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

TL;DR

This study uses the de Haas-van Alphen effect to directly probe the Fermi surface of underdoped LaFeAsO1-xFx, revealing complex topology and coexistence of superconductivity and antiferromagnetism, challenging existing theoretical models.

Contribution

First direct experimental measurement of the bulk Fermi surface in LaFeAsO1-xFx, showing reconstructed topology and coexistence of phases, which refines understanding of pairing mechanisms.

Findings

Reconstructed Fermi surface not fully described by calculations

Coexistence of superconductivity and antiferromagnetism on same Fermi surface

Enhanced effective masses in superconducting phase

Abstract

Here, we present a de Haas-van Alphen (dHvA) effect1 study on the newly discovered LaFeAsO1-xFx compounds2,3 in order to unveil the topography of the Fermi surface associated with their antiferromagnetic and superconducting phases, which is essential for understanding their magnetism, pairing symmetry and superconducting mechanism. Calculations 4 and surface-sensitive measurements 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 the Fourier spectrum of the oscillatory component superimposed onto the magnetic torque signal revealing a reconstructed Fermi surface, whose geometry is not fully described by band structure calculations. Surprisingly, several of the same frequencies, or Fermi surface cross-sectional areas, are also observed in superconducting LaFeAsO1-xFx (with a superconducting transition temperature Tc ~ 15 K). Although one could attribute this to inhomogeneous F doping, the corresponding effective masses are largely enhanced with respect to those of the antiferromagnetic compound. Instead, this implies the microscopic coexistence of superconductivity and antiferromagnetism on the same Fermi surface in the underdoped region of the phase diagram of the LaFeAsO1-xFx series. Thus, the dHvA-effect reveals a more complex Fermi surface topography than that predicted by band structure calculations4 upon which the currently proposed superconducting pairing scenarios10,11,12,13 are based, which could be at the origin of their higher Tcs when compared to their phosphide analogs.