We report on the observation of Feshbach resonances in an ultracold mixture of two fermionic species, 6 Li and 40 K. The experimental data are interpreted using a simple asymptotic bound state model and full coupled channels calculations. This unambiguously assigns the observed resonances in terms of various s-and p-wave molecular states and fully characterizes the ground-state scattering properties in any combination of spin states.PACS numbers: 34.50. 05.30.Fk Fermion pairing and Fermi superfluidity are key phenomena in superconductors, liquid 3 He, and other fermionic many-body systems. Our understanding of the underlying mechanisms is far from being complete, in particular for technologically relevant high-T c superconductors. The emerging field of ultracold atomic Fermi gases has opened up unprecedented possibilities to realize versatile and well-defined model systems. The control of interactions, offered in a unique way by Feshbach resonances in ultracold gases, is a particularly important feature. Such resonances have been used to achieve the formation of bosonic molecules in Fermi gases and to control pairing in many-body regimes [1,2,3,4,5].So far all experiments on strongly interacting Fermi systems have been based on two-component spin mixtures of the same fermionic species, either 6 Li or 40 K [1, 2]. Control of pairing is achieved via a magnetically tunable s-wave interaction between the two states. After a series of experiments on balanced spin mixtures with equal populations of the two states, recent experiments on 6 Li have introduced spin imbalance as a new degree of freedom and begun to explore novel superfluid phases [6,7]. Mixing two different fermionic species leads to unprecedented versatility and control. Unequal masses and the different responses to external fields lead to a large parameter space for experiments and promise a great variety of new phenomena [8,9,10,11,12]. The combination of the two fermionic alkali species, 6 Li and 40 K, is a prime candidate to realize strongly interacting FermiFermi systems.In this Letter, we realize a mixture of 6 Li and 40 K and identify heteronuclear Feshbach resonances [14,15,16]. This allows us to characterize the basic interaction properties. Figure 1 shows the atomic ground-state energy structure. We label the energy levels Li|i and K|j , counting the states with rising energy. The hyperfine splitting of 6 Li is (3/2)a Li hf /h = 228.2 MHz. For 40 K, the hyperfine structure is inverted and the splitting amounts to (9/2)a K hf /h = −1285.8 MHz [17]. For the low-lying states with i ≤ 3 and j ≤ 10, the projection quantum numbers are given by m Li = −i + 3/2 and m K = j − 11/2. A Li|i K|j mixture can undergo rapid decay via spin relaxation if exoergic two-body processes exist that preserve the total projection quantum number M F = m Li + m K = −i + j − 4. Whenever one of the species is in the absolute ground state and the other one is in a low-lying state (i = 1 and j ≤ 10 or j = 1 and i ≤ 3), spin relaxation is strongly suppressed [18].

We present the essential experimental steps of our all-optical approach to prepare a double-degenerate Fermi-Fermi mixture of 6 Li and 40 K atoms, which then serves as a starting point for molecule formation. We first describe the optimized trap loading procedures, the internal-state preparation of the sample, and the combined evaporative and sympathetic cooling process. We then discuss the preparation of the sample near an interspecies Feshbach resonance, and we demonstrate the formation of heteronuclear molecules by a magnetic field ramp across the resonance.