A zero-resistivity high-temperature superconductor that exhibits strange metallic behavior

Researchers at Zhejiang University and Sun Yat-Sen University have collected evidence of high-temperature superconductivity with zero resistance and strange metallic behavior in a material identified in their previous studies.

Their findings, published in Nature Physicshighlight the promise of this material for studying these rare physical properties and ultimately using them to develop innovative electronic devices.

“High-temperature superconductivity is one of the most intriguing puzzles in the field of condensed matter physics,” Prof Huiqiu Yuan, project leader of the 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). Therefore, the pursuit of high transition temperature superconductors and understanding of their mechanisms are among the best compelling targets in condensed matter physics.”

High-temperature superconductors are highly sought-after materials, as they can help develop a new class of electronics. As a result, any indication of high-temperature superconductivity often attracts considerable attention, both from researchers and electronics companies.

In 2023, a research team at Sun Yat-Sen University led by Prof Meng Weng reported signatures of superconductivity at temperatures below 80 K in La₃Ni₂O₇, a nickel material. Since 80 K is above the boiling point of liquid nitrogen, this discovery in itself was a breakthrough, but it also came with some limitations.

“A significant obstacle in Prof Wang’s initial discovery was the lack of zero resistance, the most distinguishing characteristic of a superconductor,” said Prof Yuan.

“While a major challenge is that high-temperature superconductivity in this material is only observed when subjected to a pressure of at least 14 gigapascals (about 140,000 atmospheres). Therefore, high-precision measurements and specialized high-pressure equipment are essential for further investigation.”

To examine La₃Ni₂O₇ further, researchers at Sun Yat-Sen University joined the group of Prof. Yuan at Zhejiang University’s Center for Bonded Matter (CCM), which specializes in the study of strongly bonded materials under extreme conditions.

Prof. Yuan acknowledged that the lack of zero resistance in Prof Wang’s original report could be attributed to the inhomogeneity of the sample itself, as well as the pressure conditions caused by the solid pressure of the KBr powder.

Thus, the researchers began conducting a new study investigating the properties of the material using a new sample and different pressure conditions. Specifically, Prof Yuan proposed the use of small samples and better hydrostatic pressure conditions, as these elements may represent the key to observing zero resistance.

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

“By measuring the electrical resistance of La₃Ni₂O₇ at high pressures using this method, we observed a sharp drop in resistivity when we cooled 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 gathered evidence that La₃Ni₂O₇ exhibits superconductivity at high temperature under pressure. After the publication of their work, scientists have become sure that La₃Ni₂O₇ it is indeed a high-temperature superconductor.

“The main challenge in the study of La₃Ni₂O₇ lies in its meta-stable 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 previously mentioned.”

Basically, 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 faces with a liquid medium. They attached four to five Au wires, each approximately 15 micrometers in thickness, to the surface of the sample using silver paste to ensure good electrical contact.

The researchers then applied high pressure to the sample, which compressed it and then cooled it down to several kelvins. By measuring the resistance of the sample in relation to the temperature and pressure it was at, they demonstrated its zero resistance.

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

In addition to collecting evidence of high-temperature zero-resistivity superconductivity at La₃Ni₂O₇, this latest study provides insight into the physics underlying this condition. In fact, the researchers observed distinctly strange behavior of the metal 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 after tuning by a non-thermal parameter, which cannot be described by our existing knowledge based on Landau’s theorem .” Prof. Yuan said.

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

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

“We also found that the inverse Hall coefficient undergoes a marked increase as it passes through the pressure-induced structural phase transition, suggesting that the electronic structure modification in the high-pressure phase is crucial for the emergence of superconductivity,” Prof Yuan said.

This latest study by Prof Yuan, Prof Weng and their respective teams opens up exciting new opportunities for the study of high-temperature superconductivity and its application in electronics. The researchers plan to continue investigating the physics of La₃Ni₂O₇also examining other unconventional superconductors.

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

The similarities observed between the superconductivity of various nickelates and that of other reported families of high-temperature superconductors point to the possibility that nickelates may also be superconductors at high temperatures, but potentially without requiring high pressures.

In their future studies, the researchers plan to identify more suitable candidate compounds, which may allow them to discover the key ingredients for superconductivity in terms of chemical composition and crystal structure.

“We [have] is improving 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,” Prof Meng Wang said.

More recently, evidence of superconductivity in other nickel compounds, such as La₄Ni₃O₁₀, has been found. This not only expands the family of nickel superconductors, but also provides a relatively stable compound for in-depth study. However, improving sample quality and lowering the pressure required for the superconducting transition remains a priority.”

© 2024 Science X Network