Surface electronic structure and evidence of plains-wave superconductivity in(Li0.8Fe0.2

Abstract

Li0.8Fe0.2)OHFeSe is a newly-discovered intercalated iron-selenide superconductor with a Tc above 40 K, which is much higher than the Tc of bulk FeSe (8 K). Here we report a systematic study of (Li0.8Fe0.2)OHFeSe by low temperature scanning tunneling microscopy (STM). We observed two kinds of surface terminations, namely FeSe and (Li0.8Fe0.2)OH surfaces. On the FeSe surface, the superconducting state is fully gapped with double coherence peaks, and a vortex core state with split peaks near EF is observed. Through quasiparticle interference (QPI) measurements, we clearly observed intra-and interpocket scatterings in between the electron pockets at the M point, as well as some evidence of scattering that connects Г and M points. Upon applying magnetic field, the QPI intensity of all the scattering channels are found to behave similarly. Furthermore, we studied impurity effects on the superconductivity by investigating intentionally introduced impurities and intrinsic defects. We observed that magnetic impurities such as Cr adatoms can induce in-gap states and suppress superconductivity. However, nonmagnetic impurities such as Zn adatoms do not induce visible in-gap states. Meanwhile, we show that Zn adatoms can induce in-gap states in thick FeSe films, which is believed to have an s±-wave pairing symmetry. Our experimental results suggest it is likely that (Li0.8Fe0.2)OHFeSe is a plain s-wave superconductor, whose order parameter has the same sign on all Fermi surface sections.

(Li1−xFex)OHFeSe Superconductors: Crystal Growth, Structure, and Electromagnetic Properties

“…The stable structure with large electron injection leads to high-T c superconductivity [113]. In addition, low temperature scanning tunneling microscopy (STM) suggests that the (Li 1−x Fe x )OHFeSe is a plain s-wave superconductor with strong coupling mechanism [114,115].…”

Probing the superconducting gap structure of (Li1−xFex)OHFeSe

“…24,25 However, scanning tunneling spectroscopy (STS) studies show two distinct features in the conductance spectra, suggesting the presence of multiple gaps. 26,27 Meanwhile the in-plane superfluid density obtained from muon-spin rotation (µSR) measurements is consistent with either one or two gaps, but very different behavior is seen in the out-of-plane component, which shows a much more rapid drop of the superfluid density with temperature. 28 Furthermore, inelastic neutron scattering (INS) measurements give evidence for the presence of a spin resonance peak, 29,30 consistent with a sign change of the order parameter across the Fermi surface, while a lack of a sign change was suggested from an STS study, on the basis of quasi-particle interference (QPI) results and the effect of impurities.…”

“…We note that there have been conflicting reports about the nature of the gap structure of (Li 1−x Fe x )OHFeSe, with only a single gap being resolved from ARPES measurements 24,25 , while two gaps are found from STS 26,27 and the in-plane superfluid density from µSR measurements is compatible with both single-gap and two-gap models. 28 The gap values we obtain from fitting the in-plane superfluid density are smaller than those reported from previous measurements, particularly in the case of the smaller gap.…”

“…2(a) that ∆λ(T ) for the #A samples decreases with decreasing temperature before flattening below about 0.1T c , indicating a nodeless gap structure in (Li 1−x Fe x )OHFeSe with a lack of low energy excitations, which is consistent with previous results. [24][25][26][27][28] Furthermore when fitted with a power law dependence ∆λ(T ) ∼ T n , exponents of n = 2.83 and n = 2.46 are obtained for samples #A-1 and #B-1 respectively. In the case of nodal superconductors, n = 1 for line nodes and n = 2 for point nodes is generally anticipated.…”

“…From a recent STM study it was proposed on the basis of QPI measurements, as well as examining the effects of impurities, that there is no sign change of the superconducting gap across the Fermi surface 26 , in which case such a two gap s-wave model readily explains the data. However INS measurements show evidence for a spin resonance peak, 29,30 which indicates that there is a sign change of the order parameter between regions of the Fermi surface connected by the resonance wave vector.…”