A high-temperature superconductor with zero resistance that exhibits strange metal behavior

Researchers at Zhejiang University and Sun Yat-Sen University have collected evidence of high-temperature superconductivity with no resistance and strange metal behavior in a material they identified in previous studies.

Their results, published in Natural physicsunderline the potential of this material for studying these rare physical properties and their ultimate use in developing innovative electronic devices.

“High-temperature superconductivity is one of the most fascinating puzzles in the field of solid-state physics,” Prof. Huiqiu Yuan, the project leader of this work, told Phys.org.

“It has the potential to revolutionize technology by enabling the development of superconducting electronics cooled with liquid nitrogen (above -195.8 °C or 77.4 K). Consequently, the search for superconductors with high transition temperatures and the understanding of their mechanisms are among the most pressing goals in solid-state physics.”

High-temperature superconductors are highly sought-after materials because they could support the development of a new class of electronics. Therefore, evidence of high-temperature superconductivity often attracts a lot of attention, both from researchers and electronics companies.

In 2023, a research team from Sun Yat-Sen University led by Prof. Meng Weng reported signs of superconductivity at temperatures below 80 K in La₃No₂Ö₇a nickelate material. Since 80 K is above the boiling point of liquid nitrogen, this discovery was a breakthrough in itself, but it also brought with it some limitations.

“A significant drawback of Prof. Wang’s original discovery was the lack of zero resistance, the most striking feature of a superconductor,” said Prof. Yuan.

“One of the biggest challenges is that high-temperature superconductivity is only observed in this material when it is subjected to a pressure of at least 14 gigapascals (almost 140,000 atmospheres). Therefore, high-precision measurements and special high-pressure equipment are essential for further investigations.”

To investigate La₃No₂Ö₇ In addition, the researchers from Sun Yat-Sen University joined forces with Prof. Yuan’s group at the Center for Correlated Matter (CCM) at Zhejuang University, which specializes in the study of strongly correlated materials under extreme conditions.

Prof. Yuan realized that the absence of zero resistance in Prof. Wang’s original report was due to the inhomogeneities of the sample itself as well as the pressure conditions caused by the solid pressure medium of the KBr powder.

The researchers therefore set out to conduct a new study in which they investigate the properties of the material using a new sample and different pressure conditions. Specifically, Prof. Yuan suggested using tiny samples and better hydrostatic pressure conditions, as these elements could be the key to observing zero resistance.

“Incidentally, our team recently improved the high-pressure technique by applying a liquid pressure medium in a diamond anvil cell, which is suitable for generating homogeneous (hydrostatic) pressure, and using silver paste to make electrical contacts on a tiny sample of 100 mm in length,” explained Prof. Lin Jiao, one of the co-authors of the paper and a faculty member at the Center for Correlated Matter.

“When measuring the electrical resistance of La₃No₂Ö₇ At high pressures and using this method, we observed a strong drop in resistance when cooling below 66 K, indicating the onset of superconductivity.”

The researchers cooled the material further and found that its resistance reached zero below -40 K. Their experiment thus provided evidence that La₃No₂Ö₇ demonstrates high-temperature superconductivity under pressure. After publishing their work, the scientists are convinced that La₃No₂Ö₇ is truly a high-temperature superconductor.

“The biggest challenge in studying La₃No₂Ö₇ lies in its metastable chemical composition,” said Prof. Yuan.

“This suggests that there are numerous crystal defects, phase boundaries, interfaces and the coexistence of different compositions in La.₃No₂Ö₇ even on the micrometer scale. To address these problems, tiny single crystals (approximately 100*100*20 micrometers) were measured and, as mentioned above, quasi-hydrostatic pressure was applied.”

Essentially, Prof. Yuan and his colleagues placed a small piece of a single crystal of the material between the anvils of two diamonds and filled the seal embedded in these anvil surfaces with a liquid medium. They glued four to five Au wires, each about 15 microns thick, to the surface of the sample with silver paste to ensure good electrical contact.

The researchers then placed the sample under high pressure, which compressed it, and then cooled it to a few Kelvin. By measuring the resistance of the sample as a function of temperature and pressure, they were able to prove that it had no resistance.

“Our most significant and remarkable discovery is that the resistance of La₃No₂Ö₇ begins to drop sharply when cooling below 66 K and reaches zero at about 40 K,” said Prof. Yuan. “This experiment provides decisive and convincing evidence that La₃No₂Ö₇ is a high-temperature superconductor and, alongside cuprates and iron pnictides, is one of the unconventional high-temperature superconductors.”

In addition to collecting evidence for high-temperature superconductivity with zero resistance in La₃No₂Ö₇This recent study provides insight into the physics underlying this state. In fact, the researchers observed distinct strange metal behavior in their sample under pressure, revealing a connection between this behavior and superconductivity.

“The term ‘strange metal’ refers to materials, most of which exhibit unconventional superconductivity and/or a zero-temperature quantum phase transition when tuned by a non-thermal parameter that cannot be described by our current knowledge based on the Landau theorem,” said Prof. Yuan.

“This indicates a deviation from the conventional behavior of the charge carriers, which no longer seem to act simply as electrons. A typical feature of strange metals, as we observed in La₃No₂Ö₇is a linear resistance to temperature (T-linear resistance).”

As La₃No₂Ö₇ exhibits strange metal behavior, the mechanism underlying its superconductivity should be drastically different from that described in the Bardeen-Cooper-Schrieffer (BCS) theory, which explains the typical superconductivity of simple metals and alloys. Prof. Yuan and his colleagues believe that their results could also apply to other unconventional superconductors with strange metal behavior.

“We also found that the inverse Hall coefficient increases significantly when undergoing the pressure-induced structural phase transition. This suggests that the change in electronic structure in the high-pressure phase is crucial for the occurrence of superconductivity,” said Prof. Yuan.

This recent study by Prof. Yuan, Prof. Weng and their respective teams opens up new interesting possibilities for the study of high-temperature superconductivity and its application in electronics. The researchers plan to further explore the physics of La.₃No₂Ö₇while other unconventional superconductors are also being investigated.

“Our understanding of this new family of high-temperature superconductors is still in its infancy, and much work remains to be done,” said Prof. Yuan. “As this and other studies have shown, superconductivity in nickelates appears to be extremely sensitive to atomic composition, especially when there are too few oxygen atoms.”

Observed similarities between the superconductivity of various nickelates and that of other known families of high-temperature superconductors indicate the possibility that nickelates could also be superconductors at high temperatures, but possibly without the need for high pressures.

In their next studies, the researchers plan to identify other suitable candidate compounds that could help them uncover the key factors for superconductivity in terms of chemical composition and crystal structure.

“We [have] Improving sample quality of La₃No₂Ö₇ and we are looking for other related materials as this would enable further measurements to be made, including the order parameter, the relationship between superconductivity and structural phase transition and so on,” said Prof. Meng Wang.

“Recently, evidence of superconductivity has been found in other nickel compounds such as La₄No₃Ö₁₀was found. This not only expands the family of nickelate superconductors, but also provides a relatively stable compound for in-depth studies. However, improving sample quality and reducing the pressure required for superconducting transition remain priorities.”

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