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Edit: Zenan, Source: Heart of the Machine
"Unfortunately, it's all over."
So far, a key indicator of many researches on the reproduction of room-temperature superconductivity in Korea is the observation that the material is semi-suspended in the magnetic field of a permanent magnet under normal temperature and pressure conditions.
Although observations indicate room-temperature Meissner effects in some samples, this suspension can only be considered "half" suspended, since part of the sample is still in contact with the support surface. The fact that two of the most important properties of superconductors, the Meissner effect and zero resistance, have not yet been fully demonstrated and reproduced in known quantitative measurements adds to the uncertainty of verifying whether LK-99 is a true room-temperature superconductor. Certainty.
Today, a paper submitted by Guo Kaizhen, Jia Shuang and others from the Center for Quantum Materials Science (ICQM) of Peking University to the preprint platform arXiv believes that the LK-99 sample that their team tried to synthesize is not superconducting .
Alex Kaplan, a Twitter blogger who has been following room temperature superconductivity for a long time, paid attention to this paper this morning, and believes that the results of room temperature superconductivity have basically been settled. "According to the latest research of Peking University, LK-99 is probably just an iron magnetic material, which explains its levitating properties."
At the same time, CMTC, the Center for Condensed Matter Theory at the University of Maryland, also stated, "With great sadness, we now believe the game is over. LK-99 is not a superconductor, even at room temperature (or very low temperatures). It is It’s an inferior material with very high electrical resistance. There’s no point in fighting the facts. The numbers speak.”
Next, let's see what the paper says.
论文：《 Ferromagnetic half levitation of LK-99-like synthetic samples 》
Paper link: https://arxiv.org/abs/2308.03110
Paper details
In this work, the researchers successfully synthesized LK-99-like polycrystalline samples and observed magnetic semi-levitation in some small pieces, using powder X-ray diffraction and energy-dispersive X-ray spectroscopy to verify the composition of the samples and the Korean team Work consistently. The domestic team performed magnetization and resistance measurements on the grown samples and analyzed their properties without assumptions.
Although some magnetization measurements suggest that the samples may be diamagnetic in small magnetic fields, this diamagnetic response increases with increasing magnetic fields (to the point of levitation against gravity), the Peking University researchers said. It should be noted that diamagnetism is common in many insulators and is not equivalent to the Meissner effect. In some ferromagnetic systems, when the direction of the external magnetic field is opposite to the direction of the internal magnetization of the material, it is possible to detect a signal that appears to be reverse magnetization at very small magnetic fields.
Magnetization measurements performed by the researchers on different finished products revealed a ubiquitous soft ferromagnetic component, characterized by small hysteresis loops in the field-dependent magnetization. Specifically, the field-dependent magnetization curves measured on the bulk at 100 K (without showing half-levitation) can be viewed as a superposition of ferromagnetic and linear diamagnetic behavior.
However, in small and thin samples that do show a semi-suspension state, although a small diamagnetic signal at 10 Oe can still be observed (consistent with reports from other works, the diamagnetic response is quickly suppressed by higher intensities Based on this observation and an analysis of the shape anisotropy associated with small samples, the researchers attribute the semi-levitation to the sample's ferromagnetic response to a strong external field.
In addition, as previously reported by other research institutes, the sample resistance that Peking University tried to reproduce did not show any superconducting characteristics, but instead observed semiconducting properties consistent with other reports.
Given the observations on ferromagnetic components and semiconducting properties, this study suggests that the existence of room-temperature superconductivity in LK-99 remains to be rigorously verified. The authors specifically state that the semi-suspension observations for anisotropically shaped samples should be explained by careful scrutiny of ferromagnetism. In addition to the pursuit of superconductivity, the room-temperature ferromagnetic properties in this Pb-Cu-PO system may be worthy of further understanding by physicists.
Figure 1: X-ray diffraction pattern of the sample. It can be seen that the peak position is almost the same as that of the Korean study.
Picture S1: Pb10−xCux (PO4) 6O growth product made by Peking University.
The magnetization of the non-semisuspended sample S1 was measured, and the FC and ZFC curves of magnetization versus temperature (MT) for an external magnetic field of 10 Oe both showed positive magnetic moments and obvious branches. When the magnetic field increases to 10 kOe, the FC and ZFC MT curves remain positive and coincident, as shown in Fig. 2b.
Branched morphologies in FC and ZFC curves are commonly found in ferromagnetic materials, spin-glass materials, and superconductors, with the spin-glass state being more common at lower temperatures.
Figure 2: Magnetization of sample S1.
This observation drives us to admit the existence of a ferromagnetic component. To explore this further, the researchers performed field-dependent magnetization measurements at 100 K and 300 K, as shown in Fig. 2c. The external magnetic field was increased from 0 to 70 kOe, then decreased from 70 kOe to -70 kOe, and finally increased again from -70 kOe to 70 kOe. Similar behavior was observed at both temperatures. When the magnetic field increases from 0 to 1500 Oe, the magnetization increases with the increase of the magnetic field, and then the magnetization decreases almost linearly with the increase of the magnetic field, and even becomes negative.
This phenomenon indicates that there is a large amount of insulating components in sample S1. After careful inspection of the low-field data, a clear hysteresis loop appears (Fig. 2d), further confirming the existence of the ferromagnetic phase.
Take 100 K as an example by trying to simply subtract the linear diamagnetic part of MH from the measured data (Fig. 3). After subtracting the diamagnetic background, the remainder exhibits a typical saturation above 20 kOe. Compare some diamagnetic materials with sample S1. The subtracted diamagnetic susceptibility (∼ -2 × 10^−6 emu/g) is larger than bismuth (∼ -1.6 × 10^−6 emu/g) and water (∼ -10^−7 emu/g), but less than Bismuth pyrolytic carbon (∼ -4 × 10^−6 emu/g), suggesting that this part is not from superconductivity.
Figure 3: Field-dependent magnetization of sample S1 at 100 K (black curve).
When the researchers measured the magnetization of the granular sample S2, they found that the granular sample S2 began to shake when the Nd2Fe14B magnet approached (see Figure S3). Since this sample is too small to be weighed accurately, the unit on the vertical axis in Figure 4 is directly expressed as "emu". The FC and ZFC measurements of the MT curves showed similar positive values and similar branching to sample S1. This indicates that S1 and S2 have similar magnetic compositions. However, many other samples do not respond to Nd2Fe14B magnets, some even smaller than S2. The authors suggest that this may be related to the inhomogeneity of the sample, and that it is possible to achieve a semi-suspension state when the sample has the right size, composition or shape.
Figure S3: Real-time demonstration of sample S2's response to a permanent magnet beneath the weighing paper.
Figure 4: Magnetization of sample S2.
Finally, the authors measured the magnetization of sample S3 (semi-levitated), which exhibited a semi-magnetic levitation state on a Nd2Fe14B magnet. FC measurements were first performed on the MT curves from 100 to 300 K at 10 Oe. The magnetization of the FC curve (black curve) shows a distinctly negative value in Fig. 5(a), with little change at temperatures below 300 K, which is the same result obtained by other attempts to reproduce it.
Figure 5: Magnetization of sample S3.
However, prior to the ZFC measurements of MTs, the researchers performed field-dependent magnetization measurements at 100 K, see Fig. 5(b). As the magnetic field increases from 0 to 1500 Oe, the magnetization increases from negative to positive. The black curve in Fig. 5(c) is a zoom-in view of this process. Unlike samples S1 and S2, when the magnetic field increases above 1500 Oe, the magnetization does not decrease with increasing magnetic field, but increases with a lower slope.
The researchers concluded that the semi-levitation was caused by a magnetic moment rather than a net lift force exerted on the sample . The diamagnetic response of the sample does not play a significant role in this phenomenon. However, the authors also point out that since the interpretation in this study requires the shape of the sample to be anisotropic, this means that if samples with isotropic shape can still be semi-suspended, the role of diamagnetism may still be more important.
In fact, sample S2 in this study is of this type, but as shown in Figure S3: the movement of the magnet only slightly shakes the sample, rather than levitating it. Finally, the authors note that the ferromagnetism in the processed sample appears to be strong enough to semi-suspend the sample, but not enough for the sample to be picked up from above by the same magnet against its own weight.
The paper, which appeared on arXiv today, appears to have made the "presumption of innocence" of the LK-99 falsification harder and harder for the physics community, it is believed.
In addition to Peking University's research, Nanyang Technological University and India's National Physical Laboratory (NPL) also submitted papers, all of which stated that the prepared LK-99 has not been found to have superconducting properties.
Paper address: https://arxiv.org/pdf/2308.03544.pdf
Perhaps, this wave of room temperature superconductivity can come to an end. The real room-temperature superconducting revolution will have to wait.
