China Has Strong Experimental Evidence for Superconducting Meissner Phase in LK99-like Copper Doped Lead Flourophosphate

China researchers have given new experimental evidence that finds superconducting from LK99 powders.

Copper-doped lead fluorophosphate (CSLA) was claimed by Lee et al. to exhibit superconducting properties at room temperature and atmospheric pressure. This surprising discovery generated great interest in the superconducting mechanism of this material in the scientific community. However, since only powder samples of CSLA can be synthesized at present, conventional resistivity and magnetic susceptibility measurements cannot be performed. Therefore, using a new method of microwave absorption to detect the possible superconducting phase transition in CSLA became the starting point of this study.

Through continuous wave microwave absorption spectrometry, the chinese researchers found that the CSLA sample showed significant microwave absorption peaks and hysteresis effects in which the magnetic field scanning trace was not closed under low magnetic fields (30-500 Gauss). This low magnetic field microwave absorption (LFMA) peak can be divided into three different phases: Meissner phase, vortex glass phase and normal state. The author believes that the positive derivative form of the LFMA peak and the hysteresis curve of the magnetic field scan indicate the existence of vortex excited states in CSLA, which is a typical feature in the superconducting phase. It was also observed in the experiment that as the sample rotates under an external magnetic field, the LFMA peak disappears rapidly and maintains a long-term “memory” effect, further ruling out the possibility of any ferromagnetic contribution. The LFMA peak intensity first increases and then rapidly decreases with the temperature increasing, and the turning point is about 250K, suggesting the existence of a phase transition critical temperature T_c.

There is years of extensive work by researchers in the superconducting field of Enhanced microwave absorption, larger than that in the normal state, observed in superconductors. Here is a Nature paper from 2018 for Type-II superconductors. Giant microwave absorption in fine powders of superconductors.

In order to theoretically simulate the magnetic field control and evolution dynamics of the Meissner phase and the vortex glass phase, the researchers used a quasi-one-dimensional ladder model to describe the randomly oriented one-dimensional superconducting channels in CSLA. As the magnetic flux increases, the system changes phase from the Meissner insulator state to the vortex glass state, and exhibits different circulating background current patterns. By stimulating a ring current in the middle of the ladder, the author simulated the “crawling” process in which the vortex slowly spreads outward from the middle, successfully reproducing the memory effect observed in the experiment.

This paper shows the discovering of the existence of superconducting Meissner phase and vortex glass phase in CSLA through a new method of microwave absorption spectroscopy, and successfully simulated the phase transition process and the evolution dynamics of vortices in theory. This discovery provides strong evidence for the study of the superconducting mechanism of room temperature superconducting CSLA, and also demonstrates the complex phase transition behavior of a quasi-one-dimensional strongly correlated system under microwave excitation.

A new material called copper-doped hydroxyapatite shows a surprising memory effect. This material was prepared by the research team through a special chemical synthesis method. It is the lead, copper, phosphorus and oxygen of the korean claimed room temperature superconductor LK99 material. The important thing is that it exhibits some unique magnetic properties.

The researchers used a sophisticated technique called electron paramagnetic resonance to test the new material. The technique detects the faint movement of electrons in materials. This is a very sensitive measurement method that can detect the weak magnetic response of electrons in materials. The researchers applied an external magnetic field to the sample and simultaneously irradiated the sample with microwaves. By analyzing the absorption of microwaves, information about the movement of electrons in the sample can be obtained. The test found that when the external magnetic field changes slowly, the microwave absorption intensity of this new material shows a “hysteresis” phenomenon, that is, the change in absorption intensity always lags behind the change in the magnetic field. This lag exhibits a clear “memory effect.”

What’s even more amazing is that if the researchers rotate the direction of the sample, this memory effect will be slowly “forgotten”. It cannot be reactivated even with a stronger magnetic field. But if the sample is left standing for two days, this memory effect will magically recover automatically. Researchers believe that this unique memory effect comes from the special electronic conditions in the material. They used a theory called the “quantum grid model” to successfully simulate and explain this memory effect. The model suggests that electrons in this material form a particularly fragile ordered state, similar to a glass state, that can retain its magnetic field memory but can also be destroyed by rotation. The formation temperature of this ordered state is approximately 250 Kelvin. The specific mechanism remains to be studied. This unique “memory effect” provides hope for possible applications of the material, such as new magnetic memory storage materials.

1. Microwave absorption: When microwaves pass through a material, part of the energy will be absorbed by the material, which is called microwave absorption. This can reflect the variations in the microstructure of the substance.
2. Electron Paramagnetic Resonance:Paramagnetic resonance is a technique that accurately measures unpaired electrons in a sample. In a magnetic field, electrons will undergo regular forward motion. When their frequency meets the conditions of paramagnetic resonance, they will absorb microwave energy, thereby detecting electronic signals.
3. Shielding current:Superconductors can completely repel external magnetic fields, which is the Maxwell superconducting shielding effect. The current that produces this effect is called shielding current. It circulates on the surface of the superconductor, maintaining the magnetic field at zero.
4. Vortex:When the magnetic field is strong, it will break the complete electron pairing of the superconductor and form a vortex-like current pattern. These are called vortices. The motion of the vortex reflects the excited state information of the superconductor.
5. Bistability: A system that switches between different stable states, and the state depends on history, is called bistable. Simply put, it is the memory effect.
6. Glass state:Glass is neither a solid nor a liquid. It is somewhere in between, with the rigidity of a solid and the disorder of a liquid. The glassy system has slow dynamics and takes a long time to reach equilibrium, so it exhibits memory.

Some collected QA

Q1 It is mentioned that a positive low magnetic field microwave absorption peak was observed, which is different from the negative peak of general magnetic materials. How is this criterion of positive and negative signs derived? Can it be understood more intuitively?

A1: The criteria for positive and negative peaks mainly come from the signals of copper oxide radicals measured by the author at the same time.

The electron paramagnetic resonance signal of free radicals is very strong, with an obvious negative peak appearing near 3350 Gauss. The authors can consider this peak as a reference calibration.

Relative to the free radical peak, the sign of the low magnetic field absorption peak can be determined. Since the author uses the differential curve of the microwave absorption spectrum, the positive and negative signs reflect the positive and negative changing trends of the absorption intensity as the magnetic field changes.

If the position of a peak relative to the free radical peak is on the low magnetic field side and positive, then this indicates that its absorption intensity increases with the increase of the magnetic field, which is consistent with the basic characteristics of superconducting vortex absorption.

On the contrary, if a peak is on the high magnetic field side and is negative, it means that the absorption intensity decreases with the magnetic field, which is consistent with the characteristics of preset magnetization.

So by comparing the relative positions and signs of different signal peaks, the authors can infer their physical origins. This is a relative calibration method that requires simultaneous measurement of a known reference peak.

Q2: Why choose X-band microwave for testing? Will different frequencies affect the results?

A2: The choice of X-band microwave (frequency 9.6GHz) is mainly based on the following two considerations: Microwaves of this frequency have higher detection sensitivity for superconducting samples. According to theoretical calculations, the superconducting energy gap corresponds to an excitation equivalent to the energy level of microwave photon. The X-band is just in the sensitive area for vortex excitation. The instrument system is a domestically produced E580 electron paramagnetic resonance spectrometer. Its standard microwave source and resonant cavity work in the X-band. From the perspective of experimental conditions, directly using the standard operating frequency of existing instruments can reduce variables.

Higher frequency microwaves (such as W band, Ka band) can theoretically be used for detection, and may produce resonant excitation of the superconducting energy gap to obtain better signals. However, this requires the replacement of corresponding microwave sources and cavities, which increases the difficulty of the experiment. Low frequencies such as L-band and S-band can also detect superconducting samples, but the sensitivity will be reduced.

Q3: The calculation of the lattice gauge theory model is very interesting. How was this model established? How well does it agree with the experimental results?

A3: This is a quasi-one-dimensional double-chain ladder network model, which can reflect the basic structure of the conductive channel in the sample. Its Hamiltonian uses a flow segment model, including controllable paramagnetic interactions along and across chains. The key parameter is the magnetic flux φ, which is proportional to the external magnetic field. The proportional coefficient between φ and the magnetic field includes the coherence length of the material. So changing φ is equivalent to changing the magnetic field, which can drive phase change. This paper considers the phase diagram of this model in the half-filled case. When φ is very small, the system is in a completely insulating Meissner state. At this time, the system has chiral current but the cross-chain current is zero. As φ increases to a critical value, the system enters the vortex glass state, and local cross-chain current appears, that is, a vortex is formed, but the gas state remains. This phase transition process and the current patterns of the two phases are in good agreement with the experimental results. This paper also simulated the motion of the vortex by stimulating the intermediate cross-chain current, and found the difference in the speed of vortex motion in different phases, which is consistent with the memory effect. Therefore, the model contains the key elements in the experiment, can well reproduce the experimental observation results, and also provides a microscopic mechanism for understanding the experiment.

Q4: Two phases are mentioned in the article: Meissner phase and vortex glass phase. What is the difference between these two phase states? What is their correspondence with experimental observations?

A4: The two phases observed in the experiment correspond to the Meissner phase and vortex glass in the theoretical model. Their main differences are:

The Meissner phase is insulating and has an energy gap, while the vortex glass phase has no energy gap; the Meissner phase has chiral current but no local eddy current, and the vortex glass phase has local eddy current; the Meissner phase resists The magnetism is stronger, and the diamagnetism of the vortex glass phase is weak; the vortex motion of the Meissner phase is blocked, and the vortex of the vortex glass phase can move freely. Corresponding to the experiment, the Meissner phase explains the low magnetic field saturation absorption state, while the vortex glass phase explains the hysteresis loop effect. The phase transition critical point also corresponds to the experimental temperature.

Q5: The phase transition critical magnetic field corresponds to about 500 Gauss in the experiment. What is the basis for determining this value? How is the superconducting coherence length in the sample determined? What significance does this have for understanding key parameters?

A5: Determining the theoretical critical magnetic field requires knowing the superconducting coherence length of the sample. The paper (mentioned in the original article) estimates it like this:

According to literature reports, the coherence length of similar superconducting systems is on the order of tens of nanometers; considering the possible new superconducting mechanism of the sample, the coherence length is estimated to be 100-300 nanometers; the coherence length is brought into the formula calculation to match the experimental critical magnetic field of 500 Gauss , the back calculation shows that the coherence length is about 200 nanometers. Therefore, the determination of coherence length is mainly through theoretical and experimental matching. Its value reflects the long-range coherent spatial scale of superconducting electrons, which is very important for understanding the superconducting mechanism.

Q6: You proposed in the article that the sample may be a 1D superconducting system. This conclusion is very interesting. Is there any experimental evidence to support this conclusion? Is it too far-fetched to draw the conclusion of 1D superconductivity only from theoretical analysis of the sample structure?

A6: Inferring 1D superconductivity from sample composition and structure alone requires more evidence. After looking at it, the authors’ basis for drawing this conclusion should include:

It is reported in the literature that the synthesis method can control the formation of quasi-1D structure; their XRD characterization also shows a similar chain structure; magnetic levitation video shows anisotropy, supporting quasi-1D magnetic response; microwave absorption results also show obvious magnetic anisotropy.

If further confirmation is needed, it is necessary to strengthen synthesis control and improve the quality of quasi-1D structures; prepare single crystal samples and conduct anisotropic electrical transport measurements; conduct detailed magnetic anisotropy and electrical anisotropy characterization to find more Definitive evidence of 1D superconductivity.

Q7: Why does the EPR signal of the rotating magnetic field disappear in the vortex glass phase?

A7: This is related to the special magnetism of the vortex glass phase. The so-called vortex glass phase is a glass-like intermediate phase formed by electrons in the sample. There are a large number of vortices in this phase, which are tiny rotating currents. Without rotating, these randomly distributed vortices can induce an overall weak magnetization and respond to microwave EPR signals.

But once the direction of the magnetic field is rotated, the direction of these vortices will adjust and eventually cancel out on average, thereby losing the overall net magnetization. Therefore, even if the magnetic field intensity continues to be increased, the vortex response can no longer be stimulated, so the EPR signal disappears. Only after waiting for a period of time can the magnetic interaction between the vortices slowly adjust back to a stable and orderly state, and the EPR signal can be restored.

Q8: How do you view the issue of siding involved in this?

A8: Based on the proportion of our scientific research talents and production lines, any epoch-making and useful new concept will eventually be industrialized on a large scale. Everyone makes money.