Two energy gaps and Fermi surface 'arcs' in NbSe2

Using angle-resolved photoemission spectroscopy (ARPES), 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 non-monotonic 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.