China’s Sulfur LK99 Room Temperature Superconductor Variant Has Increased Meissner Effect Signal

Chinese researchers have published a new non-peer reviewed paper on their sulfur copper doped led apatite room temperature superconductor. They have increased the Meissner effect signal.

Supplemental material from the China researchers. This describes the precise work done show they did not make measurement errors.

Background Context

In mid-2023, the Original Korean researchers discovered the LK99 – copper doped lead apatite material had superconducting properties at room temperature and room pressure. This work has been highly controversial after it triggered worldwide excitement last year. The best superconducting resistance results were for thin-film chemical vapor deposited material. No other groups have replicated the thin film results.

In January 2024, the same group of Chinese researchers detected weak magnetic signal suggesting a superconducting Meissner effect.

New Results From China- Stronger Magnetic Signals that Indicate Meissner Effect Mixed in With Other Material Phases

As reported in our previous 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 finely optimize the reaction procedure as sulfur could not be held in the bulk at overhigh temperature that enables other elements to react. In this work, the chinese researchers adjusted the synthetic procedure of SCCLA and find the [Meissner effect] signal magnitude is largely increased.

* the magnetic hysteresis effect signal is now stronger
* Indications of what they think is a weak microAmp superconducting phase that is mixed in with other phases of material
* Tablet of Compressed NanoPowders are too fragile for electrodes
* sample is an insulator before soaking in sulfur. After soaking in sulfur the resistance gets down to graphite levels

Summary : 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 is around 300 K. The RT curve manifests that SCCLA possesses a strange metal phase at large current. A weak even-in-field transverse voltage indicates the possible contribution of superconducting vortex dynamics. They therefore believe that they have made a substantial step towards room-temperature superconductivity.

Magnetic Hysteresis Effect is Stronger

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. These results strongly suggest there exists a Meissner effect in SCCLA.

Tablet from Compressed NanoPowders Are Too Fragile for Electrodes

Although the sample before soaking in sulfur is an insulator, after soaking the resistivity at room temperature is largely reduced to around 2 × 10^−5 Ω·m, close to that of natural graphite. It is thus evident that the sulfur-copper codoping plays essential role in the improvement of transport properties of the insulating apatite ionic crystal. It is also worth noting that, the tablet samples are just mechanically compressed from nano-scaled powders which is too fragile to fix electrodes, so they believe there is still large room for further improving the conductivity. Nonetheless, such considerable conductivity is still far beyond expectation, that has to be carefully comprehended.

Indications of a Weak MicroAmp Superconducting Phase Mixed in with Other Phases

As estimated by the critical magnetic field, if there is a superconducting phase, the critical current should be as small as µA, which can not be accurately detected in the current facility.

The RT curves are therefore measured with constant current of 8 mA to ensure the precision. It is found that below 20 K the derivative dR/dT exhibits a nearly linear relation suggesting a Fermi liquid behavior as in normal metals. Increasing the temperature, the derivative becomes nearly constant revealing a linear-in-temperature resistivity, which is an essential signature of strange metal. At around 230 K in the cooling curve and 304K in the heating curve, there are remarkable turning points to figure out the transition from strange metal to bad metal phase. It has been stated that, if the resistivity in bad metal can be further reduced by one or two orders, these turning points can then be recognized as the superconducting critical temperature. The thermal hysteresis of the two RT curves, probably stemming from the poor thermal conductivity of the sample, is also consistent with that of the magnetic measurements. The current–voltage (IV) curves at various temperatures are displayed in Fig. 3(b). The step of measurement is chosen to be 400 µA, so the curves are all linear obeying Ohm’s law. As an additional note, during the first test of signal-noise-ratio in IV curve which has to be reconfirmed in the future, they set a relatively small step and find the voltage does not increase within 10 mA. A possible superconducting phase might be fragile when large current is injected.

One now would be wondering why the strange metal exhibits a possible Meissner effect. They consider the diamagnetic grains are nano-scaled and stacked so the vortex should cross the margins to result in resistance of strange metal. To this end, they detect the Hall (Rxy) and longitudinal (Rxx) resistance, i.e. magnetoresistance (MR), at various temperatures. At 10 and 300 K, both Hall and MR are around 0.5%, slightly larger than those at other temperatures. It is noticed that the Hall resistance is not centrosymmetric as in the normal case.

Improved Synthesis – Better Doping Procedure

Compared to our previous synthetic procedure, the doping of sulfur not only facilitates the doping 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 may have got much stronger inter-chain interaction to activate tunneling in between.

This explains the huge improvement of electric conductivity and the emergence of strange metal. They have also conducted microwave absorption measurement and did not find visible radical signals, implying the absence of quasiparticles in normal metal. People might suspect the magnetic hysteresis is enabled by some pseudogap states, as we do not explicitly observe a zero-resistance state along with the possible Meissner effect. But since the RT curves definitely exhibit a metallic feature, they would be more preferable to think it is because the superconducting critical current is below our measurement limit. During measurements, they have noticed that the sample possesses abnormally large electric capacity and poor thermal conductivity which are inconsistent with the high electric conductivity. In particular, the capacity often induces strange sudden change between low- and high-resistance states, even if they have carefully excluded other interferences. It is more likely the sample is continuously charging
and discharging during electric measurements, hindering the detectability of zero resistance. The dielectrics of lead apatite backbone matters.