When the resistance of the conductor decreases with the temperature, i.e., when the temperature drops below the critical temperature Tc, the DC resistance suddenly disappears to zero (the resistance suddenly falls outside the minimum value of the measurement range of the instrument, the resistance can be considered to have fallen to zero), and the response of the repulsive magnetic field appears.

This phenomenon is called superconductivity, and metallic elements, alloys, and compounds with superconducting properties are called superconductors.

In 1911, the Dutch physicist Ones discovered superconductivity, and for more than a century, low-temperature superconductors such as niobium, aluminum, and niobium tri-tin have been discovered as an artificial electromagnetic material with antimagnetic, tunable, low-loss, and macroscopic quantum response properties.

In the early days, superconductors were widely used in many important fields such as strong magnets, superconducting quantum computers, and highly sensitive detectors, and nowadays, they also show bright market application prospects in the fields of storage, maglev trains, power transmission, and nuclear magnetic resonance.

And China has succeeded in mastering iron-based superconductivity technology, breaking the world record, and Japan has finally become a thing of the past.

Ancient and Modern: The Past and Present of Iron-Based Superconductors

In January 2017, Academician Zhao Zhongxian walked on the podium of the 2016 National Science and Technology Award Conference and was awarded the highest national science and technology award. "Iron-based high-temperature superconductors", "liquid nitrogen temperature region" and "55K iron-based superconductor transition temperature" seemed to come into the view of the general public for the first time.

It is hard to imagine that he has been sitting on the cold bench of superconductor research for fifty years.

Superconductivity is one of the greatest scientific discoveries of the 20th century. The contributions made by Zhongxian Zhao and his team are the independent discovery of high-temperature superconductors in the liquid nitrogen temperature region, as well as the discovery of a series of iron-based high-temperature superconductors above 50 K, and the creation of an iron-based high-temperature superconductor transition temperature of 55 K.

What kind of amazing breakthrough is this? Everything needs to be explained in detail from the development of superconductivity technology in China, and comparing the development process of superconductivity technology in China with that in China and abroad.

Superconductivity technology originated when Dutch scientist Mairin Onis used liquid nitrogen to study the electrical resistance of metals at low temperatures.

On that afternoon in April 1911, Onis was surprised to find that the temperature of metallic mercury dropped to 4.2 K - 0 K relative to minus 273.2 degrees Celsius in the thermodynamic temperature scale, and 4.2 K or minus 269 degrees Celsius - showing that the resistance value suddenly dropped beyond the minimum of the instrument's measurement range, a phenomenon in the physical sense of of resistance to zero.

Resistance down to zero? Onis was overjoyed and called this phenomenon "superconductivity". Onis himself was awarded the Nobel Prize in Physics in 1913, and the study of superconducting materials entered the limelight. Over the next hundred years, five Nobel Prizes were awarded to 10 scientists for their research on superconductivity.

Metallic mercury was the first superconductor material to be discovered, and more and more alloys and single metals were found to metamorphose into superconductors at ultra-low temperatures. 1911 to 1932, Sn, Pb, Nb and other metallic elemental superconducting materials were discovered; from 1932 to 1973, superconductivity was also found to exist in some alloy materials.

Onis' successor, the German physicist Meissner, pointed out that in addition to zero resistance, superconductors also have excellent resistance to magnetism. Experimental reports show that once single metals and alloys enter the superconducting state, they exhibit "complete resistance to magnetism" - the external magnetic field has no effect on them at all, and the magnetic induction strength inside the superconductor material plummets to zero.

The development so far, because in the superconductor no resistance, the current again through the superconductor will not occur heat loss phenomenon, can form a huge current in the wire, generate a super strong magnetic field of this working characteristic, superconductors have energy storage ring, magnetic levitation trains, superconducting generators, high-resolution nuclear magnetic resonance imaging devices and other application prospects.

When Zhao Zhongxian invites children to the China Science and Technology Museum for superconductivity science, they often ask with wide eyes and curiosity, "Grandpa, who invented the maglev train?"

Zhongxian Zhao always answered in a serious manner: "Superconductor is a macroscopic quantum phenomenon ......".

Superconducting magnetic levitation trains can achieve the goal of levitation by placing coils on the track hill, which can generate a huge repulsive force between the coils and the body. As the train is driven by the induction current generated by the cutting of the conductors on the line when the vehicle is in motion, the magnetic levitation train generates transverse guiding force over the curve and vertical levitation force according to this principle.

At the same time, superconducting transmission technology, as a new transmission technology developed by using high-density current-carrying capacity superconducting materials, a superconducting DC transmission line of about 800kV can have a transmission capacity of 16 million kW to 80 million Kw and a transmission current of 10kA~50kA, but the transmission loss is almost zero, which is a significant advantage.

Using 25 K to 30 K as the boundary, i.e., at this critical temperature limit, superconductors are classified as low-temperature superconductors and high-temperature superconductors.

Superconductors are good, but until now, they are still not as widely used as semiconductors. The fundamental reason for this is that superconductors - the low-temperature superconductors used mainly today - need to be used at very low temperatures, i.e., below -200 degrees Celsius as described above.

On the one hand, the creation of superconducting conditions requires the availability of complex deep cryogenic refrigeration technology, on the other hand, in the process of superconducting operation, when the superconducting state is transferred to the normal conduction state, the original non-lossy current will be sharply lost due to the sudden appearance of resistance, resulting in a temperature rise in a very short period of time, the safety of the high-power environment will be a problem.

Adequate refrigeration is also by no means easy, the creation and maintenance of this low-temperature environment depends on expensive liquid nitrogen, a set of cryogenic superconductor equipment operation and maintenance costs, will be a sky-high figure. Although the critical temperature of superconductors has been gradually increasing since their introduction, since 1911, more than 70 years have passed, the critical temperature of superconductivity is still stuck at the threshold of 23.2K.

In 1968, physicist McMillan proposed, on the basis of BCS superconductivity theory, that the transition temperature of superconductors in general cannot exceed 40 K. This is known as the "McMillan limit" in the physics community.

Over the years, countless physicists have challenged the limits of McMillan and "high-temperature superconductors" with superconducting temperatures exceeding 40K. Do superconductors above the liquid nitrogen temperature region really exist?

Yes, although the international physics community divides superconductors into high-temperature and low-temperature superconductors, until then, high-temperature superconductors, which break the 40 K limit of Macmillan, have been a myth.

"Iron horse glacier into the dream": a new world record! 55K iron-based superconductor transition temperature

The relative concept of electron pairing exists for high-temperature superconductors, i.e., superconductors whose critical temperature exceeds the McMillan limit, but their role is never solely due to electron-phonon interactions.

In the 1950s, superconductivity research took root in China, which was an arduous and far-sighted task for a developing country that had not yet established itself on the international stage. In 1976, he decided to conduct a research on "Exploring High Critical Temperature Superconductors".

Under the difficult conditions of scientific research, Zhao Zhongxian's team could only collect second-hand equipment from everywhere, and learn and build the equipment that was not available on the market, and now build the furnace and experimental equipment.

By 1986, the research progress of Zhongxian Zhao's team and several cutting-edge research groups in Europe and the United States almost tied - several major teams almost simultaneously obtained high-temperature superconductors above 40 K in the lanthanum-barium-copper-oxygen system, successfully breaking through the superconducting critical temperature of 40 K McMillan limit, while discovering signs of 70 K superconductivity, a great battle to climb to the pinnacle of superconductivity.

But who could have known that in 1986, when the International Business Machines Corporation Zurich Research Laboratory just published a report on the possible existence of 35K superconductivity in the lanthanum-barium-copper-oxygen system, the idea of exploring high-temperature superconductivity that had been floating in Zhao Zhongxian's mind for a long time was suddenly a flash of light.

He organized the team stationed in the laboratory overnight, sleepy in the chair, the laboratory table lying down for a nap, in the most tense moments, Zhao Zhongxian even did not close his eyes for 48 hours.

In February 1987, Zhongxian Zhao's team discovered the electrical conductivity of liquid nitrogen with a transition temperature of 92.8 K. Zhao and his co-workers independently discovered the high-temperature superconductor in the liquid nitrogen region, and at the same time announced to the international community that they had chosen to replace lanthanum with yttrium in a multiphase system based on a comprehensive consideration, and announced for the first time the elemental composition of their research results as "barium-yttrium-copper-oxygen". copper-oxygen".

The use of cheaper liquid nitrogen will greatly reduce the cost of superconductivity applications, which will make large-scale applications and in-depth scientific research on superconductivity a possibility. The research team of Zhongxian Zhao won the first prize of the National Natural Science of China in 1989 for the outstanding achievement of breaking through the liquid nitrogen temperature region for the first time in the field of superconductivity research.

Subsequently, Zhongxian Zhao's team was invited to give a talk at the American Physical Society, and as a featured speaker on the Beijing panel, Zhao spoke eloquently to a crowd of more than 3,000 people.

In the 1980s, few Chinese scientists were able to attend such a top international academic stage, and the conference, which could only accommodate more than 1100 people, was called the "rock festival of physics".

As one of the pioneers of high-temperature superconductivity research in China, Zhongxian Zhao has led China's high-temperature superconductivity research from its beginnings to its rapid development to the forefront of the world. China's high-temperature superconductivity exploration has an excellent scientific research foundation, both in terms of theoretical basis and technical level.

In 2008, after more than two decades of research, Zhongxian Zhao's team had formed a system for studying the physical mechanism of copper oxide high-temperature superconductors, and Zhao believed that high-temperature superconductivity could be achieved in a layered tetragonal system with multiple cooperative phenomena.

At this time, the research group of Japanese scientist Hideo Nishino reported a research result: the existence of superconductivity in the fluorine-doped lanthanum-iron-arsenic-oxygen system at 26 K. In 2001, Japanese scientists discovered that magnesium diboride exhibited superconducting properties near 39 K. For a while, Japanese superconductivity research gained fame.

After learning the news, Zhongxian Zhao's team, which has been following the international superconductivity research, confirmed the unlimited possibilities of the "laminar quadrilateral system with multiple cooperative phenomena".

Zhao Zhongxian chose to introduce light rare earths, a rare metal unique to China. By flexibly combining high-temperature and high-pressure material synthesis techniques, he was the first to realize a fluorine-doped praseodymium-oxo-iron-arsenic iron-based superconductor with a superconducting transition temperature of 52 K, which has long exceeded the McMillan limit.

But Zhongxian Zhao was still not satisfied with this, and in order to further confirm that iron-based superconductors are the second family of high-temperature superconductors after copper-based superconductors, he started the race again, and new superconductivity records are constantly being updated in days. Soon, Zhao's team synthesized fluorine-doped NdO-Fe-As compound above 51K and fluorine-doped SmO-Fe-As compound at 55K.

The world record for the critical temperature of this block of iron-based superconductors has been preserved to this day and no one can beat it. As the second largest family of high-temperature superconductors after copper-based superconductors, iron-based superconductors have richer physical properties and applications than copper-based superconductors, and their pairing is closer to that of conventional metal superconductors.

In January 2014, a team from the Institute of Physics of the Chinese Academy of Sciences and the University of Science and Technology of China, represented by Academicians Zhao Zhongxian, Wang Nanlin, Fang Zhong, Chen Xianhui, and Wen Haihu, won the first prize of the 2013 National Natural Science Award for their outstanding contributions to the discovery of iron-based high-temperature superconductors above 40 K and the study of some fundamental physical properties.

However, 20 major papers on iron-based superconductors had a total of 5145 SCI citations and 8 representative papers had a total of 3801 SCI citations before the declaration of the first prize of the National Natural Science of China.

Peter Hirschfeld, a professor at the University of Florida, says, "China has really entered the ranks of condensed matter physics powerhouses, and it should not surprise me, perhaps, that so many high-quality articles are coming from Beijing. "

When Zhongxian Zhao's team's work on iron-based superconductors was named the European Physical Society's "Best of 2008" and the American Physical Society's "Major Events in Physics 2008," Stanford University professor Steven Kivelson said to himself. Kivelson said, "What is striking is not that these intentional scientific results came from China, but, importantly, that they did not come from the United States."

The temporary bow of appreciation from the former leader of high-temperature superconductivity research in the United States affirms the rapid development and outstanding achievements of China in this field of superconducting materials research.

After setting a record for the transition temperature of iron-based superconductors at 55 K, the Chinese superconducting community has continued to discover new iron-based superconducting systems, such as FeSe, BaFe2As2, and other systems in which new superconductivity has been discovered, and nowadays, there are more than 3000 members of the iron-based superconducting family.

At the same time, rather than relying on intuition and experience to discover superconducting materials, in recent years, the research of Zhongxian Zhao's team has plunged deeper into the mysterious field of superconductivity mechanism.

This reminds us of Zhao Zhongxian's speech at the China Science and Technology Museum, where he personally introduced science to young children, saying, "In the future, we have to find superconductors that can be used at room temperature without liquid nitrogen, are you willing to find them?" Despite their ignorance, the children all agreed with the graying Zhao Zhongxian: "I do."

Passing on the flame from generation to generation: Superconductivity technology sets another record

The development of science and technology is the process of the birth of a giant, but also the process of the giant wheel sailing. In the past, Zhao Zhongxian set a new world record in high-temperature superconductivity research, and then there are new talents shining in the field of superconducting wire.

In 2016, the Institute of Electrical Engineering of the Chinese Academy of Sciences announced that through research on the core technologies involved in the preparation of long wires, such as microstructure control and homogeneous processing of interfacial complexes, the institute's Ma Yanwei team successfully developed the international first 100-meter-scale superconducting long wire.

Previously, whether committed to iron-based superconductivity technology in the United States, Japan or European countries, its iron-based superconducting wire are meter-level stage, how to break through the 100-meter high-performance iron-based superconducting long wire preparation technology is the focus and difficulty of the large-scale application of iron-based superconducting technology.

As early as 2014, the Institute of Electrical Engineering of the Chinese Academy of Sciences used a continuous rolling process and successfully developed Sr0. 6K0.4Fe2As2 strips with a length of 11 m. The average value of the critical point Jc of 1.84×104A/cm2 (4.2K,10T) was obtained.

High-performance superconducting long wires are the basis for the practical application of iron-based superconducting materials. On the basis of obtaining high-performance samples, the preparation of high-performance, low-cost long wires is the necessary path to large-scale applications of new superconducting materials. The successful development of practical 100-meter long iron-based superconducting wires has increased the confidence in the practical application of iron-based superconducting materials.

As of November 2020, the iron-based superconductors that have been discovered in superconductivity can be broadly classified into five major categories: type 1111, type 122, type 111, type 11, and new structural superconductor types. Scientists have recently discovered that iron-based superconductors have lower anisotropy and higher upper critical field compared to low-temperature and copper-oxygen superconductors.

In order to manufacture superconducting cables and wind superconducting magnets on a large scale, high-performance strips or wires must be developed. For iron-based superconducting materials with brittle and high hardness, superconducting wires can only be prepared by two methods: the powder loading method and the coated conductor preparation technology.

From 2021 to 2022, type 1144 iron-based superconducting materials are formally moving towards applications. type 1144 iron-based superconductor AeAFe4As4 was discovered in 2016, the structure of the crystals of type 1144 iron-based superconducting materials is very similar to that of type 122, but because the arrangement of Ae and A layers is more regular than that of type 122 iron-based superconducting materials, it avoids the local region deviation in type 122. The outstanding problems such as the stoichiometric ratio in type 122 are avoided.

The 1144 iron-based superconductor is known as the most promising iron-based superconducting material for practical use other than 122. 1144 iron-based superconductors are higher quality and easier to prepare than 122 iron-based superconductors.

However, the main problem faced by the 1144 iron-based superconductors today is how to achieve high current-carrying characteristics in the line strip, 122-type superconducting wire has entered the 100-meter-long line scale preparation stage. 1144-type iron-based superconducting wire strip generally requires the use of powder loading method, the preparation of this high-performance superconducting wire, less than the search for a suitable processing process for the preparation of high-performance line strip material.

In the history of China's superconductivity research, China has indeed accumulated profound scientific research results on superconductivity, but looking at the international scene, China's superconductivity research is actually 50 years behind the international scene, and now, with 50 years behind, China is among the international leaders. As Zhongxian Zhao said, "If room-temperature superconductors are ever discovered, there should be Chinese people in this."

Reference.

1. Iron-based superconductivity:A supernova in physics; New Materials Industry; 07, 2017
2. The former life of iron-based superconductivity; Physics; 07, 2014
3. The world's first 100-meter long iron-based superconducting wire successfully developed"; Electronics World; 2016, No. 17

Title source: 'China becomes a leader in iron-based superconductivity industry'; People's Daily Online; Nov. 14, 2017