We observe a localized phase of ultracold bosonic quantum gases in a 3-dimensional optical lattice induced by a small contribution of fermionic atoms acting as impurities in a Fermi-Bose quantum gas mixture. In particular, we study the dependence of this transition on the fermionic (40)K impurity concentration by a comparison to the corresponding superfluid to Mott-insulator transition in a pure bosonic (87)Rb gas and find a significant shift in the transition parameter. The observed shift is larger than expected based on a simple mean-field argument, which indicates that disorder-related effects play a significant role.
We report on the creation of ultracold heteronuclear molecules assembled from fermionic 40 K and bosonic 87 Rb atoms in a 3D optical lattice. Molecules are produced at a heteronuclear Feshbach resonance both on the attractive and the repulsive side of the resonance. We precisely determine the binding energy of the heteronuclear molecules from rf spectroscopy across the Feshbach resonance. We characterize the lifetime of the molecular sample as a function of magnetic field and measure between 20 and 120ms. The efficiency of molecule creation via rf association is measured and is found to decrease as expected for more deeply bound molecules.PACS numbers: 03.75. Kk, 03.75.Ss, 32.80.Pj, 34.20.Cf, There has been a long quest for production of ultracold molecules in recent years. In particular, heteronuclear molecules would open up intriguing perspectives both in view of their internal properties and their interactions. The electric dipole moment of heteronuclear molecules in their internal ground states makes them one of the best candidates for tests of fundamental physics like the search for a permanent electric dipole moment of the electron and parity violation [1] as well as for studies on the drifts of fundamental constants. In addition, polar molecules are a key for novel promising quantum computation schemes [2]. Furthermore, their large anisotropic interactions give rise to quantum magnetism [3], new types of superfluid pairing [4] and a variety of quantum phases [5]. Currently, two main routes to the production of ultracold molecules are being pursued. One approach aims at cooling thermal ensembles of molecules, e. g. using buffer gas cooling [6], Stark deceleration [7] or velocity filtering [8]. The other approach starts with ultracold atomic ensembles and assembles them into molecules by means of photoassociation [9] or Feshbach resonances [10]. In the latter case, one major issue has been the stability of these molecules. While molecules created in bosonic quantum gases have a very short collisional lifetime, bosonic molecules from two fermionic atoms are relatively stable due to the Pauli principle [11]. In other cases, as recently demonstrated for bosonic samples [12] and also expected for heteronuclear mixtures, it is favorable to produce the molecules in separated wells of optical lattices to suppress collisional inelastic losses. So far, molecules produced at Feshbach resonances have been limited to homonuclear systems.In this letter we report on the first creation of ultracold heteronuclear molecules in a 3D optical lattice at a Feshbach resonance. This approach produces ultracold molecules in the ground state of individual lattice sites. This method offers several advantages: long lifetimes allow for further manipulation towards the internal molecular ground state. Moreover, the inherent order within the lattice enables studies of new quantum phases of dipoledipole interacting systems. In particular, we perform rf association of fermionic 40 K and bosonic 87 Rb atoms close to a heteronuclear Feshba...
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