Two Energy Gaps and Fermi-Surface “Arcs” in NbSe₂

Using angle-resolved photoemission spectroscopy, we report on the direct observation of the energy gap in 2H-NbSe₂ caused by the charge-density waves (CDW). The gap opens in the regions of the momentum space connected by the CDW vectors, which implies a nesting mechanism of CDW formation. In remarkable analogy with the pseudogap in cuprates, the detected energy gap also exists in the normal state (T>T₀) where it breaks the Fermi surface into “arcs,” it is nonmonotonic as a function of temperature with a local minimum at the CDW transition temperature (T₀), and it forestalls the superconducting gap by excluding the nested portions of the Fermi surface from participating in superconductivity.