Quantum oscillations in the parent pnictide BaFe2As2: Itinerant electrons in the reconstructed state

The low-energy quasiparticle dynamics are an essential ingredient to many theories of superconductivity. The Fepnictide superconductors however, show evidence for electron itineracy 1–4 and local magnetism, 5–8 making it difficult to know which theoretical framework is most appropriate for understanding these compounds. 9 Establishing the nature of the itinerancy and magnetism in the parent compounds is therefore of fundamental importance. In the present paper we report quantum oscillation QO measurements in BaFe2As2, consistent with density-functional calculations of the antiferromagnetic ground state. We find that the spin-density wave instability does not fully gap the Fermi surface just as recently predicted 10 and quasiparticle coherence persists in the magnetically ordered ground state. In the measurements reported here on BaFe 2 As 2 we use two separate techniques, torque magnetometry and a radiofrequency contactless conductivity technique using a tunnel diode oscillator TDO, both of which have been used recently to observe oscillations in the closely related compounds LaFePO Ref. 1 and SrFe2As2. 2 We observe three small pockets comprising 1.7%, 0.7%, and 0.3%, of the paramagnetic Brillouin zone that associated with the tetragonal state and produce band-structure calculations of a reconstructed state which are in broad agreement. Furthermore we map the topology of these small pockets and extract their effective mass. The present measurements illustrate that itinerant electrons play a fundamental role in the ordered state of ternary Fe pnictides and place important limits on the topology and size of the Fermi surface in the antiferromagnetic state.