We have observed Fermi polarons, dressed spin down impurities in a spin up Fermi sea of ultracold atoms. The polaron manifests itself as a narrow peak in the impurities' rf spectrum that emerges from a broad incoherent background. We determine the polaron energy and the quasiparticle residue for various interaction strengths around a Feshbach resonance. At a critical interaction, we observe the transition from polaronic to molecular binding. Here, the imbalanced Fermi liquid undergoes a phase transition into a Bose liquid coexisting with a Fermi sea.

We report on the formation of ultracold fermionic Feshbach molecules of 23 Na 40 K, the first fermionic molecule that is chemically stable in its ground state. The lifetime of the nearly degenerate molecular gas exceeds 100 ms in the vicinity of the Feshbach resonance. The measured dependence of the molecular binding energy on the magnetic field demonstrates the open-channel character of the molecules over a wide field range and implies significant singlet admixture. This will enable efficient transfer into the singlet vibrational ground state, resulting in a stable molecular Fermi gas with strong dipolar interactions.

Ultracold quantum gases realize paradigms of condensed matter physics in pristine fashion, such as the superfluid to Mott insulator transition [1], the BEC-BCS crossover in fermionic superfluids [2,3] and the Berezinskii-Kosterlitz-Thouless transition in twodimensional Bose gases [4]. A plethora of novel manybody systems may become accessible through the advent of quantum mixtures of different atomic species. In particular, Bose-Fermi mixtures with widely tunable interactions should reveal boson mediated interactions between fermions and possibly boson induced p-wave superfluidity [5,6]. The fate of impurities in a Fermi sea [7] or a Bose condensate [8][9][10] can be studied, and new quantum phases of matter are predicted in optical lattices [11]. Furthermore, the creation of fermionic ground state molecules starting from a degenerate Bose-Fermi mixture opens up a whole new avenue of research, as this results in a Fermi gas with long-range, anisotropic dipole-dipole interactions [12]. Since the first degenerate Bose-Fermi mixture of different atomic species, 23 Na and 6 Li [13], a variety of such systems has been realized [8,[14][15][16][17][18][19][20][21][22][23]. However, so far only one mixture, 87 Rb-40 K, has allowed tunability of interspecies interactions with relative ease by means of a moderately wide (∆B ≈ 3 G) Feshbach resonance [24], and only in this case fermionic Feshbach molecules have successfully been produced [25,26].In this article, we report on the experimental realization of a new Bose-Fermi mixture of 23 Na and 40 K and the observation of over 30 s-and p-wave Feshbach resonances at low magnetic fields. We demonstrate that 23 Na is an efficient coolant for sympathetic cooling of 40 K. A pattern of wide s-wave resonances exists for most of the energetically stable hyperfine combinations, the widest being located at 138 G with a width of about 30 G in the 23 Na|F = 1, m F = 1 + 40 K|F = 9/2, m F = −5/2 hyperfine configuration. We also observe p-wave multiplet resonances that are resolved thanks to their location at low magnetic fields.In the singlet rovibrational ground state, the NaK molecule is known to have a large permanent electric dipole moment of 2.72(6) D [27,28], five times larger than that of KRb [12], and is predicted to be chemically stable against atom-atom exchange reactions [29], in contrast to KRb [30]. An ultracold gas of fermionic ground state molecules of NaK will thus be an ideal system for the study of Fermi gases with strong, long-range dipolar interactions. Indeed, the interaction energy here can be