1. Brief Introduction to the Project

Superconductivity is one of the most important and attractive fields in condensed matter physics. High temperature superconductivity not only has potential application values for future technology but also is fundamentally important for basic sciences. The discovery and development of iron-based superconductors are the most important events in the superconductivity field since the discovery of the cuprate high temperature superconductors in 1986. The members of the present project, as representatives of Chinese scientists working in this field, made important and original contributions to this field, and have gained worldwide recognitions from the international community. Their pioneer work and achievements have greatly pushed the field to a new status and stimulated enormous and unprecedented enthusiasm in the field of superconductivity. The main achievements cover the following points:

1.1.Breaking the McMillan limit of conventional BCS theory

After the initial discovery of 26K superconductivity in Fe-based compound LaFeAsO1-xFx by Hosono’s group, the members of this project used the rare-earth elements Sm, Ce to replace the non-magnetic element La, and boosted the superconducting transition temperature above 40K at ambient pressure, which, for the first time, broke the McMillan limit of conventional BCS theory. Soon after, they promoted the superconducting transition temperature up to 52 K by replacing La with Pr. Those studies demonstrated that the iron-based superconductors are a novel type of high-temperature superconductors, and stimulated enormous and unprecedented enthusiasm in the field of superconductivity.

1.2. Fabrication of series of iron-based superconductors and achieving the record of highest Tc.

By employing high pressure-high temperature (HPHT) method, the members of this project fabricated a series of Fe-based superconductors ReFeAsO1-xFx (Re = Pr, Nd, Sm,Gd) with transition temperature above 50K. By introducing extra electron-type carriers through oxygen vacancies, they synthesized a series of oxygen-deficiency iron-based superconductors ReFeAsO1-x (Re = La, Ce, Pr, Nd, Sm, Gd, Tb, Ho, Y) by HPHT method. They achieved the transition temperature of 55 K, a record of the highest Tc up to now in Fe-based superconductors. Those studies confirmed that the iron-based superconductors are a new family of high temperature superconductors in addition to the copper oxide-based superconductors. By doping Sr into the La sites, they discovered the first hole-like charge carrier dominated iron-based superconductors La1-xSrxOFeAs. In addition, they also synthesized several other new iron-based superconductors, adding new structural types to the family of the iron-based superconductors.

1.3. Fundamental physical properties investigations and identification of unconventional superconductivity.

From a combined experimental and first-principle calculation study on LaFeAsO1-xFx with different F concentrations, the members of this project identified a spin density wave (SDW) order for the parent compound and revealed competition between superconductivity and SDW instability. In particular, they predicted a stripe-type antiferromagnetic order in SDW state, which was subsequently confirmed by the neutron diffraction experiment through an international collaboration. They have been the first of growing high quality single crystals in different families and carried out the very first transport, magnetic and spectroscopy measurements. Important findings were also achieved through worldwide collaborations. The reconstruction of the band structure and the gapping of the Fermi surfaces induced by SDW transition were revealed by the optical spectroscopy study. The phase diagrams of SmO1-xFxFeAs and Ba1-xKxFe2As2 were established, and the experimental evidence for the coexistence of antiferromagnetic SDW state and superconductivity in the underdoped regime was found. Significantly, they found a linear temperature dependence of the magnetic susceptibility and a strong isotope effect on both the superconducting and SDW transition temperatures. In addition, they identified the multiband effect in the iron based superconductors. Those studies established at the very beginning the important interplay effect between superconductivity and magnetism in Fe-based compounds, which laid the foundation for understanding the unconventional superconductivity in those systems.

The above contributions were well recognized by the international community. The 8 representative publications were cited by SCI journal over 3801 times, with the highest one 823 times. 20 selected papers were cited over 5145 times. The key members of the project have given over 160 invited talks in international conference/workshops. The internationally renowned academic journals, such as Science and Physics Today, have carried out relevant news reports, where the contributions from Chinese scientists were highly remarked. For example, the Science magazine published an article entitled “New superconductors propel Chinese physicists to forefront” where it was remarked that “The torrent of results from China also signals the country’s emergence as a power in condensed matter physics, many say.” (Science 320, 432 (2008)) The work on iron-based superconductors (the contributions described here represents an important part of it) was selected as the “the breakthrough of the year 2008” by the "Science" magazine. It was also selected as the “the physics major events of 2008” by the American Physical Society, “The best of 2008” by the European Physical Society, the "Top10 significant achievements of fundamental research in China". In addition, the key members of this project were awarded the “2009 Qiu Shi outstanding science & technology team achievement award” by the Hong Kong Qiu Shi Science & Technologies Foundation.

2、Primary Discoveries

Exploring and understanding novel superconductors are of great significance not only for potential applications but also for frontier research in condensed matter physics. Since late seventy last century, superconductivity has been discovered in many correlated electronic materials, including high-Tc cuprates, but the pairing mechanisms are not compatible to the conventional BCS theory. After decades of efforts, Chinese scientists have accumulated a solid base for the research on superconductivity and moved to the forefront in the field. In February 2008, after the initial breakthrough by a Japanese group on reporting the superconductivity of 26 K in Fe-based material LaFeAsO1-xFx (Kamihara et al. J. Am. Chem. Soc. 130, 3296(2008)), the researchers in this project, as representatives of Chinese scientists working on superconductivity, reacted immediately and made important and pioneering contributions to its development.

The conventional BCS theory predicts that the superconducting transition temperature would not exceed 40 K for usual solid materials, this limit has been called as the McMillan limit. The transition temperature of Fe-based superconductor discovered by the Japanese group is only 26 K, which is below this well-known limit. Based on the long time accumulation, the members of this project immediately realized the importance of this work, a superconductor containing element Fe. They quickly worked on this and related materials and made further breakthroughs in the development of Fe-based superconductors. They not only boosted the Tc well above the McMillan limit, proving that the iron-based superconductors belong to a new family of high Tc superconductors, but also pioneered in the exploration of the fundamental properties of those materials, in particular, in identifying a spin-density-wave order and its interplay with superconductivity. Their studies revealed that the Fe-based superconductors are a new family of unconventional superconductors. Their leading researches have gained worldwide recognition and triggered a new round of worldwide exploration and investigation of high Tc superconductors.

2.1. Iron-based superconductors break through the McMillan limit.

Subject category: Superconductivity 1405070; Low temperature physics 1405075.

Before the discovery of iron-based superconductor, there already exist several superconducting systems with the transition temperature above 25 K, for example, MgB2, Cs3C60, Ba1-xKxBiO3, (Na,K)xHfNCl. Only the cuprate system exhibits a superconducting transition higher than the McMillan limit, but the superconducting mechanism of the cuprates remains unresolved. It is thus very important to explore new superconductors with Tc above this limit. The members of the project boosted the Tc of iron-based superconductor up to 55 K, making it the only exception after the cuprates that has a transition temperature well beyond the McMillan limit.

2.1.1. Breaking McMillan limitation through discovery of superconductivity with 43K critical temperature in SmFeAsO1-xFx system.

By substitution of La with rare earth magnetic element Sm, superconductivity above 40K was found in SmFeAsO1-xFx under ambient pressure. Such high value of critical temperature breaks McMillan limitation and high-Tc superconductivity is thereby achieved in iron-based superconduct under ambient pressure. It is out of the traditional BCS theory scope that substitution of non-magnetic La ions with magnetic Sm ions could lead to a substantial increase of the superconducting critical temperature. This suggests that they are unconventional high-Tc superconductors.

2.1.2. Superconductivity at 41 K and Its Competition with Spin-Density-Wave Instability in CeFeAsO1-xFx.

By replacement of La with rare earth element Ce, a series of CeFeAsO1-xFx compounds with different F concentrations were fabricated. The superconducting transition temperature Tc was found to be as high as 41 K. This was one of two entirely independent studies on rare earth element substitutions at early stage which led to a significant increase of Tc. A competing phenomenon between spin-density-wave and superconductivity was found in CeFeAsO1-xFx from transport and optical spectroscopy measurement, which is very similar to that found for LaFeAsO1-xFx system. The close proximity of the superconductivity to the spin-density-wave instability suggests that the magnetic fluctuation played a key role in the superconducting pairing mechanism. The study also revealed that the Ce 4f electrons form local moments and ordered antiferromagnetically below 4 K, which might coexist with superconductivity. This coexistence implies that the coupling between the Ce 4f moments and the itinerant Fe 3d electrons is very weak.

2.1.3 The first iron-based superconductor with Tc over 50 K.

Through experimental observation of pressure effect that the F-doping can increase superconducting transition temperature in LaFeAsO, the member of the project judged that the Tc can still be improved further. Under the knowledge that high pressure, as one new dimensionality, will bring new chance, together with the accumulation of many years of scientific experiences, they chose to fabricate and explore new superconductors by using high pressure-high temperature (HPHT) method. Fluorine-doped PrOFeAs with a transition temperature of 52 K was hence successfully synthesized, which was the first iron-based superconductor with a transition temperature above 50K.

2.2. Achieving the record of highest Tc and identification of second family of high temperature superconductors.

Subject category: Superconductivity 1405070; Low temperature physics 1405075.

The members of this project carried out systematic exploration of the Fe-based superconductors. Series of Fe-based superconductors with Tc over 50 K were synthesized by employing HPHT method and a record Tc was achieved. Novel route through oxygen vacancies without F-doping to induce superconductivity was realized. In addition, they synthesized the first hole-like charge carrier dominated superconductor, and discovered many other FeAs-based materials and superconductors with new structures.

2.2.1 Synthesis of series of iron-based superconductors ReFeAsO1-xFx (Re=Pr,Nd ,Sm, Gd) with Tc over 50K by HPHT method and creation of the highest Tc record.

By employing the HPHT method, the Fluorine-doped PrOFeAs with Tc=52 K was firstly synthesized. Taking the advantage that the HPHT synthesis takes short time in sample preparation, they quickly discovered other 1111-phase superconductors with their transition temperature above 50K, for example, the fluorine-doped NdOFeAs with a transition temperature of 52K, the fluorine-doped SmOFeAs with a transition temperature of 55K, and the fluorine-doped GdOFeAs with a transition temperature of 54K. Among them, the 55 K superconductivity achieved in fluorine-doped SmOFeAs was repeatedly confirmed as a record of Tc for the iron-based superconductors.

The temperature dependence of resistivity for the undoped SmOFeAs and the SmO0.9F0.1FeAs superconductor.

The temperature dependence of the DC-susceptibility, and differential ZFC curve for the SmO0.9F0.1FeAs superconductor.

2.2.2. Synthesis of a series oxygen-deficient iron-based superconductors ReFeAsO1-x(Re=La and rare-earth elements)by HPHT synthesis and determination of the phase diagram between Tc and lattice parameters.

By analogy with the copper-oxide superconductors, it was realized that, since the fluorine-doping is to introduce electron-type charge carriers, the oxygen vacancies may have the same effect. Then, a series of iron-based 1111-phase superconductors with oxygen vacancies without fluorine-doping REFeAsO1-x (RE = La, Ce, Pr, Nd, Sm, Gd, Tb, Ho, Y) was successfully fabricated. The optimal doping level was found at x=0.15 for REFeAsO1-x superconductors containing different rare earth elements. This allows to establish the phase diagram of the critical temperature versus the structural parameters. For the system NdFeAsO1-x, through detailed analysis on the relationship among the superconducting transition temperature, lattice constants, and oxygen concentration, the effect of the minor structural change on the superconducting transition temperature was revealed. The result is consistent with the trend given by the first principles calculations and promotes the understanding of the origin of superconductivity.

2.2.3 Synthesis of the first hole-type Fe-based superconductor.

In the early days of the new iron era, the dominantly electron-type charge carriers to the charge conduction in LaFeAsO0.9F0.1-y was identified by Hall coefficient measurement. Realizing that extra holes could be generated by the substitution of La by Sr, the hole doped new superconductors (La1-xSrx)OFeAs was successfully synthesized for the first time. The maximum superconducting transition temperature at about 25 K was observed at a doping level of x = 0.13. The dominantly hole-like charge carriers were confirmed by Hall effect measurement. Before this work, it was concluded that superconductivity may only be achieved in electron doped samples. The discovery of superconductivity in the hole-doped side opens a new avenue for exploring novel superconductors.

2.2.4 Exploration and discovery of other iron based materials and superconductors.

In exploring the new superconductors in the iron based system, the members of the project continuously fabricated and discovered some new materials or superconductors. They fabricated the superconductor Sr1-xKxFe2As2 with Tc=38 K, and determined the SDW transition temperature of about 200 K. They also synthesized the new superconductor Ca1-xNaxFe2As2 with Tc=20K, a new layered superconductor (Sr2VO3)FeAs with Tc = 37.2 K, and a new parent phase (Sr3Sc2O5)Fe2As2. These discoveries enriched the structural family of the iron based superconductors.

2.3. Fundamental physical properties investigations and identification of unconventional superconductivity.

Subject category: Superconductivity 1405070; Low temperature physics 1405075.

Not only have the members of the project contributed significantly to the materials explorations, but also made pioneer contributions in the study of the fundamental properties of those materials, in particular, in identifying a spin-density-wave order, predicting the stripe antiferromagnetic structure, and revealing its competition with the superconductivity. They also established the electronic phase diagrams for 1111 and 122 systems and found the prominent effect of the isotope exchange on both the superconducting and SDW transition temperatures. The rather high superconducting transition temperatures (well beyond McMillan limit) and strong competition to the magnetism of Fe-based superconductors prove themselves as unconventional superconductors.

2.3.1 Identification of spin-density-wave order and prediction of the stripe-type antiferromagnetic structure.

An important piece of work with respect to the understanding of the novel iron-based superconductor was made in the very early stage (before the promotion of Tc over 40 K by the rare-earth element substitution for La). From a combined experimental and first-principle calculation study on LaFeAsO1-xFx with different F concentrations, a spin-density-wave order was identified for the parent compound LaFeAsO. The superconductivity was found to compete with the SDW instability. A stripe-type or collinear-type antiferromagnetic order was predicted based on the nesting of the disconnected hole and electron Fermi surfaces. The predicted stripe-type antiferromagnetic order was confirmed by the subsequent neutron diffraction experiment through an international collaboration. Soon after, very similar competing phenomenon was found in other systems, e.g. CeFeAsO1-xFx with higher Tc. The results suggested that the competing phenomenon is a generic property of those materials. The above studies laid the foundation for investigating the interplay between superconductivity and magnetism, the key issue in iron-based superconductors.

2.3.2 Establishment of phase diagram with coexistence and competition between spin density wave and superconductivity.

Electronic phase diagram is vital to understand the mechanism of superconductivity. The complete electronic phase diagrams were firstly exposed in SmFeAsO1-xFx and Ba1-xKxFe2As2 systems through systematic measurement in electronic transport, susceptibility, lattice structure and magnetic structure. This work revealed that spin density wave and superconductivity compete with each other, but can coexist in a certain underdoped region. Such coexistence was repeated and confirmed by so many international well-known research teams with different experimental methods. This finding makes an important step in experiment towards understanding the mechanism of superconductivty.

2.3.3 Growth of high-quality single crystals and fundamental physical properties investigations.

High quality single crystals are of fundamental importance for the characterizations of physical properties. The members in this project conducted the pioneer growth of high quality single crystals in many families of iron based superconductors. Using self-flux method and under the ambient pressure, they for the first time grew the NdFeAsO0.82F0.18 single crystals with the sizes of 50-100 micrometer. Using transport technique, they measured the first set of data that intrinsically tell the upper critical field and small anisotropy of the iron based superconductors. This work represented the first physical property measurement on single crystal samples of the Fe-based superconductors.

The large-size high-quality single crystals of parent and superconducting materials in 122 family were first grown by using self-flux method. This method has been widely used in the world. The intrinsic anisotropic transport and magnetic properties of the 122 single crystals were investigated. T-linear behavior of magnetization was observed above the SDW transition temperature, which was proved to be an universal characteristic in iron-based superconductors.

The charge dynamical properties of the parent compounds AFe2As2 (A=Ba, Sr) were investigated for the first time by optical spectroscopy measurement. Both compounds were found to be quite metallic with fairly large plasma frequencies at high temperature. Upon entering the SDW state, formation of partial energy gaps was clearly observed with the presence of surprisingly two different energy scales. Meanwhile, the carrier scattering rate was even more dramatically reduced. The measurement also revealed a temperature-induced spectral weight transfer from low energy to rather high energy (or a high-energy pseudogap-like feature), yielding evidence for prominent correlation effect in the compounds.

2.3.4 Discovery of considerable isotope effect in iron-based superconductors.

They have made isotope exchange on oxygen sites and iron sites in SmFeAsO1-xFx and Ba1-xKxFe2As2. The isotope effect on superconducting transition temperature and spin-density-wave temperature was studied. The isotope-effect exponent of iron was found to be 0.4 which is much larger than that of oxygen. This indicates that electron-phonon and lattice-spin interactions should be very strong in iron-based superconductor. The results offer crucial clue to understand the mechanism of superconductivity.

3. Principal Achievers

3.1 Zhao Zhongxian, born in 1941 in Liaoning Province. Based on the initial experimental result from his own      group that superconducting transition temperature Tc of LaFeAsO1-xFx increased with hydrostatic pressure, he proposed to use light rare earth element substitution and high pressure-high temperature (HPHT) method to synthesize new iron-based superconductors. He hence led his group and successfully fabricated the first iron-based superconductor with Tc above 50K [Ref.8]. Soon after, he and his group successfully synthesized almost all iron-based superconductors ReFeAsO1-xFx (Re=Pr, Nd, Sm, Gd) with Tc above 50K and achieved the record Tc of 55 K for Fe-based superconductors. Prof. Zhao’s group also fabricated, for the first time, a series fluorine-free and  oxygen-deficient iron-based superconductors ReFeAsO1-x (Re=La, Ce, Pr, Nd, Sm ,Gd and Tb, Ho, Y). The group obtained the phase diagram of Tc vs structure parameters for Re-1111 superconductors, and identified the chemical pressure effect on the superconductivity and spin density wave order.

3.2. Chen Xianhui, born in 1963 in Hunan Province. Along with his students, he discovered the superconductivity in F-doped SmFeAsO with Tc of 43 K boosted from 26 K by replacement of magnetic ion Sm3+ for non-magnetic ion La3+ under ambient pressure, which broke the McMillan limit of conventional BCS theory. These results indicated that ion-based superconductors are the second family of unconventional high-temperature superconductors, in addition to the copper oxide-based superconductors. He and his group played a leading role in studying the phase diagram, and first obtained the electronic phase diagrams of the SmFeAsO1-xFx and Ba1-xKxFe2As2 systems, respectively, and found that the antiferromagnetic spin-density-wave (SDW) and superconductivity could coexist in underdoped region [Ref.9,11]. They also discovered the novel superconductor Ca1-xNaxFe2As2 with Tc = 20 K, and first revealed a linear temperature dependence of the magnetic susceptibility above the SDW transition temperature, which is confirmed to be a common feature in Fe-based high-temperature superconductors later on. They first reported a remarkable isotopic effect on Tc and SDW transition.

3.3. Wang Nanlin, born in 1963 in Henan Province. Along with his group, he made a number of important contributions to the development and understanding of the Fe-based superconductors. They not only boosted the Tc beyond 40 K by replacing La with the rare earth element Ce, which is higher than the normally accepted McMillan limit of conventional BCS theory, but also pioneered in the exploration of the fundamental properties of those materials. He and his group did the first comprehensive characterization of the superconducting properties of the novel Fe-based superconductor, and identified, in collaboration with their theoretical colleagues, a spin-density-wave (SDW) order in the parent compound and revealed the competition between superconductivity and SDW instability. They also worked closely with a neutron group to confirm the proposed magnetic structure in SDW order state. His group also played a leading role in probing the charge dynamical properties of both parent and superconducting compounds of Fe-based systems, revealing the SDW energy gaps in parent compounds and isotropic pairing energy gaps in superconducting compounds by optical spectroscopy technique.

3.4. Wen Haihu, born in 1964 in Anhui Province. In early March 2008, together with his group, he fabricated the first hole-doped superconductor La1-xSrxFeAsO with Tc =25 K. Using Hall effect and other transport measurements, they established that the conduction is dominated by the hole like charge carriers in the samples. This work suggests to the community that the hole-doping can induce superconductivity, and thus the hole band is necessary for the underlying pairing mechanism. In May 2008, they were among the first to fabricate the single crystals of NdFeAsO1-xFx and make the contacts using the focused-ion-beam technique. With that they succeeded in measuring the first set of transport data on the single crystal samples and reported for the first time the low anisotropy (about 5). This result helps to check the band structure calculations and stimulate the enthusiasm for the potential applications for the low anisotropy. They also determined the upper critical field of FeAs-based compound at the early stage. In the effort of searching other new FeAs-based materials, they succeeded in finding a new parent material(Sr3Sc2O5)Fe2As2.Later they succeeded in finding a new superconductor (Sr2VO3)FeAs with Tc=37.2K.

3.5.Fang Zhong, born in 1970 in Hubei Province. Based on the first-principles calculations, he and his coworkers predicted theoretically the spin-density-wave state and the stripe-type antiferromagnetic spin structure in LaOFeAs compound, which was soon confirmed by the Neutron experiment. They pointed out that the spin-density-wave state would compete with the superconducting state. They also presented a doping-dependent phase diagram of LaOMAs (M=V – Cu) family by first-principles calculations.