Pauli-Limit Breaking Superconductivity in Epitaxially Stabilized Monoclinic LaSb₂

Epitaxial growth of thin film quantum materials has been shown to allow access to novel ground states compared to their bulk counterparts. This is accomplished by imposing strain, engineering heterostructures, reducing dimensionality or even through the stabilization of novel polymorphs. In this talk, we describe our recent explorations into epitaxially grown LaSb₂. In the bulk, this material displays an orthorhombic structure wherein metallic, square-planar Sb layers are sandwiched between corrugated, insulating La-Sb spacer layers with a large c/a axis ratio. In contrast to this reported structure, high resolution diffraction on epitaxial LaSb₂ grown on MgO reveals a monoclinic structure with a shortened c-axis compared to the bulk. The structure is further explored with density functional theory calculations and it is found that the monoclinic phase is stable relative to the orthorhombic structure for epitaxially grown films. Reflection conditions along with DFT calculations have allowed the space group to be identified and a crystal structure to be proposed.

In contrast to the orthorhombic bulk crystals, which show a broad superconducting transition (T_c = 0.6K), these films display a sharp superconducting transition (T_c = 1.65) down to thickness ~2 unit cells. In-plane coherence lengths are ~100nm for thickest films, comparable to the values of the bulk crystals, while parallel critical fields that exceed the Pauli limit by a factor of 3 are observed as thickness is reduced. The interplay between strong spin-orbit coupling, dimensionality reduction and disorder will be discussed.