Unusual Spectral Behavior of Charge-Density Waves with Imperfect Nesting in a Quasi-One-Dimensional Metal

Low-temperature electronic properties of the charge-density-wave system NbSe₃ are reported from angle-resolved photoemission at 15 K. The effect of two instabilities q₁ and q₂ on the k-resolved spectral function is observed for the first time. With a pseudogap background, the gap spectra exhibit maxima at Δ₁*~110 meV and Δ₂*~45 meV. Imperfectly nested sections of the Fermi surface lack a Fermi-Dirac edge, and show the signature of a dispersion that is modified by self-energy effects. The energy scale is of the order of the effective gap 2Δ₂*. The effect disappears above T₂, suggesting a correlation with the charge-density-wave state.