Superconductivity usually manifests itself at very low temperatures, close to absolute zero, which greatly limits the practical application of materials with this ability. A team of researchers at the University of Rochester now claims to have created a superconducting material, nitrogen-doped lutetium (or lutetium) hydride, at much more accessible temperatures and pressures. Their progress could revolutionize consumer electronics or even improve the magnetic confinement of tokamaks.
A superconductor is a metal that conducts electricity without resistance. The value of such a material is obviously enormous: imagine being able to transmit electrical energy over thousands of kilometers with virtually no loss! Unfortunately, at present, superconductivity is observed only at low temperature or very high pressure, conditions that are difficult to realize in many applications. Materials specialists have been trying to overcome this limiting barrier for several years.
Ordinary superconductivity occurs at about -273.15°C. However, decades of research led in the 1980s to the discovery of so-called “high-temperature” superconductors (around -140°C) and at atmospheric pressure; these materials belong to the class of cuprates. In the 2000s, iron-based chemistries also demonstrated superconducting properties at these temperatures. More recently, two temperature records have been set: in 2015 for sulfur hydride at -70°C (at 90 GPa), then in 2018 for lanthanum decahydride at -13°C (at 188 GPa).
A mixture of rare earth elements, hydrogen and nitrogen
To date, there is no superconducting material at ambient temperature and pressure. But researchers from the University of Rochester are closest to this ultimate goal: in Nature, they report the creation of nitrogen-doped lutetium hydride, which exhibits superconductivity at 294 K (20.8 ° C) and 10 kilobars (i.e. 1 GPa). pressure. “With this material, the dawn of external superconductivity and applied technologies has come,” said Ranga Diaz, assistant professor of mechanical engineering and physics, who led the study.
True, a pressure of 1 GPa is still relatively high (given that the average pressure at sea level is 1013.25 hPa, or 10,000 times less!). However, there are widely used methods, especially in the production of microchips, to bond materials with even higher internal chemical pressures.
Hydrides, created by combining rare earth metals with hydrogen and then adding nitrogen or carbon, have been the mainstay of the work of many materials scientists for several years. These hydrides form framework structures in which rare earth metal ions act as charge donors, providing enough electrons to promote the dissociation of dihydrogen molecules; nitrogen and carbon help to stabilize the material.
Lutetium hydride sample about one millimeter in diameter under a microscope. © University of Rochester/J. Adam Fenster
The material under consideration in this new study is nitrogen-doped lutetium hydride (marked NDLH). Lutetium was a good candidate: its electronic configuration is such that it promotes the electron-phonon interaction necessary for superconductivity to occur at room temperature, explains Diaz. It remained to find a way to lower the required pressure.
For this, the team used nitrogen. Like carbon, it has a rigid atomic structure that helps stabilize the lattice within the material. This structure provides the stability necessary for the appearance of superconductivity at low pressure.
Amazing color change
So the researchers created a gas mixture of 99% hydrogen and 1% nitrogen, which they placed in a reaction chamber with a pure lutetium sample; the components were left to react for two to three days at 200°C. The resulting compound was then pressed in a diamond anvil cell. It was then that the researchers witnessed a striking visual transformation: the initially blue material took on a pink hue at the onset of superconductivity, then turned bright red as it reached the non-superconducting metallic state.
To create superconductivity, a pressure of 1 GPa was required, which is almost two orders of magnitude lower than the pressure used in their previous experiments.
Namely, that the same team has already announced in 2020 the creation of two superconducting materials – carbon disulfide hydride and yttrium superhydride – at around 15 ° C and 269 GPa, then -11 ° C and 182 GPa, respectively. However, the study, published in Nature, was heavily criticized and the journal’s editors eventually dropped it. In particular, the researchers were accused of falsifying data. To avoid further criticism, they claim they have redoubled their efforts to document their research.
If scientists are so interested in superconducting materials, it is because they have two important properties: zero electrical resistance and the expulsion of any magnetic field from within the material (a phenomenon known as the Meissner effect). These two properties suggest huge technological advances: lossless transmission of electricity, the development of maglev trains, or even improved magnetic confinement of plasma in tokamaks.
The work of Diaz and his team brings us a little closer to these achievements. Diaz is convinced that NDLH can significantly accelerate the development of tokamaks designed for nuclear fusion. Because NDLH can generate a “huge magnetic field” at room temperature, it can be used to trap plasma inside a reaction chamber.
The professor also mentions the possibility of training automatic learning algorithms on data accumulated during experiments with superconductors conducted in his laboratory; these algorithms can help identify other potential superconducting materials by testing different combinations of rare earths, hydrogen, nitrogen, and carbon. The goal is to create various superconductors. “In everyday life, we use many different metals for different purposes, so we will need different types of superconducting materials,” he concludes.