Using SSRL's beam line 5-4, researchers from Fudan University in Shanghai and SSRL have worked out the mechanism behind the formation of charge density waves in 2H-structured transition metal dichalcogenides (2H-TMD's). The results were published in the November 21, 2007 edition of Physical Review Letters.

Charge density waves (CDWs) are quantum mechanical ordering phenomena akin to superconductivity. In a normal conductive metal, electrons persist in a "sea" wherein they are evenly distributed and the ions form a perfectly periodic lattice. A charge density wave occurs under certain circumstances - such as in low dimensional materials - that cause the electrons and ions to couple together to lower their energy. A modulation of the ions' positions and "waves" in the electron sea are observed, creating new periodicities that hinder the conducting electrons, thus lowering the compound's conductivity.

Understanding CDW formation is an important step in characterizing the fundamental electronic properties of matter. The mechanism behind CDW formation is typically ascribed to Fermi surface nesting, a phenomenon wherein a material's electrons correlate with each other on the basis of their momentum and energy. However, CDW formation in 2H-TMD's appears to arise through a different mechanism. Using angle resolved photoemission spectroscopy (ARPES), the Fudan and SSRL researchers have for the first time characterized this new mechanism.

D. W. Shen, B. P. Xie, J. F. Zhao, L. X. Yang, L. Fang, J. Shi, R. H. He, D. H. Lu, H. H. Wen, D. L. Feng, "Novel Mechanism of a Charge Density Wave in a Transition Metal Dichalcogenide" Physical Review Letters 99, 216404 (2007).

To learn more about this research see the full scientific highlight at:
http://www- ssrl.slac.stanford.edu/research/highlights_archive/cdw-mechanism.html