Quantum degenerate Bose-Fermi mixture of chemically different atomic species with widely tunable interactions

Abstract

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

Heat transfer through dipolar coupling: Sympathetic cooling without contact

“…In recent years, ultracold gases of polar atoms [1][2][3][4][5][6][7] and molecules [8][9][10][11][12] with their long-range anisotropic interactions have attracted a great deal of interest for applications ranging from quantum information science [13][14][15] to condensed matter physics [16][17][18][19][20][21].…”

Observation of interspecies6Li-133Cs Feshbach resonances

“…FRs between different alkaline species have been identified for a variety of Bose-Bose [18][19][20][21], Bose-Fermi mixtures [22][23][24][25][26][27][28] as well as for the Fermi-Fermi mixture of 6 Li-40 K [29]. While the broad intraspecies FRs in 133 Cs [30,31] and 6 Li [32][33][34][35][36] are well-explored, until * weidemueller@uni-heidelberg.de now, only little is known about the interspecies interaction properties of Li and Cs.…”

Quantum-Gas Microscope for Fermionic Atoms

“…To prepare a cold cloud of fermionic atoms under the microscope, 40 K is first sympathetically cooled with 23 Na in a plugged magnetic quadrupole trap [29], centered ∼9 mm below a superpolished substrate that forms the bottom of the solid immersion lens. After removal of 23 Na, the cloud of ∼1 × 10 6 40 K atoms is magnetically transported to the substrate and trapped in a vertical lattice formed by a 1064-nm laser beam reflected off the substrate at an angle of 5.9°.…”