Unraveling the Electronic Mechanisms Driving Phase Transitions In Strongly Correlated EuO

Posted on 2020-01-06 in Events
Jan 10, 2020

Please join us for a Geological Sciences graduate student seminar this Friday January 10 at 3:30 pm in rm 155 Geology presented by Jacques Desmarais, PhD Candidate:

Unraveling the Electronic Mechanisms Driving Phase Transitions In Strongly Correlated EuO

The coherent behavior of Rare-Earth Elements (REEs) in planetary processes makes them an invaluable geochemical tracer. Despite this however, very little is known about the behavior of REEs at high pressure. What is more, REE phases have important economic applications because of their use in spintronics, in which electronic devices (e.g. computers, magnets, cell-phones, televisions, etc…) are devised by manipulating (e.g. through application of pressure) the spin and charge degrees of freedom of mineral phases. In particular, EuO has generated great interest in the field of spintronics, because of the presence of the well-localized, half-filled, perfectly ferromagnetically-coupled f-band, with 7 unpaired electrons.

We study pressure-induced isostructural electronic and NaCl-type to CsCl-type structural phase transitions in the prototypical mixed-valence and strongly correlated material EuO using the global-hybrid density functional theory. The simultaneous presence in the valence of highly localized d- and f-type bands and itinerant s- and p-type states, as well as the half-filled f-type orbital shell with seven unpaired electrons on each Eu atom, have made the description of the electronic features of this system a challenge. The electronic band structure, density-of-states, and atomic oxidation states of EuO are analyzed in the 0-52 GPa pressure range. While a first insulator-to-metal transition at about 12 GPa of pressure was already well-understood, a second, elusive, metal-metal one occurring at about 30-35 GPa of pressure is here found to be associated with a change in the population of the d-type band of Eu. In particular, d orbitals of e_g symmetry (i.e. d_{x^2 - y^2} and d_{2z^2 - x^2 - y^2}) get abruptly depopulated at the metal-metal transition. Contrary to previous deductions made from interpretation of experimental results, we find no abrupt change in oxidation state associated with the NaCl-type to CsCl-type structural phase transition occurring at about 45-50 GPa. Instead we show that the phase transitions can be explained from simple arguments of crystal field theory, in a manner that is entirely consistent with existing experimental data.