Atomic-layer Rashba-type superconductor protected by dynamic spin-momentum locking

TL;DR

This study demonstrates that atomic-layer superconductors with Rashba-type spin-orbit coupling exhibit enhanced in-plane critical magnetic fields due to dynamic spin-momentum locking, which protects superconductivity against magnetic pair-breaking effects.

Contribution

It provides the first experimental evidence that Rashba-type SOC and dynamic spin-momentum locking can significantly enhance superconductivity's resilience to magnetic fields.

Findings

In-plane critical magnetic field exceeds Pauli limit by about three times.

Dynamic spin-momentum locking suppresses pair-breaking effects.

Rashba-type SOC is confirmed in atomic-layer superconductors.

Abstract

Spin-momentum locking is essential to the spin-split Fermi surfaces of inversion-symmetry broken materials, which are caused by either Rashba-type or Zeeman-type spin-orbit coupling (SOC). While the effect of Zeeman-type SOC on superconductivity has experimentally been shown recently, that of Rashba-type SOC remains elusive. Here we report on convincing evidence for the critical role of the spin-momentum locking on crystalline atomic-layer superconductors on surfaces, for which the presence of the Rashba-type SOC is demonstrated. In-situ electron transport measurements reveal that in-plane upper critical magnetic field is anomalously enhanced, reaching approximately three times the Pauli limit at $T = 0$. Our quantitative analysis clarifies that dynamic spin-momentum locking, a mechanism where spin is forced to flip at every elastic electron scattering, suppresses the Cooper pair-breaking parameter by orders of magnitude and thereby protects superconductivity. The present result provides a new insight into how superconductivity can survive the detrimental effects of strong magnetic fields and exchange interactions.