Direct evaporative cooling of ${}^{41}\text{K}$ into a Bose-Einstein condensate

We have investigated the collisional properties of ${}^{41}\text{K}$ atoms at ultracold temperatures. To demonstrate the possibility of using ${}^{41}\text{K}$ as a coolant, a Bose-Einstein condensate of ${}^{41}\text{K}$ atoms in the stretched state $\left(F=2,m_F=2\right)$ was created by direct evaporative cooling in a magnetic trap. An upper bound of the three-body loss coefficient for atoms in the condensate was determined to be $4\left(2\right)\times {10}^{-29}{\text{cm}}^{-6}{\text{s}}^{-1}$. A Feshbach resonance in the $F=1,m_F=-1$, state was observed at 51.35(10) G, which is in good agreement with theoretical prediction.

Figure 1

The energy level diagram of ${}^{41}\text{K}$ atoms. Hyperfine splitting of the $4{{}^{2}P}_{3/2}$ state is comparable to the natural linewidth of the $D_2$ line (6.2 MHz).

Figure 2

The typical evaporative cooling trajectory for ${}^{41}\text{K}$ (squares) and ${}^{87}\text{R}\text{b}$ (circles) atoms. The efficiency of evaporative cooling $\gamma \equiv -d\left(\text{ln}D\right)/d\left(\text{ln}N\right)$ was 2.7 for ${}^{41}\text{K}$ and 3.1 for ${}^{87}\text{R}\text{b}$. Here, $D$ is the phase-space density of the gas.

Figure 3

A Bose-Einstein condensate of ${}^{41}\text{K}$ atoms obtained by direct evaporation in a magnetic trap. Absorption images of ${}^{41}\text{K}$ atoms after 40 ms expansion showed (a) a thermal distribution at $T=750\text{nK}$, (b) a bimodal distribution at $T=580\text{nK}$, and (c) an almost pure condensate of $3\times {10}^5$ atoms. The evaporative cooling was performed by driving an rf transition between hyperfine levels of ${}^{41}\text{K}$ atoms. No coolant was used in the experiment.

Figure 4

A Feshbach resonance of ${}^{41}\text{K}$ in the $|F,m_F⟩=|1,-1⟩$ state. The number of trapped atoms after 500 ms hold time (circles) showed a pronounced dip at a Feshbach resonance because of an enhanced three-body loss. The typical number density at the center of the cloud was about $1\times {10}^{13}{\text{cm}}^{-3}$. In a separate experiment, the loading rate of a crossed-beam dipole trap from a single-beam dipole trap was measured as a function of the bias magnetic field (squares). In order to determine the position and width of the resonance, the loss data were fitted with a Lorentzian curve centered at the resonance (solid line), and the loading rate data were fitted assuming that the loading rate is proportional to the elastic cross section [21]. The extracted center and the width of the resonance were 51.35(10) G and $-0.3\left(1\right)\text{G}$, respectively, which are in good agreement with theoretical predictions of Ref. [6] (51.4 and $-0.3\text{G}$, respectively).