Protection of Ising spin-orbit coupling in bulk misfit superconductors

TL;DR

This paper demonstrates that certain bulk misfit superconductors exhibit Ising superconductivity with Pauli-limit violation, despite having inversion symmetry, due to mechanisms like charge transfer and stacking effects.

Contribution

It reveals the unexpected presence of Ising protection in bulk centrosymmetric superconductors through experimental and theoretical analysis.

Findings

Bulk misfit compounds show Pauli-limit violation similar to monolayer NbSe2.

Charge transfer and stacking effects enable Ising protection in bulk materials.

The study proposes mechanisms for Ising superconductivity survival in centrosymmetric bulk compounds.

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.