Chiral charge and orbital order in 1T-TiSe2

Helical arrangements of spins are common among magnetic materials. The first material to harbor a corkscrew pattern of charge density on the other hand, was discovered only very recently [1,2]. The nature of the order parameter is of key relevance, since rotating a magnetic vector around any propagation vector trivially yields a helical pattern. In contrast, the purely scalar charge density cannot straightforwardly support a chiral state. Here we resolve this paradox by identifying the microscopic mechanism underlying the formation of the chiral charge density wave in {\it 1T}-TiSe$_2$. It is shown that the emergence of chirality is accompanied by the simultaneous formation of orbital order [3] We show that this type of combined orbital and charge order may in fact be expected to be a generic property of a broad class of charge ordered materials and discuss the prerequisites for finding chiral charge order in other materials. \\[4pt] [1] J. Ishioka, Y. H. Liu, K. Shimatake, T. Kurosawa, K. Ichimura, Y. Toda, M. Oda and S. Tanda, {\it Phys. Rev. Lett.} \textbf{105}, 176401 (2010). \newline \noindent [2] J. van Wezel and P. B. Littlewood, {\it Physics} \textbf{3}, 87 (2010). \newline \noindent [3] J. van Wezel, arXiv:1106.1930v1 (2011).