Unwrapping FeSe

FeSe appears to be an uncomplicated material, with its simple, binary crystal structure, but it shares many of the exotic properties of the highly complex iron-pnictide compounds, including unconventional superconductivity and a nearby electron nematic phase. This gives FeSe an important role in the field of iron-based superconductivity, as it provides an opportunity to study the electronic properties without the additional consideration of structural complexity and its effect on sample quality and behaviour. Recent work carried out at the EMFL labs in Nijmegen (HFML) and Toulouse (LNCMI-T) has taken advantage of high sample quality combined with high magnetic fields to reveal the complete electronic bandstructure of FeSe through detailed measurements of the Fermi surface.

Electron nematic order is a type of long range order that breaks the rotational symmetry of the crystal lattice but leaves the translational symmetry unchanged. It is widely observed in the iron-pnictide superconductors, and is thought to influence the development of superconductivity, but its origin is not yet fully understood. FeSe undergoes a structural transition from tetragonal to orthorhombic at about 90K, and below this temperature shows long range nematic order. By measuring high purity FeSe over a wide range of temperatures, using angle resolved photoemission (ARPES) above 10 K, and high magnetic field quantum oscillations at very low temperature, Matthew Watson, Amalia Coldea and their collaborators have been able to track the evolution of the Fermi surface through the 90 K transition, and fully determine the low temperature nematic Fermi surface.

Their results, which also include elastoresistivity measurements, reveal a complex electronic bandstructure with orbital-dependent renormalisation, and provide strong evidence that the nematic phase is electronically driven, stabilised by orbital-charge ordering, rather than driven by the structural instability. They also find evidence of low temperature magnetic fluctuations, which may additionally influence the superconductivity emerging from the nematic state.

In further work, in collaboration with Professor T. Shibauchi and colleagues from Tokyo and Kyoto, Watson, Coldea et al. performed extended high field magnetotransport measurements on FeSe to study quantum oscillations in more detail. Quantum oscillations effectively “unwrap” the electronic structure of a material, allowing each layer of the Fermi surface to be studied independently. By performing a novel analysis of oscillations in both the longitudinal and Hall resistivity, it was possible not only to determine the multiband electronic structure of FeSe, but to identify the electron or hole character and mobilities of the charge carriers associated with each layer.

FeSe jpg

The Fermi surface of FeSe, as calculated (using density functional theory GGA + SO) and measured, above and below the structural transition temperature Ts. The multiple layers of the real Fermi surface were fully characterised.


Emergence of the nematic electronic state in FeSe
M.D. Watson, T.K. Kim, A.A. Haghighirad, N.R. Davies, A. McCollam, A. Narayanan, S.F. Blake, Y.L. Chen, S. Ghannadzadeh, A.J. Schofield, M. Hoesch, C. Meingast, T. Wolf and A.I Coldea.
Physical Review B 91, 155106 (2015) [editor's choice]

Dichotomy between the Hole and Electron Behavior in Multiband Superconductor FeSe Probed by Ultrahigh Magnetic Fields
M.D. Watson, T. Yamashita, S. Kasahara, W. Knafo, M. Nardone, J. Beard, F. Hardy, A. McCollam, A. Narayanan, S.F. Blake, T. Wolf, A.A. Haghighirad, C. Meingast, A.J. Schofield, H. von Lohneysen, Y. Matsuda, A.I. Coldea and T. Shibauchi.
Physical Review Letters 15, 027006 (2015)