Ultracold atomic mixtures offer a multitude of opportunities for
addressing few-body physics. In recent experiments we have investigated
mixtures of K and Rb as well as spin mixtures of K. These mixtures are
produced by exploiting a total of three Feshbach resonances for
simultaneous tuning of the 39K intraspecies and the 39K-87Rb
interspecies scattering lengths [1]. Our investigation of Efimov physics
was triggered by an unexpected non-universal behavior of E_mov
resonances across isotope combinations in K+Rb . I will present our
measurements of the scattering length dependent threebody recombination
coefficient in heteronuclear mixtures of 39K-87Rb and 41K-87Rb, which do
not show any signatures of Efimov resonances [2]. In more recent
experiments we have therefore investigated homonuclear Efimov resonances
in 39K with the aim of observing multiple Efimov resonances on adjacent
Feshbach resonances. This has led to the observation of an unexpected
temperature dependence of the Efimov resonance strength, which remains
to be understood. I will present and discuss recent experimental data.
In the second part of the talk I will discuss the use of two spin
components of 39K to investigate the Bose polaron [3]. Radio frequency
spectroscopy between the two components shows the existence of a
well-defined quasiparticle state for an impurity interacting with the
BEC and allows for a measurement of its energy for attractive and
repulsive interactions. Our results show that the spectral response
consists of a well-defined quasiparticle peak at weak coupling which is
strongly broadened for increasing interaction strengths. This opens up
intriguing prospects for studying mobile impurities in a bosonic
environment, as well as strongly interacting Bose systems in genera
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