Some materials hold surprising — and possibly useful — properties: Neodymium nickel oxide is either a metal or an insulator, depending on temperature. This characteristic makes the material a potential candidate for transistors in modern electronic devices. To understand how neodymium nickel oxide makes the transition from metal to insulator and vice versa, researchers at the Paul Scherrer Institute (PSI) and the University of Geneva (UNIGE) have precisely probed the distribution of electrons in the material. By means of a sophisticated development of X-ray scattering, they were able to show that electrons in the vicinity of the material’s oxygen atoms are rearranging. The researchers have published their study in the journal Nature Communications.
Computers, smartphones, and all kinds of other electronic devices have tiny transistors as their basic elements. Up to now, these have been realised with so-called semiconductors. It’s possible that semiconductors might one day get competition from a certain class of oxide materials. Some of these materials can be switched between being an insulator and an electrically conductive metal. Thus they could also be used to build transistors.
To gain a fundamental understanding of the phase transition from metal to insulator in these materials, researchers in the team of Thorsten Schmitt, leader of the research group Spectroscopy of Novel Materials at PSI and researchers in the team of Prof. Jean-Marc Triscone in the Department of Quantum Matter Physics at UNIGE, together with scientists at the University of British Columbia in Canada, looked at one representative of this class of materials: neodymium nickel oxide (NdNiO3). Above a temperature of around 150 Kelvin (minus 123 degrees Celsius), the material is a metal and thus conducts electric current. Below this temperature, in contrast, it is an insulator and therefore non-conducting.
The mystery of the phase transition
Since the arrangement of electrons in the material is responsible for these properties, the researchers wanted first to find out which energetic states the electrons in the material take — that is, in this specific case, how the nickel and oxygen orbitals are occupied — and how the electronic structure differs in its two states, metal and insulator.
To measure the electronic structure of neodymium nickel oxide, they used the refined method of resonant inelastic X-ray scattering, (RIXS) at the Swiss Light Source (SLS) synchrotron. The researchers conducted their experiment twice — first at 300 Kelvin, far above the transition temperature and thus in the region where neodymium nickel oxide behaves like a metal. They then ran the experiment a second time at a frosty 15 Kelvin, far below the transition temperature and thus in the region where the material is an insulator. Each RIXS measurement on its own showed the researchers the electronic structure of the material in that particular state. The comparison of the two measurements revealed which changes in the electronic structure are responsible for the phase transition from metal to insulator.
Electrons are rearranging at the vicinity of the oxygen atoms
The result: During the phase transition from metal to insulator, the electronic structure of the nickel atoms remains practically the same. All action is taking place on the six oxygen atoms surrounding the nickel ions, which form an octahedron. The oxygen states are mobile (itinerant) in the metallic phase and become immobile (localized) in the insulating phase with two sizes of octathedra alternating. The electronic structure of the material differs, then, in the vicinity of the oxygen atoms, depending on the temperature; this is responsible for the metallic or, insulating properties.
Theoretical calculations have suggested for several years that the changes might not take place in the region of the nickel atoms but rather in the vicinity of the oxygen atoms. Researchers have now obtained definitive experimental proof.
Thin-film fabrication at the University of Geneva
The sample of neodymium nickel oxide material was fabricated in Geneva by collaborators at UNIGE. For RIXS measurements, it was essential to have the material available in large enough single-crystal form. Up to now, however, this can be realised only as a thin film. The challenge of the Geneva researchers lay in realising a thin film — using a suitable substrate — whose properties match those of a three-dimensional piece of the material.
The material’s phase transition between metal and insulator could be realised not only through temperature, but also through the application of pressure or of an electrical voltage. This could be exploited if such materials should one day find their way into electronics.
This successful collaboration is an example of the precious links which are uniting PSI and UNIGE. The complementarity of our researches allows us to face challenges together and obtain unequalled results based on an open and collaborative science and on a strong partnership.
This text is based on the Press release of PSI
Photos: Sara Catalano, UNIGE; Thorsten Schmitt, PSI; and Marta Gibert, UNIGE (from left to right) discussing at the ADRESS beamline of the Synchrotron Light Source SLS.
© PSI/Markus Fischer