Ising superconductivity in bulk (LaSe)_1.14(NbSe₂)_m=1,2misfit compounds

Extreme in-plane upper critical magnetic fields B_c2//ab violating the Pauli limit have been observed in the (LaSe)_1.14(NbSe₂) and (LaSe)_1.14(NbSe₂)₂ single crystals with T_c = 1.23 K and 5.7 K, respectively.  The crystals show a 2D-3D transition at the temperatures slightly below T_c with an upturn in the temperature dependence of B_c2//ab, a huge temperature dependent superconducting anisotropy and a cusp-like behavior of the angular dependence of B_c2.[1] Both misfits are characterized by a strong charge transfer from LaSe to NbSe₂. (LaSe)_1.14(NbSe₂)₂ is electronically equivalent to highly doped NbSe₂ monolayers.[2] Then, the strong upper critical field can be attributed to the Ising coupling recently discovered in atomically thin transition metal dichalcogenides with strong spin-orbit coupling and lack of inversion symmetry.[3] A similar behavior is found in (LaSe)_1.14(NbSe₂), where the charge transfer is even larger, almost filling the NbSe₂ conduction band. Here, the larger spin splitting at the top of the band leads to a stronger Ising behavior. Yet, the topmost layer exhibits entirely different electronic properties. Both misfits behave as a stack of almost decoupled superconducting atomic layers proving that Ising superconductivity can also exist in bulk materials.