Reported by the Heart of the Machine
Editors: Egg Sauce, Chen Ping
Copper-substituted lead apatite may have a Meissner effect at room temperature.
A paper on room-temperature superconductivity has once again set off a small fluctuation in the Internet.
In a recent paper, the authors have once again demonstrated the possible Meissner effect in copper-substituted lead apatite (LK-99) at room temperature.
Paper link: https://arxiv.org/pdf/2401.00999.pdf
At room temperature, lead apatite replaced with copper observed diamagnetic DC magnetization at a magnetic field of 25 Oe, with a clear divergence between zero-field cooling and field-cooled measurements, becoming paramagnetic at 200 Oe. The glass memory effect was discovered during the cooling process. The typical hysteresis loop of a superconductor is detected below 250 K, while the front and back scans of the magnetic field are asymmetrical. Our experiments show that at room temperature, there may be a Meissner effect in this material.
Since there are no instruments that can measure Meissner in the strict sense of the theoretical term, the authors take a more rigorous way of expressing the Meissner effect.
Surprisingly, nine authors come from all over the world: "People who have never given up, from different professions and different institutions, are scattered all over the world, but they are connected on the Internet because room-temperature superconductivity can achieve similar ideals. 」
People should remember the room-temperature superconductivity boom that was sparked by LK-99 last year.
LK-99 is derived from two papers published in July 2023 by a research team in South Korea. The team claims to have synthesized LK-99, a copper-doped lead apatite at room temperature at atmospheric pressure, with a critical superconducting temperature that exceeds the boiling point of water and reaches a maximum of 127 degrees Celsius.
If human beings can achieve room-temperature and atmospheric pressure superconductivity, then the energy efficiency of power grids, electronic devices, and transportation is expected to be greatly improved, and the fourth industrial revolution is expected to begin. So, after the South Korean team published their results, the entire scientific community was fascinated by the study.
Scientists from various countries have been studying LK-99, a "room-temperature superconductivity" material in South Korea, but they have not succeeded in reproducing the results claimed by the original authors' team, and there are more and more pessimistic people.
At the end of 2023, the Verification Committee of the Korean Society for Superconductivity and Cryogenics said that after several months of verification, the suspected room-temperature superconductor LK-99, previously manufactured by a South Korean research team, did not show any key characteristics of superconductivity — LK-99 showed no signs of superconductivity in a series of resistance and magnetic induction strength tests at room or low temperatures.
Contrary to what the research center said would show a sharp drop in resistivity in a single crystal sample with few impurities, a number of papers (prior to the committee's announcement) reported that impurities in the reconstituted sample, particularly copper sulfide, were responsible for the sharp drop in resistivity.
In addition, tests carried out on another single crystal sample with impurities removed showed that the so-called superconductor is a "nonconductor" that does not allow electric current to pass through.
At this point, the vigorous "room temperature superconductivity" incident seems to have come to an end.
However, Kwon Young-wan of the LK-99 research team said, "I still believe that LK-99 is a superconductor". He argues that the committee has failed to recreate room-temperature superconductors because it is impossible to prove the validity of the study in just a few months.
Researchers who have a high passion for room-temperature superconductivity have not given up.
As soon as the study was published, many people rekindled their hopes: the biggest question last time was "Can it be replicated?" and this time the answer seems to be "already there".
Of course, there are many other questions that people are concerned about, such as whether the material used in this paper is exactly the same as that of LK-99, and how do researchers measure its superconductivity? Let's take a look at what this paper says.
Essay details
Perfect diamagnetism, known as the Meissner effect, is one of the basic criteria for testing superconductors.
To demonstrate the Meissner effect, it is first necessary to observe a diamagnetic magnetization-temperature (M-T) curve that occurs below the critical temperature (Tc) and bifurcation in zero-field cooled (ZFC) and field-cooled (FC) measurements. A superconducting hysteresis back magnetization-magnetic field (M-H) cycle below the critical temperature needs to be observed at the same time.
Copper-substituted lead apatite (CSLA), also known as LK-99, is considered a novel candidate for room-temperature superconductors, but the full Meissner effect has not been reported to date. The study by Lee et al. reported that LK-99 has a great deal of diamagnetism, but according to Habamahoro et al., this diamagnetism stems from Cu_2S.
Studies have shown that in direct current (DC) measurements, a more important hysteresis loop, is still missing, and is only observed in a microwave environment.
There is no doubt that direct observation of DC hysteresis is essential, which is the main content of this work.
To prevent ferromagnetism due to excessive copper doping, an improved CSLA sample was designed and prepared for this study. The process is as follows:
Phosphate and lead sulfide are carefully mixed in an aqueous solution by a co-precipitation method. The mixture is then heated at 180°C at high pressure for 24 hours, keeping the solution pH 8. After hydrothermal treatment, the samples were calcined at 900°C for 8 hours in an argon environment. The temperature is then lowered to 500°C and calcined for 48 hours in a pure oxygen atmosphere. Subsequently, the sample is allowed to cool to room temperature in the presence of oxygen.
In this study, DC magnetization measurements were performed on the samples using the MPMS-3 SQUID, followed by M-H (magnetization-magnetic field) curve measurements at temperatures of 300 K, 250 K, 200 K, and 100 K, respectively. The sample was then cooled to 10 Kelvin and the zero-field cooling (ZFC) and field-cooled (FC) curves were measured again to determine the superconductor and glass memory effect (which is an indication of magnetic behavior).
Figure 1 shows the MT curve before and after the field scan, showing a distinct ZFC-FC bifurcation. All curves are diamagnetic at 25 Oe, while paramagnetic is present at 200 Oe, which is consistent with a lower critical field Hc1 of 30 Oe in low-field microwave absorption. The ZFC curve after initial magnetization is lower than before initial magnetization, and there are significant kinks around 100 K, suggesting a glass memory effect when the magnetic field finally sweeps at 100 K when cooled. There is also a turning point around 250 K that can be considered as a critical temperature Tc. When the curve is below 50 K, the vitreous behavior is more easily observed at 200 Oe.
The M-H curve at three temperatures is shown in Figure 2, where the signal is essentially paramagnetic in a strong magnetic field. Typical superconducting hysteresis loops can be clearly observed below 10 Oe, and hysteresis is not recognizable above 250 K. It is important to note that there is an asymmetry between the forward and backward scans, i.e., the negative peaks at zero field are sharper than the positive peaks. This asymmetry is also detected in microwave absorption.
Since the initial magnetization curve is also important, the initial magnetization curve and the first backscan curve are given in Figure 3 at different temperatures. At lower temperatures, the bifurcation point increases, and a peak appears at the low field, indicating the possible presence of a Meissner phase.
Figure 4 shows that the crystal structure of the sample is consistent with the P63/m structural characteristics of apatite.
In conclusion, diamagnetism in CSLA has been studied by M-T curves and hysteresis M-H loops and can be observed in the range of 250 K. Considering that the ZFC-FC bifurcation is above 300 K, the study believes that there is still a good chance of observing room-temperature-temperature superconductivity. The study stated that the signal in its sample was still very weak, so a scalable sample with more active ingredients had to be further synthesized.
In the discussion area of Zhihu-related issues, one of the authors, Xi Zhixi, said that the video will be released in the future:
Regardless of the results, it is believed that people will continue to pay attention to the progress of room-temperature superconductivity research.
Reference link: https://www.zhihu.com/question/637763289