Extreme in-plane upper critical magnetic fields of heavily doped quasi-two-dimensional transition metal dichalcogenides

Extreme in-plane upper critical magnetic fields B-c2//ab strongly violating the Pauli paramagnetic limit have been observed in the misfit layer (LaSe)(1.14)(NbSe2) and (LaSe)(1.14)(NbSe2)(2) single crystals with T-c = 1.23 and 5.7 K, respectively. The crystals show a two-dimensional to three-dimensional transition at temperatures slightly below T-c with an upturn in the temperature dependence of B-c2//ab, a temperature-dependent huge superconducting anisotropy and a cusplike behavior of the angular dependence of B-c2. Both misfits are characterized by a strong charge transfer from LaSe to NbSe2. As shown in our previous work, (LaSe)(1.14)(NbSe2)(2) is electronically equivalent to the highly doped NbSe2 monolayers. 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 a lack of inversion symmetry. A very similar behavior is found in (LaSe)(1.14)(NbSe2)(2), where the charge transfer is nominally twice as big, which could eventually lead to complete filling of the NbSe2 conduction band and opening superconductivity in LaSe. Whatever the particular superconducting mechanism would be, a common denominator in both misfits is that they behave as a stack of almost decoupled superconducting atomic layers, proving that Ising superconductivity can also exist in bulk materials.