Quenched nematic criticality and two superconducting domes in an iron-based superconductor

Abstract

The nematic electronic state and its associated nematic critical fluctuations have emerged as potential candidates for superconducting pairing in various unconventional superconductors. However, in most materials their coexistence with other magnetically-ordered phases poses significant challenges in establishing their importance. Here, by combining chemical and hydrostatic physical pressure in FeSe0.89S0.11, we provide a unique access to a clean nematic quantum phase transition in the absence of a long-range magnetic order. We find that in the proximity of the nematic phase transition, there is an unusual non-Fermi liquid behavior in resistivity at high temperatures that evolves into a Fermi liquid behaviour at the lowest temperatures. From quantum oscillations in high magnetic fields, we trace the evolution of the Fermi surface and electronic correlations as a function of applied pressure. We detect experimentally a Lifshitz transition that separates two distinct superconducting regions: one emerging from the nematic electronic phase with a small Fermi surface and strong electronic correlations and the other one with a large Fermi surface and weak correlations that promotes nesting and stabilization of a magnetically-ordered phase at high pressures. The lack of mass divergence suggests that the nematic critical fluctuations are quenched by the strong coupling to the lattice. This establishes that superconductivity is not enhanced at the nematic quantum phase transition in the absence of magnetic order.

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

“…This agrees with the variation of the A 1/2 coefficient (see Fig. S11) and previous studies under pressure [20], suggesting the critical nematic fluctuations could be quenched by the coupling to the lattice along certain directions in FeSe 1−x S x .…”

“…Outside the nematic phase a T 1.5 dependence of resistivity describes the data well over a large temperature range up to 120 K (see Fig. 2(a) and Fig.3(a)), in agreement with previous studies of FeSe 1−x S x under pressure [20]. Using the high-magnetic field data below T c , we extract the low-temperature resistivity in the absence of superconductivity, ρ H→0 (T).…”

“…In this paper we study the normal electronic state across the nematic transition in FeSe 1−x S x using magnetotransport studies in high-magnetic fields up to 69 T. We find that the nematic state has a non-Fermi-liquid behaviour with an unusual transverse magnetoresistance (∼ H 1.55 ), reflecting an unconventional scattering mechanism. Just outside the nematic phase, resistivity is dominated by a ∼ T 1.5 dependence, similar to studies under pressure [20]. The transverse magnetoresistance is significant inside the nematic phase and it shows an unusual change in slope at low temperatures.…”

“…Linear resistivity found at low temperatures inside the nematic state is present in the region where spin-fluctuations are likely to be present. Furthermore, the absence of superconductivity enhancement at the nematic end point in FeSe 1−x S x is supported by the lack of divergent critical fluctuations, found both with chemical pressure [3] and applied pressure [20]. It is expected that the coupling to the relevant lattice strain restricts criticality in nematic systems only to certain high symmetry directions [29,41].…”

Resistivity near a nematic quantum critical point: Impact of acoustic phonons

“…This regime in which α eff (T ) is strongly temperature dependent is particularly sizable for systems with strong nemato-elastic coupling and small Fermi energy, as it is presumably the case of the iron-based superconductors. We also contrast our theoretical results with recent experiments performed in FeSe 1−x S x [21,49], and discuss the limitations of our approach. This paper is structured as follows.…”