Revealing the Empty-State Electronic Structure of Single-Unit-Cell FeSe/SrTiO3

Atomic-Site-Dependent Pairing Gap in Monolayer FeSe/SrTiO3(001)-(√13 × √13)

The interfacial FeSe/TiO2-δ coupling induces high-temperature superconductivity in monolayer FeSe films. Using cryogenic atomically resolved scanning tunneling microscopy/spectroscopy, we obtained atomic-site dependent surface density of states, work function, and the pairing gap in the monolayer FeSe on the SrTiO3(001)-(√13 × √13)-R33.7° surface. Our results disclosed the out-of-plane Se-Fe-Se triple layer gradient variation, switched DOS for Fe sites on and off TiO5□, and inequivalent Fe sublattices, which gives global spatial modulation of pairing gap contaminants with the (√13 × √13) pattern. Moreover, the coherent lattice coupling induces strong inversion asymmetry and in-plane anisotropy in the monolayer FeSe, which is demonstrated to correlate with the particle-hole asymmetry in coherence peaks. These results disclose delicate atomic-scale correlations between pairing and lattice-electronic coupling in the Bardeen-Cooper-Schrieffer to Bose-Einstein condensation crossover regime, providing insights into understanding the pairing mechanism of multiorbital superconductivity.

Lateral Quantum Confinement Effect on High-TC Superconducting FeSe Monolayer

Despite decades of research in spatially confined superconducting systems to understand the modification of superconductivity from reduced length scales, the investigation of the quantum confinement effect on high-temperature superconductors remains an outstanding challenge. Here, we report scanning tunneling spectroscopy measurements on laterally confined FeSe monolayers on SrTiO3 substrates, which are formed by epitaxially growing FeSe films with a coverage less than one unit cell. Comparing to the uniform regions of FeSe monolayers, the peninsula regions at the monolayer boundary exhibit reduced Fermi energy and undiminished superconductivity, leading to a putative crossover from a Bardeen-Cooper-Schrieffer state to a Bose-Einstein condensate state. In isolated FeSe monolayer islands, superconductivity is shown to exist in samples of smaller volume in contrast to conventional superconductors, while the validity of Anderson's criterion remains fulfilled. Our work reveals lateral quantum confinement effects in unconventional superconductors to enrich the understanding of high-temperature superconductivity in low-dimensional systems.

Charge order driven by multiple-Q spin fluctuations in heavily electron-doped iron selenide superconductors

Intertwined spin and charge orders have been widely studied in high-temperature superconductors, since their fluctuations may facilitate electron pairing; however, they are rarely identified in heavily electron-doped iron selenides. Here, using scanning tunneling microscopy, we show that when the superconductivity of (Li_0.84Fe_0.16OH)Fe_1-xSe is suppressed by introducing Fe-site defects, a short-ranged checkerboard charge order emerges, propagating along the Fe-Fe directions with an approximately 2a_Fe period. It persists throughout the whole phase space tuned by Fe-site defect density, from a defect-pinned local pattern in optimally doped samples to an extended order in samples with lower T_c or non-superconducting. Intriguingly, our simulations indicate that the charge order is likely driven by multiple-Q spin density waves originating from the spin fluctuations observed by inelastic neutron scattering. Our study proves the presence of a competing order in heavily electron-doped iron selenides, and demonstrates the potential of charge order as a tool to detect spin fluctuations.

Anisotropic Gap Structure and Sign Reversal Symmetry in Monolayer Fe(Se,Te)

The iron-based superconductors are an ideal platform to reveal the enigma of the unconventional superconductivity and potential topological superconductivity. Among them, the monolayer Fe(Se,Te)/SrTiO₃(001), which is proposed to be topological nontrivial, shows interface-enhanced high-temperature superconductivity in the two-dimensional limit. However, the experimental studies on the superconducting pairing mechanism of monolayer Fe(Se,Te) films are still limited. Here, by measuring the quasiparticle interference in monolayer Fe(Se,Te)/SrTiO₃(001), we report the observation of the anisotropic structure of the large superconducting gap and the sign change of the superconducting gap on different electron pockets. The results are well consistent with the "bonding-antibonding" s_±-wave pairing symmetry driven by spin fluctuations in conjunction with spin-orbit coupling. Our work is of basic significance not only for a unified superconducting formalism in the iron-based superconductors, but also for understanding of topological superconductivity in high-temperature superconductors.

Orbital-Selective High-Temperature Cooper Pairing Developed in the Two-Dimensional Limit

For multiband superconductors, the orbital multiplicity yields orbital differentiation in normal-state properties and can lead to orbital-selective spin-fluctuation Cooper pairing. The orbital-selective phenomenon has become increasingly pivotal in clarifying the pairing "enigma", particularly for multiband high-temperature superconductors. Meanwhile, in one-unit-cell (1-UC) FeSe/SrTiO₃, since the standard electron-hole Fermi pocket nesting scenario is inapplicable, the actual pairing mechanism is subject to intense debate. Here, by measuring high-resolution Bogoliubov quasiparticle interference, we report observations of highly anisotropic magnetic Cooper pairing in 1-UC FeSe. Theoretically, it is important to incorporate orbitally selective effects of electronic correlations within a spin-fluctuation pairing calculation, where the d_xy orbital becomes coherence-suppressed. The resulting pairing gap is compatible with the experimental findings, which suggests that high-T_c Cooper pairing with orbital selectivity applies to 2D-limit 1-UC FeSe. Our findings imply the general existence of orbital selectivity in iron-based superconductors and the universal significance of electron correlations in high-T_c superconductors.

Observation of Distinct Spatial Distributions of the Zero and Nonzero Energy Vortex Modes in (Li_{0.84}Fe_{0.16})OHFeSe

The energy and spatial distributions of vortex bound state in superconductors carry important information about superconducting pairing and the electronic structure. Although discrete vortex states, and sometimes a zero energy mode, had been observed in several iron-based superconductors, their spatial properties are rarely explored. In this study, we used low-temperature scanning tunneling microscopy to measure the vortex state of (Li,Fe)OHFeSe with high spatial resolution. We found that the nonzero energy states display clear spatial oscillations with a period corresponding to bulk Fermi wavelength; while in contrast, the zero energy mode does not show such oscillation, which suggests its distinct electronic origin. Furthermore, the oscillations of positive and negative energy states near E_{F} are found to be clearly out of phase. Based on a two-band model calculation, we show that our observation is more consistent with an s_{++} wave pairing in the bulk of (Li, Fe)OHFeSe, and superconducting topological states on the surface.

Observation of robust edge superconductivity in Fe(Se,Te) under strong magnetic perturbation

The iron-chalcogenide high temperature superconductor Fe(Se,Te) (FST) has been reported to exhibit complex magnetic ordering and nontrivial band topology which may lead to novel superconducting phenomena. However, the recent studies have so far been largely concentrated on its band and spin structures while its mesoscopic electronic and magnetic response, crucial for future device applications, has not been explored experimentally. Here, we used scanning superconducting quantum interference device microscopy for its sensitivity to both local diamagnetic susceptibility and current distribution in order to image the superfluid density and supercurrent in FST. We found that in FST with 10% interstitial Fe, whose magnetic structure was heavily disrupted, bulk superconductivity was significantly suppressed whereas edge still preserved strong superconducting diamagnetism. The edge dominantly carried supercurrent despite of a very long magnetic penetration depth. The temperature dependences of the superfluid density and supercurrent distribution were distinctively different between the edge and the bulk. Our Heisenberg modeling showed that magnetic dopants stabilize anti-ferromagnetic spin correlation along the edge, which may contribute towards its robust superconductivity. Our observations hold implication for FST as potential platforms for topological quantum computation and superconducting spintronics.

Preformed Cooper Pairs in Layered FeSe-Based Superconductors

Superconductivity arises from two distinct quantum phenomena: electron pairing and long-range phase coherence. In conventional superconductors, the two quantum phenomena generally take place simultaneously, while in the underdoped high- T_{c} cuprate superconductors, the electron pairing occurs at higher temperature than the long-range phase coherence. Recently, whether electron pairing is also prior to long-range phase coherence in single-layer FeSe film on SrTiO_{3} substrate is under debate. Here, by measuring Knight shift and nuclear spin-lattice relaxation rate, we unambiguously reveal a pseudogap behavior below T_{p}∼60  K in two kinds of layered FeSe-based superconductors with quasi2D nature. In the pseudogap regime, a weak diamagnetic signal and a remarkable Nernst effect are also observed, which indicates that the observed pseudogap behavior is related to superconducting fluctuations. These works confirm that strong phase fluctuation is an important character in the 2D iron-based superconductors as widely observed in high-T_{c} cuprate superconductors.

Reversible Superconductor-Insulator Transition in (Li, Fe)OHFeSe Flakes Visualized by Gate-Tunable Scanning Tunneling Spectroscopy

Electric field control of charge carrier density provides a key in situ technology to continuously tune the ground states and map out the phase diagram of correlated electron systems in one device. This technique is highly expected to be combined with the modern state-of-the art spectroscopic probes, such as angle-resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy (STM/S), to efficiently address these states and the underlying physics. However, it is extremely difficult and not successful so far, mainly because the fabrication process of such devices makes them prohibitive for surface probes. Here, by using a solid Li-ion conductor (SIC) as gate dielectric, we have successfully developed gate-tunable STM/S and visualized the superconductor-insulator transition (SIT) in a thin flake of single crystal (Li, Fe)OHFeSe at the nanoscale. The gate-controlled Li-ion injection first enhances the superconductivity and then drives the flake into an inhomogeneous insulating state, where superconductivity is totally suppressed. This process can be reversed by applying an opposite gate voltage. Importantly, the atomically resolved images allow us to identify the critical role that the injected Li ions play in the tuning process. Our results not only provide clear evidence of the microscopic mechanism of the tunable superconductivity and SIT in the SIC-based (Li, Fe)OHFeSe devices, but also establish SIC-gating STM as a powerful tool for investigating the complicated phase diagram of correlated electron system spectroscopically in a single sample with the field-effect approach.

Correlation effect on the electronic properties of pair-checkerboard AFM monolayer FeSe: a first-principles study

We presented a study of the influence of on-site Coulomb correlation U and Hund coupling J on the electronic properties of monolayer FeSe in a pair-checkerboard antiferromagnetic (PAFM) state based on Hubbard-corrected density functional theory (DFT  +  U). The results demonstrated the Hubbard-U have a strong influence on the electronic bands and lattice structure of monolayer FeSe in PAFM state. The in-plane lattice constant changes under U  >  1.85 eV has been identified under full structure optimization. Furthermore, the Hund coupling J has significant influence on the electronic bands and exhibits a remarkable orbital selective behavior at U  =  0. Additionally, our results also demonstrated the Hund coupling J has a weaker effect on electronic properties of FeSe when U gets larger. These results are useful for elucidating the possible role of correlation strength on the electronics band and magnetic ground state of monolayer FeSe.

Enhanced Superconducting State in FeSe/SrTiO_{3} by a Dynamic Interfacial Polaron Mechanism

The observation of substantially enhanced superconductivity of single-layer FeSe films on SrTiO_{3} has stimulated intensive research interest. At present, conclusive experimental data on the corresponding electron-boson interaction is still missing. Here we use inelastic electron scattering spectroscopy and angle resolved photoemission spectroscopy to show that the electrons in these systems are dressed by the strongly polarized lattice distortions of the SrTiO_{3}, and the indispensable nonadiabatic nature of such a coupling leads to the formation of dynamic interfacial polarons. Furthermore, the collective motion of the polarons results in a polaronic plasmon mode, which is unambiguously correlated with the surface phonons of SrTiO_{3} in the presence of the FeSe films. A microscopic model is developed showing that the interfacial polaron-polaron interaction leads to the superconductivity enhancement.

Asymmetrically optimized structure in a high-T c single unit-cell FeSe superconductor

We report asymmetric Se heights in a single unit-cell (UC) FeSe on SrTiO₃(0 0 1) substrate with the highest superconducting transition temperature (T _c) among the Fe-based superconductors revealed by total-reflection high-energy positron diffraction measurements. Among various iron-based superconductors, this single UC FeSe on the SrTiO₃(0 0 1) has been the best material to achieve the highest-T _c above 50 K. We found the asymmetric Se heights of 1.44  ±  0.03 and 1.33  ±  0.03 Å from the single Fe layer by the intensity analysis based on dynamical diffraction theory. The average Se height results in 1.39  ±  0.04 Å, corresponding to the optimum value for Fe-based superconductors. In addition, the average of bond angles of Se-Fe-Se, 107.2  ±  1.1 and 111.5  ±  1.2° becomes 109.3  ±  1.6°, which is close to the optimum value of 109.5° for a regular tetrahedron. Thus, this single UC FeSe is found to have asymmetrically optimized structure. Based on our first-principles calculations, the asymmetry does not change the bandwidth whereas it splits the electron bands at the M point only at the bottom. These calculations suggest that at low electron doping, the structural asymmetry is expected to lead to exotic properties of non-centrosymmetric superconductivity, whereas after a certain amount of electron doping, average anion height plays an important role for high-T _c.

Light induced non-volatile switching of superconductivity in single layer FeSe on SrTiO3 substrate

The capability of controlling superconductivity by light is highly desirable for active quantum device applications. Since superconductors rarely exhibit strong photoresponses, and optically sensitive materials are often not superconducting, efficient coupling between these two characters can be very challenging in a single material. Here we show that, in FeSe/SrTiO₃ heterostructures, the superconducting transition temperature in FeSe monolayer can be effectively raised by the interband photoexcitations in the SrTiO₃ substrate. Attributed to a light induced metastable polar distortion uniquely enabled by the FeSe/SrTiO₃ interface, this effect only requires a less than 50 µW cm^-2 continuous-wave light field. The fast optical generation of superconducting zero resistance state is non-volatile but can be rapidly reversed by applying voltage pulses to the back of SrTiO₃ substrate. The capability of switching FeSe repeatedly and reliably between normal and superconducting states demonstrate the great potential of making energy-efficient quantum optoelectronics at designed correlated interfaces.

Realization of continuous electron doping in bulk iron selenides and identification of a new superconducting zone

It is known that iron selenide superconductors exhibit unique characteristics distinct from iron pnictides, especially in the electron-doped region. However, a comprehensive study of continuous carrier doping and the corresponding crystal structures of FeSe is still lacking, mainly due to the difficulties in controlling the carrier density in bulk materials. Here we report the successful synthesis of a new family of bulk Lix(C3N2H10)0.37FeSe, which features a continuous superconducting dome harboring Lifshitz transition within the wide range of 0.06 ⩽ x ⩽ 0.68 . We demonstrate that with electron doping, the anion height of FeSe layers deviates linearly away from the optimized values of pnictides and pressurized FeSe. This feature leads to a new superconducting zone with unique doping dependence of the electronic structures and strong orbital-selective electronic correlation. Optimal superconductivity is achieved when the F e 3 d t 2 g orbitals have almost the same intermediate electronic correlation strength, with moderate mass enhancement between 3 ~ 4 in the two separate superconducting zones. Our results shed new light on achieving unified mechanism of superconductivity in iron-based superconductors.

Direct imaging of electron transfer and its influence on superconducting pairing at FeSe/SrTiO3 interface

The exact mechanism responsible for the significant enhancement of the superconducting transition temperature (Tc) of monolayer iron selenide (FeSe) films on SrTiO3 (STO) over that of bulk FeSe is an open issue. We present the results of a coordinated study of electrical transport, low temperature electron energy-loss spectroscopy (EELS), and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) measurements on FeSe/STO films of different thicknesses. HAADF-STEM imaging together with EELS mapping across the FeSe/STO interface shows direct evidence of electrons transferred from STO to the FeSe layer. The transferred electrons were found to accumulate within the first two atomic layers of the FeSe films near the STO substrate. An additional Se layer is also resolved to reside between the FeSe film and the TiO x -terminated STO substrate. Our transport results found that a positive backgate applied from STO is particularly effective in enhancing Tc of the films while minimally changing the carrier density. This increase in Tc is due to the positive backgate that "pulls" the transferred electrons in FeSe films closer to the interface and thus enhances their coupling to interfacial phonons and also the electron-electron interaction within FeSe films.

Correlation of Fe-Based Superconductivity and Electron-Phonon Coupling in an FeAs/Oxide Heterostructure

Interfacial phonons between iron-based superconductors (FeSCs) and perovskite substrates have received considerable attention due to the possibility of enhancing preexisting superconductivity. Using scanning tunneling spectroscopy, we studied the correlation between superconductivity and e-ph interaction with interfacial phonons in an iron-based superconductor Sr_{2}VO_{3}FeAs (T_{c}≈33  K) made of alternating FeSC and oxide layers. The quasiparticle interference measurement over regions with systematically different average superconducting gaps due to the e-ph coupling locally modulated by O vacancies in the VO_{2} layer, and supporting self-consistent momentum-dependent Eliashberg calculations provide a unique real-space evidence of the forward-scattering interfacial phonon contribution to the total superconducting pairing.

How Correlated is the FeSe/SrTiO_{3} System?

Recent observation of ∼10 times higher critical temperature in a FeSe monolayer compared with its bulk phase has drawn a great deal of attention because the electronic structure in the monolayer phase appears to be different than bulk FeSe. Using a combination of density functional theory and dynamical mean field theory, we find electronic correlations have important effects on the predicted atomic-scale geometry and the electronic structure of the monolayer FeSe on SrTiO_{3}. The electronic correlations are dominantly controlled by the Se-Fe-Se angle either in the bulk phase or the monolayer phase. But the angle sensitivity increases and the orbital differentiation decreases in the monolayer phase compared to the bulk phase. The correlations are more dependent on Hund's J than Hubbard U. The observed orbital selective incoherence to coherence crossover with temperature confirms the Hund's metallic nature of the monolayer FeSe. We also find electron doping by oxygen vacancies in SrTiO_{3} increases the correlation strength, especially in the d_{xy} orbital by reducing the Se-Fe-Se angle.

Superconductivity across Lifshitz transition and anomalous insulating state in surface K-dosed (Li0.8Fe0.2OH)FeSe

In iron-based superconductors, understanding the relation between superconductivity and electronic structure upon doping is crucial for exploring the pairing mechanism. Recently, it was found that, in iron selenide (FeSe), enhanced superconductivity (T_c of more than 40 K) can be achieved via electron doping, with the Fermi surface only comprising M-centered electron pockets. By using surface K dosing, scanning tunneling microscopy/spectroscopy, and angle-resolved photoemission spectroscopy, we studied the electronic structure and superconductivity of (Li_0.8Fe_0.2OH)FeSe in the deep electron-doped regime. We find that a Γ-centered electron band, which originally lies above the Fermi level (E_F), can be continuously tuned to cross E_F and contribute a new electron pocket at Γ. When this Lifshitz transition occurs, the superconductivity in the M-centered electron pocket is slightly suppressed, and a possible superconducting gap with a small size (up to ~5 meV) and a dome-like doping dependence is observed on the new Γ electron pocket. Upon further K dosing, the system eventually evolves into an insulating state. Our findings provide new clues to understand superconductivity versus Fermi surface topology and the correlation effect in FeSe-based superconductors.

High-temperature superconductivity in one-unit-cell FeSe films

Since the dramatic enhancement of the superconducting transition temperature (T _c) was reported in a one-unit-cell FeSe film grown on a SrTiO₃ substrate (1-UC FeSe/STO) by molecular beam epitaxy (MBE), related research on this system has become a new frontier in condensed matter physics. In this paper, we present a brief review on this rapidly developing field, mainly focusing on the superconducting properties of 1-UC FeSe/STO. Experimental evidence for high-temperature superconductivity in 1-UC FeSe/STO, including direct evidence revealed by transport and diamagnetic measurements, as well as other evidence from scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES), are overviewed. The potential mechanisms of the enhanced superconductivity are also discussed. There are accumulating arguments to suggest that the strengthened Cooper pairing in 1-UC FeSe/STO originates from the interface effects, specifically the charge transfer and coupling to phonon modes in the TiO₂ plane. The study of superconductivity in 1-UC FeSe/STO not only sheds new light on the mechanism of high-temperature superconductors with layered structures, but also provides an insight into the exploration of new superconductors by interface engineering.

Enhanced superconductivity accompanying a Lifshitz transition in electron-doped FeSe monolayer

The origin of enhanced superconductivity over 50 K in the recently discovered FeSe monolayer films grown on SrTiO₃ (STO), as compared to 8 K in bulk FeSe, is intensely debated. As with the ferrochalcogenides A_xFe_2−ySe₂ and potassium-doped FeSe, which also have a relatively high-superconducting critical temperature (T_c), the Fermi surface (FS) of the FeSe/STO monolayer films is free of hole-like FS, suggesting that a Lifshitz transition by which these hole FSs vanish may help increasing T_c. However, the fundamental reasons explaining this increase of T_c remain unclear. Here we report a 15 K jump of T_c accompanying a second Lifshitz transition characterized by the emergence of an electron pocket at the Brillouin zone centre, which is triggered by high-electron doping following in situ deposition of potassium on FeSe/STO monolayer films. Our results suggest that the pairing interactions are orbital dependent in generating enhanced superconductivity in FeSe.

The origin of superconductivity enhancement in FeSe monolayer grown on SrTiO₃ compared to bulk FeSe is still a debated issue. Here, Shi et al. report a further 15 K jump of T_c accompanying a second Lifshitz transition triggered by electron doping in FeSe/SrTiO₃ monolayer films.

FeTe1-xSex monolayer films: towards the realization of high-temperature connate topological superconductivity

We performed angle-resolved photoemission spectroscopy studies on a series of FeTe_1-xSe_x monolayer films grown on SrTiO₃. The superconductivity of the films is robust and rather insensitive to the variations of the band position and effective mass caused by the substitution of Se by Te. However, the band gap between the electron- and hole-like bands at the Brillouin zone center decreases towards band inversion and parity exchange, which drive the system to a nontrivial topological state predicted by theoretical calculations. Our results provide a clear experimental indication that the FeTe_1-xSe_x monolayer materials are high-temperature connate topological superconductors in which band topology and superconductivity are integrated intrinsically.

Dispersing artifacts in FT-STS: a comparison of set point effects across acquisition modes

Fourier-transform scanning tunnelling spectroscopy (FT-STS), or quasiparticle interference, has become an influential tool for the study of a wide range of important materials in condensed matter physics. However, FT-STS in complex materials is often challenging to interpret, requiring significant theoretical input in many cases, making it crucial to understand potential artifacts of the measurement. Here, we compare the most common modes of acquiring FT-STS data and show through both experiment and simulations that artifact features can arise that depend on how the tip height is stabilized throughout the course of the measurement. The most dramatic effect occurs when a series of dI/dV maps at different energies are acquired with simultaneous constant current feedback; here a feature that disperses in energy appears that is not observed in other measurement modes. Such artifact features are similar to those arising from real physical processes in the sample and are susceptible to misinterpretation.

Dumbbell Defects in FeSe Films: A Scanning Tunneling Microscopy and First-Principles Investigation

The properties of iron-based superconductors (Fe-SCs) can be varied dramatically with the introduction of dopants and atomic defects. As a pressing example, FeSe, parent phase of the highest-Tc Fe-SC, exhibits prevalent defects with atomic-scale "dumbbell" signatures as imaged by scanning tunneling microscopy (STM). These defects spoil superconductivity when their concentration exceeds 2.5%. Resolving their chemical identity is a prerequisite to applications such as nanoscale patterning of superconducting/nonsuperconducting regions in FeSe as well as fundamental questions such as the mechanism of superconductivity and the path by which the defects destroy it. We use STM and density functional theory to characterize and identify the dumbbell defects. In contrast to previous speculations about Se adsorbates or substitutions, we find that an Fe-site vacancy is the most energetically favorable defect in Se-rich conditions and reproduces our observed STM signature. Our calculations shed light more generally on the nature of Se capping, the removal of Fe vacancies via annealing, and their ordering into a √5 × √5 superstructure in FeSe and related alkali-doped compounds.

Observation of Double-Dome Superconductivity in Potassium-Doped FeSe Thin Films

We report on the emergence of two disconnected superconducting domes in alkali-metal potassium- (K-)doped FeSe ultrathin films grown on graphitized SiC(0001). The superconductivity exhibits hypersensitivity to K dosage in the lower-T_{c} dome, whereas in the heavily electron-doped higher-T_{c} dome it becomes spatially homogeneous and robust against disorder, supportive of a conventional Cooper-pairing mechanism. Furthermore, the heavily K-doped multilayer FeSe films all reveal a large superconducting gap of ∼14  meV, irrespective of film thickness, verifying the higher-T_{c} superconductivity only in the topmost FeSe layer. The unusual finding of a double-dome superconducting phase is a step towards the mechanistic understanding of superconductivity in FeSe-derived superconductors.

Anomalous correlation effects and unique phase diagram of electron-doped FeSe revealed by photoemission spectroscopy

FeSe layer-based superconductors exhibit exotic and distinctive properties. The undoped FeSe shows nematicity and superconductivity, while the heavily electron-doped K_xFe_2−ySe₂ and single-layer FeSe/SrTiO₃ possess high superconducting transition temperatures that pose theoretical challenges. However, a comprehensive study on the doping dependence of an FeSe layer-based superconductor is still lacking due to the lack of a clean means of doping control. Through angle-resolved photoemission spectroscopy studies on K-dosed thick FeSe films and FeSe_0.93S_0.07 bulk crystals, here we reveal the internal connections between these two types of FeSe-based superconductors, and obtain superconductivity below ∼46 K in an FeSe layer under electron doping without interfacial effects. Moreover, we discover an exotic phase diagram of FeSe with electron doping, including a nematic phase, a superconducting dome, a correlation-driven insulating phase and a metallic phase. Such an anomalous phase diagram unveils the remarkable complexity, and highlights the importance of correlations in FeSe layer-based superconductors.

Electron doping is a powerful way to induce quantum phase transitions in materials and explore exotic states of matter. Here, Wen et al. present carefully-controlled potassium dosing in FeSe films and FeSe_0.93S_0.07 bulk, which enhances superconductivity and induces other anomalous phases, revealing a complex phase diagram.