The deformation of a Fermi surface is a fundamental phenomenon leading to a plethora of exotic quantum phases. Understanding these phases, which play crucial roles in a wealth of systems, is a major challenge in atomic and condensed-matter physics. Here, we report on the observation of a Fermi surface deformation in a degenerate dipolar Fermi gas of erbium atoms. The deformation is caused by the interplay between strong magnetic dipole-dipole interaction and the Pauli exclusion principle. We demonstrate the many-body nature of the effect and its tunability with the Fermi energy. Our observation provides basis for future studies on anisotropic many-body phenomena in normal and superfluid phases.PACS numbers: 03.75. Ss, 37.10.De, 51.60.+a, 67.85.Lm The Fermi-liquid theory, formulated by Landau in the late 50's, is one of the most powerful tools in modern condensedmatter physics [1]. It captures the behavior of interacting Fermi systems in the normal phase, such as electrons in metals and liquid 3 He [2]. Within this theory the interaction is accounted by dressing the fermions as quasi-particles with an effective mass and an effective interaction. The ground state is the so-called Fermi sea, in which the quasi-particles fill one-by-one all the states up to the Fermi momentum, k F . The Fermi surface (FS), which separates occupied from empty states in k-space, is a sphere of radius k F for isotropically interacting fermions in uniform space. The FS is crucial for understanding system excitations and Cooper pairing in superconductors. When complex interactions act, the FS can get modified. For instance, strongly-correlated electron systems violates the Fermi-liquid picture, giving rise to a deformed FS, which spontaneously breaks the rotational invariance of the system [3]. Symmetry-breaking FSs have been studied in connection with electronic liquid crystal phases [4] and Pomeranchuk instability [5] in solid state systems. Particularly relevant is the nematic phase, in which anisotropic behaviors spontaneously emerge and the system acquires an orientational order, while preserving its translational invariance. This exotic phase has recently been observed by transport and thermodynamics studies in ruthenates [6], in high-transitiontemperature superconductors such as cuprates [7], and in other systems [3].A completely distinct approach to study FSs is provided by ultracold quantum gases. These systems are naturally free from impurities and crystal structures, realizing a situation close to the ideal uniform case. Here, the shape of the FS can directly reveal the fundamental interactions among particles. Studies of FSs in strongly interacting Fermi gases have been crucial in understanding the BEC-to-BCS crossover, where the isotropic s-wave (contact) interaction causes a broadening of the always-spherical FS [8]. Recently, Fermi gases with anisotropic interactions have attracted remarkable attention in the context of p-wave superfluidity [9, 10] and dipolar physics [11]. Many theoretical studies have focused on dipol...

We report on the realization of a mixture of fermionic 161 Dy and fermionic 40 K where both species are deep in the quantum-degenerate regime. Both components are spin-polarized in their absolute ground states, and the low temperatures are achieved by means of evaporative cooling of the dipolar dysprosium atoms together with sympathetic cooling of the potassium atoms. We describe the trapping and cooling methods, in particular the final evaporation stage, which leads to Fermi degeneracy of both species. Analyzing cross-species thermalization we obtain an estimate of the magnitude of the inter-species s-wave scattering length at low magnetic field. We demonstrate magnetic levitation of the mixture as a tool to ensure spatial overlap of the two components. The properties of the Dy-K mixture make it a very promising candidate to explore the physics of strongly interacting mass-imbalanced Fermi-Fermi mixtures. arXiv:1810.13437v1 [cond-mat.quant-gas]