Ising superconductivity in bulk layered non-centrosymmetric 4H-NbSe2

TL;DR

This paper demonstrates that bulk 4H-NbSe2, which breaks inversion symmetry, exhibits Ising superconductivity characterized by high in-plane critical fields and valley-selective spin polarization, expanding the understanding of superconductivity in layered materials.

Contribution

It reports the discovery of Ising superconductivity in bulk 4H-NbSe2, a non-centrosymmetric polymorph, supported by experimental measurements and theoretical analysis.

Findings

In-plane critical field exceeds Pauli limit

Superconductivity is anisotropic with respect to magnetic field orientation

Valley-selective spin polarization confirmed by calculations

Abstract

Transition metal dichalcogenides exhibit multiple polymorphs that enable the exploration of diverse quantum states, including valley-selective spin polarization, the valley Hall effect, Ising superconductivity, and nontrivial topology. Monolayer 2$H$-NbSe$_2$ is a promising candidate for realizing Ising superconductivity due to its spin-split, out-of-plane spin-polarized states arising from inversion symmetry breaking and strong spin-orbit coupling. In contrast, bulk 2$H$-NbSe$_2$ retains inversion symmetry and lacks spin splitting, limiting its suitability for hosting Ising superconductivity. Here, we report the growth of high-quality single crystals of the acentric bulk superconducting polymorph, 4$H$-NbSe$_2$, which intrinsically breaks the inversion symmetry and supports valley-selective spin-polarized states. Magnetization and resistivity measurements reveal anisotropic superconductivity, with the in-plane upper critical field exceeding the Pauli limit, while out-of-plane fields suppress superconductivity more rapidly, before reaching the Pauli limit, which strongly suggests the presence of Ising pairing. First-principles calculations and symmetry analysis confirm significant valley-selective spin splitting with out-of-plane spin polarization, further supporting the emergence of Ising superconductivity in 4$H$-NbSe$_2$. These results establish 4$H$-NbSe$_2$ as a robust bulk platform to investigate Ising superconductivity and valley-selective phenomena in transition-metal dichalcogenides.