A new step in understanding the metal-insulator transition of nickelates
Research in materials physics is one of the cornerstones of our company's technological development. Just as a better understanding of the mode of action of COVID-19 appears to us today to be an indispensable prerequisite for the development of effective treatments and vaccines, an understanding at the atomic scale of the phenomena originating at the heart of materials is an indispensable step for their subsequent use in innovative and practical devices. It is in this context that Yajun Zhang, Alain Mercy and Philippe Ghosez, from the Department of Theoretical Physics of Materials of the CESAM research unit (University of Liège), have just made an important discovery.
ertain complex oxides called nickelates - with the chemical formula RNiO3, where R is an atom of the rare earths family - crystallize in the form of a perovskite structure and have the unique property of being able to change abruptly and spontaneously from the state of insulator to that of electrical conductor at a characteristic temperature, TMIT, which can vary from 0 to 600 Kelvin depending on the compound. Synthesized for the first time in 1971, these materials have since generated growing interest, but the origin of their novel behaviour remains widely debated to this day.
In 2017, Alain Mercy, Jordan Bieder and Philippe Ghosez, from the CESAM research unit, had already highlighted , on the basis of ab initio calculations (a complex theoretical approach based on the equations of quantum mechanics and electromagnetism), that the phase transition of nickelates is similar to a so-called "Peierls" transition but of a new type insofar as it is induced by the rotation of the oxygen cages inherent to such compounds of perovskite structure.
In collaboration with experimenters from the University of Twente, they were able to confirm the validity of this theory and its practical usefulness in modulating the transition temperature by directly influencing the amplitude of rotation of the oxygen cages .
More recently, Yajun Zhang, Alain Mercy and Philippe Ghosez studied the behaviour of artificial nanostructures alternating layers of nickelate compounds with distinct transition temperatures. In collaboration with researchers from the University of Geneva, EPFL and the Flatiron Institute in New York, they were able to show that the range of the metal-insulator transition of these compounds is controlled by a high interface energy between the insulating and metallic phases, the range of this effect being much greater than that of the structural coupling between the phases. This discovery, recently published in the journal Nature Materials , represents a new and important step towards exploiting these compounds in devices operating around room temperature.
 Structurally triggered metal-insulator transition in rare-earth nickelates. A. Mercy, J. Bieder, J. Iniguez and Ph. Ghosez, Nature Communications 8, 1677 (2017).
 Geometric design of metal-insulator transitions in perovskite nickelates for room-temperature optical switching. Z. Liao, N. Gauquelin, R. J. Green, K. Müller-Kaspary, I. Lobato, L. Li, S. Van Aert, J. Verbeeck, M. Huijben, M. N. Grisola, V. Rouco, R. El Hage, J. E. Villegas, A. Mercy, M. Bibes, Ph. Ghosez, G. A. Sawatzky, G. Rijnders and G. Koster, PNAS 115, 9515(2018).
 Length-scales of interfacial coupling between metal-insulator phases in oxides. C. Dominguez, A.B. Georgescu, B. Mundet, Y. Zhang, J. Fowlie, A. Mercy, S. Catalano, T. Duncan, T.L. Alexander, Ph. Ghosez, A. Georges, A. Millis, M. Gibert and J.-M. Triscone, Nature Materials (2020)
Image 2 - Understanding at the atomic scale the couplings that can exist between the different electronic and structural degrees of freedom within materials is an indispensable step for their subsequent use in innovative and practical devices.