  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.
