Independent experiments have been unable confirm the claims that, even outside controlled conditions, the materials are capable of transmitting electricity without energy loss
Superconductors expel magnetic fields, causing the phenomenon of levitation
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Temperatures continue to rise inside and outside the laboratories looking for superconducting materials with zero resistance, capable of conducting electricity without losing any energy in the form of heat. On March 8 of this year, a team led by physicist Ranga Dias of the University of Rochester, USA, published a scientific article in the journal Nature describing the synthesis of the first superconductor that functions at room temperature. According to the study, the compound LuNH — a material based on the silvery-white metal lutetium mixed with nitrogen and hydrogen — presents zero resistance at 294 Kelvin (K), equivalent to 21 degrees Celsius (°C).
The supposed achievement, far from being proven for now, would have revolutionary potential. The superconductors currently available — used in MRI machines, large particle accelerators, and some high-speed magnetic trains — only work when cooled to extremely low temperatures close to absolute zero (0 K), which is an expensive process. Absolute zero kelvin is equivalent to negative hundreds of degrees Celsius.
The most important parameter of a superconductor is the critical temperature (Tc), usually given on the Kelvin scale, below which a material presents zero resistance to an electric current. This is what makes the material an electrical superconductor. Another defining characteristic is the ability to expel magnetic fields, a property that can be used to produce the phenomenon of levitation.
According to the Nature article, LuNH had to be subjected to pressure 10,000 times greater than the Earth’s atmosphere to act as a perfect conductor of electricity. Even so, since the 1911 discovery of superconductivity in mercury at -269 °C (4.15 K), no one else has ever claimed to have identified a superconductor with such a high Tc. No one other than Dias himself, that is, in a previous article published in Nature in October 2020. In that paper, written with colleagues from the University of Nevada and his team in Rochester, the physicist claimed to have produced a compound made of carbon, hydrogen, and sulfur that became a superconductor at 15 ºC if subjected to 2.6 million times the pressure of Earth’s atmosphere.
No energy is lost in the form of heat when an electric current passes through a superconductor
Six months after publication, there are growing cracks in the reputation of LuNH as a room-temperature superconductor. “I’m a little skeptical about this material. In this field of science, the first result is always provisional until it is confirmed by other labs,” says physicist Wilson Ortiz, head of the superconductivity and magnetism group at the Federal University of São Carlos (UFSCar). No independent research group has managed to reproduce the experiment — in the laboratory or computationally — that indicated its superconductivity at 21 °C.
The situation surrounding the previous study from three years ago involving the compound made of carbon, hydrogen, and sulfur is even more uncomfortable. In September last year, Nature retracted the 2020 article by Dias’s team. “Following publication [of the article], questions were raised regarding the manner in which the data in this paper have been processed and analyzed,” the journal’s editors wrote in a notice explaining the decision.
To make matters worse, in August 2023 the prestigious journal Physical Review Letters (PRL) retracted a 2021 article by Dias’s group and its colleagues from Nevada. The retracted paper did not relate specifically to superconductivity, but to the electrical properties of manganese disulfide (MnS2), which can behave either as an insulator or as a metal.
Four independent teams of experts reviewed the PRL paper and expressed “serious doubts” about the data presented in a figure showing the material’s electrical resistance curves, according to the retraction note issued by the journal. All 10 authors of the paper agreed with the journal’s verdict, with the exception of Dias, who has recently refused to give interviews about his work.
J. Adam Fenster / Universidade de RochesterPhysicist Ranga Dias of the University of Rochester, who claims to have produced superconductors that work at room temperatureJ. Adam Fenster / Universidade de Rochester
The University of Rochester physicist was born in Sri Lanka and completed his doctorate at Washington State University in 2013. He did a postdoctorate at Harvard University before being hired as a professor at the University of Rochester in 2017. Together with physicist Ashkan Salamat of the University of Nevada, with whom he worked on many studies, Dias founded the company Unearthly Materials in 2020, selling (supposed) superconductors that function at room temperature under modest pressures.
“The most recent article by Ranga Dias’s team does not properly explain the composition of this LuNH material nor the method used to obtain it,” points out Luiz Eleno, a materials engineer from the Lorena School of Engineering at the University of São Paulo (EEL-USP). In partnership with researchers from Sapienza University in Rome (Italy), the University of Cambridge (United Kingdom), and the Technical University of Graz (Austria), Eleno and Pedro Ferreira, a physical engineer studying his PhD under his supervision, published a study in Nature Communications on September 4 in which they ruled out any possibility that a combination of the elements lutetium, hydrogen, and nitrogen can form a superconducting material with characteristics similar to those described by the Rochester group.
The study involving the Brazilians was not experimental in nature. They did not attempt to reproduce the same material and results in a laboratory as presented by Dias — although some international groups did, also without success. Instead, they used computational modeling to virtually design the material and attempt to predict its electronic and magnetic properties and other parameters using existing knowledge of solid-state physics and various modern computing techniques, such as artificial intelligence and machine learning.
They thus simulated different temperature and pressure conditions, using a cluster of computers to create different versions of crystals based on the combination of chemical elements used by the Rochester team. The ultimate objective of the computational experiment was to determine whether any compound made of these ingredients could produce a candidate material for a superconductor at room temperature.
In total, 200,000 compounds with different atomic structures were simulated on the computer. Of just over 150 materials that appeared to be stable when subjected to an electrical current, the group identified 52 that showed potential to behave as superconductors, but none at temperatures close to the temperatures at which human beings live. “According to the simulations, the compound that performed best could only transmit a current without energy loss at 40 K, or -233.15 ºC,” says Eleno.
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It was a spontaneous study, planning for which began as soon as the Rochester group’s article appeared in Nature last March. At the time, Ferreira was at the Technological University of Graz, working with Christoph Heil’s team on another topic in the field of superconductivity. However, given the enormous interest and controversy sparked by Dias’s study, they decided to concentrate their efforts on the LuNH compound.
“If it really were a superconductor at room temperature as they claimed, it would be the scientific discovery of the century,” says Ferreira, who was lead author of the Nature Communications paper and is studying his PhD with a fellowship from FAPESP. “That’s why we started our study so quickly. We knew that every major research group searching for new materials, especially in the area of superconductors, would be doing the same.”
The old and perhaps slightly pretentious adage that high-temperature superconductors are the Holy Grail of physics (something highly sought after, but never found) continues to be repeated. And it makes sense, despite the fact there are so many other potentially revolutionary objectives in various scientific fields, including physics itself. The discovery of a material that acts as a superconductor at room temperature (and pressure) would boost new applications in several fields, such as quantum computing, transport, and of course, electricity transmission, in addition to generating unparalleled energy savings.
“A lot of money is spent on refrigeration systems that use liquid helium to keep superconductors below their Tc,” points out Pascoal Pagliuso, a physicist from the University of Campinas (UNICAMP) who specializes in condensed matter physics. In theory, an electric current would circulate infinitely in a superconducting material — as long as the temperature and pressure conditions that lead to the occurrence of zero electrical resistance are maintained. In an experiment carried out in the UK, a current was maintained in a superconducting ring for two and a half years, only ending when a strike delayed the delivery of liquid helium needed to keep the material below its critical temperature.
In addition to the practical problems, there are also unresolved theoretical issues in high-temperature superconductivity. The Bardeen–Cooper–Schrieffer (BCS) theory provides a basis for understanding the emergence of zero electrical resistance in classical superconductors, which generally operate in extremely cold conditions. It states that atoms and electrons vibrate in coordination within the structural mesh of superconducting crystals without causing a loss of energy when electricity passes through them. “But in many superconductors with a higher critical temperature, we do not know the microscopic mechanism that results in superconductivity and BCS theory cannot explain these cases,” says Ortiz.
Léo Ramos Chaves / Revista Pesquisa FAPESPMagnetic resonance imaging scanners use superconductors maintained at extremely low temperaturesLéo Ramos Chaves / Revista Pesquisa FAPESP
The passing or blocking of an electric current is a quantum phenomenon that can be explained in simplified terms as follows: Electrons, as their name suggests, are the particles responsible for conducting electricity within materials. In insulators, they are so close to the nucleus of the atoms that they cannot move and thus cannot make a current flow. In materials that conduct electricity but are not superconductors, electrons can move and transmit part — but not all — of an electrical current.
Some, however, collide with the nuclei of the atoms, which have a positive charge and attract negatively charged electrons. These collisions cause energy to be lost in the form of heat. “That is why the wires of a conductive metal, like copper, get hot,” explains Mauro Doria, a physicist from the Federal University of Rio de Janeiro (UFRJ) who specializes in superconductivity, magnetism, and fluids. “Its electrical resistance is not zero.” In materials that transmit electrical currents but are not perfect superconductors, at least 15% of the energy is dissipated as heat.
Ranga Dias’s team is not the first to claim it has discovered superconductivity at room temperature. Several others have made the same assertion in the past, all unproven. In parallel with the discussions and controversies surrounding the studies by the Rochester group, South Korean researchers from the Center for Quantum Nanoscience in Seoul made an even more spectacular claim in a preprint shared on the arXiv repository last July. They wrote that they had created a compound of copper, lead, phosphorus, and oxygen called LK-99, which offers zero resistance to an electric current at ambient temperatures and pressures.
Despite garnering a great deal of attention, the paper was not accepted for publication by any journals, and experts said it lacked scientific rigor. “They made interpretative errors in both the text and the figures that they claim indicated the occurrence of superconductivity,” says Pagliuso. “It was very hurried and amateurish.” When a compound reaches the temperature at which it becomes a superconductor, its resistance to an electric current drops abruptly to zero. “The Korean study showed a large sudden drop in electrical resistance, but it did not appear to reach zero in the data provided.”
Perhaps unsurprisingly, the difficulties in finding materials that allow an electric current to flow completely freely with zero resistance in conditions similar to the natural environment once led Argentine physicist Elbio Dagotto of the University of Tennessee to declare that high-temperature superconductivity was “the Vietnam of theoretical physics.”
BCS, a theory for superconductivity Coordinated interaction of electrons and atoms allows current to pass through a material with no energy loss
Alexandre Affonso / Revista Pesquisa FAPESP
Presented in 1957 by American physicists John Bardeen (1908–1991), Leon Cooper, and John Schrieffer (1931–2019), BCS theory (named after the final initials of the three scientists) is currently the best scientific explanation for the microscopic mechanism behind superconductivity. Although it does not explain the mechanisms that enable electrical conductivity with zero resistance in materials that act as superconductors at higher temperatures, the theory from the middle of last century introduces a fundamental concept for a general understanding of this quantum phenomenon: the formation of Cooper pairs.
A Cooper pair is the counterintuitive binding of two electrons that in certain materials, under certain favorable conditions — such as temperatures close to absolute zero or extremely high pressures — allow an electric current to be conducted without losing any energy. Because they have a negative charge, electrons should repel each other rather than being attracted. However, when a material becomes superconductive, the pairs of electrons flowing within the structure of a crystal move very close together (but without joining or colliding) and begin to interact, like a quasi-particle, with the atoms of the material, which have a positive charge.
The Cooper pairs move toward and away from the atoms in an orderly manner — a collective excitation or vibration called a phonon — and travel through the structural makeup of the crystal without causing collisions. In simple terms, it is the collisions between electrons and the nuclei of atoms that make materials such as copper wires or silicon chips perform as conductors or semiconductors, losing some of their energy in the form of heat. “BCS theory is complete in its description of superconductivity, but it does not really explain how Cooper pairs form in high-temperature superconductors,” says UNICAMP physicist Pascoal Pagliuso. Today, there is more or less a consensus that in addition to BCS theory, which won the three scientists the 1972 Nobel Prize in Physics, new theories need to be developed to explain the emergence of superconductivity at higher temperatures.
Project Ab initio study of superconducting and topological systems (nº 20/08258-0); Grant Mechanism Doctoral (PhD) Fellowship; Supervisor Luiz Tadeu Fernandes Eleno (USP); Beneficiary Pedro Pires Ferreira; Investment R$384,888.43.
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