On the interpretation of Hahn echo measurements in electron spin resonance scanning tunneling microscopy

Electron spin resonance scanning tunneling microscopy (ESR-STM) has become a powerful tool for probing spin dynamics and coherence of individual atoms and molecules on surfaces. In this work, we perform Rabi oscillation and Hahn echo pulse protocols on individual iron phthalocya-nine (FePc) molecules on MgO/Ag(001) using ESR-STM. While Hahn echo protocols are widely used to extract spin coherence times, we show that in ESR-STM they are particularly susceptible to misinterpretation due to tunneling electrons generated by the applied radio-frequency (RF) and direct current (DC) voltage. The RF voltage not only drives the spin, but simultaneously probes and relaxes it in combination with the constant DC readout. This consequently leads to an exponential decay that reflects spin relaxation (T$_1$) rather than intrinsic phase coherence (T$_2$). We moreover show that varying both delay times in the refocusing pulse sequence is a reliable way to ensure a coherent nature of the echo signal. The extracted decay for the latter protocol suggests that T$^{Hahn}_2$ is ≈ 30 ns and is thus closer to the decoherence time T$^{Rabi}_2$ observed in Rabi oscillation measurements. ... mehrThis is significantly shorter than values reported in previous echo measurements. Our findings underscore the need for caution in interpreting T$_2$ times from Hahn echo and Carr-Purcell protocols in ESR-STM and provide practical criteria for distinguishing true spin echoes from tunneling-induced relaxometry signals.