We discovered the chirality of charge density waves (CDW) in 1T-TiSe2 by using scanning tunnelling microscopy (STM) and optical ellipsometry. We found that the CDW intensity becomes Ia1:Ia2:Ia3=1:0.7±0.1:0.5±0.1, where Iai (i =1, 2, 3) is the amplitude of the tunnelling current contributed by the CDWs. There were two states, in which the three intensity peaks of the CDW decrease \textit{clockwise} and \textit{anticlockwise} when we index each nesting vector in order of intensity in the Fourier transformation of the STM images. The chirality in CDW results in the three-fold symmetry breaking. Macroscopically, two-fold symmetry was indeed observed in optical measurement. We propose the new generalized CDW chirality H_{CDW} \equiv {\boldmath q_1} \cdot ({\boldmath q_2}\times {\boldmath q_3}), where {\boldmath q_i} are the nesting vectors, which is independent of the symmetry of components. The nonzero HCDW - the triple-{\boldmath q} vectors do not exist in an identical plane in the reciprocal space - should induce a real-space chirality in CDW system.
@article{arxiv.1001.3196,
title = {Chiral charge-density-waves},
author = {J. Ishioka and Y. H. Liu and K. Shimatake and T. Kurosawa and K. Ichimura and Y. Toda and M. Oda and S. Tanda},
journal= {arXiv preprint arXiv:1001.3196},
year = {2015}
}