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

TL;DR

This paper investigates how Hahn echo measurements in ESR-STM can be misinterpreted due to tunneling electron effects, proposing methods to reliably extract spin coherence times.

Contribution

It demonstrates that RF-induced tunneling effects can distort Hahn echo signals and offers practical criteria to accurately measure spin coherence times in ESR-STM.

Findings

Hahn echo signals in ESR-STM are susceptible to tunneling electron effects.

Varying delay times in refocusing sequences helps confirm true coherence.

Estimated T2 coherence time is approximately 30 ns, shorter than previous reports.

Abstract

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 phthalocyanine (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) voltage. The RF voltage not only drives the spin, but simultaneously probes and relaxes it, which consequently leads to an exponential decay that reflects spin relaxation rather than intrinsic phase coherence. 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 T2 is approximately 30 ns and is thus closer to the decoherence time observed in Rabi oscillation measurements. This is significantly shorter than values reported in previous echo measurements. Our findings underscore the need for caution in interpreting T2 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.