XFEL: Transition metal insulators: The origin of colour

In a theoretical study, researchers have explained the vibrant colours of two compounds whose electronic properties seemingly prohibit such colouring. The hues exhibited by the two insulators originate from transitions in the spins of the electrons, which modify the way the materials absorb and reflect light in such a way as to create the bright colours. The theoretical framework employed by the team promises new insights in fields such as optoelectronics or in the study of qubits, the quantum bits used in quantum computers. 

Although colour is a familiar phenomenon, it is sometimes challenging to explain how the hues of certain materials come about. This is the case with insulators that contain transition metals. In these compounds, the energy gap between the valence band, in which the electrons are tightly bound to the atoms, and the conduction band, in which the electrons can move freely, is larger than the highest energy of photons of visible light—meaning that these materials should not absorb visible light. As the colour of a compound is complementary to the wavelengths it absorbs, we should thus perceive these insulators as being transparent instead of coloured. 
A team of researchers including the head of the European XFEL Theory group, Alexander Lichtenstein, now used two complementary theoretical methods to study the origin of colour in two typical transition metal insulators: nickel(II) oxide (NiO)—a green compound used in the production of ceramics and nickel steel as well as in thin-film solar cells, nickel–iron batteries, and fuel cells—and manganese(II) fluoride (MnF₂), a pink material employed in the manufacture of special kinds of glass and lasers.

While both theoretical approaches successfully explained the green colour of NiO, the first one failed to account for the pink colour of MnF₂. “Whereas the first method is a so-called perturbative theory, which can describe only excitations in the d-shell of transition metals that conserve the electron spin, the second is a locally exact theory, which includes complicated vertex corrections from higher-order spin-flip processes,” explains Alexander Lichtenstein, who is also a professor at Universität Hamburg. “These additional contributions are key mechanisms that modify the response of the materials to visible light and thus explain why they are coloured, although their electronic configuration would seem to forbid this at first sight.” In addition, the team was able to disentangle processes that control the brightness of the charge excitations—called “excitons”—that are responsible for the colouring.

Understanding such excitons and their spin mechanisms is important for research in optoelectronics, for example, or in the study of qubits for use in quantum computers. The team’s theoretical work provides a means to explicitly calculate the parameters observed in such systems, marking a step change in their description and thus their understanding.

Alongside Alexander Lichtenstein, the team included researchers from Radboud University (The Netherlands), the National Renewable Energy Laboratory (USA), King’s College London (UK), and the Australian National University (Australia). The work was published in Nature Communications.

Reference:
 “A theory for colors of strongly correlated electronic systems”, Swagata Acharya et al., Nat. Commun. 14, 5565 (2023). DOI:10.1038/s41467-023-41314-6

Scientific contact:
Prof. Alexander Lichtenstein
E-mail: alichten@physnet.uni-hamburg.de
Tel.: +49-40-42838-2393

Media contact:
Dr Bernd Ebeling
Tel.: +49-40-8998-6921
E-mail: bernd.ebeling@xfel.eu