Structures, charge density wave, and superconductivity of noncentrosymmetric 4Ha−NbSe2

Abstract

We report the crystal and electronic structures, charge density wave (CDW), and superconductivity of polycrystalline 4Ha−NbSe2 studied by x-ray diffraction, ab initio calculations, electrical resistivity, magnetization, and specific heat. 4Ha−NbSe2 has no central inversion symmetry (space group P−6m2), and the stacking-fault structure was observed in the crystal lattice. The weak CDW transition at TCDW∼43K thermodynamically is of second order. 4Ha−NbSe2 has a superconducting critical temperature Tc∼6.4 K, which is a moderately coupled BCS superconductor with two-gap s+s-wave-like pairing. The upper critical field μ0Hc2(0) 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 Nb4d orbitals dominate the bands near the Fermi level with different degree of hybridization between the Nb 4d and Se 4p orbitals in 2H- and 4Ha−NbSe2, and the band structures of 4Ha−NbSe2 favor a higher TCDW. The higher TCDW but lower Tc compared to 2H−NbSe2 leads us to conclude that the two collective electronic states are competitive on the Fermi surface in this system. The large μ0Hc2(0) derived from 4Ha−NbSe2 is expected to stimulate the explorations for high-field superconductor applications from a noncentrosymmetric structural strategy.

Source-reported materials — not catalogue approval

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