An international group of researchers has discovered unusual quantum properties that were concealed in magnetite—the oldest magnetic material known to human beings.
This latest research has exposed the presence of low-energy waves that specify the crucial role of electronic communications with the crystal lattice. This provides another step to completely interpret the metal-insulator phase transition mechanism in magnetite and, specifically, to learn about the critical behavior and dynamical properties of this material in the purlieu of the transition temperature.
Magnetite (FeO4) is a standard mineral, whose robust magnetic characteristics were already recognized in ancient Greece. At first, this mineral was mainly employed in compasses and later used in several other devices, like data recording tools.
Magnetite is also extensively used in catalytic processes. The properties of magnetite also help animals to detect magnetic fields—for instance, birds are known to use this mineral in their navigation.
Moreover, physicists are extremely fascinated by magnetite because it exhibits an unusual phase transition—dubbed after the Dutch chemist Verwey—at around a temperature of 125 K. This Verwey transition happens to be the first metal-to-insulator phase transformation to be seen historically.
At the time of this highly complicated process, a change occurs in the electrical conductivity by as much as two orders of magnitude and the structure of the crystal is reorganized.
A transformation mechanism was proposed by Verwey based on the site of electrons on iron ions, which causes the emergence of a periodic spatial distribution of Fe3+ and Fe2+ charges at low temperatures.
In the recent past, the Verwey hypothesis was confirmed by sophisticated calculations and structural studies, which exposed a relatively more complicated pattern of charge distribution (16 non-equivalent locations of iron atoms) and demonstrated the presence of orbital order.
Polarons form the underlying parts of this charge-orbital ordering. Polarons are quasiparticles that occur when the crystal lattice is locally deformed by the electrostatic communication of a charged particle (hole or electron) traveling in the crystal.
With regard to magnetite, the polarons assume the form of trimerons—that is, complexes composed of three iron ions—in which the inner atom contains more numbers of electrons when compared to the other two outer atoms.
Published in the Nature Physics journal, the latest study was performed by researchers from several top research centers worldwide. The aim of this study is to experimentally reveal the excitations that play a role in the charge-orbital order of magnetite and explain them through sophisticated theoretical techniques.
Edoardo Baldini, Carina Belvin, Ilkem Ozge Ozel, and Nuh Gedik carried out the experimental part at MIT. Samples of magnetite were produced at the AGH University of Science and Technology (Andrzej Kozłowski), and the theoretical investigations were carried out in a number of locations.
These locations include the Jagiellonian University and the Max Planck Institute (Andrzej M. Oleś), the Institute of Nuclear Physics of the Polish Academy of Sciences (Przemysław Piekarz and Krzysztof Parlinski), Northeastern University (Gregory Fiete), the University of Rome “La Sapienza” (José Lorenzana), the Technical University in Ostrava (Dominik Legut), and the University of Texas at Austin (Martin Rodriguez-Vega).
At the Institute of Nuclear Physics of the Polish Academy of Sciences, we have been conducting studies on magnetite for many years, using the first-principles calculation method. These studies have indicated that the strong interaction of electrons with lattice vibrations (phonons) plays an important role in the Verwey transition.
Przemysław Piekarz, Professor, The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences
The MIT researchers quantified the optical reaction of magnetite in the extreme infrared for a number of temperatures. Later, they used an ultra-short laser pulse (pump beam) to irradiate the crystal and also used a delayed probe pulse to quantify the variation in the far-infrared absorption.
This is a powerful optical technique that enabled us to take a closer view at the ultrafast phenomena governing the quantum world.
Nuh Gedik, Professor and Head of the Research Group, MIT
These measurements exposed the presence of low-energy excitations of the trimeron order, which equates to charge oscillations attached to a lattice deformation. When two coherent modes approach the Verwey transition, their energy reduces to zero, denoting their crucial behavior close to this transformation.
Using sophisticated theoretical models, the researchers were able to explain the recently identified excitations as a coherent tunneling of polarons. Using density functional theory (DFT), the researchers calculated the energy obstacle for the tunneling process and other model parameters on the basis of the quantum-mechanical description of crystals and molecules.
The researchers also used the Ginzburg-Landau model to confirm the role of these waves in the Verwey transition. Lastly, the calculations also dismissed other plausible explanations for the witnessed phenomenon, including the traditional phonons and orbital excitations.
The discovery of these waves is of key importance for understanding the properties of magnetite at low temperatures and the Verwey transition mechanism. In a broader context, these results reveal that the combination of ultrafast optical methods and state-of-the-art calculations makes it possible to study quantum materials hosting exotic phases of matter with charge and orbital order.
Dr Edoardo Baldini and Ms Carina Belvin, Study Lead Authors, MIT
The results, thus obtained, led to many crucial conclusions. Firstly, in magnetite, the trimeron order has rudimentary excitations with extremely low energy and absorbs radiation in the far-infrared area of the electromagnetic spectrum.
Secondly, such excitations are overall fluctuations of lattice and charge deformations exhibiting crucial behavior and, therefore, play a role in the Verwey transition. Lastly, the outcomes provide a better understanding of the dynamical properties and cooperative mechanism that lie at the origin of this intricate phase transition.
“As for the plans for the future of our team, as part of the next stages of work we intend to focus on conducting theoretical calculations aimed at better understanding the observed coupled electronic-structural waves,” concluded Professor Piekarz.
Baldini, E., et al. (2020) Discovery of the soft electronic modes of the trimeron order in magnetite. Nature Physics. doi.org/10.1038/s41567-020-0823-y.