Multifrequency ESR Characterization of the [VO(TPP)] Nanomagnet

[VO(TPP)], a promising vanadyl-based molecular nanomagnet qubit candidate, has been shown to have impressive coherence times T₂,^1,2 on the order of 1 μs. We employ a homebuilt electron-spin resonance (ESR) apparatus to characterize the system's behavior across a variety of frequencies in the microwave regime, using both continuous wave (cw) and pulsed ESR. Strong hyperfine coupling (A_z = 480 MHz) in this S = 1/2, I = 7/2 nanomagnet generates narrow hyperfine-split lines that we characterize with ESR in the Zeeman regime. Using the CPMG pulse sequence and diluting [VO(TPP)] to 2% in a nonmagnetic structural analogue, we have found T₂ to be greater than 8 μs at multiple frequencies for several ESR spin transitions. Clock Transitions provide an opportunity to further enhance coherence by minimizing the decohering behavior of dipolar fluctuations;³ the spin Hamiltonian for [VO(TPP)] suggests the presence of clock transitions in L-band and in the sub-GHz regime.¹ By mapping out the frequency-field relation using multifrequency ESR at low frequency, we seek to demonstrate a precision characterization of the spin Hamiltonian, locate avoided crossings, and investigate whether these crossings demonstrate enhanced coherence.

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

²C. Bonizzoni, et. al., npj Quantum Inf 6, 68 (2020).

³M. Shiddiq, et. al., Nature 531, 348–351 (2016).