Interface-Induced Zeeman-Protected Superconductivity in Ultrathin Crystalline Lead Films

Abstract

Two-dimensional (2D) superconducting systems are of great importance for exploring exotic quantum physics. The recent development of fabrication techniques has stimulated studies of high-quality single-crystalline 2D superconductors, where intrinsic properties give rise to unprecedented physical phenomena. Here, we report the observation of Zeeman-type spin-orbit interaction protected superconductivity (Zeeman-protected superconductivity) in 4-monolayer (ML) to 6-ML crystalline Pb films grown on striped incommensurate Pb layers on Si(111) substrates by molecular beam epitaxy. An anomalously large in-plane critical field far beyond the Pauli limit is detected, which can be attributed to the Zeeman-protected superconductivity due to the in-plane inversion symmetry breaking at the interface. Our work demonstrates that, in superconducting heterostructures, the interface can induce Zeeman-type spin-orbit interactions and modulate the superconductivity.

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Plain Language Summary

Superconductors are exotic materials, capable of conducting electricity with zero resistance. One intriguing line of research is to see if superconductivity can exist in ultrathin materials that are effectively two dimensional and to observe what novel behaviors arise in such devices. The results of such research could lead, for example, to novel superconductive nanoscale devices. Here, we experimentally demonstrate how a particular type of superconductivity can arise in a lead film just a few atoms thick by tweaking the interface between the film and an underlying substrate.

Specifically, we observe the onset of Zeeman-protected superconductivity, where spin-orbit interactions orient the spin of the electrons perpendicular to the film and protect the superconductivity against the in-plane magnetic field. The lead film is grown on a silicon substrate. Between the film and the substrate is a one-atom-thick layer known as a striped incommensurate (SIC) layer, where mismatches between the lattice of lead and silicon create a special phase of lead. We detect that the superconductivity is immune to an anomalously large magnetic field in the plane of the lead film—a clear sign of Zeeman-protected superconductivity. Our calculations show that distortions in the SIC lattice extend to the neighboring thin lead film and induce this type of superconductivity.

Our work shows how Zeeman-protected superconductivity can be induced by engineering the interface in crystalline 2D heterostructures. These findings not only provide a new platform to create this type of superconductivity but also might stimulate further studies on the interface modulation of unconventional superconductivity in these systems.