In the field of iron-based superconductors, one of the frontier studies is about the pairing mechanism. The recently discovered (Li1−xFex)OHFeSe superconductor with the transition temperature of about 40 K provides a good platform to check the origin of double superconducting gaps and high transition temperature in the monolayer FeSe thin film. Here we report a scanning tunnelling spectroscopy study on the (Li1−xFex)OHFeSe single crystals. The tunnelling spectrum mimics that of the monolayer FeSe thin film and shows double gaps at about 14.3 and 8.6 meV. Further analysis based on the quasiparticle interference allows us to rule out the d-wave gap, and for the first time assign the larger (smaller) gap to the outer (inner) Fermi pockets (after folding) associating with the dxy (dxz/dyz) orbitals, respectively. The gap ratio amounts to 8.7, which demonstrates the strong coupling mechanism in the present superconducting system.

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]

During the last two decades, organic and polymer lightemitting devices (OLEDs/PLEDs) have been the subject of intense academic and industrial research because of their potential applications in full-color flat-panel displays and solid-state lighting. [1][2][3] Recently, phosphorescent PLEDs, in which heavy metal complexes (such as Ir and Pt complexes) are doped into appropriate polymer host materials, have attracted increasing interest owing to the possible full utilization of both singlet and triplet excitons combined with solution processability.[4] Many studies [5] have shown that in order to realize high device efficiency, in phosphorescent PLEDs, the triplet energy state (E T ) of the polymer host should be located at higher energy than that of the guest complexes. Otherwise, triplet excitons on the guest could undergo back-transfer to the triplet state of the host, and as a result the host polymer would become a luminescence quencher of triplet emitters.[6] Since most conjugated polymers have a low-lying triplet state, so far only red-phosphorescent PLEDs with a conjugated polymer as the host have shown high device efficiency. [7] For green-and blue-light-emitting triplet emitters [8] with typically a higher triplet state than that of conjugated polymers, [9] the devices with the conjugated polymer host are poor. Therefore for green-and blue-light-emitting triplet devices, typically nonconjugated poly(vinylcarbazole) (PVK) has been used as the polymer host, since PVK has a relatively high triplet energy state (2.5 eV).[5c] Since PVK is a nonconjugated polymer with high resistivity, typically, the operating voltage of these devices is relatively high. Therefore in order to obtain phosphorescent PLEDs with high efficiency and low power consumption, it is crucially important to develop a wide-bandgap conjugated polymer host with high-lying triplet energy state.The most widely used polymer hosts for fluorescent guest dyes are poly(2,7-fluorene)s and their derivatives, owing to their wide bandgap, high photoluminescence (PL) quantum efficiency, and excellent conductivity.[10] However, many authors have reported that poly(2,7-fluorene)s are not a good host for blue-and green-light-emitting triplet complexes because of their low-lying triplet energy state (2.15-2.3 eV).[5b,5d,6a] Surprisingly, despite poly(2,7-fluorene)s having been investigated for many years, there have been almost no reports on a fluorene homopolymer with fluorene units linked at the 3,6 position. As poly(3,6-carbazole)s [8,11] and poly(3,6-silafluorene)s [12] show higher triplet energy levels than their 2,7-counterparts, we thought it of interest to synthesize poly(3,6-fluorene)s and investigate their photophysical properties. The group of van Dijken and Brunner reported a 3,6-spirofluorene copolymer and oligomers with carbazole. [8a,13] Recently, Mo et al. [14] reported synthesis of poly(3,6-fluorene) by Ni-catalyzed coupling, but their polymer showed a strange and extremely broad PL spectra in the film and neither detailed photophysical p...