Orbital-assisted Peierls state in NaTiSi2O6

Does the quasi–one-dimensional titanium pyroxene NaTiSi2O6 exhibit the novel orbital-assisted Peierls state? We calculate its groundstate properties by three methods: Monte Carlo simulations, a spin-orbital decoupling scheme and a mapping onto a classical model. The results show univocally that for the spin and orbital ordering to occur at the same temperature—an experimental observation—the crystal field needs to be small and the orbitals are active. We also find that fluctuations in the spin-orbital sector drive the transition, explaining why bandstructure methods fail to find it. The conclusion that NaTiSi2O6 shows an orbital assisted Peierls transition is therefore inevitable.