Invited talk

Single-molecule electron-spin resonance with atomic force microscopy
Lisanne Sellies, R. Spachtholz, S. Bleher, J. Eckrich, P. Scheuerer, J. Repp

Institute for Experimental and Applied Physics, University of Regensburg, 93040 Regensburg, Germany

Recently, we combined the high energy resolution of electron spin resonance (ESR) with the spatial resolution offered by atomic force microscopy (AFM). This ESR-AFM technique relies on driving electron spin transitions between the non-equilibrium triplet state levels of a single molecule. First, the molecule is brought to its triplet excited state, after which the decay of this state is read-out, using a recently developed electronic pump-probe scheme [1]. Since the three non-degenerate triplet states have typically different lifetimes, the resulting triplet decay can be used as a measure for driving resonant transitions between two of these states [2,3]. This is illustrated in figure 1 for a single pentacene molecule.

The ESR-AFM spectra feature a sub-nanoelectronvolt energy resolution. Thereby, molecules only differing in their isotopic configuration can be distinguished. Moreover, due to the minimally invasive nature of the ESR-AFM technique, the electron spins of pentacene can be coherently manipulated over tens of microseconds, likely not limited by the detection method but by the molecular properties [4]. The signatures of these coherent manipulations depend on the orientation of the individual molecules, illustrating the selection rules at play.

After introducing ESR-AFM, recent results obtained by ESR-AFM will be presented.

Figure 1: A Schematic of the setup, by which we probe the ESR-AFM signal of an individual pentacene molecule. B ESR-AFM spectrum of the TX-TZ transition of pentacene. A normalized AFM signal was measured (ΔFnorm), which can be calibrated against the triplet population. Figure adapted from [4].

[1] J. Peng et al., Science, 373, 452 (2021)
[2] J. Köhler et al., Nature, 363, 242 (1993)
[3] J. Wrachtrup et al., Nature, 363, 244 (1993)
[4] L. Sellies et al., arXiv:2212.12244 (2022)