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Register a new userChemical composition control at the substrate interface as the key for FeSe thin film growth
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Yukiko Obata
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The strong fascination exerted by the binary compound of FeSe demands reliable engineering protocols and more effective approaches towards inducing superconductivity in FeSe thin films. Our study addresses the peculiarities in pulsed laser deposition which determine FeSe thin film growth and focuses on the film/substrate interface, the tendency for domain matching epitaxial growth but also the disadvantage of chemical heterogeneity. We propose that homogenization of the substrate surface improves the control of stoichiometry, texture, and nanostrain in a way that favors superconductivity even in ultrathin FeSe films. The controlled interface in FeSe/Fe/MgO demonstrates the proof-of-principle.
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The significantly enhanced superconducting transition temperature ($T_c$) of an FeSe monolayer on SrTiO$_3$(001) substrate has attracted extensive attention in recent years. Here, based on first-principles electronic structure calculations, we propose another candidate substrate LaO(001) for the epitaxial growth of FeSe monolayer to realize superconductivity. Our calculations show that for the optimal adsorption structure of FeSe monolayer on LaO(001), the stripe antiferromagnetic state and the dimer antiferromagnetic state are almost energetically degenerate, indicating the existence of strong magnetic fluctuation that is beneficial to the appearance of superconductivity. According to the Bader charge analysis, the calculated electron doping from the LaO substrate to the FeSe monolayer is about 0.18 electrons per Fe atom, even larger than that in case of FeSe/SrTiO$_3$(001). Since LaO was also reported to be a superconductor with $T_c$ ~ 5 K, it may have a superconducting proximity effect on the epitaxial FeSe film and vice versa. These results suggest that LaO would be an interesting substrate to study the interface-related superconductivity.
Fermi surface topology and pairing symmetry are two pivotal characteristics of a superconductor. Superconductivity in one monolayer (1ML) FeSe thin film has attracted great interest recently due to its intriguing interfacial properties and possibly high superconducting transition temperature (Tc) over 77 K. Here, we report high-resolution measurements of the Fermi surface and superconducting gaps in 1ML FeSe using angle-resolved photoemission spectroscopy (ARPES). Two ellipse-like electron pockets are clearly resolved overlapping with each other at the Brillouin zone corner. The superconducting gap is nodeless but moderately anisotropic, which put strong constraints on determining the pairing symmetry. The gap maxima locate along the major axis of ellipse, which cannot be explained by a single d-wave, extended s-wave, or s$pm$ gap function. Four gap minima are observed at the intersection of electron pockets suggesting the existence of either a sign change or orbital-dependent pairing in 1ML FeSe.
Nematic state, where the system is translationally invariant but breaks the rotational symmetry, has drawn great attentions recently due to experimental observations of such a state in both cuprates and iron-based superconductors. The mechanism of nematicity that is likely tied to the pairing mechanism of high-Tc, however, still remains controversial. Here, we studied the electronic structure of multilayer FeSe film by angle-resolved photoemission spectroscopy (ARPES). We found that the FeSe film enters the nematic state around 125 K, while the electronic signature of long range magnetic order has not been observed down to 20K indicating the non-magnetic origin of the nematicity. The band reconstruction in the nematic state is characterized by the splitting of the dxz and dyz bands. More intriguingly, such energy splitting is strong momentum dependent with the largest band splitting of ~80meV at the zone corner. The simple on-site ferro-orbital ordering is insufficient to reproduce the nontrivial momentum dependence of the band reconstruction. Instead, our results suggest that the nearest-neighbor hopping of dxz and dyz is highly anisotropic in the nematic state, the origin of which holds the key in understanding the nematicity in iron-based superconductors.
We report the successful growth of tetragonal FeS film with one or two unit-cell (UC) thickness on SrTiO3(001) substrate by molecular beam epitaxy. Large lattice constant mismatch with the substrate leads to high density of defects in single UC FeS, while it has been significantly reduced in double UC thick film due to the lattice relaxation. The scanning tunneling spectra on the surface of FeS thin film reveal the electronic doping effect of single UC FeS from the substrate. In addition, at the Fermi level, the energy gaps of approximate 1.5 meV are observed in films of both thicknesses at 4.6 K and below. The absence of coherence peaks of gap spectra may be related to the preformed Cooper-pairs without phase coherence.
The high temperature superconductivity in single-unit-cell (1UC) FeSe on SrTiO3 (STO)(001) and the observation of replica bands by angle-resolved photoemission spectroscopy (ARPES) have led to the conjecture that the coupling between FeSe electron and the STO phonon is responsible for the enhancement of Tc over other FeSe-based superconductors1,2. However the recent observation of a similar superconducting gap in FeSe grown on the (110) surface of STO raises the question of whether a similar mechanism applies3,4. Here we report the ARPES study of the electronic structure of FeSe grown on STO(110). Similar to the results in FeSe/STO(001), clear replica bands are observed. We also present a comparative study of STO (001) and STO(110) bare surfaces, where photo doping generates metallic surface states. Similar replica bands separating from the main band by approximately the same energy are observed, indicating this coupling is a generic feature of the STO surfaces and interfaces. Our findings suggest that the large superconducting gaps observed in FeSe films grown on two different STO surface terminations are likely enhanced by a common coupling between FeSe electrons and STO phonons.
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