At the 20 14 and 10 National Science and Technology Awards Conference, the first prize of National Natural Science, which has been vacant for many years, was won by the iron-based superconducting research team. With the spread of news reports, the word "iron-based high temperature superconductivity" has been paid attention to again. Since 2008, when the research upsurge of iron-based high-temperature superconductivity started in the field of condensed matter physics, the scientific research of iron-based superconductivity has entered the sixth year, and tens of thousands of research papers on iron-based superconductivity have been published. Up to February 20 13, among the top 20 cited papers in the field of iron-based superconductivity research in the world, 9 papers are from China. Iron-based superconductivity is still one of the frontier sciences in the basic research of condensed matter physics, attracting the attention of many outstanding scientists in the world. Why is iron-based superconductivity so special? What kind of important influence does its discovery have on basic physics research? What role did China people play in the torrent of iron-based superconductivity?

This has to start with mysterious and weird superconductors. 19 1 1 On April 8th, 2008, Anis et al. of Leiden Laboratory in the Netherlands used liquid helium, the last gas they just liquefied, to study the resistance of metals at low temperature. When they cooled the metal mercury to 4.2 K (0 K corresponds to minus 273.2 degrees Celsius in the thermodynamic temperature scale, and 4.2 K corresponds to minus 269 degrees Celsius), they found that its resistance suddenly dropped. Anis called this physical phenomenon superconductivity, which means superconductivity. He won the Nobel Prize in Physics at 19 13. After the discovery of mercury, the first superconductor metal, it was found that many simple metals and their alloys were superconductors at low temperatures. 1933, German physicist meissner pointed out that superconductors are different from ideal metal conductors. In addition to zero resistance, they have another independent magical property-complete diamagnetism. Once the superconductor enters the superconducting state, it is like practicing a "golden bell and iron cloth shirt". The external magnetic field can't get in at all, and the magnetic induction intensity inside the material is zero. At the same time, zero resistance and diamagnetism are the double standards to judge superconductors. Only by these two superb skills can superconductors have a series of high-voltage application prospects. Using superconducting materials with zero resistance instead of conventional metal materials with resistance can save a lot of heat loss caused by power transmission; A superconducting generator, a transformer and an energy storage ring can be arranged; Strong magnetic field can be realized in a small space, so as to obtain high-resolution nuclear magnetic resonance imaging, study the physical properties under extreme conditions, develop safe and high-speed maglev trains and so on. However, the popularity of such a powerful superconductor in people's daily life is far less than that of semiconductors. This is because semiconductors can be used at room temperature, but superconductors often need a very low temperature environment (below their superconducting critical temperature), which generally depends on expensive liquid helium to maintain, which greatly increases the cost of superconducting applications. The key to solve this problem is to find superconductors with higher critical temperature, especially room temperature superconductors-this is the ultimate dream of all superconducting researchers.

In the process of exploring new superconductors, physicists also undertake another important scientific task-explaining why electrons can "walk unimpeded" in solid materials from the microscopic level. Many top intelligent physicists in the world, including Einstein, Bohr and Feynman, tried to accomplish this task, and most of them failed. After waiting for a long time of 46 years, the microscopic theory of conventional metal superconductivity was successfully established in the hands of three American physicists in 1957, and this theory was named BCS theory (Badin, Cooper and schrieffer) after them. According to BCS theory, in addition to the well-known Coulomb repulsion, there is also a weak attraction interaction for free electrons in conventional metal alloys. Because the atoms in solid materials always vibrate near the equilibrium position, the positively charged atoms formed by the nucleus and its internal electrons will attract and interact with the negatively charged electrons passing by. If two electrons move in opposite directions (opposite momentum), then their interaction with the surrounding atoms can be equivalent to a weak attraction interaction between them, just like two dancers throwing balls at each other on ice, which leads to the pairing of electrons in the material. Pairs of electrons are also called Cooper pairs. If all Cooper pairs keep in step in the process of motion, the paired electrons will cancel each other even if they are hindered by motion, so that the energy loss of the whole paired free electron group can be guaranteed to be zero, thus achieving zero resistance state. Although BCS theory solved the superconducting mechanism of conventional metal alloys so beautifully with the idea of "pairing electrons for nothing", its innovative and bold idea was not accepted by people until it was confirmed by experiments, and it was awarded the Nobel Prize in Physics in 1972. With the guidance of theory, it seems that superconductors with higher critical temperatures can also follow the diagram. However, excited experimental physicists only found 23.2 K superconductivity in germanium niobate alloy, and the road of superconductivity exploration has lasted for more than 60 years, just like a tortoise pacing, the road is long, with Xiu Yuan and Xiu Yuan. Where is the dawn? Once again, theoretical physicists mercilessly poured a large basin of cold water-under the framework of BCS theory, the critical temperature of all superconductors has a theoretical upper limit of 40 K, which is called McMillan limit.

Experimental physicists have been working hard. Little by little surprises constantly arouse everyone's excitement. Research shows that many simple materials in the periodic table are superconductors at low temperature, and some can be superconducting even if high pressure is needed. After these simple materials alloys are alloyed, the critical temperature will be higher, and they are collectively called "metal alloy superconductors". Electrons in some metal compounds can be superconducting even if they are "heavy", so they are classified as "heavy fermion superconductors". Compounds of C60 and alkali metals, and even some organic materials are superconductors, which are classified as "organic superconductors". What is more gratifying is that many metal oxides, such as titanium oxide, niobium oxide, bismuth oxide, ruthenium oxide and cobalt oxide, which are generally considered to have poor conductivity, are also superconductors. Superconducting, almost everywhere! Because "all roads lead to superconductivity", physicists have begun a bolder exploration to find possible superconductivity in copper oxide ceramic materials, which are usually regarded as insulators. From 1986, Dawn finally broke through the fog. Bernoz and Miao Lei, two engineers of IBM in Zurich, Switzerland, found that there may be 35 K superconductivity in the La-Ba-Cu-O system. Although the critical temperature has not yet exceeded 40 K, 35 K is a new record of the critical temperature of all superconductors at that time. For this, Bernoz and Miao Lei won the 1987 Nobel Prize in Physics. Thus began a battle to climb the peak of superconductivity, including many China people and China scientists. 1In February 1987, Zhu Jingwu and Wu Maokun of the University of Houston and Zhao Zhongxian of the Institute of Physics of the Chinese Academy of Sciences independently discovered that there was a critical temperature above 90 K in the YBCO system, and superconducting research successfully broke through the liquid nitrogen temperature zone (the boiling point of liquid nitrogen was 77 K) for the first time. Using cheaper liquid nitrogen will greatly reduce the application cost of superconductivity, making it possible for large-scale application and in-depth scientific research of superconductivity. Therefore, Zhao Zhongxian's research team won the first prize of National Natural Science Award 1989. In the next decade, the critical temperature record of superconductivity rose like a rocket. At present, the superconductor with the highest critical temperature in the world is the mercury-barium-calcium-copper-oxygen system (at atmospheric pressure 135 K, at high pressure 164 K), which was created by Zhu Jingwu's research group in 1994. Because the critical temperature of copper oxide superconductors far exceeds the McMillan limit of 40 K, they are collectively called "high temperature superconductors" (the high temperature here is actually only relative to the lower critical temperature of metal alloy superconductors). Soon, people also realized that copper oxide high temperature superconductors (or copper-based superconductors) could not be described by the traditional BCS superconducting microscopic theory. In order to obtain such a high critical temperature, it is not enough to form paired electrons only by atomic thermal vibration as an intermediate. In addition, it is found that heavy fermion superconductors, organic superconductors and some oxide superconductors cannot be described by BCS theory. Although the concept of electronic pairing still holds, there are many strange pairing methods, matching media and matching methods. These superconductors are also called unconventional superconductors, which are different from the conventional metal alloy superconductors that can be described by BCS theory. In 200 1 year, Japanese scientists discovered 39 K superconductivity in magnesium diboride. Many electrons in this superconducting material participate in superconducting electron pairing, which is also called multi-band superconductor. Magnesium diboride is a conventional superconductor with the highest critical temperature, which is only one step away from the upper limit of 40 K, but it can't leave the limit of 40 K no matter how doped or pressurized.

Room temperature superconductivity has stagnated in the world record of 164 K, and it is no longer difficult to move up half a step. When people try to popularize the application technology of high temperature superconducting high voltage in liquid nitrogen temperature area on a large scale, they find it is actually "ugly and useless". Copper oxide is a kind of ceramic material in essence, with weak mechanical properties, lack of flexibility and ductility, too small critical current density in physics, and it is easy to lose superconductivity and heat up quickly when carrying large current. Scientists have worked hard for more than 20 years. Although copper oxide superconducting coils have begun to enter the market, most of the applications of superconducting high voltage are still on conventional metal alloy superconductors. However, the weak current application of copper-based superconductivity is also developing rapidly. Superconducting quantum interferometer made of copper-based superconductivity is the most sensitive magnetic detection technology in the world at present, and superconducting microwave devices made of copper oxide superconducting films are moving towards commercialization and marketization. In the future, there may be a quantum computer with superconducting bits as the unit-a high-speed computer based on the principle of quantum mechanics. Because copper-based superconductors are the most special among unconventional superconductors, they also have very important basic research value. The microscopic mechanism of high temperature superconductivity has become one of the jewels in the crown of condensed matter physics. The challenge is far more difficult than expected. It is found that many novel physical phenomena in high-temperature superconductors may be beyond the understanding of the existing physical theory system, and the most troublesome thing is that there is a strong correlation effect between electrons in this kind of materials, which becomes a strong correlation system. After nearly 30 years of struggle, there are few research conclusions about copper-based superconductors, and more are full of controversy and confusion. Theoretically, it is almost an idiotic dream to guide the search for superconductors with higher critical temperature, while experimental physicists can only find a needle in a haystack by experience and feeling.

On March 1 -5, 2008, a group of China scientists who are active in the forefront of superconducting research gathered at the Institute of Physics, Chinese Academy of Sciences, where a "seminar on situation assessment of HTS mechanism research" was being held to discuss the lost future of HTS research and try to identify the breakthrough point of copper-based HTS research. At this time, the superconducting laboratory and the extreme condition laboratory in the same building have quietly walked in the forefront of superconducting research and reform. On February 23, 2008, Hideo Nishino's research group reported that there was 26 K superconductivity in the fluorine-doped La-Fe-As-O system. Chinese scientists synthesized this material as soon as they learned the news and studied its physical properties. Among them, researchers from the Institute of Physics of Chinese Academy of Sciences and the Chinese University of Science and Technology obtained a series of high-quality samples by rare earth substitution method, and were pleasantly surprised to find that their critical temperatures exceeded 40 K. After optimizing the synthesis method, a high critical temperature of 55 K could be obtained. A new generation of high-temperature superconductors-iron-based high-temperature superconductors was born. This time, it took less than three months from the discovery of new superconductors to the critical temperature breaking the Macmillan limit, and the new superconducting records were constantly updated almost in days. In the following years, new iron-based superconducting systems such as iron arsenide and iron selenide were continuously discovered. Typical precursors are La-Fe-As-O, Ba-Fe-As, Li-Fe-As, Fe-Se, etc. These materials can be doped in different ways at almost all atomic positions to obtain superconductivity. The number of iron-based superconducting family members is roughly estimated to be more than 3000 (many of which have yet to be discovered), and it is the largest superconducting family discovered so far. The discovery of iron-based high-temperature superconductors undoubtedly injected an unprecedented "cardiotonic agent" into the research of high-temperature superconductors that was almost depressed at that time, and the superconducting research that lasted for more than 100 years has since given off a new round of youthful vitality.

As the second largest family of high-temperature superconductors after copper-based superconductors, iron-based superconductors have richer physical properties and greater potential application value. It has a similar relationship with copper-based superconductor, and its crystal structure, magnetic structure and electronic phase diagram are very similar. But from the electronic structure, it belongs to a multi-band superconductor like magnesium diboride; Its matrix is more metallic, which is completely different from the insulating copper oxide matrix (copper oxide is metallic only after doping); The latest research results confirm that the concept of electron pairing is still applicable, which may be similar to copper-based superconductors in pairing medium, but closer to traditional metal superconductors in pairing mode; Generally speaking, iron-based superconductors are more like a bridge between copper-based superconductors and traditional metal superconductors. Through years of experience and technical accumulation in copper-based superconductivity research, the progress of iron-based superconductivity research has been greatly accelerated. The research results since the new century can almost rival the research results of copper-based superconductivity in the past 30 years, and even surpass the previous understanding of high-temperature superconductivity research in some aspects. With this bridge, the research road of HTS is no longer like a castle in the air, but a path to follow, and the mystery of HTS micro-mechanism is slowly revealed. In application, iron-based superconductors are easier to be processed into wires and strips because of their metallic characteristics, and the upper critical magnetic field/critical current they can carry is equivalent to or even superior to that of copper-based superconductors. Of course, the preparation of iron-based superconducting materials needs arsenic compounds and alkali metals or alkaline earth metals in most cases, which is very toxic and extremely sensitive to air, which puts forward higher requirements for the preparation technology and safety of materials. In the weak current application of superconductivity, iron-based superconductivity is still in its infancy, which is far from the mature weak current application of copper-based superconductivity. From the point of view of materials, iron-based superconductors are more flexible and changeable, which greatly expands the research space of high-temperature superconductors. Many experimental phenomena can also be compared in different systems, thus drawing a more universal conclusion. As mentioned above, superconductivity can be achieved by doping elements with different valence states or even the same valence state in almost any atomic position of iron-based parent materials, and the superconductivity of materials in different systems changes with the evolution of external pressure. More interestingly, Japanese scientists also found that parent materials soaked in various wines can also be superconducting. It's really "the meaning of drunkenness is not in wine, but also in superconductivity"! The discovery of iron-based superconductors has greatly inspired the confidence of the explorers of superconducting materials. As the Japanese scientist who discovered magnesium diboride said, "I believe that all materials in the world are likely to become superconductors. As long as enough carriers or external conditions such as strong pressure or low temperature are introduced, there is hope to achieve superconductivity!"

The discovery of high-temperature superconductivity in iron-containing compounds is a breakthrough in itself, because it is generally believed that iron ions are magnetic and will greatly destroy superconductivity. Unexpectedly, doping magnetic ions such as cobalt and nickel into the iron arsenide matrix will induce superconductivity. The discovery of iron-based superconductivity proves that magnetism and superconductivity can actually "coexist peacefully", and the discovery of new superconductors often breaks away from convention. The discovery of new superconductors requires opportunities, luck and long-term experience accumulation. Hideo Nishino of Japan originally did not study superconductivity. His research group has been trying to find transparent conductive oxide materials, and in 2006, it was unexpectedly found that there was about 3 K superconductivity in La-Fe-P-O materials. Later, he realized that superconductivity might also exist in La-Fe-As-O compounds, and they obtained a new superconductor of 26 K by doping fluorine. In fact, La-Fe-P-O material was discovered by German scientists as early as 1990, and similar iron phosphide, cobalt phosphide and ruthenium phosphide were also reported by the same research group in 1995. By 2000, the rare earth-iron arsenide with the same crystal structure was also successfully prepared. Similarly, Chinese scientists have successfully broken through the McMillan limit by using the substitution effect of rare earths. It is precisely because of their keen insight into superconducting research all the year round, as well as years of experimental equipment and talent reserve, that they can seize the important opportunity at the first time.