Orbital switching in a frustrated magnet.

The orbital is one of the four fundamental degrees of freedom in a solid, besides spin, charge and lattice. In transition metal compounds, it is usually the d orbitals which play deciding roles in determining the crystallographic and physical properties. Here we report the discovery of a unique structural transition in single crystals of the spin-1/2 quasi-kagomé antiferromagnet volborthite, Cu(3)V(2)O(7)(OH)(2)·2H(2)O, whereby the unpaired electron 'switches' from one d orbital to another upon cooling. This is not a conventional orbital order-disorder transition, but rather an orbital switching that has not previously been observed. The structural transition is found to profoundly affect the magnetic properties of volborthite, because magnetic interactions between Cu spins in the kagomé lattice are considerably modified by the orbital switching. This finding provides us with an interesting example to illustrate the intimate interplay between the orbital degree of freedom and competing magnetic interactions in a frustrated magnet.