Enhancing Coherence with Clock Transitions in Molecular Nanomagnets and Silica Defects

Clock transitions (CTs) in molecular nanomagnets (MNMs), which occur at avoided level crossings, enhance quantum coherence lifetimes T₂ because the transition becomes immune to the decohering effects of magnetic field fluctuations to first order. Using a custom frequency-tunable electron-spin resonance (ESR) spectrometer I examine two types of CTs at zero field, and demonstrate coherence enhancements at the CT in each system. In the first, a vanadyl-based S = 1/2, I = 7/2 molecular nanomagnet with strong hyperfine coupling (A_z = 480 MHz)¹, I observe a narrow-band CT at zero field in the sub-GHz regime, and demonstrate high-precision characterization of the spin Hamiltonian and avoided crossing generating the CT effect. Continuous wave (CW) ESR spectra near the CT clearly demonstrate an avoided crossing that exhibits Zeeman splitting as the frequency is tuned upward. Pulsed ESR reveals a coherence time approaching 2 μs at the CT. In another set of systems, defect-rich silica glasses, I observe CT behavior with coherence times up to 16 μs². In this system CT behavior exists across a broad frequency range of 3.5–5.5 GHz in borosilicate and aluminosilicate glasses, but not in a variety of other defect-rich silica-based glasses. Since boron and aluminum have the same valence and are acceptors when substituted for silicon, I suggest the observed CT behavior could be generated by a spin-1 boron vacancy center within the borosilicate glass, and similarly, an aluminum vacancy center in the aluminosilicate glass. In each system I examine the CT for remaining nonmagnetic limitations on coherence using multiple pulse sequences, as well as controlled variations of field and temperature, and provide insight into how these limitations on coherence can be used as part of a feedback loop for generating better MNMs through rational chemical design.

¹T. Yamabayashi et al., J. Am. Chem. Soc. 140, 12090-12101 (2018).

²B. C. Sheehan et al., Appl. Phys. Lett. 125, 254003 (2024).