Sub-Kelvin electron spin resonance on quantum spin liquid candidates*

Theoretical [1] and experimental [2] advancements have established ESR as a useful probe of exotic excitations in quantum and frustrated magnets. However, relatively few groups have been able to study such materials with magnetic resonance below the 1 to 4 Kelvin limit of most spectrometers.

To overcome this barrier and measure the low energy collective spin dynamics of several model magnetic systems, including quantum spin liquid candidates, we designed and implemented a continuous wave ESR spectrometer inside a dilution refrigerator reaching a base temperature of 30 mK. Microwave resonance structures, known as loop gap resonators (LGRs), were utilized for their flexible design and ability to achieve strong hybridization and filling factors even with samples of limited size. This experimental platform provides an opportunity to directly probe a material’s dynamic susceptibility by monitoring the cavity enhanced response, as previously outlined [3], and offers a new supplemental method to study strongly correlated spin dynamics.

References:

[1] Zhu-Xi Luo, Ethan Lake, Jia-Wei Mei, and Oleg A. Starykh, Spinon magnetic resonance of quantum spin liquids, Phys. Rev. Lett. 120, 037204 (2018); arXiv:1706.01597 (2017).

[2] K. Yu. Povarov, A.I. Smirnov, O.A. Starykh, S.V. Petrov, A. Ya. Shapiro, Modes of magnetic resonance in the spin liquid phase of Cs2CuCs4, - Phys. Rev. Lett. 107, 037204 (2011); arxiv:1101.5275 (2011).

[3] Yikai Yang, et al. "Cavity-Magnon-Polariton spectroscopy of strongly hybridized electro-nuclear spin excitations in LiHoF4." arXiv preprint arXiv:2309.05051 (2023).