Spin-exchanging interactions govern the properties of strongly correlated electron systems such as many magnetic materials. When orbital degrees of freedom are present, spin exchange between different orbitals often dominates, leading to the Kondo effect, heavy fermion behaviour or magnetic ordering. Ultracold ytterbium or alkaline-earth ensembles have attracted much recent interest as model systems for these effects, with two (meta-) stable electronic configurations representing independent orbitals. We report the observation of spin-exchanging contact interactions in a two-orbital SU(N )-symmetric quantum gas realized with fermionic 173 Yb. We find strong inter-orbital spinexchange by spectroscopic characterization of all interaction channels and demonstrate SU(N = 6) symmetry within our measurement precision. The spin-exchange process is also directly observed through the dynamic equilibration of spin imbalances between ensembles in separate orbitals. The realization of an SU(N )-symmetric two-orbital Hubbard Hamiltonian opens the route to quantum simulations with extended symmetries and with orbital magnetic interactions, such as the Kondo lattice model. arXiv:1403.4761v3 [cond-mat.quant-gas] 21 May 2015

We employ radio-frequency spectroscopy to investigate a polarized spin-mixture of ultracold 6 Li atoms close to a broad Feshbach scattering resonance. Focusing on the regime of strong repulsive interactions, we observe well-defined coherent quasiparticles even for unitarity-limited interactions. We characterize the many-body system by extracting the key properties of repulsive Fermi polarons: the energy E+, the effective mass m * , the residue Z and the decay rate Γ. Above a critical interaction, E+ is found to exceed the Fermi energy of the bath while m * diverges and even turns negative, thereby indicating that the repulsive Fermi liquid state becomes energetically and thermodynamically unstable.Landau's idea of mapping the behavior of impurity particles interacting with a complex environment into quasiparticle properties [1] plays a fundamental role in physics and materials science, from helium liquids [2] and colossal magnetoresistive materials [3,4] to polymers and proteins [5,6]. In the field of ultracold gases, the impurity problem and the associated concept of polaron quasiparticle have attracted over the last decade a growing interest [7][8][9][10]. Initiated with the investigation of polarized Fermi gases in the BEC-BCS crossover [11][12][13][14][15][16], the study of polaron physics has been extended to mass-imbalanced [17,18], low-dimensional fermionic systems [19], and also to bosonic environments [20][21][22]. The polaron properties are fundamentally relevant for understanding the more complex scenario of partially-polarized and balanced Fermi mixtures: the impurity limit exhibits some of the critical points of the full phase diagram, whose topology we can thus learn about by investigating polarized systems [8,16].While researchers initially focused on attractive interactions [14,15], more recently they have explored novel quasiparticles associated with repulsive interactions: these repulsive polarons [23][24][25][26][27] are centrally important for realizing repulsive many-body states [23,24,28,29] and therein exploring itinerant ferromagnetism [30][31][32]. In particular, if the polaron energy exceeds the Fermi energy of the surrounding medium, a fullyferromagnetic phase is favored against the paramagnetic Fermi liquid [23][24][25]27]. However, short-ranged strong repulsion always require an underlying weakly-bound molecular state, into which the system may rapidly decay [31,33], making the repulsive polaron an excited manybody state, whose theoretical and experimental investigation are challenging. In three dimensions, repulsive Fermi polarons have been first unveiled in a 6 Li -40 K mixture at a comparatively narrow Feshbach resonance [17], but they lack observation in the universal, broad * scazza@lens.unifi.it resonance case, for which the decay rate is expected to be the largest [10].In this Letter we report on reverse radio-frequency (RF) spectroscopy [17,34,35] experiments to unveil the existence and characterize the properties of repulsive polarons in a polarized Fermi mixture of lithium ...

We report on the experimental observation of a novel interorbital Feshbach resonance in ultracold (173)Yb atoms. This opens up the possibility of tuning the interactions between the (1)S(0) and (3)P(0) metastable state, both possessing zero total electronic angular momentum. The resonance is observed at experimentally accessible magnetic field strengths and occurs universally for all hyperfine state combinations. We characterize the resonance in the bulk via interorbital cross thermalization as well as in a three-dimensional lattice using high-resolution clock-line spectroscopy. Our measurements are well described by a generalized two-channel model of the orbital-exchange interactions.