Abstract
Niobium diselenide has long served as a prototype of two-dimensional charge ordering, believed to arise from an instability of the electronic structure analogous to the one-dimensional Peierls mechanism. Despite this, various anomalous properties have recently been identified experimentally, which cannot be explained by Peierls-like weak-coupling theories. Here, we consider instead a model with strong electron-phonon coupling, taking into account both the full momentum and orbital dependence of the coupling matrix elements. We show that both are necessary for a consistent description of the full range of experimental observations. We argue that NbSe2 is typical in this sense, and that any charge-ordered material in more than one dimension will generically be shaped by the momentum and orbital dependence of its electron-phonon coupling as well as its electronic structure. The consequences will be observable in many charge-ordered materials, including cuprate superconductors.
Highlights
Niobium diselenide has long served as a prototype of two-dimensional charge ordering, believed to arise from an instability of the electronic structure analogous to the one-dimensional Peierls mechanism
Kinks in the density of states (DOS) observed by planar tunnelling experiments have been interpreted to arise from a gap of D 1⁄4 35 meV8,9, while only much smaller gaps between 2 and 5 meV were seen by high-precision angleresolved photoemission spectroscopy (ARPES) experiments[4,10], and older studies even reported no gap at all in ARPES and resistivity measurements[11,12]
We find that the phonon modes are softened over a broad range of wave vectors, and that including the presence of strong phonon fluctuations leads to a suppression of the transition temperature, implying a range of intermediate temperatures dominated by incoherent, fluctuating, charge order and a corresponding pseudogap
Summary
Niobium diselenide has long served as a prototype of two-dimensional charge ordering, believed to arise from an instability of the electronic structure analogous to the one-dimensional Peierls mechanism. Despite a lack of nesting, the limited matching of states at EF acts in unison with the momentum dependence of the electron–phonon coupling to select out the CDW-ordering vector, while the orbital characters of the bands naturally explain why a CDW gap appears primarily in only one of the Fermi surface pockets.
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