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  Borisenko, S.V.; Kordyuk, A.A.; Zabolotnyy, V.B.; Inosov, D.S.; Evtushinsky, D.; Büchner, B.; Yaresko, A.N.; Varykhalov, A.; Follath, R.; Eberhardt, W.; Patthey, L.; Berger, H.: Two Energy Gaps and Fermi-Surface "Arcs" in NbSe₂. Physical Review Letters 102 (2009), p. 166402/1-4

10.1103/PhysRevLett.102.166402

Abstract:
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.