High temperature superconductivity remains one of the most enduring mysteries in condensed matter physics, while also potentially enabling revolutionizing technologies, since it offers the tantalizing prospect of realizing superconducting electronics cooled by liquid nitrogen (above -195.8 °C or 77. 4 K), rather than using helium. The two known families of high-temperature superconductors that can operate at temperatures above 50 K without needing to apply pressure are the copper-oxide (cuprate) and iron-based superconductors. These high-temperature superconductors contain certain building blocks, that is specifically arranged atomic layers – formed from either copper-oxide (CuO₂) for the former family, or iron-pnictogen/chalcogen (e.g. FeAs/FeSe) for the latter. For a long time, scientists have sought to find new high temperature superconductors containing nickel instead of copper or iron. In 2019, superconductivity was found in a series known as the ‘infinite layer nickelates’, but only in thin-films several atoms thick rather than in bulk material form. Also, the transition temperatures (T_c) are only up to around 20 K [1], or 30 K under pressure [2],  still some way short of the copper and iron-based materials.

Last year, scientists from Sun Yat-Sen University made the exciting discovery of signatures of superconductivity in a bulk nickelate material La₃Ni₂O₇ at temperatures below 80 K [3]. The catch however, is that the high-temperature superconductivity is only seen when the material is compressed by at least 14 gigapascals of pressure (nearly 140,000 atmospheres). In order for the scientific community to be fully convinced that a material is a superconductor, several experimental observations are required, one of which is that high precision measurements must show that the electrical resistance drops to zero, which was not achieved in the initial study. The requirements of high pressure present additional experimental challenges, one of which being that it is difficult to ensure that the pressure applied is hydrostatic, such that the material feels the same pressure from all directions.

To tackle this problem, scientists from the Center for Correlated Matter at Zhejiang University measured La₃Ni₂O₇ samples from the group of Sun Yat-Sen University using a different high-pressure technique. By using a liquid medium to transmit the pressure, they were able to apply a much more hydrostatic high-pressure environment to very small samples (see left of Fig. 1) . When they measured the electrical resistance of the material at high pressures using this method, they found that the resistance begins to sharply drop upon cooling below 66 K, signaling that the material has become superconducting, and when the material is cooled further, the resistance smoothly reaches zero (see right of Fig. 1) [4]. This experiment confirms high-temperature superconductivity under pressure in La₃Ni₂O₇. By measuring the resistance under different pressures, they mapped out the phase diagram shown in Fig. 2.

In their pressure study, the team were able to make additional insights into the physics of the high-temperature superconductivity. In a typical metal, the charge carriers that transport an electrical current can be thought to behave very much like electrons, even though there are strong interactions between the carriers. The only difference is that they may behave a bit more sluggishly or nimbly, seemingly having a different mass to electrons moving in free space. If the metal then becomes a superconductor at low temperatures, these carriers feel an attraction to each other, and form so-called Cooper pairs that can travel unimpeded through a material.

In stark contrast however, the charge carriers of high-temperature superconductors seem to be fundamentally different from those of a typical metal. In particular, for electrons moving through a material, one expects that the electrical resistance R grows quadratically when the temperature T is increased (R∝T²). On the other hand, many different high temperature superconductors show a ‘strange metal’ phase when warming above the superconducting transition, where instead the resistance is proportional to temperature (R ∝T). Such a strange metal is also found in La₃Ni₂O₇ under pressure, as shown in Fig. 1, where the measured resistance follows a straight line when warming up. This suggests that the charge carriers no longer seem to simply behave like electrons, and as a result the superconducting fluid that forms below the transition should also be drastically different from typical superconductors.

 This study reveals a close association between strange metallicity and superconductivity in La₃Ni₂O₇. At the pressure where the superconducting transition temperature is highest, the strange metal behaviour is found over the widest temperature range, and when additional pressure is applied, both the transition temperature and linear rate-of-increase of the resistance are suppressed together. Similar associations have also been found in the copper and iron-based materials, highlighting a common thread between these and the new nickelate family of high temperature superconductors.

 Further information about this underlying fabric of charge carriers can be found from a different type of electrical transport measurement, namely measurements of the Hall resistance. These measurements make use of the Hall effect, whereby a magnetic field is applied to a material that has a flowing electrical current, which deflects the charge carriers sideways. This leads to a voltage across the material in a direction perpendicular to the current, and the size of this induced voltage corresponds to the number of charge carriers. From measurements of the Hall resistance under pressure, the scientists found that there is a pronounced increase in the number of charge carriers at pressures above 15 GPa, which is right around the threshold pressure at which superconductivity emerges (see Fig. 2). These findings suggest that this rearrangement of the electronic structure is an important precursor for superconductivity in La₃Ni₂O₇, which may provide key clues for finally revealing how the unconventional superconductivity occurs.

 Finally, our understanding of this new family of high temperature superconductors is still in its infancy, and there is still much to be done. As borne out in this and other studies, the superconductivity in nickelates appears to be extremely sensitive to the atomic composition, especially when there are deficiencies in the number of oxygen atoms. And there are also still questions about the exact structural arrangement that gives rise to the superconductivity. Most tantalizingly, the commonality between the superconductivity of these nickelates and the other two families of high-temperature superconductors leads to hopes that high temperature superconductivity can eventually be realized in nickel-based materials without the need to apply high pressures. Such materials exploration is currently being embarked on by many research teams around the world.

[1] D. F. Lee et al. Nature 572, 624–627 (2019).

[2] N. N. Wang et al. Nature Communications 13, 4367 (2022).

[3] H. L. Sun et al. Nature 621, 493–498 (2023).

[4] Y. N. Zhang et al. Nature Physics (2024). URL: https://www.nature.com/articles/s41567-024-02515-y