Prof. Andrzej M. Oleś from the JU Institute of Theoretical Physics participated in an international research project on the Verwey transition in magnetite (Fe3O4). The studies highlight the strength of ultrafast pump-probe experiments and first-principles calculations in uncovering the critical character of collective modes associated with charge-orbital-lattice order.
The Verwey transition in magnetite (Fe3O4) was investigated as part of an international research project involving physicists from the Jagiellonian University (Prof. Andrzej M. Oleś), Institute of Nuclear Physics of the Polish Academy of Sciences (PAS, Dr Przemysław Piekarz), University of Science and Technology (AGH), USA, Italy, and Czech Republic. The results of joint research were published in Nature Physics. They demonstrate for the first time the existence of soft electronic modes at low temperatures near the Verwey transition.
Magnetite, the first magnetic material already discovered in ancient Greece, is used in compasses and in other technological appliances. Evert Verwey observed the dramatic drop of the electric conductivity by two orders of magnitude at temperature ~125 K and proposed the mechanism of this transition (called Verwey transition), i.e., charge ordering on the iron (Fe) ions. In recent years, it was found that the fundamental building blocks of the charge-orbital order are three-site small polarons called trimerons. The electron density is largest at the central trimeron Fe ion, as shown by the largest orbital in the image (red/gray spheres stand for oxygen and other Fe ions). So far, however, collective modes of this complex order have not been detected, and thus complete understanding of the dynamics of the Verwey transition was lacking.
In the present studies, Dr Edoardo Baldini and Dr Carina A Belvin from the Massachusetts Institute of Technology (MIT, USA) performed the optical measurements using the pump-probe technique, which revealed critical softening related to the Verwey transition due to the trimeron order. High-quality magnetite samples were prepared by Prof. Andrzej Kozłowski (AGH). Prof. Jose Lorenzana from the University La Sapienza in Rome proposed that these excitations can be described by oscillations of the charge-orbital-lattice order using a model of coherent polaron tunneling. In the calculations performed in Kraków, the researchers used the density functional theory for a tunneling process involving charge carriers localised on the central Fe sites of the trimerons and obtained the potential energy barrier for their coherent hopping. In order to compute the characteristic energy of atomic vibrations coupled to tunneling electrons, the ab initio method invented by Prof. Krzysztof Parlinski (PAS) and his software PHONON were employed. By estimating the bonding-antibonding splitting arising from the superposition of the two states created by tunneling through the barrier, its energy was found to be close to the observed energies of oscillations. The polaron motion becomes slower when temperature increases towards the Verwey transition, reducing relative polaron splitting, and the energies as well as the amplitudes of excitations dramatically decrease. The critical behavior of the experimentally observed collective modes was successfully reproduced within the time-dependent Ginsburg-Landau theory.
The current studies highlight the strength of ultrafast pump-probe experiments and first-principles calculations in uncovering the critical character of collective modes associated with charge-orbital-lattice order. Similar spectroscopic studies can be performed to shed more light on the nature of joint charge-orbital excitations in other strongly correlated systems.