Localization of Bosonic Atoms by Fermionic Impurities in a Three-Dimensional Optical Lattice

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 ${}^{40}\mathrm{K}$ impurity concentration by a comparison to the corresponding superfluid to Mott-insulator transition in a pure bosonic ${}^{87}\mathrm{Rb}$ 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.

Figure 1

Schematic localization scenarios. (a) Pure bosonic superfluid in an optical lattice. (b) Shift of the effective potential depth due to fermionic impurities. (c) Localization by interfering paths of the bosonic wave function scattered by randomly distributed fermionic impurities. (d) Localization due to percolation. A random fermion distribution hampers the establishment of a coherent connection and causes the localization of bosonic ensembles in superfluid “islands.” (e) Mott-insulator transition induced by a uniform distribution of attractive fermionic impurities, resulting in an effectively deeper lattice potential for the bosons.

Figure 2

Time of flight absorption images of the bosonic component 15 ms after switching off the lattice and trap potentials. (The colors and pseudo-3D representation encode the particle column density integrated along the imaging direction.) (a) Pure bosonic ensemble for three different lattice depths. (b) Fermi-Bose mixture for the same lattice depths and 20% fermionic impurity concentration. The reduction of interference contrast visible in the images shows the onset of localization, which is shifted to a lower lattice depth in the presence of fermionic impurities.

Figure 3

Visibility $\mathcal{V}$ (a) and central peak width (b) of the bosonic interference pattern for different fermionic impurity concentrations.

Figure 4

Observed shift of the localization transition as a function of fermionic impurity concentration.