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 $(LaSe)_{1.14}(NbSe_2)$ and $(LaSe)_{1.14}(NbSe_2)_2$ single crystals with $T_c$ = 1.23 K 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 $NbSe_2$. As shown in our previous work, $(LaSe)_{1.14}(NbSe_2)$ is electronically equivalent to the highly doped $NbSe_2$ 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 $(LaSe)_{1.14}(NbSe_2)_2$, where the charge transfer is nominally twice as big, which could eventually lead to complete filling of the $NbSe_2$ 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.