariant hydrodynamics, where the mean square cloud size $\langle{\mathbf{r}}^2\rangle=\langle x^2+y^2+z^2\rangle$ expands ballistically (like a noninteracting gas) and the energy-averaged bulk viscosity is consistent with zero, $0.00(0.04)\,\hbar\,n$, with $n$ the density. In contrast, the aspect ratios of the cloud exhibit anisotropic ``elliptic" flow with an energy-dependent shear viscosity. Tuning away from resonance, we observe conformal symmetry breaking, where $\langle{\mathbf{r}}^2\rangle$ deviates from ballistic flow.   

Local observation of pair-condensation in a Fermi gas at unitarity

Ultracold Fermi gases near a Feshbach resonance provide a means to investigate the physics of strongly interacting quantum systems. Through the use of spatially resolved Bragg spectroscopy we are able to measure the homogenous density-density response function of a Fermi gas at unitarity. The resulting Bragg response provides a clear signature of pair-condensation at temperatures below the superfluid transition temperature. The method used to obtain the local measurement is generalizable to any homogenous parameter which satisfies the local density approximation, providing a new tool that can be used where techniques such as the inverse-Abel transform are no longer applicable.   

Dynamics of pair correlations in a strongly interacting polarized Fermi gas

Radio-frequency (rf) spectroscopy has been used to measure the chemical potential, the molecular binding energy, the quasiparticle dispersion relation, and the contact of Fermi gases in equilibrium. Here we present a dynamic study of Fermi-degenerate $^{40}$K that is initially fully polarized in a superposition of two hyperfine states. A magnetic field gradient, combined with transverse spin diffusion, causes the gas to demagnetize. We present results of time-resolved rf spectroscopy, used to observe both contact dynamics at unitarity and the formation of molecular dimers below the Feshbach resonance. The time variation of contact shows how an ideal gas dynamically evolves into a strongly correlated mixture. Molecular dynamics allow us to delineate a metastable regime of repulsive atomic interactions.   

Periodically driven cold fermions in integer Quantum Hall regime

We present theoretical results for ultracold fermions in the presence of a constant artificial magnetic field but time-periodically modulated optical lattices. Besides the familiar integer quantum Hall effect in the stroboscopic limit, we show a series of Floquet topological phases that have no static analog and are unique to periodically driven systems. A hallmark of these phases is the appearance of robust, counter-propagating edge modes. We elucidate the nature of these edge states by considering different driving protocols and then discuss their experimental signatures.