Electronic Nematic States Tuned by Isoelectronic Substitution in Bulk FeSe1−xSx

Abstract

Isoelectronic substitution is an ideal tuning parameter to alter electronic states and correlations in iron-based superconductors. As this substitution takes place outside the conducting Fe planes, the electronic behaviour is less affected by the impurity scattering experimentally and relevant key electronic parameters can be accessed. In this short review, I present the experimental progress made in understanding the electronic behaviour of the nematic electronic superconductors, FeSe1−xSx. A direct signature of the nematic electronic state is in-plane anisotropic distortion of the Fermi surface triggered by orbital ordering effects and electronic interactions that result in multi-band shifts detected by ARPES. Upon sulphur substitution, the electronic correlations and the Fermi velocities decrease in the tetragonal phase. Quantum oscillations are observed for the whole series in ultra-high magnetic fields and show a complex spectra due to the presence of many small orbits. Effective masses associated to the largest orbit display non-divergent behaviour at the nematic end point (x ∼ 0.175(5)), as opposed to critical spin-fluctuations in other iron pnictides. Magnetotransport behaviour has a strong deviation from the Fermi liquid behaviour and linear T resistivity is detected at low temperatures inside the nematic phase, where scattering from low energy spin-fluctuations are likely to be present. The superconductivity is not enhanced in FeSe1−xSx and there are no divergent electronic correlations at the nematic end point. These manifestations indicate a strong coupling with the lattice in FeSe1−xSx and a pairing mechanism likely promoted by spin fluctuations.

Enhanced superconducting pairing strength near a nonmagnetic nematic quantum critical point

“…In this context, the iron-chalcogenide superconductor FeSe is an important material [13][14][15]. Below T s ≈ 90 K, FeSe exhibits a unique nematic order, which does not accompany a long-range magnetic order, unlike other ironbased superconductors [16].…”

Relationship Between Nematicity, Antiferromagnetic Fluctuations, and Superconductivity in FeSe1−xSx Revealed by NMR

“…The nematic phase in FeSe can also be suppressed with the isovalent S substitution for Se in FeSe 1−x S x as shown in Figure 1A taken from Ref. [25] based on data from Refs. [26,27], where T s decreases to zero at the critical x value, x c ~0.17.…”

“…As shown in Figure 1A, T c first increases up to 10 K around x = 0.09 making a maximum and then decreases gradually at higher x without showing any clear change in T c around x c [18,26,30]. Nevertheless, the considerable change in the size and anisotropy of the SC gap is observed at the nematic QPT in spectroscopic-imaging scanning tunneling microscopy [26,30,31], thermal conductivity [32], and specific heat [33] measurements, implying different SC states inside (SC1) and outside (SC2) nematic states [25]. In addition, signatures of the crossover between Bardeen-Cooper-Schrieffer and Bose-Einstein-Condensate superconductivities at the nematic QPT were recently reported by laser-excited angle-resolved photoemission spectroscopy (ARPES) measurements [34].…”

Evolution of lattice, spin, and charge properties across the phase diagram of FeSe$_{1-x}$S$_x$

“…and increases by only 10% for x ≤ 0.20. For x > 0.20 v(xy) Fincreases significantly towards FeS[25]. Generally, v(xy) F in FeSe is smaller than v (xy) F…”