Nickel oxide: new hope for high-temperature superconductivity!

Although the critical temperature of La3Ni2O7 has just broken through the liquid nitrogen temperature zone and requires 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!

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

On July 12, 2023, Nature published a major achievement from Chinese scientists: the discovery of pressure-induced superconductivity at around 80 K in nickel oxides (Figure 1) [1]. After 36 years, scientists finally discovered the second family of unconventional superconductors that break through liquid nitrogen temperature (77 K) after copper oxides, igniting new hope for the research on the mechanism and application of high-temperature superconductivity!

Figure 1: Nature paper: Superconductivity at nearly 80 K under high pressure discovered in nickel oxides [1]

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The triple “ceiling” of superconductivity research

Since the Dutch physicist Kamerlingh Onnes discovered superconductivity in 1911, superconductivity research has become one of the most popular areas in the field of physics. Over the past century, people's in-depth exploration of superconductivity has not only continuously promoted the rapid development of materials science and the continuous progress of technological science, but also enabled us to 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 condensed matter physics research [2].

Superconducting materials have two magical properties: absolute zero resistance and complete anti-magnetism. Their essence is the macroscopic quantum condensation state of roaming electrons inside the material. Because of this, superconductivity is useful in almost all fields involving electricity and magnetism. For example, in strong electric and magnetic applications: lossless superconducting cables, high-efficiency superconducting current limiters, motors, energy storage systems, etc. High-field superconducting magnets are the core technology of controlled nuclear fusion, nuclear magnetic resonance functional imaging, high-energy particle accelerators, etc. They can also be used for high-speed maglev trains, magnetic induction heating smelting, sewage treatment, mineral processing, etc. In weak electric and magnetic fields: superconducting single-photon detectors and superconducting quantum interference devices are the guarantee of quantum precision measurement; superconducting microwave and terahertz devices can provide high-performance and high-security communications; superconducting high-frequency resonant cavities are the heart of particle accelerators; superconducting quantum bits are the basic units of high-speed quantum computer chips [3]. It can be said that in the next generation of scientific and technological revolution, superconducting materials will definitely be one of the well-deserved stars (Figure 2).

Figure 2: Some typical applications of superconducting materials

However, despite the huge potential for superconducting applications, we do not see superconducting household appliances everywhere in our daily lives. The application of superconductivity in power grid systems is limited to demonstration projects, and superconducting applications in basic science and cutting-edge technology are even more out of reach for ordinary people. The reason is that almost all of the thousands of superconducting materials discovered so far are "not very easy to use"! There are three main 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. They must achieve superconductivity at a sufficiently low temperature, a not too high magnetic field, and a not particularly large current density. Once a certain critical parameter is broken, the material may instantly change from zero resistance to a state of resistance, which of course is not easy to use. The latter two of the three critical parameters determine its scope of application scenarios, and the critical temperature is the biggest bottleneck of application, because low temperature means high refrigeration costs.

How low is the critical temperature of a superconductor? The superconducting temperature of the first discovered superconductor, metallic mercury, is 4.2 K, equivalent to about -269 °C, which is lower than the average surface temperature of Pluto. The highest superconducting temperature of a single metal under normal pressure is niobium, which is only 9 K (-264 °C) [4]. For this reason, scientists have been working hard to increase the critical temperature of superconducting materials in the 117 years of superconductivity research, among which the "triple ceiling" is the key breakthrough target.

The first ceiling is 40 K (-233 ℃), also known as the McMillan limit . In 1957, three American scientists, Bardeen, Cooper, and Schrieffer, proposed a microscopic theory of metal and alloy superconductors, which was later named the BCS theory after them [5]. The theory holds that electrons in metal materials can pair up with the help of energy quanta generated by atomic lattice vibrations, namely "phonons". The paired electrons further achieve phase coherence and condense into a macroscopic whole, far exceeding the scale of the atomic lattice, thereby achieving lossless current. Based on the BCS theory, Eliashberg proposed a superconducting critical temperature model based on strong electron-phonon coupling [6]. McMillan further simplified the relationship between the superconducting critical temperature and the electron-phonon coupling strength [7]. Anderson et al. further inferred that under the condition that the atomic lattice is not unstable, the superconducting critical temperature has an upper limit of 40 K [8], which was later called the "McMillan limit". The McMillan limit is actually only applicable to superconductors based on the electron-phonon coupling mechanism under normal pressure (also known as "conventional superconductors"). If high pressure is applied, the stability of the atomic lattice will be greatly improved, and it is entirely possible for the critical temperature of conventional superconductors to exceed 40 K. If superconductivity is not formed by the electron-phonon coupling mechanism, then it is naturally not limited to 40 K. These superconductors are collectively referred to as "unconventional superconductors." Interestingly, in the more than 70 years since the discovery of superconductivity, despite the discovery of a large number of normal-pressure superconductors, the McMillan limit has been like an indestructible curse, and this first "ceiling" has been difficult to break through (Figure 3) [3].

Figure 3: Critical temperature of conventional superconducting materials and the “McMillan limit” [3]

Superconductivity at 35 K was discovered in the La-Ba-Cu-O system[10]. Subsequently, in early 1987, the Zhao Zhongxian team from China and the Zhu Jingwu team from the United States discovered superconductivity at 93 K in the Y-Ba-Cu-O system[11,12]. The McMillan limit and liquid nitrogen temperature were simultaneously broken! Copper oxide materials are considered to be "high-temperature superconductors". They have multiple material systems, such as La, Bi, Y, Hg, Tl, etc., all of which are unconventional superconductors[13]. The highest superconducting temperature of copper oxides at normal pressure is the Hg-Ba-Ca-Cu-O system, which is 134 K, and can be further increased to 165 K under high pressure[14]. In 2008, the second high-temperature superconducting family - iron-based superconductors was discovered, mainly including several types of compounds such as Fe-As, Fe-Se and Fe-S[15]. Chinese scientists also discovered that iron-based superconducting materials can break through the McMillan limit. The highest superconducting temperature of Fe-As-based bulk can reach 55 K, and the superconducting temperature of FeSe single-layer film can reach 65 K, both of which are unconventional superconductors [16]. However, although the material system of the iron-based superconductor family is far more than that of copper oxides, the critical temperature of iron-based superconductors has not yet exceeded the liquid nitrogen temperature (Figure 4) [3].

Figure 4: Discovery time of iron-based superconductors and their critical temperature [3]

The third ceiling is room temperature, which is generally defined as 300 K (27 °C) in condensed matter physics . There is no doubt that if the superconducting critical temperature can break through room temperature, then there will be no refrigeration cost in practical applications, and the large-scale application of superconducting materials will also remove the biggest obstacle. However, ideals are full, but reality is very skinny. The record for the highest critical temperature of normal-pressure superconducting materials discovered so far is still the Hg-Ba-Ca-Cu-O system, which is 134 K. However, scientists have discovered through years of research that high pressure is one of the "magic weapons" to increase the superconducting critical temperature. For example, some non-metallic elements are not superconducting under normal pressure, but can become superconductors under high pressure [17]; and the superconducting temperature of existing metal elements can be further increased under pressure. Among them, the recently discovered scandium has a critical temperature of 36 K under high pressure, which is the highest among single-element superconductors [18]. Theory predicts that if hydrogen can be metallized under high pressure, it can rely on strong phonon vibrations and electroacoustic coupling to achieve

Superconductivity has been discovered in the CSH ternary system (such as the quartz crystal, etc.), but they all rely on high pressure conditions of millions of atmospheric pressures (above 100 GPa) [20]. Such harsh conditions obviously do not have much application value. In 2020, the Dias team in the United States claimed to have achieved 288 K "room temperature superconductivity" at 267 GPa in the CSH ternary system. Later, it could not withstand the widespread doubts of its peers and the paper was retracted at the end of 2022 [21]. In March 2023, the Dias team again claimed to have achieved 294 K "near-normal pressure room temperature superconductivity" at 1 GPa in the Lu-NH ternary system [22]. However, it was widely questioned by scientists. The so-called room temperature superconductivity observed is likely due to problems with experimental measurements and errors in data analysis [23-26]. Therefore, even with the use of high pressure as a weapon, the ceiling of room temperature superconductivity still exists, and normal pressure room temperature superconductivity is still the "holy grail" that has not been achieved in the field of superconductivity so far (Figure 5).

Figure 5: The exploration process of “room temperature superconductivity” in metal hydrides[20]

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Dilemma of high-temperature superconductor research

Since the only superconductor that can break through the liquid nitrogen temperature zone under normal pressure is copper oxide, can we understand its microscopic mechanism to help us find superconductors with higher temperatures? And can we achieve large-scale industrial applications due to the reduction of refrigeration costs?
The reality is rather pessimistic. The microscopic mechanism of unconventional superconducting materials, including copper oxides, iron-based superconductors and heavy fermion superconductors, is still a "hard nut to crack" in condensed matter physics. The difficulty lies in the complexity and variability of experimental phenomena, which even go beyond the existing theoretical framework. In particular, the so-called "strongly correlated electron" effect needs to be considered, that is, the interaction between electrons cannot be simply ignored or considered approximately, and magnetic and electrical interactions are equally important. For example, the energy gap function of conventional superconductors is generally an isotropic s-wave, but in copper oxide superconductors it is an anisotropic d-wave, which is completely different. The multi-material system of iron-based superconductors may be an important bridge to reveal the mechanism of high-temperature superconductivity, because the energy gap function of iron-based superconductors is mainly s± waves, which is between copper oxides and conventional superconductors, and the same is true in physical and chemical properties (Figure 6) [3]. The solution to the microscopic mechanism of high-temperature superconductivity must ultimately rely on the development and improvement of many-body quantum theory, that is, the so-called "new paradigm" of condensed matter physics.

Figure 6: Iron-based superconductors are a bridge between copper-based high-temperature superconductors and conventional superconductors [3]

So, what exactly limits the large-scale application of high-temperature superconducting materials? Not all copper oxide superconductors can break through 77 K. In fact, many systems are below 40 K. They are just called "high-temperature superconductors" because they belong to the copper oxide family. The only superconducting systems above 77 K are Bi, Y, Tl, and Hg. The latter two cannot be truly industrialized because Hg and Tl are both highly toxic elements, extremely sensitive to air, and have variable structural components. In this way, only Bi and Y are left. However, as transition metal oxides, they are naturally brittle, and it is impossible to directly prepare wires like metal alloys. Scientists have invented the powder sheath method, pulse deposition method, chemical plating method, etc., using the flexibility of metal sheaths and substrates to overcome this problem. However, the introduction of a method will inevitably bring more new problems, which makes everyone very anxious. More than 30 years have passed, and now the high-temperature superconducting tape of the ReBaCuO system has barely reached the standard of large-scale industrialization [27].

It is precisely because copper oxide superconducting materials are "good-looking but not practical" that scientists have been working hard to search for new high-temperature superconducting materials, and iron-based superconductors were discovered. The critical temperature of the Fe-Se and Fe-S families in iron-based superconductors is low, and the critical current density is not high, which makes them unsuitable for high-voltage applications. Although the Fe-As system can reach a critical temperature of 30-55 K, it also places more stringent requirements on the material preparation process because of the toxicity of As and the presence of alkali metals or alkaline earth metals such as Na, K, Ca, Sr, and Ba. The research on iron-based superconductor wire and strip is still in its early stages, and the current carrying capacity needs to be further improved. The production capacity is also limited to the hundred-meter level (Figure 7) [28].

Figure 7: Current carrying performance of different superconducting wires under high field conditions [3]

Figure 8: Superconducting systems and typical structures in transition metal compounds [3]

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Nickel oxide superconductors bring new hope

Figure 9: Schematic diagram of nickel oxide thin film superconductor and its electron pairing [38]
The idea of ​​searching for nickel-based superconductors is inspired by years of research on high-temperature superconductivity in copper oxides. People believe that if +1-valent Ni is realized in nickel oxides, it will be similar to the +2-valent copper electron configuration, and it is possible to find unconventional superconductivity.

The reproducibility of the samples was very poor. In addition, the critical temperature was not high. Nickel-based superconductors initially attracted the attention of many theorists, but few experimental teams in the world were willing to follow up in a timely manner. Later, people discovered that there was an "invisible hand" in the CaH2 reduction process, that is, the H element was likely to enter the material and effectively reduce the orbital coupling of Ni and Nd, realizing d-wave superconductivity. Superconductivity is only likely to occur under specific H content conditions (Figure 10) [39]. Although this is not related to metal hydride high-pressure superconductivity, it also has the same meaning. Nickel-based superconductors have similar d-wave pairing components in copper oxides, strong spin fluctuations and dispersion, similar Fermi surface structures, etc., so they are considered to be the best reference system for studying the microscopic mechanism of copper oxide superconductivity.

Figure 10. Reduction process and H ion state of nickel oxide superconducting film [37,39]

With the increase of pressure, it gradually transforms into a metallic state and is accompanied by a structural phase transition, forming a structure similar to the octahedron in copper oxide, but the details are different. The research team observed a resistance onset transition temperature of 78-80 K and a magnetic susceptibility drop temperature of 77 K, as well as the corresponding magnetic field suppression superconducting transition phenomenon and linear resistance behavior in the normal state (Figure 12). Theoretical analysis shows that the +2.5 valence of Ni ions plays a unique role. Its two different d orbitals affect the correlated electronic states in the c direction and the ab plane, respectively, to achieve unconventional superconductivity. From this point of view, nickel-based superconductors and multi-orbital iron-based superconductors have the same effect!

Superconductivity brings new hope - 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 in iron-based superconductors, scientists have already accumulated rich experience and profound understanding. With the help of nickel-based superconductors, the mystery of the high-temperature superconductivity mechanism will be solved faster.

Figure 13: More than 100 years of exploration of superconducting materials [3]

Indeed, in the history of superconducting research, surprises are always "unexpected" and "reasonable". Although the "triple ceiling" seems to be full of difficulties, no ceiling can stop scientists from exploring bravely (Figure 13). We believe that more new superconducting materials will appear in the future. They 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 have more physical mechanisms that have not yet been discovered.

I hope everyone can read more books on superconductivity and experience the eternal charm of superconductivity (Figure 14) [3]!

Figure 14. “Superconductivity’s Little Era”: The Past, Present and Future of Superconductivity [3]

References

[1] https://www.nature.com/articles/s41586-023-06408-7

[2] Zhou X et al., Nat. Rev. Phys. 2021, 3: 462.

[3] Luo Huiqian, The “Little Era” of Superconductivity: The Past, Present and Future of Superconductivity (Tsinghua University Press, 2022).

[4] van Delft D and Kes P. Physics Today 2010, 63(9): 38.

[5] Bardeen J, Cooper LN, Schrieffer J R. Phys. Rev. 1957, 106 (1): 162.ibid, 108 (5): 1175.

[6] Eliashberg G M. Sov. Phys. JETP,1960, 11(13):696.

[7] McMillan WL and Rowell J M. Phys. Rev. Lett.,1965, 14: 108.

[8] Anderson PW., National Academy of Sciences. Biographical Memoirs V.81. Washington, DC: The National Academies Press, 2002.

[9] Steglich F et al. Phys. Rev. Lett., 1979,43:1892.

[10] Bednorz JG and Müller K AZ Phys. B, 1986, 64: 189.

[11] Zhao Zhongxian et al. Science Bulletin, 1987, 32: 412-414.

[12] Wu MK et al. Phys. Rev. Lett., 1987, 58:908.

[13] Cava R JJ Am. Ceram. Soc., 2000, 83(1):5.

[14] Schrieffer JR, Brooks J S. Handbook of High-Temperature Superconductivity, Springer, 2007.

[15] Chen X. et al., Nat. Sci. Rev. 2014, 1: 371.

[16] Liu X. et al., J. Phys.: Condens. Matter 2015, 27:183201.

[17] Lorenz B and Chu C W. High Pressure Effects on Superconductivity, Frontiers in Superconducting Materials, AV Narlikar (Ed.), Springer Berlin Heidelberg 2005, p459.

[18] Ying J. et al., Phys. Rev. Lett. 2023, 130: 256002.

[19] Drozdov AP et al. Nature, 2015, 525:73.

[20] Zhong X et al., The Innovation 2022, 3(2): 100226.

[21] E. Snider et al., Nature 2020, 586: 373.

[22] N. Dasenbrock-Gammon et al., Nature 2023, 615: 244.

[23] https://www.nature.com/articles/s41586-023-06162-w

[24] Xing X. et al. arXiv: 2303.17587.

[25] Peng D. et al., arXiv: 2307.00201.

[26] Xie F. et al., Chin. Phys. Lett.2023, 40: 057401.

[27] MacManus-Driscoll JL and Wimbush SC Nat. Rev. Mater.2021, 6: 587.

[28] Hosono H et al. Mater. Today, 2018, 21: 278.

[29] Wu. W et al., Nat. Commun.2014, 5: 5508.

[30] Cheng JG et al., Phys. Rev. Lett. 2015, 114: 117001.

[31] Bao JK et al., Phys. Rev. X 2015, 5: 011013.

[32] Mu QG et al., Phys. Rev. B 2017, 96: 140504.

[33] Liu ZY et al., Phys. Rev. Lett.2022, 128: 187001.

[34] Yajima T. et al., J. Phys. Soc. Jpn. 2012, 81: 103706.

[35] Ortiz, BR et al., Phys. Rev. Materials 2019, 3:094407.

[36] Goodenough JB, Longo M., Crystal and solid state physics, Springer-Verlag, 1970.

[37] Li D. et al., Nature 2019, 572: 624.

[38] Gu Q, Wen H H., The Innovation, 2022, 3(1): 100202.

[39] Ding X. et al., Nature 2023, 615: 50.

[40] Liu Z. et al., Sci. China-Phys. Mech. Astron.2023, 66: 217411.

This article is supported by the Science Popularization China Starry Sky Project

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.

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