A high-temperature superconductor with no resistance that exhibits strange metallic behavior

Researchers from Zhejiang University and Sun Yat-Sen University have gathered evidence of high-temperature superconductivity without resistance and strange metallic behavior in a material identified in their previous studies.

Their findings, published in Natural physicshighlight the promise of this material to study these rare physical properties and ultimately use them to develop innovative electronic devices.

“Superconductivity at high temperatures is one of the most intriguing puzzles in the field of condensed matter physics,” Prof. Huiqiu Yuan, the project leader of this work, told Phys.org.

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

High-temperature superconductors are highly sought-after materials because they can support the development of a new class of electronics. As a result, any evidence of high-temperature superconductivity often attracts significant 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 La₃Ni₂O₇, a bulk nickel material. Because 80 K is above the boiling point of liquid nitrogen, this discovery was a breakthrough in itself, but it also came with some limitations.

“A major drawback of Prof. Wang’s initial discovery was the absence of zero resistance, the most distinguishing feature of a superconductor,” said Prof. Yuan.

“Although a major challenge is that high-temperature superconductivity in this material is only observed when exposed to a pressure of at least 14 gigapascals (nearly 140,000 atmospheres). Therefore, highly accurate measurements and specialized high-pressure equipment are essential for further research.”

To investigate La₃Ni₂O₇ Furthermore, the researchers from Sun Yat-Sen University collaborated with the group of Prof. Yuan at Zhejuang University’s Center for Correlated Matter (CCM), which specializes in the study of highly correlated materials under extreme conditions.

Prof. Yuan recognized that the absence of zero resistance in Prof. Wang’s first report could be attributed to the inhomogeneities of the sample itself, as well as pressure conditions caused by the solid pressure medium of KBr powder.

The researchers therefore wanted to conduct a new investigation into the properties of the material using a new sample and different pressure conditions. Specifically, Prof. Yuan suggested using small samples and better hydrostatic pressure conditions, as these elements could hold the key to observing zero drag.

“Coincidentally, our team has recently improved the high-pressure technique by applying a liquid pressure medium in a diamond anvil cell, suitable for creating homogeneous (hydrostatic) pressure, and by making electrical contacts on a small sample of 100 mm length using silver paste ,” Prof. Lin Jiao, one of the paper’s co-corresponding authors and a faculty member at the Center for Correlated Matter, explained.

“When measuring the electrical resistance of La₃Ni₂O₇ At high pressure, we observed a sharp drop in resistance with this method upon cooling below 66 K, indicating the onset of superconductivity.”

The researchers further cooled the material and found that its resistance reached zero below -40 K. Their experiment thus gathered evidence that La₃Ni₂O₇ exhibits superconductivity at high temperatures under pressure. After publishing their paper, the scientists have become confident that La₃Ni₂O₇ is truly a superconductor at high temperatures.

“The main challenge in studying La₃No₂O₇ lies in the metastable chemical composition,” said Prof. Yuan.

“This leads to the expectation of numerous crystal defects, phase boundaries, interfaces and the coexistence of different compositions within La₃Ni₂O₇ even on a micrometer scale. To address these issues, small single crystals (approximately 100*100*20 micrometers) were measured and quasi-hydrostatic pressure was applied as mentioned previously.”

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

The researchers then applied high pressure to the sample, compressing it and then cooling it to a few Kelvin. By measuring the resistance of the sample in combination with the temperature and pressure it was at, they demonstrated zero resistance.

“Our most significant and remarkable discovery is that the resistance of La₃Ni₂O₇ starts to drop sharply upon cooling below 66 K and reaches zero around 40 K,” said Prof. Yuan. “This experiment provides crucial and compelling evidence that La₃Ni₂O₇ is a high-temperature superconductor, joining the ranks of unconventional high-temperature superconductors alongside cuprates and iron pnictides.”

In addition to collecting evidence of high-temperature superconductivity without resistance in La₃Ni₂O₇this recent study provides insight into the physics underlying this condition. In fact, the researchers clearly observed strange metal behavior in their sample under pressure, revealing a link 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 to a non-thermal parameter, which cannot be described by our existing knowledge based on Landau’s theorem,” said Prof Yuan.

‘This suggests a deviation from the conventional behavior of charge carriers, which no longer appear to function simply as electrons. A typical feature of foreign metals, as we observed in La₃Ni₂O₇is a linear resistance versus temperature (T-linear resistance).”

Like La₃Ni₂O₇ exhibits strange metallic behavior, the mechanism underlying the superconductivity should differ drastically from that described by the Bardeen-Cooper-Schrieffer (BCS) theory, which explains the typical superconductivity of simple metals and alloys. Professor Yuan and his colleagues believe that their findings could also apply to other unconventional superconductors that exhibit strange metallic behavior.

“We also found that the inverse Hall coefficient undergoes a pronounced increase while passing through the pressure-induced structural phase transition, suggesting that the change of 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 continue investigating the physics of La₃Ni₂O₇while other unconventional superconductors are also being investigated.

“Our understanding of this new family of high-temperature superconductors is still in the early stages and there is still much work to be done,” said Prof. Yuan. “As shown in this and other studies, superconductivity in nickelates appears to be extremely sensitive to atomic composition, especially when there are deficiencies in the number of oxygen atoms.”

Observed similarities between the superconductivity of various nickelates and that of other reported families of superconductors at high temperatures 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 more suitable candidate compounds, allowing them to discover the key ingredients for superconductivity in terms of chemical composition and crystal structure.

“We [have] has improved the sample quality of La₃Ni₂O₇ and searching for other related materials, as this would allow more 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 has been found for superconductivity in other nickelate compounds, such as La₄Ni₃O₁₀, is found. This not only expands the family of nickelate superconductors, but also provides a relatively stable compound for in-depth research. However, improving sample quality and reducing the pressure required for the superconducting transition remains a priority.”

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