Correlated insulators with strong spin-orbit coupling have recently emerged as a rich playground for unconventional low-temperature orders and exotic states of matter. Double perovskites with 5d1 electronic configuration, in particular, have garnered significant attention for their potential to realize high-rank multipole ordering, which, unlike traditional dipolar magnetic orders, often evade direct experimental observation and are consequently referred to as “hidden orders”
In our study, we employ several ab-initio techniques, including Density Functional Theory, Dynamical Mean Field Theory, Jahn-Teller and phonon analysis, as well as calculations of intersite exchange interactions using a many-body force-theorem method to study the "hidden"-quadrupolar and magnetic transitions of the 5d1 double perovskite Ba2MgReO6 (BMRO).
This multifaceted approach effectively captures the dual-phase transition observed in BMRO —the emergence of an anti-ferro quadrupolar phase at higher temperatures and the subsequent development of canted antiferromagnetic (cAFM) phase. Moreover, we find that these phases result from the intricate interplay between electronic superexchange mechanisms and electron-lattice interactions, with intersite elastic couplings playing a pivotal role.
Additionally, our research explains the behavior of the low-temperature ordered phases in response to applied pressure. We observe that both the quadrupolar and cAFM phases are robust with respect to hydrostatic pressure; however, non-hydrostatic effects strongly impact their stability, particularly by suppressing the quadrupolar transition in favor of a purely collinear antiferromagnetic order.