Protection of Ising spin-orbit coupling in bulk misfit superconductors
Tomáš Samuely, Darshana Wickramaratne, Martin Gmitra, T. Jaouen, O. Šofranko, Dominik Volavka, M. Kuzmiak, Jozef Haniš, P. Szabó, Claude Monney, Geoffroy Kremer, Patrick Le Fèvre, F. Bertran, Tristan Cren, Shunsuke Sasaki, Laurent Cario, Matteo Calandra, I. I. Mazin, P. Samuely
Abstract
Ising superconductivity is present due to the combined effect of broken-inversion symmetry and spin-orbit coupling that locks the spins out of plane, features that are associated with two-dimensional materials. We show that bulk misfit superconductors, ${(\mathrm{LaSe})}_{1.14}(\mathrm{Nb}{\mathrm{Se}}_{2})$ and ${(\mathrm{LaSe})}_{1.14}{(\mathrm{Nb}{\mathrm{Se}}_{2})}_{2}$, comprising monolayers and bilayers of $\mathrm{Nb}{\mathrm{Se}}_{2}$, exhibit unexpectedly strong Ising protection with a Pauli-limit violation comparable to monolayer $\mathrm{Nb}{\mathrm{Se}}_{2}$. We establish these misfit compounds as Ising superconductors using complementary experimental methods in combination with first-principles calculations. A concerted effect of charge-transfer, defects, reduction of interlayer hopping, and stacking enables Ising superconductivity in these compounds and therefore provides a possible pathway to design of bulk superconductors that are resilient to magnetic fields.