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This article comes from the official account of Wechat: ID:fanpu2019, author: Luo Huiqian

Although the critical temperature of La3Ni2O7 has just broken through the liquid nitrogen temperature zone and needs the help of high pressure, this discovery undoubtedly brings new hope for high temperature superconductors-more superconductors or even high temperature superconductors are likely to appear in nickel-based materials.

Written by Luo Huiqian (Institute of Physics, Chinese Academy of Sciences)

On July 12, 2023, Nature released a major achievement from Chinese scientists: the discovery of pressure-induced superconductivity of about 80 K in nickel oxides (figure 1) [1]. After a lapse of 36 years, scientists have finally discovered the second kind of unconventional superconducting family that breaks through the liquid nitrogen temperature (77 K) after copper oxide, which ignites a new hope for the study of the mechanism and application of high temperature superconductivity.

Figure 1:Nature thesis: the phenomenon of superconductivity of nearly 80K found in nickel oxides under high pressure [1] 1. The triple "ceiling" of the study of superconductivity since the discovery of superconductivity by Dutch physicist Camelin Onis in 1911, the study of superconductivity has become one of the enduring directions in the field of physics. For more than a hundred years, the in-depth exploration of superconductivity has not only continuously promoted the rapid development of material science and the continuous progress of technical science, but also made us have a deeper understanding of various interactions in matter. In particular, the study of correlated quantum effects may give birth to a new paradigm for the study of condensed matter physics [2].

Superconducting materials have two magical properties: absolute zero resistance and complete diamagnetism, and their essence is the macroscopic quantum condensed state of itinerant electrons in the material. Because of this, superconductivity has the opportunity to display its talents in almost all fields involving electricity and magnetism. For example, in high-intensity magnetic applications, such as superconducting cables with or without loss, high-efficiency superconducting current limiter, motor, energy storage system, etc., high-field superconducting magnet is the core technology of controllable nuclear fusion, nuclear magnetic resonance functional imaging, high-energy particle accelerator and so on. it can also be used in high-speed maglev train, magnetic induction heating smelting, sewage treatment, mineral processing and so on. In the aspect of weak electricity and weak magnetism: superconducting single photon detector and superconducting quantum interferometer are the guarantee of quantum precision measurement; superconducting microwave and terahertz devices can provide high performance and high secret communication; superconducting high frequency resonant cavity is the heart of particle accelerator; superconducting qubit is the basic unit of high speed quantum computer chip [3]. It can be said that in the next generation of technological revolution, superconducting materials must be one of the well-deserved stars (figure 2).

Figure 2: some typical applications of superconducting materials however, despite the great potential of superconducting applications, superconducting household appliances can not be found everywhere in our daily life, and the application of superconducting in power grid systems is limited to demonstration projects. the application of superconductivity in basic science and cutting-edge technology is even more remote to ordinary people. The reason is that almost all of the thousands of superconducting materials found so far are "not very easy to use". There are three critical parameters that limit the application of superconductivity: critical temperature, critical magnetic field and critical current density. In other words, superconducting materials are not very ideal, and they can only achieve superconductivity at sufficiently low temperature, not too high magnetic field and not particularly high current density. Once a certain critical parameter is broken, the material may instantly change from zero resistance to resistance, which, of course, is not easy to use. The latter two of the three critical parameters determine the scope of its application scenario, and the critical temperature is the biggest bottleneck of the application, because low temperature means high refrigeration cost.

How low is the critical temperature of a superconductor? The first superconductor, metallic mercury, has a superconducting temperature of 4.2K, equivalent to about-269 ℃, which is lower than the average surface temperature of Pluto. The highest superconducting temperature at atmospheric pressure is niobium, which is only 9 K (- 264 ℃) [4]. For this reason, scientists have been trying to increase the critical temperature of superconducting materials during the 117 years of superconducting research, among which the "triple ceiling" is the key breakthrough goal.

The first heavy ceiling is 40K (- 233℃), also known as the McMillan limit. In 1957, three American scientists Bardeen, Cooper and Shriver put forward the microscopic theory of metal and alloy superconductors, which was later named BCS theory [5]. The theory holds that electrons in metal materials can be paired with the energy quantum produced by atomic lattice vibration-"phonons", and the paired electrons can further achieve phase coherence and condense into a macroscopic whole. far beyond the scale of the atomic lattice, so as to achieve lossless current. Based on BCS theory, Eliashberg proposed a superconducting critical temperature model based on strong electron-phonon coupling [6]. McMillan (Macmillan) further simplified the relationship between superconducting critical temperature and electron-phonon coupling strength [7]. Anderson et al further inferred that there is an upper limit of 40K for superconducting critical temperature when the atomic lattice is not unstable, which is later called "McMillan limit". In fact, the Macmillan limit is only applicable to superconductors (also known as "conventional superconductors") based on the electron-phonon coupling mechanism at atmospheric pressure. If high pressure is applied, the stability of the atomic lattice will be greatly improved. It is entirely possible that the critical temperature of conventional superconductors exceeds 40 K. If it is not the superconductivity formed by the electron-phonon coupling mechanism, then there is no need to be limited to 40K at all. These superconductors are collectively referred to as "unconventional superconductors". Interestingly, in the more than 70 years after the discovery of superconductivity, although a large number of atmospheric superconductors were found, the McMillan limit was like an unbreakable curse. this first "ceiling" has been difficult to break (figure 3) [3].

Figure 3: critical temperature of conventional superconductors and "McMillan limit" [3] the second heaviest ceiling is the boiling point of liquid nitrogen, that is, 77 K (- 196 ℃). The critical temperature of conventional superconductors is often very low, and the superconducting temperature of most metal alloys is below 20K under normal pressure. This means that in order to use superconducting materials, there must be a sufficiently low temperature environment. For example, the most widely used superconducting materials Nb-Ti and Nb3Sn need to be cooled by liquid helium. Helium is a rare gas and the mineral resources of helium in the world are extremely uneven, and the cooling cost is very high. If the critical temperature of superconductivity breaks through the boiling point of liquid nitrogen (77 K), it can be used in the temperature range of liquid nitrogen, and nitrogen, as the most abundant gas in nature, is one of the most economical choices as refrigerating medium. At atmospheric pressure, unconventional superconductors can only break through the liquid nitrogen temperature zone because they are not bound by the McMillan limit. The first unconventional superconductor discovered in history is the heavy fermion material CeCu2Si2, whose critical temperature is only 0.5K (1978) [9]. In 1986, Bednorz and M ü ller of Switzerland discovered superconductivity of 35 K in the La-Ba-Cu-O system [10]. In early 1987, the Zhao Zhongxian team from China and the Paul Chu team from the United States discovered the superconductivity of 93 K in the Y-Ba-Cu-O system, and the McMillan limit and liquid nitrogen temperature were broken at the same time. Copper oxide materials are considered as "high temperature superconductors". They all belong to unconventional superconductors with many material systems, such as La system, Bi system, Y system, Hg system, Tl system and so on. The highest superconducting temperature in copper oxide is Hg-Ba-Ca-Cu-O system at atmospheric pressure, which is 134K, which can be further raised to 165K [14] under high pressure. In 2008, the second family of high temperature superconductors-iron-based superconductors was discovered, mainly including Fe-As-based, Fe-Se-based and Fe-S-based compounds [15]. Chinese scientists have also found that iron-based superconductors can break through the McMillan limit, and the highest superconducting temperature of Fe-As-based blocks can reach 55K and the superconducting temperature of FeSe monolayer thin films can reach 65K, which are all unconventional superconductors. However, although the iron-based superconducting family has far more material systems than copper oxides, the critical temperature of iron-based superconductors has not exceeded the liquid nitrogen temperature so far (figure 4) [3].

Figure 4: discovery time and critical temperature of Fe-based superconductors [3] the third heavy ceiling is room temperature, which is generally defined as 300K (27 ℃) in condensed matter physics. There is no doubt that if the superconducting critical temperature can break through the room temperature, then there can be no cooling cost in practical application, and the large-scale application of superconducting materials will clear the biggest obstacle. However, the ideal is very plump, but the reality is very skinny. At present, the record of the highest critical temperature of atmospheric superconducting materials is still Hg-Ba-Ca-Cu-O system, that is, 134K. However, after years of research, scientists have found that high pressure is one of the "magic weapons" to increase the critical temperature of superconductivity. For example, if some non-metallic elements are not superconducting at atmospheric pressure, they can become superconductors under high pressure [17]. The existing metal elemental superconducting temperature can be further increased under pressure, among which scandium recently found that the critical temperature at high pressure is 36K, which is the highest for elemental superconductors [18]. The theory predicts that if hydrogen can be metallized under high pressure, it will be possible to achieve superconductivity at room temperature depending on strong phonon vibration and electro-acoustic coupling. In 2015, the high voltage superconductivity of 202 K was discovered in H3S, which opened the journey of exploring room temperature superconductivity of high pressure hydride [19]. Superconductivity was subsequently found in a series of metal hydrides (such as LaH10, YH6, ThH10, SnH12, CaH6, etc.), but all depend on the high pressure condition of million atmospheric pressure (more than 100GPa) [20]. Obviously, such harsh conditions will not have much application value. In 2020, the American Dias team claimed to achieve 288K "room temperature superconductivity" in the C-S-H ternary system at 267GPa. Later, it could not stand the widespread questioning of its peers, and the paper was withdrawn at the end of 2022 [21]. In March 2023, the Dias team once again claimed to achieve 294K "near-atmospheric room temperature superconductivity" under 1 GPa in the Lu-N-H ternary system [22]. However, scientists generally questioned that the so-called room temperature superconductivity observed was probably due to problems in experimental measurements and errors in data analysis [23-26]. Therefore, even with the sharp weapon of high pressure, the ceiling of room temperature superconductivity still exists, while atmospheric pressure room temperature superconductivity is still the "holy grail" that has not been won in the field of superconductivity (figure 5).

Figure 5: the exploration history of metal hydride "room temperature superconductivity" [20] 2. The dilemma of high temperature superconductivity since copper oxide is the only superconductor that can break through the liquid nitrogen temperature region at atmospheric pressure. so can we understand its microscopic mechanism and help us find higher-temperature superconductors? Is it possible to realize large-scale industrial application because of the reduction of refrigeration cost?

The truth is rather pessimistic. The microscopic mechanism of not only copper oxides, but also unconventional superconductors including iron-based superconductors and heavy fermion superconductors, is still an "old problem" in condensed matter physics. The difficulty is reflected in the complexity and variability of the experimental phenomena, even beyond the existing theoretical framework, especially the need to consider the so-called "strong correlation electron" effect, that is, the electron-electron interaction can not be simply ignored or approximately considered. magnetic and electrical interactions are equally important. For example, the energy gap function of a conventional superconductor is generally an isotropic s-wave, but when it comes to a copper oxide superconductor, it is an anisotropic d-wave. The multi-material system of iron-based superconductivity may be an important bridge to reveal the mechanism of high temperature superconductivity, because the energy gap function of iron-based superconductivity is mainly s ±wave, which is between copper oxide and conventional superconductor. the same is true in physical and chemical properties (figure 6) [3]. The solution of the microscopic mechanism of high temperature superconductivity must ultimately depend on the development and improvement of multibody quantum theory, that is, the so-called establishment of a "new paradigm" of condensed matter physics.

Figure 6: iron-based superconductors are the bridge between copper-based high-temperature superconductors and conventional superconductors [3] so what limits the large-scale application of high-temperature superconductors? Not all copper oxide superconductors can break 77 K, and even many systems are lower than 40 K, just because they belong to the copper oxide family and are collectively referred to as "high temperature superconductors". The superconducting systems above 77K are only Bi system, Y system, Tl system and Hg system. Because Hg and Tl are both highly toxic elements, they are extremely sensitive to air, and their structures and components are changeable, so they can not be applied industrially. In this way, only Bi system and Y system are left, but as transition metal oxides, they are naturally fragile, so it is impossible to directly prepare wires like metal alloys. Scientists have invented powder casing method, pulse deposition method, chemical coating method and so on, with the help of the flexibility of metal casing and substrate to overcome this problem. However, the introduction of a method will inevitably bring more new problems and make everyone worried. More than 30 years later, the high temperature superconducting tapes of ReBaCuO system barely reach the standard of scale industrialization [27].

It is precisely because copper oxide superconductors are "useless" that scientists keep trying to search for new high temperature superconductors, and iron-based superconductors have been discovered. The Fe-Se and Fe-S families of Fe-based superconductors have low critical temperature and low critical current density, so they are not suitable for high-power applications. Although Fe-As system can reach the critical temperature of 30-55 K, but also because of the toxicity of As, including Na, K, Ca, Sr, Ba and other alkali metals or alkaline earth metals, it puts forward more stringent requirements for the preparation process of materials. At present, the research of iron-based superconductor tapes is still in the initial stage, the current-carrying capacity needs to be further improved, and the production capacity is limited to the 100-meter level (figure 7) [28].

Figure 7: current-carrying performance of different superconductor tapes at high field [3] in high field applications, unconventional superconducting materials are the best candidates. Because the critical magnetic field of conventional superconductors such as Nb-Ti, Nb3Sn and MgB2 is not high (all < 25 T), the increase of magnetic field will quickly restrain the critical current density and critical temperature. Superconductors such as copper oxides and iron-based materials can maintain good current-carrying properties under high magnetic fields (< 40 T) (figure 7). Therefore, looking for more unconventional superconductors with high critical temperature is one of the ways for large-scale application of superconductors.

A simple idea is that in transition metal compounds, in addition to the unconventional superconductivity of copper-based and iron-based materials, is it possible for other element-based superconductors? Yes, you can! In 2014, the first Cr-based superconductor CrAs was discovered by Chinese scientists with a critical temperature of 2K (pressure of 8 kbar) [29]. The following year, the first Mn-based superconductor MnP was discovered by Chinese scientists with a critical temperature of 1K (pressure of 8 GPa) [30]. Subsequently, related superconductors such as K2Cr3As3, KCr3As3 and KMn6Bi5 were also discovered [31-33], while Ti-based superconductors were found in Ba1 − xNaxTi2Sb2O [34]. In 2022, a kind of V-based superconductor AV3Sb5 (A = K, Rb, Cs) with cage structure was discovered [35]. Co and Ni, located between Cu and Fe, are also looking forward to discovering unconventional superconducting families (figure 8).

Figure 8: superconducting systems and typical structures in transition metal compounds [3] 3. Nickel oxide superconductivity brings new hope as early as 80 years in the last century, when Bednorz and M ü ller were searching for superconductivity in oxides, they noticed SrFeO3 and LaNiO3, because they may have metal conductivity, not traditional insulators. Limited by the material preparation conditions at that time, they did not find the first iron-based or nickel-based superconductor, but switched to copper-based materials. In 2019, the American Hwang team achieved superconductivity of about 15 K in Nd0.8Sr0.2NiO2 thin film samples. The first nickel-based superconductor was framed as LnNiO2 (Ln is a rare earth element), one less than the LaNiO3 sought that year [37].

Figure 9: schematic diagram of nickel oxide thin film superconductors and their electron pairing [38] the idea of looking for nickel-based superconductors is the inspiration brought by years of research on copper oxide high temperature superconductors. It is believed that if + 1-valent Ni is realized in nickel oxides, it is similar to the arrangement of + 2-valent copper electrons, it is also possible to find unconventional superconductivity and even reproduce some complex electronic state behaviors such as d-wave energy gaps (figure 9) [38]. But the problem is that the structure of LnNiO2 is metastable, so it is difficult to obtain single crystal samples directly. Therefore, the researchers prepared the thin film samples from Nd0.8Sr0.2NiO3 and reduced the valence state of Ni with the help of CaH2. The perovskite structure which is not superconducting becomes the Nd0.8Sr0.2NiO2 superconductor with infinite layer structure. The secret of this "reduction process" has not been mastered by scientists in the field for a time, resulting in poor repeatability of the sample. Coupled with the fact that the critical temperature is not high, nickel-based superconductivity has attracted the attention of many theorists at first, but few international experimental teams are willing to follow up in time. Later, it was found that there is an "invisible hand" in the CaH2 reduction process, that is, H element may enter into the material, effectively reduce the orbital coupling between Ni and Nd, and achieve d-wave superconductivity, which is easy to appear only in a specific H content (Fig. 10) [39]. Although this has nothing to do with metal hydride high-pressure superconductivity, it also has the taste of the same goal. Nickel-based superconductors have similar d-wave pairing components in copper oxides, strong spin fluctuation and dispersion, and similar Fermi surface structure, so they are considered as the best reference system for the study of superconducting micro-mechanism of copper oxides.

Figure 10. The reduction process and H ion state of nickel oxide superconducting thin films Ni-based oxide thin films can further increase the critical temperature from 15 K to more than 30 K (pressure 12 GPa) under high pressure. However, single crystal or bulk superconductivity has not been realized in LnNiO2 system, and only a few research groups can obtain thin film superconducting samples. People rely on other nickel oxide materials under high pressure to find unconventional superconductivity, such as La2NiO4, La3Ni2O7, La4Ni3O10 and other structural systems. Among them, the average valence state of Ni in La3Ni2O7 system is + 2.5, which is different from the expected valence of + 1, and is not favored at first [40]. On July 12, 2023, Professor Wang Meng and his collaborators from the School of Physics, Sun Yat-sen University published a paper on Nature, announcing the discovery of high pressure-induced superconductivity of about 80 K (pressure 14 GPa) in La3Ni2O7 single crystal samples. Finally, a new breakthrough has been made in the study of nickel-based superconductors (figure 11) [1]! La3Ni2O7 material is an insulator at atmospheric pressure and low temperature. With the increase of pressure, it gradually changes to metal state and accompanied by a structural phase transition, forming a structure similar to octahedron in copper oxide, but the details are different. The team observed the resistance onset transition temperature from 78 to 80 K, and the magnetic susceptibility drop temperature at 77 K, as well as the corresponding magnetic field inhibiting the superconducting transition and the linear resistance behavior of the normal state (figure 12). Theoretical analysis shows that the + 2.5 valence of Ni ion plays a unique role. Its two different d orbitals affect the c direction and the associated electronic states in the ab plane respectively, thus realizing unconventional superconductivity. From this point of view, there are similarities between nickel-based superconductors and multi-track iron-based superconductors.

Figure 11. Nearly 80 K superconductivity induced by high voltage in La3Ni2O7 [1]

Figure 12. Structure and electronic phase diagram of La3Ni2O7 under high pressure [1] although the critical temperature of La3Ni2O7 has just broken through the liquid nitrogen temperature zone and needs the help of high pressure, this discovery undoubtedly brings new hope for high temperature superconductivity-more superconductors, even high temperature superconductors, are likely to appear in nickel-based materials. After 37 years of research in the field of copper oxide superconductors and 15 years of research on iron-based superconductors, scientists have accumulated rich experience and profound understanding. With the help of nickel-based superconductors, the process of solving the mystery of high temperature superconductivity will be accelerated.

Figure 13: more than a hundred years of exploration of superconducting materials [3] indeed, in the history of superconducting research, surprises are always "unexpected" and "reasonable". Although it is difficult to have a "triple ceiling", no ceiling can stop scientists from exploring bravely (figure 13). We believe that more new superconducting materials will emerge in the future, which may have the strength to break through the critical temperature ceiling again, or have comprehensive critical parameters that are more suitable for large-scale applications, or more undiscovered physical mechanisms.

I hope you can read more books on superconductivity and feel the eternal charm of superconductivity!

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