The amplitude of the radio‑frequency (RF) bias applied to a single‑atom spin device does more than drive electron‑spin resonance (ESR); it also selects the transport regime that the system occupies and determines the mix between field‑like torque (FLT) and damping‑like torque (DLT). In the semiclassical limit, the periodic RF voltage sweeps the junction through its static transport landscape. When the RF trajectory approaches a charge‑transition threshold, the DLT that results from spin‑polarized charge transfer is strongly amplified, whereas the FLT that originates from virtual charge fluctuations remains finite deeper in the Coulomb‑blockade (CB) region. The RF amplitude therefore serves as a single control knob that sets the ESR power, the transport regime, and the FLT/DLT ratio.

The microscopic mechanism was described with a Redfield quantum‑master equation. In this framework, the oscillating bias enters explicitly through the electronic reservoirs, generating photon‑assisted tunnelling sidebands whose strength is weighted by Bessel functions. This approach captures sequential tunnelling at lowest order and describes how the RF bias modulates the tunnel rates, but it does not include cotunnelling processes that may become important deep in the CB regime.

Transport through the impurity is classified into three regimes by a dimensionless detuning parameter ​µ = (eVDC – ε)/VRF (or (ε+U – eVDC)/VRF), where ε is the impurity level, U the charging energy, eVDC the direct current bias, and VRF the RF amplitude. The regimes are separated by the boundaries ​µ = ±1, corresponding to the RF trajectory crossing a charge‑transition threshold. When ​µ < –1 the system is in the sequential regime; when ​µ > 1 it remains in the CB regime throughout the cycle; and when | ​µ | ≤ 1 the trajectory alternates between the sequential and CB regimes each RF cycle.