Fermi surface with Dirac fermions in CaFeAsF determined via quantum oscillation measurements

TL;DR

This study reveals the Fermi surface of CaFeAsF, a 1111 iron-arsenide, showing Dirac fermions with nontrivial Berry phase, small carrier density, and unusual magnetotransport properties, advancing understanding of its electronic structure.

Contribution

First complete determination of the Fermi surface in CaFeAsF's antiferromagnetic state, identifying Dirac fermions and topological features in a 1111 iron-arsenide compound.

Findings

Fermi surface includes Dirac electron cylinders and a hole cylinder.

Electron cylinders carry a nontrivial Berry phase of π.

Carrier density is extremely low, around 10^{-3} per Fe.

Abstract

Despite the fact that 1111-type iron arsenides hold the record transition temperature of iron-based superconductors, their electronic structures have not been studied much because of the lack of high-quality single crystals. In this study, we completely determine the Fermi surface in the antiferromagnetic state of CaFeAsF, a 1111 iron-arsenide parent compound, by performing quantum oscillation measurements and band-structure calculations. The determined Fermi surface consists of a symmetry-related pair of Dirac electron cylinders and a normal hole cylinder. From analyses of quantum-oscillation phases, we demonstrate that the electron cylinders carry a nontrivial Berry phase $\pi$. The carrier density is of the order of 10$^{-3}$ per Fe. This unusual metallic state with the extremely small carrier density is a consequence of the previously discussed topological feature of the band structure which prevents the antiferromagnetic gap from being a full gap. We also report a nearly linear-in-$B$ magnetoresistance and an anomalous resistivity increase above about 30 T for $B \parallel c$, the latter of which is likely related to the quantum limit of the electron orbit. Intriguingly, the electrical resistivity exhibits a nonmetallic temperature dependence in the paramagnetic tetragonal phase ($T >$ 118 K), which may suggest an incoherent state. Our study provides a detailed knowledge of the Fermi surface in the antiferromagnetic state of 1111 parent compounds and moreover opens up a new possibility to explore Dirac-fermion physics in those compounds.