Transition metal dichalcogenides (TMDs) and similar layered materials have attracted significant attention in recent years due to their novel electronic and optical properties. Various periodic lattice distortion (PLD) states related to the charge density wave (CDW) state have been observed in TMDs. The CDW state has attracted a lot of attention due to being a bulk quantum state of electrons, which is quite often rivaling the superconducting state. Very early theoretical work implied that CDW states occur due to very special arrangement of electronic states in a material called Fermi nesting conditions. Such conditions occur not in real space, but in the momentum space of electrons. Application of high pressure would usually suppress the nesting conditions and destroy the PLD state and its accompanying CDW state. The Geophysical Laboratory's Jianjun Ying and Viktor Struzhkin in collaboration with Elena R. Margine, Hari Paudyal in Binghamton University-SUNY, Christoph Heil in University of Oxford, and Xiao-jia Chen in HPSTAR, have found an unusual pressure induced PLD in a TMDs-related material SnSe2. Their work is published in Physical Review Letters. They found that √3  × √3  ×1 superlattice appears above 17 GPa in SnSe2 material, contrary to decomposition into Sn3Se4 and Se as proposed by the preceding theoretical calculations for SnSe2. The authors have found that the Fermi surface nesting and the strong electron-phonon coupling occur under pressure at the same momentum wave vector in SnSe2, which goes in line with theoretical expectations for the creation of the PLD and CDW states. Notably, this observation is in strong contrast to findings in other metallic TMDs, where the Fermi surface nesting was found to play a less significant role in the creation of the CDW state. Thus, the new PLD state created against all expectations by applying pressure will provide new playground to study the intricate mechanisms governing the emergence of PLD and CDW states and their rich physics in TMD-related materials.

Caption: Atomic displacements in the √3  × √3  ×1 H1-supercell for the two soft phonon modes leading to the formation of the H2-1 and H2-2 phases. The length of each arrow is proportional to the magnitude of the atomic displacement. (Image provided by Elena Margine)