School of Physics and Astronomy, University of St. Andrews（圣安德鲁斯大学物理与天文学院）；Institute of Physics, University of Greifswald（格赖夫斯瓦尔德大学物理研究所）；Diamond Light Source（钻石光源）
Luke C. Rhodes, Matthias Eschrig, Timur K. Kim, Matthew D. Watson
The nature and origin of electronic nematicity remains a significant challenge in our understanding of the iron-based superconductors. This is particularly evident in the iron chalcogenide, FeSe, where it is currently unclear how the experimentally determined Fermi surface near the M point evolves from having two electron pockets in the tetragonal state to exhibiting just a single electron pocket in the nematic state. This has posed a major theoretical challenge, which has become known as the missing electron pocket problem of FeSe, and is of central importance if we wish to uncover the secrets behind nematicity and superconductivity in the wider iron-based superconductors. Here, we review the recent experimental work uncovering this nematic Fermi surface of FeSe from both ARPES and STM measurements, as well as current theoretical attempts to explain this missing electron pocket of FeSe, with a particular focus on the emerging importance of incorporating the $d_{xy}$ orbital into theoretical descriptions of the nematic state. Furthermore, we will discuss the consequence this missing electron pocket has on the theoretical understanding of superconductivity in this system and present several remaining open questions and avenues for future research.
We measure the electronic structure of FeSe from within individual orthorhombic domains. Enabled by an angle-resolved photoemission spectroscopy beamline with a highly focused beamspot (nano-ARPES), we identify clear stripe-like orthorhombic domains in FeSe with a length scale of approximately 1-5~$μ$m. Our photoemission measurements of the Fermi surface and band structure within individual domains reveal a single electron pocket at the Brillouin zone corner. This result provides clear evidence for a one-electron pocket electronic structure of FeSe, observed without the application of uniaxial…
Superconductivity originates from pairing of electrons. Pairing channel on Fermi surface and pairing glue are thus two pivotal issues for understanding a superconductor. Recently, high-temperature superconductivity over 40 K was found in electron-doped FeSe superconductors including K$_x$Fe$_{2-y}$Se$_2$, Li$_{0.8}$Fe$_{0.2}$OHFeSe, and 1 monolayer FeSe thin film. However, their pairing mechanism remains controversial. Here, we studied the systematic evolution of electronic structure in potassium-coated FeSe single crystal. The doping level is controlled precisely by in situ evaporating potass…
The origin of spontaneous electronic nematic ordering provides important information for understanding iron-based superconductors. Here, we analyze a scenario where the $d_{xy}$ orbital strongly contributes to nematic ordering in FeSe. We show that the addition of $d_{xy}$ nematicity to a pure $d_{xz}/d_{yz}$ order provides a natural explanation for the unusual Fermi surface and correctly reproduces the strongly anisotropic momentum dependence of the superconducting gap. We predict a Lifshitz transition of an electron pocket mediated by temperature and sulphur doping, whose signatures we discu…
High-Tc pairing mechanism absent of hole-pockets in heavily electron-doped FeSe is one of the key unsolved problems in Fe-based superconductors. Here, this problem is attacked by focusing on the higher-order many-body effects neglected in conventional Migdal-Eliashberg formalism. We uncover two significant many-body effects for high-Tc superconductivity: (i) Due to the "vertex correction", the dressed multiorbital Coulomb interaction acquires prominent orbital dependence for low-energy electrons. The dressed Coulomb interaction not only induces the orbital fluctuations, but also magnifies the …
We report high resolution ARPES measurements of detwinned FeSe single crystals. The application of a mechanical strain is used to promote the volume fraction of one of the orthorhombic domains in the sample, which we estimate to be 80$\%$ detwinned. While the full structure of the electron pockets consisting of two crossed ellipses may be observed in the tetragonal phase at temperatures above 90~K, we find that remarkably, only one peanut-shaped electron pocket oriented along the longer $a$ axis contributes to the ARPES measurement at low temperatures in the nematic phase, with the expected po…
We measured the electronic-structure of FeSe_xTe_1-x above and below Tc. In the normal state we find multiple bands with remarkably small values for the Fermi energy, E_F. Yet,below Tc we find a superconducting gap, Delta, that is comparable in size to E_F, leading to a ratio Delta/E_F~0.5 that is much larger than found in any previously studied superconductor. We also observe an anomalous dispersion of the coherence peak which is very similar to the dispersion found in cold Fermi-gas experiments and which is consistent with the predictions of the BCS-BEC crossover theory.
The electronic structure of the enigmatic iron-based superconductor FeSe has puzzled researchers since spectroscopic probes failed to observe the expected electron pocket at the $Y$ point in the 1-Fe Brillouin zone. It has been speculated that this pocket, essential for an understanding of the superconducting state, is either absent or incoherent. Here, we perform a theoretical study of the preferred nematic order originating from nearest-neighbor Coulomb interactions in an electronic model relevant for FeSe. We find that at low temperatures the dominating nematic components are of inter-orbit…
To understand the nematicity in Fe-based superconductors, nontrivial $k$-dependence of the orbital polarization ($ΔE_{xz}(k)$, $ΔE_{yz}(k)$) in the nematic phase, such as the sign reversal of the orbital splitting between $Γ$- and X,Y-points in FeSe, provides significant information. To solve this problem, we study the spontaneous symmetry breaking with respect to the orbital polarization and spin susceptibility self-consistently. In FeSe, due to the sign-reversing orbital order, the hole- and electron-pockets are elongated along the $k_y$- and $k_x$-axes respectively, consistently with experi…
In the 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 (Tc over 40K) can be achieved via electron doping, with the Fermi surface only comprising M-centered electron pockets. Here by utilizing surface potassium dosing, scanning tunneling microscopy/spectroscopy (STM/STS) and angle-resolved photoemission spectroscopy (ARPES), we studied the electronic structure and superconductivity of (Li0.8Fe0.2OH)Fe…
We reproduce the electronic properties of FeSe in the high-temperature phase within an ab initio framework that includes screened Fock exchange and local dynamical correlations. We robustly capture the experimental band structure, as long as the system is in the Hund's metal phase. In particular, we account for the shrinking of the Fermi pockets and the sinking below the Fermi level of the hole pocket with $xy$ orbital character. This entails the elusive correct estimate of the Sommerfeld coefficient, and supports the interpretation of non-compensated Fermi pockets seen in ARPES in terms of su…
We study the structure of the d-wave orbital order in FeSe in light of recent STM and ARPES data, which detect the shapes of hole and electron pockets in the nematic phase. The geometry of the pockets indicates that the sign of the orbital order $Γ= \langle d^{\dagger}_{xz} d_{xz}- d^{\dagger}_{yz} d_{yz}\rangle$ is different between hole and electron pockets $(Γ_h$ and $Γ_e$). We argue that this sign change cannot be reproduced if one solves for the orbital order within mean-field approximation, as the mean-field analysis yields either no orbital order, or order with the same sign of $Γ_e$ an…
The FeSe nematic phase has been the focus of recent research on iron based superconductors (IBSs) due to its unique properties. A number of electronic structure studies were performed to find the origin of the phase. However, such attempts came out with conflicting results and caused additional controversies. Here, we report results from angle resolved photoemission and X-ray absorption spectroscopy studies on FeSe with detwinning by a piezo stack. We have fully resolved band dispersions with orbital characters near the Brillouin zone corner which reveals absence of a Fermi pocket at the Y poi…
Superconductivity emerges in proximity to a nematic phase in most iron-based superconductors. It is therefore important to understand the impact of nematicity on the electronic structure. Orbital assignment and tracking across the nematic phase transition prove to be challenging due to the multiband nature of iron-based superconductors and twinning effects. Here, we report a detailed study of the electronic structure of fully detwinned FeSe across the nematic phase transition using angle-resolved photoemission spectroscopy. We clearly observe a nematicity-driven band reconstruction involving d…
The lifting of $d_{xz}$-$d_{yz}$ orbital degeneracy is often considered a hallmark of the nematic phase of Fe-based superconductors, including FeSe, but its origin is not yet understood. Here we report a high resolution Angle-Resolved Photoemission Spectroscopy study of single crystals of FeSe, accounting for the photon-energy dependence and making a detailed analysis of the temperature dependence. We find that the hole pocket undergoes a fourfold-symmetry-breaking distortion in the nematic phase below 90~K, but in contrast the changes to the electron pockets do not require fourfold symmetry-b…
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. Intrigu…
A rich variety of nematic/smectic orders in Fe-based superconductors is an important unsolved problem in strongly correlated electron systems. A unified understanding of these orders has been investigated for the last decade. In this article, we explain the $B_{1g}$ symmetry nematic transition in FeSe$_{1-x}$Te$_x$, the $B_{2g}$ symmetry nematicity in AFe$_2$As$_2$ (A=Cs, Rb), and the smectic state in BaFe$_2$As$_2$ based on the same framework. We investigate the quantum interference mechanism between spin fluctuations by developing the density wave equation. The observed rich variety of nemat…
The pairing glue of high-$T_{\rm c}$ superconductivity in heavily electron-doped (e-doped) FeSe, in which hole-pockets are absent, has been an important unsolved problem. Here, we focus on a heavily e-doped bulk superconductor Li$_{1-x}$Fe$_x$OHFeSe ($T_{\rm c} \sim 40$K). We construct a multiorbital model beyond the rigid band approximation and analyze the spin and orbital fluctuations by taking both vertex corrections (VCs) and self-energy into consideration. Without e-doping ($x=0$), the ferro-orbital order without magnetism in FeSe is reproduced by the VCs.The orbital order quickly disappe…
We use density functional theory to study the structure and the band structure of the monolayer FeSe deposited on the SrTiO$_3$ substrate with the additional layer of Se between them. Top of the SrTiO$_3$ is formed by the double TiO layer with and without oxygen vacancies. Several structures with different arrangements of the additional Se atoms above the double TiO layer is considered. Equilibrium structures were found and the band structures for them were obtained. Near the $Γ=(0,0,0)$ point of the Brillouin zone, the hole Fermi surface pockets persist and, additionally, an electron pocket a…
Iron selenide (FeSe) is an iron-based superconductor which shows unique properties, including strongly anisotropic superconducting gap, paramagnetism in undoped compound and extremely small Fermi pocket size. In this work, we demonstrate that the sizes of electron and hole pockets in FeSe monolayer become much smaller than those in bulk. The Fermi energy is in the order of a few meV and can be fine-tuned by the thickness of graphene layers underneath. Despite the low carrier density, the FeSe monolayers grown on trilayer or multi-layer graphene are superconducting. The superconducting gap size…
Magnetoresistivity \r{ho}xx and Hall resistivity \r{ho}xy in ultra high magnetic fields up to 88T are measured down to 0.15K to clarify the multiband electronic structure in high-quality single crystals of superconducting FeSe. At low temperatures and high fields we observe quantum oscillations in both resistivity and Hall effect, confirming the multiband Fermi surface with small volumes. We propose a novel and independent approach to identify the sign of corresponding cyclotron orbit in a compensated metal from magnetotransport measurements. The observed significant differences in the relativ…
An exact particle-hole transformation is discovered in a local-moment model for a single layer of heavily electron-doped FeSe. The model harbors hidden magnetic order between the iron d_xz and d_yz orbitals at the wavenumber (pi,pi). It potentially is tied to the magnetic resonances about the very same Neel ordering vector that have been recently discovered in intercalated FeSe. Upon electron doping, the local-moment model successfully accounts for the electron-pocket Fermi surfaces observed experimentally at the corner of the two-iron Brillouin zone in electron-doped FeSe, as well as for isot…
FeSe is argued as a superconductor in the Bardeen-Cooper-Schrieffer Bose-Einstein-condensation crossover regime where the superconducting-gap size and the superconducting transition temperature Tc are comparable to the Fermi energy. In this regime, vortex bound states should be well quantized and the preformed pairs above Tc may yield a pseudogap in the quasiparticle-excitation spectrum. We performed spectroscopic-imaging scanning tunneling microscopy to search for these features. We found Friedel-like oscillations near the vortex, which manifest the quantized levels, whereas the pseudogap was…
There is growing evidence that the superconducting semimetal FeSe ($T_c\sim8$ K) is in the crossover regime between weak-coupling Bardeen-Cooper-Schrieffer (BCS) and strong-coupling Bose-Einstein-condensate (BEC) limits. We report on longitudinal and transverse thermal conductivities, $κ_{xx}$ and $κ_{xy}$, respectively, in magnetic fields up to 20 T. The field dependences of $κ_{xx}$ and $κ_{xy}$ imply that a highly anisotropic small superconducting gap forms at the electron Fermi-surface pocket whereas a more isotropic and larger gap forms at the hole pocket. Below $\sim1.0$ K, both $κ_{xx}$…
We report the results of our in-depth analysis of spectroscopic and thermodynamic properties of a multi-orbital metal, like FeSe, which first develops a nematic order and then undergoes a transition into a superconducting state, which co-exists with nematicity. We analyze the angular dependence of the gap function and specific heat $C_V (T)$ of such nematic superconductor. We specifically address three issues: (i) angular dependence of the gap in light of the competition between nematicity-induced $s$-$d$ mixture and orbital transmutation of low-energy excitations in the nematic state, (ii) th…
FeSe is a unique member of the family of iron-based superconductors, not only because of the high values of $T_{c}$ in FeSe monolayer, but also because in bulk FeSe superconductivity emerges inside a nematic phase without competing with long-range magnetic order. Near $T_{c}$, superconducting order necessarily has $s+d$ symmetry, because nematic order couples linearly the $s$-wave and $d$-wave harmonics of the superconducting order parameter. Here we argue that the near-degeneracy between $s$-wave and $d$-wave pairing instabilities in FeSe, combined with the sign-change of the nematic order pa…
We use high-resolution angle-resolved photoemission spectroscopy to map the three-dimensional momentum dependence of the superconducting gap in FeSe. We find that on both the hole and electron Fermi surfaces, the magnitude of the gap follows the distribution of $d_{yz}$ orbital weight. Furthermore, we theoretically determine the momentum dependence of the superconducting gap by solving the linearized gap equation using a tight binding model which quantitatively describes both the experimental band dispersions and orbital characters. By considering a Fermi surface only including one electron po…
Iron chalcogenide superconductors are multi-band systems with strong electron correlations. Here we use angle-resolved photoemission spectroscopy to study band dependent correlation effects in single-layer FeSe/Nb:BaTiO3/KTaO3, a new iron chalcogenide superconductor with non-degenerate electron pockets and interface-enhanced superconductivity. The non-degeneracy of the electron bands helps to resolve the temperature dependent evolution of different bands. With increasing temperature, the single layer FeSe undergoes a band-selective localization, in which the coherent spectral weight of one ele…
The thermal conductivity of the iron-based superconductor FeSe was measured at temperatures down to 50 mK in magnetic fields up to 17 T. In zero magnetic field, the electronic residual linear term in the T = 0 limit, κ_0/T, is vanishingly small. Application of a magnetic field H causes no increase in κ_0/T initially. Those two observations show that there are no zero-energy quasiparticles that carry heat and therefore no nodes in the superconducting gap of FeSe. The full field dependence of κ_0/T has the classic shape of a two-band superconductor, such as MgB2: it rises exponentially at very l…
The origin of enhanced superconductivity over 50 K in the recently discovered FeSe monolayer films grown on SrTiO$_{3}$ (STO), as compared to 8 K in bulk FeSe, is intensely debated. As with the ferrochalcogenides A$_{x}$Fe$_{2-y}$Se$_{2}$ 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 uncle…
The origin of the 90 K nematic transition in the chalcogenide FeSe, which displays no magnetic order down to T=0, remains a major puzzle for a unifying theory for the iron-based superconductors. We analyze this problem in light of recent experimental data which reveal very small Fermi pockets in this material. We show that the smallness of the Fermi energy leads to a near-degeneracy between magnetic fluctuations and fluctuations in the charge-current density-wave channel. While the two fluctuation modes cooperate to promote the same preemptive Ising-nematic order, they compete for primary orde…
FeSe exhibits a novel ground state in which superconductivity coexists with a nematic order in the absence of any long-range magnetic order. Here we report an angle-resolved photoemission study on the superconducting gap structure in the nematic state of FeSe$_{0.93}$S$_{0.07}$, without the complication caused by Fermi surface reconstruction induced by magnetic order. We found that the superconducting gap shows a pronounced 2-fold anisotropy around the elliptical hole pocket near the Z point of the Brillouin zone, with gap minima at the endpoints of its major axis, while no detectable gap was …
Superconductivity in FeSe has recently attracted a great deal of attention because it emerges out of an electronic nematic state of elusive character. Here we study both the electronic normal state and the superconducting gap structure using heat-capacity measurements on high-quality single crystals. The specific-heat curve, from 0.4 K to Tc = 9.1 K, is found to be consistent with a recent gap determination using Bogoliubov quasiparticle interference [P. O. Sprau et al., Science 357, 75 (2017)], however only if nodes are introduced on either the electron or the hole Fermi-surface sheets. Our a…
Iron pnictides are the only known family of unconventional high-temperature superconductors besides cuprates. Until recently, it was widely accepted that superconductivity is spin-fluctuation driven and intimately related to their fermiology, specifically, hole and electron pockets separated by the same wave vector that characterizes the dominant spin fluctuations, and supporting order parameters (OP) of opposite signs. This picture was questioned after the discovery of a new family, based on the FeSe layers, either intercalated or in the monolayer form. The critical temperatures there reach ~…
The electronic structure and magnetism of a new magnetic intercalation compound (Li0.8Fe0.2)OHFeSe are investigated theoretically. The electronic structure calculations predict that the Fe in the (Li,Fe)OH intercalated layer is in +2 valence state, i.e. there is electron doping to the FeSe layer, resulting in the shrinking of the Fermi surface (FS) pocket around Gamma and a strong suppression of dynamical spin susceptibility at M in comparison with the bulk FeSe compound. The ground state of the FeSe layer is a striped antiferromagnetic (SAFM) metal, while the (Li,Fe)OH layer displays a very w…
Following the discovery of the potentially very high temperature superconductivity in monolayer FeSe we investigate the doping effect of Se vacancies in these materials. We find that Se vacancies pull a vacancy centered orbital below the Fermi energy that absorbs most of the doped electrons. Furthermore we find that the disorder induced broadening causes an effective hole doping. The surprising net result is that in terms of the band structure Se vacancies behave like hole dopants rather than electron dopants. Our results exclude Se vacancies as the origin of the large electron pockets measure…
The large anisotropy in the electronic properties across a structural transition in several correlated systems has been identified as the key manifestation of electronic nematic order, breaking rotational symmetry. In this context, FeSe is attracting tremendous interest, since electronic nematicity develops over a wide range of temperatures, allowing accurate experimental investigation. Here we combine angle-resolved photoemission spectroscopy and theoretical calculations based on a realistic multi-orbital model to unveil the microscopic mechanism responsible for the evolution of the electroni…
We derive the effective action for the collective spin modes in iron-based superconductors. We show that, due to the orbital-selective nature of spin fluctuations, the magnetic and nematic instabilities are controlled by the degrees of orbital nesting between electron and hole pockets. Within a prototypical three-pockets model the hole-electron orbital mismatch is found to boost spin-nematic order. This explains the enhancement of the nematic order in FeSe as compared to 122 compounds, and its suppression under pressure, where the emergence of the second hole pocket compensates the orbital mis…
Single-layer FeSe film on SrTiO3(001) was recently found to be the champion of interfacial superconducting systems, with a much enhanced superconductivity than the bulk iron-based superconductors. Its superconducting mechanism is of great interest. Although the film has a simple Fermi surface topology, its pairing symmetry is unsettled. Here by using low-temperature scanning tunneling microscopy (STM), we systematically investigated the superconductivity of single-layer FeSe/SrTiO3(001) films. We observed fully gapped tunneling spectrum and magnetic vortex lattice in the film. Quasi-particle i…
Bulk FeSe is a special iron-based material in which superconductivity emerges inside a well-developed nematic phase. We present a microscopic model for this nematic superconducting state, which takes into account the mixing between $s-$wave and $d-$wave pairing channels and the changes in the orbital spectral weight promoted by the sign-changing nematic order parameter. We show that nematicity gives rise to a $\cos2θ$ variation of the pairing gap on the hole pocket that agrees with ARPES and STM data for experimentally-extracted Fermi surface parameters. We further argue that, in BCS theory, $…
FeSe is an intriguing iron-based superconductor. It presents an unusual nematic state without magnetism and can be tuned to increase the critical superconducting temperature. Recently it has been observed a noteworthy anisotropy of the superconducting gaps. Its explanation is intimately related to the understanding of the nematic transition itself. Here we show that the spin-nematic scenario driven by orbital-selective spin-fluctuations provides a simple scheme to understand both phenomena. The pairing mediated by anisotropic spin modes is not only orbital selective but also nematic, leading t…
We present a combined analysis of neutron scattering and photoemission measurements on superconducting FeSe(0.5)Te(0.5). The low-energy magnetic excitations disperse only in the direction transverse to the characteristic wave vector (1/2,0,0), whereas the electronic Fermi surface near (1/2,0,0) appears to consist of four incommensurate pockets. While the spin resonance occurs at an incommensurate wave vector compatible with nesting, neither spin-wave nor Fermi-surface-nesting models can describe the magnetic dispersion. We propose that a coupling of spin and orbital correlations is key to expl…
Unconventional symmetry breaking due to nonlocal order parameters has attracted considerable attention in many strongly correlated metals. Famous examples are the nematic order in Fe-based superconductors and the star-of-David charge density order in kagome metals. Such exotic symmetry breaking in metals is a central issue of modern condensed matter physics, while its theoretical foundation is still unclear in comparison with the well-established theory of superconductivity. To overcome this difficulty, here we introduce the "form factor" that generalizes the nonlocal order parameter into the …
In several Fe-based superconductors, slight $C_4$ symmetry breaking occurs at $T^*$, which is tens of Kelvin higher than the structural transition temperature $T_S$. In this "hidden" nematic state at $T_S<T<T^*$, the orthorhombicity is tiny [$ϕ=(a-b)/(a+b) \ll 0.1$%], but clear evidences of bulk phase transition have been accumulated. To explain this long-standing mystery, we propose the emergence of antiferro-bond (AFB) order with the antiferro wavevector ${\bf q}=(0,π)$ at $T=T^*$, by which the characteristic phenomena below $T^*$ are satisfactorily explained. This AFB order originates from …
Nematic phase intertwines closely with high-Tc superconductivity in iron-based superconductors. Its mechanism, which is closely related to the pairing mechanism of superconductivity, still remains controversial. Comprehensive characterization of how the electronic state reconstructs in the nematic phase is thus crucial. However, most experiments focus only on the reconstruction of band dispersions. Another important characteristic of electronic state, the spectral weight, has not been studied in details so far. Here, we studied the spectral weight transfer in the nematic phase of FeSe$_{0.9}$S…
We report the crystal structure, superconductivity and normal state properties of two iron-based materials, Li0.15(C3H10N2)0.32FeSe(P-4) and Lix(C3H10N2)0.32FeSe(P4/nmm, 0.25<x<0.4) with superconducting transition temperature from 40~46K. The determined crystal structures revealed a coupling between Li concentration and the oritation of 1,2-Diaminopropane molecules within the hyper expanded FeSe layers. Further fitting on resistivity in terms of the Lawence-Doniach model suggests the two superconductors belong to the quasi-two dimonsional system. With increasing temperature, a differences in c…
Here we report the electronic structure of FeS, a recently identified iron-based superconductor. Our high-resolution angle-resolved photoemission spectroscopy studies show two hole-like ($α$ and $β$) and two electron-like ($η$ and $δ$) Fermi pockets around the Brillouin zone center and corner, respectively, all of which exhibit moderate dispersion along $k_z$. However, a third hole-like band ($γ$) is not observed, which is expected around the zone center from band calculations and is common in iron-based superconductors. Since this band has the highest renormalization factor and is known to be…
The iron-based superconductors are characterized by multiple-orbital physics where all the five Fe 3$d$ orbitals get involved. The multiple-orbital nature gives rise to various novel phenomena like orbital-selective Mott transition, nematicity and orbital fluctuation that provide a new route for realizing superconductivity. The complexity of multiple-orbital also asks to disentangle the relationship between orbital, spin and nematicity, and to identify dominant orbital ingredients that dictate superconductivity. The bulk FeSe superconductor provides an ideal platform to address these issues be…
The observation of substantially enhanced superconductivity of single-layer FeSe films on SrTiO3 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 SrTiO3, and the indispensable non-adiabatic nature of such a coupling leads to the formation of dynamic interfacial polarons. Furthermore, the…
Mechanism of unconventional superconductivity in FeSe has been intensely scrutinized recently because of a variety of exotic properties unprecedented for other iron-based superconductors. A central unanswered question concerns the origin of the interaction that causes the nematic transition at $T_s=90\,K$ without accompanying magnetic order. Elucidating the nature of spin excitations in the normal state is a key to addressing this issue. Here we report, from inelastic neutron-scattering measurements in FeSe single crystals, that high-energy spin excitation spectra of FeSe exhibit characteristi…
We perform a theoretical study of the leading pairing instabilities and the associated superconducting gap functions within the spin-fluctuation mediated pairing scenario in the presence of spin-orbit coupling (SOC). Focussing on iron-based superconductors (FeSCs), our model Hamiltonian consists of a realistic density functional theory (DFT)-derived ten-band hopping term, spin-orbit coupling, and electron-electron interactions included via the multi-orbital Hubbard-Hund Hamiltonian. We perform an extensive parameter sweep and investigate different doping regimes including cases with only hole-…
We model a single layer of heavily electron-doped FeSe by spin-1/2 moments over a square lattice of iron atoms that include the 3d xz and 3d yz orbitals, at strong on-site Coulomb repulsion. Above half filling, we find emergent hole bands below the Fermi level at the center of the one-iron Brillouin zone in a half metal state characterized by hidden magnetic order and by electron-type Fermi surface pockets at wavenumbers that double the unit cell along the principal axes. "Replicas" of the emergent hole bands exist at lower energy in the two-iron Brillouin zone. Exact calculations with two mob…
The momentum dependence of the nematic order parameter is an important ingredient in the microscopic description of iron-based high-temperature superconductors. While recent reports on FeSe indicate that the nematic order parameter changes sign between electron and hole bands, detailed knowledge is still missing for other compounds. Combining angle-resolved photoemission spectroscopy (ARPES) with uniaxial strain tuning, we measure the nematic band splitting in both FeSe and BaFe$_2$As$_2$ without interference from either twinning or magnetic order. We find that the nematic order parameter exhi…
We present a detailed comparison of the electronic structure of BaFe2As2 in its paramagnetic and antiferromagnetic (AFM) phases, through angle-resolved photoemission studies. Using different experimental geometries, we resolve the full elliptic shape of the electron pockets, including parts of dxy symmetry along its major axis that are usually missing. This allows us to define precisely how the hole and electron pockets are nested and how the different orbitals evolve at the transition. We conclude that the imperfect nesting between hole and electron pockets explains rather well the formation …
The iron-based superconductors that contain FeAs layers as the fundamental building block in the crystal structures have been rationalized in the past using ideas based on the Fermi Surface nesting of hole and electron pockets when in the presence of weak Hubbard $U$ interactions. This approach seemed appropriate considering the small values of the magnetic moments in the parent compounds and the clear evidence based on photoemission experiments of the required electron and hole pockets. However, recent results in the context of alkali metal iron selenides, with generic chemical composition $A…
The missing hole packets near the Brillouin zone center render unique electronic structures to the heavily electron doped FeSe-based superconductors with Tc above 40 K. It challenges the existing scenario accounting for the nature of superconductivity in the iron-based family. Yet, one hurdle that has to be overcome is the materials complexity in the rather limited number of compounds. Here we report the growth of heavily electron doped Li-NH3 intercalated FeSe single crystals that are free of such complexities and allow access to the intrinsic superconducting properties. Our results show extr…
Since the discovery of high temperature superconductivity in iron pnictides in early 2008, many iron-based superconductors with different structures have been discovered, with the highest transition temperature to date being 57 K. By the end of 2010, another kind of new superconductor, the Fe-based chalcogenide K1-xFe2-ySe2 was discovered. A naive counting of the electrons in the system would lead to a conclusion that the system is heavily electron overdoped (~0.4 e/Fe). Band structure calculations further support this speculation and predict that the hole pockets which are found in the iron p…
