Experimental investigation on the crystal structure and superconductivity of germanium-intercalated 2H–NbSe2 system

We report the structural and superconducting properties of Ge-intercalated 2H–NbSe₂ polycrystals. Ge_xNbSe₂ samples with nominal 0 ≤ x ≤ 0.1 crystallize in the space group P6₃/mmc with Ge disorderedly occupying the interlayer Se₆ octahedral interstices. Superconducting critical temperature T_c monotonically decreases from 7.2 K in NbSe₂ to 4.9 K in Ge_0.1NbSe₂. Studies on resistivity, magnetization and specific heat derive the superconducting- and normal-state parameters, indicating that the suppression of the two-gap superconductivity is mostly caused by the lowered electron-phonon coupling parameter λ_e-p and density of states at the Fermi level N(E_F). Surprisingly, the upper critical field H_c2 and irreversible field H_irr of the low Ge-level samples are enhanced compared with those of the undoped one, which may ascribe to the electron scattering and vortex pinning by nonmagnetic Ge. This study suggests the feasibility for improving high-field performance by slight impurity doping and advances the understanding of superconductivity in transition-metal dichalcogenides.