We formulate and study an effective Hamiltonian for low-energy Kramers doublets of d1-ions on a square lattice. We find that the system exhibits a magnetically hidden order in which the expectation values of the local spin and orbital moments both vanish. The order parameter responsible for a time-reversal symmetry breaking has a composite nature and is a spin-orbital analog of a magnetic octupole. We argue that such a hidden order is realized in the layered perovskite Sr2VO4.
@article{arxiv.0906.2733,
title = {Magnetically Hidden Order of Kramers Doublets in $d^1$ Systems: Sr$_2$VO$_4$},
author = {George Jackeli and Giniyat Khaliullin},
journal= {arXiv preprint arXiv:0906.2733},
year = {2009}
}