Repulsive Atomic Gas in a Harmonic Trap on the Border of Itinerant Ferromagnetism

Alongside superfluidity, itinerant (Stoner) ferromagnetism remains one of the most well-characterized phases of correlated Fermi systems. A recent experiment has reported the first evidence for novel phase behavior on the repulsive side of the Feshbach resonance in a two-component ultracold Fermi gas. By adapting recent theoretical studies to the atomic trap geometry, we show that an adiabatic ferromagnetic transition would take place at a weaker interaction strength than is observed in experiment. This discrepancy motivates a simple nonequilibrium theory that takes account of the dynamics of magnetic defects and three-body losses. The formalism developed displays good quantitative agreement with experiment.

Figure 1

(a) Atoms remaining after the hold time, (b) rms cloud size, (c) release energy, and (d) loss rate with the dimensionless interaction parameter $k_F^{0}a$ shown on the primary $x$ axis in the mean-field case (green or dark gray), with fluctuation corrections (red or dark gray), and with defects (dashed magenta line). The dotted blue line shows the trend in the small $k_F^{0}a$ limit, the vertical cyan line at $k_F^{0}a=\pi /2$ is the Stoner criterion, and the experimental points of Ref. 10 are also highlighted.

Figure 2

The momentum distribution $n_k/N$ of a half net polarized atomic gas in (a) the mean-field case and (b) with fluctuation corrections. The cyan or gray filled curve shows the population of majority spin particles and the blue or dark gray filled curve shows the minority spin particles. The dotted curves show the expected distribution for the majority spin particles in a harmonic trap, $n_k\propto k^2(2\mu -k^2{)}^{3/2}$, and simply due to the density of states, $n_k\propto k^2$. The inset curves show the radial distribution of atoms (orange or gray, primary $y$ axis) and magnetization (blue or dark gray, secondary $y$ axis).