Adjustable coupling and <i>in-situ</i> variable frequency probe with loop-gap resonators for cw and pulse electron paramagnetic resonance spectroscopy up to X-band

In standard electron paramagnetic resonance (EPR) spectroscopy, the frequency of an experiment is set and the spectrum is acquired using magnetic field as the independent variable. There are cases in which it is desirable instead to fix the field and tune the frequency such as when studying atomic-clock transitions at avoided level crossings in molecular nanomagnets and other spin qubits [1,2,3]. We have designed and tested an adjustable frequency and variable coupling EPR probe with loop-gap resonators (LGRs) that works at a temperature down to 1.8 K [4]. The frequency is tuned by adjusting the height of a dielectric piece of sapphire inserted into the gap of an LGR; coupling of the microwave antenna is varied with the height of the antenna above the LGR. We demonstrate the operation of our probe with continuous wave EPR by mapping out avoided crossings for the Ni₄ single-molecule magnet to determine the tunnel splittings with high precision.
[1] M. Shiddiq et al., Nature 531, 348 (2016)
[2] G. Wolfowicz et al., Nat. Nanotechnol., 8, 561 (2013)
[3] C. A. Collet et al., Magnetochemistry, 5,4 (2019)
[4] G. Joshi et al., arXiv:1910.10901