Se Deficiency in Unconventional Charge Density Wave 1T-TiSe2-δ
S. H. Huang1,2*, G. J. Shu1, Woei Wu Pai1, H. L. Liu2, F. C. Chou1
1Center for Condensed Matter Science, National Taiwan University, Taipei, Taiwan
2Department of Physics, National Taiwan Normal University, Taipei, Taiwan
* presenting author:Song-Hsun Huang,
The normal state properties (semimetal vs. semiconductor) of 1T-TiSe2 have been the subject of debate. A systematic study of polycrystalline 1T-TiSe2-δ with controlled Se loss level indicates that a critical Se deficiency level of δ ~0.12 is closely tied to the occurrence of an unconventional charge density wave (CDW) phase. The level of Se vacancy defect and temperature determines whether 1T-TiSe2-δ should be categorized as a semiconductor, a semimetal, or an excitonic insulator. The second order structural phase transition and resistivity anomaly below ~200 K are found to be most pronounced in samples with δ ~0.12, instead of being Se vacancy free. An interpretation using band picture of an extrinsic p-type semiconductor is proposed to explain the evolution of electronic structures for 1T-TiSe2-δ, from the low doping bound (δ ~0.08) of typical semiconducting behavior to the high doping bound (δ ~0.17) showing typical metallic behavior before decomposition. The CDW phase transition is also sensitive to both the Se deficiency level and temperature.

Keywords: charge density wave (CDW)