Tenth anniversary of the discovery of iron-based high temperature superconductors
编者按：
新型超导体的发现常常为物理研究带来科学突破的机遇。2008年发现的新一类高温超导体家族——铁基高温超导体，由于其独特的物理性质和在应用方面的潜在优势，激发了世界范围的研究热潮，是过去十年凝聚态物理研究的热点之一。
自铁基高温超导发现以来，中国科学家一直走在铁基高温超导研究的最前沿，不断取得重要成果，在国内外学术界产生了重大的影响。不仅发现并合成了多种新型超导材料，而且发展并利用各种实验测量手段对铁基高温超导进行了全方位的研究，极大加深了对高温超导机理的理解，推动了实验技术和高温超导应用领域的发展。
在铁基高温超导体发现十周年之际，本刊特组织专题，邀约国内相关学者对铁基高温超导体新材料、物理性质及机理以及应用等方面的发展现状和创新研究进行了介绍。希望通过对十年来铁基高温超导体研究进展的总结，可以进一步加速新超导体材料的探索和物理机理的理解，并促进铁基高温超导体的应用。
2018, 67 (20): 207102.
doi: 10.7498/aps.67.20181401
Abstract +
We perform an in-plane optical spectroscopy measurement on iron-based superconductor Li0.8Fe0.2ODFeSe single crystal. At room temperature, the low frequency optical conductivity shows an incoherent characteristic; the Drude component is absent. With temperature decreasing, the Drude component develops and narrows rapidly. A well-defined plasma edge is observed in reflectance spectrum at temperature below 100 K, indicating a dramatically reduced scattering rate. The spectral weight contributed from free carriers is even smaller than that of FeSe single crystal. A number of phonon modes are visible in the measured spectra. We also observe clear spectral change below 160 cm-1 at 10 K, associated with the formation of superconducting energy gap in the superconducting state. The energy scale of the superconducting gap is comparable to the value measured by angle-resolved photoemission spectroscopy technique. Like FeSe and other iron pnictides, a clear temperature-induced spectral weight transfer at high energy is observed for Li0.8Fe0.2ODFeSe, indicating the presence of strong correlation effect.
2018, 67 (20): 207401.
doi: 10.7498/aps.67.20181818
Abstract +
Since the discovery of iron-based superconductors in 2008, it has been a hot topic to research the pairing mechanism of superconductivity. Scanning tunneling microscopy (STM) can be used to detect the electronic information in nano-scale, hence, it is an important tool to do research on superconductivity. In recent 10 years, many valuable works have been carried out by STM in iron-based superconductors. In this paper, we try to make a brief introduction of the STM works in iron-based superconductors. Since the iron-based superconductors have multiple bands and superconducting gaps, the Fermi surface topology can change significantly among different materials. There are some evidences to prove a nodeless s-wave pairing in the optimally-doped iron-based superconductors with both electron and hole pockets by STM experiments. Furthermore, it has been demonstrated that FeSe-based materials with only electron pockets also have a sign-change order parameter, which provides a robust evidence for the unified picture of the electron pairing in iron-based superconductors. Besides, STM experiments provide fruitful information about the novel electronic properties including the electronic nematicity, shallow band effect, and possible topological superconductivity. Finally, we also give perspectives about the STM studies in iron based superconductors.
2018, 67 (20): 207407.
doi: 10.7498/aps.67.20181543
Abstract +
Like the superconductivities in other unconventional superconductors, high-temperature superconductivity in the iron pnictide often emerges after the static antiferromagnetic order has been suppressed, and is always accompanied by strong spin fluctuations. Therefore, understanding the magnetism and its origin could be an important premise for ascertaining the microscopic mechanism of iron-based superconductivity. Neutron scattering, as a powerful tool for studying magnetic ordering and spin dynamics in condensed matters, plays an essential role in understanding the relationship between magnetism and superconductivity in iron-based superconductors. In this paper, we review the neutron scattering results for iron pnictides, including static magnetic structures, magnetic phase transitions, spin excitations and electronic nematicity, and discuss their relationship with superconductivity.
2018, 67 (20): 207402.
doi: 10.7498/aps.67.20181256
Abstract +
With high transition temperature Tc (~38 K), high upper critical field Hc2 ( 100 T), superior transport Jc (~106 A/cm2) and extremely small anisotropy (1.5-2.0), the 122-type iron-based superconductors show great promise in high-field applications such as next-generation high energy physics accelerator and high-field magnetic resonance imaging (MRI). Power-in-tube (PIT) method is widely adopted to fabricate the iron-based superconducting wires and tapes due to low cost and easiness of large-scale fabrication. In the past few years, substantial efforts have been made to improve the transport performances of 122-type iron-based superconducting wires and tapes by ex-situ PIT technique. In this review, the recent progress of 122-type iron-based superconducting wires and tapes is presented. Firstly, we focus on the techniques for fabricating high-performance 122-type wires and tapes. We also discuss the key factors affecting the final performances of wires and tapes during the PIT process, including the preparation of high-quality precursor, the effect of chemical doping, the improvement of core density and grain connection. Recently, due to the improving of degree of c-axis texture and connectivity of grains, the transport Jc value of 122/Ag tapes reached 1.5105 A/cm2 at 4.2 K and 10 T, which exceeds the practical level of 105 A/cm2 and demonstrates their promise in high-field applications. Then, the progress of practical application of 122-type wires and tapes is summarized. In order to reduce the fabrication cost and improve the mechanical strengths of superconducting wires and tapes, an additional outer sheath such as Fe, Cu and stainless steel was used in combination with Ag. Besides, a favourable transport Jc was also obtained in the Cu-, or Fe-sheathed 122 tapes. For round wires, the highest Jc value reached 3.8104 A/cm2 in Cu/Ag composite sheathed wires at 4.2 K and 10 T, obtained by the hot-isostatic-press technology. From the viewpoint of practicality, the fabrication of multifilamentary wires and tapes is an indispensable step. The 7-, 19-and 114-filament 122 wires and tapes were successfully fabricated by the PIT method, and these multifilamentary tapes exhibited weak field dependence of Jc. Based on the experience of high-performance short samples and multifilamentary wires process, the scalable rolling process has been used to produce the first 115-m-long 7-filament Sr1-xKxFe2As2/Ag superconducting tape, confirming the great potential for large-scale manufacture. Moreover, the mechanical property, anisotropy and superconducting joint of 122 tapes are also studied. Finally, a perspective for the future development of 122-type wires and tapes in practical applications is given.
2018, 67 (20): 207403.
doi: 10.7498/aps.67.20181393
Abstract +
The 112-type (Ca, RE)FeAs2 (RE=rare earth) superconductors are very special among the iron-based superconductors for their particular crystal structures with arsenic chain configuration and attractive electronic phase diagram with the coexistence of superconductivity and antiferromagnetism upon carrier doping, while the chemical phases are absent for the low doping level or undoped parent compound. Here we report the single crystal growth method and physical characterizations for the newly discovered Eu 112 type parent compound EuFeAs2. The single crystal of EuFeAs2 is grown by high temperature solution method through using CsCl as the flux under the constant temperature of 800℃ with the molar ratio of the starting materials Eu:Fe:As:CsCl=1:1:4:18. The as-grown crystal is shinyplatelike piece with a typical size of 1 mm1 mm0.2 mm, and quite stable in air. The chemical composition of EuFeAs2 crystal is confirmed by energy-dispersive X-ray spectroscopy. The single crystal X-ray diffraction analysis at room temperature indicates that EuFeAs2 crystallizes into an orthorhombic crystal structure with the space group Imm2 (No. 44), and the refined lattice parameters are a=21.285(9) , b=3.9082(10) , c=3.9752(9) , which are different from those of the Ca 112 compound, but similar to those of unique zigzag As-As chain configuration presented in the layered crystal structure. Electrical resistivity measurements show three anomalies near 110 K, 98 K, and 46 K. The former two anomalies with relatively high temperature imply that the structural and antiferromagnetic transitions are related to Fe2+ sublattice, which is similar to other iron-based parent compounds. The low temperature anomaly at 46 K is attributed to the antiferromagnetic transition of Eu2+ sublattice, which is also confirmed by the corresponding transition observed in the direct current magnetic susceptibility measurement. The magnetic susceptibility of EuFeAs2 exhibits obvious anisotropy blow 46 K when the magnetic field is parallel or perpendicular to the bc plane, while the exact orientation of the Eu2+ moment needs further studying. The discovery of EuFeAs2 provides a new platform for further studying the unique crystal structure and electronic state phase diagrams in the 112-type iron-based superconducting family, and may shed new light on the correlations between superconductivity and magnetism.
2018, 67 (20): 207404.
doi: 10.7498/aps.67.20181319
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Among the iron-based superconductors, the structural simplest FeSe and its derived materials have received much attention in recent years due to the great tunability of the superconducting transition temperature (Tc). The relatively low Tc 8.5 K of FeSe can be raised to over 40 K via the interlayer intercalations such as AxFe2-ySe2 (A=K, Rb, Cs, Tl), Lix(NH3)yFe2Se2, and (Li1-xFex)OHFeSe. Although the monolayer FeSe/SrTiO3 is reported to have a Tc as high as 65 K, none of the Tc values of these FeSe-derived bulk materials has exceeded 50 K at ambient pressure so far. In order to explore other routes to further enhance Tc of FeSe-based materials, we recently performed the detailed high-pressure study of two intercalated FeSe high-Tc superconductors, namely (Li0.84Fe0.16)OHFe0.98Se and Li036(NH3)yFe2Se2, by using a cubic anvil cell apparatus. We find that the applied high pressure first suppresses the superconducting phase (denoted as SC-I) and then induces a second high-Tc superconducting phase (denoted as SC-Ⅱ) above a critical pressure Pc (~5 GPa for (Li0.84Fe0.16)OHFe0.98Se and 2 GPa for Li036(NH3)yFe2Se2). The highest Tc values in the SC-Ⅱ phases of these two compounds can reach~52 K and 55 K, respectively, the latter of which is the highest in the FeSe-based bulk materials, and is very close to the highest Tc of FeAs-based high-Tc superconductors. Our high-precision resistivity data of (Li0.84Fe0.16)OHFe0.98Se also uncover a sharp transition of the normal state from Fermi liquid for SC-I to non-Fermi liquid for SC-Ⅱ phase. In addition, the reemergence of high-Tc SC-Ⅱ phase under pressure is found to be accompanied with a concurrent enhancement of electron carrier density. Interestingly, we find a nearly parallel scaling behavior between Tc and the inverse Hall coefficient for the SC-Ⅱ phases of both (Li0.84Fe0.16)OHFe0.98Se and Li0.36(NH3)yFe2Se2. In the case without structural transition below 10 GPa, the observed enhancement of carrier density in SC-Ⅱ should be ascribed to an electronic origin presumably associated with pressure-induced Fermi surface reconstruction. Our work demonstrates that high pressure offers a distinctive means to further raise the maximum Tc values of intercalated FeSe-based materials.
2018, 67 (20): 207406.
doi: 10.7498/aps.67.20181355
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The key structural unit of iron-based superconductors (FeSCs) is the Fe2X2 (X refers to a pnictogen or a chalcogen element) layer which stacks alternately along the crystallographic c axis with other spacer layers. This structural feature makes it possible to find FeSCs via rational material design. In this paper, we first review the crystal structure of FeSCs along with the relevant progress. Then we summarize several rules for designing the intergrowth structures. The rules include the following points. 1) Lattice match between the intergrowth layers should be good enough. Quantitatively, the lattice mismatch, defined as =2(aA-aB)/(aA + aB), where aA and aB are respectively the lattice parameters of the two constituent compounds, should be no larger than~2%. 2) The charge transfer between the intergrowth layers is mostly essential, which acts as the glue that combines the constituent layers together. Such a charge transfer also induces the extra charge carriers in the superconducting key layer to give rise to superconductivity without extrinsic doping (so-called self doping). 3) For the structure with similar yet crystallographically distinct sites, one needs to avoid forming solid solutions. 4) Each intergrowth layer is preferably thermodynamically stable. 5) The designed structure can be preliminary evaluated with the hard and soft acids and bases conception and ab initio calculations. Following these empirical rules, we introduce and analyze five examples, namely, (Li0.8Fe0.2OH)FeSe, Ba2Ti2Fe4As4O, 42214-type Ln4Fe2As2Te1-xO4 (Ln=Pr, Sm, Gd), 1144-type AkAeFe4As4 (Ak=K, Rb, Cs; Ae=Ca, Sr, Eu), and 12442-type AkCa2Fe4As4F2 and AkLn2Fe4As4O2 (Ak=K, Rb, Cs; Ln=Nd-Ho). For the last 12442-type compounds, we also discuss the unusual relation between superconducting transition temperature and crystallographic parameters. We conclude that the structural-design approach may serve as an effective route, not only for discovering new FeSCs but also for exploring other relevant functional materials with similar crystal structures.
2018, 67 (20): 207408.
doi: 10.7498/aps.67.20181496
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Since the high-Tc superconductivity in iron-based superconductors was found in 2008, numerous new iron-based superconductors have been discovered. Of them, FeSe-based superconductors receive the most attention due to their unique properties. Here, we briefly introduce the structure and physical properties of two newly found FeSe-based superconductors, i.e. (Li, Fe) OHFeSe and (CTA)x FeSe. The former is synthesized by the hydrothermal method, while the latter is synthesized by electrochemical intercalation method. Moreover, we also introduce the tuning of electronic properties of FeSe by electric-double-layer and solid-ion-conductor based transistors.
2018, 67 (20): 207409.
doi: 10.7498/aps.67.20181651
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The discovery of Fe-based superconductor in 2018 opened an illustrious chapter in the history of high temperature superconductors. Over the past ten years, many progresses on experiments, theories and applications have been achieved in the studies of Fe-based superconductors, which have greatly enriched the basic knowledge on the superconductivity of high temperature (Tc) superconductors and laid a solid foundation for uncovering superconducting mechanism of high-Tc superconductors and expanding their applications. In this review article, we present some important progresses and new phenomena/physics exhibited in the pressurized Fe-based superconductors, including pressure-induced superconductivity, pressure-induced reemergence of superconductivity, pressure-enhanced superconducting temperature, the prediction on the highest superconducting temperature for Fe-based superconductors via high pressure studies, the effect of the separated phase structure on the superconductivity and the discovery of a bi-critical point between antiferromagnetic and superconducting phases. It is expected that these high pressure experimental results on Fe-based superconductors, together with the results reported in the same issue through other experimental and theoretical methods, can aid to outline a more complete physical picture for a more comprehensive and deeper understanding on Fe-based superconductors.
2018, 67 (20): 207411.
doi: 10.7498/aps.67.20181692
Abstract +
Since the discovery of high-temperature superconductivity in cuprates, finding more unconventional superconductors and understanding their superconducting pairing mechanism has been an important theme in condensed matter physics. Recently, ternary Cr-based superconductors A2Cr3As3 (A=K, Rb, Cs) and ACr3As3 (A=K, Rb) were reported, which own quasi-one-dimensional crystal structure, containing[(Cr3As3)-] linear chains. A2Cr3As3 belongs to P6m2 space group, and ACr3As3 crystallizes in a centrosymmetric structure with the space group P63/m. Many experiments, such as nuclear magnetic resonance, London penetration depth, show that A2Cr3As3 is an unconventional superconductor. However, these A2Cr3As3 compounds are extremely unstable in air. Here, we study the superconducting gap of the air-stable RbCr3As3 single crystal, using ultralow-temperature thermal conductivity measurement. The resistivity of RbCr3As3 single crystal shows a superconducting transition temperature Tczero at 6.6 K. The normal-state resistivity data from 20 K to 8 K are fitted to (T)=0 + AT2, which gives a residual resistivity of 0=781 cm. Then, the thermal conductivity of RbCr3As3 single crystal is measured at temperature down to 80 mK and in magnetic fields up to 9 T. In zero field, residual linear term 0/T=7.5 WK-2cm-1 is observed, which is about 24% of its normal-state value, suggesting nodes in the superconducting gap. At low field, the 0/T of RbCr3As3 shows a relatively faster field dependence than single-gap s-wave superconductors. These results reveal that RbCr3As3 is likely an unconventional superconductor with superconducting gap nodes, although the exact superconducting gap symmetry and structure for this quasi-one-dimensional superconductor needs further investigation.
2018, 67 (20): 207412.
doi: 10.7498/aps.67.20181701
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FeSe-based superconductors, as an important part of the family of iron-based superconducting materials, have attracted intensive research interest in the field of condensed matter physics. The exploration and preparation of such superconducting materials is the basis for studying their physical properties. At present, the exploration of FeSe-based superconducting materials mainly focuses on intercalated materials and epitaxial single-layer FeSe films. Among them, the intercalated FeSe-based superconducting materials have unique properties and are numerous in variety. This paper introduces a series of FeSe-based high-temperature superconducting materials discovered in recent years, covering KxFe2Se2, AxNH3FeSe, LiOHFeSe and organic molecular intercalation FeSe, etc., their properties and impacts are also briefly described.
2018, 67 (20): 207413.
doi: 10.7498/aps.67.20181768
Abstract +
Copper oxide superconductors and iron-based superconductors are two important families of high temperature superconductors. Their high-temperature superconductivity mechanism is a long-standing issue and still in hot debate in the field of condensed matter physics. The extensive and in-depth exploration of iron-based superconductors and their comparative study with copper oxide high-temperature superconductors are of great significance for the development of new quantum theory, the solution of high-temperature superconducting mechanism, the exploration of new superconductors and practical applications of superconductors. The macroscopic properties of materials are determined by their microscopic electronic structure. Revealing the microscopic electronic structure of high temperature superconductors is fundamental for understanding high temperature superconductivity. Angle-resolved photoelectron spectroscopy, due to its unique simultaneous energy, momentum and even spin resolving ability, has become the most direct and powerful experimental tool for detecting the microscopic electronic structure of materials, and has played an important role in the study of iron-based high-temperature superconductors. The revealing and discovery of the Fermi surface topology, superconducting energy gap and its symmetry, three-dimensionality, orbital selectivity, and electronic coupling mode in different iron-based superconductor systems provide an important basis for identifying and proposing new theory of iron-based superconductivity to solve high temperature superconductivity mechanism.
2018, 67 (20): 207414.
doi: 10.7498/aps.67.20181586
Abstract +
111-typed iron based superconductors have three members: LiFeAs, NaFeAs and LiFeP. The family of LiFeAs itself does not show any long range magnetic order but become superconductor without chemical doping. NaFeAs displays the separation of structural and magnetic transition, suitable to investigate the origin of the two transitions. LiFeP has been proved to be a nodal superconductor. The structure of 111 compounds consists of[FeAs/P] layers intercalated with two alkali metal layers, which makes single crystals easy to be cleaved into two equal counterparts with non-polar surface and thus is favored by the surface characterization techniques, such as the research of angleresolved photoemission experiment and scanning tunneling microscope measurement. Up to now, fruitful results have been achieved about the study of 111 family. In this paper, we summarize recent progresses on this family.
2018, 67 (20): 207410.
doi: 10.7498/aps.67.20181638
Abstract +
High-quality superconducting single crystals and thin films play an important role in the basic research and application of high-Tc superconductivity. In these two aspects, iron-based superconductors feature the merit of rich physical phenomena and high superconducting critical parameters (including the transition temperature Tc, the upper critical field Hc2 and the critical current density Jc). By developing ion-exchange and ion-de-intercalation method, we successfully synthesize a series of high-quality and sizable (Li,Fe)OHFeSe and FeSe single crystal samples. We observe Ising spin nematicity (below Tsn), and the universal linear relationship between Tc and Tsn in FeSe single crystals, indicating that the superconductivity is closely related to the spin nematicity driven by stripe antiferromagnetic spin fluctuations. In (Li,Fe)OHFeSe single crystals, we observe the coexistence of an AFM state (below Tafm~125 K) together with the SC state. We explain the coexistence by electronic phase separation, similar to that in high-Tc cuprates and iron arsenides, and establish a complete phase diagram for (Li,Fe)OHFeSe system. Here, we also make a brief introduction about our latest progress in growing a high-quality single-crystalline superconducting film of (Li,Fe)OHFeSe. The film is prepared by a hydrothermal epitaxial method. The high crystalline quality of the film is demonstrated by x-ray diffraction results, showing a single (001) orientation with a small crystal mosaic of 0.22 in terms of the full width at half maximum of the rocking curve, as well as an excellent in-plane orientation by the -scan of (101) plane. Its bulk superconducting transition temperature Tc of 42.4 K is characterized by both zero electrical resistance and diamagnetization measurements. Based on systematic magnetoresistance measurements, the values of upper critical field Hc2 are estimated at 79.5 T and 443 T for the magnetic field perpendicular and parallel to the ab plane, respectively. Moreover, a large critical current density Jc of a value over 0.5 MA/cm2 is achieved at~20 K. Such a (Li,Fe)OHFeSe film is not only important for the fundamental research for understanding the high-Tc mechanism, but also promises the high-Tc superconductivity applications, especially in high-performance electronic devices and large scientific facilities such as superconducting accelerator.
2018, 67 (20): 207101.
doi: 10.7498/aps.67.20181455
Abstract +
Iron-based superconductors and topological quantum states have been two important research frontiers in condensed matter physics in recent years. It is a very significant question whether the nontrivial topological phenomena can occur in iron-based superconductors. In this paper, the basic characteristics of the electronic structure of iron-based superconducting are analyzed from three aspects:crystal symmetry, effective model near the high symmetry points in Brillouin zone, and spin-orbit coupling interaction. On this basis, this paper focuses on how the nontrivial topological quantum states occur in the normal state, the states with long-range order near superconducting state and the superconducting state. Furthermore, the relevant theoretical models and results are introduced in detail, the related experimental progress is reviewed, and the development in this field is prospected.
2018, 67 (20): 207415.
doi: 10.7498/aps.67.20181681
Abstract +
We report on the observation of a superconducting gap of about 14-15 meV, significantly enlarged compared with the value of 2.2 meV for bulk FeSe, in monolayer FeSe film interfaced with MgO epitaxial on SrTiO3(001) substrate by using the scanning tunneling microscopy. While the MgO exhibits the same work function as SrTiO3 substrate, the gap magnitude is in coincidence with that of surface K-doped two-unit-cell FeSe film on SrTiO3(001), suggesting that the interface enhanced superconductivity might be attributed to cooperation of interface charge transfer driven by band bending with interface electron-phonon coupling as discovered at FeSe/TiO2 interfaces. On the other hand, the observation of such an enlarged superconducting gap, complementary to our previous transport observation of an onset superconducting transition temperature of 18 K in monolayer FeSe film on a bulk MgO substrate, implies that FeSe/MgO interface is likely to be a new interface high-temperature superconducting system, providing a new platform for investigating the mechanism of interface hightemperature superconductivity.
2018, 67 (20): 207405.
doi: 10.7498/aps.67.20181541
Abstract +
The similarities between the Fe-based superconductors and cuprate superconductors imply a possible unified picture of high temperature superconductivity. However, various chemical doping effects in Fe-based superconductors can lead to qualitatively similar phase diagrams that show diverse and complicated details, which pose great challenges of establishing a unified picture. Studying how chemical doping affects the electronic structure and superconductivity, and finding the real universal control parameter for superconductivity, are very important for establishing a unified picture and revealing the mechanism of high temperature superconductivity. In this article, we review a series of angle resolved photoemission studies on the chemical doping effect in Fe-based superconductors, involving both type I Fe-based superconductors with both electron and hole Fermi pockets, and type Ⅱ Fe-based superconductors with only electron Fermi pockets, and involving chemical doping of hetero-valent doping, isovalent doping, and chemical doping at different sites in unit cell. Comprehensive studies and analysis are conducted from various aspects of doping effects, including Fermi surfaces, impurity scattering, and electron correlation, and their roles in evolving the superconductivity. Electron correlation is found to be a universal electronic parameter behind the diverse phase diagrams of Fe-based superconductors, which naturally explains the qualitatively similar phase diagrams of various Fe-base superconductors despite of doping them in different ways. The electron correlation in Fe-based superconductors is closely related to both the carrier type of dopant and the lattice structure parameters, such as bond length. The different impurity scattering effects and different structures may affect the optimal Tc and thus leading to the diversity and complexity in the phase diagram. Fermi surface topology and its evolution with doping may play a secondary role in determining Tc. In order to enhance the Tc, one needs to optimize a moderate electronic correlation while minimizing the impurity scattering in the Fe-anion layer. Our results explain many puzzles and controversies and provide a new view for understanding the phase diagrams, resistivity behaviors, superconducting properties, etc. Our findings also strongly challenge the weak coupling theories based on the Fermi surface nesting, but favors the strong-coupling pairing scenario, where the competition between the electron kinetic energy and the local correlation interactions is a driving parameter of superconducting phase diagram. Like the t-J model of cuprates, in the picture of local antiferromagnetic exchange pairing, superconductivity appears in Fe-based superconductor when the electron correlation strength is at a moderate level. If the correlation is too weak, the system cannot exhibit superconductivity and remains metallic at low temperature. If the correlation is too strong, magnetic order appears in type I Fe-based superconductor, while type Ⅱ Fe-based superconductor shows a bandwidth-control correlated insulating state. The control parameter of the phase diagram is carrier doping for cuprates, but electron correlation strength for Fe-based superconductors. Our experimental results give a unified understanding of iron-based superconductors as a bandwidth-controlled system.
