We report the crystal and electronic structures, charge density wave (CDW), and superconductivity of polycrystalline $4H_{\mathrm{a}}\text{−}{\mathrm{NbSe}}_2$ studied by x-ray diffraction, ab initio calculations, electrical resistivity, magnetization, and specific heat. $4H_{\mathrm{a}}\text{−}{\mathrm{NbSe}}_2$ has no central inversion symmetry (space group $P\text{−}6m2$), and the stacking-fault structure was observed in the crystal lattice. The weak CDW transition at $T_{\mathrm{CDW}}\sim 43\mathrm{K}$ thermodynamically is of second order. $4H_{\mathrm{a}}\text{−}{\mathrm{NbSe}}_2$ has a superconducting critical temperature $T_{\mathrm{c}}\sim 6.4$ K, which is a moderately coupled BCS superconductor with two-gap $s+s$-wave-like pairing. The upper critical field ${\mu }_0{H}_{\mathrm{c}2}\left(0\right)$ is as high as 26.5 T which far exceeds the Pauli paramagnetic limit and this violation is mostly attributed to the breaking of the central inversion symmetry. Second magnetization peaks can be observed in the system with weak flux pinning. Electronic structure calculations reveal that the $\text{Nb}4d$ orbitals dominate the bands near the Fermi level with different degree of hybridization between the Nb $4d$ and Se $4p$ orbitals in $2H$- and $4H_{\mathrm{a}}\text{−}{\mathrm{NbSe}}_2$, and the band structures of $4H_{\mathrm{a}}\text{−}{\mathrm{NbSe}}_2$ favor a higher $T_{\mathrm{CDW}}$. The higher $T_{\mathrm{CDW}}$ but lower $T_{\mathrm{c}}$ compared to $2H-{\mathrm{NbSe}}_2$ leads us to conclude that the two collective electronic states are competitive on the Fermi surface in this system. The large ${\mu }_0{H}_{\mathrm{c}2}\left(0\right)$ derived from $4H_{\mathrm{a}}\text{−}{\mathrm{NbSe}}_2$ is expected to stimulate the explorations for high-field superconductor applications from a noncentrosymmetric structural strategy.

• Received 25 September 2023
• Revised 5 December 2023
• Accepted 6 December 2023

DOI:https://doi.org/10.1103/PhysRevB.108.224518