Ising Superconductivity in Noncentrosymmetric Bulk NbSe₂

Ising superconductivity allows in-plane upper critical magnetic fields to vastly surpass Pauli limit by locking the antiparallel electron spins of Cooper pairs in the out-of-plane direction. It was first explicitly demonstrated in fully two-dimensional monolayers of transition metal dichalcogenides with large spin-orbit coupling and broken inversion symmetry. Since then, several studies have shown that it can be present in layered bulk materials, too. In our previous study, we have clarified the underlying microscopic mechanism of Ising superconductivity in bulk, based on a reduced electronic coupling between superconducting layers due to intercalation by insulating layers and restricted inversion symmetry. But earlier studies suggest that, in some transition metal dichalcogenide polytypes, the Pauli paramagnetic limit is violated even without intercalation. Here, using heat capacity measurements we unambiguously demonstrate that the pristine noncentrosymmetric bulk 4H_a-NbSe₂ polytype significantly violates the Pauli limit. The band structure parameters obtained from ab initio calculations using the experimentally determined crystal structure are used in the theoretical model, which provides the microscopic mechanism of the Ising protection based solely on broken inversion symmetry.