A new species of polaron in quantum oxides


A team of researchers led by Cesare Franchini has yielded a significant breakthrough: the discovery of a novel type of particle in materials that the researchers named Spin-orbital Jahn-Teller bipolaron. | New publication in "Nature Communications".

A team of researchers led by Cesare Franchini at the University of Vienna, in collaboration with European and American institutions including the University of Bologna, the University of Parma, the Collège de France, Brown University, and Ohio State University, has yielded a significant breakthrough: the discovery of a novel type of particle in materials that the researchers named Spin-orbital Jahn-Teller bipolaron. Published in the Nature Communications journal, the study investigates the entanglement of different quantum phenomena in a complex nanomaterial.

The ongoing technological progress of our society is intricately linked to the continual advancement and comprehension of nanomaterials. Transition metal oxides, among these materials, play a pivotal role across a spectrum of applications, including photovoltaics, superconductive magnets, data storage, sensing, catalysis, and environmental remediation. Furthermore, they exhibit a diverse array of intriguing quantum effects stemming from the delicate balance of interactions between the materials’ electrons and ions. The recent study led by the University of Vienna sheds new light on the complexities of these quantum phenomena, unveiling the emergence of a novel particle species that significantly alters material properties.

The properties of transition metal oxide crystals are significantly influenced by two distinct types of electrical interactions: the interactions between electrons themselves and the interactions between electrons and the nuclei of the atoms comprising the crystal. In scenarios where both these interactions are strong enough, as commonly observed in transition metal oxides, certain electrons undergo a transformation into entities known as polarons. "I often liken electrons and polarons to birds and elephants navigating a forest: while birds may flit around, barely disturbing a few leaves, polarons - like elephants - displace bushes and tree branches around themselves as they traverse through dense vegetation", says Lorenzo Celiberti, first author of the study.

While polarons have been known since the early 20th century, recent advancements in both experimental and theoretical techniques have unveiled new variants of these particles. Despite their familiarity with the diverse range of polarons, researchers were consistently surprised by the unique blend of properties displayed by the polaron they discovered in a specific transition metal oxide called BNOO.
Initially, it was believed that the formation of polarons in materials like BNOO would be highly improbable. However, calculations performed by the researchers in Vienna and Paris clearly reveal the presence of polarons in BNOO, thus validating the interpretation of the experiments conducted by their colleagues in Italy and the US.
While trying to better understand the nature of the newly found particle, the researchers discovered that their particle is characterized by the balance of two competing phenomena. "If I were to represent this polaron symbolically, I would likely choose the yin-yang symbol, with the white side symbolizing the Jahn-Teller effect and the black side representing spin-orbit coupling", explains Lorenzo Celiberti. The Jahn-Teller effect and the spin-orbit coupling are two long known quantum effects. If they relative strength is similar they can give rise to a variety of different scenarios. In BNOO, in particular, they compete and try to cancel each other. However, in this struggle between opposing forces of comparable strength, neither emerges triumphant, resulting in a new equilibrium that characterizes the polaron. To underline the dynamical nature of this equilibrium researchers named the new particle as Spin-orbital Jahn-Teller bipolaron.

The unexpected discovery of polarons exhibiting distinctive properties in BNOO enhances our comprehension of this class of materials, which remains relatively limited compared to other nanomaterials. Furthermore, understanding the intricate interplay of diverse quantum phenomena within these particles holds the potential for harnessing them in the creation of novel, efficient technologies.

Original publication: L. Celiberti et al., "Spin-orbital Jahn-Teller bipolarons", Nature Communications 15, 2429 (2024) DOI:10.1038/s41467-024-46621-0

Scientific Contact
Lorenzo Celiberti, MSc
Computational Materials Physics, Faculty of Physics
Kolingasse 14-16
1090 Wien
T: +43-1-4277-514 01

Localized spin-orbital Jahn-Teller polaron (red and blue) emerging from the sea of valence electrons (white). © Lorenzo Celiberti.