Protection of Ising spin-orbit coupling in bulk misfit superconductors

Abstract

Low-dimensional materials have remarkable properties that are distinct from their bulk counterparts. A paradigmatic example is Ising superconductivity that occurs in monolayer materials such as NbSe2 which show a strong violation of the Pauli limit. In monolayers, this occurs due to a combination of broken inversion symmetry and spin-orbit coupling that locks the spins of the electrons out-of-plane. Bulk NbSe2 is centrosymmetric and is therefore not an Ising superconductor. We show that bulk misfit compound superconductors, (LaSe)1.14(NbSe2) and (LaSe)1.14(NbSe2)2, comprised of monolayers and bilayers of NbSe2, exhibit unexpected Ising protection with a Pauli-limit violation comparable to monolayer NbSe2, despite formally having inversion symmetry. We study these misfit compounds using complementary experimental methods in combination with first-principles calculations. We propose theoretical mechanisms of how the Ising protection can survive in bulk materials. We show how some of these mechanisms operate in these bulk compounds due to a concerted effect of charge-transfer, defects, reduction of interlayer hopping, and stacking. This highlights how Ising superconductivity can, unexpectedly, arise in bulk materials, and possibly enable the design of bulk superconductors that are resilient to magnetic fields.

14 pages, 4 figures, 4 extended data items

References in corpus (3)

• Review of U-based Ferromagnetic Superconductors: Comparison between UGe2, URhGe, and UCoGe
• Pristine and intercalated transition metal dichalcogenide superconductors
• Transition from three- to two-dimensional Ising superconductivity in few-layer NbSe2 by proximity effect from van der Waals heterostacking

Cited by in corpus (5)

• Ising superconductivity in noncentrosymmetric bulk NbSe2
• Stacking of charge-density waves in 2H-NbSe${}_2$ bilayers
• Orbital Fulde-Ferrell-Larkin-Ovchinnikov state in 2H-NbS2 flakes
• Single-gap two-band superconductivity well above the Pauli limit in non-centrosymmetric TaIr${}_2$B${}_2$
• Pressure-induced reentrant superconductivity in a misfit layered compound $(\mathrm{SnS}{)}_{1.15}(\mathrm{Ta}{\mathrm{S}}_2)$