Evidence for Dirac Nodes from Quantum Oscillations in SrFe${}_2$As${}_2$

Abstract

We present a detailed study of quantum oscillations in the antiferromagnetically ordered pnictide compound SrFe${}_2$As${}_2$ as the angle between the applied magnetic field and crystalline axes is varied. Our measurements were performed on high quality single crystals in a superconducting magnet, and in pulsed magnetic fields up to 60 T, allowing us to observe orbits from several small Fermi surface pockets. We extract the cyclotron effective mass $m^{\star }$ and frequency $F$ for these orbits and track their values as the field is rotated away from the c-axis. While a constant ratio of $m^{\star }/F$ is observed for one orbit as expected for a parabolic band, a clear deviation is observed for another. We conclude that this deviation points to an orbit derived from a band with Dirac dispersion near the Fermi level.

5 pages, 4 figures