LK-99

LK-99

3D structure
Identifiers
3D model (JSmol)	Interactive image
InChI InChI=1S/Cu.6H3O4P.O.9Pb/c;6*1-5(2,3)4;;;;;;;;;;/h;6*(H3,1,2,3,4);;;;;;;;;;/q+2;;;;;;;-2;9*+2/p-18 Key: KZSIWLDFTIMUEG-UHFFFAOYSA-A
SMILES [Pb+2].[Pb+2].[Pb+2].[Pb+2].[Pb+2].[Pb+2].[Pb+2].[Pb+2].[Pb+2].[Cu+2].O=P([O-])([O-])[O-].O=P([O-])([O-])[O-].O=P([O-])([O-])[O-].O=P([O-])([O-])[O-].O=P([O-])([O-])[O-].O=P([O-])([O-])[O-].[O-2]
Properties
Chemical formula	CuO25P6Pb9
Molar mass	2514.2 g·mol−1
Appearance	grey black solid
Density	≈6.699 g/cm3
Structure
Crystal structure	hexagonal
Space group	P63/m, No. 176
Lattice constant	a = 9.843 Å, c = 7.428 Å
Lattice volume (V)	623.2 Å3
Formula units (Z)	1
Related compounds
Related compounds	Oxypyromorphite (lead apatite)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

LK-99 (from Lee‒Kim‒1999) is a potential room-temperature superconductor with a gray‒black appearance. It has a hexagonal structure slightly modified from lead‒apatite, by introducing small amounts of copper. The material was first discovered and manufactured by a team of researchers including Sukbae Lee (이석배) and Ji-Hoon Kim (김지훈) from Korea University. The team claims it functions as a superconductor at ambient pressure and below 400 K (127 °C; 260 °F).

As of 3 August 2023, the synthesis of LK-99 and observation of its superconductivity at any temperature have not been peer-reviewed or independently replicated. The announcement was widely shared and the reaction by the scientific world was mainly skeptical due to the extraordinary nature of the claims, the repeated failure of previous claims of room-temperature superconductors, and errors and inconsistencies in the pre-published papers. Independent teams are attempting to replicate the South Korean team's work, with results expected in August 2023 owing to the straightforward method of producing the material.

The initial studies announcing the discovery were uploaded to the open-access repository of electronic preprints arXiv. Lee claimed that the uploaded preprint papers were incomplete, while coauthor Hyun-Tak Kim (김현탁) stated that one of the papers contained defects.

Chemical properties

The chemical composition of LK-99 is approximately Pb₉Cu(PO₄)₆O such that—compared to pure lead-apatite (Pb₁₀(PO₄)₆O)—approximately one quarter of Pb(II) ions in position 2 of the apatite structure are replaced by Cu(II) ions.

Synthesis

Lee et al. provide a method for chemical synthesis of LK-99 material by producing lanarkite from a 1:1 molar mixing of lead(II) oxide (PbO) and lead(II) sulfate (Pb(SO₄)) powders, then heating at 725 °C (1,000 K; 1,340 °F) for 24 hours:

PbO + Pb(SO₄) → Pb₂(SO₄)O

Additionally, copper(I) phosphide (Cu₃P) was produced by mixing copper (Cu) and phosphorus (P) powders in a 3:1 molar ratio in a sealed tube under a vacuum and heated to 550 °C (820 K; 1,000 °F) for 48 hours:

Cu + P → Cu₃P

Lanarkite and copper phosphide crystals were ground into a powder, placed in a sealed tube under a vacuum, and heated to 925 °C (1,200 K; 1,700 °F) for between 5‒20 hours:

Pb₂(SO₄)O + Cu₃P → Pb_10-xCu_x(PO₄)₆O + S (g), where (0.9 < x < 1.1)

Physical properties

(a) Diamagnetic susceptibility measurements of LK-99, (b) sample of LK-99 partially levitating over large magnet

The material is claimed to be a room-temperature superconductor. The original published articles do not claim to have seen definitive features of superconductivity, zero resistance and the Meissner effect, but show the material exhibiting strong diamagnetic properties, including a video of sample of the material partially levitating on top of a large magnet, which is correlated with superconductivity.

As many materials can spuriously seem like potential candidates for high-temperature superconductivity, in addition to a zero-resistance mode and a clear Meissner effect, researchers generally also demonstrate other expected properties such as flux pinning, AC magnetic susceptibility, the Josephson effect, a temperature-dependent critical field and current, or a sudden jump in specific heat around the critical temperature. As of August 1, none of these have been observed by the original experiment or attempted replications.

Proposed mechanism for superconductivity

Partial replacement of Pb²⁺ ions (measuring 133 picometres) with Cu²⁺ ions (measuring 87 picometres) is said to cause a 0.48% reduction in volume, creating internal stress inside the material. The internal stress is claimed to cause a heterojunction quantum well between the Pb(1) and oxygen within the phosphate ([PO₄]³⁻) generating a superconducting quantum well (SQW).

Lee et al. claim to show LK-99 exhibits a response to a magnetic field (potentially due to the Meissner effect) when chemical vapor deposition is used to apply LK-99 to a non-magnetic copper sample. Pure lead-apatite is an insulator, but Lee et al. claim copper-doped lead-apatite forming LK-99 is a superconductor, or at higher temperatures, a metal. They do not claim to have observed any change in behavior across a transition temperature.

The paper's mechanisms were based on a 2021 paper by Hyun-Tak Kim describing a novel "BR-BCS" theory of superconductivity combining a classical theory of metal-insulator transitions with the standard Bardeen–Cooper–Schrieffer theory of superconductivity. They also use ideas from the theory of hole superconductivity by J.E.Hirsch, another controversial work.

On 1 August 2023, three independent groups published analyses of LK-99 with density functional theory (DFT). Sinéad Griffin of Lawrence Berkeley National Laboratory analyzed it with the Vienna Ab initio Simulation Package, showing that its structure would have correlated isolated flat bands, one of the signatures of high-transition-temperature superconductors. Si and Held found similar flat bands and conjectured that LK-99 is a Mott or charge transfer insulator, that electron or hole doping is needed to make it (super)conducting.

Compound name

The name LK-99 is from the initials of discoverers Sukbae Lee and Ji-Hoon Kim, and the year of discovery (1999). The pair had originally been working with Professor Tong-Shik Choi (최동식) at Korea University in the 1990s.

In 2008, researchers from Korea University founded the Quantum Energy Research Centre (퀀텀 에너지연구소; also known as Q-Centre). Lee would later become CEO of Q-Centre, and Kim would become director of research and development (R&D) at Q-Centre.

Publication history

In 2020, an initial paper was submitted to Nature, but it was rejected. Similarly-presented research on room-temperature superconductors by Ranga P. Dias had been published in Nature earlier that year, and received with skepticism—Dias's paper would subsequently be retracted in 2022 after its data was found to have been falsified.

In 2020, Lee and Ji-Hoon Kim filed a patent application. A second patent application (additionally listing Young-Wan Kwon), was filed in 2021, which was published on 3 March 2023. On 4 April 2023, a Korean trademark application for "LK-99" was filed by the Q-Centre.

In February 2023, the Q-Centre published a video on YouTube claiming to show the magnetic properties of a thin layer of LK-99 thermally deposited on a copper plate.

Scholarly articles and preprints

A series of academic publications summarizing initial findings came out in 2023, with a total of seven authors across four publications.

On 31 March 2023, a Korean-language paper, "Consideration for the development of room-temperature ambient-pressure superconductor (LK-99)", was submitted to the Korean Journal of Crystal Growth and Crystal Technology. It was accepted on 18 April, but was not widely read until three months later.

On 22 July 2023, two preprints appeared on arXiv. One listed Young-Wan Kwon, former Q-Centre CTO, as third author. A second preprint listed as third author Hyun-Tak Kim, former principal researcher at the Electronics & Telecommunications Research Institute and professor at the College of William & Mary. On 23 July, the findings were also submitted to APL Materials for peer review.

On 28 July 2023, Kwon presented the findings at a symposium held at Korea University. That same day, Yonhap News Agency published an article quoting an official from Korea University as saying that Kwon was no longer in contact with the University. The article also quoted Lee saying that Kwon had left the Q-Centre Research Institute four months previously; that the academic papers on LK-99 were not finished; and that the papers had been uploaded to arXiv without the other authors' permission.

On 31 July 2023, a group led by Kapil Kumar published a preprint on arXiv documenting their replication attempts, which confirmed the structure using X-ray crystallography (XRD) but failed to find diamagnetism or levitation.

On 1 August 2023, a Q-Centre representative told SBS News that the original samples referenced in the paper would be released to the world for verification soon.

Authors

Author credit and affiliation matrix:

Author Affiliation	Lee, Sukbae (이석배)	Kim, Ji-Hoon (김지훈)	Kim, Hyun-Tak (김현탁)	Im, Sungyeon (임성연)	An, SooMin (안수민)	Kwon, Young-Wan (권영완)	Auh, Keun Ho (오근호)	Choi, Tong-Shik (최동식)
HYU							Professor Emeritus
KU‒KIST						'former Research Professor†
W&M			Professor
Q-Centre (주)퀀텀에너지연구소	CEO	R&D Director		Yes	Yes	'former CTO	CTO
Patent (2020)	1	2
Patent (2021)	1	2				3
Lee & Kim+ (18 April 2023)	1	2		3	4	5	6	Acknowledged
Lee & Kim+ (22 July 2023.a)	1	2		Acknowledged	Acknowledged	3	Acknowledged
Lee & Kim+ (22 July 2023.b)	1	2	3	4	5	Acknowledged	6	Acknowledged

Response

Materials scientists and superconductor researchers responded with skepticism. The highest-temperature superconductors known at the time of publication had a critical temperature of 250 K (−23 °C; −10 °F) at pressures of over 170 gigapascals (1,680,000 atm; 24,700,000 psi). The highest-temperature superconductors at atmospheric pressure (1 atm) had a critical temperature of at most 150 K (−123 °C; −190 °F).

On 2 August 2023, the The Korean Society of Superconductivity and Cryogenics established a verification committee as a response to the controversy and unverified claims of LK-99, in order to arrive at conclusions over these claims. The verification committee is headed by Prof. Kim Chang-Young of Seoul National University and consists of members of the university, Sungkyunkwan University and Pohang University of Science and Technology. Upon formation, the verification committee did not agree that the two 22 July arXiv papers by Lee et al. nor the publicly available videos at the time supported the claim of LK-99 being a superconductor.

As of 2 August 2023, the measured properties do not prove that LK-99 is a superconductor as the published material does not fully explain how the LK-99's magnetisation can change, demonstrate its specific heat capacity, or demonstrate it crossing its transition temperature. An alternative explanation for LK-99's stated partial magnetic levitation could be solely from non-superconductive diamagnetism.

Public response

The claims of a room temperature superconductor in the 22 July papers by Lee et al. went viral on social media platforms the following week, including Twitter. Despite interest from commentators, scientists interviewed by the press remained skeptical.

A video from Huazhong University of Science and Technology uploaded on 1 August 2023 apparently showing a micrometre-sized sample of LK-99 levitating went viral on Chinese social media. Gathering millions of views, it became the second most viewed video on Bilibili the next day. A researcher from the Chinese Academy of Sciences refused to comment on the video for the press, dismissing the claim as "ridiculous". Public excitement grew after the video made its way to western social media, with a prediction market briefly putting the chance of successful replication at 60%. As the topic of LK-99 trended on Twitter for days at the start of August, users began to create memes about "floating rocks" and suggested backing stocks in superconductors. It was concurrently reported in the press that the study had caused a surge in Korean and Chinese technology stocks.

Replication attempts

As of 3 August 2023, the experiment has not been successfully replicated, despite the initial experiments being completed in 2020. No replication attempts have yet been peer-reviewed. After the July 2023 publications release, independent groups reported that they had begun attempting to reproduce the synthesis, with initial results expected within weeks. However while positive results can come quickly, negative results are slow, as "falsification needs to verify all possibilities, and it will take a lot of time."

The first attempts that published results did not observe levitation or diamagnetism, and their samples had high resistivity. None have published tests of flux pinning or specific heat capacity.

Some unpublished results shared brief videos of their work which received large public of attention. On 1 August 2023, a team at Huazhong University of Science and Technology in China reported producing tiny flakes that showed diamagnetic levitation, on their second attempt. On 2 August 2023, a team around Prof. Sun Yue at Southeast University, claimed to have measured zero resistance in a flake of LK-99 up to a temperature of 110 K in an online video. Doubts were expressed by experts that their results had what looked like a large measurement artefact, did not show the expected dropoff to zero resistance, were quite noisy and were unable to measure resistance below 10 µΩ, which is high for measurements of superconductors.

Theoretical studies

In the initial papers, the theoretical explanations for potential mechanisms of superconductivity in LK-99 were incomplete. Later analyses by other labs have added further simulations and theoretical evaluations of the material's electronic properties from first principles.

Group	Date	Country	Status	Result	References	Notes
Chinese Academy of Sciences (SYNL)	2023-07-29	China	Theoretical study available	First-principles study of the electronic structure of LK-99 and other variants. Expresses no opinion on room-temp superconductivity, but suggests gold-doped lead apatite may have stronger effects.		Junwen Lai, et al., arXiv.
Lawrence Berkeley National Laboratory	2023-08-01	United States	Theoretical study available	DFT analysis on a simplified 3D structure explores possible electronic structure that could be favorable for superconductivity, suggests slightly decreased lattice constant.		Sinéad Griffin, arXiv.
Northwest University & TU Wien	2023-08-01	China Austria	Theoretical study available	Similar results from DFT analysis. Conjectures superconductivity might be possible, but only when LK-99 is doped, and that diamagnetism without superconductivity is unlikely.		Liang Si & Karsten Held, arXiv.
UC Boulder, National Renewable Energy Laboratory, & King's College	2023-08-01	United States United Kingdom	Theoretical study available	Similar results from DFT analysis. Conjectures this class of material (weak interaction of Copper‒Oxygen, while minimising hybridisation) shows promise for high temperature superconductivity, regardless of realisation of LK-99.		Rafal Kurleto, et al., arXiv.

Further reading

• 최동식 (17 May 1994). 초전도 혁명의 이론적 체계 [Theoretical Framework of the Superconducting Revolution] (in Korean). ISBN 89-7641-276-1. Archived from the original on 27 July 2023. Retrieved 27 July 2023.
• "…최동식…". Donga Science (Interview with T.S. Chair) (in Korean): 106‒107. September 1993.
• Krivovichev, Sergey V.; Burns, Peter C. (17 October 2002). "Crystal chemistry of lead oxide phosphates: crystal structures of Pb₄O(PO₄)₂, Pb₈O₅(PO₄)₂ and Pb₁₀(PO₄)₆O". Zeitschrift für Kristallographie – Crystalline Materials. Munich: Oldenbourg Wissenschaftsverlag (published 1 May 2003). doi:10.1524/zkri.218.5.357.20732. The compound Pb₁₀(PO₄)₆O has been designated 'oxypyromorphite' … Pb₁₀(PO₄)₆O crystallizes with an apatite-type structure. The structure contains a single O atom that is not part of a PO₄ tetrahedron; it has a site occupancy factor of 0.25 and is located on the 6₃ axis.
• Lowe, Derek (26 July 2023). "Breaking Superconductor News". Chemical News. In the Pipeline (blog). American Association for the Advancement of Science. Archived from the original on 26 July 2023. Retrieved 26 July 2023 – via Science.org.
• Orf, Darren (27 July 2023). "Scientists Claim They Found the Holy Grail of Superconductors". Energy. Popular Mechanics. Retrieved 28 July 2023.
• Harris, Margaret (27 July 2023). "Have scientists in Korea discovered the first room-temperature, ambient-pressure superconductor?". Superconductivity blog. Physics World. Institute of Physics. Archived from the original on 28 July 2023. Retrieved 29 July 2023.
• Wu, Yuewei; Sun, Wenya; Li, Ruiyang (28 July 2023). 南大教授谈韩国室温超导：不像超导，正重复实验—新闻 [NU professor talks about room temperature superconductivity found in South Korea: Unlike superconductivity, experiments are being repeated]. ScienceNet.cn (broad overview) (in Chinese). Archived from the original on 31 July 2023. Retrieved 31 July 2023.

External links

• Official website "Archived copy". Archived from the original on 1 August 2023. Retrieved 26 July 2023.{{cite web}}: CS1 maint: archived copy as title (link) CS1 maint: bot: original URL status unknown (link)
• Magnetic Property Test of LK-99 Film (video). Quantum Energy Research Centre. 26 January 2023. Retrieved 25 July 2023 – via Youtube.- This effect is a conductive copper plate induced by a magnetic.
• Kim, Hyun-Tak (25 July 2023). Superconductor Pb_10-xCu_x(PO₄)₆O showing levitation at room temperature and atmospheric pressure and mechanism (video). Retrieved 25 July 2023 – via ScienceCast."Archived copy". Archived from the original on 31 July 2023. Retrieved 31 July 2023.{{cite web}}: CS1 maint: archived copy as title (link) CS1 maint: bot: original URL status unknown (link)