Jahn-Teller effect and spin-orbit coupling in t_2g systems

In this talk we will discuss a very general problem of interplay between the Jahn-Teller (JT) effect and the spin-orbit coupling (SOC). It will be shown that the result of competition strongly depends on the specific situation: for some cases, such as d⁴ and d⁵, but also d² configurations, strong SOC may sometimes reverse, but finally completely suppresses JT distortion, whereas in other cases, notably d³, SOC generates JT effect in the configuration which is usually considered as “orbitally dead”.