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We study an interplay between the orbital degeneracy and the spin-orbit coupling (SOC) giving rise to spin-orbital entangled states in concentrated systems (cooperative Jahn-Teller [JT] effect). As a specific example, we analyze the interaction of electrons occupying triply degenerate single-ion t2g levels with trigonal vibrations (the t - T problem). A more general problem of the electron-lattice interaction involving both tetragonal and trigonal vibrations is also considered. It is shown that the result of such interaction crucially depends on the occupation of t2g levels leading to either the suppression or the enhancement of the JT effect by the SOC. © 2022 American Physical Society.

The electronic and magnetic properties of clinopyroxene CaMnGe2O6 were studied using density function calculations within the GGA + U approximation. It is shown that anomalous ferromagnetic ordering of neighboring chains is due to a “common-enemy” mechanism. Two antiferromagnetic exchange couplings between nearest neighbors within the Mn-Mn chain and interchain coupling via two GeO4 tetrahedra suppress antiferromagnetic exchange via single GeO4 tetrahedron and stabilize ferromagnetic ordering of Mn chains. © 2019, Pleiades Publishing, Inc.