Critical Velocity for Superfluid Flow across the BEC-BCS Crossover

Critical velocities have been observed in an ultracold superfluid Fermi gas throughout the BEC-BCS crossover. A pronounced peak of the critical velocity at unitarity demonstrates that superfluidity is most robust for resonant atomic interactions. Critical velocities were determined from the abrupt onset of dissipation when the velocity of a moving one-dimensional optical lattice was varied. The dependence of the critical velocity on lattice depth and on the inhomogeneous density profile was studied.

Figure 1

Onset of dissipation for superfluid fermions in a moving optical lattice. (Inset) Schematic of the experiment in which two intersecting laser beams produced a moving optical lattice at the center of an optically trapped cloud (trapping beams not shown). Number of fermion pairs which remained in the condensate $N_c$ after being subjected to a $V_0=0.2E_F$ deep optical lattice for 500 ms, moving with velocity $v_L$, at a magnetic field of 822 G ($1/k_{F}a=0.15$). An abrupt onset of dissipation occurred above a critical velocity $v_c$, which we identify from a fit to Eq. (1).

Figure 2

Critical velocities throughout the BEC-BCS crossover. A pronounced maximum was found at resonance. Data are shown for a $V_0=0.2E_F$ deep lattice, held for $t=500\mathrm{ms}$. The solid line is a guide to the eye.

Figure 3

Effects of density inhomogeneity on the critical velocity. A configuration in which the lattice beams ($80\mu \mathrm{m}$) were larger than the trapped sample ($37\mu \mathrm{m}$) results in loss in the condensate number $N_c$ at significantly lower velocity. Data are shown for a $V_0=0.15E_F$ deep optical lattice held for 200 ms at a magnetic field of 822 G.

Figure 4

Critical velocities at different lattice depths. The results show $v_c$ to be a decreasing function of lattice depth $V_0$. In the limit of low $V_0$, $v_c$ converges to a maximum value of 0.25 $v_F$. Data were taken near resonance, at 822 G ($1/k_{F}a=0.15$) for hold times $t=250$, 500, 1000, 2000 ms (squares, diamonds, circles, triangles).

Figure 5

Number of pairs that remained in the condensate $N_c$ (solid circles) and thermal component $N_{\mathrm{th}}$ (open circles) after being held in a $V_0=0.35E_F$ deep optical lattice moving with velocity $v_L=6\mathrm{mm}/\mathrm{s}$ for a variable hold time. The thermal component shows a linear increase (dashed line), whereas $N_{\mathrm{cond}}$ showed an accelerated loss, and is fit to a quadratic function (solid line).