Upgrading a low-temperature scanning tunneling microscope for electron spin resonance experiments

We describe the upgrade of a helium-3 STM to an electron spin resonance enabled apparatus [1]. We are able to transmit RF power to the tunnel junction at frequencies of up to 30 GHz. Our apparatus is benchmarked via magnetic field sweep ESR on the model system TiH/MgO/Ag(100)[2] for which we find a magnetic moment of (1.004±0.001) μ_B. We can chose a DC mode for regular STM operation or an ultra-fast mode for pump-probe spectroscopy or the reading of spin-states. In both modes, simultaneous radiofrequency excitation is possible, which we add via a resistive pick-off tee to the bias voltage path. We discuss our transmission, indicate potential pitfalls in the upgrade and demonstrate how to synchronize the arrival times of RF and DC pulses coming from different paths to the STM junction, a prerequisite for future pulsed ESR experiments.
[1] Natterer et al., arXiv:1810.03887 (2018)
[2] Yang et al., PRL 119, 227206 (2017)