Adjustable coupling and in-situ variable frequency EPR probe with loop-gap resonators for spectroscopy up to X-band

TL;DR

This paper introduces an adjustable frequency and coupling EPR probe with loop-gap resonators capable of operating down to 1.8 K, allowing precise in-situ tuning for spectroscopy up to X-band frequencies.

Contribution

The authors developed a novel cryogenic EPR probe with in-situ adjustable frequency and coupling, enabling flexible spectroscopy at low temperatures and a wide frequency range.

Findings

Achieved more than 1 GHz frequency tuning range.

Successfully mapped avoided crossings in Ni4 single-molecule magnet.

Demonstrated high-precision determination of tunnel splittings.

Abstract

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 avoided level crossings. 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. 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 antenna above the LGR. Both coupling antenna and dielectric are located within the cryogenic sample chamber, but their motion is controlled with external micrometers located outside the cryostat. The frequency of the LGR can be adjusted by more than 1 GHz. To cover a wide range of frequencies, different LGRs can be designed to cover frequencies up to X-band. We demonstrate the operation of our probe by mapping out avoided crossings for the Ni$_4$ single-molecule magnet to determine the tunnel splittings with high precision.