Evolution of the low-temperature Fermi surface of superconducting FeSe1−xSx across a nematic phase transition

Abstract

The existence of a nematic phase transition in iron-chalcogenide superconductors poses an intriguing question about its impact on superconductivity. To understand the nature of this unique quantum phase transition, it is essential to study how the electronic structure changes across this transition at low temperatures. Here, we investigate the evolution of the Fermi surfaces and electronic interactions across the nematic phase transition of FeSe 1−x S x using Shubnikov-de Haas oscillations in high magnetic fields up to 45 T in the low temperature regime down to 0.4 K. Most of the Fermi surfaces of FeSe 1−x S x monotonically increase in size except for a prominent low frequency oscillation associated with a small, but highly mobile band, which disappears at the nematic phase boundary near x~0.17, indicative of a topological Lifshitz transition. The quasiparticle masses are larger inside the nematic phase, indicative of a strongly correlated state, but they become suppressed outside it. The experimentally observed changes in the Fermi surface topology, together with the varying degree of electronic correlations, will change the balance of electronic interactions in the multi-band system FeSe 1−x S x and promote different k z-dependent superconducting pairing channels inside and outside the nematic phase.

Anomalous high-magnetic field electronic state of the nematic superconductors FeSe1−xSx

“…A magnetic field can induce changes in scattering and/or field-induced Fermi-surface effects in the limit when the cyclotron energy is close to the Zeeman energy. The smallest inner bands of FeSe 1−x S x shift in energy as a function of composition x (and temperature [3]), as shown in Figs.1(k-o). Furthermore, Hall effect in ironbased superconductors can be affected by the spin fluctuations that induce mixing of the electron and hole currents [24].…”

“…Furthermore, it also manifests in x ∼ 0.07 inside the nematic A phase but it disappears for higher x 0.1. As T c and the upper critical field inside the nematic phase for different x remain close to that of FeSe [3,21], the changes in the resistivity slope in high magnetic fields are likely driven by field-induced effects that influence scattering and/or the electronic structure.…”

“…2(a)). It is worth mentioning that inside the nematic B phase the quantum oscillations are dominated by a low-frequency pocket with light-mass that disappears at the nematic end point [3]. Thus, the behaviour of R H is linked to the disappearance of a small 3D hole pocket center at the Z-point in FeSe below T s and its re-emergence in the nematic B phase with x substitution around x ∼ 0.11, as found in ARPES studies [15] and sketched in Fig.1(m).…”

“…Our study reveals anomalous transport inside the nematic phase, driven by the subtle interplay between the changes in the electronic structure of a multi-band system and the unusual scattering processes affected by large magnetic fields and disorder.Magnetic field is a unique tuning parameter that can suppress superconductivity to reveal the normal low-temperature electronic behavior of many unconventional superconductors [1,2]. High-magnetic fields can also induce new phases of matter, probe Fermi surfaces and determine the quasi-particle masses from quantum oscillations in the proximity of quantum critical points [1,3]. In unconventional superconductors, close to antiferromagnetic critical regions, an unusual scaling between a linear resistivity in temperature and magnetic fields was found [4,5].…”

“…To establish the role played by different competing interactions on nematicity and superconductivity, an ideal route is provided by the isoelectronic substitution of selenium by sulphur ions in FeSe 1−x S x [15]. This tuning parameter suppresses nematicity and it leads to changes in the electronic structure, similar to the temperature effects, with the Fermi surface becoming isotropic in the tetragonal phase and the electronic correlations becoming weaker [3,6,15,16]. As nematicity is suppressed, it creates ideal conditions to explore a potential nematic critical point [17] in the absence of magnetism.…”