Orbital-assisted Peierls state in

¹ Institute-Lorentz for Theoretical Physics, Universiteit Leiden, P.O. Box 9506, 2300 RA Leiden, The Netherlands
² Institute for Molecules and Materials, Radboud Universiteit Nijmegen, P.O. Box 9010, 6500 GL Nijmegen, The Netherlands

Received: 23 June 2006
Accepted: 21 July 2006

Abstract

Does the quasi–one-dimensional titanium pyroxene 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 shows an orbital assisted Peierls transition is therefore inevitable.