Optic phonons and the soft mode in 2H-NbSe2

Abstract

We present an investigation of the lattice dynamics of the charge-density-wave compound 2H-NbSe2. We analyze the precise nature of the wave vector dependent electron-phonon coupling (EPC) and derive the bare dispersion of the charge-density-wave (CDW) soft phonon mode using inelastic x-ray scattering combined with ab-initio calculations. Experimentally, phonon modes along the Γ - M line, i.e. q = (h, 0, 0) with 0 <= h <= 0.5, with the same longitudinal symmetry (Σ1) as the CDW soft mode were investigated up to 32 meV. In agreement with our calculations we observe significant EPC in the optic modes at h <= 0.2. We analyze the EPC in the optic as well as acoustic modes and show that the q dependences stem from scattering processes between two bands at the Fermi surface both having Nb 4d character. Finally, we demonstrate that the soft mode dispersion at T = 33 K (= {T_{CDW}}) can be well described on the basis of a strongly q dependent EPC matrix element and an acoustic-like bare phonon dispersion in agreement with observations near room temperature.

accepted as regular article in Physical Review B

References in corpus (4)

• Extended Phonon Collapse and the Origin of the Charge-Density-Wave in NbSe${}_2$
• Charge density wave and superconducting dome in TiSe2 from electron-phonon interaction
• A quantum phase transition from triangular to stripe charge order in NbSe${}_2$
• Fermi surface nesting in several transition metal dichalcogenides

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