Stronger Meissner Effect with China’s LK99 Variant Room Temperature Superconductor

Copper-substituted lead apatite, aka LK-99, was claimed to be room-temperature superconductor, but due to the complicated components and structures, the reproduction is still controversial. Chinese researchers have some replication of the Korean work and have extended with better procedures and research. They have updated a prior paper with new synthesis (instructions about how to make it) and better measurements.

They modified the synthetic procedure of SCCLA to codope both sulfur and copper into lead apatite, and the structural characterization reveals a directional stacking mechanism. The magnetic and electric properties of SCCLA have been comprehensively investigated. The hysteresis MH loops can be observed up to 250 K, and the ZFC–FC bifurcation occurs as well. The RT curve manifests that SCCLA possesses a strange-metal phase at large current and a second-order phase transition occurs at around 230 K during cooling. Further synthesis of lead-free sample is then performed which provides us even stronger diamagnetism and smaller resistance at low temperature. They therefore believe that they have made a substantial step towards room-temperature superconductivity.

The researchers on this paper are associated with 9 University departments or institutions (South China, Wuhan, Beijing, Japa, Fuzhou, Hefei).

One can see that, from −300 to 300 Oe, the MH curves exhibit notable hysteresis effect up to 250 K. In particular, the hysteresis at 150 K is pretty obvious, exceeding the highest critical temperature of known superconductors at ambient pressure. This phenomenon has been reported in our previous paper, but the quality of the present data is largely increased, further eliminating the possibility of measurement faults. At 300 and 350 K, the hysteresis can not be detected. As stated before, these results strongly suggest there exists a Meissner effect in SCCLA.

Considering copper is a sulfophilic element, the participation of sulfur in the synthesis benefits the substitution of copper, which does originally not favor the ionic crystal structure of lead apatite. As reported in the previous China work, this sulfur-copper codoped lead apatite (SCCLA) manifests a weak Meissner effect at near room temperature. In order to further enhance the effect, they have to either finely optimize the reaction procedure as sulfur could not be held in the bulk at overhigh temperature that enables other elements to react, or alternatively design and synthesize some lead-free frameworks. In this work, they adjust the synthetic procedure of SCCLA and find the signal magnitude is largely increased.

Work suggests the lead does not matter in the superconductivity, and a lead-free sample was made. Lead-free is more fragile than that with lead, the role of lead might merely be to enhance the robustness of structure. Interestingly, below 40 K, the magnetic moment drops dramatically by more than one order. The different FC moment with different holders at low temperature may be due to the orientational difference of sample under magnetic field, as the crystal grains have been directionally stacked. By MH curve at 10 K, it is also observed that the diamagnetism is greatly enhanced, far exceeding the normal diamagnetism in metals, which can only be explained by superconductivity.

Compared to the previous synthetic procedure, the doping of sulfur not only facilitates the performance of copper, but also changes the topology of 1D ionic channel of apatite. From head-to-tail to side-by-side, the quasi-1D copper-sulfur lattice in apatite may have got much stronger inter-chain interaction to activate tunneling in between. The alkali metals (they added small amounts of silver) may also play roles in connecting the crystal grains. This explains the huge improvment of electric conductivity and the emergence of Meissner effect.

Observation of diamagnetic strange-metal phase in sulfur-copper codoped lead apatite

By codoping sulfur and copper into lead apatite, the crystal grains are directionally stacked and the room-temperature resistivity is reduced from insulating to 2 × 10−5 Ω·m. The resistance-temperature curve exhibits a nearly linear relationship at low temperature suggesting the presence of strange-metal phase, and a second-order phase transition is then observed at around 230 K during cooling the samples. A possible Meissner effect is present in dc magnetic measurements. Further hydrothermal lead-free synthesis results in smaller resistance and stronger diamagnetism, demonstrating the essential component might be sulfur-substituted copper apatite and the alkalis matter as well. A clear pathway towards superconductivity in this material is subsequently benchmarked.

What is In the Updated Paper

1. The updated content is generally an optimization of the previous paper, removing the IV curve with little information, as well as the magnetoresistance and Hall that everyone thinks are of little significance, and adding the magnetism and electricity of two new samples. Measurement data.

2. Sample 2 is a parallel sample of the previous set of samples, but it is made more meticulously: the feeding is more precise and the purification is more thorough. Material purification is a difficult task to quantify. It is difficult to quantify that 99% and 99.9% can be achieved by washing more times, but the difference can indeed be seen from the measurement. It is obvious that the resistance transition of the purer sample 2 looks more significant, which is a typical second-order phase transition.

3. Sample 3 is based on the previous set of samples with the lead removed. The main purpose is to verify whether lead has any impact. From the perspective of magnetism and electricity, it does not have much impact. At present, lead seems to mainly play a role in stabilizing the structure, because lead apatite is the most stable and resistant to burning among all apatites.

4. The decrease in resistance of sample 3 is probably caused by replacing sodium with potassium.

5. Sample 3 has ZFC and FC bifurcated below 250K, and ZFC is diamagnetic, similar to sample 1. Moreover, stronger diamagnetism appears below 40K, which is difficult to explain by mechanisms other than superconductivity.

6. The conductivity was measured by the indium pressure method. The results were similar to those of sample 1, and also showed linear exotic metal characteristics. But using the silver glue method, the resistance showed an obvious jump from large to small around 250K, but the measured jitter was more significant. At low temperatures, the resistance is obviously less than the measurement limit and fluctuates up and down the zero axis, which is very suspicious of zero resistance.

7. The silver electrode turned black obviously after the measurement, indicating that it reacted with the excess sulfur in the sample. According to Mr. Chen’s analysis, it is likely that the silver took away the sulfur from the apatite ion channel, turning the entire material into an electron-rich state, causing the superconductivity to become more obvious.

8. Considering that new phenomena that are difficult to explain have appeared in the experiment, it is not appropriate to publish a new article directly, so it will be updated based on the previous version. When more complete data is produced in the future, I will choose to write a new article and submit it officially.