A high-frequency optical phonon mode of SrTiO3 (STO) was found to assist the high-temperature superconductivity observed recently at the interface between monolayer FeSe and STO substrate. However, the origin of this mode is not clear. Through first-principles calculations, we find that there is a novel polar phonon mode on the surface layers of the STO substrate, which does not exist in the STO crystals. The oxygen vacancies near the FeSe/STO interface drives the dispersion of this phonon mode to be flat and lowers its energy, whereas the charge transfer between STO substrate and FeSe monolayer further reduces its energy to 81 meV. This energy is in good agreement with the experimental value fitted by Lee et al. for the phonon mode responsible for the observed replica band separations and the increased superconducting gap. The oxygen-vacancy-induced flat and polar phonon mode provides clues for understanding the origin of high Tc superconductivity at the FeSe/STO interface.
Various Fe-vacancy orders have been reported in tetragonal Fe1-xSe single crystals and nanowires/nanosheets, which are similar to those found in alkali metal intercalated A1-xFe2-ySe2 superconductors. Here we report the in-situ angle-resolved photoemission spectroscopy study of Fe-vacancy disordered and ordered phases in FeSe multi-layer thin films grown by molecular beam epitaxy. Low temperature annealed FeSe films are identified to be Fe-vacancy disordered phase and electron doped. Further long-time low temperature anneal can change the Fe-vacancy disordered phase to ordered phase, which is …
We apply high resolution scanning tunneling microscopy to study intrinsic defect states of bulk FeSe. Four types of intrinsic defects including the type I dumbbell, type II dumbbell, top-layer Se vacancy and inner-layer Se-site defect are extensively analyzed by scanning tunneling spectroscopy. From characterized depression and enhancement of density of states measured in a large energy range, the type I dumbbell and type II dumbbell are determined to be the Fe vacancy and Se$_\mathrm{Fe}$ defect, respectively. The top-layer Se vacancy and possible inner-layer Se-site vacancy are also determin…
We have carried out a detailed study to investigate the existence of an insulating parent phase for FeSe superconductor. The insulating Fe4Se5 with specific Fe-vacancy order shows a 3D-Mott variable range hopping behavior with a Verwey-like electronic correlation at around 45 K. The application of the RTA process at 450 celcius degree results in the destruction of Fe-vacancy order and induces more electron carriers by increasing the Fe3+ valence state. Superconductivity emerges with Tc ~ 8K without changing the chemical stoichiometry of the sample after the RTA process by resulting in the addi…
In contrast to its bulk crystals, the FeSe film or layer exhibits better superconductivity performance, which attract much interest in its fundamental research as well as potential application. In present work, transition from insulator to superconductor and significant enhancement of superconductivity were achieved in the high quality (00l) oriented FeSe films via controlling the thickness as well as the Fe/Se ratio. The highest Tc up to 15.2 K (almost 2 times higher than those of bulk crystals) and Hc2 up to 35.5 T are obtained in our atmosphere-stable FeSe thin film with practical thickness…
To study the microscopic electronic and magnetic interactions in the substoichiometric iron chalcogenide FeSe$_{1-x}$ which is observed to superconduct at x~1/8 up to $T_c$=27 K, we use first principles methods to study the Se vacancy in this nearly magnetic FeSe system. The vacancy forms a ferrimagnetic cluster of eight Fe atoms, which for the ordered x=1/8 alloy leads to half metallic conduction. Similar magnetic clusters are obtained for FeTe$_{1-x}$ and for BaFe$_2$As$_2$ with an As vacancy, although neither of these are half metallic. Based on fixed spin density results, we suggest the lo…
Monolayer FeSe on SrTiO$_3$ superconducts with reported $T_\mathrm{c}$ as high as 100 K, but the dramatic interfacial $T_\mathrm{c}$ enhancement remains poorly understood. Oxygen vacancies in SrTiO$_3$ are known to enhance the interfacial electron doping, electron-phonon coupling, and superconducting gap, but the detailed mechanism is unclear. Here we apply scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) to FeSe/SrTiO$_3$ to image the diffusion of selenium into SrTiO$_3$ to an unexpected depth of several unit cells, consistent with the simultaneous…
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 Fe…
We report on the local electronic structure of oxygen incorporated FeTe and FeSe films and how this relates to superconductivity observed in these films. In the case of FeTe, intially grown films are measured to be non-superconducting, but become superconducting following oxygen incorporation. In FeSe the opposite happens, initially grown films are measured to be superconducting, but experience a quenching of superconductivity following oxygen incorporation. Total Fluorescence Yield (TFY) X-ray absorption experiments show that oxygen incorporation changes the initial Fe valence state in both t…
Several superconducting transition temperatures in the range of 30-40 K were reported in the recently discovered intercalated FeSe sytem (A1-xFe2-ySe2, A = K, Rb, Cs, Tl). Although the superconducting phases were not yet conclusively decided, more than one magnetic phase with particular orders of iron vacancy and/or potassium vacancy were identified, and some were argued to be the parent phase. Here we show the discovery of the presence and ordering of iron vacancy in nonintercalated FeSe (PbO-type tetragonal β-Fe1-xSe). Three types of iron-vacancy order were found through analytical electron …
We investigate the local density of states in the vicinity of a native dumbbell defect arising from an Fe vacancy in FeSe single crystals. The tunneling spectra close to the impurity display two bound states inside the superconducting gap, equally spaced with respect to zero energy but asymmetric in amplitude. Using spin-polarized density functional theory (DFT) calculations on realistic slab models with Fe vacancy, we show that such a defect does not induce a local magnetic moment. Therefore, the dumbbell defect is considered as non-magnetic. Thus, the in-gap bound states emerging from a non-…
By using DFT+U, we explore the formation of oxygen vacancies and their impact on the electronic and magnetic structure in strained bulk LaNiO3 and (LaNiO3)$_1$/(LaAlO3)$_1$(001) superlattices. For bulk LaNiO3, we find that epitaxial strain induces a substantial anisotropy in the oxygen vacancy formation energy. In particular, tensile strain promotes the selective reduction of apical oxygen, which may explain why the recently observed superconductivity of infinite-layer nickelates is limited to strained films. For (LaNiO3)$_1$/(LaAlO3)$_1$(001) superlattices, the simulations reveal that the NiO…
We report on the observation of high-temperature ($T_\textrm{c}$) superconductivity and magnetic vortices in single-unit-cell FeSe films on anatase TiO$_2$(001) substrate by using scanning tunneling microscopy. A systematic study and engineering of interfacial properties has clarified the essential roles of substrate in realizing the high-$T_\textrm{c}$ superconductivity, probably via interface-induced electron-phonon coupling enhancement and charge transfer. By visualizing and tuning the oxygen vacancies at the interface, we find their very limited effect on the superconductivity, which exclu…
Compared to experiment, the calculated bulk lattice constant of FeSe is too small in LDA, PBEsol and PBE type exchange-correlation (xc) functionals even though the mismatch decreases from LDA to PBE. In bulk SrTiO3 (STO) on the other hand LDA over-binds, PBE under-binds and PBEsol agrees best with experiment. With the errors in opposite directions, the strain in the FeSe monolayer on STO depends on the xc functional and, especially in the non-magnetic state, is strongly overestimated. In this work we investigate the influence of the xc functional and magnetism in density functional calculation…
The transition from insulator to superconductor was achieved in the non-superconducting FeSe ultrathin film via simple Mg coating technique in present work. It was found that in non-superconducting FeSe ultrathin film without Mg coating, insulating \b{eta}-Fe1-xSe phase with iron-vacancy disorders is the main phase and more likely to be the parent phase of FeSe superconducting system. Proper Mg coating on the surface of FeSe films can lead to Mg entering FeSe crystal lattice to fill up these Fe vacancies rather than replace Fe. Simultaneously, additional electron doping is introduced and the e…
The 2DEG at the LaAlO3/SrTiO3 interface promises to add a new dimension to emerging electronic devices due to its high tunability. Defects in the form of Oxygen vacancies in titanate surfaces and interfaces, on the other hand, play a key role in the emergence of novel phases. Based on an effective model, we study the influence of Oxygen vacancies on superconductivity and ferromagnetism at LaAlO3/SrTiO3 interface. Using the Bogoliubov-de Gennes formulation in conjunction with Monte-Carlo simulation, we find a clustering of the Oxygen vacancies at the interface that favours formation of coexisti…
First principles band-structure calculations for large supercells of Ba$_{2}$CuO$_{4-δ}$ and La$_{2}$CuO$_{4-δ}$ with different distributions and concentrations of oxygen vacancies show that the effective doping on copper sites strongly depends on where the vacancy is located. A vacancy within the Cu layer produces a weak doping effect while a vacancy located at an apical oxygen site acts as a stronger electron dopant on the copper layers and gradually brings the electronic structure close to that of La$_{2-x}$Sr$_x$CuO$_{4}$. These effects are robust and only depend marginally on lattice dist…
It was observed recently (K. Shigekawa et al, PNAS 116, 2470 (2019)) that while monolayer iron chalcigenide FeSe on SrTiO3 (STO) substrate has a very high critical temperature, its chemical and structural "twin" material FeS=STO has a very low Tc if any. To explain this the substrate interfacial phonon model of superconductivity in iron chalcogenides is further developed. The main glue is the oxygen ion (60mev) vibrations longitudinal optical (LO) mode. The mode propagates mainly in the TiO2 layer adjacent to the monolayer (and genrally present also in similar highly polarized ionic crystals l…
We are examining the possibility of the formation of charge order in the high-temperature superconductor $\mathrm{YBa_2Cu_3O_{7-δ}}$ (YBCO) due to interaction between the charged oxygen vacancies or di-vacancies. The molecular dynamics method is used to analyze the displacement fields around these defects. The distribution of displacements around a single charged oxygen vacancy and di-vacancy, determination of binding energy of oxygen vacancy in di-vacancy demonstrate that there is, in principle, the possibility of the charge order formation in YBCO by charged oxygen vacancies or di-vacancies.…
The record of superconducting transition temperature(Tc) has long been 56K for the iron-based high temperature superconductors(Fe-HTS's). Recently, in single layer FeSe films grown on SrTiO3 substrate, signs for a new 65K Tc record are reported. Here with in-situ photoemission measurements, we substantiate the presence of the spin density wave(SDW) in FeSe films, a key ingredient of Fe-HTS that was missed in FeSe before, which weakens with increased thickness or reduced strain. We demonstrate that the superconductivity occurs when the electrons transferred from the oxygen-vacant substrate supp…
The dream of room temperature superconductors has inspired intense research effort to find routes for enhancing the superconducting transition temperature (Tc). Therefore, single-layer FeSe on a SrTiO3 substrate, with its extraordinarily high Tc amongst all interfacial superconductors and iron based superconductors, is particularly interesting, but the mechanism underlying its high Tc has remained mysterious. Here we show through isotope effects that electrons in FeSe couple with the oxygen phonons in the substrate, and the superconductivity is enhanced linearly with the coupling strength atop…
We report very large bandgap enhancement in SrTiO3 (STO) films (fabricated by pulsed laser deposition below 800 °C), which can be up to 20% greater than the bulk value, depending on the deposition temperature. The origin is comprehensively investigated and finally attributed to Sr/Ti antisite point defects, supported by density functional theory calculations. More importantly, the bandgap enhancement can be utilized to tailor the electronic and magnetic phases of the two-dimensional electron gas (2DEG) in STO-based interface systems. For example, the oxygen-vacancy-induced 2DEG (2DEG-V) at the…
We study the electron-phonon coupling strength near the interface of monolayer and bilayer FeSe thin films on SrTiO$_3$, BaTiO$_3$, and oxygen-vacant SrTiO$_3$ substrates, using ab initio methods. The calculated total electron-phonon coupling strength $λ=0.2\text{--}0.3$ cannot account for the high $T_c\sim 70$ K observed in these systems through the conventional phonon-mediated pairing mechanism. In all of these systems, however, we find that the coupling constant of a polar oxygen branch peaks at $\mathbf{q}=0$ with negligible coupling elsewhere, while the energy of this mode coincides with …
Recently, the signs of both superconducting transition temperature (Tc) beyond 60 K and spin density wave (SDW) have been observed in FeSe thin film on SrTiO3 (STO) substrate, which suggests a strong interplay between superconductivity and magnetism. With the first-principles calculations, we find that the substrate-induced tensile strain tends to stabilize the SDW state in FeSe thin film by enhancing of the next-nearest-neighbor superexchange antiferromagnetic interaction bridged through Se atoms. On the other hand, we find that when there are oxygen vacancies in the substrate, the significan…
Mono- and multilayer FeSe thin films grown on SrTiO$_\mathrm{3}$ and BiTiO$_\mathrm{3}$ substrates exhibit a greatly enhanced superconductivity over that found in bulk FeSe. A number of proposals have been advanced for the mechanism of this enhancement. One possibility is the introduction of a cross-interface electron-phonon ($e$-$ph$) interaction between the FeSe electrons and oxygen phonons in the substrates that is peaked in the forward scattering (small ${\bf q}$) direction due to the two-dimensional nature of the interface system. Motivated by this, we explore the consequences of such an …
Single unit cell films of iron selenide (1UC FeSe) grown on SrTiO3 (STO) substrates have recently shown superconducting energy gaps opening at temperatures close to the boiling point of liquid nitrogen (77 K), a record for iron-based superconductors. Towards understanding why Cooper pairs form at such high temperatures, a primary question to address is the role, if any, of the STO substrate. Here, we report high resolution angle resolved photoemission spectroscopy (ARPES) results which reveal an unexpected and unique characteristic of the 1UC FeSe/STO system: shake-off bands suggesting the pre…
The theory of EPI with strong forward scattering peak (FSP)[1-3] is recently applied in [4-5] in studying high Tc(~100 K) in a FeSe grown on SrTiO3 [4] and TiO2 [6]. The EPI is due to a long-range dipolar electric field created by the high-energy oxygen vibrations (E0~90 meV) at the interface [4-5]. Besides obtaining new results we also correct some misleading results from the recent literature. We show that the mean-field critical temperature Tc0 is an interplay between the maximal pairing potential and the FSP-width qc. For Tc0~100 K the gap (G) is G~16 meV in agreement with ARPES experiment…
