Structural and Electronic Instabilities of Transition Metal Chalcogenides

Abstract

The electronic instabilities of low-dimensional metals leading to their structural modulations are often discussed in terms of Fermi surface nesting [1]. One-dimensional (1D) metals are susceptible to a charge density wave (CDW) formation due to the electronic instability associated with their nested Fermi surfaces. A CDW phenomenon also occurs in certain two-dimensional metals [2], although individual Fermi surfaces exhibit no nesting. This finding led to the concept of hidden Fermi surface nesting [3]: the combined Fermi surfaces of such a system are decomposed into a set of hidden 1D Fermi surfaces, and the nesting associated with the hidden surfaces is responsible for the CDW instability. The CDW instability driven by Fermi surface nesting (regular or hidden) refers to the tendency that a low-dimensional metal lowers its electronic energy by opening a band gap at the Fermi level. This is analogous in nature to the first-order Jahn-Teller instability in molecular chemistry [4]: a molecule with partially filled degenerate HOMO’s tends to undergo a symmetry-lowering distortion which splits the levels into the filled and empty ones with an energy gap between the two. A molecule with no partially filled degenerate HOMO’s can also undergo a symmetry-lowering distortion, as exemplified by the second-order Jahn-Teller instability [4]. Likewise, the structural modulation of a low-dimensional metal can originate from the lowering of the energy levels lying well below the Fermi level. Such a structural modulation is not explained in terms of the concept of Fermi nesting. In this work, the origin of the structural modulations in several transition metal chalcogenide metals is classified.