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

Abstract

Extreme in-plane upper critical magnetic fields ${B}_{\mathrm{c}2//\mathrm{ab}}$ strongly violating the Pauli paramagnetic limit have been observed in the misfit layer ${(\mathrm{LaSe})}_{1.14}(\mathrm{Nb}{\mathrm{Se}}_{2})$ and ${(\mathrm{LaSe})}_{1.14}{(\mathrm{Nb}{\mathrm{Se}}_{2})}_{2}$ single crystals with ${T}_{\mathrm{c}}=1.23$ and 5.7 K, respectively. The crystals show a two-dimensional to three-dimensional transition at temperatures slightly below ${T}_{\mathrm{c}}$ with an upturn in the temperature dependence of ${B}_{\mathrm{c}2//\mathrm{ab}}$, a temperature-dependent huge superconducting anisotropy and a cusplike behavior of the angular dependence of ${B}_{\mathrm{c}2}$. Both misfits are characterized by a strong charge transfer from LaSe to ${\mathrm{NbSe}}_{2}$. As shown in our previous work, ${(\mathrm{LaSe})}_{1.14}{(\mathrm{Nb}{\mathrm{Se}}_{2})}_{2}$ is electronically equivalent to the highly doped ${\mathrm{NbSe}}_{2}$ 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 ${(\mathrm{LaSe})}_{1.14}(\mathrm{Nb}{\mathrm{Se}}_{2})$, where the charge transfer is nominally twice as big, which could eventually lead to complete filling of the ${\mathrm{NbSe}}_{2}$ 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.