Ultracold gases of highly magnetic atoms such as erbium offer an ideal platform for investigating novel aspects of many-body quantum phenomena in the presence of dipole-dipole interactions. In my talk I will present two different examples that we have been recently exploring in our experiment in Innsbruck.
First, I will present our observation of a roton mode on a dipolar BEC, the roton being an elementary excitation of minimal energy at finite momentum, similar to the case of superfluid helium He-II . In contrast to He-II, the roton mode in a dipolar BEC does not require strong interactions, but arises from the long-range and anisotropic nature of the dipolar interactions. First predicted in 2003 , it has long remained elusive to observation. In our experiment we have first observed the population of the roton mode after an interaction quench on an elongated BEC of Er166 via the apparition of side peaks in the momentum distribution of the atomic cloud. We have probed the scaling of the momentum and energy of the roton mode thanks to a detailed study of the peak characteristics and to a comparison with parameter free theory predictions .
Second, I will present our realisation of a tunable spin mixture of fermionic Er167. We have performed high resolution Feshbach spectroscopy of the two lowest spin states and identify a comparatively broad interspin resonance which allows interactions tuning. In the vincinity of the resonance, we have precisely determine the variation of the interspin scattering length with the magnetic field via lattice modulation spectroscopy and studied the collisionnal behavior of the mixture. This system offers for instance an ideal platform to study BEC-BCS crossover physics, or lattice spin physics, in presence of dipolar interactions .
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 Santos, L., Shlyapnikov, G. V. and Lewenstein, M. Roton-maxon spectrum and stability of trapped dipolar Bose-Einstein condensates. Phys. Rev. Lett. 90, 250403 (2003).
 Chomaz, L., Bijnen, R.M.W., Petter, D., Faraoni, G., Baier, S., Becher, J.H., Mark, M.J., Wächtler, F., Santos, L. and Ferlaino, F., 2018. Observation of roton mode population in a dipolar quantum gas. Nature Physics, 14 (5), p.442 (2018).
 Baier, S., D. Petter, J. H. Becher, A. Patscheider, G. Natale, L. Chomaz, M. J. Mark, and F. Ferlaino. Realization of a strongly interacting fermi gas of dipolar atoms. arXiv preprint arXiv:1803.