Charge-density-wave mechanism in the2H−NbSe2family: Angle-resolved photoemission studies

Abstract

We report high-resolution angle resolved photoemission studies of $2H-{\mathrm{NbSe}}_2$ and $2H-{\mathrm{TaS}}_2.$ Results are compared to those of $2H-{\mathrm{TaSe}}_2.$ We find that the normal state electronic structures in this family of materials have the same basic characteristics. Their Fermi surface consists of a pocket around Γ and another one around K. They all exhibit a saddle band near $E_F$ along Γ-K. A finite energy gap is observed in the saddle band region and on the Fermi surface around K in $2H-{\mathrm{TaS}}_2$ and $2H-{\mathrm{TaSe}}_2.$ Gapping on the Fermi surface around Γ was not observed. We suggest that the charge density waves in this family of materials originate from a combination of the partial nesting of the Fermi surface around K and the saddle band.