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Register a new userLaO as a candidate substrate for realizing superconductivity in FeSe epitaxial film
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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.
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A recent experiment reported the first rare-earth binary oxide superconductor LaO ($T_c $ $sim$ 5 K) with a rock-salt structure [K. Kaminaga et al., J. Am. Chem. Soc. 140, 6754 (2018)]. Correspondingly, the underlying superconducting mechanism in LaO needs theoretical elucidation. Based on first-principles calculations on the electronic structure, lattice dynamics, and electron-phonon coupling of LaO, we show that the superconducting pairing in LaO belongs to the conventional Bardeen-Cooper-Schrieffer (BCS) type. Remarkably, the electrons and phonons of the heavy La atoms, instead of those of the light O atoms, contribute most to the electron-phonon coupling. We further find that both the biaxial tensile strain and the pure electron doping can enhance the superconducting $T_c$ of LaO. With the synergistic effect of electron doping and tensile strain, the $T_c$ could be even higher, for example, 11.11 K at a doping of 0.2 electrons per formula unit and a tensile strain of $4%$. Moreover, our calculations show that the superconductivity in LaO thin film remains down to the trilayer thickness with a $T_c$ of 1.4 K.
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.
Single-layer FeSe films with extremely expanded in-plane lattice constant of 3.99A are fabricated by epitaxially growing FeSe/Nb:SrTiO3/KTaO3 heterostructures, and studied by in situ angle-resolved photoemission spectroscopy. Two elliptical electron pockets at the Brillion zone corner are resolved with negligible hybridization between them, indicating the symmetry of the low energy electronic structure remains intact as a free-standing single-layer FeSe, although it is on a substrate. The superconducting gap closes at a record high temperature of 70K for the iron based superconductors. Intriguingly, the superconducting gap distribution is anisotropic but nodeless around the electron pockets, with minima at the crossings of the two pockets. Our results put strong constraints on the current theories, and support the coexistence of both even and odd parity spin-singlet pairing channels as classified by the lattice symmetry.
Cryogenic scanning tunneling microscopy is employed to investigate the stoichiometry and defects of epitaxial FeSe thin films on SrTiO3(001) substrates under various post-growth annealing conditions. Low-temperature annealing with an excess supply of Se leads to formation of Fe vacancies and superstructures, accompanied by a superconductivity (metal)-to-insulator transition in FeSe films. By contrast, high-temperature annealing could eliminate the Fe vacancies and superstructures, and thus recover the high-temperature superconducting phase of monolayer FeSe films. We also observe multilayer FeSe during low-temperature annealing, which is revealed to link with Fe vacancy formation and adatom migration. Our results document very special roles of film stoichiometry and help unravel several controversies in the properties of monolayer FeSe films.
There is an ongoing debate about the relative importance of structural change versus doping charge carriers on the mechanism of superconductivity in Fe-based materials. Elucidating this issue is a major challenge since it would require a large number of samples where structure properties or the carrier density is systematically varied. FeSe, with its structural simplicity, is an ideal platform for addressing this question. It has been demonstrated that the superconductivity in this material can be controlled through crystal lattice tuning, as well as electronic structure manipulation. Here, we apply a high-throughput methodology to FeSe to systematically delineate the interdependence of its structural and electronic properties. Using a dual-beam pulsed laser deposition, we have generated FeSe films with a marked gradient in the superconducting transition temperature (below 2 K < Tc < 12 K) across 1 cm width of the films. The Tc gradient films display ~ 1% continuous stretch and compression in the out-of-plane and in-plane lattice constants respectively, triggering the continuous enhancement of superconductivity. Combining transport and angular-resolved photoemission measurements on uniform FeSe films with tunable Tc from 3 K to 14 K, we find that the electron carrier density is intimately correlated with Tc, i.e., it increases by a factor of 6 and ultimately surpasses the almost constant hole concentration. Our transmission electron microscope and band structure calculations reveal that rather than by shifting the chemical potential, the enhanced superconductivity is linked to the selective adjustment of the dxy band dispersion across the Fermi level by means of reduced local lattice distortions. Therefore, such novel mechanism provides a key to understand discrete superconducting phases in FeSe.
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