Stronger Meissner Effect with China’s LK99 Variant Room Temperature Superconductor
By Brian Wang
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.
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