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Here, we present a de Haas-van Alphen (dHvA) effect1 study on the newlydiscovered LaFeAsO1-xFx compounds2,3 in order to unveil the topography of theFermi surface associated with their antiferromagnetic and superconductingphases, which is essential for understanding their magnetism, pairing symmetryand superconducting mechanism. Calculations 4 and surface-sensitivemeasurements 5,6,7 provided early guidance, but lead to contradictory results,generating a need for a direct experimental probe of their bulk Fermi surface.In antiferromagnetic LaFeAsO1-xFx 8,9 we observe a complex pattern in theFourier spectrum of the oscillatory component superimposed onto the magnetictorque signal revealing a reconstructed Fermi surface, whose geometry is notfully described by band structure calculations. Surprisingly, several of thesame frequencies, or Fermi surface cross-sectional areas, are also observed insuperconducting LaFeAsO1-xFx (with a superconducting transition temperature Tc~ 15 K). Although one could attribute this to inhomogeneous F doping, thecorresponding effective masses are largely enhanced with respect to those ofthe antiferromagnetic compound. Instead, this implies the microscopiccoexistence of superconductivity and antiferromagnetism on the same Fermisurface in the underdoped region of the phase diagram of the LaFeAsO1-xFxseries. Thus, the dHvA-effect reveals a more complex Fermi surface topographythan that predicted by band structure calculations4 upon which the currentlyproposed superconducting pairing scenarios10,11,12,13 are based, which could beat the origin of their higher Tcs when compared to their phosphide analogs.

Fermi-surface of underdoped LaFeAsO1-xFx as determined by the Haas-van Alphen-effect