On the Remarkable Superconductivity of FeSe and Its Close Cousins

Abstract

Emergent electronic phenomena in iron-based superconductors have been at the forefront of condensed matter physics for more than a decade. Much has been learned about the origins and intertwined roles of ordered phases, including nematicity, magnetism, and superconductivity, in this fascinating class of materials. In recent years, focus has been centered on the peculiar and highly unusual properties of FeSe and its close cousins. This family of materials has attracted considerable attention due to the discovery of unexpected superconducting gap structures, a wide range of superconducting critical temperatures, and evidence for nontrivial band topology, including associated spin-helical surface states and vortex-induced Majorana bound states. Here, we review superconductivity in iron chalcogenide superconductors, including bulk FeSe, doped bulk FeSe, FeTe1−xSex, intercalated FeSe materials, and monolayer FeSe and FeTe1−xSex on SrTiO3. We focus on the superconducting properties, including a survey of the relevant experimental studies, and a discussion of the different proposed theoretical pairing scenarios. In the last part of the paper, we review the growing recent evidence for nontrivial topological effects in FeSe-related materials, focusing again on interesting implications for superconductivity.

Spatially dispersing Yu-Shiba-Rusinov states in the unconventional superconductor FeTe0.55Se0.45

“…he putative s ± superconductor FeTe 0.55 Se 0.45 is peculiar because it has a low Fermi energy and an unusually low and inhomogeneous superfluid density [1][2][3][4][5][6][7] . It has been predicted to host a topological superfluid and Majorana zeromode states [8][9][10] .…”

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

“…Furthermore, the isoelectronic substitution can access the experimental manifestations around a putative nematic critical point, undisturbed by the presence of a magnetic critical point, as found in other systems, like BaFe 2 (As 1−x P x ) 2 [11]. The parent compound of this family, FeSe, displays a nematic electronic phase and a tetragonal-to-orthorhombic transition below 90 K [12] and no long range magnetic order was detected despite a rich spectrum of low and high-energy spin fluctuations [13][14][15].…”

“…Another way to assess the proximity to the crossover is to check whether the size of the Cooper pair, given by the coherence length ξ ab , is smaller than the mean inter-particle spacing 1/k F and ξk F ≪ 1 [15,161]. Using the in-plane coherence length for FeSe of ξ 4.6-5.7 nm [42,45] and the values of k F ∼ 0.038-0.15 for the hole bands (Figure 4), it suggests that ξk F ∼ 1.75-8.55 is large and the Cooper pairs are quite extended suggesting that the superconductivity of FeSe need to be understood considering its multi-band effects.…”

“…Using the in-plane coherence length for FeSe of ξ 4.6-5.7 nm [42,45] and the values of k F ∼ 0.038-0.15 for the hole bands (Figure 4), it suggests that ξk F ∼ 1.75-8.55 is large and the Cooper pairs are quite extended suggesting that the superconductivity of FeSe need to be understood considering its multi-band effects. Further aspects of the pairing mechanism of FeSe and other iron-chalcogenides are discussed in detail in recent reviews [15,37].…”

“…The availability of single crystals of these materials have allowed intense interest and study of their physical properties, summarized in recent reviews in Refs. 15,[35][36][37]. Furthermore, by combining physical and chemical pressures, the relative position of the nematic electronic phase in relation to the spin-density wave phase can be varied and thus the influence of two competing electronic phases on superconductivity can be disentangled [38,39].…”