Phononensoftening in the Spinel-Vanadate MgV2O4

In the vanadium spinel MgV₂O₄ the t2g electrons of the vanadium ion are triply degenerate. Lowering the temperature, this degeneracy is partly overcome by a cooperative Jahn-Teller distortion at T ~ 65 K, where the crystal symmetry is lowered from cubic to tetragonal. We mapped out the acoustic phonon branches in both phases and found several anomalous softening effects, which point towards an anharmonic interaction of the lattice with the orbital electrons. These interactions are concentrated at very specific points of reduced momentum transfer. As a rigorous theoretical model is not yet available, we review several competing models which describe certain limiting cases of the involved orbital physics.