A material levitates in a magnetic field. /CFP
Chinese researchers have found evidence which overturns a claim made by a group of South Korean researchers that they have created a room-temperature, ambient-pressure superconducting material.
The researchers from the Institute of Physics under Chinese Academy of Sciences stated in a study that the newly synthesized material dubbed LK-99 cannot show superconductivity at room temperature, and pointed out the Cu2S impurity contained in LK-99 is the "most likely" reason leading to the false claim.
The researchers said they noticed that the LK-99 sample reported contained a certain amount of Cu2S impurity, which may have caused the so-called superconducting behavior.
To determine whether the superconducting-like transition observed is intrinsic for LK-99 or caused by the Cu2S impurity, the researchers studied the transport and magnetic properties of Cu2S and the mixture of LK-99 and Cu2S.
They further synthesized two kinds of LK-99 with different Cu2S content, and respectively measured their resistance, diamagnetism and other parameters, and compared them with the pure Cu2S.
"We observed a sharp superconducting-like transition and a thermal hysteresis behavior in the resistivity and magnetic susceptibility. However, we did not observe zero-resistivity below the transition temperature," said the researchers in their study published on the open-access preprint platform arXiv on Tuesday.
Their findings of resistivity and magnetization "strongly suggest that the superconductivity-like behavior in LK-99 is caused by the structural phase transition of the impurity Cu2S," the study said.
Worldwide attempt for verification
Superconductors are materials that have zero electric resistance and can expel magnetic fields, through which electricity can pass without losing energy.
Such materials are one of the most sought-after materials by researchers for their huge potential and wide applications such as nuclear magnetic resonance instruments and maglev trains.
Conventional superconducting materials are metals and can only show superconductivity under extreme conditions, such as ultra-low temperatures and high pressures.
In late July, a South Korean team led by experts from Quantum Energy Research Center posted two papers on arXiv, claiming that they synthesized the world's first room-temperature superconducting material under ambient pressure, known as LK-99, a modified-lead apatite crystal structure doping with copper.
"The superconductivity of LK-99 proved with the critical temperature, zero-resistivity, critical current, critical magnetic field, and Meissner effect," said the researchers.
Their discovery soon went viral, arousing excitement and skepticism.
Research teams worldwide have attempted to replicate the material, but have yet to confirm the superconductivity of LK-99.
In South Korea, experts from the Korean Society of Superconductivity and Cryogenics set up a verification committee to assess the authenticity, and ended up with the conclusion that LK-99 "is not a superconductor as the material presents no negative resistance, called Meissner effect," Yonhap News Agency reported on August 3.
"We can't say the material shows the exact Meissner effect (based on an experiment in the paper)," Yonhap reported citing an official from the institute, adding the authors have refused to submit a sample for its test.
The researchers from the National Physical Laboratory of India said in their study on July 31 that they "do not approve the reported appearance of bulk superconductivity in LK-99," but will synthesize more samples for verification. On the same day, a research team from China's Beihang University said the room-temperature resistance of the LK-99 sample is not zero, and no magnetic levitation was observed, saying the material is similar to a semiconductor rather than a superconductor.
A study by researchers at Southeast University said they didn't observe Meissner effect, but observed zero resistance in their LK-99 samples at above 110 K (−163 degrees Celsius), indicating the material can be "a possible candidate for searching high-temperature superconductor."
Researchers from Peking University saw "half levitation" in some small flaky fragments, but without Meissner effect or zero resistance, which they believe "do not exhibit superconductivity."
Earlier, a team from Huazhong University of Science and Technology uploaded a video on the Chinese social media platform Bilibili, showing the synthesized LK-99 crystals can be magnetically levitated at a larger angle than the one by the South Korea team at room temperature.
However, they also stated that the material does not possess zero resistance, and its overall behavior is similar to a semiconductor curve. Still, the researchers said "it is expected to realize the true potential of room temperature, non-contact superconducting magnetic levitation in near future."