Abstract
The connection between the Fermi surface and charge-density wave (CDW) order is revisited in 2H-TaSe2. Using angle-resolved photoemission spectroscopy, ab initio band structure calculations, and an accurate tight-binding model, we develop the empirical k-resolved susceptibility function, which we use to highlight states that contribute to the susceptibility for a particular q-vector. We show that although the Fermi surface is involved in the peaks in the susceptibility associated with CDW order, it is not through conventional Fermi surface nesting, but rather through finite energy transitions from states located far from the Fermi level. Comparison with monolayer TaSe2 illustrates the different mechanisms that are involved in the absence of bilayer splitting.
Highlights
The question of whether nesting instabilities of the Fermi surface (FS) can drive charge-density-wave (CDW) formation in real materials has been the topic of numerous experimental and theoretical investigations for many years.1–3 In cases of apparently well-nested FSs, subsequent inspection of the real part of the generalized susceptibility, which is the relevant quantity in assessing instabilities in the electronic system, and its imaginary counterpart can rule against FS nesting being the primary driving force.4 In concert with instabilities in the electronic system, lattice effectsmust be considered on an equal footing.5The analysis of the electronic susceptibility of a material is central in determining whether an electronic instability that may be due to FS nesting is capable of driving some associated ordering phenomena
Using angle-resolved photoemission spectroscopy, ab initio band-structure calculations, and an accurate tightbinding model, we develop the empirical k-resolved susceptibility function, which we use to highlight states that contribute to the susceptibility for a particular q vector
The q landscape of the real and imaginary parts of the susceptibility is compared, and a peak that survives in both parts is taken as evidence that FS nesting may play a role in emergent phenomena that occurs at that wave vector
Summary
The question of whether nesting instabilities of the Fermi surface (FS) can drive charge-density-wave (CDW) formation in real materials has been the topic of numerous experimental and theoretical investigations for many years. In cases of apparently well-nested FSs, subsequent inspection of the real part of the generalized susceptibility, which is the relevant quantity in assessing instabilities in the electronic system, and its imaginary counterpart (which is not) can rule against FS nesting being the primary driving force. In concert with instabilities in the electronic system, lattice effects (through the softening of phonon modes associated with the CDW). State-of-theart band-structure results firmly rule out the FS nesting model, whereas some recent high-resolution angle-resolved photoemission (ARPES) measurements contradict the theory, suggesting a primary role for the FS via its experimental autocorrelation map.. State-of-theart band-structure results firmly rule out the FS nesting model, whereas some recent high-resolution angle-resolved photoemission (ARPES) measurements contradict the theory, suggesting a primary role for the FS via its experimental autocorrelation map.13,19 We address this controversy directly through complementary ARPES measurements and ab initio band-structure calculations. We show that this concept explains both the temperature dependent ARPES spectral function, as well as why the material has courted controversy for so long. We suggest that similar careful inspection of the k-resolved susceptibility function in other materials will be capable of discriminating between different models of charge, spin, or superconducting order
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