Kalendra, Vidmantas, Justinas Turčak, Jūras Banys, John J.L. Morton, and Mantas Šimėnas. “X- and Q-Band EPR with Cryogenic Amplifiers Independent of Sample Temperature.” Journal of Magnetic Resonance 346 (January 2023): 107356.
https://doi.org/10.1016/j.jmr.2022.107356.
Inspired by the success of NMR cryoprobes, we recently reported a leap in X-band EPR sensitivity by equipping an ordinary EPR probehead with a cryogenic low-noise microwave amplifier placed closed to the sample in the same cryostat [Šime˙ nas et al. J. Magn. Reson. 322, 106876 (2021)]. Here, we explore, theoretically and experimentally, a more general approach, where the amplifier temperature is independent of the sample temperature. This approach brings a number of important advantages, enabling sensitivity improvement irrespective of sample temperature, as well as making it more practical to combine with ENDOR and Q-band resonators, where space in the sample cryostat is often limited. Our experimental realisation places the cryogenic preamplifier within an external closed-cycle cryostat, and we show CW and pulsed EPR and ENDOR sensitivity improvements at both X- and Q-bands with negligible dependence on sample temperature. The cryoprobe delivers signal-to-noise ratio enhancements that reduce the equivalent pulsed EPR measurement time by 16Â at X-band and close to 5Â at Q-band. Using the theoretical framework we discuss further improvements of this approach which could be used to achieve even greater sensitivity.