Quantum Degenerate Dipolar Fermi Gas

We report the first quantum degenerate dipolar Fermi gas, the realization of which opens a new frontier for exploring strongly correlated physics and, in particular, quantum liquid crystalline phases. A quantum degenerate Fermi gas of the most magnetic atom ${}^{161}\mathrm{Dy}$ is produced by laser cooling to $10\mu \mathrm{K}$ before sympathetically cooling with ultracold, bosonic ${}^{162}\mathrm{Dy}$. The temperature of the spin-polarized ${}^{161}\mathrm{Dy}$ is a factor $T/T_F=0.2$ below the Fermi temperature $T_F=300\mathrm{nK}$. The cotrapped ${}^{162}\mathrm{Dy}$ concomitantly cools to approximately $T_c$ for Bose-Einstein condensation, thus realizing a novel, nearly quantum degenerate dipolar Bose-Fermi gas mixture. Additionally, we achieve the forced evaporative cooling of spin-polarized ${}^{161}\mathrm{Dy}$ without ${}^{162}\mathrm{Dy}$ to $T/T_F=0.7$. That such a low temperature ratio is achieved may be a first signature of universal dipolar scattering.

Figure 1

(a) Electronic energy level structure for bosonic ${}^{162}\mathrm{Dy}$ (nuclear spin $I=0$) and fermionic ${}^{161}\mathrm{Dy}$ ($I=5/2$), including laser cooling and trapping transitions. (b) Additional ground and excited-state hyperfine structure exists for ${}^{161}\mathrm{Dy}$ ($F=I+J$, where $J=8$ is the total electronic angular momentum and primes denote the excited states). Shown is the 32-MHz-wide transition at 412 nm used for the transverse cooling, Zeeman slower, capture MOT, and imaging beams. (c) Blue-detuned, narrowline (1.8 kHz-wide) MOT cooling transition at 741 nm. (d) Spectra of 421-nm ${}^{162}\mathrm{Dy}$ and ${}^{161}\mathrm{Dy}$ (hyperfine) transitions including relative detunings of each MOT and Zeeman slower (ZS) laser. (e) Transition and detuning spectra for MOT on a 741-nm line. (f) Sketch of dual species 1064-nm crossed optical dipole trap (ODT) along with magnetic field $B$ and gravity $g$ orientation. The orthogonal ODT1 and ODT2 beams and the imaging (421-nm) beam are in the $\widehat{\rho }$ plane.

Figure 2

(a) Single shot time-of-flight absorption image at $t=6\mathrm{ms}$. (b) Average of six images. Density integrations versus $\widehat{\rho }$ (c) and $\widehat{z}$ (d). The green curve (larger amplitude) is a Gaussian fit to the data’s wings (radius $\sigma =20\mu \mathrm{m}$), while the red curve (smaller amplitude) is a fit to a Thomas-Fermi distribution. Data are consistent with a Thomas-Fermi distribution of $T/T_F=0.21(5)$. The Fermi velocity and temperature are $5.6(2)\mathrm{mm}/\mathrm{s}$ and 306(20) nK, respectively, and the gas temperature is 64(16) nK. The degenerate Fermi gas contains $6.0(6)\times {10}^3$ atoms at peak density $4(1)\times {10}^{13}{\mathrm{cm}}^{-3}$.

Figure 3

(a) Three measures [Maxwell-Boltzmann (dark points), Fermi-Dirac (light-gray points), fugacity (light-grey, square points)] of ${}^{161}\mathrm{Dy}$ $\widehat{\rho }$ temperature-to-Fermi temperature $T_F$ as trap population decreases due to forced evaporation of the spin-polarized Bose-Fermi mixture. (b) Single isotope evaporative cooling of spin-polarized fermionic ${}^{161}\mathrm{Dy}$ to below $T_F$. (c) The dipolar Bose-Fermi mixture remains in thermal contact throughout the evaporation sequence as measured by fits of TOF densities to Gaussian or TF distributions. The orange dashed line demarcates the boundary below which the temperature of ${}^{161}\mathrm{Dy}$ is lower than $T_F$. Likewise, the purple dashed line demarcates the temperature below which $T_c$ for ${}^{162}\mathrm{Dy}$ Bose degeneracy would be reached given the trap frequencies and population at 19 s. (d) Trap populations of the spin-polarized Bose-Fermi mixture versus evaporation time.