Superconductors conduct electric current with no resistance when cooled below a certain critical temperature. They have many applications where strong magnetic fields are needed, for example in hospital scanners and particle accelerators.
Although superconductivity was discovered more than 100 years ago, the synthesis of new superconductors based on theoretical forecast has so far remained elusive.
The computational design of superconductors is challenging because only “conventional” superconductors are understood well enough by theory with predictive power.
Determining the critical temperature below which the material becomes superconducting also needs a lot of computing power.
Moreover, the materials' stability and superconductivity tend to be inversely correlated so that many proposed superconducting materials are not stable enough to exist, and those that do form are poor superconductors.
In 2010, a team of scientists predicted iron tetraboride (FeB4) to be a superconductor. This new material is made out iron and boron, two cheap elements, and it should exhibit a brand-new crystal structure.
Scanning electron microscopy image of the Iron Tetraboride sample as synthesised in a high-pressure multi anvil apparatus. Credit: N. Dubrovinskaia.
The team of scientists was led by Natalia Dubrovinskaia and Leonid Dubrovinsky from the University of Bayreuth (Germany) and also comprised scientists from Max Planck Institute for Chemical Physics of Solids, Dresden, and Technische Hochschule Wildau, Wildau (Germany); National Institute of Chemical Physics and Biophysics, Tallinn (Estonia); Università degli Studi di Milano, Milano (Italy); University of Antwerp, Antwerp (Belgium); Binghamton University, Vestal (USA), and the European Synchrotron ESRF in Grenoble (France).
The synthesis of iron tetraboride was a particular research success of Huiyang Gou who devoted himself to this challenge as a postdctoral Humboldt Research Fellow at the University of Bayreuth. Iron tetraboride, which does not occur in nature, was produced by compressing the ingredients to pressures above 80.000 bar and high temperatures of 1500 °C. “Here at the University of Bayreuth, we have at our disposal unique instruments for synthesising materials under extreme conditions of pressure and temperature”, says Huiyang Gou.
This new material is surprisingly hard, close to that of diamond
The scientists first proved that the material is indeed superconducting. They then came to Grenoble where single crystal X-ray diffraction at the ESRF confirmed the theoretically predicted atomic structure of iron tetraboride.
The team was surprised to discover that the material is also highly incompressible and superhard with a nanoindentation hardness of 65 GPa, close to that of diamond.
“The only way to study the elastic behaviour of FeB4 was synchrotron-based compressibility measurements in a diamond anvil cell. These experiments were carried out at the ESRF, which provides a unique opportunity for full structural refinement at Megabar pressure range.” says Michael Hanfland, an ESRF scientist who took part in the study.
The diamond anvil cell used on ID09. Both diamonds are visible at the centre of the elements.
“This compound opens a new class of highly desirable materials combining advanced mechanical properties and superconductivity. Although superconductors persist in being one of the most challenging classes of materials to develop, our results demonstrate the possibility of designing them ‘from scratch’”, summarises Natalia Dubrovinskaia. She adds, that “our follow-up studies should answer questions on what defines superconductivity in this material and how it can be improved”.
The development of superhard superconductors opens the door to designing new superconducting nano- and micro-electromechanical systems requiring high-strength superconducting elements. It also enables the observation of new fundamental effects using high-pressure cryogenic micro-tools, or nano-probes for electrical measurements at very low temperatures, manufactured from such advanced materials.
Principal publication and authors
Huiyang Gou, Natalia Dubrovinskaia, Elena Bykova, Alexander A. Tsirlin, Deepa Kasinathan, Walter Schnelle, Asta Richter, Marco Merlini, Michael Hanfland, Artem M. Abakumov, Dmitry Batuk, Gustaaf Van Tendeloo, Yoichi Nakajima, Aleksey N. Kolmogorov, Leonid Dubrovinsky. Discovery of a superhard iron tetraboride superconductor, Phys. Rev. Lett., 111, 157002 (2013).
- Kolmogorov et al. Phys. Rev. Lett. 105, 217003 (2010).
- F. Occelli, P. Loubeyre, R. Letoullec, Nat. Mater. 2, 151 (2003).