Spin-valley locking allowing for in-plane upper critical magnetic fields vastly surpassing Pauli limit was first demonstrated in fully two-dimensional monolayers like of NbSe₂ [1] with large spin-orbit coupling and broken inversion symmetry. Surprisingly, this Ising superconductivity can be present in layered bulk materials, too. We have clarified the underlying microscopic mechanism of Ising superconductivity in bulk, based on a reduced interlayer coupling between superconducting layers due to intercalation by insulating layers and restricted inversion symmetry in misfit compounds of (LaSe)_1.14(NbSe₂)m=1,2 [2,3]. Here, we show that in some transition metal dichalcogenide polytypes Pauli paramagnetic limit is violated even without intercalation. By the specific heat measurements up to 35 T we show that that in the pristine non-centrosymmetric bulk 4Ha-NbSe₂ polytype the in-plane upper critical magnetic field exceeds the Pauli limit three times. Ab initio band structure calculations based on experimentally determined crystal structure confirm the spin-valley locking. The theoretical model provides the microscopic mechanism of the Ising protection based solely on broken inversion symmetry [4]. Moreover, the low temperature specific heat measurements in magnetic field applied at different angles with respect to the basal plane and low temperature scanning tunnelling microscopy and spectroscopy are used to analyze the unconventional superconducting order parameter.

References:

[1] X. Xi et al., Nat. Phys. 12, 139 (2016).
[2] P. Samuely et al., Phys. Rev. B 104, 224507 (2021).
[3] T. Samuely et al., Phys. Rev. B 108, L220501 (2023).
[4] D. Volavka et al., arXiv:2501.08867

*corresponding author: e-mail: samuely@saske.sk