Can negative temperature be a superconductor

New temperature record for superconductors

An important step towards superconductivity at room temperature: For the first time, researchers have discovered a material that can conduct electricity loss-free at minus 23 degrees - a new temperature record. The maximum value for previous high-temperature superconductors was 50 degrees lower. The lanthanum hydride that has now been tested could be a representative of materials that may even become superconducting at room temperature, as the researchers report in the specialist journal “Nature”.

Whether graphene, copper compounds such as cuprates or some metal hydrides: some materials can conduct electricity with almost no resistance - they are superconductors. After this exotic property was initially only known at temperatures close to absolute zero, researchers have now discovered high-temperature superconductors that even become resistance-free at "only" minus 70 degrees, such as hydrogen sulfide.

However, the low temperatures required for the previous superconductors are a major hurdle for practical use. There are already first attempts with superconductor cables in urban power grids, but the need for cooling makes such approaches extremely complex and expensive.

Structure of the superconducting lanthanum hydride. © Drozdov et al

Lanthanum in a hydrogen cage

Now an experimental breakthrough gives hope: researchers led by Mikhael Eremets from the Max Planck Institute for Chemistry in Mainz have now discovered a material that has to be cooled down even less than the previous record holder, hydrogen sulfide. Based on their experience with this material, they have now tested another metal hydride under high pressure for its superconductivity: lanthanum hydride (LaH3).

"Such hydrides have a clathrate-like structure in which the host atom - calcium, yttrium or lanthanum - sits in the center of a cage made of hydrogen atoms," the researchers explain. Under high pressure, these “cages” can form a structure that makes superconductivity possible - at least that's the theory.

Transition already at minus 23 degrees

In the experiment, the researchers clamped a sample of the lanthanum hydride in a hydrogen atmosphere in a diamond punch press and put it under gradually increasing pressure. This changed the binding structure of the material and the particularly hydrogen-rich lanthanum hydride LaH was formed at around 170 gigapascals - a pressure more than a million times higher than in the earth's atmosphere10.

The decisive factor, however, is that the material already showed a first transition to the superconducting state at 250 Kelvin - around minus 23 degrees Celsius - it lost its resistance to electron conduction and the critical temperature threshold for this could be influenced by external magnetic fields - both features are considered characteristic signs for superconductivity. A third feature, the displacement of a magnetic field from within, the researchers were only unable to detect because of the small size of the sample.

Room temperature superconductors are getting closer

"As far as we know, this is the highest critical temperature that has ever been reported in a superconducting material," state Eremets and his team. “Our study is a further step towards superconductivity at room temperature.” According to the researchers, yttrium superhydrides, but also certain carbon compounds, could be promising candidates for such superconductors with even higher critical temperatures.

The scientists are also already working on reducing the pressure required to produce these superhydrides. “Our next goal is to generate samples that can be synthesized at high pressure, but still remain superconducting even at normal pressure,” explains co-author Vitali Prakapenka from the University of Chicago. (Nature, 2019; doi: 10.1038 / s41586-019-1201-8)

Source: Nature, University of Chicago

23 May 2019

- Nadja Podbregar