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تسجيل مستخدم جديدAn extensive impurity-scattering study on the pairing symmetry of monolayer FeSe films on SrTiO3
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Determination of the pairing symmetry in monolayer FeSe films on SrTiO3 is a requisite for understanding the high superconducting transition temperature in this system, which has attracted intense theoretical and experimental studies but remains controversial. Here, by introducing several types of point defects in FeSe monolayer films, we conduct a systematic investigation on the impurity-induced electronic states by spatially resolved scanning tunneling spectroscopy. Ranging from surface adsorption, chemical substitution to intrinsic structural modification, these defects generate a variety of scattering strength, which renders new insights on the pairing symmetry.
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Charge transfer and electron-phonon coupling (EPC) are proposed to be two important constituents associated with enhanced superconductivity in the single unit cell FeSe films on oxide surfaces. Using high-resolution electron energy loss spectroscopy
combined with first-principles calculations, we have explored the lattice dynamics of ultrathin FeSe films grown on SrTiO3. We show that, despite the significant effect from the substrate on the electronic structure and superconductivity of the system, the FeSe phonons in the films are unaffected. The energy dispersion and linewidth associated with the Fe- and Se-derived vibrational modes are thickness- and temperature-independent. Theoretical calculations indicate the crucial role of antiferromagnetic correlation in FeSe to reproduce the experimental phonon dispersion. Importantly, the only detectable change due to the growth of FeSe films is the broadening of the Fuchs-Kliewer (F-K) phonons associated with the lattice vibrations of SrTiO$_3$(001) substrate. If EPC plays any role in the enhancement of film superconductivity, it must be the interfacial coupling between the electrons in FeSe film and the F-K phonons from substrate rather than the phonons of FeSe.
The intriguing role of nematicity in iron-based superconductors, defined as broken rotational symmetry below a characteristic temperature, is an intensely investigated contemporary subject. Nematicity is closely connected to the structural transition
, however, it is highly doubtful that the lattice degree of freedom is responsible for its formation, given the accumulating evidence for the observed large anisotropy. Here we combine molecular beam epitaxy, angle-resolved photoemission spectroscopy and scanning tunneling microscopy together to study the nematicity in multilayer FeSe films on SrTiO3. Our results demonstrate direct connection between electronic anisotropy in momentum space and standing waves in real space at atomic scale. The lifting of orbital degeneracy of dxz/dyz bands gives rise to a pair of Dirac cone structures near the zone corner, which causes energy-independent unidirectional interference fringes, observed in real space as standing waves by scattering electrons off C2 domain walls and Se-defects. On the other hand, the formation of C2 nematic domain walls unexpectedly shows no correlation with lattice strain pattern, which is induced by the lattice mismatch between the film and substrate. Our results establish a clean case that the nematicity is driven by electronic rather than lattice degrees of freedom in FeSe films.
Single monolayer FeSe film grown on Nb-doped SrTiO$_3$(001) substrate shows the highest superconducting transition temperature (T$_C$ $sim$ 100 K) among the iron-based superconductors (iron-pnictide), while T$_C$ of bulk FeSe is only $sim$ 8 K. Antif
erromagnetic spin fluctuations were believed to be crucial in iron-pnictides, which has inspired several proposals to understand the FeSe/SrTiO$_3$ system. Although bulk FeSe does not show the antiferromagnetic order, calculations suggest that the parent FeSe/SrTiO$_3$ films are AFM. Experimentally, due to lacking of direct probe, the magnetic state of FeSe/SrTiO$_3$ films remains mysterious. Here, we report the direct evidences of the antiferromagnetic order in the parent FeSe/SrTiO$_3$ films by the magnetic exchange bias effect measurements. The phase transition temperature is $geq$ 140 K for single monolayer film. The AFM order disappears after electron doping.
Synthesis of monolayer FeSe on SrTiO3, with greatly enhanced superconductivity compared to bulk FeSe, remains difficult. Lengthy annealing within a certain temperature window is always required to achieve superconducting samples as reported by differ
ent groups around the world, but the mechanism of annealing in inducing superconductivity has not been elucidated. We grow FeSe films on SrTiO3 by molecular beam epitaxy and adjust the stoichiometry by depositing additional small amounts of Fe atoms. The monolayer films become superconducting after the Fe deposition without any annealing, and show similar superconducting transition temperatures as those of the annealed films in transport measurements. We also demonstrate on the 5-unit-cell films that the FeSe multilayer can be reversibly tuned between the non-superconducting $sqrt{5} times sqrt{5}$ phase with Fe-vacancies and superconducting $1 times 1$ phase. Our results reveal that the traditional anneal process in essence removes Fe vacancies and the additional Fe deposition serves as a more efficient way to achieve superconductivity. This work highlights the significance of stoichiometry in the superconductivity of FeSe thin films and provides an easy path for superconducting samples.
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