3d transition metal oxides (or oxides containing a transition metal ion with an unfilled 3d sub-shell) have been of much interest to condensed matter scientists, owing to the rich interplay between the spin and orbital motion of electrons in these materials. In contrast, 4d and 5d transition metal oxides have not been explored in detail until now, because of weak electron correlations, compared to 3d transition metal oxides.

However, with the discovery of effects of strong spin-orbit coupling (or SOC) in a 5d transition metal oxide, the tide turned for 4d and 5d transition metal compounds, which are now known to give rise to a rich variety of exotic spin-orbit entangled states depending on the d-orbital electron configuration, chemical bonds, and lattice geometry.

In a new study published in the Journal of the Physical Society of Japan, an international team of researchers took stock of the emerging 4d and 5d transition metal oxides, exploring the exotic phases induced by SOC in these materials and the possibility of their realization. On one hand, the researchers highlighted several novel phenomena expected to emerge from theoretical considerations but requires development of novel materials for their realization. On the other, they took note of several experimentally observed unusual behaviors that are lacking in realistic theories. Finally, they provided speculations about unknown exotic phases yet to be discovered.

The study shows that 4d and 5d transition metal compounds are a mine of novel quantum phases waiting to be discovered and explored, thus providing a fertile ground for future research in condensed matter physics.