arXiv subjects

J. A. Joseph

Publications and source records attributed to J. A. Joseph.

4 recordsLinked to original sources

arXiv2015

Two-field optical methods to control magnetic Feshbach resonances

Using an optically-trapped mixture of the two lowest hyperfine states of a $^6$Li Fermi gas, we observe two-field optical tuning of the narrow Feshbach resonance by up to 3 G and an increase in spontaneous lifetime near the broad resonance from $0.5$ ms to $0.4$ s. We present a new model of light-induced loss spectra, employing continuum-dressed basis states, that agrees in shape and magnitude with measurements for both broad and narrow resonances.

cond-mat.quant-gas↗

arXiv2014

Shear viscosity of a universal Fermi gas near the superfluid phase transition

We precisely measure the shear viscosity for a resonantly interacting Fermi gas as a function of temperature, from nearly the ground state through the superfluid phase transition at a critical temperature $T_c$. Using an iterative method to invert the data, we extract the {\it local} shear viscosity coefficient $α_S(θ)$ versus reduced temperature $θ$, revealing previously hidden features. We find that $α_S$ begins to decrease rapidly with decreasing $θ$ well above $T_c$, suggesting that preformed pairs play an important role. Further, we observe that the derivative $α_S'(θ)$ has a maximum at $T_c$. We compare the local data to several microscopic theories. Finally, we determine the local ratio of the shear viscosity to the entropy density.

cond-mat.quant-gas↗

arXiv2014

Anomalous minimum in the shear viscosity of a Fermi gas

We measure the static shear viscosity $η$ in a two-component Fermi gas near a broad collisional (Feshbach) resonance, as a function of interaction strength and energy. We find that $η$ has both a quadratic and a linear dependence on the interaction strength $1/({k_{FI}a})$, where $a$ is the s-wave scattering length and $k_{FI}$ is the Fermi wave vector for an ideal gas at the trap center. For energies above the superfluid transition, the minimum in $η$ as a function of interaction strength is significantly shifted toward the BEC side of resonance, to $1/(k_{FI}a)\simeq 0.25$.

cond-mat.quant-gas↗

arXiv2013

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

We precisely test scale invariance and 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, $0.005(0.016)\,\hbar\,n$, 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.

cond-mat.quant-gas↗