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

• Study on the lattice structure of Ge x NbSe 2 shows that Ge atoms disorderedly occupy the interlayer Se 6 octahedral interstices. • The two-gap superconductivity was suppressed with increasing the content of Ge. • The decrease of T c arises from the lowered electron-phonon coupling and density of states. • Upper critical field H c2 and irreversible field H irr were enhanced in the low Ge-level samples. • The improvement of H c2 and H irr is attributed to the boosted electron scattering and strengthened vortex pinning. We report the structural and superconducting properties of Ge-intercalated 2H–NbSe 2 polycrystals. Ge x NbSe 2 samples with nominal 0 ≤ x ≤ 0.1 crystallize in the space group P 6 3 / mmc with Ge disorderedly occupying the interlayer Se 6 octahedral interstices. Superconducting critical temperature T c monotonically decreases from 7.2 K in NbSe 2 to 4.9 K in Ge 0.1 NbSe 2. 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. Ge atoms are disorderedly located at the interlayer Se 6 octahedral holes in Ge x NbSe 2 , resulting in the enlarged cell dimension and axial ratio c / a. Superconducting transition temperature T c decreases due to the reduced λ e-p and N (E F). H c2 and H irr are enhanced in the low Ge-level samples due to the boosted electron scattering and vortex pinning. [Display omitted] [ABSTRACT FROM AUTHOR]

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