Date:

New type of coupled electronic-structural waves discovered in magnetite

An international team of scientists uncovered exotic quantum properties hidden in magnetite, the oldest magnetic material known to mankind.

The study reveals the existence of low-energy waves that indicate the important role of electronic interactions with the crystal lattice. This is another step to fully understand the metal-insulator phase transition mechanism in magnetite, and in particular to learn about the dynamical properties and critical behavior of this material in the vicinity of the transition temperature.

- Advertisement -

Magnetite (Fe3O4) is a common mineral, whose strong magnetic properties were already known in ancient Greece. Initially, it was used mainly in compasses, and later in many other devices, such as data recording tools. It is also widely applied to catalytic processes. Even animals benefit from the properties of magnetite in detecting magnetic fields – for example, birds are known to use it in navigation.

Physicists are also very interested in magnetite because around a temperature of 125 K it shows an exotic phase transition, named after the Dutch chemist Verwey. This Verwey transition was also the first phase metal-to-insulator transformation observed historically. During this extremely complex process, the electrical conductivity changes by as much as two orders of magnitude and a rearrangement of the crystal structure takes place. Verwey proposed a transformation mechanism based on the location of electrons on iron ions, which leads to the appearance of a periodic spatial distribution of Fe2+ and Fe3+ charges at low temperatures.

In recent years, structural studies and advanced calculations have confirmed the Verwey hypothesis, while revealing a much more complex pattern of charge distribution (16 non-equivalent positions of iron atoms) and proving the existence of orbital order. The fundamental components of this charge-orbital ordering are polarons – quasiparticles formed as a result of a local deformation of the crystal lattice caused by the electrostatic interaction of a charged particle (electron or hole) moving in the crystal. In the case of magnetite, the polarons take the form of trimerons, complexes made of three iron ions, where the inner atom has more electrons than the two outer atoms.

The new study, published in the journal Nature Physics, was carried out by scientists from many leading research centers around the world. Its purpose was to experimentally uncover the excitations involved in the charge-orbital order of magnetite and describe them by means of advanced theoretical methods. The experimental part was performed at MIT (Edoardo Baldini, Carina Belvin, Ilkem Ozge Ozel, Nuh Gedik); magnetite samples were synthesized at the AGH University of Science and Technology (Andrzej Kozlowski); and the theoretical analyses were carried out in several places: the Institute of Nuclear Physics of the Polish Academy of Sciences (Przemyslaw Piekarz, Krzysztof Parlinski), the Jagiellonian University and the Max Planck Institute (Andrzej M. Oles), the University of Rome “La Sapienza” (Jose Lorenzana), Northeastern University (Gregory Fiete), the University of Texas at Austin (Martin Rodriguez-Vega), and the Technical University in Ostrava (Dominik Legut).

- Advertisement -

“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,” explains Prof. Przemyslaw Piekarz. “These studies have indicated that the strong interaction of electrons with lattice vibrations (phonons) plays an important role in the Verwey transition.”

The scientists at MIT measured the optical response of magnetite in the extreme infrared for several temperatures. Then, they illuminated the crystal with an ultrashort laser pulse (pump beam) and measured the change in the far-infrared absorption with a delayed probe pulse. “This is a powerful optical technique that enabled us to take a closer view at the ultrafast phenomena governing the quantum world,” says Prof. Nuh Gedik, head of the research group at MIT.

The measurements revealed the existence of low-energy excitations of the trimeron order, which correspond to charge oscillations coupled to a lattice deformation. The energy of two coherent modes decreases to zero when approaching the Verwey transition – indicating their critical behavior near this transformation. Advanced theoretical models allowed them to describe the newly-discovered excitations as a coherent tunneling of polarons. The energy barrier for the tunneling process and other model parameters were calculated using density functional theory (DFT), based on the quantum-mechanical description of molecules and crystals. The involvement of these waves in the Verwey transition was confirmed using the Ginzburg-Landau model. Finally, the calculations also ruled out other possible explanations for the observed phenomenon, including conventional 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,” say Dr. Edoardo Baldini and Ms. Carina Belvin of MIT, the lead authors of the article. “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.”

The obtained results lead to several important conclusions. First, the trimeron order in magnetite has elementary excitations with a very low energy, absorbing radiation in the far-infrared region of the electromagnetic spectrum. Second, these excitations are collective fluctuations of charge and lattice deformations that exhibit critical behavior and are thus involved in the Verwey transition. Finally, the results shed new light on the cooperative mechanism and dynamical properties that lie at the origin of this complex 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,” concludes Prof. Piekarz.

The Henryk Niewodniczanski Institute of Nuclear Physics (IFJ PAN) is currently the largest research institute of the Polish Academy of Sciences. The broad range of studies and activities of IFJ PAN includes basic and applied research, ranging from particle physics and astrophysics, through hadron physics, high-, medium-, and low-energy nuclear physics, condensed matter physics (including materials engineering), to various applications of methods of nuclear physics in interdisciplinary research, covering medical physics, dosimetry, radiation and environmental biology, environmental protection, and other related disciplines. The average yearly yield of the IFJ PAN encompasses more than 600 scientific papers in the Journal Citation Reports published by the Clarivate Analytics.

The part of the Institute is the Cyclotron Centre Bronowice (CCB) which is an infrastructure, unique in Central Europe, to serve as a clinical and research centre in the area of medical and nuclear physics. IFJ PAN is a member of the Marian Smoluchowski Kraków Research Consortium: “Matter-Energy-Future” which possesses the status of a Leading National Research Centre (KNOW) in physics for the years 2012-2017. In 2017 the European Commission granted to the Institute the HR Excellence in Research award. The Institute is of A+ Category (leading level in Poland) in the field of sciences and engineering.

THE HENRYK NIEWODNICZANSKI INSTITUTE OF NUCLEAR PHYSICS POLISH ACADEMY OF SCIENCES

Header Image – Illustration of the newly discovered charge fluctuations in the trimeron order of magnetite triggered by a laser beam. Credit : Ambra Garlaschelli and MIT

- Advertisement -
spot_img
Mark Milligan
Mark Milligan
Mark Milligan is multi-award-winning journalist and the Managing Editor at HeritageDaily. His background is in archaeology and computer science, having written over 7,500 articles across several online publications. Mark is a member of the Association of British Science Writers (ABSW), the World Federation of Science Journalists, and in 2023 was the recipient of the British Citizen Award for Education, the BCA Medal of Honour, and the UK Prime Minister's Points of Light Award.
spot_img

Mobile Application

spot_img

Related Articles

Archaeologists search crash site of WWII B-17 for lost pilot

Archaeologists from Cotswold Archaeology are excavating the crash site of a WWII B-17 Flying Fortress in an English woodland.

Roman Era tomb found guarded by carved bull heads

Archaeologists excavating at the ancient Tharsa necropolis have uncovered a Roman Era tomb guarded by two carved bull heads.

Revolutionary war barracks discovered at Colonial Williamsburg

Archaeologists excavating at Colonial Williamsburg have discovered a barracks for soldiers of the Continental Army during the American War of Independence.

Pleistocene hunter-gatherers settled in Cyprus thousands of years earlier than previously thought

Archaeologists have found that Pleistocene hunter-gatherers settled in Cyprus thousands of years earlier than previously thought.

Groundbreaking study reveals new insights into chosen locations of pyramids’ sites

A groundbreaking study, published in the journal Communications Earth & Environment, has revealed why the largest concentration of pyramids in Egypt were built along a narrow desert strip.

Soldiers’ graffiti depicting hangings found on door at Dover Castle

Conservation of a Georgian door at Dover Castle has revealed etchings depicting hangings and graffiti from time of French Revolution.

Archaeologists find Roman villa with ornate indoor plunge pool

Archaeologists from the National Institute of Cultural Heritage have uncovered a Roman villa with an indoor plunge pool during excavations at the port city of Durrës, Albania.

Archaeologists excavate medieval timber hall

Archaeologists from the University of York have returned to Skipsea in East Yorkshire, England, to excavate the remains of a medieval timber hall.