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

Advancing Superconductivity with Interface Engineering

The development of superconducting materials has attracted significant attention not only for their improved performance, such as high transition temperature (TC), but also for the exploration of their underlying physical mechanisms. Recently, considerable efforts have been focused on interfaces of materials, a distinct category capable of inducing superconductivity at non-superconducting material interfaces or augmenting the TC at the interface between a superconducting material and a non-superconducting material. Here, two distinct types of interfaces along with their unique characteristics are reviewed: interfacial superconductivity and interface-enhanced superconductivity, with a focus on the crucial factors and potential mechanisms responsible for enhancing superconducting performance. A series of materials systems is discussed, encompassing both historical developments and recent progress from the perspectives of technical innovations and the exploration of new material classes. The overarching goal is to illuminate pathways toward achieving high TC, expanding the potential of superconducting parameters across interfaces, and propelling superconductivity research toward practical, high-temperature applications.

Phase diagram simulations incorporating the gap anisotropy with AFM spin and charge density wave under spin-orbital coupling in Fe-based superconductors

For over a decade, iron-based superconductors (IBSCs) have been the subject of intense scientific research, yet the underlying principle of their pairing mechanism remains elusive. To address this, we have developed a simulation tool that reasonably predicts the regional superconducting phase diagrams of key IBSCs, incorporating factors such as anisotropic superconducting gap, spin-orbital coupling, electron-phonon coupling, antiferromagnetism, spin density wave, and charge transfer. Our focus has been on bulk FeSe, LiFeAs, NaFeAs, and FeSe films on SrTiO3 substrates. By incorporating angle-resolved photoemission spectroscopy (ARPES) data to fine-tune the electron concentration in the superconducting state, our simulations have successfully predicted the theoretical superconducting transition temperature (Tc) of these compounds, closely matching experimental results. Our research not only aids in identifying patterns and establishing correlations with Tc but also provides a simulation tool for potentially predicting high-pressure phase diagrams.

Preliminary Tc Calculations for Iron-Based Superconductivity in NaFeAs, LiFeAs, FeSe and Nanostructured FeSe/SrTiO3 Superconductors

Many theoretical models of iron-based superconductors (IBSC) have been proposed, but the superconducting transition temperature (T_c) calculations based on these models are usually missing. We have chosen two models of iron-based superconductors from the literature and computed the T_c values accordingly; recently two models have been announced which suggest that the superconducting electron concentration involved in the pairing mechanism of iron-based superconductors may have been underestimated and that the antiferromagnetism and the induced xy potential may even have a dramatic amplification effect on electron-phonon coupling. We use bulk FeSe, LiFeAs and NaFeAs data to calculate the T_c based on these models and test if the combined model can predict the superconducting transition temperature (T_c) of the nanostructured FeSe monolayer well. To substantiate the recently announced xy potential in the literature, we create a two-channel model to separately superimpose the dynamics of the electron in the upper and lower tetrahedral plane. The results of our two-channel model support the literature data. While scientists are still searching for a universal DFT functional that can describe the pairing mechanism of all iron-based superconductors, we base our model on the ARPES data to propose an empirical combination of a DFT functional for revising the electron-phonon scattering matrix in the superconducting state, which ensures that all electrons involved in iron-based superconductivity are included in the computation. Our computational model takes into account this amplifying effect of antiferromagnetism and the correction of the electron-phonon scattering matrix, together with the abnormal soft out-of-plane lattice vibration of the layered structure. This allows us to calculate theoretical T_c values of LiFeAs, NaFeAs and FeSe as a function of pressure that correspond reasonably well to the experimental values. More importantly, by taking into account the interfacial effect between an FeSe monolayer and its SrTiO₃ substrate as an additional gain factor, our calculated T_c value is up to 91 K and provides evidence that the strong T_c enhancement recently observed in such monolayers with T_c reaching 100 K may be contributed from the electrons within the ARPES range.

Type-II quantum spin Hall effect in two-dimensional metals

The quantum spin Hall (QSH) effect has been observed in topological insulators and long quantum wells using spin-orbit coupling as the probe, but it has not yet been observed in a metal. An experiment is proposed to measure the different Type-II QSH effect of an electron or hole in a two-dimensional (2D) metal by using the previously unexplored but relativistically gauge-invariant form of the generated 2D QSH Hamiltonian. Instead of using the electric field in the surface of the spin-polarized bands of a topological insulator or across the quantum well width as the probe, ones uses an applied azimuthal vector potential and an applied radial electric field as the tools to generate a spontaneously quantized spin current in an otherwise spin unpolarized 2D metal. A long cylindrical solenoid lies normally through the inner radius of a 2D metallic Corbino disk. The currentISsurrounding the solenoid produces an azimuthal magnetic vector potential but no magnetic field in the disk. In addition, a radial electric field is generated across the disk by imposing either a potential differenceΔvor a radial charge currentIacross its inner and outer radii. Combined changes inISand in eitherΔvorIgenerate spontaneously quantized azimuthal charge and spin currents. The experiment is designed to measure these quantized azimuthal charge and spin currents in the disk consistently. The quantum Hamiltonians for both experiments are solved exactly. A method to control the Joule heating is presented, which could potentially allow the Type-II QSH measurements to be made at room temperature.

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.

Van Der Waals Epitaxial Growth and Phase Transition of Layered FeSe2 Nanocrystals

Layered iron chalcogenides (FeX, X = S, Se, Te) provide excellent platforms to study intertwined phase transitions, superconductivity, and magnetism. However, layered iron dichalcogenides (FeX₂ , X = S, Se, Te) are rarely reported and their intrinsic properties are still unknown. Here, phase-pure layered iron diselenide (FeSe₂ ) nanocrystals are epitaxially grown on mica by the sublimed-salt-assisted chemical vapor deposition method at atmospheric pressure. The layered atomic structure of FeSe₂ is confirmed by X-ray diffraction and atomic-resolution scanning transmission electron microscopy. Electrical transport shows that the layered FeSe₂ is a metal with high conductivity and a phase transition at ≈11 K. The phase transition manifests itself as a kink in the temperature-dependent resistivity, as well as anomalous magnetoresistance (MR) appearing around the phase-transition temperature. The MR changes from negative to positive, accompanied by large hysteresis near the phase-transition temperature upon cooling. The negative MR and hysteresis might originate from magnetic field suppression scattering of spin fluctuations and competition of magnetic interactions induced by the phase transition, respectively. Layered iron dichalcogenide will be potential candidate to explore novel quantum phenomena and other applications.

Atomically Thin Superconductors

In recent years, atomically thin superconductors, including atomically thin elemental superconductors, single layer FeSe films, and few-layer cuprate superconductors, have been studied extensively. This hot research field is mainly driven by the discovery of significant superconductivity enhancement and high-temperature interface superconductivity in single-layer FeSe films epitaxially grown on SrTiO₃ substrates in 2012. This study has attracted tremendous research interest and generated more studies focusing on further enhancing superconductivity and finding the origin of the superconductivity. A few years later, research on atomically thin superconductors has extended to cuprate superconductors, unveiling many intriguing properties that have neither been proposed or observed previously. These new discoveries challenge the current theory regarding the superconducting mechanism of unconventional superconductors and indicate new directions on how to achieve high-transition-temperature superconductors. Herein, this exciting recent progress is briefly discussed, with a focus on the recent progress in identifying new atomically thin superconductors.

On the Remarkable Superconductivity of FeSe and Its Close Cousins

Emergent electronic phenomena in iron-based superconductors have been at the forefront of condensed matter physics for more than a decade. Much has been learned about the origins and intertwined roles of ordered phases, including nematicity, magnetism, and superconductivity, in this fascinating class of materials. In recent years, focus has been centered on the peculiar and highly unusual properties of FeSe and its close cousins. This family of materials has attracted considerable attention due to the discovery of unexpected superconducting gap structures, a wide range of superconducting critical temperatures, and evidence for nontrivial band topology, including associated spin-helical surface states and vortex-induced Majorana bound states. Here, we review superconductivity in iron chalcogenide superconductors, including bulk FeSe, doped bulk FeSe, FeTe1−xSex, intercalated FeSe materials, and monolayer FeSe and FeTe1−xSex on SrTiO3. We focus on the superconducting properties, including a survey of the relevant experimental studies, and a discussion of the different proposed theoretical pairing scenarios. In the last part of the paper, we review the growing recent evidence for nontrivial topological effects in FeSe-related materials, focusing again on interesting implications for superconductivity.

Interfacial Superconductivity in FeSe Ultrathin Films on SrTiO_{3} Probed by In Situ Independently Driven Four-Point-Probe Measurements

We investigated the superconducting transport properties of the one-unit-cell FeSe ultrathin films epitaxially grown on undoped SrTiO_{3}(001) (STO) with a well-defined surface structure by in situ independently-driven four-point-probe measurements. Our results unambiguously revealed the detection of the two-dimensional electrical conduction of the films without parallel conduction through the underlying substrate, both in the normal and superconducting states. The monolayer film exhibited a superconducting transition at an onset temperature of 40 K. Surprisingly, the onset of superconductivity was constantly observed at 40 K even for three- and five-unit-cell-thick FeSe films, even though the normal resistivity decreased with increasing thickness. These results agree with the picture of the interfacial superconductivity, where only the FeSe/STO interface and/or the adjacent first layer of FeSe becomes superconducting while the upper layers stay in the normal metallic state. The observed T_{c} is much lower than that reported by a previous in situ transport measurement for FeSe/Nb:STO but consistent with the results obtained by ex situ measurements for FeSe-undoped STO with a capping layer. This suggests that the capping layer is not an essential factor to limit T_{c}. We rather propose that the charge transfer from the doped substrate has a key role to achieve the higher temperature superconductivity in the one-unit-cell FeSe.

Zero-energy bound states in the high-temperature superconductors at the two-dimensional limit

Majorana zero modes (MZMs) that obey the non-Abelian statistics have been intensively investigated for potential applications in topological quantum computing. The prevailing signals in tunneling experiments "fingerprinting" the existence of MZMs are the zero-energy bound states (ZEBSs). However, nearly all of the previously reported ZEBSs showing signatures of the MZMs are observed in difficult-to-fabricate heterostructures at very low temperatures and additionally require applied magnetic field. Here, by using in situ scanning tunneling spectroscopy, we detect the ZEBSs upon the interstitial Fe adatoms deposited on two different high-temperature superconducting one-unit-cell iron chalcogenides on SrTiO3(001). The spectroscopic results resemble the phenomenological characteristics of the MZMs inside the vortex cores of topological superconductors. Our experimental findings may extend the MZM explorations in connate topological superconductors toward an applicable temperature regime and down to the two-dimensional (2D) limit.

Bulk Superconductivity and Role of Fluctuations in the Iron-Based Superconductor FeSe at High Pressures

The iron-based superconductor FeSe offers a unique possibility to study the interplay of superconductivity with purely nematic as well magnetic-nematic order by pressure (p) tuning. By measuring specific heat under p up to 2.36 GPa, we study the multiple phases in FeSe using a thermodynamic probe. We conclude that superconductivity is bulk across the entire p range and competes with magnetism. In addition, whenever magnetism is present, fluctuations exist over a wide temperature range above both the bulk superconducting and the magnetic transitions. Whereas the magnetic fluctuations are likely temporal, the superconducting fluctuations may be either temporal or spatial. These observations highlight similarities between FeSe and underdoped cuprate superconductors.

Spectroscopic Imaging of Quasiparticle Bound States Induced by Strong Nonmagnetic Scatterings in One-Unit-Cell FeSe/SrTiO_{3}

The absence of holelike Fermi pockets in the heavily electron-doped iron selenides (HEDISs) challenges the s_{±}-wave pairing originally proposed for iron pnictides, which consists of opposite signs of the gap function on electron and hole pockets. While the HEDIS compounds have been investigated extensively, a consistent description of the superconducting pairing therein is still lacking. Here, by in situ scanning tunneling spectroscopy and theoretical calculations, we study the effects of strong scatterings from nonmagnetic Pb adatoms on the epitaxially grown HEDIS, one-unit-cell FeSe/SrTiO_{3}(001). Systematic tunneling spectra measured on the Pb adatoms show comprehensive signals of quasiparticle bound states, which can be well explained theoretically within the sign-reversing pairing scenarios. The finding implies that, in addition to previously detected phonons, spin fluctuations play an important role in driving the Cooper pairing in FeSe/SrTiO_{3}(001). The sign reversal in the gap function we revealed here is a significant ingredient in a unified understanding of the high-temperature superconductivity in HEDISs.

Detection of Bosonic Mode as a Signature of Magnetic Excitation in One-Unit-Cell FeSe on SrTiO3

A "fingerprint" of Cooper pairing mediated by collective bosonic excitation mode is the reconstruction of the quasiparticle-density-of-states (DOS) spectrum with an additional "dip-hump" structure located outside the superconducting coherence peak. Here, we report an in situ scanning tunneling spectroscopy study of one-unit-cell (1-UC) FeSe film on a SrTiO₃(001) substrate. In the quasiparticle-DOS spectrum, the bosonic excitation mode characterized by the dip-hump structure is detected outside the larger superconducting gap. Statistically, the excitation mode shows an anticorrelation with pairing strength in magnitude and yields an energy scale upper-bounded by twice the superconducting gap. The observation coincides with the characteristics of magnetic resonance in cuprates and iron-based superconductors. Furthermore, the local response of superconducting spectra to magnetically distinct Se defects all exhibits the induced in-gap quasiparticle bound states, indicating an unconventional sign-reversing pairing over the Fermi surface in 1-UC FeSe. These results clarify the magnetic nature of the bosonic excitation mode and reveal a signature of electron-magnetic-excitation coupling in 1-UC FeSe/SrTiO₃(001) besides the previously established pairing channel of electron-phonon interaction.

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.

Role of Electric Fields on Enhanced Electron Correlation in Surface-Doped FeSe

Electron-doped high-T_{c} FeSe reportedly has a strong electron correlation that is enhanced with doping. It has been noticed that significant electric fields exist inevitably between FeSe and external donors along with electron transfer. However, the effects of such fields on the electron correlation are yet to be explored. Here we study potassium- (K-) dosed FeSe layers using density-functional theory combined with dynamical mean-field theory to investigate the roles of such electric fields on the strength of the electron correlation. We find, very interestingly, that the electronic potential-energy difference between the topmost Se and Fe atomic layers, generated by local electric fields of ionized K atoms, weakens the Se-mediated hopping between Fe d orbitals. Since it is the dominant hopping channel in FeSe, its reduction narrows the Fe d bands near the Fermi level, enhancing the electron correlation. This effect is orbital dependent and occurs in the topmost FeSe layer only. We also find the K dosing may increase the Se height, enhancing the electron correlation further. These results shed new light on the comprehensive study of high-T_{c} FeSe and other low-dimensional systems.

Tip-induced or enhanced superconductivity: a way to detect topological superconductivity

Topological materials, hosting topological nontrivial electronic band, have attracted widespread attentions. As an application of topology in physics, the discovery and study of topological materials not only enrich the existing theoretical framework of physics, but also provide fertile ground for investigations on low energy excitations, such as Weyl fermions and Majorana fermions, which have not been observed yet as fundamental particles. These quasiparticles with exotic physical properties make topological materials the cutting edge of scientific research and a new favorite of high tech. As a typical example, Majorana fermions, predicted to exist in the edge state of topological superconductors, are proposed to implement topological error-tolerant quantum computers. Thus, the detection of topological superconductivity has become a frontier in condensed matter physics and materials science. Here, we review a way to detect topological superconductivity triggered by the hard point contact: tip-induced superconductivity (TISC) and tip-enhanced superconductivity (TESC). The TISC refers to the superconductivity induced by a non-superconducting tip at the point contact on non-superconducting materials. We take the elaboration of the chief experimental achievement of TISC in topological Dirac semimetal Cd₃As₂ and Weyl semimetal TaAs as key components of this article for detecting topological superconductivity. Moreover, we also briefly introduce the main results of another exotic effect, TESC, in superconducting Au₂Pb and Sr₂RuO₄ single crystals, which are respectively proposed as the candidates of helical topological superconductor and chiral topological superconductor. Related results and the potential mechanism are conducive to improving the comprehension of how to induce and enhance the topological superconductivity.

Unconventional superconductivity in magic-angle graphene superlattices

The behaviour of strongly correlated materials, and in particular unconventional superconductors, has been studied extensively for decades, but is still not well understood. This lack of theoretical understanding has motivated the development of experimental techniques for studying such behaviour, such as using ultracold atom lattices to simulate quantum materials. Here we report the realization of intrinsic unconventional superconductivity-which cannot be explained by weak electron-phonon interactions-in a two-dimensional superlattice created by stacking two sheets of graphene that are twisted relative to each other by a small angle. For twist angles of about 1.1°-the first 'magic' angle-the electronic band structure of this 'twisted bilayer graphene' exhibits flat bands near zero Fermi energy, resulting in correlated insulating states at half-filling. Upon electrostatic doping of the material away from these correlated insulating states, we observe tunable zero-resistance states with a critical temperature of up to 1.7 kelvin. The temperature-carrier-density phase diagram of twisted bilayer graphene is similar to that of copper oxides (or cuprates), and includes dome-shaped regions that correspond to superconductivity. Moreover, quantum oscillations in the longitudinal resistance of the material indicate the presence of small Fermi surfaces near the correlated insulating states, in analogy with underdoped cuprates. The relatively high superconducting critical temperature of twisted bilayer graphene, given such a small Fermi surface (which corresponds to a carrier density of about 10¹¹ per square centimetre), puts it among the superconductors with the strongest pairing strength between electrons. Twisted bilayer graphene is a precisely tunable, purely carbon-based, two-dimensional superconductor. It is therefore an ideal material for investigations of strongly correlated phenomena, which could lead to insights into the physics of high-critical-temperature superconductors and quantum spin liquids.