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 report on the observation of interspecies Feshbach resonances in an ultracold, optically trapped mixture of Rb and Cs atoms. In a magnetic field range up to 300 G we find 23 interspecies Feshbach resonances in the lowest spin channel and 2 resonances in a higher channel of the mixture. The extraordinarily rich Feshbach spectrum suggests the importance of different partial waves in both the open and closed channels of the scattering problem along with higher-order coupling mechanisms. Our results provide, on one hand, fundamental experimental input to characterize the Rb-Cs scattering properties and, on the other hand, identify possible starting points for the association of ultracold heteronuclear RbCs molecules.

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.

A new method is presented for a single pulse laser patterning of metallic thin films. This process is used to form submicron, variable width metallic wires. By employing laser desorption of a physisorbed buffer gas, a grating of gold has been patterned on a Ru(100) substrate under UHV conditions. Upon annealing, the physisorbed layer desorbs and the patterned metallic clusters softly land and strongly attach to the substrate. This versatile technique can be employed with practically any metallic element or molecular species.

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The interaction of potassium atoms on top of Cr 2 O 3 ͑0001͒/Cr͑110͒ has been studied using work-function ͑⌬⌽͒, temperature programmed desorption ͑TPD͒, and optical second-harmonic generation ͑SHG͒ measurements. Potassium grows via the completion of a first layer, followed by a second layer in the form of two-dimensional ͑2D͒ islands, and at higher coverage 3D clusters are formed. This growth model is supported by and consistent with the results obtained from all three methods. Work-function data suggest that annealing at temperatures above 350 K results in the formation of a surface potassium oxide compound, provided the potassium coverage is higher than 0.5 monolayers ͑ML͒. Diffusion of alkali-metal atoms on an oxide surface is reported here over distances of several micrometers. This was measured using optical SH diffraction from coverage gratings that were generated by laser-induced thermal desorption. The activation energy for surface diffusion of potassium on Cr 2 O 3 ͑0001͒/Cr͑110͒ has been determined to be 11Ϯ0.5 kcal/mol with a preexponential factor D 0 ϭ10 5 cm 2 /sec in the coverage range of 1.5-2.5 ML, dropping to 9 kcal/mol and D 0 ϭ3 ϫ10 3 cm 2 /sec at a coverage of 3.0 ML. These results are consistent with the diffusion of atoms in the third layer, on top of two-dimensional potassium islands in the second layer, the activation energy represent the barrier for descending from the 2D islands.

Patterning of metallic clusters on surfaces is demonstrated by utilizing a buffer layer assisted laser patterning technique (BLALP). This method has been employed in order to measure the diffusion of AFM and STM characterized size selected gold nanoclusters (5-10 nm diameter), over Ru(100) and p(1 x 2)-O/Ru(100) surfaces. Optical linear diffraction from gold cluster coverage gratings was utilized for the macroscopic diffusion measurements. The clusters were found to diffuse on the surface intact without significant coalescence or sintering. The barrier for metastable gold nanocluster diffusion on the surface is thought to be lower than the energy required for surface wetting. The apparent activation energy for diffusion was found to depend on the cluster size, increasing from 6.2 +/- 0.4 kcal/mol for 5 nm clusters to 10.6 +/- 0.5 kcal/mol for 9 nm clusters. The macroscopic diffusion of gold nanoclusters has been studied on the p(1 x 2)-O/Ru(100) surface as well, where surface diffusion was found to be rather insensitive to the clusters size with activation energy of 5.5 +/- 1 kcal/mol. The difference between the two surfaces is discussed in terms of a better commensurability (higher level of friction) of the gold facets at the contact area with the clean Ru(100) than in the case of the oxidized surface.

Sublimative desorption is a process in which desorption of multilayers of an adsorbate precedes melting and surface diffusion. Here we report on the desorption kinetics of Xe atoms from multilayer coverage studied using temperature programmed desorption and optical diffraction methods. It is found that decay of the diffraction peak intensities from multilayer coverage grating during surface heating cannot be explained as one-dimensional diffusion process. Instead, the diffraction signal follows Xe desorption, as deduced from simultaneous linear diffraction and desorption measurements. This observation suggests that no macroscopic two-dimensional melting and diffusion occur in the case of multilayers of Xe before the onset for desorption. It is concluded that Xe atoms undergo sublimative desorption from the topmost layers. Similar results were obtained in the case of water multilayers on Ru͑100͒. These results suggest that on solid surfaces the desorption of multilayers is thermodynamically favorable over surface melting or diffusion.