Fermi surface nesting in several transition metal dichalcogenides

H Berger,B Büchner,D S Inosov,A A Kordyuk,V B Zabolotnyy,D V Evtushinsky,S V Borisenko,R Follath

doi:10.1088/1367-2630/10/12/125027

Abstract

By means of high-resolution angle-resolved photoelectron spectroscopy (ARPES), we have studied the fermiology of 2H transition metal dichalcogenide polytypes TaSe2, NbSe2 and Cu0.2NbS2. The tight-binding model of the electronic structure, extracted from ARPES spectra for all three compounds, was used to calculate the Lindhard function (bare spin susceptibility), which reflects the propensity to charge density wave (CDW) instabilities observed in TaSe2 and NbSe2. We show that though the Fermi surfaces of all three compounds possess an incommensurate nesting vector in the close vicinity of the CDW wave vector, the nesting and ordering wave vectors do not exactly coincide, and there is no direct relationship between the magnitude of the susceptibility at the nesting vector and the CDW transition temperature. The nesting vector persists across the incommensurate CDW transition in TaSe2 as a function of temperature despite the observable variations of the Fermi surface geometry in this temperature range. In Cu0.2NbS2, the nesting vector is present despite different doping levels, which leads us to expect a possible enhancement of the CDW instability with Cu intercalation in the CuxNbS2 family of materials.

Highlights

In 1955, Peierls [1] suggested that in one-dimensional metals, a spontaneous formation of periodic lattice distortions (PLD) and charge density waves (CDW) can be energetically favorable under certain conditions
This kind of symmetry breaking, which happens upon cooling at a certain transition temperature TCDW, is known as a Peierls phase transition
The phase transition would be preceded by the softening of a phonon mode, until it ‘freezes’ at TCDW, giving rise to the PLD with the same wave vector q

Summary

CDW vectors qn

M are well known from experiments [2, 3]. The question that we want to address here is whether the Fermi surface geometry observed in the normal state by ARPES possesses the nesting properties that could explain the transition to the CDW state upon cooling. The tight-binding parameters were found by solving an overdetermined system of 15 equations relating the Fermi momenta and velocities of the model to the experimentally measured ones, so that the resulting tight-binding model best reproduces both the Fermi surface contours and the experimental dispersion in the vicinity of the Fermi level. Such a fitting procedure has been applied to the Fermi surface maps of 2H-TaSe2 independently at three temperatures: 107 K (in the incommensurate CDW state), 180 K and 290 K (both in the normal state).

The sharp peak seen near the

Inner barrels