Sign reversal of the order parameter in (Li1−xFex)OHFe1−yZnySe

Abstract

Iron pnictides are the only known family of unconventional high-temperature superconductors besides cuprates. Until recently, it was widely accepted that superconductivity is driven by spin fluctuations and intimately related to the fermiology, specifically, hole and electron pockets separated by the same wavevector that characterizes the dominant spin fluctuations, and supporting order parameters (OP) of opposite signs 1,2 . This picture was questioned after the discovery of intercalated or monolayer form of FeSe-based systems without hole pockets, which seemingly undermines the basis for spin-fluctuation theory and the idea of a signchanging OP [3][4][5][6][7][8][9][10][11] . Using the recently proposed phase-sensitive quasiparticle interference technique, here we show that in LiOH-intercalated FeSe compound the OP does change sign, albeit within the electronic pockets. This result unifies the pairing mechanism of iron-based superconductors with or without the hole Fermi pockets and supports the conclusion that spin fluctuations play the key role in electron pairing.In iron pnictides, it has been widely perceived that superconductivity is driven by spin fluctuations, which supports the sign reversal between order parameters (OP) on the electron and hole pockets 1,2 . The discovery of superconductivity in intercalated or monolayer FeSe at a critical temperature of the order of 40 K rekindled interest in Fe-based superconductivity and sent many theorists back to the drawing board [3][4][5][6][7][8][9][10][11]

Quasiparticle interference and symmetry of superconducting order parameter in strongly electron-doped iron-based superconductors

“…However, distinguishing between these alternatives was beyond the resolution of the experiment. In any case, the situation is somewhat more complicated than anticipated in [28], since the effect of spin-orbit interaction on pairing needs to be taken into account as well. Moreover, recent observation of Majorana zero modes in (Li 0.84 Fe 0.16 )OHFeSe [29] suggests possible broader implications of the SOC for the Cooper-pairing in electron doped intercalated iron-based superconductors.…”

“…Moreover, recent observation of Majorana zero modes in (Li 0.84 Fe 0.16 )OHFeSe [29] suggests possible broader implications of the SOC for the Cooper-pairing in electron doped intercalated iron-based superconductors. Note that in contrast to [28], no Zn substitution was used in [29]. The amount of Zn, however, is relatively small (less than 2%).…”

s+is superconductivity with incipient bands: Doping dependence and STM signatures

“…This method has been recently successfully applied to confirm the signchanging nature of the order parameter in FeSe 73 , where the superconducting gaps are also extremely anisotropic. More recently, the method has been also applied to (Li 1−x Fe x )OHFe 1−y Zn y Se with only electron Fermi surface pockets 74 . For the 122 doped systems, such an experiment has not been yet performed; however, one would expect that such a test is feasible in these compounds as well, given the availability of the high-quality STM data 75,76 .…”

Extensive impurity-scattering study on the pairing symmetry of monolayer FeSe films onSrTiO3

“…With the observed in-gap bound states on substitutional Zn impurities, the authors concluded a signchanging order parameter and pointed out that the spin fluctuation plays a key role in electron pairing [21]. It is also worth mentioning that recent high-resolution angle-resolved photoemission spectroscopy (ARPES) measurements on monolayer FeSe revealed two overlapped ellipse-like electron pockets and a peculiar nodeless anisotropic superconducting gap in each of the electron pockets [22].…”

“…Theoretical studies have proposed various pairing symmetries for such systems with only electron pockets, such as plain s-wave pairing [9][10][11] An experimental pathway to obtain information on pairing symmetry is to study atomic-scale impurity scattering by using scanning tunneling microscopy/spectroscopy (STM/STS) [18,20,21]. It is well known in theory that only magnetic impurities are pair breakers in the plain s-wave superconductors [18], whereas if the pairing order parameter is sign-changing both magnetic and nonmagnetic impurities can induce in-gap states and suppress superconductivity [19].…”