We discover that hcp phases of Fe and Fe 0.9 Ni 0.1 undergo an electronic topological transition at pressures of about 40 GPa. This topological change of the Fermi surface manifests itself through anomalous behavior of the Debye sound velocity, c/a lattice parameter ratio and Mössbauer center shift observed in our experiments. First-principles simulations within the dynamic mean field approach demonstrate that the transition is induced by many-electron effects. It is absent in one-electron calculations and represents a clear signature of correlation effects in hcp Fe. Iron is the most abundant element on our planet. It is one of the most important technological materials and, at the same time, one of the most challenging elements for the modern theory. As a consequence, the study of iron and iron-based alloys has been a focus of experimental and computational research over the past decades. Recently, investigations of phase relations and physical properties of iron and its alloys at high pressure led to new exciting discoveries including evidence for a body-centred-cubic (bcc) phase of iron-nickel alloy at conditions of the Earth's core [1]and the observation of superconductivity in the high-pressure hexagonal close packed (hcp) phase of iron in the pressure range 15-30 GPa and at temperatures below 2 K [2]. While the structural properties of iron and iron-nickel alloys at pressures below 100 GPa are well established [3], their electronic and magnetic properties are still debated. The α-phases (bcc) of Fe and Fe 0.9 Ni 0.1 are ferromagnetic at ambient conditions, but an 49 accurate description of the electronic structure of α-Fe and its high-temperature magnetism require a proper treatment of the many-electron effects [4,5]. The γ-phases 51 (face-centered cubic, fcc) are believed to have complex incommensurate magnetic ground 52 states [6], which are still not reproduced by theory [7]. The importance of correlation 53 effects for the description of the αto γ-phase transition in Fe at elevated temperature and ambient pressure has been recently underlined [8]. The ε-phases (hcp) of Fe and Fe 0.9 Ni 0.1 55 were previously believed to be nonmagnetic [9]; however recent theoretical work showed
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