The electronic structure of the nematic materials Sr₃Ru₂O₇ and Ca(Co[subscript(x)]Fe[subscript(1-x)])₂As₂

Abstract

We investigated the electronic structure of the two nematic materials Sr₃Ru₂O₇ and Ca(Fe₀.₉₇Co₀.₀₃As)₂ using spectroscopic – imaging scanning tunneling microscopy (SI-STM) and angle resolved photoemission spectroscopy (ARPES). – – – Sr₃Ru₂O₇ is an itinerant metamagnet that shows a putative quantum critical endpoint at 8 Tesla, submersed by the formation of a nematic electronic phase. Using ARPES, we identified at least 5 Fermi pockets in agreement with quantum oscillation measurements. Surprisingly, we found Fermi velocities up to an order of magnitude lower than in single layer Sr₂RuO₄ and up to 35 times lower than predicted by ab initio calculations. Many bands are confined in an energy range of only ∼10 meV below the Fermi level. This, as well as distinct peak-dip-hump shapes of the spectra with a characteristic energy of around ∼5 meV indicate strong correlations and a possible nontrivial mechanism that is absent in single layer Sr₂RuO₄ and connected to the nematicity. The quasiparticle interference of one of the bands was detected by SI-STM, which was also used to measure subatomic features with the symmetries of the relevant Ru d orbitals. – – – In the second mate- rial, the iron-based high-temperature superconductor Ca(Fe[subscript(1-x)]Co[subscript(x)]As)₂, we discovered electronic nematic nano-pattern in its under-doped ‘parent’ state. We spectroscopically imaged this state in real space over large areas and across domain boundaries that change the directionality of the nano-pattern by 90°. We propose that oriented, dimer-shaped electronic nematogens are responsible for this pattern, in striking contrast to what has been expected and observed in electronic nematic materials. The dimers consist of two Gaussian conductance peaks separated by about 8 a[subscript(FeFe)]. Unidirectionality also shows in the quasiparticle interference pattern of the delocalized electrons. The dispersion is in agreement with scattering from the α₂ band discovered by ARPES but has distinct C₂ symmetry, not inconsistent with a C₄-symmetric band scattered by the proposed dimers.